Obesity and its associated medical complications including type II diabetes, cardiovascular disease, dyslipidemia, and cancer account for more than 300,000 deaths per year in the United States. Obesity treatment strategies often do not result in adequate sustained weight loss, and the prevalence and severity of obesity in the U.S. and many other countries is progressively increasing. Recent surveys classify roughly 1/3 of all Americans as obese. The complexity of the obesity condition results from the interaction between environmental and predisposed genetic factors. A more thorough understanding of the molecular mechanisms underlying the pathogenesis of obesity and regulation of energy metabolism is essential for the development of effective therapies. Energy homeostasis is maintained by tissues and organs that sense and respond to the nutritional status of the body. At the central level, the hypothalamus is the primary component of the nervous system in interpreting adiposity or nutrient related inputs;it delivers hormonal and behavioral responses with the ultimate purpose of regulating energy intake and consumption. At the molecular level, enzymes called nutrient or energy sensors mimic the role of the tissues involved in energy balance. Two key energy/nutrient sensors, mTOR (mammalian target of rapamycin) and AMPK (AMP activated kinase) - in addition to their extensive metabolic roles in the peripheral tissues - are involved in the control of food intake in the hypothalamus. The enzyme Sirt1 is an evolutionarily conserved NAD+-dependent deacetylase involved in many biological processes including cellular differentiation, apoptosis, metabolism, and aging. Because of its dependence on NAD+, Sirt1 also functions as a nutrient/redox sensor. Thus, we hypothesized that Sirt1 could play a role in the control of energy balance, perhaps by influencing the mTOR or AMPK pathways, at the level of the hypothalamus. Our preliminary results indicate that pharmacological inhibition of hypothalamic Sirt1 or siRNA mediated knock down of Sirt1 in the arcuate nucleus (ARC) of the hypothalamus decreases food intake and body weight gain. Therefore, this proposal provides a novel view assigned to Sirt1 as a cellular energy sensor regulating energy balance at the hypothalamic level. Sirt1 links nutrient availability to energy homeostasis at the hypothalamic level, and this effect depends -at least in part- on the fasting induced and Sirt1 mediated regulation of FoxO1 deacetylation and mTOR signaling. Based on these findings we will:
Aim 1 : Determine the targets and mechanism of action hypothalamic Sirt1 signaling regulating energy balance. To this purpose we will determine whether: 1. Sirt1 regulates the anorexigenic POMC as well as the orexigenic Agouti-related peptide (AgRP) and neuropeptide Y (NPY) through FoxO1 in a nutrient dependent manner. 2. Sirt1 deacetylates and inhibits Stat3 in a nutrient sensitive manner 3. Sirt1 represses mTOR signaling at the hypothalamic level.
Aim 2 : Determine the role of hypothalamic Sirt1 in the regulation of the Prohormone Convertases 1 and 2. To this purpose we will test the following hypotheses: 1. Sirt1 regulates PC1 and PC2 in N43 hypothalamic cells 2. Sirt1 regulates the PC's in POMC and AgRP neurons in the arcuate nucleus. 3. The regulation of PC2 causes a decrease in 1-MSH levels and aggravates the DIO condition.
Aim 3 : Determine the role of hypothalamic Sirt1 in the diet-induced obesity (DIO) condition. To this purpose we will investigate the following hypotheses: 1. Food intake of DIO rats is sensitive to inhibition of hypothalamic Sirt1. 2. Exposure to High Fat Diet (HFD) alters hypothalamic Sirt1 expression, signaling, and Sirt1 co- substrate/activator NAD+ levels. 3. The inhibitory effect of Sirt1 on mTOR signaling in the DIO condition is one of the causes of obesity.
According to the World Health Organization, obesity in developed countries has reached epidemic proportions. There are globally more than 1 billion adults overweight, with around 300 million of them clinically obese. As a consequence, the study of obesity has become a priority in medical research. Still, the search for effective anti-obesity drugs has been unsuccessful. Initially, the discovery of the hormone leptin raised a lot of expectations, given its capacity to decrease appetite and body weight of obese leptin deficient mice. However, it was soon established that most obese humans and rodents have high levels of plasma leptin, which fail to reduce appetite and body weight. Many aspects of this leptin-resistant state are still unknown. A better understanding of leptin molecular mechanisms affecting neural circuits is essential to develop new strategies for the treatment of obesity. Very recently, the discovery of nutrient sensors took a big stage in the obesity research. Aside from mTOR (mammalian target of rapamycin) and AMPK (AMP activated kinase), known to be important energy/nutrient sensors, the enzyme Sirt1, known to be involved in many biological processes including cellular differentiation, apoptosis, metabolism, and aging, now has another potential role as energy sensor. Therefore, this proposal provides a novel view assigned to Sirt1 as a cellular energy sensor regulating energy balance at the hypothalamic level.
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