Regulation of P450. b. competitive-inhibitor but not substrate. c. non-competitive inhibition. 2. Covalent binding to heme or protein

Transcription

1 10. Inhibition of P450 Regulation of P450 a. competitive-2 chemicals metabolized by same isoform b. competitive-inhibitor but not substrate c. non-competitive inhibition 11. Induction of P Non-covalent binding to heme; macrolide antibiotics 2. Covalent binding to heme or protein a. induction less important than inhibition b. usually leads to protection c. may lead to tolerance d. chemicals that are resistant to B/T can cause prolonged induction e. Adverse effects 1. porphyria f. classes of inducers 1. 3MC---->20 fold increase in CYP1A1 and 1A2 2. transcriptional activation---->mrna stabilization---> increased protein syn. 3. without inducer, transcription normally repressed

2 4. Aryl hydrocarbon receptor --normally complexed in 1:2 ratio with HSP90 which dissociates upon binding with ligand allowing phosphorylation by tyrosine kinase. --activated receptor enters nucleus and forms a heterodimer complex with Ah-receptor nuclear translocator (ARnT). --inside nucleus Ah-receptor/ARnT complex undergoes phosphorylation by PKC, then binds DRE/XRE [5'-TXGCGTG-3']-located up to 1000 bp from initiation start site. (multiple copies) --similar to steroid receptor except SR has zinc finger binding domains and form homodimers, whereas Ah-ARNT is heterodimer with a basic Helix-Loop-Helix domain near N-terminus. basic region binds DNA

3 a. in mice Ah receptor is encoded by single gene with 4 allelic variants 1. Ah d (104 kd) low afffinity (DBA/2; AKR and 129) "nonresponsive" 2. 3 high affinity receptors a. Ah b-1 (95 kd)--c57 b. Ah b-2 (104 kd)--balb/c C3H and A c. Ah b-3 (105 kd)--molf/ei b. regulation by other receptors 1. 4S-binding protein = glycine methyltransferase 2. GRE in CYP1A1 3. ARE in DT diaphorase 5'-GTGACAAAGC-3' a. regulates UGT1.6; ALDH (class 3); GST A1 (Y α ) b. not regulated by AP1 (dimerized jun and fos proteins) c. CYP1A2 also has XRE 1. induction occurs at lower doses of inducer 2. involves RNA stabilization 3. requires hepatic growth factors (not inducible in extrahepatic tissues) 2. Phenobarbital-inducible (CYP2B1/2) a. inducers drugs, OC pesticides, food additives (BHT), polyhalogenated aromatics (HCB) b. 2-4 fold increases in CYP2A1, 2C6, 3A % decrease in CYP2C % increase in cytochrome b5 and NADPH-p450 reductase epoxide hydrolase aldehyde dehydrogenase GST and UDPGT

4 c. mechanism 1. no specific reaction; or receptor; or structural consistency 2. Barbie Box (15 bp enhancer element)--5'aaag-3' --in absence of inducer a repressor protein binds the Barbie bos and impedes transcription --in presence of inducer--2nd TF increases transcription --conserved in bacteria 3. PCN-inducible--CYP3A1/2 only in rats a. inducers--dexamethazone, macrolides, imidazoles b. CYP3A2 contains 3 Barbie boxes (induced by PB) c. mechanism 1. induction not mediated by glucocorticoid receptor a. stabilization of protein by macrolide b. inhibition of camp-dependent phosphorylation at Ser Isoniazid inducible 2-5 fold increase in CYP2E1 a. other inducers, ETOH, acetone, pyrazole, pyridine, ketoconazole, fasting, uncontroled diabetes (10 fold increases) b. mechanism 1. stabiliztion of protein against degradation 2. stabiliztion of mrna 3. increased efficiency of mrna translation c. inducer-dependent regulation 1. diabetes--mrna stabilization 2. fasting-increased transcription 3. acetone blocks camp-dependent phos. at Ser Fibric Acid-inducible CYP4A1/2/3 in male rats

5 a. inducers-- perflourodecanoic acid; phthalate esters; 2,4 D; hypolipidemic drugs; aspirin (NSAIDs) b. mechanism 12. Sex differences 13. Assays 1. PP activation receptor (PPAR) is TF a. member of steroid receptors b. regulates fatty acyl CoA oxidase; bifunctional enzyme; and fatty acid binding protein 2. binding to PPAR results in formation of heterodimer with Retenoid X Receptor (activated by 9-cis-retinoic acid) 3. Ligand bound heterodimer of PPAR/RXR binds PPRE 4. response element contains a DR1 sequences PuGGTCA N PuGGTCA 5. Steroselective binding to PPAR 6. Androsterone and AA are physiological ligands for PPAR a. Male specific (rats) CYP2A2, 2C11, 2C13, 3A2, 4A2 b. Female specific CYP2C12 1. increased in female CYP2A1, 2C7, 2E1 c. differences due to Growth Hormone d. no marked sex differences in dogs, monkeys or humans CYP1A--EROD, MROD CYP2B-PROD, benzyloxyrod CYP2C11-- 2α-OH of testosterone CYP2A1--7α-OH of test CYP2B1/2--16β-OH of test. CYP3A1/2 2β, 6β,15β-OH of test CYP2E1 p-nitrophenol--oh; aniline 4-OH; chlozoxazone 6-OH CYP4A 11 or 12 OH of lauric acid