Date on Master's Thesis/Doctoral Dissertation

12-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Pharmacology and Toxicology

Degree Program

Pharmacology and Toxicology, PhD

Committee Chair

Hein, David

Committee Member

Hong, Kyung

Committee Member

Conklin, Daniel

Committee Member

Hill, Bradford

Committee Member

Hood, Joshua

Author's Keywords

hepatocytes; insulin resistance; diabetes; metabolic syndrome

Abstract

Heterocyclic amines (HCAs) are mutagens generated when cooking meat for prolonged periods of time or until well-done. Recent epidemiological studies reported significant associations between dietary HCA exposure and insulin resistance and type II diabetes. However, no previous studies have examined if HCAs, independent of meat consumption, contributes to pathogenesis of insulin resistance or metabolic disease. It is well known that HCAs require hepatic bioactivation by cytochrome P450 1A2 (CYP1A2) and N-acetyltransferase 2 (NAT2). NAT2 expresses a well-defined genetic polymorphism in humans that, depending on the combination of NAT2 alleles, correlate to rapid, intermediate, or slow acetylator phenotypes that exhibit differential metabolism of aromatic amines and HCAs. We hypothesize that HCAs will induce insulin resistance and disrupt energy homeostasis in human hepatocytes, and that the effect will be dependent on NAT2 genetic polymorphism. HCA treatment on human hepatocytes induced insulin resistance, glucose production, and expression of several key genes involved in gluconeogenesis, and these effects were more apparent in rapid NAT2 acetylator hepatocytes than in slow. Additionally, HCAs lead to neutral lipid accumulation and increased levels of triglycerides, cholesterol, and free fatty acids, in addition to dysregulated expression of genes involved in lipid homeostasis. Potential mechanisms of HCA-induced insulin resistance and disrupted energy homeostasis were also explored. HCAs increased JNK activity, a well-known mechanism of hepatic insulin resistance, and blocking JNK activity restored insulin signaling in HCA-treated hepatocytes. Additionally, HCAs led to increased reactive oxygen species and induction of TNF, indicating oxidative stress and inflammation as potential mechanisms of insulin resistance. An RNA-sequencing study also indicated endoplasmic reticulum stress as a potential mechanism. Taken together, the findings presented here indicate that HCAs lead to insulin resistance and dysregulation of energy homeostasis in human hepatocytes, and that these outcomes are differentially affected by NAT2 genetic polymorphism. These studies provide insight into the novel role of HCAs in the development of conditions associated with metabolic syndrome and fatty liver, and elucidate the crucial need to consider the gene-environmental interactions when investigating these conditions.

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