Susceptibility to drug-induced apoptosis correlates with differential modulation of Bad, Bcl-2 and Bcl-x(L) protein levels

To define the responses of apoptotic regulatory proteins to different chemo therapeutic agents, we investigated the expression of Bcl-2 family gene pro ducts, the release of cytochrome c, and the activation of pro-caspase-3 dur ing apoptosis induced by Taxol and Thiotepa, in the MCF-7 breast carcinoma and the HL-60 leukemia cell lines. The earliest event induced by drug expos ure was increase in Bad protein levels, followed by Bcl-2 down-regulation, cytochrome c release, and Bcl-x(L) and Bar up-regulation. Bak accumulation was a late event. Activation of pro-caspase-3 and cleavage of Bcl-2 protein occurred in the HL-60 cells only, and followed the cytochrome c release. T he overall responses were qualitatively similar in both cell types, but MCF -7 cells treated with Taxol showed a significant delay in apoptosis, correl ating with early up-regulation of Bcl-2 and delayed release of cytochrome c , We conclude that Bad up-regulation is an early indicator of a cellular re sponse that will lead to cell death, but may be modulated by survival mecha nisms, which cumulatively govern the ultimate susceptibility to apoptosis.