We investigated the role of some key regulators of cell cycle in the activation of caspases during apoptosis of insulinsecreting cells after sustained depletion of GTP by a specific inosine 5 0 -monophosphate dehydrogenase inhibitor, mycophenolic acid (MPA). p21Waf1/Cip1 was significantly increased following MPA treatment, an event closely correlated with the time course of caspase activation under the same conditions. MPA-induced p21Waf1/Cip1 was not mediated by p53, since p53 mass was gradually reduced over time of MPA treatment. The increment of p21 Waf1/Cip1 by MPA was further enhanced in the presence of a pan-caspase inhibitor, indicating that the increased p21Waf1/Cip1 may occur prior to caspase activation. This notion of association of p21Waf1/Cip1 accumulation with caspase activation and apoptosis was substantiated by using mimosine, a selective p21Waf1/Cip1 inducer independent of p53. Mimosine, like MPA, also increased p21 Waf1/Cip1 , promoted apoptosis and simultaneously increased the activity of caspases. Furthermore, knocking down of p21 Waf1/Cip1 transfection of siRNA duplex inhibited caspase activation and apoptosis due to GTP depletion. In contrast to p21Waf1/Cip1 , a reduction in p27 Kip1 occurred in MPA-treated cells. These results indicate that p21Waf1/Cip1 may act as an upstream signal to block mitogenesis and activate caspases which in turn contribute to induction of apoptosis.