chemical biologicalcarcinogens csbrp

Bladder Carcinoma

Benzidine

Tobacco leavesBenzpyrenes

Cycasin

Aflotoxin B1

Polycyclic hydrocarbons

Polycyclic hydrocarbons

Percival PottScrotal cancer

Chemical CarcinogensBiological Carcinogens

Dr.CSBR.Prasad, M.D.,

Carcinogens• Agents causing cancer (thro’ genetic damage)

• Classification• Chemical• Physical• Hormonal• Biological

• History:• 18th Century Sir Percival Pott • Scrotal skin cancer in chimney sweepers• Exposure to soot

Chemical Carcinogen

• Synthetic • Naturally occurring• Directly acting (no metabolic activity)

• Indirectly acting (procarcinogens, needs metabolic activity)

• Initiators (irreversible DNA damage)

• Promoters (reversible non DNA damage)

Initiation

• Induction of mutation in genome of cells• Initiated cells are not transformed cells• They have no growth autonomy/unique

phenotype• In contrast to normal cells, can give rise to

tumors when appropriately stimulated by promoting agents

Promotion

• Process of tumor induction in a previously initiated cell by chemicals.

• The effect of promoters is short lived and reversible

• They do not affect DNA• Non tumorigenic by themselves

Chemical CarcinogensInitiation and Promotion

Major Chemical Carcinogens• Direct-Acting Carcinogens

– Alkylating Agents • Anticancer drugs (cyclophosphamide , chlorambucil , nitrosoureas) • β-Propiolactone, Dimethyl sulfate, Diepoxybutane

– Acylating Agents • 1-Acetyl-imidazole, 2-Dimethylcarbamyl chloride

• Procarcinogens That Require Metabolic Activation – Polycyclic and Heterocyclic Aromatic Hydrocarbons

• Benz(a)anthracene, Benzo(a)pyrene, Dibenz(a,h)anthracene, • 3-Methylcholanthrene, 7,12-Dimethylbenz(a)anthracene

– Aromatic Amines, Amides, Azo Dyes • 2-Naphthylamine (β-naphthylamine), Benzidine, 2-Acetylaminofluorene,

Dimethylaminoazobenzene (butter yellow)– Natural Plant and Microbial Products

• Aflatoxin B1, Griseofulvin, Cycasin, Safrole, Betel nuts

• Others • Nitrosamine and amides, Vinyl chloride, nickel, chromium, Insecticides,

fungicides, Polychlorinated biphenyls

Concept of initiation and promotion sequence

• Initiation - exposure of cells to sufficient dose of carcinogen

• Potenial to induce tumor• Initiation alone is insufficient• Permanent DNA damage- mutation.• Rapid and irreversible• Promoters can induce tumor in an initiated

cell, by themselves are not tumorigenic

i

• The cellular changes resulting from application of promoters do not affect DNA directly and are reversible

Property of direct acting & ultimate carcinogens

• Highly reactive electrophiles (electron deficient) react with nucleophilic (electron rich) sites in the cell

• Non enzymatic reactions - covalent adducts b/n carcinogen & nucleotide DNA

• Electron rich sites in cell:– DNA– RNA– Proteins

Epoxides

• From hydrocarbons• Structure of Cyclic ether• Highly reactive chemicals• Can form adducts with many cellular

elements

Adducts

• An adduct (from the Latin adductus, "drawn toward") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components

• Covalent adducts: can damage DNA, RNA and proteins

Metabolic action of carcinogens

• Most carcinogens need activation to form ultimate carcinogens

• Other metabolic pathways detoxify • Carcinogenic potency

– Inherent reactivity of electrophilic derivative– Balance b/n metabolic activation and

inactivation

Metabolic action of carcinogens

• Carcinogenesis is regulated in part by polymorphism in the genes that encode the genes

• Age, sex and nutritional status determine the internal dose of toxicants

Molecular targets of chemical carcinogens

• Mutations affecting oncogenes, tumor suppressor genes and genes that regulate apoptosis and genes involved in DNA repair

• DNA is the primary target• No single or unique alteration with initiation• Each class of carcinogen produces limited pattern

of DNA damage• Each carcinogen produces molecular ‘fingerprint’

that links specific chemical with their mutational effect

Initiated cell

• Unrepaired DNA alterations are essential first step in initiating tumor

• Damaged DNA template must replicate to be inheritable and permanent

• Quiescent cells may never be affected by carcinogens, unless mitogenically stimulated

• Concurrent exposure to viruses, parasites, hormones induce proliferation

Chemical Carcinogen - Mechanism

• Reactive electrophiles– Electron deficient substances– Binds with electron rich portions of cells (DNA)

• Target molecules– DNA – mutation - carcinogenesis

• Initiated cellUnrepaired DNA damage

One cycle of proliferation

Irreversible damage

Vulnerable to promotion

Promotion of Carcinogenesis

• Phenols, artificial sweeteners

• No sudden change• Need sufficient time and dose• Changes reversible• Enhance the effect of carcinogens• Acts through growth factor pathway

Promotion of carcinogenesis

• Tumor promotion steps- multiple steps. Proliferation of preneoplastic cells Malignant conversion Tumor progression (depends on cells &

stroma)

Promotion of carcinogenesis

• The effect of promoters are pleiotropic• Induction of cell proliferation is a sine qua

none phenomenon of promoters• They act via signal transduction pathways

ex.,protein kinase C,• Activation of PKC - series of

phosphorylations- cell proliferation and differentiation

Promotion of carcinogenesis

• Involved in clonal expansion and aberrant differentiation of initiated cells

Carcinogenic chemicals

• Direct acting alkylating agents• Polycyclic aromatic hydrocarbons• Aromatic amines and azo dyes• Naturally occurring carcinogens• Nitrosamines and amides• Miscellaneous agents

Carcinogenic chemicals Direct acting alkylating agents

• Activation dependent, weak carcinogens• Used as anticancer drugs• Induce lymphoid neoplasms, leukemia etc.,• Powerful immunosuppressive agents• Eg: Cyclophosphamide, Busulfan• Interact with DNA and damage

Carcinogenic chemicalsPolycyclic aromatic hydrocarbons

• Source: Combustion of smoke, smoked meat, animal fat processing broiled meat, smoked fish

• Induce lung and bladder cancer• Most potent carcinogens known• Require metabolic activation• Skin paint - skin cancer• Subcutaneous injection - sarcoma

Carcinogenic chemicalsAromatic amines & azo dyes

• Source: food coloring agents Eg: Butter yellow, Scarlet red• Beta naphthylaimine – rubber industry -

bladder cancer

Carcinogenic chemicalsNaturally occurring carcinogens

• Source: moldy grains, peanuts, rice - aspergillus flavus - aflatoxin B1.

• Potent hepatocarcinogen• Correlates with increased incidence in china

/ Africa

Carcinogenic chemicalsNitrosamines and amides

• Source: Nitrostable amines and nitrates used as food preservative - bacteria convert them to nitrites

• Induce gastric cancers

Carcinogenic chemicalsmiscellaneous agents

• Asbestos - Bronchogenic ca. Mesothelioma,GI cancers

• Asbestos + smoking = many fold risk ca lung• Vinyl chloride - hemangiosarcoma liver• Chromiun & nickle - Ca lung - volatile

industrial environment pollutant• Arsenic – skin cancer• Hormones –endometrial cancer

Tests for Chemical Carcinogenecity

• Experimental induction– Animals: Initiator……Promoter

• Tests for mutagenicity (Ames Test)

Ames test

• In vitro test for carcinogenicity testing• Ability of potential carcinogens to induce

mutations in selected strains of salmonella typhi murium

Ames test

Katsusaburō Yamagiwawas the first to prove chemical carcinogenesis

Johannes Fibiger Katsusaburō Yamagiwa

Physical Carcinogens

• Radiation• UV rays of sunlight• Ionizing radiation

– Medical– Occupational

• Non radiation• Mechanical irritation

– Stone in the bladder

Radiation

• Mechanism– DNA repair by formation of

pyrimidine dimers• Normally taken care by

nucleotide excision repair (NER) – Recognition of DNA repair– Incision of strands on both sides

of damage– Removal of damages

nucleotides– Synthesis of nucleotide patch– Ligation

– Mutation in oncogenes & suppressor genes

Hormonal Carcinogens

• Contraceptives– Liver adenoma

• Anabolic steroids– Hepatic tumors

• Estrogen– Breast cancer– Endometrial cancer

Biological Carcinogens

Biological Carcinogens• Oncogenic Viruses

– DNA• Human Papilloma Virus Papilloma, Ca. of cervix, skin• EB virus Lymphoma, Nasopharyngeal carcinoma• HHV 8 Kaposi sarcoma, B cell lymphoma• Hepatitis B Hepatocellular carcinoma• Pox virus Molluscum contagiosum, Papilloma

– RNA• HTLV 1 Adult T cell Leukemia/Lymphoma• HTLV 2 T cell variant of Hairy cell leukemia• Hepatitis C Hepatocellular carcinoma

• Oncogenic Bacteria• H. pylori Gastric lymphoma, Adenocarcinoma

Human papilloma virus

• HPV- 6,11-genital warts• HPV-16,18- invasive squamous cell ca• Early viral genes E6&E7 are oncogeneic• E7 binds to Rb and displaces E2F• E6 inactivates TP53,degrades BAX

Viral oncogenesis

• Replication• Integration• Oncogene/suppressor

gene

EBV

• LMP1-proliferation of Bcells via CD40• Promotes BCL2-reduced apoptosis• EBNA2-activates cyclinD• Bcells escape immuneregulation due to lack

of Ag expression• Acquire myc mutation leading to neoplasm

Hepatitis B

• HBV—leads to chronic liver cell injury and proliferation

• Predisposing to mutations• HBx encoded by HBV acts on NFkB

pathway and signal transduction pathways

Viral oncogenesis

H. pylori

Adenocarcinoma Chronic gastritis

AtrophyIntestinal metaplasiaDysplasiaCarcinoma

LymphomaGastritisB cell proliferationGenetic abnormalityMALToma

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