Antagonist-occupied human progesterone B-receptors activate transcription without binding to progesterone response elements and are dominantly inhibited by A-receptors.

Abstract

When antagonist-occupied steroid receptors have agonist-like effects, the clinical consequences are grave. We present evidence that human progesterone B-receptors (hPRB) when occupied by progesterone antagonists, inappropriately activate transcription by an unusual mechanism that does not require the canonical progesterone response element (PRE). In HeLa cells cotransfected with a PRE-tk-chloramphenicol acetyltransferase reporter and a hPRB expression vector, strong transcription is seen not only when receptors are activated by the agonist R5020, but also in the presence of the three antiprogestins, RU486, ZK112993, and ZK98299. Human PRB occupied by ZK98299 do not bind to a PRE, suggesting that the transcriptional stimulation is independent of DNA binding. Indeed, a tk-chloramphenicol acetyltransferase promoter-reporter lacking the PRE loses transcriptional activation by the agonist, but retains transactivation by the three antagonists. The PRE-independent antagonist-induced transcription requires that hPRB have an intact DNA-binding domain, but hPR target gene specificity is not required, because a hPRB mutant that binds an estrogen response element still activates transcription. It appears that antagonist-occupied hPR activate transcription without binding to a PRE, perhaps by interacting with tethering proteins instead. Even a gene that is not a normal progesterone target could be aberrantly activated. Human cells contain equimolar amounts of hPRB and the N-terminally truncated natural isotype, hPRA. Unlike hPRB, hPRA are not transcriptionally activated by progesterone antagonists. We, therefore, tested the effects of antagonists when the two receptor isotypes are coexpressed and found that A-receptors can annul the inappropriate transcription by B-receptors. Thus, when both receptor forms are present, the hPRA phenotype is dominant. Moreover, pure hPRB/hPRA heterodimers, produced by fos/jun leucine zipper domain-hPR chimeras, also have the inactive transcriptional phenotype of hPRA. Our studies suggest not only that the two hPR isotypes are functionally quite different, but also that some of the agonist-like transcriptional effects of antagonist-occupied B-receptors proceed through novel mechanisms.

Progesterone-regulated genes in the ovulation process: ADAMTS-1 and cathepsin L proteases

“…A similar pattern of rapid yet transient induction of PR is observed in primary cultures of rat granulosa cells in response to LH (9,10). Additionally, both isoforms of PR (PRA and PRB), differentially translated from a single transcript (11)(12)(13)(14), are induced by LH, with PRA being the prevalent translated product (9). The significance of ovarian PR was demonstrated by the targeted disruption of the PR gene; PR knockout (PRKO) female mice lacking PR (PRA and PRB) fail to ovulate, even in response to exogenous hormones, and are completely infertile (15,16).…”

Progesterone Receptor A and B Isoforms in the Human Breast and Its Disorders

“…Human PR exists as two isoforms, A and B. Altered ratios of PR isoform expression have been reported to be closely associated with modulations of various progesterone actions, [21][22][23][24] but the precise functions of PRA and PRB have not been clearly characterized. In the great majority of progesterone-responsive cells, PRB is a dominant activator of progesterone-responsive target genes, whereas PRA may inhibit this PRB activity.…”

“…In addition, several investigators have suggested that one PR isoform could modulate the function of the other isoform. 21,24,25) PRA, but not PRB, has been demonstrated to inhibit gene transcription induced by other families of steroid receptors, including glucocorticoid, androgen, and mineralocorticoid receptors. 25) This inhibition is not only induced by progestins, but also by some antiprogestins.…”

Potential roles of decidual prolactin in early pregnancy

“…In contrast, expression of the progesterone receptor persists in the endometrial stromal compartment and is particularly evident in the perivascular tissues. Both oestrogen and progesterone receptors exist in more than one isoform (Tung et al, 1993;Enmark et al, 1997). A novel oestrogen receptor isoform, oestrogen receptor β, was cloned from a rat prostate cDNA library (Kuiper et al, 1996).…”