Calcium regulates the PI3K-Akt pathway in stretched osteoblasts

Low-intensity ultrasound increases FAK, ERK-1/2, and IRS-1 expression of intact rat bones in a noncumulative manner

BACKGROUND

Low-intensity pulsed ultrasound stimulation (LIPUS) reportedly increases osteogenesis in fracture models but fails in intact bone, suggesting LIPUS does not act on mechanotransduction and growth factor pathways of intact bone.

QUESTIONS/PURPOSES

We asked whether daily 20-minute LIPUS applied to intact tibias would act on bone proteins involved in mechanotransduction (focal adhesion kinase [FAK], and extracellular signal-regulated kinase-1/2 [ERK-1/2]), and growth factor signaling (insulin receptor substrate-1 [IRS-1]) pathways at 7, 14, and 21 days of treatment.

METHODS

Immunoblotting was performed to detect FAK, ERK-1/2, and IRS-1 expression and activation from the stimulated intact tibias at 7, 14, and 21 days of daily 20-minute LIPUS.

RESULTS

LIPUS increased FAK expression (at 7 days), ERK-1/2 (at 14 days), and IRS-1 (at 7 days), but expression decreased 7 days later, indicating a noncumulative effect of LIPUS. As only FAK expression was detected at 21 days, these observations suggest LIPUS influences nuclear reactions that may be modulated by a major cellular mechanism preferentially inhibiting IRS-1 expression and not FAK expression. Increased ERK-1/2 expression at 14 days suggests the differing mechanisms for promoting ERK-1/2, FAK, and IRS-1 syntheses. IRS-1 expression behaved similarly to FAK expression; therefore, LIPUS may modulate growth factor pathways. LIPUS increased sustained FAK and ERK-1/2 activation, but not IRS-1, suggesting sustained ERK-1/2 activation is not the result of mechanically induced growth factor activation.

CONCLUSIONS

LIPUS acts on mechanotransduction and growth factor pathways in intact bone in a noncumulative manner. Clinical relevance These data suggest LIPUS applied to intact bone acts on proteins involved in osteogenesis.

Cyclic mechanical stretch decreases cell migration by inhibiting phosphatidylinositol 3-kinase- and focal adhesion kinase-mediated JNK1 activation

Epithelial cell migration during wound healing requires coordinated signaling pathways that direct polarization of the leading and trailing ends of the cells, cytoskeletal organization, and remodeling of focal adhesions. These inherently mechanical processes are disrupted by cyclic stretch (CS), but the specific signaling molecules involved in this disruption are not well understood. In this study, we demonstrate that inhibition of phosphatidylinositol 3-kinase (PI3K) or expression of a dominant-negative form of PI3K caused inhibition of airway epithelial cell wound closure. CS caused a sustained decrease in activation of PI3K and inhibited wound healing. Expression of constitutively active PI3K stimulated translocation of Tiam1 to the membrane, increased Rac1 activity, and increased wound healing of airway epithelial cells. Increased Rac1 activity resulted in increased phosphorylation of JNK1. PI3K activation was not regulated by association with focal adhesion kinase. Restoration of efficient cell migration during CS required coexpression of constitutively active PI3K, focal adhesion kinase, and JIP3.

Mechano-transduction in osteoblastic cells involves strain-regulated estrogen receptor alpha-mediated control of insulin-like growth factor (IGF) I receptor sensitivity to Ambient IGF, leading to phosphatidylinositol 3-kinase/AKT-dependent Wnt/LRP5 receptor-independent activation of beta-catenin signaling

The capacity of bones to adjust their mass and architecture to withstand the loads of everyday activity derives from the ability of their resident cells to respond appropriately to the strains engendered. To elucidate the mechanisms of strain responsiveness in bone cells, we investigated in vitro the responses of primary mouse osteoblasts and UMR-106 osteoblast-like cells to a single period of dynamic strain. This stimulates a cascade of events, including activation of insulin-like growth factor I receptor (IGF-IR), phosphatidylinositol 3-kinase-mediated phosphorylation of AKT, inhibition of GSK-3β, increased activation of β-catenin, and associated lymphoid-enhancing factor/T cell factor-mediated transcription. Initiation of this pathway does not involve the Wnt/LRP5/Frizzled receptor and does not culminate in increased IGF transcription. The effect of strain on IGF-IR is mimicked by exogenous des-(1–3)IGF-I and is blocked by the IGF-IR inhibitor H1356. Inhibition of strain-related prostanoid and nitric oxide production inhibits strain-related (and basal) AKT activity, but their separate ectopic administration does not mimic it. Strain-related IGF-IR activation of AKT requires estrogen receptor α (ERα) with which IGF-1R physically associates. The ER blocker ICI 182,780 increases the concentration of des-(1–3)IGF-I necessary to activate this cascade, whereas estrogen inhibits both basal AKT activity and its activation by des-(1–3)IGF-I. These data suggest an initial cascade of strain-related events in osteoblasts in which strain activates IGF-IR, in association with ERα, so initiating phosphatidylinositol 3-kinase/AKT-dependent activation of β-catenin and altered lymphoid-enhancing factor/T cell factor transcription. This cascade requires prostanoid/nitric oxide production and is independent of Wnt/LRP5.

TRPV4 channels mediate cyclic strain-induced endothelial cell reorientation through integrin-to-integrin signaling

Cyclic mechanical strain produced by pulsatile blood flow regulates the orientation of endothelial cells lining blood vessels and influences critical processes such as angiogenesis. Mechanical stimulation of stretch-activated calcium channels is known to mediate this reorientation response; however, the molecular basis remains unknown. Here, we show that cyclically stretching capillary endothelial cells adherent to flexible extracellular matrix substrates activates mechanosensitive TRPV4 (transient receptor potential vanilloid 4) ion channels that, in turn, stimulate phosphatidylinositol 3-kinase-dependent activation and binding of additional beta1 integrin receptors, which promotes cytoskeletal remodeling and cell reorientation. Inhibition of integrin activation using blocking antibodies and knock down of TRPV4 channels using specific small interfering RNA suppress strain-induced capillary cell reorientation. Thus, mechanical forces that physically deform extracellular matrix may guide capillary cell reorientation through a strain-dependent "integrin-to-integrin" signaling mechanism mediated by force-induced activation of mechanically gated TRPV4 ion channels on the cell surface.

Different magnitudes of tensile strain induce human osteoblasts differentiation associated with the activation of ERK1/2 phosphorylation

Mechanical factors are related to periprosthetic osseointegration following total hip arthroplasty. However, osteoblast response to strain in implanted femurs is unclear because of the absence of accurate stress-measuring methods. In our study, finite element analysis was performed to calculate strain distribution in implanted femurs. 0.8-3.2% tensile strain was then applied to human osteoblasts. Higher magnitudes of strain enhanced the expression of osteocalcin, type I collagen, and Cbfa1/Runx2. Lower magnitudes significantly increased ALP activity. Among these, type I collagen expression increased with the activation of ERK1/2 phosphorylation in a strain-magnitude-dependent manner. Our study marks the first investigation of osteoblast response at different magnitudes of periprosthetic strain. The results indicate that the functional status of human osteoblasts is determined by strain magnitude. The strain distribution in the proximal region of implanted femur should be improved for osseointegration.

FOXO transcription factors are mechanosensitive and their regulation is altered with aging in the respiratory pump

The mechanical regulation of the forkhead box O (FOXO) subclass of transcription factors in the respiratory pump and its implication in aging are completely unknown. We investigated the effects of diaphragm stretch on three FOXO isoforms, Foxo1, Foxo3a, and Foxo4, in normal mice at different ages. We tested the hypotheses that 1) FOXO activities are regulated in response to diaphragm stretch and 2) mechanical properties of aging diaphragm are altered, leading to altered regulation of FOXO with aging. Our results showed that stretch downregulated FOXO DNA-binding activity by a mechanism that required Akt and IKK activation in young mice but that these pathways lost their mechanosensitivity with age. This aberrant regulation of FOXO with aging was associated with altered viscoelasticity, compliance, and extensibility of the aged diaphragm. Curiously, the dramatic decrease of the nuclear content of Foxo1 and Foxo3a, the two isoforms associated with muscle atrophy, with aging correlated with higher basal activation of Akt and IKK signaling in diaphragms of old mice. In contrast, the stability of Foxo4 in the nucleus became dependent on JNK, which is strongly activated in aged diaphragm. This finding suggests that Foxo4 was responsible for the FOXO-dependent transcriptional activity in aging diaphragm. Our data support the hypothesis that aging alters the mechanical properties of the respiratory pump, leading to altered mechanical regulation of the stretch-induced signaling pathways controlling FOXO activities. Our study supports a mechanosensitive signaling mechanism that is responsible for regulation of the FOXO transcription factors by aging.

Capacitative calcium entry and proliferation of human osteoblast-like MG-63 cells

Adult bone tissue is continuously being remodelled and bone mass is maintained by a balance between osteoclastic bone resorption and osteoblastic bone formation. Alteration of osteoblastic cell proliferation may account in part for lack of balance between these two processes in bone loss of osteoporosis. There is calcium (Ca2+) control in numerous cellular functions; however, involvement of capacitative Ca2+ entry (CCE) in proliferation of bone cells is less well investigated.

OBJECTIVES

The study described here was aimed to investigate roles of CCE in the proliferation of osteoblast-like MG-63 cells.

MATERIALS AND METHODS

Pharmacological characterizations of CCE were undertaken in parallel, with evaluation of the expression of transient receptor potential canonical (TRPC) channels and of cell proliferation.

RESULTS

Intracellular Ca2+ store depletion by thapsigargin induced CCE in MG-63 cells; this was characterized by a rapid transient increase of intracellular Ca2+ followed by significant CCE, induced by conditions that stimulated cell proliferation, namely serum and platelet-derived growth factor. Inhibitors of store-operated Ca2+ channels (2-APB and SKF-96365) prevented CCE, while voltage-dependent Ca2+ channel blockers had no effect. Expression of various TRPC channels was shown in the cells, some having been shown to be responsible for CCE. Voltage-dependent Ca2+ channel blockers had no effect on osteoblast proliferation while thapsigargin, 2-APB and SKF-96395, inhibited it. Cell cycle analysis showed that 2-APB and SKF-96395 lengthen the S and G2/M phases, which would account for the reduction in cell proliferation.

CONCLUSIONS

Our results indicate that CCE, likely attributed to the activation of TRPCs, might be the main route for Ca2+ influx involved in osteoblast proliferation.

Inhibition of phosphatidylinositol 3-kinase causes cell death in rat osteoblasts through inactivation of Akt

Previous evidences indicated that phosphatidylinositol 3-kinase (PI3-kinase) is an important regulatory molecule that is involved in the cell growth and survival, and inhibition of the PI3-kinase activity enhances apoptotic cell death. However, the relationship between PI3-kinase activity and osteoblasts, capable of new bone formation, remained unknown. In the present study, pharmacological inhibitor of PI3-kinase LY294002 was used to observe the role of the PI3-kinase in the growth of rat osteoblasts. To identify its molecular mechanism, Western blots analysis and immunocytochemistry were applied to examine changes of Akt phosphorylation and its distribution. Our data showed that inhibition of PI3-kinase activity significantly triggered the decrease of cell growth, cell apoptosis and loss of mitochondrial membrane potential (Deltapsi(m)). Osteoblastic dysfunction stimulated by LY294002 was accompanied by inactivation of Akt and its redistribution. In all these results demonstrated that inhibition of PI3-kinase induced apoptotic cell death, which was mediated by inactivation of Akt pathway in rat osteoblasts.

Cyclic Pressure Stimulates DNA Synthesis through the PI3K/Akt Signaling Pathway in Rat Bladder Smooth Muscle Cells

Previous studies demonstrated that the bladder exhibited severe tissue remodeling following spinal cord injury. In such pathological bladders, uninhibited non-voiding contractions subject bladder cells to cyclic oscillations of intravesical pressure. We hypothesize that cyclic pressure is a potential trigger for tissue remodeling in overactive bladder. Using a custom-made setup, rat bladder smooth muscle cells (SMC) in vitro were exposed to cyclic hydrostatic pressure (40 cm H2O) at either 0.1 Hz or 0.02 Hz frequency for up to 24 h. When compared to static control and cells exposed to 0.02-Hz cyclic pressure, SMC exposed to 0.1-Hz cyclic pressure contained significantly (p < 0.05) higher amounts of DNA. We confirmed that the increase in DNA was due to increased cell proliferation, indicated by increased BrdU incorporation, but not due to decreased apoptosis rates in response to cyclic pressure. In addition, significant (p < 0.05) elevation of Akt phosphorylation in SMC following exposure to cyclic pressure and lack of pressure-induced SMC hyperplasia in the presence of PI3K inhibitors, wortmannin and LY294002, indicated the involvement of the PI3K/Akt pathway in the proliferative response of SMC to cyclic pressure. We concluded that chronic exposure to intravesical pressure oscillation may be a potential trigger for bladder tissue remodeling.

Phosphoinositide 3-kinase, Src, and Akt modulate acute ventilation-induced vascular permeability increases in mouse lungs

To determine the role of phosphoinositide 3-OH kinase (PI3K) pathways in the acute vascular permeability increase associated with ventilator-induced lung injury, we ventilated isolated perfused lungs and intact C57BL/6 mice with low and high peak inflation pressures (PIP). In isolated lungs, filtration coefficients (K(f)) increased significantly after ventilation at 30 cmH(2)O (high PIP) for successive periods of 15, 30 (4.1-fold), and 50 (5.4-fold) min. Pretreatment with 50 microM of the PI3K inhibitor, LY-294002, or 20 microM PP2, a Src kinase inhibitor, significantly attenuated the increase in K(f), whereas 10 microM Akt inhibitor IV significantly augmented the increased K(f). There were no significant differences in K(f) or lung wet-to-dry weight (W/D) ratios between groups ventilated with 9 cmH(2)O PIP (low PIP), with or without inhibitor treatment. Total lung beta-catenin was unchanged in any low PIP isolated lung group, but Akt inhibition during high PIP ventilation significantly decreased total beta-catenin by 86%. Ventilation of intact mice with 55 cmH(2)O PIP for up to 60 min also increased lung vascular permeability, indicated by increases in lung lavage albumin concentration and lung W/D ratios. In these lungs, tyrosine phosphorylation of beta-catenin and serine/threonine phosphorylation of Akt, glycogen synthase kinase 3beta (GSK3beta), and ERK1/2 increased significantly with peak effects at 60 min. Thus mechanical stress activation of PI3K and Src may increase lung vascular permeability through tyrosine phosphorylation, but simultaneous activation of the PI3K-Akt-GSK3beta pathway tends to limit this permeability response, possibly by preserving cellular beta-catenin.

Differential activation of the PI 3-kinase effectors AKT/PKB and p70 S6 kinase by compound 48/80 is mediated by PKCα

The secretagogue compound 48/80 (c48/80) is a well known activator of calcium mediated processes and PKCs, and is a potent inducer of mast cell degranulation. As the latter process is a phosphoinositide 3-kinase (PI 3-kinase) mediated event, we wished to address whether or not c48/80 was an activator of PI 3-kinases. The data presented here reveal that c48/80 is an effective activator of PI 3-kinases as judged by the increased phosphorylation of PKB and p70(S6K) in fibroblasts in a PI 3-kinase dependent fashion. Compound 48/80 effectively translocates PKB to the plasma membrane and induces phosphorylation at serine 473 (S473), detected by fluorescence imaging of fixed cells. At higher concentrations the secretagogue is inhibitory towards PKB phosphorylation on S473. Conversely, p70(S6K) phosphorylation on T389 is unaffected at high doses. We provide evidence that the differential effect on the two PI 3-kinase effectors is due to activation of PKCalpha by c48/80, itself a PI 3-kinase dependent process. We conclude that compound 48/80 is an effective activator of PI 3-kinase dependent pathways, leading to the activation of effectors including PKB/Akt, p70(S6K) and PKCalpha. The latter is only activated by higher doses of c48/80 resulting in an inhibition of the c48/80 induced PKB phosphorylation, thus explaining the observed biphasic activation profile for PKB in response to this secretagogue.

Phosphatidylinositol 3-kinase/Akt auto-regulates PDGF-BB-stimulated interleukin-6 synthesis in osteoblasts

It has been reported that platelet-derived growth factor (PDGF)-BB stimulates the synthesis of interleukin (IL)-6 in osteoblasts. In the present study, we investigated whether the phosphatidylinositol 3-kinase (PI3K)/Akt is involved in the PDGF-BB-induced IL-6 synthesis in osteoblast-like MC3T3-E1 cells. PDGF-BB markedly induced the phosphorylation of Akt and GSK-3beta. Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-O-octadecylcarbonate, significantly amplified the synthesis of IL-6 by PDGF-BB. The PDGF-BB-induced GSK-3beta phosphorylation was suppressed by the Akt inhibitor. The IL-6 synthesis stimulated by PDGF-BB was markedly enhanced by LY294002 and wortmannin, inhibitors of PI3K. Wortmannin and LY294002 suppressed the PDGF-BB-induced phosphorylation of Akt and GSK-3beta. Taken together, these results strongly suggest that PI3K/Akt negatively regulates the PDGF-BB-stimulated IL-6 synthesis in osteoblasts.

Regulation of Foxo-1 and the angiopoietin-2/Tie2 system by shear stress

Transcription factor Foxo-1 can be inactivated via Akt-mediated phosphorylation. Since shear stress activates Akt, we determined whether Foxo-1 and the Foxo-1-dependent, angiogenesis-related Ang-2/Tie2-system are influenced by shear stress in endothelial cells. Expression of Foxo-1 and its target genes p27Kip1 and Ang-2 was decreased under shear stress (6dyn/cm(2), 24h), nuclear exclusion of Foxo-1 by phosphorylation increased. eNOS and Tie2 were upregulated. No effects on Ang-1 expression were detected. In conclusion, Foxo-1 and Ang-2/Tie2 are part of the molecular response to shear stress, which may regulate angiogenesis.

Calcium signalling and calcium transport in bone disease

Calcium transport and calcium signalling mechanisms in bone cells have, in many cases, been discovered by study of diseases with disordered bone metabolism. Calcium matrix deposition is driven primarily by phosphate production, and disorders in bone deposition include abnormalities in membrane phosphate transport such as in chondrocalcinosis, and defects in phosphate-producing enzymes such as in hypophosphatasia. Matrix removal is driven by acidification, which dissolves the mineral. Disorders in calcium removal from bone matrix by osteoclasts cause osteopetrosis. On the other hand, although bone is central to management of extracellular calcium, bone is not a major calcium sensing organ, although calcium sensing proteins are expressed in both osteoblasts and osteoclasts. Intracellular calcium signals are involved in secondary control including cellular motility and survival, but the relationship of these findings to specific diseases is not clear. Intracellular calcium signals may regulate the balance of cell survival versus proliferation or anabolic functional response as part of signalling cascades that integrate the response to primary signals via cell stretch, estrogen, tyrosine kinase, and tumor necrosis factor receptors.

Mechanisms of Disease: autosomal dominant and recessive polycystic kidney diseases

Autosomal dominant polycystic kidney disease and autosomal recessive polycystic kidney disease are the best known of a large family of inherited diseases characterized by the development of renal cysts of tubular epithelial cell origin. Autosomal dominant and recessive polycystic kidney diseases have overlapping but distinct pathogeneses. Identification of the causative mutated genes and elucidation of the function of their encoded proteins is shedding new light on the mechanisms that underlie tubular epithelial cell differentiation. This review summarizes recent literature on the role of primary cilia, intracellular calcium homeostasis, and signaling involving Wnt, cyclic AMP and Ras/MAPK, in the pathogenesis of polycystic kidney disease. Improved understanding of pathogenesis and the availability of animal models orthologous to the human diseases provide an excellent opportunity for the development of pathophysiology-based therapies. Some of these have proven effective in preclinical studies, and clinical trials have begun.

Wnt/beta-catenin signaling is a normal physiological response to mechanical loading in bone

A preliminary expression profiling analysis of osteoblasts derived from tibia explants of the high bone mass LRP5 G171V transgenic mice demonstrated increased expression of canonical Wnt pathway and Wnt/beta-catenin target genes compared with non-transgenic explant derived osteoblasts. Therefore, expression of Wnt/beta-catenin target genes were monitored after in vivo loading of the tibia of LRP5 G171V transgenic mice compared with non-transgenic mice. Loading resulted in the increased expression of Wnt pathway and Wnt/beta-catenin target genes including Wnt10B, SFRP1, cyclin D1, FzD2, WISP2, and connexin 43 in both genotypes; however, there was a further increased in transcriptional response with the LRP5 G171V transgenic mice. Similar increases in the expression of these genes (except cyclin D1) were observed when non-transgenic mice were pharmacologically treated with a canonical Wnt pathway activator, glycogen synthase kinase 3beta inhibitor and then subjected to load. These in vivo results were further corroborated by in vitro mechanical loading experiments in which MC3T3-E1 osteoblastic cells were subjected to 3400 microstrain alone for 5 h, which increased the expression of Wnt10B, SFRP1, cyclin D1, FzD2, WISP2, and connexin 43. Furthermore, when MC3T3-E1 cells were treated with either glycogen synthase kinase 3beta inhibitor or Wnt3A to activate Wnt signaling and then subjected to load, a synergistic up-regulation of these genes was observed compared with vehicle-treated cells. Collectively, the in vivo and in vitro mechanical loading results support that Wnt/beta-catenin signaling is a normal physiological response to load and that activation of the Wnt/beta-catenin pathway enhances the sensitivity of osteoblasts/osteocytes to mechanical loading.

Arachidonic acid influences intracellular calcium handling in human osteoblasts

The effect of arachidonic acid (AA) on intracellular Ca(2+) concentration ([Ca(2+)]i) in human osteoblasts MG63 was studied. AA caused a concentration-dependent increase in [Ca(2+)]i, mainly due to inward Ca(2+) transport from extracellular environment. Moreover, AA in Ca(2+) -free medium produced a small, transient increase of [Ca(2+)]i, indicating that AA may also trigger Ca(2+) release from intracellular stores. Because the [Ca(2+)]i response to AA was inhibited by the cyclooxygenase (COX) inhibitor indomethacin, we tested the effect of prostaglandins (PGs), products of COX pathway. PGs E1 and E2 caused an increase in [Ca(2+)]i, which, however, was far lower than that obtained with AA. The [Ca(2+)]i response to AA was not inhibited by nifedipine, suggesting that AA did not activate a voltage-dependent Ca(2+) channel. Our results indicate that AA could modulate [Ca(2+)]i in MG63 human osteoblasts, where it may influence Ca(2+) transport across both plasma and endoplasmic membranes. Furthermore, they suggest that osteoblast activity may be modulated by AA.

Global gene expression analysis in the bones reveals involvement of several novel genes and pathways in mediating an anabolic response of mechanical loading in mice

To identify the genes and signal pathways responsible for mechanical loading-induced bone formation, we evaluated differential gene expression on a global basis in the tibias of C57BL/6J (B6) mice after four days of four-point bending. We applied mechanical loads to the right tibias of the B6 mice at 9 N, 2 Hz for 36 cycles per day, with the left tibias used as unloaded controls. RNA from the tibias was harvested 24 h after last stimulation and subjected to microarray. Of the 20,280 transcripts hybridized to the array, 346 were differentially expressed in the loaded bones compared to the controls. The validity of the microarray data was established with the increased expression of bone-related genes such as pleiotrophin, osteoglycin, and legumain upon four-point bending and confirmation of increased expression of selected genes by real-time PCR. The list of differentially expressed genes includes genes involved in cell growth, differentiation, adhesion, proteolysis, as well as signaling molecules of receptors for growth factors, integrin, Ephrin B2, endothelin, and adhesion G protein coupled receptor. Pathway analyses suggested that 28 out of the 346 genes exhibited a direct biological association. Among the biological network, fibronectin and pleitrophin function as important signaling molecules in regulating periosteal bone formation and resorption in response to four-point bending. Furthermore, some expressed sequence tags (ESTs) with no prior known function have been identified as potential mediators of mechanotransduction signaling pathways. Further studies on these previously unknown genes will improve our understanding of the molecular pathways and mechanisms involved in bone's response to mechanical stress.

Bi-phasic activation of eNOS in response to uni-axial cyclic stretch is mediated by differential mechanisms in BAECs

We investigated the signaling mechanism of stretch-induced NO (Nitric oxide) production in bovine arterial endothelial cells (BAECs). BAECs cultured on an elastic silicone chamber coated with fibronectin were subjected to uni-axial cyclic stretch (1 Hz, 20% in length) and the amount of produced NO was measured by a cGMP assay. NO production increased in a bi-phasic manner and peaked at 5 min and 20 min after stretch onset. Correspondingly, the activities of endothelial nitric oxide synthase (eNOS) and Akt/PKB (measured by phosphorylation at serine 1,177 and serine 473, respectively), showed two peaks over time. Application of Gd(3+), a potent SA channel blocker, and depletion of external Ca(2+) exclusively inhibited the first peaks of eNOS and Akt activity, but exerted little effect on the second peak. On the other hand, the PI3K inhibitors, Wortmannin, LY294002, almost completely inhibited the second peak but not the first. These results suggest that up-regulation of eNOS in response to cyclic stretch was mediated by two distinct pathways, [Ca(2+)](i) increases via the SA channel in an early phase (partially Akt/PKB), and PI3K-Akt/PKB pathways in a late phase.

Phosphatidylinositol 3-kinase/Akt plays a role in sphingosine 1-phosphate-stimulated HSP27 induction in osteoblasts

We previously reported that p38 mitogen-activated protein (MAP) kinase plays a part in sphingosine 1-phosphate-stimulated heat shock protein 27 (HSP27) induction in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) is involved in the induction of HSP27 in these cells. Sphingosine 1-phosphate time dependently induced the phosphorylation of Akt. Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-O-octadecylcarbonate, reduced the HSP27 induction stimulated by sphingosine 1-phosphate. The sphingosine 1-phosphate-induced phosphorylation of GSK-3beta was suppressed by Akt inhibitor. The sphingosine 1-phosphate-induced HSP27 levels were attenuated by LY294002 or wortmannin, PI3K inhibitors. Furthermore, LY294002 or Akt inhibitor did not affect the sphingosine 1-phosphate-induced phosphorylation of p38 MAP kinase and SB203580, a p38 MAP kinase inhibitor, had little effect on the phosphorylation of Akt. These results suggest that PI3K/Akt plays a part in the sphingosine 1-phosphate-stimulated induction of HSP27, maybe independently of p38 MAP kinase, in osteoblasts.

Roles of MAP kinases in the regulation of bone matrix gene expressions in human osteoblasts by oscillatory fluid flow

We investigated the effects of oscillatory flow in regulating the gene expressions of type I collagen (COL1, the main component of human bone tissues) and osteopontin (OPN, the key gene for calcium deposition) in human osteoblast-like (MG-63) cells, and the roles of mitogen-activated protein kinases (MAPKs) in this regulation. The cells were subjected to oscillatory flow (0.5 +/- 4 dyn/cm(2)) or kept under static condition for various time periods (15 min, 30 min, 1 h, 2 h, 4 h, 8 h, and 16 h). Oscillatory flow caused significant up-regulations of both COL1 and OPN gene expressions over the 16 h of study, and a transient activation of MAPKs was starting at 15 min and declining to basal level in 2 h. The flow-induction of COL1 was blocked by an ERK inhibitor (PD98059) and reduced by a JNK inhibitor (SP600125), whereas that of OPN was abolished by PD98059. Analysis of the cis-elements in the COL1 and OPN promoters suggests the involvement of transacting factors Elk-1 and AP-1 in the transcription regulation. The ERK inhibitor (PD98059) blocked Elk-1 phosphorylation, as well as COL1 and OPN gene expression. The JNK inhibitor (SP600125) abolished c-jun phosphorylation and COL1 expression. These results suggest that the flow-induction of OPN was mediated through the ERK-Elk1-OPN pathway, and that COL1 was regulated by both the ERK-Elk1-COL1 and JNK-c-JUN-COL1 pathway.

The pilus and porin of Neisseria gonorrhoeae cooperatively induce Ca2+ transients in infected epithelial cells

Purified pili and porin from Neisseria quickly mobilize calcium (Ca(2+)) stores in monocytes and epithelial cells, ultimately influencing host cell viability as well as bacterial intracellular survival. Here, we examined the Ca(2+) transients induced in human epithelial cells during infection by live, piliated N. gonorrhoeae. Porin induced an influx of Ca(2+) from the extracellular medium less than 60 s post infection. The porin-induced transient is followed by a pilus-induced release of Ca(2+) from intracellular stores. The timing of these events is similar to that observed using purified proteins. Interestingly, the porin-induced Ca(2+) flux is required for the pilus-induced transient, indicating that the pilus-induced Ca(2+) release is, itself, Ca(2+) dependent. Several lines of evidence indicate that porin is present on pili. Moreover, pilus retraction strongly influences the porin- and pilus-induced Ca(2+) fluxes. These and other results strongly suggest that the pilus and porin cooperate to modulate calcium signalling in epithelial cells, and propose a model to explain how N. gonorrhoeae triggers Ca(2+) transients in the initial stages of pilus-mediated attachment.

Identification of Pip4k2beta as a mechanical stimulus responsive gene and its expression during musculoskeletal tissue healing

To investigate the mechano-transduction system of cells, we identified genes responsive to a cyclic mechanical stimulus. MC3T3.E1 cells were cultured on a computer-controlled vacuum-pump-operated device designed to provide a cyclic mechanical stimulus. A maximum elongation of 15% of membrane at 10 cycles/min (3 s extension followed by 3 s relax per cycle) was repeated for 48 h. By means of a differential display, the gene expression pattern of cells exposed to the stimulus was compared with that of unexposed cells. As a result, a gene fragment that was exclusively expressed in mechanically stressed cells was identified. By using expressed sequence tag walking together with the oligo-capping method, this gene was identified as phosphatidylinositol 4-phosphate 5-kinase type II beta (initially known as Pip5k2beta but now reclassified as Pip4k2beta). The specific up-regulation of Pip4k2beta upon mechanical stimulus was also confirmed by using another apparatus, viz. a computer-controlled linearized-stepping motor system. To examine the involvement of the cyclic mechanical stimulus in the regulation of Pip4k2beta expression in musculoskeletal tissue, we created an Achilles tendon transection model in rabbits. The temporal expression of Pip4k2beta was assessed by means of a quantitative reverse-transcribed polymerase chain reaction. In the gastrocnemius muscle, expression of Pip4k2beta rapidly decreased 1 week after transection but was restored to normal levels at 4 weeks. In the Achilles tendon, however, expression remained decreased until 4 weeks after transection. We suggest that the expression of Pip4k2beta can be used as a marker for cells receiving a suitable mechanical stimulus.

Calcium oscillation and phosphatidylinositol 3-kinase positively regulate integrin alpha(IIb)beta3-mediated outside-in signaling

The frequency of calcium oscillation reveals the platelet activation status, however, the biological significance of the periodic calcium responses and methods of communication with other integrin-mediated signals are not clear. RGD-containing disintegrin rhodostomin coated substrates were employed to enhance platelet spreading and calcium oscillation through direct binding and clustering of the receptor integrin alpha(IIb)beta3. The results showed that the activation of phosphatidylinositol 3-kinase (PI3-K) and internal calcium pathways were crucial for alpha(IIb)beta3 outside-in signaling. PI3-K antagonists wortmannin and LY294002 inhibited disintegrin substrates and induced platelet spreading and calcium oscillation. At the same time, pretreatment of platelets with the microsomal calcium-ATPase inhibitor thapsigargin to deplete internal calcium stores severely impaired the calcium oscillation as well as PI3-K activation and spreading on disintegrin substrates. Because inhibition of one pathway could inhibit the other, our data indicates that PI3-K and calcium oscillation are synergistically operated and form a positive-feedback regulation in integrin alpha(IIb)beta3-mediated outside-in signaling.

Traumatic injury activates protein kinase B/Akt in cultured astrocytes: role of extracellular ATP and P2 purinergic receptors

Protein kinase B/Akt is a key signaling molecule that regulates cell survival, growth, and metabolism, and inhibits apoptosis. Traumatic brain injury (TBI) activates Akt, and Akt has been implicated in neuronal survival after TBI, but little is known about injury-induced Akt activation in astrocytes, cells that exhibit hypertrophic and hyperplastic responses to CNS injury. Here we have investigated the effect of mechanical strain on Akt activation in primary cultures of rat cortical astrocytes growing on deformable Silastic membranes. When astrocytes were subjected to mechanical strain (50 msec; 5-7.5 mm displacement), we observed an increase in phosphorylation of serine 473, a key indicator of Akt activation. Akt phosphorylation was increased at 3 min postinjury, was maximal from 5 to 10 min, and declined gradually thereafter. Akt activation was also dependent on the severity of the injury. Stretch-induced Akt phosphorylation was attenuated by blocking calcium influx and phosphoinositide 3-kinase (PI3K), an upstream activator of Akt. In addition, we found that ATP is rapidly released after mechanical strain and that the P2 purinergic receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',5'disulfonate (PPADS) attenuated trauma-induced Akt activation. We conclude that mechanical strain causes activation of Akt in astrocytes via stimulation of P2 receptors. This suggests that P2 receptor/Akt signaling promotes astrocyte survival and growth, and this process may play a role in the generation of reactive gliosis after TBI.

Akt mediates mechanical strain-induced collagen production by mesangial cells

Increased glomerular hydrostatic pressure is an important determinant of glomerulosclerosis and can be modeled by in vitro exposure of mesangial cells to cyclic mechanical strain. Stretched mesangial cells increase extracellular matrix protein production, the hallmark of glomerulosclerosis. Recent data indicate that the serine/threonine kinase Akt may be involved in matrix modulation. Thus, Akt activation and matrix synthesis in stretched mesangial cells were studied. Exposure of mesangial cells to 1 Hz cyclic strain led to prompt Akt activation, which was biphasic to 24 h. Activation was dependent on signaling through phosphatidylinositol-3-kinase and required EGF receptor transactivation. Inhibition of signaling through the PDGF receptor, Src kinase, or cytoskeletal disruption failed to prevent strain-induced Akt activation. Collagen type 1A1 transcript expression, promoter activation, and protein secretion were increased by stretch at 24 h and were dependent on phosphatidylinositol-3 kinase. Overexpression of dominant-negative Akt inhibited strain-induced collagen 1A1 production. Conversely, overexpression of constitutively active Akt led to increased collagen 1A1 upregulation and secretion. Finally, Akt activation was observed in the glomeruli of remnant rat kidneys, a model marked by increased intraglomerular pressure. The authors conclude that mechanical strain induces Akt activation in mesangial cells through a mechanism requiring phosphatidylinositol-3-kinase and EGF receptor transactivation. Type 1 collagen production is dependent on Akt and can be induced by Akt overexpression. Akt activation is observed in remnant kidneys in vivo. Thus, the role of Akt in progression of chronic hemodynamic glomerular disease is worthy of further exploration.

Molecular Mechanism of Apoptosis Induced by Mechanical Forces

In all biological systems, a balance between cell proliferation/growth and death is required for normal development as well as for adaptation to a changing environment. To affect their fate, it is essential for cells to integrate signals from the environment. Recently, it has been recognized that physical forces such as stretch, strain, and tension play a critical role in regulating this process. Despite intensive investigation, the pathways by which mechanical signals are converted to biochemical responses is yet to be completely understood. In this review, we will examine our current understanding of how mechanical forces induce apoptosis in a variety of biological systems. Rather than being a degenerative event, physical forces act through specific receptor-like molecules such as integrins, focal adhesion proteins, and the cytoskeleton. These molecules in turn activate a limited number of protein kinase pathways (p38 MAPK and JNK/SAPK), which amplify the signal and activate enzymes (caspases) that promote apoptosis. Physical forces concurrently activate other signaling pathways such as PIK-3 and Erk 1/2 MAPK, which modulate the apoptotic response. The cell phenotype and the character of the physical stimuli determine which pathways are activated and, consequently, allow for variability in response to a specific stimulus in different cell types.

Mechanotransduction by integrin is essential for IL-6 secretion from endothelial cells in response to uniaxial continuous stretch

We previously reported that uniaxial continuous stretch in human umbilical vein endothelial cells (HUVECs) induced interleukin-6 (IL-6) secretion via IkappaB kinase (IKK)/nuclear factor-kappaB (NF-kappaB) activation. The aim of the present study was to clarify the upstream signaling mechanism responsible for this phenomenon. Stretch-induced IKK activation and IL-6 secretion were inhibited by application of alpha(5)beta(1) integrin-inhibitory peptide (GRGDNP), phosphatidylinositol 3-kinase inhibitor (LY-294002), phospholipase C-gamma inhibitor (U-73122), or protein kinase C inhibitor (H7). Although depletion of intra- or extracellular Ca(2+) pool using thapsigargin (TG) or EGTA, respectively, showed little effect, a TG-EGTA mixture significantly inhibited stretch-induced IKK activation and IL-6 secretion. An increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) upon continuous stretch was observed even in the presence of TG, EGTA, or GRGDNP, but not in a solution containing the TG-EGTA mixture, indicating that both integrin activation and [Ca(2+)](i) rise are crucial factors for stretch-induced IKK activation and after IL-6 secretion in HUVECs. Furthermore, while PKC activity was inhibited by the TG-EGTA mixture, GRGDNP, LY-294002, or U-73122, PLC-gamma activity was retarded by GRGDNP or LY-294002. These results indicate that continuous stretch-induced IL-6 secretion in HUVECs depends on outside-in signaling via integrins followed by a PI3-K-PLC-gamma-PKC-IKK-NF-kappaB signaling cascade. Another crucial factor, [Ca(2+)](i) increase, may at least be required to activate PKC needed for NF-kappaB activation.

Role of PTEN and Akt in the regulation of growth and apoptosis in human osteoblastic cells

Cancer cells are characterized by either an increased ability to proliferate or a diminished capacity to undergo programmed cell death. PTEN is instrumental in regulating the balance between growth and death in several cell types and has been described as a tumor suppressor. The chromosome arm on which PTEN is located is deleted in a subset of human osteosarcoma tumors. Therefore, we predicted that the loss of PTEN expression was contributing to increased Akt activation and the subsequent growth and survival of osteosarcoma tumor cells. Immunoblot analyses of several human osteosarcoma cell lines and normal osteoblasts revealed relatively abundant levels of PTEN. Furthermore, stimulation of cell growth or induction of apoptosis in osteosarcoma cells failed to affect PTEN expression or activity. Therefore, routine regulation of osteosarcoma cell growth and survival appears to be independent of changes in PTEN. Subsequently, the activation of a downstream target of PTEN activity, the survival factor Akt, was analyzed. Inappropriate activation of Akt could bypass the negative regulation by PTEN. Analyses of Akt expression in several osteosarcoma cell lines and normal osteoblasts revealed uniformly low basal levels of phosphorylated Akt. The levels of phosphorylated Akt did not increase following growth stimulation. In addition, osteosarcoma cell growth was unaffected by inhibitors of phosphatidylinositol-3 kinase, an upstream activator of the Akt signaling pathway. These data further suggest that the Akt pathway is not the predominant signaling cascade required for osteoblastic growth. However, inhibition of PTEN activity resulted in increased levels of Akt phosphorylation and enhanced cell proliferation. These data suggest that while abundant levels of PTEN normally maintain Akt in an inactive form in osteoblastic cells, the Akt signaling pathway is intact and functional.

Calcium restriction allows cAMP activation of the B-Raf/ERK pathway, switching cells to a cAMP-dependent growth-stimulated phenotype

cAMP can be either mitogenic or anti-mitogenic, depending on the cell type. We demonstrated previously that cAMP inhibited the proliferation of normal renal epithelial cells and stimulated the proliferation of cells derived from the cysts of polycystic kidney disease (PKD) patients. The protein products of the genes causing PKD, polycystin-1 and polycystin-2, are thought to regulate intracellular calcium levels, suggesting that abnormal polycystin function may affect calcium signaling and thus cause a switch to the cAMP growth-stimulated phenotype. To test this hypothesis, we disrupted intracellular calcium mobilization by treating immortalized mouse M-1 collecting duct cells and primary cultures of human kidney epithelial cells with calcium channel blockers and by lowering extracellular calcium with EGTA. Calcium restriction for 3-5 h converted both cell types from a normal cAMP growth-inhibited phenotype to an abnormal cAMP growth-stimulated phenotype, characteristic of PKD. In M-1 cells, we showed that calcium restriction was associated with an elevation in B-Raf protein levels and cAMP-stimulated, Ras-dependent activation of B-Raf and ERK. Moreover, the activity of Akt, a negative regulator of B-Raf, was decreased by calcium restriction. Inhibition of Akt or phosphatidylinositol 3-kinase also allowed cAMP-dependent activation of B-Raf and ERK in normal calcium. These results suggest that calcium restriction causes an inhibition of the phosphatidylinositol 3-kinase/Akt pathway, which relieves the inhibition of B-Raf to allow the cAMP growth-stimulated phenotypic switch. Finally, M-1 cells stably overexpressing an inducible polycystin-1 C-terminal cytosolic tail construct were shown to exhibit a cAMP growth-stimulated phenotype involving B-Raf and ERK activation, which was reversed by the calcium ionophore A23187. We conclude that disruption of calcium mobilization in cells that are normally growth-inhibited by cAMP can derepress the B-Raf/ERK pathway, thus converting these cells to a phenotype that is growth-stimulated by cAMP.

Nongenomic androgen activation of phosphatidylinositol 3-kinase/Akt signaling pathway in MC3T3-E1 osteoblasts

Androgens have important effects on the bone metabolism. However, the effect and mechanism of androgen action on the osteoblasts remains unknown. Here we showed that androgens increase phosphorylation and nuclear translocation of Akt. siRNA-AR prevented androgen-induced Akt activation in MC3T3-E1 cells. This suggests that nongenomic androgen activation of Akt is mediated by androgen receptor in osteoblasts.

INTRODUCTION

Androgens have important effects on the human skeleton in both males and females. However, the mechanism of androgen action on bone metabolism remains unknown. The aims of this study were to determine the effect and mechanism of androgen action on the osteoblast cells.

MATERIALS AND METHODS

Here we showed that 5alpha-dihydrotestosterone (DHT) accelerates cell growth of the MC3T3-E1 cell line in a time- and dose-dependent manner. The specific phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor LY294002 and kinase-deficient Akt mutant can repress the androgen effect on MC3T3-E1 cells. Western blot analysis showed that DHT, 17beta-estradiol, and testosterone (T) induce a rapid and transient phosphorylation of Akt in MC3T3-E1 cells. This activation reached to a plateau after 15 minutes and gradually diminished after 60 minutes of DHT treatment.

RESULTS

Fluorescence microscopy showed a distinct increase in immunostaining intensity in the nuclear interior after androgen treatment but no change in the subcellular distribution of Akt when the cells were pretreated with hydroxyflutamide (HF) or LY294002. In addition, small interfering RNA against androgen receptor (siRNA-AR) prevented DHT-induced Akt phosphorylation and cell growth.

CONCLUSION

These findings represents the first physiological finding to indicate how steroid hormones such as androgens can mediate the nuclear localization of Akt/PKB in osteoblasts that has previously mainly been linked to growth factor-induced events occurring at the plasma membrane level.

Taurine prevents the ischemia-induced apoptosis in cultured neonatal rat cardiomyocytes through Akt/caspase-9 pathway

Activated Akt kinase has been proposed as a central role in suppressing apoptosis by modulating the activities of Bcl-2 family proteins and/or caspase-9. To study the mechanism underlying the anti-apoptotic effect of taurine, the interaction between taurine and Akt/caspase-9 pathway was examined using a simulated ischemia model with cultured rat neonatal cardiomyocytes sealed in closed flasks. Taurine (20mM) treatment attenuated simulated ischemia-induced decline in the activity of Akt. Although taurine treatment had no effect on the expression of Bcl-2 in mitochondria and the level of cytosolic cytochrome c, it inhibited ischemia-induced cleavage of caspases 9 and 3. Moreover, adenovirus transfer of the dominant negative form of Akt objected taurine-mediated anti-apoptotic effects, cancelling the suppression of caspase-9 and caspase-3 activities by taurine. These findings provide the first evidence that taurine inhibits ischemia-induced apoptosis in cardiac myocytes with the increase in Akt activities, by inactivating caspase-9.

Luminal flow induces eNOS activation and translocation in the rat thick ascending limb. II. Role of PI3-kinase and Hsp90

Endothelial nitric oxide synthase (eNOS) regulates NaCl absorption by the thick ascending limb of the loop of Henle (THAL). We found that augmenting luminal flow induces eNOS activation and translocation to the apical membrane of THALs (Ortiz PA, Hong NJ, and Garvin JL. Am J Physiol Renal Physiol 287: F274-F280, 2004). In other cells, eNOS activation by shear stress is mediated by phosphatidylinositol 3-OH kinase (PI3)-kinase. We hypothesized that luminal flow induces eNOS activation via PI3-kinase. Pretreatment of THALs with wortmannin, a PI3-kinase inhibitor, significantly reduced flow-induced nitric oxide (NO) release by 75% (from 53.6 +/- 6 to 13.2 +/- 5.7 pA/mm). Increasing luminal flow from 0 to 20 nl/min induced eNOS translocation to the apical membrane, whereas in the presence of wortmannin eNOS translocation was prevented (basolateral = 32 +/- 2%, middle = 38 +/- 1%, apical = 30 +/- 1%, n = 5, not significant vs. no flow). We next studied which PI3-kinase product mediates eNOS translocation. Addition of PI(3,4,5)P(3) (5 microM) in the absence of flow increased NO levels (P < 0.05) and induced eNOS translocation to the apical membrane (from 40 +/- 4 to 60 +/- 2% of total eNOS, n = 6, P < 0.05). Incubation with PI(3,4)P(2) or PI(4,5)P(2) did not change eNOS localization. We next tested whether heat shock protein (Hsp)90 is involved in eNOS translocation. The Hsp90 inhibitor geldanamycin blocked flow-induced eNOS translocation to the apical membrane (n = 6). Flow also induced translocation of Hsp90 to the apical membrane (from 35 +/- 2 to 57 +/- 2%; P < 0.05) in a PI3-kinase-dependent manner. We conclude that luminal flow induces eNOS translocation and activation in the THAL via PI3-kinase and that Hsp90 is involved in eNOS translocation to the apical membrane.

Calcium signaling pathway involving calcineurin regulates interleukin-8 gene expression through activation of NF-kappaB in human osteoblast-like cells

Involvement of aberrant IL-8 production by osteoblasts was demonstrated in pathogenesis of inflammatory joint diseases. We thus investigated intracellular signaling pathways leading to IL-8 expression in human osteoblast-like HOS-TE85 cells. It was demonstrated that Ca2+ signaling pathway involving calcineurin regulates IL-8 gene expression through activation of a transcription factor, NF-kappaB.

INTRODUCTION

Involvement of aberrant interleukin (IL)-8 production by osteoblasts was demonstrated in pathogenesis of inflammatory joint diseases. However, intracellular signaling pathways leading to IL-8 expression in osteoblasts have been poorly explored. Because a variety of external stimuli was shown to increase intracellular Ca2+ in osteoblasts, we investigated effects of Ca(2+)-ionophore and phorbol-myristate-acetate (Ion/PMA) on IL-8 expression in human osteoblast-like HOS-TE85 cells and compared the effects with those elicited by TNF-alpha.

MATERIALS AND METHODS

HOS-TE85 cells were treated with Ion/PMA or TNF-alpha in the presence and absence of calcineurin inhibitors (CnI), cyclosporin A, and FK506. IL-8 mRNA levels and its promoter activities were examined by Northern blot and luciferase reporter analyses, respectively. Electrophoretic mobility shift assay (EMSA) was used to evaluate DNA binding activities of transcription factors such as NF-kappaB. Degradation of IkappaB, a cytoplasmic NF-kappaB-inhibitory protein, was examined by Western blot analysis.

RESULTS

Ion/PMA and TNF-alpha induced IL-8 mRNA expression. Interestingly, CnI attenuated the induction by Ion/PMA, but not that by TNF-alpha. Promoter activity was also increased by both stimuli, and only the Ion/PMA-dependent increase was suppressed by CnI. Introduction of mutations in the promoter demonstrated that one NF-kappaB site was responsible for the suppression by CnI. EMSA revealed that this site binds with NF-kappaB containing p65 that was activated by Ion/PMA and TNF-alpha and that CnI inhibited only Ion/PMA-dependent NF-kappaB activation. Accordingly, CnI blocked only Ion/PMA-dependent degradation of IkappaB-alpha. In addition, the basal and Ion/PMA-dependent IL-8 promoter activities were enhanced by co-transfection of constitutively active calcineurin.

CONCLUSION

These results show that the Ca2+ signaling pathway involving calcineurin regulates IL-8 gene expression through activation of NF-kappaB in human osteoblast-like cells.

The expanding role of PI3-kinase in bone

Phosphatidylinositol-3-kinases (PI3-Ks) play an important role in signal transduction and have been implicated in mediating a broad range of cellular responses. There are three classes of PI3-Ks [I (a and b subclasses), II, and III] with different substrate specificities and different modes of regulation. In osteoclasts, PI3-K has been shown to be a critical downstream effector from at least three cell-surface receptors, c-fms [the receptor for colony-stimulating factor 1 (CSF-1)], alphaVB3 integrin, and RANK [receptor activator of nuclear factor-kB (NF-kB)]. Furthermore, PI3-K is known to partner with the cytoplasmic tyrosine kinase c-src in mediating the effects of activated c-fms. The effector actions of PI3-K are diverse, including influencing osteoclast survival and activity, mediating actin remodeling and motility, and regulation of attachment structures. Less is known about the roles of PI3-K in osteoblasts. However, recent evidence suggests a role for PI3-K in osteoblast differentiation and survival. The classification, structure, function, and regulation of PI3-Ks will be reviewed here, with particular emphasis on the role of PI3-K in bone.

Akt signaling in skeletal muscle: regulation by exercise and passive stretch

Akt/protein kinase B is a serine/threonine kinase that has emerged as a critical signaling component for mediating numerous cellular responses. Contractile activity has recently been demonstrated to stimulate Akt signaling in skeletal muscle. Whether physiological exercise in vivo activates Akt is controversial, and the initiating factors that result in the stimulation of Akt during contractile activity are unknown. In the current study, we demonstrate that treadmill running exercise of rats using two different protocols (intermediate high or high-intensity exhaustive exercise) significantly increases Akt activity and phosphorylation in skeletal muscle composed of various fiber types. To determine if Akt activation during contractile activity is triggered by mechanical forces applied to the skeletal muscle, isolated skeletal muscles were incubated and passively stretched. Passive stretch for 10 min significantly increased Akt activity (2-fold) in the fast-twitch extensor digitorum longus (EDL) muscle. However, stretch had no effect on Akt in the slow-twitch soleus muscle, although there was a robust phosphorylation of the stress-activated protein kinase p38. Similar to contraction, stretch-induced Akt activation in the EDL was fully inhibited in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin, whereas glycogen synthase kinase-3 (GSK3) phosphorylation was only partially inhibited. Stretch did not cause dephosphorylation of glycogen synthase on GSK3-targeted sites in the absence or presence of wortmannin. We conclude that physiological exercise in vivo activates Akt in multiple skeletal muscle fiber types and that mechanical tension may be a part of the mechanism by which contraction activates Akt in fast-twitch muscles.