Pharmacological estrogen administration causes a FSH-independent osteo-anabolic effect requiring ER alpha in osteoblasts.

Postmenopausal osteoporosis is characterized by declining estrogen levels, and estrogen replacement therapy has been proven beneficial for preventing bone loss in affected women. While the physiological functions of estrogen in bone, primarily the inhibition of bone resorption, have been studied extensively, the effects of pharmacological estrogen administration are still poorly characterized. Since elevated levels of follicle-stimulating hormone (FSH) have been suggested to be involved in postmenopausal bone loss, we investigated whether the skeletal response to pharmacological estrogen administration is mediated in a FSH-dependent manner. Therefore, we treated wildtype and FSHß-deficicent (Fshb(-/-)) mice with estrogen for 4 weeks and subsequently analyzed their skeletal phenotype. Here we observed that estrogen treatment resulted in a significant increase of trabecular and cortical bone mass in both, wildtype and Fshb(-/-) mice. Unexpectedly, this FSH-independent pharmacological effect of estrogen was not caused by influencing bone resorption, but primarily by increasing bone formation. To understand the cellular and molecular nature of this osteo-anabolic effect we next administered estrogen to mouse models carrying cell specific mutant alleles of the estrogen receptor alpha (ERα). Here we found that the response to pharmacological estrogen administration was not affected by ERα inactivation in osteoclasts, while it was blunted in mice lacking the ERα in osteoblasts or in mice carrying a mutant ERα incapable of DNA binding. Taken together, our findings reveal a previously unknown osteo-anabolic effect of pharmacological estrogen administration, which is independent of FSH and requires DNA-binding of ERα in osteoblasts.

DNA binding by estrogen receptor-alpha is essential for the transcriptional response to estrogen in the liver and the uterus.

The majority of the biological effects of estrogens in the reproductive tract are mediated by estrogen receptor (ER)alpha, which regulates transcription by several mechanisms. Because the tissue-specific effects of some ERalpha ligands may be caused by tissue-specific transcriptional mechanisms of ERalpha, we aimed to identify the contribution of DNA recognition to these mechanisms in two clinically important target organs, namely uterus and liver. We used a genetic mouse model that dissects DNA binding-dependent vs. independent transcriptional regulation elicited by ERalpha. The EAAE mutant harbors amino acid exchanges at four positions of the DNA-binding domain (DBD) of ERalpha. This construct was knocked in the ERalpha gene locus to produce ERalpha((EAAE/EAAE)) mice devoid of a functional ERalpha DBD. The phenotype of the ERalpha((EAAE/EAAE)) mice resembles the general loss-of-function phenotype of alphaER knockout mutant mice with hypoplastic uteri, hemorrhagic ovaries, and impaired mammary gland development. In agreement with this phenotype, the expression pattern of the ERalpha((EAAE/EAAE)) mutant mice in liver obtained by genome-wide gene expression profiling supports the observation of a near-complete loss of estrogen-dependent gene regulation in comparison with the wild type. Further gene expression analyses to validate the results of the microarray data were performed by quantitative RT-PCR. The analyses indicate that both gene activation and repression by estrogen-bound ERalpha rely on an intact DBD in vivo.