The progesterone receptor A (PR-A) is one of three known isoforms of the progesterone receptor (PR), the main biological target of the endogenous progestogen sex hormone progesterone. The other isoforms of the PR include the PR-B and PR-C.
PR-A is 164 residues shorter than PR-B in humans and anywhere from 128-165 residues shorter in other organisms. Each isoform binds its natural ligand, progesterone, but also demonstrates the ability to bind a number of other agonists including norethindrone, a synthetic progestin.
Expression and overexpression PR-A and PR-B are generally expressed in equal ratios, but PR-A is expressed in larger amounts in uterine stromal cells normally. A spike in PR-A expression in the myometrium has been observed to initiate parturition in placental mammals.
PR-A is the isoform most commonly observed to be overexpressed in human breast cancer. Currently PR is estimated by immunohistochemistry and earlier was quantified by standardized radio-ligand binding assays developed by New England Nuclear and Wittliff.Fisher, B., Redmond, C., Brown, A., Wickerham, D. L., Wolmark, N., Allegra, J. C., Escher, G., Lippman, M., Savlov, E., Wittliff, J. L. and Fisher, E. R. et al. Influence of Tumor Estrogen and Progesterone Receptor Levels on the Response to Tamoxifen and Chemotherapy in Primary Breast Cancer. J. Clin. Oncol., 1:227-241, 1983. https://pubmed.ncbi.nlm.nih.gov/6366135/ Patients with PR-A rich carcinomas, as opposed to patients with PR-B rich carcinomas, have faster recurrence rates.
See also Membrane progesterone receptor
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