Obesity is associated with multi-organ insulin resistance, which is an important risk factor for type 2 diabetes, atherogenic dyslipidemia, and cardiovascular disease. The complex mechanisms responsible for obesity- induced multi-organ insulin resistance are not clear. We recently conducted a series of focused studies that provide evidence for a novel mechanism that links adipocyte nicotinamide adenine dinucleotide (NAD) biosynthesis with increased PPAR? (peroxisome proliferator-activated receptor ?) serine-273 (S273) phosphorylation and multi-organ insulin resistance. First, we found that adipose tissue gene expression of nicotinamide phosphoribosyltransferase (NAMPT), which is a key regulator of NAD biosynthesis, and NAD content were decreased in both obese rodents and obese people, compared with their healthy non-obese counterparts. Second, we observed severe insulin resistance in adipose tissue, liver and skeletal muscle, and increased plasma free fatty acid (FFA) concentration, independent of whole-body adiposity, in adipocyte- specific Nampt knockout mice. The insulin resistance phenotype was completely normalized by administration of a key NAD precursor, nicotinamide mononucleotide. Third, adipocyte-specific Nampt deletion increased the phosphorylation of PPAR? S273 and decreased adipose tissue gene expression of obesity-linked specific targets of PPAR? S273 phosphorylation, including a key insulin-sensitizing adipokine, adiponectin. Consistently, loss of NAMPT markedly decreased plasma concentrations of adiponectin, which could contribute to the development of multi-organ insulin resistance. Based on these data, we hypothesize that impaired NAMPT-mediated NAD biosynthesis in adipocytes increases PPAR? acetylation and S273 phosphorylation, which plays a causative role in the pathogenesis of adipocyte dysfunction and multi-organ insulin resistance. Thus, NAD biosynthesis in adipocytes could be a key therapeutic target for obesity-induced multi-organ insulin resistance. To test these hypotheses, we will determine 1) adipose tissue NAMPT gene expression, NAD content and PPAR? S273 phosphorylation in metabolically-normal obese (MNO) and metabolically-abnormal obese (MAO) subjects before and after weight loss, 2) the mechanisms by which NAD regulates PPAR? S273 phosphorylation in human and mouse adipocytes, and 3) whether enhanced NAD biosynthesis in adipocytes encounters the pathophysiology of obesity-induced insulin resistance in mice and human adipose tissue explants. These studies will provide new mechanistic and therapeutic insights into a potential novel mechanism involved in the pathogenesis of multi-organ insulin resistance in obese people.
Obesity is associated with insulin resistance, which is an important risk factor for type 2 diabetes and cardiovascular disease. In the proposed studies, we will aim to understand the mechanisms of obesity- associated insulin resistance and to provide a new treatment for insulin resistance.
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