Androgen receptor stimulates bone sialoprotein (BSP) gene transcription via cAMP response element and activator protein 1/glucocorticoid response elements

Exogenous PTH 1-34 Attenuates Impaired Fracture Healing in Endogenous PTH Deficiency Mice via Activating Indian Hedgehog Signaling Pathway and Accelerating Endochondral Ossification

Fracture healing is a complicated, long-term, and multistage repair process. Intermittent administration of parathyroid hormone (PTH) has been proven effective on intramembranous and endochondral bone formation during the fracture healing process, however, the mechanism is unclear. In this study, we investigated the role of exogenous PTH and endogenous PTH deficiency in bone fracture healing and explored the mechanism by using PTH knockout (PTH^-/-) mice and ATDC5 cells. In a mouse femur fracture model, endogenous PTH deficiency could delay endochondral ossification whereas exogenous PTH promotes accumulation of endochondral bone, accelerates cartilaginous callus conversion to bony callus, enhances maturity of bony callus, and attenuates impaired fracture healing resulting from endogenous PTH deficiency. In fracture callus tissue, endogenous PTH deficiency could inhibit chondrocyte proliferation and differentiation whereas exogenous PTH could activate the IHH signaling pathway to accelerate endochondral ossification and rescue impaired fracture healing resulting from endogenous PTH deficiency. In vitro, exogenous PTH promotes cell proliferation by activating IHH signaling pathway on ATDC5 cells. In mechanistic studies, by using ChIP and luciferase reporter assays, we showed that PTH could phosphorylate CREB, and subsequently bind to the promoter of IHH, causing the activation of IHH gene expression. Therefore, results from this study support the concept that exogenous PTH 1-34 attenuates impaired fracture healing in endogenous PTH deficiency mice via activating the IHH pathway and accelerating endochondral ossification. Hence, the investigation of the mechanism underlying the effects of PTH treatment on fracture repair might guide the exploration of effective therapeutic targets for fracture.

Adrenomedullin promotes the odontogenic differentiation of dental pulp stem cells through CREB/BMP2 signaling pathway

Adrenomedullin (AM) could promote the proliferation, the odontogenic differentiation and inhibit the apoptosis of dental pulp stem cells (DPSCs). AM in combination with DPSCs may be an effective strategy for pulp repair. However, there was no report on the mechanisms of AM in the odontogenic differentiation of DPSCs. The aim of this study is to investigate the molecular mechanisms through which AM promotes the odontogenic differentiation of DPSCs. Freshly extracted wisdom teeth were obtained from 27 patients. Cells at passage 3 to passage 5 were used in this study. DPSCs were treated with or without 10-7 M AM in Dulbecco's modified Eagle's medium culture, and then the accumulated calcium deposition was analyzed after 21 days by using alizarin red S staining. Odontogenic differentiation markers were determined by western blot analysis and quantitative real-time PCR. Western blot analysis results showed that AM had the capability of promoting the odontogenic differentiation of DPSCs and AM could enhance the phosphorylation of CREB and up-regulate the expression of BMP2. H89 is a CREB inhibitor which can inhibit the odontogenic differentiation of DPSCs through inhibiting the phosphorylation of CREB. Noggin could inhibit the odontogenic differentiation of DPSCs through inhibiting the activity of BMP2. These results indicated that AM could promote the odontogenic differentiation of DPSCs by upregulating the expression of BMP2 through the CREB signaling pathway.

Osthole Enhances Osteogenesis in Osteoblasts by Elevating Transcription Factor Osterix via cAMP/CREB Signaling In Vitro and In Vivo

Anabolic anti-osteoporotic agents are desirable for treatment and prevention of osteoporosis and fragility fractures. Osthole is a coumarin derivative extracted from the medicinal herbs Cnidium monnieri (L.) Cusson and Angelica pubescens Maxim.f. Osthole has been reported with osteogenic and anti-osteoporotic properties, whereas the underlying mechanism of its benefit still remains unclear. The objective of the present study was to investigate the osteopromotive action of osthole on mouse osteoblastic MC3T3-E1 cells and on mouse femoral fracture repair, and to explore the interaction between osthole-induced osteopromotive effect and cyclic adenosine monophosphate (cAMP) elevating effect. Osthole treatment promoted osteogenesis in osteoblasts by enhancing alkaline phosphatase (ALP) activity and mineralization. Oral gavage of osthole enhanced fracture repair and increased bone strength. Mechanistic study showed osthole triggered the cAMP/CREB pathway through the elevation of the intracellular cAMP level and activation of the phosphorylation of the cAMP response element-binding protein (CREB). Blockage of cAMP/CREB downstream signals with protein kinase A (PKA) inhibitor KT5720 partially suppressed osthole-mediated osteogenesis by inhibiting the elevation of transcription factor, osterix. In conclusion, osthole shows osteopromotive effect on osteoblasts in vitro and in vivo. Osthole-mediated osteogenesis is related to activation of the cAMP/CREB signaling pathway and downstream osterix expression.

The transcription factor cyclic adenosine 3',5'-monophosphate response element-binding protein enhances the odonto/osteogenic differentiation of stem cells from the apical papilla

AIM

To investigate the role of cAMP response element-binding protein (CREB) in the regulation of odonto/osteogenic differentiation of stem cells from the apical papilla (SCAPs).

METHODOLOGY

Stem cells from the apical papilla were obtained from human impacted third molars (n = 15). Isolated SCAPs were transfected with CREB overexpressing/silenced lentivirus. Transfected cells were stained with alizarin red to investigate mineralized nodule formation. The expression of the mineralization-related genes, alkaline phosphatase (ALP), collagen type I (Col I), runt-related transcription factor 2 (RUNX2), osterix (OSX) and osteocalcin (OCN), was determined by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Protein expression of the odontogenic-related marker dentine sialoprotein (DSP) and the osteogenic-related marker RUNX2 was measured by Western blotting analysis. One-way analysis of variance (anova) and Student's t-test were used for statistical analysis (a = 0.05).

RESULTS

The overexpression of CREB enhanced mineralized nodule formation and up-regulated (P < 0.05) the mRNA levels of odonto/osteogenic-related markers, including ALP, Col I, RUNX2, OSX and OCN, and also increased (P < 0.05) the protein expression of DSP and RUNX2. In contrast, the silencing of CREB inhibited (P < 0.05) the mineralization capacity of the SCAPs and decreased (P < 0.05) the expression of odonto/osteogenic-related markers.

CONCLUSION

Up-regulation of CREB expression promoted odonto/osteogenic differentiation of SCAPs and provided a potential method for the regeneration of the dentine-pulp complex.

Small-molecule hormones: molecular mechanisms of action

Small-molecule hormones play crucial roles in the development and in the maintenance of an adult mammalian organism. On the molecular level, they regulate a plethora of biological pathways. Part of their actions depends on their transcription-regulating properties, exerted by highly specific nuclear receptors which are hormone-dependent transcription factors. Nuclear hormone receptors interact with coactivators, corepressors, basal transcription factors, and other transcription factors in order to modulate the activity of target genes in a manner that is dependent on tissue, age and developmental and pathophysiological states. The biological effect of this mechanism becomes apparent not earlier than 30-60 minutes after hormonal stimulus. In addition, small-molecule hormones modify the function of the cell by a number of nongenomic mechanisms, involving interaction with proteins localized in the plasma membrane, in the cytoplasm, as well as with proteins localized in other cellular membranes and in nonnuclear cellular compartments. The identity of such proteins is still under investigation; however, it seems that extranuclear fractions of nuclear hormone receptors commonly serve this function. A direct interaction of small-molecule hormones with membrane phospholipids and with mRNA is also postulated. In these mechanisms, the reaction to hormonal stimulus appears within seconds or minutes.

Transcriptional regulation of BMP2 expression by the PTH-CREB signaling pathway in osteoblasts

Intermittent application of parathyroid hormone (PTH) has well established anabolic effects on bone mass in rodents and humans. Although transcriptional mechanisms responsible for these effects are not fully understood, it is recognized that transcriptional factor cAMP response element binding protein (CREB) mediates PTH signaling in osteoblasts, and that there is a communication between the PTH-CREB pathway and the BMP2 signaling pathway, which is important for osteoblast differentiation and bone formations. These findings, in conjunction with putative cAMP response elements (CREs) in the BMP2 promoter, led us to hypothesize that the PTH-CREB pathway could be a positive regulator of BMP2 transcription in osteoblasts. To test this hypothesis, we first demonstrated that PTH signaling activated CREB by phosphorylation in osteoblasts, and that both PTH and CREB were capable of promoting osteoblastic differentiation of primary mouse osteoblast cells and multiple rodent osteoblast cell lines. Importantly, we found that the PTH-CREB signaling pathway functioned as an effective activator of BMP2 expression, as pharmacologic and genetic modulation of PTH-CREB activity significantly affected BMP2 expression levels in these cells. Lastly, through multiple promoter assays, including promoter reporter deletion, mutation, chromatin immunoprecipitation (ChIP), and electrophoretic mobility shift assay (EMSA), we identified a specific CRE in the BMP2 promoter which is responsible for CREB transactivation of the BMP2 gene in osteoblasts. Together, these results demonstrate that the anabolic function of PTH signaling in bone is mediated, at least in part, by CREB transactivation of BMP2 expression in osteoblasts.

The effects of hormonal contraceptives on bone turnover markers and bone health

Sex hormones are important regulators of bone metabolism. As hormonal contraceptives contain either oestrogens or progestins, or a combination thereof, it is conceivable that these widely used agents have an effect on bone metabolism and bone health. The main users of hormonal contraceptives, adolescent girls and young women, are still building bone and accruing bone mass and may therefore be particularly susceptible to the effects of hormonal contraceptives on bone. Despite these concerns, the effects of hormonal contraceptives on bone health are still poorly understood. As biochemical markers of bone turnover have been proven useful tools in the assessment and monitoring of bone metabolism, we reviewed the effects of combined and gestagen-only hormonal contraceptives on bone turnover markers and related effects on bone mineral density and fracture risk in premenopausal women, as documented in the literature until January 2009.

The orphan nuclear receptor SHP is a positive regulator of osteoblastic bone formation

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) interacts with a diverse array of transcription factors and regulates a variety of cellular events such as cell proliferation, differentiation, and metabolism. However, the role of SHP in bone formation has not yet been elucidated. SHP expression is significantly increased during osteoblast differentiation, and its expression is partially regulated by bone morphogenetic protein 2 (BMP-2), which plays an important role in bone formation. In our study, inhibition of SHP expression significantly repressed BMP-2-induced osteoblast differentiation and ectopic bone formation. In accordance with these in vitro and in vivo results, osteoblast differentiation in SHP(-/-) mice primary osteoblasts was significantly repressed, and the mice showed decreased bone mass resulting from decreased numbers of osteoblasts. Finally, SHP physically interacts and forms a complex with runt-related transcription factor 2 (Runx2) on the osteocalcin gene promoter, and overexpression of SHP increased Runx2 transactivity via competition with histone deacetylase 4 (HDAC4), an enzyme that inhibits DNA binding of Runx2 to its target genes. Taken together, these results indicate that SHP acts as a novel positive regulator of bone formation by augmenting osteoblast differentiation through regulation of the transcriptional activity of Runx2.

The SIE, SRE, CRE, and FAP-1 four intracellular signal pathways between stimulus and the expression of c-fos promoter

c-fos gene has a close relationship with the osteoblasts. Mechanical signal effect on osteoblasts would change the expression level of c-fos. Authors introduce the signal pathways of four cis-response elements on the promoter of c-fos, that is, CRE (cAMP responsive element), FAP-1 (Fbs-AP-1 site), SRE (serum response element), and SIE (sis-inducible element), as the regulatory mechanism for c-fos gene expression following various stimuli.

Induction of Kruppel-like factor 5 expression by androgens results in increased CXCR4-dependent migration of prostate cancer cells in vitro

Advanced prostate cancers preferentially metastasize to bone, suggesting that this tissue produces factors that provide a suitable microenvironment for prostate cancer cells. Recently, it has become clear that even in antiandrogen-resistant cancers, the androgen receptor (AR)-signaling axis is required for prostate cancer progression. Therefore, we hypothesized that AR may be involved in the regulation of pathways that are responsible for the homing of prostate cancer cells to select microenvironments. In support of this hypothesis, we have determined that chemokine (C-X-C motif) receptor 4 (CXCR4), the receptor for the chemokine CXCL12, is up-regulated in prostate cancer cells in response to androgens. Given that the levels of CXCL12 are elevated at sites of known prostate cancer metastases such as bone, these results suggest that androgens may influence prostate cancer metastasis. Specifically, we demonstrate that androgens increase the levels of both CXCR4 mRNA and functional protein in LNCaP prostate cancer cells. Importantly, androgens enhanced the migration of LNCaP cells toward a CXCL12 gradient, an effect that could be blocked by the specific CXCR4 antagonist AMD3100. Interestingly, CXCR4 is not directly regulated by androgens but rather is positively up-regulated by Krüppel-like factor 5 (KLF5), a transcription factor that we have shown to be an early, direct target of AR. Further, KLF5 is both required and sufficient for androgen-mediated CXCR4 expression and migration toward CXCL12. Taken together, these findings demonstrate that AR can utilize the CXCL12/CXCR4 axis through induction of KLF5 expression to promote prostate cancer progression and highlight the potential utility of CXCR4 antagonists as prostate cancer therapeutics.

Nicotine suppresses bone sialoprotein gene expression

BACKGROUND AND OBJECTIVE

Tobacco smoking is a risk factor for periodontitis and osteoporosis. Nicotine is a major component of tobacco, and has been reported to inhibit proliferation and differentiation of osteoblasts. Bone sialoprotein (BSP) is a mineralized tissue-specific protein expressed by differentiated osteoblasts that appears to function in the initial mineralization of bone. The purpose of this study was to determine the effects of nicotine on bone metabolism.

MATERIAL AND METHODS

We used rat osteobast-like UMR106 and ROS 17/2.8 cells and rat stromal bone marrow RBMC-D8 cells. To determine the molecular basis of the transcriptional regulation of the BSP gene by nicotine, we conducted Northern hybridization, transient transfection analyses with chimeric constructs of the BSP gene promoter linked to a luciferase reporter gene and gel mobility shift assays.

RESULTS

Nicotine (250 microg/mL) decreased the BSP mRNA levels at 12 and 24 h in UMR106 and ROS 17/2.8 cells. From transient transfection assays using various sized BSP promoter-luciferase constructs, nicotine decreased the luciferase activities of the construct, including the promoter sequence nucleotides -116 to +60, in UMR106 and RBMC-D8 cells. Nicotine decreased the nuclear protein binding to the cAMP response element (CRE), fibroblast growth factor 2 response element (FRE) and homeodomain protein-binding site (HOX) at 12 and 24 h.

CONCLUSION

This study indicates that nicotine suppresses BSP transcription mediated through CRE, FRE and HOX elements in the proximal promoter of the rat BSP gene.

Androgens and bone

PURPOSE OF REVIEW

The present review will focus on the most important recent findings with respect to skeletal androgen action. Many studies have indicated that part of the androgen action may be related to the conversion of androgens into estrogens. Therefore, some of the most recent findings of skeletal estrogen action relevant for male skeletal physiology will also be discussed.

RECENT FINDINGS

Androgens and estrogens stimulate bone formation and inhibit bone resorption. Sex steroids may interact with different receptors, target cells and other bone anabolic pathways. Androgen receptor and estrogen receptor signalling appear to be important for male bone formation during growth. Sex steroid signalling may involve genomic and nongenomic pathways, interaction with mechanical loading, the growth hormone/insulin-like growth factor-I axis and/or other bone anabolic pathways. Estrogen receptor alpha in osteoclasts appears to regulate bone resorption in women but not men, whereas androgen receptor signalling in osteoblasts may only partly regulate bone resorption in males.

SUMMARY

The latest developments indicate that androgens and estrogens are important for male bone metabolism and homeostasis and therefore selective estrogen receptor alpha and androgen receptor signalling remain interesting drug targets for the stimulation of bone formation and male skeletal integrity.

Bone sialoprotein and its transcriptional regulatory mechanism

BACKGROUND AND OBJECTIVE

Bone sialoprotein is a mineralized tissue-specific noncollagenous protein that is glycosylated, phosphorylated and sulfated. The temporo-spatial deposition of bone sialoprotein into the extracellular matrix of bone, and the ability of bone sialoprotein to nucleate hydroxyapatite crystal formation, indicates a potential role for bone sialoprotein in the initial mineralization of bone, dentin and cementum. Bone sialoprotein is also expressed in breast, lung, thyroid and prostate cancers.

MATERIAL AND METHODS

We used osteoblast-like cells (rat osteosarcoma cell lines ROS17/2.8 and UMR106, rat stromal bone marrow RBMC-D8 cells and human osteosarcoma Saos2 cells), and breast and prostate cancer cells to investigate the transcriptional regulation of bone sialoprotein. To determine the molecular basis of the transcriptional regulation of the bone sialoprotein gene, we conducted northern hybridization, transient transfection analyses with chimeric constructs of the bone sialoprotein gene promoter linked to a luciferase reporter gene and gel mobility shift assays.

RESULTS

Bone sialoprotein transcription is regulated by hormones, growth factors and cytokines through tyrosine kinase, mitogen-activated protein kinase and cAMP-dependent pathways. Microcalcifications are often associated with human mammary lesions, particularly with breast carcinomas. Expression of bone sialoprotein by cancer cells could play a major role in the mineral deposition and in preferred bone homing of breast cancer cells.

CONCLUSION

Bone sialoprotein protects cells from complement-mediated cellular lysis, activates matrix metalloproteinase 2 and has an angiogenic capacity. Therefore, regulation of the bone sialoprotein gene is potentially important in the differentiation of osteoblasts, bone matrix mineralization and tumor metastasis. This review highlights the function and transcriptional regulation of bone sialoprotein.

AP1 binding site is another target of FGF2 regulation of bone sialoprotein gene transcription

Bone sialoprotein (BSP) is an early marker of osteoblast differentiation. We previously reported that fibroblast growth factor 2 (FGF2) regulates BSP gene transcription via FGF2 response element (FRE) in the proximal promoter of rat BSP gene. We here report that activator protein 1 (AP1) binding site overlapping with glucocorticoid response element (GRE) AP1/GRE in the rat BSP gene promoter is another target of FGF2. Using the osteoblastic cell line ROS17/2.8, we determined that BSP mRNA levels increased by 10 ng/ml FGF2 at 6 and 12 h. Runx2 protein levels increased by FGF2 (10 ng/ml) at 3 h. Treatment of ROS17/2.8 cells with FGF2 (10 ng/ml, 12 h) increased luciferase activities of constructs including -116 to +60 and -938 to +60 of the rat BSP gene promoter. Effects of FGF2 abrogated in constructs included 2 bp mutations in the FRE and AP1/GRE elements. Luciferase activities induced by FGF2 were blocked by tyrosine kinase inhibitor herbimycin A, src-tyrosine kinase inhibitor PP1 and MAP kinase kinase inhibitor U0126. Gel shift analyses showed that FGF2 increased binding of FRE and AP1/GRE elements. Notably, the AP1/GRE-protein complexes were supershifted by Smad1 and c-Fos antibodies, c-Jun and Dlx5 antibodies disrupted the complexes formation, on the other hand AP1/GRE-protein complexes did not change by Runx2 antibody. These studies demonstrate that FGF2 stimulates BSP gene transcription by targeting the FRE and AP1/GRE elements in the rat BSP gene promoter.