Coronary heart disease, diabetes, and obesity remain major clinical problems worldwide and elevated plasma triglyceride (TG) levels constitute an independent risk factor. Discovered in 2001, apolipoprotein AV (apoAV) is a protein synthesized and secreted by the liver and is found to be inversely proportional to plasma TG levels. Mice and humans lacking functional apoAV have markedly elevated plasma TG while mice with high expression of human apoAV have dramatically decreased plasma TG. Curiously, plasma concentrations of apoAV are extremely low compared with other apolipoproteins such as apoB and apoAI. It has fascinated investigators for years that such a low circulating apolipoprotein can exert such a profound effect on plasma TG. We believe we have some novel insight to this question and this involves apoAV's action on the formation and secretion of chylomicrons (CM) by the enterocytes. Our preliminary data show that: 1) apoAV knockout (KO) animals were significantly more efficient in the absorption and lymphatic transport of both triglyceride (TG) an cholesterol than WT animals; 2) apoAV-KO animals have shorter CM appearance time than WT animals; 3) apoAV-KO animals release more pre-CM transport vesicles (PCTV) from endoplasmic reticulum (ER) than WT animals; 4) apoAV-KO animals secrete more apoB48 (therefore more CM particles since there is one apoB48 per particle) into lymph than WT animals; 5) apoAV is associated with CM in lymph during active fat absorption; and 6) apoAV is present in bile. We hypothesize that apoAV secreted by the liver regulates the intracellular formation and secretion of chylomicrons by the enterocytes and this regulation is mediated via biliary delivery and uptake of apoAV from the intestinal lumen. We will test these hypotheses in two specific aims.
AIM 1 : To determine the molecular mechanisms underlying the increased lymphatic transport of TG and cholesterol by apoAV-KO mice. There are four subaims to investigate the molecular mechanism of how apoAV modulates the formation and secretion of CM by the small intestine. The role of apoAV in CM and VLDL pathways for the transport of esterified FA will be determined. Taking advantage of our ability to label and isolate intestinal CM, the role of apoAV on CM metabolism will also be studied.
AIM 2 : Using recombinant apoAV in apoAV-KO animals, we will determine the effect of restoring apoAV in the circulation on intestinal CM transport. We will determine the effect of restoring hepatic production of apoAV to knockout animals (thus restoring both circulating and biliary apoAV) on intestinal CM transport. Lastly, we will determine directly the role of biliary apoAV in the lumen of apoAV-KO animals on lipid and apolipoprotein secretion by the gut, and study the uptake and lymphatic transport of 35S methionine labeled apoAV by the intestine will be studied.
Mice and humans lacking functional apoAV have markedly elevated plasma TG while mice with high expression of human apoAV have dramatically decreased plasma TG. Since apoAV is produced almost exclusively by the liver and its production is highly upregulated by partial hepatectomy, we hypothesized in our proposal that apoAV is produced in the liver to regulate the production of chylomicrons by the gastrointestinal tract so as not to overwhelm the liver when its function is compromised. Thus the proposed research is not only of interest to our understanding of the gastrointestinal physiology of apoAV but it may also provide insights and potential targets for the clinical management of hypertriglyceridemia.
