Cancer resistance in amphibians.

While spontaneous tumours may occasionally develop in inbred and isogenic strains of Xenopus laevis, the South African clawed toad, they are extremely rare in natural and laboratory populations. Only two amphibian neoplasms, the renal adenocarcinoma of Rana pipiens and the lymphosarcoma of Xenopus laevis, have been extensively explored. Amphibians are resistant to the development of neoplasia, even following exposure to "direct-acting" chemical carcinogens such as N-methyl-N-nitrosourea, that are highly lymphotoxic, thus diminishing immune reactivity. Regenerative capacity in adults, and a dramatic metamorphosis which remodels much of the larval body to produce the adult form, are unique to amphibian vertebrates, and the control mechanisms involved may protect against cancer. For example, naturally rising corticosteroid titres during metamorphosis will impair some T-cell functions, and the removal of T-regulatory (suppressor) functions inhibits the induction of altered-self tolerance. Altered-self tolerance is not as effectively induced in adult Xenopus laevis as in mammals, so cancer cells with new antigenicity are more likely be rejected in amphibians. Amphibian immunocytes tend to undergo apoptosis readily in vitro, and, unlike mammalian immunocytes, undergo apoptosis without entering the cell cycle. Cells not in the cell cycle that die from nuclear damage (apoptosis), will have no opportunity to express genetic instability leading to cell transformation. We suggest that all these factors, rather than any one of them, may reduce susceptibility to cancer in amphibians.