Chemical carcinogenesis

12 important questions on Chemical carcinogenesis

Why is smoking deathly?

Because of the tabacco. Tabaco has a lot of PAHs (polycyclic aromatic hydrocarbons), which act as carcinogens. Orally ingested tobacco is very rapidly metabolized by the hepatic cytochrome p450 via the first pass effect.

What is DNA adduct?

A piece of DNA covalently bonded to a cancer-causing chemical.

What kinds of carcinogens can be present in food?

  1. Naturally present carcinogens in plants (fytotoxins)
  2. Formed by micro-organisms present in decaying food (mycotoxins)
  3. synthetic carcinogens (dioxins)
  4. products formed during heating up food (pyrolytic and pyrosynthetic products) (BBQ)
  5. carcinogens formed from nitrite and other precursors in the stomach by low pH
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Methods for Detecting DNA Adducts

32P-postlabeling, immunoassays, and mass spectrometry (MS)

Why is aflatoxine B1 a very potent carcinogen?

Aflatoxin is produced by Aspargillus flavus and is found in peanuts. The metabolic activation of aflatoxin B1 to the 8,9-epoxide produces a highly electrophile molecule which either is hydrolytically inactivated or else can react with DNA bases and result in covalent binding. An impotant way of detoxification is GST-mediated conjugation with glutathione.

Stages of Chemical Carcinogenesis?

Initiation, where a genetic mutation is introduced; promotion, where the mutated cells are induced to proliferate; and progression, where malignant characteristics are acquired.

Explain the classical carcinogenesis model

Cancer can be divided into three stages; initiation, promotion and progression.
Initiation: gene mutation --> changed function of gene. No morphological changes yet.
Promotion: Circumstances are created in which cells are more susceptible for mutations or in which the initiated cells have a growth advantage. There are no mutations of karyotypic changes, promotion happens by interference with cellular signaling. This is a reversible process since no additional mutations occur.
Progression: karyotypic instability or instability by change of DNA-methylation occurs --> metastases, hyperproliferation and loss of control by cellular environment.

Explain the non-classical model of carcinogenesis

Once a cell is exposed to a carcinogen it can either excrete of metabolize it. If the compound is metabolized two mechanisms are possible.
  • The genotoxic model gives direct binding to DNA and causes genomic damage, one carcinogenic molecule would thus be sufficient to cause a mutation.
  • The non-genotoxic mechanism gives inflammation, immunosupression, ROS, receptor activation and epigenetic silencing. The carcinogen thus gives its effect by binding to molecules other than DNA and causes an altered signal transduction.  

Why is benzo[a]pyrene (BP) a genotoxic human carcinogen?

Its diol epoxide metabolites can react and bind to DNA thereby causing DNA-adduct-derived mutations in cancer susceptibility genes. The first formed epoxide is rapidly hydrolyzed. The second formed epoxide is not, due to its orientation. It is therefore, and due to its stability, that this is not the one causing carcinogenic mutations. If BP binds to a guanine, the guanine is not able to form three hydrogen bridges anymore, due to this it cannot basepair anymore to cytosine. This results in guanine binding to adenosine, a possible carcinogenic mutation.

What are two examples of ABC family proteins that are transporters?

BCRP (breast cancer resistance protein) and MRP2 (multidrug resistance protein 2) --> causing excretion of toxic substances out of the body.

Explain the non-genotoxic model of carcinogenesis in detail

The non-genotoxic mechanisms involved xeinobiotics which bind to other molecules than DNA, and thereby causes altered signal transduction (e.g inflammation, immunosupression, ROS, receptor activation, epigenetic silencing).
Epigenetics are also involved in chemical carcinogenesis. Epigenetics is are alternate phenotypic states that are not based in differences in genotype and which are potentially reversible. In general they are stably maintained during cell division. These changes are often histone modification, acetylation and DNA methylation.

Explain how extrapolation is used in non-genotoxic (with a threshold) cancer studies

Extrapolation is a principle which refers to the approval of a biosimilar for use in an indication held by the reference product, but not directly studied in a comparative clinical trial with the biosimilar. Extrapolation from an animal experiment to a clinical trial usually takes three things into account; the interspecies differences, the intraspecies differences and the duration of the expierment. Most this extrapolation is in total is a factor of one hundred.

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