ENVR 132 TOXC 142 BIOC142 Biochemical Molecular Toxicology Induction of Metabolism by Toxicants Instructor Edward LeCluyse Ph D e mail edl cellzdirect com Induction Definitions and Principles The process of increasing the amount or the activity of a protein A homeostatic mechanism for regulating enzyme production in a barrier organ such as the liver intestine kidney In enzymology an inducer usually combines with and deactivates activates a regulatory protein which leads to increased gene expression P450 Enzyme Induction Induction can cause marked increases in P450 composition 20 fold and chemical clearance or bioactivation As a result induction can increase tolerance to some toxicants while enhancing the toxicity of others Induction can decrease the therapeutic effect of drugs by increasing the rate and pattern of metabolism Xenobiotics are known to induce enzymes that play a major or no role in their biotransformation e g omeprazole vs ethanol Internal Exposure to Natural and Man made Chemicals drugs industrial chemicals pesticides pollutants alkaloids cigarette smoke cruciferous vegetables indole 3 carbinol secondary plant metabolites toxins produced by molds plants and animals pyrolysis products in cooked food Types of P450 Inducers Many prototypical inducers of specific families 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 may also suppress another family e g BNF Induction of Rat Liver P450 Enzymes by Prototypical Inducers In Vivo In Vivo Induction in Male Rats P450 Enzyme Inducer CYP1A BNF CYP2B PB CYP3A CYP4A Control Activity Induced Activity 152 27 3 320 183 24 4 1 460 180 PCN 2 460 780 12 693 2 255 CLO 489 52 10 693 620 CYP1A EROD CYP2B PROD CYP3A testosterone 6 hydroxylation CYP4A lauric acid 12 hydroxylation Induction and Inhibition of P450 in Mice Treated with PB or SKF525A 14C methyl aminopyrine Unwanted Side effects of Enzyme 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 peroxisomes Serrum triazolam ng ml Rifampin Effects on Triazolam Disposition Rifampin Placebo Villikka et al Clin Pharmacol Ther 1997 61 8 14 Consequences of Cytochrome P450 Enzyme Induction Increased toxic effect Acetaminophen Bromobenzene CCl4 Alcohol 3 MC Phenobarbital 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 naphthalene Effects of Inducers on Rodent Liver Physiology and Function Acetaminophen Metabolism and Toxicity HN COCH3 60 HN COCH3 O O C O2 H OH HO 35 OH CYP2E CYP1A CYP3A N COCH3 OH HN O COCH3 S O3 H induced by ethanol isoniazid phenobarbital Protein adducts Oxidative stress Toxicity O NAPQI N acetyl p benzoquinone imine Endocrine Disruption UG T1 A Molecular Mechanisms of P450 Enzyme Induction Mechanisms of P450 Induction Receptor mediated transcriptional activation Receptor A macromolecule with which a hormone drug or other chemical interacts to produce a characteristic effect Two key features chemical recognition signal transduction Ligand A chemical that exhibits specific binding to a receptor mRNA stabilization Protein stabilization Enzyme Induction General mechanism of enzyme induction Drug DR cytosol DR nucleus Nuclear Receptor Gene transcription mRNA Phase1 Phase 2 transporters protein activity Co regulation of Target Genes by NR s Complementary roles of NR s in protection against xenobiotic exposure Increased expression of the hepatic genes involved in drug metabolism and excretion e g CYP s UGT s GST s transporters These target genes represent redundant but distinct layers of defense There are distinct and overlapping species differences in response to activators of NR s Receptors Involved in the Regulation of CYP Gene Expression Transcription factor Dimerization partner AHR ARNT CAR Examples of ligands Genes Regulated Dioxins non ortho PCBs some PAHs bilirubin etc CYP1A CYP1B GST UGT NQO RXR Phenobarbital PB TCPOBOP chlorinated pesticides ortho PCBs androstanol androstenol inhibits CYP2B CYP3A GST ABC transporters PXR SXR RXR PB ortho PCBs organochlorine pesticides dexamethasone pregnenalone corticosterone bile acids lithocholic acid CYP3A CYP2B CYP7A repression GST ABC transporters PPAR RXR Fibrate drugs phthalate esters linoleic acid arachidonic acid CYP4A CYP7A repression CYP8B LXR LXR RXR Cholesterol 24 S hydroxycholesterol CYP7A ABC transporters LXR FXR RXR Bile acids chenodeoxycholic acid Represses CYP7A CYP8B CYP27A ER ER Structurally diverse xenoestrogens CYP19 Coordinate Regulation of P450 s UGT s and Transporters by NR s UGT s MRP3 Modified from Kast H R et al J Biol Chem 277 2908 2915 2002 Aryl Hydrocarbon Receptor AhR Aryl hydrocarbon receptor AHR is a basic helixloop helix bHLH protein belonging to the PerArnt Sim PAS family of transcription factors It transcriptionally induces expression of hepatic CYP1A1 CYP1A2 and CYP1B1 as well as several other genes including some phase II metabolizing enzymes AHR ligands include PAHs and TCDD AhR Signaling Pathway Nucleus Cytoplasm X 90 90 X X X 90 90 90 90 90 90 L L L AhR L or L Arnt L From Anne Mullen Advanced Pharmacology McMaster University Ontario CA AhR Signaling Pathway AhR Arnt heterodimer Increased expression CYP1A1 protein mRNA Translation IC XRE promoter gene CYP1A1 Increased expression of other gene products AF 1 DBD LBD AF 2 Amino Carboxy Modulators interact with some cofactors Binding to response elements of target genes LBD 5 DBD 3 Monomers NR LBD RXR LBD DBD DBD 5 Ligand and coactivator binding pockets 3 5 RXR Heterodimers ROR TLX ERR NGFI B Translocase activity NR LBD NR LBD DBD DBD 3 Homodimers GR ER RXR COUP TF HNF4 Rev Erb GCNF PXR CAR PPAR LXR FXR RAR 5 3 n n n DRn IRn ERn CYP2B Response elements CYP3A Response elements NR1s DRs CYP2B6 CYP2b10 CYP2B1 CYP2B2 TGTACT n 4 TGACCC TGTACT n 4 TGACCT TCTACT n 4 TGACCT TGTACT n 5 TGACCT CYP3A4 CYP3A2 CYP3A23 CYP3A2 NR2s CYP2B6 CYP2b10 CYP2B1 CYP2B2 TGAACT n 3 TGACCC TGACCT n 3 TGAGCT TGACCT n 4 TGAGTT
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