Functional & molecular study of intetinal cholesterol transporters & absorption

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Zetia (Ezetimibe) inhibits cholesterol absorption via inhibition of NPC1-L1 mediated cholesterol transport pathway(s) in the enterocytes. We demonstrated that CD36 also plays a role in cholesterol absorption. It has also been demonstrated that the scavenger receptor SR-BI as another putative cholesterol transporter on intestinal brush border membrane. We hypothesize that SR-BI and CD36 are cholesterol transporters in the proximal and distal intestine, respectively, working in conjunction with NPC1-L1 (may be intracellular) to mediate cholesterol uptake from the lumen to intracellular compartments of the enterocytes where chylomicron assembly occurs. Aim 1: We will determine: 1) if the lack of SR-BI, CD36, or NPC1L1 alters the site and/or efficiency of cholesterol absorption (by varying the site or dose of infusion); and 2) if the counterpart transporters are upregulated to maintain cholesterol absorption in the absence of one or the other transporter protein. In lymph fistula mice, we will also determine if Ezetimibe remains effective in inhibiting cholesterol uptake by the small intestine in these knockout animals. Aim 2: First, we will determine if sub-effective doses of Ezetimibe, in combination with sub-effective doses of either ursodeoxycholate or Pluronic F-68 (F-68), inhibits intestinal cholesterol absorption in the rat. Also, we will determine the same phenomenon also works in the mouse. Second, in the knockout mice models described above, we will determine if Ezetimibe or ursodeoxycholate or F-68 alone or in combination is/are effective in inhibiting cholesterol absorption by the small intestine. Aim 3: We will take advantage of the ability of Ezetimibe to inhibit intestinal cholesterol, and the ability of Pluronic L-81 to prevent chylomicron formation to identify the pathway and quantify the amount of cholesterol exiting the enterocytes through efflux to the apical side as well as secreted through the basolateral membrane as triglyceride-rich lipoproteins. Using a combination of both Ezetimibe and Pluronic L-81, we will be able to study both the uptake and efflux of cholesterol by the small intestine. Completion of the proposed studies will provide us with new insights into how cholesterol uptake and lymphatic transport are regulated by the various transporters in the gut. These new data may also provide us with new information in the clinical management of hypercholestermic patients.