Presentation on theme: "Oral benzo[a]pyrene, immunosuppression, and tumors: role of the three CYP1 enzymes Daniel W Nebert, MD Department of Pediatrics, Division of Human Genetics."— Presentation transcript:
Oral benzo[a]pyrene, immunosuppression, and tumors: role of the three CYP1 enzymes Daniel W Nebert, MD Department of Pediatrics, Division of Human Genetics Center for Environmental Genetics Department of Environmental Health University of Cincinnati Medical Center Boston University, March 5, 2oo7
OUTLINE of the TALK Route-of-administration, dose, target organ, and cell-type-specific gene expression (including metabolism) are all critical in environmentally-caused malignancies CYP1 inducibility also in humans = AHR Paradoxical studies in knockout mice The human CYP, mouse Cyp superfamily Intro: the [Ah] gene battery in the mouse
Signal Received by cell Response Polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene (BaP) Received by the cell Induction of an enzyme that metabolizes the stimulus Tryptophan Received in bacteria Induction of tryptophan pyrrolase 80,000-fold PHENOTYPE PERTURBATION
50 25 75 LACK of CYP1A1 induction autosomal recessive “Resp” “Nonresp” 50 2575 3MC was one of eight PAHs tested
Genetics of “Ah-responsiveness” Ah b Ah b x Ah d Ah d F 1 Ah b Ah d Ah b Ah d x Ah b Ah b Ah b Ah b : Ah b Ah d Ah b Ah d x Ah b Ah d F 2 Ah b Ah b : Ah b Ah d : - Ah b Ah d : Ah d Ah d Ah b Ah d x Ah d Ah d Ah b Ah d : Ah d Ah d
B6-D2 Difference in CYP1A1 Inducibility: the Shot Heard ’Round the World Has resulted in >400 publications by DwN Lab Top 1% “most cited” in pharmacology and toxicology field by ISI––since 1st survey 1964-78 Numerous national and international awards Hundreds, if not thousands, of labs have also entered AH receptor/CYP1 field of research
Metabolic activation Detoxication CYP1’s XME Receptors Xenobiotic-Metabolizing Enzymes (XMEs) 1968 - 99: CYPs are BAD
B6-D2 Difference in CYP1 Inducibility: PAH-induced in utero lethality; teratogenesis PAH-induced malignancies of certain tissues, following various routes-of-administration (ROAs) PAH-induced mutagenesis (Ames test) PAH-induced marrow toxicity; immunosuppression PAH-induced athersclerosis; resistance to EtOH Basis for identifying the AH receptor PAH-induced ovarian toxicity; uroporphyria –– Crit Rev Toxicol 1989; 20: 153
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; “dioxin”) TCDD is poorly metabolized, ~36,000 times more potent than BaP in inducing rat liver CYP1A1 activity (A. Poland, 1972)
This 1974 study says the Cyp1 structural genes same, but regulatory gene (a receptor?) is different? In 1994 study, B6 high-affinity AHR: Pro-474; D2 poor-affinity AHR: Leu-474 “Ah-Resp” “Ah-NonResp” ED 50 15-20X different
SEVEN TO TEN OUT OF EVERY 100 CIGARETTE SMOKERS DEVELOP LUNG CANCER WHY DON’T THE OTHER 90%+ SMOKERS DEVELOP LUNG CANCER..?
>50-fold difference in lung cancer susceptibility between cigarette smokers of two ethnic groups..!!
Could this be like cigarette smokers with lung cancer? Could this be like cigarette smokers who don’t get cancer? Cigarette-pack-years? Highly Sensitive (HS) Highly Resistant (HR)
Dissociation constant K d = TCDD-binding affinity, det’d by Scatchard plot analysis of HUMAN placental cytosolic samples (N=115) N= 37 N = 78 Highest affinity Poorest affinity Differences in affinity also at least 15-20X
DetoxicationMetabolic activation 1979 [Scheme, of course, fashioned after what was known of the ER]
Definition of EICOSANOID: “Any of the many dozens of physiologically active substances derived from arachidonic acid––including the prostaglandins, leukotrienes, prostacyclins, and thromboxanes ––involved in many critically important life functions”
(COX-1) (COX-2) There are >115 eicosanoids EpoxyEicosaTrienoic acids HydroxyEicosaTriEnoic HydroPeroxyEicosaTetraEnoic prostaglandins, prostacyclins, thromboxanes, leukotrienes
PROPERTIES OF AH RECEPTOR Affects many pathways ( EGFR, PKC, p21 RAS, MAPK, Src, Wnt/ -catenin, Myc, Myb, Fos, p27, p53, RB1 binding, slowing at G 1 /S & G 2 /M boundaries ) Ubiquitous; expressed in utero, placenta ( even in mollusk, sea squirt, Drosophila, Caenorhabditis elegans ) Ahr(-/-) k.o. mouse––lower viability, fertility; defective ( A-V ) vasculature in liver, heart, kidney Quite likely that AHR uses various endogenous ligands (ELs) in different cell types
PROPERTIES OF CYP1A1 Constitutive activity nil; PAH-inducible ; metabolizes PAHs Ubiquitous; expressed in utero, even 12-h ovum Cyp1a1(-/-) knockout mouse viable, fertile No mutants of Cyp1a1 gene alter PAH activity ( steroid hydroxylases? eicosanoid metabolism? )
PROPERTIES OF CYP1A2 Not detectable until neonatal period High basal levels in liver; PAH-inducible in liver, lung, brain, GI tract, pancreas; VERY low in spleen, thymus; nil in kidney; metabolizes aryl and alkyl amines Cyp1a2(-/-) knockout mouse viable, fertile In human, no DNA variant in CYP1A2 gene so far can explain >60X differences in liver
PROPERTIES OF CYP1B1 High basal levels in blood vessels, GI tract, skin, all endocrine tissues, spleen, marrow, thymus, tumors; PAH-inducible; metabolizes PAHs Expressed in placenta; in utero ( adrenal cortex ) Cyp1b1(-/-) knockout mouse viable, fertile (Glaucoma, when combined with ablation of Tyr gene ) Mutations in human CYP1B1 gene causes primary congenital glaucoma (buphthalmos)
Hypothesis: If the Ahr gene, or any of the Cyp1a1, Cyp1a2, Cyp1b1 genes were genetically removed, the mice should be protected against chemical substrates that bind to AHR or that each of the enzymes metabolically activates Thanks especially to: Tim Dalton, Shige Uno, Nadine Dragin
Cyp1a1/1a2(-/-) line has been made loxP sites in 3' UTR of Cyp1a1 and Cyp1a2 genes Cyp1a1 -- spacer region -- Cyp1a2 (6.2 kb) (13,456 bp) (6.7 kb) Cyp1a1/1a2(-/-) Cre-loxP excision of ~25 kb; inter -chromosomal..!
Cyp1a1(-/-) Cyp1a1/1b1(-/-) Cyp1a1/1b1(-/-) has greater BaP body burden but shows less toxicity than Cyp1a1(-/-)..!
Cyp1(+/+) Cyp1a1(-/-) 1a1/1b1(-/-) 2.2 + 1.6 56 + 5.7 165 + 46 BaP ( ng/ml ) in whole blood; 5 da oral BaP Cyp1a1(-/-) is 25X and Cyp1a1/1b1(-/-) is 75X more BaP in blood, compared with Cyp1(+/+) wild-type..!!
Cyp1(+/+) Cyp1b1(-/-) Cyp1a2(-/-) Cyp1a2/1b1(-/-) Cyp1a1/1a2(-/-) Cyp1a1(-/-) 1a1/1a2/1b1(-/-) Cyp1a1/1b1(-/-) Oral BaP, 125 mg/kg/day Clinical outcome: Healthy for months, years Die within one month Wild-type phenotype Blood BaP levels (ng/ml) after 5 days feeding: 1.5-6.050-60160-180 Bone marrow, thymus, spleen: NormalNear normal Severe aplastic anemia
Metabolism by CYP1B1 required; GREATER BaP body burden..!! WHY does lack of CYP1B1 revert Cyp1a1(-/-) back to near-normal, to wild-type ? Cyp1a1/1b1(-/-) Cyp1(+/+) No CYP1A1 in GI tract or liver CYP1A1 inducible
CONCLUSIONS (oral BaP) Cyp1a1(-/-) -mediated (oral) BaP problems are largely ablated by lack of CYP1B1 ( in spleen, thymus, bone marrow ) Oral BaP-induced CYP1A1 in GI tract and/or liver is beneficial to the mouse BaP metabolism in vitro or cell culture studies DO NOT reflect what happens in the intact animal receiving oral BaP
GENERAL RULE OF BIG PHARMA Any candidate drug that shows inducibility of CYP1A1/1A2/1B1 (“AHR activation”) is regarded as hazardous, potentially cancer-causing Such candidate drugs––usually abandoned immediately, without further cost to the company
Oral BaP dosages 12.5 mg/kg/day immunosuppression still seen; altered ALT, AST; BaP-DNA adducts; Cyp1a1(-/-) dies within 4-6 months 125 mg/kg/day immunosuppression; Cyp1a1(-/-) die within 1 month 1.25 mg/kg/day immunosuppression still seen ( lymphocytopenia ); BaP-DNA adducts
BaP, 12.5 mg/kg/day die 4-6 mo instead of 1 month; however, at 6-9 weeks, … (!!) GenotypeOutcome Cyp1(+/+) wild-typeHealthy Cyp1a1(-/-) Duodenum/jejunum CA Cyp1a2(-/-)Healthy Cyp1b1(-/-)Healthy Cyp1a1/1a2(-/-) Duodenum/jejunum CA Cyp1a1/1b1(-/-) Preputial gland CA Cyp1a2/1b1(-/-)Healthy Cyp1a1/1a2/1b1(-/-) Preputial gland CA
Unique “duodenal intraepithelial neoplasm” (DIN) in lack of CYP1A1 but presence of CYP1B1
But: sometimes invasive; sometimes also in proximal jejunum
BaP, 12.5 mg/kg/day die 4-6 mo instead of 1 month; however, … (!!) GenotypeOutcome Cyp1(+/+) wild-typeHealthy Cyp1a1(-/-) Duodenum/jejunum CA Cyp1a2(-/-)Healthy Cyp1b1(-/-)Healthy Cyp1a1/1a2(-/-) Duodenum/jejunum CA Cyp1a1/1b1(-/-) Preputial gland CA Cyp1a2/1b1(-/-)Healthy *Cyp1a1/1a2/1b1(-/-) Preputial gland CA *But, … other problems exist in triple k.o. … … … …
Genetic crosses Total number of pups Observ. number of pups Expectd number of pups 2 value P value aabb aaBb aaBb aabb Aabb AaBb AaBb aaBb Aabb aaBb 2643058.2510.85<0.001 Aabb Aabb aaBb aaBb aabb aabb 11 01.0 Generation of triple-k.o. pups shows embryolethality 7 combinations, female left, male right; A = Cyp1a1/1a2 allele; B = Cyp1b1 Embryolethality – incomplete penetrance; survivors are fertile
Phenotype of Cyp1a1/1a2/1b1(-/-) Hydrocephalus – incomplete penetrance Embryolethality – incomplete penetrance Hermaphroditism – incomplete penetrance Small in size, but some survive, are fertile; smaller litters Cystic ovary – incomplete penetrance
Other details of Cyp1(-/-) triple k.o. Decreased response to i.p. inflammatory challenge by zymosan Metabonomics; cDNA microarray expression: perturbations in “lipid, cell, cation, anion” genes What is the common denominator between alterations in inflammatory response and cation/anion homeostasis?
Arachidonic acid cascade: eicosanoids AHR must have as its E.L. one or several of these >115 eicosanoids CYP1 enzymes must be critical in formation and degradation of specific eicosanoids Perturbations in eicosanoids also consistent with A-V shunts, proliferation, apoptosis, and development ( & toxicity ) of many types
hCYP1A1_CYP1A2 Locus in Bacterial Artificial Chromosomes (BACs) BAC-D: no expression of hCYP1A2 mRNA/protein/enz.activity BAC-H: Normal Expression (basal; inducible) of hCYP1A1 & 1A2 mRNA/protein/enz. activity (liver, lung, kidney, intestine, brain, spleen, pancreas, testis, ovary) 53 kb 23.3 kb 90 kb
Other studies to be published Now have a liver-specific Cyp1a1(-/-) knockout line i.p. versus p.o. BaP Now have the Cyp1a1/1a2(-/-) double and Cyp1a1/1a2/1b1(-/-) triple k.o. oral BaP Now have a GI tract-specific Cyp1a1(-/-) knockout line i.p. versus p.o. BaP Yes, hCYP1A1_1A2 behaves like mouse Cyp1a1/1a2(+/+) i.p. versus p.o. BaP
CONCLUSIONS (of the Entire Talk) “Inducible” CYP1 act. not necessarily bad The intact animal can exhibit different results from in vitro or cell culture studies Route-of-administration, dose, target organ, and cell-type-specific gene expression (including metabolism) are all critical in environmentally-caused malignancies Loss of all three Cyp1 genes, loss of AHR fundamental defects in eicosanoid action