Many veterans suffer from obesity, an illness that results from a cumulative excess of energy intake over energy expenditure. To combat this health issue, it is important to identify novel regulators of energy homeostasis that are amenable to pharmacological intervention. Our long-term goal is to elucidate how energy homeostasis is regulated by tankyrase (TNKS), a Golgi-associated enzyme that modifies substrate proteins through poly-ADP-ribosylation.
The aim of this application is to investigate the role of tankyrase in regulating energy expenditure with an emphasis on fuel substrate partitioning between organs as well as adiponectin secretion from adipocytes. To this end, we have used the gene- trapping technique in mice to create a tankyrase deficient (TNKS -/-) model. We found that TNKS -/- mice have increased energy expenditure, reduced adiposity, and greater food intake than wild-type controls. Intriguingly, their plasma levels of adiponectin, particularly the bioactive high molecular-weight form, are robustly increased. Consistent with the known effect of adiponectin, muscle in TNKS -/- mice shows an upregulation of genes that mediate the catabolism of triglycerides and fatty acids. On the other hand, liver in these mice shows an upregulation of genes that promote triglyceride release. Based on these and other observations, we hypothesize that the heightened energy expenditure in TNKS -/- mice entails increased hepatic secretion of triglycerides, which are channeled to the muscle for catabolism in response to hyperadiponectinemia. In this working model, the lipid flux from liver to muscle is at the expense of storage in adipose tissue, resulting in the lean phenotype of TNKS -/- mice despite a compensatory increase in food intake. To test our working hypothesis, Aim 1 will determine the extent to which TNKS deficiency increases the production and clearance of circulating triglycerides in vivo as well as the activity of muscle to hydrolyze triglycerides and oxidize fatty acids ex vivo.
Aim 2 will evaluate the prediction that TNKS -/- mice, owing to increased energy expenditure, are protected from obesity and lipotoxicity when challenged with a high fat diet.
Aim 3 will investigate the adipose-specific roles of TNKS in adiponectin secretion and energy homeostasis. Preliminary studies have shown that the hyperadiponectinemia of TNKS -/- mice is a post-transcriptional effect and can be recapitulated by knocking down TNKS in 3T3-L1 adipocytes. We will apply [35S] pulse-chase analysis to 3T3-L1 adipocytes to test the prediction that TNKS knockdown stabilizes adiponectin in the secretory pathway. We will also use the conditional knockout approach to determine the extent to which adipose-specific ablation of TNKS recapitulates the hyperadiponectinemia and other metabolic manifestations of TNKS -/- mice. The proposed studies are expected to shed light on the roles of TNKS in adiponectin secretion and energy homeostasis as well as the partitioning of fuel substrates between catabolism and storage. Outcomes from this application may provide the foundation for additional studies that validate TNKS as a pharmacological target for anti-obesity therapy.
Dietary calorie intake that exceeds energy expenditure is stored in the adipose tissue as fat. A long-term consequence of excessive caloric intake is obesity, which is a major risk factor for type 2 diabetes, heart attack and other serious complications. According to a recent survey, obesity affects as many as 37% of women and 33% of men among the population served by VA medical facilities. Therefore, amelioration of obesity is important to the health of many veterans. Using a mouse model to explore the regulation of adiposity, we have observed a lean phenotype when a cellular protein called tankyrase is inactivated. Our data indicate that these tankyrase-deficient mice are lean because of heightened energy expenditure. We propose to investigate the molecular mechanisms whereby lack of tankyrase function leads to heightened energy expenditure. Our long-term goal is to establish tankyrase as a pharmacological target in the treatment of obesity.