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Human amylin (10 microM) significantly inhibited RINm5F islet beta-cell proliferation and evoked apoptosis associated with typical degenerative ultrastructural changes and DNA fragmentation, whereas rat amylin did not. Time course analysis showed that human amylin elicited apoptosis in a passage-dependent manner. Expression of the apoptosis-related genes p53, bcl-2 and WAF1/CIP1 was examined using Northern blots. mRNAs corresponding to p53 and to p21WAF/CIP1 were remarkably increased following human amylin treatment, whereas no change in bcl-2 was detected. Our data suggest a role of p53 and p21 in human amylin-induced beta-cell apoptosis. Furthermore, cells with higher proliferative potential (lower passage) were found to be more susceptible to apoptosis and to induction of p53, suggesting that beta-cells with different proliferation rates respond differently to human amylin, and that human amylin may be more toxic to proliferating cells.

When cells progressing in G1 phase are irradiated with UV light, two damage checkpoint pathways are activated: CHK1–Cdc25A and p53–p21WAF1/CIP1, both targeting Cdk2 but the latter inducing long lasting inactivation. In similarly irradiated S phase cells, however, p21WAF1/CIP1-dependent checkpoint is largely inactive. We report here that p21-dependent checkpoint can effectively be activated and induce a prolonged S phase arrest with similarly extended inactivation of Cdk2 by association of p21 if mid-S phase cells are damaged with a base-modifying agent instead of UV light, indicating that the poor utilization of p21-dependent checkpoint is not an innate property of S phase cells.