Status of p53, p21, mdm2, pRb proteins, and DNA methylation in gonocytes of control and gamma-irradiated rats during testicular development

In fetal and newborn rat testes, gonocytes, which stop cycling for about 8 days, become highly radiosensitive. The presence of p53, p21, mdm2, and pRb , which are involved in cell cycle, apoptosis control, or both, were studie d by immunohistochemistry to determine if their expression is related to th is radiosensitivity. A strong cytoplasmic expression of p53 and p21 was det ected. Cytoplasmic expression of p53 occurred only in arrested gonocytes, w hereas that of p21 was observed before and after the block. P21 was found t o colocalize with mitochondria. No expression of mdm2 was detected and pRb was present only when the gonocytes started cycling again. In animals expos ed to 1.5 Cy of gamma-irradiation at Day 19 postcoitum, p53 expression was prolonged in time, whereas no change was observed in p21 amounts and locali zation, compared with controls. Using antibodies against 5-methyl cytosine, it was shown that gonocyte DNA passed from a hypomethylated to a methylate d status 1 day after gonocytes stopped cycling. A prolonged survival of gon ocytes after exposure to radiation was followed by their progressive apopto sis, which finally involved the entire gonocyte population between Days 6 a nd 12 postpartum. The elevated but delayed sensitivity of gonocytes to geno toxic stress may be related to the unusual expression of p53 and p21, which may itself be related to the large DNA methylation changes.