Heparanase Regulates Response to Proteasomal Inhibition in Myeloma

Abstract 4902

Multiple myeloma is an incurable hematologic malignancy. In patients, even after several rounds of high-dose chemotherapy, myeloma tumor cells survive, proliferate aggressively and re-establish the active disease. Understanding the molecular mechanisms that aid myeloma cells in evading chemotherapy is therefore critical for curing this deadly disease. Studies from our lab have established heparanase, an endoglycosidase expressed highly by myeloma tumor cells, as a master regulator of aggressive myeloma disease where it drives tumor growth, angiogenesis, invasion, and osteolysis. However, the effect of heparanase on response to chemotherapy is unknown. Addressing this question, we have made two striking discoveries: i) treatment with proteasomal inhibitors dramatically elevates heparanase expression and secretion in myeloma, and ii) high heparanase expression significantly reduces therapy-induced apoptosis and cell death in myeloma. Proteasomal inhibition by either bortezomib or MG132 dramatically elevated the expression of heparanase and its secretion into the conditioned media. When this conditioned media was added to tumor cells that had not been exposed to proteosome inhibitors, the heparanase was readily taken up by the cells. This is consistent with previous studies demonstrating that exogenous heparanase can be taken up by myeloma tumor cells resulting in activation of aggressive tumor behavior.

Next, to test whether heparanase influences chemoresistance in myeloma, we evaluated the effect of bortezomib on inducing apoptosis of myeloma cells expressing either high (HPSE-high) or low (HPSE-low) levels of heparanase. Apoptotic cells were identified by staining for Annexin V and cell death was assessed by staining with propidium iodide. Results revealed that following treatment with bortezomib, HPSE-high cells had a significantly higher percentage of viable cells (Annexin V negative, PI negative) and a significantly smaller fraction of apoptotic cells (Annexin V positive) compared to HPSE-low cells. Similarly, even when myeloma cells were grown on fibronectin, cells expressing high levels of heparanase exhibited significantly decreased bortezomib-induced cell death. Consistent with the above findings, we found that several well established chemoresistant myeloma cell lines such as LR5, Dox 6 and Dox 40 all expressed high levels of heparanase. Together the above findings demonstrate that tumors respond to proteosome inhibitors by upregulating expression and secretion of heparanase and that tumor cells having high levels of heparanase resist proteosome inhibitor-induced apoptosis. These results indicate that heparanase may play an important role in chemoresistance of myeloma cells and that heparanase inhibitors used in tandem with other drugs may improve initial patient outcome and reduce incidence of relapse.