P-Glycoprotein Protects Leukemia Cells Against Caspase-Dependent, but not Caspase-Independent, Cell Death

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Abstract
A major problem with treating patients with cancer by traditional chemotherapeutic regimes is that their tumors often develop a multidrug resistant (MDR) phenotype and subsequently become insensitive to a range of different chemotoxic drugs. One cause of MDR is overexpression of the drug-effluxing protein, P-glycoprotein.It is now apparent that P-glycoprotein may also possess a more generic antiapoptotic function that protects P-glycoprotein–expressing cancer cells and normal cells from cell death. Herein we show that cells induced to express P-glycoprotein either by drug selection or by retroviral gene transduction with MDR1 cDNA are resistant to cell death induced by a wide range of death stimuli, such as FasL, tumor necrosis factor (TNF), and ultraviolet (UV) irradiation, that activate the caspase apoptotic cascade.However, P-glycoprotein–expressing cells were not resistant to caspase-independent cell death mediated by pore-forming proteins and granzyme B.MDR P-glycoprotein–expressing cells were made sensitive to caspase-dependent apoptosis by the addition of anti–P-glycoprotein antibodies or verapamil, a pharmacological inhibitor of P-glycoprotein function. Clonogenic assays showed that P-glycoprotein confers long-term resistance to caspase-dependent apoptotic stimuli but not to caspase-independent cell death stimuli. This study has confirmed a potential novel physiological function for P-glycoprotein and it now remains to dissect the molecular mechanisms involved in the inhibition of capsase-dependent cell death by P-glycoprotein.