In vivo and In vitro Studies on the Protective Effects of Silymarin against the Homocysteine-induced Hepatic ER stress

초록

Hyperhomocysteinemia, a condition of elevated blood homocysteine (Hcy) levels, is known to a risk factor for cardiovascular diseases, and is found in patients with liver diseases. Silymarin, an antioxidant flavonoid mixture isolated from the milk thistle (Silybum marianum L.), was used as a hepatoprotective agent. However, the effect of silymarin on Hcy-induced toxicity in liver is not well known. Therefore, we investigated the effects of silymarin on Hcy-induced endoplasmic reticulum (ER) stress in HepG2 cells. When cells incubated with 5 mM Hcy, splicing of X-box binding protein-1 (XBP-1) mRNA, expression of C/EBP homologous protein (CHOP) mRNA and protein, and phosphorylation of c-Jun and c-Jun N-terminal kinase (JNK) were increased. Pretreatment of cells with silymarin prevented the Hcy-induced CHOP protein expression and phosphorylation of c-Jun and JNK, but not XBP-1 splicing and CHOP mRNA expression. Furthermore silymarin induced the expression and activity of cystathionine β-synthase (CBS), an enzyme metabolizes Hcy to cystathionine in HepG2 cells. Hepatic ER stress also induced by the high-methionine (HM) diet fed rat. Oral silymarin administration for 30 days with HM diet prevented HM diet-induced increase of phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α), c-Jun, and JNK in the liver. Additionally, silymarin administration prevented HM diet-induced hepatic CBS expression and activity. Hcy-induced accumulation of total cholesterol was reduced in the pretreatment of silymarin in HepG2 cells. Silymarin administration prevented HM diet-induced increase in the concentrations of plasma triglycerides. In conclusion, silymarin has a hepatoprotective effect against Hcy-induced ER stress and partially regulates the Hcy metabolism.