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Figure 1. Most triglycerides are resynthesized in the enterocyte by the monoglyceride pathway. The phosphatidic acid pathway normally accounts for approximately 20% of enterocytic triglycerides. However, Caco-2 cells triglycerides are primarily esterified by the latter path- way. Broken arrow, ACYL CoA as substrate.

Figure 1 Most triglycerides are resynthesized in the enterocyte by the monoglyceride pathway. The phosphatidic acid pathway normally accounts for approximately 20% of enterocytic triglycerides. However, Caco-2 cells triglycerides are primarily esterified by the latter path- way. Broken arrow, ACYL CoA as substrate.

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cell line, has permitted researchers to uncover much infor- mation about the composition, structure, distribution, and function of intestinal lipoproteins. Such studies offer many advantages over conventional techniques, such as the rapid assessment of fatty acid, cholesterol, and phospholipid up- take and metabolism; the opportunity to elucidate the molecular mechanisms of lipoprotein formation and assem- bly; and the advantage of using human rather than animal tissues. The intent of this review is to summarize the current state of knowledge regarding the regulation and metabolism of lipids and lipoproteins in Caco-2 cells. ABSTRACT Caco-2 cells, an intestinal cell line derived from a human colorectal carcinoma that spontaneously differentiates under standard culture conditions, lends it- self to the in vitro study of human gut in view of its efficient intestinal transport processes. Among its multi- ple biological functions are those related to the absorp- tion, transport, and metabolism of lipids and lipoproteins. Despite their intestinal origin, confluent Caco-2 cell monolayers primarily express L-FABP for the uptake of apical dietary long chain fatty acids, incorporating them into triglycerides by the glycerol 3-phosphate pathway, and assembling very-low-density lipoprotein, high-density lipoprotein, and low-density lipoprotein. The monoacyl- glycerol pathway is inactive in Caco-2 cells. Furthermore, the secretion of newly synthesized triglyceride-rich lipoproteins is very restricted, despite abundant produc- tion of apolipoprotein (apo) B. The regulation of apoB synthesis and its mRNA editing at the enterocyte level has been intensively examined in Caco-2 cells. Luminal fatty acids, calcium ion, as well as vitamins and hormones are known to modulate the apoB-48/apoB-100 at the tran- scriptional and/or translational level. The regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and acyl-CoA: (cholesterol acyltransferase), the key enzymes governing intracellular cholesterol handling, have also been exten- sively examined in Caco-2 cells. In many respects this cell line provides an excellent in vitro model for the investiga- tion of intestinal lipoprotein metabolism; however, their limited secretion capacity remains a potential drawback to comparisons with the in vivo physiological state. — Levy, E., Mehran, M., Seidman, E. Caco-2 cells as a model for intestinal lipoprotein synthesis and secretion. FASEB J. 9, 626-635 (1995)
cell line, has permitted researchers to uncover much infor- mation about the composition, structure, distribution, and function of intestinal lipoproteins. Such studies offer many advantages over conventional techniques, such as the rapid assessment of fatty acid, cholesterol, and phospholipid up- take and metabolism; the opportunity to elucidate the molecular mechanisms of lipoprotein formation and assem- bly; and the advantage of using human rather than animal tissues. The intent of this review is to summarize the current state of knowledge regarding the regulation and metabolism of lipids and lipoproteins in Caco-2 cells. ABSTRACT Caco-2 cells, an intestinal cell line derived from a human colorectal carcinoma that spontaneously differentiates under standard culture conditions, lends it- self to the in vitro study of human gut in view of its efficient intestinal transport processes. Among its multi- ple biological functions are those related to the absorp- tion, transport, and metabolism of lipids and lipoproteins. Despite their intestinal origin, confluent Caco-2 cell monolayers primarily express L-FABP for the uptake of apical dietary long chain fatty acids, incorporating them into triglycerides by the glycerol 3-phosphate pathway, and assembling very-low-density lipoprotein, high-density lipoprotein, and low-density lipoprotein. The monoacyl- glycerol pathway is inactive in Caco-2 cells. Furthermore, the secretion of newly synthesized triglyceride-rich lipoproteins is very restricted, despite abundant produc- tion of apolipoprotein (apo) B. The regulation of apoB synthesis and its mRNA editing at the enterocyte level has been intensively examined in Caco-2 cells. Luminal fatty acids, calcium ion, as well as vitamins and hormones are known to modulate the apoB-48/apoB-100 at the tran- scriptional and/or translational level. The regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and acyl-CoA: (cholesterol acyltransferase), the key enzymes governing intracellular cholesterol handling, have also been exten- sively examined in Caco-2 cells. In many respects this cell line provides an excellent in vitro model for the investiga- tion of intestinal lipoprotein metabolism; however, their limited secretion capacity remains a potential drawback to comparisons with the in vivo physiological state. — Levy, E., Mehran, M., Seidman, E. Caco-2 cells as a model for intestinal lipoprotein synthesis and secretion. FASEB J. 9, 626-635 (1995)
Figure 2. Scheme of lipid and apo processing for the formation of chylomicrons and VLDL
Figure 2. Scheme of lipid and apo processing for the formation of chylomicrons and VLDL
F, fetal calf serum; ND, not done. TABLE 2. Fatty acid regulation of apoB in Caco-2 cells the amount of secreted labeled apoB was also observed (41, 42). It seems therefore that more newly synthesized apoB is destined for translocation and secretion. The regulation o! apoB production by fatty acids occurs by a posttranscrip- tional mechanism, because apoB secretion increased without a simultaneous change in cellular apoB mRNA expressior 42, 45). This regulation may reflect translational control of poB synthesis or other posttranscriptional control mechan- sms, such as an alteration in the rate of intracellular apoB legradation (46). Similar studies in hepatocytes have previ- usly demonstrated that rapid intracellular degradation of ascent apoB occurs in the endoplasmic reticulum and sug-
F, fetal calf serum; ND, not done. TABLE 2. Fatty acid regulation of apoB in Caco-2 cells the amount of secreted labeled apoB was also observed (41, 42). It seems therefore that more newly synthesized apoB is destined for translocation and secretion. The regulation o! apoB production by fatty acids occurs by a posttranscrip- tional mechanism, because apoB secretion increased without a simultaneous change in cellular apoB mRNA expressior 42, 45). This regulation may reflect translational control of poB synthesis or other posttranscriptional control mechan- sms, such as an alteration in the rate of intracellular apoB legradation (46). Similar studies in hepatocytes have previ- usly demonstrated that rapid intracellular degradation of ascent apoB occurs in the endoplasmic reticulum and sug-
TABLE 1. Utilization of the Caco-2 cell line as a model for study of metabolic processes in enterocytes
TABLE 1. Utilization of the Caco-2 cell line as a model for study of metabolic processes in enterocytes
“Medium lipids in this study were associated with VLDL. Most of the labeled triglyceride (90%) found in the medium was recovered in the VLDL raction. ND, not done. TABLE 3. Caco-2 cells are inefficient at secreting lipids Although the intestine plays an important role in the ab- sorption and biosynthesis of cholesterol, the regulation of the enzymes responsible for intracellular cholesterol homeostasis remains unclear. Enterocytes have to regulate luminal cholesterol flux (from bile, cell desquamation, and diet), their internal cholesterol synthesis, and various lipoprotein interactions. By using Caco-2 cells, Field et al. (77) studied the mechanisms by which luminal cholesterol regulates cholesterol synthesis. They demonstrated that HMG-CoA reductase regulation occurs primarily at the posttranscrip- tional level. Indeed, the decrease in HMG-CoA reductase mass and activity was mediated by a decline in the transla- tional efficiency of mRNA and an elevation in the rate of degradation of the protein. Both enzymes, ACAT and HMG- CoA reductase. were also shown to be regulated by lipoproteins in Caco-2 cells (78). LDL and HDL suppressed HMG-CoA reductase and stimulated ACAT activity. On the other hand, HDL; stimulated HMG-CoA reductase while reducing ACAT activity. These studies clearly demonstrated that cholesterol transported by lipoproteins exerted a modulatory effect on the two enzymes in Caco-2 cells. Caco-2 cells were also used as an in vitro model to study the hormonal regulation of ACAT. Decreased ACAT activity was induced by insulin (79). Caco-2 enterocytes were also used to investigate the secretion of cholesteryl ester transfer pro- tein (LT'P-1), considered to play a fundamental role in sterol metabolism. Caco-2 cells secreted functional LTP-1, which possessed molecular identity with plasma LTP-1 (80). The secretion process of LTP-1 proceeded in a polar fashion from the basolateral membrane and was controled by fatty acids. excess cholesterol, fatty acyl-CoA, or mevalonolactone (/4). Cholestery] ester formation may be necessary for normal triglyceride transport by the intestine. The assembly of chylomicrons probably requires an appropriate rate of cholesterol esterification. However, much investigation is needed because of controversy on this issue. The role of pancreas-derived cholesterol esterase in the intestine does not diminish the major importance of ACAT. On the con- trary, the hypothesis suggesting a cooperation between ACAT and cholesterol esterase has stood the test of time (75). Bhat and Brockman (75) suggested that pancreatic cholesterol esterase catalyzes the esterification of cholesterol on the cell surface and its hydrolysis in the inner leaflet of the plasma membrane, thereby allowing intracellularly liberated free cholectern] tn he reectemfhed hv ACAT The elecant
“Medium lipids in this study were associated with VLDL. Most of the labeled triglyceride (90%) found in the medium was recovered in the VLDL raction. ND, not done. TABLE 3. Caco-2 cells are inefficient at secreting lipids Although the intestine plays an important role in the ab- sorption and biosynthesis of cholesterol, the regulation of the enzymes responsible for intracellular cholesterol homeostasis remains unclear. Enterocytes have to regulate luminal cholesterol flux (from bile, cell desquamation, and diet), their internal cholesterol synthesis, and various lipoprotein interactions. By using Caco-2 cells, Field et al. (77) studied the mechanisms by which luminal cholesterol regulates cholesterol synthesis. They demonstrated that HMG-CoA reductase regulation occurs primarily at the posttranscrip- tional level. Indeed, the decrease in HMG-CoA reductase mass and activity was mediated by a decline in the transla- tional efficiency of mRNA and an elevation in the rate of degradation of the protein. Both enzymes, ACAT and HMG- CoA reductase. were also shown to be regulated by lipoproteins in Caco-2 cells (78). LDL and HDL suppressed HMG-CoA reductase and stimulated ACAT activity. On the other hand, HDL; stimulated HMG-CoA reductase while reducing ACAT activity. These studies clearly demonstrated that cholesterol transported by lipoproteins exerted a modulatory effect on the two enzymes in Caco-2 cells. Caco-2 cells were also used as an in vitro model to study the hormonal regulation of ACAT. Decreased ACAT activity was induced by insulin (79). Caco-2 enterocytes were also used to investigate the secretion of cholesteryl ester transfer pro- tein (LT'P-1), considered to play a fundamental role in sterol metabolism. Caco-2 cells secreted functional LTP-1, which possessed molecular identity with plasma LTP-1 (80). The secretion process of LTP-1 proceeded in a polar fashion from the basolateral membrane and was controled by fatty acids. excess cholesterol, fatty acyl-CoA, or mevalonolactone (/4). Cholestery] ester formation may be necessary for normal triglyceride transport by the intestine. The assembly of chylomicrons probably requires an appropriate rate of cholesterol esterification. However, much investigation is needed because of controversy on this issue. The role of pancreas-derived cholesterol esterase in the intestine does not diminish the major importance of ACAT. On the con- trary, the hypothesis suggesting a cooperation between ACAT and cholesterol esterase has stood the test of time (75). Bhat and Brockman (75) suggested that pancreatic cholesterol esterase catalyzes the esterification of cholesterol on the cell surface and its hydrolysis in the inner leaflet of the plasma membrane, thereby allowing intracellularly liberated free cholectern] tn he reectemfhed hv ACAT The elecant
“TG, triglyceride; FC, free cholesterol; CE, cholesteryl ester; PL, phospholipid. (-), not done. TABLE 4. Lipoprotein synthesis by Caco-2 cells*
“TG, triglyceride; FC, free cholesterol; CE, cholesteryl ester; PL, phospholipid. (-), not done. TABLE 4. Lipoprotein synthesis by Caco-2 cells*
Related topics:
PhysiologyLong Chain Fatty AcidMedical PhysiologyLipoprotein(a)Caco 2 CellBiochemistry and cell biology
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