Regulation of intestinal triglyceride metabolism and molecular mechanism

Aki Uchida, Purdue University

Abstract

As obesity has reached epidemic proportions, globally, the prevalence of metabolic syndrome has also increased. Metabolic syndrome is associated with excess body fat and high blood triglyceride (TG) concentrations, both in fasting and postprandial states. The small intestine is the portal for systemic delivery of energy dense dietary fat, which significantly contributes to blood TG concentrations and fat storage in adipose tissue. When a high fat (HF) meal is consumed, dietary fat in the form of TG is hydrolyzed to fatty acids (FAs) and monoglycerides in the lumen of the small intestine and absorbed into the absorptive cells of the intestine, enterocytes. Once in enterocytes, FAs have multiple potential fates such as incorporation into TG for storage or secretion, and FA oxidation. Altering the fate of FAs in enterocytes thus has the potential to improve blood TG concentrations and whole body energy balance. The objective of this dissertation was to investigate how obesity, drugs, and genetics regulate the process of dietary fat absorption. We investigated whether obesity affects intestinal TG metabolism—more specifically TG storage in and secretion from the intestine. In order to understand the complex and dynamic nature of dietary fat absorption, we gathered data at multiple times before and post-acute dietary fat challenge in diet-induced obese (DIO) and leptin-deficient (ob/ob) mice. We found greater levels of TG storage in the intestine of HF-fed DIO mice compared to chow-fed lean mice in the fed state, but similar levels of TG storage after fasting. In addition, we found similar TG storage in the intestine of lean and DIO mice at multiple time points after an acute dietary fat challenge. Unexpectedly, we identified remarkably lower TG secretion from both DIO and ob/ob mice compared to lean controls in response to an acute dietary fat challenge. Our results characterized intestinal phenotypes of obesity mouse models, which have not been previously done. Furthermore, we investigated the contribution of the intestinal effects of fenofibrate, a hypolipidemic drug, in lowering postprandial blood TG levels. We found that the hypotriglyceridemic actions of fenofibrate in the postprandial state of HF-fed mice include a decrease in supply of TG for secretion by the small intestine. A decreased supply of TG for secretion was due in part to the decreased dietary fat absorption and increased intestinal fatty acid oxidation in fenofibrate compared to vehicle treated HF-fed mice. These results demonstrate that the small intestine plays a critical role in the hypotriglyceridemic effects of fenofibrate. Lastly, we investigated the role of acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) and DGAT2 in intestinal TG biosynthesis. We hypothesized that DGAT1 and DGAT2 synthesizes TG for distinct functions in enterocytes. Using our mouse models with intestinal over-expression of either Dgat1 or Dgat2 mRNA, we were able to drive the ratio of Dgat1:Dgat2 either higher or lower than the 5:1 ratio in the intestine of WT mice. We found that in Dgat2Int mice, which have a lower ratio of Dgat1:Dgat2 (close to one) compared to WT mice, TG storage in response to dietary fat challenges is unchanged, but TG secretion of normal sized chylomicrons (CMs) is enhanced. We found that in Dgat1-/- mice, which have only DGAT2 present, TG storage in response to dietary fat is enhanced, and TG secretion is blunted with smaller sized CMs being secreted. These results suggest that DGAT1 and DGAT2 work together to govern number and size of CMs for TG secretion. We also found that in Dgat1Int mice, which have much higher ratio of Dgat1:Dgat2, TG storage in response to dietary fat is reduced, with no effect on TG secretion or CM size. We speculate that the imbalance between TG storage and secretion as well as the unaccounted TGs in these models may be due to altered FA oxidation in enterocytes. We found that after a chronic HF feeding in Dgat2Int models, which have a lower ratio of Dgat1:Dgat2 compared to WT mice, mRNA levels of genes related to FA oxidation in the small intestine were remarkably lower. Together, these findings suggest that increased intestinal Dgat2 mRNA levels may decrease FAs targeted for oxidation, subsequently increasing substrates for synthesis of TG for secretion and systemic delivery. The results from this study have allowed us to propose a new role of DGAT1 and DGAT2, which is in conjunction with the two-step model for TG-rich lipoprotein synthesis. We theorize that in the small intestine DGAT1 is responsible for the synthesis of TG in the ER lumenal lipid droplets, whereas DGAT2 is responsible for the synthesis of TG in primordial lipoproteins and cytoplasmic lipid droplets. The results presented in this dissertation provide evidence that dietary fat absorption and intestinal TG metabolism is a highly regulated process. The long term objectives of these studies are to identify new mechanistic details of this dietary fat absorption that will aid in the development of clinical treatments for the prevention and treatment of obesity and related metabolic diseases.

Degree

Ph.D.

Advisors

Buhman, Purdue University.

Subject Area

Biochemistry|Nutrition|Physiology

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