Propolin H from Taiwanese propolis induces G1 arrest in human lung carcinoma cells

Propolis, a natural product collected by honeybee, has been reported to exert a wide spectrum of biological functions. In this study, we have isolated a novel component, namely, propolin H, and investigated its effects in human carcinoma cells. Propolin H inhibited the proliferation of human lung carcinoma cell lines in MTT assay, and a significant G1 arrest was observed to occur in a dose-dependent manner at 24 h of exposure in H460 cells. After treatment with propolin H in H460 cells, the content of the CDK inhibitor p21Waf1/Cip1 protein increased in correlation with the elevation in p53 levels. Western blot analysis of G1 regulatory proteins further revealed a decrease in cyclin-dependent kinase 2 (CDK2) and CDK4 and an increase in cyclin E. The CDKs kinase activities assay showed that propolin H has inhibited CDK2 and CDK4 kinase activities. Accordingly, coimmunoprecipitations revealed an increased association of both CDK2 and CDK4 immunoreactive protein with the p21Waf1/Cip1 protein complex under propolin H-treated conditions. Additionally, we found that propolin H enhanced the expression of p21Waf1/Cip1 in p53-mutant and p53-null lung carcinoma cell lines, following the induction of G1 arrest. Together, these findings suggest that the induction of p21Waf1/Cip1 expression occurred through p53-dependent and -independent pathways in propolin H-treated cells. Propolin H exerts its significantly growth inhibitory effects and may have therapeutic applications.