Effect of progesterone on cell proliferation in the MXT mouse hormone-sensitive mammary neoplasm

The transplantable MXT mouse mammary tumor is an experimental model for which the growth is modulated by ovarian hormones; because of scant knowledge about the effects of progesterone (Pg) alone on cancer cells, the present investigation was made. By measuring the percentage of cells with labeled nuclei after tritiated thymidine ([3H]dThd) injection 1 hour prior to sacrifice [dThd labeling index (TLI)], a study was made of the effect (in vivo) of a near physiologic dose of Pg (125 micrograms ip) on cell proliferation in uteri (as the control Pg target organ) and on tumors of spayed (C57BL female X DBA/2F male)F1 mice. Pg induced a significant and transient rise in TLI, which increased from the 12th to the 24th hour after its injection. This mitogenic effect on tumors was comparable to that elicited by 0.25 microgram 17 beta-estradiol injected under similar conditions. In vivo brief exposure of castrated mice to Pg induced a significant mitogenic effect on MXT tumors. These observations might have important consequences for a better understanding of the endocrine mechanisms involved in human hormone-dependent breast cancer.