Expression of Bcl-xL Can Confer a Multidrug Resistance Phenotype

It has been suggested that genes that regulate apoptotic cell death may play an important role in determining the sensitivity of tumor cells to chemotherapy. We have recently cloned a member of the bcl-2 family, bcl-x. To test whether bcl-xL exprassion affects the sensitivity of tumor cells to chemotherapy, we have created stable cell lines overexpressing bcl-xL and have tested these cells for resistance to cell death induced by metabolic inhibitors and chemotherapeutic agents. Bcl-xL expression dramatically reduces the cytotoxicity of bleomycin, cisplatin, etoposide, vincristine, hygromycin B, and mycophenolic acid for up to 4 days in culture. BcI-xL does not prevent cells from undergoing cell cycle arrest in response to these drugs, but rather prevents treated cells from undergoing apoptosis. Cell-cycle analysis on cells treated with the chemotherapeutic agents bleomycin, cisPOPTOSIS, or programmed cell death, is emerging as an important biologic mechanism that maintains homeostasis in complex organisms.’ Unlike necrosis, apoptotic cell death is characterized by morphologic and biochemical changes such as plasma membrane blebbing, chromosomal condensation, and DNA fragmentation at nucleosomal interval~.’.~ Apoptosis can be induced by a multitude of stimuli, including growth factor withdrawal, DNA damage, and oncogene activation.‘“ Indeed, recent evidence suggests that apoptosis may provide defense against oncogenic progression as a result of oncogene activation or tumor suppressor inactivation.”” One of the major obstacles to successful chemotherapy is that tumors frequently are resistant to certain forms of chemotherapy or develop resistance during the course of initial therapy. Furthermore, tumors that initially respond to an antineoplastic drug and then acquire resistance often concurrently become resistant to a broad spectrum of drugs. One explanation for multidrug resistance in some tumors is the increased activity of the p-glycoprotein, which is a product of the mdrl gene and an energy-dependent multidrug transporter that acts to decrease the cellular influx and increase the cellular efflux of many drugs.” However, amplification of mdrl accounts for only a small percentage of cancers with a multidrug resistance phenotype. Other mechanisms for drug resistance include mutations in topoisomerase and mutations in ~53.’~”~ The recent observation that at least some antineoplastic drugs used in chemotherapy induce tumor cells to undergo apoptosis has led investigators to examine whether dysregulation of genes that control apoptosis can contribute to drug resistance. Consistent with this hypothesis, overexpression of bcl-2 has been shown to protect tumor cell lines from the toxicity of some chemotherapeutic agents.’6-’0 Although a correlation between high bcl-2 expression in acute myeloid leukemia cells and poor response to chemotherapy has been reported, the relationship between bcl-2 and cell death in human cancers is not completely clear.’l We have recently cloned a gene, bcl-x, whose product shares homology to bc1-2.” Bcl-x has two alternatively spliced forms, bcl-x, and bcl-xs. Initial studies showed that bcl-xl can inhibit cell death induced by growth factor withdrawal similarly to bcl-2. In contrast, bcl-xs uses an alternative 5’ splice donor to splice out 63 of the most conserved A