Proteosome Inhibitor Treatment Down-Regulates S-Phase Kinase-Associated Protein 2 Causing Inhibition of Proliferation and Induces Apoptosis in Primary Effusion Lymphoma.

Proteosome inhibition is a novel approach for treating malignancy and has been approved for clinical use. The proteosome is the primary proteolytic mechanism in eukaryotic cells and inhibition of its catalytic activity initiates a cascade of events affecting cell cycle and apoptotic activities. These activities ultimately lead to cell cycle arrest and apoptosis in malignant cells however, the normal counterpart of these cells are spared. In this study, we used a panel of primary effusion lymphoma cell lines (BC1, BC3, BCBL1 and HBL6) to study the effects of proteosome inhibitor, MG132 on cell proliferation and apoptosis. Our data showed that proteosome inhibitor MG132 decreased cell viability as well as induced apoptosis in a dose dependent manner ranging from 0.5–10μM. Furthermore, treatment with 2.5μM MG132 for 24hours induced 41% apoptosis in BC1, 51% in BC3, 41% in BCBL1 and 48% in HBL6 cell lines as detected by annexinV/PI dual staining. S-phase kinase-associated protein 2 (skp-2) is a proto-oncogene and over expressed in various types of tumors. We sought to determine the role of Skp-2 following proteosome inhibition in PELs. MG132 treatment of PEL cell lines resulted in down-regulation of SKP-2 protein in a dose dependent manner whereas the expression of p-27 was up-regulated demonstrating an inverse relationship between these two proteins. Furthermore, MG132 treatment of PELs led to conformational changes in Bax protein and translocation to the mitochondria leading to the loss of mitochondrial membrane potential with subsequent release of cytochrome c from mitochondria into cytosol. Cytochrome c release caused activation of caspase-3 followed by polyadenosin-5′-diphosphate-ribose polymerase (PARP) cleavage. In addition, proteosome inhibitor treatment also caused down-regulation of inhibitor of apoptosis protein, XIAP. Taken together, our findings show that proteosome inhibition causes down-regulation of skp-2, up-regulation of p-27, inhibition of proliferation as well as caspase-dependent apoptosis in primary effusion lymphoma cells suggesting a role of proteosome inhibitors in the treatment of these aggressive cancers.