Novel molecular regulators of adipose tissue metabolism

Adipose tissue is distributed across the body as a characteristic depot to serve timely energy demands. Adipocytes are the functional units of adipose tissue (AT) maintain glucose and lipid homeostasis, a robust physiological system regulated precisely by a complex network of molecular stimuli to achieve the energy demands in both fed and fast state. Adipocytes are most versatile cells which efficiently synthesise fat, store fatty acids safely and release them upon requirement through three well-coordinated processes termed lipogenesis, lipid storage and lipolysis. These processes are highly sensitive to nutritional and hormonal stimuli and respond via proteins and/ or miRNA regulation. Dysregulation leads to various metabolic complications like obesity, insulin resistance (IR), type 2 diabetes (T2D), cardiovascular disorders (CVD), metabolic syndrome and cancer. AT related complications are physiologically interlinked and can affect other metabolic organs like liver, heart, muscle etc. Hence, it is important to understand the mechanisms regulating adipocyte differentiation, lipid storage, lipolysis and signalling to improve our understanding of adipose tissue in metabolism. In the first study, we investigated the expression of miR-192* in visceral adipose tissue (VAT) of cohort 1 where we found a negative correlation between miR192* with serum triglyceride (TG) and positive correlation with high-density lipoprotein (HDL) levels. In cohort 2 miR-192* negatively correlated with the body mass index (BMI). Overexpression of miR192* in cultured adipocytes revealed reduced expression of the adipocyte differentiation marker proteins perilipin 1 and adipocyte protein. miR-192* expression in adipocytes reduced TG accumulation and affected many genes of the lipogenic pathway. In study II, we discovered that miR-221-3p targets and downregulates the expression of ANGPTL8 under inflammatory stimuli in cultured adipocytes. We observed a positive correlation between mRNA expressions of ANGPTL8 and insulin sensitive genes in subcutaneous (SAT) and visceral AT (VAT). The significant negative correlation was found between ANGPTL8 and miR-221-3p expression in a morbidly obese subgroup of SAT samples (cohort III, n=22) before bariatric surgery, which interestingly disappeared after 2-year post-surgery weight loss resulting in a significant reduction of miR-221-3p. ANGPTL8 negatively correlated with the AT inflammatory marker phospholipase A2 G7, while miR-221-3p showed a significant positive correlation with this inflammatory marker. In study III, ANGPTL8 knockdown in 3T3-L1 adipocytes suggested that ANGPTL8 inhibits intracellular lipolysis in adipocytes. Depletion of Angptl8 results in an altered cellular phospholipid composition. In summary, studies from this thesis revealed roles of two new microRNAs miR192* and miR221-3p as regulators of adipocyte metabolism. and addressed the intracellular function of insulin-regulated ANGPTL8 in adipocytes.