The cDNA for a novel truncated progesterone receptor (PR-M) was previously cloned from human adipose and aortic cDNA libraries. The predicted protein sequence contains 16 unique N-terminal amino acids, encoded by a sequence in the distal third intron of the progesterone receptor PR gene, followed by the same amino acid sequence encoded by exons 4 through 8 of the nuclear PR. Thus, PR-M lacks the N terminus A/B domains and the C domain for DNA binding, whereas containing the hinge and hormone-binding domains. In this report, we have localized PR-M to mitochondria using immunofluorescent localization of a PR-M-green fluorescent protein (GFP) fusion protein and in Western blot analyses of purified human heart mitochondrial protein. Removal of the putative N-terminal mitochondrial localization signal obviated association of PR-M with mitochondria, whereas addition of the mitochondrial localization signal to green fluorescent protein resulted in mitochondrial localization. Immunoelectron microscopy and Western blot analysis after mitochondrial fractionation identified PR-M in the outer mitochondrial membrane. Antibody specificity was shown by mass spectrometry identification of a PR peptide in a mitochondrial membrane protein isolation. Cell models of overexpression and gene silencing of PR-M demonstrated a progestin-induced increase in mitochondrial membrane potential and an increase in oxygen consumption consistent with an increase in cellular respiration. This is the first example of a truncated steroid receptor, lacking a DNA-binding domain that localizes to the mitochondrion and initiates direct non-nuclear progesterone action. We hypothesize that progesterone may directly affect cellular energy production to meet the increased metabolic demands of pregnancy.

As an example, progesterone levels correlate with increased epithelial cell proliferation, but there is discordance between the dividing cells and the cells with nuclear progesterone receptor expression. The release of paracrine growth factors from nuclear receptor-positive cells has been postulated as a mechanism, since in vitro studies show a lack of growth effect by progesterone in breast epithelial cells lacking nuclear receptors. This study examined possible nongenomic effects of progesterone in breast epithelia by using MCF-10A cells known to lack nuclear progesterone receptor expression. Treatment for 30 -60 min with progesterone or the progestin, R5020, increased mitochondrial activity as shown by an increase in mitochondrial membrane potential (hyperpolarization) with a concordant increase in total cellular ATP. The reaction was inhibited by a specific progesterone receptor antagonist and not affected by the translation inhibitor cycloheximide. Progestin treatment inhibited apoptosis induced by activation of the FasL pathway, as shown by a decrease in sub-G 1 cell fraction during fluorescence-activated cell sorting and a decrease in caspase 3/7 levels. Progestin treatment did not alter the cell cycle over 48 h. Our study demonstrates a nongenomic action of progesterone on benign breast epithelial cells, resulting in enhanced cellular respiration and protection from apoptosis.progesterone; MCF-10A cells; mitochondria; apoptosis; cellular respiration PREVIOUS STUDIES HAVE SHOWN a discordance between the proliferative action of progesterone in the breast and the content of nuclear progesterone receptors (PR-B and PR-A) in breast epithelial cells. As examples, the mitotic rate of breast epithelial cells is greatest in the progesterone-dominant luteal phase (44). Exogenous administration of estrogen plus progestin results in greater breast epithelial proliferation than estrogen alone (15, 18). The progesterone receptor knockout (PRKO) mouse, an excellent model for the function of nuclear PR in mammary gland development, supports a proliferative role. PR-B is primarily responsible for gland development, with PRKO-B mice showing less extensive ductal development and lack of alveolar terminal end buds (7,29). In contrast to these physiological effects, very few breast epithelial cells appear to express nuclear PR. In immunocytochemical studies of human breast tissue, from 6 to 13% of ductal epithelial cells express PR (6, 37), with the two PR isoforms, B and A, typically localizing in the same cell (14). Immunocytochemical analysis of proliferation by detection of Ki67 antigen in human breast or autoradiographs of [ 3 H]thymidine incorporation in rodent mammary samples show nuclear PR-expressing cells to be nonproliferating. Adjacent cells lacking nuclear PR expression have greater mitotic activity (23). This disassociation between nuclear PR expression and proliferation led to the hypothesis that nuclear PR-expressing cells regulate proliferation of adjacent cells via the control of paracrine factors (4).This ...

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