Kinetin inhibits proliferation of hepatic stellate cells by interrupting cell cycle and induces apoptosis by down-regulating ratio of Bcl-2/Bax

Liver fibrosis is an important health problem that can further progress into cirrhosis or liver cancer, and result in significant morbidity and mortality. Inhibiting proliferation and inducing apoptosis of hepatic stellate cells (HSCs) may be the key point to reverse liver fibrosis. At present, anti-fibrosis drugs are rare. Kinetin is a type of plant-derived cytokinin which has been reported to control differentiation and induce apoptosis of human cells. In this study, the HSCs were incubated with different concentrations of kinetin. The proliferation of rat HSCs was measured by MTT assay, cell cycle and apoptosis were analyzed by flow cytometry, and the apoptosis was examined by TUNEL method. The expression of Bcl-2 and Bax proteins was detected by immunocytochemistry staining. It was found that kinetin could markedly inhibit proliferation of HSCs. In a concentration range of 2 to 8 mu g/mL, the inhibitory effects of kinetin on proliferation of HSCs were increased with the increased concentration and the extension of time (P < 0.01). Flow cytometry indicated that kinetin could inhibit the DNA synthesis from G0/G1 to S phase in a dose-dependent manner (P < 0.01). The apoptosis rates of the HSCs treated with 8, 4 and 2 mu g/mL kinetin (25.62%+/- 2.21%, 15.31%+/- 1.9% and 6.18%+/- 1.23%, respectively) were increased significantly compared with the control group (3.81%+/- 0.93%) (P < 0.01). All the DNA frequency histogram in kinetin-treated groups showed obvious hypodiploid peak (sub-G1 peak), and with the increase of kinetin concentrations, the apoptosis rate of HSCs also showed a trend of increase. It was also found that kinetin could down-regulate the expression of Bcl-2, and up-regulate the expression of Bax, leading to the decreased ratio of Bcl-2/Bax significantly. The kinetin-induced apoptosis of HSCs was positively correlated with the expression of Bax, and negatively with the expression of Bcl-2. It was concluded that kinetin can inhibit activation and proliferation of HSCs by interrupting the cell cycle at G1/S restriction point and inducing apoptosis of HSCs via reducing the ratio of Bcl-2/Bax.