Effects of perfluorinated compounds (PFCs) on metabolic tissues and the benefits of caloric restriction
The CDC states that there has been a dramatic increase in obesity from 1990 to 2010. Type-II diabetes and obesity prevalence are increasing worldwide. Often, obesity and Type-II diabetes are concurrent, and predispose individuals to development of fatty liver disease, referred to as Non-alcoholic fatty liver disease (NAFLD). Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are two commonly studied perfluorinated compounds (PFC's) that are considered environmental toxicants that have the potential to elicit diabetic and NAFLD phenotypes. This dissertation presents novel findings of gaps within the literature to date. Traditionally, diabetes and obesity have been discussed in regard with genetics, diet and old age. Now, risk factors that also need to be considered are environmental chemicals. We found that PFOS elicits an insulin-resistant phenotype in adult mice, where they were not utilizing glucose as readily compared to the controls. The effect of PFOS on therapeutic management interventions has not previously been looked at. Here, we show that PFOS interferes with the Metformin-induced decrease in glucose. We also found a vast increase in the hepatic triglycerides with PFOS exposure. In this thesis, PFOS was administered in a sub-chronic low dose (100&mgr;g/kg) daily to mice fed ad libitum or placed on caloric restriction (CR) for five weeks. In the cohort we generated, we observed that PFOS exposure increased hepatic lipid content in mature male mice fed ad libitum and dampened the observed CR-induced decrease in hepatic lipids. PFOS administration did not affect glucose tolerance in ad libitum fed mice, but did interfere with CR-induced improvement of glucose tolerance. This was further associated with suppression of IRS-1 mRNA expression in liver. As hepatic lipid content is closely tied to insulin and glucagon signaling for hepatic glucose production, it was determined whether the observed effects in vivo were due to PFOS-stimulated hepatic glucose production. Using cultured mouse hepatocytes under low glucose conditions, it was evaluated whether PFOS could enhance glucagon-stimulated glucose production. PFOS stimulated hepatocyte glucose production and also enhanced glucagon-induced glucose production. Furthermore, in HEPG2 cells, PFOS exposure (25 and 250 µM) significantly increased glucose output and AiCar, which suppresses glucose production, was ineffective in the presence of PFOS. These findings provide a mechanistic explanation for the decreased glucose tolerance in our in vivo mouse cohort. PFOS increases glucose output from the in vitro models, even when challenged with metformin or AiCar, as well as, decreases the glucose utilization in the in vivo mouse study suggesting that it has a glucagon-like effect. PFOA is a perfluorinated carboxylic acid also commonly found in the environment. According to the EPA, low levels of PFOA are widely distributed in environmental media (Houde et al., 2011; Gewertz et al., 2013) and in the blood of the general United States population. The EPA states that PFOA is a known liver toxicant, development toxicant, and carcinogen in rodents. It has been detected in human liver, kidney and adipose tissue ranging from 0.3 to 3.8ng/g with the highest concentrations within the liver (Maestri et al., 2006). In another study, PFOA is considered an obesogen to mid-aged mice where insulin and leptin levels were altered at a very low concentration (Hines et al., 2009). PFOA is a Potent activator of peroxisome-proliferator activated receptor-alpha (Ppar-&agr;) contributing to oxidative stress and activation of fatty acid oxidation pathways in hepatocytes. Given its persistence, the purpose of this study was to evaluate whether PFOA treatment affects fatty acid oxidation, lipid synthesis, and antioxidant response gene expression in adipose tissue in adult male mice. Utilizing an exposure paradigm characterized by the Environmental Protection Agency, adult male mice were treated with 1.0 or 3.0 mg PFOA ammonium salt/kg for 7 days. Adipose tissue was collected and total RNA was isolated. Analysis of mRNA was completed by quantitative PCR. For the most part, gene expression in adipose tissue from vehicle- and PFOA-treated mice was similar. Literature lacks data on PFOA in adipose tissue and in human health, which continue to be discovered. Given its persistence, longer exposure periods and protein expression changes should be examined.
Deanna M Salter,
"Effects of perfluorinated compounds (PFCs) on metabolic tissues and the benefits of caloric restriction"
Dissertations and Master's Theses (Campus Access).