Parathyroid hormone induces interleukin-6 heterogeneous nuclear and messenger RNA expression in murine calvarial organ cultures

The Ex Vivo Organ Culture of Bone

The ex vivo organ culture of bone provides many of the advantages of both the whole organism and isolated cell strategies and can deliver valuable insight into the network of processes and activities that are fundamental to bone and cartilage biology. Through maintaining the bone and/or cartilage cells in their native environment, this model system provides the investigator with a powerful experimental protocol to address specific facets of skeletal growth and development. In this chapter, we outline the basic protocols and possible readouts of organ culture models to replicate; (a) linear bone growth (murine metatarsal culture model), (b) bone and cartilage metabolism (murine femoral head culture model), (c) bone response to mechanical stimulation (bovine trabecular core culture model), and (d) bone resorption and formation (murine calvaria culture model).

Models of ex vivo explant cultures: applications in bone research

Ex vivo explant culture models are powerful tools in bone research. They allow investigation of bone and cartilage responses to specific stimuli in a controlled manner that closely mimics the in vivo processes. Because of limitations in obtaining healthy human bone samples the explant growth of animal tissue serves as a platform to study the complex physico-chemical properties of the bone. Moreover, these models enable preserving important cell-cell and cell-matrix interactions in order to better understand the behaviour of cells in their natural three-dimensional environment. Thus, the use of bone ex vivo explant cultures can frequently be of more physiological relevance than the use of two-dimensional primary cells grown in vitro. Here, we describe isolation and ex vivo growth of different animal bone explant models including metatarsals, femoral heads, calvaria, mandibular slices and trabecular cores. We also describe how these explants are utilised to study bone development, cartilage and bone metabolism, cancer-induced bone diseases, stem cell-driven bone repair and mechanoadaptation. These techniques can be directly used to understand mechanisms linked with bone physiology or bone-associated diseases.

Parathyroid hormone-related protein drives a CD11b+Gr1+ cell-mediated positive feedback loop to support prostate cancer growth

In the tumor microenvironment, CD11b(+)Gr1(+) bone marrow-derived cells are a predominant source of protumorigenic factors such as matrix metalloproteinases (MMP), but how distal tumors regulate these cells in the bone marrow is unclear. Here we addressed the hypothesis that the parathyroid hormone-related protein (PTHrP) potentiates CD11b(+)Gr1(+) cells in the bone marrow of prostate tumor hosts. In two xenograft models of prostate cancer, levels of tumor-derived PTHrP correlated with CD11b(+)Gr1(+) cell recruitment and microvessel density in the tumor tissue, with evidence for mediation of CD11b(+)Gr1(+) cell-derived MMP-9 but not tumor-derived VEGF-A. CD11b(+)Gr1(+) cells isolated from mice with PTHrP-overexpressing tumors exhibited relatively increased proangiogenic potential, suggesting that prostate tumor-derived PTHrP potentiates this activity of CD11b(+)Gr1(+) cells. Administration of neutralizing PTHrP monoclonal antibody reduced CD11b(+)Gr1(+) cells and MMP-9 in the tumors. Mechanistic investigations in vivo revealed that PTHrP elevated Y418 phosphorylation levels in Src family kinases in CD11b(+)Gr1(+) cells via osteoblast-derived interleukin-6 and VEGF-A, thereby upregulating MMP-9. Taken together, our results showed that prostate cancer-derived PTHrP acts in the bone marrow to potentiate CD11b(+)Gr1(+) cells, which are recruited to tumor tissue where they contribute to tumor angiogenesis and growth.

Immunoregulation of bone remodelling

Remodeling, a continuous physiological process maintains the strength of the bones, which maintains a delicate balance between bone formation and resorption process. This review gives an insight to the complex interaction and correlation between the bone remodeling and the corresponding changes in host immunological environment and also summarises the most recent developments occuring in the understanding of this complex field. T cells, both directly and indirectly increase the expression of receptor activator of nuclear factor kB ligand (RANKL); a vital step in the activation of osteoclasts, thus positively regulates the osteoclastogenesis. Though various cytokines, chemikines, transcription factors and co-stimulatory molecules are shared by both skeletal and immune systems, but researches are being conducted to establish and analyse their role and / or control on this complex but vital process. The understanding of this part of research may open new horizons in the management of inflammatory and autoimmune diseases, resulting into bone loss and that of osteoporosis also.

Osteoimmunopathology in HIV/AIDS: A Translational Evidence-Based Perspective

Infection with the human immunodeficiency virus-1 (HIV) and the resulting acquired immune deficiency syndrome (AIDS) alter not only cellular immune regulation but also the bone metabolism. Since cellular immunity and bone metabolism are intimately intertwined in the osteoimmune network, it is to be expected that bone metabolism is also affected in patients with HIV/AIDS. The concerted evidence points convincingly toward impaired activity of osteoblasts and increased activity of osteoclasts in patients with HIV/AIDS, leading to a significant increase in the prevalence of osteoporosis. Research attributes these outcomes in part at least to the ART, PI, and HAART therapies endured by these patients. We review and discuss these lines of evidence from the perspective of translational clinically relevant complex systematic reviews for comparative effectiveness analysis and evidence-based intervention on a global scale.

NMR-based metabolic profiling identifies biomarkers of liver regeneration following partial hepatectomy in the rat

Tissue injury and repair are often overlapping consequences of disease or toxic exposure, but are not often considered as distinct processes in molecular studies. To establish the systemic metabolic response to liver regeneration, the partial hepatectomy (PH) model has been studied in the rat by an integrated metabonomics strategy, utilizing (1)H NMR spectroscopy of urine, liver and serum. Male Sprague-Dawley rats were subjected to either surgical removal of approximately two-thirds of the liver, sham operated (SO) surgery, or no treatment (n = 10/group) and samples collected over a 7 day period. A number of urinary metabolic perturbations were observed in PH rats compared with SO and control animals, including elevated levels of taurine, hypotaurine, creatine, guanidinoacetic acid, betaine, dimethylglycine and bile acids. Serum betaine and creatine were also elevated after PH, while levels of triglyceride were reduced. In the liver, triglycerides, cholesterol, alanine and betaine were elevated after PH, while choline and its derivatives were reduced. Upon examining the dynamic pattern of urinary response (the 'metabolic trajectory'), several metabolites could be categorized into groups likely to reflect perturbations to different processes such as dietary intake or hepatic 1-carbon metabolism. Several of the urinary perturbations observed during the regenerative phase of the PH model have also been observed after exposure to liver toxins, indicating that hepatic regeneration may make a contribution to the systemic alterations in metabolism associated with hepatotoxicity. The observed changes in 1-carbon and lipid metabolism are consistent with the proposed role of these pathways in the activation of a regenerative response and provide further evidence regarding the utility of urinary NMR profiles in the detection of liver-specific pathology. Biofluid (1)H NMR-based metabolic profiling provides new insight into the role of metabolism of liver regeneration, and suggests putative biomarkers for the noninvasive monitoring of the regeneration process.

Hyperlipidemia impairs osteoanabolic effects of PTH

Epidemiological and in vitro studies have suggested that hyperlipidemia/oxidized phospholipids adversely affect bone. We recently found that oxidized phospholipids attenuate PTH-induced cAMP and immediate-early gene (IEG) expression in MC3T3-E1 cells, raising concerns that clinical hyperlipidemia may attenuate osteoanabolic effects of PTH in vivo. Thus, we studied whether intermittent PTH treatment has differential osteoanabolic effects in wildtype (C57BL/6) and hyperlipidemic (LDLR(-/-)) mice. Consistent with our previous in vitro studies, induction of IEGs in calvarial tissue, 45 min after a single dose of recombinant hPTH(1-34), was attenuated in LDLR(-/-) mice compared with C57BL/6 mice. Daily hPTH(1-34) injections for 5 wk significantly increased total and cortical BMD and BMC, assessed by pQCT, in C57BL/6 mice. However, this induction was completely abrogated in LDLR(-/-) mice. Similarly, PTH(1-34) failed to increase BMD in another hyperlipidemic mouse model, ApoE(-/-) mice. Histomorphometric analysis showed that trabecular bone of both mice responded similarly to PTH(1-34). Structural parameters improved significantly in response to PTH(1-34) in both mouse strains, although to a lesser degree in LDLR(-/-) mice. With PTH(1-34) treatment, osteoblast surface trended toward an increase in C57BL/6 mice and increased significantly in LDLR(-/-) mice. PTH(1-34) did not alter resorption parameters significantly, except for the eroded surface (ES/BS), which was reduced in the C57BL/6 but not in the LDLR(-/-) mice. These results show that PTH(1-34) has adverse effects on cortical bones of the hyperlipidemic mice, suggesting that the therapeutic effects of PTH may be compromised in the presence of hyperlipidemia.

Interleukin-18 is regulated by parathyroid hormone and is required for its bone anabolic actions

Interleukin-18 (IL-18) can regulate osteoblast and osteoclast function. We have identified, using cDNA microarray technology, that IL-18 expression is increased in UMR 106-01 rat osteoblastic cells in response to parathyroid hormone (PTH) treatment. Confirmation of these data using real-time reverse transcription-PCR showed that steady-state levels of IL-18 mRNA increased by 2 h (3-fold), peaked by 4 h (10-fold), and had diminished after 12 h (4.4-fold) and that this regulation was via the protein kinase A signaling pathway and did not involve activation of the PKC signal cascade. PTH regulation of IL-18 was confirmed at the protein level, and analysis of differentiating primary rat calvarial osteoblasts verified that both IL-18 mRNA and protein are regulated by PTH in primary rat osteoblasts. Promoter reporter assays revealed that PTH regulated the upstream IL-18 promoter and induced the exon 1 containing 1.1-kb IL-18 mRNA transcript in primary osteoblast cells. The in vivo physiological role of IL-18 in the anabolic actions of PTH on bone was then assessed using IL-18 knock-out mice. Female IL-18 null mice and wild-type littermate controls were injected with vehicle or 8 microg/100 g of human 1-38 PTH for 4 weeks. In IL-18 knock-out animals the anabolic effect of PTH (determined by bone mineral density changes in the proximal tibia) was abolished in trabecular bone but not in the cortical component. These data characterize the PTH regulation of IL-18 expression in osteoblastic cells and suggest that this cytokine is involved in the anabolic actions of PTH.

Prostanoids induce egr1 gene expression in cementoblastic OCCM cells

BACKGROUND AND OBJECTIVE

Prostanoids that activate protein kinase C signaling are potent anabolic stimulators of cementoblastic OCCM cells. Using cDNA subtractive hybridization, we identified early growth response gene-1 (egr1) as a prostanoid-induced gene. Egr1, a zinc-finger transcription factor expressed during tooth development, regulates cell growth and differentiation. We hypothesize that Egr1 may mediate part of the prostanoid-induced anabolic effect in cementoblasts. Our objective was to characterize prostanoid-induced egr1 gene expression in OCCM cells.

MATERIAL AND METHODS

Total RNA and proteins were assayed by northern blot and western immunoblot assays.

RESULTS

Prostaglandin E2-, prostaglandin F2alpha- and fluprostenol-induced egr1 mRNA levels peaked at 0.5 h and returned to baseline by 4 h. Prostaglandin F2alpha and fluprostenol more potently induced egr1 compared with prostaglandin E2. The phorbol ester, phorbol 12-myristate 13-acetate, which activates protein kinase C signaling, induced egr1 mRNA levels 66-fold over the control, whereas forskolin (a cAMP-protein kinase A activator) and ionomycin (a calcium activator) had no effect. Protein kinase C inhibition significantly inhibited prostaglandin E2-, prostaglandin F2alpha- and fluprostenol-induced egr1 mRNA levels. Finally, prostanoids maximally induced Egr1 protein at 1 h.

CONCLUSION

egr1 is a primary response gene induced by prostaglandin E2, prostaglandin F2alpha and fluprostenol in OCCM cells through protein kinase C signaling, suggesting that Egr1 may be a key mediator of anabolic responses in cementoblasts. Cementum is vital for periodontal organ maintenance and regeneration. Periodontal ligament fibers (Sharpey's fibers) insert into bone and cementum, thereby supporting the tooth in the alveolus (1). If the periodontal organ is lost, its regeneration requires cementoblast differentiation in order to form new cementum for periodontal ligament fiber insertion. Early attempts to regenerate cementum have proven difficult and rarely generate sufficient tissue (2). A better understanding of the molecular and cellular regulators that promote cementoblast differentiation is critical for developing targeted periodontal regeneration.

Genistein modulates the effects of parathyroid hormone in human osteoblastic SaOS-2 cells

Genistein and parathyroid hormone (PTH) are anabolic agents that stimulate bone formation through their direct actions in osteoblastic cells. In the present study, we aimed to determinewhether genistein modulates the actions of PTH in human osteoblastic SaOS-2 cells in an oestrogen-depleted condition. The present results showed that genistein (10−8to 10−6m) induced alkaline phosphatase (ALP) activity and osteoprotegrin (OPG) expression in SaOS-2 cells in a dose-dependent manner. These effects could be completely abolished by co-treatment with oestrogen antagonist ICI 182780 (7α-[9-[(4,4,5,5,5-pentafluoropentyl)sulfonyl]nonyl]-estra-1,3,5(10)-triene-3,17β-diol). Genistein (at 1μm) could stimulate the mRNA expression of receptor activator of NF-κB ligand (RANKL). As OPG and RANKL are known to modulate osteoclastogenesis, the ability of genistein to modulate OPG and RANKL expression in SaOS-2 cells suggested that it might modulate osteoclastogenesis through its direct actions on osteoblastic cells. PTH (at 10nm) stimulated ALP activity, induced RANKL mRNA expression and suppressed OPG mRNA expression in SaOS-2 cells, confirming its bi-directional effects on osteoblastic cells. Pre-treatment of SaOS-2 cells with genistein andoestrogen not only enhanced PTH-induced ALP activity, but also attenuated PTH up regulation ofRANKL mRNA expression and PTH down regulation of OPG mRNA expression. Taken together, the present study provides the first evidence that genistein could modulate the actions of PTH in human osteoblastic SaOS-2 cells in an oestrogen-depleted condition.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

Parathyroid hormone controls receptor activator of NF-kappaB ligand gene expression via a distant transcriptional enhancer

RANKL, a protein essential for osteoclast development and survival, is stimulated by parathyroid hormone (PTH) via a PTH receptor 1/cyclic AMP (cAMP)/protein kinase A (PKA)/CREB cascade, exclusively in osteoblastic cells. We report that a bacterial artificial chromosome-based transcriptional reporter construct containing 120 kb of RANKL 5'-flanking region was stimulated by dibutyryl-cAMP in stromal/osteoblastic cells, but not other cell types. Full cAMP responsiveness was dependent upon a conserved 715-bp region located 76 kb upstream from the transcription start site, which we identified by sequential deletion analysis and by comparison of human and mouse genomic sequences in silico. This region contained conserved consensus sequences which bound CREB and the osteoblast-specific transcription factor Runx2, and when mutated blunted cAMP responsiveness. Overexpression of Runx2 potentiated cAMP responsiveness of the endogenous RANKL gene in a cell-type-specific manner. Lastly, PTH responsiveness of the endogenous RANKL gene was abrogated in mice from which we deleted this conserved upstream region. Thus, PTH responsiveness of the RANKL gene is determined by a distant regulatory region that responds to cAMP in a cell-type-specific manner and Runx2 may contribute to such cell-type specificity.

TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics

Parathyroid hormone (PTH) and tumor necrosis factoralpha (TNFalpha) are bone resorptive agents that upregulate interleukin-6 (IL-6) and RANKL production by osteoblasts. IL-6 mRNA expression induced by PTH is rapid and transient in osteoblasts both in vitro and in vivo. This study found that IL-6 secretion induced by PTH is also rapid and transient. The induction of RANKL mRNA by PTH is also rapid and transient although with an extended time course compared to that of IL-6 mRNA. In contrast, the effects of TNFalpha are biphasic. During the first 2 h of stimulation with TNFalpha, the responses are similar to those induced by PTH. This is followed by a period of relatively low IL-6 and RANKL mRNA levels and little IL-6 secretion. A late phase of increased IL-6 and RANKL mRNA expression occurs 12-24 h after stimulation with TNFalpha leading to a significant increase in IL-6 secretion. A similar biphasic pattern of activation of p38 MAP kinase is induced by TNFalpha. p38alpha/beta activation is required for the increased RANKL mRNA during the early phase of stimulation by TNFalpha but not in the late phase. In contrast, p38alpha/beta activation is not required for increased IL-6 mRNA or IL-6 protein secretion in either the early or late phases of stimulation by TNFalpha. Blocking the increases in IL-6 transcription completely eliminates IL-6 secretion induced during the early phases of stimulation by either PTH or TNFalpha. Consistent with the dependence on transcription, IL-6 mRNA is rapidly degraded with half-lives of 10-14 min following stimulation with either PTH or TNFalpha. In contrast to IL-6, RANKL mRNA is substantially more stable with half-lives of 40-60 min. Taken together, our results show that TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics. The more extensive effect of TNFalpha likely reflects that TNFalpha stimulates IL-6 production and bone resorption in pathological situations. In contrast, the less extensive effect of PTH likely reflects that it acts in physiological situations where it is important to minimize the potential adverse effects of high levels of IL-6 on bone and/or surrounding tissues.

Stimulation of amphiregulin expression in osteoblastic cells by parathyroid hormone requires the protein kinase A and cAMP response element-binding protein signaling pathway

Parathyroid hormone (PTH), an anabolic agent for bone metabolism, has profound effects on gene expression in the osteoblast. Recently, we identified that amphiregulin (AR), an EGF-like ligand, is an immediate early gene for PTH treatment and has an important role in bone metabolism. In the present report, by using different PTH peptide fragments, protein kinase activators, and inhibitors, we have demonstrated that PTH regulates amphiregulin in a cAMP-protein kinase A (PKA)-dependent manner both in vitro and in vivo. We found that the phosphorylation of cAMP-response element (CRE)-binding protein (CREB) preceded AR transcription after PTH treatment. Moreover, luciferase reporter assays revealed that the binding of phosphorylated CREB to a conserved CRE site in the AR promoter plays an important role in basal, PTH-induced, and prostaglandin E2 (PGE2)-induced AR expression in osteoblastic cells. In summary, our data suggest that PTH-induced AR mRNA expression is mediated primarily through cAMP-PKA-CREB signaling.

Parathyroid hormone induces the NR4A family of nuclear orphan receptors in vivo

Parathyroid hormone (PTH) has both anabolic and catabolic effects on bone metabolism, although the molecular mechanisms mediating these effects are largely unknown. Among the transcription factors induced by PTH in osteoblasts are the nerve growth factor-inducible factor B (NR4A; NGFI-B) family of orphan nuclear receptors: Nurr1, Nur77, and NOR-1. PTH induces NR4A members through the cAMP-protein kinase A (PKA) pathway in vitro. We report here that PTH rapidly and transiently induced expression of all three NR4A genes in PTH-target tissues in vivo. In calvaria, long bones, and kidneys, NR4A induction was maximal 0.5-1 h after a single intraperitoneal (i.p.) injection of 80 microg/kg PTH. Nur77 demonstrated the highest expression, followed, in order, by Nurr1 and NOR-1. In calvaria and long bone, PTH-induced expression of each NR4A gene was detectable at 10 microg/kg i.p. with maximum induction at 40-80 microg/kg. PTH (3-34) did not induce NR4A mRNA levels in calvaria, long bone, and kidney in vivo, confirming our in vitro results that NR4A genes are induced primarily through the cAMP-PKA pathway. The magnitude of PTH-induced NR4A expression was comparable in vivo and in vitro. However, NR4A mRNA levels peaked and returned to baseline faster in vivo. Both in vivo and in vitro, PTH induced NR4A pre-mRNA levels suggesting that induction of these genes is, at least in part, through activation of mRNA synthesis. The in vivo induction of the NR4A family members by PTH suggests their involvement in, at least some, PTH-induced changes in bone metabolism.

Role of interleukin-6 in murine airway responses to ozone

This study sought to examine the role of interleukin-6 (IL-6) in ozone (O(3))-induced airway injury, inflammation, and hyperresponsiveness (AHR). Subacute (72 h) exposure to 0.3 ppm O(3) significantly elevated bronchoalveolar lavage fluid (BALF) protein, neutrophils, and soluble TNF receptors (sTNFR1 and sTNFR2) in wild-type C57BL/6 (IL-6(+/+)) mice; however, all four outcome indicators were significantly reduced in IL-6-deficient (IL-6(-/-)) compared with IL-6(+/+) mice. Acute O(3) exposure (2 ppm for 3 h) increased BALF protein, KC, macrophage inflammatory protein(MIP)-2, eotaxin, sTNFR1, and sTNFR2 in IL-6(+/+) mice. However, MIP-2 and sTNFR2 were not significantly increased following O(3) exposure in IL-6(-/-) mice. Increases in BALF neutrophils induced by O(3) (2 ppm for 3 h) were also significantly reduced in IL-6(-/-) vs. IL-6(+/+) mice. Airway responsiveness to methacholine was measured by whole body plethysmography before and following acute (3 h) or subacute (72 h) exposure to 0.3 ppm O(3). Acute O(3) exposure caused AHR in both groups of mice, but there was no genotype-related difference in the magnitude of O(3)-induced AHR. AHR was absent in mice of either genotype exposed for 72 h. Our results indicate that IL-6 deficiency reduces airway neutrophilia, as well as the levels of BALF sTNFR1 and sTNFR2 following acute high dose and/or subacute low-dose O(3) exposure, but has no effect on O(3)-induced AHR.

Impaired liver regeneration in mice lacking methionine adenosyltransferase 1A

Methionine adenosyltransferase (MAT) is an essential enzyme because it catalyzes the formation of S-adenosylmethionine (SAMe), the principal biological methyl donor. Of the two genes that encode MAT, MAT1A is mainly expressed in adult liver and MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic SAMe content and spontaneously develop hepatocellular carcinoma. The current study examined the influence of chronic hepatic SAMe deficiency on liver regeneration. Despite having higher baseline hepatic staining for proliferating cell nuclear antigen, MAT1A knockout mice had impaired liver regeneration after partial hepatectomy (PH) as determined by bromodeoxyuridine incorporation. This can be explained by an inability to up-regulate cyclin D1 after PH in the knockout mice. Upstream signaling pathways involved in cyclin D1 activation include nuclear factor kappaB (NFkappaB), the c-Jun-N-terminal kinase (JNK), extracellular signal-regulated kinases (ERKs), and signal transducer and activator of transcription-3 (STAT-3). At baseline, JNK and ERK are more activated in the knockouts whereas NFkappaB and STAT-3 are similar to wild-type mice. Following PH, early activation of these pathways occurred, but although they remained increased in wild-type mice, c-jun and ERK phosphorylation fell progressively in the knockouts. Hepatic SAMe levels fell progressively following PH in wild-type mice but remained unchanged in the knockouts. In culture, MAT1A knockout hepatocytes have higher baseline DNA synthesis but failed to respond to the mitogenic effect of hepatocyte growth factor. Taken together, our findings define a critical role for SAMe in ERK signaling and cyclin D1 regulation during regeneration and suggest chronic hepatic SAMe depletion results in loss of responsiveness to mitogenic signals.

Expression of inducible cAMP early repressor is coupled to the cAMP-protein kinase A signaling pathway in osteoblasts

We previously showed that parathyroid hormone (PTH) induces inducible cAMP early repressor (ICER) in osteoblastic cells and mouse calvariae. PTH signaling in osteoblastic cells is transduced by PTH receptor 1, which is coupled to cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling pathways. In the present study, we examined the role of these pathways in mediating PTH-induced ICER mRNA and protein expression in osteoblastic MC3T3-E1 cells. Using RT-PCR, we found that PTH(1-34), forskolin (FSK), and 8-bromo-cAMP (8Br-cAMP) induced ICER expression, while phorbol myristate acetate (PMA), ionomycin, and PTH(3-34) did not. Similar results were found for the induction of ICER protein. PKA inhibition by H89 markedly reduced PTH- and FSK-induced ICER expression, while PKC depletion by PMA had little effect. We also tested ICER induction by other osteotropic signaling agonists. Other cAMP-PKA pathway activators, such as PTH-related protein (PTHrP), induced ICER expression, while agents that signal through other pathways did not. PTHrP maximally induced ICER mRNA at 2-4 h, which then returned to baseline by 10 h. Finally, PTH, FSK, and PTHrP induced ICER in cultured mouse calvariae and osteoblastic ROS 17/2.8, UMR-106, and Pyla cells. We conclude that ICER expression in osteoblasts requires activation of the cAMP-PKA signaling pathway.

Termination of immediate-early gene expression after stimulation by parathyroid hormone or isoproterenol

cAMP/PKA signaling transiently stimulates mRNA expression of immediate-early genes, including IL-6 and c-fos. We confirmed that these mRNAs are transiently stimulated by parathyroid hormone (PTH) in ROS 17/2.8 osteoblastic cells. Consistent with the role for cAMP/PKA signaling in this response, PTH induces transient cAMP elevation, PKA activation, and cAMP-responsive element-binding protein (CREB) phosphorylation. Our goal was to determine whether termination of immediate-early gene expression is due to receptor desensitization or cAMP degradation. The approaches used were 1) inhibition of PTH receptor desensitization with G protein-coupled receptor kinase 2 (GRK2) antisense oligonucleotides or antisense plasmids, 2) sustained activation of adenyl cyclase with forskolin, and 3) inhibition of cAMP degradation with 3-isobutyl-1-methylxanthine. These experiments show that mechanisms downstream of receptor desensitization and cAMP degradation are primarily responsible for termination of PKA activity, CREB phosphorylation, and immediate-early gene expression. Similar conclusions were also obtained in response to PTH in a second osteoblastic cell line (MC3T3-E1) and in response to isoproterenol in NIH3T3 fibroblasts. This conclusion may therefore reflect a general mechanism for termination of immediate-early gene expression after induction by cAMP/PKA.

Parathyroid hormone induces RGS-2 expression by a cyclic adenosine 3',5'-monophosphate-mediated pathway in primary neonatal murine osteoblasts

Parathyroid hormone (PTH) is a promising anabolic agent for the treatment of osteoporosis. However, PTH is also potently catabolic. To help delineate the molecular mediators of PTH's opposing effects on skeletal metabolism, we have examined PTH-induced regulator of G-protein signaling-2 (RGS-2) expression and function in murine osteoblasts. RGS proteins are GTPase-activating proteins (GAPs) that regulate GTP-binding protein-coupled receptor (GPCR) signaling by enhancing the intrinsic GTPase activity of Galpha subunits. We found that 10 nmol/L PTH maximally induced RGS-2 mRNA in murine MC3T3-E1 cells, rat Py1a and ROS-17/2.8 cells, primary mouse osteoblasts (MOB cells), and mouse calvariae organ culture at 1-2 h posttreatment. PTH signaling through its receptor, PTHR1, is coupled to cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling pathways. We examined the effect of selective signaling agonists and antagonists on RGS-2 expression in MOB cells to determine which pathway(s) mediates PTH-induced RGS-2 expression. Although selective activation of all three pathways led to RGS-2 expression, cAMP-PKA activation with 10 nmol/L PTH and 10 micromol/L forskolin elicited the strongest induction. Similarly, RGS-2 mRNA expression was most strongly inhibited by the PKA inhibitor, H89 (10-30 micromol/L). The phorbol ester, PMA (1 micromol/L), which activates the PKC pathway, and ionomycin (1 micromol/L), which activates the calcium pathway, produced small but detectable elevations in RGS-2 mRNA levels. Overnight treatment with 1 micromol/L PMA to deplete PKC did not affect subsequent RGS-2 induction by PTH, but significantly inhibited PMA-induced RGS-2 expression. Treatment with 1-100 nmol/L PTH(3-34), which does not activate cAMP-PKA signaling, did not induce RGS-2 expression. MOB cells pretreated with 3 microg/mL cycloheximide produced sustained RGS-2 mRNA levels 2 h after 10 nmol/L PTH treatment. Actinomycin D (5 microg/mL) completely blocked 10 nmol/L PTH-induced RGS-2 expression. Finally, we tested the effect of RGS-2 overexpression on PTH- and fluprostenol-induced interleukin (IL)-6 promoter activity in MOB cells. PTH induces IL-6 through PKA activation, whereas fluprostenol induces IL-6 through PKC activation. We found that RGS-2 overexpression significantly inhibited IL-6 promoter activity following fluprostenol treatment, but not following PTH treatment. We conclude that RGS-2 is a PTH-induced primary response gene in murine osteoblasts that is induced mainly through the cAMP-PKA pathway and specifically inhibits Galphaq-coupled receptors.

Involvement of PKC-beta in PTH, TNF-alpha, and IL-1 beta effects on IL-6 promoter in osteoblastic cells and on PTH-stimulated bone resorption

Protein kinase C (PKC) has been shown to be activated by parathyroid hormone (PTH) in osteoblasts. Prior evidence suggests that this activation mediates responses leading to bone resorption, including production of the osteoclastogenic cytokine interleukin-6 (IL-6). However, the importance of specific PKC isozymes in this process has not been investigated. A selective antagonist of PKC-beta, LY379196, was used to determine the role of the PKC-beta isozyme in the expression of IL-6 in UMR-106 rat osteoblastic cells and in bone resorption in fetal rat limb bone organ cultures. PTH, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1 beta (IL-1 beta) induced translocation of PKC-alpha and -beta(I) to the plasma membrane in UMR-106 cells within 5 min. The stimulation of PKC-beta(I) translocation by PTH, TNF-alpha or IL-1 beta was inhibited by LY379196. In contrast, LY379196 did not affect PTH, TNF-alpha-, or IL-1 beta-stimulated translocation of PKC-alpha. PTH, TNF-alpha, and IL-1 beta increased luciferase expression in UMR-106 cells transiently transfected with a -224/+11 bp IL-6 promoter-driven reporter construct. The IL-6 responses were also attenuated by treatment with LY379196. Furthermore, LY379196 inhibited bone resorption elicited by PTH in fetal rat bone organ cultures. These results indicate that PKC-beta(I) is a component of the signaling pathway that mediates PTH-, TNF-alpha-, and IL-1 beta-stimulated IL-6 expression and PTH-stimulated bone resorption.

Stimulation of interleukin-6 promoter by parathyroid hormone, tumor necrosis factor alpha, and interleukin-1beta in UMR-106 osteoblastic cells is inhibited by protein kinase C antagonists

To investigate the level at which protein kinase C (PKC) regulates expression of interleukin-6 (IL-6) in osteoblastic cells, effects of several PKC antagonists and PKC down-regulation by phorbol ester were studied in UMR-106 osteoblastic cells that had been transiently transfected with a -224/+11-base pair (bp) IL-6 promoter coupled to a luciferase reporter. Parathyroid hormone (PTH) elicited a dose-dependent stimulation of the IL-6 promoter expression, with significant increases produced by 5 h of treatment with concentrations of PTH as low as 10(-14) M. The increase in IL-6 promoter expression was inhibited by the PKC antagonists GF109203X, 30 nM to 1 microM, and calphostin C, 250 nM. Prior down-regulation of PKC with 100 nM phorbol-12,13-dibutyrate (PDBU) for 48 h inhibited the PTH effect as well as the smaller stimulatory effects elicited by tumor necrosis factor alpha (TNF-alpha), 10(-9)-10(-8) M, and by IL-1beta, 1-10 ng/ml. In contrast to these findings, the stimulatory effects of PTH, TNF-alpha, and IL-1beta on the IL-6 promoter expression were enhanced by staurosporine. Treatment with GF109203X or down-regulation of PKC with PDBU prevented the stimulatory effects of staurosporine. PKC activity was increased by staurosporine. The findings with staurosporine are consistent with our earlier observations that this agent enhances the calcium signaling and bone resorption elicited by PTH. The studies support the role of PKC in the stimulatory effects of PTH, TNF-alpha, and IL-1beta on IL-6 expression.

ADAMTS-1: A cellular disintegrin and metalloprotease with thrombospondin motifs is a target for parathyroid hormone in bone

PTH stimulates bone formation in animals and humans, and the expressions of a number of genes have been implicated in the mediation of this effect. To discover new bone factors that initiate and support this phenomenon we used differential display RT-PCR and screened for genes that are selectively expressed in osteoblast-enriched femoral metaphyseal primary spongiosa of young male rats after a single s.c. injection of human PTH-(1-38) (8 microg/100 g). We show that one of the messenger RNAs that is up-regulated in bone is ADAMTS-1, a new member of the ADAM (A disintegrin and metalloprotease) gene family containing thrombospondin type I motifs. ADAMTS-1 consists of multiple domains common to ADAM family of proteins, including pro-, metalloprotease-like, and disintegrin-like domains. However, unlike other ADAMs, ADAMTS-1 does not possess a transmembrane or cytoplasmic domain and is a secreted protein. Northern blot analysis confirmed that ADAMTS-1 was up-regulated in both metaphyseal (14- to 35-fold) and diaphyseal (4.2-fold) bone 1 h after PTH-(1-38) injection and returned to control levels by 24 h. We also analyzed the regulation of ADAMTS-1 in response to various PTH/PTH-related peptide (PTHrP) analogs and found that PTH-(1-31) and PTHrP-(1-34), which activate the protein kinase A (PKA) pathway, induce ADAMTS-1 expression 1 h after injection, whereas PTH-(3-34) and PTH-(7-34), which do not activate the PKA pathway, did not regulate expression. To investigate the effect of other osteotropic agents, we analyzed ADAMTS-1 expression after a single dose of PGE2 (6 mg/kg) and found that it was up-regulated 1 h after injection and returned to control levels by 6 h. In vitro ADAMTS-1 is expressed in primary osteoblasts and osteoblastic cell lines, but was not detectable in osteoclasts generated from macrophage colony-stimulating factor/receptor activator of NF-kappaB ligand/transforming growth factor-beta1-treated bone marrow cells. Treatment of UMR 106 osteosarcoma cells with PTH, PGE2, forskolin, or (Bu)2cAMP increased ADAMTS-1 expression 7-, 4-, 5-, and 5-fold, respectively. Also, in vitro treatment with 1alpha,25-dihydroxyvitamin D3 increased ADAMTS-1 expression 3-fold. Tissue distribution analysis showed that ADAMTS-1 is expressed at high levels in many tissues, including the heart, lung, liver, skeletal muscle, and kidney. Taken together, these results demonstrate that ADAMTS-1 is specifically up-regulated in bone and osteoblasts by the osteotropic agents PTH, PTHrP, and PGE2 possibly via the cAMP/PKA pathway. We speculate that the rapid and transient increase in ADAMTS-1 expression may contribute to some of the effects of PTH on bone turnover.

Estrogen modulates parathyroid hormone-induced interleukin-6 production in vivo and in vitro

Interleukin (IL)-6 promotes osteoclastogenesis and is thought to play a role in the bone loss that follows estrogen withdrawal. In vitro studies have demonstrated that IL-6 is produced in response to PTH by cells in the osteoblast lineage and that PTH-induced bone resorption is inhibited by a neutralizing antibody to the IL-6 receptor. In addition, we have recently reported that IL-6 plays a role in PTH-induced bone resorption in humans with chronic PTH excess and in experimental animals during the short-term infusion of PTH. In the current study, we examined whether estrogen withdrawal augments PTH-induced IL-6 production. When cultured in the absence of estrogen, human osteosarcoma cells (Saos-2) treated with PTH demonstrated significantly greater release of IL-6 than cells grown under estrogen-replete conditions, 30-fold vs. 15-fold (P = 0.005). A similar effect but of lesser magnitude was seen with primary human osteoblasts. In vivo, PTH induced IL-6 production was also increased in the estrogen-deficient state (ovx) such that at the end of a 5-day PTH infusion, the mean circulating level of IL-6 was significantly higher in ovx vs. sham/ovx mice (60.1 vs. 16.9 pg/ml; P < 0.0001). The greater increase in circulating levels of IL-6 in PTH-treated ovx mice was paralleled by a greater rise in bone resorption markers with the mean level of urine collagen cross-links in the PTH-treated ovx group being more than 2.5-fold higher than in the PTH-treated sham/ovx animals (236 vs. 88.5 microg/mmol creatinine, P < 0.0001). Mean serum collagen cross-link values were 17.4 microg/liter in PTH-treated ovx vs. 7.4 microg/liter in PTH-treated sham/ovx animals (P < 0.0001). Treatment of animals with estrogen prevented the exaggerated response to PTH infusion such that the increase in both circulating levels of IL-6 and bone turnover markers in estrogen-treated animals were similar to those observed in sham/ovx animals and significantly lower than those in PTH-treated ovx animals. These findings may help to explain the increased skeletal sensitivity to the resorbing effects of PTH seen in the estrogen-deficient state.

Protein kinase C involvement in interleukin-6 production by parathyroid hormone and tumor necrosis factor-alpha in UMR-106 osteoblastic cells

The cytokine interleukin-6 (IL-6) is increased in bone and bone cells by several resorptive stimuli, including parathyroid hormone (PTH), IL-1beta, and tumor necrosis factor-alpha (TNF-alpha). The current studies were designed to determine the contribution of the protein kinase C (PKC) signaling pathway to the effects of these three agents to increase IL-6 in UMR-106 rat osteoblastic cells. Cells were pretreated with vehicle (dimethylsulfoxide [DMSO]) or the phorbol ester, phorbol 12,13-dibutyrate (PDB; 300 nM) for 48 h to down-regulate phorbol-sensitive PKC isozymes. Either PTH (0.1-10 nM), IL-1beta (0.1-10 nM), or TNF-alpha (5 nM and 10 nM) was then added for 24 h in the continued presence of vehicle or PDB. PKC isozymes were visualized by Western immunoblotting and IL-6 was determined by bioassay. PDB pretreatment caused a partial down-regulation of the conventional alpha-PKC and betaI-PKC isozymes and complete down-regulation of the novel delta-isoenzyme and epsilon-isozymes but it had no effect on the atypical zeta-PKC isozyme. PDB pretreatment reduced IL-6 responses to 5 nM and 10 nM PTH by 61% and 33%, respectively, reduced IL-6 responses to 5nM and 10 nM TNF-a by 54% and 42%, respectively, and failed to inhibit the IL-6 responses to 0.1-10 nM IL-1beta. The PDB pretreatment protocol significantly enhanced PTH-stimulated cyclic adenosine monophosphate (cAMP) production. The PKC inhibitor calphostin C also decreased IL-6 responses to PTH. Thus, in this osteoblast cell line, the PKC pathway is an important component of the signaling pathway for the IL-6 production stimulated by PTH and TNF-alpha but not that from IL-1beta.

Differential stimulation by PGE(2) and calcemic hormones of IL-6 in stromal/osteoblastic cells

Formation of osteoclast-like cells in mouse bone marrow cultures induced by either 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)), parathyroid hormone (PTH) or prostaglandin E(2) (PGE(2)), respectively, shows partial dependence on interleukin-6 receptor (IL-6R) activation. This suggests that locally produced IL-6 could be relevant for osteoclast formation. Therefore, we evaluated the effects of 1,25-(OH)(2)D(3), PTH, and PGE(2) on IL-6 production in stromal/osteoblastic cell lines. It appeared that these bone resorptive factors differed widely in their ability to modulate IL-6 mRNA expression and, consequently, protein synthesis in each of the cell lines studied. While 1,25-(OH)(2)D(3) was marginally effective only in ST2 cells, and PTH caused a 2- to 20-fold increase in IL-6 levels MC3T3-E1 and UMR-106 cells, PGE(2) enhanced IL-6 production in the ST2 and MC3T3-E1 cell line by two to three orders of magnitude, respectively, and also induced IL-6 in fibroblastic L929 cells. PGE(2)-stimulated IL-6 release from mesenchymal cells seems to be important for autocrine/paracrine control of osteoclast formation in health and disease.

Parathyroid hormone regulation of the rat collagenase-3 promoter by protein kinase A-dependent transactivation of core binding factor alpha1

Previously we showed that the activator protein-1 site and the runt domain binding site in the collagenase-3 promoter act cooperatively in response to parathyroid hormone (PTH) in the rat osteoblastic osteosarcoma cell line, UMR 106-01. Our results of the expression pattern of core binding factor alpha1 (Cbfa1), which binds to the runt domain site, indicated that there is no change in the levels of Cbfa1 protein or RNA under either control conditions or after PTH treatment. The importance of posttranslational modification of Cbfa1 in the signaling pathway for PTH-induced collagenase-3 promoter activity was analyzed. PTH stimulation of collagenase-3 promoter activity was completely abrogated by protein kinase A (PKA) inhibition. To determine the role of PKA activity with respect to Cbfa1 activation (in addition to its known activity of phosphorylating cAMP-response element-binding protein to enhance c-fos promoter activity), we utilized the heterologous Gal4 transcription system. PTH stimulated the transactivation of activation domain-3 in Cbfa1 through the PKA site. In vitro phosphorylation studies indicated that the PKA site in the wild type activation domain-3 is a substrate for phosphorylation by PKA. Thus, we demonstrate that PTH induces a PKA-dependent transactivation of Cbfa1, and this transactivation is required for collagenase-3 promoter activity in UMR cells.

Parathyroid hormone inhibits collagen synthesis and the activity of rat col1a1 transgenes mainly by a cAMP-mediated pathway in mouse calvariae

We examined the effect of parathyroid hormone and various signaling molecules on collagen synthesis and chloramphenicol acetyltransferase activity in cultured transgenic mouse calvariae carrying fusion genes of the rat Col1a1 promoter and the chloramphenicol acetyltransferase reporter. After 48 h of culture, parathyroid hormone, forskolin, dibutyryl cAMP, 8-bromo cAMP, and phorbol myristate acetate inhibited transgene activity, while the calcium ionophore ionomycin had no effect. Pretreatment of calvariae with the phosphodiesterase inhibitor isobutylmethylxanthine potentiated the inhibitory effect of 1 nM parathyroid hormone on transgene activity and collagen synthesis. Parathyroid hormone further inhibited transgene activity and collagen synthesis in the presence of phorbol myristate acetate. Parathyroid hormone inhibition of transgene activity and collagen synthesis was not affected by indomethacin or interleukin-6. After 48 h of culture, parathyroid hormone inhibited chloramphenicol acetyltransferase activity by 50-85% in cultured calvariae carrying transgenes having progressive 5' upstream deletions of promoter DNA down to -1683 bp. These data show that the inhibitory effect of parathyroid hormone on Col1a1 expression in mouse calvariae is mediated mainly by the cAMP signaling pathway. Prostaglandins and IL-6 are not local mediators of the parathyroid hormone response in this model. Finally, regions of the Col1a1 promoter downstream of -1683 bp are sufficient for parathyroid hormone inhibition of the Col1a1 promoter.