Polyunsaturated Fatty Acid Suppression of Hepatic Fatty Acid Synthase and S14 Gene Expression Does Not Require Peroxisome Proliferator-activated Receptor α
1997; Elsevier BV; Volume: 272; Issue: 43 Linguagem: Inglês
10.1074/jbc.272.43.26827
ISSN1083-351X
AutoresBing Ren, Annette P. Thelen, Jeffrey M. Peters, Frank J. Gonzalez, Donald Β. Jump,
Tópico(s)Metabolism, Diabetes, and Cancer
ResumoDietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor α (PPARα) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARα-deficient mouse to examine the role of PPARα in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2(8-fold) and suppressed mRNAFAS and mRNAS14 by ≥807 in wild type mice. In PPARα-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARα in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARα-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARα and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARα and is involved in the PUFA-mediated suppression of lipogenic gene expression. Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor α (PPARα) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARα-deficient mouse to examine the role of PPARα in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2(8-fold) and suppressed mRNAFAS and mRNAS14 by ≥807 in wild type mice. In PPARα-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARα in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARα-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARα and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARα and is involved in the PUFA-mediated suppression of lipogenic gene expression. Dietary polyunsaturated fatty acids (PUFAs), 1The abbreviations used are: PUFA, polyunsaturated fatty acids; PPAR, peroxisome proliferator-activated receptor; AOX, acyl-CoA oxidase; RXR, retinoid X receptor; FAS, fatty acid synthase; CYP4A2, cytochrome P450 4A2; PPRE, peroxisome proliferator response element; TK, thymidine kinase; CAT, chloramphenicol acetyltransferase; PCR, polymerase chain reaction. 1The abbreviations used are: PUFA, polyunsaturated fatty acids; PPAR, peroxisome proliferator-activated receptor; AOX, acyl-CoA oxidase; RXR, retinoid X receptor; FAS, fatty acid synthase; CYP4A2, cytochrome P450 4A2; PPRE, peroxisome proliferator response element; TK, thymidine kinase; CAT, chloramphenicol acetyltransferase; PCR, polymerase chain reaction. in particular highly unsaturated n-3 fatty acids, suppress hepatic lipogenesis and triglyceride synthesis/secretion while inducing peroxisomal and microsomal fatty acid oxidation (1Clarke S.D. Jump D.B. Annu. Rev. 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A. 1993; 90: 2160-2164Crossref PubMed Scopus (850) Google Scholar, 22Schoonjans K. Steals B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar, 23Kliewer S.A. Forman B.M. Blumberg B. Ong E.S. Borgmeyer U. Mangelsdorf D.J. Umesono K. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7355-7359Crossref PubMed Scopus (1269) Google Scholar, 24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar).Peroxisome proliferators encompass a wide variety of compounds including hypolipidemic drugs (WY14,643, gemfibrozil, and clofibrate), plasticizers (di-(2-ethyhexyl)phthalate), steroids (dehyrdoepiandrosterone and dehyrdoepiandrosterone-sulfate), and dietary fatty acids (25Lalwani N.D. Reddy M.K. Qureshi S.A. Sirtori C.R. Abiko Y. Reddy J.K. Hum. Toxicol. 1983; 2: 27-48Crossref PubMed Scopus (122) Google Scholar, 26Reddy J.K. Mannaerts G.P. Annu. Rev. 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A. 1992; 89: 4653-4657Crossref PubMed Scopus (796) Google Scholar, 21Keller H. Dreyer C. Medin J. Mahfoudi A. Ozato K. Wahli W. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 2160-2164Crossref PubMed Scopus (850) Google Scholar, 22Schoonjans K. Steals B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar, 23Kliewer S.A. Forman B.M. Blumberg B. Ong E.S. Borgmeyer U. Mangelsdorf D.J. Umesono K. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7355-7359Crossref PubMed Scopus (1269) Google Scholar, 24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar, 25Lalwani N.D. Reddy M.K. Qureshi S.A. Sirtori C.R. Abiko Y. Reddy J.K. Hum. Toxicol. 1983; 2: 27-48Crossref PubMed Scopus (122) Google Scholar, 26Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (363) Google Scholar, 27Issemann I. Green S. Nature. 1990; 347: 645-650Crossref PubMed Scopus (3021) Google Scholar, 28Green S. Wahli W. Mol. Cell. Endocrinol. 1994; 100: 149-153Crossref PubMed Scopus (157) Google Scholar, 29Kliewer S.A. Umesono K. Noonan D.J. Heyman R.A. Evans R.M. Nature. 1992; 358: 771-774Crossref PubMed Scopus (1514) Google Scholar, 30Muerhoff A.S. Griffin K.J. Johnson E.F. J. Biol. Chem. 1992; 267: 19051-19053Abstract Full Text PDF PubMed Google Scholar, 31Gearing K.L. Crickmore A. Gustafsson J.A. Cell. 1994; 79: 1147-1156Abstract Full Text PDF PubMed Scopus (3093) Google Scholar, 32Keller H. Givel F. Perroud M. Wahli W. Mol. Endocrinol. 1995; 9: 794-804Crossref PubMed Google Scholar, 33Isseman I. Prince R.A. Tugwood J.D. Green S. J. Mol. Endocrinol. 1993; 11: 37-47Crossref PubMed Scopus (283) Google Scholar). PPARs also have inhibitory effects on gene transcription. For example, apolipoprotein CIII and transferrin gene expression is inhibited by PPAR-RXR competition for an HNF-4 binding site within the promoters of these genes (34Hertz R. Bishara-Shieban J. Bar-Tana J. J. Biol. Chem. 1995; 270: 13470-13475Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, 35Hertz R. Seckbach M. Zakin M.M. Bar-Tana J. J. Biol. Chem. 1996; 271: 218-224Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). PPARα has also been shown to interfere with thyroid hormone action by sequestering RXRα, a factor required for thyroid hormone receptor binding to DNA (36Juge-Aubry C.E. Gorla-Bajszczak A. Pernin A. Lemberger T. Wahli W. Burger A.G. Meier C.A. J. Biol. Chem. 1995; 270: 18117-18122Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar, 37Chu R. Madison L.D. Lin Y. Kopp P. Rao M.S. Jameison J.L. Reddy J.K. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11593-11597Crossref PubMed Scopus (89) Google Scholar, 38Ren B. Thelen A. Jump D.B. J. Biol. Chem. 1996; 271: 17167-17173Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar).The fact that PPARs are activated by fatty acids in conjunction with the known effects of PUFA on peroxisomal and microsomal fatty acid oxidation suggest that PUFA regulation of these pathways might utilize a common transcriptional mediator, i.e. PPAR. While several PPAR subtypes (α, ॆ (also known as δ, Nuc1, FAAR), γ1, and γ2) have been identified in rodents, PPARα is the predominant form in rodent liver (22Schoonjans K. Steals B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar, 23Kliewer S.A. Forman B.M. Blumberg B. Ong E.S. Borgmeyer U. Mangelsdorf D.J. Umesono K. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7355-7359Crossref PubMed Scopus (1269) Google Scholar, 24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar, 28Green S. Wahli W. Mol. Cell. Endocrinol. 1994; 100: 149-153Crossref PubMed Scopus (157) Google Scholar, 31Gearing K.L. Crickmore A. Gustafsson J.A. Cell. 1994; 79: 1147-1156Abstract Full Text PDF PubMed Scopus (3093) Google Scholar, 39Huang Q. Alvares K. Chu R. Bradfield C.A. Reddy J.K. J. Biol. Chem. 1994; 269: 8493-8497Abstract Full Text PDF PubMed Google Scholar). Recent gene targeting studies clearly demonstrate that PPARα is required for the pleiotropic response to peroxisome proliferators including an increase in hepatic mRNAs encoding peroxisomal and microsomal enzymes (40Lee S.S.T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz K.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1492) Google Scholar).This report examines the role of PPARα in PUFA-regulation of mRNAs encoding hepatic lipogenic, microsomal and peroxisomal enzymes. In this work, we assessed PUFA regulation of theS14 gene and fatty acid synthase, models for lipogenic gene expression, and acyl-CoA oxidase (AOX) and cytochrome P450A2 (CYP4A2), enzymes involved in peroxisomal and microsomal fatty acid oxidation, respectively. The recently developed PPARα-null mouse (40Lee S.S.T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz K.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1492) Google Scholar) was used to determine whether PPARα mediates PUFA regulation of hepaticAOX, CYP4A2, S14, and FASgene expression. This work shows that while PPARα is required for the PUFA-mediated induction of both AOX and CYP4A2gene expression, it is not required for the PUFA-mediated inhibition of either S14 or FAS gene expression. These and other studies indicate that PUFA regulation of hepatic gene transcription involves at least two distinct pathways, a PPARα-dependent and a PPARα-independent pathway.DISCUSSIONPPARα is the predominant PPAR subtype expressed in rat liver and it plays a central role in the induction of hepatic peroxisomal and microsomal fatty acid oxidation (24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar, 40Lee S.S.T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz K.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1492) Google Scholar). Since several peroxisomal, microsomal and lipogenic enzymes are regulated by PUFA at the pretranslational level, we tested the hypothesis that dietary PUFA regulate hepatic fatty acid oxidation and de novolipogenesis through a common mediator, i.e. PPARα. Interestingly, all studies reporting on fatty acid regulation of PPARα have been carried out by over expressing receptors in established cell lines. No studies have directly examined the role PPARα may have in fatty acid-regulated hepatic gene transcription. The PPARα-null mouse allows such an analysis. Coupling this genetic approach with other studies has allowed us to show for the first time that 1) PPARα is required for PUFA-mediated induction of hepatic mRNAAOX and mRNACYP4A2 (Fig. 1); 2) PPARα is not required for PUFA-mediated suppression of mRNAS14 or mRNAFAS (Fig. 1); 3) while 18:2 (n-6), 18:3 (n-6 and n-3), 20:4 (n-6) and 20:5 (n-3) suppress mRNAS14 and mRNAFAS, only 20:5 (n-3) induces mRNAAOX in primary hepatocytes (Fig. 4); 4) while gemfibrozil induces hepatic mRNAAOX, it has little or no effect on mRNAS14 or mRNAFAS (Fig. 3). Taken together, these studies indicate that PUFA control of peroxisome/microsomal fatty acid oxidation and de novo lipogenesis in rat liver does not involved PPARα as a common mediator. The differential effect of specific fatty acids, i.e. 18:2, 18:3 (n-3 andn-6), 20:4 (n-6), versus gemfibrozil underscores the lack of coordinate regulation of these pathways in rat liver. Such studies indicate that PUFA regulates at least two pathways in liver, one involves PPARα and controls expression of genes encoding proteins involved in peroxisomal and microsomal fatty acid oxidation. The other mechanism is PPARα-independent and is involved in the PUFA-mediated suppression of lipogenic gene expression.PUFA suppress hepatic mRNAS14 and mRNAFAS levels by inhibiting gene transcription (15Blake W.L. Clarke S.D. J. Nutr. 1990; 120: 1727-1729Crossref PubMed Scopus (119) Google Scholar, 16Jump D.B. Clarke S.D. MacDougald O.A. Thelen A. Proc. Natl. Acad. Aci. U. S. A. 1993; 90: 8454-8458Crossref PubMed Scopus (143) Google Scholar, 17Jump D.B. Clarke S.D. Thelen A. Liimatta M. J. Lipid Res. 1994; 35: 1076-1084Abstract Full Text PDF PubMed Google Scholar). From the data reported above, this inhibitory mechanism does not require PPARα. Although the mechanism of PUFA induction of hepatic mRNAAOX and mRNACYP4A2 has not been established, the following studies implicate transcription as the principal mode of PUFA regulation of AOX andCYP4A: 1) peroxisomal proliferators rapidly induce transcription of genes encoding AOX, the bifunctional enzyme, thiolase and CYP4A subtypes 1–3 (26Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (363) Google Scholar, 40Lee S.S.T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz K.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1492) Google Scholar); 2) PPARα is required for the induction of these genes (40Lee S.S.T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz K.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1492) Google Scholar); 3) PPARα binds PPREs as PPAR/RXR heterodimers in the promoters of these genes and stimulates transcription of cis-linked reporter genes (20Gottlicher M. Widmark E. Li Q. Gustafsson J.A. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 4653-4657Crossref PubMed Scopus (796) Google Scholar, 21Keller H. Dreyer C. Medin J. Mahfoudi A. Ozato K. Wahli W. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 2160-2164Crossref PubMed Scopus (850) Google Scholar, 22Schoonjans K. Steals B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar, 23Kliewer S.A. Forman B.M. Blumberg B. Ong E.S. Borgmeyer U. Mangelsdorf D.J. Umesono K. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7355-7359Crossref PubMed Scopus (1269) Google Scholar, 24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar, 25Lalwani N.D. Reddy M.K. Qureshi S.A. Sirtori C.R. Abiko Y. Reddy J.K. Hum. Toxicol. 1983; 2: 27-48Crossref PubMed Scopus (122) Google Scholar, 26Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (363) Google Scholar, 27Issemann I. Green S. Nature. 1990; 347: 645-650Crossref PubMed Scopus (3021) Google Scholar, 28Green S. Wahli W. Mol. Cell. Endocrinol. 1994; 100: 149-153Crossref PubMed Scopus (157) Google Scholar, 29Kliewer S.A. Umesono K. Noonan D.J. Heyman R.A. Evans R.M. Nature. 1992; 358: 771-774Crossref PubMed Scopus (1514) Google Scholar, 30Muerhoff A.S. Griffin K.J. Johnson E.F. J. Biol. Chem. 1992; 267: 19051-19053Abstract Full Text PDF PubMed Google Scholar, 31Gearing K.L. Crickmore A. Gustafsson J.A. Cell. 1994; 79: 1147-1156Abstract Full Text PDF PubMed Scopus (3093) Google Scholar, 32Keller H. Givel F. Perroud M. Wahli W. Mol. Endocrinol. 1995; 9: 794-804Crossref PubMed Google Scholar, 33Isseman I. Prince R.A. Tugwood J.D. Green S. J. Mol. Endocrinol. 1993; 11: 37-47Crossref PubMed Scopus (283) Google Scholar, 38Ren B. Thelen A. Jump D.B. J. Biol. Chem. 1996; 271: 17167-17173Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar); 4) fatty acids activate PPARα and stimulate transcription of cis-linked reporter genes (20Gottlicher M. Widmark E. Li Q. Gustafsson J.A. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 4653-4657Crossref PubMed Scopus (796) Google Scholar, 21Keller H. Dreyer C. Medin J. Mahfoudi A. Ozato K. Wahli W. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 2160-2164Crossref PubMed Scopus (850) Google Scholar, 22Schoonjans K. Steals B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar, 23Kliewer S.A. Forman B.M. Blumberg B. Ong E.S. Borgmeyer U. Mangelsdorf D.J. Umesono K. Evans R.M. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7355-7359Crossref PubMed Scopus (1269) Google Scholar, 24Braissant O. Foufelle F. Scotto C. Dauca M. Wahli W. Endocrinology. 1996; 137: 354-366Crossref PubMed Scopus (1737) Google Scholar, 25Lalwani N.D. Reddy M.K. Qureshi S.A. Sirtori C.R. Abiko Y. Reddy J.K. Hum. Toxicol. 1983; 2: 27-48Crossref PubMed Scopus (122) Google Scholar, 26Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (363) Google Scholar, 27Issemann I. Green S. Nature. 1990; 347: 645-650Crossref PubMed Scopus (3021) Google Scholar, 28Green S. Wahli W. Mol. Cell. Endocrinol. 1994; 100: 149-153Crossref PubMed Scopus (157) Google Scholar, 29Kliewer S.A. Umesono K. Noonan D.J. Heyman R.A. Evans R.M. Nature. 1992; 358: 771-774Crossref PubMed Scopus (1514) Google Scholar, 30Muerhoff A.S. Griffin K.J. Johnson E.F. J. Biol. Chem. 1992; 267: 19051-19053Abstract Full Text PDF PubMed Google Scholar, 31Gearing K.L. Crickmore A. Gustafsson J.A. Cell. 1994; 79: 1147-1156Abstract Full Text PDF PubMed Scopus (3093) Google Scholar, 32Keller H. Givel F. Perroud M. Wahli W. Mol. Endocrinol. 1995; 9: 794-804Crossref PubMed Google Scholar, 33Isseman I. Prince R.A. Tugwood J.D. Green S. J. Mol. Endocrinol. 1993; 11: 37-47Crossref PubMed Scopus (283) Google Scholar) (Fig. 5); and 5) PPARα is required for the PUFA induction of hepatic mRNAAOX and mRNACYP4A2 (Fig. 1).Previous efforts to examine the involvement of PPARα in PUFA regulation of lipogenic gene expression showed that the cis-regulatory targets for PUFA and PPAR in the S14 promoter (38Ren B. Thelen A. Jump D.B. J. Biol. Chem. 1996; 271: 17167-17173Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar, 45Jump D.B. Ren B Clarke S Thelen A. Prostaglandins Leukot. Essent. Fatty Acids. 1995; 52: 107-111Abstract Full Text PDF PubMed Scopus (47) Google Scholar) did not converge. Analysis of stearoyl-CoA desaturase 1 gene expression indicated that peroxisome proliferators/PPAR induced by PUFA-suppressed transcription (46Miller C.W. Ntambi J.M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 9443-9448Crossref PubMed Scopus (216) Google Scholar). Such studies argued against PPAR as a mediator of PUFA effects on lipogenic gene transcription. However, the over expression of receptors does not necessarily reflect physiologically relevant processes. The use of the PPARα-null mouse allows us to directly evaluate the role PPARα plays in PUFA regulation of hepatic gene expression. In contrast to (+/+) mice, hepatic mRNAAOX and mRNACYP4A2 was not significantly induced in PPARα (−/−) mice by the PUFA diet indicating a requirement for PPARα in the PUFA-mediated induction of these enzymes. 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Fukami M.H. Kryvi H. Christiansen E.N. Biochim. Biophys. Acta. 1988; 962: 122-130Crossref PubMed Scopus (74) Google Scholar). This apparent conflict can be reconciled by the fact that primary hepatocytes have a high capacity for fatty acid oxidation, triglyceride synthesis and very low density lipoprotein secretion (7Rustan A.C. Nossen J.O. Christiansen E.N. Drevon C.A. J. Lipids Res. 1988; 29: 1417-1426Abstract Full Text PDF PubMed Google Scholar). We speculate that these pathways prevent intracellular fatty acids from accumulating to levels that activate PPARα. Interestingly, 20:5 (n-3) was the only PUFA tested here that activated PPARα. 20:5 (n-3) is reported to be poorly oxidized in mitochondria and poorly incorporated into complex lipids, such as triglycerides (7Rustan A.C. Nossen J.O. Christiansen E.N. Drevon C.A. J. Lipids Res. 1988; 29: 1417-1426Abstract Full Text PDF PubMed Google Scholar). 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