Stage-dependent genetically-based deformities of the regenerating newt limb from 4-nitroquinoline-N-oxide mutagenesis: potential embryonic regulation of cancer.

ABSTRACT

The effect of the mutagenic carcinogen 4-nitroquinoline-N-oxide (4NQO) on limb regeneration was studied in adult Triturus cristatus newts. Delayed limb regeneration was observed when a 10- to 15-micrograms crystal of 4NQO was implanted at the late dedifferentiation or late bud stage. Additionally, 4NQO administration at these stages caused developmental deformities in skeletal elements of the subsequent regenerates. In contrast, 4NQO implanted at the wound healing stage did not affect skeletal morphogenesis. These critical stages of limb regeneration affected by carcinogen exposure (but not other manipulations) represent mechanisms distinct from disruption of basement membrane deposition. Secondary regeneration, initiated by reamputation of the 4NQO-treated abnormal regenerates 4-5 mm proximal to the site of carcinogen implantation, produced normal regenerates, setting a limit on the diffusion distance of 4NQO in the limb. Distal reamputation through regenerates at the level of the abnormality resulted in regeneration of the original bone deformity, suggesting that the teratogenic effect of 4NQO is mediated via heritable mutational events in blastema cells. These results extend a previously published conclusion that 4-NQO is mutagenic in non-regenerative tissue to include regenerative tissue as well. However, in spite of mutagenic activity, squamous carcinomas were not induced by 4NQO exposure at any stage of regeneration. Therefore, the resistance of regenerative tissue to 4'-NQO carcinogenesis is not due to resistance to mutations, but rather to other mechanisms perhaps similar to those which regulate malignant cells in murine embryos.