Suppression of adipocyte hyperplasia through SIRT1mediated inhibition of cMyc function
Authors: Mazloum Nayef Ali 
Published: June 2019

Yasser Majeed1, Aisha Madani1, Muneera Vakayil1, Maha Agha1, Houari Abdesselem2, Mohamed ElRayess3, Moataz Basha4, Nasrin Mesaeli1, Michael Bonkowski5, David A Sinclair5, Nayef A. Mazloum1 1. Weill Cornell Medicine-Qatar, Doha, Qatar 2. Qatar Biomedical Research Institute, Doha, Qatar 3. Anti-Doping Lab-Qatar, Doha, Qatar, 4. Hamad Medical Corporation, Doha, Qatar 5. Harvard Medical School, Boston, MA, USA. Global estimates from the World Health Organization (WHO) suggest that obesity has approximately tripled since 1975 and that over 600 million adults and 300 million children/adolescents were obese. Obesity is a major risk factor for Type 2 Diabetes (T2D) and associated disorders that affect the cardiovascular system. Insulin resistance, insufficient insulin secretion and increased blood glucose levels (hyperglycemia) are characteristic features of T2D. Together with other cardio-metabolic complications observed in T2D, hyperglycemia results in vascular disease and increased morbidity and mortality. There is therefore a worldwide health impact of T2D and its complications. Locally, the incidence of obesity and T2D in Qatar ranks among the highest in the world and is a serious public health burden. Genetic predisposition and environmental factors such as intake of calorie-rich food and a sedentary lifestyle make significant contributions to the obesity epidemic. White adipose tissue (WAT) plays a key role in the pathophysiology of obesity and its associated co-morbidities. WAT is distributed either subcutaneously or within different depots in the intra-abdominal cavity surrounding or near vital organs (viscera). WAT mass expansion observed in obesity is due to enlarged fat cell volume (hypertrophy) and increased cell number (hyperplasia). Hence, understanding how WAT function is dysregulated is essential to understanding the pathophysiology of obesity and designing improved therapeutics to treat obesity and its metabolic complications. There is little known about the molecular mechanism that drives adipocyte hyperplasia in obesity. The NAD-dependent protein deacetylase sirtuin-1 (SIRT1), a master regulator of mammalian metabolism, maintains proper metabolic functions in many tissues counteracting obesity. Our laboratory has shown that differentiated adipocytes are hyperplastic when the expression of SIRT1 is stably reduced in mouse 3T3-L1 preadipocytes. This phenotype is concomitant with altered adipocyte metabolism and increased inflammation. We also show that SIRT1 is critical in the regulation of proliferation of preadipocytes. By employing quantitative proteomics studies, we provided evidence that the molecular pathway downstream of the c-Myc proto-oncogene is affected to drive enhanced proliferation in SIRT1-silenced preadipocytes cells. Our data suggest that c-Myc is transcriptionally activated upon SIRT1 reduction leading to lower levels of p27 (cyclin-dependent kinase inhibitor) and the activation of the CDK2 (cyclin-dependent kinase 2). Remarkably, differentiated SIRT1-silenced preadipocytes show enhanced mitotic clonal expansion (MCE) phenotype along with reduced levels of p27, as well as elevated levels of c-Myc and the adipogenesis transcription factor C/EBPβ. c-Myc activation and enhanced proliferation phenotype were also observed to be SIRT1-dependent in mouse embryo fibroblasts (MEFs) and human SW872 preadipocytes. Interestingly, the stable reduction of both SIRT1 and c-Myc expression in 3T3-L1 preadipocytes did not lead to the adipocyte hyperplasia phenotype, which further confirms that SIRT1 suppresses adipocytes hyperplasia through c-Myc inhibition. Current studies are focused on investigating the in vivo relevance of Sirt1-c-myc interaction in mouse models of diet-induced obesity. We are also isolating preadipocytes from WAT collected from insulin sensitive (IS) and insulin resistant (IR) obese subjects to evaluate differences in their proliferation rates and adipogenic potential and to understand if these might be linked to altered SIRT1 and C-Myc functions. Better understanding of the molecular mechanisms of adipocyte hyperplasia will open new venues towards understanding obesity.