Differential mechanisms of transcriptional regulation of the mouse osteocalcin gene by Jun family members

Titanium with nanotopography attenuates the osteoclast-induced disruption of osteoblast differentiation by regulating histone methylation

The bone remodeling process is crucial for titanium (Ti) osseointegration and involves the crosstalk between osteoclasts and osteoblasts. Considering the high osteogenic potential of Ti with nanotopography (Ti Nano) and that osteoclasts inhibit osteoblast differentiation, we hypothesized that nanotopography attenuate the osteoclast-induced disruption of osteoblast differentiation. Osteoblasts were co-cultured with osteoclasts on Ti Nano and Ti Control and non-co-cultured osteoblasts were used as control. Gene expression analysis using RNAseq showed that osteoclasts downregulated the expression of osteoblast marker genes and upregulated genes related to histone modification and chromatin organization in osteoblasts grown on both Ti surfaces. Osteoclasts also inhibited the mRNA and protein expression of osteoblast markers, and such effect was attenuated by Ti Nano. Also, osteoclasts increased the protein expression of H3K9me2, H3K27me3 and EZH2 in osteoblasts grown on both Ti surfaces. ChIP assay revealed that osteoclasts increased accumulation of H3K27me3 that represses the promoter regions of Runx2 and Alpl in osteoblasts grown on Ti Control, which was reduced by Ti Nano. In conclusion, these data show that despite osteoclast inhibition of osteoblasts grown on both Ti Control and Ti Nano, the nanotopography attenuates the osteoclast-induced disruption of osteoblast differentiation by preventing the increase of H3K27me3 accumulation that represses the promoter regions of some key osteoblast marker genes. These findings highlight the epigenetic mechanisms triggered by nanotopography to protect osteoblasts from the deleterious effects of osteoclasts, which modulate the process of bone remodeling and may benefit the osseointegration of Ti implants.

Protein phosphatase 1 regulatory subunit 18 suppresses the transcriptional activity of NFATc1 via regulation of c-fos

The transcription factor NFATc1 and its binding partner AP-1 (a complex containing c-fos and c-Jun) play a central role in osteoclast differentiation. NFATc1 and AP-1 promote the expression of target genes such as Acp5, Ctsk and also auto-regulate NFATc1 expression as well. We previously reported that protein phosphatase 1 regulatory subunit 18 (PPP1r18) is a negative regulator of osteoclast bone resorption by inhibiting cell attachment to bone matrix. We also reported that PPP1r18 potentially regulates NFATc1 expression during osteoclast differentiation. To further explore this, in this study we have examined the effect of PPP1r18 on NFATc1 expression and activity by overexpressing PPP1r18 during the early stage of osteoclast differentiation. We found that PPP1r18 suppressed NFATc1 expression through inhibition of the transcriptional activity of NFATc1. Since PPP1r18 does not regulate NFATc1 directly, we next explored the involvement of AP-1. Our data showed that c-fos phosphorylation and nuclear localization were reduced by PPP1r18 overexpression. Further experiments showed that overexpression of c-fos together with PPP1r18 rescued NFATc1 expression and transcriptional activity. Moreover, c-fos activity inhibition by PPP1r18 was canceled by mutation of the phosphatase binding site of PPP1r18. Taken together, PPP1r18-regulated phosphatase activity targets c-fos phosphorylation and suppresses subsequent NFATc1 expression and activity.

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PPP1r18 suppresses osteoclast differentiation.

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PPP1r18 suppresses c-fos phosphorylation and nuclear localization.

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PPP1r18 suppresses NFAT via c-fos.

Nfil3, a target of the NACA transcriptional coregulator, affects osteoblast and osteocyte gene expression differentially

Intermittent administration of PTH(1-34) has a profound osteoanabolic effect on the skeleton. At the cellular level, osteoblasts and osteocytes are two crucial cell types that respond to PTH stimulation in bone. The transcriptional cofactor Nascent polypeptide Associated Complex and coregulator alpha (NACA) is a downstream target of the PTH-Gαs-PKA axis in osteoblasts. NACA functions as a transcriptional cofactor affecting bZIP factor-mediated transcription of target promoters in osteoblasts, such as Osteocalcin (Bglap2). Here, we used RNA-Seq and ChIP-Seq against NACA in PTH-treated MC3T3-E1 osteoblastic cells to identify novel targets of the PTH-activated NACA. Our approach identified Nuclear factor interleukin-3-regulated (Nfil3) as a target promoter of this pathway. Knockdown of Naca reduced the response of Nfil3 to PTH(1-34) stimulation. In silico analysis of the Nfil3 promoter revealed potential binding sites for NACA (located within the ChIP fragment) and CREB. We show that following PTH stimulation, phosphorylated-CREB binds the proximal promoter of Nfil3 in osteoblasts. The activity of the Nfil3 promoter (-818/+182 bp) is regulated by CREB and this activation relies on the presence of NACA. In addition, we show that knockdown of Nfil3 enhances the expression of osteoblastic differentiation markers in MC3T3-E1 cells while it represses osteocytic marker gene expression in IDG-SW3 cells. These results show that the PTH-induced NACA axis regulates Nfil3 expression and suggest that NFIL3 acts as a transcriptional repressor in osteoblasts while it exhibits differential activity as an activator in osteocytes.

Dephosphorylation of the transcriptional cofactor NACA by the PP1A phosphatase enhances cJUN transcriptional activity and osteoblast differentiation

The transcriptional cofactor nascent polypeptide-associated complex and co-regulator α (NACA) regulates osteoblast maturation and activity. NACA functions, at least in part, by binding to Jun proto-oncogene, AP-1 transcription factor subunit (cJUN) and potentiating the transactivation of AP-1 targets such as osteocalcin (Bglap) and matrix metallopeptidase 9 (Mmp9). NACA activity is modulated by phosphorylation carried out by several kinases, but a phosphatase regulating NACA's activity remains to be identified. Here, we used affinity purification with MS in HEK293T cells to isolate NACA complexes and identified protein phosphatase 1 catalytic subunit α (PP1A) as a NACA-associated Ser/Thr phosphatase. NACA interacted with multiple components of the PP1A holoenzyme complex: the PPP1CA catalytic subunit and the regulatory subunits PPP1R9B, PPP1R12A and PPP1R18. MS analysis revealed that NACA co-expression with PPP1CA causes dephosphorylation of NACA at Thr-89, Ser-151, and Thr-174. NACA Ser/Thr-to-alanine variants displayed increased nuclear localization, and NACA dephosphorylation was associated with specific recruitment of novel NACA interactants, such as basic transcription factor 3 (BTF3) and its homolog BTF3L4. NACA and PP1A cooperatively potentiated cJUN transcriptional activity of the AP-1-responsive MMP9-luciferase reporter, which was abolished when Thr-89, Ser-151, or Thr-174 were substituted with phosphomimetic aspartate residues. We confirmed the NACA-PP1A interaction in MC3T3-E1 osteoblastic cells and observed that NACA phosphorylation status at PP1A-sensitive sites is important for the regulation of AP-1 pathway genes and for osteogenic differentiation and matrix mineralization. These results suggest that PP1A dephosphorylates NACA at specific residues, impacting cJUN transcriptional activity and osteoblast differentiation and function.

The PTH-Gαs-protein kinase A cascade controls αNAC localization to regulate bone mass

The binding of PTH to its receptor induces Gα(s)-dependent cyclic AMP (cAMP) accumulation to turn on effector kinases, including protein kinase A (PKA). The phenotype of mice with osteoblasts specifically deficient for Gα(s) is mimicked by a mutation leading to cytoplasmic retention of the transcriptional coregulator αNAC, suggesting that Gαs and αNAC form part of a common genetic pathway. We show that treatment of osteoblasts with PTH(1-34) or the PKA-selective activator N(6)-benzoyladenosine cAMP (6Bnz-cAMP) leads to translocation of αNAC to the nucleus. αNAC was phosphorylated by PKA at serine 99 in vitro. Phospho-S99-αNAC accumulated in osteoblasts exposed to PTH(1-34) or 6Bnz-cAMP but not in treated cells expressing dominant-negative PKA. Nuclear accumulation was abrogated by an S99A mutation but enhanced by a phosphomimetic residue (S99D). Chromatin immunoprecipitation (ChIP) analysis showed that PTH(1-34) or 6Bnz-cAMP treatment leads to accumulation of αNAC at the Osteocalcin (Ocn) promoter. Altered gene dosages for Gα(s) and αNAC in compound heterozygous mice result in reduced bone mass, increased numbers of osteocytes, and enhanced expression of Sost. Our results show that αNAC is a substrate of PKA following PTH signaling. This enhances αNAC translocation to the nucleus and leads to its accumulation at target promoters to regulate transcription and affect bone mass.

Enhancement of Flow-Induced AP-1 Gene Expression by Cyclosporin A Requires NFAT-Independent Signaling in Bone Cells

Growing evidence suggests that aging compromises the ability of the skeleton to respond to anabolic mechanical stimuli. Recently, we reported that treating senescent mice with Cyclosporin A (CsA) rescued aging-related deficits in loading-induced bone formation. Given that the actions of CsA are often attributed to inhibition of the calcineurin/NFAT axis, we hypothesized that CsA enhances gene expression in bone cells exposed to fluid flow, by inhibiting nuclear NFATc1 accumulation. When exposed to flow, MC3T3-E1 osteoblastic cells exhibited rapid nuclear accumulation of NFATc1 that was abolished by CsA treatment. Under differentiation conditions, intermittent CsA treatment enhanced gene expression of late osteoblastic differentiation markers and activator protein 1 (AP-1) family members. Superimposing flow upon CsA further enhanced expression of the AP-1 members Fra-1 and c-Jun. To delineate the contribution of NFAT in this response, cells were treated with VIVIT, a specific inhibitor of the calcineurin/NFAT interaction. Treatment with VIVIT blocked flow-induced nuclear NFATc1 accumulation but did not recapitulate the CsA-mediated enhancement of flow-induced AP-1 component gene expression. Taken together, our study is the first to demonstrate that CsA enhances mechanically-induced gene expression of AP-1 components in bone cells, and suggests that this response requires calcineurin-dependent mechanisms that are independent of inhibiting NFATc1 nuclear accumulation.

αNAC interacts with histone deacetylase corepressors to control Myogenin and Osteocalcin gene expression

In the nucleus of differentiated osteoblasts, the DNA-binding αNAC protein acts as a transcriptional coactivator of the Osteocalcin gene. Chromatin immunoprecipitation-microarray assays (ChIP-chip) showed that αNAC binds the Osteocalcin promoter but also identified the Myogenin promoter as an αNAC target. Here, we confirm these array data using quantitative ChIP and further detected that αNAC binds to these promoters in myoblasts. Since these genes are differentially regulated during osteoblastogenesis or myogenesis, these results suggest cell- and promoter-context specific functions for αNAC. We hypothesized that αNAC dynamically recruits corepressors to inhibit Myogenin expression in cells committing to the osteoblastic lineage or to inhibit Osteocalcin transcription in differentiating myoblasts. Using co-immunoprecipitation assays, we detected complexes between αNAC and the corepressors HDAC1 and HDAC3, in myoblasts and osteoblasts. Sequential ChIP confirmed HDAC1 recruitment by αNAC at the Osteocalcin and Myogenin promoters. Interaction with the corepressors was detectable in pre-osteoblasts and in myoblasts but disappeared as the cells differentiate. Treatment with an HDAC inhibitor caused de-repression of Osteocalcin expression in myoblasts. Overexpression of αNAC in myoblasts inhibits expression of Myogenin and differentiation. However, overexpression of an N-terminus truncated αNAC mutant allowed myoblasts to express Myogenin and differentiate, and this mutant did not interact with HDAC1 or HDAC3. This study identified an additional DNA-binding target and novel protein-protein interactions for αNAC. We propose that αNAC plays a role in regulating gene transcription during mesenchymal cell differentiation by differentially recruiting corepressors at target promoters.

The tumor suppressor RASSF10 is upregulated upon contact inhibition and frequently epigenetically silenced in cancer

The Ras association domain family (RASSF) comprises a group of tumor suppressors that are frequently epigenetically inactivated in various tumor entities and linked to apoptosis, cell cycle control and microtubule stability. In this work, we concentrated on the newly identified putative tumor suppressor RASSF10. Methylation analysis reveals RASSF10 promoter hypermethylation in lung cancer, head and neck (HN) cancer, sarcoma and pancreatic cancer. An increase in RASSF10 methylation from normal tissues, primary tumors to cancer cell lines was observed. Methylation was reversed by 5-aza-2'-deoxycytidine treatment leading to reexpression of RASSF10. We further show that overexpression of RASSF10 suppresses colony formation in cancer cell lines. In addition, RASSF10 is upregulated by cell-cell contact and regulated on promoter level as well as endogenously by forskolin, protein kinase A (PKA) and activator Protein 1 (AP-1), linking RASSF10 to the cAMP signaling pathway. Knockdown of the AP-1 member JunD interfered with contact inhibition induced RASSF10 expression. In summary, we found RASSF10 to be epigenetically inactivated by hypermethylation of its CpG island promoter in lung, HN, sarcoma and pancreatic cancer. Furthermore, our novel findings suggest that tumor suppressor RASSF10 is upregulated by PKA and JunD signaling upon contact inhibition and that RASSF10 suppresses growth of cancer cells.

Up-regulation of human CYP2J2 in HepG2 cells by butylated hydroxyanisole is mediated by c-Jun and Nrf2

Cytochrome P450 2J2 oxidizes arachidonic acid to a series of epoxyeicosatrienoic acid (EET) isomers in human tissues. EETs regulate numerous homeostatic processes, including cytoprotective and proliferative responses against injurious stresses. There is little information currently available on the factors that regulate CYP2J2, but strategies to activate expression could use the beneficial effects of EETs in cells. The basic leucine zipper (bZIP) transcription factor c-Jun has been shown previously to maintain CYP2J2 expression in human HepG2 cells; c-Jun forms transcriptionally active dimers with the antioxidant-inducible bZIP factor Nrf2. In the present study, we tested the hypothesis that CYP2J2 expression may be activated in cells by c-Jun/Nrf2 heterodimers. Treatment of HepG2 cells with butylated hydroxyanisole elicited concentration- and time-dependent activation of CYP2J2 expression, as well as the bZIP factors Nrf2 and c-Jun; chromatin immunoprecipitation assays revealed a pronounced increase in binding of these bZIP factors to the CYP2J2 5'-flank. Transient transfection analysis using deletion constructs and gel-shift assays were consistent with a role for the -105/-88 region of CYP2J2 in c-Jun/Nrf2 responsiveness. Using a series of mutant expression plasmids, we identified c-Jun as the critical partner in CYP2J2 transactivation. Coimmunoprecipitation experiments confirmed the importance of the leucine zipper region of Nrf2 in the enhancement of c-Jun-dependent transactivation of CYP2J2. Agents that activate CYP2J2 expression may offer a new approach to using the beneficial effects of EETs in cells.

Corosolic acid stimulates osteoblast differentiation by activating transcription factors and MAP kinases

Recently, the use of anabolic agents to enhance bone mass has been a source of interest. Previous work by us suggested that corosolic acid (2alpha-hydroxyursolic acid), an active component of banaba leaves (Lagerstroemia speciosa L.), potentially stimulates the differentiation of mouse osteoblasts. Therefore, the present study investigated whether corosolic acid stimulates osteoblast differentiation, and its possible mechanisms of action. At low concentrations (up to 5 microm), corosolic acid significantly stimulated osteoblast differentiation and mineralization without cytotoxicity. Corosolic acid induced NF-kappaB and MAP kinase activity at an early stage of osteoblast differentiation and increased the activity of the transcription factor AP-1 during late-stage osteoblast differentiation. These results suggest that the anabolic effects of corosolic acid upon osteoblast differentiation could result from its activation of transcription factors and MAP kinases.

Nuclear alpha NAC influences bone matrix mineralization and osteoblast maturation in vivo

Nascent-polypeptide-associated complex and coactivator alpha (alpha NAC) is a protein shuttling between the nucleus and the cytoplasm. Upon phosphorylation at residue serine 43 by integrin-linked kinase, alpha NAC is translocated to the nuclei of osteoblasts, where it acts as an AP-1 coactivator to increase osteocalcin gene transcription. To determine the physiological role of nuclear alpha NAC, we engineered a knock-in mouse model with a serine-to-alanine mutation at position 43 (S43A). The S43A mutation resulted in a decrease in the amount of nuclear alpha NAC with reduced osteocalcin gene promoter occupancy, leading to a significant decrease in osteocalcin gene transcription. The S43A mutant bones also expressed decreased levels of alpha(1)(I) collagen mRNA and as a consequence had significantly less osteoid tissue. Transient transfection assays and chromatin immunoprecipitation confirmed the alpha(1)(I) collagen gene as a novel alpha NAC target. The reduced quantity of bone matrix in S43A mutant bones was mineralized faster, as demonstrated by the significantly reduced mineralization lag time, producing a lower volume of immature, woven-type bone characterized by poor lamellation and an increase in the number of osteocytes. Accordingly, the expression of the osteocyte differentiation marker genes DMP-1 (dentin matrix protein 1), E11, and SOST (sclerostin) was increased. The accelerated mineralization phenotype was cell autonomous, as osteoblasts isolated from the calvaria of S43A mutant mice mineralized their matrix faster than did wild-type cells. Thus, inhibition of alpha NAC nuclear translocation results in an osteopenic phenotype caused by reduced expression of osteocalcin and type I collagen, accelerated mineralization, and immature woven-bone formation.

Stimulatory effect of undecylenic acid on mouse osteoblast differentiation

Natural compounds with bone-forming (or anabolic) activity have been recently focused on in bone research. The present study investigated the effect of undecylenic acid (UA) on osteoblast differentiation in mouse osteoblastic MC3T3-E1 subclone 4 cells and primary mouse calvarial cells. Low concentrations of UA (up to 5 microM) exhibited no cytotoxicity and significantly increased the expression and activity of alkaline phosphatase (early differentiation marker of osteoblast) and calcium deposition with the induction of expression of the osteocalcin gene in both cells. Interestingly, at low concentration of UA, the induction of NF-kappaB p65 translocation into nucleus and the up-regulation of AP-1 and NFATc1 transcript levels were also observed, suggesting that the stimulatory effect of UA on osteoblast differentiation could be mediated through the activation of transcription factors. Additionally, although the patterns of UA-induced activation of MAP kinases (JNK and p38) were not completely consistent with the increase of both ALP activity and calcium deposition by UA, MAP kinases might be partially involved in the biological function of UA during the early and late stages of osteoblast differentiation.

FIAT is co-expressed with its dimerization target ATF4 in early osteoblasts, but not in osteocytes

FIAT represses osteocalcin gene transcription by heterodimerizing with ATF4 and preventing it from binding to DNA. We report here the expression profiles of FIAT and ATF4 during osteoblastogenesis. Messenger RNA levels for the osteoblast transcriptional regulators Satb2, Runx2, Fiat, and Atf4 were quantified using real-time reverse-transcription PCR (RT-qPCR) and respective protein levels monitored by immunodetection in differentiating primary osteoblast cultures. Satb2, Fiat, and Atf4 mRNA levels remained constant throughout the differentiation sequence, whereas Runx2 transcript levels were significantly increased by 12 days post-confluency. Using immunofluorescence, the SATB2, RUNX2, and ATF4 signals appeared to increase as a function of time in culture. FIAT protein expression was readily detected in early cultures, but signal intensity decreased thereafter. When immunoblotting was used to quantify the relative amounts of FIAT and ATF4 proteins, the expression levels of the two proteins were found to be inversely correlated. The decrease in FIAT protein levels coincided with increased binding of ATF4 to the osteocalcin gene promoter, and with increased osteocalcin expression measured by RT-qPCR or immunoblotting. Immunohistochemistry of long bones from mice at E16.5 and 2 days post-natal revealed that both proteins are initially expressed in osteoblasts. In adult bone, FIAT was detected in osteocytes, while ATF4 expression was observed in active osteoblasts and lining cells, but not in osteocytes. Taken together, these data support the idea that a stoichiometric excess of ATF4 over FIAT in mature osteoblasts releases ATF4 from sequestration by FIAT, thereby allowing ATF4 homodimerization and subsequent transactivation of the osteocalcin gene.

Multiple facets of junD gene expression are atypical among AP-1 family members

JunD is a versatile AP-1 transcription factor that can activate or repress a diverse collection of target genes. Precise control of junD expression and JunD protein-protein interactions modulate tumor angiogenesis, cellular differentiation, proliferation and apoptosis. Molecular and clinical knowledge of two decades has revealed that precise JunD activity is elaborated by interrelated layers of constitutive transcriptional control, complex post-transcriptional regulation and a collection of post-translational modifications and protein-protein interactions. The stakes are high, as inappropriate JunD activity contributes to neoplastic, metabolic and viral diseases. This article deconvolutes multiple layers of control that safeguard junD gene expression and functional activity. The activity of JunD in transcriptional activation and repression is integrated into a regulatory network by which JunD exerts a pivotal role in cellular growth control. Our discussion of the JunD regulatory network integrates important open issues and posits new therapeutic targets for the neoplastic, metabolic and viral diseases associated with JunD/AP-1 expression.