Obesity causes insulin resistance, which increases the risk of Type 2 Diabetes (T2D) and Coronary Heart Disease (CHD). Although weight loss is a primary goal in clinical management of obesity, long-term weight loss is rarely sustained. Therefore, it is imperative to understand pathways that might ameliorate the impact of obesity on T2D and CHD. We discovered that transgenic expression of Cholesteryl Ester Transfer Protein (Cetp) protects against high-fat diet (HFD)-induced insulin resistance in female mice in the setting of obesity. Cetp traffics triglyceride and cholesteryl ester (CE) between plasma lipoproteins, culminating in delivery of CE to the liver for secretion as bile - termed reverse cholesterol transport (RCT). Cetp inhibitors raise high- density lipoprotein (HDL) but fail to reduce risk for CHD, which may indicate that Cetp has non-HDL functions. Mice lack Cetp, so we used transgenic mice expressing Cetp to define how Cetp impacts insulin resistance. Using the hyperinsulinemic-euglycemic clamp technique, we found that Cetp protects against insulin resistance in females, but not males following 4 weeks of HFD. This protection was despite a similar degree of obesity. The objective of this proposal is to understand how Cetp protects against insulin resistance in diet-induced obese mice. Our discovery represents a novel and beneficial function of Cetp with regard to obesity pathophysiology. Despite high-fat feeding, Cetp expression preserved insulin signaling pathways in liver and muscle and insulin-mediated downregulation of mRNA for gluconeogenic genes Phosphoenopyruvate Carboxykinase (Pck1) and Glucose-6-Phosphatase (G6pc). We found that Cetp increased mRNA of the bile acid signaling molecule Small Heterodimer Partner (Shp) and increased estrogen signaling in liver. We hypothesize that Cetp promotes bile acid and estrogen signaling in liver to protect against diet-induced insulin resistance. Understanding how Cetp protects against diet-induced insulin resistance may yield novel therapeutic targets that lessen obesity's impact on CHD and T2D.
In Aim 1, we will determine the mechanism of female-specificity in Cetp-mediated protection from HFD-induced insulin resistance. We expect that liver deletion of estrogen receptor alpha will diminish Cetp-mediated protection from HFD-induced insulin resistance.
In Aim 2, we will determine the hepatic bile acid signaling pathways contributing to Cetp-mediated protection from HFD-induced insulin resistance. We expect that liver knockdown of Shp will abrogate Cetp- mediated protection from HFD-induced insulin resistance. Accomplishing the proposed work will 1) identify mechanisms responsible for sex differences that prevent metabolic disease and 2) identify liver bile acid signaling pathways downstream of Cetp that protect against diet-induced insulin resistance. Additionally, the work proposed in this application will support a critical nex step in my training as a physician-scientist whose career aims to make therapeutic connections between obesity and diabetes.
The dramatic rise in obesity in the United States has increased the burden of Type 2 Diabetes and other metabolic diseases, and the goal of the NIH/NIDDK is to reduce this burden. The goal of this proposal is to discover liver pathways that lessen insulin resistance in the setting of obesity, which will help reverse the impact of obesity on risk for Type 2 Diabetes and other metabolic diseases. This proposal also identifies pathways that mediate sex-differences in metabolic disease, which is also an NIH priority area.