FRINK Linked Data Fragments server

Matches in SemOpenAlex for { <https://semopenalex.org/work/W2063841526> ?p ?o ?g. }

• W2063841526 endingPage "28290" @default.
• W2063841526 startingPage "28285" @default.
• W2063841526 abstract "Retinoid x receptor α (RXRα) serves as an active partner of peroxisome proliferator-activated receptor (PPARα). In order to dissect the functional role of RXRα and PPARα in PPARα-mediated pathways, the hepatocyte RXRα-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRα and PPARα deficiency are different in several aspects. At the basal untreated level, RXRα deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARα-null mice. Fasting-induced PPARα activation was drastically prevented in the absence of hepatocyte RXRα. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRα. Hepatocyte RXRα deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRα-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRα. These data suggested that in comparison to PPARα, hepatocyte RXRα has its unique role in lipid homeostasis and that the effect of RXRα, -β, and -γ is redundant in certain aspects. Retinoid x receptor α (RXRα) serves as an active partner of peroxisome proliferator-activated receptor (PPARα). In order to dissect the functional role of RXRα and PPARα in PPARα-mediated pathways, the hepatocyte RXRα-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRα and PPARα deficiency are different in several aspects. At the basal untreated level, RXRα deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARα-null mice. Fasting-induced PPARα activation was drastically prevented in the absence of hepatocyte RXRα. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRα. Hepatocyte RXRα deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRα-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRα. These data suggested that in comparison to PPARα, hepatocyte RXRα has its unique role in lipid homeostasis and that the effect of RXRα, -β, and -γ is redundant in certain aspects. peroxisome proliferator-activated receptor α retinoid X receptor acyl-CoA oxidase liver fatty acid-binding protein medium chain acyl-CoA dehydrogenase cytochrome P450 4A1 apolipoprotein A-I apolipoprotein G-III Peroxisome proliferators including herbicides, plasticizers, hypolipidemic drugs (fibrates), and leukotriene D4 inhibitors play a crucial role in hepatocyte proliferation. The most potent peroxisome proliferator is Wy14,643. These agents cause profound peroxisome proliferation in hepatocytes resulting in hepatomegaly and hepatoma and a rapid transcription of genes encoding the enzymes involved in fatty acid metabolism (for reviews, see Refs. 1Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (369) Google Scholar, 2Desvergne B. Wahli W. Bauerle P. Inducible Transcription. 1. Birkhäuser, Boston, MA1995: 142-176Google Scholar, 3Dreyer C.K. Krey G Keller H. Givel F. Helftenbein G. Wahli W. Cell. 1992; 68: 879-887Abstract Full Text PDF PubMed Scopus (1214) Google Scholar, 4Fan C.-Y. Pan J. Usuda N. Yeldandi A.V. Rao M.S. Reddy J.K. Hepatology Elsewhere. 1999; 29: 606-608Crossref PubMed Scopus (19) Google Scholar). Peroxisome proliferators exert their pleitropic responses via PPARα,1 a member of the nuclear hormone receptor superfamily (5Issemann I. Green S. Nature. 1990; 347: 645-650Crossref PubMed Scopus (3059) Google Scholar, 6Schmidt A. Endo N. Rutledge S.J. Vogel R. Shinar D. Rodan G.A. Mol. Endocrinol. 1992; 6: 1634-1641Crossref PubMed Scopus (366) Google Scholar, 7Göttlicher M. Widmark E. Li Q. Gustafsson J-Å Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 4653-4657Crossref PubMed Scopus (800) Google Scholar, 8Sher T. Yi H.F. McBride O.W. Gonzalez F.J Biochemistry. 1993; 32: 5598-5604Crossref PubMed Scopus (452) Google Scholar, 9Chen F. Law S.W. O'Malley B.W. Biochem. Biophys. Res. Commun. 1993; 196: 671-677Crossref PubMed Scopus (138) Google Scholar, 10Kliewer 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 (1281) Google Scholar). Besides peroxisome proliferator, PPARα can also be activated by certain conditions such as starvation, high fat diet, and diabetes mellitus under which increased fatty acids are delivered to the liver (11Kroetz D. Yook P. Costet P. Bianchi P. Pineau T. J. Biol. Chem. 1998; 273: 31581-31589Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar, 12Leone T.C. Weinheimer C.J. Kelly D.P. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7473-7478Crossref PubMed Scopus (824) Google Scholar, 13Kersten S. Seydoux J. Peters J.M. Gonzalez F.J. Desvergne B. Wahli W. J. Clin. Inves. 1999; 103: 1489-1498Crossref PubMed Scopus (1371) Google Scholar). RXRs are the required active heterodimeric partners of PPARs (14Mangelsdorf D.J. Borgmeyer U. Heyman R.A. Zhou J.Y. Ong E.S. Oro A.E. Kakizuka A. Evans R.M. Genes Dev. 1992; 6: 329-344Crossref PubMed Scopus (1069) Google Scholar). Thus, RXR, PPAR, and their ligands are all actively involved in regulating liver gene expression, fatty acid metabolism, lipid transport, and hepatocyte proliferation. Among the three types of RXR, RXRα is the predominant one expressed in the liver. Absence of PPARα expression in knockout mice prevents the induction of hepatocyte peroxisome proliferation and of fatty acid synthesizing enzymes and β oxidizing enzymes by Wy14,643 (15Lee S.S.-T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz D.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1506) Google Scholar, 16Peters J.M. Hennuyer N. Staels B. Fruchart J.-C. Fievet C. Gonzalez F.J. Auwerx J. J. Biol. Chem. 1997; 272: 27307-27312Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 17Aoyama T. Peters J.M. Iritiani N. Nakajima T. Furihata K. Hashimoto T. Gonzalez F.J. J. Biol. Chem. 1998; 273: 5678-5684Abstract Full Text Full Text PDF PubMed Scopus (754) Google Scholar). In addition, PPARα deficiency leads to elevated serum cholesterol levels in young adult mice and increased serum triglyceride levels and steatosis in aging mice (18Costet P. Legerdre C. More J. Edgar A. Galtier P. Pineau T. J. Biol. Chem. 1998; 273: 29577-29585Abstract Full Text Full Text PDF PubMed Scopus (367) Google Scholar). There is no in vivo model available with which to compare the role of RXRα with PPARα, because of embryonic lethality caused by a fetal cardiac phenotype in RXRα-null mice (19Sucov H.M. Dyson E. Gumeringer C.L. Price J. Chien K.R. Evans R.M. Genes Dev. 1994; 8: 1007-1018Crossref PubMed Scopus (541) Google Scholar, 20Kastner P. Grondona J.M. Mark M. Gansmuller A. LeMeur M. Decimo D. Vonesch J.L. Dolle P. Chambon P. Cell. 1994; 78: 987-1003Abstract Full Text PDF PubMed Scopus (610) Google Scholar, 21Kastner P. Mark M. Leid M. Gansmuller A. Chin W. Grondona J.M. Decimo D. Krezel W. Dierich A. Chambon P. Genes Dev. 1996; 10: 80-92Crossref PubMed Scopus (273) Google Scholar). RXRβ and RXRγ-null mice have no apparent consequence on the liver (21Kastner P. Mark M. Leid M. Gansmuller A. Chin W. Grondona J.M. Decimo D. Krezel W. Dierich A. Chambon P. Genes Dev. 1996; 10: 80-92Crossref PubMed Scopus (273) Google Scholar). Furthermore, RXRα is strongly implicated in postnatal liver physiology and regulation of liver gene (22Ge R. Rhee M. Malik S. Karathanasis S.K. J. Biol. Chem. 1994; 269: 13185-13192Abstract Full Text PDF PubMed Google Scholar, 23Vu D.N. Schoonjans K. Kosykh V. Dallongeville J. Heyman R.A. Staels B. Auwerx J. Mol. Cell. Biol. 1996; 16: 3350-3360Crossref PubMed Google Scholar, 24Hertz R. Nikodem V. Ben I.A. Berman I. Bar-Tana J. Biochem. J. 1996; 319: 248-341Crossref Scopus (32) Google Scholar, 25Poirier H. Braissant O. Niot I. Wahli W. Besnard P. FEBS Lett. 1997; 412: 480-484Crossref PubMed Scopus (51) Google Scholar, 26Schoonjans K. Peinado O.J. Lefebvre A.M. Heyman R.A. Briggs M. Deeb S. Staels B. Auwerx J. EMBO J. 1996; 15: 5336-5348Crossref PubMed Scopus (1026) Google Scholar, 27Sohlenius A.K. Wigren J. Backstreom K. Andersson K. DePierre J.W. Biochim. Biophys. Acta. 1995; 1258: 257-264Crossref PubMed Scopus (14) Google Scholar, 28Westin S. Sonneveld E. van der Leede B.M. van der Saag P.T. Gustafsson J.A. Mode A. Mol. Cell. Endocrinol. 1997; 129: 169-179Crossref PubMed Scopus (27) Google Scholar). To understand the biological role of RXRα in the liver, we have generated hepatocyte-specific RXRα knockout mice using a cre/loxP recombination system (29Wan Y.-J.Y. An D. Cai Y. Repa J.J. Chen H.-P. Flores M. Postic C. Magnuson M.A. Chen J. Chien K.R. French S. Mangelsdorf D.J. Sucov H.M. Mol. Cell. Biol. 2000; 20: 4436-4444Crossref PubMed Scopus (204) Google Scholar). In this study, we further characterized the impact of RXRα in PPARα-mediated pathways. A line of mice in which the RXRα gene is conditionally mutated by introduction of loxP sites into introns flanking exon 4 of the RXRα gene was provided by Dr. K. Chien (University of California, San Diego) (30Chen J. Kubalak S.W. Chien K.R. Development. 1998; 125: 1943-1949Crossref PubMed Google Scholar). This modified “floxed” allele is fully functional, in those animals which are homozygous for this allele are normal and viable. Moreover, this mutated allele was used to specifically ablate RXRα function in the cardiomyocyte lineage (30Chen J. Kubalak S.W. Chien K.R. Development. 1998; 125: 1943-1949Crossref PubMed Google Scholar). To abolish RXRα function in the hepatocytes, the albumin promoter/enhancer was employed to express cre recombinase. Dr. M. A. Magnuson (Vanderbilt Medical Center) provided this albumin-cre transgenic line, which provides liver-specific expression. Heptocyte-specific RXRα knockout was established by crossing albumin-cre transgene with the RXRα flox/flox background (29Wan Y.-J.Y. An D. Cai Y. Repa J.J. Chen H.-P. Flores M. Postic C. Magnuson M.A. Chen J. Chien K.R. French S. Mangelsdorf D.J. Sucov H.M. Mol. Cell. Biol. 2000; 20: 4436-4444Crossref PubMed Scopus (204) Google Scholar). PPARα-null mice were generously provided by Dr. Frank Gonzalez (15Lee S.S.-T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz D.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1506) Google Scholar). 4-Chloro-6-(2,3-sylidine)-pyrimidinylthio)acetic acid (Wy14,643) was purchased commercially (ChemSyn Science Laboratories, Lenexa, KS). Pelleted mouse chow, which was composed of 21.4% protein, 55% carbohydrates, 4% fat, 6.7% ash, 4% fiber, and less than 10% moisture, was commercially prepared containing either 0.0% (control) or 0.1% (w/w) Wy14,643 (Bioserv, Frenchtown, NJ). For all the experiments 10–16-week-old male mice were used. Mice were fed either control or Wy14,643 diet ad libitum for 10 days. For the starvation experiment, mouse chow was removed from mice for 48 h. Animals were housed in groups of two or three in plastic microisolator cages at 25 °C with a 12-h light/12-h dark cycle. At the end of the treatment, animals were weighed and anesthetized with pentobarbital (60 mg/kg, intraperitoneally). Blood samples were obtained by intracardiac puncture. Blood triglycerides and cholesterol levels were determined by automated analysis. The liver was removed immediately, weighed, frozen in liquid nitrogen, and processed for RNA extraction. Part of liver was fixed by formalin and 1.5% glutaraldehyde for light and electron microscopy analysis, respectively. Molecular aspects of hepatocyte-specific RXRα mutation were evaluated by Northern blotting analysis of RNA levels in the liver for the expression of PPARα target genes. The gene probes used were apoA-I andapoC-III (provided by Dr. J. Auwerx), liver fatty acid-binding protein (provided by Dr. J. Gordon), malic enzyme (provided by Dr. G. Brent), acyl-CoA oxidase(31Miyazawa S. Hayashi H. Hijikata M. Ishii N. Furuta S. Kagamiyama H. Osumi T. Hashimoto T. J. Biol. Chem. 1987; 262: 8131-8137Abstract Full Text PDF PubMed Google Scholar) (provided by Dr. T. Osumi), medium chain acyl-CoA dehydrogenase (12Leone T.C. Weinheimer C.J. Kelly D.P. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7473-7478Crossref PubMed Scopus (824) Google Scholar) (provided by Dr. D. Kelly), CYP4A1(32Hardwick L.P. Song B.-J. Huberman E. Gonzalez F.J. J. Biol. Chem. 1987; 262: 801-810Abstract Full Text PDF PubMed Google Scholar) (provided by Dr. F. Gonzalez), and catalase (33Quan F. Korneluk R.G. Gravel R.A. Nucleic Acids Res. 1986; 14: 5321-5335Crossref PubMed Scopus (174) Google Scholar) (purchased from American Type Culture Collection). For Northern analysis, hepatocyte and liver total RNA was extracted by the guanidinium isothiocyanate method (34Chomczynski P. Sauhi N. Anal. Biochem. 1987; 162: 156-159Crossref PubMed Scopus (63232) Google Scholar). Twenty μg of total RNA per lane was resolved by electrophoresis on a 1.2% agarose gels containing 2.2 m formaldehyde and then transferred to nylon membranes by capillary blotting. cDNA fragments were labeled by random priming and hybridized to membranes in 7% (w/v) SDS, 0.5m sodium phosphate, pH 6.5, 1 mm EDTA, and 1 mg/ml bovine serum albumin at 68 °C overnight. The membranes were washed twice in 1% SDS, 50 mm NaCl, and 1 mmEDTA at 68 °C for 15 min each and autoradiographed using intensifying screens. Four animals from each group were studied for each gene. The amount of mRNA expressed was quantitated by densitometry and then normalized with the level of 18 S rRNA to obtain mean and standard deviation. Statistical relevance of discrepancies between groups was evaluated by Student's t test. Prolonged starvation induces dramatic changes in metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by fatty acid oxidation in the liver. It has been demonstrated that PPARα mediates the adaptive response to fasting (11Kroetz D. Yook P. Costet P. Bianchi P. Pineau T. J. Biol. Chem. 1998; 273: 31581-31589Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar, 12Leone T.C. Weinheimer C.J. Kelly D.P. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7473-7478Crossref PubMed Scopus (824) Google Scholar, 13Kersten S. Seydoux J. Peters J.M. Gonzalez F.J. Desvergne B. Wahli W. J. Clin. Inves. 1999; 103: 1489-1498Crossref PubMed Scopus (1371) Google Scholar). To analyze the role of RXRα in fasting-activated PPARα pathways, PPARα-null mic, hepatocyte RXRα-deficient mice, and wild-type controls were deprived from food for 48 h and then the expression of PPARα target genes in the livers was examined by Northern hybridization. In mice fed the control diet, PPARα deficiency caused a reduction in the level of acyl-CoA oxidase (AOX) and cytochrome P450 4A1 (CYP4A1) mRNA encoding the key enzymes involved in fatty acid β- and ω-oxidation pathways (Fig. 1). PPARα deficiency also resulted in a decreased expression of liver fatty acid-binding protein (LFABP) mRNA and a weak induction of apoA-I mRNA. In comparison, RXRα deficiency resulted in inhibition of expression of CYP4A1 and LFABP mRNA. The level of AOX and medium chain acyl-CoA dehydrogenase (MCAD) mRNA remained unchanged. The most striking difference between the PPARα- and RXRα-deficient mice was that the expression of apoA-I and apoC-III mRNA was markedly increased in the absence of RXRα, whereas the induction was very weak, if there was any, in the PPARα-null mice (Fig. 1). In wild-type mice, starvation caused significant induction of the PPARα target gene except for the apoC-III gene (Fig. 1). PPARα deficiency completely abolished PPARα target gene activation induced by starvation. The reduced expression of AOX , MCAD , CYP4A1, and LFABP genes in fasted PPARα-null mice suggested that the transcription of these genes was dependent on PPARα in the fasting state. RXRα deficiency had a similar effect; starvation induced PPARα activation was prevented in the absence of RXRα. Starvation only caused a weak induction of MCAD and CYP4A1 mRNA in RXRα-deficient mice (1.8- and 4-fold induction, respectively) compared with wild-type mice (10- and 20-fold induction, respectively); this weak effect caused by starvation probably was due to the presence of RXRβ and -γ. These data unambiguously proved that in vivo in the hepatocyte, the effect of PPARα and RXRα is coupled. In addition, hepatocyte RXRα has a unique effect in regulating the expression of apolipoprotein genesin vivo. To further analyze the role of RXRα in PPARα/RXRα-mediated pathways; the expression of the PPARα/RXRα target genes was examined in Wy14,643-treated mice. Wild type and RXRα-deficient mice were treated with Wy14,643 (0.1%, w/w) for 10 days. Total liver RNA was extracted for analyzing the expression of PPARα target genes. The results of two representative mouse liver samples from two mice are shown in Fig.2. Consistent with the data demonstrated in Fig. 1, the basal AOX, MCAD, and malic enzyme mRNA level remained unchanged in mutant mouse livers. After Wy14,643 treatment, the expression of AOX, MCAD, and malic enzyme mRNA (× 50, × 20, and × 50, respectively) in the wild-type mouse livers was significantly induced (Fig. 2). In contrast, the inductions were markedly reduced due to hepatocyte RXRα deficiency (only 2–5-fold induction). The expression of the catalase gene was not affected by Wy14,643 in wild-type and mutant mouse livers (Fig. 2). These data indicate that at the physiological level, PPARα/RXRα or RXRα/RXRα do not regulate the basal transcription of the AOX , MCAD, and malic enzyme gene in vivo and that only exogenous ligand (Wy14,643)-activated PPARα/RXRα can regulate the expression of these genes. The weak residual inducibility of these genes by Wy14,643 in the mutant mouse livers may be due to the presence of RXRβ and -γ. In contrast to the AOX , MCAD, and malic enzymegenes, the basal transcription of the CYP4A1 andLFABP genes can be controlled by PPARα/RXRα at the physiological level. CYP4A1 and LFABP mRNA level was reduced about 3-fold in RXRα-deficient mouse livers compared with the wild-type livers (Fig. 3). However, the inducibility of these two genes by Wy14,643 remained the same in mutant mouse livers (Fig. 3). After Wy14,643 administration, there was a 50- and 10-fold induction of CYP4A1 and LFABP mRNA level, respectively, in both wild-type and mutant mouse livers. These data suggest that the basal transcription of the CYP4A1 and LFABP genes is constitutively regulated by PPARα/RXRα or RXRα/RXRα through endogenous ligands such as polyunsaturated fatty acids or 9-cis-retinoic acid in vivo. Therefore, in the absence of RXRα, these genes are expressed at a reduced level. However, when pharmacological levels of exogenous ligands are present, the availability of RXRβ and -γ is sufficient to mediate the inductive effect of Wy14,643. To further understand the role of RXRα in regulating cholesterol and lipid homeostasis, the expression of apoA-I and apoC-III mRNA was examined in Wy14,643-treated mice. In normal cells, PPARα agonists suppress the expression of these genes. RXRα is involved in the basal transcription of the apolipoprotein genes because the basal mRNA levels in normally fed mice were increased in the absence of RXRα (Fig. 4). However, the inhibitory effect of Wy14,643 on apolipoprotein gene expression remained in the absence of hepatocyte RXRα. Taken together, the expression pattern of these PPARα target genes can be divided into two groups. In the first group exemplified by theAOX , MCAD, and malic enzyme genes, these genes' basal mRNA level remains unchanged in mutant mouse liver, but the inducibility of the gene by Wy14,643 is decreased remarkably. In the second group, which includes the CYP4A1 , LFABP , apoA-I, andapoC-III genes, the basal mRNA level is altered in the absence of RXRα, but the regulatory effect of Wy14,643 on gene expression remains unchanged in mutant mouse liver. As a hypolipidemic drug (1Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (369) Google Scholar, 2Desvergne B. Wahli W. Bauerle P. Inducible Transcription. 1. Birkhäuser, Boston, MA1995: 142-176Google Scholar, 3Dreyer C.K. Krey G Keller H. Givel F. Helftenbein G. Wahli W. Cell. 1992; 68: 879-887Abstract Full Text PDF PubMed Scopus (1214) Google Scholar, 4Fan C.-Y. Pan J. Usuda N. Yeldandi A.V. Rao M.S. Reddy J.K. Hepatology Elsewhere. 1999; 29: 606-608Crossref PubMed Scopus (19) Google Scholar), Wy14,643 reduces serum cholesterol and triglyceride level. These effects were tested in the hepatocyte RXRα-deficient mice. As shown in Fig. 5, basal serum triglyceride and cholesterol levels were elevated in the RXRα-deficient mice, which is consistent with the Northern data (Figs. 1 and 4) demonstrating the induction of apoA-I and apoC-III mRNA in the mutant mouse livers. Administration of Wy14,643 reduced serum triglyceride and cholesterol level not only in wild-type but also in mutant mice. Therefore, Wy14,643 still can exert its hypolipidemic effect even when RXRα is not expressed in the hepatocyte. It is well characterized that Wy14,643 causes liver enlargement due to hypertrophy and hyperplasia (hepatomegaly) of hepatocytes (1Reddy J.K. Mannaerts G.P. Annu. Rev. Nutr. 1994; 14: 343-370Crossref PubMed Scopus (369) Google Scholar, 2Desvergne B. Wahli W. Bauerle P. Inducible Transcription. 1. Birkhäuser, Boston, MA1995: 142-176Google Scholar, 3Dreyer C.K. Krey G Keller H. Givel F. Helftenbein G. Wahli W. Cell. 1992; 68: 879-887Abstract Full Text PDF PubMed Scopus (1214) Google Scholar, 4Fan C.-Y. Pan J. Usuda N. Yeldandi A.V. Rao M.S. Reddy J.K. Hepatology Elsewhere. 1999; 29: 606-608Crossref PubMed Scopus (19) Google Scholar). Furthermore, clofibrate- and Wy14,643-induced hepatomegaly is not found in PPARα-null mice (15Lee S.S.-T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz D.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1506) Google Scholar). In our system, the data was reproducible where Wy14,643 also produced a marked increase in liver weight in the wild-type mouse. The liver/body weight ratio of the wild-type mice increased 2.4-fold after 10 days of Wy14,643 feeding compared with mice fed a standard control diet (TableI). In contrast, the liver/body ratio of hepatocyte RXRα-deficient mouse only increased by 1.6-fold after Wy14,643 treatment. Therefore, the hepatomegaly caused by treatment with the peroxisome proliferator was partially prevented when RXRα was absent.Table ILiver/body weight ratio of wild-type (RXRα +/+) and hepatocyte RXRα-deficient (RXRα −/−) miceRXRα (+/+)RXRα (−/−)ControlWy14,643ControlWy14,643Liver/body weight0.048 ± 0.0050.118 ± 0.0101-ap < 0.05.0.046 ± 0.0050.080 ± 0.0081-ap < 0.05.Mice were fed either a control diet or 0.1% Wy14,643 for 10 days. Results are the mean ± S.D. of four determinations.1-a p < 0.05. Open table in a new tab Mice were fed either a control diet or 0.1% Wy14,643 for 10 days. Results are the mean ± S.D. of four determinations. Using light and electron microscopy, the liver morphology of the wild-type and RXRα-deficient mice was evaluated (Fig. 6). Compared with wild-type mouse livers, RXRα-deficient mouse livers had normal morphology under light and electron microscope (Fig. 6,a-d). Treatment of wild-type mice with Wy14,643 resulted in pale pink staining of enlarged cells which had increased homogeneous cytoplasm. The cytoplasmic rough endoplasmic reticulum was strikingly reduced (Fig. 6 e). Furthermore, the number and size of peroxisome were significantly increased after the administration of Wy14,643 as demonstrated by electron microscopy (Fig. 6 f). In contrast, under light microscopy, the mutant mouse liver contain both normal and enlarged cells after administration of Wy14,643 (Fig.6 g). Electron microscopy revealed that Wy14,643 still induced hepatocyte peroxisome proliferation in RXRα-deficient mice (Fig. 6 h). Using biochemical and morphological analyses, we have analyzed the hepatic role of RXRα and demonstrated both essential and redundant effects of RXRα in RXRα/PPARα-mediated pathways. Hepatocyte RXRα is crucial for basal lipid and cholesterol homeostasis since serum cholesterol and triglyceride levels are elevated in normally fed mice lacking RXRα. RXRα deficiency can partially prevent the hepatomegaly effect of peroxisome proliferator. Hepatocyte RXRα is essential for maintaining the physiological level of CYP4A1, LFABP, apoA-I, and apoC-III. Hepatocyte RXRα deficiency also significantly prevents starvation and Wy14,643-induced PPARα activation. Mice with hepatocyte RXRα deficiency are unable to increase the capacity for cellular fatty acid utilization in the context of short-term starvation. However, hepatocyte RXRα deficiency neither prevents hepatocyte peroxisome proliferation nor the hypolipidemic effect of the peroxisome proliferators. Since the RXRα, -β, and -γ genes are expressed in different types of liver cells including parenchyma, endothelial, Kupffer, and stellate cells (35Ulven S.M. Natarajan V. Holven K.B. Lvdal T. Berg T. Blomhoff R. Eur. J. Cell Biol. 1998; 77: 111-116Crossref PubMed Scopus (37) Google Scholar, 36Ohata M. Yamauchi M. Takeda K. Toda G. Kamimura S. Motomura K. Xiong S. Tsukamot H. Exp. Mol. Pathol. 2000; 68: 13-20Crossref PubMed Scopus (23) Google Scholar), the presence of RXRα in the liver cells other than hepatocytes as well as the redundant role of RXRs could explain why hepatocyte RXRα-deficient mice are still responsive to Wy14,643. The hepatocyte-specific RXRα-deficient mice allow us to compare the functional roles of RXRα with PPARα. Phenotype comparison between the hepatocyte RXRα-deficient and PPARα-null mice (15Lee S.S.-T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz D.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1506) Google Scholar, 16Peters J.M. Hennuyer N. Staels B. Fruchart J.-C. Fievet C. Gonzalez F.J. Auwerx J. J. Biol. Chem. 1997; 272: 27307-27312Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 17Aoyama T. Peters J.M. Iritiani N. Nakajima T. Furihata K. Hashimoto T. Gonzalez F.J. J. Biol. Chem. 1998; 273: 5678-5684Abstract Full Text Full Text PDF PubMed Scopus (754) Google Scholar, 18Costet P. Legerdre C. More J. Edgar A. Galtier P. Pineau T. J. Biol. Chem. 1998; 273: 29577-29585Abstract Full Text Full Text PDF PubMed Scopus (367) Google Scholar) is summarized in Table II. In PPARα knockout mice, basal serum cholesterol level is elevated to the same extent (1.6-fold induction) as in the hepatocyte-specific RXRα knockout mice. However, young adult male PPARα-null mice have normal serum triglyceride and apoC-III level (16Peters J.M. Hennuyer N. Staels B. Fruchart J.-C. Fievet C. Gonzalez F.J. Auwerx J. J. Biol. Chem. 1997; 272: 27307-27312Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 18Costet P. Legerdre C. More J. Edgar A. Galtier P. Pineau T. J. Biol. Chem. 1998; 273: 29577-29585Abstract Full Text Full Text PDF PubMed Scopus (367) Google Scholar). Serum triglyceride level only elevates in aged animals (6–12-month-old), and the level is higher in females (2-fold induction) than males (1.5-fold induction) (18Costet P. Legerdre C. More J. Edgar A. Galtier P. Pineau T. J. Biol. Chem. 1998; 273: 29577-29585Abstract Full Text Full Text PDF PubMed Scopus (367) Google Scholar). In contrast, in hepatocyte RXRα-deficient mice, a 1.7-fold induction of serum triglyceride level and a remarkable induction ofapoC-III gene expression were observed in 2-month-old male mice. The early induction in serum triglyceride level defines the unique and important role of hepatocyte RXRα in controlling lipid homeostasis. It is possible that the effect of RXRα in regulatingapoC-III gene expression and serum triglyceride level is mediated through dimerization with PPARγ rather than PPARα.Table IIPhenotype comparison between hepatocyte-specific RXRα-deficient and PPARα-null micePhenotypesHepatocyte RXRα-deficient mousePPARα-null mouseHepatomegalyYesNoPeroxisome proliferation induced by PPYesNoSerum cholesterol levelInduced (1.6×)Induced (1.6×)Serum triglyceride level in young adult male miceInduced (1.7×)No changeSerum triglyceride level in aged mice (6–12-month-old)Not doneInduced (higher in females (2×) than males (1.5×))Gene expressionHepatocyte RXRα-deficient mousePPARα-null mouseBasal levelRegulation by PPRegulation by fastingBasal levelRegulation by PPRegulation by fastingAcyl-CoA oxidaseNo changeWeak inductionNo inductionDecreaseNo inductionInhibitionMedium chain acyl-CoA dehydrogenaseNo changeWeak inductionWeak inductionNo changeNo inductionInhibitionMalic enzymeNo changeWeak inductionNo response to fastingDecreaseNo inductionNo response to fastingCYP4A1DecreaseInductionWeak inductionDecreaseNo inductionNo inductionLiver fatty acid-binding proteinDecreaseInductionInhibitionDecreaseNo inductionInhibitionApoA-IIncreaseInhibitionNo inductionIncreaseNo inhibitionNo inductionApoC-IIIIncreaseInhibitionNo response to fastingNo changeNo inhibitionNo response to fasting Open table in a new tab In PPARα-null mice, peroxisome proliferators such as clofibrate and Wy14,643 are completely unable to induce hepatomegaly and hepatocyte peroxisome proliferation, and have no effect in regulating the expression of PPARα target genes including AOX , bifunctional enzymes, CYP4A1 , CYP4A3, LFABP , apoA-I, and apoC-III(15Lee S.S.-T. Pineau T. Drago J. Lee E.J. Owens J.W. Kroetz D.L. Fernandez-Salguero P.M. Westphal H. Gonzalez F.J. Mol. Cell. Biol. 1995; 15: 3012-3022Crossref PubMed Scopus (1506) Google Scholar, 16Peters J.M. Hennuyer N. Staels B. Fruchart J.-C. Fievet C. Gonzalez F.J. Auwerx J. J. Biol. Chem. 1997; 272: 27307-27312Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 17Aoyama T. Peters J.M. Iritiani N. Nakajima T. Furihata K. Hashimoto T. Gonzalez F.J. J. Biol. Chem. 1998; 273: 5678-5684Abstract Full Text Full Text PDF PubMed Scopus (754) Google Scholar). These data suggest that the effect of PPARα is unique in peroxisome proliferator-mediated pathways, and that PPARβ and -γ cannot replace PPARα. In contrast, in vivo, the roles of RXRα, -β, and -γ appear to be at least partially redundant. Based on our results, the PPARα/RXRα target genes can be categorized into several groups. The first group of genes includesAOX and malic enzyme. The basal transcriptional rate of these genes is controlled by PPARα, but not by RXRα. The second group of genes is CYP4A1 , LFABP, andapoA-I. Within this group, the basal transcriptional rate of the genes is constitutively maintained by PPARα as well as by RXRα through endogenous ligands. The third group of genes include apoC-III. The basal transcriptional rate of the apoC-III gene is controlled by RXRα, but not by PPARα. Since RXRα controls the basal transcription of the CYP4A1 , LFABP, andapoA-I genes, but has no effect on the AOX , MCAD ,and malic enzyme genes, these data suggest that in vivo at the physiological level RXRα is crucial for microsomal ω-hydroxylation of fatty acids, fatty acid transport, and cholesterol and fatty acid homeostasis, whereas RXRα may only become important for AOX- and MCAD-mediated fatty acid β-oxidation and malic enzyme-mediated lipogenesis when pharmacological dose of PPARα ligand is employed. Even though RXRβ and -γ are able to substitute RXRα, the total amount of RXRs is critical in mediating the action of RXRs because in the absence of RXRα, fatty acid is not utilized efficiently in response to starvation and Wy14,643 cannot fully exert its effects. RXR dimerizes with more than 10 different kinds of receptor. Activation one of these RXR-mediated pathways might alter other pathways in opposite directions. When the pool of RXRs is decreased, many RXR-mediated regulatory pathways may be impaired. Based on our data, it seems that the level of RXR, rather than the type of RXR, has a major impact in mediating the effect of peroxisome proliferator. It is crucial to understand the regulation of the RXR genes. RXR can be freely activated in permissive heterodimers with PPAR (37Kliewer S.A. Umesono K. Noonam D.J. Heyman R.A. Evans R.M. Nature. 1992; 358: 771-774Crossref PubMed Scopus (1525) Google Scholar) although it also can be silent in nonpermissive heterodimers with the thyroid hormone receptor or the vitamin D receptor (38Blumberg B. Evens R.M. Genes Dev. 1998; 12: 3149-3155Crossref PubMed Scopus (287) Google Scholar). It would be interesting to test if 9-cis-retinoic acid has the same effect as Wy14,643 on RXRα-deficient mice. 9-cis-Retinoic acid can activate RXR/RAR and RXR/RXR, and that would further deprive the availability of RXR to PPARα. Therefore, challenge the mutant mice with 9-cis-retinoic acid may produce more phenotypes. Taken together, nuclear factors might have unique, redundant, synergistic, or antagonistic effects. These effects depend on the relative level of the receptors, presence of hormones, or the pathological condition. Comprehension of the regulation of liver gene transcription provides insight into the understanding of the molecular mechanisms leading to liver physiology, function, development, and differentiation, as well as proliferation. We thank Drs. Frank Gonzalez, Kenneth Chien, and Mark Magnuson for providing PPARα knockout mice, mice which carrying floxed RXRα alleles, and albumin-cre transgenic mice, respectively. We also thank all the investigators listed under “Experimental Procedures” for providing cDNA clones." @default.
• W2063841526 created "2016-06-24" @default.
• W2063841526 creator A5019924930 @default.
• W2063841526 creator A5033298543 @default.
• W2063841526 creator A5036116332 @default.
• W2063841526 creator A5040178560 @default.
• W2063841526 creator A5043642740 @default.
• W2063841526 creator A5051995125 @default.
• W2063841526 creator A5088941449 @default.
• W2063841526 date "2000-09-01" @default.
• W2063841526 modified "2023-10-05" @default.
• W2063841526 title "Peroxisome Proliferator-activated Receptor α-mediated Pathways Are Altered in Hepatocyte-specific Retinoid X Receptor α-deficient Mice" @default.
• W2063841526 cites W1509781249 @default.
• W2063841526 cites W1552471169 @default.
• W2063841526 cites W1573804402 @default.
• W2063841526 cites W1672804368 @default.
• W2063841526 cites W1972298894 @default.
• W2063841526 cites W1975962309 @default.
• W2063841526 cites W1978125779 @default.
• W2063841526 cites W1979761686 @default.
• W2063841526 cites W1991909025 @default.
• W2063841526 cites W1999580573 @default.
• W2063841526 cites W2001260034 @default.
• W2063841526 cites W2001396094 @default.
• W2063841526 cites W2002652075 @default.
• W2063841526 cites W2019899393 @default.
• W2063841526 cites W2040460019 @default.
• W2063841526 cites W2054363438 @default.
• W2063841526 cites W2060052903 @default.
• W2063841526 cites W2066092624 @default.
• W2063841526 cites W2068669347 @default.
• W2063841526 cites W2077782383 @default.
• W2063841526 cites W2102106861 @default.
• W2063841526 cites W2103116278 @default.
• W2063841526 cites W2107752044 @default.
• W2063841526 cites W2115248104 @default.
• W2063841526 cites W2115494923 @default.
• W2063841526 cites W2116470680 @default.
• W2063841526 cites W2119016586 @default.
• W2063841526 cites W2121914083 @default.
• W2063841526 cites W2126640439 @default.
• W2063841526 cites W2140161647 @default.
• W2063841526 cites W2152201022 @default.
• W2063841526 cites W2158550325 @default.
• W2063841526 cites W2169027640 @default.
• W2063841526 cites W2338770850 @default.
• W2063841526 cites W2399414035 @default.
• W2063841526 cites W4294216491 @default.
• W2063841526 doi "https://doi.org/10.1074/jbc.m000934200" @default.
• W2063841526 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/10866995" @default.
• W2063841526 hasPublicationYear "2000" @default.
• W2063841526 type Work @default.
• W2063841526 sameAs 2063841526 @default.
• W2063841526 citedByCount "70" @default.
• W2063841526 countsByYear W20638415262012 @default.
• W2063841526 countsByYear W20638415262013 @default.
• W2063841526 countsByYear W20638415262014 @default.
• W2063841526 countsByYear W20638415262015 @default.
• W2063841526 countsByYear W20638415262018 @default.
• W2063841526 countsByYear W20638415262019 @default.
• W2063841526 countsByYear W20638415262020 @default.
• W2063841526 countsByYear W20638415262021 @default.
• W2063841526 countsByYear W20638415262022 @default.
• W2063841526 crossrefType "journal-article" @default.
• W2063841526 hasAuthorship W2063841526A5019924930 @default.
• W2063841526 hasAuthorship W2063841526A5033298543 @default.
• W2063841526 hasAuthorship W2063841526A5036116332 @default.
• W2063841526 hasAuthorship W2063841526A5040178560 @default.
• W2063841526 hasAuthorship W2063841526A5043642740 @default.
• W2063841526 hasAuthorship W2063841526A5051995125 @default.
• W2063841526 hasAuthorship W2063841526A5088941449 @default.
• W2063841526 hasBestOaLocation W20638415261 @default.
• W2063841526 hasConcept C104317684 @default.
• W2063841526 hasConcept C126322002 @default.
• W2063841526 hasConcept C127078168 @default.
• W2063841526 hasConcept C128821507 @default.
• W2063841526 hasConcept C130241916 @default.
• W2063841526 hasConcept C134018914 @default.
• W2063841526 hasConcept C170493617 @default.
• W2063841526 hasConcept C185592680 @default.
• W2063841526 hasConcept C187345961 @default.
• W2063841526 hasConcept C188413054 @default.
• W2063841526 hasConcept C202751555 @default.
• W2063841526 hasConcept C2776200302 @default.
• W2063841526 hasConcept C2910266098 @default.
• W2063841526 hasConcept C55493867 @default.
• W2063841526 hasConcept C62478195 @default.
• W2063841526 hasConcept C63932345 @default.
• W2063841526 hasConcept C68484354 @default.
• W2063841526 hasConcept C71924100 @default.
• W2063841526 hasConcept C758759 @default.
• W2063841526 hasConcept C86339819 @default.
• W2063841526 hasConcept C86803240 @default.
• W2063841526 hasConcept C95444343 @default.
• W2063841526 hasConceptScore W2063841526C104317684 @default.
• W2063841526 hasConceptScore W2063841526C126322002 @default.
• W2063841526 hasConceptScore W2063841526C127078168 @default.
• W2063841526 hasConceptScore W2063841526C128821507 @default.

• next