Progesterone inhibits human breast cancer cell growth through transcriptional upregulation of the cyclin-dependent kinase inhibitor p27Kip1 gene.

The effects of progesterone derivatives on breast cancer development are still controversial, probably accounting for their biphasic, opposed effects on mammary cell-cycle regulation. Here, we demonstrate in vitro that the growth-inhibitory effects of progesterone on breast cancer T-47D cells require the transcriptional upregulation of the cyclin-dependent kinase inhibitor p27(Kip1) (p27) gene. A statistical analysis of human tumor biopsies further indicates that p27 mRNA levels correlate to progesterone receptor (PR) levels. Moreover, p27 gene expression is inversely associated with tumor aggressiveness, and is a prognostic factor of favorable disease outcome. Thus, progesterone derivatives selectively activating the p27 gene promoter could be promising drugs against breast cancer progression.