Jiafu Ou

The liver X receptors (LXRs) are members of the nuclear hormone receptor superfamily that are bound and activated by oxysterols. These receptors serve as sterol sensors to regulate the transcription of gene products that control intracellular cholesterol homeostasis through catabolism and transport. In this report, we describe a novel LXR target, the sterol regulatory element-binding protein-1c gene (SREBP-1c), which encodes a membrane-bound transcription factor of the basic helix-loop-helix-leucine zipper family. SREBP-1c expression was markedly increased in mouse tissues in an LXR-dependent manner by dietary cholesterol and synthetic agonists for both LXR and its heterodimer partner, the retinoid X receptor (RXR). Expression of the related gene products, SREBP-1a and SREBP-2, were not increased. Analysis of the mouse SREBP-1c gene promoter revealed an RXR/LXR DNA-binding site that is essential for this regulation. The transcriptional increase in SREBP-1c mRNA by RXR/LXR was accompanied by a similar increase in the level of the nuclear, active form of the SREBP-1c protein and an increase in fatty acid synthesis. Because this active form of SREBP-1c controls the transcription of genes involved in fatty acid biosynthesis, our results reveal a unique regulatory interplay between cholesterol and fatty acid metabolism.

Transcription of the gene encoding sterol regulatory element-binding protein 1c (SREBP-1c) is known to be activated by insulin in the liver. The resultant SREBP-1c protein activates transcription of the genes required for fatty acid synthesis. Here, we use SREBP-1c promoter reporter constructs to dissect the mechanism of insulin activation in freshly isolated rat hepatocytes. The data show that a complete insulin response (increase of 6- to 11-fold) requires two binding sites for liver X receptors (LXRs), which are nuclear receptors that are activated by oxygenated sterols. Disruption of these binding sites did not lower basal transcription but severely reduced the response to insulin. In contrast, disruption of the closely linked binding sites for SREBPs and nuclear factor Y lowered basal transcription drastically but still permitted a 4- to 7-fold increase in response to insulin. Arachidonic acid, an inhibitor of LXR activation, blocked the response to insulin. We conclude that insulin activates the SREBP-1c promoter primarily by increasing the activity of LXRs, possibly through production of a ligand that activates LXRs or their heterodimerizing partner, the retinoid X receptor. Nuclear SREBPs and nuclear factor Y play permissive roles.

Sterol regulatory element-binding protein-1c (SREBP-1c) enhances transcription of genes encoding enzymes of unsaturated fatty acid biosynthesis in liver. SREBP-1c mRNA is known to increase when cells are treated with agonists of liver X receptor (LXR), a nuclear hormone receptor, and to decrease when cells are treated with unsaturated fatty acids, the end products of SREBP-1c action. Here we show that unsaturated fatty acids lower SREBP-1c mRNA levels in part by antagonizing the actions of LXR. In cultured rat hepatoma cells, arachidonic acid and other fatty acids competitively inhibited activation of the endogenous SREBP-1c gene by an LXR ligand. Arachidonate also blocked the activation of a synthetic LXR-dependent promoter in transfected human embryonic kidney-293 cells. In vitro, arachidonate and other unsaturated fatty acids competitively blocked activation of LXR, as reflected by a fluorescence polarization assay that measures ligand-dependent binding of LXR to a peptide derived from a coactivator. These data offer a potential mechanism that partially explains the long-known ability of dietary unsaturated fatty acids to decrease the synthesis and secretion of fatty acids and triglycerides in livers of humans and other animals.

Sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that increase the synthesis of fatty acids as well as cholesterol in animal cells. All three SREBP isoforms (SREBP-1a, -1c, and -2) are subject to feedback regulation by cholesterol, which blocks their proteolytic release from membranes. Previous data indicate that the SREBPs are also negatively regulated by unsaturated fatty acids, but the mechanism is uncertain. In the current experiments, unsaturated fatty acids decreased the nuclear content of SREBP-1, but not SREBP-2, in cultured human embryonic kidney (HEK)-293 cells. The potency of unsaturated fatty acids increased with increasing chain length and degree of unsaturation. Oleate, linoleate, and arachidonate were all effective, but the saturated fatty acids palmitate and stearate were not effective. Down-regulation occurred at two levels. The mRNAs encoding SREBP-1a and SREBP-1c were markedly reduced, and the proteolytic processing of these SREBPs was inhibited. When SREBP-1a was produced by a cDNA expressed from an independent promoter, unsaturated fatty acids reduced nuclear SREBP-1a without affecting the mRNA level. There was no effect when the cDNA encoded a truncated version that was not membrane-bound. When administered together, sterols and unsaturated fatty acids potentiated each other in reducing nuclear SREBP-1. In the absence of fatty acids, sterols did not cause a sustained reduction of nuclear SREBP-1, but they did reduce nuclear SREBP-2. We conclude that unsaturated fatty acids, as well as sterols, can down-regulate nuclear SREBPs and that unsaturated fatty acids have their greatest inhibitory effects on SREBP-1a and SREBP-1c, whereas sterols have their greatest inhibitory effects on SREBP-2. Sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that increase the synthesis of fatty acids as well as cholesterol in animal cells. All three SREBP isoforms (SREBP-1a, -1c, and -2) are subject to feedback regulation by cholesterol, which blocks their proteolytic release from membranes. Previous data indicate that the SREBPs are also negatively regulated by unsaturated fatty acids, but the mechanism is uncertain. In the current experiments, unsaturated fatty acids decreased the nuclear content of SREBP-1, but not SREBP-2, in cultured human embryonic kidney (HEK)-293 cells. The potency of unsaturated fatty acids increased with increasing chain length and degree of unsaturation. Oleate, linoleate, and arachidonate were all effective, but the saturated fatty acids palmitate and stearate were not effective. Down-regulation occurred at two levels. The mRNAs encoding SREBP-1a and SREBP-1c were markedly reduced, and the proteolytic processing of these SREBPs was inhibited. When SREBP-1a was produced by a cDNA expressed from an independent promoter, unsaturated fatty acids reduced nuclear SREBP-1a without affecting the mRNA level. There was no effect when the cDNA encoded a truncated version that was not membrane-bound. When administered together, sterols and unsaturated fatty acids potentiated each other in reducing nuclear SREBP-1. In the absence of fatty acids, sterols did not cause a sustained reduction of nuclear SREBP-1, but they did reduce nuclear SREBP-2. We conclude that unsaturated fatty acids, as well as sterols, can down-regulate nuclear SREBPs and that unsaturated fatty acids have their greatest inhibitory effects on SREBP-1a and SREBP-1c, whereas sterols have their greatest inhibitory effects on SREBP-2. sterol regulatory element-binding protein cleaved nuclear form of SREBP N-acetyl-Leu-Leu-norleucinal bovine serum albumin fetal calf serum human embryonic kidney-293 cells herpes simplex virus phosphate-buffered saline SREBP cleavage-activating protein thymidine kinase polyacrylamide gel electrophoresis Ingestion of n-3 and n-6 polyunsaturated fatty acids in place of saturated fatty acids shifts the pattern of fat metabolism in liver from storage to oxidation (1Jump D.B. Clarke S.D. Annu. Rev. Nutr. 1999; 19: 63-90Crossref PubMed Scopus (544) Google Scholar). Genes involved in fatty acid oxidation are induced, and genes involved in fatty acid synthesis or lipogenesis are repressed (2Price P.T. Nelson C.M. Clarke S.D. Curr. Opin. Lipidol. 2000; 11: 3-7Crossref PubMed Scopus (172) Google Scholar, 3Sessler A.M. Ntambi J.M. J. Nutr. 1998; 128: 923-926Crossref PubMed Scopus (201) Google Scholar). The increase in genes responsible for oxidation is achieved in part by activation of peroxisome proliferator-activated receptors (4Forman B.M. Chen J. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 4312-4317Crossref PubMed Scopus (1844) Google Scholar, 5Kliewer S.A. Sundseth S.S. Jones S.A. Brown P.J. Wisely G.B. Koble C.S. Devchand P. Wahli W. Willson T.M. Lenhard J.M. Lehmann J.M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 4318-4323Crossref PubMed Scopus (1859) Google Scholar, 6Jump D.B. Thelen A. Ren B. Mater M. Prostaglandins, Leukot. Essent. Fatty Acids. 1999; 60: 345-349Abstract Full Text PDF PubMed Scopus (39) Google Scholar) and in part by conversion of polyunsaturated fatty acids to eicosanoids (1Jump D.B. Clarke S.D. Annu. Rev. Nutr. 1999; 19: 63-90Crossref PubMed Scopus (544) Google Scholar). Neither of these two pathways is required for the decreased expression of lipogenic genes elicited by unsaturated fatty acids (6Jump D.B. Thelen A. Ren B. Mater M. Prostaglandins, Leukot. Essent. Fatty Acids. 1999; 60: 345-349Abstract Full Text PDF PubMed Scopus (39) Google Scholar). New candidates for fatty acid-mediated regulation of lipogenic genes have emerged in the form of sterol regulatory element-binding proteins (SREBPs).1 SREBPs are a unique family of membrane-bound transcription factors that activate genes involved in the synthesis of cholesterol and fatty acids and their uptake from plasma lipoproteins (7Brown M.S. Goldstein J.L. Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (2918) Google Scholar, 8Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1085) Google Scholar). The regulation of SREBPs has been studied most intensively in fibroblast-like cultured cells such as Chinese hamster ovary cells and human embryonic kidney (HEK)-293 cells. These studies showed that the SREBPs are synthesized as membrane-bound precursors averaging 1150 amino acids in length. The NH2-terminal domain of ∼480 amino acids is a transcription factor of the basic helix-loop-helix-leucine zipper family. This is followed by a 90-amino acid membrane attachment domain consisting of two membrane-spanning helices separated by a short hydrophilic loop. The third domain is a COOH-terminal regulatory domain of ∼590 amino acids. The SREBPs are oriented in a hairpin fashion with their NH2-terminal and COOH-terminal domains facing the cytosol and the short hydrophilic loop projecting into the lumen of the endoplasmic reticulum. Immediately after synthesis in the endoplasmic reticulum, the SREBPs form complexes with SREBP cleavage-activating protein (SCAP), a polytopic membrane protein that escorts them to the Golgi complex (9Sakai J. Nohturfft A. Goldstein J.L. Brown M.S. J. Biol. Chem. 1998; 273: 5785-5793Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 10Nohturfft A. DeBose-Boyd R.A. Scheek S. Goldstein J.L. Brown M.S. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11235-11240Crossref PubMed Scopus (191) Google Scholar, 11DeBose-Boyd R.A. Brown M.S. Li W.-P. Nohturfft A. Goldstein J.L. Espenshade P.J. Cell. 1999; 99: 703-712Abstract Full Text Full Text PDF PubMed Scopus (248) Google Scholar). Here the SREBPs are cleaved sequentially by two proteases that release the NH2-terminal domain into the cytosol. The released fragment travels to the nucleus, where it binds to sterol regulatory elements located in the 5′-flanking regions of more than 20 genes involved in lipid synthesis and uptake. Target genes include the low density lipoprotein receptor and the cholesterologenic enzymes 3-hydroxy-3-methylglutaryl CoA synthase and reductase as well as the lipogenic enzymes acetyl-CoA carboxylase, fatty acid synthetase, and stearoyl CoA desaturase (8Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1085) Google Scholar, 12Kim J.B. Spiegelman B.M. Genes Dev. 1996; 10: 1096-1107Crossref PubMed Scopus (824) Google Scholar, 13Magana M.M. Lin S.S. Dooley K.A. Osborne T.F. J. Lipid Res. 1997; 38: 1630-1638Abstract Full Text PDF PubMed Google Scholar, 14Sul H.S. Wang D. Ann. Rev. Nutr. 1998; 18: 331-351Crossref PubMed Scopus (232) Google Scholar, 15Edwards P.A. Ericsson J. Annu. Rev. Biochem. 1999; 68: 157-185Crossref PubMed Scopus (387) Google Scholar, 16Horton J.D. Shimomura I. Curr. Opin. Lipidol. 1999; 10: 143-150Crossref PubMed Scopus (268) Google Scholar). The proteolytic processing of SREBPs is under feedback control by cholesterol. Thus, when sterols accumulate in cells, the SCAP·SREBP complex fails to move to the Golgi, and SREBPs are not processed (17Nohturfft A. Yabe D. Goldstein J.L. Brown M.S. Espenshade P.J. Cell. 2000; 102: 315-323Abstract Full Text Full Text PDF PubMed Scopus (272) Google Scholar). The nuclear SREBPs are rapidly degraded by a proteasomal process, and the synthesis of sterols and fatty acids (primarily 18:1 unsaturates) declines. Three isoforms of SREBP have been identified (8Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1085) Google Scholar). Two of these proteins, designated SREBP-1a and SREBP-1c, are derived from a single gene through use of alternate transcriptional start sites producing different forms of exon 1 that are spliced to a exon The SREBP-1a has a NH2-terminal activation and it is a transcriptional The SREBP-1c has a short activation and it to activate transcription J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). The third SREBP-2, derived from a is to the and it a activation the genes for the SREBPs the two isoforms are more in fatty acid whereas on the cholesterol (8Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1085) Google Scholar, 16Horton J.D. Shimomura I. Curr. Opin. Lipidol. 1999; 10: 143-150Crossref PubMed Scopus (268) Google Scholar, J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). The of SREBP-1a to isoforms markedly different cells. In most the SREBP-1c I. J.D. Goldstein J.L. Brown M.S. J. 1997; 99: PubMed Scopus Google but the is in cultured cells I. J.D. Goldstein J.L. Brown M.S. J. 1997; 99: PubMed Scopus Google Scholar). The of is to the of SREBP-1a and (8Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1085) Google Scholar). as unsaturated fatty acids are of SREBP have to fatty acids feedback effects on SREBP In these studies have regulatory but the have been In the of these M. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) showed that with sterols in reducing the of nuclear and in Chinese hamster ovary cells. without sterols no Osborne T.F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) that unsaturated fatty acids not saturated fatty without sterols decreased the of in of cultured cells and the mRNA for an SREBP J. Clarke S.D. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) in n-6 polyunsaturated fatty acids to and a in in liver cells. n-6 polyunsaturated fatty acids decreased the of mRNA for SREBP-1, but they did not to transcription of the gene as by nuclear In J. Clarke S.D. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) no inhibitory effect on expression with M. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) that of a in n-3 polyunsaturated fatty acids decreased and in the of The mRNA for SREBP-1c was reduced and that of was reduced but was no effect on In with the of J. Clarke S.D. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google M. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) that effect in their The current studies were to a of the effects of fatty acids on the of mRNA and protein for all three isoforms of SREBP in human cells in The that unsaturated fatty acids the of in the absence of In the inhibitory effect of the fatty acids increased with increasing chain length and degree of unsaturation. The in was by a in the mRNAs encoding SREBP-1a and when the of mRNA for SREBP-1a was through acid reduced that fatty acids have at the of SREBP-1a protein as well as at the of acid did not have an effect on cells that were to a truncated form of SREBP-1a that the without a for that the effect is at the of proteolytic processing of the SREBP-1a In the absence of sterols, unsaturated fatty acids effect on the of mRNA or the We from from and from fatty acids from and from bovine serum albumin from and from and from and human cDNA control from human acids of the SREBP-1a and human acids were as Brown M.S. Goldstein J.L. Cell. Full Text PDF PubMed Scopus Google Scholar). fetal calf serum was by J.L. Brown M.S. PubMed Scopus Google Scholar). were from (9Sakai J. Nohturfft A. Goldstein J.L. Brown M.S. J. Biol. Chem. 1998; 273: 5785-5793Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, Brown M.S. Goldstein J.L. J. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). calf serum was by a of the of and J. Lipid Res. Full Text PDF PubMed Google Scholar). of serum was with of and of at for 20 followed by a on the was with of to under a of with of and phosphate-buffered saline were at The of fatty acids in the serum was with the Fatty the of cholesterol and were as M. S. Brown M.S. Goldstein J.L. PubMed Scopus Google Scholar, Chem. 19: PubMed Scopus Google Scholar). In of fetal calf the of fatty acids was reduced from to the of cholesterol was reduced from to and the of was reduced from to or of each fatty acid was by the fatty acid in and it with the of of The was under with and at for with at in was to with and in at from in under is an expression that human SREBP-1a under control of the J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). human SREBP-1a at the by two of the and at the by three of the This was as Brown M.S. Goldstein J.L. J. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) was with and to a fragment that the and but not the of the of to amino acids of the and to amino acid of human was to the of The chain was and and a fragment encoding amino acids of SREBP-1a was The two were to which was in encoding the nuclear form of human SREBP-1a acids designated was by of to the was the The expression was in to the Two different of the were of human embryonic kidney (HEK)-293 cells were on and cultured in at in and with on the cells were with and with with in the absence or of fatty acids sterols cholesterol in a of as in the with fatty acids sterols at for was to the at a of the cells were by in the and the from was and at for at The was by in at after which the was in at 1 1 and a of that and The was through a 20 and at at for The was in of at 1 1 and the of This was at for 1 and at for at in a The is designated as the nuclear The of the was at for at after which the was in of at 1 1 and the of and designated as the membrane cells were for with the the as J. Goldstein J.L. Brown M.S. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). of nuclear and membrane was with a of nuclear or membrane was with at for of the proteins were to and to The were with as in the were with or the to the were with were to at for the The of the nuclear was with the of was from of of cells the followed by and gel of was with with and to electrophoresis in a and to The cDNA for human and were by a fragment from J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) and a fragment from J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). The cDNA for human was from cDNA were with a The were with the for at with at for followed by at for and at to with for the The of was the The cDNA for human SREBP-1a and SREBP-1c as for synthesis were by chain the and J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). The to SREBP-1a and The to SREBP-1c and The human SREBP-1a fragment to of exon and of exon The human SREBP-1c fragment to of of exon and of exon that exon is to SREBP-1a and The were into the of with was with as I. J.D. Goldstein J.L. Brown M.S. J. 1997; 99: PubMed Scopus Google Scholar). of from each were with the a for the mRNA of human I. J.D. Goldstein J.L. Brown M.S. J. 1997; 99: PubMed Scopus Google Scholar) the in the In the the of the to for and SREBP-1. with were separated on polyacrylamide and the were and to by and an SREBP-1c was The to SREBP-1a and SREBP-1c were on with the cells, a SREBP-1a was to the SREBP-1a as a fragment and SREBP-1a as a The of mRNA in each was to for the SREBP In studies from the regulation of SREBP processing was in cultured cells that were in which is of low density lipoprotein cholesterol, to the of low density lipoprotein by J.L. Brown M.S. PubMed Scopus Google Scholar). not fatty acids, which are to serum more in the current studies a that and The of fatty acids in serum and serum were and 1 the content of SREBPs in membrane and nuclear of cells that were for in serum or serum with or without sterols or as not SREBP-1a and -1c, use the to to the of When the cells were in the form of in and the cleaved form in nuclear 1 of sterols of to an increase in the form in and a in the nuclear form When arachidonate was the nuclear form of also but was no of the In the of sterols the nuclear form but the When the were in was increased as with that in serum When sterols were in the but was a in The of arachidonate without a of the When arachidonate and sterols were was also but the In sterols regulated processing in a that was to that of 1 and in to the with SREBP-1, arachidonate did not effect the nuclear form of arachidonate a that was to that of sterols In sterols and arachidonate no effect and sterols arachidonate was to sterols and The data of 1 indicate a in the of sterols and arachidonate to and SREBP-2. were to a fatty acid such as arachidonate was In sterols in the absence of fatty acids. arachidonate reduced the of but it did not reduce the content of The data that the of sterols to in serum is on the of fatty acids in the the effects of sterols and fatty acids all in were with cells that were in serum and sterols or fatty acids. that the of fatty acids to reduce increased in to chain length and degree of unsaturation. Thus, palmitate no but a reduced but a effect of and were more effective. and were also of these fatty acids a effect on with the of which produced a the effects of these fatty acids on the mRNA for SREBP-1a and -1c, an We the to for the that the of SREBP-1a mRNA was than that of fatty acids reduced the SREBP-1a and The of fatty acids increased with increasing chain length and degree of in with the effects on the nuclear In not that saturated and fatty acids to in length to the mRNAs or the of the nuclear the of required for unsaturated fatty acids to the mRNA and the nuclear the of the to arachidonate the cells were from to and the cells were 20 was to the at the in the of was when arachidonate was and the by at and at There was in after The form of SREBP-2, but not SREBP-1, increased with after of We a in the mRNAs for SREBP-1a and SREBP-1c that was after arachidonate and and in The for the of the We no in the which was by in in the absence of fatty acids, sterols cause a in when at sterol was with a the cells were from to serum sterols in the absence or of were at different and SREBPs were by and and were and the and after of sterols in nuclear proteins were markedly reduced and The low the and by and it at The of was with an increase in the mRNAs encoding SREBP-1a and the cells have unsaturated fatty acids as a of in The after the unsaturated fatty acids and the mRNA This is by arachidonate in the the sterols and fatty acids on of mRNAs and cells for in serum with of arachidonate in the absence or of The mRNAs and protein were by and and the are in the mRNA for which for of the mRNA in the cells. The for SREBP-1c were not The of sterols the of SREBP-1a mRNA and the form of the protein The of arachidonate reduced the mRNA and the protein under The with the nuclear form of did not for the mRNA and the In the absence of sterols did not increase the increase in mRNA and levels. The of arachidonate in the of sterols to a in than it did in the absence of sterols In the of sterols, the nuclear protein at 20 In the absence of sterols, at together, the data of the that arachidonate two effects on the SREBP-1a but it decreased the of nuclear protein to an that was than the reduction in the This was most in the of sterols 20 the mRNA and the of by than in the absence of sterols, arachidonate the of to nuclear Thus, at 20 arachidonate the of by than whereas the of nuclear protein by the that arachidonate of in the of studied the of SREBP-1a that was produced by under the control of the promoter, which to transcriptional regulation by cells were in the absence or of and and nuclear were to electrophoresis and with an the In the absence of SREBP-1a was and the nuclear form was of increasing of arachidonate and it did not the mRNA of the SREBP-1a a control in the cells, that arachidonate decreased the mRNA of SREBP-1a as by arachidonate by proteolytic processing of the the fatty acid have no effect when cells a truncated that the membrane-spanning cells with a cDNA encoding an truncated version of SREBP-1a with a at This protein at which to the of after it has been processed by the and proteases Brown M.S. Goldstein J.L. J. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). from cells the nuclear form of SREBP-1a as by with an the The of nuclear protein did not when increasing of arachidonate were and the mRNA was not by arachidonate the SREBP-1a was decreased by arachidonate under the an of the that arachidonate the proteolytic processing of SREBP-1, effect on the of the COOH-terminal fragment that is when is cleaved by cells with a cDNA encoding SREBP-1a with a at the and an at the and nuclear were to electrophoresis and with the and When the cells were in the the cleaved which with the in The nuclear the NH2-terminal which with the in The of the and COOH-terminal were markedly when the cells were with arachidonate whereas the in the The mRNA was by arachidonate as by the not as a the data in indicate that is regulated in a different fashion than in cells. In these cells sterols are the of SREBP-2, and they by affecting proteolytic processing with in the of the other is regulated by a of the of sterols and unsaturated fatty acids, and regulation in mRNA fatty as well as an reduction in proteolytic processing fatty acids and and other The mechanism by which unsaturated fatty acids SREBP-1a and mRNA in in cells to These two mRNAs use different that are separated by at I. J.D. Goldstein J.L. Brown M.S. J. 1997; 99: PubMed Scopus Google Scholar, J. Goldstein J.L. Brown M.S. PubMed Scopus Google Scholar). The two different with different regions and start These into a and they are The fatty acids on a single transcription factor that fatty acids transcription through an on a that is to also the of the two as for liver by J. Clarke S.D. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). Fatty acids did not the of mRNA encoded by a This was by a different than the of the two and it all of the it did all of the of the SREBP-1a and The mechanism for the protein regulation of SREBP-1a and in proteolytic This from the that fatty acids down-regulate when it is produced from a encoding the but not when it the truncated nuclear form of This is by the that the of the other of SREBP the membrane-bound COOH-terminal fragment is also decreased by acid We not regulation of by fatty acids is by which the effects of sterols on SREBP fatty acids have a effect with sterols but is by a effect on a single regulatory to The of fatty acids processing is not by these In to the reduction in mRNA for SREBP-1a and SREBP-1c, fatty acids did not reduce the mRNA for SREBP-2. the content of did not an increase in the form of in the when arachidonate was These the that unsaturated fatty acids but the reduction in is the mRNA and the cells to of the the in cells to other cells, most is in the have a in mRNA and protein when are with fatty acids, or when are in polyunsaturated fatty acids, but is which fatty acids are most effective, and the mechanism has not been in as the liver is the of synthesis of fatty acids and as in polyunsaturated fatty acids reduce plasma of cholesterol and (1Jump D.B. Clarke S.D. Annu. Rev. Nutr. 1999; 19: 63-90Crossref PubMed Scopus (544) Google Scholar, P.T. Nelson C.M. Clarke S.D. Curr. Opin. Lipidol. 2000; 11: 3-7Crossref PubMed Scopus (172) Google Scholar, 3Sessler A.M. Ntambi J.M. J. Nutr. 1998; 128: 923-926Crossref PubMed Scopus (201) Google the feedback regulation of SREBPs by fatty acids in liver We and for and with the

The current paper describes a line of cultured rat hepatoma cells (McA-RH7777 cells) that mimics the behavior of rat liver by producing an excess of mRNA for sterol regulatory element-binding protein 1c (SREBP-1c) as opposed to SREBP-1a. These two transcripts are derived from a single gene by use of alternative promoters that are separated by many kilobases in the genome. The high level of SREBP-1c mRNA is abolished when cholesterol synthesis is blocked by compactin, an inhibitor of 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase that inhibits cholesterol synthesis. Levels of SREBP-1c mRNA are restored by mevalonate, the product of the HMG CoA reductase reaction, and by ligands for the nuclear hormone receptor LXR, including 22(R)-hydroxycholesterol and T0901317. These data suggest that transcription of the SREBP-1c gene in hepatocytes requires tonic activation of LXR by an oxysterol intermediate in the cholesterol biosynthetic pathway. Reduction of this intermediate lowers SREBP-1c levels, and this in turn is predicted to lower the rates of fatty acid biosynthesis in liver.