The plant alkaloid cryptolepine induces p21WAF1/CIP1 and cell cycle arrest in a human osteosarcoma cell line

We previously established a bioassay method to screen for compounds that activate the promoter activity of p21WAF1/CIP1, a potent inhibitor of cyclin-dependent kinases, in a p53-independent manner. As an activator of p21WAF1/CIP1 promoter activity, we isolated cryptolepine (CLP: 5-methyl indolo (2,3b)-quiniine), an indoloquinoline alkaloid, from the traditional Ayurvedic medicinal plant Sida cordifolia. We show here that CLP induces the expression of p21WAF1/CIP1 with growth arrest in p53-mutated human osteosarcoma MG63 cells. Four micromolar of CLP completely inhibited the growth of MG63 cells and caused G2/M-phase arrest. CLP up-regulated the expression of p21WAF1/CIP1 at both mRNA and protein levels in a dose-dependent manner. Using several mutant p21WAF1/CIP1 promoter constructs, we found that the CLP-responsive element is an Sp1 site at -82 relative to the transcription start site of the p21WAF1/CIP1 promoter. These findings suggest that CLP arrests the growth of MG63 cells by activating the p21WAF1/CIP1 promoter through the specific Sp1 site in a p53-independent manner. In addition, CLP-mediated cell cycle arrest was reduced by the knockout of the p21WAF1/CIP1 gene in human colon cancer HCT116 cells, suggesting that the cell cycle arrest by CLP was at least partially mediated through the induction of p21WAF1/CIP1 expression. Although we need further study of chemotherapeutic effect in vivo, these results raise the possibility that CLP might be a suitable chemotherapeutic agent for treatment of osteosarcoma.