Progesterone is one of the central regulators of female reproduction. In breast development, progesterone is involved in the formation of lobular-alveolar structures and also affects differentiation in the breast by modulation of milk protein synthesis. In human endometrium, progesterone directs glandular differentiation and glycogenesis, as well as stromal proliferation and development of predecidual cells. The cellular effects of progesterone are mediated through progesterone receptors (PR). PR, a protein with 946 amino acids, is a ligand-activated transcription factor member of the steroid receptor super family of nuclear receptors. The functional structure is similar to that of estrogen receptor (ER), with considerable sequence homology in the DNA-binding central domain. The presence of a functional ER is required for PR synthesis in the cell. PR is predominantly expressed in female sex steroid responsive tissues such as the mammary gland, uterus and ovary but is also found in other tissues such as endocrine cells of the Langerhans' islets. PR exists in two isoforms, PR-A and PR-B, transcribed from two promoters by a single gene. The two PR isoforms are identical except that PR-A lacks 164 amino acids contained at the N-terminal end of PR-B. In human tissues PR-A and PR-B are usually co-expressed equivalently. Predominant expression of one isoform occurs in some tissues and circumstances. PR-A is the major isoform in the uterine stroma, and PR-B is the predominant isoform in the endometrial glands. Normal human breast expresses PR-A and PR-B to equal extent.
PR is predominantly expressed in tumours of female sex steroid responsive tissues such as the mammary gland, endometrium and the ovary. About half of the breast carcinomas are ER+/PR+. A small fraction (<5%) is ER-/PR+. About half of the non-mucinous ovarian carcinomas are also PR+. From other PR-expressing tumours, meningiomas, various pancreatic neoplasms such as solid-pseudopapillary tumour and endocrine tumours, and salivary gland neoplasms are worth mentioning. The ER and PR status has been used for over 20 years as a predictor of breast carcinoma responsiveness to endocrine therapy and as a prognostic indicator for early recurrence. Up to 75% of ER+/PR+ breast carcinomas respond positively to endocrine treatment. ER+/PR- tumours are less responsive, and thus PR status adds information to ER-status. In combination the two predict benefit from endocrine therapy both in adjuvant setting and in advanced disease. In breast cancer predominance of one isoform, namely PR-B, is common. The majority of endometrial carcinomas express only one isoform.
The applications of antibodies to PR are similar to those against ER, i.e. diagnosis of PR-positive tumours (often metastasis) and prediction of therapeutic response of breast carcinoma (see description of ER).
Uterine cervix is an appropriate positive tissue control - almost all columnar epithelial cells, basal squamous epithelial cells and most of the stromal cells must show a strong and distinct nuclear staining reaction with only a minimal cytoplasmic staining.
Tonsil is an appropriate negative tissue control – no nuclear staining reaction should be seen.
In concordance with the observations seen in previous PR assessments, it is highly advisable to include both PR negative and PR positive breast tumours and well characterized PR negative tissues as tonsil in the validation/verification process of the PR assay and meticulously monitor the PR expression results and metrics produced in the laboratory.
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