CD95-mediated apoptosis in Burkitt's lymphoma B-cells is associated with Pim-1 down-regulation

Abstract

B-cells of the high-grade non-Hodgkin lymphoma Burkitt's lymphoma (BL) overexpress survival oncoproteins, including the proviral integration site for Moloney murine leukaemia virus kinase (Pim)-1, and become apoptosis resistant. Activated death receptor CD95 after ligation with anti-CD95 monoclonal antibody (mAb) resulted in the regression of BL via induction of apoptosis, suggesting a decrease of survival protein expression. Here, CD95-mediated apoptotic pathways in BL B-cell lines (Raji and Daudi) following treatment with anti-CD95 mAb was investigated with the cause-and-effects on pim-1 gene expression, in comparison with leukemic cell line (K562) used as CD95-negative cells. Immunohistochemical staining for CD95 and Pim-1 was performed, and the effects of anti-CD95 mAb on apoptotic signalling using western blotting, on caspase activity and cell survival of BL B-cell and leukemic cell lines were determined. We showed that Raji cells expressed more CD95 receptors than Daudi cells. Half of each population underwent apoptosis accompanied by decreased cell viability after anti-CD95 mAb treatment. Distinct extrinsic and intrinsic CD95-mediated apoptotic pathways in Raji and Daudi cells were revealed by high caspase activity and mitochondrial outer membrane permeabilization, respectively. We observed decreased Pim-1 transcript and protein expression levels with increased heat-shock protein (Hsp)70 and decreased Hsp90 expression in anti-CD95 mAb-treated cells. Throughout the study, K562 cells did not undergo apoptosis upon anti-CD95 mAb treatment. Pim-1 knockdown following to stable transfection with plasmid vectors induced apoptosis and decreased viability of BL and K562 cells. Therefore, CD95-mediated apoptosis induces Pim-1 down-regulation in BL B-cells, but Pim-1 down-regulation cannot fully eradicate BL and leukaemia.