Importance of Niemann-Pick C1-Like 1 in Intestinal Cholesterol Transport and Vascular Reactivity

Cardiovascular disease is the leading cause of death worldwide. An increase in plasma LDL-C is associated with an increased risk for CVD progression and events. The drug ezetimibe reduces dietary and biliary cholesterol absorption in humans by about 50% leading to an approximate 15% reduction in plasma LDL-C. Though Niemann-Pick C1 Like 1 (NPC1L1) has been shown to be the major target of ezetimibe; the exact role that NPC1L1 plays in the cholesterol absorption process remains controversial with some studies indicating a role in cholesterol uptake from the lumen and others indicating a role in intracellular trafficking.;The goal of the first part of this dissertation was to determine if NPC1L1 is required for the uptake of cholesterol from the intestinal lumen and/or for the transport of cholesterol to the intracellular site of esterification. Findings from our studies indicate that there is an alternative cholesterol absorption pathway whereby cholesterol can enter enterocytes independent of NPC1L1. Furthermore, additional findings show that any cholesterol taken up by enterocytes via this NPC1L1-independent pathway can be esterified similarly to the NPC1L1-dependent pathway. Taken together, these findings indicate that although most cholesterol is absorbed via the NPC1L1-dependent pathway in mice, NPC1L1 is not absolutely required for either the uptake or esterification of cholesterol.;The goal of the second part of this dissertation was to identify an ezetimibe-mediated protection against post-prandial vascular dysfunction. The post-prandial state is known to reduce vascular function in individuals with CVD, diabetes and metabolic syndrome and increase the risk for CV events. Additionally, post-prandial vascular function is slightly reduced in normolipemic individuals. Studies looking at the role of ezetimibe on vascular function in humans and animals have been inconclusive with some studies indicating no effect and others showing protection. Findings from these studies indicate that ezetimibe treatment protects wild-type mice from decreased post-prandial vascular reactivity. Further findings show that this protection is associated with an alteration in post-prandial triglyceride-rich lipoprotein composition; the ratio of both triglyceride and cholesterol to ApoB are reduced in the post-prandial TRL fraction in the presence of ezetimibe indicating a reduction in lipoprotein size. Additionally, electron microscopy indicated an overall reduction in TRL particle size after ezetimibe treatment. Finally, the amount of [ 3H]triolein associated with the aorta two hours after oral gavage was reduced in the presence of ezetimibe.;Taken together, the findings from this dissertation have a two-fold implication in the field of cardiovascular research. First, these studies indicate an alternative cholesterol absorption pathway, which is not susceptible to the current cholesterol absorption inhibitor, ezetimibe. Secondly, we demonstrated that reducing dietary and biliary cholesterol absorption in the absence of a statin improved the post-prandial vascular response, altered TRL composition and reduced TAG associated with the aorta. The development of a drug to inhibit both the NPC1L1-dependent and NPC1L1-independent cholesterol absorption pathways may reduce cholesterol absorption in humans beyond that exhibited by ezetimibe and lead to both a further reduction in plasma LDL-C and a more pronounced protection against reduced post-prandial vascular function.