Estrogen regulated transcription of the non-estrogen-regulated hamster uteroglobin gene in MCF-7 cells.

To study the estrogen regulated transcription of the uteroglobin (UG) gene, the founding member of the secretoglobin family widely expressed in many different mammalian species, we re-created functional estrogen response elements (EREs) in the UG gene promoter from a species where UG expression is not regulated by estrogens: the hamster Mesocricetus auratus (Ma), to ascertain if the lack of functional EREs is the real cause of its estrogen insensitivity. Functional EREs in the hamster promoter, including the consensus ERE (cERE), failed to respond to an appropriate estrogen stimulus compared with its estrogen regulated ortholog from the brown hare Lepus capensis (Lc). As the nucleotide sequence is the only difference between genetic constructs from these two species, we suspected that the UG promoter from the hamster probably contains cis-acting genetic elements that negatively impairs the estrogen-regulated transcription mediated by the functional ERE. Accordingly, we prepared chimeric DNA constructs which eventually allowed to identify a region located 29 base pairs (bp) downstream of the ERE as responsible for the lack of estrogen-responsiveness of the Ma-UG gene in the breast cancer cell line MCF-7. This region contains the sequence ACACCCC which has been identified as the core sequence of the Sp/ Krüppel-like factor (KLF) family of transcription factors. This finding is relevant, not only due to the observation on a novel mechanism that control estrogen-induced transcription, but also because it may encourage further investigation for better defining specific genes with an ERE that do not respond to estrogen signaling in MCF-7 cells, a cell line widely employed as an in vitro model in breast cancer research.

Expression profiles of the Wnt/ß-catenin signaling-related extracellular antagonists during proliferation and differentiation in human osteoblast-like cells.

Bone formation is a dynamic process directed by osteoblast activity. The transition from the proliferation to differentiation stage during osteoblast maturation involves the downregulation of the Wnt/ß-catenin signaling pathway, and extracellular antagonists are important for the regulation of Wnt signaling. However, the expression levels of Wnt antagonists in these stages of human osteoblast maturation have not been fully elucidated. Therefore, the aim of the present study was to investigate the expression levels of extracellular Wnt antagonists during proliferation and differentiation in osteoblast-like cell lines. The results demonstrated an overlap between the differential expression of secreted Frizzled-related protein (SFPR)2, SFRP3, SFRP4 and Dickkopf (DKK) 2 genes during the differentiation stage in the MG-63 and Saos-2 cells. Furthermore, high expression levels of DKK3 in MG-63 cells, Wnt inhibitory factor 1 (WIF1) in Saos-2 cells and DKK4 in hFOB 1.19 cells during the same stage (differentiation), were observed. The upregulated expression levels of Wnt antagonists were also correlated with the high expression of anxin 2 during the differentiation stage. These findings suggested that Wnt-related antagonists could modulate the Wnt/ß-catenin signaling pathway. By contrast, DKK1 was the only gene that was found to be upregulated during the proliferation stage in hFOB 1.19 and Saos-2 cells. To the best of our knowledge, the present study provides, for the first time, the expression profile of Wnt antagonists during the proliferation stage and the initial phases of differentiation in osteoblast-like cell lines. The current results offer a basis to investigate potential targets for bone-related Wnt-signaling modulation in bone metabolism research.

The Non-Aromatic Δ5-Androstenediol Derivative of Dehydroepiandrosterone Acts as an Estrogen Agonist in Neonatal Rat Osteoblasts through an Estrogen Receptor α-related Mechanism.

Purpose: It has been proposed that DHEA influences bone formation through, bioconversion to 17ß-estradiol; however, DHEA is converted to Δ5-androstenediol (Δ5-Adiol), a metabolite with estrogenic potential involved in diverse biological process. To gain new insight into the role of Δ5-Adiol in bone cells, we examined DHEA and Δ5-Adiol effects in neonatal rat and human hFOB1.19 osteoblasts. Methods: Osteoblast activity was assessed by analyzing proliferation, alkaline phosphatase activity, and expression of OSX and ALPL. We also examined binding affinities for osteoblast-ER and transcriptional activation of human (h)ERα, hERß or hAR in U2-OS cells. Results: The most striking finding was that Δ5-Adiol had greater stimulatory effect than DHEA on rat osteoblast proliferation and differentiation, as well as ALPL expression in human osteoblasts. Interestingly, the Δ5-Adiol or DHEA-induced effects were not precluded with letrozole or trilostane, consistent with bioconversion of DHEA to Δ5-Adiol due to elevated expression of Hsd17b1 in neonatal rat osteoblasts, suggesting a high level of 17ß-hydroxysteroid dehydrogenase type 1 activity. Conversely, Δ5-Adiol and DHEA-induced proliferative effects were inhibited with ICI 182780 alone or combined with trilostane, which correlates with the higher binding affinity of Δ5-Adiol for ER compared to DHEA. Furthermore, Δ5-Adiol showed a greater relative agonist activity for hERα than for hERß or hAR. Conclusion: This study is the first to show that a bioactive DHEA derivative stimulates E2-dependent osteoblast activities, including proliferation and differentiation in rat and human osteoblasts, through ERα-related mechanisms.

The synthetic progestin, gestodene, affects functional biomarkers in neonatal rat osteoblasts through an estrogen receptor-related mechanism of action.

BACKGROUND: Clinical studies have shown that gestodene (GDN), a potent third-generation synthetic progestin, affects bone resorption. However, its mode of action in bone cells is not fully understood. The aim of this study was to establish whether GDN affects bone directly or through its bioconversion to other metabolites with different biological activities. METHODS: In this study, we investigated the effects of GDN and its A-ring reduced metabolites on proliferation, differentiation, and mineralization of calvarial osteoblasts isolated from neonatal rat and their capacity to displace [3H]-E2 at ER binding sites. RESULTS: In contrast to progesterone, gestodene did exert significant effects on osteoblast activities. The most striking finding was the observation that the A-ring reduced derivatives 3ß,5α-tetrahydro-GDN and 3α,5α-tetrahydro-GDN, though to a lesser extent, had greater stimulatory effects on the osteoblast activity than those observed with GDN. The effects on osteoblast proliferation and differentiation induced by GDN-reduced derivatives were abolished by the antiestrogen ICI 182780, consistent with their binding affinities for the estrogen receptor. In addition, the presence of a 5α-reductase inhibitor or inhibitors of aldo-keto hydroxysteroid dehydrogenases abolished the GDN-induced enhancement of osteoblast differentiation. These results indicated that GDN is metabolized to the A-ring reduced metabolites with estrogen-like activities and through this mechanism, GDN may affect the osteoblast activity. CONCLUSION: Together, the data suggest that synthetic progestins derived from 19-nortestosterone such as GDN, have beneficial effects on bone due to their biotransformation into metabolites with intrinsic estrogenic activity.

Neonatal rat osteoblasts bioconvert testosterone to non-phenolic metabolites with estrogen-like effects on bone cell proliferation and differentiation.

Testosterone (T) restores bone mass loss in postmenopausal women and osteoporotic men mainly through its bioconversion to estradiol (E2). In target tissues, T is also biotransformed to the A-ring-reduced metabolites 3α,5α-androstanediol (3α,5α-diol) and 3ß,5α-androstanediol (3ß,5α-diol), which are potent estrogen receptor (ER) agonists; however, their biological role in bone has not been completely elucidated. To assess if osteoblasts bioconvert T to 3α,5α-diol and to 3ß,5α-diol, we studied in cultured neonatal rat osteoblasts the metabolism of [14C]-labeled T. In addition, the intrinsic estrogenic potency of diols on cell proliferation and differentiation in neonatal calvarial rat osteoblasts was also investigated. Osteoblast function was assessed by determining cell DNA, cell-associated osteocalcin, and calcium content, as well as alkaline phosphatase activity and Alp1 gene expression. The results demonstrated that diols were the major bioconversion products of T, with dihydrotestosterone being an obligatory intermediary, thus demonstrating in the rat osteoblasts the activities of 5α-steroid reductase and 3α- and 3ß-hydroxysteroid dehydrogenases. The most important finding was that 3ß,5α- and 3α,5α-diols induced osteoblast proliferation and differentiation, mimicking the effect of E2. The observation that osteoblast differentiation induced by diols was abolished by the presence of the antiestrogen ICI 182,780, but not by the antiandrogen 2-hydroxyflutamide, suggests that diols effects are mediated through an ER mechanism. The osteoblast capability to bioconvert T into diols with intrinsic estrogen-like potency offers new insights to understand the mechanism of action of T on bone cells and provides new avenues for hormone replacement therapy to maintain bone mass density.

Bioconversion of norethisterone, a progesterone receptor agonist into estrogen receptor agonists in osteoblastic cells.

A number of clinical studies have demonstrated that norethisterone (NET), a potent synthetic progestin, restores postmenopausal bone loss, although its mode of action on bone cells is not fully understood, while the effect of naturally occurring progesterone in bone has remained controversial. A recent report claims that the potent effects of NET on osteoblastic cell proliferation and differentiation, mimicking the action of estrogens, are mediated by non-phenolic NET derivatives. To determine whether osteoblasts possess the enzymes required to bioconvert a progesterone receptor (PR) agonist into A-ring reduced metabolites with affinity to bind estrogen receptor (ER), we studied the in vitro metabolism of [(3)H]-labeled NET in cultured neonatal rat osteoblasts and the interaction of its metabolic conversion products with cytosolic -osteoblast ER, employing a competition analysis. Results indicated that NET was extensively bioconverted (36.4%) to 5 alpha-reduced metabolites, including 5 alpha-dihydro NET, 3 alpha,5 alpha-tetrahydro NET (3 alpha,5 alpha-NET) and 3beta,5 alpha-tetrahydro NET (3beta,5 alpha-NET), demonstrating the activities of 5 alpha-steroid reductase and two enzymes of the aldo-keto reductases family. Expression of Srd5a1 in neonatal osteoblast was well demonstrated, whereas Srd5a2 expression was not detected. The most striking finding was that 3beta,5 alpha-NET and 3 alpha,5 alpha-NET were efficient competitors of [(3)H]-estradiol for osteoblast ER binding sites, exhibiting affinities similar to that of estradiol. The results support the concept that the interplay of 5 alpha-steroid reductase and aldo-keto reductases in osteoblastic cells, acting as an intracrine modulator system is capable to bioconvert a PR agonist into ER agonists, offering an explanation of the molecular mechanisms NET uses to enhance osteoblastic cell activities.

The effects of synthetic 19-norprogestins on osteoblastic cell function are mediated by their non-phenolic reduced metabolites.

The key role of estrogens on osteoblastic cell function is well documented; however, the role of progesterone (P) and synthetic progestins remains controversial. While several reports indicate that P has no significant effects on bone cells, a number of clinical studies have shown that 19-norprogestins restore postmenopausal bone loss. The mechanisms by which 19-norprogestins induce estrogen-like effects on bone cells are not fully understood. To assess whether the actions of 19-norprogestins on osteoblasts are mediated by their non-phenolic metabolites, we studied the effects of norethisterone (NET), levonorgestrel (LNG), and two of their A-ring reduced derivatives upon cell proliferation and differentiation in neonatal rat osteoblasts. Osteoblast function was assessed by determining cell DNA, cell-associated osteocalcin and calcium content, alkaline phosphatase activity, and mineral deposition. P failed to induce changes on osteoblasts, while NET and LNG exerted a number of actions. The most striking finding was that the 3beta,5alpha- and 3alpha,5alpha-tetrahydro derivatives of NET and LNG induced osteoblast proliferation and differentiation with higher potency than those exerted by their parent compounds, mimicking the effects of estradiol. Interestingly, osteoblast differentiation and mineral deposition induced by NET and LNG were abolished by finasteride, a 5alpha-reductases inhibitor, while the potent effect on osteoblast proliferation induced by progestin derivatives was abolished by a steroidal antiestrogen. Results demonstrate that A-ring reduced derivatives of NET and LNG exhibit intrinsic estrogen-like potency on rat osteoblasts, offering a plausible explanation for the mechanism of action of 19-norprogestins in bone restoration in postmenopausal women and providing new insights for hormone replacement therapy research.