Induction of caspase-dependent death by proteasome targeted therapy in glioblastoma

New therapeutic options are needed for glioblastoma, a deadly disease with a median survival of only 14 months with current treatment. The proteasome inhibitor bortezomib (BTZ) shows efficacy in cancers like myeloma, but its clinical utility in other cancer types has been more limited. Newer proteasome inhibitors such as marizomib (MRZ) have unique inhibitory and death inducing properties that have not been well examined in GBM. Additionally, targeting other components of the ubiquitin-proteasome system is possible, but has not been explored in GBM. Questions also still remain about the ability of BTZ and MRZ to be delivered to brain tumors in a relevant orthotopic system. The goal of this study was to determine the kinetics and mechanism of death induced by proteasome inhibitors in GBM and to compare the ability of BTZ and MRZ to cause proteasome inhibition in orthotopic brain tumors in order to establish a framework for the use of these drugs in the clinic by identifying potential biomarkers of efficacy and enabling design of a combination treatment strategy for potentiation of cell death in GBM. Using strategies that inhibited multiple proteasome components, I determined that inhibition of the standard proteasome by BTZ and MRZ was sufficient for optimal targeting of the ubiquitin-proteasome system in GBM. I then determined that both BTZ and MRZ induced caspase-dependent death in GBM cells that was dependent upon activation of caspase 9. Using an orthotopic xenograft model of GBM, I found that both BTZ and MRZ increased levels of the proteasome substrates p21 and p27 in intracranial tumors, with MRZ exerting slightly stronger effects, indicating that these drugs do affect brain tumors. Examination of cleaved caspase 3 and lamin A as markers of apoptosis in brain tumors from mice showed increased cell death after treatment with BTZ or MRZ and the histone deacetylase inhibitor vorinostat. Together, this data clarifies the optimal strategy for proteasome targeting in GBM, clarifies the hierarchy of caspase induction by proteasome inhibitors, and provides evidence that proteasome inhibitors can reach brain tumors where they exert functional effects and increase death in combination with vorinostat