C-Myc- Dependent Stabilization of Hif-1alpha in MM: Therapeutic Implications

Bone marrow (BM) angiogenesis has been associated with multiple myeloma (MM) progression. Here we demonstrate that oncogenic c-Myc, also prominently associated with adverse prognosis in MM, regulates Hif-1α promoter activity, Hif-1alpha protein levels, and consequently the expression of pro-angiogenic target genes such as VEGF in MM cells. As observed with c-Myc knockdown, anti-MM agents with known anti-angiogenic activity including bortezomib, lenalidomide, and enzastaurin also downregulate Hif-1alpha and VEGF. Moreover, tyrophostin adaphostin induces similar effects, associated with anti-angiogenic activity, in a zebrafish model. The correlation between c-Myc, Hif-1alpha and tumor angiogenesis was further confirmed in a xenograft mouse model of human MM. Mechanistically, our in vitro data show that oncogenic c-Myc stabilizes caspase-8, and thereby protects Hif-1alpha against caspase-8- dependent degradation and attenuation of VEGF secretion. These sequelae also contribute to enhanced VEGF secretion triggered by MM cell-bone marrow stromal cell binding. Indeed, conditioned media from adaphostin- treated co-cultures inhibit endothelial cell growth and tubule formation. In addition, HIF1alpha gene demonstrated significant increased expression (p=0.02, Mann Whitney test) in tumor cells versus plasma cells derived from healthy donors. Importantly, when tested on a dataset of 559 patients, a significant link between HIF1alpha overexpression and poor prognosis was found (p=0,0069). These data further point to the possible relevance of HIF1alpha in the progression of a subset of MM patients. In summary, our data provide strong evidence of a key role of Hif-1alpha in MM angiogenesis and the therapeutic potential of specific Hif-1alpha and c-Myc inhibitors. Moreover they demonstrate additional biologic sequelae of novel MM therapies.