Estrogen Receptor Isoform-Specific Induction of Progesterone Receptors in Human Osteoblasts

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Abstract
Estrogen induction of progesterone receptor (PR) expression may be important to bone physiology because progesterone has been implicated in the control of bone formation and resorption. Although PR gene expression can be induced in osteoblasts by estrogen signaling through the estrogen receptor (ER) a isoform, it is unknown whether the ER-beta isoform is involved in this regulation. The effect of estrogen on PR expression was examined in human fetal osteoblast (hFOB) cell lines stably transfected with either ER-alpha or ER-beta. Estrogen treatment of hFOB/ER-a cells induced PR messenger RNA (mRNA) steady-state levels after 24 h and protein levels after 48 h, as established by competitive reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting. Interestingly, no induction of PR expression was observed in the hFOB/ER-beta cells during this period. However, PR mRNA was induced progressively after 48 h of treatment with estrogen with maximum levels achieved at 12 days posttreatment. ER protein also was increased after 12 days of treatment. Both A and B isoforms of PR (PRA and PRB) were induced by estrogen in the hFOB/ER-a cells as well as much later in hFOB/ER-beta cells. The pure antiestrogen ICI 182,780 prevented PR induction by estrogen in both cell lines. An ER-beta-selective antagonist R, R-tetrahydrochrysene (THC) abolished the induction of PR mRNA in hFOB/ER-beta but not in hFOB/ER-a cells, verifying that the response in the former cell line was ER-beta-mediated. Transient cotransfection of hFOB cells with ER-a or ER-beta together with either a human PRA or PRB promoter linked to a reporter plasmid revealed that although the PRB promoter was stimulated equally by estrogen activation of either ER isoform, PRA was activated preferentially by ER-alpha. Together, these results show that although estrogen can up-regulate endogenous PR gene expression in osteoblasts via both ER isoforms, ER-alpha is the predominant inducer.