Molecular Mechanisms Determining the Fate of Intestinal Triacylglycerol

Dietary fat provides essential nutrients, contributes to energy intake and regulates blood lipid levels. These functions are important to health; however, when dysregulated they contribute to dyslipidemia and increase risk for development of obesity, diabetes and cardiovascular disease. ^ Dietary fat absorption is efficiently mediated by the small intestine. The digested products of dietary fat in the gut lumen are taken up by enterocytes, the absorptive cells of the intestine, where they are re-synthesized to triacylglycerol (TAG). The resulting TAG is packaged onto chylomicrons (CMs) for secretion into the circulation, contributing to postprandial blood lipid levels. When levels of dietary fat are high, TAG is also packaged into cytoplasmic lipid droplets (CLDs) for temporary storage within enterocytes. The CLDs increase and then decrease overtime in response to fat consumption, indicating that the stored TAG is mobilized for secretion or to the other fates at later time points. The intestinal metabolism of CMs and CLDs together regulate the rate and the amount of TAG secreted into the circulation. The objective of this dissertation work is to explore the mechanisms through which the TAG is partitioned or mobilized to certain metabolic fates in enterocytes. ^ First, we investigated the effect of endurance exercise on genes of intestinal lipid metabolism using Otsuka Long-Evans Tokushima Fatty (OLETF), an obese and diabetic rat model. We found that exercise training in these animals resulted in parallel upregulations of genes involved in TAG anabolic and catabolic processes and promoted mitochondrial biosynthesis in enterocytes compared to sedentary rats. We proposed that these changes lead to a more efficient fatty acid oxidation in the intestine and a consequent reduction of intestinal TAG secretion in this model. Overall, this work highlights that endurance exercise training programs intestinal lipid metabolism, contributing to the beneficial effect of endurance exercise on improving obesity and metabolic disease. ^ Nest, we investigated the differential roles of acyl CoA: diacylglycerol acyltransferase 1 (Dgat1) and Dgat2, in regulating dietary fat absorption. Mice with intestine-specific overexpression of Dgat1 (Dgat1 Int) or Dgat2 (Dgat2Int), or lack of Dgat1 (Dgat1–/–) were previously reported to have different intestinal phenotypes in response to fat consumption and altered susceptibilities to obesity and hepatic steatosis; the underlying mechanism(s) is unknown. By conducting an ultrastructural analysis on enterocytes from these Dgat mouse models in response to fat consumption, we found that Dgat1 and Dgat2 altered intracellular TAG distribution for CM and CLD synthesis. Based on the observations in the study, Dgat1 is proposed to preferentially synthesize TAG for the subcellular pool that promotes CM expansion in the ER lumen and thus limits TAG storage in CLDs. In addition, Dgat2 is proposed to preferentially synthesize TAG for the subcellular pool that determines the number of CMs generated in the ER lumen and for storage in CLDs. In this study, we provide the mechanism of how intestinal Dgat1 and Dgat2 exert regulatory effects on postprandial blood lipid levels and whole-body physiology. Overall, this work demonstrates non-redundant cellular roles of Dgat1 and Dgat2 in dietary fat absorption. ^ Lastly, we investigated the regulation of lipophagy, where CLDs are targeted by autophagy and catabolized in acidic lysosomes, in enterocytes of Dgat1–/– mice. We found an increased number of autophagic vesicle (AV) and abnormal TAG accumulation within AVs in enterocytes of Dgat1–/– compared to WT mice, suggesting an impaired AV turnover and an inefficient lipophagy by Dgat1 deficiency. In addition, we identified that this impaired lipophagy process was due to a lysosome dysfunction, as indicated by the decreased mRNA levels of genes involved in lysosome acidification and higher lysosome pH in enterocytes of Dgat1–/– compared to WT mice. Furthermore, we found alterations in cellular lipid composition and levels of reactive oxygen species (ROS) in enterocytes of Dgat1 –/– compared to WT mice. These changes may possibly contribute to the lysosome dysfunction seen in Dgat1 –/– mice. Based on the results in the study, we propose that the lysosome dysfunction limits lipid supply from the storage pool for secretion, resulting in a greater intestinal TAG storage and a reduced rate of intestinal TAG secretion seen in Dgat1 –/– mice. Together, this study highlights that lysosome function plays a critical role in lipophagy and that lipophagy may serve as a potential target for treating postprandial hyperlipidemia and its related diseases. ^ The findings presented in this dissertation expand the current knowledge of regulation of dietary fat absorption. The proposed models generated from these studies provide novel therapeutic strategies for managing postprandial blood lipid levels and preventing obesity and its related diseases.