Chemical carcinogenesis

Transcription

1 Chemical carcinogenesis 林 嬪 嬪 研 究 員 國 衛 院 環 職 組 NHRI 學 程, June,

2 Outline - Definition - Multistage carcinogenesis - Mechanisms of action of chemical carcinogenesis - Test systems for carcinogenicity assessment 2

3 Definition Carcinogen: a physical or chemical agent that causes or induces neoplasia. Genotoxic: carcinogens that interact with DNA resulting in mutation Nongenotoxic: carcinogens that modify gene expression but do not damage DNA Casarett & Doull s Toxicology: the basic science of poisons. 7 th ed. 3

4 Multistage carcinogenesis 1. Initiation (genotoxic) Genetic changes (mutation and deletion) 2. Promotion (non-genotoxic) Involves the selective clonal expansion of initiated cells to produce a preneoplastic lesion; nongenotoxic 3. Progression (genotoxic) Additional DNA damage (chromosomal aberration and translocation) 4

5 Anais da Academia Brasileira de Ciências (2007) 79(4):

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7 Initiation Nature Reviews Cancer 2005, 5, 113 7

8 Features of genotoxic and nongenotoxic carcinogens Genotoxic carcinogens Mutagenic Can be complete carcinogen Tumorigenicity is dose dependent No theoretical threshold Nongenotoxic carcinogens Nonmutagenic Threshold, reversible Tumorigenicity is dose dependent May function at tumor promotion stage No direct DNA damage Species, strain, tissue specificity Nature Reviews Cancer 2005, 5, 113 8

9 Genotoxic carcinogens: initiated tumors by producing DNA damage. Direct-acting carcinogens: do not require metabolic activation or chemical modification to induce cancer. For example, nitrogen mustard, some chemotherapeutic chemicals. Indirect-acting genotoxic carcinogens: require metabolic activation (from procarcinogen, proximate carcinogen, to ultimate carcinogen). For example, benzo[a]pyrene, alfatoxin B1, NNK. 9

10 Summary of events involved in chemical carcinogenesis 10

11 Metabolism is required for some carcinogens (initiator) activation Anais da Academia Brasileira de Ciências (2007) 79(4):

12 Phase I and phase II enzymes 12

13 Oxidation catalyzed by cytochrome P450 13

14 Metabolic activation of carcinogens Procarcinogen Proximate carcinogen Ultimate carcinogen 14 Nature Reviews Cancer 2005, 5, 113

15 Example of phase II reaction : glutathione conjugation (by glutathione-s-transferase) Acetyl 15

16 Metabolic activation and detoxification (glucuronic acid conjugation) of tobacco-specific nitrosamine: NNK Activation by CYP2A Detoxification by conjugation 16

17 H Covalent binding between ultimate carcinogens and endogenous macromolecules Ultimate carcinogens: electrophiles Endogenous macromolecules (DNA, RNA, protein and lipids): nucleophiles.... O H Pauling Electronegativity Scale H Li Be B C N O F.... F : Na Mg Al Si P S Cl Electronegativity increases from left to right in the periodic table. Electronegativity decreases going down a group. 17

18 In general, soft electrophiles prefer to react with soft nucleophiles (low charge-toradius ratio in both). Whereas hard electrophiles react more readily with hard electrophiles. 18

19 Covalent binding with DNA: AFB1 and AAF 19

20 Covalent binding with DNA: BaP 20

21 One cell cycle is necessary to lock in mutation: G to T transversion mutation following BPDE-DNA adduct formation 21

22 Structures of 8-oxoG-containing base pairs and of several nucleosides of guanine oxidation products Missmatched! 22 Nature, 447, (21 June 2007)

23 Metabolic activation of carcinogens and cancer risk 23

24 Metabolic activation of benzo[a]pyrene Activator: cytochrome p4501a1 (CYP1A1) 24

25 CYP1A1 SNP Low activity High activity Allele Protein Nucleotide changes Trivial name Effect CYP1A1*1A CYP1A1.1 None Wild - type CYP1A1*1B CYP1A C>T CYP1A1*1C CYP1A G>A CYP1A1*2A CYP1A T>C ( Msp I) m1 CYP1A1*2B CYP1A A>G; 3801T>C ( Msp I) I462V CYP1 A1*2C CYP1A A>G m2 I462V CYP1A1*3 CYP1A T>C m3 CYP1A1*4 CYP1A C>A m4 T461N CYP1A1*5 CYP1A C>A R464S CYP1A1*6 CYP1A G>T M331I CYP1A1*7 2346_2347insT frame shift CYP1A1*8 CYP1A T>A I448N CYP1A1*9 CYP1 A C>T R464C CYP1A1*10 CYP1A C>T R477W CYP1A1*11 CYP1A C>G P492R 25

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28 Low ratio of NNAL-Gluc/free NNAL increases urothelial carcinoma risk Individual susceptibility (metabolic index): Total free (NNAL-Gluc)/free NNAL Detoxification index: Total free (NNAL-Gluc) /free NNAL Cancer risk 28

29 Common forms of DNA damage Abasic site Strand breaks (single and double) DNA cross-link Hydoxylated (oxidized ) base Bulky adducts 29

30 DNA repair Base excision repair Nucleotide excision repair 30

31 DNA substitution mutations are of two types Transitions are interchanges of two-ring purines (A G) or of one-ring pyrimidines (C T). Transversions are interchanges of purine for pyrimidine bases. 31

32 The presence of 8-oxoG (O) in DNA causes G-to-T transversions O: 8-oxoG The actions of AP endonuclease, deoxyribophosphate lyase, DNA polymerase and DNA ligase 8-oxoG DNA glycosylase DNA glycosylase MutY 32 Nature, 447, (21 June 2007)

33 Tumor promotion AhR: aryl hydrocarbon receptor 33 Nature Reviews Cancer 2005, 5, 113

34 Nongenotoxic carcinogens: not involve direct binding, damage, or interaction of the chemical or its metabolites with DNA. Proposed mode of action: Cytotoxicity, Receptor mediated : CAR, PPARa, and aryl hydrocarbon receptor (AhR), Hormonal, Epigenetic change, Oxidative stress 34

35 Pathogenesis of preneoplastic lesions leading to hepatocellular carcinoma in animal models. DENA: diethylnitrosamine (initiator) Arch Toxicol (2008) 82:

36 Receptor mediated : CAR, PPARa, and aryl hydrocarbon receptor (AhR) CAR, PXR, PPAR, LXR, and FXR belong to NR1 (a nuclear receptor family): share a common heterodimerization partner, retinoid X- receptor (RXR); Highly expressed in liver 36

37 Ligand-mediated PPAR activation Uncertainty: 1.Species difference in sensitivity to PPs exists, with humans appearing to be more resistant to the toxic effects of these compounds. 2.The peroxisome proliferation and regulation of peroxisomal enzymes do not appear to be as highly inducible in the human liver Peroxisome proliferators are classified as rodent-specific hepatocarcinogens.

38 AhR (Aryl hydrocarbon receptor; dioxin receptor) - Ligand-activated transcription factor - Transcriptionally regulate cytochrome P450 family 1 1. Location: chromosome 7 2. Protein Size: 105 KDa 3. mrna size: 6.6Kb 38

39 AhR Activation Mechanism 39 From Wikipedia,

40 Hypothetical model of the role of AhR in tumor initiation, promotion and progression. Carcinogenesis,

41 CYP1A1 involves in metabolic activation of benzo[a]pyrene Benzo[a]pyrene carcinogenicity is lost in mice lacking the aryl hydrocarbon receptor (Shimizu et al., PNAS, 2000) 41

42 Epithelial to mesenchymal transition (EMT) One consequence is sequestration of b-catenin, thereby preventing its nuclear translocation. Over-expression of master regulators of EMT, such as Snail and Slug (and others) lead to down-regulation of E-cadherin, hence allowing nuclear translocation of b-catenin. In association with transcription factors (TF) of the TCF/LEF family, b-catenin induces transcription of proliferative and mesenchymal genes 42

43 Potential role of the AhR in EMT and tumor progression NFAT: nuclear factor of activated T cells Autotaxin exhibit lysophospholipase D activity LPA: lysophosphatidic acid Carcinogenesis,

44 Epigenetic changes Numerous potential environmental factors can lead to the formation of CSCs from normal SCs and/or progenitor cells. Anais da Academia Brasileira de Ciências (2007) 79(4): Toxicol Sci March; 120(suppl_1): S192 S

45 Hormonal Estrogens Synthesis, Distribution, Metabolism ER Ligands Metabolites Elimination /or Protection ER-mediated Signal Transduction Cell Differentiation Growth (Spontaneous Mutations) JNCI Monograph, 2000 Non ER-mediated Signal transduction DNA Adducts Ox. DNA Damage Lipid Peroxidation Redox Imbalance Altered Transcription (?) Initiation and Progression of 45 Cancer

46 Oxidative metabolism of estrogens 2-OHCE and 4-OHCE : 2-hydroxy and 4-hydroxy catechol estrogens; COMT : catechol-o-methyltransferase; GSH: glutathione 46 JNCI Monograph, 2000

47 47 JNCI Monograph, 2000

48 Oxidative stress Cancers 2010, 2(2), J Cancer Sci Ther 3:

49 Selected mechanisms responsible for liver tumor promotion in humans and animal model Arch Toxicol (2008) 82:

50 - Test systems for carcinogenicity assessment Method Short term Mutagenesis assays Transformation in cell culture Medium term Qualitative and quantitative analysis of preneoplasia Long term Chronic bioassay in animals Time frame Several weeks 1-3 months 2-8 months months 50

51 Mouse model for skin carcinogenesis and genetic changes Molecular and Biochemical Toxicology 4 th ed

52 The report identifies agents, substances, mixtures, and exposure circumstances that are known or reasonably anticipated to cause cancer in humans. Known To Be Human Carcinogen: There is sufficient evidence of carcinogenicity from studies in humans,* which indicates a causal relationship between exposure to the agent, substance, or mixture, and human cancer. (*This evidence can include traditional cancer epidemiology studies, data from clinical studies, and/or data derived from the study of tissues or cells from humans exposed to the substance in question, which can be useful for evaluating whether a relevant cancer mechanism is operating in humans.) Reasonably Anticipated To Be Human Carcinogen: There is limited evidence of carcinogenicity from studies in humans,* which indicates that causal interpretation is credible, but that alternative explanations, such as chance, bias, or confounding factors, could not adequately be excluded, or there is sufficient evidence of carcinogenicity from studies in experimental animals, which indicates there is an increased incidence of malignant and/or a combination of malignant and benign tumors (1) in multiple species or at multiple tissue sites, or (2) by multiple routes of exposure, or (3) to an unusual degree with regard to incidence, site, or type of tumor, or age at onset, or there is less than sufficient evidence of carcinogenicity in humans or laboratory animals; however, the agent, substance, or mixture belongs to a well-defined, structurally related class of substances whose members are listed in a previous Report on Carcinogens as either known to be a human carcinogen or reasonably anticipated to be a human carcinogen, or there is convincing relevant information that the agent acts through 52 mechanisms indicating it would likely cause cancer in humans.

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55 Agents Classified by the IARC Monographs, Volumes Group 1 Carcinogenic to humans 107 agents Group 2A Probably carcinogenic to humans 63 Group 2B Possibly carcinogenic to humans 271 Group 3 Not classifiable as to its carcinogenicity to humans 509 Group 4 Probably not carcinogenic to humans 1 55

56 Adapted from Table 4 in Cogliano et al. (2011) available at: 56

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