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

Dysfunction of poly (ADP-ribose) glycohydrolase suppresses osteoclast differentiation in RANKL-stimulated RAW264 cells

Poly (ADP-ribose) glycohydrolase (PARG) is an enzyme that mainly degrades poly (ADP-ribose) (PAR) synthesized by poly (ADP-ribose) polymerase (PARP) family proteins. Although PARG is involved in many biological phenomena, including DNA repair, cell differentiation, and cell death, little is known about the relationship between osteoclast differentiation and PARG. It has also not been clarified whether PARG is a valuable target for therapeutic agents in the excessive activity of osteoclast-related bone diseases such as osteoporosis. In the present study, we examined the effects of PARG inhibitor PDD00017273 on osteoclast differentiation in RANKL-induced RAW264 cells. PDD00017273 induced the accumulation of intracellular PAR and suppressed the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells. PDD00017273 also downregulated osteoclast differentiation marker genes such as Trap, cathepsin K (Ctsk), and dendrocyte expressed seven transmembrane protein (Dcstamp) and protein expression of nuclear factor of activated T cells 1 (NFATc1), a master regulator of osteoclast differentiation. Taken together, our findings suggest that dysfunction of PARG suppresses osteoclast differentiation via the PAR accumulation and partial inactivation of the NFATc1.

Possible Action of Olaparib for Preventing Invasion of Oral Squamous Cell Carcinoma In Vitro and In Vivo

Despite recent advances in treatment, the prognosis of oral cancer remains poor, and prevention of recurrence and metastasis is critical. Olaparib is a PARP1 inhibitor that blocks polyADP-ribosylation, which is involved in the epithelial–mesenchymal transition (EMT) characteristic of tumor recurrence. We explored the potential of olaparib in inhibiting cancer invasion in oral carcinoma using three oral cancer cell lines, HSC-2, Ca9-22, and SAS. Olaparib treatment markedly reduced their proliferation, migration, invasion, and adhesion. Furthermore, qRT-PCR revealed that olaparib inhibited the mRNA expression of markers associated with tumorigenesis and EMT, notably Ki67, Vimentin, β-catenin, MMP2, MMP9, p53, and integrin α2 and β1, while E-Cadherin was upregulated. In vivo analysis of tumor xenografts generated by injection of HSC-2 cells into the masseter muscles of mice demonstrated significant inhibition of tumorigenesis and bone invasion by olaparib compared with the control. This was associated with reduced expression of proteins involved in osteoclastogenesis, RANK and RANKL. Moreover, SNAIL and PARP1 were downregulated, while E-cadherin was increased, indicating the effect of olaparib on proteins associated with EMT in this model. Taken together, these findings confirm the effects of olaparib on EMT and bone invasion in oral carcinoma and suggest a new therapeutic strategy for this disease.

Okamoto model for necrosis and its expansions, CD38-cyclic ADP-ribose signal system for intracellular Ca2+ mobilization and Reg (Regenerating gene protein)-Reg receptor system for cell regeneration

In pancreatic islet cell culture models and animal models, we studied the molecular mechanisms involved in the development of insulin-dependent diabetes. The diabetogenic agents, alloxan and streptozotocin, caused DNA strand breaks, which in turn activated poly(ADP-ribose) polymerase/synthetase (PARP) to deplete NAD+, thereby inhibiting islet β-cell functions such as proinsulin synthesis and ultimately leading to β-cell necrosis. Radical scavengers protected against the formation of DNA strand breaks and inhibition of proinsulin synthesis. Inhibitors of PARP prevented the NAD+ depletion, inhibition of proinsulin synthesis and β-cell death. These findings led to the proposed unifying concept for β-cell damage and its prevention (the Okamoto model). The model met one proof with PARP knockout animals and was further extended by the discovery of cyclic ADP-ribose as the second messenger for Ca2+ mobilization in glucose-induced insulin secretion and by the identification of Reg (Regenerating gene) for β-cell regeneration. Physiological and pathological events found in pancreatic β-cells have been observed in other cells and tissues.

Tankyrase inhibitor XAV-939 enhances osteoblastogenesis and mineralization of human skeletal (mesenchymal) stem cells

Tankyrase is part of poly (ADP-ribose) polymerase superfamily required for numerous cellular and molecular processes. Tankyrase inhibition negatively regulates Wnt pathway. Thus, Tankyrase inhibitors have been extensively investigated for the treatment of clinical conditions associated with activated Wnt signaling such as cancer and fibrotic diseases. Moreover, Tankyrase inhibition has been recently reported to upregulate osteogenesis through the accumulation of SH3 domain-binding protein 2, an adaptor protein required for bone metabolism. In this study, we investigated the effect of Tankyrase inhibition in osteoblast differentiation of human skeletal (mesenchymal) stem cells (hMSCs). A Tankyrase inhibitor, XAV-939, identified during a functional library screening of small molecules. Alkaline phosphatase activity and Alizarin red staining were employed as markers for osteoblastic differentiation and in vitro mineralized matrix formation, respectively. Global gene expression profiling was performed using the Agilent microarray platform. XAV-939, a Tankyrase inhibitor, enhanced osteoblast differentiation of hBMSCs as evidenced by increased ALP activity, in vitro mineralized matrix formation, and upregulation of osteoblast-related gene expression. Global gene expression profiling of XAV-939-treated cells identified 847 upregulated and 614 downregulated mRNA transcripts, compared to vehicle-treated control cells. It also points towards possible changes in multiple signaling pathways, including TGFβ, insulin signaling, focal adhesion, estrogen metabolism, oxidative stress, RANK-RANKL (receptor activator of nuclear factor κB ligand) signaling, Vitamin D synthesis, IL6, and cytokines and inflammatory responses. Further bioinformatic analysis, employing Ingenuity Pathway Analysis identified significant enrichment in XAV-939-treated cells of functional categories and networks involved in TNF, NFκB, and STAT signaling. We identified a Tankyrase inhibitor (XAV-939) as a powerful enhancer of osteoblastic differentiation of hBMSC that may be useful as a therapeutic option for treating conditions associated with low bone formation.

PARP1 Hinders Histone H2B Occupancy at the NFATc1 Promoter to Restrain Osteoclast Differentiation

Induction of nuclear factor of activated T cell cytoplasmic 1 (NFATc1) by macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) is essential for macrophage differentiation into osteoclasts (OCs), but the underlying mechanisms remain unclear. The ability of poly(ADP-ribose) polymerase 1 (PARP1) to poly-ADP-ribosylate NFATc1 in T cells prompted us to investigate the PARP1 and NFATc1 interaction during osteoclastogenesis. However, extensive studies failed to directly link PARP1 to NFATc1. A combination of transcriptomics and proteomics studies was then used to identify PARP1 targets under these conditions. These unbiased approaches in conjunction with site-directed mutagenesis studies revealed that PARP1 inhibited NFATc1 expression and OC formation by ADP-ribosylating histone H2B at serine 7 and decreasing the occupancy of this histone variant at the NFATc1 promoter. The anti-osteoclastogenic function of PARP1 was confirmed in vivo in several mouse models of PARP1 loss-of-function or gain-of-function, including a novel model in which PARP1 was conditionally ablated in myeloid cells. Thus, PARP1 ADP-ribosylates H2B to negatively regulate NFATc1 expression and OC differentiation. © 2019 American Society for Bone and Mineral Research.

Loss of PARP-1 attenuates diabetic arteriosclerotic calcification via Stat1/Runx2 axis

Accelerated atherosclerotic calcification is responsible for plaque burden, especially in diabetes. The regulatory mechanism for atherosclerotic calcification in diabetes is poorly characterized. Here we show that deletion of PARP-1, a main enzyme in diverse metabolic complications, attenuates diabetic atherosclerotic calcification and decreases vessel stiffening in mice through Runx2 suppression. Specifically, PARP-1 deficiency reduces diabetic arteriosclerotic calcification by regulating Stat1-mediated synthetic phenotype switching of vascular smooth muscle cells and macrophage polarization. Meanwhile, both vascular smooth muscle cells and macrophages manifested osteogenic differentiation in osteogenic media, which was attenuated by PARP-1/Stat1 inhibition. Notably, Stat1 acts as a positive transcription factor by directly binding to the promoter of Runx2 and promoting atherosclerotic calcification in diabetes. Our results identify a new function of PARP-1, in which metabolism disturbance-related stimuli activate the Runx2 expression mediated by Stat1 transcription to facilitate diabetic arteriosclerotic calcification. PARP-1 inhibition may therefore represent a useful therapy for this challenging complication.

Role of APD-Ribosylation in Bone Health and Disease

The transfer of adenosine diphosphate (ADP)-ribose unit(s) from nicotinamide adenine dinucleotide (NAD⁺) to acceptor proteins is known as ADP-ribosylation. This post-translational modification (PTM) unavoidably alters protein functions and signaling networks, thereby impacting cell behaviors and tissue outcomes. As a ubiquitous mechanism, ADP-ribosylation affects multiple tissues, including bones, as abnormal ADP-ribosylation compromises bone development and remodeling. In this review, we describe the effects of ADP-ribosylation in bone development and maintenance, and highlight the underlying mechanisms.

Identification of miR-708-5p in peripheral blood monocytes: Potential marker for postmenopausal osteoporosis in Mexican-Mestizo population

IMPACT STATEMENT

This is the first study in which hsa-miR-708-5p has been identified in peripheral blood monocytes (osteoclast precursors) and associated with postmenopausal osteoporosis through small RNA-Sequencing, in an Admixed Mexican Mestizo population. By conducting in silico and bioinformatic analyzes, we identified target genes and important signaling pathways involved in bone metabolism pointing hsa-miR-708-5p as a candidate marker for osteoporosis in Mexican population. These approaches provide a landscape of the post-transcriptional regulation, which can be useful for the management of postmenopausal osteoporosis along with the potential use of microRNAs as markers for its early detection.

From macrophage to osteoclast - How metabolism determines function and activity

Osteoclasts are specialised cells that resorb bone and develop from the monocyte/macrophage lineage. While there is a wealth of information on the regulation of macrophage function through metabolic activity, the connection between osteoclast differentiation and metabolism is less well understood. Recent data show that mitochondria participate in switching macrophages from an inflammatory phenotype towards differentiation into osteoclasts. Additionally, it was found that reactive oxygen species (ROS) actively take place in osteoclast differentiation by acting as secondary signalling molecules. Bone resorption is an energy demanding process and differentiating osteoclasts triggers the biogenesis of mitochondria. In addition, the activity of specific OXPHOS components of macrophages and osteoclasts is differentially regulated. This review summarises our knowledge on macrophage-mediated inflammation, its impact on a cell's metabolic activity and its effect on osteoclast differentiation.

Inflammatory osteolysis: a conspiracy against bone

There are many causes of inflammatory osteolysis, but regardless of etiology and cellular contexts, the osteoclast is the bone-degrading cell. Thus, the impact of inflammatory cytokines on osteoclast formation and function was among the most important discoveries advancing the treatment of focal osteolysis, leading to development of therapeutic agents that either directly block the bone-resorptive cell or do so indirectly via cytokine arrest. Despite these advances, a substantial number of patients with inflammatory arthritis remain resistant to current therapies, and even effective anti-inflammatory drugs frequently do not repair damaged bone. Thus, insights into events such as those impacted by inflammasomes, which signal through cytokine-dependent and -independent mechanisms, are needed to optimize treatment of inflammatory osteolysis.

Cell fate regulation by chromatin ADP-ribosylation

ADP-ribosylation is an evolutionarily conserved complex posttranslational modification that alters protein function and/or interaction. Intracellularly, it is mainly catalyzed by diphtheria toxin-like ADP-ribosyltransferases (ARTDs), which attach one or several ADP-ribose residues onto target proteins. Several specific mono- and poly-ADP-ribosylation binding modules exist; hydrolases reverse the modification. The best-characterized ARTD family member, ARTD1, regulates various DNA-associated processes. Here, we focus on the role of ARTD1-mediated chromatin ADP-ribosylation in development, differentiation, and pluripotency, and the recent development of new methodologies that will enable more insight into these processes.

Poly-ADP-ribosylation-mediated degradation of ARTD1 by the NLRP3 inflammasome is a prerequisite for osteoclast maturation

Evidence implicates ARTD1 in cell differentiation, but its role in skeletal metabolism remains unknown. Osteoclasts (OC), the bone-resorbing cells, differentiate from macrophages under the influence of macrophage colony-stimulating factor (M-CSF) and receptor-activator of NF-κB ligand (RANKL). We found that M-CSF induced ADP-ribosyltransferase diphtheria toxin-like 1 (ARTD1) auto-ADP-ribosylation in macrophages, a modification that marked ARTD1 for cleavage, and subsequently, for degradation upon RANKL exposure. We established that ARTD1 proteolysis was NLRP3 inflammasome-dependent, and occurred via the proteasome pathway. Since ARTD1 is cleaved at aspartate²¹⁴, we studied the impact of ARTD1 rendered uncleavable by D214N substitution (ARTD1^D214N) on skeletal homeostasis. ARTD1^D214N, unlike wild-type ARTD1, was resistant to cleavage and degradation during osteoclastogenesis. As a result, ARTD1^D214N altered histone modification and promoted the abundance of the repressors of osteoclastogenesis by interfering with the expression of B lymphocyte-induced maturation protein 1 (Blimp1), the master regulator of anti-osteoclastogenic transcription factors. Importantly, ARTD1^D214N-expressing mice exhibited higher bone mass compared with controls, owing to decreased osteoclastogenesis while bone formation was unaffected. Thus, unless it is degraded, ARTD1 represses OC development through transcriptional regulation.

ARTD1 regulates osteoclastogenesis and bone homeostasis by dampening NF-κB-dependent transcription of IL-1β

While ADP-ribosyltransferase diphtheria toxin-like 1 (ARTD1, formerly PARP1) and its enzymatic activity have been shown to be important for reprogramming and differentiation of cells, such as during adipogenesis, their role and mechanism in regulating osteoclastogenesis and bone homeostasis are largely unknown. Here, in cell culture-based RANKL-induced osteoclastogenesis models, we show that silencing of ARTD1 or inhibition of its enzymatic activity enhances osteoclast differentiation and function. As a consequence of ARTD1 silencing or inhibition, the recruitment of p65/RelA to the IL-1β promoter, which is associated with transcriptionally active histone marks, IL-1β expression and inflammasome-dependent secretion of IL-1β are enhanced. This subsequently promotes sustained induction of the transcription factor Nfatc1/A and osteoclastogenesis in an autocrine manner via the IL-1 receptor. In vivo, Artd1-deficient mice display significantly decreased bone mass as a consequence of increased osteoclast differentiation. Accordingly, the expression of osteoclast markers is enhanced in mutant compared to wild-type mice. Together, these results indicate that ARTD1 controls osteoclast development and bone remodelling via its enzymatic activity by modulating the epigenetic marks surrounding the IL-1β promoter and expression of IL-1β and subsequently also Nfatc1/A.

Poly(ADP-ribose) in the bone: from oxidative stress signal to structural element

Contrary to common perception bone is a dynamic organ flexibly adapting to changes in mechanical loading by shifting the delicate balance between bone formation and bone resorption carried out by osteoblasts and osteoclasts, respectively. In the past decades numerous studies demonstrating production of reactive oxygen or nitrogen intermediates, effects of different antioxidants, and involvement of prototypical redox control mechanisms (Nrf2-Keap1, Steap4, FoxO, PAMM, caspase-2) have proven the central role of redox regulation in the bone. Poly(ADP-ribosyl)ation (PARylation), a NAD-dependent protein modification carried out by poly(ADP-ribose) polymerase (PARP) enzymes recently emerged as a new regulatory mechanism fine-tuning osteoblast differentiation and mineralization. Interestingly PARylation does not simply serve as a signaling mechanism during osteoblast differentiation but also couples it to osteoblast death. Even more strikingly, the poly(ADP-ribose) polymer likely released from succumbed cells at the terminal stage of differentiation is incorporated into the bone matrix representing the first structural role of this versatile biopolymer. Moreover, this new paradigm explains why and how osteodifferentiation and death of cells entering this pathway are closely coupled to each other. Here we review the role of reactive oxygen and nitrogen intermediates as well as PARylation in osteoblast and osteoclast differentiation, function, and cell death.

NLRP3 mediates osteolysis through inflammation-dependent and -independent mechanisms

Activating-mutations in NOD-like receptor (NLR) family, pyrin domain-containing 3 (NLRP3) cause neonatal-onset multisystem inflammatory disease. However, the ontogeny of skeletal anomalies in this disorder is poorly understood. Mice globally expressing the D301N mutation in Nlrp3 (D303N in human) model the human phenotype, including systemic inflammation and skeletal deformities. To gain insights into the skeletal manifestations, we generated mice in which the expression of D301N Nlrp3 (Nlrp3( D301N)) is restricted to myeloid cells. These mice exhibit systemic inflammation and severe osteopenia (∼ 60% lower bone mass) similar to mice globally expressing the knock-in mutation, consistent with the paradigm of innate immune-driven cryopyrinopathies. Because systemic inflammation may indirectly affect bone homeostasis, we engineered mice in which Nlrp3( D301N) is expressed specifically in osteoclasts, the cells that resorb bone. These mice also develop ∼ 50% lower bone mass due to increased osteolysis, but there is no systemic inflammation and no change in osteoclast number. Mechanistically, aside from its role in IL-1β maturation, Nlrp3( D301N) expression enhances osteoclast bone resorbing ability through reorganization of actin cytoskeleton while promoting the degradation of poly(ADP-ribose) polymerase 1, an inhibitor of osteoclastogenesis. Thus, NLRP3 inflammasome activation is not restricted to the production of proinflammatory mediators but also leads to cytokine-autonomous responses.

PARP-1 and YY1 are important novel regulators of CXCL12 gene transcription in rat pancreatic beta cells

Despite significant progress, the molecular mechanisms responsible for pancreatic beta cell depletion and development of diabetes remain poorly defined. At present, there is no preventive measure against diabetes. The positive impact of CXCL12 expression on the pancreatic beta cell prosurvival phenotype initiated this study. Our aim was to provide novel insight into the regulation of rat CXCL12 gene (Cxcl12) transcription. The roles of poly(ADP-ribose) polymerase-1 (PARP-1) and transcription factor Yin Yang 1 (YY1) in Cxcl12 transcription were studied by examining their in vitro and in vivo binding affinities for the Cxcl12 promoter in a pancreatic beta cell line by the electrophoretic mobility shift assay and chromatin immunoprecipitation. The regulatory activities of PARP-1 and YY1 were assessed in transfection experiments using a reporter vector with a Cxcl12 promoter sequence driving luciferase gene expression. Experimental evidence for PARP-1 and YY1 revealed their trans-acting potential, wherein PARP-1 displayed an inhibitory, and YY1 a strong activating effect on Cxcl12 transcription. Streptozotocin (STZ)-induced general toxicity in pancreatic beta cells was followed by changes in Cxcl12 promoter regulation. PARP-1 binding to the Cxcl12 promoter during basal and in STZ-compromised conditions led us to conclude that PARP-1 regulates constitutive Cxcl12 expression. During the early stage of oxidative stress, YY1 exhibited less affinity toward the Cxcl12 promoter while PARP-1 displayed strong binding. These interactions were accompanied by Cxcl12 downregulation. In the later stages of oxidative stress and intensive pancreatic beta cell injury, YY1 was highly expressed and firmly bound to Cxcl12 promoter in contrast to PARP-1. These interactions resulted in higher Cxcl12 expression. The observed ability of PARP-1 to downregulate, and of YY1 to upregulate Cxcl12 promoter activity anticipates corresponding effects in the natural context where the functional interplay of these proteins could finely balance Cxcl12 transcription.

RANKL up-regulates brain-type creatine kinase via poly(ADP-ribose) polymerase-1 during osteoclastogenesis

Receptor activator of nuclear factor κB ligand (RANKL) is the key regulator for osteoclast formation and function. During osteoclastogenesis, RANKL-stimulated signals differentially modulate expression of a large number of proteins. Using proteomics approaches, we identified that brain-type cytoplasmic creatine kinase (Ckb) was greatly induced in mature osteoclasts. Ckb has been shown to contribute to osteoclast function. However, the mechanisms of Ckb regulation and the contribution of other isoforms of creatine kinase during RANKL-induced osteoclastogenesis are unknown. We found that Ckb was the predominant isoform of creatine kinase during osteoclastogenesis. Real-time PCR confirmed that RANKL induced ckb mRNA expression by over 40-fold in primary mouse bone marrow macrophages and Raw 264.7 cells. The RANKL-responsive region was identified within the -0.4- to -0.2-kb 5'-flanking region of the ckb gene. Affinity binding purification followed by mass spectrometry analysis revealed that poly(ADP-ribose) polymerase-1 (PARP-1) bound to the -0.4/-0.2-kb fragment that negatively regulated expression of ckb in response to RANKL stimulation. Electrophoretic mobility shift assays with PARP-1-specific antibody located the binding site of PARP-1 to the TTCCCA consensus sequence. The expression of PARP-1 was reduced during RANKL-induced osteoclastogenesis, concurrently with increased expression of Ckb. Consistently, knockdown of PARP-1 by lentivirus-delivered shRNA enhanced ckb mRNA expression. The activity of PARP-1 was determined to be required for its inhibitory effect on the ckb expression. In summary, we have demonstrated that PARP-1 is a negative regulator of the ckb expression. Down-regulation of PARP-1 is responsible for the up-regulation of ckb during RANKL-induced osteoclastogenesis.

Specific expression of BMP2/4 ortholog in biomineralizing tissues of corals and action on mouse BMP receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta superfamily, and have been identified by their ability to induce bone formation in vertebrates. The biomineral-forming process, called biomineralization, is a widespread process, present in all kingdoms of living organisms and among which stony corals are one of the major groups of calcifying animals. Here, we report the presence of a BMP2/4 ortholog in eight species of adult corals. The synthesis of such a protein by the calcifying epithelium of corals suggests that coral BMP2/4 plays a role in skeletogenesis, making BMP the first common protein involved in biomineralization among Eumetazoans. In addition we show that recombinant coral BMP2/4 is able to inhibit human BMP2-induced osteoblastic differentiation in mesenchymal C2C12 cells. We suggest that this inhibition results from a competition between coral BMP2/4 and human BMP2, indicating conservation of binding affinity of BMP and its receptor during evolution from corals to vertebrates. Further studies are needed to understand interactions between coral BMP2/4 and its receptors, and, thus, the action of BMP2/4 in adult corals.