The molecular mechanistic effects of a grape seed procyanidin extract in regulating hepatic lipid metabolism
AuthorDowning, Laura Elizabeth
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Hypertriglyceridemia is an independent risk factor for cardiovascular disease (CVD), the leading cause of mortality in the United States. Previous studies have shown that a grape seed procyanidin extract (GSPE) exerts potent triglyceride-lowering effects in normolipidemic rodents through acting as a co-agonist ligand for the farnesoid x receptor (Fxr), repressing lipogenesis while also increasing bile acid (BA) and subsequent cholesterol synthesis. The aim of the studies presented herein was to elucidate the underlying molecular mechanisms responsible for these physiological effects in both normolipidemic and hyperlipidemic states.Along with the Fxr-dependent mechanism described above, previous studies have shown increased Cpt1a and Apoa5 gene expression following GSPE treatment, suggesting increased fatty acid catabolism. Because histone deacetylaces (HDACs) exert epigenetic control on fatty acid catabolism and dietary flavonoids have proven to be effective HDAC inhibitors, we assessed whether GSPE increases fatty acid catabolism and lowers serum triglyceride (TG) levels via HDAC inhibition and subsequent modulation of Pparα phosphorylation and target gene expression. Studies were carried out in liver tissue from normolipidemic male C57BL/6 mice administered 250 mg/kg GSPE for 14 hours. We found that GSPE treatment led to inhibition of HDAC2 and 3 activity and increased histone acetylation. Pparα gene expression was increased, and although Pparα protein expression was decreased, phosphorylated Pparα protein levels were increased over two-fold, while expression of Pparα target-genes involved in fatty acid catabolism were also up-regulated. Additionally, expression of Fgf21, a hormone and Pparα target, demonstrated an eight-fold increase in gene expression, while serum levels were increased seven-fold. Such changes in gene and protein expression were accompanied by a 28% decrease in serum TG levels. Collectively these results suggest, for the first time, that GSPE may lower serum TG via HDAC inhibition, a process that allows increased phosphorylation of Pparα, resulting in increased fatty acid catabolism.Because hypertriglyceridemia is causally associated with CVD, we next assessed the hypolipidemic effects of GSPE on fructose-induced dyslipidemia. Prior to GSPE treatment, rats were fed either a high-fructose diet, which elevated serum TG and liver fat accumulation, or a standard chow control diet. Following the initial 8-week period of fructose feeding, rats continued on their respective diets while also receiving either GSPE (250 mg/kg) or vehicle (water) for 7 days. Rats administered GSPE on the high fructose diet showed a 41% decrease in serum TG levels compared to rats administered vehicle on the high fructose diet, as well as increased fecal cholesterol excretion. GSPE treatment in the presence of fructose repressed the expression of key genes involved in TG synthesis, including sterol regulatory element binding protein 1c (Srebp-1c), while robustly increasing the expression of genes involved in cholesterol synthesis, particularly the rate-limiting enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (Hmgcr). Expression of genes involved in BA and cholesteryl ester synthesis and export were not increased. Such results indicate that, in the presence of dietary fructose, GSPE diverts lipogenic substrates into cholesterol synthesis in order to maintain hepatic lipid homeostasis, subsequently secreting the cholesterol via a non-biliary route into the plasma, where it is ultimately sent to the intestine for excretion via trans-intestinal cholesterol excretion (TICE). Because excessive fructose intake has been causally linked with the CVD risk factors hypertriglyceridemia and fatty liver, these results suggest that GSPE treatment may be beneficial in mitigating the negative metabolic effects induced by a high fructose diet.In conclusion, the results presented in this thesis suggest that GSPE lowers serum TG in vivo through multiple mechanisms, including epigenetic regulation via modulation of HDAC and Pparα activity and also via modulation of cholesterol synthesis and excretion, depending on the physiological state. Because HDAC inhibitors are gaining recognition as hypolipidemic agents and GSPE effectively lowers serum TG in a hyperlipidemic state, further studies are warranted to assess the therapeutic effects of this natural product in treating CVD-associated risk factors.