Detailed definition
The progesterone receptor (PR) is a member of the nuclear receptor superfamily, structurally related to the estrogen, androgen, and glucocorticoid receptors. Two main isoforms, PR-A and PR-B, are produced from the same gene by alternative promoter use. PR-A is generally a weaker transactivator than PR-B and can repress PR-B function; their relative expression varies by tissue. PR expression is itself regulated by estrogen — estrogen induces PR transcription, which is why progesterone is most active in estrogen-primed tissues like endometrium and breast. In the endometrium, progesterone binding to PR drives the proliferative-to-secretory transition that is the basis of endometrial protection in HRT.
Why it matters in menopause
Knowing that progesterone needs estrogen-primed PR to act effectively explains why postmenopausal women can take progesterone alone without much effect, but adding estrogen brings PR back online. It also explains why progesterone-only contraception (mini-pills, implants) can fail in low-estrogen states.
Related terms
Sources
External references: Wikipedia.