Differential roles of p21^Waf1 and p27^Kip1 in modulating chemosensitivity and their possible application in drug discovery studies

In this study, the differential role of the cyclin-dependent kinase (CDK) inhibitors p21^Waf1 and p27^Kip1 in cell cycle regulation was proposed for use in screening natural or synthetic compounds for cell cycle-dependent (particularly M phase-dependent) antineoplastic activity. p21^Waf1 or p27^Kip1 was ectopically expressed with an ecdysone-inducible mammalian expression system in a human colon adenocarcinoma cell line. Induction of p21^Waf1 or p27^Kip1 expression inhibited the activities of CDK2 and completely arrested cells at G₁ phase of the cell cycle by p27^Kip1 and at G₁ and G₂ phases by p21^Waf1. We examined the sensitivity of these cells to several antineoplastic agents known to be cell cycle-dependent or -independent. Substantially increased resistance to cell cycle-dependent antineoplastic agents was found in the cells when the expression of p21^Waf1 or p27^Kip1 was induced. In contrast, only a desensitization to cell cycle-independent antineoplastic agents was found in the cells arrested by p21^Waf1 or p27^Kip1. Because p21^Waf1induces an additional block at G₂ phase that inhibits cell entry into M phase, we further examined the difference between p21^Waf1- and p27^Kip1-induced cells in their sensitivity to D-24851, a novel M phase-dependent compound. We found that induction of p21^Waf1 after exposure of the cells to D-24851 conferred stronger resistance than did induction of p27^Kip1. Taken together, our results suggest that the differential effect of p21^Waf1 and p27^Kip1 on cell cycle regulation may be advantageous for screening chemical libraries for novel antineoplastic candidates that are cell cycle-dependent, and M phase-dependent in particular.