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Advances in Biology and Therapy of Multiple Myeloma

Abstract

Even during this past year, further advances have been made in understanding the molecular genetics of the disease, the mechanisms involved in the generation of myeloma-associated bone disease and elucidation of critical signaling pathways as therapeutic targets. New agents (thalidomide, Revimid, Velcade) providing effective salvage therapy for end-stage myeloma, have broadened the therapeutic armamentarium markedly.

As evidenced in Section I by Drs. Kuehl and Bergsagel, five recurrent primary translocations resulting from errors in IgH switch recombination during B-cell development in germinal centers involve 11q13 (cyclin D1), 4p16.3 (FGFR3 and MMSET), 6p21 (cyclin D3), 16q23 (c-maf), and 20q11 (mafB), which account for about 40% of all myeloma tumors.

Based on gene expression profiling data from two laboratories, the authors propose 5 multiple myeloma (MM) subtypes defined by the expression of translocation oncogenes and cyclins (TC molecular classification of MM) with different prognostic implications. In Section II, Drs. Barillé-Nion and Bataille review new insights into osteoclast activation through the RANK Ligand/OPG and MIP-1 chemokine axes and osteoblast inactivation in the context of recent data on DKK1. The observation that myeloma cells enhance the formation of osteoclasts whose activity or products, in turn, are essential for the survival and growth of myeloma cells forms the basis for a new treatment paradigm aimed at reducing the RANKL/OPG ratio by treatment with RANKL inhibitors and/or MIP inhibitors.

In Section III, Dr. Fenton reviews apoptotic pathways as they relate to MM therapy. Defects in the mitochrondrial intrinsic pathway result from imbalances in expression levels of Bcl-2, Bcl-XL and Mcl-1. Mcl-1 is a candidate target gene for rapid induction of apoptosis by flavoperidol. Antisense oglionucleotides (ASO) lead to the rapid induction of caspace activity and apoptosis, which was potentiated by dexamethasone. Similar clinical trials with Bcl-2 ASO molecules alone and in combination with doxorubicin and dexamethasone or thalidomide showed promising results.

The extrinsic pathway can be activated upon binding of the ligand TRAIL. OPG, released by osteoblasts and other stromal cells, can act as a decoy receptor for TRAIL, thereby blocking its apoptosis-inducing activity. MM cells inhibit OPG release by stromal cells, thereby promoting osteoclast activation and lytic bone disease (by enhancing RANKL availability) while at the same time exposing themselves to higher levels of ambient TRAIL. Thus, as a recurring theme, the relative levels of pro- versus anti-apoptotic molecules that act in a cell autonomous manner or in the milieu of the bone marrow microenvironment determine the outcome of potentially lethal signals.

In Section IV, Dr. Barlogie and colleagues review data on single and tandem autotransplants for newly diagnosed myeloma. CR rates of 60%–70% can be reached with tandem transplants extending median survival to ~7 years. Dose adjustments of melphalan in the setting of renal failure and age > 70 may be required to reduce mucositis and other toxicities in such patients, especially in the context of amyloidosis with cardiac involvement.

In Total Therapy II the Arkansas group is evaluating the role of added thalidomide in a randomized trial design. While data are still blinded as to the contribution of thalidomide, the overriding adverse importance of cytogenetic abnormalities, previously reported for Total Therapy I, also pertain to this successor trial. In these two-thirds of patients without cytogenetic abnormalities, Total Therapy II effected a doubling of the 4-year EFS estimate from 37% to 75% (P < .0001) and increased the 4-year OS estimate from 63% to 84% (P = .0009).

The well-documented graft-vs-MM effect of allotransplants can be more safely examined in the context of non-myeloablative regimens, applied as consolidation after a single autologous transplant with melphalan 200 mg/m², have been found to be much better tolerated than standard myeloablative conditioning regimens and yielding promising results even in the high-risk entity of MM with cytogenetic abnormalities.

For previously treated patients, the thalidomide congener Revimid and the proteasome inhibitor Velcade both are active in advanced and refractory MM (~30% PR).

Gene expression profiling (GEP) has unraveled distinct MM subtypes with different response and survival expectations, can distinguish the presence of or future development of bone disease, and, through serial investigations, can elucidate mechanisms of actions of new agents also in the context of the bone marrow microenvironment. By providing prognostically relevant distinction of MM subgroups, GEP should aid in the development of individualized treatment for MM.

W. Michael Kuehl, MD,^* and P. Leif Bergsagel, MD

Multiple myeloma (MM), currently an incurable malignancy that often is preceded by premalignant monoclonal gammopathy of undetermined significance (MGUS), has a yearly incidence of nearly 14,000 in the US.¹ For both MGUS and MM, the incidence is markedly age dependent, about 2-fold higher in American blacks than Caucasians, and significantly higher in males.² The roles of genetic background and environment are poorly defined, although there may be clustering within families.³

MM Is a Plasmablast/Plasma Cell Tumor of Postgerminal Center B Cells
Most B cell tumors, including MM, involve germinal center (GC) or post-GC B cells⁴. Germinal center B cells uniquely modify their DNA through sequential rounds of somatic hypermutation and antigen selection, and also by immunoglobulin heavy chain (IgH) switch recombination. Post-GC B cells can generate plasmablasts (PB) that have successfully completed somatic hypermutation and IgH switching before migrating to the bone marrow (BM), where stromal cells enable terminal differentiation into long-lived plasma cells (PC). Although PC can be generated from either pre-GC or post-GC B cells, premalignant nonIgM MGUS and malignant MM are post-GC clonal tumors with phenotypic features of PB/PC, and are distributed at multiple sites in the bone marrow. A critical feature shared by MGUS and MM is the presence of a substantial tumor mass despite an extremely low rate of proliferation, usually with less than 1% of tumor cells synthesizing DNA until late stages of MM.⁵

Stages of Multiple Myeloma
There are a number of clinically defined stages for MM tumors.⁶ A clonal PC neoplasm must expand to about 10⁹ cells before it produces enough immunoglobulin to be recognized as a monoclonal Ig "spike" (M-Ig) by serum electrophoresis. For premalignant MGUS, which typically is asymptomatic and stable, the M-Ig is 3 g/dl and the tumor cells comprise no more than 10% of the mononuclear cells in the BM. However, depending on the level of M-Ig (a surrogate measure of the number of tumor cells), 0.6%–3% per year of patients with non-IgM MGUS progress to MM expressing the same M-Ig.⁷ At present, there are no unequivocal genetic or phenotypic markers that distinguish MGUS from MM tumor cells, so it is not possible to predict if and when a particular MGUS tumor will progress to MM.⁴ Also, it remains unclear to what extent intrinsic genetic or epigenetic changes in the MGUS tumor cell versus extrinsic changes in nontumor cells (e.g., immune cells) are responsible for this progression. Primary amyloidosis, which accounts for about 4000 deaths per year in the United States, appears to represent premalignant MGUS that is symptomatic because of pathological deposits of the M protein (generally the intact or fragmented Ig light chain) in various tissues.⁸ MM is distinguished from MGUS by having a BM tumor cell content of > 10%. Smoldering MM (SMM), which has a stable intramedullary tumor cell content of > 10% but no osteolytic lesions or other complications of malignant MM, has a high probability of progressing to frankly malignant MM, which is distinguished by having osteolytic bone lesions and/or an increasing tumor mass. Further progression of MM is associated with increasingly severe secondary features (lytic bone lesions, anemia, immunodeficiency, renal impairment), and in a fraction of patients, the occurrence of tumor in extramedullary locations. Extramedullary MM is a more aggressive tumor that often is called secondary or primary plasma cell leukemia, depending on whether preceding intramedullary myeloma has been recognized. Human MM cell lines (HMCL) can sometimes be generated, but usually only from extramedullary tumors.

Oncogene Dysregulation by Ig Translocation: A Hallmark of B Cell Tumors
A seminal event in most kinds of B cell tumors (chronic lymphocytic leukemia being a major exception) is dysregulation of an oncogene that is juxtaposed near a strong Ig enhancer as a result of translocation to the IgH locus (14q32), or (less often) to an IgL locus (, 2p11, or , 22q11).⁹ These translocations appear to be mediated mainly by errors in 1 of 3 B cell specific DNA modification mechanisms: V(D)J recombination that occurs during early B cell development, IgH switch recombination, and somatic hypermutation, the latter 2 processes occurring mainly in germinal center B cells. The mechanism(s) by which double-stranded DNA breaks are generated in the partner chromosome are poorly understood. However, for many kinds of B cell tumors, there is a consistent Ig translocation that involves only 1 major partner (e.g., cyclin D1 at 11q13 in mantle cell lymphoma or bcl-2 at 18q21 in follicular lymphoma).

Translocations involving either an IgH or IgL locus are frequent in myeloma
The combination of karyotypic complexity, an inability to efficiently perform conventional cytogenetics on low proliferative tumors, and the telomeric location of some translocation partners delayed the identification of Ig translocations in MGUS and MM. An important initial step in solving this problem was the identification and cloning of IgH translocation breakpoints in HMCL.¹⁰ The subsequent application of interphase fluorescence in situ hybridization (FISH) using specific genetic probes to identify karyotypic abnormalities even in nondividing cells that have a PC phenotype enabled analysis of primary MGUS and MM tumors.^11,12 Many recent studies have shown that many MM tumors have an IgH translocation that nonrandomly involves one of many potential chromosomal partners (for example, Bergsagel and Kuehl,¹⁰ Avet-Loiseau et al,¹³ and Fonseca et al^14,15). The prevalence of IgH translocations varies with the stage of disease: 46%–48% in MGUS or SMM, 55%–73% in intramedullary MM, 85% in primary plasma cell leukemia, and > 90% in HMCL. Limited studies indicate a much lower prevalence of Ig translocations: 11% in MGUS, 17% in advanced MM tumors, and 23% in HMCL.^4,15 The prevalence of Ig translocations is even lower, and appears to be no more than 2%–3% based on studies of advanced MM tumors and HMCL.^16,17 Importantly, although all 34 HMCL fully analyzed have either an IgH or Ig translocation, nearly 50% of MGUS tumors and at least 26% of advanced MM tumors have neither an IgH nor an IgL translocation.^4,15

Recurrent Chromosomal Partners for Ig Translocations
Apart from c-myc at 8q24 (see below), there are 5 well-defined recurrent chromosomal partners (oncogenes) that are involved in IgH translocations in MGUS and MM: 11q13 (cyclin D1), 4p16.3 (FGFR3 & MMSET), 6p21 (cyclin D3), 16q23 (c-maf), and 20q11 (mafB), with at least the latter 3 also involved in Ig translocations.¹⁰ Together the combined prevalence of these 5 IgH translocation partners is about 40%, with approximately 15% 11q13, 15% 4p16, 3% 6p21, 5% 16q23, and 2% 20q11.^4,13,16–19 The t(4;14) translocation is unusual in that it appears to dysregulate 2 potential oncogenes, FGFR3 on der (14) and MMSET on der (4), but FGFR3 on der (14) is lost or not expressed in about 20% of MM tumors that have a t(4;14) translocation.^20–22 There is conflicting data regarding the issue of whether the prevalence of t(4;14) and t(14;16) translocations is the same or much lower in MGUS/SMM compared to MM. However, the apparently lower incidence of 4p16 and/or 16q23 in MGUS/SMM compared to MM may be due to these translocations resulting in de novo MM without preceding MGUS, or a more rapid progression of MGUS to MM, an hypothesis supported by the fact that patients with translocations involving 4p16 or 16q23 have an extremely poor prognosis.^13,14,21,23

Primary Versus Secondary Translocations in MM
Primary translocations occur as early and perhaps initiating events during tumor pathogenesis, whereas secondary translocations occur as progression events.¹⁰ For B cell tumors, most primary translocations appear to be simple reciprocal translocations that juxtapose a partner chromosomal locus (and oncogene) and one of the Ig enhancers, and usually are mediated by 1 of the 3 B-cell–specific DNA modification mechanisms described above. For MM, most translocations involving the 5 recurrent translocation partners described above appear to be primary translocations that occurred from errors in IgH switch recombination (possibly less often errors in somatic hypermutation) during B cell development in germinal centers. Occasionally, however, it appears that secondary translocations might involve 1 or more of these 5 recurrent partners^4,15–17 (C Cultraro and A Gabrea, unpublished). In addition, there are rare tumors that have independent translocations involving 2 of these 5 recurrent partners; all combinations have been documented except translocations that involve both 6p21 and 11q13^4,15 (C Cultraro and A Gabrea, unpublished). For normal PC and PC tumors, it appears that B-cell–specific DNA modification mechanisms are inactive. Therefore, unless one of these mechanisms could be reactivated, secondary translocations in PC tumors would be mediated by other kinds of recombination mechanisms that do not specifically target the Ig loci but could involve an Ig locus. In contrast to primary translocations, secondary translocations usually are complex, unbalanced translocations or insertions, often involving 3 different chromosomes and sometimes with associated inversion, deletion, duplication, or amplification. Primary translocations should be present in all tumor cells in both MGUS and MM, whereas secondary translocations are expected to be less frequent in MGUS than in MM, and might be present in only a subset of MGUS or MM tumor cells. Obviously, however, the only definitive way to distinguish primary from secondary translocations would be to document the time(s) at which translocations occur during the progression of individual tumors. In the absence of this definitive test, the criteria described above provide some help in distinguishing primary from secondary translocations.

Dysregulation of Myc: A Paradigm for Late Secondary Translocations in MM
Similar to other kinds of B cell tumors, translocations that dysregulate c-myc represent an important pathogenic event in MM.⁴ Chromosomal translocations that dysregulate c-myc by juxtaposing it with one of the three Ig loci represent an essentially invariant and apparently primary event in human Burkitt’s lymphoma and murine plasmacytoma tumors. The nontranslocated c-myc allele is not expressed, corresponding to the absence of c-myc expression, in resting germinal center B cells and terminally differentiated plasma cells. Strikingly, L-myc (1 HMCL) or 1 c-myc allele is expressed selectively in all informative HMCL, consistent with cis-dysregulation of L-myc or 1 c-myc allele in all HMCL. In addition, by our analysis of published^24,25 gene expression profiling, N-myc (which is not expressed in normal PB or PC) is expressed in 2 of 82 primary MM tumors. Three-color FISH analyses of metaphase chromosomes show that nearly 90% of HMCL and 50% of advanced MM tumors have similar karyotypic abnormalities involving c-myc, L-myc (1 HMCL), or N-myc (1 tumor). Simple, reciprocal t(8;14) and t(8;22) translocations are infrequent. Most karyotypic abnormalities are complex translocations and insertions that often are nonreciprocal, and frequently involve 3 different chromosomes. Karyotypic abnormalities involving c-, L-, or N-myc often do not include association with an Ig enhancer, which suggests that secondary translocations can dysregulate c-myc by juxtaposition to non-Ig enhancers. By interphase FISH analyses, it is reported that the c-myc locus is rearranged in 3% of MGUS/SMM tumors, 10% of MM tumors with a low tumor mass, and 19% of MM tumors with a high tumor mass (ß2 microglobulin > 3), and frequently is heterogeneous within a tumor.²⁶ Cloned t(8;14) translocation/insertion breakpoints usually do not occur at the IgH sites targeted by the 3 B-cell–specific DNA modifications. All of these results support a model for MM in which dysregulation of c-, L-, or N-myc occurs as a late progression event that is mediated by secondary translocations not involving the 3 B-cell–specific DNA modification mechanisms.

Promiscuous Partners for Ig Translocations
Interphase FISH studies suggest that approximately 20% of MM tumors have IgH translocations not involving 1 of the recurrent loci cited above.^13,27 Similarly, spectral karyotypic (SKY) analyses of metaphase chromosomes from 150 advanced MM tumors show that 15% of tumors have IgH translocations that do not involve a myc gene or one of the five recurrent partners described above.^16,17 Most of these novel chromosomal loci are involved in only one MM tumor, and none are involved in more than three of the 150 MM tumors analyzed. It is unclear to what extent these promiscuous partners are generated by primary translocations versus secondary translocations. However, it is notable that IgH translocations involving the recurrent 5 chromosomal loci are found mainly in nonhyperdiploid tumors, whereas translocations involving other partners are represented to a similar extent in hyperdiploid and nonhyperdiploid tumors.^16,17,27 More importantly, the identity and significance of the promiscuous partners (with the exception of IRF-4 at 6p25) remain enigmatic.

Dysregulation of Cyclin D1, 2, or 3: A Possible Unifying Oncogenic Event in MM
In terms of proliferation, MGUS and MM seem closer to normal, nonproliferating PC than to normal, but highly proliferating PB, for which 30% or more of the cells can be in S phase. Surprisingly, however, our analysis⁴⁶ of combined gene expression profiling data from two laboratories^24,25 shows that the expression level of cyclin D1, cyclin D2, or cyclin D3 mRNA in MM (and MGUS based on analysis of a limited number of samples) is relatively high, comparable to the levels of cyclin D2 mRNA expressed in normal proliferating PB, and distinctly different from normal PC. Normal hematopoietic cells, including normal B lymphocytes and PB, predominantly express cyclin D2, usually together with lower levels of cyclin D3, but do not express cyclin D1 (reviewed in Shaughnessy et al²⁸). Given the lack of cyclin D1 expression in normal lymphocytes and the occurrence of Ig translocations that dysregulate cyclin D1 or cyclin D3 in a subset of MM tumors it seems apparent that virtually all MM tumors dysregulate at least 1 of the cyclin D genes. From cyclin D1 transgenic mice and cyclin D transfection experiments, it is known that overexpression of cyclin D is insufficient by itself to cause cell cycle progression. Instead, it has been suggested that the overexpression of cyclin D renders a cell more sensitive to growth activating signals and/or less sensitive to growth inhibitory signals.²⁹ The apparent universal enhanced expression/dysregulation of one of the cyclin D genes in low proliferative MM tumors seems consistent with what is known about the effect of dysregulated cyclin D expression in these model systems.

Five MM Subtypes Defined by Expression of Translocation Oncogenes and Cyclin D
In addition to determining the expression level of cyclin D1, 2, and 3, gene expression profiling can effectively identify MM tumors that overexpress the oncogenes dysregulated by the 5 recurrent IgH translocations: 11q13 (cyclin D1); 6p21 (cyclin D3); 4p16 (MMSET and usually FGFR3); 16q23 (c-maf); and 20q11 (mafB)²⁵ (and unpublished Affymetrix Hu95Av2 and Hu133A+B data from J. Shaughnessy lab). Unsupervised hierarchical cluster analysis of microarray gene expression profiles (Affymetrix Hu95Av2) confirms that there are a minimum of 5 distinct groups, with only limited overlap of the different groups. These groups (Table 1) can be distinguished based on the Ig translocation present, and cyclin D expression (TC classification): Group TC1 tumors (18%) express high levels of either cyclin D1 or cyclin D3 as a result of an Ig translocation; Group TC2 tumors (43%) ectopically express low to moderate levels of cyclin D1 despite the absence of a t(11;14) translocation; Group TC3 tumors (17%) are a mixture, with most expressing cyclin D2, but a few expressing only very low levels of cyclin D2 and/or cyclin D3; Group TC4 tumors (15%) express high levels of cyclin D2, and also MMSET (and in most cases FGFR3) as a result of a t(4;14) translocation; Group TC5 tumors (7%) express the highest levels of cyclin D2, and also high levels of either c-maf or mafB, consistent with the possibility that both maf transcription factors up-regulate the expression of cyclin D2.

Table 2

Table 2

Figure 1

View larger version (63K): [in this window] [in a new window]   Figure 1. Two pathways of generating monoclonal gammopathy of undetermined significance (MGUS)/multiple myeloma (MM) tumors. Tumors can be generated either with (+) or without (-) the occurrence of a primary translocation, the former being mainly nonhyperdiploid and the latter mainly hyperdiploid. Dysregulation of a cyclin D gene appears to be an early event in each case. Secondary translocations can occur in both kinds of tumors. As indicated, tumors in the former pathway selectively generate stromal independent MM cell lines, but the dashed arrows suggest the possibility that the latter pathway might generate cell lines.

Figures 1

Molecular Phenotypes Predict Prognosis and Response to Existing Therapies
In addition to tumor mass and secondary features that represent a host response to malignant MM (anemia, bone disease, immunodeficiency, etc.), intrinsic properties of the tumor cell are also informative in predicting prognosis and response to existing therapies. For example, it has been well documented that an unfavorable outcome is associated with each of the following: increased plasma cell labeling index, the generation of tumor cells with an abnormal karyotype (perhaps a surrogate for increased proliferation), hypodiploidy compared to hyperdiploidy, monosomy of chromosome 13/13q, and monosomy of chromosome 17/deletion of p53.^5,14,27,38 It also has been reported but not confirmed that activating mutations of K-Ras (but not N-Ras) represent an adverse prognostic factor.³³ More recently, it has become clear that specific IgH translocations, which represent early if not initiating events in tumorigenesis, also have a profound prognostic significance^14,23 (Table 1). In particular, patients with tumors that have a t(4;14) translocation (TC4) have a substantially shortened survival with either standard or intense therapy, and patients with a t(14;16) (TC5) have a similarly poor if not worse prognosis. By contrast, patients with tumors that have a t(11;14) translocation (TC1) appear to have a marginally better survival following conventional chemotherapy but apparently a remarkably better response to intense therapy. These results suggest that the TC classification, based on translocation and cyclin D expression, and supported by hierarchical cluster analysis, may be a clinically useful way to classify patients into groups that have distinct subtypes of MM (and MGUS) tumors. In practice it would be very hard to get sufficient patients with either 6p21 or 20q11 translocations for a meaningful analysis. Therefore, given the similar pathogenesis and shared pattern of gene expression for tumors with 11q13 and 6p21 (TC1) and tumors with 16q23 and 20q11 (TC5) we suggest that in each case the patients be considered as part of the same group.

Identification of Novel Therapeutic Strategies
The critical role of cyclin D dysregulation in the pathogenesis of MM highlights the importance of the cyclin D/RB pathway, and suggests that there may be a therapeutic window in targeting this pathway³⁹ for all molecular subtypes of MM. For example, epigenetic silencing of CDK inhibitor (INK4A) mRNA expression might be reversed by histone deacetylase inhibitors (SAHA, depsipeptide), or inhibitors of DNA methyl transferase (5 aza-2'deoxy-cytidine).⁴⁰ To target cyclin D per se, there are a number of possible strategies including modulation of mRNA translation, posttranslational modifications, enzyme function, and perhaps even expression of cyclin D mRNA. First, the cyclin D mRNA is under strict translational control, and agents that inhibit the translation have been identified (e.g., desferroxamine, eicosapentaenoic acid).^41,42 Second, cyclin D and many other cell cycle regulated proteins are posttranslationally regulated by ubiquitination and proteasomal degradation, which might provide another therapeutic target.^43,44 Third, important steps downstream of cyclin D are dependent on kinases (CDKs) that may be targeted by selective kinase inhibitors.^29,39 Finally, as suggested above, it is particularly intriguing that the TC2 group, which includes nearly half of MM tumors, may be absolutely dependent on an interaction with BM stromal cells for the ectopic expression of cyclin D1 that appears to be a critical property for these tumor cells. Clearly we need to elucidate the mechanism that is responsible for the ectopic expression of cyclin D1 in the TC2 group, but there may already be data with new emerging therapies that will help to address this issue. Recently, there have been promising therapeutic results with both thalidomide and its derivatives, and also bortezomib (PS-341), a proteasome inhibitor.⁴⁵ Both kinds of treatment appear to target the MM tumor cells but also the interaction of tumor cells with the bone marrow microenvironment, and in each case an as yet undefined subset of patients may be more responsive. It is possible, for example, that the TC2 subgroup is selectively targeted. In any case, for this and for other treatments, it obviously is critically important to determine the response among the different TC groups, and to study the changes in protein levels of cell cycle proteins induced by treatment.

Additional specificity may be achieved by targeting the genes directly dysregulated. This seems to be especially true in the case of the t(4;14) where two enzymes are overexpressed: FGFR3, a tyrosine kinase receptor, and MMSET, which has homology to histone methyltransferases. As a surface receptor, FGFR3 may be targeted by monoclonal antibodies, and as a tyrosine kinase, by selective tyrosine kinase inhibitors. Preclinical studies have validated FGFR3 as a therapeutic target in t(4;14) MM, and plans for a clinical trial are under way (S Trudel and L Bergsagel, unpublished). Histone methyltransferases are being developed, and studies are under way to validate MMSET as a target in MM.

Concluding Thoughts
The past year has seen the synthesis of a number of different observations into a clearer picture of the molecular pathogenesis of MM. There appear to be two pathways of pathogenesis that are associated with specific cytogenetic features,^27,31 one that is hyperdiploid, and usually lacks primary Ig translocations (TC2 and perhaps some TC3), and one that is nonhyperdiploid and has primary Ig translocations (TC1, TC4, TC5, and perhaps some TC3). Subtypes are identified on the basis of the Ig translocation and cyclin D expression (TC classification). This dichotomy between hyperdiploidy and translocation superficially appears very similar to what is seen in ALL,⁴⁷ and the analogy deserves further scrutiny. The TC classification identifies clinically important molecular subtypes of MM with different prognoses and with unique responses to different treatments (e.g., high-dose therapy (HDT) and 11q13-TC1, FGFR3 inhibitor and 4p16-TC4). Prospective studies will be required to validate this classification for use in clinical trials. Almost all of the preclinical studies of new agents in MM are based on the induction of apoptosis at 24 or 48 hours in 1 or 2 cell lines (primarily 8226 and MM.1, both with 16q23 translocations— TC5). It now is clear that there are a minimum of 5 different molecular subtypes of MM, and an effort should be made to include the more common ones in preclinical studies (e.g., at a minimum, U266 for 11q13-TC1, H929 for 4p16-TC4, and 8226 or MM.1 for maf-TC5). It also is obvious that we desperately need a model for the large group of patients (TC2) for which we do not appear to have representative cell lines. For the reasons stated above, this may depend on developing models that are able to reproduce or replace critical features of the host BM microenvironment. Importantly, we need to gain a more comprehensive understanding of the molecular phenotype of each subtype of MM tumor, including identification of changes that remain essential for survival and growth of the tumor, thereby representing potential therapeutic targets. Although much remains to be learned, it seems essential that we rapidly incorporate what we have already learned about the molecular biology of myeloma into the clinical arena.

II. NEW INSIGHTS IN THE BIOLOGY AND TREATMENT OF MYELOMA-INDUCED BONE DISEASE

Sophie Barillé-Nion, PhD, and Régis Bataille, MD, PhD^*

MM is a disorder in which malignant plasma cells accumulate in the BM and usually produce a monoclonal immunoglobulin (Ig) of IgG or IgA type. MM is responsible for about 1% of all cancer-related deaths in Western countries and epidemiological studies have shown that at least one third of MM emerge from a preexisting benign monoclonal plasma cell disorder, i.e., MGUS.¹ One prominent feature in MM is the occurrence of skeletal events including bone pain, pathological fractures secondary to lytic bone lesions, and hypercalcemia. Up to 80% of patients with MM present with bone pain, and over 70% of the patients will develop pathologic fractures during the course of their disease. Bone lesions and hypercalcemia correlate directly with the presence of the total mass of myeloma cells and have prognostic value. The excessive bone resorption is an early symptom in MM and a hallmark of malignancy in individuals with MGUS.²

Bone Disease in MM
Myeloma cells grow in the BM, where the microenvironment supports their growth and protects them from apoptosis. The accumulation of myeloma cells within the BM is associated with increased rates of bone turnover. Histomorphometric analyses of bone biopsies from patients with MM have shown that an unbalanced bone remodeling formation was the characteristic feature of patients with osteolytic bone lesions, which on one hand increased osteoclastic resorption and on the other hand lowered bone formation.^3–5

Increased osteoclastic resorption
A significant increase both in the recruitment of new osteoclasts and in single osteoclast activity occurs in the close vicinity of myeloma cells, suggesting that bone disease results from local production of an osteoclast activating factor (OAF) secreted by either myeloma cells or BM stromal cells. Recently, two kinds of factors have been identified as such OAF: RANKL/OPG system and the chemokine macrophage inflammatory protein-1 (MIP-1).

RANKL/OPG:Evidence from gene-deleted and transgenic mice indicates that generation of activated osteoclasts from monocytic precursors is controlled by coordinate expression of the RANKL (receptor activation of NF-B ligand, also known as TRANCE, OPGL, TNFSF11) and its decoy receptor osteoprotegerin (OPG, TNFRSF11B). RANKL is expressed by osteoblastic cells and binds to its receptor (RANK) present on osteoclastic cells triggering differentiation and activation signals in osteoclast precursors, thereby promoting bone resorption.⁶ Osteoprotegerin (OPG) is a naturally occurring factor that antagonizes the effects of RANKL, thereby preserving bone integrity.⁷ Therefore, the ratio between RANKL and OPG (RANKL/OPG) is determining to regulate osteoclast activity and bone resorption. Moreover, it has been suggested that the interactive network of bone-resorbing and antiresorptive cytokines and hormones converges at the RANKL/OPG system. RANKL/OPG then serves as the final common effector system to regulate osteoclast formation from precursors in the BM and its subsequent activation. Because the RANKL/OPG system is likely to play a pivotal role in the control of bone resorption, this axis was evaluated in MM-induced osteolysis. Recent studies have shown that myeloma cells are able to induce increased RANKL expression and decreased OPG production in the BM environment.^8–10 First, an overexpression of RANKL has been observed in BM biopsies from patients with MM. RANKL is overexpressed in stromal cells at the interface of MM with normal BM elements, rather than in myeloma cells. RANKL also may be produced by myeloma cells in some patients as described by Heider et al.¹¹ Of note, in contrast to human myeloma cell lines, RANKL was detected in the murine myeloma cell line 5T2MM.¹² In vitro coculture experiments have indicated that myeloma cells were able to induce RANKL expression in stromal/osteoblastic cells in part through cell-to-cell contact involving the integrin VLA-4 and in part through a soluble factor. Several OAF, including IL-1ß, IL-6, and tumor necrosis factor (TNF)-, have been reported as overproduced by stroma in response to MM. However, the RANKL overexpression seems unrelated to these cytokines since the addition of blocking antibodies against IL-1ß, IL-6, and TNF- in cocultures did not prevent RANKL upregulation.⁸ The soluble RANKL-inducing factors involved in MM are still unidentified but may implicate IL-7, which is produced by myeloma cells.¹³ Such abnormalities of RANKL overexpression in bone environment participate in the pathogenesis of various osteolytic diseases especially osteolytic metastasis in breast cancer, in which the parathyroid hormone (PTH)-related peptide plays a major role.¹⁴ In addition to increased expression of RANKL, MM-infiltrated BM exhibit decreased production of the natural RANKL inhibitor OPG. Two mechanisms have been involved in that process. First, a decrease of OPG production by stromal cells has been described as induced by MM cells.^8,9 Second, myeloma cells sequestrate OPG, internalize, and degrade this factor within the lysosomal compartment. This process is dependent on physical interactions between OPG and heparane sulfates present on syndecan-1 highly expressed on myeloma cells.¹⁵ Both mechanisms may contribute to low local and systemic OPG levels observed in patients with MM.^8,16 In summary, inhibition of OPG production at both transcriptional and posttranscriptional levels by myeloma cells associated with increased expression of RANKL in BM deeply disrupt RANKL/OPG ratio in favor of the osteoclastogenic factor RANKL. Finally, the main role of RANKL/OPG axis deregulation in MM-induced osteolysis is highlighted by the high potency of RANKL inhibitors such as OPG or RANK-Fc to prevent both excessive osteoclast development and lytic bone lesion occurrence in different murine myeloma models.^9,12,17 Furthermore, as discussed below, disruption of RANKL/OPG axis may promote tumor progression, since treatment of mice with RANKL antagonists decreased tumor burden.

Chemokines MIP-1: Two different groups have recently shown that the chemokines MIP-1 and -ß significantly participate in myeloma-induced bone disease. The first group found that MIP-1 was overproduced in myeloma BM¹⁸ and the second that both MIP-1 and MIP-1ß were secreted by myeloma cells.¹⁹ The chemokines MIP-1 belong to the RANTES family and act as chemoattractants and activators of monocytes. Both osteoclast precursors and stromal cells express the chemokine receptor for MIP-1 and MIP-1ß (CCR5). Data demonstrated that MIP-1 as well as MIP-1ß induce expression of RANKL in stromal cells and consequently enhance osteoclast formation and resorbing activity. In line with these results, administration of neutralizing anti-MIP-1 antibodies to 5TGM1 myeloma–bearing mice limited development of osteolytic lesions and intact RANK/RANKL signaling is necessary since MIP-1 had no effect in RANK null mutant mice.²⁰ Choi et al²¹ recently cloned the human MIP-1 promotor and characterized the transcription factor (TF) that controls MIP-1 expression in MM cells. They reported that the ratio of both alternatively spliced variants of the TF acute myeloid leukemia-1 (AML-1), AML-1A and AML-1B, regulates MIP-1, and that abnormal expression of these TF in MM correlates with increased MIP-1 expression. The clinical correlation between severity of bone lesions and MIP-1 production by MM cells corroborates these results. In addition, the gene expression profile study of 92 primary MM indicated that the MIP-1 gene is overexpressed in osteolytic MM.²² Furthermore, because the chemokine receptor, CCR5, is also expressed by MM cells, MIP-1 and MIP-1ß may act on MM cells in an autocrine paracrine fashion. In fact, it has been recently shown that MIP-1 triggers migration and signaling cascades mediating survival and proliferation in MM cells.²³ In addition to their osteoclast-inductive capacity, MIP-1 and MIP-1ß have other biologic activities that may be relevant to clinical features of patients with MM. In fact, these chemokines have been suggested to be potent modulators of hematopoiesis: MIP-1 inhibited early erythropoiesis²⁴ and MIP-1ß increased apoptosis in pre-B cells.²⁵ Therefore, MIP-1 and -ß are pluripotent chemokines that may play important roles in the pathogenesis of several clinical features of MM including not only destructive bone lesions, but also suppression of erythropoiesis, of B lymphopoiesis and of immunoglobulin production.

Direct interaction with osteoclasts: Several studies have demonstrated that myeloma cells enhanced osteoclast formation and activity through osteoblastic cells (i.e. RANKL and MIP-1). Moreover, studies on mice demonstrate the dependence of myeloma cells on osteoclast activity and highlight the importance of the myeloma-osteoclast loop for sustaining the disease process. But direct interactions between myeloma cells and osteoclasts remain unclear. Like this, the chemokine MIP-1, in addition to acting through osteoblastic cells to enhance osteoclast activity, may also be a potent osteoclastogenic factor that acts directly on osteoclast precursors that express CCR5 to induce late stage differentiation.¹⁸ Furthermore, the gene coding for Gas6, the ligand for the receptor tyrosine kinase Tyro-3, is overexpressed in plasma cells.²⁶ As Tyro-3 is expressed on mature osteoclasts and involved in stimulation of osteoclastic bone resorption,²⁷ overproduction of its ligand by plasma cells may be at the origin of strong direct interactions between myeloma cells and osteoclasts. These observations suggest that an interdependence could truly exist between myeloma cells and osteoclasts but further data are needed to sustain this hypothesis.

Decreased bone formation
Histomorphometric studies and biochemical indicators of bone turnover in MM have shown that although osteoclast number and function are increased in MM, the key difference in vivo between the presence and absence of lytic lesions is that osteoblasts are fewer and less active in patients with lytic lesions. In the early stages of MM, bone formation is increased reflecting the coupling of resorption to formation. However, as the disease progresses, bone formation is decreased and this leads to an uncoupling resorption and formation and rapid bone loss.⁵ This suggests that myeloma cells could first stimulate osteoblastic function during the early stages of the disease then inhibit it or even be toxic for these cells during overt expansion of the tumor. Few inhibiting interactions between osteoblasts and MM have been described so far. Recently, Shaughnessy et al²⁸ reported the production of the potential osteoblast inhibitor DKK1 by myeloma cells. Actually, DKK1 can block Wnt signaling, an important pathway involved in osteoblast differentiation and function, and its overexpression in MM is associated with lytic bone disease. However, further experiments need to be done to confirm these data. Other potential means for the interplay between osteoblasts and myeloma cells could be through homophilic binding by the neural cell adhesion molecule NCAM/CD56. On the one hand, NCAM is known to be overexpressed by MM cells mainly of kappa subtype,²⁹ in correlation with the presence of lytic bone lesions.³⁰ Conversely, the lack of or weak expression of NCAM by MM cells delineates a subset of MM at diagnosis mainly characterized by a lambda light chain subtype, a lower osteolytic potential and a trend for malignant cells to circulate in the peripheral blood.³¹ Of note, NCAM is also strongly expressed by human osteoblasts.^30,32 Thereby, NCAM-NCAM homophilic binding between CD56⁺/NCAM-positive MM cells and osteoblasts may induce a decrease in osteoblast function as we previously described for osteocalcin production.³² On the other hand, such negative interactions lacked in CD56^-/NCAM negative MM.

Biochemical Markers of Bone Turnover
Classic biochemical markers of bone turnover remain poor predictive parameters in MM. Therefore, as RANKL/OPG axis and MIP-1 are particularly involved in the biology of bone resorption, their study as bone markers could be useful. First, median OPG serum levels were lower in patients with MM at the time of diagnosis than in healthy age- and sex-matched controls. Moreover, OPG levels were correlated with serum levels of carboxy-terminal propeptide of type I procollagen (PICP bone formation marker) but not with clinical stage or survival.^16,33 Second, serum levels of sRANKL were elevated in patients with MM and correlated with bone disease. The ratio sRANKL/OPG was also increased and correlated with markers of bone resorption (TRACP-5b, NTX), osteolytic lesions, and markers of disease activity (ß2-microglobulin but not CRP).³⁴ Furthermore, Abe et al found that MIP-1 production by MM cells in BM correlated with the severity of bone lesions.¹⁹ In conclusion, these markers may have a clinical utility.

New Therapeutical Approaches in Myeloma-Induced Osteolysis
The development of lytic bone lesions is a major cause of morbidity in patients with MM. However, the therapeutic arsenal available to control excessive bone resorption remains insufficient, despite the emergence of new bisphosphonates. The recent description of the RANKL/OPG system and its main role in bone remodeling regulation has opened new avenues in the therapeutic approach of excessive bone resorption. In the close future, MIP-1 inhibitors could also represent a new effective therapeutical target to treat MM-induced bone disease. The 5T mouse model using the murine MM lines (5T2, 5T33, and its derived subclone 5TGM1) and the SCID-hu model have provided important tools for validation of in vitro observations and for preclinical studies. MM growth in the BM microenvironment was observed together with the appearance of osteolytic destruction of the human bone. The effect of potential antiresorptive drugs on bone disease and tumor growth has been evaluated in these models. These studies demonstrate that using bisphosphonates or blocking RANKL and the MIP-1 axis strongly affect not only bone resorption but also tumor development.

Bisphosphonates
Bisphosphonates, which are potent inhibitors of bone resorption, are widely used in MM-associated hypercalcemia. Placebo-controlled studies, generally including patients with stage III MM, have shown that bisphosphonates, mainly clodronate, pamidronate, and zoledronate, contribute to the long-term control of bone disease.^35–39 They reduced the incidence of skeletal events, prevented hypercalcemia, alleviated bone pain, and improved the patient’s quality of life. But they neither induced bone lesion healing nor improved the survival of patients, certainly because of their use at a too advanced stage of the disease (Table 3). Even though in vitro studies demonstrated that nitrogen-containing bisphosphonates induce apoptosis using human MM cell lines⁴⁰ and in vivo use of pamidronate or zoledronate in the SCID-hu model and zoledronate in 5T2^17,41 halted MM bone resorption and decreased tumor burden, there is no proof so far that bisphosphonates really improve survival in vivo in patients. Interestingly, it has been recently shown that both pamidronate and zoledronate stimulate OPG production by primary human osteoblasts.⁴² These observations strongly argue for the early use of bisphosphonates in MM to prevent bone disease and slow down tumor development.

MIP-1 inhibitors
Because the chemokines MIP-1 and MIP-1ß on the one hand are among the leading candidates for MM-derived factors that enhance osteoclast differentiation and function and on the other hand may also be involved in the development of major clinical features of MM such as anemia and hypogammaglobulinemia, inhibition of their production or activities could be a novel and powerful therapeutic target in MM. Such inhibitors could come from antibodies blocking MIP-1 chemokines or their receptors, i.e., small-molecule nonpeptide receptor (CCR5) antagonists or modified chemokines.⁴⁹

Conclusion
RANKL and OPG play an essential role in osteoclast formation and activation, and various bone tumors act through that system to trigger bone resorption. As described in this review and summarized in Figure 3, the interaction of MM with stroma results in deregulation of the RANKL/OPG axis, both in increasing RANKL and decreasing OPG. This disruption of the RANKL/OPG ratio in the bone environment increases osteoclast activity, triggers bone destruction, and promotes tumor growth. Moreover, the chemokines MIP-1 and MIP-1ß produced by myeloma tumor also enhance osteoclast activity both through RANKL expression in bone environment and direct effect on osteoclast precursors leading to increased bone resorption. Finally, in vivo use of osteoclast inhibitors (bisphosphonates or specific inhibitors of RANKL) halted MM-induced bone resorption and resulted in inhibition of myeloma growth and survival. These observations demonstrate a strong interdependence between myeloma cells and osteoclasts: myeloma cells enhance the formation of osteoclasts, whose activity or products, in turn, are essential for the survival and growth of myeloma cells. In line with this concept, a recent study has shown that IL-6 and osteopontin highly produced by osteoclasts played a central role in survival and growth of myeloma cells.^50–52 Indeed, the use of effective osteoclast inhibitors in vivo could break down this vicious circle and both suppress bone resorption and decrease tumor growth. It is tempting to speculate that interfering with BM cultivation by myeloma cells may inhibit the development of myeloma especially in early or premalignant stages (MGUS). Altogether, these observations strongly suggest that reducing the RANKL/OPG ratio by treatment with RANKL inhibitors and/or MIP inhibitors should provide a high therapeutic interest to decrease both bone resorption and tumor burden in MM.

View larger version (26K): [in this window] [in a new window]   Figure 3. Biological mechanisms involved in the excessive bone resorption in multiple myeloma (MM). (A) MM tumor induces enhanced osteoclast differentiation and activity directly and through stromal cells/osteoblasts and (B) decreases bone formation.

Robert G. Fenton, MD, PhD^*

Cancer cells arise through the progressive acquisition of mutations that deregulate cell cycle checkpoints, inactivate DNA repair mechanisms, disrupt apoptosis pathways, and alter host-tumor interactions allowing invasion and metastasis.¹ Because of the acquisition of genetic lesions that perturb normal cell physiology, the hypothesis has been put forth that tumor cells differ from normal cells in their apoptotic burden.² In normal cells, genetic abnormalities are "sensed," and signaling pathways are activated that lead to cell cycle arrest or the induction of apoptosis. The "rheostat"’ that regulates activity of the core cell death machinery (i.e., the threshold of stimuli required to induce apoptosis) is set at a low level to ensure that renegade cells with genetic lesions conferring a growth advantage are destroyed. As tumor cells evolve, they acquire (through the process of mutation and selection) a myriad of mechanisms enabling them to survive even in the face of death signals that should lead to their demise.^3,4

Because of this increased apoptotic burden, it can be hypothesized that tumor cells live on the precipice of apoptosis: even partial inhibition of antiapoptotic mechanisms operative in tumor cells would be expected to render them vulnerable to cell death.⁵ Therefore, therapies designed to downregulate antiapoptotic pathways would be expected to enhance the demise of cancer cells without affecting normal cells. This hypothesis will be testable when therapeutic agents are developed that specifically target the key regulatory elements of the apoptotic machinery. The Bcl-2 antisense oligonucleotide Genasense may represent the first of this class of agents, as discussed below. Note also that since most cytotoxic drugs exert their antitumor effects by activation of apoptosis pathways,⁴ interventions that inhibit antiapoptosis mechanisms in tumor cells should overcome many forms of drug resistance.

We believe that MM fits this paradigm. From the earliest stages of proliferation and differentiation in the lymph node germinal center, incipient MM cells accumulate genetic lesions including translocations involving the Ig-H switch region, and gross chromosomal abnormalities leading to aneuploidy.⁶ Late in the disease, MM cells lose the requirement for the BM microenvironment as they acquire additional mutations involving oncogenes such as myc, ras, and p53.⁷ One would hypothesize that even during the earliest stages of disease (MGUS), genetic lesions result in an increased apoptotic burden, and the surviving cells must have developed antiapoptotic mechanisms to counterbalance the death signals.^8,9 This concept is consistent with the notion that while MM is a disease of deregulated proliferation, early in the disease the labeling index is low (< 1%), and an increased survival of malignant plasma cells may be a more important factor for the initial expansion of malignant plasma cells in the BM.⁷

Apoptosis is induced through two distinct yet intertwined pathways: (1) the extrinsic or death receptor pathway composed of tumor necrosis factor (TNF)-family receptors and ligands and (2) the intrinsic pathway in which the release of mitochondrial constituents regulates caspase activation. Although the mitochondrial pathway of apoptosis is the main topic of this review, it should be noted that death receptors 4 and 5 (DR4, DR5) are expressed on MM cell lines and primary MM isolates, and can efficiently activate the extrinsic pathway after binding of the ligand TRAIL.^10,11 This leads to the rapid activation of caspase 8 that can either directly activate effector caspases 3, 6, or 7 or cleave Bid, leading to amplification of the apoptotic signal by recruiting the mitochondrial pathway.¹² Regulation of TRAIL-induced death can also occur at the level of the BM microenvironment, as OPG released by osteoblasts and other stromal cells can act as a decoy receptor for TRAIL, thereby blocking its apoptosis-inducing activity.¹³ MM cells inhibit OPG release by stromal cells, thereby promoting osteoclast activation and lytic bone disease (by enhancing RANKL availability), while at the same time exposing themselves to higher levels of ambient TRAIL. As a recurring theme, the relative levels of proapoptotic versus antiapoptotic molecules that act in a cell autonomous manner or in the milieu of the BM microenvironment determine the outcome of potentially lethal signals.

Regulation of the Mitochondrial Pathway of Apoptosis
Myeloma cells are exposed to multiple noxious stimuli with the potential to induce apoptosis, such as chromosomal instability or hypoxia, and those induced by different forms of therapy, which work through mechanisms that are as varied as the therapeutic agents themselves (e.g., dexamethasone, melphalan, thalidomide, Velcade). Resistance to apoptosis in these cases ultimately rests on the ability of the MM cells to prevent activation of the mitochondrial pathway of apoptosis. Whether noxious signals activate this pathway is determined by members of the Bcl-2 family, and understanding the molecular functions of these proteins is required for the design of novel therapeutics to overcome the resistance to apoptosis exhibited by MM cells. Bcl-2 family members are divided into 3 functional groups; these encode 1 or more Bcl-2 homology domains (BH1-BH4) and act as inhibitors or inducers of the mitochondrial apoptosis pathway (Figure 4). Antiapoptotic family members (Bcl-2, Bcl-XL, Mcl-1) are localized to the outer mitochondrial membrane (OMM) via a hydrophobic carboxy-terminal tail, and regulate the release of apoptotic molecules from the intermembrane space.¹⁴ The apoptosis inducers (e.g., Bax and Bak) encode BH1, BH2, and BH3 domains, and can be induced by apoptotic signals to homo-oligomerize and form pores in the outer membrane, thus permitting efflux of apoptosis-inducing molecules including cytochrome c, dATP, SMAC/Diablo, and AIF.^14,15 The mechanism by which mitochondrial outer membrane permeabilization (MOMP) allows efflux of apoptogenic proteins is controversial; however, it has been shown that Bax oligomers can form pores in liposomes that permit the passage of cytochrome c.¹⁶ Under normal growth conditions, Bak is tethered to the mitochondrial outer membrane, while Bax translocates to the mitochondria in response to apoptosis-induced conformational changes that unmask its carboxy-terminal hydrophobic domain.¹⁷ Cells isolated from Bax/Bak double knock-out mice exhibit a dramatic resistance to the induction of apoptosis by many different noxious insults, including overexpression of BH3-only proteins.¹⁸

View larger version (27K): [in this window] [in a new window]   Figure 4. Structural domains of Bcl-2 family members. Transmembrane (TM) domains mediate insertion into the mitochondrial outer membrane. The Bcl-2-homology (BH) domains 1, 2, and 3 of antiapoptotic family members form a hydrophobic binding pocket for BH3-only proteins. Mcl-1 encodes a unique amino terminal domain of 180 amino acids, which may be involved in functions that are unique to Mcl-1. The multidomain killers Bax and Bak can oligomerize to form pores in the outer mitochondrial membrane in a process regulated by BH3-only family members.

Figure 5

Figure 5

View larger version (26K): [in this window] [in a new window]   Figure 5. The induction of apoptosis by diverse death stimuli occurs through the transcriptional or posttranscriptional regulation of BH3-only proteins. Levels of Noxa and Puma are transcriptionally upregulated by p53 in response to DNA damage. Cytokines and growth factors act as survival factors by inhibiting the proapoptotic activities of Bad and Bim through changes in the state of protein phosphorylation. Loss of cell adherence to extracellular matrix can lead to a form of apoptosis termed "anoikis," and this is regulated by BH3 proteins that associate with actin filaments (Bmf) or microtubules (Bim). The caspase 8-mediated cleavage of Bid to form tBid acts as a link between the extrinsic and intrinsic pathways of apoptosis. BH3-only proteins translocate to the mitochondria, where they can bind to antiapoptotic (Bcl-2, Bcl-XL, Mcl-1) or proapoptotic (Bax, Bak) Bcl-2 family members to regulate mitochondrial outer membrane permeability (MOMP) and the release of cytochrome c, apoptosis-inducing factor (AIF), and SMAC/Diablo (see Figure 6). Bax is cytosolic until death signals induce a conformational change followed by its insertion into the outer mitochondrial membrane with formation of oligomers. Recent studies demonstrate that the level of Ca+2 stores in the endoplasmic reticulum (ER) and subsequent uptake of Ca+2 by the mitochondria play an important role in determining the threshold for apoptosis. This cyclical ER-mitochondrial flux of Ca+2 is regulated in part by Bax, Bak, Bcl-2 and other family members that are localized to these organelles. Abbreviations: IMS, inter-membrane space; SMAC, second mitochondrial activator of caspases; PKA, protein kinase A

Recent elegant experiments have led to a revised model for the regulation of the mitochondrial pathway of apoptosis by BH3-only proteins (Figure 6).^28,32 A critical aspect of this model is based on the relative affinities of distinct BH3-only proteins for antiapoptotic versus proapoptotic Bcl-2 family members, and the relative abundance of each class of protein within the cell. Letai et al have determined the binding affinity of Bcl-2 for a variety of peptides encoding the functional alpha-helical regions of killer protein BH3 domains, and have determined which of these peptides can directly induce cytochrome c release from purified mitochondria.³² BH3-domain peptides from Bid and Bim directly induced MOMP and cytochrome c release through a process that required Bax or Bak. A BH3-domain peptide from BAD could not induce cytochrome release directly. However the BAD peptide did bind to Bcl-2 with high affinity and could displace the lower affinity Bid peptide. A model was proposed that some BH3-only proteins (e.g., Bid and Bim) directly target Bax and Bak and induce pore formation; apoptosis can be averted if BH3-only proteins are bound by antiapoptotic Bcl-2 family members. A second class of BH3 proteins, represented by BAD, cannot directly induce pore formation, but can occupy the hydrophobic binding pocket of Bcl-2 (and presumably Bcl-XL or Mcl-1) thus enabling subthreshold levels of Bid to target Bax or Bak.

View larger version (19K): [in this window] [in a new window]   Figure 6. Model for the regulation of the mitochondrial pathway of apoptosis by Bcl-2-family members. 1. In the absence of apoptotic stimuli, Bak molecules reside in the outer mitochondrial membrane but do not oligomerize to form pores; Mcl-1 may associate with Bak to block Bak self-association. 2. Apoptosis-inducing signals promote the movement of BH3-only proteins to the outer mitochondrial membrane (OMM), where some BH3-only family members (e.g., Bid, Bim) transiently associate with Bak or Bax to induce a conformational change resulting in formation of homo-oligomers and induction of apoptosis. 3. Antiapoptotic proteins such as Mcl-1 and Bcl-XL (or Bcl-2, not shown) may be overexpressed in multiple myeloma (MM) cells and sequester BH3-only proteins and prevent their interaction with Bak or Bax, thus overcoming cell death signals. Inhibition of expression of antiapoptotic proteins in MM cells could overcome this tumor survival mechanism. 4. Antiapoptotic effects of Mcl-1 and Bcl-XL can be overcome by excess BH3-only proteins. 5. Resistance to apoptosis may be reversed by the development of low molecular weight, cell-permeable drugs (D) that bind to the hydrophobic BH1-3 pocket of Mcl-1 and/or Bcl-XL, thus preventing the sequestration of BH3-only proteins. The latter are then free to interact with Bak and Bax with the subsequent formation of pores and induction of apoptosis. Efforts are in progress to identify novel compounds that specifically target Mcl-1 (D1) or Bcl-XL (D2).

The Mitochondrial Pathway in MM
In MM, defects in programmed cell death pathways are frequently caused by imbalances in expression levels of the Bcl-2 family of proteins. It has become clear that every nucleated cell requires protection by at least one prosurvival Bcl-2 homologue, and that the abundance of these guardians regulates tissue homeostasis.¹⁵ MM cells conform to this model; they express Bcl-2, Bcl-XL, and Mcl-1, and both clinical and in vitro data suggest important roles for these proteins in maintaining MM cell survival and in clinical resistance to therapy.^35,36 Although aberrant switch recombination leading to the activation of multiple translocation partners plays an important role in the pathogenesis of MM, these translocations do not include Bcl-2 family members.⁶ Nevertheless, Bcl-2 is expressed in many (but not all) MM cell lines and primary clinical isolates.^36,37 Bcl-XL is expressed in most cell lines and clinical isolates, and was detected more often at the time of patient relapse and correlated with resistance to chemotherapy.³⁵ Mcl-1 appears to be expressed in virtually all MM cell lines and in all clinical isolates thus far examined by numerous groups.^37–40 In a comparison of the expression of Mcl-1 in MM cells from 150 patients to that of plasma cells from 31 normal volunteers, a statistically significant increase in the myeloma cells was demonstrated (J. Shaughnessy et al, personal communication). Thus it appears that most MM cells express some level of Bcl-2, Bcl-XL, and Mcl-1, and it remains to be determined if these are purely overlapping in function, or also have distinct activities to promote tumor cell survival in the face of a myriad of apoptotic stimuli.

Experimental approaches to ablate expression of Mcl-1, Bcl-2, and Bcl-XL have begun to address this issue. A number of laboratories have correlated the induction of MM cell apoptosis with decreased expression of Mcl-1.^37,38,40 Mcl-1 mRNA and protein have a short half-life, and inhibition of Mcl-1 synthesis led to the rapid induction of apoptosis of MM cells.⁴¹ The main apoptosis-inducing activity of the CDK-inhibitor flavopiridol can be linked to inhibition of CDK9/cyclin T1 with subsequent inhibition of transcription elongation; Mcl-1 was shown to be a candidate target gene for the rapid induction of apoptosis by flavopiridol.^41,42 The critical role for Mcl-1 as a survival factor in MM has been demonstrated in vitro using antisense oligonucleotides (ASO) to specifically inhibit Mcl-1 expression. Mcl-1 ASO led to the rapid induction of caspase activity and apoptosis (within 3 hours in some cases) when used as a single agent, and killing was potentiated by the addition of dexamethasone.^38,40 In the latter study, ASO-mediated inhibition of Bcl-XL or Bcl-2 did not induce apoptosis as single agents, even though expression of the molecular targets was shown to be significantly reduced. However, addition of dexamethasone to Bcl-2 ASO-treated cells did promote apoptosis in some cell lines.⁴⁰ Others have demonstrated that in MM cell lines with a low level of Bcl-2 expression, apoptosis can be induced using the specific Bcl-2 ASO G3139, and that killing was potentiated by dexamethasone or taxol.⁴³ A second study cultured purified primary MM cells with high concentrations of G3139 (10 µM, corresponding to 56 µg/mL; in clinical studies serum concentrations during the 7-day infusion were 3–7 µg/mL) and demonstrated a significant reduction of Bcl-2 RNA and protein in most patients.⁴⁴ G3139 alone was not toxic to these cells, but it enhanced killing by doxorubicin and dexamethasone. It remains to be clarified if inhibition of Bcl-XL can promote MM cell death as only 1 study has critically examined this question.⁴⁰ No studies have systematically evaluated the role of Bfl-1/A1 in primary MM cell isolates.

A number of signal transduction pathways have been shown to regulate the expression of antiapoptotic Bcl-2 family members in MM cells, and the role of IL-6 has been closely scrutinized. STAT3 was recently shown to be constitutively activated in primary MM cells, and was shown to induce the upregulation of Bcl-XL in the U266 cell line.⁴⁵ A number of groups have determined that Mcl-1 is upregulated by IL-6, perhaps through the activity of the STAT3 pathway.^37,46 However the role of STAT3 was based on experiments using the tyrosine kinase inhibitor AG490, whose specificity and mechanism of action are not at all clear. Our group demonstrated that approximately one third of MM cell lines and primary MM cells responded to IL-6 with the upregulation of Mcl-1, while in the other two thirds of cases, Mcl-1 was expressed at high levels in the absence of IL-6, without further increase up