ENVR 132/TOXC 142/BIOC142Biochemical & Molecular ToxicologyInduction of Metabolism by ToxicantsInstructor:Edward LeCluyse, Ph.D.e-mail: [email protected]: Definitions and Principles• The process of increasing the amount or theactivity of a protein.• A homeostatic mechanism for regulating enzymeproduction in a barrier organ, such as the liver,intestine, kidney.• In enzymology, an inducer usually combines withand deactivates/activates a regulatory proteinwhich leads to increased gene expression.P450 Enzyme Induction• Induction can cause marked increases in P450composition (>20-fold) and chemical clearance orbioactivation.• As a result, induction can increase tolerance to sometoxicants while enhancing the toxicity of others.• Induction can decrease the therapeutic effect of drugsby increasing the rate and pattern of metabolism.• Xenobiotics are known to induce enzymes that play amajor or no role in their biotransformation (e.g.,omeprazole vs. ethanol).Internal Exposure to Natural andMan-made Chemicals• drugs• industrial chemicals• pesticides• pollutants• alkaloids• cigarette smoke• cruciferous vegetables(indole-3-carbinol)• secondary plantmetabolites• toxins produced bymolds, plants, andanimals• pyrolysis products incooked foodTypes of P450 Inducers• Many “prototypical” inducers of specificfamilies or subfamilies of P450 enzymes– CYP1A inducers: 3-MC, BNF, omeprazole, TCDD– CYP3A inducers: rifampin, dexamethasone,troglitazone– CYP2B inducers: phenobarbital, PCBs, phenytoin– CYP2E1 inducers: ethanol, isoniazid• Some overlap in “specificities” of inducers• An inducer for one family of enzymes mayalso suppress another family (e.g., BNF)Induction of Rat Liver P450 Enzymes byPrototypical Inducers In VivoCLOPCNPBBNFInducer10,693 ± 620489 ± 52CYP4A12,693 ± 2,2552,460 ± 780CYP3A1,460 ± 18024 ± 4CYP2B3,320 ± 183152 ± 27CYP1AInduced ActivityControl ActivityIn Vivo Induction in Male RatsP450 EnzymeCYP1A, EROD; CYP2B, PROD; CYP3A, testosterone 6β-hydroxylation;CYP4A, lauric acid 12-hydroxylation.Induction and Inhibition of P450 in Mice Treatedwith PB or SKF525A: [14C-methyl]aminopyrineUnwanted Side-effects ofEnzyme Induction• Failure of drug therapy• Drug tolerance with auto-induction• Xenobiotic toxicity potentiated• Chemical carcinogenesis potentiated• Endogenous substrate metabolism perturbed• Proliferation of cellular ER and peroxisomesSerrum triazolam (ng/ml)Rifampin Effects on Triazolam DispositionVillikka et al., Clin Pharmacol Ther 1997;61:8-14. Rifampin PlaceboConsequences of CytochromeP450 Enzyme Induction• Increased toxic effect– Acetaminophen Alcohol, 3-MC– Bromobenzene, CCl4Phenobarbital• Increased bioactivation– Cyclophosphamide Macrolides, pesticides• Increased tumor formation– Altered disposition of endogenous substrates• Altered cellular and physiological function– proliferation of peroxisomes and SER– increased liver weight– endocrine disruption• Porphyria, chloracne– PCDDs, azobenzenes, biphenyls (PCBs), naphthaleneEffects of Inducers on Rodent LiverPhysiology and FunctionAcetaminophen Metabolism and Toxicity~60%~35%CYP2E*CYP1A CYP3A NAPQIN-acetyl-p-benzoquinone imine*induced by ethanol, isoniazid, phenobarbital Protein adducts,Oxidative stress,ToxicityEndocrine DisruptionUGT1AMolecular Mechanisms of P450Enzyme InductionMechanisms of P450 Induction• Receptor-mediated transcriptional activation– Receptor• A macromolecule with which a hormone, drug, or otherchemical interacts to produce a characteristic effect.– Two key features:• chemical recognition• signal transduction– Ligand: A chemical that exhibits specific binding toa receptor.• mRNA stabilization• Protein stabilizationEnzyme InductionGeneral mechanism of enzyme inductionproteinactivitymRNAGene transcriptionDrugNuclear ReceptorDR cytosol DR nucleusPhase1Phase 2 transportersEnzyme InductionGeneral mechanism of enzyme inductionproteinactivitymRNAGene transcriptionXNuclear ReceptorXR cytosol XR nucleusPhase1Phase 2 transporterscytoplasmnucleusHepatocyteCo-regulation of Target Genes by NR’s• Complementary roles of NR’s in protection againstxenobiotic exposure.• Increased expression of the hepatic genes involved indrug metabolism and excretion (e.g., CYP’s, UGT’s,GST’s, transporters).• These target genes represent redundant but distinctlayers of defense.• There are distinct and overlapping species differencesin response to activators of NR’s.Receptors Involved in the Regulation ofCYP Gene ExpressionModified from Kast, H. R. et al. J. Biol. Chem. 277:2908-2915, 2002Coordinate Regulation of P450’s, UGT’s andTransporters by NR’sUGT’sMRP3Aryl Hydrocarbon Receptor(AhR)• Aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix (bHLH) protein belonging to the Per-Arnt-Sim (PAS) family of transcription factors• It transcriptionally induces expression of hepaticCYP1A1, CYP1A2, and CYP1B1 , as well asseveral other genes, including some phase IImetabolizing enzymes• AHR ligands include PAHs and TCDDAhR Signaling PathwayCytoplasmNucleus9090XAhRLL9090XL9090XLL9090XLorArntFrom: Anne Mullen, Advanced Pharmacology, McMaster University, Ontario, CAAhR Signaling PathwayXRE promoter gene(CYP1A1)TranslationIncreased expressionCYP1A1 proteinIncreased expression ofother gene products+AhR/ArntheterodimermRNAIC+AminoCarboxyAF-1 DBD LBD AF-2Modulators interactwith some cofactorsBinding to responseelements of target genesLigand and coactivatorbinding pocketsTranslocaseactivity5’ 3’ 5’ 3’ 5’ 3’Monomers RXR Heterodimers HomodimersLBDDBDNR-LBDRXR-LBDDBD DBD DBD DBDNR-LBD NR-LBDRORTLXERRNGFI-BPXRCARPPARLXRFXRRARGRERRXRCOUP-TFHNF4Rev-ErbGCNFNuclear Hormone Receptors5’ 3’ nnn DRn IRn ERnCYP2B Response elementsCYP2B6 TGTACT n=4 TGACCCCYP2b10 TGTACT n=4 TGACCTCYP2B1 TCTACT n=4 TGACCTCYP2B2 TGTACT n=5 TGACCTNR1sCYP2B6 TGGACT n=4 TGAACCCYP2b10 TCAACT n=4 TGACACCYP2B1 TCAACT n=4 TGACAC CYP2B2 TCAACT n=4 TGACACNR2sNR3CYP2B6 TGGACT n=4 TGACCC CYP3A Response elements CYP3A4 TGAACT n=3 TGACCC CYP3A2 TGACCT n=3 TGAGCT CYP3A23 TGACCT n=4 TGAGTT CYP3A2 TGAACT n=3 TGAACT DRsCYP3A4 TGAAAT n=6 GGTTCACYP3A4 TGAACT n=6 AGGTCACYP3A23 TTAACT n=6 AGGTCACYP3A5 TGAACT n=6 AGGTAACYP3A7 TTAACT n=6 AGGTCACYP3A7 TGAAAT n=6 AGTTCAERs Other Genes