Apolipoprotein C-III (apoC-III) is an exchangeable lipoprotein produced by both the liver and intestine. The plasma concentration of apoC-III is elevated in type 1 diabetics and is an independent predictor of cardiovascular disease. ApoC-III is found on triglyceride-rich lipoproteins and high-density lipoproteins, and acts to raise plasma triglyceride levels through the inhibition of lipoprotein lipase activity, and by interfering with lipoprotein clearance by the liver. While significant progress has been made in understanding the mechanisms that govern these effects, the role of apoC-III in intestinal lipoprotein synthesis and secretion remains undefined. We have recently observed that whole body overexpression of apoC-III in apoC-III transgenic (Tg) mice results in inhibited intestinal TAG secretion from the enterocyte into lymph coupled with the retention of free fatty acid (FFA) and monoacylglycerol (MAG) precursors and a decrease in TAG within the enterocyte. This inhibitory effect of apoC-III in the intestinal cell is in contrast to studies in hepatocytes, which have correlated apoC-III expression with increased VLDL secretion. Our overall hypothesis is that apoC-III plays a unique role in the intestine and liver in lipoprotein synthesis and secretion. We will test the 3 possible mechanisms by which increased apoC-III would lead to inhibited TAG synthesis (that apoC-III directly inhibits 1) MGAT and/or DGAT enzyme activity;2) FFA trafficking to the ER;or 3) the activation of FFA to fatty acyl- CoA) and will delineate a mechanistic role for apoC-III in intestinal lipid absorption and secretion. We will also investigate the functional importance of apoC-III in the intestine versus the liver using adenovirus to knockdown and overexpress apoC-III in mouse liver. Finally, we will investigate the role of human apoC-III polymorphisms on intestinal TAG secretion in primary mouse enteroid cultures. The completion of these aims will provide key knowledge about the role of apoC-III in mediating intestinal lipoprotein secretion and the potential of intestinal apoC-III as a therapeutic target.
Previous studies of apoC-III have focused on its roles in both liver and plasma to contribute to hyperlipidemia. Its expression in intestine coupled with our discovery that overexpression inhibits TAG secretion suggest that there is a unique and unknown role for apoC-III in fat absorption. Given that fat absorption and intestinal lipoprotein secretion contribute to both cardiovascular disease and diabetes, this is a significant gap in our knowledge. The studies proposed here will investigate the mechanisms by which apoC-III acts in this tissue, thus contributing significantly to our understanding of the intestine, fat absorptin, and apoC-III and will be crucial for the development of drugs that might directly target intestinal apoC-III expression thus inhibiting TAG secretion.
