ABROGATION OF MITOCHONDRIAL CYTOCHROME-C RELEASE AND CASPASE-3 ACTIVATION IN ACQUIRED MULTIDRUG-RESISTANCE

Acquired multidrug resistance to anti-cancer agents has been associate d with overexpression of the P-glycoprotein and other members of the A TP-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 t o 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 wer e obtained when SCC-25/CDDP cells were exposed to the structurally and functionally unrelated antimetabolite l-beta-D-arabinofuranosyl-cytos ine (ara-C). Other cells selected for resistance to doxorubicin or vin cristine also exhibited overexpression of Bcl-x(L) and failed to respo nd to CDDP and ara-C with activation of caspase-3. The results further demonstrate that multidrug-resistant cells exhibit a block in the rel ease of mitochondrial cytochrome c into the cytosol and that this effe ct 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 i s because of inhibition of mitochondrial cytochrome c release, These f indings demonstrate that cells respond to diverse classes of anti-canc er drugs with overexpression of Bcl-x(L) and that this response repres ents another mechanism of acquired multidrug resistance.