Parathyroid hormone regulates transforming growth factor beta1 and beta2 synthesis in osteoblasts via divergent signaling pathways

Tmem119 is involved in bone anabolic effects of PTH through enhanced osteoblastic bone formation in mice

The intermittent administration of parathyroid hormone (PTH) exerts potent bone anabolic effects, which increase bone mineral density (BMD) and reduce fracture risk in osteoporotic patients. However, the underlying mechanisms remain unclear. Tmem119 has been proposed as a factor that is closely linked to the osteoblast phenotype, and we previously reported that PTH enhanced the expression of Tmem119 in mouse osteoblastic cells. However, roles of Tmem119 in the bone anabolic effects of PTH in vivo remain unknown. We herein investigated the roles of Tmem119 in bone anabolic effects of PTH using Tmem119-deficient mice. Tmem119 deficiency significantly reduced PTH-induced increases in trabecular bone volume and cortical BMD of femurs. Effects of Tmem119 deficiency on bone mass seemed predominant in female mice. Histomorphometric analyses with calcein labeling showed that Tmem119 deficiency significantly attenuated PTH-induced increases in the rates of bone formation and mineralization as well as numbers of osteoblasts. Moreover, Tmem119 deficiency significantly blunted PTH-induced decreases in phosphorylation of β-catenin and increases in alkaline phosphatase activity in osteoblasts. In conclusion, the present results indicate that Tmem119 is involved in bone anabolic effects of PTH through osteoblastic bone formation partly related to canonical Wnt-β-catenin signaling in mice.

Accelerated Bone Loss in Transgenic Mice Expressing Constitutively Active TGF-β Receptor Type I

Transforming growth factor beta (TGF-β) is a key factor mediating the intercellular crosstalk between the hematopoietic stem cells and their microenvironment. Here, we investigated the skeletal phenotype of transgenic mice expressing constitutively active TGF-β receptor type I under the control of Mx1-Cre (Mx1;TβRI^CA mice). μCT analysis showed decreased cortical thickness, and cancellous bone volume in both femurs and mandibles. Histomorphometric analysis confirmed a decrease in cancellous bone volume due to increased osteoclast number and decreased osteoblast number. Primary osteoblasts showed decreased ALP and mineralization. Constitutive TβRI activation increased osteoclast differentiation. qPCR analysis showed that Tnfsf11/Tnfrsf11b ratio, Ctsk, Sufu, and Csf1 were increased whereas Runx2, Ptch1, and Ptch2 were decreased in Mx1;TβRI^CA femurs. Interestingly, Gli1, Wnt3a, Sp7, Alpl, Ptch1, Ptch2, and Shh mRNA expression were reduced whereas Tnfsf11/Tnfrsf11b ratio was increased in Mx1;TβRI^CA mandibles. Similarly, osteoclast-related genes were increased in Mx1;TβRI^CA osteoclasts whereas osteoblast-related genes were reduced in Mx1;TβRI^CA osteoblasts. Western blot analysis indicated that SMAD2 and SMAD3 phosphorylation was increased in Mx1;TβRI^CA osteoblasts, and SMAD3 phosphorylation was increased in Mx1;TβRI^CA osteoclasts. CTSK was increased while RUNX2 and PTCH1 was decreased in Mx1;TβRI^CA mice. Microindentation analysis indicated decreased hardness in Mx1;TβRI^CA mice. Our study indicated that Mx1;TβRI^CA mice were osteopenic by increasing osteoclast number and decreasing osteoblast number, possibly by suppressing Hedgehog signaling pathways.

Osteoblastic monocyte chemoattractant protein-1 (MCP-1) mediation of parathyroid hormone's anabolic actions in bone implicates TGF-β signaling

Parathyroid hormone (PTH) is necessary for the regulation of calcium homeostasis and PTH (1-34) was the first approved osteoanabolic therapy for osteoporosis. It is well established that intermittent PTH increases bone formation and that bone remodeling and several cytokines and chemokines play an essential role in this process. Earlier, we had established that the chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2), was the most highly stimulated gene in rat bone after intermittent PTH injections. Nevertheless, MCP-1 function in bone appears to be complicated. To identify the primary cells expressing MCP-1 in response to PTH, we performed in situ hybridization of rat bone sections after hPTH (1-34) injections and showed that bone-lining osteoblasts are the primary cells that express MCP-1 after PTH treatment. We previously demonstrated MCP-1's importance by showing that PTH's anabolic effects are abolished in MCP-1 null mice, further implicating a role for the chemokine in this process. To establish whether rhMCP-1 peptide treatment could rescue the anabolic effect of PTH in MCP-1 null mice, we treated 4-month-old wild-type (WT) mice with hPTH (1-34) and MCP-1^-/- mice with rhMCP-1 and/or hPTH (1-34) for 6 weeks. Micro-computed tomography (μCT) analysis of trabecular and cortical bone showed that MCP-1 injections for 6 weeks rescued the PTH anabolic effect in MCP-1^-/- mice. In fact, the combination of rhMCP-1 and hPTH (1-34) has a synergistic anabolic effect compared with monotherapies. Mechanistically, PTH-enhanced transforming growth factor-β (TGF-β) signaling is abolished in the absence of MCP-1, while MCP-1 peptide treatment restores TGF-β signaling in the bone marrow. Here, we have shown that PTH regulates the transcription of the chemokine MCP-1 in osteoblasts and determined how MCP-1 affects bone cell function in PTH's anabolic actions. Taken together, our current work indicates that intermittent PTH stimulates osteoblastic secretion of MCP-1, which leads to increased TGF-β signaling, implicating it in PTH's anabolic actions.

Parathyroid hormone-dependent bone formation requires butyrate production by intestinal microbiota

Parathyroid hormone (PTH) is a critical regulator of skeletal development that promotes both bone formation and bone resorption. Using microbiota depletion by wide-spectrum antibiotics and germ-free (GF) female mice, we showed that the microbiota was required for PTH to stimulate bone formation and increase bone mass. Microbiota depletion lowered butyrate levels, a metabolite responsible for gut-bone communication, while reestablishment of physiologic levels of butyrate restored PTH-induced anabolism. The permissive activity of butyrate was mediated by GPR43 signaling in dendritic cells and by GPR43-independent signaling in T cells. Butyrate was required for PTH to increase the number of bone marrow (BM) regulatory T cells (Tregs). Tregs stimulated production of the osteogenic Wnt ligand Wnt10b by BM CD8+ T cells, which activated Wnt-dependent bone formation. Together, these data highlight the role that butyrate produced by gut luminal microbiota plays in triggering regulatory pathways, which are critical for the anabolic action of PTH in bone.

Regulatory T cells are expanded by Teriparatide treatment in humans and mediate intermittent PTH-induced bone anabolism in mice

Teriparatide is a bone anabolic treatment for osteoporosis, modeled in animals by intermittent PTH (iPTH) administration, but the cellular and molecular mechanisms of action of iPTH are largely unknown. Here, we show that Teriparatide and iPTH cause a ~two-threefold increase in the number of regulatory T cells (Tregs) in humans and mice. Attesting in vivo relevance, blockade of the Treg increase in mice prevents the increase in bone formation and trabecular bone volume and structure induced by iPTH Therefore, increasing the number of Tregs is a pivotal mechanism by which iPTH exerts its bone anabolic activity. Increasing Tregs pharmacologically may represent a novel bone anabolic therapy, while iPTH-induced Treg increase may find applications in inflammatory conditions and transplant medicine.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Osteoblastogenesis regulation signals in bone remodeling

Bone remodeling is essential for adult bone homeostasis. The failure of this process often leads to the development of osteoporosis, a present major global health concern. The most important factor that affects normal bone remodeling is the tightly controlled and orchestrated regulation of osteoblasts and osteoclasts. The present review summarized the recent discoveries related to osteoblast regulation from several signals, including transforming growth factor-β, bone morphogenetic proteins, Wnt signal, Notch, Eph-Ephrin interaction, parathyroid hormone/parathyroid hormone-related peptide, and the leptin-serotonin-sympathetic nervous systemic pathway. The awareness of these mechanisms will facilitate further research that explores bone remodeling and osteoporosis. Future investigations on the endogenous regulation of osteoblastogenesis will increase the current knowledge required for the development of potential drug targets in the treatment of osteoporosis.

Changes in the contents of enzymatic immature, mature, and non-enzymatic senescent cross-links of collagen after once-weekly treatment with human parathyroid hormone (1-34) for 18 months contribute to improvement of bone strength in ovariectomized monkeys

Improvements in total content of enzymatic cross-linking, the ratio of hydroxylysine-derived enzymatic cross-links, and non-enzymatic advanced glycation end product cross-link formation from once-weekly administration of hPTH(1-34) for 18 months in OVX cynomolgus monkeys contributed to the improvement of bone strength.

INTRODUCTION

Parathyroid hormone (PTH) is used for the treatment of osteoporosis. To elucidate the contribution of material properties to bone strength after once-weekly treatment with hPTH(1-34) in an ovariectomized (OVX) primate model, the content of collagen and enzymatic immature, mature, and non-enzymatic cross-links, collagen maturity, trabecular architecture, and mineralization in vertebrae were simultaneously estimated.

METHODS

Adult female cynomolgus monkeys were divided into four groups (n = 18-20 each) as follows: SHAM group, OVX group, and OVX monkeys given once-weekly subcutaneous injections of hPTH(1-34) either at 1.2 or 6.0 μg/kg (low- or high-PTH groups) for 18 months. The content of collagen, enzymatic and non-enzymatic cross-linking pentosidine, collagen maturity, trabecular architecture, mineralization, and cancellous bone strength of vertebrae were analyzed.

RESULTS

Low-PTH and high-hPTH treatments increased the content of enzymatic immature and mature cross-links, bone volume (BV/TV), and trabecular thickness, and decreased pentosidine, compared with the OVX group. Stepwise logistic regression analysis revealed that BV/TV, the content of total enzymatic cross-links, and calcium content independently affected ultimate load (model R (2) = 0.748, p < 0.001) and breaking energy (model R (2) = 0.702, p < 0.001). BV/TV was the most powerful and enzymatic cross-link content was the second powerful determinant of both ultimate load and breaking energy. The most powerful determinant of stiffness was the enzymatic cross-link content (model R (2) = 0.270, p < 0.001).

CONCLUSION

Once-weekly preventive administration of hPTH(1-34) increased the total contents of immature and mature enzymatic cross-links, which contributed significantly to vertebral cancellous bone strength.

Antagonist minigenes identify genes regulated by parathyroid hormone through G protein-selective and G protein co-regulated mechanisms in osteoblastic cells

Parathyroid hormone (PTH) is the major hormone regulating bone remodeling. Binding of PTH to the PTH1 receptor (PTH1R), a heterotrimeric G protein coupled receptor (GPCR), can potentially trigger multiple signal transduction pathways mediated through several different G proteins. In this study, we employed G protein antagonist minigenes inhibiting Gα(s), Gα(q) or Gα₁₂ to selectively dissect out which of these G proteins were responsible for effects of PTH(1-34) in targeted signaling and osteogenesis arrays consisting of 159 genes. Among the 32 genes significantly regulated by 24h PTH treatment in UMR-106 osteoblastic cells, 9 genes were exclusively regulated through G(s), 6 genes were solely mediated through G(q), and 3 genes were only controlled through G₁₂. Such findings support the concept that there is some absolute specificity in downstream responses initiated at the G protein level following binding of PTH to the PTH1R. On the other hand, 6 PTH-regulated genes were regulated by both G(s) and G(q), 3 genes were regulated by both G(s) and G₁₂, and 3 genes were controlled by G(s), G(q) and G₁₂. These findings indicate potential overlapping or sequential interactions among different G protein-mediated pathways. In addition, two PTH-regulated genes were not regulated through any of the G proteins examined, suggesting that additional signaling mechanisms may be involved. Selectivity was largely maintained over a 2-48-hour time period. The minigene effects were mimicked by downstream inhibitors. The dissection of the differential effects of multiple G protein pathways on gene regulation provides a more complete understanding of PTH signaling in osteoblastic cells.

Parathyroid hormone-related protein promotes epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is an important process that contributes to renal fibrogenesis. TGF-beta1 and EGF stimulate EMT. Recent studies suggested that parathyroid hormone-related protein (PTHrP) promotes fibrogenesis in the damaged kidney, apparently dependent on its interaction with vascular endothelial growth factor (VEGF), but whether it also interacts with TGF-beta and EGF to modulate EMT is unknown. Here, PTHrP(1-36) increased TGF-beta1 in cultured tubuloepithelial cells and TGF-beta blockade inhibited PTHrP-induced EMT-related changes, including upregulation of alpha-smooth muscle actin and integrin-linked kinase, nuclear translocation of Snail, and downregulation of E-cadherin and zonula occludens-1. PTHrP(1-36) also induced EGF receptor (EGFR) activation; inhibition of protein kinase C and metalloproteases abrogated this activation. Inhibition of EGFR activation abolished these EMT-related changes, the activation of ERK1/2, and upregulation of TGF-beta1 and VEGF by PTHrP(1-36). Moreover, inhibition of ERK1/2 blocked EMT induced by either PTHrP(1-36), TGF-beta1, EGF, or VEGF. In vivo, obstruction of mouse kidneys led to changes consistent with EMT and upregulation of TGF-beta1 mRNA, p-EGFR protein, and PTHrP. Taken together, these data suggest that PTHrP, TGF-beta, EGF, and VEGF might cooperate through activation of ERK1/2 to induce EMT in renal tubuloepithelial cells.

Is Wnt signalling the final common pathway leading to bone formation?

Since the discovery of the link between mutations in the LRP5 gene and human bone mass, considerable progress has been made in our understanding of Wnt signalling and bone formation. The connection between canonical Wnt signalling and bone formation is convincing, and there is evidence of interaction between the Wnt signalling pathway and key growth factors, transcriptional factors and systemic hormones. More recently, the role of the non-canonical pathway in bone metabolism has also started to be explored as well as potential bone-gut interactions. This review focuses on the role of the Wnt pathway in osteoblast differentiation as well as the interplay between Wnt signalling and other pathways involved in bone formation.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Parathyroid hormone synergizes with non-cyclic AMP pathways to activate the cyclic AMP response element

Parathyroid hormone (PTH) activates multiple signaling pathways following binding to the PTH1 receptor in osteoblasts. Previous work revealed a discrepancy between cAMP stimulation and CRE reporter activation of truncated PTH peptides, suggesting that additional signaling pathways contribute to activation of the CRE. Using a CRE-Luciferase reporter containing multiple copies of the CRE stably transfected into the osteoblastic cell line Saos-2, we tested the ability of modulators of alternative pathways to activate the CRE or block the PTH-induced activation of the CRE. Activators of non-cyclic AMP pathways, that is, EGF (Akt, MAPK, JAK/STAT pathways); thapsigargin (intracellular calcium pathway); phorbol myristate acetate (protein kinase C, PKC pathway) induced minor increases in CRE-luciferase activity alone but induced dramatic synergistic effects in combination with PTH. The protein kinase A (PKA) inhibitor H-89 (10 microM) almost completely blocked PTH-induced activation of the CRE-reporter. Adenylate cyclase inhibitors SQ 22536 and DDA had profound and time-dependent biphasic effects on the CRE response. The MAPK inhibitor PD 98059 partially inhibited basal and PTH-induced CRE activity to the same degree, while the PKC inhibitor bisindolylmaleimide (BIS) had variable effects. The calmodulin kinase II inhibitor KN-93 had no significant effect on the response to PTH. We conclude that non-cAMP pathways (EGF pathway, calcium pathway, PKC pathway) converge on, and have synergistic effects on, the response of a CRE reporter to PTH.

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.

Parathyroid hormone activates phosphoinositide 3-kinase-Akt-Bad cascade in osteoblast-like cells

To understand the molecular basis underlying the anabolic action of parathyroid hormone (PTH) on bone, the anti-apoptotic action of PTH on osteoblast-like cells was investigated. Since Akt is a key protein kinase for cell survival, we focused on a possible involvement of Akt in the anti-apoptotic action of PTH. Human osteoblast-like MG-63 cells cultured without serum were treated with PTH. Western blot analysis revealed that PTH rapidly phosphorylated Akt and induced its nuclear translocation. The phosphorylation of pro-apoptotic protein Bad was also increased by PTH, leading to its inactivation. The PTH-dependent activation of Akt was also detected in other osteoblastic cell lines, SaOS-2 and ROS 17/2.8. The pretreatment of MG-63 cells with either one of inhibitors for phosphoinositide 3-kinase (PI3K), wortmannin or LY294002 prevented Akt and Bad phosphorylation. Furthermore, co-immunoprecipitation analysis revealed that PTH receptor (PTH-1R) directly interacted with p85, a regulatory subunit of PI3K, in a PTH-dependent manner. Serum withdrawal induced the apoptosis of MG-63 cells, and PTH prevented the apoptosis, which was inhibited by PI3K inhibitors. These results demonstrate the presence of a novel PTH/PTH receptor signaling cascade consisting of PTH-1R, PI3K, Akt and Bad and that this cascade can work as an anti-apoptotic signaling pathway in osteoblast-like cells.

Up-regulation of endogenous RGS2 mediates cross-desensitization between Gs and Gq signaling in osteoblasts

Regulator of G protein signaling (RGS) proteins limit G protein signals. In this study, we investigated the role of RGS2 in the control of G protein signaling cascades in osteoblasts, the cells responsible for bone formation. Expression of RGS2 was up-regulated in primary cultures of mouse calvarial osteoblasts by parathyroid hormone-related peptide (PTHrP)-(1-34), which stimulates G(s) signaling. RGS2 was also up-regulated by extracellular ATP, which selectively activates G(q), as well as by forskolin and phorbol myristate acetate, which activate targets downstream of G(s) and G(q), respectively. To assess the role of endogenous RGS2, we characterized G(s) and G(q) signaling in osteoblasts derived from wild type and rgs2(-/-) mice. Under control conditions, nucleotide-stimulated calcium release, endothelin-stimulated accumulation of inositol phosphates, and PTHrP-stimulated cAMP accumulation were equivalent in osteoblasts isolated from wild type and rgs2(-/-) mice. Thus, basal levels of endogenous RGS2 do not appear to regulate G(s) or G(q) signaling in osteoblasts. Interestingly, forskolin treatment of wild type but not rgs2(-/-) osteoblasts suppressed both endothelin-stimulated accumulation of inositol phosphates and nucleotide-stimulated calcium release, indicating that up-regulation of RGS2 by G(s) signaling desensitizes G(q) signals. Furthermore, pretreatment with ATP suppressed PTHrP-dependent cAMP accumulation in wild type but not rgs2(-/-) osteoblasts, implying that up-regulation of RGS2 by G(q) signaling desensitizes G(s) signals. Our findings demonstrate that endogenously expressed RGS2 can limit G(s) signaling. Moreover, up-regulation of RGS2 contributes to cross-desensitization of G(s)- and G(q)-coupled signals.

Parathyroid hormone activates PKC-delta and regulates osteoblastic differentiation via a PLC-independent pathway

PTH exerts major effects upon bone by activating PTH/PTHrP receptors (PTH1Rs) expressed on osteoblasts. The PTH1R is capable of engaging multiple signaling pathways in parallel, including Gs/adenylyl cyclase (AC), Gq/phospholipase C/protein kinase C (PLC/PKC) and a distinct mechanism, involving activation of PKC via a PLC-independent pathway, that depends upon ligand determinants within the PTH(29-34) sequence. The involvement of PLC-dependent vs. PLC-independent PKC activation in PTH action was studied in clonal PTH1R-expressing murine calvarial osteoblasts ("Wt9") using two signal-selective analogs, [G1,R19]hPTH(1-28) and [G1,R19]hPTH(1-34). Both analogs lack PLC signaling but differ in their capacity to activate the PLC-independent PKC pathway. Both hPTH(1-34) and [G1,R19]hPTH(1-34), but not [G1,R19]hPTH(1-28), increased differentiation of Wt9 cells during a 16-day alternate-daily treatment protocol. Wt9 cells expressed PKC-betaI, -delta, -epsilon and -zeta, none of which exhibited net translocation to membranes in response to hPTH(1-34) or either analog. hPTH(1-34) induced activation of membrane-associated PKC-delta, however, and a time- and concentration-dependent increase in cytosolic [phospho-Thr505]PKC-delta which was maximal within 40 s at 100 nM in both Wt9 cells and primary osteoblasts. This response was mimicked by [G1,R19]hPTH(1-34) but not by [G1,R19]hPTH(1-28). Increased expression of bone sialoprotein (BSP) and osteocalcin (OC) mRNAs induced by PTH(1-34) and [G1,R19]hPTH(1-34) in Wt9 cells was blocked by rottlerin, a PKC-delta inhibitor. We conclude that PTH1Rs activate PKC-delta by a PLC-independent, PTH(29-34)-dependent mechanism that promotes osteoblastic differentiation.

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.

Influence of brain injury on early posttraumatic bone metabolism

BACKGROUND

Various clinical studies and observations demonstrate enhanced osteogenesis in patients sustaining traumatic brain injury. It is presumed that the induction of this process starts early after trauma. The purpose of our study was to investigate humoral markers of bone metabolism during the early posttraumatic period, with special regard to traumatic brain injury.

METHODS

Serum concentrations of biochemical markers of bone metabolism (calcium, inorganic phosphorus, carboxyl-terminal propeptide of type 1 procollagen, pyridinoline cross-linked telopeptide domain of type 1 collagen, Ostase, osteocalcin, intact parathyroid hormone, and calcitonin) were measured in three different groups of 80 patients during the first posttraumatic week. Patients were categorized into three groups: group I, fractures only; group II, isolated traumatic brain injury; and group III, traumatic brain injury in combination with fractures.

RESULTS

Osteocalcin levels were significantly lower in the presence of traumatic brain injury (p < .05). Elevated pyridinoline cross-linked telopeptide domain of type 1 collagen levels expressed enhanced bone resorption in all groups, but levels were significantly higher in the absence of traumatic brain injury (p < .05). Intact parathyroid hormone levels were significantly higher on days 0 and 1 in the combined presence of traumatic brain injury plus fractures.

CONCLUSION

These results demonstrate an imbalance of bone formation and resorption parameters in patients with traumatic brain injury during the early posttraumatic period, suggesting a central regulation in bone formation. The lower levels of osteocalcin detected in this study may play an important role in patients with brain injury and the later development of posttraumatic heterotopic ossification.

1,25-dihydroxyvitamin D3 stimulated protein kinase C phosphorylation of type VI adenylyl cyclase inhibits parathyroid hormone signal transduction in rat osteoblastic UMR 106-01 cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) treatment of osteoblastic cells was shown previously to attenuate Parathyroid hormone (PTH) response by inhibiting adenylyl cyclase (AC) activity. In this study, we have investigated the mechanism by which 1,25(OH)(2)D(3) inhibits AC in rat osteoblastic UMR 106-01 cells. 1,25(OH)(2)D(3) treatment inhibited both PTH and forskolin-stimulated AC activity by 25%-50% within 12 min in a concentration-dependent manner suggesting a direct inhibition of the AC enzyme. Treatment with 25(OH)D(3) had no effect on basal or stimulated AC activity. We determined the profile of AC subtypes expressed in UMR cells and found AC VI to be the dominant subtype accounting for 50% of AC mRNA. Since AC VI can be inhibited by protein kinase C (PKC) phosphorylation, we examined 1,25(OH)(2)D(3) activation of various PKC isoforms. 1,25(OH)(2)D(3) increased the membrane translocation of PKC-betaI, -delta, and -zeta with a concomitant increase in PKC activity. The translocation of PKC-betaI and -delta was blocked by the PLC inhibitor U73122 whereas that of PKC-zeta was abolished by the PI-3 kinase inhibitor wortmannin. The attenuation of cAMP production by 1,25(OH)(2)D(3) was antagonized by the PKC inhibitors Go6850, calphostin C, and wortmannin, but not by a calmodulin kinase II (CaMKII) inhibitor. Treatment with 1,25(OH)(2)D(3) for 20 min increased AC VI phosphorylation by 10.8-fold and this was blocked partially by Go6850 and partially by wortmannin but was unaffected by CaMKII inhibitor. These results demonstrate that 1,25(OH)(2)D(3) activation of PKC isoforms leads to phosphorylation of AC VI and inhibition of PTH-activation of this pathway in osteoblasts.

Increased expression of G11α in osteoblastic cells enhances parathyroid hormone activation of phospholipase C and AP-1 regulation of matrix metalloproteinase-13 mRNA

In osteoblasts parathyroid hormone (PTH) stimulates the PTH/PTH-related peptide (PTHrP) receptor (PTH1R) that couples via G(s) to adenylyl cyclase stimulation and via G(11) to phospholipase C (PLC) stimulation. We have investigated the effect of increasing G(11)alpha levels in UMR 106-01 osteoblastic cells by transient transfection with cDNA encoding G(11)alpha on PTH stimulation of PLC and protein kinase C (PKC) as well as PTH regulation of mRNA encoding matrix metalloproteinase-13 (MMP-13). Transfection with G(11)alpha cDNA resulted in a 5-fold increase in PTH-stimulated PLC activity with no change in PTH-stimulated adenylyl cyclase. PTH-induced translocation of PKC-betaI, -delta, and -zeta to the cell membrane and PKC-zeta to the nucleus was also increased. Increased G(11)alpha protein resulted in increased stimulation of MMP-13 mRNA levels at all doses of PTH. There was a 2.5 +/- 0.35 fold increase in maximal PTH-stimulation of c-jun mRNA and smaller but significant increases in c-fos accompanied by increased basal and PTH-stimulated AP-1 binding in cells expressing increased G(11)alpha. Runx-2 mRNA and protein levels were not significantly increased by increased G(11)alpha expression. The increase in PTH stimulation of c-jun, c-fos, and MMP-13 in G(11)alpha-transfected cells were all blocked by bisindolylmaleimide I, a selective inhibitor of PKC. These results demonstrate that regulation of the PLC pathway through the PTH1R is significantly increased by elevating expression of G(11)alpha in osteoblastic cells. This leads to increased PTH stimulation of MMP-13 expression by increased stimulation of AP-1 factors c-jun and c-fos.

Parathyroid hormone: a double-edged sword for bone metabolism

Parathyroid hormone (PTH) is the major hormone regulating calcium metabolism. It is also the only FDA-approved drug for osteoporosis treatment that stimulates bone formation when injected daily. However, continuous infusion of PTH causes severe bone loss in line with its known catabolic effects. Many studies to understand the dual effects of PTH have been carried out, and in recent years a growing number of molecular and cellular mechanisms underlying these effects have emerged. Here, we outline the present knowledge and conclude that the kinetics of administration and subsequent signaling probably account for the divergent actions of the hormone.

Regulation of gelatinases expression by cytokines, endotoxin, and pharmacological agents in the human osteoarthritic knee

We examined the amount of gelatinases (matrix metalloproteinase-2 and -9 [MMP-2 and MMP-9] in a series of chondral, meniscal, and synovial cultures of early osteoarthritis (OA) after treatment with or without catabolic cytokines. These included interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS), and pharmacological agents, including plasmin/serine proteinase antagonist aprotinin, protein synthesis inhibitor cycloheximide, and protein kinase C (PKC) inhibitors staurosporine, H7, and Gö6976 for investigation of their effects on MMP-2 and -9 production in OA. Gelatin zymography revealed that IL-alpha, TNF-alpha, and LPS could elevate MMP-2 secretion in all tissue cultures and also increase MMP-9 production in all synovial and some meniscal cultures. In contrast, aprotinin, cycloheximide, staurosporine, H7, and Gö6976 could suppress MMP-2 secretion in all tissue cultures and also decrease MMP-9 production in all synovial and some meniscal cultures. Our data indicate that catabolic cytokines and LPS may promote tissue destruction and disintegration of extracellular matrix in early OA. Agents that target on the PKC pathway, plasmin/serine proteinase or protein synthesis for MMP-2 and -9 in early OA may inhibit the production of MMPs. These findings might contribute to the design of more efficacious therapies.

Semiquantitative mRNA measurements of osteoinductive growth factors in human iliac-crest bone: expression of LMP splice variants in human bone

Although osteotropic growth factors are known to play an important role in bone metabolism, knowledge about their expression in relation to age, sex and smoking remains limited. In this study we report mRNA levels of the recently discovered Lim mineralization protein splice variants (LMP-1, LMP-2, LMP-3) and the established osteotropic growth factors BMP-2, BMP-6, BMP-7, TGF-beta, IGF-I, IGF-II and b-FGF in human iliac crest bone. Standardized bone biopsy specimens were obtained from the iliac crest during graft harvesting in 62 patients (38 males, 24 females, mean age 44.7 years, range 13-78 years) undergoing spinal surgery. Samples were immediately stored in liquid nitrogen for PCR analysis. Semi-quantitative RT-PCR was performed for TGF-beta, IGF-I, IGF-II, BMP2, BMP-6, BMP7, bFGF, LMP-1, LMP-2 and LMP-3 using beta-actin as internal standard. Triplicate measurements were made of each growth factor and beta-actin. mRNA for all examined growth factors was detected in 69% of the specimens. The lowest degree of detection was present for b-FGF and BMP-2, both of which were found in 85% of the specimens. LMP-1 was detected in 98% of the specimens. LMP-2 in 94% and LMP-3 in 27%, respectively. LMP-1 was generally expressed in higher amounts than LMP-2 and LMP-3. Nondetectable levels of the growth factors were more frequent in the >60-year-old males compared with >60-year-old females ( P < 0.05) and <60-year-old males ( P < 0.01). LMP-1 expression was more variable among young individuals, but mean values were similar between age groups. TGF-beta, BMP-2 and BMP-7 values did not differ between age groups, but generally a higher variation was found among older patients. IGF-I values were significantly higher ( P < 0.05) in males over 60 years, whereas the highest level of bFGF mRNA was present in males younger than 20 years ( P < 0.05). In addition, regression analysis revealed correlation between BMP-2 and BMP-7 (R2 = 0.74, P < 0.0005), LMP-2 and BMP-2 (R2 = 0.27, P < 0.0005) and LMP-2 and bFGF (R2 = 0.40, P < 0.0005). In conclusion, we have demonstrated expression of LMP-1 and LMP-2 in human bone. LMP-1 was expressed in higher amounts and showed a higher degree of variation among young individuals. LMP-2 was correlated to a number of other growth factors, suggesting that LMPs may also play a role in human bone metabolism. Higher variation in the expression of TGF-beta, BMP-2 and BMP-7 was found in the older age groups, but whether or not this can be correlated to age-related changes in bone turnover requires further studies.

Parathyroid hormone-Smad3 axis exerts anti-apoptotic action and augments anabolic action of transforming growth factor beta in osteoblasts

Although several studies indicated that parathyroid hormone (PTH) exerted anabolic action on bone, its precise mechanisms have been unknown. On the other hand, transforming growth factor beta (TGF-beta), abundantly stored in bone matrix, stimulates bone formation with a local injection in rodents. Although our previous study suggested that Smad3 is an important molecule for the stimulation of bone formation, no reports have been available about the effects of PTH on Smad3. In this present study, we examined the effects of PTH on Smad3 and the physiological significance in mouse osteoblastic cells. PTH promoted the expression of Smad3 mRNA within 10 min and the protein level in a dose-dependent manner in MC3T3-E1 and rat osteoblastic UMR-106 cells. Protein kinase A (PKA) activator as well as protein kinase C (PKC) activators increased Smad3 protein level, and both PKA and PKC inhibitors antagonized PTH-induced Smad3, indicating that PTH promotes the production of Smad3 through both PKA and PKC pathways. Next, we examined anti-apoptotic effects of PTH and Smad3 in these cells, employing trypan blue, transferase-mediated nick end labeling, and Hoechst staining. Pretreatment with PTH or overexpression of Smad3 decreased the number of apoptotic cells induced by dexamethasone and etoposide. Moreover, a dominant negative mutant, Smad3DeltaC, abrogated PTH-induced anti-apoptotic effects. On the other hand, PTH augmented TGF-beta-induced transcriptional activity. Furthermore, PTH enhanced TGF-beta-induced production of type I collagen, whereas it did not affect TGF-beta-reduced proliferation in MC3T3-E1 cells. These observations indicated that PTH amplified the anabolic effects of TGF-beta by accelerating the transcriptional activity of Smad3. In conclusion, we first demonstrated that PTH-Smad3 axis exerts anti-apoptotic effects in osteoblasts and reinforces the anabolic action by TGF-beta in osteoblasts. Hence, PTH-Smad3 axis might be involved in the bone anabolic action of PTH.

Glucose-dependent insulinotropic peptide stimulates proliferation and TGF-beta release from MG-63 cells

Glucose-dependent insulinotropic peptide (GIP) is known to modulate alkaline phosphatase activity and collagen type I message in osteoblastic-like cells. GIP effects on cell proliferation are not known. We report that GIP dose dependently stimulated 3H-thymidine incorporation in the osteoblastic-like cell line MG-63. Furthermore, GIP increased message and secretion of transforming growth factor beta (TGF-beta), an agent known to regulate osteoblastic proliferation and differentiation. However, when GIP was added to MG-63 cells concurrently with a TGF-beta neutralizing antibody, there was no effect on 3H-thymidine incorporation in these cells. These data demonstrate that GIP stimulates osteoblastic-like cell proliferation but that this effect is not mediated by TGF-beta.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Tissue culture models for studies of hormone and vitamin action in bone cells

Osteoporosis is a major health care concern and levies a serious financial burden on the world health care system. For this reason, many physicians and scientists are engaged in research to better understand and treat this disease. To this end, numerous in vitro bone cell models have been developed to explore the cellular and molecular mechanisms of skeletal biology and for the identification and characterization of new drug targets and therapies. In this chapter, we review many of these cellular models as tools to study the hormonal regulation of bone metabolism. In particular, we pay special attention to new human bone cell models, since these have the greatest relevance to osteoporosis research and drug discovery. These new models include (1) the use of peripheral blood mononuclear cells as progenitors of osteoclasts and primary cultures of mesenchymal stem cells as precursors of osteoblasts; (2) the development of conditionally immortalized preosteoclastic and osteoblastic cell lines using temperature-sensitive large T-antigens; and (3) the establishment of the first osteocytic cell lines. Thus, we now have at our disposal many good in vitro models to investigate the regulation of bone resorption and formation by hormones, vitamins and drugs. These models should accelerate our understanding of bone physiology and pathophysiology as well as our ability to develop important new therapies to prevent and treat skeletal diseases.

Parathyroid Hormone, Its Fragments and Their Analogs for the Treatment of Osteoporosis

The susceptibility to traumatic fracturing of osteopenic bones, and the spontaneous fracturing of osteoporotic bones by normal body movements caused by the microstructural deterioration and loss of bone, are currently treated with antiresorptive drugs, such as the bisphosphonates, calcitonin, estrogens, and selective estrogen receptor modulators. These antiresorptive agents target osteoclasts and, as their name indicates, reduce or stop bone resorption. They cannot directly stimulate bone formation, increase bone mass above normal values in ovariectomized rat models, or improve microstructure. However, there is a family of agents - the parathyroid hormone (PTH) and some of its fragments and their analogs - which directly stimulate bone growth and improve microstructure independently from impairing osteoclasts. These drugs are about to make their clinical debut in treating patients with osteoporosis and, probably not too far in the future, for accelerating fracture healing. They stimulate osteoblast accumulation and bone formation in three ways via signals from the type 1 PTH/PTH-related protein (PTHR1) receptors on proliferatively inactive preosteoblasts, osteoblasts, osteocytes and bone-lining cells. The receptor signals shut down the proliferative machinery in preosteoblasts and push their maturation to osteoblasts, cause the osteoblastic cells to make and secrete several factors that stimulate the extensive proliferation of osteoprogenitors without PTHRI receptors, stimulate the reversion of bone-lining cells to osteoblasts, and extend osteoblast lifespan and productivity by preventing them from suicidally initiating apoptosis. The first of the PTHs to reach the clinic will be teriparatide [recombinant human (h)PTH-(1-34)], which was recommended for approval in 2001 by the US Food and Drug Administration Endocrinology and Metabolic Drugs Advisory Committee for the treatment of postmenopausal osteoporosis. Teriparatide has been shown to considerably increase cancellous and cortical bone mass, improve bone microstructure, prevent fractures and thus provide benefits that cannot be provided by current antiresorptive drugs, when administered subcutaneously at a daily dose of 20 microg for no longer than 2 years to patients with osteoporosis.

Stimulation of protein kinase C activity in cells expressing human parathyroid hormone receptors by C- and N-terminally truncated fragments of parathyroid hormone 1-34

The parathyroid hormone (PTH) fragment PTH(1-34) stimulates adenylyl cyclase, phospholipase C (PLC), and protein kinase C's (PKCs) in cells that express human, opossum, or rodent type 1 PTH/PTH-related protein (PTHrP) receptors (PTHR1s). Certain carboxyl (C)-terminally truncated fragments of PTH(1-34), such as human PTH(1-31) [hPTH-(1-31)NH2], stimulate adenylyl cyclase but not PKCs in rat osteoblasts or PLC and PKCs in mouse kidney cells. The hPTH(1-31)NH2 peptide does fully stimulate PLC in HKRK B7 porcine renal epithelial cells that express 950,000 transfected hPTHR1s per cell. Amino (N)-terminally truncated fragments, such as bovine PTH(3-34) [bPTH(3-34)], hPTH(3-34)NH2, and hPTH(13-34), stimulate PKCs in Chinese hamster ovary (CHO) cells expressing transfected rat receptors, opossum kidney cells, and rat osteoblasts, but an intact N terminus is needed to stimulate PLC via human PTHR1s in HKRK B7 cells. We now report that the N-terminally truncated analogs bPTH(3-34)NH2 and hPTH(13-34)OH do activate PKC via human PTHR1s in HKRK B7 cells, although less effectively than hPTH(1-34)NH2 and hPTH(1-31)NH2. Moreover, in a homologous human cell system (normal foreskin fibroblasts), these N-terminally truncated fragments stimulate PKC activity as strongly as hPTH(1-34)NH2 and hPTH(1-31)NH2. Thus, it appears that unlike their opossum and rodent equivalents, hPTHR1s can stimulate both PLC and PKCs when activated by C-terminally truncated fragments of PTH(1-34). Furthermore, hPTHR1s, like the PTHR1s in rat osteoblasts, opossum kidney cells, and rat PTHR1-transfected CHO cells also can stimulate PKC activity by a mechanism that is independent of PLC. The efficiency with which the N-terminally truncated PTH peptides stimulate PKC activity depends on the cellular context in which the PTHR1s are expressed.