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

Effect of osteoclast co-culture on the differentiation of human mesenchymal stem cells grown on bone graft granules

Traditional approaches to bone repair are currently being integrated with innovative tissue-engineering techniques, as researchers and clinicians shift their treatment focus toward regenerating functional tissue rather than just filling a defect to provide structural support. Cells are expanded and incorporated into implantable systems in hopes of enhancing the bone-forming capabilities of traditional bone graft substitutes. The present study examined how osteoclasts might be used to stimulate the differentiation of human mesenchymal stem cells (hMSCs) into bone forming cells. The two cell types were co-cultured on a resorbable, three-dimensional bone graft substitute. Osteoclasts were seeded prior to the addition of hMSCs, as well as simultaneously, to determine if resorption of the scaffold would have any bearing on observed response by hMSCs. When seeded directly with hMSCs on the 3-D substrates, the osteoclasts had an increase in TRAP expression over time if seeded simultaneously. The co-culture setup had a positive influence on the proliferation of hMSCs. Late stage osteoblast differentiation markers (bone sialoprotein) were positively affected by direct co-culture with osteoclasts. The addition of RANKL to the culture medium for osteoclastogenesis appears to be a factor in the observed responses by hMSCS, but is not the only factor influencing the MSCs. Osteoclasts were shown to have an influence on the development of mesenchymal stem cells into osteoblasts when cultured in vitro. Findings from this study, coupled with the knowledge obtained from our previous work, will aid in the development of a clinically viable mesenchymal stem cell based bone graft system.

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.

Gene expression profile of the bone microenvironment in human fragility fracture bone

Osteoporosis (OP) is a common age-related systemic skeletal disease, with a strong genetic component, characterised by loss of bone mass and strength, which leads to increased bone fragility and susceptibility to fracture. Although some progress has been made in identifying genes that may contribute to OP disease, much of the genetic component of OP has yet to be accounted for. Therefore, to investigate the molecular basis for the changes in bone causally involved in OP and fragility fracture, we have used a microarray approach. We have analysed altered gene expression in human OP fracture bone by comparing mRNA in bone from individuals with fracture of the neck of the proximal femur (OP) with that from age-matched individuals with osteoarthritis (OA), and control (CTL) individuals with no known bone pathology. The OA sample set was included because an inverse association, with respect to bone density, has been reported between OA and the OP individuals. Compugen H19K oligo human microarray slides were used to compare the gene expression profiles of three sets of female samples comprising, 10 OP-CTL, 10 OP-OA, and 10 OA-CTL sample pairs. Using linear models for microarray analysis (Limma), 150 differentially expressed genes in OP bone with t scores >5 were identified. Differential expression of 32 genes in OP bone was confirmed by real time PCR analysis (p<0.01). Many of the genes identified have known or suspected roles in bone metabolism and in some cases have been implicated previously in OP pathogenesis. Three major sets of differentially expressed genes in OP bone were identified with known or suspected roles in either osteoblast maturation (PRRX1, ANXA2, ST14, CTSB, SPARC, FST, LGALS1, SPP1, ADM, and COL4A1), myelomonocytic differentiation and osteoclastogenesis (TREM2, ANXA2, IL10, CD14, CCR1, ADAM9, CCL2, CTGF, and KLF10), or adipogenesis, lipid and/or glucose metabolism (IL10, MARCO, CD14, AEBP1, FST, CCL2, CTGF, SLC14A1, ANGPTL4, ADM, TAZ, PEA15, and DOK4). Altered expression of these genes and others in these groups is consistent with previously suggested underlying molecular mechanisms for OP that include altered osteoblast and osteoclast differentiation and function, and an imbalance between osteoblastogenesis and adipogenesis.

YY1 and FoxD3 regulate antiretroviral zinc finger protein OTK18 promoter activation induced by HIV-1 infection

OTK18 is a C2H2 type zinc finger protein involved in the regulation of HIV-1 replication in human mononuclear phagocytes. Previously, we reported OTK18 expression in brain perivascular macrophages but not in microglia in HIV encephalitis brain. We have cloned the OTK18 promoter region proximal to the transcriptional start site and determined the region responsible (-884/+1) for the basal transcriptional activity in a microglia cell line. Sequential deletion mutation analyses reveal three important response elements: Yingyang-1 (YY1; -805/-777), an HIV-1 response element for promoter activation; FoxD3 (-743/-725), a negative regulatory element; and Ets response element (-725/-707), a basal transcriptional activity response element. HIV-1 infection-induced upregulation of YY1 and c-Ets-1 protein, binding to the promoter region as determined by immunoblotting and chromatin immunoprecipitation and polymerase chain reaction (PCR) assays, and induction of YY1 was also observed in virus-infected monocyte-derived macrophages. Silencing of FoxD3 and YY1 in the cell line by small interfering RNA duplexes specific to these molecules significantly up- and downregulated basal OTK18 promoter activity in FoxD3 and YY1 response element-dependent manners, respectively. On the other hand, infection of primary cultured human microglia significantly reduced YY1 expression and induced FoxD3 as determined by immunoblotting and reverse transcription real-time PCR. These data suggest that HIV-1 induces OTK18 expression through a YY1-mediated manner in human macrophages, although its gene expression is suppressed by FoxD3 upregulation and YY1 downregulation in human microglia. This mechanism may explain the perivascular macrophage-specific expression of OTK18 in HIV encephalitis brains.

Promoter sequence variants of LIGHT are associated with female vascular dementia

LIGHT (homologous to L ymphotoxins, exhibits I nducible expression, and competes with herpes simplex virus G lycoprotein D for H erpes virus entry mediator, a receptor expressed by T lymphocytes) is implicated in the inflammation by disrupted T cell homeostasis, primarily at a transcriptional level. We investigated the association of LIGHT promoter with ischemic stroke and vascular dementia induced by such inflammation. We determined transcription factor binding sites altered by promoter SNPs using transcription factor prediction programs. Six common haplotypes composed of the selected SNPs (C-770T, G-607T, G-543A, and A-399G) were used for the assay of reporter activity. The most frequent haplotype construct, CGGA, induced the highest luciferase activity. The haplotype TTGA showed the lowest expression with 0.39-fold activity (P < 0.001) of CGGA. The substitution from C to T at the locus of C-770T (TGGA) decreased the reporter activity by 47% (P < 0.001). The SNPs and haplotypes were further investigated to see their association with ischemic stroke and vascular dementia in 455 controls and 478 patients. Significant association with vascular dementia was shown in the allele T of C-770T (odds ratio [OR] = 1.54; P < 0.05) and the haplotype TTGA (OR = 10.59; P < 0.05) in females. We concluded that the allele T of C-770T and the haplotype TTGA of the promoter SNPs in LIGHT gene might decrease the expression of LIGHT and subsequently increase the susceptibility to vascular dementia in females.

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.