The impact of environmental metabolic disruptors on PPARgamma transcriptional regulation of adipocyte differentiation and function
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Metabolic homeostasis is controlled, in part, by a family of proteins called nuclear receptors through which lipophilic hormones and hormone-like molecules regulate gene expression. One such nuclear receptor is peroxisome proliferator activated receptor γ (PPARγ). Its activation is essential for white, brite (brown-in-white) and brown adipogenesis, adipocyte function, mature adipocyte maintenance, and insulin sensitivity. PPARγ activation regulates energy homeostasis by both promoting storage of excess energy as lipids in white adipocytes and stimulating energy dissipation in brite and brown adipocytes. Accumulation of white adipocytes significantly increases the risk of obesity and metabolic syndrome. On the other hand, brown and brite adipocytes potentially counteract metabolic disease-related symptoms. The adipocyte differentiation and function as well as insulin sensitizing activities of PPARγ are regulated separately through differential post-translational modifications and/or co-regulator recruitment, with ligands having distinct abilities to activate each of PPARγ’s functions. These provide mechanisms by which a ligand could induce adipogenesis without stimulating PPARγ’s health promoting functions (i.e. insulin sensitivity, energy dissipation). The central hypothesis of this dissertation is that compared to therapeutic PPARγ ligands (i.e. rosiglitazone), environmental PPARγ ligands will activate a distinct PPARγ transcriptional program that disrupts adipose and metabolic homeostasis. Two study aims were developed to test and refine this central hypothesis. The first aim identified genes and pathways that differentiate environmental PPARγ ligands from endogenous and therapeutic chemicals. In primary mouse bone marrow multipotent stromal cells and 3T3-L1 cells, the environmental PPARγ ligands tributyltin (TBT, an antifouling agent and plasticizer) and triphenyl phosphate (TPhP, an organophosphate flame retardant) induced transcriptomic profiles that were distinct from rosiglitazone. All ligands induced adipogenesis; yet, only rosiglitazone strongly enriched pathways related to brown fat differentiation and mitochondrial processes and induced brite adipocyte gene markers (Cidea, Elovl3, Ucp1). Using the transcriptional profiles from 3T3-L1 adipocytes differentiated in the presence of 76 different chemicals, a taxonomy was built to identify environmental chemicals as PPARγ-modifying chemicals distinct from known PPARγ-modifying therapeutics. The second aim investigated the role of phosphorylation of PPARγ in defining environmental ligand-induced changes in adipocyte differentiation and function. In differentiated 3T3-L1 cells, rosiglitazone and TPhP both induced adipogenesis through PPARγ, but only rosiglitazone enhanced mitochondrial biogenesis and mitochondrial respiration, which contribute to healthy energy expenditure. Rosiglitazone, but not TPhP, protected PPARγ from phosphorylation at Ser-273. However, in 3T3-L1 cells in which PPARγ cannot be phosphorylated, TPhP was able to induce mRNA expression of a suite of brite adipocyte genes. In male C57BL/6J mice fed either a low or high fat diet, TPhP caused a significant decrease in brite adipocyte gene expression (Elovl3, Ucp1) in mature adipocytes from inguinal adipose tissue. Together, these studies support our hypothesis that environmental PPARγ ligands (i.e. TBT and TPhP) skew adipocyte differentiation toward white adipogenesis at the expense of brite adipogenesis, potentially because of differential post-translational modification of PPARγ.