Myeloma Exhibits Dependence on Atypical Glucose Transporters: Targeting MCL-1 Through GLUT4 Inhibition

Abstract 1850

Multiple Myeloma (MM) is an incurable heterogeneous B cell malignancy characterized by hyperdiploidy and/or aberrant chromosomal translocations warranting strategies that can target diverse molecular subtypes. One such unifying feature in MM entails an increased avidity for glucose that forms the basis for clinical imaging of myeloma by ¹⁸fluoro-deoxyglucose positron emission tomography. We are however not yet able to target abnormal glucose utilization for therapy. Given that glucose entry is a key rate-limiting step in glycolysis we performed an unbiased multi-cell line gene-expression profiling study combined with functional knock-down experiments to determine critical glucose transporters facilitating glucose entry in myeloma. These studies revealed a critical dependence of myeloma on the insulin-responsive glucose transporter GLUT4, and GLUTs 8 and 11. Our data demonstrate that myeloma cells exhibit a significant reliance on constitutively cell surface-localized GLUT4 for basal glucose consumption and viability. The functional significance of these transporters was further bolstered by detection of similar patterns of expression in myeloma patient samples. We also tested the therapeutic utility of targeting plasma-membrane localized GLUT4 in MM cell lines and patient samples with the FDA approved HIV protease inhibitor ritonavir that has an off-target inhibitory effect on GLUT4, providing proof of principle that GLUTs can be targeted for therapeutic benefit.

In this study we have investigated the mechanism by which GLUT4 suppression elicits cell death in myeloma. GLUT4 contributes to a significant fraction of glucose entry in myeloma while GLUTs 8 and 11 appear to have a significant impact on myeloma cell viability relating to alternative enigmatic subcellular functions not reliant on glucose entry per se. To further elucidate the mechanisms associated with cell death upon glucose deprivation in response to GLUT4 suppression, we examined expression levels of key apoptotic effectors in three myeloma cell lines expressing GLUT 1, 4, 8 or 11 shRNAs. Suppression of GLUT4 appears to selectively engage an apoptotic cascade involving suppression of MCL-1 and BCL-xL in addition to an increase in pro-apoptotic BAX expression. We do not detect up-regulation of PUMA or NOXA that have previously been demonstrated to play a role in glucose-deprivation induced cell death in other cell types. The suppression of MCL-1 is associated with an upstream activation of GSK-3. Expression of an MCL-1 ubiquitination-resistant mutant prevents reduction of MCL-1 protein upon GLUT4 knockdown, and reverses the toxicity elicited by GLUT4 suppression. In addition we do not detect the appearance of the cleaved pro-apoptotic fragment of MCL-1. These observations suggest glucose-deprivation induced cell death in response to GLUT4 reduction is mediated by ubiquitination and proteasomal degradation of MCL-1.

In sum we have discovered a novel myeloma specific method to target glucose entry and elicit apoptosis via suppression of MCL-1 expression. MCL-1 is a key effector of chemo-resistance in MM with levels of expression correlating to disease severity. Therefore strategies that can target MCL-1 will have significant utility in treatment of this fatal malignancy. These observations provide further rationale for the development of GLUT4-specific biologics to target aberrant glucose metabolism in myeloma and potentially other glucose-driven cancers.