Oleate Potentiates Oxysterol Inhibition of Transcription from Sterol Regulatory Element-1-regulated Promoters and Maturation of Sterol Regulatory Element-binding Proteins

Abstract

Activation of genes containing SRE-1 (sterol regulatory element1) sequences is known to be under the regulation of sterols through modulation of the proteolytic maturation of SREBPs (SRE-1-binding proteins). Previous work has demonstrated SREBP-mediated transcriptional activation of genes encoding enzymes of sterol and fatty acid biosynthesis. Because synthesis of both sterols and C18 fatty acids are required for cell growth, in the absence of exogenous supplements of these lipids, we examined the hypothesis that fatty acid can also be regulatory in SREBP maturation. Our data indicate that C18 fatty acids can potentiate the biological activities of a typical, regulatory sterol: 25-hydroxycholesterol. Inhibition of C18 fatty acid synthesis in cells cultured in serum-free medium renders them resistant to killing by 25-hydroxycholesterol. Repression of expression of reporter constructs driven by promoters bearing SRE-1 element(s) by 25-hydroxycholesterol is increased by C18 fatty acid supplementation. C18 fatty acids also increase the inhibitory effect of 25-hydroxycholesterol on proteolytic maturation and nuclear localization of SREBPs. Furthermore, we also show that C18 fatty acid supplementation can enhance the inhibitory effect of 25-hydroxycholesterol on sterol and fatty acid biosynthesis. These results demonstrate that maximal down-regulation of SREBP maturation and the consequent repression of SRE-1 promoters occurs in response to both a regulatory sterol and fatty acid. Activation of genes containing SRE-1 (sterol regulatory element1) sequences is known to be under the regulation of sterols through modulation of the proteolytic maturation of SREBPs (SRE-1-binding proteins). Previous work has demonstrated SREBP-mediated transcriptional activation of genes encoding enzymes of sterol and fatty acid biosynthesis. Because synthesis of both sterols and C18 fatty acids are required for cell growth, in the absence of exogenous supplements of these lipids, we examined the hypothesis that fatty acid can also be regulatory in SREBP maturation. Our data indicate that C18 fatty acids can potentiate the biological activities of a typical, regulatory sterol: 25-hydroxycholesterol. Inhibition of C18 fatty acid synthesis in cells cultured in serum-free medium renders them resistant to killing by 25-hydroxycholesterol. Repression of expression of reporter constructs driven by promoters bearing SRE-1 element(s) by 25-hydroxycholesterol is increased by C18 fatty acid supplementation. C18 fatty acids also increase the inhibitory effect of 25-hydroxycholesterol on proteolytic maturation and nuclear localization of SREBPs. Furthermore, we also show that C18 fatty acid supplementation can enhance the inhibitory effect of 25-hydroxycholesterol on sterol and fatty acid biosynthesis. These results demonstrate that maximal down-regulation of SREBP maturation and the consequent repression of SRE-1 promoters occurs in response to both a regulatory sterol and fatty acid. Oxygenated sterols, such as 25-hydroxycholesterol, are down-regulators of cholesterol biosynthesis (1Kandutsch A.A. Chen H.W. Heininger H.J. Science. 1978; 201: 498-501Crossref PubMed Scopus (404) Google Scholar). 25-Hydroxycholesterol down-regulates the rate-limiting step in this pathway, 3-hydroxy-3-methyl-glutaryl (HMG) 1The abbreviations used are: HMG3-hydroxy-3-methyl-glutarylSREsterol regulatory elementSREBPsterol regulatory element-binding proteinSCAPSREBP cleavage-activating proteinCoAcoenzyme ACHOChinese hamster ovaryFAFfatty acid-freeBSAbovine serum albuminHDA2-hexadecynoic acidPBSphosphate-buffered salineCATchloramphenicol acetyltransferaseLDLlow density lipoprotein. coenzyme A reductase, through several mechanisms but also has a pleiotropic effect on transcription of all the genes of the cholesterol biosynthetic pathway that have been studied to date (2Rudney H. Panini S.R. Curr. Opin. Lipidol. 1993; 4: 230-237Crossref Scopus (17) Google Scholar). This transcriptional control is mediated through the regulated two-step proteolysis (3Brown M.S. Goldstein J.L. Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (3027) Google Scholar) of transcription factors called SREBPs, which are synthesized as proteins localized to the endoplasmic reticulum. SREBP proteolysis to the soluble, mature transcription factors is inhibited by 25-hydroxycholesterol at the first cleavage. Genes that are transcriptionally activated by SREBPs possess specific recognition elements for these proteins, called SREs (2Rudney H. Panini S.R. Curr. Opin. Lipidol. 1993; 4: 230-237Crossref Scopus (17) Google Scholar). These include many of the genes of the cholesterol biosynthetic pathway and the gene encoding the LDL receptor (4Bennett M.K. Lopez J.M. Sanchez H.B. Osborne T.F. J. Biol. Chem. 1995; 270: 25578-25583Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar). In addition, the genes encoding two of the enzymes of fatty acid biosynthesis (4Bennett M.K. Lopez J.M. Sanchez H.B. Osborne T.F. J. Biol. Chem. 1995; 270: 25578-25583Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar, 5Lopez J.M. Bennett M.K. Sanchez H.B. Rosenfeld J.M. Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 1049-1053Crossref PubMed Scopus (249) Google Scholar) and HMG-CoA reductase (6Vallett S.M. Sanchez H.B. Rosenfeld J.M. Osborne T.F. J. Biol. Chem. 1996; 271: 12247-12253Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar) also contain sequences recognized by SREBPs that differ from the classically defined SRE. 3-hydroxy-3-methyl-glutaryl sterol regulatory element sterol regulatory element-binding protein SREBP cleavage-activating protein coenzyme A Chinese hamster ovary fatty acid-free bovine serum albumin 2-hexadecynoic acid phosphate-buffered saline chloramphenicol acetyltransferase low density lipoprotein. At least under some experimental conditions, it is possible to demonstrate regulation of promoters for these genes encoding enzymes of fatty acid biosynthesis, as well as those for cholesterol biosynthesis, by expression of SREBPs. Such studies, utilizing promoter reporter constructs or measurement of mRNA levels for specific enzymes of fatty acid biosynthesis, have been performed both in cultured cells (4Bennett M.K. Lopez J.M. Sanchez H.B. Osborne T.F. J. Biol. Chem. 1995; 270: 25578-25583Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar,5Lopez J.M. Bennett M.K. Sanchez H.B. Rosenfeld J.M. Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 1049-1053Crossref PubMed Scopus (249) Google Scholar, 7Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (687) Google Scholar, 8Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (614) Google Scholar) and transgenic mice (7Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (687) Google Scholar, 9Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M.S. Goldstein J.L. J. Clin. Invest. 1996; 98: 1575-1584Crossref PubMed Scopus (699) Google Scholar) with truncated, soluble SREBP-1a and SREBP-1c. However, these observations create something of a metabolic paradox because they suggest that fatty acid biosynthesis can be inhibited by sterols, a regulatory mechanism that would lead to inhibition of cell growth when exogenous sources of sterols were high but fatty acids were poor. Somatic cell mutants of CHO cells, resistant to killing by 25-hydroxycholesterol in the absence of lipoprotein cholesterol, have been extremely useful in further defining the role of SREBPs in the regulation of cholesterol biosynthesis (10Leonard S. Sinensky M. Biochim. Biophys. Acta. 1988; 947: 101-112Crossref PubMed Scopus (23) Google Scholar, 11Chang T.Y. Hasan M.T. Chin J. Chang C.C.Y. Spillane D.M. Chen J. Curr. Opin. Lipidol. 1997; 8: 65-71Crossref PubMed Scopus (29) Google Scholar). The mutants characterized to date, fall into two classes (3Brown M.S. Goldstein J.L. Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (3027) Google Scholar): class 1 mutants are sterol-resistant because they produce a truncated SREBP-2 that serves as a constitutive, mature transcription factor, and class 2 mutants are defective in a protein called SCAP (SREBP-cleavage activatingprotein), which appears to transduce the oxysterol-mediated inhibition of SREBP proteolysis. Class 2 mutants carry out the normally oxysterol-regulated first proteolytic cleavage constitutively. Many oxysterol-resistant mutants were isolated long before the mechanism of transcriptional regulation of cholesterol biosynthesis, particularly the role of the SREs and SREBPs, was understood. Early somatic cell genetic characterization indicated that there was genetic diversity in such mutants and that some were recessive (13Sinensky M. Armagast S. Mueller G. Torget R. Proc. Natl. Acad. Sci. U. S. A. 1980; 77: 6621-6623Crossref PubMed Scopus (13) Google Scholar, 14Sinensky M. Logel J. Torget R. J. Cell. Physiol. 1982; 113: 314-319Crossref PubMed Scopus (12) Google Scholar). The class 1 and 2 mutants, whose defects are now known, are dominant (or co-dominant) mutations. Our laboratory reported (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar) a novel, recessive 25-hydroxycholesterol-resistant mutant (crB). This mutant had a genetic defect in fatty acid elongation that was required for both resistance to killing and down-regulation of mRNA levels transcribed from cholesterol biosynthetic genes. Lack of regulation of the activity of the rate-limiting enzyme in cholesterol synthesis, HMG-CoA reductase, as well as its mRNA, was observed. Supplementation of these cells with a mix of stearate and oleate restored cell growth to normal, but most of the regulation of HMG-CoA reductase by 25-hydroxycholesterol and partial restoration of cell growth could be produced by oleate supplementation alone. These results suggest a solution to the metabolic paradox outlined above, which is that oxysterols can lose activity as regulators of SRE promoters when cells are starved for C18 fatty acids, particularly oleate. In this report we further explore the molecular basis for these observations and test the hypothesis that oleate can modulate the transcriptional regulatory activity of 25-hydroxycholesterol. The crB mutant turned out to be difficult to analyze for such initial studies because, as will be described elsewhere, its genotype appears to be complex. Instead, we have made use of a potent inhibitor of fatty acid elongation, hexadecynoic acid, in the current study. IgG-2A4 and IgG-7D4 cells, which produce monoclonal antibodies SREBP-1 and SREBP-2, respectively, were purchased from American Type Culture Collection (catalog numbers CRL-2121 and CRL-2198). All cell culture reagents were obtained from Life Technologies, Inc. Fatty acid-free bovine serum albumin (FAF-BSA, catalog number A6003) was from Sigma. Stearate and oleate were obtained from and 25-hydroxycholesterol was from Inc. (catalog number was from Life was from The and were by of acid was a of J. M. cells were in medium containing and at and cells were produced by cells with 2 of and of or by the a The cells were Technologies, in were isolated and for activity as described activity were and in containing 25-hydroxycholesterol were at a density of in on the cells were with phosphate-buffered saline and in or containing fatty acids and sterols as described in the a the was and the cells were for The were and with as described (13Sinensky M. Armagast S. Mueller G. Torget R. Proc. Natl. Acad. Sci. U. S. A. 1980; 77: 6621-6623Crossref PubMed Scopus (13) Google Scholar). of chloramphenicol acetyltransferase cells were in at in culture catalog number a the medium was and the cells were with The cells were cultured for in medium containing (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar) serum in the or absence of 1 25-hydroxycholesterol and with or stearate and oleate to The cells were with and with 1 1 and 1 Chang PubMed Scopus Google Scholar) before for 2 in The was and cell was by the cells in of The cell was to a of by the of of and by at at for of in was to of cell and the were with of chloramphenicol containing 2 in were out at for to 2 before of the chloramphenicol and with 1 of the was in of and to The were in the were to The of produced was by cells were at 2 culture in the medium was and the cells were with The cells were for in containing and in the or absence of sterols and fatty acids as described in the and were as described by J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). was a of the nuclear and were with D.M. Scholar) and to on the proteins were to The were with in saline containing by a with protein IgG-7D4 or IgG-2A4 at in The were with and for 1 with in The were with and was performed to the cells were at 1 culture in for The cells were with and for in with 1 25-hydroxycholesterol and as was to and the for were with as described (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar). were in at for 1 and were into The was with and the fatty acids were into by were under and the fatty acids were in of into and fatty acids was by The protein of was by the cells with and a protein as have that cells with inhibitor of fatty acid elongation, stearate and oleate with (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar). have also that a somatic cell mutant defective in elongation of is resistant to the of 25-hydroxycholesterol in medium (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google we that C18 fatty acid synthesis was for 25-hydroxycholesterol cells resistant to 25-hydroxycholesterol. when cells were with the of 25-hydroxycholesterol was with cells with supplementation by was effect on the growth of the These results observations that C18 fatty acid synthesis has a role in 25-hydroxycholesterol Because inhibition of C18 fatty acid synthesis with renders cells resistant to killing by 25-hydroxycholesterol in lipoprotein it that also produce resistance of genes containing SREs to down-regulation by 25-hydroxycholesterol, as This has been the with 25-hydroxycholesterol-resistant somatic cell C18 fatty acid levels sterol we examined the of and fatty acid supplementation on we LDL receptor promoter which the LDL receptor promoter and has been demonstrated to be regulated by sterols Goldstein J.L. Brown M.S. J. Biol. Chem. Full Text PDF PubMed Google Scholar). into cells, transcription of this is when in medium and when the cells are with 25-hydroxycholesterol Supplementation of the medium with fatty acids and to or with had or effect on the expression of the reporter However, when the medium was with 25-hydroxycholesterol and fatty acids, expression of the reporter gene was by with cells in the medium fatty acids the of cells with inhibited the down-regulation of the LDL receptor promoter by 25-hydroxycholesterol a in the down-regulation produced by 25-hydroxycholesterol from to Furthermore, the effect of on the response to 25-hydroxycholesterol could be by the medium with stearate and oleate. Supplementation of cells with these fatty acids in in the of activity of 25-hydroxycholesterol, which is to the of down-regulation produced by 25-hydroxycholesterol in the absence of inhibition of fatty acid elongation a in transcriptional regulation of the LDL receptor promoter by 25-hydroxycholesterol. examined the SRE-1 was to fatty acid of sterol down-regulation or sequences of the SRE-1 were for this this we promoter which LDL receptor SRE-1 sequences the promoter and has been to be regulated by sterols Brown M.S. Goldstein J.L. J. Biol. Chem. 1988; Full Text PDF PubMed Google Scholar). cells with a response to 25-hydroxycholesterol to that of cells with the LDL receptor promoter The down-regulation of activity was further by C18 fatty cells with a control a SRE down-regulation when in medium with 25-hydroxycholesterol or C18 fatty acids and 25-hydroxycholesterol also Brown M.S. Goldstein J.L. J. Biol. Chem. 1988; Full Text PDF PubMed Google Scholar). These results indicate that the SRE-1 element is the of the LDL receptor promoter that is for the effect of C18 fatty acids on the down-regulation of transcription by 25-hydroxycholesterol. also that of the effect of the fatty acids on the of 25-hydroxycholesterol on the LDL receptor promoter could be with oleate supplementation is for sterol regulation of the LDL receptor promoter in cells with were for in with and sterols and or stearate oleate as described for the of Because regulation of LDL receptor transcription by oxysterols SRE-1 has been demonstrated to be mediated by SREBPs, it that fatty acid supplementation would enhance the of nuclear SREBP levels produced by 25-hydroxycholesterol. test this nuclear were from cells in with or fatty acids in the or absence of of nuclear were to and monoclonal antibodies to SREBP-1 and the of cells cultured in the absence of sterols the mature of SREBP-1 and SREBP-2, the of cells in medium with sterols of the mature of SREBP Supplementation with fatty acids the nuclear levels of the mature SREBP-1 or with the results from the reporter of cells with C18 fatty acids the activity of 25-hydroxycholesterol in the levels of SREBP-1 and in nuclear Because the reported mechanism nuclear SREBP levels is through inhibition of proteolytic these data are with C18 fatty acids or a a role in the proteolytic of SREBPs in response to that the of fatty acid supplementation on proteolytic of SREBPs was most demonstrated in in which fatty acid elongation had been inhibited with the proteolytic is to be The results of such that C18 fatty acid supplementation the activity of 25-hydroxycholesterol in nuclear SREBP Furthermore, the in the nuclear levels of the mature SREBPs in cells with the of fatty acids and 25-hydroxycholesterol is for by a increase in the of SREBP in the This is with a effect of fatty acid supplementation and 25-hydroxycholesterol on SREBP proteolytic maturation. of stearate and oleate for of regulation of nuclear SREBP levels that the regulatory response is specific for oleate fatty acids enhance sterol inhibition of SREBP-1 and SREBP-2 proteolytic maturation. and proteins were isolated from cells cultured for in containing and with and sterols and C18 fatty acids as described for of proteins and of nuclear proteins were by as described in the to supplementation the of nuclear SREBP-1 and SREBP-2 levels in response to proteins were from cells cultured for in with and sterols 25-hydroxycholesterol and and or stearate oleate as indicated in the protein were to with monoclonal antibodies IgG-7D4 and IgG-2A4 as described in the to The metabolic of the of 25-hydroxycholesterol and fatty acids on SREBP would be a regulation of fatty acid synthesis and sterol synthesis by exogenous sterols and down-regulation of both sterol and fatty acid synthesis by the of on activity of mature SREBP in fatty acid starved cells, would fatty acid synthesis to under exogenous sources of but fatty acids, were in to cell both are it would be that both would be test these we examined the effect of 25-hydroxycholesterol on of into sterols and fatty acids in cells in the or absence of exogenous oleate. The results indicate that with 1 25-hydroxycholesterol, a that SREBP and transcription from the LDL receptor promoter sterol synthesis fatty acid synthesis is inhibited supplementation the inhibition of both sterol and fatty acid synthesis by of into of were at 1 culture and in for The medium was to 25-hydroxycholesterol and oleate to fatty acid for to the of a the into was as described under The results are the of in a were at 1 culture and in for The medium was to 25-hydroxycholesterol and oleate to fatty acid for to the of a the into was as described under The results are the of Because fatty acids are to they with cholesterol, the of the is that the biosynthetic for these be studies have (4Bennett M.K. Lopez J.M. Sanchez H.B. Osborne T.F. J. Biol. Chem. 1995; 270: 25578-25583Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar, 5Lopez J.M. Bennett M.K. Sanchez H.B. Rosenfeld J.M. Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 1049-1053Crossref PubMed Scopus (249) Google Scholar, 7Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (687) Google Scholar, 8Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (614) Google Scholar) that sterols can be regulatory for fatty acid synthesis through transcriptional control mediated by A role for SREBP-1 in fatty biosynthesis is also with in fatty acid synthesis in of SREBP-1 mice H. Shimomura I. Hammer R.E. J. Goldstein J.L. Brown M.S. Horton J.D. J. Clin. Invest. 1997; PubMed Scopus Google Scholar). These studies are for of the cholesterol and fatty acid biosynthetic by The of the fatty acid synthesis and cholesterol synthesis would be regulation of by fatty that this can was indicated in a from laboratory (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar) of a somatic cell mutant in elongation of to which was defective in down-regulation of encoding enzymes of sterol biosynthesis. that this mutant required both stearate and oleate for growth, that cells could be to supplementation with both fatty In the of these studies, we also that the regulatory to fatty acids could be in a medium that is of C18 fatty The serum supplements or serum-free medium described be used to these Many of the studies on regulation of SREBP by oxysterols have been performed in a medium with a serum for J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). Such medium of fatty acids to serum which would the fatty acid of the response of SREBP or LDL receptor promoter regulation to that we in the current In the current we demonstrate that inhibition of proteolytic cleavage of SREBPs 1 and 2 is by fatty acid supplementation. this of SREBP by 25-hydroxycholesterol and fatty acids also as of SRE-1 driven sterol synthesis, and 25-hydroxycholesterol with such inhibition of fatty acid elongation with has the effect of fatty acid supplementation on transcriptional repression by 25-hydroxycholesterol. The results with the fatty acid supplements indicate that of the effect on transcription is mediated by oleate. This is with a of the of stearate and oleate on SREBP have also that oleate supplementation can most of the regulation of coenzyme A reductase by 25-hydroxycholesterol in the crB mutant (15Panini S.R. Lutz R.J. Wenger L. Miyake J. Leonard S. Andalibi A. Lusis A.J. Sinensky M. J. Biol. Chem. 1990; 265: 14118-14126Abstract Full Text PDF PubMed Google Scholar). is that this restoration of regulation with fatty acids or or with fatty acids or of SREBP by sterol oleate is in the of of the transcriptional control of the synthesis of these two by SREBPs. A class of regulatory somatic cell mutants, by is defective in the proteolytic step of SREBP maturation J. Brown M.S. Goldstein J.L. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). these mutants, which mature SREBPs, are for both cholesterol and oleate Chin J. Chang Proc. Natl. Acad. Sci. U. S. A. 1978; PubMed Scopus Google Scholar, J. Chang J. Biol. Chem. Full Text PDF PubMed Google Scholar). This is because of the role of SREBPs in transcriptional of genes of the cholesterol biosynthetic pathway, but also the gene that the enzyme that stearate to H. Horton J.D. Hammer R.E. Shimomura I. Brown M.S. Goldstein J.L. J. Clin. Invest. 1996; 98: 1575-1584Crossref PubMed Scopus (699) Google Scholar). A mutant defective in is oleate Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google that expression of this enzyme is required for oleate synthesis and cell growth in the absence of exogenous oleate. the growth for the mutants defective in SREBP include the reported regulation of fatty acid synthesis by SREBPs, as well (4Bennett M.K. Lopez J.M. Sanchez H.B. Osborne T.F. J. Biol. Chem. 1995; 270: 25578-25583Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar). would that cholesterol and oleate the two metabolic required for cell growth whose biosynthesis is regulated by SREBPs. that this for these is for of regulation of SREBP maturation by both of these metabolic This mechanism that synthesis of oleate will in the of exogenous which in the absence of such control could oleate synthesis a growth have demonstrated such a effect on fatty acid synthesis in response to 25-hydroxycholesterol in I. we also a in the of inhibition of fatty acid and sterol synthesis in response to oleate This is with a specific regulation of fatty acid synthesis, by fatty acids, in to the has also been that regulation of fatty acid synthesis in by particularly in is mediated by J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (614) Google Scholar). is possible that a of oleate oleate is the is the of which is cholesterol oleate J.L. Brown M.S. J. Biol. Chem. Full Text PDF PubMed Google Scholar). However, it that this is the because have been reported to enhance down-regulation of LDL I. J. Biol. Chem. Full Text PDF PubMed Google Scholar) and HMG-CoA reductase mRNA levels I. J. Biol. Chem. Full Text PDF PubMed Google Scholar, R. Full Text PDF PubMed Scopus Google Scholar) by exogenous of activation of oleate to possible step in its is into the 2 of to acid. This is by which is on the of the endoplasmic J. of and Scholar). This is the localization as which is to be in sterols J. A. Brown M.S. Goldstein J.L. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) through its the of SCAP can such as well as The response of SREBP maturation to and fatty acids is of studies on the effect of on serum cholesterol in both S.M. J. PubMed Scopus Google Scholar) and J.L. J. 1990; Full Text PDF PubMed Google Scholar) indicate when oleate is the of cholesterol levels are and cholesterol levels are has been E. P. A. J. 1996; 89: Scopus Google Scholar) that on oleate produce oleate LDL that is control LDL in cholesterol is to that these of oleate be with its on SREBP and cholesterol