PLECKSTRIN-HOMOLOGY LIKE DOMAIN FAMILY A MEMBER 1 (PHLDA1)/T-CELL DEATH ASSOCIATED GENE 51 (TDAG51) AS A METABOLIC MODULATOR THAT IMPACTS LIVER FUNCTION AND ADIPOSITY

Abstract

The endoplasmic reticulum (ER) is a specialized organelle responsible for facilitating the synthesis, folding and maturation of proteins. Conditions that severely impair protein folding, alter intracellular calcium or redox signaling, as well as other factors that impact ER function leads to ER stress. ER stress triggers a sequence of signal transduction cascades called the unfolded protein response (UPR) designed to restore ER homeostasis. It is well established that ER stress and a dysfunctional UPR cause metabolic disturbances which contribute to obesity, fatty liver, and insulin resistance. Hepatocytes play a critical role in energy homeostasis as well as lipid and carbohydrate metabolism. Overnutrition dysregulates lipogenesis and beta-oxidation resulting in hepatic lipid accumulation and the impairment of hepatocyte function leading to ER stress-induced cellular pathways that modulate autophagy, apoptosis, and fibrosis. Deficiency of the ER stress-response gene, T-cell death associated gene 51 (TDAG51), in mice promotes the development of high fat diet (HFD)-induced obesity, fatty liver, and hepatic insulin resistance (IR). The focal point of my doctoral studies represents the axis between hepatic TDAG51 protein expression and resulting ER stress-related metabolic syndrome. Based on this information, the first aim of my doctoral studies was to examine hepatic TDAG51 protein stability under conditions of liver injury in mice and humans. We report that hepatic TDAG51 protein levels are reduced in multiple mouse models of liver steatosis and injury as well as in liver biopsies from patients with non-alcoholic steatohepatitis, compared to normal controls. Several mechanisms were explored to examine the mode of TDAG51 protein degradation. Studies in this thesis report the anti-obesogenic and hepatoprotective effects of liver-specific TDAG51/PHLDA1 overexpression in several mouse models of obesity. Delivery of a liver-specific adeno-associated virus (AAV) increased hepatic expression of a TDAG51-GFP fusion protein in wild type, whole-body TDAG51 knockout mice (TDAG51-/-) and leptin-deficient (ob/ob) mice. Increased hepatic TDAG51 protein levels improved insulin sensitivity while reducing body weight and fatty liver in HFD fed TDAG51-/- mice and in ob/ob mice. These findings support the role of TDAG51/PHLDA1 as an important modulator of lipid metabolism contributing to liver function and whole-body energy metabolism. Indeed, as reported here, forced AAV expression of TDAG51-GFP reduced lipogenic gene expression, increased beta-oxidation and lowered hepatic and serum triglycerides, findings consistent with reduced liver weight. In vitro studies also reflect the lipid-lowering effect of TDAG51 overexpression in oleic acid-treated Huh7 cells. In the second aim of this thesis, we determined the effects of liver-specific and whole-body TDAG51-/- in mouse models of fasting-induced autophagy. We hypothesize that the blunted ER stress response may be responsible for the attenuation of autophagy resulting in the impairment of lipid metabolism observed in liver-specific and whole-body TDAG51-/- mice while AAV-TDAG51-GFP restoration reduces lipid accumulation and promotes autophagosomal formation. Thirdly, we examined histological outcomes of methionine-choline deficient diet-induced steatohepatitis and fibrosis in liver-specific and whole-body TDAG51-/- compared to wild-type controls. In the absence of hepatic TDAG51 protein, ER stress mediated pathways resulting in apoptosis and fibrosis were attenuated in methionine-choline deficient diet fed liver-specific and whole-body TDAG51-/- mice compared to wild-type controls. Lastly, our research group has shown that the loss of TDAG51 affects the expression of cellular factors responsible for adipogenesis including the peroxisome proliferator-activated receptor gamma and growth differentiation factor 10 (GDF10). In this thesis, we report that GDF10 levels are significantly lowered in children with obesity and this effect correlates with cholesterol impairment. Taken together, studies in this thesis indicate that TDAG51 acts as an effector of ER stress activation to modulate hepatocyte lipid metabolism and adiposity. These studies suggest that targeting TDAG51 protein could be a relevant therapeutic strategy in the treatment and management of cardiometabolic diseases, particularly MAFLD-related obesity, and insulin resistance.