Hyperhomocysteinemia induces hepatic cholesterol biosynthesis and lipid accumulation via activation of transcription factors

Departments of 1 Physiology and 2 Animal Science, 3 National Centre for Agri-Food Research in Medicine, St. Boniface Hospital Research Centre, 4 Centre for Research and Treatment of Atherosclerosis, Faculty of Medicine, and 5 Department of Internal Medicine, University of Manitoba, Winnipeg, Manitoba, Canada Submitted 29 October 2004 ; accepted in final form 29 December 2004 Hyperhomocysteinemia is an independent risk factor for cardiovascular disorders. Elevated plasma homocysteine (Hcy) concentration is associated with other cardiovascular risk factors. We previously reported that Hcy stimulated cholesterol biosynthesis in HepG2 cells. In the present study, we investigated the underlying mechanisms of Hcy-induced hepatic cholesterol biosynthesis in an animal model. Hyperhomocysteinemia was induced in Sprague-Dawley rats by feeding a high-methionine diet for 4 wk. The mRNA expression and the enzyme activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase were significantly increased in livers of hyperhomocysteinemic rats. There were marked hepatic lipid accumulation and an elevation of plasma cholesterol concentration in hyperhomocysteinemic rats. Three transcription factors, namely, sterol regulatory element-binding protein-2 (SREBP-2), cAMP response element-binding protein (CREB), and nuclear factor Y (NF-Y) were activated in livers of hyperhomocysteinemic rats. Upon Hcy treatment of hepatocytes, there was a significant increase in HMG-CoA reductase mRNA expression in these cells. The activation of SREBP-2, CREB, and NF-Y preceded the increase in HMG-CoA reductase expression in Hcy-treated cells. Pretreatment of hepatocytes with inhibitors for transcription factors not only blocked the activation of SREBP-2, CREB, and NF-Y but also attenuated Hcy-induced HMG-CoA reductase mRNA expression. These results suggested that hyperhomocysteinemia-induced activation of SREBP-2, CREB, and NF-Y was responsible for increased cholesterol biosynthesis by transcriptionally regulating HMG-CoA reductase expression in the liver leading to hepatic lipid accumulation and subsequently hypercholesterolemia. In conclusion, the stimulatory effect of Hcy on hepatic cholesterol biosynthesis may represent an important mechanism for hepatic lipid accumulation and cardiovascular disorder associated with hyperhomocysteinemia. homocysteine; 3-hydroxy-3-methylglutaryl coenzyme A reductase; cAMP response element-binding protein; sterol regulatory element-binding protein-2;