Changes in endonuclease activity during growth and regression of the Shionogi mammary carcinoma

Autodigestion of nuclei isolated from the androgen-dependent Shionogi mouse mammary carcinoma revealed the presence of endogenous endonuclease activity which was stimulated by Ca2+ and Mg2+ and generated a repeating pattern of DNA fragments with an average monomeric size of 179 base pairs. The nuclear concentration of endonuclease activity declined rapidly after castration to a level 45% of that measured in tumours from non-castrated hosts. Most of the endonuclease activity (80%) could be extracted in a soluble form after sonication of the nuclei and subsequent centrifugation. The reduced concentration of endonuclease activity in regressing tumours following castration was not due to changes in chromatin template conformation and could be restored to normal levels within 1 day of androgen treatment. Examination of DNA synthetic activity and the concentration of nuclear androgen receptors during tumour regression indicated that the decrease in endonuclease activity paralleled the declining rate of DNA synthesis but was preceded by a loss of nuclear androgen receptors. Together these results suggest that the endonuclease activity of the Shionogi carcinoma is more likely involved with androgen-stimulated cell proliferation rather than with the autophagic mechanism responsible for tumour regression and cell death.