Apoptosome-independent pathway for apoptosis: Biochemical analysis of APAF-1 defects and biological outcomes

Induction and execution of apoptosis programs are generally believed to be mediated through a hierarchy of caspase activation. By using two cellular variants obtained from the L1210 cell line (L1210/S and L1210/0), we have shown previously that staurosporine induces apoptotic cell death through both caspase-dependent and caspase-independent pathways. Both pathways normally coexisted in L12101S cells, whereas L121010 cells lacked the ability to activate caspases despite the confirmed presence of both procaspase-3 and -9. Here we show that this defect in caspase activation is not due to mechanisms such as an absence of cytochrome c release, the expression of non-functional caspases, or the presence of an endogenous inhibitor but results from the loss of apoptosis protease activator protein-1 (APAF-1) expression. This absence of APAF-1 protein results from multiple alterations at both genomic and transcriptional levels. However, although this lack of APAF-1 delays the apoptotic program, it does not hamper its execution. Importantly, in these cells, apoptosis develops not only in an APAF-1-independent way but also in the absence of caspase-3 and -9 activation. Altogether these findings provide evidence that apoptosis may occur through alternative signaling pathways independent of APAF-1 expression and totally dissociated from any caspase processing. Therefore, the L1210/0 variant subline provides a valuable tool for the elucidation of these pathways.