Ginsenoside Rh2-mediated G1 Phase Cell Cycle Arrest in Human Breast Cancer Cells Is Caused by p15 Ink4B and p27 Kip1 -dependent Inhibition of Cyclin-dependent Kinases

Present study was undertaken to gain insights into the mechanism of cell cycle arrest by ginseng saponin ginsenoside Rh2 (Rh2) using MCF-7 and MDA-MB-231 breast cancer cells. Cell viability and cell cycle distribution were determined by trypan blue dye exclusion assay and flow cytometry, respectively. Immunoblotting was performed to determine changes in protein levels. Knockdown of desired protein was achieved by transfection with small interfering RNA (siRNA). Rh2 treatment significantly inhibited viability of both cells in a concentration-dependent manner, which correlated with G(0)/G(1) phase cell cycle arrest. Rh2-mediated cell cycle arrest was accompanied by down-regulation of cyclin-dependent kinases (Cdk) and cyclins leading to decreased interaction between cyclin D1 and Cdk4/Cdk6 and increased recruitment of p15 (Ink4B) and p27 (Kip1) to cyclin D1/Cdk4 and cyclin D1/Cdk6 complexes. In addition, Rh2 treatment markedly reduced the levels of phosphorylated retinoblastoma protein (P-Rb) and decreased transcriptional activity of E2F1 in luciferase reporter assay. Rh2-induced cell cycle arrest was significantly attenuated by knockdown of p15 (Ink4B) and/or p27 (Kip1) proteins. Rh2-mediated cell cycle arrest in human breast cancer cells is caused by p15 (Ink4B) and p27 (Kip1) -dependent inhibition of kinase activities of G(1)-S specific Cdks/cyclin complexes.