Evidence that TRPC3 is a molecular component of the 1alpha,25(OH)2D3-activated capacitative calcium entry (CCE) in muscle and osteoblast cells

TRPC expression in human periodontal ligament cells and the periodontal tissue of periodontitis mice: a preliminary study

BACKGROUND

Transient receptor potential canonical (TRPC) channels are non-selective cationic channels with permeability to Ca2+ and Na+. Despite their importance, there are currently few studies on TRPC in the periodontal ligament (PDL) and bone cells in the dental field. To provide biological information regarding TRPC in PDL cells and periodontal tissue, we evaluated TRPC channels expression in the osteoblast differentiation of PDL cells and periodontitis-induced tissue. Human PDL cells were cultured in osteogenic differentiation media for 28 days, and the expression of Runx2, osteocalcin (OCN), and TRPC1, 3, 4, and 6 was evaluated by real-time PCR. In ligature-induced periodontitis mice, the alveolar bone and osteoid areas, the osteoclast number, and the expression of Runx2, OCN, TRPC3, and TRPC6 was evaluated by H&E staining, TRAP staining, and immunohistochemistry, respectively.

RESULTS

In the PDL cell differentiation group, TRPC6 expression peaked on day 7 and TRPC3 expression generally increased during differentiation. During the 28 days of periodontitis progression, alveolar bone loss and osteoclast numbers increased compared to the control group during the experimental period and the osteoid area increased from day 14. TRPC6 expression in the periodontitis group increased in the PDL area and in the osteoblasts compared to the control group, whereas TRPC3 expression increased only in the PDL area on days 7 and 28.

CONCLUSIONS

These results indicate changes of TRPC3 and TRPC6 expression in PDL cells that were differentiating into osteoblasts and in periodontitis-induced tissue, suggesting the need for research on the role of TRPC in osteoblast differentiation or periodontitis progression.

Voltage-Gated Calcium Channels in Nonexcitable Tissues

The identification of a gain-of-function mutation in CACNA1C as the cause of Timothy syndrome, a rare disorder characterized by cardiac arrhythmias and syndactyly, highlighted roles for the L-type voltage-gated Ca2+ channel CaV1.2 in nonexcitable cells. Previous studies in cells and animal models had suggested that several voltage-gated Ca2+ channels (VGCCs) regulated critical signaling events in various cell types that are not expected to support action potentials, but definitive data were lacking. VGCCs occupy a special position among ion channels, uniquely able to translate membrane excitability into the cytoplasmic Ca2+ changes that underlie the cellular responses to electrical activity. Yet how these channels function in cells not firing action potentials and what the consequences of their actions are in nonexcitable cells remain critical questions. The development of new animal and cellular models and the emergence of large data sets and unbiased genome screens have added to our understanding of the unanticipated roles for VGCCs in nonexcitable cells. Here, we review current knowledge of VGCC regulation and function in nonexcitable tissues and cells, with the goal of providing a platform for continued investigation.

Vitamin D and Muscle Sarcopenia in Aging

Sarcopenia, the age-dependent decline of muscle mass and performance, is a common condition among elderly population and is related to numerous adverse health outcomes. Due to the effect of sarcopenia on quality of life, disability, and mortality, a greater awareness is important in order to correctly recognize the condition both in community and geriatric settings. Research on sarcopenia prevention and treatment is growing quickly, but many questions are still unanswered. The core of the sarcopenia state includes quantitative and qualitative declines of skeletal muscle. These two aspects should therefore be considered when designing and examining preventive and therapeutic interventions. The role of vitamin D in skeletal muscle metabolism has been highlighted in recent years. The interest arises from the important findings of studies indicating multiple impacts of vitamin D on this tissue, which can be divided into genomic (direct impacts) and non-genomic impacts (indirect impacts). Another important dimension to be considered in the study of vitamin D and muscle fiber metabolism is associated with different expressions of the vitamin D receptor, which differs in muscle tissue, depending on age, gender, and pathology. Vitamin D inadequacy or deficiency is related to muscle fiber atrophy, elevated risk of chronic musculoskeletal pain, sarcopenia, and falls. This review describes the effect of vitamin D in skeletal muscle tissue function and metabolism and includes discussion of possible mechanisms in skeletal muscle.

STIM2 regulates both intracellular Ca2+ distribution and Ca2+ movement in skeletal myotubes

Stromal interaction molecule 1 (STIM1) along with Orai1 mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism in various tissues including skeletal muscle. However, the role(s) of STIM2, a homolog of STIM1, in skeletal muscle has not been well addressed. The present study, first, was focused on searching for STIM2-binding proteins from among proteins mediating skeletal muscle functions. This study used a binding assay, quadrupole time-of-flight mass spectrometry, and co-immunoprecipitation assay with bona-fide STIM2- and SERCA1a-expressing rabbit skeletal muscle. The region for amino acids from 453 to 729 of STIM2 binds to sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a). Next, oxalate-supported ⁴⁵Ca²⁺-uptake experiments and various single-myotube Ca²⁺ imaging experiments using STIM2-knockdown mouse primary skeletal myotubes have suggested that STIM2 attenuates SERCA1a activity during skeletal muscle contraction, which contributes to the intracellular Ca²⁺ distribution between the cytosol and the SR at rest. In addition, STIM2 regulates Ca²⁺ movement through RyR1 during skeletal muscle contraction as well as SOCE. Therefore, via regulation of SERCA1a activity, STIM2 regulates both intracellular Ca²⁺ distribution and Ca²⁺ movement in skeletal muscle, which makes it both similar to, yet different from, STIM1.

A focus on extracellular Ca2+ entry into skeletal muscle

The main task of skeletal muscle is contraction and relaxation for body movement and posture maintenance. During contraction and relaxation, Ca²⁺ in the cytosol has a critical role in activating and deactivating a series of contractile proteins. In skeletal muscle, the cytosolic Ca²⁺ level is mainly determined by Ca²⁺ movements between the cytosol and the sarcoplasmic reticulum. The importance of Ca²⁺ entry from extracellular spaces to the cytosol has gained significant attention over the past decade. Store-operated Ca²⁺ entry with a low amplitude and relatively slow kinetics is a main extracellular Ca²⁺ entryway into skeletal muscle. Herein, recent studies on extracellular Ca²⁺ entry into skeletal muscle are reviewed along with descriptions of the proteins that are related to extracellular Ca²⁺ entry and their influences on skeletal muscle function and disease.

Hypothalamic Vitamin D Improves Glucose Homeostasis and Reduces Weight

Despite clear associations between vitamin D deficiency and obesity and/or type 2 diabetes, a causal relationship is not established. Vitamin D receptors (VDRs) are found within multiple tissues, including the brain. Given the importance of the brain in controlling both glucose levels and body weight, we hypothesized that activation of central VDR links vitamin D to the regulation of glucose and energy homeostasis. Indeed, we found that small doses of active vitamin D, 1α,25-dihydroxyvitamin D3 (1,25D3) (calcitriol), into the third ventricle of the brain improved glucose tolerance and markedly increased hepatic insulin sensitivity, an effect that is dependent upon VDR within the paraventricular nucleus of the hypothalamus. In addition, chronic central administration of 1,25D3 dramatically decreased body weight by lowering food intake in obese rodents. Our data indicate that 1,25D3-mediated changes in food intake occur through action within the arcuate nucleus. We found that VDR colocalized with and activated key appetite-regulating neurons in the arcuate, namely proopiomelanocortin neurons. Together, these findings define a novel pathway for vitamin D regulation of metabolism with unique and divergent roles for central nervous system VDR signaling. Specifically, our data suggest that vitamin D regulates glucose homeostasis via the paraventricular nuclei and energy homeostasis via the arcuate nuclei.

Elevation of extracellular Ca2+ induces store-operated calcium entry via calcium-sensing receptors: a pathway contributes to the proliferation of osteoblasts

Aims

The local concentration of extracellular Ca²⁺ ([Ca²⁺]_o) in bone microenvironment is accumulated during bone remodeling. In the present study we investigated whether elevating [Ca²⁺]_o induced store-operated calcium entry (SOCE) in primary rat calvarial osteoblasts and further examined the contribution of elevating [Ca²⁺]_o to osteoblastic proliferation.

Methods

Cytosolic Ca²⁺ concentration ([Ca²⁺]_c) of primary cultured rat osteoblasts was detected by fluorescence imaging using calcium-sensitive probe fura-2/AM. Osteoblastic proliferation was estimated by cell counting, MTS assay and ATP assay. Agonists and antagonists of calcium-sensing receptors (CaSR) as well as inhibitors of phospholipase C (PLC), SOCE and voltage-gated calcium (Cav) channels were applied to study the mechanism in detail.

Results

Our data showed that elevating [Ca²⁺]_o evoked a sustained increase of [Ca²⁺]_c in a dose-dependent manner. This [Ca²⁺]_c increase was blocked by TMB-8 (Ca²⁺ release inhibitor), 2-APB and BTP-2 (both SOCE blockers), respectively, whereas not affected by Cav channels blockers nifedipine and verapamil. Furthermore, NPS2143 (a CaSR antagonist) or U73122 (a PLC inhibitor) strongly reduced the [Ca²⁺]_o-induced [Ca²⁺]_c increase. The similar responses were observed when cells were stimulated with CaSR agonist spermine. These data indicated that elevating [Ca²⁺]_o resulted in SOCE depending on the activation of CaSR and PLC in osteoblasts. In addition, high [Ca²⁺]_o significantly promoted osteoblastic proliferation, which was notably reversed by BAPTA-AM (an intracellular calcium chelator), 2-APB, BTP-2, TMB-8, NPS2143 and U73122, respectively, but not affected by Cav channels antagonists.

Conclusions

Elevating [Ca²⁺]_o induced SOCE by triggering the activation of CaSR and PLC. This process was involved in osteoblastic proliferation induced by high level of extracellular Ca²⁺ concentration.

The roles of vitamin D in skeletal muscle: form, function, and metabolism

Beyond its established role in bone and mineral homeostasis, there is emerging evidence that vitamin D exerts a range of effects in skeletal muscle. Reports of profound muscle weakness and changes in the muscle morphology of adults with vitamin D deficiency have long been described. These reports have been supplemented by numerous trials assessing the impact of vitamin D on muscle strength and mass and falls in predominantly elderly and deficient populations. At a basic level, animal models have confirmed that vitamin D deficiency and congenital aberrations in the vitamin D endocrine system may result in muscle weakness. To explain these effects, some molecular mechanisms by which vitamin D impacts on muscle cell differentiation, intracellular calcium handling, and genomic activity have been elucidated. There are also suggestions that vitamin D alters muscle metabolism, specifically its sensitivity to insulin, which is a pertinent feature in the pathophysiology of insulin resistance and type 2 diabetes. We will review the range of human clinical, animal, and cell studies that address the impact of vitamin D in skeletal muscle, and discuss the controversial issues. This is a vibrant field of research and one that continues to extend the frontiers of knowledge of vitamin D's broad functional repertoire.

TRP channels in normal and dystrophic skeletal muscle

TRP proteins constitute non-selective cation-permeable ion channels, most of which are permeable to Ca²⁺. In skeletal muscle, several isoforms of the TRPC (Canonical), TRPV (Vanilloid) and TRPM (Melastatin) subfamilies are expressed. In particular, TRPC1, C3 and C6, TRPV2 and V4, TRPM4 and TRPM7 have been consistently found in cultured myoblasts or in adult muscles. These channels seem to directly or indirectly respond to membrane stretch or to Ca²⁺ stores depletion; some isoforms might also constitute unregulated Ca²⁺ leak channels. Their function is largely unknown. TRPC1 and C3 have been involved in muscle development, in particular in myoblasts migration and differentiation. TRPC1 and V4 might allow a basal influx of Ca²⁺ at rest. Their lack has consequences on muscle fatigue. TRPV2 seems to be stretch-sensitive. It localizes mainly in intracellular pools at rest, and translocates to the plasma membrane upon IGF-1 stimulation. TRP channels seem to be involved in the pathophysiology of muscle disorders. In particular in Duchenne muscular dystrophy, the lack of the cytoskeletal protein dystrophin induces a disregulation of several ion channels leading to an abnormal influx of Ca²⁺. We discuss here, the possible involvement of TRP channels in this abnormal influx of Ca²⁺.

VDR activation of intracellular signaling pathways in skeletal muscle

The purpose of this article is to review the activation of signal transduction pathways in skeletal muscle cells by the hormone 1α,25(OH)(2)-vitamin D(3) [1α,25(OH)(2)D(3)], focusing on the role of the vitamin D receptor (VDR). The hormone induces fast, non transcriptional responses, involving stimulation of the transmembrane second messenger systems adenylyl cyclase/cAMP/PKA, PLC/DAG+IP(3)/PKC, Ca(2+) messenger system and MAPK cascades. Short treatment with 1α,25(OH)(2)D(3) induces reverse translocation of the VDR from the nucleus to plasma membranes. Accordingly, a complex is formed in the caveolae between the VDR and TRCP3, integral protein of capacitative Ca(2+) entry (CCE), suggesting an association between both proteins and a functional role of the VDR in 1α,25(OH)(2)D(3) activation of CCE. Stimulation of tyrosine phosphorylation cascades by 1α,25(OH)(2)D(3) have demonstrated the formation of complexes between Src and the VDR. Through these mechanisms, 1α,25(OH)(2)D(3) plays an important function in contractility and myogenesis.

TRPC3 cation channel plays an important role in proliferation and differentiation of skeletal muscle myoblasts

During membrane depolarization associated with skeletal excitation-contraction (EC) coupling, dihydropyridine receptor [DHPR, a L-type Ca(2+) channel in the transverse (t)-tubule membrane] undergoes conformational changes that are transmitted to ryanodine receptor 1 [RyR1, an internal Ca(2+)-release channel in the sarcoplasmic reticulum (SR) membrane] causing Ca(2+) release from the SR. Canonical-type transient receptor potential cation channel 3 (TRPC3), an extracellular Ca(2+)-entry channel in the t-tubule and plasma membrane, is required for full-gain of skeletal EC coupling. To examine additional role(s) for TRPC3 in skeletal muscle other than mediation of EC coupling, in the present study, we created a stable myoblast line with reduced TRPC3 expression and without alpha1((S))DHPR (MDG/TRPC3 KD myoblast) by knock-down of TRPC3 in alpha1((S))DHPR-null muscular dysgenic (MDG) myoblasts using retrovirus-delivered small interference RNAs in order to eliminate any DHPR-associated EC coupling-related events. Unlike wild-type or alpha1((S))DHPR-null MDG myoblasts, MDG/TRPC3 KD myoblasts exhibited dramatic changes in cellular morphology (e.g., unusual expansion of both cell volume and the plasma membrane, and multi-nuclei) and failed to differentiate into myotubes possibly due to increased Ca(2+) content in the SR. These results suggest that TRPC3 plays an important role in the maintenance of skeletal muscle myoblasts and myotubes.

Protein phosphatases: possible bisphosphonate binding sites mediating stimulation of osteoblast proliferation

We investigated the existence of a bisphosphonate (BP) target site in osteoblasts. Binding assays using [³H]-olpadronate ([³H]OPD) in whole cells showed the presence of specific, saturable and high affinity binding for OPD (K(d)=1.39 ± 0.33 μM) in osteoblasts. [³H]OPD was displaced from its binding site by micromolar concentrations of lidadronate, alendronate and etidronate (K(d)=1.42 ± 0.15 μM, 2.00 ± 0.2 μM and 2.4 ± 0.4 μM, respectively), and by millimolar concentrations of the non-permeant protein phosphatase (PP) substrates p-nitrophenylphosphate and α-naphtylphosphate. PP inhibitors orthovanadate, NaF or vpb(bipy) did not displace [³H]OPD. As expected, specific OPD binding was detected in the plasma membrane of ROS 17/2.8 cells, although significant BP binding was also found intracellularly. Moreover, OPD increased DNA synthesis in these cells with a temporal profile similar to the protein tyrosine phosphatase (PTP) inhibitors, Na₃VO₄ and vpb(bipy); but different from a general PP inhibitor (NaF). The stimulatory effect of OPD and PTP inhibitors on osteoblast proliferation was inhibited by the protein tyrosine kinase inhibitors genistein and geldanamycin. These results provide new evidence on the existence of a BP target in osteoblastic cells, presumably a PTP, which may be involved in the stimulatory action of BPs on osteoblast proliferation.

Expression of transient receptor potential (TRP) channels in human and murine osteoblast-like cells

The preservation of bone mass relies on adequate proliferation, differentiation, secretion of matrix proteins and rate of apoptosis of the bone-forming osteoblasts. Although growing body of evidence indicates that the transient receptor potential (TRP) channels play important roles in numerous cellular functions, limited information is available about the TRP channels in osteoblasts. Here, we inventoried the gene expression and addressed some roles of the TRP channels in various osteoblast-like cells. The transcripts of canonical TRP (TRPC) channels were revealed for TRPC1, TRPC3, TRPC4 and TRPC6 in human MG-63, SaOS and U2 OS osteoblasts while transcripts for TRPC2, TRPC4, TRPC6 and TRPC7 were observed in the murine MC3T3 osteoblasts. PCR products were shown for the melastatin-related TRP (TRPM) channels TRPM4, TRPM6, TRPM7 and TRPM8 in all cell lines. The TRPM1 was specifically expressed by murine MC3T3 cells while the TRPM3 transcripts were revealed solely in human osteoblast-like cells. Transcripts for TRPV2 and TRPV4 were shown in osteoblastic cells. By interfering RNA approaches, the TRPC1 channels in osteoblasts were shown to be responsible for the capacitative calcium entry (CCE) and for the stimulation of cell proliferation by platelet-derived growth factor. On the other hand, interfering RNA-mediated abrogation of the expression of TRPM7, known as calcium and magnesium channels, resulted in the reduction of both basal and growth factor-stimulated osteoblastic cell proliferation. Our results provide the first complete reference for the gene expression of TRP channels in osteoblasts and point to their importance in cell proliferation.

ATP modulation of mitogen activated protein kinases and intracellular Ca2+ in breast cancer (MCF-7) cells

In the breast tumor cell line MCF-7, extracellular nucleotides induce transient elevations in intracellular calcium concentration ([Ca(2+)](i)). In this study we show that stimulation with ATP or UTP sensitizes MCF-7 cells to mechanical stress leading to an additional transient Ca(2+) influx. ATP> or =ATPgamma-S> or =UTP>>>ADP=ADPbeta-S elevate [Ca(2+)](i), proving the presence of P2Y(2)/P2Y(4) purinergic receptor subtypes. In addition, cell stimulation with ATP, ATPgamma-S or UTP but not ADPbeta-S induced the phosphorylation of ERK1/2, p38 and JNK1/2 mitogen activated protein kinases (MAPKs). The use of Gd(3+), La(3+) or a Ca(2+)-free medium, inhibited ATP-dependent stress activated Ca(2+) (SAC) influx, but had no effect on MAPK phosphorylation. ATP-induced activation of MAPKs was diminished by two PI-PLC inhibitors and an IP(3) receptor antagonist. These results evidence an ATP-sensitive SAC influx in MCF-7 cells and indicate that phosphorylation of MAPKs by ATP is dependent on PI-PLC/IP(3)/Ca(2+)(i) release but independent of SAC influx in these cells, differently to other cell types.

Endothelin1-induced Ca(2+) mobilization is altered in calvarial osteoblastic cells of Cx43(+/- ) mice

During bone remodeling, osteoblastic (OB) cells have a central role leading to the production of extracellular matrix and its subsequent mineralization. As revealed by human physiopathologies, the OB differentiation process is essential for the control of calcium metabolism and normal bone formation. Moreover, accumulating data in the field of bone development suggest that connexin 43 (Cx43)-mediated gap junctional communication plays an important role in OB differentiation and function. Since Ca(2+) has a central role in OB physiology, the aim of the present study was to investigate the hypothetical involvement of Cx43 in OB calcium homeostasis. We performed measurements of intracellular calcium activity ([Ca(2+)]( i )) by a cytofluorimetric method using Fluo-4 as a calcium indicator and endothelin-1 (ET-1) as a physiological calcium-mobilizing factor on cultured OB cells isolated from calvaria of Cx43(+/-) and Cx43(+/+) mice. Partial deletion of the Cx43 gene induced a significant decrease in the [Ca(2+)]( i ) rise elicited by ET-1. This reduction was not correlated to a decrease or a modification of ET receptor subtype expression as assessed by real-time reverse-transcription polymerase chain reaction. Pharmacological investigations led us to demonstrate that the significant difference in [Ca(2+)]( i ) peak amplitude during the ET-1 action was associated with decreased calcium influx involving L-type voltage-sensitive calcium channels, whereas calcium release from intracellular stores and implication of phospholipase C were not affected by the reduced expression of Cx43. In conclusion, our data demonstrate for the first time that the Cx43 level of expression and/or function is able to modulate the [Ca(2+)]( i ) mobilization in OB cells.

Endothelin-1 inhibits human osteoblastic cell differentiation: Influence of connexin-43 expression level

Gap junctional intercellular communication (GJIC) permits coordinated cellular activities during developmental and differentiation processes. In bone, the involvement of the gap junctional protein, connexin-43 (Cx43), and of GJIC in osteoblastic differentiation and mineralization of the extracellular matrix has been previously demonstrated. Former studies have shown that endothelin-1 (ET-1) was also implicated in the control of osteoblastic proliferation and differentiation. However, depending on the cellular models, ET-1 has been shown to decrease or increase osteoblastic differentiation markers. As no data were available on the ET-1 effect on GJIC and Cx43 expression in osteoblastic cells, we analyzed here the possible crosstalk between Cx43 and ET-1 in a human cell line (hFOB 1.19) which displays different Cx43 expression levels and phenotypes when cultured at 33.5 or 39 degrees C. The presence of ET-1 (10(-8) M) for 2-12 days of culture did not significantly alter the proliferation rate of hFOB cells whatever their phenotype. In contrast, ET-1 induced a differential inhibitory effect on the biochemical differentiation markers (alkaline phosphatase activity and osteocalcin expression) with a significant reduction in the differentiated phenotype at 39 degrees C, whereas no effects were measured at 33.5 degrees C. The inhibitory effect was linked to a decrease of GJIC and of Cx43 both at transcriptional and protein levels. Altogether, our results suggest that Cx43 expression level could influence the action of ET-1 on human osteoblastic cell differentiation. Our data also indicate that the gap junctional protein could play a pivotal role in the response of osteoblasts to mitogenic factors implicated in bone pathologies.

Ca2+ channels and chronic hypoxia

Many chronic lung diseases are associated with prolonged exposure to alveolar hypoxia, resulting in the development of pulmonary hypertension. While the exact mechanisms underlying the pathogenesis of hypoxic pulmonary hypertension remain poorly understood, a key role for changes in Ca2+ homeostasis has emerged. Intracellular Ca2+ concentration controls a variety of pulmonary vascular cell functions, including contraction, gene expression, growth, barrier function and synthesis of vasoactive substances. Several studies indicate that prolonged exposure to hypoxia causes alterations in the expression and activity of several Ca2+ handling pathways in pulmonary arterial smooth muscle cells. In contrast, the effect of chronic hypoxia on Ca2+ homeostasis in pulmonary arterial endothelial cells is relatively unexplored. In this review, we discuss data from our laboratory and others describing the effects of prolonged hypoxia on pulmonary vascular smooth muscle and endothelial cell Ca2+ homeostasis and the various Ca2+ channels and handling pathways involved in these responses. We will also highlight future directions of investigation that might improve our understanding of the response of pulmonary vascular cells to chronic hypoxia.

Mechanism and functional significance of TRPC channel multimerization

Ca(2+) signaling regulates many important physiological events within a diverse set of living organisms. In particular, sustained Ca(2+) signals play an important role in controlling cell proliferation, cell differentiation and the activation of immune cells. Two key elements for the generation of sustained Ca(2+) signals are store-operated and receptor-operated Ca(2+) channels that are activated downstream of phospholipase C (PLC) stimulation, in response to G-protein-coupled receptor or growth factor receptor stimulation. One goal of this review is to help clarify the role of canonical transient receptor potential (TRPC) proteins in the formation of native store-operated and native receptor-operated channels. Toward that end, data from studies of endogenous TRPC proteins will be reviewed in detail to highlight the strong case for the involvement of certain TRPC proteins in the formation of one subtype of store-operated channel, which exhibits a low Ca(2+)-selectivity, in contrast to the high Ca(2+)-selectivity exhibited by the CRAC subtype of store-operated channel. A second goal of this review is to highlight the growing body of evidence indicating that native store-operated and native receptor-operated channels are formed by the heteromultimerization of TRPC subunits. Furthermore, evidence will be provided to argue that some TRPC proteins are able to form multiple channel types.

Modulation of ERK 1/2 and p38 MAPK signaling pathways by ATP in osteoblasts: involvement of mechanical stress-activated calcium influx, PKC and Src activation

There is evidence that extracellular nucleotides, acting through multiple P2 receptors, may play an important role in the regulation of bone metabolism by activating intracellular signaling cascades. We have studied the modulation of mitogen-activated protein kinase (MAPK) signaling pathways and its relationship to changes in intracellular calcium concentration ([Ca(2+)](i)) induced by ATP in ROS-A 17/2.8 osteoblastic cells. ATP and UTP (10 microM) increased [Ca(2+)](i) by cation release from intracellular stores. We have found that when the cells are subsequently subjected to mechanical stress (medium perturbation), a transient calcium influx occurs. This mechanical stress-activated calcium influx (MSACI) was not observed after ADP stimulation, indicating that P2Y(2) receptor activation is required for MSACI. In addition, ERK 1/2 and p38 MAPK were activated by ATP in a dose- and time-dependent manner. This activation was almost completely blocked using neomycin (2.5mM), an inhibitor of phosphoinositide-phospholipase C (PI-PLC), Ro 318220 (1 microM), a protein kinase C (PKC) inhibitor, and PP1 (50 microM), a potent and selective inhibitor of the Src-family tyrosine kinases. Ca(2+)-free extracellular medium (containing 0.5mM EGTA) and the use of gadolinium (5 microM), which suppressed MSACI, prevented ERK 1/2 and p38 phosphorylation by ATP. Altogether, these results represent the first evidence to date suggesting that P2Y(2) receptor stimulation by ATP in osteoblasts sensitizes mechanical stress activated calcium channels leading to calcium influx and a fast activation of the ERK 1/2 and p38 MAPK pathways. This effect also involves upstream mediators such as PI-PLC, PKC and Src family kinases.