Multiple, Independently Regulated Pathways of Cholesterol Transport across the Intestinal Epithelial Cells

The present study provides a new understanding about the mechanisms involved in cholesterol absorption by the intestinal cells. Contrary to general belief, our data show that newly absorbed cholesterol is neither immediately available for secretion with apoB lipoproteins nor exclusively secreted as part of chylomicrons. Based on our data, cholesterol transport by enterocytes can be broadly classified into two independently modulated, apoB-dependent and -independent, pathways. Cholesterol secretion by the apoB-dependent pathway is induced by oleic acid, is repressed by microsomal triglyceride transfer protein inhibitors, and occurs only with larger apoB-containing lipoproteins. ApoB-independent pathways do not require microsomal triglyceride transfer protein and involve efflux mediated by ABCA1, high density lipoprotein assembly, and possibly other unknown mechanisms. There are at least two different metabolic pools of cholesterol. The newly absorbed and pre-absorbed cholesterol are preferentially secreted via apoB-independent and apoB-dependent pathways, respectively. In contrast to compartmentalization for secretion, these two metabolic pools are equally accessible for cellular esterification. The esterified cholesterol is mainly secreted by the apoB-dependent pathway, whereas both the pathways are involved in the secretion of free cholesterol. Thus, enterocytes transport exogenous cholesterol by several independently regulated pathways raising the possibility that targeting of apoB-independent pathways may result in selective inhibition of cholesterol transport without affecting triglyceride transport. The present study provides a new understanding about the mechanisms involved in cholesterol absorption by the intestinal cells. Contrary to general belief, our data show that newly absorbed cholesterol is neither immediately available for secretion with apoB lipoproteins nor exclusively secreted as part of chylomicrons. Based on our data, cholesterol transport by enterocytes can be broadly classified into two independently modulated, apoB-dependent and -independent, pathways. Cholesterol secretion by the apoB-dependent pathway is induced by oleic acid, is repressed by microsomal triglyceride transfer protein inhibitors, and occurs only with larger apoB-containing lipoproteins. ApoB-independent pathways do not require microsomal triglyceride transfer protein and involve efflux mediated by ABCA1, high density lipoprotein assembly, and possibly other unknown mechanisms. There are at least two different metabolic pools of cholesterol. The newly absorbed and pre-absorbed cholesterol are preferentially secreted via apoB-independent and apoB-dependent pathways, respectively. In contrast to compartmentalization for secretion, these two metabolic pools are equally accessible for cellular esterification. The esterified cholesterol is mainly secreted by the apoB-dependent pathway, whereas both the pathways are involved in the secretion of free cholesterol. Thus, enterocytes transport exogenous cholesterol by several independently regulated pathways raising the possibility that targeting of apoB-independent pathways may result in selective inhibition of cholesterol transport without affecting triglyceride transport. Due to a significant positive correlation between cholesterol absorption and plasma cholesterol levels (1Kesaniemi Y.A. Miettinen T.A. Eur. J. Clin. Invest. 1987; 17: 391-395Crossref PubMed Scopus (161) Google Scholar, 2Wilson M.D. Rudel L.L. J. Lipid Res. 1994; 35: 943-955Abstract Full Text PDF PubMed Google Scholar, 3McGill Jr., H.C. Am. J. Clin. Nutr. 1979; 32: 2664-2702Crossref PubMed Scopus (116) Google Scholar), cholesterol absorption has been the subject of intense research. Cholesterol absorption is defined as the transfer of cholesterol from the intestinal lumen to the mesenteric or thoracic lymph duct (2Wilson M.D. Rudel L.L. J. Lipid Res. 1994; 35: 943-955Abstract Full Text PDF PubMed Google Scholar). Early studies resulted in the identification and characterization of enzymes that hydrolyze cholesterol esters in the intestinal lumen, and an appreciation of the role bile acids play in the solubilization and cellular uptake of cholesterol (4Phan C.T. Tso P. Front Biosci. 2001; 6: D299-D319Crossref PubMed Google Scholar, 5Tso P. Karlstad M.D. Bistrian B.R. DeMichele S.J. Am. J. Physiol. 1995; 268: G568-G577Crossref PubMed Google Scholar, 6Carey M.C. Small D.M. Bliss C.M. Annu. Rev. Physiol. 1983; 45: 651-677Crossref PubMed Scopus (643) Google Scholar). From these studies, it was established that the dietary cholesterol esters are hydrolyzed in the intestinal lumen, and free cholesterol is solubilized in the bile salt micelles and is taken up by intestinal epithelial cells (2Wilson M.D. Rudel L.L. J. Lipid Res. 1994; 35: 943-955Abstract Full Text PDF PubMed Google Scholar, 4Phan C.T. Tso P. Front Biosci. 2001; 6: D299-D319Crossref PubMed Google Scholar, 5Tso P. Karlstad M.D. Bistrian B.R. DeMichele S.J. Am. J. Physiol. 1995; 268: G568-G577Crossref PubMed Google Scholar, 6Carey M.C. Small D.M. Bliss C.M. Annu. Rev. Physiol. 1983; 45: 651-677Crossref PubMed Scopus (643) Google Scholar). Currently, it is believed that the uptake of cholesterol by the enterocytes is the rate-limiting step in cholesterol absorption (7Dawson P.A. Rudel L.L. Curr. Opin. Lipidol. 1999; 10: 315-320Crossref PubMed Scopus (96) Google Scholar). This is supported by the observation that a man who ate 25 eggs a day did not develop hypercholesterolemia because he absorbed less cholesterol (8Kern Jr., F. N. Engl. J. Med. 1991; 324: 896-899Crossref PubMed Scopus (104) Google Scholar). After uptake, enterocytes esterify cholesterol, a process mediated by acyl-coenzyme A:cholesterol acyltransferases (9Chang T.Y. Chang C.C. Cheng D. Annu. Rev. Biochem. 1997; 66: 613-638Crossref PubMed Scopus (441) Google Scholar, 10Chang T.Y. Chang C.C. Lin S. Yu C. Li B.L. Miyazaki A. Curr. Opin. Lipidol. 2001; 12: 289-296Crossref PubMed Scopus (211) Google Scholar), and package it into chylomicrons for basolateral secretion into the mesenteric lymph. The major sources of cholesterol for absorption are of exogenous and endogenous origins. The exogenous source of cholesterol is food, whereas endogenous cholesterol is derived from bile, desquamated cells, and biosynthesis. Endogenous cholesterol, especially biliary cholesterol, enters the intestinal lumen in association with bile salts and is immediately available for absorption. On the other hand, there is a time delay for the absorption of dietary cholesterol because it needs to be solubilized in bile salt micelles. The differences observed in the metabolism of biliary and dietary cholesterol have been explained based on the differences in the solubilization and uptake (2Wilson M.D. Rudel L.L. J. Lipid Res. 1994; 35: 943-955Abstract Full Text PDF PubMed Google Scholar). It is possible that after entering the cells these two pools are handled differently. The intracellular mechanisms for cholesterol packaging in chylomicrons are poorly understood because most of the animal studies involve measurement of radiolabeled cholesterol in the feces and thoracic duct lymph or blood. Recent advances in cellular models to study intestinal lipid absorption have resulted in newer attempts toward the understanding of cholesterol absorption by enterocytes. In and Jr., Biochem. J. PubMed Scopus Google Scholar, J. Lipid Res. 1995; Full Text PDF PubMed Google have that cells preferentially plasma cholesterol with lipoproteins and that secretion is by The plasma cholesterol is to the by intracellular a process by oleic S. J. Lipid Res. 1995; Full Text PDF PubMed Google Scholar), and is by cholesterol from absorption or cellular biosynthesis. the mechanisms involved in the transport of cholesterol, and observed that intestinal cells absorbed cholesterol and by several mechanisms. for the of apoB high with and microsomal triglyceride transfer oleic density density have been 1995; PubMed Scopus Google Scholar, A. J. 1997; 32: PubMed Scopus Google Scholar). was from of the of from acid, oleic and from was from triglyceride transfer was a from of the of to of by and at a was This was and in micelles for with in and of to micelles oleic acid, and and The was of micelles to to of micelles Small D.M. Med. PubMed Scopus (104) Google Scholar). with cells from the in high with and and cells from to on at a density of of these cells, other day for J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, N. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar, A. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar). two different and the cells of on the and of with or without on the basolateral for studies, cells with cholesterol for as with two the was from the and the basolateral After cells and with and on the and on the basolateral to density and the was for the measurement of and radiolabeled cholesterol. of the to with density in J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). the density to of and by with of and and of density The to and the was This was and chylomicrons J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). The with of and and and the was The with and and the was by This the The of the into and in in of Cholesterol with most of the was to study the secretion of cholesterol with apoB lipoproteins. After of with cells with for in the of to secretion of cholesterol with chylomicrons. the secretion of cholesterol with with to density of of cholesterol secretion was by the of N. C. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google on the basolateral and of not because of in S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar, C. A. N. S. Biochem. Res. PubMed Scopus Google Scholar), cells with 25 with for The basolateral and on the and on the basolateral After cells and with on the and with on the basolateral after and for and density After cells with with and of and from the and to the of and J. Biochem. Physiol. PubMed Scopus Google Scholar). of or was of and at with After of was and for of was and and the was with a The was with of and with the from cells, and in of and by on to free cholesterol and esters after and by with enterocytes by the of J. Full Text PDF PubMed Google as by PubMed Scopus Google and and J. Lipid Res. 1999; Full Text Full Text PDF PubMed Google Scholar). to of the from and the and with for with of the was and with with The in a at with After the was and with with and in as After the and and in with enterocytes with of at with and with at After enterocytes at for and the with After cells for with oleic acid, and micelles in the and of the of the enterocytes and for density was by the of Biochem. PubMed Scopus Google Scholar). and in the and in different density a as 1995; PubMed Scopus Google Scholar, A. J. 1997; 32: PubMed Scopus Google Scholar). of on the and of is believed that dietary cholesterol enters the as part of chylomicrons by the intestinal cells. have that of high of oleic and secretion by cells J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, N. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar, A. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar). study the of on the of cellular of cholesterol, cells with in the of or on the There significant differences in the cellular of cholesterol both the secretion of radiolabeled cholesterol by these cells secretion of larger apoB-containing lipoproteins by cells, that cholesterol neither the nor the of cholesterol secreted with cells studies that not cellular uptake and secretion of cholesterol. was to the possibility that have the secretion of cholesterol. cholesterol secretion by cells was and of cholesterol with chylomicrons was by density and with the of cholesterol in Cholesterol was present at a in the of the cholesterol was in the as in the with of cholesterol of was present in that to In contrast to the of the of cholesterol of in larger lipoproteins studies that the of the secreted cholesterol was in the whereas significant of cholesterol in the that larger lipoproteins. In with that the of cholesterol in the not to not the of cholesterol present with lipoproteins was to the of in the these in the of on the basolateral Cholesterol was mainly present at a density of to in the of the secreted cholesterol was in the as it was in the with In two and and to and cholesterol was in two and and to and of secreted cholesterol was with larger lipoproteins. studies that the of the secreted cholesterol was not with larger it was with these lipoproteins the cholesterol secretion in was to cells, these studies to enterocytes the of cholesterol to cells. in free cholesterol was mainly present in enterocytes with cholesterol for and with to and After the to density The secreted cholesterol was in and and of the secreted cholesterol was present in the and the of secreted cholesterol. studies that intestinal cells cholesterol in two that can be based on of Cholesterol in and that of cholesterol secreted with lipoproteins. is for cholesterol secretion cells with for and with or for and cholesterol levels in the basolateral apoB secretion by without affecting secretion with the secretion of apoB by on secretion of cholesterol that the secretion of cholesterol by the cholesterol studies that apoB and cholesterol secretion has on The in apoB and cholesterol secretion are by the inhibition of of oleic and on the secretion of and cholesterol by with with and with with with with with and with with with in a new that cells did not apoB as chylomicrons and the of apoB was in density density lipoproteins as has been N. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar). the secretion of apoB as part of and chylomicrons and with our studies J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, N. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar). of secreted In the of the secretion of apoB with and chylomicrons was apoB was mainly present in lipoproteins at the of secreted cholesterol in different lipoprotein There was cholesterol in larger lipoprotein in the of cells and that these cells apoB as lipoproteins and these did not of cholesterol. The of the secreted cholesterol was in the that apoB the secretion of cholesterol in larger apoB lipoproteins and on the of cholesterol secreted with lipoproteins and In cells, with apoB secretion in larger there was a significant in the secretion of cholesterol in lipoprotein and that on the secretion of cholesterol studies that apoB was secreted as part of cholesterol was not with these Thus, it that of larger lipoproteins is for the secretion of cholesterol with apoB lipoproteins. the of cholesterol secretion in enterocytes enterocytes not with lipid cholesterol was present in two and and and of enterocytes with lipid micelles the of cholesterol in the and In significant of cholesterol present in and to and chylomicrons. In the of the secretion of cholesterol in and was and the of cholesterol in and that these on the of cholesterol present in and that the of larger lipoproteins is for the secretion of cholesterol with and secretion is in the of In of lipoprotein has on the cholesterol secretion in to Cholesterol of cholesterol in the basolateral was cholesterol efflux to the secretion of cholesterol of lipoprotein secretion, cells for and in the and of an efflux N. C. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google with to cholesterol secretion by on as by Biochem. PubMed Scopus Google Scholar, J. 1995; Full Text Full Text PDF PubMed Scopus Google in not that cholesterol in the by without affecting the of cholesterol secreted with apoB lipoproteins cholesterol secretion the not the role of cholesterol induced cholesterol efflux pathway by and acid, have been to the of in cells S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar, C. A. N. S. Biochem. Res. PubMed Scopus Google Scholar). of cells with and the of cholesterol and was in the of that cholesterol secretion by and was in the of that there is cholesterol in because these cells did not studies that efflux mechanisms to cholesterol secretion by cells. Cholesterol for with to for the secretion of cholesterol with chylomicrons the of secretion of two different cholesterol newly absorbed and newly absorbed cholesterol, cells with radiolabeled cholesterol for the time study the metabolism of pre-absorbed cholesterol, cells with for and for time in the newly and pre-absorbed cellular pools of cholesterol. the cellular of newly absorbed cholesterol with time to uptake On the other hand, the pre-absorbed cellular cholesterol with of the basolateral that the of newly absorbed cholesterol of the pre-absorbed pools at time The secretion of newly absorbed cholesterol was especially significant at time at cellular levels of pre-absorbed cholesterol the newly absorbed cholesterol the basolateral of newly absorbed cholesterol with pre-absorbed cholesterol The cellular of to newly absorbed cholesterol with most to uptake of radiolabeled cholesterol The secreted of newly absorbed cholesterol, and the between and newly absorbed cholesterol was data that cells newly absorbed cholesterol, and secretion is of the present in the cellular the pools of cholesterol secreted with apoB and without apoB lipoproteins of pre-absorbed cholesterol newly absorbed cholesterol at time In of newly absorbed and pre-absorbed cholesterol up to This in of newly absorbed cholesterol with and the of newly absorbed cholesterol at The to newly absorbed cholesterol in apoB lipoproteins was in the to at and and to at In was and to at From these studies that the pre-absorbed cholesterol was preferentially secreted with apoB and the newly absorbed cholesterol was mainly secreted with these lipoproteins. of and the transport of cholesterol the cells. It is that cholesterol in and esterified about differences in the transport of free and esterified cholesterol, in different lipoprotein that cells secreted free cholesterol mainly in and In these esterified cholesterol of the cholesterol the of free and esterified cholesterol in apoB lipoproteins and on the free and esterified cholesterol present in the and In esterified cholesterol of the cholesterol. the secretion of free and esterified cholesterol in apoB lipoproteins on the secretion of free and esterified cholesterol in and of efflux by only free cholesterol in not studies the of free and esterified cholesterol secretion by apoB-independent and apoB-dependent pathways. cholesterol pools for cellular the of and newly absorbed free and esterified cholesterol in cells at different time to observed for cellular cholesterol the of newly absorbed free cholesterol with time On the other hand, the of pre-absorbed free cholesterol with time in cells. for the of different cholesterol pools The of both the newly and pre-absorbed cholesterol with time the of of pre-absorbed cholesterol to be that of newly absorbed cholesterol. of the cellular esterified cholesterol that about of the pre-absorbed cholesterol was in the esterified at and to at of the newly absorbed cholesterol was in esterified and to at studies that cellular mechanisms do not between pre-absorbed and newly absorbed cholesterol pools and that both the pools are accessible for esterification. different metabolic pools of free and esterified cholesterol in apoB lipoproteins and at The of pre-absorbed free cholesterol in lipoprotein about of newly absorbed cholesterol In of newly absorbed free cholesterol of cholesterol esters in different that cholesterol esters derived from pre-absorbed cholesterol derived from newly absorbed cholesterol in apoB lipoproteins In the of cholesterol esters derived from newly absorbed cholesterol studies that free and esterified cholesterol mainly derived from the newly absorbed cholesterol In apoB lipoproteins free and esterified cholesterol from the pre-absorbed cholesterol the of esterified cholesterol in different In apoB of both newly and pre-absorbed cholesterol was in the esterified In only of esterified cholesterol. studies that of the free cholesterol is secreted of whereas cholesterol esters are secreted mainly with lipoproteins. Cholesterol by studies a new understanding about the mechanisms involved in the transport of cholesterol by intestinal cells. of the cells cholesterol by of lipoproteins and cholesterol by efflux pathways. cells are in to from the basolateral M.C. Am. J. Physiol. 1995; PubMed Google Scholar), these cells up cholesterol from the intestinal lumen and cholesterol as part of chylomicrons. Contrary to the general belief, the data that newly absorbed cholesterol is neither immediately available for secretion with apoB lipoproteins nor exclusively secreted as part of chylomicrons. Based on our data, cholesterol transport by enterocytes can be broadly classified into two independently apoB-dependent and apoB-independent pathways. pathways show toward the free and esterified cholesterol. The apoB-dependent pathway the esterified cholesterol, whereas both pathways free cholesterol. it is believed that cholesterol is mainly as part of there are in the the of cholesterol in lymph lipoproteins other chylomicrons. J. Lipid Res. Full Text PDF PubMed Google have that in and lymph lipoproteins and of cholesterol, respectively. The transport of cholesterol in lipoproteins to in and to in that in lipoproteins may be to of the of lymph lipoproteins by high cholesterol J. Lipid Res. Full Text PDF PubMed Google Scholar). and Rudel Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar, Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google have that cholesterol is present in both and lipoprotein in that and of the exogenous and endogenous cholesterol is on In cholesterol was present in lymph lipoproteins that and did not apoB Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar). that these lipoproteins have been derived from plasma by Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar, Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar). C. D. Am. J. Clin. Nutr. 2001; PubMed Scopus Google that of the newly absorbed cholesterol was not with lipoproteins in these studies to our that cholesterol both apoB-dependent and apoB-independent pathways. the apoB-dependent pathway to cholesterol transport the The apoB-independent pathways may be of in the and high cholesterol It is that cholesterol absorption is in the of apoB lipoprotein and In two with microsomal triglyceride transfer cholesterol absorption was to be of the Lin J. Full Text PDF PubMed Scopus Google Scholar). In apoB C.M. A. Jr., J. Clin. Invest. 1995; PubMed Scopus Google have cholesterol absorption. data are to that apoB lipoprotein is the only present for cholesterol transport the intestinal epithelial cells. there be other for the to cholesterol absorption in the of apoB lipoprotein have the possibility that may be for apoB-independent secretion inhibition of apoB lipoprotein by has on apoB-independent cholesterol secretion and It is possible that apoB-independent secretion is by the to the of in the of apoB lipoprotein and Lipid may cholesterol from cellular and for apoB-independent pathways. of Cholesterol with of cholesterol secretion with apoB lipoproteins are different from that of triglyceride transport. have that newly are secreted with chylomicrons in cells J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). In cholesterol taken up by these cells is not immediately available for secretion with chylomicrons enterocytes cholesterol from the of these cells of cholesterol with chylomicrons. The mechanisms of and secretion of cholesterol can in studies cholesterol secretion into the a after in that and secretion C. D. Am. J. Clin. Nutr. 2001; PubMed Scopus Google Scholar). Thus, a significant time occurs from the time of cellular cholesterol uptake to secretion with apoB lipoproteins. It is that cells two of apoB-containing J. J. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, N. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar). In the of apoB is secreted as by cells, whereas it is secreted on larger lipoproteins after the of free data to that apoB lipoproteins are as lipoprotein that are to larger lipoproteins by Full Text Full Text PDF PubMed Scopus Google Scholar, J. P. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar, J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). show that apoB lipoproteins do not levels of cholesterol. cholesterol is secreted with larger apoB lipoproteins that is for apoB-dependent transport the secretion with larger that cholesterol is the of lipoproteins into larger lipoproteins. of Cholesterol by ApoB-independent observed that cholesterol was present in the The apoB-independent cholesterol secretion was not to not and was observed in and in or not have observed cholesterol in lipoproteins in J. Lipid Res. 1995; Full Text PDF PubMed Google and in the lymph of Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar, Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google and J. Lipid Res. Full Text PDF PubMed Google Scholar). In of cholesterol was in C. D. Am. J. Clin. Nutr. 2001; PubMed Scopus Google Scholar). Based on these studies, that apoB-independent pathways to cholesterol transport the intestinal epithelial cells. possible for apoB-independent transport of cholesterol is the efflux Cholesterol efflux mediated by is in cholesterol secretion because of inhibition by and by In our studies, process to of cholesterol the data about cholesterol absorption have been in cholesterol absorption is by in studies C. 2001; Full Text Full Text PDF PubMed Scopus (116) Google Scholar, T.A. B.L. S. P.A. Am. J. Full Text Full Text PDF PubMed Scopus Google and in other studies J. D. C. C. J. C. S. A. PubMed Scopus Google Scholar). In of the cholesterol is absorbed by the efflux pathway A. C. J. Full Text Full Text PDF PubMed Scopus Google Scholar). Thus, that cholesterol efflux pathway in enterocytes can to the absorption of cholesterol. that efflux pathways are in cholesterol transfer for of the pathway that to apoB-independent cholesterol transport is the intracellular and secretion of by the Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar, J. Clin. Invest. PubMed Scopus Google Scholar, T.A. J. Clin. Invest. PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). has been with and cholesterol in and D. The and of Scholar, N. A. S. J. Lipid Res. 1997; Full Text PDF PubMed Google Scholar). This is to the of to transport cholesterol in the studies are to cholesterol absorption with from the of enterocytes to the intestinal lumen has been as a for the cholesterol absorption J. Li PubMed Scopus Google Scholar). Recent studies have that do not efflux from the to bile salt micelles and other S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar, C. A. N. S. Biochem. Res. PubMed Scopus Google Scholar). have not In our data and that of S. J. Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar, C. A. N. S. Biochem. Res. PubMed Scopus Google that efflux may play a role in the of cholesterol from the intestinal lumen to the of apoB-independent cholesterol secretion may be a to plasma cholesterol to plasma cholesterol is to and secretion of apoB lipoproteins. are A. A. Front Biosci. PubMed Scopus Google Scholar). data that a to plasma cholesterol be to the apoB-independent cholesterol transport is to result in inhibition of cholesterol it to triglyceride that as a of the inhibition of apoB lipoprotein of Cholesterol and absorbed and pre-absorbed pools may dietary and biliary cholesterol, respectively. observed that these two metabolic pools of cholesterol are for The newly absorbed and pre-absorbed are preferentially secreted via apoB-independent and apoB-dependent pathways, respectively. The secretion of newly absorbed cholesterol by an apoB-independent pathway is in with the studies of and Rudel Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google Scholar, Rudel L.L. J. Lipid Res. 1983; Full Text PDF PubMed Google who that of exogenous cholesterol in lymph lipoproteins. The secretion of two metabolic pools the compartmentalization of cholesterol in different intracellular The newly absorbed cholesterol is into the plasma as by J. Lipid Res. 1995; Full Text PDF PubMed Google Scholar). This is available for efflux in the from both the and basolateral it to efflux with time because it is to other intracellular it available for secretion with apoB-containing lipoproteins. In contrast to the secretion, these two metabolic pools are equally accessible for esterification. This is in with studies on the of plasma cholesterol by newly and absorbed cholesterol J. Lipid Res. 1987; Full Text PDF PubMed Google Scholar). The cholesterol may be to the of at least two enzymes that have to different cholesterol pools (9Chang T.Y. Chang C.C. Cheng D. Annu. Rev. Biochem. 1997; 66: 613-638Crossref PubMed Scopus (441) Google Scholar, 10Chang T.Y. Chang C.C. Lin S. Yu C. Li B.L. Miyazaki A. Curr. Opin. Lipidol. 2001; 12: 289-296Crossref PubMed Scopus (211) Google Scholar). In have that cholesterol transport by enterocytes is a regulated process mechanisms. pathways mainly transport cholesterol esters and pre-absorbed free cholesterol. It is induced by oleic and by Cholesterol secretion by pathway is on the of larger lipoproteins that cholesterol is to these lipoproteins at of ApoB-independent pathways are involved in the secretion of newly absorbed cholesterol and may play a role in the of cellular free cholesterol. have that efflux to It is that mechanisms to apoB-independent cholesterol pathways may as new to plasma cholesterol

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