Asiatic Acid Promotes p21WAF1/CIP1 Protein Stability through Attenuation of NDR1/2 Dependent Phosphorylation of p21WAF1/CIP1 in HepG2 Human Hepatoma Cells

Previous studies have suggested anti-tumor effects of asiatic acid in some human cancer cell lines. This agent isreported to increase the levels of p21WAF1/CIP1 in human breast cancer cell lines. However, the molecular mechanismshave not been established. Here we report that asiatic acid up-regulates p21WAF1/CIP1 protein expression but notthe level of p21WAF1/CIP1 mRNA in HepG2 human hepatoma cells. Furthermore, we found that the asiatic acidinduced increase of p21WAF1/CIP1 protein was associated with decreased phosphorylation (ser-146) of p21WAF1/CIP1.Knockdown of NDR1/2 kinase, which directly phosphorylates p21WAF1/CIP1 protein at ser-146 and enhances itsproteasomal degradation, increased the levels of p21WAF1/CIP1 protein and eliminated the regulation of p21WAF1/CIP1 stability by asiatic acid. At the same time, the expression of NDR1/2 kinase decreased during treatment withasiatic acid in HepG2 cells. Moreover, asiatic acid inhibited the proliferation of HepG2 cells, this being attenuatedby knockdown of p21WAF1/CIP1. In conclusion, we propose that asiatic acid inhibits the expression NDR1/2 kinaseand promotes the stability of p21WAF1/CIP1 protein through attenuating NDR1/2 dependent phosphorylation ofp21WAF1/CIP1 in HepG2 cells.