Involvement of upstream stimulatory factors 1 and 2 in RANKL-induced transcription of tartrate-resistant acid phosphatase gene during osteoclast differentiation

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Hematopoietic upstream stimulating factor 1 deficiency is associated with increased atherosclerosis susceptibility in LDL receptor knockout mice

Total body upstream stimulatory factor 1 (USF1) deficiency in mice is associated with brown adipose tissue activation and a marked protection against the development of obesity and atherosclerotic lesions. Functional expression of USF1 has also been detected in monocytes and monocyte-derived macrophages. In the current study we therefore tested whether selective hematopoietic USF1 deficiency can also beneficially impact the development of atherosclerosis. For this purpose, LDL receptor knockout mice were transplanted with bone marrow from USF1 knockout mice or their wild-type littermate controls and subsequently fed a Western-type diet for 20 weeks to stimulate atherosclerotic lesion development. Strikingly, absence of USF1 function in bone marrow-derived cells was associated with exacerbated blood leukocyte (+ 100%; P < 0.01) and peritoneal leukocyte (+ 50%; P < 0.05) lipid loading and an increased atherosclerosis susceptibility (+ 31%; P < 0.05). These effects could be attributed to aggravated hyperlipidemia, i.e. higher plasma free cholesterol (+ 33%; P < 0.001) and cholesteryl esters (+ 39%; P < 0.001), and the development of hepatosteatosis. In conclusion, we have shown that hematopoietic USF1 deficiency is associated with an increased atherosclerosis susceptibility in LDL receptor knockout mice. These findings argue against a contribution of macrophage-specific USF1 deficiency to the previously described beneficial effect of total body USF1 deficiency on atherosclerosis susceptibility in mice.

Catalpol suppresses osteoclastogenesis and attenuates osteoclast-derived bone resorption by modulating PTEN activity

Excessive activation of osteoclast activity is responsible for many bone diseases, such as osteoporosis, rheumatoid arthritis, periprosthetic osteolysis, and periodontitis. Natural compounds that inhibit osteoclast formation and/or function have therapeutic potential for treating these diseases. Catalpol, a bioactive iridoid extracted from a traditional herbal medicine Rehmannia glutinosa, exhibits various pharmacological properties, including anti-inflammatory, antioxidant, antidiabetic, and antitumor effects. However, its effects on osteoclast formation and function remain unknown. In the present study, we showed that catalpol inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclast formation and bone resorption, as well as the expression of osteoclast-related marker genes. The investigation of molecular mechanisms showed that catalpol upregulated phosphatase and tensin homolog (PTEN) activity by reducing its ubiquitination and degradation, subsequently suppressing RANKL-induced NF-κB and AKT signaling pathways, leading to an inhibition on NFATc1 induction. Furthermore, catalpol protected mice against inflammation- and ovariectomy-induced bone loss by inhibiting osteoclast activity in vivo. These results suggest that catalpol might be developed as a promising candidate for treating osteoclast-related bone diseases.

Stachydrine prevents LPS-induced bone loss by inhibiting osteoclastogenesis via NF-κB and Akt signalling

Osteoclast overactivation‐induced imbalance in bone remodelling leads to pathological bone destruction, which is a characteristic of many osteolytic diseases such as rheumatoid arthritis, osteoporosis, periprosthetic osteolysis and periodontitis. Natural compounds that suppress osteoclast formation and function have therapeutic potential for treating these diseases. Stachydrine (STA) is a bioactive alkaloid isolated from Leonurus heterophyllus Sweet and possesses antioxidant, anti‐inflammatory, anticancer and cardioprotective properties. However, its effects on osteoclast formation and function have been rarely described. In the present study, we found that STA suppressed receptor activator of nuclear factor‐κB (NF‐κB) ligand (RANKL)‐induced osteoclast formation and bone resorption, and reduced osteoclast‐related gene expression in vitro. Mechanistically, STA inhibited RANKL‐induced activation of NF‐κB and Akt signalling, thus suppressing nuclear factor of activated T cells c1 induction and nuclear translocation. In addition, STA alleviated bone loss and reduced osteoclast number in a murine model of LPS‐induced inflammatory bone loss. STA also inhibited the activities of NF‐κB and NFATc1 in vivo. Together, these results suggest that STA effectively inhibits osteoclastogenesis both in vitro and in vivo and therefore is a potential option for treating osteoclast‐related diseases.

Tartrate-Resistant Acid Phosphatase Deficiency in the Predisposition to Systemic Lupus Erythematosus

OBJECTIVE

Mutations in the ACP5 gene, which encodes tartrate-resistant acid phosphatase (TRAP), cause the immuno-osseous disorder spondyloenchondrodysplasia, which includes as disease features systemic lupus erythematosus (SLE) and a type I interferon (IFN) signature. Our aims were to identify TRAP substrates, determine the consequences of TRAP deficiency in immune cells, and assess whether ACP5 mutations are enriched in sporadic cases of SLE.

METHODS

Interaction between TRAP and its binding partners was tested by a yeast 2-hybrid screening, confocal microscopy, and immunoprecipitation/Western blotting. TRAP knockdown was performed using small interfering RNA. Phosphorylation of osteopontin (OPN) was analyzed by mass spectrometry. Nucleotide sequence analysis of ACP5 was performed by Sanger sequencing or next-generation sequencing.

RESULTS

TRAP and OPN colocalized and interacted in human macrophages and plasmacytoid dendritic cells (PDCs). TRAP dephosphorylated 3 serine residues on specific OPN peptides. TRAP knockdown resulted in increased OPN phosphorylation and increased nuclear translocation of IRF7 and P65, with resultant heightened expression of IFN-stimulated genes and IL6 and TNF following Toll-like receptor 9 stimulation. An excess of heterozygous ACP5 missense variants was observed in SLE compared to controls (P = 0.04), and transfection experiments revealed a significant reduction in TRAP activity in a number of variants.

CONCLUSION

Our findings indicate that TRAP and OPN colocalize and that OPN is a substrate for TRAP in human immune cells. TRAP deficiency in PDCs leads to increased IFNα production, providing at least a partial explanation for how ACP5 mutations cause lupus in the context of spondyloenchondrodysplasia. Detection of ACP5 missense variants in a lupus cohort suggests that impaired TRAP functioning may increase susceptibility to sporadic lupus.

Receptor activator for nuclear factor-κB ligand signaling promotes progesterone-mediated estrogen-induced mammary carcinogenesis

Breast cancer is a leading cause of cancer-related death in women. Prolonged exposure to the ovarian hormones estrogen and progesterone increases the risk of breast cancer. Although estrogen is known as a primary factor in mammary carcinogenesis, very few studies have investigated the role of progesterone. Receptor activator for nuclear factor-κB (NF-κB) ligand (RANKL) plays an important role in progesterone-induced mammary carcinogenesis. However, the molecular mechanism underlying RANKL-induced mammary carcinogenesis remains unknown. In our current study, we show that RANKL induces glioma-associated oncogene homolog 1 (GLI-1) in estrogen-induced progesterone-mediated mammary carcinogenesis. In vivo experiments were carried out using ACI rats and in vitro experiments were carried out in MCF-7 cells. In ACI rats, mifepristone significantly reduced the incidence of mammary tumors. Likewise, mifepristone also inhibited the proliferation of MCF-7 cells. Hormone treatments induced RANKL, receptor activator of NF-κB (RANK), and NF-κB in a protein kinase B-dependent manner and inhibited apoptosis by activation of anti-apoptotic protein Bcl2 in mammary tumors and MCF-7 cells. Mechanistic studies in MCF-7 cells reveal that RANKL induced upstream stimulatory factor-1 and NF-κB, resulting in subsequent activation of their downstream target GLI-1. We have identified that progesterone mediates estrogen-induced mammary carcinogenesis through activation of GLI-1 in a RANKL-dependent manner.

Conditional testing of multiple variants associated with bone mineral density in the FLNB gene region suggests that they represent a single association signal

BACKGROUND

Low bone mineral density (BMD) is a primary risk factor for osteoporosis and is a highly heritable trait, but appears to be influenced by many genes. Genome-wide linkage studies have highlighted the chromosomal region 3p14-p22 as a quantitative trait locus for BMD (LOD 1.1 - 3.5). The FLNB gene, which is thought to have a role in cytoskeletal actin dynamics, is located within this chromosomal region and presents as a strong candidate for BMD regulation. We have previously identified significant associations between four SNPs in the FLNB gene and BMD in women. We have also previously identified associations between five SNPs located 5' of the transcription start site (TSS) and in intron 1 of the FLNB gene and expression of FLNB mRNA in osteoblasts in vitro. The latter five SNPs were genotyped in this study to test for association with BMD parameters in a family-based population of 769 Caucasian women.

RESULTS

Using FBAT, significant associations were seen for femoral neck BMD Z-score with the SNPs rs11720285, rs11130605 and rs9809315 (P = 0.004 - 0.043). These three SNPs were also found to be significantly associated with total hip BMD Z-score (P = 0.014 - 0.026). We then combined the genotype data for these three SNPs with the four SNPs we previously identified as associated with BMD and performed a conditional analysis to determine whether they represent multiple independent associations with BMD. The results from this analysis suggested that these variants represent a single association signal.

CONCLUSIONS

The SNPs identified in our studies as associated with BMD appear to be part of a single association signal between the FLNB gene and BMD in our data. FLNB is one of several genes located in 3p14-p22 that has been identified as significantly associated with BMD in Caucasian women.

The liver X receptor promotes macrophage differentiation and suppresses osteoclast formation in mouse RAW264.7 promyelocytic leukemia cells exposed to bacterial lipopolysaccharide

Lipopolysaccharide (LPS), the principal component of Gram-negative bacterial cell walls, is a stimulator of osteoclastogenesis and thus a key factor in inflammatory bone loss. We have recently reported that the important cholesterol and inflammatory regulator, liver X receptor (LXRα/β), can potently inhibit osteoclast formation from bone marrow-derived osteoclast precursors in a bacterial/LPS environment. In this manuscript, we further studied the effect of the LXR agonist GW3965 on osteoclast differentiation in RAW264.7 promyelocytic leukemia cells exposed to LPS. We found that not only did activation of the LXR potently inhibit the formation of TRAP-positive osteoclast-like cells, but promoted a population of TRAP-negative mononuclear cells with high phagocytic activity. We observed reduced expression of the osteoclast markers TRAP/Acp5, Ctsk, Calcr and Oscar after 3-4days of GW3965 treatment, coinciding with an increase in the expression of the anti-osteoclastogenic factor Irf8. Expression of the macrophage/phagocytic marker Cd68 was increased, however the "classical" macrophage markers F4/80 and Cd14 and the "alternatively" activated macrophage markers Tgfβ and Il10 were not altered. Further, activation of LXR increased the expression of the macrophage survival gene AIM/SPα, a known LXR target gene, and osteoclast/macrophage-related markers (Mitf, Pu.1, Usf1/2, Ostm1 and Mfr). Although Akt phosphorylation was reduced, GW3965 seemed to act independently of MAPKs (p38, ERK, JNK) and NFκB, and had no inhibitory effect on cytokine expression (Tnfα, Il6, or Il1β). Our results indicate that activation of the LXR not only inhibits the differentiation of osteoclast-like cells from RAW264.7 cells in a bacterial/LPS environment, but is also involved in the fate determination of myeloid progenitor cells into macrophages with high phagocytic capacity.

Activation of liver X receptor (LXR) inhibits receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation in an LXRβ-dependent mechanism

Bone destruction is the major pathological process in many bone metabolic diseases and is a result of increased osteoclast formation and bone resorption. The liver X receptors (α,β), important regulators of cholesterol metabolism and inflammatory signaling, have recently been observed to play a role in both physiological and pathological bone turnover. However, the relationship between liver X receptors (LXR) and osteoclast differentiation/formation remains unknown. Here, we report that the LXR ligand GW3965 is able to clearly and potently inhibit the formation of mature osteoclasts from receptor activator of nuclear factor κB ligand (RANKL)-stimulated human and murine osteoclast precursors. This results in a significant inhibition of bone resorption. We observed that GW3965 significantly inhibited expression of the osteoclast markers tartrate-resistant acid phosphatase, cathepsin K, osteoclast-associated receptor (OSCAR), and calcitonin receptor, appearing to act in an NFATc1/p38/microphthalmia-associated transcription factor (MITF)-dependent mechanism, independently of receptor activator of nuclear factor κB or c-Fos and not directly involving the NFκB pathways. GW3965 was less effective in RAW264.7 monocyte/macrophage cells, which are more committed into the osteoclast lineage. Also, GW3965 seemed to act differently depending on the source of the progenitor cells as it had no effect on calvarial osteoclasts, compared with marrow or blood-derived monocytes. As these effects were abolished in osteoclast precursors derived from LXRβ(-/-) mice, we suggest that GW3965 acts via an LXRβ-dependent mechanism. Taken together, our results suggest that the LXR can act as an important inhibitor of RANKL-mediated osteoclast differentiation.

Biology and clinical significance of tartrate-resistant acid phosphatases: new perspectives on an old enzyme

Type 5 tartrate-resistant acid phosphatase (TRAP) has been a clinically relevant biomarker for about 50 years. It has always been a reliable and specific cytochemical marker for hairy cell leukemia and for differentiated cells of monocytic lineage. Only recently has the test for serum TRAP activity been accepted as sensitive and specific enough for clinical use as a marker of osteoclasts and bone resorption. This has come about through steady advances in knowledge about TRAP enzymology, structure, function, and molecular regulation and a consequent appreciation that TRAP isoforms 5a and 5b have very different clinical significance. As a measure of osteoclast number and bone resorption, TRAP 5b has diagnostic and prognostic applications in osteoporosis, cancers with bone metastasis, chronic renal failure, and perhaps other metabolic and pathologic bone diseases. Serum TRAP 5a, on the other hand, has no relationship to bone metabolism but seems instead to be a measure of activated macrophages and chronic inflammation. Exploration of the real clinical usefulness of serum TRAP 5a for diagnosis and disease management in a wide variety of chronic inflammatory diseases is only now beginning. This perspective traces the important basic scientific developments that have led up to the refinement of serum TRAP isoform immunoassays and their validation as biomarkers of disease. Many unanswered questions remain, providing a wealth of opportunity for continued research of this multifaceted enzyme.

Complex regulation of tartrate-resistant acid phosphatase (TRAP) expression by interleukin 4 (IL-4): IL-4 indirectly suppresses receptor activator of NF-kappaB ligand (RANKL)-mediated TRAP expression but modestly induces its expression directly

Interleukin 4 (IL-4) inhibits receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast formation and functional activity in a STAT6-dependent manner. IL-4 down-regulates expression of tartrate-resistant acid phosphatase (TRAP) in mature osteoclasts. To determine whether IL-4 regulates TRAP promoter activity, RAW264.7 cells were transfected with a TRAP promoter-luciferase reporter. Treatment with IL-4 alone modestly enhanced TRAP luciferase activity. However, IL-4 suppressed the ability of RANKL to up-regulate TRAP-luciferase activity, suggesting that IL-4 has multiple effects on TRAP transcription. IL-4 also reduced the RANKL-induced association of RNA polymerase II with the TRAP gene in osteoclasts. The TRAP promoter contains a STAT6-binding motif, and STAT6 bound to the endogenous TRAP promoter after IL-4 treatment. To determine the impact of STAT6 binding, we transfected cells with STAT6VT, a constitutively active STAT6 mutant. STAT6VT alone up-regulated TRAP-luciferase activity; this effect was abrogated by mutating the STAT6 binding site in the minimal TRAP promoter. STAT6VT did not inhibit the potent up-regulation of TRAP promoter activity caused by overexpression of NFATc1, PU.1, and microphthalmia transcription factor, downstream targets of macrophage colony-stimulating factor and RANKL. IL-4 down-regulated the expression of c-Fos and NFATc1 in mature osteoclasts. Knockdown of NFATc1 by short interfering RNA caused TRAP expression to be down-regulated, and ectopic expression of NFATc1 abrogated the IL-4-induced down-regulation of TRAP. These results suggest that STAT6 plays two distinct roles in TRAP expression. The IL-4-induced activation of STAT6 mediates suppression of the RANKL-induced TRAP promoter activity indirectly by inhibiting NFATc1 expression. However, in the absence of RANKL and osteoclast differentiation, STAT6 binds the TRAP promoter after IL-4 treatment and directly enhances TRAP expression.

Proteomic profile of osteoclast membrane proteins: identification of Na+/H+ exchanger domain containing 2 and its role in osteoclast fusion

Osteoclast formation and bone resorption are multiple processes that involve the participation of specialized membrane structures and their associated proteins. In this study, we used an MS to analyze the profile of proteins associated with osteoclast membranes and focused on the function of channel proteins in osteoclast differentiation and function. We filtered out with a SEQUEST score greater than 10 and a peptide hit number of more than 2, resulting in the identification of 499 proteins that were commonly found in both macrophages and osteoclasts, 96 proteins selectively found in osteoclasts, and 179 proteins selectively found in macrophages. The proteins that were selectively found in osteoclasts were classified based on their localizations: plasma membrane (17%), ER/Golgi and lysosome/endosome (15%), mitochondrion (18%), nucleus (13%), cytosol (19%), and unknown (18%). Proteins associated with osteoclast function such as v-ATPase, IGF2R, TRAP, and cathepsin K were found in osteoclasts as previously shown. We found several ion channel proteins such as Ank and Nhedc2 and signaling molecules such as Dock5 and RAB-10 in osteoclasts. Inhibition of the Na(+)/H(+) exchanger family by amiloride suppressed RANKL-induced osteoclast fusion and bone resorption. In addition, shRNA for Nhedc2 inhibited osteoclast differentiation. Our results provide a proteomic profile of osteoclast membrane proteins and identify Nhedc2, which is probably associated with proton transport in osteoclasts, as a regulator of osteoclast function.

Poly(adp-ribose) polymerase-1 regulates Tracp gene promoter activity during RANKL-induced osteoclastogenesis

The Tracp gene encodes an acid phosphatase strongly upregulated during osteoclastogenesis on RANKL treatment. Using the mouse osteoclastic model RAW264.7, we studied Tracp gene expression, and we identified PARP-1 as a transcriptional repressor negatively regulated by RANKL during osteoclastogenesis.

INTRODUCTION

The Tracp gene encodes an acid phosphatase strongly expressed in differentiated osteoclasts. TRACP enzyme has a dual role and is involved in (1) the regulation of the biological activity of the bone matrix phosphoproteins osteopontin and bone sialoprotein and (2) the intracellular collagen degradation. Based on our previous work on Tcirg1 gene expression, and using data available in the literature, we focused on a 200-bp sequence located upstream the Tracp gene transcriptional start to identify binding activities.

MATERIALS AND METHODS

We first performed siRNA transfections and RAW264.7 cell treatment with an inhibitor of poly(ADP-ribose) polymerase-1 (PARP-1) activity. After EMSA and supershift experiments, we measured the promoter activity of wildtype and mutant constructs throughout the osteoclastic differentiation.

RESULTS

We first showed that depleting PARP-1 mRNA in the pre-osteoclastic cell line RAW264.7 results in an increase of both matrix metalloproteinase 9 and TRACP mRNA expression (3.5- and 2.5-fold, respectively). Moreover, in response to 3-aminobenzamide treatment, we measured a weak stimulation of MMP9 mRNA expression, whereas up to a 2-fold enhancement above the control condition of TRACP mRNA expression was observed. We next identified in the -839/-639 Tracp promoter region a PARP-1 binding site, and supershift experiments showed the interaction of a PARP-1 binding activity with the Tracp promoter sequence -830/-808. Finally, RAW264.7 cell transfection with a promoter construct mutated for this PARP-1 interacting sequence showed the functionality of this site within intact pre-osteoclastic cells.

CONCLUSIONS

In this study, we provide evidence that the transcriptional activity of the Tracp gene, in pre-osteoclastic cells, is negatively regulated by the binding of PARP-1 protein to a potential consensus sequence located in its promoter region. Taken together with our previous results related to the control of Tcirg1 gene expression, our data suggest that PARP-1 exerts a pivotal role in the basal repression of genes that are upregulated during RANKL-induced osteoclastogenesis.

Differential binding of poly(ADP-Ribose) polymerase-1 and JunD/Fra2 accounts for RANKL-induced Tcirg1 gene expression during osteoclastogenesis

We studied Tcirg1 gene expression on RANKL-induced osteoclastic differentiation of the mouse model RAW264.7 cells. We identified a mechanism involving PARP-1 inhibition release and JunD/Fra-2 binding, which is responsible for Tcirg1 gene upregulation.

INTRODUCTION

The Tcirg1 gene encodes the a3 isoform of the V-ATPase a subunit, which plays a critical role in the resorption activity of the osteoclast. Using serial deletion constructs of the Tcirg1 gene promoter, we performed a transcriptional study to identify factor(s) involved in the regulation of the RANKL-induced gene expression.

MATERIALS AND METHODS

The promoter activity of serial-deletion fragments of the Tcirg1 gene promoter was monitored throughout the RAW264.7 cells differentiation process. We next performed sequence analysis, EMSA, UV cross-linking, qPCR, and gel supershift experiments to identify the factor(s) interacting with the promoter.

RESULTS

A deletion of the -1297-1244 region led to the disappearance of the RANKL-induced promoter activity. EMSA experiments showed the binding of two factors that undergo differential binding on RANKL treatment. Supershift experiments led us to identify the dimer JunD/Fra-2 as the binding activity associated with the -1297/-1268 Tcirg1 gene promoter sequence in response to RANKL. Moreover, we observed poly(ADP-ribose) polymerase-1 (PARP-1) binding to an adjacent site (-1270/-1256), and this interaction was disrupted after RANKL treatment.

CONCLUSIONS

We provide data that identify junD proto-oncogene (JunD) and Fos-related antigen 2 (Fra-2) as the activator protein-1 (AP-1) factors responsible for the RANKL-induced upregulation of the mouse Tcirg1 gene expression. Moreover, we identified another binding site for PARP-1 that might account for the repression of Tcirg1 gene expression in pre-osteoclastic cells.

RANKL treatment releases the negative regulation of the poly(ADP-ribose) polymerase-1 on Tcirg1 gene expression during osteoclastogenesis

The Tcirg1 gene encodes the osteoclast-specific a3 isoform of the V-ATPase a subunit. Using the mouse osteoclastic model RAW264.7 cells, we studied Tcirg1 gene expression, and we identified PARP-1 as a transcriptional repressor negatively regulated by RANKL during osteoclastogenesis.

INTRODUCTION

The TCIRG1 gene encodes the a3 isoform of the V-ATPase a subunit, and mutations at this locus account for approximately 60% of infantile malignant osteopetrosis cases. Using RAW264.7 cells as an osteoclastic differentiation model, we undertook a transcriptional study of the mouse Tcirg1 gene focused on the 4-kb region upstream of the transcription starting point.

MATERIALS AND METHODS

The promoter activity of serial-deletion fragments of the Tcirg1 gene promoter was monitored throughout the RAW264.7 cell differentiation process. We next performed EMSA, UV cross-linking, affinity purification, mass spectrometry analysis, gel supershift, and siRNA transfection experiments to identify the factor(s) interacting with the promoter.

RESULTS

The -3946/+113 region of the mouse Tcirg1 gene displayed a high basal promoter activity, which was enhanced by RANKL treatment of RAW264.7 cells. Constructs deleted up to -1589 retained this response to RANKL. A deletion up to -1402 induced a 3-fold enhancement of the basal activity, whereas RANKL response was not affected. EMSA experiments led us to identify within the -1589/-1402 region, a 10-nucleotide sequence, which bound a nuclear protein present in nondifferentiated RAW264.7 cells. This interaction was lost using nuclear extracts derived from RANKL-treated cells. Affinity purification followed by mass spectrometry analysis and gel supershift assay allowed the identification of poly(ADP-ribose) polymerase-1 (PARP-1) as this transcriptional repressor, whereas Western blot experiments revealed the cleavage of the DNA-binding domain of PARP-1 on RANKL treatment. Finally, both PARP-1 depletion after siRNA transfection and RAW264.7 cell treatment by an inhibitor of PARP-1 activity induced an increase of a3 mRNA expression.

CONCLUSIONS

We provide evidence that the basal transcription activity of the Tcirg1 gene is negatively regulated by the binding of PARP-1 protein to its promoter region in mouse pre-osteoclast. On RANKL treatment, PARP-1 protein is cleaved and loses its repression effect, allowing an increase of Tcirg1 gene expression that is critical for osteoclast function.

Modulation of osteoclastogenesis induced by nucleoside reverse transcriptase inhibitors

Osteopenia is a common and debilitating side-effect of HAART, yet little is known concerning the effects of HAART on bone metabolism. We reported previously that zidovudine (AZT) stimulates osteoclastogenesis in vitro and causes osteopenia in mice. Here, we confirmed that the AZT-induced osteoclastogenesis is dependent on RANKL in that osteoclastogenesis is blocked by osteoprotegestin. Alendronate, which is used for the treatment of osteopenia and osteoporosis, failed to inhibit AZT-induced osteoclastogenesis in vitro. Osteoclastogenesis in vitro was not affected by tumor necrosis factor-alpha. Two other NRTI drugs, ddl and 3TC, also induced osteoclastogenesis in vitro and induced osteopenia in mice. The osteopenia was associated with an elevation of parameters of osteoclasts, but not with osteoblasts. Combinations of the NRTIs did not result in additive or synergistic effects in vitro or in vivo. Finally, AZT induced osteoclastogenesis of human osteoclast precursors in a RANKL-dependent manner. This in vitro osteoclastogenesis assay using human peripheral blood mononuclear cells could be useful in evaluating bone turnover and the risk of developing osteopenia in AIDS patients on HAART.

Cloning and characterization of osteoclast precursors from the RAW264.7 cell line

Osteoclasts are bone-resorbing cells that differentiate from macrophage precursors in response to receptor activator of NF-kappaB ligand (RANKL). In vitro models of osteoclast differentiation are principally based on primary cell cultures, which are poorly suited to molecular and transgene studies because of the limitations associated with the use of primary macrophage. RAW264.7 is a transfectable macrophage cell line with the capacity to form osteoclast-like cells. In the present study, we have identified osteoclast precursors among clones of RAW264.7 cells. RAW264.7 cell were cloned by limiting dilution and induced to osteoclast differentiation by treatment with recombinant RANKL. Individual RAW264.7 cell clones formed tartrate resistant acid phosphatase (TRAP)-positive multinuclear cells to various degrees with RANKL treatment. All clones tested expressed the RANKL receptor RANK. Each of the clones expressed the osteoclast marker genes TRAP and cathepsin-K mRNA with RANKL treatment. However, we noted that only select clones were able to form large, well-spread, TRAP-positive multinuclear cells. Clones capable of forming large TRAP-positive multinuclear cells also expressed beta3 integrin and calcitonin receptor mRNAs and were capable of resorbing a mineralized matrix. All clones tested activated NF-kappaB with RANKL treatment. cDNA expression profiling of osteoclast precursor RAW264.7 cell clones demonstrates appropriate expression of a large number of genes before and after osteoclastic differentiation. These osteoclast precursor RAW264.7 cell clones provide a valuable model for dissecting the cellular and molecular regulation of osteoclast differentiation and activation.

Detection of novel skeletogenesis target genes by comprehensive analysis of a Runx2(-/-) mouse model

Runx2 is an essential factor for skeletogenesis and heterozygous loss causes cleidocranial dysplasia in humans and a corresponding phenotype in the mouse. Homozygous Runx2-deficient mice lack hypertrophic cartilage and bone. We compared the expression profiles of E14.5 wildtype and Runx2(-/-) murine embryonal humeri to identify new transcripts potentially involved in cartilage and bone development. Seventy-one differentially expressed genes were identified by two independent oligonucleotide-microarray hybridizations and quantitative RT-PCR experiments. Gene Ontology analysis demonstrated an enrichment of the differentially regulated genes in annotations to terms such as extracellular, skeletal development, and ossification. In situ hybridization on E15.5 limb sections was performed for all 71 differentially regulated genes. For 54 genes conclusive in situ hybridization results were obtained and all of them showed skeletal expression. Co-expression with Runx2 was demonstrated for 44 genes. While 41 of the 71 differentially expressed genes have a known role in bone and cartilage, we identified 21 known genes that have not yet been implicated in skeletal development and 9 entirely new transcripts. Expression in the developing skeleton was demonstrated for 21 of these genes.

Sp1 and Sp3 regulate the basal transcription of receptor activator of nuclear factor kappa B ligand gene in osteoblasts and bone marrow stromal cells

Receptor activator of nuclear factor kappa B ligand (RANKL), a potent regulator of osteoclast formation and function, is expressed by osteoblasts and bone marrow stromal cells. However, the molecular mechanism underlying RANKL expression in osteoblast/stromal cells remains largely unclear. Here, we characterized the molecular mechanism controlling RANKL basal transcription in osteoblast/stromal cells. We cloned a 1,103-bp murine RANKL promoter (from -953 to +150, relative to the transcription start site). Using a series of deletion mutants of the 1,103-bp promoter, we identified a 100-bp region (-154 to -54) mediating RANKL basal transcription in both osteoblasts and bone marrow stromal cells. Electrophoretic mobility shift assay (EMSA) using five overlapping oligonucleotides (Probes 1-5) spanning the 100-bp region showed that Probes 1 and 2 specifically bound nuclear proteins with high affinity from both cell types. Computer analysis revealed that Probes 1 and 2 contain a putative Sp1-binding site. Supershift assays with Sp1 and Sp3 antibodies confirmed that the nuclear proteins binding to Probes 1 and 2 are Sp1 and Sp3. Functionally, the mutation of the Sp1/Sp3 site in Probe 1 profoundly reduced the basal promoter activity while the mutation of the one in Probe 2 resulted in moderate reduction in the basal promoter activity. Moreover, the mutation of both sites abrogated the RANKL basal promoter activity, indicating that Sp1 and Sp3 play a key role in the RANKL basal transcription in osteoblasts and bone marrow stromal cells.

A customized retroviral vector confers marker gene expression in osteoclast lineage cells

Osteoclasts play a seminal role in many skeletal diseases and therefore are candidates for cell-based gene delivery systems to treat disorders of bone. As an initial step toward developing osteoclast-mediated gene delivery systems, we have made and analyzed a customized Molony-Murine leukemia virus (MMLV)-based retroviral vector containing elements of the osteoclast-specific tartrate-resistant acid phosphatase (TRAP) gene. RAW 264.7 cells were transduced with the customized vector (E3) and differentiated along macrophage or osteoclast lineages. E3 contained a truncated form of the human nerve growth factor receptor (NGFR) as a reporter gene. NGFR expression increased with RANK-ligand (RANK-L) treatment but not with macrophage (gamma-IFN/LPS treatment) differentiation. Enhanced NGFR expression peaked 48 h after RANK-L treatment. Electrophoretic mobility shift assays (EMSA) analysis of the TRAP gene regulatory elements in E3 identified a single 27 bp DNA probe, which specifically bound protein from RANK-L-treated cells. DNA sequence revealed AP-1 binding sites, and analysis with mutant probes implied that the sites were functional. EMSA supershift analysis identified Fos protein interacting with the 27 bp probe. In summary, insertion of sequence -962 to -868 from the TRAP gene into the U3 region of the MMLV LTR confers RANK-L induced retroviral gene expression via Fos family protein interaction at AP-1 sites.

YY1 is involved in RANKL-induced transcription of the tartrate-resistant acid phosphatase gene in osteoclast differentiation

Receptor activator of nuclear factor kappa B (NF-kappaB) ligand (RANKL), a critical activator of osteoclast differentiation, plays a pivotal role in tartrate-resistant acid phosphatase (TRAP) gene expression. Previously, we showed that upstream stimulatory factors (USF) 1 and 2 are implicated in the RANKL-induced TRAP transcriptional activation via a 12-bp USF binding site in the TRAP promoter. In that study, we also demonstrated that a RANKL-induced nuclear protein binds to a 50-bp oligonucleotide (Oligo IV) corresponding to a distinct TRAP promoter region. Here we report the identification and functional characterization of the nuclear protein binding to Oligo IV. We identified a 21-bp sequence CTGTTTATGATGGCGAGGGGG in Oligo IV that specifically binds the RANKL-induced nuclear protein from RAW264.7 cells by performing a series of competition assays. Computer analysis of the 21-bp sequence revealed that the sequence contains a putative Yin Yang 1 (YY1) binding site overlapped with a putative activator protein-2 (AP-2) binding site. Competition and supershift assays indicated that the nuclear protein binding to the 21-bp sequence is YY1, not AP-2. Functionally, mutation of the YY1-binding site resulted in a reduction in the RANKL-induced TRAP transcription in RAW264.7 cells, demonstrating that YY1 positively regulates RANKL-induced TRAP transcriptional activation. In conclusion, our data demonstrated that YY1 plays a functional role in RANKL-mediated TRAP gene expression during osteoclast differentiation.

Analysis of Distinct Tartrate-resistant Acid Phosphatase Promoter Regions in Transgenic Mice

The tartrate-resistant acid phosphatase (TRAP) is present in multiple tissues, including kidney, liver, lung, spleen, and bone. Recent study of (TRAP) gene expression has provided evidence for distinct promoters within the (TRAP) gene, suggesting that the gene has alternative, tissue-preferred mRNA transcripts. Examination of endogenous (TRAP) exon 1B and 1C mRNA transcripts revealed tissue-preferred transcript abundance with increased exon 1B transcripts detected in liver and kidney and increased exon 1C transcripts detected in bone and spleen. In this investigation, we have made transgenic mice that express a marker gene driven by two candidate promoters, designated BC and C, within the (TRAP) gene. The BC and C promoters are 2.2 and 1.6 kb, respectively, measured from the translation initiation site. Evaluation of BC transgenic lines demonstrated robust expression in multiple tissues. In contrast, significant transgene expression was not detected in C transgenic lines. Evaluation of transgene mRNAs in BC transgenic lines revealed that virtually all expression was in the form of B transcripts, suggesting that the tissue-preferred pattern of endogenous (TRAP) was not replicated in the BC transgenic line. Likewise, osteoclastogenic cultures from BC, but not C, transgenic bone marrow cells expressed the transgene following receptor activator of NFkappaB ligand/macrophage colony-stimulating factor stimulation. In conclusion, when compared with the 2.2-kb BC portion of the (TRAP) promoter region, the 1.6-kb C portion does not account for significant gene expression in vivo or in vitro; production of the bone- and spleen-preferred (TRAP) C transcript must depend on regulatory elements outside of the 2.2-kb promoter. As the majority of currently investigated transcription factors that influence transcriptional regulation of osteoclast gene expression bind within the 1.6-kb C portion of the (TRAP) promoter, it is likely that transcription binding sites outside of the 2.2-kb region will have profound effects on regulation of the gene in vivo and in vitro.

Upstream stimulatory factors stimulate transcription through E-box motifs in the PF4 gene in megakaryocytes

Platelet factor 4 (PF4) is expressed during megakaryocytic differentiation. We previously demonstrated that the homeodomain proteins (myeloid ecotropic integration site 1 [MEIS1], Pbx-regulating protein 1 [PREP1], and pre-B-cell leukemia transcription factors [PBXs]) bind to the novel regulatory element tandem repeat of MEIS1 binding element [TME] and transactivate the rat PF4 promoter. In the present study, we investigated and identified other TME binding proteins in megakaryocytic HEL cells using mass spectrometry. Among identified proteins, we focused on upstream stimulatory factor (USF1) and USF2 and investigated their effects on the PF4 promoter. USF1 and 2 bound to the E-box motif in the TME and strongly transactivated the PF4 promoter. Furthermore, physiologic bindings of USF1 and 2 to the TME in rat megakaryocytes were demonstrated by the chromatin immunoprecipitation (ChIP) assay. Interestingly, the E-box motif in the TME was conserved in TME-like sequences of both the human and mouse PF4 promoters. USF1 and 2 also bound to the human TME-like sequence and transactivated the human PF4 promoter. Expressions of USF1 and 2 were detected by reverse-transcriptase-polymerase chain reaction (RT-PCR) in the human megakaryocytes derived from CD34+ cells. Thus, these studies demonstrate that the novel TME binding transcription factors, USF1 and 2, transactivate rat and human PF4 promoters and may play an important role in megakaryocytic gene expression.

Glucose up-regulates thrombospondin 1 gene transcription and transforming growth factor-beta activity through antagonism of cGMP-dependent protein kinase repression via upstream stimulatory factor 2

Thrombospondin 1 (TSP1) transcription is stimulated by glucose, resulting in increased TGF-beta activation and matrix protein synthesis. We previously showed that inducible expression of the catalytic domain of cGMP-dependent protein kinase (PKG) inhibits glucose-regulated TSP1 transcription and transforming growth factor (TGF)-beta activity in stably transfected rat mesangial cells (RMCs(tr/cd)). However, the molecular mechanisms by which PKG represses glucose-regulated TSP1 transcription are unknown. Using a luciferase-promoter deletion assay, we now identify a single region of the human TSP1 promoter (-1172 to -878, relative to the transcription start site) that is responsive to glucose. Further characterization of this region identified an 18-bp sequence that specifically binds nuclear proteins from mesangial cells. Moreover, binding is significantly enhanced by high glucose treatment and is reduced by increased PKG activity. Gel mobility shift and supershift assays show that the nuclear proteins binding to the 18-bp sequence are USF1 and -2. USF1 and USF2 bound to the endogenous TSP1 promoter using a chromatin immunoprecipitation assay. Glucose stimulates nuclear USF2 protein accumulation through protein kinase C, p38 MAPK, and extracellular signal-regulated kinase pathways. Increased PKG activity down-regulates USF2 protein levels and its DNA binding activity under high glucose conditions, resulting in inhibition of glucose-induced TSP1 transcription and TGF-beta activity. Overexpression of USF2 reversed the inhibitory effect of PKG on glucose-induced TSP1 gene transcription and TGF-beta activity. Taken together these data present the first evidence that USF2 mediates glucose-induced TSP1 expression and TSP1-dependent TGF-beta bioactivity in mesangial cells, suggesting that USF2 is an important transcriptional regulator of diabetic complications.

AZT enhances osteoclastogenesis and bone loss

A variety of metabolic complications have been reported to be associated with highly active antiretroviral therapy (HAART), including osteopenia and osteoporosis. In this study, we determine the effects of zidovudine (AZT), a nucleoside reverse transcriptase inhibitor, on osteoclastogenesis in a cultured mouse macrophage preosteoclast cell line (RAW264.7), in mouse primary bone marrow macrophage-monocyte precursors, and on bone mineral density in mice. The results indicate that AZT induces an increase in osteoclastogenesis in the mouse preosteoclast cell line and in mouse bone marrow osteoclast precursors in the presence of RANKL. This increased osteoclastogenesis is dependent upon the concentration of AZT. AZT increases the promoter activity of tartrate-resistant acid phosphatase (TRAP) and the binding and function of the nuclear transcription protein, NF-kappaB, in RAW264.7 cells. Therefore, the effect of AZT is mediated, at least in part, by enhancing RANKL-mediated osteoclastogenesis. Bone mineral density (BMD) in AZT-treated mice is decreased and histopathology shows marked osteopenia. These results support an important role of AZT-stimulated osteoclastogenesis in HAART-induced osteopenia.

Molecular Characterization and Role of Bovine Upstream Stimulatory Factor 1 and 2 in the Regulation of the Prostaglandin G/H Synthase-2 Promoter in Granulosa Cells

The transcriptional activation of the prostaglandin G/H synthase-2 (PGHS-2) gene in granulosa cells is required for ovulation. To directly study the ability of upstream stimulatory factor 1 (USF1) and USF2 to trans-activate the bovine PGHS-2 promoter in granulosa cells, USF1 or USF2 expression vectors were cotransfected with the PGHS-2/luciferase (LUC) chimeric construct, -149/-2PGHS-2.LUC. Results revealed that overexpression of USF1 or USF2 caused a marked and significant increase in basal and forskolin-inducible promoter activities (p<0.05), and these effects were dependent on the presence of a consensus E-box cis-element within the promoter fragment. Co-transfections with different N- and C-terminal truncated USF mutants led to significant reductions in promoter activation, as compared with full-length constructs (p<0.05), thus allowing identification of putative bovine USF functional domains. Overexpression of a USF2 truncated mutant lacking the first 220 residues (U2Delta1-220) acted as a dominant negative mutant and blocked endogenous and USF-stimulated PGHS-2 promoter activation. Interestingly, transfections with U2Delta1-220 blocked the forskolin-dependent induction of PGHS-2 mRNA in granulosa cells, whereas transfections with full-length USF2 increased PGHS-2 transcript levels. Immunoblot analyses confirmed overexpression of full-length and truncated USF proteins, and electrophoretic mobility shift assays (EMSAs) and supershift EMSAs established that the observed effects were dependent on specific interactions between USF proteins and the consensus E-box cis-element. Stimulation of cells with forskolin increased, whereas treatment of extracts with phosphatase decreased USF binding activities to the E-box. Thus, this study presents for the first time direct evidence for a role of USF proteins in the regulation of the PGHS-2 promoter in preovulatory granulosa cells.

Mitf-PU.1 interactions with the tartrate-resistant acid phosphatase gene promoter during osteoclast differentiation

It has been postulated that the transcription factors micropthalmia associated factor (Mitf) and PU.1 interact with the tartrate-resistant acid phosphatase (TRAP) gene promoter and activate TRAP gene expression in osteoclasts. However, studies on the interaction of these factors with the TRAP promoter employing nuclear extracts from osteoclasts and osteoclast precursors have not been reported. We therefore treated murine mononuclear phagocyte cells with various cytokines to generate cultures of osteoclasts and macrophagic cells with high or low potential to form osteoclasts. The presence of Mitf and PU.1 in nuclear extracts from these cultures and the ability of these factors to bind to the TRAP promoter was then assessed. We demonstrate that Mitf and a related factor, TFE3, are present in nuclear extracts from all cultures and bind the TRAP promoter. While PU.1 is present in nuclear extracts from all cultures, it does not significantly interact with a putative binding site in the TRAP promoter. These results suggest Mitf and PU.1 interactions with the TRAP promoter are not responsible for the specific activation of TRAP gene expression in osteoclasts.