The cyclin-dependent kinase inhibitor flavopiridol potentiates the effects of topoisomerase I poisons by suppressing rad51 expression in a p53-dependent manner

The results of a phase I clinical trial of the topoisomerase I (Topo 1) poison CPT-11 followed by the cyclin-dependent kinase inhibitor flavopiridol in patients with advanced solid tumors indicate that patients whose tumors were wild-type, but not mutant, for p53 obtained the most clinical benefit from this combination therapy. We elected to elucidate the mechanistic basis for this effect in isogenic-paired HCT116 colon cancer cells that were either wild-type (+/+) or null (-/-) for p53. With the combination therapy of SN-38 (the active metabolite of CPT-11) followed by flavopiridol, the induction of apoptosis was 5-fold greater in the p53+/+ cells compared with the p53-/- cells. This sequential treatment induced phosphorylation of p53 at Ser(15), which interacted with Rad(51), a DNA repair protein involved in homologous recombination. Rad(51) bound to p53-Ser(15) within the first 5 hours of combination therapy, and then was transcriptionally suppressed at 24 hours by flavopiridol only in p53+/+ cells. Microarray analysis also revealed suppression of Rad51 in a p53-dependent manner. Depletion of Rad51 by small interfering RNA (siRNA) sensitized both p53+/+ and p53-/- cells to SN-38-induced apoptosis with increase of gamma H2AX, a marker of DNA damage. Conversely, overexpression of Rad51. rescued p53+/+ cells from SN -> F-induced apoptosis. Because flavopiridol inhibits Cdk9, we found that inhibition of Cdk9 by DRB; or by siRNA could recapitulate the flavopiridol effects, with suppression of Rad51 and induction of apoptosis only in p53+/+ cells. In conclusion, after DNA damage by Topo I poisons, flavopiridol targets homologous recombination through a p53-dependent down-regulation of Rad51, resulting in enhancement of apoptosis.