Effects of cAMP on intercellular coupling and osteoblast differentiation

Vasoactive intestinal peptide exerts an osteoinductive effect in human mesenchymal stem cells

Several neuropeptides present in bone tissues, produced by nerve fibers and bone cells, have been reported to play a role in regulating the fine-tuning of osteoblast and osteoclast functions to maintain bone homeostasis. This study aims to characterize the influence of the neuropeptide vasoactive intestinal peptide (VIP) on the differentiation process of human mesenchymal stem cells (MSCs) into osteoblasts and on their anabolic function. We describe the mRNA and protein expression profile of VIP and its receptors in MSCs as they differentiate into osteoblasts, suggesting the presence of an autocrine signaling pathway in these cells. Our findings reveal that VIP enhances the expression of early osteoblast markers in MSCs under osteogenic differentiation and favors both bone matrix formation and proper cytoskeletal reorganization. Finally, our data suggest that VIP could be exerting a direct modulatory role on the osteoblast to osteoclast signaling by downregulating the receptor activator of nuclear factor-κB ligand/osteoprotegerin ratio. These results highlight the potential of VIP as an osteoinductive differentiation factor, emerging as a key molecule in the maintenance of human bone homeostasis.

Bone regenerative potential of the selective sphingosine 1-phosphate receptor modulator siponimod: In vitro characterisation using osteoblast and endothelial cells

The repair of critical bone defects remains a significant therapeutic challenge. While the implantation of drug-eluting scaffolds is an option, a drug with the optimal pharmacological properties has not yet been identified. Agents acting at sphingosine 1-phosphate (S1P) receptors have been considered, but those investigated so far do not discriminate between the five known S1P receptors. This work was undertaken to investigate the potential of the specific S1P_1/5 modulator siponimod as a bone regenerative agent, by testing in vitro its effect on cell types critical to the bone regeneration process. hFOB osteoblasts and HUVEC endothelial cells were treated with siponimod and other S1P receptor modulators and investigated for changes in intracellular cyclic AMP content, viability, proliferation, differentiation, attachment and cellular motility. Siponimod showed no effect on the viability and proliferation of osteoblasts and endothelial cells, but increased osteoblast differentiation (as shown by increased alkaline phosphatase activity). Furthermore, siponimod significantly increased endothelial cell motility in scratch and transwell migration assays. These effects on osteoblast differentiation and endothelial cell migration suggest that siponimod may be a potential agent for the stimulation of localised differentiation of osteoblasts in critical bone defects.

Regulation of MC3T3-E1 differentiation by actin cytoskeleton through lipid mediators reflecting the cell differentiation stage

Actin cytoskeleton is reported to be related in various functions of osteoblast, bone-forming cell. However the function of actin cytoskeleton in osteoblasts is not fully understood, since bone formation is derived from extracellular interactions of functional proteins produced from osteoblasts, including osteocalcin (Ocn), and it is a result of closely and complex organized sequence of biochemical events. In this study, we showed that actin cytoskeleton of MC3T3-E1 cells functioned in recognition of cell condition and regulation of extracellular matrix mineralization, bone formation. Maturation of MC3T3-E1 cells by 14 days of culture reduced F-actin filaments, while induced expression of Ocn mRNA known as late stage differentiation marker and matrix mineralization, terminal stage of cell differentiation. The disruption of actin cytoskeleton with Cyto D in immature MC3T3-E1 cells significantly increased expression of Ocn mRNA in 24 h. Both PTX-induced inhibition of signal transduction through GPCRs and celecoxib-induced suppression of lipid mediators in immature MC3T3-E1 cells reduced actin filaments and suppressed matrix mineralization. Furthermore, addition of lipid mediators extracted from culture mediums of differentiated MC3T3-E1 cells by Bligh-Dyer method induced actin cytoskeleton reorganization and matrix mineralization change in MC3T3-E1 cells. Taken together, our data suggest that actin cytoskeleton of MC3T3-E1 cells regulates activation of developmental pathway reflecting cell differentiation stages through lipid mediators. The function we identified is important for bone formation tightly regulated by mechanical stress, since actin cytoskeleton is also known as a mechanosensor of osteoblasts.

Osteochondromyxoma: Review of a rare carney complex criterion

Osteochondromyxoma is an extremely rare bone tumor associated with 1% of Carney complex patients and constitutes one of its 11 diagnostic criteria. This narrative review of osteochondromyxoma is based on a search of all references to the topic in PubMed, Web Of Science, SCOPUS, ScienceDirect, and JSTOR databases. Special attention was focused on case reports, leading to a review encompassing the case reports to date, as well as related animal model studies. This review covers the current understanding of osteochondromyxoma, highlighting its variability while providing consensus on the most common clinical presentation, pathological findings, and genetic features of this rare bone tumor.

Influence of Parathyroid Hormone-Loaded PLGA Nanoparticles in Porous Scaffolds for Bone Regeneration

Biodegradable poly(lactide-co-glycolide) (PLGA) nanoparticles, containing human parathyroid hormone (PTH (1–34)), prepared by a modified double emulsion-solvent diffusion-evaporation method, were incorporated in porous freeze-dried chitosan-gelatin (CH-G) scaffolds. The PTH-loaded nanoparticles (NPTH) were characterised in terms of morphology, size, protein loading, release kinetics and in vitro assessment of biological activity of released PTH and cytocompatibility studies against clonal human osteoblast (hFOB) cells. Structural integrity of incorporated and released PTH from nanoparticles was found to be intact by using Tris-tricine SDS-PAGE. In vitro PTH release kinetics from PLGA nanoparticles were characterised by a burst release followed by a slow release phase for 3–4 weeks. The released PTH was biologically active as evidenced by the stimulated release of cyclic AMP from hFOB cells as well as increased mineralisation studies. Both in vitro and cell studies demonstrated that the PTH bioactivity was maintained during the fabrication of PLGA nanoparticles and upon release. Finally, a content of 33.3% w/w NPTHs was incorporated in CH-G scaffolds, showing an intermittent release during the first 10 days and, followed by a controlled release over 28 days of observation time. The increased expression of Alkaline Phosphatase levels on hFOB cells further confirmed the activity of intermittently released PTH from scaffolds.

Age-related changes in gap junctional intercellular communication in osteoblastic cells

Aging demonstrates deleterious effects upon the skeleton which can predispose an individual to osteoporosis and related fractures. Despite the well-documented evidence that aging decreases bone formation, there remains little understanding whereby cellular aging alters skeletal homeostasis. We, and others, have previously demonstrated that gap junctions--membrane-spanning channels that allow direct cell-to-cell conductance of small signaling molecules--are critically involved in osteoblast differentiation and skeletal homeostasis. We examined whether the capacity of rat osteoblastic cells to form gap junctions and respond to known modulators of gap junction intercellular communication (GJIC) was dependent on the age of the animal from which they were isolated. We observed no effect of age upon osteoblastic Cx43 mRNA, protein or GJIC. We also examined age-related changes in PTH-stimulated GJIC. PTH demonstrated age-dependent effects upon GJIC: Osteoblastic cells from young rats increased GJIC in response to PTH, whereas there was no change in GJIC in response to PTH in osteoblastic cells from mature or old rats. PTH-stimulated GJIC occurred independently of changes in Cx43 mRNA or protein expression. Cholera toxin significantly increased GJIC in osteoblastic cells from young rats compared to those from mature and old rats. These data demonstrate an age-related impairment in the capacity of osteoblastic cells to generate functional gap junctions in response to PTH, and suggest that an age-related defect in G protein-coupled adenylate cyclase activity at least partially contributes to decreased PTH-stimulated GJIC.

Rapid changes in connexin-43 in response to genotoxic stress stabilize cell-cell communication in corneal endothelium

PURPOSE

To determine how corneal endothelial (CE) cells respond to acute genotoxic stress through changes in connexin-43 (Cx43) and gap junction intercellular communication (GJIC).

METHODS

Cultured bovine CE cells were exposed to mitomycin C or other DNA-damaging agents. Changes in the levels, stability, binding partners, and trafficking of Cx43 were assessed by Western blot analysis and immunostaining. Live-cell imaging of a Cx43-green fluorescent protein (GFP) fusion protein was used to evaluate internalization of cell surface Cx43. Dye transfer and fluorescent recovery after photobleaching (FRAP) assessed GJIC.

RESULTS

After genotoxic stress, Cx43 accumulated in large gap junction plaques, had reduced zonula occludens-1 binding, and displayed increased stability. Live-cell imaging of Cx43-GFP plaques in stressed CE cells revealed reduced gap junction internalization and degradation compared to control cells. Mitomycin C enhanced transport of Cx43 from the endoplasmic reticulum to the cell surface and formation of gap junction plaques. Mitomycin C treatment also protected GJIC from disruption after cytokine treatment.

DISCUSSION

These results show a novel CE cell response to genotoxic stress mediated by marked and rapid changes in Cx43 and GJIC. This stabilization of cell-cell communication may be an important early adaptation to acute stressors encountered by CE.

Gap Junctions and Biophysical Regulation of Bone Cells

Communication between osteoblasts, osteoclasts, and osteocytes is integral to their ability to build and maintain the skeletal system and respond to physical signals. Various physiological mechanisms, including nerve communication, hormones, and cytokines, play an important role in this process. More recently, the important role of direct, cell-cell communication via gap junctions has been established. In this review, we demonstrate the integral role of gap junctional intercellular communication (GJIC) in skeletal physiology and bone cell mechanosensing.

Regulation of the connexin 43 promoter in the brook trout testis: role of the thyroid hormones and cAMP

Gap junctions are critical for spermatogenesis. They are composed of integral proteins, the connexins. In mammals, a loss of Cx43 expression results in the inhibition of spermatogenesis. We have shown that Cx43 is expressed in the Sertoli cells of rainbow trout and that cAMP and triiodothyronine (T(3)) regulate testicular Cx43 expression in brook trout testis. The objective of this study was to determine if cAMP and T(3) act at the level of the cx43 promoter to regulate its expression. A 607 bp 5' flanking sequence of the cx43 promoter was obtained by Genome Walking. A TATA box was predicted to be located between positions -36 and -30 relative to the transcriptional initiation site. 5'-Rapid amplification of cDNA ends indicated a single transcriptional start site. Single C/EBP (-164 to -156) and tr-beta (-112 to -107) response elements were identified and electrophoretic mobility shift assays indicated the presence of competitive protein binding sites at each region. Immortalized rainbow trout gonadal cell line (RTG-2) which express cx43 and tr-beta transcripts were transfected with a vector containing the Cx43 promoter inserted into a luciferase expression vector. Transactivation of the reporter genes was stimulated by either cAMP or T(3). Sequential deletion and point mutations in either the C/EBP or tr-beta response element indicated that T(3) but not cAMP directly induced luciferase transactivation of the luciferase gene by acting on different sites of the Cx43 promoter. Together, these data indicate that T(3) stimulates cx43 expression via direct regulation of gene transcription.

Modulatory effects of cAMP and PKC activation on gap junctional intercellular communication among thymic epithelial cells

BACKGROUND

We investigated the effects of the signaling molecules, cyclic AMP (cAMP) and protein-kinase C (PKC), on gap junctional intercellular communication (GJIC) between thymic epithelial cells (TEC).

RESULTS

Treatment with 8-Br-cAMP, a cAMP analog; or forskolin, which stimulates cAMP production, resulted in an increase in dye transfer between adjacent TEC, inducing a three-fold enhancement in the mean fluorescence of coupled cells, ascertained by flow cytometry after calcein transfer. These treatments also increased Cx43 mRNA expression, and stimulated Cx43 protein accumulation in regions of intercellular contacts. VIP, adenosine, and epinephrine which may also signal through cyclic nucleotides were tested. The first two molecules did not mimic the effects of 8-Br-cAMP, however epinephrine was able to increase GJIC suggesting that this molecule functions as an endogenous inter-TEC GJIC modulators. Stimulation of PKC by phorbol-myristate-acetate inhibited inter-TEC GJIC. Importantly, both the enhancing and the decreasing effects, respectively induced by cAMP and PKC, were observed in both mouse and human TEC preparations. Lastly, experiments using mouse thymocyte/TEC heterocellular co-cultures suggested that the presence of thymocytes does not affect the degree of inter-TEC GJIC.

CONCLUSIONS

Overall, our data indicate that cAMP and PKC intracellular pathways are involved in the homeostatic control of the gap junction-mediated communication in the thymic epithelium, exerting respectively a positive and negative role upon cell coupling. This control is phylogenetically conserved in the thymus, since it was seen in both mouse and human TEC preparations. Lastly, our work provides new clues for a better understanding of how the thymic epithelial network can work as a physiological syncytium.

Prostaglandin promotion of osteocyte gap junction function through transcriptional regulation of connexin 43 by glycogen synthase kinase 3/beta-catenin signaling

Gap junction intercellular communication in osteocytes plays an important role in bone remodeling in response to mechanical loading; however, the responsible molecular mechanisms remain largely unknown. Here, we show that phosphoinositide-3 kinase (PI3K)/Akt signaling activated by fluid flow shear stress and prostaglandin E(2) (PGE(2)) had a stimulatory effect on both connexin 43 (Cx43) mRNA and protein expression. PGE(2) inactivated glycogen synthase kinase 3 (GSK-3) and promoted nuclear localization and accumulation of beta-catenin. Knockdown of beta-catenin expression resulted in a reduction in Cx43 protein. Furthermore, the chromatin immunoprecipitation (ChIP) assay demonstrated an association of beta-catenin with the Cx43 promoter, suggesting that beta-catenin could regulate Cx43 expression at the level of gene transcription. We have previously reported that PGE(2) activates cyclic AMP (cAMP)-protein kinase A (PKA) signaling and increases Cx43 and gap junctions. Interestingly, the activation of PI3K/Akt appeared to be independent of the activation of PKA, whereas both PI3K/Akt and PKA signaling inactivated GSK-3 and increased beta-catenin translocation. Together, these results suggest that shear stress, through PGE(2) release, activates both PI3K/Akt and cAMP-PKA signaling, which converge through the inactivation of GSK-3, leading to the increase in nuclear accumulation of beta-catenin. beta-Catenin binds to the Cx43 promoter, stimulating Cx43 expression and functional gap junctions between osteocytes.

Interaction between human umbilical vein endothelial cells and human osteoprogenitors triggers pleiotropic effect that may support osteoblastic function

Osteogenesis occurs in striking interaction with angiogenesis. There is growing evidence that endothelial cells are involved in the modulation of osteoblast differentiation. We hypothesized that primary human umbilical vein endothelial cells (HUVEC) should be able to modulate primary human osteoprogenitors (HOP) function in an in vitro co-culture model. In a previous study we demonstrated that a 3 day to 3 week co-culture stimulates HOP differentiation markers such as Alkaline Phosphatase (ALP) activity and mineralization. In the present study we addressed the effects induced by the co-culture on HOP within the first 48 hours. As a prerequisite, we validated a method based on immuno-magnetic beads to separate HOP from HUVEC after co-culture. Reverse transcription-real time quantitative PCR studies demonstrated up-regulation of the ALP expression in the co-cultured HOP, confirming previous results. Surprisingly, down-regulation of runx2 and osteocalcin was also shown. Western blot analysis revealed co-culture induced down-regulation of Connexin43 expression in both cell types. Connexin43 function may be altered in co-cultured HOP as well. Stimulation of the cAMP pathway was able to counterbalance the effect of the co-culture on the ALP activity, but was not able to rescue runx2 mRNA level. Co-culture effect on HOP transcriptome was analyzed with GEArray cDNA microarray showing endothelial cells may also modulate HOP extracellular matrix production. In accordance with previous work, we propose endothelial cells may support initial osteoblastic proliferation but do not alter the ability of the osteoblasts to produce extracellular mineralizing matrix.

Pathophysiological roles of gap junction in glomerular mesangial cells

Glomerular mesangial cells (MCs) are specialized vascular smooth muscle cells that play a critical role in the control of glomerular hemodynamics. One of the intriguing features of MCs is their extraordinary abundance in gap junctions (GJs). It has long been speculated that GJs may bridge MCs together and provide the mesangium with the characteristics of a functional syncytium. Accumulating scientific evidence supports this idea. GJs are reported to be critically involved in important physiological processes like tubuloglomerular feedback and glomerular filtration. In addition, GJs are implicated in the control of many cellular processes of MCs, including growth, differentiation and survival. This article summarizes the current knowledge on the roles of GJs in glomerular pathophysiology.

Synergistic effects of PDGF-BB and cAMP-elevating agents on expression of connexin43 in mesangial cells

The gap junction plays an important role in the regulation of cell growth, migration, and differentiation. Platelet-derived growth factor (PDGF) is reported to be a potent inhibitor of gap junctional intercellular communication (GJIC). Short-term exposure of cells to PDGF causes rapid and transient disruption of GJIC without altering connexin43 (Cx43) protein level. In this study, we investigated long-term effects of PDGF-BB on Cx43 expression in mesangial cells (MCs). Exposure of MCs to PDGF-BB affected neither the Cx43 protein level nor GJIC. However, in the presence of cAMP-elevating agents, PDGF-BB dramatically increased the expression of Cx43, which was accompanied by obviously augmented membrane distribution of Cx43 and functional GJIC. The increased expression of Cx43 was closely correlated with reduction in alpha-actin, a dedifferentiation marker of MCs. The effect of PDGF on Cx43 was largely prevented by inhibitors of phosphatidylinositol 3'-kinase or mitogen-activated protein kinase, but not by inhibition of protein kinase C. Exposure of MCs to PDGF-BB caused elevation in intracellular cAMP, and it was abolished by indomethacin, a cyclooxygenase inhibitor. However, indomethacin did not affect the synergistic effect. In addition, PDGF-BB also did not affect the degradation of Cx43. With the use of MCs transfected with a Cx43 promoter-luciferase vector, cooperative activation of Cx43 promoter by PDGF and cAMP was found. Together, our data reveal, for the first time, unexpected synergy between PDGF-BB and cAMP-elevating agents in the induction of Cx43 and MC differentiation. Regulation of GJIC could be an important mechanism via which PDGF modulates MC phenotypes.

A mouse model for the Carney complex tumor syndrome develops neoplasia in cyclic AMP-responsive tissues

Carney complex is an autosomal dominant neoplasia syndrome characterized by spotty skin pigmentation, myxomatosis, endocrine tumors, and schwannomas. This condition may be caused by inactivating mutations in PRKAR1A, the gene encoding the type 1A regulatory subunit of protein kinase A. To better understand the mechanism by which PRKAR1A mutations cause disease, we have developed conventional and conditional null alleles for Prkar1a in the mouse. Prkar1a(+/-) mice developed nonpigmented schwannomas and fibro-osseous bone lesions beginning at approximately 6 months of age. Although genotype-specific cardiac and adrenal lesions were not seen, benign and malignant thyroid neoplasias were observed in older mice. This spectrum of tumors overlaps that seen in Carney complex patients, confirming the validity of this mouse model. Genetic analysis indicated that allelic loss occurred in a subset of tumor cells, suggesting that complete loss of Prkar1a plays a key role in tumorigenesis. Similarly, tissue-specific ablation of Prkar1a from a subset of facial neural crest cells caused the formation of schwannomas with divergent differentiation. These observations confirm the identity of PRKAR1A as a tumor suppressor gene with specific importance to cyclic AMP-responsive tissues and suggest that these mice may be valuable tools not only for understanding endocrine tumorigenesis but also for understanding inherited predispositions for schwannoma formation.

Effect of carvedilol on Ca2+ movement and cytotoxicity in human MG63 osteosarcoma cells

Carvedilol is a useful cardiovascular drug for treating heart failure, however, the in vitro effect on many cell types is unclear. In human MG63 osteosarcoma cells, the effect of carvedilol on intracellular Ca2+ concentrations ([Ca2+]i) and cytotoxicity was explored by using fura-2 and tetrazolium, respectively. Carvedilol at concentrations greater than 1 microM caused a rapid rise in [Ca2+]i in a concentration-dependent manner (EC50=15 microM). Carvedilol-induced [Ca2+]i rise was reduced by 60% by removal of extracellular Ca2+. Carvedilol-induced Mn2+-associated quench of intracellular fura-2 fluorescence also suggests that carvedilol induced extracellular Ca2+ influx. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which the increasing effect of carvedilol on [Ca2+]i was inhibited by 50%. Conversely, pretreatment with carvedilol to deplete intracellular Ca2+ stores totally prevented thapsigargin from releasing more Ca2+. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca2+ mobilizer)-induced, but not carvedilol-induced, [Ca2+]i rise. Pretreatment with phorbol 12-myristate 13-acetate and forskolin to activate protein kinase C and adenylate cyclase, respectively, did not alter carvedilol-induced [Ca2+]i rise. Separately, overnight treatment with 0.1-30 microM carvedilol inhibited cell proliferation in a concentration-dependent manner. These findings suggest that in human MG63 osteosarcoma cells, carvedilol increases [Ca2+]i by stimulating extracellular Ca2+ influx and also by causing intracellular Ca2+ release from the endoplasmic reticulum and other stores via a phospholipase C-independent manner. Carvedilol may be cytotoxic to osteoblasts.

Developmental expression patterns and regulation of connexins in the mouse mammary gland: expression of connexin30 in lactogenesis

The mammary gland reaches a fully differentiated phenotype at lactation, a stage characterized by the abundant expression of beta-casein. We have investigated the expression and regulation of gap junction proteins (connexins, Cx) during the various developmental stages of mouse mammary gland. Immunohistochemical analysis, with specific antibodies, reveals that Cx26 and Cx32 are expressed and confined to the cell borders of luminal epithelial cells in all developmental stages of the gland. Cx26 and Cx32 expression, at the mRNA and protein levels, increases in pregnancy and peaks in lactation. Whereas Cx43 mRNA decreases in pregnancy and lactation, the functional activity of Cx43 protein, which has been localized to myoepithelial cells, is regulated (through phosphorylation) during pregnancy and peaks during lactation. Cx30 mRNA and proteins have, for the first time, been detected in mammary gland epithelia. Using reverse transcription/polymerase chain reaction and sequencing techniques, we show that Cx30 is abundant in pregnant and lactating mammary gland. Cx30 protein levels have not been detected in the mammary gland prior to day 15 of pregnancy, whereas maximum expression occurs at the onset of lactation. In mouse mammary cells in culture, Cx30 is epithelial-cell-specific and is induced by lactogenic hormones. These data identify a novel player in mammary differentiation and suggest a potential role for Cx30 in the fully differentiated gland.

Strategies for directing the differentiation of stem cells into the osteogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in bone reconstruction and bone tissue engineering. This will require well-defined and efficient protocols for directing the differentiation of stem cells into the osteogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages on transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying osteogenesis and bone development, and facilitate the genetic manipulation of stem cells for therapeutic applications. The development of pharmokinetic and cytotoxicity/genotoxicity screening tests for bone-related biomaterials and drugs could also use protocols developed for the osteogenic differentiation of stem cells. This review critically examines the various strategies that could be used to direct the differentiation of stem cells into the osteogenic lineage in vitro.

Effect of celecoxib on Ca2+ movement and cell proliferation in human osteoblasts

In human osteoblasts, the effect of the widely prescribed cyclooxygenase-2 inhibitor celecoxib on intracellular Ca(2+) concentrations ([Ca(2+)](i)) and cell proliferation was explored by using fura-2 and the tetrazolium assay, respectively. Celecoxib at concentrations greater than 1microM caused a rapid rise in [Ca(2+)](i) in a concentration-dependent manner ( EC 50= 10 microM). Celecoxib-induced [Ca(2+)](i) rise was reduced by 90% by removal of extracellular Ca(2+), and by 30% by l-type Ca(2+) channel blockers. Celecoxib-induced Mn(2+)-associated quench of intracellular fura-2 fluorescence also suggests that celecoxib-induced extracellular Ca(2+) influx. In Ca(2+)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a monophasic [Ca(2+)](i) rise, after which the increasing effect of celecoxib on [Ca(2+)](i) was greatly inhibited. Conversely, pretreatment with celecoxib to deplete intracellular Ca(2+) stores totally prevented thapsigargin from releasing more Ca(2+). U73122, an inhibitor of phoispholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca(2+) mobilizer)-induced, but not celecoxib-induced, [Ca(2+)](i) rise. Pretreatment with phorbol 12-myristate 13-acetate and forskolin to activate protein kinase C and adenylate cyclase, respectively, partly inhibited celecoxib-induced [Ca(2+)](i) rise in Ca(2+)-containing medium. Separately, overnight treatment with 1-100microM celecoxib inhibited cell proliferation in a concentration-dependent manner. These findings suggest that in human osteoblasts, celecoxib increases [Ca(2+)](i) by stimulating extracellular Ca(2+) influx and also by causing intracellular Ca(2+) release from the endoplasmic reticulum via a phospholiase C-independent manner. Celecoxib may be cytotoxic at higher concentrations.

Effects of mechanical strain on the function of Gap junctions in osteocytes are mediated through the prostaglandin EP2 receptor

Osteocytes embedded in the matrix of bone are thought to be mechanosensory cells that translate mechanical strain into biochemical signals that regulate bone modeling and remodeling. We have shown previously that fluid flow shear stress dramatically induces prostaglandin release and COX-2 mRNA expression in osteocyte-like MLO-Y4 cells, and that prostaglandin E2 (PGE2) released by these cells functions in an autocrine manner to regulate gap junction function and connexin 43 (Cx43) expression. Here we show that fluid flow regulates gap junctions through the PGE2 receptor EP2 activation of cAMP-dependent protein kinase A (PKA) signaling. The expression of the EP2 receptor, but not the subtypes EP1,EP3, and EP4, increased in response to fluid flow. Application of PGE2 or conditioned medium from fluid flow-treated cells to non-stressed MLO-Y4 cells increased expression of the EP2 receptor. The EP2 receptor antagonist, AH6809, suppressed the stimulatory effects of PGE2 and fluid flow-conditioned medium on the expression of the EP2 receptor, on Cx43 protein expression, and on gap junction-mediated intercellular coupling. In contrast, the EP2 receptor agonist butaprost, not the E1/E3 receptor agonist sulprostone, stimulated the expression of Cx43 and gap junction function. Fluid flow conditioned medium and PGE2 stimulated cAMP production and PKA activity suggesting that PGE2 released by mechanically stimulated cells is responsible for the activation of cAMP and PKA. The adenylate cyclase activators, forskolin and 8-bromo-cAMP, enhanced intercellular connectivity, the number of functional gap junctions, and Cx43 protein expression, whereas the PKA inhibitor, H89, inhibited the stimulatory effect of PGE2 on gap junctions. These studies suggest that the EP2 receptor mediates the effects of autocrine PGE2 on the osteocyte gap junction in response to fluid flow-induced shear stress. These data support the hypothesis that the EP2 receptor, cAMP, and PKA are critical components of the signaling cascade between mechanical strain and gap junction-mediated communication between osteocytes.

ECM-induced gap junctional communication enhances mammary epithelial cell differentiation

The relationship between gap junctional intercellular communication (GJIC) and mammary cell (CID-9) differentiation in vitro was explored. CID-9 cells differentiate and express beta-casein in an extracellular matrix (ECM)- and hormone-dependent manner. In response to interaction with the ECM, cells in culture modulated the expression of their gap junction proteins at the transcriptional and post-translational levels. In the presence of EHS-matrix, connexins (Cx)26, 32 and 43 localized predominantly to the plasma membrane, and enhanced GJIC [as measured by Lucifer Yellow (LY) dye transfer assays] was noted. Inhibition of GJIC of cells on EHS-matrix with 18 alpha glycyrrhetinic acid (GA) resulted in reversible downregulation of beta-casein expression. In the presence of cAMP, cells cultured on plastic expressed beta-casein, upregulated Cx43 and Cx26 protein levels and enhanced GJIC. This was reversed in the presence of 18 alpha GA. cAMP-treated cells plated either on a non-adhesive PolyHEMA substratum or on plastic supplemented with function-blocking anti-beta 1 integrin antibodies, maintained beta-casein expression. These studies suggest that cell-ECM interaction alone may induce differentiation through changes in cAMP levels and formation of functional gap junctions. That these events are downstream of ECM signalling was underscored by the fact that enhanced GJIC induced partial differentiation in mammary epithelial cells in the absence of an exogenously provided basement membrane and in a beta 1-integrin- and adhesion-independent manner.

Intercellular calcium communication regulates platelet aggregation and thrombus growth

The ability of platelets to form stable adhesion contacts with other activated platelets (platelet cohesion or aggregation) at sites of vascular injury is essential for hemostasis and thrombosis. In this study, we have examined the mechanisms regulating cytosolic calcium flux during the development of platelet-platelet adhesion contacts under the influence of flow. An examination of platelet calcium flux during platelet aggregate formation in vitro demonstrated a key role for intercellular calcium communication (ICC) in regulating the recruitment of translocating platelets into developing aggregates. We demonstrate that ICC is primarily mediated by a signaling mechanism operating between integrin alpha IIb beta 3 and the recently cloned ADP purinergic receptor P2Y12. Furthermore, we demonstrate that the efficiency by which calcium signals are propagated within platelet aggregates plays an important role in dictating the rate and extent of thrombus growth.

Fluid shear stress remodels expression and function of junctional proteins in cultured bone cells

We tested the hypothesis that fluid shear stress (tau) modifies the expression, function, and distribution of junctional proteins [connexin (Cx)43, Cx45, and zona occludens (ZO)-1] in cultured bone cells. Cell lines with osteoblastic (MC3T3-E1 cells) and osteocytic (MLO-Y4 cells) phenotypes were exposed to tau-values of 5 or 20 dyn/cm(2) for 1-3 h. Immunostaining indicated that at 5 dyn/cm(2), the distribution of Cx43, Cx45, and ZO-1 was moderately disrupted at cell membranes; at 20 dyn/cm(2), disruption was more severe. Intercellular coupling was significantly decreased at both shear stress levels. Western blots showed the downregulation of membrane-bound Cx43 and ZO-1 and the upregulation of cytosolic Cx43 and Cx45 at different levels of shear stress. Similarly, Northern blots revealed that expression of Cx43, Cx45, and ZO-1 was selectively up- and downregulated in response to different shear stress levels. These results indicate that in cultured bone cells, fluid shear stress disrupts junctional communication, rearranges junctional proteins, and determines de novo synthesis of specific connexins to an extent that depends on the magnitude of the shear stress. Such disconnection from the bone cell network may provide part of the signal whereby the disconnected cells or the remaining network initiate focal bone remodeling.

Extracellular NAD+: a novel autocrine/paracrine signal in osteoblast physiology

Intercellular communication allows co-ordination of cell metabolism and sensitivity to extracellular stimuli. In bone cells, paracrine stimulation and cell-to-cell coupling through gap junctions induce the formation of complex intercellular networks, which favours the intercellular exchange of nutrients and second messengers, ultimately controlling the process of bone remodelling. The importance of local factors in bone remodelling is known since many years. Bone cells secrete and respond to a variety signals, among which include prostaglandins, cytokines, growth factors, and ATP. We here report evidence that extracellular NAD(+) is a novel extracellular signal stimulating osteoblast differentiation. We found that HOBIT human osteoblastic cells, which are known to express ADP-ribosyl cyclase/CD38 activity, respond to micromolar concentrations of extracellular NAD(+) with oscillatory increases of the cytosolic Ca(2+) concentration. The initial Ca(2+) response was followed by a time-dependent inhibition of cell growth, the appearance of an epithelial morphology, and by an increase of alkaline phosphatase and osteocalcin expression. Under resting condition HOBIT cells release NAD(+) in the extracellular medium and the release is significantly potentiated by mechanical stimulation. Taken together these results point to NAD(+) as a novel autocrine/paracrine factor involved in stimulation and maintenance of the osteoblast differentiated phenotype.

Behavior of junction channels between rat glomus cells during normoxia and hypoxia

The activity of gap junction channels between cultured and clustered carotid body glomus cells of the rat was studied with dual voltage clamping during normoxia (PO(2) 300 Torr) and hypoxia induced by sodium dithionite (Na(2)S(2)O(4)) or 100% N(2). Na(2)S(2)O(4) reduced the saline PO(2) to approximately 10 Torr, whereas 100% N(2) reduced ambient O(2) to approximately 60 Torr. The following observations were made. 1) In normoxia, the intercellular macroconductance (G(j) = 3.0 +/- 1.01 ns, mean +/- SE) was changed unevenly (increased and decreased) under hypoxic conditions by either agent, although N(2) produced the largest changes. 2) The intercellular microconductances of the channels (g(j) = 104.44 +/- 10.16 pS under normoxic conditions) significantly decreased in 100% N(2) but showed depressions and enhancements in Na(2)S(2)O(4). 3) The conductance of single-junction channels (SChs), calculated as g(j) variance/mean g(j), yielded a mean of approximately 17.6 pS. Larger values were obtained with manual measurements of the data (approximately 34 pS). Hypoxic hypoxia (induced by 100% N(2)) significantly depressed the conductance of SChs when calculated from digitized records or from manual measurements. Hypoxia induced by Na(2)S(2)O(4) did not significantly change junctional conductance. 4) The number of intercellular channels, calculated as g(j)/SCh g(j), had a mean of approximately 452 (range 1 to 2,471). During N(2)-induced hypoxia, this number significantly decreased to approximately 84 but remained unchanged during Na(2)S(2)O(4) hypoxia. 5) The mean open time of junction channels varied from 4 to 30 ms in different experiments, having an overall mean of mu = 11.33 +/- 0.33 ms. This value was significantly reduced by 100% N(2) but was not changed by Na(2)S(2)O(4). 6) Intracellular calcium ([Ca(2+)](i)), 46.2 +/- 4.84 nM under normoxia, significantly increased to 77.32 +/- 11.27 nM with Na(2)S(2)O(4) and to 66.39 +/- 11.64 nM with 100% N(2). It is concluded that 100% N(2) uncouples glomus cells by significantly reducing intercellular macro- and microconductances. Hypoxia induced by Na(2)S(2)O(4) had variable effects. The coupling effects of hypoxia may depend on, or be aided by, increases in [Ca(2+)](i) and/or intracellular pH changes. However, secreted transmitters and ATP plus the effects of hypoxia on second messengers and other cytoplasmic components may also play an important role in this phenomenon.