Secretion and expression regulation of progesterone receptor membrane component1 (PGRMC1) in breast cancer cells

Abstract

In women breast cancer still is the most prevalent cancer and one of the leading causes of death. Progesterone receptor membrane component-1 (PGRMC1) highly more expressed in cancerous breast tissue than in benign surrounding breast tissue may be involved in tumorigenesis and increase breast cancer risk. In recent investigations it could be shown that estrogens and certain synthetic progestogens can induce an increased proliferation rate in breast cancer cells via PGRMC1 suggesting a possible importance of the kind of estrogen and progestogen in terms of breast cancer risk when used as hormone therapy in the postmenopause. However, the detailed mechanisms through which PGMRC1 mediates proliferative effects elicited by progestogens and regulating its expression are still little known. It remained elusive whether PGRMC1 is secreted by breast cancer cells into plasma and thus might be used for cancer risk screening. Here we analyzed PGRMC1 expression and the proliferative effect of progestogens in various breast cancer cell lines. In BM cells (endogenous estrogen receptor (ER-α) and PGMRC1), medroxyprogesterone acetate (MPA), norethisterone (NET), levonorgestrel (LNG) and drospirenone (DRSP) significantly increased the proliferation. However, these progestogens didn’t obviously alter the proliferation of SUM225CWN cells (only endogenous PGRMC1). In MCF-7 breast cancer cells, the presence of PGRMC1 can sensitize E2-induced PS2 mRNA levels, an estrogen response element. Interestingly it could be shown for the first time that certain progestogens such as NET also were able to induce an enhanced PS2 expression in MCF-7/PGRMC1 cells. No effect was found for progesterone. Addition of NET to E2 did not obviously alter the PS2 mRNA levels as compared with E2 only. This effect of E2 and NET could be blocked by antagonists of ER-α, PGRMC1 and CK2, a kinase which is involved in cell proliferation, the effect being highest for the ER-α-antagonist. In order to elucidate the possible involvement of RANK/RANKL, its expression was investigated in various breast cancer cell lines. However, the expression of RANK and RANKL couldn’t be detected in these cell lines using western blot. Estradiol and progestogens didn’t significantly change cell proliferation and RANKL expression in ER-α negative, RANKL overexpressing cells. In co-cultures of MDA-MB-231 and MCF-7/PGRMC1 cells with osteoblast-like cells no induction of RANKL expression was found in the breast cancer cells. Thus the possible involvement of RANK/RANKL in the PGRMC1 mediated progestogenic effect remains unclear. To elucidate probable mechanisms on the influence on endogenous PGRMC1 expression, MCF-7 cells were transfected with a mimic of miRNA let-7i, since let-7i is considered as a tumor suppressor to inhibit malignant growth of cancer cells. We demonstrated that let-7i targets PGRMC1 and inhibited endogenous PGRMC1 mRNA expression. Further, an analysis of PGRMC1 expression in the supernatant of breast cancer cells by western blot revealed that PGRMC1 could be secreted by these cells. However, first investigations with plasma samples from women with and without breast cancer weren’t successful in detecting PGRMC1.

In summary we conclude from these results that ER-α may play an important role in the signal transduction of PGRMC1 activated by estradiol and progestogens, and PGRMC1 may participate in the ER-α/PS2 pathway. PGRMC1 expression might be regulated by a complex mechanism that might involve the suppressive action of miRNAs, e.g. let-7i, which could become a therapeutic target. Moreover, PGRMC1 might be a potential predictive biomarker for breast cancer risk especially in postmenopausal women using hormone therapy.