What is the Origin of PR-A Isoform?

The definitive identification of the two main progesterone receptor isoforms, PR-A and PR-B, resulted from molecular biology studies in the late 1980s that revealed alternative transcription start sites on the PGR gene. This discovery provided a molecular explanation for the varied and complex effects of progesterone observed across different reproductive and non-reproductive tissues. The nomenclature distinguishes the shorter 'A' form from the longer, canonical 'B' form.

What is the Mechanism of PR-A Isoform?

Both PR-A and PR-B bind progesterone with similar high affinity and then dimerize to interact with progesterone response elements (PREs) on the target DNA. However, the absence of the N-terminal domain in PR-A fundamentally changes its transcriptional activity, often leading it to repress the full agonist action of PR-B. The specific ratio of PR-A to PR-B within a target cell is a critical determinant of that tissue's overall sensitivity and physiological response to circulating progesterone, a key factor in reproductive health and cancer biology.

PR-A Isoform

Meaning

PR-A, or Progesterone Receptor A isoform, is one of the two primary protein variants transcribed from the single progesterone receptor gene (PGR). It is a truncated version of the full-length PR-B isoform, specifically lacking a critical N-terminal activation domain essential for full function. Functionally, PR-A often acts as a dominant negative regulator, capable of binding progesterone but frequently repressing the transcriptional activity of both PR-B and other steroid receptors, depending entirely on the cellular context.

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How Do Testosterone and Progesterone Interact to Influence Female Breast Health?

Testosterone and progesterone create a vital balance in breast tissue, with progesterone driving maturation and testosterone ensuring structural stability.

∞ Breast Cancer
∞ Progesterone Receptor
∞ Menstrual Cycle Hormonal Changes