Date of Graduation

2017

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Biochemistry

Committee Chair

Frank B Hillgartner

Committee Co-Chair

Roberta Leonardi

Committee Member

Peter Mathers

Committee Member

Joseph McFadden

Committee Member

Peter Stoilov

Abstract

The incidence of obesity has increased at an alarming rate during the past thirty years. Current therapies for the treatment and prevention of obesity and metabolic syndrome are ineffective and/or produce undesirable side effects. Fibroblast growth factor 21 (FGF21) is a liver derived hormone that mediates adaptive changes in hepatic glucose and lipid metabolism by elevating energy expenditure and reducing lipid storage. Pharmacological administration of FGF21 analogs in both animal and human models induces striking reversals of obesity and type 2 diabetes. However, the activity of FGF21 analogs exhibit short half-lives in blood circulation and are unsuitable for current treatment. Therefore it is imperative that mechanisms mediating the activity of endogenous FGF21 be understood.;FGF21 expression is enhanced during starvation, high-fat low-carbohydrate (HF-LC) ketogenic diet, type 2 diabetes, and obesity. Starvation induces a high glucagon to insulin metabolic hormone ratio in the portal vein. Type 2 diabetics also exhibit chronically high glucagon to insulin ratios in blood circulation. Furthermore, infusions with high glucagon, low insulin hormone concentrations increase the production and secretion of splanchnic FGF21 in man. The mechanisms by which glucagon and insulin work cooperatively to selectively increase hepatic FGF21 expression and secretion is currently unknown.;To understand the molecular mechanisms mediating the induction of FGF21 expression in the liver, we recapitulated the conditions of starvation in primary rat hepatocytes by treating cultures with glucagon and insulin using concentrations similar to that observed in the portal vein during starvation. We then utilized a reverse genetics approach to identify two conserved amino acid response elements (AAREs) on the FGF21 promoter that were critical in mediating the effects of starvation on FGF21 gene expression. We further defined the transcription factor, activating transcription factor 4 (ATF4), as the primary transcription fact responsible in stimulating FGF21 gene transcription during glucagon plus insulin treatment. We then characterized the glucagon and insulin signaling pathways mediating the cooperative actions of glucagon plus insulin on FGF21 gene expression using loss-of-function and gain-of-function techniques, and determined that both the PI3K/Akt/mTORC1 and cAMP/PKA signaling pathways were responsible for inducing FGF21 expression. Lastly, we applied these novel findings to further define the mechanisms other stimuli, such as chenodeoxycholic acid (CDCA), employ to induce FGF21 expression. Understanding the fundamental mechanisms by which these two hormones work together provided the scientific community with in-depth knowledge on pathways mediating the endogenous expression of hepatic FGF21 and also identified additional targets for drug development for the treat of metabolic syndrome.

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