Abstract
Multiple myeloma (MM) is an incurable hematopoietic neoplasm derived from plasma cells, and existing in the bone marrow. Recent developments in the field of myeloma onco-biology have enabled the use of proteasome inhibitors (PIs) as key drugs for MM. PIs can increase cell sensitivity to endoplasmic reticulum stress, leading to apoptosis of myeloma cells. PI cannot kill all myeloma cells, however; one reason of this might be activation of autophagy via hypoxic stress in the bone marrow microenvironment. Hypoxia-inducible gene(s) that regulate autophagy may be novel therapeutic target(s) for PI-resistant myeloma cells. Here, a hypoxia-inducible glycolytic enzyme hexokinase-2 (HK2) was demonstrated to contribute by autophagy activation to the acquisition of an anti-apoptotic phenotype in myeloma cells. We found that hypoxic stress led to autophagy activation accompanied by HK2 upregulation in myeloma cells. Under hypoxic conditions, HK2 knockdown inhibited glycolysis and impaired autophagy, inducing apoptosis. The cooperative effects of a PI (bortezomib) against immunodeficient mice inoculated with HK2-knocked down myeloma cells were examined and significant tumor reduction was observed. An HK2 inhibitor, 3-bromopyruvate (3-BrPA), also induced apoptosis under hypoxic rather than normoxic conditions. Further examination of the cooperative effects between 3-BrPA and bortezomib on myeloma cells revealed a significant increase in apoptotic myeloma cells. These results strongly suggested that HK2 regulates the activation of autophagy in hypoxic myeloma cells. Cooperative treatment using PI against a dominant fraction, and HK2 inhibitor against a minor fraction, adapted to the bone marrow microenvironment, may lead to deeper remission for refractory MM.