title: 2-Phenylphenanthridinone and related compounds: aryl hydrocarbon receptor agonists and suicide inactivators of P4501A1.
abstract: The effects of several 2-substituted phenanthridinones (2-nitro-, 2-t-butyl-, 2-bromo-, 2-phenyl-, 2-ethyl-, 2-methoxy-, 2-iodo-, 2-n-butyl-, 2-chloro-, 2-trifluoromethyl-, 2-fluoro-, 2-isopropyl-, and 2-methyl) and phenanthridinone as ligands for the rat liver cytosolic aryl hydrocarbon (Ah) receptor were determined using a competitive binding assay and 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) as the radioligand. The competitive binding IC50 values varied from 317 (2-trifluoromethyl-) to 5870 nM (2-methoxyphenanthridinone); the relative low Ah receptor binding affinities for these compounds were paralleled by their weak activity as inducers of ethoxyresorufin O-deethylase (EROD) activity in rat hepatoma H4IIE cells; however, there was not a correlation between their structure-binding and structure-induction relationship. In cells cotreated with 1 nM TCDD plus different concentrations (0.01-10 microM) of the 2-substituted phenanthridinones, several of these compounds inhibited induction of EROD activity by TCDD; 2-t-butyl- and 2-phenylphenanthridinone (2-PP) were the most active compounds, causing a > 80% reduction in the induced response. 2-PP was selected as a model to further investigate the mechanism of this inhibitory response. The results of interactive studies in rat hepatoma H4IIE cells cotreated with 2-PP plus TCDD or [3H]TCDD showed that 2-PP did not inhibit formation of the nuclear Ah receptor complex or induction of CYP1A1 mRNA levels or CYP1A1 protein. In contrast, incubation of 2-PP with either rat hepatoma H4IIE cells treated with TCDD or hepatic microsomes from TCDD-treated rats resulted in a rapid loss of EROD activity. In parallel experiments, [3H]2-PP was incubated with hepatic microsomes from TCDD-treated rats and analysis by denaturing electrophoresis showed that [3H]2-PP formed a covalent adduct with a 50- to 55-kDa protein and thus acted as a suicide inactivator of CYP1A1.
authors.affiliation: Department of Veterinary Physiology and Pharmacology, Texas A & M University, College Station 77843-4466.
authors.forename: H
authors.initials: H
authors.lastname: Liu
authors.forename: M
authors.initials: M
authors.lastname: Santostefano
authors.forename: S
authors.initials: S
authors.lastname: Safe
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D019363:Cytochrome P-450 CYP1A1; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis*; D003600:Cytosol / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects; D007202:Indicators and Reagents; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008114:Liver Neoplasms, Experimental / Q000201:enzymology; D015394:Molecular Structure; D010088:Oxidoreductases / Q000096:biosynthesis*; D010617:Phenanthridines / Q000138:chemical synthesis / Q000494:pharmacology*; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon / Q000037:antagonists & inhibitors*; D013329:Structure-Activity Relationship; D014407:Tumor Cells, Cultured
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007202:Indicators and Reagents; D010617:Phenanthridines; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; C089169:2-phenylphenanthridinone; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D019363:Cytochrome P-450 CYP1A1