Phosphorylation of human progesterone receptors at serine-294 by mitogen-activated protein kinase signals their degradation by the 26 S proteasome. Proc Natl Acad Sci USA

Department of Medicine, University of Colorado, Denver, Colorado, United States
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 02/2000; 97(3):1032-7. DOI: 10.1073/pnas.97.3.1032
Source: PubMed


Ligand-dependent down-regulation that leads to rapid and extensive loss of protein is characteristic of several nuclear steroid receptors, including human progesterone receptors (PRs). In breast cancer cells, >95% of PRs are degraded 6 h after the start of progestin treatment. The mechanism for down-regulation is unknown. We examined the role of PR phosphorylation by mitogen-activated protein kinases (MAPKs) in this process. Lactacystin and calpain inhibitor I, specific inhibitors of the 26S proteasome, blocked progestin-induced down-regulation, and ubiquitinated conjugates of PR accumulated in cells. Ligand-dependent PR degradation was also blocked by specific inhibition of p42 and p44 MAPKs. To define the targets of phosphorylation by this kinase, two serine/proline MAPK consensus sites on PR were mutated. We demonstrate that mutation of PR serine-294 to alanine (S294A) specifically and completely prevents ligand-dependent receptor down-regulation. We also find that rapid, ligand-independent degradation of immature PR intermediates occurs by a proteasome-mediated pathway. These results demonstrate that PR destruction, by either of two alternate routes, is mediated by the 26S proteasome. Specifically, down-regulation of mature PRs occurs by a mechanism in which ligand binding activates PR phosphorylation by MAPKs at a unique serine residue, which then targets the receptors for degradation.

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    • "These receptor modifications dramatically alter PR function, receptor localization and turnover, and promoter selectivity. The PR can be phosphorylated basally in the absence of the hormonal ligand, but is potently modified after ligand treatment, in response to local growth factors or in a cell cycle-dependent manner [12,13,15-17] (G. Dressing and C. Lange, unpublished data). "
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    ABSTRACT: The ovarian steroid hormone, progesterone, and its nuclear receptor, the progesterone receptor, are implicated in the progression of breast cancer. Clinical trial data on the effects of hormone replacement therapy underscore the importance of understanding how progestins influence breast cancer growth. The progesterone receptor regulation of distinct target genes is mediated by complex interactions between the progesterone receptor and other regulatory factors that determine the context-dependent transcriptional action of the progesterone receptor. These interactions often lead to post-translational modifications to the progesterone receptor that can dramatically alter receptor function, both in the normal mammary gland and in breast cancer. This review highlights the molecular components that regulate progesterone receptor transcriptional action and describes how a better understanding of the complex interactions between the progesterone receptor and other regulatory factors may be critical to enhancing the clinical efficacy of anti-progestins for use in the treatment of breast cancer.
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    • "Interestingly, hormone-dependent down-regulation that leads to rapid and extensive loss of receptor is characteristic of other nuclear steroid receptors, including human progesterone receptors (PRs). In particular, in breast cancer cells, it has been demonstrated that PR is phosphorylated by ERK in Ser294 and degraded by a 26S proteasome-mediated pathway 6 hrs after treatment with progestin [18]. These results indicate that steroid hormones evoke surprising similar mechanisms to trigger receptor down-regulation. "
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