PHL 472 Chemical Carcinogens Abdelkader Ashour, Ph.D. 4 th Lecture

II.Epigenetic (non-genotoxic) Carcinogens  No direct chemical reactivity with DNA  They are non-mutagenic  Usually act as tumor promoters  There are no common chemical structural features between these chemicals  Their carcinogenic potential is generally lower than that of genotoxic carcinogens Classification of Carcinogens According to the Mode of Action, Based on Reactivity with DNA

Epigenetic Carcinogens, Mechanisms 1.Prolonged stimulation of cell proliferation, via chronic cytotoxicity or increased secretion trophic hormones 2.Inhibition of apoptosis in cells with DNA damage 3.Impairment of DNA-replication fidelity and DNA-repairing machinery 4.Dysregulated gene expression  Altered DNA methylation status in the genes that control cell growth and differentiation 5.Induction of metabolizing enzymes 6.Dysregulated cell signaling via receptor- or non-receptor-mediated pathways 7.Persistent immunosuppression, leading to compromised immunosurveillance 8.Oxidative Stress  Indirect DNA damage  Induction of cell proliferation signaling cascades

Epigenetic Carcinogens, Mechanisms Cell Replication is Essential for Multistage Carcinogenesis  Decreases time available for DNA repair  Converts repairable DNA damage into non-repairable mutations  Necessary for chromosomal aberrations, insertions, deletions and gene amplification  Clonally expands existing cell populations  Examples: Epidermal growth factor, hepatocyte growth factor, estrogens

Epigenetic Carcinogens, Mechanisms Apoptosis  Programmed Cell Death (Apoptosis): Active, orderly and cell-type-specific death distinguishable from necrotic cell death (passive process):  Induced in normal and cancer cells  Non-random event  Result of activation of a cascade of biochemical, gene expression and morphological events  Tissue and cell specific  Growth factors and mitogens inhibit apoptosis

Epigenetic Carcinogens, Mechanisms Alteration of Gene Expression  Nuclear (hormone-like) receptors  Kinase cascades  Calcium-mediated signaling  Transcription factors  Gene methylation status (hypo  enhanced gene expression; hyper  gene silencing)  The next four slides are just for your own information

Intracellular Receptors  These receptors could be cytosolic or nuclear  Several biologic signals are sufficiently lipid-soluble to cross the plasma membrane and act on intracellular receptors.  Examples of such ligands include corticosteroids, mineralocorticoids, sex steroids, vitamin D, and thyroid hormone. They can stimulate the transcription of genes in the nucleus by  binding to nuclear receptors  This binding of hormone exposes a normally hidden domain of the receptor protein, thereby permitting the latter to bind to a particular nucleotide sequence on a gene and to regulate its transcription.  End result is an alteration in gene transcription and therefore protein synthesis  Actions: slow-acting (hours), long lasting

Nuclear Receptors, an example  Mechanism of glucocorticoid action.  A heat-shock protein, hsp90, binds to the glucocorticoid receptor polypeptide in the absence of hormone and prevents folding into the active conformation of the receptor.  Binding of a hormone ligand (steroid) causes dissociation of the hsp90 stabilizer and permits conversion of glucocorticoid receptor to the active configuration.  The active glucocorticoid receptor binds to a particular nucleotide sequence on a gene  altered transcription of certain genes

Kinase-linked Receptors, Activation of Ras following binding of a hormone (e.g., EGF) to an RTK. 1.The adapter protein GRB2 binds to a specific phosphotyrosine on the activated RTK and to Sos, which in turn interacts with the inactive Ras·GDP. 2.The guanine nucleotide – exchange factor (GEF) activity of Sos then promotes formation of active Ras·GTP.  Note that Ras is tethered to the membrane by a farnesyl anchor

Kinase-linked Receptors, Kinase cascade that transmits signals downstream from activated Ras protein 1.Activated Ras binds to the N-terminal domain of Raf, a serine/threonine kinase. 2.Raf binds to and phosphorylates MEK, a dual- specificity protein kinase that phosphorylates both tyrosine and serine residues. 3.MEK phosphorylates and activates MAP kinase, another serine/threonine kinase. 4.MAP kinase phosphorylates many different proteins, including nuclear transcription factors, that mediate cellular responses.

Chemical Carcinogens, Representative Members

I.Interaction with DNA  A great body of information indicates that interaction with DNA is the critical factor in chemical carcinogenesis.  Several distinct sorts of data have been gathered. Relevant findings are as follows: 1.In general, carcinogens are mutagens, indicating that they have the potential to interact with DNA. 2.Within groups of related carcinogenic chemicals, carcinogenic potency correlates best with ability to interact with DNA. 3.Patients with DNA repair defects, such as xeroderma pigmentosum (defect in repair of damage induced by UV and bulky aromatic chemicals), have increased incidence of cancer. Modifying Factors in Chemical Carcinogenesis

II.Environment: The most impressive feature of cancer epidemiology is a high degree of geographic variability in the incidence of specific forms of cancer. This can easily be seen if one compares incidences between countries or between regions within a country III.Genetic factors: They influence some specific cancers, this influence is a major one. The sorts of genetic involvement which have been described are: 1.Single gene - probably directly involved in carcinogenesis. Example: retinoblastoma. 2.Single gene - predisposes to cancer. Example: xeroderma pigmentosum, a DNA repair defect 3.Familial predisposition, probably polygenic. Example: increased incidence of breast cancer in women whose mother or sister have had breast cancer Modifying Factors in Chemical Carcinogenesis  Environment vs. Genetic factors :  Some of the most productive studies that have been used were analyses of changes in cancer incidence occurring when groups of people emigrate from one country to another  In such studies (next slide), genetic factors are essentially held constant, and effects of environment can be observed  In most cases, dramatic changes in cancer incidence are seen in the immigrant populations, and such changes generally lead to a cancer incidence similar to that of the natives in the immigrants' new homeland

IV.Biological behaviors of the chemical carcinogen: 1.Site of action: Chemicals can act both locally and distally, e.g., benzo(a)pyrene painting causes skin tumor, whereas DMBA painting causes tumors of the skin and breast and also leukemia 2.Tissue responsiveness: There appears to be a great variation in tissue responsiveness  2-naphthylamine  bladder tumor; urethane  lung tumor; zinc  testis tumor; tin and nickel  sarcoma, etc 3.Species specificity: 2-naphthylamine causes bladder cancer in man, dog and hamster, but only liver cancer in mouse and no effect in rats. 4.Sex specificity: Hepatocarcinogens are more effective in male rats  Female reproductive history: Late age at first pregnancy is associated with enhanced risk of breast cancer, while zero or low parity is associated with increased risk of ovarian cancer 5.Age: Many carcinogens are ineffective as transplacental carcinogens at preimplantation but more effective after organogenesis begins, and more so at postnatal life before immune system develops Modifying Factors in Chemical Carcinogenesis

IV.Biological behaviors of the chemical carcinogen: 6.Diet: Diet greatly influences the effect of carcinogens e.g., caloric restriction in general reduces cancer incidence (and vice versa). Phenylalanine- and cysteine-deficient diets reduce breast cancer in mice. Azodye induced liver tumors in rats are enhanced in the presence of vitamin B 6 but decrease in the presence of B 2  The most common mechanism of diet-associated carcinogenesis in humans is the action of major dietary constituents (mainly fat and carbohydrate) as promoting agents 7.Dose responsiveness. Carcinogen effect also appears to be dose dependent, additive and irreversible. Large single dose or fractional doses appear to induce the same incidence of tumors 8.Latency. Carcinogenesis requires time. The latent period could be shortened by means of large doses, but a certain minimum period called the "absolute minimum period of latency" is required  The long latent period raises the question of whether factors other than true carcinogens might act during the latent interval  Both in vivo and in vitro results suggest that transient short exposure to carcinogen causes irreversible changes, but this must be followed by several cell divisions before neoplastic cells become detectable

Modifying factors in chemical carcinogenesis V.Life style  Unhealthy lifestyle habits such as: excess alcohol consumption; inhalation of tobacco and related products; the ingestion of certain foods and their contamination by mycotoxins (such as aflatoxin B 1 ; a complete carcinogen); are responsible for higher incidences of certain types of neoplasias in a number of population groups VI.Immune system  Immune system may have a protective role in tumor development (i.e., preventing tumor formation)  Small accumulations of tumor cells may develop and because of their possession of new antigenic potentialities provoke an effective immunological reaction with regression of the tumor  Mice with induced immunodeficiencies showed a high susceptibility to virally induced tumors and a greater tendency to develop spontaneous lymphomas compared with immunocompetent mice  At the same time, the immune system also may function to promote or select tumor variants with reduced immunogenicity, thereby providing developing tumors with a mechanism to escape immunologic detection and elimination.  This is called: tumor-sculpting actions of the immune system on developing tumors

Modifying factors in chemical carcinogenesis VII.Inflammation  Inflammation caused by uncertain aetiology (e.g. ulcerative colitis, pancreatitis, etc) is one the modifying factors in chemical carcinogenesis  Inflammation orchestrates the microenvironment around tumours, contributing to proliferation, survival and migration.  Cancer cells use selectins, chemokines and their receptors for invasion, migration and metastasis.  On the other hand, many cells of the immune system contribute to cancer immunology, suppressing cancer