Programmed cell death by bcl-2-dependent and independent mechanisms in B lymphoma cells

Programmed cell death (PCD) or apoptosis is a common form of cellular demise during embryogenesis, tumorigenesis and clonal selection in the immune system. The bcl-2 proto-oncogene has been recently implicated as a potential physiological regulator of the PCD pathway. Gene transfer studies have shown that overexpression of bcl-2 blocks apoptosis mediated by several stimuli in cultured cell lines and promotes the survival of B and T lymphocytes in transgenic mice. However, it remains unclear whether under normal conditions bcl-2 is responsible for controlling cell death. We have investigated the role of bcl-2 in the antimembrane IgM (mIgM)-induced apoptotic death of WEHI-231 B cell lymphoma, a model that mimics clonal deletion of immature B cells by antigen. Signalling of mIgM receptors triggered downregulation of both bcl-2 RNA and protein, and induced apoptosis in WEHI-231 B cells. This effect appeared to be specific since (i) the levels of beta 2-microglobulin and beta-actin RNA remain unchanged and (ii) signalling of the apoptosis-resistant B cell lymphoma line BAL-17 with anti-mu was not associated with downregulation of bcl-2 RNA. However, stable expression of bcl-2 by transfection did not rescue WEHI-231 B cells from apoptosis, yet WEHI-231 cells overexpressing bcl-2 were more resistant to programmed cell death induced by heat-shock.(ABSTRACT TRUNCATED AT 250 WORDS)