Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physiology and Pharmacology

Supervisor

Dr. Rommel Tirona

Abstract

Non-alcoholic fatty liver disease (NAFLD) is defined as lipid accumulation within hepatocytes (steatosis) in the absence of excess alcohol consumption. It is the most common liver disease in the western world, affecting one third of the general adult population with particularly high prevalence in obesity and type 2 diabetes. NAFLD is a disease continuum originating with simple hepatic steatosis that can progress to non-alcoholic steatohepatitis (NASH) with fibrosis and potentially cirrhosis, which places patients at risk for hepatocellular carcinoma. Unfortunately, there are not yet specific pharmacologic agents to treat NAFLD and so its management involves treatment of comorbidities, but there are significant efforts to develop new drugs to reverse disease course and prevent progression.

While the liver is the major organ of drug elimination, little is known regarding the effect of NAFLD on hepatic drug metabolism in humans. The most important drug metabolizing enzyme is cytochrome P450 (CYP) 3A4, which metabolizes medications widely prescribed in NAFLD patients including those to treat hypertension, type 2 diabetes and dyslipidemia. We tested the hypothesis that CYP3A4 expression and activity are altered in NAFLD.

In the first study, we evaluated the use of predictive endogenous plasma biomarkers of drug metabolic function against the gold-standard midazolam pharmacokinetic phenotyping approach for the assessment of constitutive CYP3A activity in healthy subjects. There was a lack of association between levels of the CYP3A derived metabolites 4β-hydroxycholesterol and 6β-hydroxycortisol with midazolam pharmacokinetics indicating that these biomarkers have limited utility within the context of relatively narrow enzyme activity variability in healthy individuals.

CYP3A activity was next examined in patients with biopsy-proven NAFLD in comparison to healthy control subjects using midazolam pharmacokinetic phenotyping and biomarker approaches. We demonstrated, for the first time, that in vivo CYP3A activity is reduced in patients with simple steatosis or NASH and that fibrosis was also associated with lower enzyme function. Likewise, experimental NAFLD in mice and cultured hepatoma cells was associated with lower CYP3A4 expression.

In the third study, mouse and cell models of NAFLD were used to define a novel regulatory pathway involved in the observed reduction of CYP3A4 expression and activity. We found that NAFLD causes hepatic upregulation of the metabolic hormone, fibroblast growth factor 21 (FGF21), stimulating a canonical cellular phospho-signaling pathway. FGF21 acted on the liver to decrease the nuclear localization and activity of the pregnane X receptor, a key transcriptional regulator of CYP3A4 gene expression.

These findings provide novel insights to altered drug metabolism in NAFLD and a mechanistic basis for studies aimed to optimize pharmacotherapy and drug development for this common liver disease.


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