Chemical Carcinogenesis - Toxicology slide 3

Taught by Dr. Thomas Visalli; 1:48min

Tumor

space occupying lesion that may or may nor be neoplastic

Neoplasm

an independent growth of tissue with abnormal regulation of gene expression; benign or malignant

Benign neoplasm

a non-cancerous tumor that grows slowly and does not spread to other parts of the body

Malignant Neoplasm

has the ability to metastasize; an abnormal growth of cells that has the potential to invade and spread to other parts of the body

Metastases

secondary growths of cells from the primary neoplasm

Cancer

a subset of malignant neoplastic lesions(neoplasms); Malignant Neoplasm

carcinogen

an agent that causes or induces neoplasia

what is the long definition of a carcinogen?

An agent whose administration to previously untreated animals leads to a statistically significant increased incidence of neoplasms of one or more histogenetic types compared with the incidence in appropriate untreated animals

carcinoma

malignant tumor arising in the epithelium; usually spreads via lymphatic system

what is the most common form of cancer?

carcinoma

sarcoma

malignant tumor arising in muscle or connective tissue; usually spreads via circulation

carcinogenesis can be caused by what kind of chemicals?

• organic

  • polycyclic aromatic hydrocarbons (PAHs)

  • Nitrosamines

• inorganic

  • cadmium

  • nickel

  • lead

  • arsenic

• Hormonal

  • over production or derangement 

• Mixtures (defined/undefined)

  • tobacco smoke

  • Diesel exhausts

• Diet

  • excessive caloric intake

  • excessive alcohol intake

  • aflatoxin B₁

what is a common trait in almost all the carcinogenic molecules

they contain a nitrogen group, nitrogen is very good at interacting with DNA

what is the most often mechanism of chemical carcinogenesis

covalent interaction of chemicals with biological macromolecules(DNA) is required for chemical induction of cancer

pro-carcinogens

compounds that require metabolic alteration(activation) to a metabolite that is capable of covalent binding

Ultimate carcinogens

Highly reactive metabolites; final form of the carcinogen 

what is caner in basic terms?

dysregulated cell growth

Mutagenesis

structural DNA alteration and cell proliferation that “fixes”(establishes) the DNA damage; The damage will become part of the DNA from now on and other cells will have it too

example of mutagenesis

DNA adduct formation- when a toxin binds to DNA and will cause errors in the DNA during replication, future copies of that cell will also have the errors

Mechanisms of chemical carcinogenesis 

• inhibition of DNA methylation

• macromolecular adducts

• Free radicals

• crosslinking of DNA strands

• Thymine dimers

• Single and Double strand breaks

macromolecular adducts

covalent binding of the ultimate carcinogen to DNA, this usually happens in the the N⁷ position of guanine

what is the most nucleophilic site in DNA?

N⁷ position of guanine

adducts

the combined molecule of the toxin and usually DNA

what can adducts be produced?

on bases, sugars, and phosphate backbone

Free Radicals

elements which possess an unpaired electron

Free Radicals are created during?

metabolism of carcinogens

Free Radicals reacts with?

DNA to cause structural changes in bases

Inhibition of DNA methylation

preventing the methylation of genes on DNA 

Inhibition of DNA methylation with result in what happening?

Causes excessive transcription of a rarely transcribed gene 

In normal environment, genes that are hypomethylated

actively transcribed

In normal environment, genes that are hypermethylated

rarely transcribed

Crosslinking of DNA strands

crossing reaction of 2 opposing bases, can result in errors when that DNA is replicated; results from some chemical compounds

Thymine dimers

when 2 thymine components interact which can cause replication issues where errors can occur; common result of UV exposure

Single and double strand breaks

breaks the strands in DNA, messing with how it interacts with replication resulting in possible errors 

what does this diagram display/represent?

visual representations of the different mechanisms of carcinogenesis

what are the 2 types of mechanisms of DNA repair

• Repair pathways

• Tolerance mechanisms

Repair pathways

• Damage removed or revered

  • adducts removed and new bases inserted

what are the 2 repair pathways

• Base Excision Repair

• Nucleotide Excision Repair

Base Excision Repair

repaired a small amount of damage; Removal of a single altered base with a low molecular weight adduct

Nucleotide Excision Repair

• Repairs a larger amount of damage

  • Removal of a base with a large adduct, or adduct effecting multiple bases

Potential for resultant mutations when large amount are repaired

Tolerance mechanisms

Damage circumvented and not repaired

what does this diagram tell you?

Even though damage to DNA is a concern the rate of repair is still far greater in the cell, still, the more repair that is done the more likely mutations can happen

T/F carcinogenesis develops very quickly

False, it develops slowly; think of how fast it take for smokes to get chancer, etc

why does carcinogenesis develop slowly?

Long latency period between initial exposure to carcinogen and ultimate development of malignant neoplasia

Process of Neoplastic Development

• Stage 1: initiation

  • Structural changes in DNA observed

• Stage 2: promotion

  • No structural changes

  • Altered expression of genes

• stage 3: Progression

  • Structural changes in DNA observed 

Initiation (Stage 1)

Requires one or more rounds of cell division for “fixation” of the process resulting in a initiated cell if the genetic change sticks; spontaneous imitation of cells in a variety of tissue is very common

Initiation (Stage 1) - reversable of irreversible?

irreversible

what does Irreversible mean in the context of process of Neoplastic development?

even if the chemical that caused the damage is not present anymore, the damage will remain 

what does reversible mean in the context of process of Neoplastic development?

If the chemical is removed, the damage would leave with it

T/F All initiated cells survive

False, not all survive; apoptosis, cell get repaired, immune system fights the cell, etc

Promotion (stage 2)

promoting agents that do not directly interact with the cell which depends on continued administration of the promoting agent

Initiation (Stage 1) - reversable of irreversible?

reversible

what happened when the promoter agent is removed?

Regression may occur; apoptosis of affected cells. Re-modeling of affected cells

what are examples of promoters?

• Chemicals that may interact with receptors, mimicking their natural ligands

• initiate cascades of promoting agents that exert effects on gene expression

Some promoters inhibit apoptosis

Relative potency of a promoter is measured?

by its effectiveness in causing an expansion of cell progeny of the initiated population

T/F The effect of the promotor agent is proportional to the number of receptors occupied by that chemical “ligand” and withdrawal of the promoting agent returns system to its original state

True

Progression (stage 3)

exposure of a substance that fosters continuing evolution of unstable cells, causes higher rate of cellular growth, alters bodies response to hormones usually leading to continued cell division, Ectopic hormone production

Ectopic hormone production

hormones that are produced in the wrong location or should be produced at the time

what can progression (stage 3) lead to?

invasive malignant disease

summary of initiation agent

(incomplete carcinogen) a chemical capable only of initiating cells

summary of promoting agent

a chemical capable of causing the expansion of initiated cell clones

summary of progressor agent

a chemical capable of converting an initiated cell or a cell in the stage of promotion to a potentially malignant cell

complete carcinogen

a chemical possessing the capability of inducing cancer from normal cells, usually possessing properties of initiating, promoting, and progressor agents

even though our body is able to take care of damaged cells why does cancer still take place

The more a person get exposed to carcinogens(of any or all stages) the more it builds up and cancer taking place, people are exposed to chemicals all the time and additional exposure for different lifestyle choices can increase the chances of cancer 

what does this diagram show you?

A visual representation of the different stages and how cancer can arise

what does this diagram show you?

A visual representation of the different stages and how cancer can arise

What does this diagram display?

explains neoplastic development in another way

what is a examples of cancer suppressor gene?

p53 genes

Epidemiology

Study of distribution and determinants of disease from observation rather than through controlled experimentation

how do we determine if a chemical causes carcinogenesis in humans?

epidemiology but it often is insufficient, rarely possible to identify a single chemical as sole carcinogen; there are numerous environmental variables, there is a long time interval between exposure and neoplasm development

International Agency for Research on Cancer (IARC)

provided system of definitions of relative carcinogenicity

what are the definitions provided by the IARC?

• Sufficient Evidence

• Limited Evidence

• Inadequate Evidence

Sufficient Evidence

causal relationship between agent and human cancer

Limited Evidence

Causal interpretation is credible by alternative explanation can’t be excluded

Inadequate Evidence

Few pertinent data. Available studies don’t exclude chance, bias, variables

since epidemiology alone is not sufficient, what else is done?

Laboratory studies, in vitro and in vivo; also used to determine carcinogenicity of potential therapeutic compounds 

examples of lifestyle carcinogenesis

• alcoholic beverages

  • metabolized into acetaldehyde. contributes to progression

• Dietary intake

  • carbohydrates and lipids may be promoting agents

• Aflatoxins

  • complete carcinogens

• Tobacco Use

what does this chart show?

things that a person can do that can help in preventing cancer

(I- Initialization, Pr- promotion, Pg - progression)

how are assays classified?

• short-term tests

• medium-term tests

• long-term tests

short-term testing

identify carcinogenic agents based on their capacity to induce mutations in DNA in cells in vitro or in vivo

• e.g.

  • in silico analysis (SAR - structure activity relationship) - computer program that looks at structure and tells u if carcinogenic (DEREK M-case)

  • Ames test - gold standard for screening, required test for pharmaceutical compounds

Ames test

the test consists of salmonella typhinmurium cells which are deficient in DNA repair and lack ability to grow in absence of histidine, these cells are treated with the suspected carcinogen, if the cell grows on the plate with the suspected chemical that means a mutation took place allowing it to grow; 80% accuracy 

what is S9 extract?

liver extract from rates that is put in the mixture that contains a lot of the metabolizing enzymes; allows you to know if the chemical needs to be metabolized first to be carcinogenic or no, you can run both with and without S9 to know both

Example of in Vivo, short-term test

Dominant lethal assay

male mice exposed to carcinogen, mated with untreated female mice, % pregnancy determined against a control population

Long-term in vivo study

• Chronic 2-year study - gold standard

  • 50 rodents/sex/dose level at 3 dose level

  • FDA must approve protocol

  • comparing a control group of the mice and 1 that was exposed to a chemical and see the spectrum of tumors and their incidence

• the use of transgenic mice (p53 knockouts) can also be employed

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