p27(Kip1) induces drug resistance by preventing apoptosis upstream of cytochrome c release and procaspase-3 activation in leukemic cells

The cyclin-dependent kinase inhibitor p27(Kip1) has been implicated as a drug resistance factor in tumor cells grown as spheroids or confluent monolayers. Here, we show that p27(Kip1) overexpression also induces resistance to drug-induced apoptosis and cytotoxicity in human leukemic cells growing in suspension. The anti-apoptotic effect of p27(Kip1) is not restricted to DNA-damaging agents but extends to the tubulin poison vinblastin, agonistic anti-Fas antibodies and macromolecule synthesis inhibitors. To further identify at which level this protein interferes with the cell death pathway, we investigated its influence on caspase activation and mitochondrial changes. Exposure of mock-transfected U937 cells to 50 μM etoposide activates procaspase-3 and the long isoform of procaspase-2 and induces mitochondrial potential decrease and cytochrome c release from mitochondria to the cytosol. All these events are prevented by p27(Kip1) overexpression. p27(Kip1) does not modulate Bcl-2, Bcl-X(L), Mcl-1 and Bax protein level in leukemic cells but suppresses Mcl-1 expression decrease observed in mock-transfected U937 cells undergoing etoposide-induced cell death. We conclude that p27(Kip1) prevents cell death upstream of the final pathway common to many apoptotic stimuli that involves cytochrome c release from mitochondria and activation of downstream caspases.