Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Medical Sciences

Major Professor

Kay-Pong Yip, Ph.D.

Committee Member

Caralina Marin De Evsikova, Ph.D.

Committee Member

Michel Murr, M.D.

Committee Member

Timo Rieg, Ph.D.

Committee Member

Thomas Taylor-Clark, Ph.D.

Keywords

Adiponectin, Non-Alcoholic Fatty Liver Disease, Rat Model, Stearoyl-CoA Desaturase, Sterol Regulatory Element Binding Protein

Abstract

Obesity is a growing epidemic in the United States with significant co-morbidities. Non-Alcoholic Fatty Liver Disease (NAFLD) is a prevalent manifestation of obesity that can lead to cirrhosis. Roux-en-Y gastric bypass (RYGB) results in substantial long-term weight loss and resolution of obesity-related metabolic diseases. There appears to be a weight-independent molecular mechanism for the improvement of diabetes mellitus and NAFLD after RYGB, which is poorly understood. Obesity is associated with chronic inflammation that accompanies the hepatic steatosis. Through unknown mechanisms, RYGB in humans increases serum levels of the fat-derived adipocytokine, adiponectin. Adiponectin (an anti-inflammatory cytokine) is known to have a protective role in animal models of alcoholic and non-alcoholic liver injury. Clarifying the link in signaling between NAFLD, inflammatory signaling, and lipogenesis would enhance our insight into the pathogenesis of the metabolic syndrome and warrants further exploration. The accompanying resolution of metabolic disorders after gastric bypass gives a unique opportunity to elucidate the associated mechanisms and molecular pathways.

We hypothesized that RYGB would attenuate obesity-induced steatosis and inflammatory changes in the livers of obese rats. These investigations would lend insight into alterations in the hepatic fatty acid synthesis and fatty acid oxidation signaling pathways after gastric bypass. We set out to test our hypothesis in obese rats via three specific aims: (1) Establish a rat model of RYGB to study obesity-related liver injury, (2) Investigate the mechanisms through which RYGB improves inflammation, (3) Investigate the mechanisms through which RYGB improves hepatic steatosis.

Rat models for obesity and RYGB may be used to elucidate mechanistic pathways and develop less invasive treatments for obesity and its related co-morbidities. However, the rat model for RYGB is difficult to successfully reproduce and is frequently associated with high operative mortality. Our first challenge was to establish this rat model of RYGB so that we could use it in future studies. Initially, we followed published protocols for rat RYGB with minimal success. Subsequently, we identified several key factors that improved survival, developing a model with consistent survival of over 90% and tracking our outcomes. To help other researchers, we detailed techniques to improve survival of these obese rats undergoing RYGB in order to more effectively establish this model. We further demonstrated that our rat RYGB model had the necessary endpoints of sustained weight loss and resolution of steatosis. This rat model of RYGB provides a valuable opportunity to further elucidate the pathogenesis of NAFLD to pave the road for future therapies for this disease.

Next, we used this rat RYGB model to investigate the mechanisms through which RYGB improves inflammation. Obesity is associated with chronic inflammation and RYGB is thought to improve this. Adiponectin has anti-inflammatory properties and is increased with weight loss. Tumor necrosis factor (TNF)-α is a pro-inflammatory cytokine that negatively regulates adiponectin. We hypothesized that the weight loss and steatosis resolution after RYGB would alter the interplay of TNF-α and adiponectin signaling in the postoperative period. Using our rat model of RYGB, we examined TNF-α and adiponectin signaling in serum, adipose tissue, and liver tissue. Contrary to what we were expecting, we found that RYGB in obese rats did not increase anti-inflammatory adiponectin signaling in the immediate postoperative period but was associated with decreased pro-inflammatory TNF-α signaling in the adipose tissue. During this period, pro-inflammatory signaling might play a more important role than adiponectin. Notwithstanding, the results of this present study demonstrate exciting and novel changes in the balance of pro-inflammatory and anti-inflammatory signaling after RYGB and lay the framework for future investigations into these metabolic changes after gastric bypass.

Finally, we investigated the mechanisms through which RYGB improves hepatic steatosis. We hypothesized that either lipogenesis would be down-regulated or fatty acid oxidation would be upregulated or both of these would occur. We aimed to examine major cell signaling pathways of hepatic lipogenesis and fatty acid oxidation using our rat model of RYGB. We confirmed that RYGB resolved steatosis, reduced hepatic triglycerides, and downregulated hepatic lipogenesis controlled by multiple components in the stearoyl-CoA desaturase (SCD) 1 signaling pathway. This novel finding is important since SCD1 plays a pivotal role in triglyceride synthesis. In this model, the SIRT1-PPARα/PGC1α signaling pathway and fatty acid oxidation were not upregulated at 9 weeks after RYGB. Whether this fatty acid oxidation pathway plays a greater role at a different time point after RYGB needs to be determined to assess the temporal relationship between gastric bypass and fatty acid oxidation changes. These studies demonstrate the downregulation of multiple components of the triglyceride synthesis pathway, which warrant further investigation to increase our insight into the impact of gastric bypass on NAFLD. These novel findings indicate that RYGB improves steatosis by downregulating lipogenesis without upregulating fatty acid oxidation in obese rats.

Download

Share

COinS