The role of adhesion receptors in the pathogenesis of multiple myeloma

Multiple myeloma: an update of developments in targeted therapy

Multiple myeloma continues to be an incurable illness. Improvement in our understanding of the disease is leading to new therapeutic modalities and the definition of new targeted therapy. Several pathways have been identified as areas for targeted therapy, and this review will discuss key molecules in some detail. New agents and molecules with biologic modulator activity appear to be changing the natural history of the disease. Agents such as thalidomide or its analogs, arsenic trioxide or bortezomib, hold great promise; however, much is yet to be learned on how to integrate these agents into disease management.

Stable changes in mesenchymal stromal cells from multiple myeloma patients revealed through their responses to Toll-like receptor ligands and epidermal growth factor

In human multiple myeloma (MM), the tumor cells exhibit strict dependence on bone marrow (BM) stromal elements. It has been suggested that, in turn, MM cells modify multipotent stromal cells (MSCs), diverting them to support the myeloma. We investigated MM-derived MSCs by comparing their toll-like receptor (TLR) responses to those of MSCs derived from healthy controls. We now report that MM-derived MSCs manifested intact proliferation responses and IL-6 secretion and their adipose and osteogenic differentiation responses to TLR ligands were also similar to those of healthy controls, ranging from augmentation to inhibition. However, MM-derived MSCs were found to be defective in IL-8 secretion and ERK1/2 phosphorylation following TLR-2 activation. Moreover, MM-derived MSCs failed to respond to EGF by elevation of ERK1/2 phosphorylation. The persistence of these changes in extensively cultured MM-derived MSCs, suggests that these cells are stably, if not irreversibly modified.

The thalidomide saga

Over the past 50 years, thalidomide has been a target of active investigation in both malignant and inflammatory conditions. Although initially developed for its sedative properties, decades of investigation have identified a multitude of biological effects that led to its classification as an immunomodulatory drug (IMiD). In addition to suppression of tumor necrosis factor-alpha (TNF-alpha), thalidomide effects the generation and elaboration of a cascade of pro-inflammatory cytokines that activate cytotoxic T-cells even in the absence of co-stimulatory signals. Furthermore, vascular endothelial growth factor (VEGF) and beta fibroblast growth factor (bFGF) secretion and cellular response are suppressed by thalidomide, thus antagonizing neoangiogenesis and altering the bone marrow stromal microenvironment in hematologic malignancies. The thalidomide analogs, lenalidomide (CC-5013; Revlimid) and CC-4047 (Actimid), have enhanced potency as inhibitors of TNF-alpha and other inflammatory cytokines, as well as greater capacity to promote T-cell activation and suppress angiogenesis. Both thalidomide and lenalidomide are effective in the treatment of multiple myeloma and myelodysplastic syndromes for which the Food and Drug Administration granted recent approval. Nonetheless, each of these IMiDs remains the subject of active investigation in solid tumors, hematologic malignancies, and other inflammatory conditions. This review will explore the pharmacokinetic and biologic effects of thalidomide and its progeny compounds.

Thalidomide and its derivatives: new promise for multiple myeloma

BACKGROUND

The combination of melphalan and prednisone has been accepted as standard treatment for multiple myeloma (MM) because most studies demonstrate only minimal survival benefit of combination chemotherapy regimens when compared with melphalan and prednisone. Despite modest gains with more intensive myeloablative regimens for certain subgroups, myeloma remains incurable. In 1999, investigators at the University of Arkansas reported the promising results of a phase II study of thalidomide in patients with resistant MM. Since then various trials of thalidomide alone, and in combination, have been tested in patients with resistant, and more recently, untreated MM. In addition, preliminary results of phase I studies of the immunomodulatory derivatives (IMiDs) of thalidomide have recently been reported.

METHODS

The author reviewed and reports the results of clinical trials of thalidomide and the IMiDs, as well as the pharmacology, mechanism of action, and toxicity of these agents.

RESULTS

Thalidomide has demonstrated significant activity in both resistant and previously untreated multiple myeloma. Combination therapy with dexamethasone increases response rate, even in patients previously resistant to both drugs given as single agents, indicating probable synergy. More recent studies of thalidomide-dexamethasone in previously untreated patients are encouraging. IMiD-3 is active in resistant myeloma and has a toxicity profile different from that of thalidomide.

CONCLUSIONS

Many studies have confirmed the activity of thalidomide in MM, as well as more responses with dexamethasone. Newer thalidomide derivatives with reduced toxicity are promising. Thalidomide-dexamethasone may now represent the treatment of choice for previously untreated patients. Further studies with these and other novel agents early in the course of myeloma, followed by rational programs of dose intensification, may improve complete remission rates and the frequency of long-term control.

CD106 and activated-CD29 are expressed on myelomatous bone marrow plasma cells and their downregulation is associated with tumour progression

Malignant plasma cells (PC) from multiple myeloma (MM) patients characteristically home to the bone marrow (BM). High numbers of tumour cells are found in the peripheral blood (PB) only at end-stage disease (secondary plasma cell leukaemia, PCL) in a minority of patients. Using flow cytometric and fluorescence in situ hybridization (FISH) analysis, a high percentage of tumoral BM PC from untreated patients was found to express CD106. In addition, these cells also expressed an activated form of CD29, as determined using the CD29 activation reporter monoclonal antibody HUTS-21. Adhesion-binding experiments showed that CD106+-activated CD29+ BM PC from these patients adhered to fibronectin (FN) in a CD29/CD49d-dependent manner. In contrast, marrow PC from progressive patients and BM or circulating malignant cells from secondary PCL patients expressed lower levels or were negative for CD106 and activated CD29, respectively, with a decreased or zero ability to adhere to FN. The expression of constitutive CD29 and CD49d, however, was similar during disease progression. We conclude that BM myelomatous cells co-express CD106 and a functionally active form of CD29. Moreover, our results suggest that the loss of expression and/or function of these antigens are associated with the progression of MM and may explain the exit of tumoral cells from the BM.

Nontraditional cytotoxic therapies for relapsed/refractory multiple myeloma

Multiple myeloma remains an incurable disease, with median survival rates of 4-6 years even with aggressive, high-dose chemotherapy, bone marrow transplantation, and intensive supportive care. Additionally, multiple myeloma is primarily a disease of the elderly, many of whom cannot tolerate aggressive chemotherapy. Thus, newer treatments with good safety profiles are needed to improve the quality of responses and, hopefully, to translate into prolonged progression and overall survival. The pathophysiology of multiple myeloma is complex, involving many pathways and interactions among cytokines, adhesion molecules, angiogenesis, and mechanisms of resistance, which, taken together, provide multiple targets for novel therapeutic modalities. Agents currently under investigation for treating multiple myeloma include thalidomide and its successors, PS-341, and arsenic trioxide. Thalidomide and immunomodulatory drugs both exhibit activity against multiple myeloma by affecting different levels of the immune system. PS-341 is a proteasome inhibitor that halts the cell cycle, resulting in apoptosis; it also inhibits a key transcription factor and may have antiangiogenic activity. Arsenic trioxide activates multicellular mechanisms to induce apoptosis, inhibit angiogenesis, and stimulate immune responses. Preclinical and early clinical data suggest that combination regimens should be pursued, given the different mechanisms of action of these compounds on the immune system and their non-overlapping toxicities at low dosages.

Myeloid cell factor-1 is a critical survival factor for multiple myeloma

Multiple myeloma (MM) is characterized by the accumulation of malignant plasma cells in the bone marrow caused primarily by failure of normal homeostatic mechanisms to prevent the expansion of postgerminal center plasma cells. We have examined the molecular mechanisms that promote the survival of MM cells and have identified a key role for myeloid cell factor-1 (Mcl-1), an antiapoptotic member of the Bcl-2 family. These experiments were initiated by the observation that MM cells were exquisitely sensitive to culture in the presence of actinomycin D: caspase activation occurred within 3 hours of treatment and cells were not protected by interleukin-6, the main MM cell growth and survival factor. Actinomycin D-induced apoptosis was blocked by proteasome inhibitors, suggesting that a labile protein was required for MM cell survival. Further analysis demonstrated that Mcl-1 was likely to be the labile factor governing MM cell survival. Mcl-1 protein levels decreased rapidly after culture in the presence of actinomycin D in concordance with effector caspase activation, but addition of proteasome inhibitors reversed the loss of Mcl-1 and maintained cell viability. The levels of other antiapoptotic proteins, including Bcl-2 and members of the inhibitors-of-apoptosis family, were unaffected by these interventions. Furthermore, Mcl-1 antisense oligonucleotides caused a rapid down-regulation of Mcl-1 protein levels and the coincident induction of apoptosis, whereas overexpression of Mcl-1 delayed actinomycin D-induced apoptosis with kinetics that correlated with expression levels of Mcl-1. These data indicate that Mcl-1 expression is required for the survival of MM cells and may represent an important target for future therapeutics.

The heterogeneity shown by human plasma cells from tonsil, blood, and bone marrow reveals graded stages of increasing maturity, but local profiles of adhesion molecule expression

Plasma cells (PCs) are the final B-cell differentiation stage. Recent evidence reveals relevant functional differences within the PC compartment. In rodents, early PCs formed in secondary lymphoid tissues show enhanced apoptosis and short life span, whereas PCs present in a final destination organ, such as the bone marrow (BM), have reached a stable prolonged survival state. BM PCs arrive at this organ as a circulating precursor whose cellular nature remains uncertain. An initial aim of this study was to characterize this circulating cell. We hypothesized that antibody-secreting cells detectable in the human blood after immunization might be a candidate precursor. These cells were obtained from the blood of volunteers immunized 6 days earlier with tetanus toxoid (tet), and they were unambiguously identified as PCs, as demonstrated by their expression of the CD38(h) phenotype, by morphology, by immunoglobulin (Ig) intracytoplasmic staining, and by IgG-tet-secreting capacity in vitro. In addition, by using the common CD38(h) feature, human PCs from tonsil (as a possible source of early PCs), from blood from tet-immunized donors (as the putative precursors of BM PCs), and from BM (as a deposit organ) have been purified and their phenotypes compared. The results show that a variety of differentiation molecules, proteins involved in the control of apoptosis, the B-cell transcription factors, positive regulatory domain I-binding factor 1/B lymphocyte-induced maturation protein 1 and B cell-specific activating protein and, at least partially, the chemokine receptor CXCR4 were expressed by human PCs following a gradient of increasing maturity in the direction: tonsil-->blood-->BM. However, PCs from these different organs showed a local pattern of adhesion molecule expression. These observations are discussed in light of the complex physiology of the human PC compartment.

Arsenic trioxide in multiple myeloma: rationale and future directions

Multiple myeloma remains an incurable malignancy with a median survival that does not exceed 3 years. At least one third of patients with multiple myeloma fail to respond to induction chemotherapy, and those who initially achieve remission eventually relapse and require additional therapy. Recent reports demonstrating the efficacy of arsenic trioxide in acute promyelocytic leukemia have prompted a revival in the clinical use of this compound. The achievement of clinical responses marked by molecular conversion of the malignant phenotype and remissions in patients who had failed to respond to multiple courses of conventional chemotherapy provided the impetus to explore its use in multiple myeloma. Properties that favor the use of arsenic trioxide are its ability to target selectively malignant cells for apoptosis through enhancementof reactive oxygen species, to induce differentiation, and to inhibit angiogenesis. Multiple events involved in the pathogenesis of multiple myeloma coincide with pathways targeted by arsenic trioxide, and early results have suggested that clinical responses and safety in patients are promising with advanced disease.

Abnormalities of bone marrow mesenchymal cells in multiple myeloma patients

BACKGROUND

The importance of the bone marrow microenvironment in multiple myeloma is receiving increasing attention. Recent studies have suggested the importance of cytokine production and cell-cell contact by bone marrow stromal cells in the survival of myeloma cells.

METHODS

In the current study, the authors examined bone marrow mesenchymal progenitor cell (MPC) cultures derived from eight multiple myeloma patients (mean age, 58 years) and nine normal donors (mean age, 61 years), with emphasis on cell surface antigens, cytokine, and growth factor expression.

RESULTS

The authors have found, based on analysis of cellular receptors, growth factors, and cytokine expression, that myeloma MPCs are phenotypically and functionally distinguishable from normal donor MPCs. Immunofluorescence analysis of MPC monolayers shows that myeloma MPC cultures expressed reduced cell surface vascular cell adhesion molecule-1 and fibronectin, in contrast with the strong expression found on normal donor MPCs. Furthermore, a subset of myeloma MPCs strongly express intracellular receptor for hyaluronan-mediated motility, whereas normal MPCs do not. Cytokine expression in bone marrow MPC cultures was examined by reverse transcription-polymerase chain reaction and enzyme linked immunosorbent assay. Bone marrow MPCs constitutively express interleukin (IL)-1beta, IL-6, granulocyte colony-stimulating factor (G-CSF), granulocyte macrophage (GM)-CSF, stem cell factor (SCF), and tumor necrosis factor (TNF)-alpha. In comparison to normal MPCs, multiple myeloma MPCs express increased basal levels of IL-1beta and TNF-alpha. In vitro exposure of MPC cultures to dexamethasone resulted in the down-regulation of IL-6, G-CSF, and GM-CSF in both normal and myeloma MPC cultures. However, dexamethasone treatment significantly increased expression of SCF-1 in myeloma MPCs.

CONCLUSIONS

In myeloma, bone marrow stromal cells provide paracrine factors, through cytokine production and cell-cell contact, which play a role in plasma cell growth and survival. The authors' data indicate differences in bone marrow MPCs, which may be biologically relevant to the growth and survival of myeloma plasma cells.