title: Heme and heme biosynthesis intermediates induce heme oxygenase-1 and cytochrome P450 2A5, enzymes with putative sequential roles in heme and bilirubin metabolism: different requirement for transcription factor nuclear factor erythroid- derived 2-like 2.
abstract: Cytochrome P450 2A5 (CYP2A5) oxidizes bilirubin to biliverdin and represents a putative candidate for maintaining bilirubin at safe but adequate antioxidant levels. Curiously, CYP2A5 is induced by both excessive heme and chemicals that inhibit heme synthesis. We hypothesized that heme homeostasis is a key modifier of Cyp2a5 expression via transcription factor nuclear factor erythroid-derived 2-like 2 (Nrf2) and characterized the coordination of CYP2A5 and heme oxygenase-1 (HMOX1) responses using wild-type and Nrf2(-/-) primary mouse hepatocytes. HMOX1 was rapidly elevated by exogenous hemin, thereby limiting the transactivation of Cyp2a5 until high heme (> 5µM) exposure. Nrf2 was mandatory for CYP2A5 but not for HMOX1 induction by heme. CYP2A5 was intensively and HMOX1 moderately elevated in heme synthesis blockades by succinylacetone and N-methyl protoporphyrin IX, and Nrf2 partially mediated the induction of CYP2A5. Immunoelectron microscopy revealed that CYP2A5 is targeted Nrf2 dependently both to the endoplasmic reticulum (ER) and mitochondria. However, excessive heme increased CYP2A5 predominantly in the ER. Phenobarbital, dibutyryl-cAMP, and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) overexpression stimulate heme biosynthesis and induce CYP2A5. Acute but not chronic CYP2A5 induction by phenobarbital required Nrf2, whereas CYP2A5 induction by dibutyryl-cAMP and PGC-1α was potentiated by Nrf2 knockout. Collectively, heme homeostasis is established as a crucial regulator of hepatic Cyp2a5 expression mediated via Nrf2 activation, whereas Nrf2 is redundant for Hmox1 induction by heme. Similar subcellular targeting and coordination of CYP2A5 and HMOX1 responses suggest favorable conditions for enhanced CYP2A5-mediated bilirubin maintenance in altered heme homeostasis that predisposes to oxidative stress.
authors.affiliation: Department of Pharmacology and Toxicology, Institute of Biomedicine, University of Oulu, 90014 Oulu, Finland.
authors.forename: Virpi
authors.initials: V
authors.lastname: Lämsä
authors.forename: Anna-Liisa
authors.initials: AL
authors.lastname: Levonen
authors.forename: Raija
authors.initials: R
authors.lastname: Sormunen
authors.forename: Masayuki
authors.initials: M
authors.lastname: Yamamoto
authors.forename: Jukka
authors.initials: J
authors.lastname: Hakkola
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000096:biosynthesis* / Q000235:genetics; D001663:Bilirubin / Q000378:metabolism*; D065637:Cytochrome P-450 CYP2A6; D000072467:Cytochrome P450 Family 2; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000648:ultrastructure; D004790:Enzyme Induction; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006418:Heme / Q000378:metabolism*; D051547:Heme Oxygenase-1 / Q000096:biosynthesis* / Q000235:genetics; D006427:Hemin / Q000494:pharmacology; D022781:Hepatocytes / Q000378:metabolism*; D006537:Heptanoates / Q000494:pharmacology; D006706:Homeostasis / Q000187:drug effects; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008928:Mitochondria / Q000187:drug effects / Q000648:ultrastructure; D051267:NF-E2-Related Factor 2 / Q000378:metabolism*; D011524:Protoporphyrins / Q000494:pharmacology
chemical_list: D006537:Heptanoates; D051267:NF-E2-Related Factor 2; C495636:Nfe2l2 protein, mouse; D011524:Protoporphyrins; D006418:Heme; C020804:succinylacetone; D006427:Hemin; C028842:N-methylprotoporphyrin IX; D001189:Aryl Hydrocarbon Hydroxylases; C585520:Cyp2a5 protein, mouse; D065637:Cytochrome P-450 CYP2A6; D000072467:Cytochrome P450 Family 2; D051547:Heme Oxygenase-1; D001663:Bilirubin