Experimental model for the study of carcinogenensis

1. Experimental model for the study of carcinogenesis
Presented by
Md. Eram Hosen
Dept. of Genetic Engineering & Biotechnology,
University of Rajshahi

2. • A number of models for the study of carcinogenesis have been developed over the years.
Historically, two of the most useful ones have been the initiation-promotion model of
mouse skin carcinogenesis (the ‘‘skin-painting’’ model) and the induction of liver
cancers in rats.
• The classic model of carcinogenesis is the single application of an initiating agent such as a
polycyclic aromatic hydrocarbon followed by the continuous application of a promoting
agent like TPA to the backs of shaved mice. Much of what we know about tumor initiation,
promotion, and progression has come from this model system.

3. • Initiation and promotion during mouse skin carcinogenesis produce multiple benign
squamous papillomas. A few squamous cell carcinomas eventually arise from the
papillomas over many months.
• However, malignant conversion can be speeded up by exposure of papilloma
bearing mice to mutagens, which activates oncogenes such as H-ras and causes loss of
tumor suppressor genes such as p53, as noted above.

4. • The mouse skin carcinogenesis model is also a useful one in which to study the
role of diet and chemopreventive agents in carcinogenesis.
• For example, calorie-restricted diets have been shown to reduce the number and
size of papillomas during and following promotion with TPA in DMBA-initiated
SENCAR mice.
• Furthermore, the latency period for occurrence of carcinomas was increased and
the total number of carcinomas was decreased.
• Application of apigenin,a plant alkaloid,retinoic acid, and prostratin, a
nonpromoting phorbol ester have been shown to inhibit the promotion phase
(appearance of papillomas) of mouse skin carcinogenesis.
• Multistage carcinogenesis has also been observed for liver tissue.

5. Peraino et al. Experiment
• He observed that a 3-week exposure of rats to AAF in the diet produced only a small number
of hepatomas after several months, but if the animals were subsequently treated with
phenobarbital for several months after carcinogen feeding was discontinued, a high incidence
of hepatomas was noted.
Kitagawa et al.Experiment
• Similar results have been obtained.who fed rats a non-hepatocarcinogenic dose of 2-methyl-
N,Ndimethyl-4-aminoazobenzene for 2 to 6 weeks, and then a dietary administration of
phenobarbital for 70 weeks. By 72 weeks, many large hepatocellular carcinomas had developed
in the phenobarbital-treated animals, whereas only a few small tumor nodules were observed
in the rats not given phenobarbital. Thus, the action of phenobarbital appears to be
analogous to that of TPA in the mouse skin system—that is, it ‘‘fixes’’ the damage to cells
induced by an initiating agent and causes a clone of cells arising from a damaged cell to
proliferate.

6. • However, whereas TPA stimulates DNA synthesis and hyperplasia in skin,
phenobarbital produces only a transient and relatively small increase in DNA synthesis in
liver.
• Perhaps that is all that is needed to fix the carcinogenic damage and to allow for
the initial proliferation of a damaged clone of cells. Once the damaged clone is present, it
could undergo alteration due to its genetic instability and gradually progress to a
detectable malignant tumor. This idea is supported by the experiments of Pitot et al.

7. Pitot et al.Experiment
• He treated rats with a single dose of diethylnitrosamine by intubation 24 hours after
partial hepatectomy (partial removal of the liver), which stimulates DNA synthesis and
cell proliferation in the remaining tissue. If the animals were then treated, starting 8
weeks later, with phenobarbital in the diet for 6 months, many small, phenotypically
heterogeneous foci characterized by glucose-6phosphatase–deficient areas, ATPase-
deficient areas, and g-glutamyltranspeptidase-containing areas developed in the liver.
• Many of these animals also had hepatomas, for which the enzyme-altered foci appear to
represent the early stage of neoplastic development. Thus in this case, phenobarbital
appears to have stimulated the replication of dormant initiated cells, which, in the absence
of the promoter, would not have proliferated. If each enzyme-altered focus observed in
these experiments were a clone derived from a single cell, about 104 to 105 cells in the liver
were ‘‘initiated’’ by diethylnitrosamine, and a very small number of these subsequently
underwent clonal proliferation during phenobarbital feeding. Thus the conversion of these
abnormal foci, or early nodules, as they have been called, to a malignant neoplasm is a rare
event.

8. • Newer models of carcinogenicity have involved the use of knock-out or knock-in
rodent models, in which various oncogenes, tumor-suppressor genes, or
susceptibility genes have been engineered into or out of rodent embryos
(usually mice). This process has enabled the definition of some of the genes that are
key to various steps in the tumor-initiation promotion and progression steps.
• These tumor models are now being superceded by conditional genetic knock-out
models in mice that allow for the controlled expression of oncogenes or tumor
suppressor genes in a way that more closely mimics ‘‘spontaneously’’ arising human
cancers (Table 2–2).

9. • Conditional gene expression in the mouse has been achieved by mutations induced
by FLP/ FRT or Cre/lox P site-specific recombination systems, regulatable
oncogene expression, and retroviral gene transfer in transgenic mice that express an
avian retroviral receptor.
• These models provide for the induction of somatic mutations in tissue-specific and
time-sequenced way. These models more closely mimic human cancer
development, which involves the activation of oncogenes and inactivation of
tumor suppressor genes over time. They enable investigators to determine the
contribution that individual mutations make to the various stages of tumor
development.
• These models also provide a way to validate various targets for anticancer drug
development.