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An Organotypic High Throughput System for Characterization of Drug Sensitivity of Primary Multiple Myeloma Cells

In this work we describe a novel approach that combines ex vivo drug sensitivity assays and digital image analysis to estimate chemosensitivity and ...

H. Lee Moffitt Cancer Center and Research Institute

Viral interleukin-6 (vIL-6) is a homolog of cellular IL-6 that is encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) genome. vIL-6 binds to the IL-6 signal transducer gp130 without the cooperation of the IL-6 high affinity receptor to induce STAT3 DNA binding and cell proliferation. Although vIL-6 is believed to be important in the pathogenesis of KSHV-induced diseases, its secretion and post-translational modifications have not previously been characterized. Pulse-chase analysis revealed that the half-time of vIL-6 secretion is approximately 8-fold longer than that of human IL-6. Yet, the vIL-6 signal sequence targets human IL-6 secretion to nearly wild-type levels. Surprisingly, vIL-6 was not secreted from a cell line that does not express gp130 but expression of human gp130 in these cells enabled the secretion of vIL-6. Consistent with this observation, complete maturation of gp130 N-glycans is inhibited by vIL-6 coexpression, suggesting that the binding of the receptor to vIL-6 occurs intracellularly in early or pre-Golgi compartments. Furthermore, a vIL-6 mutant containing an endoplasmic reticulum retention signal is not secreted but does still induce receptor activation and signaling. Secreted vIL-6 is completely glycosylated at both possible N-glycosylaton sites and contains a large proportion of immature high-mannose glycans that is not typical of cytokines. These findings suggest that vIL-6 may induce gp130 signaling by an exclusively autocrine mechanism that relies on intracellular binding to its receptor. During KSHV infection, vIL-6 may only induce signaling in KSHV-infected cells to benefit the viral life cycle and promote oncogenic transformation.

Kaposi's sarcoma-associated herpesvirus-encoded viral interleukin-6 is secreted and modified differently than human interleukin-6: evidence for a unique autocrine signaling mechanism.

The bone marrow microenvironment facilitates the survival, differentiation, and proliferation of hematopoietic cells. These cells are supported by fibroblast-like bone marrow stromal cells, osteoblasts, and osteoclasts which secrete soluble factors and extracellular matrix proteins that mediate these functions. This rich environment serves as a safe haven not only for normal and malignant hematopoietic cells, but also for epithelial tumor cells that metastasize to bone, offering protection from chemotherapeutic agents by common mechanisms. Soluble factors produced in the bone marrow, such as stromal cell-derived factor-1 and interleukin-6, mediate homing, survival, and proliferation of tumor cells, and integrin-mediated adhesion sequesters tumor cells to this protective niche. Environment-mediated drug resistance includes a combination of soluble factors and adhesion, and can be subdivided into soluble factor-mediated drug resistance and cell adhesion-mediated drug resistance. Because it is induced immediately by the microenvironment and is independent of epigenetic or genetic changes caused by the selective pressure of drug exposure, environment-mediated drug resistance is a form of de novo drug resistance. In this form of drug resistance, tumor cells are transiently and reversibly protected from apoptosis induced by both chemotherapy and physiologic mediators of cell death. This protection allows tumor cells to survive the insult of chemotherapy, leading to minimal residual disease, and thereby increases the probability for the development of acquired drug resistance.

The bone marrow microenvironment as a tumor sanctuary and contributor to drug resistance.

Using a previously described method for the functionalization of glass substrates with glycosaminoglycans (GAGs), in vitro experimental comparison of adhesion levels of cancer cells to glycosaminoglycan-modified substrates was performed with non-treated and heparin-treated human cancer cells of different metastatic activity. For both non-treated and heparin-treated cells, our results indicate that heparan sulfate is the preferred substrate for adhesion while keratan sulfate shows anti-adhesive properties. The observed net effect of heparin is a cell-dependent reduction in the adhesion figures. Overall, our results suggest that tissues with higher composition of heparan sulfate chains may be preferential metastatic targets and indicate that the effective use of heparin as anti-metastatic or anti-inflammatory agent may also depend on glycosaminoglycan composition of the affected organs.

Static adhesion of cancer cells to glass surfaces coated with glycosaminoglycans.

The bone marrow microenvironmental components interleukin (IL)-6 and fibronectin (FN) individually influence the proliferation and survival of multiple myeloma (MM) cells; however, in vivo, these effectors most likely work together. We examined signaling events, cell cycle progression, and levels of drug response in MM cells either adhered to FN via beta1 integrins, stimulated with IL-6, or treated with the two combined. Although G(1)-S cell cycle arrest associated with FN adhesion was overcome when IL-6 was added, the cell adhesion-mediated drug resistance (CAM-DR) was maintained in the presence of IL-6. Concomitant exposure of MM cells to IL-6 and FN adhesion revealed a dramatic increase in signal transducers and activators of transcription 3 (STAT3) phosphorylation, nuclear translocation, and DNA binding, compared with either IL-6 or FN adhesion alone in four MM cell lines. Importantly, this increase in STAT3 activation correlated with a novel association between STAT3 and gp130 in cells adhered to FN before stimulation with IL-6, relative to nonadherent cells. Taken together, these results suggest a mechanism by which collaborative signaling by beta1 integrin and gp130 confers an increased survival advantage to MM cells.

Beta1 integrin adhesion enhances IL-6-mediated STAT3 signaling in myeloma cells: implications for microenvironment influence on tumor survival and proliferation.

Environment-mediated drug resistance is a form of de novo drug resistance that protects tumour cells from the initial effects of diverse therapies. Surviving foci of residual disease can then develop complex and permanent acquired resistance in response to the selective pressure of therapy. Recent evidence indicates that environment-mediated drug resistance arises from an adaptive, reciprocal signalling dialogue between tumour cells and the surrounding microenvironment. We propose that new therapeutic strategies targeting this interaction should be applied during initial treatment to prevent the emergence of acquired resistance.

Environment-mediated drug resistance: a major contributor to minimal residual disease.

Hematopoietic stroma promotes resistance to immune control by APO2L/TRAIL in multiple myeloma (MM) cells in part by increasing synthesis of the anti-apoptotic protein c-FLIP. Here, we tested whether bortezomib can reverse the APO2L/TRAIL environmental mediated-immune resistance (EM-IR).

Bortezomib restores stroma-mediated APO2L/TRAIL apoptosis resistance in multiple myeloma.

GM-CSF, IL-3, and IL-5 are proinflammatory cytokines that control the production and function of myeloid and lymphoid cells. Their receptors are composed of a ligand-specific alpha subunit and a shared common signal-transducing beta subunit (beta common receptor or GM-CSFR beta [beta(c)]). The pleiotropic nature of biologic outcomes mediated by beta(c) and the presence of large, uncharacterized regions of its cytoplasmic domain suggest that much remains to be learned about its downstream signaling pathways. Although some previous work has attempted to link beta(c) with NF-kappaB activation, a definitive mechanism that mediates this pathway has not been described and, to date, it has not been clear whether the receptor can directly activate NF-kappaB. We demonstrate that NF-kappaB activation by beta(c) is dependent on TNFR-associated factor 6 (TRAF6) and that association of TRAF6 with beta(c) requires a consensus-binding motif found in other molecules known to interact with TRAF6. Furthermore, point mutation of this motif abrogated the ability of beta(c) to mediate NF-kappaB activation and reduced the viability of an IL-3-dependent hematopoietic cell line. Because this receptor plays a key role in hematopoiesis and the beta(c) cytoplasmic domain identified in this work mediates hematopoietic cell viability, this new pathway is likely to contribute to immune cell biology. This work is significant because it is the first description of a TRAF6-dependent signaling pathway associated with a type I cytokine receptor. It also suggests that TRAF6, a mediator of TNFR and TLR signaling, may be a common signaling intermediate in diverse cytokine receptor systems.

Mark Meads

Department of Cancer Imaging and Metabolism

H. Lee Moffitt Cancer Center and Research Institute

An Organotypic High Throughput System for Characterization of Drug Sensitivity of Primary Multiple Myeloma Cells