Abrogation of mitochondrial cytochrome c release and caspase-3 activation in acquired multidrug resistance

Acquired multidrug resistance to anti-cancer agents has been associated with overexpression of the P-glycoprotein and other members of the ATP-binding cassette superfamily. The present studies demonstrate that SCC-25 cells selected for resistance to the alkylating agent cisplatin (CDDP) overexpress the anti-apoptotic Bcl-x(L) protein. In contrast to parental cells, the SCC-25/CDDP-resistant variant failed to exhibit activation of caspase-3, cleavage of protein kinase C delta, and other characteristics of apoptosis in response to CDDP. Similar results were obtained when SCC-25/CDDP cells were exposed to the structurally and functionally unrelated antimetabolite l-beta-D-arabinofuranosyl-cytosine (ara-C). Other cells selected for resistance to doxorubicin or vincristine also exhibited overexpression of Bcl-x(L) and failed to respond to CDDP and ara-C with activation of caspase-3. The results further demonstrate that multidrug-resistant cells exhibit a block in the release of mitochondrial cytochrome c into the cytosol and that this effect is dependent on overexpression of Bcl-x(L). The demonstration that lysates from the resistant cells respond to the addition of cytochrome c with activation of caspase-3 confirms that the block in apoptosis is because of inhibition of mitochondrial cytochrome c release, These findings demonstrate that cells respond to diverse classes of anti-cancer drugs with overexpression of Bcl-x(L) and that this response represents another mechanism of acquired multidrug resistance.