Thus screening for PGRMC1 expression might be of interest to identify women with a higher expression of PGRMC1 and who might thus be susceptible for breast cancer development under hormone replacement therapy (HRT). The data presented are very important in terms of the positive results of progesterone and breast cancer risk in clinical studies so far. Of further interest is if synthetic progestins that are used for HRT are different in their stimulation of PGRMC1. Abstract Objective

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... influences mammary gland development and probably breast cancer tumorigenesis. We investigated receptor membrane-initiated actions of progesterone in MCF-7 breast cancer cells via progesterone receptor membrane component 1 (PGRMC1). Design&Method The expression of PGRMC1 in breast cancer was verified by immune fluorescent analysis of paraffin sections. MCF-7 cells were transfected with PGRMC1 (wild type) or PGRMC1 variants in which potential interaction domains were mutated. These cells were stimulated with a membrane-impermeable progesterone:BSA:fluoresceinisothiocyanate conjugate (P4:BSA-FITC, 10 -6 M) or unconjugated progesterone (P4, 10 -6 M) in the presence or absence of the progesterone receptor blocker RU486 (10 -6 M) followed by measuring the intracellular ATP content or by employing the MTT assay. Additionally, the effects on the expression of vascular endothelial growth factor A (VEGF-A) was determined using qRT-PCR. Results PGRMC1 is perinuclearly localised in breast cancer cells. Western Blot analysis suggests that PGRMC1 is phosphorylated at serine 180. MCF-7-PGRMC1(S180A) cells with this amino acid changed show an approximately 35% increase in proliferation after seven days of incubation with P4:BSA-FITC compared to MCF-7 control and MCF-7-PGRMC1 (wild type) cells. This effect cannot be blocked by co-inbubation with RU486. Incubation with P4 reduced proliferation of MCF-7-PGRMC1 cells by approximately 10% compared to untreated controls. The proliferative effect of P4:BSA-FITC on MCF-7-PGRMC1(S180A) cells peaks at a concentration of 10 -6 M, higher concentrations act