Persons with fasting plasma tiglycerides (TG) above 250 mg/dl have a two-fold greater risk of cardiovascular disease. The nutritional manipulation of increasing the consumption of fish oils high in omega-3 fatty acids (w3-FA) presumably decreases this disease risk by inhibiting hepatic VLDL-TG production. The long term goal is to establish the intracellular mechanisms by which dietary fats of different fatty acid composition interact with hormonal stimuli to modulate hepatic lipogeneis.
The specific aim i s to demonstrate that w3-FA enrichment of rat liver cell membranes causes increased responsiveness to the antilipogenic horonmal stimuli and decreased sensitivity to lipogenic signals. The hormone models will be the adenylate cyclase agonist glucagon and the phosphoinositide-Ca++-release agonist, vassopressin. The shift in hepatic horonmal responsiveness will be revealed by: (1) decreased fatty acid synthetase (FAS) gene expression (de novo path), (2) lower phosphatidic acid phosphohydrolase (PAP) activity (TG path), and (3) increased partitioning of FFA into oxidation and ketogenesis. Comparisons will include w3-, w6-, and w9-FA. The approach will demonstrate: (1) that dietary w3-FA suppress FAS synthesis by decreasing FAS-mRNA abundance as determined by hybridization with a specific cDNA for FAS-mRNA; and that w3-FA decrease liver PAP maximal activity by inhibiting PAP synthesis; (2) that glucagon inhibition of FAS expression is greatest in cultured hepatocytes enriched with w3-FA (20:5, 22:6) in comparison to w6-FA (18:2, 20:4) or w9-FA (18:1); hepatocyte phospholipid enrichment is achieved by culturing cells in presence of specific FA; (3) with hepatocytes freshly isolated from donors adapted to dietary w3-, w6-, or w9-FA, that w3-FA alter the kinetics of 1- 14C-oleate partitioning away from TG and toward ketones, and that the greater ketogenic capacity with w3-FA is due to enhanced mitochondrial FFA uptake; and (4) that the amount of glucagon needed to achieve half-max stimulation of adenylate cyclase, cAMP, and glycogenolysis is lowest in w3-FA rich liver cells, and the amount of vassopressin needed to achieve half-max cytosolic Ca++and glycogen phosphorylase activation is increased by w3-FA. The approach will elucidate the mechanism of the hypolipemic action of w3-FA as well as yield essential information regarding nutritional control of gene expression.
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