Non-alcoholic fatty liver disease (NAFLD) impacts 25% of the world's population and is expected to become the leading cause for liver transplantation; however, there are currently no recommended pharmaceutical interventions available for treatment. Exposure to environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), can elicit hepatopathologies in rodents that resemble NAFLD in humans. Most, if not all, of the toxic effects of exposure to TCDD are mediated by the aryl hydrocarbon receptor (AHR). Exposure to TCDD can also alter cholesterol homeostasis in mice and is implicated in increased prevalence of metabolic syndrome in humans, which are also leading risk factors for NAFLD development. Therefore, the primary goal of this dissertation was to gain a better understanding of the connection between AHR-mediated signaling, dysregulation of cholesterol and TCDD-induced liver injury in an effort to discover potential therapeutic options for treating NAFLD in the future. First, a genome-wide association study (GWAS) was carried out to characterize regions of the genome that might be linked to TCDD-induced metabolic and physiological changes in mice. The goal of this GWAS was to identify key genes and/or pathways implicated in liver injury. Linear regression implicated 7 genes, 2 of which were novel, that were significantly associated with TCDD liver burden. Notably, TCDD-induced changes in body weight were associated with HMG-CoA reductase (Hmgcr), which encodes the rate limiting step for cholesterol biosynthesis. Secondly, a mouse study was undertaken to deduce the role of HMGCR in modulating TCDD-induced liver injury. This study used simvastatin, a competitive inhibitor of HMGCR, in the presence and absence of TCDD to determine if inhibition of cholesterol synthesis could affect TCDD-induced hepatosteatosis. Interestingly, simvastatin co-treatment was found to be protective against AHR-mediated steatosis in both sexes. However, co-treatment induced sex-specific injury by increasing hepatic glycogen in females and exacerbating TCDD-induced liver injury in males. These results suggest that people who take statins are at greater risk of toxicant-induced liver injury. Finally, single-nuclei RNA sequencing (snRNAseq) was utilized in mouse liver to gain a mechanistic understanding for statin-induced changes to TCDD-induced liver injury. Although co-treatment did not significantly alter liver pathology compared to TCDD alone, it was shown to alter relative proportions of distinct liver cell (sub)types and promote decreased immune cell infiltration. While these changes could be protective against liver injury, statin co-treatment was also associated with wasting and death, suggesting that taking statins as a treatment for NAFLD may lead to adverse consequences. The results outlined in this dissertation provide insight on the role of cholesterol homeostasis in liver injury. Furthermore, while statin drugs have been suggested as a possible treatment for NAFLD, this work demonstrates how statins directly impact liver cell (sub)types proportions during toxicant-induced liver injury and suggests that they may have unintentional side effects in vulnerable populations. Although the impact of statins will need to be further evaluated in human NAFLD, we hope that the outcomes of this research can be used to inform the development of new treatment options for NAFLD patients.
Read
