Modulation of caspase activation and p27 Kip1 degradation in the p53-induced apoptosis in IW32 erythroleukemia cells

Abstract

To examine the p53-mediated biological activities and signalling pathways, we generated stable transfectants of the p53-null IW32 murine erythroleukemia cells expressing the temperature-sensitive p53 mutant DNA, tsp53 val135. Two clones with different levels of p53 protein expression were selected for further characterization. At permissive temperature, clone 1–5 cells differentiated along the erythroid pathway, and clone 3–2 cells that produced greater levels (3.5-fold) of p53 underwent apoptosis. Apoptosis of 3–2 cells was accompanied by mitochondrial cytochrome c release and caspase activation as well as by cleavage of caspase substrates. Bax protein was induced to a similar extent in these clones by wild-type p53; expression of p21 Cip1/Waf1 and p27 Kip1 proteins was also increased. However, significantly lesser extent of induction for both CDK inhibitors was detected in the apoptotic 3–2 clone. The general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD.fmk) blocked the p53-induced apoptosis in 3–2 cells, with a concomitant elevation of p27 Kip1, suggesting that p27 Kip1 protein underwent caspase-dependent proteolysis in the apoptotic 3–2 cells. Together these results linked a pathway involving cytochrome c release, caspase activation and p27 Kip1 degradation to the p53-induced apoptosis in IW32 erythroleukemia cells.