Overexpression of progesterone receptor membrane component 1: possible mechanism for increased breast cancer risk with norethisterone in hormone therapy

Objective: Clinical trials have demonstrated an increased risk of breast cancer during estrogen/norethisterone (NET) therapy. With this in mind, the effects of estrogen/NET combination on the proliferation of breast cancer cells overexpressing the progesterone receptor membrane component 1 (PGRMC1) were examined. The same combination was used for the first time in a mouse xenograft model to determine its effects on tumor development. Methods: MCF-7 cells were stably transfected with PGRMC1 expression plasmid (WT-12 cells) or empty vector control (pcDNA-3HA). NET, medroxyprogesterone acetate (MPA), and progesterone were tested alone and sequentially and continuously combined with estradiol (E-2). Six-week-old nude mice were inoculated with E2 pellets 24 hours before the injection of tumor cells into both flanks (n = 5-6 mice per group). After 8 days, animals were inoculated with a NET pellet or with placebo pellets, and tumor volumes were recorded twice a week. Results: NET alone significantly increased the proliferation of WT-12 cells, MPA was effective only at the two highest concentrations, and progesterone had no effect. The twofold to threefold E-2-induced increase (10(-10) M) was not significantly influenced by the addition of the various progestogens. In contrast, 10(-12) M E-2 had no effect; however, addition of MPA and NET triggered a significant proliferative response. In vivo, a sequential combination of NET and E-2 also significantly increased the tumor growth of WT-12 cells; empty vector cells did not respond to NET. Conclusions: We have demonstrated for the first time that an E-2/NET combination increases the proliferation of PGRMC1-overexpressing breast cancer cells, both in vivo and in vitro. Our results suggest that undetected tumor cells overexpressing PGRMC1 may be more likely to develop into frank tumor cells in women undergoing E-2/NET hormone therapy.