Peroxisome proliferator-activated receptor alpha (PPAR-alpha) is the major regulator of fatty acid oxidation in the liver. PPARalpha is also the target of fibrate drugs used to treat abnormal plasma lipid levels. PPAR-alpha is a type II nuclear receptor (its subcellular location does not depend on ligand binding). PPAR-alpha forms heterodimers with Retinoid X receptor alpha (RXR-alpha), another type II nuclear receptor. PPAR-alpha is activated by binding fatty acid ligands, especially polyunsaturated fatty acids having 18-22 carbon groups and 2-6 double bonds. The PPAR-alpha:RXR-alpha heterodimer binds peroxisome proliferator receptor elements (PPREs) in and around target genes. Binding of fatty acids and synthetic ligands causes a conformational change in PPAR-alpha such that it releases the corepressors and binds coactivators (CBP-SRC-HAT complex, ASC complex, and TRAP-Mediator complex) which initiate transcription of the target genes. Target genes of PPAR-alpha participate in fatty acid transport, fatty acid oxidation, triglyceride clearance, lipoprotein production, and cholesterol homeostasis.
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The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that belongs to the basic helix-loop-helix/PER-ARNT-SIM family of DNA binding proteins and controls the expression of a diverse set of genes. Two major types of environmental compounds can activate AHR signaling: halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polycyclic aromatic hydrocarbons (PAH) such as benzo(a)pyrene. Unliganded AHR forms a complex in the cytosol with two copies of 90kD heat shock protein (HSP90AB1), one X-associated protein (AIP), and one p23 molecular chaperone protein (PTGES3). After ligand binding and activation, the AHR complex translocates to the nucleus, disassociates from the chaperone subunits, dimerises with the aryl hydrocarbon receptor nuclear translocator (ARNT) and transactivates target genes via binding to xenobiotic response elements (XREs) in their promoter regions. AHR targets genes of Phase I and Phase II metabolism, such as cytochrome P450 1A1 (CYP1A1), cytochorme P450 1B1 (CYP1B1), NAD(P)H:quinone oxidoreductase I (NQO1) and aldehyde dehydrogenase 3 (ALHD3A1). This is thought to be an organism's response to foreign chemical exposure and normally, foreign chemicals are made less reactive by the induction and therefore increased activity of these enzymes (Beischlag et al. 2008).
AHR itself is regulated by the aryl hydrocarbon receptor repressor (AHRR, aka BHLHE77, KIAA1234), an evolutionarily conserved bHLH-PAS protein that inhibits both xenobiotic-induced and constitutively active AHR transcriptional activity in many species. AHRR resides predominantly in the nuclear compartment where it competes with AHR for binding to ARNT. As a result, there is competition between AHR:ARNT and AHRR:ARNT complexes for binding to XREs in target genes and AHRR can repress the transcription activity of AHR (Hahn et al. 2009, Haarmann-Stemmann & Abel 2006).
MED1 is a component of each of the various Mediator complexes, that function as transcription co-activators. The MED1-containing compolexes include the DRIP, ARC, TRIP and CRSP compllexes.
Peroxisome proliferator receptor elements bind heterodimers containing a peroxisome proliferator receptor and a retinoic acid receptor. The consensus sequence is TGAMCTTTGNCCTAGWTYYG.
Peroxisome proliferator receptor elements bind heterodimers containing a peroxisome proliferator receptor and a retinoic acid receptor. The consensus sequence is TGAMCTTTGNCCTAGWTYYG.
The NPAS2 gene is transcribed to yield mRNA and the mRNA is translated to yield protein. Transcription of NPAS2 is enhanced by the RORA:Coactivator complex and repressed by the REV-ERBA:Corepressor complex.
The ALAS1 gene is transcribed to yield mRNA and the mRNA is translated in the cytosol to yield precursor protein. The ALAS1 precursor is imported into the mitochodrial matrix where it catalyzes the synthesis of 5-aminolevulinate from glycine and succinyl-CoA as part of heme biosynthesis.
The CYP1A1 gene is transcribed to yield mRNA and the mRNA is translated to yield protein (van der Meer et al. 2010, Rakhshandehroo et al. 2009). The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. AHR acts as a sensor of polyaromatic chemicals, which upon activation, translocates to the nucleus and binds with the aryl hydrocarbon receptor nuclear translocator (ARNT) to transactivate target genes via binding to xenobiotic response elements (XREs) in their promoter regions. AHR targets genes of Phase I and Phase II metabolism, such as cytochrome P450 1A1 (CYP1A1) (Beischlag et al. 2008). AHR is regulated by the aryl hydrocarbon receptor repressor (AHRR, aka BHLHE77, KIAA1234) that inhibits both xenobiotic-induced and constitutively active AHR transcriptional activity in many species (Hahn et al. 2009, Haarmann-Stemmann & Abel 2006).
PPAR-alpha is activated by binding polyunsaturated fatty acids especially those having 18-22 carbon groups and 2-6 double bonds. These ligands bind the C-terminal region of PPAR-alpha and include linoleic acid, linolenic acids, arachidonic acid, and eicosapentaenoic acid. The fibrate drugs are also agonists of PPAR-alpha. Binding of a ligand causes a conformational change in PPAR-alpha so that it recruits coactivators. By analogy with the closely related receptor PPAR-gamma, PPAR-alpha probably binds TBL1 and TBLR1, which are responsible for recruiting the 19S proteasome to degrade corepressors during the exchange of corepressors for coactivators. The coactivators belong to the CBP-SRC-HAT complex (CBP/p300, SRC1, SRC2, SRC3, CARM1, SWI/SNF, BAF60C, PRIC320, and PRIC285), the ASC complex (PRIP/ASC2, PIMT), and the TRAP-DRIP-ARC-MEDIATOR complex (TRAP130, PBP/TRAP220). The coactivators contain LXXLL motifs (Nuclear Receptor Boxes) that interact with the AF-2 region in nuclear receptors such as PPAR-alpha.
In the absence of activating ligands of PPAR-alpha, the PPAR-alpha:RXR-alpha heterodimers recruit corepressors NCoR1, NCoR2(SMRT), and histone deacetylases (HDACs) to genes regulated by PPAR-alpha. The corepressors maintain chromatin at the gene in an inactive conformation and prevent expression of the gene.
Peroxisome proliferator-activated receptor alpha (PPAR-alpha) is a type II nuclear receptor (its subcellular location is independent of ligand binding) related to PPAR-beta/delta and PPAR-gamma. PPAR-alpha is expressed highly in the liver where if functions to control lipid metabolism, especially fatty acid oxidation. PPAR-alpha forms heterodimers with Retinoid X receptor alpha (RXR-alpha). The heterodimers bind peroxisome proliferator receptor elements (PPREs) in and around genes regulated by PPAR-alpha.
The PPARA gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, BMAL1:CLOCK (ARNTL:CLOCK) heterodimers bind the scond intron of the PPARA gene and activate transcription of PPARA.
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AHR itself is regulated by the aryl hydrocarbon receptor repressor (AHRR, aka BHLHE77, KIAA1234), an evolutionarily conserved bHLH-PAS protein that inhibits both xenobiotic-induced and constitutively active AHR transcriptional activity in many species. AHRR resides predominantly in the nuclear compartment where it competes with AHR for binding to ARNT. As a result, there is competition between AHR:ARNT and AHRR:ARNT complexes for binding to XREs in target genes and AHRR can repress the transcription activity of AHR (Hahn et al. 2009, Haarmann-Stemmann & Abel 2006).
dimer:12Zn2+:HMGCR
geneProliferator Receptor Element
(PPRE)Annotated Interactions
dimer:12Zn2+:HMGCR
geneProliferator Receptor Element
(PPRE)Binding of a ligand causes a conformational change in PPAR-alpha so that it recruits coactivators. By analogy with the closely related receptor PPAR-gamma, PPAR-alpha probably binds TBL1 and TBLR1, which are responsible for recruiting the 19S proteasome to degrade corepressors during the exchange of corepressors for coactivators. The coactivators belong to the CBP-SRC-HAT complex (CBP/p300, SRC1, SRC2, SRC3, CARM1, SWI/SNF, BAF60C, PRIC320, and PRIC285), the ASC complex (PRIP/ASC2, PIMT), and the TRAP-DRIP-ARC-MEDIATOR complex (TRAP130, PBP/TRAP220). The coactivators contain LXXLL motifs (Nuclear Receptor Boxes) that interact with the AF-2 region in nuclear receptors such as PPAR-alpha.
PPAR-alpha forms heterodimers with Retinoid X receptor alpha (RXR-alpha). The heterodimers bind peroxisome proliferator receptor elements (PPREs) in and around genes regulated by PPAR-alpha.