The metabolic syndrome is a state of insulin resistance characterized by multiple derangements in lipid homeostasis, leading to dyslipidemia, atherosclerosis, hepatic steatosis, and cholesterol gallstones. The factors that drive these derangements are unknown but must be determined in order to effectively treat the metabolic syndrome. The transcription factors FoxO1 and Sterol Regulatory Element Binding Protein (SREBP)-1c are key regulators of insulin action. FoxO1 promotes transcription of the gluconeogenic enzymes, but also the enzymes necessary for VLDL secretion and biliary cholesterol secretion. SREBP-1c, on the other hand, induces the lipogenic enzymes. In normal livers, insulin suppresses FoxO1 and activates SREBP-1c. In the metabolic syndrome, insulin fails to suppress FoxO1 but SREBP-1c is paradoxically increased. This raises the question, what drives SREBP-1c in the presence of insulin resistance? Our preliminary data show that the livers of Liver Insulin Receptor Knockout (LIRKO) mice, which are unresponsive to insulin, show a rapamycin- sensitive increase in SREBP-1c in response to dietary carbohydrates. These data indicate the existence of an insulin-independent signaling pathway that could potentially allow the excessive consumption of carbohydrates to activate SREBP-1c and lipogenesis, even in the presence of insulin resistance. The overarching goal of this proposal is to identify the key driver of lipid metabolism in the insulin resistant state. We hypothesize that FoxO1, which fails to be suppressed by insulin, drives dyslipidemia, atherosclerosis and gallstones; but that SREBP-1c, induced by nutrients, drives lipogenesis and steatosis.
Our aims are to (1) determine the extent to which FoxO1 and SREBP-1c promote atherosclerosis, steatosis, and cholesterol gallstones by knocking down FoxO1 or reconstituting SREBP-1c expression in the livers of LIRKO mice; and (2) to define the insulin-independent signaling pathways by which nutrients can activate SREBP-1c and lipogenesis. We expect to find that FoxO1 and SREBP-1c define two distinct metabolic signaling pathways that are both necessary for the full complement of derangements present in the metabolic syndrome.
The metabolic syndrome affects approximately one third of adults in the United States. Much of the morbidity and mortality associated with this disorder can be ascribed to derangements in lipid homeostasis: dyslipidemia, atherosclerosis, cholesterol gallstones, and hepatic steatosis. The studies here will identify the key regulatory molecules that drive these derangements, and ultimately lead to more rational and effective therapies for the metabolic syndrome.
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