Strong evidence has implicated the circadian clock network as a key integrator of behavior and metabolism, and recent studies have shown that genetic disruption of core clock genes leads to diabetes mellitus and obesity. Conversely, high fat diet disrupts circadian rhythms of locomotor activity as well as 24 hr cycles of clock and clock controlled gene expression. At the molecular level, growing evidence has uncovered extensive interactions between circadian and metabolic transcription networks that may begin to explain the physiological connections between circadian rhythms, lipid and glucose metabolism. Recently our lab has established exciting new evidence to show that NAD+ biosynthesis and NAD+ vary across the light-dark cycle, leading us to hypothesisze that NAD+ functions as an oscillating metabolite linking circadian and metabolic cycles. One major regulator of NAD+ biosynthesis is the rate-limiting enzyme nicotinamide phosphoribosyltransferase (Nampt), which varies from dawn to dusk in the liver and is controlled in turn at the transcriptional level by 24 hr cycling of CLOCK/BMAL1. Alterations in Nampt/NAD+ modulate the nutrient-responsive deacetylase SIRT1, which plays an important role in both hepatic glucose synthesis and in the regulation the circadian clock. Thus, the CLOCK/BMAL1-NAMPT/NAD+-SIRT1 pathway comprises a novel metabolic feedback loop that integrates daily cycles of activity, feeding and glucose homeostasis. The goal of this proposal is to test the hypothesis that circadian disruption leads to metabolic disturbances through alterations in the NAMPT-NAD+-SIRT1 pathway in liver. The enclosed aims will address the role of the CLOCK/BMAL1 activity in SIRT1 mediated control of hepatic gluconeogenesis as well as the role of the NAD+-SIRT1 pathway in the pathogenesis of the metabolic disorder seen in circadian mutant mice. These studies will further our understanding of the molecular mechanism underlying the interconnection between circadian rhythm and metabolism.
One of the major crises that threatens to negate many of the recent advances in public health is the unremitting escalation of obesity and diabetes mellitus. However, deciphering the causitive factors in this epidemic has thwarted the best efforts of leading metabolic, behavioral and epidemiologic investigators. The studies proposed in the present application will establish in detail the basis of the internal circadian clock transcription network in metabolic pathobiology and diabetes mellitus.
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