Carcinogenesis/Mutagenesis

Cancer History

Hippocrates termed carcinos/carcinomas for non-ulcer and ulcer-forming tumors

Carcinogen

Physical/chemical agent that increases risk of cancer

Mutagen

Physica/chemical agent that modifies genetic material

Chemical carcinogenesis

Induction/enhancement of neoplasia by chemicals

Ex. radioactivity, industrial chemical, asbestos

Are all mutagens, carcinogens?

NO: not all change to DNA (mutagens) causes cancer (carcinogen)

Neoplasm

• Heritably altered

• Autonomous growth of tissue

• Benign or malignant

Metastases

Secondary growths of cells from primary neoplasm

Cancer

Malignant neoplasms

Tumor

Space-occupying lesion, may or may not be neoplastic

John Hill

Discovered connection between nasal cancer and tobacco snuff

Percival Pott

Discovered link between scrotum cancer and soot/coal tar (chimney sweeps)

Rehn

Discovered link between bladder tumors and aniline dye

First chemical carcinogenesis induction:

1918 at Tokyo University: coal tar to rabbit ears

James and Elizabeth Miller

• Groundwork for chemical carcinogenesis:

  • 1st researchers to show a chemical caused cancer in rats

    • Binding with proteins in the liver (covalent binding)

• Proposed that chemicals require metabolic activation

Oncogene

Mutated form a gene causing normal cells to grow out of control (cancer cells)

Causes a cell to go from normal → cancerous

Protooncogenes

Genes that control how often a cell divides

Become oncogenes if mutated

What cells are most susceptible to cancer?

Rapidly dividing cells (targets of oncogenes), most division/growth

Tumor Suppressor Gene

Genes that slow down division, repair DNA, and induce apoptosis

What happens when tumor suppressor genes don’t work?

Cells can grow out of control, leading to cancer

Direct-acting carcinogens

• “Ultimate” carcinogens

• Electrophilic, bind to DNA

Precarcinogens

Carcinogens that require bioactivation to lead to cancer

Examples of direct acting carcinogens

Nitrosamides, nitrosoureas, epoxides

Examples of precarcinogens

Polycyclic aromatic hydrocarbons, aflatoxin B, safrole, nitrosamines

Polycyclic Aromatic Hydrocarbons

Produced from burning organic materials (leaves, garbage, etc.)

Aflatoxin comes from…

Aspergillus fungus (mold) found in humid climates, rice, grain, etc.

Benzo[a]pyrene

Metabolizes to toxic intermediate by P450 enzymes

Epoxide at the top of ring = carcinogenic, causing tumors and cancer

Covalently binds to DNA

Nongenotoxic Carcinogens

Do NOT damage DNA, but enhance the growth of tumors caused by genotoxic carcinogens or by other mechanisms

Cocarcinogens and promoters

Cocarcinogens

Increase concentration of initiator

Promoters

Increase effects of initiators during two-stage process of carcinogenesis

Example of a cocarcinogen

Tobacco smoke and catechols

Examples of promoters

Phorbal esters and Dioxin

Two-stage process of carcinogenesis:

• Stimulate cell proliferation

• Inhibition intercellular communication

• Immunosuppression

Cocarcinogens vs. Promoters

Increase concentration vs. enhance effect

Stages of carcinogenesis

1. Initiation - beginning of growth

2. Promotion - growth of cancer

3. Progression - metastases

Initiation

• Irreversible (geno/phenotype is established)

• AFTER cell division

• No threshold

Promotion

• Reversible dependent on continued administration

• Dose response and maximal effect exist

Sub-threshold dose of initiator

Produces FEW tumors

Sub-threshold dose of initiator, followed by promoter

MANY tumors

Promoter alone

VERY few tumors (initiator must come 1st)

Solid state carcinogens

Foreign body reaction

Examples of solid state carcinogens

Asbestos, plastics, metals, glasses

Metals and metalloids

Arsenic, cadmium, chromium

What metal is toxic to humans, not animals?

Arsenic

Epigenetic Agents

Agents that alter gene expression without changing DNA itself

Examples of epigenetic agents

• Immunosuppressive xenobiotics

• Asbestos

• Hormones (estrogen-dependent cancer)

• Promoters (esters, Dioxin, Phenobarbital)

Aflatoxin

Toxic mold from aspergillus fungi

Targets LIVER

Alcohol

Main target organ is the liver

Tobacco Smoke

Main target organ are the lungs

Arsenic target organ

Skin (blistering, blackening of skin)

Asbestos target organ

Lungs

Benzene target

Bone marrow

Cadmium targets

Lung and prostate

Mutations

Additions or deletions of base pairs; OR substitution of incorrect base pair in DNA

Transition Mutations

Switching purine - purine (A &G), pyrimidine - pyrimidine (C & T)

Transversion Mutations

Switching purine to pyrimidine, vice versa)

Purines

A and G

Pyrimidines

C and T

Point Mutations

Missense and nonsense (changing codons)

Frameshift mutations

Changing DNA frame by adding or deleting a base pair

ALL amino acids change

Missense Mutation

Replacing 1 nucleotide, resulting in changed amino acid

MAY or MAY NOT affect protein

Nonsense Mutation

Switching 1 amino acid to a STOP codon

Chromosomal Mutations

Deletion, duplication, inversion, insertion, translocation

Happen OVER TIME, location on chromosome

Short-term Tests

• Bacterial mutagenesis (Ames)

• Mammalian mutagenesis

• DNA repair

• Chromosome integrity

• Cell transformation

Limited Tests

• Skin tumors in mice

• Pulmonary tumors in mice

• Altered Foci in rodent liver

• Breast cancer in female rats

Long-term test animals

• Rats, mice, hamsters of both sexes

• Dogs and primates

  • Large and long lives

Advantages of using rats, mice, hamsters:

Small, short life, available, well-characterized

Disadvantages of rats, mice, and hamsters

Genetically different, lower phylogenetic order

Long-term Tests

• Inception/duration:

  • Shortly after weaning

• Route of administration

• Dose (2-3) with control

Observations and Examinations

• Gross/microscopic review of weak/dead animals

• Onset, location, size, and growth of tissue

• Weigh organs (will be heavier)

• Histological exam

• # of tumors, tumor-bearing animals, onset

Tumor Incidence

• Increased # of tumor-bearing animals (most common)

• Unusual tumors

• Increased # of tumors per animal (cocarcinogenicity)

• Dose-response

• Reproducibility (strain/species)

Evaluating Risk

• Genotoxic (no threshold)

• Non-genotoxic (no-effect dose level)

Chloroform is both epigenetic and genotoxic

Ames Test

Uses bacteria to test for mutations/chemicals (bacteria grows quicker)

Positive = chemical is mutagenic and can be a carcinogen

S-9 Fraction

Supernatant of liver tissues with metabolic enzymes (since chemical carcinogens must be metabolized)

NEED this, or else wouldn’t be able to determine if chemical or natural mutagen

Ortho-phenyl phenol

Preservative (food processing, fungicide etc); known carcinogen

Aniline

Metabolites produce oxygen deficits/hemoglobin damage (Buffalo Company made dye from this)

Mutagen

Aflatoxin

Mycotoxin naturally produced by Aspergillus mold

Found in crops (corn, peanuts, seeds, etc) in tropical/humid condition

Aflatoxin mechanism

Metabolized by P450 enzymes to form epoxide intermediate that binds to DNA (G → T transversion)

Actinomycin D

Chemo medication/antibiotic produced by Streptomyces bacteria in soil

Mechanisms of Actinomycin D

1. Intercalates DNA between G and C

2. Induces apoptosis

Nitroso Dimethylamine

• Found in contaminated water, cured meats, cigarettes, meds

  • Added to meats to prevent botulinum toxin

Nitroso dimethylamine mechanism

Destabilize and alkylate base pairs → DNA breakage

Acridine Orange

Fluorescent dye in microscopy, anti-malaria drug

Produced by boiling coal tar

Acridine Orange mechanism

Binds to DNA due to positive charge (Mutagen)