Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Pharmaceutical Sciences

Specialization

Pharmacology and Toxicology

Department

Pharmaceutical Sciences

First Advisor

Angela L. Slitt

Abstract

Perfluoroalkyl substances (PFAS), a family of environmental toxicants, are ubiquitous in the environment and the human population. PFAS are manmade chemicals that have been widely used in manufacturing since the 1940s. PFAS are extremely resistant to degradation leading to their accumulation within the environment and the general population. The most concentrated PFAS member present in the general population, perfluorooctanesulfonic acid (PFOS), has been linked to suppressed vaccine response, high serum cholesterol, low fetal birth weight, thyroid disease, increased markers of liver injury, and even certain types of cancers. PFOS was voluntarily phased out of manufacturing due to its bioaccumulutive and toxic properties by the year 2015. In 2016, the EPA dramatically lowered the federal health advisory level of PFOS in drinking water to 70ppt due to the emerging evidence of its toxicity. After the phase out of PFOS, replacement PFAS members perfluorononanoic acid (PFNA) and perfluorohexanesulfonic acid (PFHxS) took its place and began to rise in environmental and human serum concentrations. Currently, there is no federal health advisory level in place for PFNA or PFHxS. The relative toxicity of these replacement PFAS compounds is still being evaluated in the literature and by regulatory officials.

PFAS, including PFOS, PFNA, and PFHxS, are known to augment hepatic lipid accumulation and steatosis in animal studies. However, there is a lack of knowledge concerning their potential role in the increasing incidence of nonalcoholic fatty liver disease (NAFLD) in the global population. It is difficult to study the potential link between PFAS exposure and NAFLD due to the lack of an accurate and widely accepted serum biomarker for the NAFLD. PFAS levels are typically measured through serum samples, however NAFLD is often diagnosed via liver biopsy. Obtaining human liver biopsy data that is matched to serum samples characterized for PFAS concentration has proven prohibitively difficult. The vast majority of studies that have evaluated PFAS exposure in the liver have utilized standard chow diets, high doses, and acute exposures. In the human population, NAFLD is most commonly induced by poor diet and lifestyle over time. The present work aimed to understand how environmentally relevant exposure to PFAS affect the onset and progression of NAFLD in the presence of a poor diet over a subchronic or chronic timeframe. Furthermore, this work sought to uncover the mechanistic drivers of these PFAS-diet interactions.

In manuscript 1, male C57BL6 mice were fed with either a low fat diet (10% kcal from fat) or a moderately high fat diet (45% kcal from fat) with or without PFOS or PFNA 0.0003% w/w in feed for 12 weeks. Proteomics and transcriptomics were utilized to explore the mechanistic pathways driving the liver pathology. The aim of this study was to assess and explore the impact of diet-PFAS interactions on the onset of NAFLD using a subchronic, low-dose PFAS exposure. In addition, we sought to compare the toxicity of PFOS to the unregulated PFNA. In manuscript 2, male C57BL6 mice were fed with either a low-fat diet (11% kcal from fat) or a high fat (58% kcal from fat) high carbohydrate (42g/L) diet with or without PFOS or PFHxS, in feed (0.0003% w/w) for 29 weeks. Sera lipidomics, as well as hepatic proteomics, gene expression, and pathology were used to assess diet-PFAS interactions. The aim of manuscript 2 was to assess the impact of PFAS-diet interactions on the progression of nonalcoholic fatty liver (NAFL) to the inflammatory stage, nonalcoholic steatohepatitis (NASH) using a chronic low-dose PFAS exposure. Moreover, we compared the relative toxicity of PFOS to an unregulated replacement, PFHxS.

In manuscript 1, PFASs were augmented the onset of fatty liver only in combination with a low fat diet. Yet in manuscript 2, PFASs worsened macrovesicular steatosis and inflammation within the high fat high carbohydrate diet relative to combination with a low fat diet. Both studies revealed that diet composition as well as slight alterations in PFAS structure exert significant influence on PFAS tissue partitioning and excretion, liver pathology, lipids, and the resulting hepatic biochemical signature. Finally, diet-PFAS interactions may produce differential outcomes depending on the severity and stage of NAFLD.

Available for download on Thursday, December 17, 2020

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