Mechanisms of obesity-linked muscle atrophy and n-3 fatty acids

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PROJECT SUMMARY Obesity is a clinical issue of epidemic proportion in the Veteran community and across the country. It adversely affects almost every organ and causes metabolic dysfunction including chronic inflammation, insulin resistance and diabetes, dyslipidemias, and cardiovascular disease. An underappreciated complication of obesity is that it causes muscle atrophy. This wasting leads to muscle weakness and frailty which increases the incidence of falls, reduces the quality of life, and is associated with a higher incidence of co-morbidities and risks of mortality. Evidence from animals and patients indicates that obesity decreases muscle size and strength by impairing myogenesis (i.e., fiber repair) and by attenuating protein synthesis and accelerating proteolysis. Previous studies have documented activation of multiple proteolytic systems - the ubiquitin-proteasome, autophagy and caspase-3 systems - in obese db/db mice and recent studies by our lab indicate that these responses can be duplicated by incubating cultured C2C12 myotubes with palmitate, one of the body’s most abundant saturated fatty acids. Palmitate accelerates the rate of protein degradation in C2C12 myotubes by reducing insulin signaling and stimulating all three of the proteolytic systems that are up-regulated in obese mice. We have extended these findings by recently demonstrating that palmitate-induced ER stress also contributes to the production of atrophy-related gene and protein expression. Myostatin (MSTN), a member of the BMP/TGFß superfamily, contributes to protein degradation by inducing atrophy-related proteolytic enzymes and the myokine is increased in muscle of obese patients and animals. New preliminary data demonstrate that palmitate induces MSTN mRNA. Based on these findings, we propose to investigate whether obesity contributes to MSTN expression in myotubes and skeletal muscle in vivo by activating the C/EBP and CREB transcription factors via two newly identified signaling mechanisms linked to ER stress and Akt- regulated phosphodiesterases (PDE3/4), respectively (Aim 1). MSTN increases the expression of atrophy- related proteins, in part, by impairing insulin and IGF-1 signaling through Akt, which activates the catabolic FoXO transcription factors), however, it remains unknown how MSTN reduces Akt activity. In Aim 2, we will investigate whether MSTN-related suppression of microRNA-29 and increased expression of one of its targets - phosphatase and tensin homolog (PTEN) contributes to Akt dysregulation and increased FoxO-mediated gene expression in skeletal muscle and myotubes. In Aim 3, we will test whether dietary provision of omega-3 polyunsaturated fatty acids (n-3 PUFA) to db/db mice reverses obesity-related dysfunctional MSTN signaling and protein catabolic responses that lead to muscle atrophy. Recent animal and patient studies provide evidence that administration of n-3 PUFA can produce muscle-sparing effects in cancer, kidney failure and other atrophy-associated conditions. Our preliminary data demonstrate that a n-3 PUFA, docosahexaenoic acid (DHA), prevents the palmitate-induced changes in MSTN expression and Akt-FoxO and ER stress signaling that are linked to muscle atrogene expression. We propose that these beneficial effects are achieved by reversing obesity-related dysfunctional signaling that produce MSTN and other protein catabolic responses which ultimately lead to muscle atrophy. Results from Aim 3 will provide new preclinical evidence to support the feasibility of using of n-3 PUFA as a cost-effective therapy to counter the adverse effects of obesity on skeletal muscle mass in Veterans.