Cancer biology

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Cancer biology

• Chemicals, Risk Cancer 5
• David R. Bell

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Cancer

• Characterised by uncontrolled growth
• Growth is the sum of cell division and cell death
• Tumour swelling
• Benign tumour no spread of cancer cells
• Malignant tumour cancer spread of cancer cell
• Spread of cancer cells is metastasis

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Effects of cancer

• Benign
• Swelling mass squeezes a vital organ, eg brain
• Production of hormone, insulinoma
• Bleeding, obstruction, eg of gut
• Malign
• Spread of tumour
• Breaking through the basement membrane
• Cachexia- a wasting disease
• Anaemia, widespread infections

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Cancer histology

• Metaplasia
• Replacement of normal differentiated tisssue by
  another (differentiated) type
• Anaplasia
• Loss of differentiated phenotype
• Dysplasia
• Loss of tissue organisation
• Hyperplasia
• Increase in cell division

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Types of tumour

• Epithelial
• Papilloma (skin)- benign
• Adenoma- benign
• Carcinoma- malignant
• Mesenchyme
• Benign- Fibroma, lipoma, osteoma, etc
• Malignant- Sarcoma
• Mixed
• teratoma

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Does cancer progress ?
Model originally set up dosing mouse skin
DMBA
Vehicle control- 6 months
0 tumours
DMBA
Croton oil- 6 months
Multiple tumours
DMBA
Croton oil- 6 months
0 tumours
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Promotion

• Use subcarcinogenic dose of initiating agent
• The promoting agent is not carcinogenic per se
• The promoting agent enhances the effect of the
  genotoxic initiating agent
• Long delay possible between administration of
  initiating agent and promoting agent
• Promotion is reversible

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The active ingredient

• TPA, or tetradecanoyl phorbol acetate
• TPA causes hyperplasia in mouse skin
• TPA activates protein kinase C
• Cell-signalling receptor
• Diacyl-glycerol is an endogenous activator
• Inappropriate activation of programme of cell
  growth

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Skin promotion is complex
Initiation Urethane
Phase I promotion 1x TPA, wounding, A23187
Phase II promotion Mezerein
Adenoma, carcinoma
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Mechanisms of promotion

• Skin promotors
• Okadaic acid
• Inhibitor of protein phosphatases
• TCDD
• Activates the Ah receptor
• Wounding

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Promotion in different tissues

• Colon
• Bile acids, high fat diet
• Bladder
• Saccharin
• Mammary gland
• Hormones, high fat diet

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Hepatic promotion

• Well characterised agents such as peroxisome
  proliferators, phenobarbital
• PB induces multiple lesions which express GST
• Very low progression
• Peroxisome proliferators induce few lesions,
  which do not express GST
• High progression

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Initiator-specific promotion

• After initiation with either DEN (Di-ethyl
  nitrosamine) or 2-AAF, phenobarbital promotes
  liver tumour growth
• After initiation with 2-AAF, the peroxisome
  proliferator Wy-14,643 fails to promote
• After initiation with DEN, Wy-14,643 promotes
  liver tumour growth

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Progression

• Progression from adenoma to carcinoma is
  frequently very poor
• The addition of a genotoxic carcinogen greatly
  enhances the rate of formation of carcinomas
• This stage is known as progression

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A model for cancer
Initiation
Promotion
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Promotion
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Progression Metastasis
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Screening for carcinogens

• Animal tests
• 50 animals per group
• Maximal tolerated dose (MTD), and 25
• Cost 250 000
• Time 2 years (rat or mouse)

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Problems with bioassay

• The US National Toxicology Programme has tested gt
  300 compounds, with selection based solely on
  environmental relevance
• Approx 66 of all chemicals cause cancer
• Is this a problem with MTD ?
• e.g. at very high doses, there may be tissue
  damage leading to hyperplasia
• Weak carcinogens may lead to longer lifespans in
  the test animals

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Ames assay

• Relies on the concept that DNA-damaging, or
  mutagenic, agents will cause cancer
• Uses a disabled Salmonella Typhimurium, with
  defective cell wall, allowing easy import of
  chemicals
• Look for mutations in genes for Histidine
  biosynthesis
• Mutant genes will allow survival and growth of
  the bacteria

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Ames assay 2
His- Salmonella
Histidine Deficient plates
Plate chemical
Control plate
Benefits two day assay very cheap
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Ames III

• Ames test detects direct acting mutagens
• Add liver homogenate (the S9, 9000g supernatant)
  to cells to detect mutagens which require
  activation
• Some chemicals are mutagenic, but not
  carcinogenic
• Many chemicals are carcinogenic, but not mutagenic

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Big blue mouse/ rat
A major shortfall of Ames test is that it uses
bacteria, Instead of eukaryotic cells. Mice were
engineered with transgenic phage l
genome, containing a functional lacZ gene,
regulated by the lacI Repressor ( 1300 bp).
Mutations in the lacI gene result derepression
of the lacZ gene. Expose the mice to chemicals,
then isolate DNA from various tissues, and add
phage packaging extract. The phage DNA will be
packaged, producing white phage. Mutant phage
will be blue. Count the ratio of blue white
phage. Sequence the mutant genes.
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Big blue mouse/ rat

• Take account of pharmacokinetics, and metabolism
  of foreign compounds
• Mutation increase does not always correlate
  simply with carcinogenicity
• Mutation increase appears to be a prerequisite
  for the action of genotoxic carcinogens
• No detection of non-genotoxins

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How useful are screening tests ?

• Essential to prevent human epidemiology
• Animal screening tests currently detect 66 of
  all chemicals as carcinogens
• Humans are exposed to thousands of chemicals in
  the diet
• Necessary to develop a rational approach to this
  plethora of carcinogens