Related adhesion focal tyrosine kinase and the epidermal growth factor receptor mediate the stimulation of mitogen-activated protein kinase by the G-protein-coupled P2Y2 receptor. Phorbol ester or [Ca2+]i elevation can substitute for receptor activation

Tri-nucleotide receptors play a critical role in epithelial cell wound repair

The cornea plays a major role in the refraction of light to the retina. Therefore, the integrity and transparency of the corneal epithelium are critical to vision. Following injury, a combination of rapid signal transduction events and long-term cell migration are essential for wound closure. We have demonstrated previously that injury resulted in the release of nucleotides that induce the propagation of a Ca²⁺ wave to neighboring cells. This suggests that nucleotides and their receptors are critical components of wound healing. Epidermal growth factor (EGF) and integrins also have been shown to play a role in injury. In this study, we demonstrate that pretreatment of cells with ATP and UTP inhibited the immediate wound response, while BzATP, ADP, and UDP did not affect this response. Tri-nucleotide pretreatment also reduced the EGF induced Ca²⁺ response. Additionally, lower EC₅₀ concentrations of ATP and UTP triggered migration of cells that was enhanced further with EGF and was inhibited by the tripeptide, RGD. Results indicate that the desensitization induced by ATP and UTP was specific. While ADP and UDP cause a homologous desensitization of their own signal, they did not cause an inhibition of the wound response nor does BzATP. Neither Ca²⁺ wave propagation nor cell migration occurred in response to β,γ-MeATP. Together these results lead us to hypothesize that corneal epithelial wound repair is mediated by both P2Y₂ and P2Y₄ receptors.

P2Y2 nucleotide receptors enhance alpha-secretase-dependent amyloid precursor protein processing

The amyloid precursor protein (APP) is proteolytically processed by beta- and gamma-secretases to release amyloid beta, the main component in senile plaques found in the brains of patients with Alzheimer disease. Alternatively, APP can be cleaved within the amyloid beta domain by alpha-secretase releasing the non-amyloidogenic product sAPP alpha, which has been shown to have neuroprotective properties. Several G protein-coupled receptors are known to activate alpha-secretase-dependent processing of APP; however, the role of G protein-coupled nucleotide receptors in APP processing has not been investigated. Here it is demonstrated that activation of the G protein-coupled P2Y2 receptor (P2Y2R) subtype expressed in human 1321N1 astrocytoma cells enhanced the release of sAPP alpha in a time- and dose-dependent manner. P2Y2 R-mediated sAPP alpha release was dependent on extracellular calcium but was not affected by 1,2-bis(2-aminophenoxy)ethane-N,N,N,-trimethylammonium salt, an intracellular calcium chelator, indicating that P2Y2R-stimulated intracellular calcium mobilization was not involved. Inhibition of protein kinase C (PKC) with GF109203 or by PKC down-regulation with phorbol ester pre-treatment had no effect on UTP-stimulated sAPP alpha release, indicating a PKC-independent mechanism. U0126, an inhibitor of the mitogen-activated protein kinase pathway, partially inhibited sAPPalpha release by UTP, whereas inhibitors of Src-dependent epidermal growth factor receptor transactivation by P2Y2Rs had no effect. The metalloprotease inhibitors phenanthroline and TAPI-2 and the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethylketone also diminished UTP-induced sAPP alpha release. Furthermore, small interfering RNA silencing of an endogenous adamalysin, ADAM10 or ADAM17/TACE, partially suppressed P2Y2R-activated sAPP alpha release, whereas treatment of cells with both ADAM10 and ADAM17/TACE small interfering RNAs completely abolished UTP-activated sAPP alpha release. These results may contribute to an understanding of the non-amyloidogenic processing of APP.

Mitogen-activated protein kinase mediates purinergic-enhanced nerve growth factor-induced neurite outgrowth in PC12 cells

In 1999, we reported new observations that several compounds, including ATP, enhance neurite expression in PC12 cells when coapplied with nerve growth factor (NGF). Because purinergic and NGF signaling have several potential interfaces in PC12 cells, a series of experiments was conducted to elucidate the signal mediators contributing to the enhancement. Activities of selected kinases were measured and Western blots evaluated mitogen-activated protein kinase (MAPK) active and nonactive isoforms in lysates of the treated PC12 cells. In terms of purinergic potency, ATP and beta,gamma-methylene ATP elicited the greatest neurite-enhancing effect, whereas adenosine and alpha,beta-methylene ATP elicited the smallest. The effectiveness of a nonhydrolyzable analog such as beta,gamma-methylene ATP indicates that a nonmetabolic process is responsible. In response to ATP, NGF, or NGF + ATP, MAPK activity (measured by 32P incorporation) was maximal within 2 hr and remained statistically elevated over control levels throughout the 24 hr monitored. At maximal 32P incorporation, MAPK activity in response to ATP, NGF, and NGF + ATP was two-, four-, and sixfold higher, respectively, than control values; the observed increase was qualitatively confirmed using Western blots. Short-term inhibition experiments with protein kinase C and MAPK indicated that MAPK transduces the enhancing signal. We conclude from these experiments that ATP coapplied with NGF increases PC12 neurite expression by elevation of MAPK activity, likely by P2 receptor activation, and suggest that combination therapies with NGF and its enhancing adjunct compounds may be plausible for certain degenerative neurological disorders.

P2Y2 nucleotide receptor up-regulation in submandibular gland cells from the NOD.B10 mouse model of Sjögren's syndrome

Sjögren's syndrome (SS) is an autoimmune disease that specifically targets exocrine glands, including salivary glands, and results in an impairment of secretory function. P2Y(2) nucleotide receptors for extracellular ATP and UTP are up-regulated in response to stress or injury in a variety of tissues including submandibular glands (SMGs) [Ahn JS, Camden JM, Schrader AM, Redman RS, Turner JT. Reversible regulation of P2Y(2) nucleotide receptor expression in the duct-ligated rat submandibular gland. Am J Physiol Cell Physiol 2000;279:C286-94; Hou M, Malmsjö M, Möller S, Pantev E, Bergdahl A, Zhai X-H, et al. Increase in cardiac P2X(1)- and P2Y(2)-receptor mRNA levels in congestive heart failure. Life Sci 1999;65:1195-206; Kishore BK, Wang Z, Rab H, Haq M, Soleimani M. Upregulation of P2Y(2) purinoceptor during ischemic reperfusion injury (IRI): possible relevance to diuresis of IRI. J Am Soc Nephrol 1998;9:581 (abstract); Koshiba M, Apasov S, Sverdlov V, Chen P, Erb L, Turner JT, et al. Transient up-regulation of P2Y(2) nucleotide receptor mRNA expression is an immediate early gene response in activated thymocytes. Proc Natl Acad Sci U S A 1997;94:831-6; Turner JT, Landon LA, Gibbons SJ, Talamo BR. Salivary gland P2 nucleotide receptors. Crit Rev Oral Biol Med 1999;10:210-24; Seye CI, Gadeau AP, Daret D, Dupuch F, Alzieu P, Capron L, et al. Overexpression of the P2Y(2) purinoceptor in intimal lesions of the rat aorta. Arterioscler Thromb Vasc Biol 1997;17:3602-10; Seye C, Kong Q, Erb L, Garrad RC, Krugh B, Wang M, et al. Functional P2Y(2) nucleotide receptors mediate uridine 5'-triphosphate-induced intimal hyperplasia in collared rabbit carotid arteries. Circulation 2002;106:2720-6].

OBJECTIVE

To assess whether P2Y(2) receptor expression is up-regulated in SMGs of the NOD.B10 mouse model of primary SS as compared to SMGs of normal C57BL/6 mice.

DESIGN

SMG cells were isolated from normal C57BL/6 and diseased NOD.B10 mice. P2Y(2) receptor mRNA expression was determined by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization, whereas functional P2Y(2) receptor activity was analyzed by measuring UTP-induced increases in [Ca(2+)](i).

RESULTS

In contrast to SMG cells from C57BL/6 mice, SMG cells from 4- to 19-week-old NOD.B10 mice exhibited increased P2Y(2) receptor mRNA localized to both ductal and acinar cell types. The levels of mRNA for other uridine nucleotide receptors, i.e., P2Y(4) and P2Y(6) receptors, showed no significant differences between SMG cells of C57BL/6 and NOD.B10 mice, suggesting that only the P2Y(2) receptor was up-regulated in NOD.B10 mice. Moreover, P2Y(2) receptor activity in SMG cells from NOD.B10 mice increased with age (i.e., disease progression).

CONCLUSION

P2Y(2) receptor up-regulation in SMGs is associated with the SS phenotype in NOD.B10 mice, which encourages further attempts to determine the role of this pathway in the development of SS.

Met5-enkephalin-induced cardioprotection occurs via transactivation of EGFR and activation of PI3K

Our previous studies indicated that opioid-induced cardioprotection occurs via activation of mitochondrial ATP-sensitive K(+) (K(ATP)) channels. However, other elements of the Met(5)-enkephalin (ME) cardioprotection pathway are not fully characterized. In the present study, we investigated the role of tyrosine kinase, MAPK, and phosphatidylinositol 3-kinase (PI3K) signaling in ME-induced protection. Ca(2+)-tolerant, adult rabbit cardiomyocytes were isolated by collagenase digestion and subjected to simulated ischemia for 180 min. ME was administered 15 min before the 180 min of simulated ischemia; blockers were administered 15 min before ME. Cell death was assessed by trypan blue as a function of time. The epidermal growth factor receptor (EGFR) kinase inhibitor AG-1478 (250 nM) blocked ME-induced protection, but the inactive analog AG-9 (100 microM) did not. Treatment with herbimycin (1 microM) completely eliminated ME-induced protection. To verify that ME activates EGFR and to determine the involvement of Src, Western blotting of EGFR was performed after ME administration with and without herbimycin A. ME resulted in herbimycin-sensitive robust phosphorylation of EGFR at Tyr(992) and Tyr(1068). Administration of the selective MAPK inhibitor PD-98059 (10 nM) and the specific MEK1/2 inhibitor U-0126 (10 microM) also inhibited ME-induced cardioprotection. ME-induced ERK1/2 phosphorylation was significantly reduced by PD-98059, the EGFR kinase inhibitor PD-153035 (10 microM), and chelerythrine (2 microM). The PI3K inhibitor LY-294002 (20 microM) abrogated ME-induced protection, and ME-induced Akt phosphorylation at Ser(473) was suppressed by LY-294002, PD-153035, and chelerythrine. We conclude that ME-induced cardioprotection is mediated via Src-dependent EGFR transactivation and activation of the PI3K and MAPK pathways.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

Mechanisms of the ATP potentiation of hyposmotic taurine release in Swiss 3T3 fibroblasts

Reducing osmolarity by 35% increased (3)H-taurine efflux from Swiss 3T3 fibroblasts from 0.5% to a peak of 5.7%. The presence of ATP (10-100 microM; EC(50) 1.5 microM) increased taurine efflux up to 10%, and decreased the set point for hyposmotically stimulated taurine release (HTR). ATP potentiation was mimicked by UTP, reduced by addition of suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and unaffected by ADP, beta,gamma-methylene-ATP (beta,gamma-ATP) or 2-methylthio-ATP (Me-ATP), suggesting its mediation by purinergic P2Y(2) and P2Y(4) metabotropic receptors. Under isosmotic conditions ATP increased the cytosolic [Ca(2+)] ([Ca(2+)](i)) markedly, but did not increase taurine release. HTR was independent of external Ca(2+) but was reduced (by 56-59%) by BAPTA-AM, thapsigargin-induced depletion of intracellular Ca(2+) stores, or phospholipase C (PLC) inhibition. Blockade of calmodulin (CaM) or calmodulin kinase II (CaMKII) reduced HTR by 54% and 76%, respectively. The ATP-mediated potentiation was prevented fully by all these treatments. HTR was reduced by 30-50% by blockers of protein tyrosine kinases (AG18), phosphoinositide 3-kinase (PI3K) (wortmannin), p21rho (toxin B), p21rho-kinase (Y27632) and the stress-activated kinase p38 (PD169316). ATP-mediated potentiation was reduced similarly by these blockers. Simultaneous inhibition of PI3K and CaMKII abolished HTR. Altogether, these results suggest a modulatory effect of ATP, probably exerted by a potentiation of the Ca(2+)-dependent fraction of HTR. This fraction has as signalling elements a PLC-dependent [Ca(2+)](i) increase, resulting from Ca(2+) released from thapsigargin-sensitive internal stores, followed by activation of CaM/CaMKII reactions. The Ca(2+)/ATP effect operates only when the Ca(2+)-independent, tyrosine kinase-mediated pathway is already activated. Suggested elements of cross-talk between the two pathways are PLC, PI3K and CaMKII.

Neuropeptide Y-evoked proliferation of retinal glial (Muller) cells

BACKGROUND

Glial cells in human retinas and in fibrocellular membranes from patients with proliferative vitreoretinopathy (PVR) have been described to upregulate their expression of Y1 receptors for neuropeptide Y (NPY) (Soler et al.: Glia 39:320, 2002). However, it is unknown whether Y1 receptor activation causes proliferation of retinal glial cells. We investigated whether NPY exerts a proliferation-stimulating effect on retinal glial cells, and compared the NPY-evoked signaling with the signaling of purinergic P2Y receptors.

METHODS

Proliferation assays using bromodeoxyuridine were carried out on primarily cultured Muller glial cells of the guinea pig, in the absence and presence of blockers of Y1 receptors, of receptor tyrosine kinases (RTKs), of mitogen-activated protein kinases (MAPKs) and of phosphatidylinositol-3 kinase (PI3K).

RESULTS

NPY exerted a biphasic effect on Muller cell proliferation. At low concentrations (0.1 ng/ml and 1 ng/ml) it decreased the proliferation rate of the cells, while at higher concentration (100 ng/ml) it increased Muller cell proliferation. The NPY-evoked proliferation was mediated by Y1 receptor stimulation and by activation of the p44/p42 MAPKs and partially of the p38 MAPK. Moreover, Y1 receptor-induced activation of PI3K as well as transactivations of the platelet-derived and the epidermal growth factor RTKs were necessary for full mitogenic effect of NPY. Y1 and P2Y receptors share partially common signal transduction pathways in Muller cells.

CONCLUSION

It is suggested that NPY may be involved in stimulation of retinal glial cell proliferation during PVR when it is released at higher amounts into the injured retina.

The P2Y2 nucleotide receptor mediates vascular cell adhesion molecule-1 expression through interaction with VEGF receptor-2 (KDR/Flk-1)

UTP stimulates the expression of pro-inflammatory vascular cell adhesion molecule-1 (VCAM-1) in endothelial cells through activation of the P2Y(2) nucleotide receptor P2Y(2)R. Here, we demonstrated that activation of the P2Y(2)R induced rapid tyrosine phosphorylation of vascular endothelial growth factor receptor (VEGFR)-2 in human coronary artery endothelial cells (HCAEC). RNA interference targeting VEGFR-2 or inhibition of VEGFR-2 tyrosine kinase activity abolishes P2Y(2)R-mediated VCAM-1 expression. Furthermore, VEGFR-2 and the P2Y(2)R co-localize upon UTP stimulation. Deletion or mutation of two Src homology-3-binding sites in the C-terminal tail of the P2Y(2)R or inhibition of Src kinase activity abolished the P2Y(2)R-mediated transactivation of VEGFR-2 and subsequently inhibited UTP-induced VCAM-1 expression. Moreover, activation of VEGFR-2 by UTP leads to the phosphorylation of Vav2, a guanine nucleotide exchange factor for Rho family GTPases. Using a binding assay to measure the activity of the small GTPases Rho, we found that stimulation of HCAEC by UTP increased the activity of RhoA and Rac1 (but not Cdc42). Significantly, a dominant negative form of RhoA inhibited P2Y(2)R-mediated VCAM-1 expression, whereas expression of dominant negative forms of Cdc42 and Rac1 had no effect. These data indicate a novel mechanism whereby a nucleotide receptor transactivates a receptor tyrosine kinase to generate an inflammatory response associated with atherosclerosis.

Sensitization of Epidermal Growth Factor-induced Signaling by Bradykinin Is Mediated by c-Src

Communication between receptor tyrosine kinase (RTK)- and G protein-coupled receptor (GPCR)-mediated signaling systems has received increasing attention in recent years. Here, we report that activation of G protein-coupled bradykinin B2 receptor induces an up-regulation of cellular responses mediated by epidermal growth factor receptor (EGFR) and provide essential mechanistic characteristics of this sensitization process. EGF, which failed to evoke detectable amount of calcium increase and neurotransmitter release when administrated alone in primary cultures of rat adrenal chromaffin cells and PC12 cells, became capable of inducing these responses specifically after bradykinin pretreatment. Both EGFR and non-receptor tyrosine kinase p60Src, whose kinase activities were required in the sensitization, were found to be enriched in cholesterol-rich lipid rafts. Bradykinin caused activation of p60Src and Src-dependent phosphorylation of the EGFR on Tyr-845 in lipid rafts, as well as recruitment of phospholipase C (PLC) gamma1 to the rafts. Depletion of cholesterol by methyl-beta-cyclodextrin disrupted the raft localization of EGFR and Src, as well as bradykinin-induced translocation of PLCgamma1. Furthermore, sensitization, which was impaired by cholesterol depletion, was restored by repletion of cholesterol. Therefore, we suggest that lipid rafts are essential participants in the regulation of receptor-mediated signal transduction and cross-talk via organizing signaling complexes in membrane microdomains.

Src homology 3 binding sites in the P2Y2 nucleotide receptor interact with Src and regulate activities of Src, proline-rich tyrosine kinase 2, and growth factor receptors

Many G protein-coupled receptors activate growth factor receptors, although the mechanisms controlling this transactivation are unclear. We have identified two proline-rich, SH3 binding sites (PXXP) in the carboxyl-terminal tail of the human P2Y(2) nucleotide receptor that directly associate with the tyrosine kinase Src in protein binding assays. Furthermore, Src co-precipitated with the P2Y(2) receptor in 1321N1 astrocytoma cells stimulated with the P2Y(2) receptor agonist UTP. A mutant P2Y(2) receptor lacking the PXXP motifs was found to stimulate calcium mobilization and serine/threonine phosphorylation of the Erk1/2 mitogen-activated protein kinases, like the wild-type receptor, but was defective in its ability to stimulate tyrosine phosphorylation of Src and Src-dependent tyrosine phosphorylation of the proline-rich tyrosine kinase 2, epidermal growth factor receptor (EGFR), and platelet-derived growth factor receptor. Dual immunofluorescence labeling of the P2Y(2) receptor and the EGFR indicated that UTP caused an increase in the co-localization of these receptors in the plasma membrane that was prevented by the Src inhibitor PP2. Together, these data suggest that agonist-induced binding of Src to the SH3 binding sites in the P2Y(2) receptor facilitates Src activation, which recruits the EGFR into a protein complex with the P2Y(2) receptor and allows Src to efficiently phosphorylate the EGFR.

Substance P protects spiral ganglion neurons from apoptosis via PKC-Ca2+-MAPK/ERK pathways

In the current study, we have investigated the ability of substance P (SP) to protect 3-day-old (P3) rat spiral ganglion neurons (SGNs) from trophic factor deprivation (TFD)-induced cell death. The presence of SP high affinity neurokinin-1 receptor (NK1) transcripts was detected in the spiral ganglion and the NK1 protein localized to SGNs both ex vivo and in vitro. Treatment with SP increased cytoplasmic Ca2+ in SGNs, further arguing for the presence of functional NK1 on these neurons. Both SP and the agonist [Sar9,Met(O2)11]-SP significantly decreased SGN cell death induced by TFD, with no effect on neurite outgrowth. The survival promoting effect of SP was blocked by the NK1 antagonist, WIN51708. Both pan-caspase inhibitor BOC-D-FMK and SP treatments markedly reduced activation of caspases and DNA fragmentation in trophic factor deprived-neurons. The neuroprotective action of SP was antagonised by specific inhibitors of second messengers, including 1.2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) to chelate cytosolic Ca2+, the protein kinase C (PKC) inhibitors bisindolylmaleimide I, Gö6976 and LY333531 and the MAPK/ERK inhibitor U0126. In contrast, nifedipine, a specific inhibitor of l-type Ca2+ channel, and LY294002, a phosphatidylinositol-3-OH kinase (PI3K) inhibitor, had no effect on SP trophic support of SGNs. Moreover, activation of endogenous PKC by 4 beta-phorbol 12-myristate 13-acetate (PMA) also reduced the loss of trophic factor-deprived SGNs. Thus, NK1 expressed by SGNs transmit a survival-promoting regulatory signal during TFD-induced SGN cell death via pathways involving PKC activation, Ca2+ signalling and MAPK/ERK activation, which can be accounted for by an inhibition of caspase activation.

Activation of P2Y2 receptor induces c-FOS protein through a pathway involving mitogen-activated protein kinases and phosphoinositide 3-kinases in HeLa cells

The effects of P2Y2 purinoceptor activation on c-Fos expression and the signaling pathways evoked by extracellular ATP/UTP in HeLa cells were investigated. We found that P2Y2 activation induced c-Fos protein and phosphorylated the extracellular signal-regulated kinases 1 and 2 (ERK1/2). The P2Y2-stimulated c-Fos induction was partly blocked (a) by U73122, a phospholipase C inhibitor, (b) by Gö6976, a conventional PKC inhibitor, (c) by PD098059, a mitogen-activated protein kinase kinase inhibitor, and, moreover, (d) by the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin. When Gö6976 and PD098059, or Gö6976 and wortmannin, were combined there was a totally inhibition of P2Y2-induced c-Fos increase. Either U73122 or Gö6976 did not inhibit ERK1/2 phosphorylation induced by ATP/UTP, while it was inhibited by LY294002 (or wortmannin) and by staurosporine. Additionally, wortmannin inhibited the cytosol-to-membrane translocation of PKC- epsilon induced by ATP/UTP. These data indicated that agonist-induced PI3K and downstream PKC- epsilon activation mediated the effect of ATP/UTP on ERK1/2 activation. To test the biological consequences of ERK1/2 activation, the effect of P2Y2 on cell functions were examined. P2Y2 stimulation increased cell proliferation and this effect was attenuated by PD098059 in a dose-dependent manner, thereby indicating that the ERK pathway mediates mitogenic signaling by P2Y2. In conclusion, the activation of conventional PKCs through P2Y2 receptor acts in concert with ERK and PI3K/PKC- epsilon pathways to induce c-Fos protein and HeLa cell proliferation.

Epidermal-growth-factor receptor and metalloproteinases mediate thromboxane A2-dependent extracellular-signal-regulated kinase activation

The signalling pathways that link G-protein-coupled receptors to mitogen-activated protein kinases involve receptor and non-receptor tyrosine kinases and protein kinase C (PKC). We explored the pathways that are implicated in the thromboxane (TX) A(2)-dependent activation of extracellular-signal-regulated protein kinase (ERK) and the role of the two TX receptor (TP) isoforms, TP alpha and TP beta. ERK activation by IBOP, a TX analogue, was dependent on epidermal-growth-factor receptor (EGFR) in TP alpha- or TP beta-transfected cells and in human aortic smooth muscle cells (hASMCs), since AG1478, a selective inhibitor of tyrosine phosphorylation of the EGFR, strongly blocked ERK and EGFR phosphorylation. In addition, EGFR transactivation leading to ERK activation involved matrix metalloproteinases (MMPs), since BB2516, an inhibitor of MMP, decreased ERK and EGFR phosphorylation in TP alpha- or TP beta-transfected cells. Moreover, we showed that both isoforms activate ERK phosphorylation in an Src-kinase-dependent manner, whereas PKC was mainly implicated in ERK activation and EGFR phosphorylation by TP beta. In hASMCs, we showed that ERK activation depended on both pertussis-sensitive and -insensitive G alpha-proteins. We demonstrated further that EGFRs, PKC, Src kinase and MMPs are involved in ERK activation by TX. The results of the present study highlight a role for MMPs and PKC in EGFR transactivation triggered by the TPs and demonstrate this mechanism for the first time in primary cells, i.e. hASMCs.

Dependence of gonadotropin-releasing hormone-induced neuronal MAPK signaling on epidermal growth factor receptor transactivation

The hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), utilizes multiple signaling pathways to activate extracellularly regulated mitogen-activated protein kinases (ERK1/2) in normal and immortalized pituitary gonadotrophs and transfected cells expressing the GnRH receptor. In immortalized hypothalamic GnRH neurons (GT1-7 cells), which also express GnRH receptors, GnRH, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) caused marked phosphorylation of ERK1/2. This action of GnRH and PMA, but not that of EGF, was primarily dependent on activation of protein kinase C (PKC), and the ERK1/2 responses to all three agents were abolished by the selective EGF receptor kinase inhibitor, AG1478. Consistent with this, both GnRH and EGF increased tyrosine phosphorylation of the EGF receptor. GnRH and PMA, but not EGF, caused rapid phosphorylation of the proline-rich tyrosine kinase, Pyk2, at Tyr(402). This was reduced by Ca(2+) chelation and inhibition of PKC, but not by AG1478. GnRH stimulation caused translocation of PKC alpha and -epsilon to the cell membrane and enhanced the association of Src with PKC alpha and PKC epsilon, Pyk2, and the EGF receptor. The Src inhibitor, PP2, the C-terminal Src kinase (Csk), and dominant-negative Pyk2 attenuated ERK1/2 activation by GnRH and PMA but not by EGF. These findings indicate that Src and Pyk2 act upstream of the EGF receptor to mediate its transactivation, which is essential for GnRH-induced ERK1/2 phosphorylation in hypothalamic GnRH neurons.

Specific diadenosine pentaphosphate receptor coupled to extracellular regulated kinases in cerebellar astrocytes

In this study, we show specific intracellular responses evoked by the stimulation of astrocytes with the P1,P5-di(adenosine-5')pentaphosphate, Ap5A. The stimulation of astrocytes with micromolar concentrations of the dinucleotide elicited rapid increases in intracellular calcium concentration ([Ca2+]i), showing an EC50 value of 15.27 +/- 0.61 micro m. Moreover, the stimulation of cells with nanomolar concentrations of Ap5A, unable to induce calcium responses, increased the phosphorylated forms of extracellular-signal regulated kinase 1/2 (ERK) with an EC50 value of 9.8 +/- 2.4 nm. The maximal activation was observed at 100 nm Ap5A, which was similar to that produced by epidermal growth factor (EGF) under the same experimental conditions. The present data reported here indicate that Ap5A mediated these effects by interacting with a specific receptor, not yet identified, which was different from the P2Y1 and P2Y2/P2Y4 receptors present in all individual astrocytes.

Role of PKC and MAPK in cytosolic PLA2 phosphorylation and arachadonic acid release in primary murine astrocytes

Although Group IV cytosolic phospholipase A2 (cPLA2) in astrocytes has been implicated in a number of neurodegenerative diseases, mechanisms leading to its activation and release of arachidonic acid (AA) have not been clearly elucidated. In primary murine astrocytes, phorbol myristate acetate (PMA) and ATP stimulated phosphorylation of ERK1/2 and cPLA2 as well as evoked AA release. However, complete inhibition of phospho-ERK by U0126, an inhibitor of mitogen-activated protein kinase kinase (MEK), did not completely inhibit PMA-stimulated cPLA2 and AA release. Epidermal growth factor (EGF) also stimulated phosphorylation of ERK1/2 and cPLA2[largely through a protein kinase C (PKC)-independent pathway], but EGF did not evoke AA release. These results suggest that phosphorylation of cPLA2 due to phospho-ERK is not sufficient to evoke AA release. However, complete inhibition of ATP-induced cPLA2 phosphorylation and AA release was observed when astrocytes were treated with GF109203x, a general PKC inhibitor, together with U0126, indicating the important role for both PKC and ERK in mediating the ATP-induced AA response. There is evidence that PMA and ATP stimulated AA release through different PKC isoforms in astrocytes. In agreement with the sensitivity of PMA-induced responses to PKC down-regulation, prolonged treatment with PMA resulted in down-regulation of PKCalpha and epsilon in these cells. Furthermore, PMA but not ATP stimulated rapid translocation of PKCalpha from cytosol to membranes. Together, our results provided evidence for an important role of PKC in mediating cPLA2 phosphorylation and AA release in astrocytes through both ERK1/2-dependent and ERK1/2-independent pathways.

Polyamines regulate beta-catenin tyrosine phosphorylation via Ca(2+) during intestinal epithelial cell migration

Polyamines are essential for early mucosal restitution that occurs by epithelial cell migration to reseal superficial wounds after injury. Normal intestinal epithelial cells are tightly bound in sheets, but they need to be rapidly disassembled during restitution. beta-Catenin is involved in cell-cell adhesion, and its tyrosine phosphorylation causes disassembly of adhesion junctions, enhancing the spreading of cells. The current study determined whether polyamines are required for the stimulation of epithelial cell migration by altering beta-catenin tyrosine phosphorylation. Migration of intestinal epithelial cells (IEC-6 line) after wounding was associated with an increase in beta-catenin tyrosine phosphorylation, which decreased the binding activity of beta-catenin to alpha-catenin. Polyamine depletion by alpha-difluoromethylornithine reduced cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)), prevented induction of beta-catenin phosphorylation, and decreased cell migration. Elevation of [Ca(2+)](cyt) induced by the Ca(2+) ionophore ionomycin restored beta-catenin phosphorylation and promoted migration in polyamine-deficient cells. Decreased beta-catenin phosphorylation through the tyrosine kinase inhibitor herbimycin-A or genistein blocked cell migration, which was accompanied by reorganization of cytoskeletal proteins. These results indicate that beta-catenin tyrosine phosphorylation plays a critical role in polyamine-dependent cell migration and that polyamines induce beta-catenin tyrosine phosphorylation at least partially through [Ca(2+)](cyt).

The role of Aktand RAFTK in beta1 integrin mediated survival of precursor B-acute lymphoblastic leukemia cells

In this study, we report that the related adhesion focal tyrosine kinase RAFTK, is an upstream kinase in beta1 integrin mediated activation of Akt. Stimulation through beta1 integrins by fibronectin reversed apoptosis induced by adriamycin. Inhibitors of phosphatidylinositol 3-kinase (PI3 kinase)/Akt (LY 294002), tyrosine kinases (Herbimycin-A) and the cytotoxic agent adriamycin induced apoptosis of REH cells. beta1 integrin ligation induced activation of Akt, and tyrosine phosphorylation of RAFTK and FAK, but not SYK in REH cells. This suggested that RAFTK and FAK activation might be linked to Akt activation. Evidence that RAFTK is a modulator of Akt came from phorbol myristic acetate (PMA) stimulation. RAFTK and Akt were activated but FAK was not. Using fibroblasts from FAK -/- mice, which express high levels of RAFTK, fibronectin plating enhanced Akt activation. Pretreatment of REH cells with a P13 kinase/Akt inhibitor LY 294002 did not inhibit RAFTK tyrosine phosphorylation showing that RAFTK is upstream of P13k/Akt. Further evidence for a link between RAFTK tyrosine phosphorylation and Akt activation was the observation that the p85 subunit of P13 kinase associated with RAFTK following integrin ligation in REH cells. These results suggest that RAFTK plays an anti-apoptotic role through the activation of Akt.

Calcium-sensing receptor-mediated activation of phospholipase C-gamma1 is downstream of phospholipase C-beta and protein kinase C in MC3T3-E1 osteoblasts

Elevated extracellular calcium (Ca(e)) stimulates both chemotaxis and mitogenesis of MC3T3-E1 osteoblasts via a calcium-sensing receptor (CasR). Ca(e)-mediated chemotaxis of these bone-forming cells is dependent on phospholipase C (PLC) and blocked by the Gi-protein inhibitor pertussis toxin. In this study, we examine the signaling mechanisms by which the CasR stimulates PLC activity in MC3T3-E1 osteoblasts. We found that elevated Ca(e) stimulated PLC-gamma1 tyrosine phosphorylation in a time-dependent and Ca(e)-concentration-dependent manner. The maximal increase in PLC-gamma1 tyrosine phosphorylation was observed 3-5 min after increasing Ca(e) by 3.2 mmol/L from 1.8 mmol/L. Elevated Ca(e) also promoted a rapid increase in both inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], a second messenger formed by PLC-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate, and cytosolic free calcium ([Ca+2]i). The kinetics of the CasR-mediated increases in Ins(1,4,5)P3 and [Ca+2]i and the sensitivity of the Ca(e)-stimulated elevation in [Ca+2]i to U73122 (a PLC inhibitor) together suggest that the osteoblast CasR is coupled via Gq to PLC-beta. U73122 blocked the Ca(e)-promoted, but not PDGF-promoted, PLC-gamma1 tyrosine phosphorylation, suggesting that the activation of PLC-beta is upstream of PLC-gamma1 activation. Inhibition of protein kinase C (PKC) disrupted Ca(e)-stimulated tyrosine phosphorylation of PLC-gamma1. In addition, exposure to pertussis toxin or exogenous activation of protein kinase A (PKA) inhibited PLC-gamma1 tyrosine phosphorylation in response to Ca(e). The results indicate that: (a) the osteoblast CasR activates PLC-gamma1 downstream of PLC-beta in a PKC-dependent manner; (b) PKA is a negative regulator of Ca(e)-promoted PLC-gamma1 phosphorylation; and (c) Gq and Gi are both involved in the CasR-mediated phosphorylation of PLC-gamma1.

Interaction between ATP and nerve growth factor signalling in the survival and neuritic outgrowth from PC12 cells

In a previous study we used P2 receptor antagonists to inhibit diverse responses that nerve growth factor (NGF) promotes and coordinates in PC12 cells and we suggested that P2 receptors partake in the NGF signalling cascade. In this paper, we examine the direct role of extracellular P2 receptor agonists as neurotrophic factors. ATP and 2-Cl-ATP promote neurite regeneration after priming PC12 cells with NGF and the effect is dose-dependent, with an EC(50) of about 5 and 3 microM, respectively. The number of cell clumps bearing neurites was maximally induced in day 1 and it was maintained up to about one week by ATP, or up to at least 2 weeks by 2-Cl-ATP. The involvement of P1 receptors or intracellular inosine in these actions was excluded, whereas various antagonists of P2 receptors were inhibitory. Moreover, NGF and ATP caused a direct up-regulation of P2X(2), P2X(3), P2X(4) and P2Y(2), but not P2Y(4) receptor proteins under neurite-regenerating conditions, as well as extracellular signal-regulated kinase (Erk)1-2 tyrosine/threonine phosphorylation and activation. Finally, ATP, 2-Cl-ATP and ATPgammaS enhanced neurite initiation evoked by sub-optimal NGF concentrations and ATP and 2-Cl-ATP fully sustained survival of PC12 cells after serum deprivation. Our results establish that P2 receptor agonists can behave as neurotrophic factors for neuronal cells and suggest a potential interplay between ATP and NGF in the signalling pathways triggered on their target cells.

Role of EGF Receptor and Pyk2 in endothelin-1-induced ERK activation in rat cardiomyocytes

G protein-coupled receptor (GPCR)-evoked signal transduction pathways leading to the activation of extracellular signal-regulated kinases (ERK) are quite different among cell types. In cardiomyocytes, much attention has been focused on the activation of protein kinase C (PKC) or mobilization of intracellular Ca(2+) ([Ca(2+)](i)), however, the contributions of tyrosine kinases are controversial. In the present study, we characterized the signaling pathways involving tyrosine kinases, Pyk2 and epidermal growth factor receptor (EGFR), and their contribution to ERK activation in cultured cardiomyocytes. We initially investigated the potential involvement of [Ca(2+)](i) and PKC on the activation of these kinases in endothelin-stimulated cardiomyocytes. Interestingly, activation of Pyk2 was abrogated by chelating [Ca(2+)](i) or by downregulation of PKC, whereas transactivation of EGFR was solely dependent on PKC. By using a compound that selectively interferes with EGFR (AG1478), c-Src (PP1), or disrupts actin cytoskeleton (cytochalasin D), we demonstrated that cytochalasin D completely inhibited the activation of Pyk2, but not that of EGFR, whereas AG1478 did not inhibit the activation of Pyk2, indicating that transactivation of EGFR and signaling pathways involving Pyk2 were distinct pathways. Furthermore, activation of ERK and Shc, and c- fos gene expression were significantly inhibited by AG1478 but not by cytochalasin D or PP1. Overexpression of deletion mutant of EGFR attenuated the activation of ERK. These facts demonstrated the existence of two distinct tyrosine kinase pathways requiring Pyk2 or EGFR downstream from GPCR in cardiomyocytes. EGFR was Ca(2+)-independently activated and predominantly contributed to Shc/ERK/c- fos activation, while Pyk2 or c-Src contributed less to it.

Peptide YY as a growth factor for intestinal epithelium

Peptide YY is an abundant distal gut hormone that may play a significant role in intestinal epithelial proliferation. Gut epithelial cells express specific receptors for PYY, PYY induces proliferation in intestinal cells in vivo and in vitro, and the Y1 receptor subtype couples to mitogenic signaling pathways. In addition to proposed physiologic effects on gut mucosal maintenance, PYY proliferative effects may be hypothesized to contribute to pathophysiologic consequences of stimulated growth.

Metabotropic glutamate receptor 5-induced phosphorylation of extracellular signal-regulated kinase in astrocytes depends on transactivation of the epidermal growth factor receptor

G-protein-coupled receptors (GPCRs) induce the phosphorylation of mitogen-activated protein (MAP) kinase by actions on any of a number of signal transduction systems. Previous studies have revealed that activation of the G(q)-coupled metabotropic glutamate receptor 5 (mGluR5) induces phosphorylation of the MAP kinase extracellular signal-regulated kinase 2 (ERK2) in cultured rat cortical astrocytes. We performed a series of studies to determine the mechanisms underlying mGluR5-induced phosphorylation of MAP kinase in these cells. Interestingly, our studies suggest that mGluR5-mediated ERK2 phosphorylation is dependent on the activation of G(alphaq) but is not mediated by the activation of phospholipase Cbeta1, activation of protein kinase C, or increases in intracellular calcium. Studies with peptide inhibitors suggest that this response is not dependent on G(betagamma) subunits. However, the activation of ERK2 was dependent on activation of the epidermal growth factor (EGF) receptor and activation of a Src family tyrosine kinase. Furthermore, activation of mGluR5 induced an association of this receptor and the EGF receptor, suggesting the formation of a signaling complex involved in the activation of ERK2. These data suggest that mGluR5 increases ERK2 phosphorylation in astrocytes by a novel mechanism involving the activation of G(alphaq) and both receptor and nonreceptor tyrosine kinases but that is independent of the activation of phospholipase Cbeta1.

Diversity of G protein-coupled receptor signaling pathways to ERK/MAP kinase

One of the most intriguing examples of cross talk between signaling systems is the interrelationship between G protein-coupled receptor and growth factor receptor pathways leading to activation of the ERK/MAP kinase phosphorylation cascade. This review focuses on the mechanism of this cross talk, denoting primarily signaling components known to occur in the G protein-coupled receptor branch of the MAP kinase pathways in neural cells. Recent evidence is presented on the existence of a plethora of pathways, due to the multiplicity of G protein-coupled receptors, their differential interaction with heterotrimeric G protein isoforms, various effectors and second messengers. In light of this rich diversity, the review will discuss different points of convergence of G protein-coupled receptor and growth factor receptor pathways that may feature a requirement for growth factor receptor transactivation, receptor internalization and scaffolds to assemble receptor, adaptor and anchoring proteins into multiprotein complexes.

HIV-1 gp120 and chemokine activation of Pyk2 and mitogen-activated protein kinases in primary macrophages mediated by calcium-dependent, pertussis toxin-insensitive chemokine receptor signaling

Human immunodeficiency virus type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry. It was recently demonstrated that HIV-1 glycoprotein 120 (gp120) elevated calcium and activated several ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. This study shows that chemokine receptor engagement by both CCR5-dependent (R5) and CXCR4-dependent (X4) gp120 led to rapid phosphorylation of the focal adhesion-related tyrosine kinase Pyk2 in macrophages. Pyk2 phosphorylation was also induced by macrophage inflammatory protein-1beta (MIP-1beta) and stromal cell-derived factor-1alpha, chemokine ligands for CCR5 and CXCR4. Activation was blocked by EGTA and by a potent blocker of calcium release-activated Ca++ (CRAC) channels, but was insensitive to pertussis toxin (PTX), implicating CRAC-mediated extracellular Ca++ influx but not Galpha(i) protein-dependent mechanisms. Coreceptor engagement by gp120 and chemokines also activated 2 members of the mitogen-activated protein kinase (MAPK) superfamily, c-Jun amino-terminal kinase/stress-activated protein kinase and p38 MAPK. Furthermore, gp120-stimulated macrophages secreted the chemokines monocyte chemotactic protein-1 and MIP-1beta in a manner that was dependent on MAPK activation. Thus, the gp120 signaling cascade in macrophages includes coreceptor binding, PTX-insensitive signal transduction, ionic signaling including Ca++ influx, and activation of Pyk2 and MAPK pathways, and leads to secretion of inflammatory mediators. HIV-1 Env signaling through these pathways may contribute to dysregulation of uninfected macrophage functions, new target cell recruitment, or modulation of macrophage infection.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

mu-Opioid receptor-mediated ERK activation involves calmodulin-dependent epidermal growth factor receptor transactivation

Phosphorylation of the MAPK isoform ERK by G protein-coupled receptors involves multiple signaling pathways. One of these pathways entails growth factor receptor transactivation followed by ERK activation. This study demonstrates that a similar signaling pathway is used by the mu-opioid receptor (MOR) expressed in HEK293 cells and involves calmodulin (CaM). Stimulation of MOR resulted in both epidermal growth factor receptor (EGFR) and ERK phosphorylation. Data obtained with inhibitors of EGFR Tyr kinase and membrane metalloproteases support an intermediate role of EGFR activation, involving release of endogenous membrane-bound epidermal growth factor. Previous studies had demonstrated a role for CaM in opioid signaling based on direct CaM binding to MOR. To test whether CaM contributes to EGFR transactivation and ERK phosphorylation by MOR, we compared wild-type MOR with mutant K273A MOR, which binds CaM poorly, but couples normally to G proteins. Stimulation of K273A MOR with [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin (10-100 nm) resulted in significantly reduced ERK phosphorylation. Furthermore, wild-type MOR stimulated EGFR Tyr phosphorylation 3-fold more than K273A MOR, indicating that direct CaM-MOR interaction plays a key role in the transactivation process. Inhibitors of CaM and protein kinase C also attenuated [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin-induced EGFR transactivation in wild-type (but not mutant) MOR-expressing cells. This novel pathway of EGFR transactivation may be shared by other G protein-coupled receptors shown to interact with CaM.

Modulation of PKCδ tyrosine phosphorylation and activity in salivary and PC-12 cells by Src kinases

Protein kinase C (PKC) δ becomes tyrosine phosphorylated in rat parotid acinar cells exposed to muscarinic and substance P receptor agonists, which initiate fluid secretion in this salivary cell. Here we examine the signaling components of PKCδ tyrosine phosphorylation and effects of phosphorylation on PKCδ activity. Carbachol- and substance P-promoted increases in PKCδ tyrosine phosphorylation were blocked by inhibiting phospholipase C (PLC) but not by blocking intracellular Ca2+ concentration elevation, suggesting that diacylglycerol, rather than d- myo-inositol 1,4,5-trisphosphate production, positively modulated this phosphorylation. Stimuli-dependent increases in PKCδ activity in parotid and PC-12 cells were blocked in vivo by inhibitors of Src tyrosine kinases. Dephosphorylation of tyrosine residues by PTP1B, a protein tyrosine phosphatase, reduced the enhanced PKCδ activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKCδ activation. Two PKCδ regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKCδ tyrosine phosphorylation. These results demonstrate that PKCδ activity is positively modulated by tyrosine phosphorylation in parotid and PC-12 cells and suggest that PLC-dependent effects of secretagogues on salivary cells involve Src-related kinases.

Extracellular nucleotide signaling: a mechanism for integrating local and systemic responses in the activation of bone remodeling

Bone turnover occurs at discreet sites in the remodeling skeleton. The focal nature of this process indicates that local cues may facilitate the activation of bone cells by systemic factors. Nucleotides such as adenosine triphosphate (ATP) are locally released, short-lived, yet potent extracellular signaling molecules. These ligands act at a large family of receptors-the P2 receptors, which are subdivided into P2Y and P2X subtypes based on mechanism of signal transduction. Nucleotides enter the extracellular milieu via non-lytic and lytic mechanisms where they activate multiple P2 receptor types expressed by both osteoblasts and osteoclasts. In this review the release of ATP by bone cells is discussed in the context of activation of bone remodeling. We provide compelling evidence that nucleotides, acting via P2Y receptors, are potent potentiators of parathyroid hormone-induced signaling and transcriptional activation in osteoblasts. The provision of a mechanism to induce activation of osteoblasts above a threshold attained by systemic factors alone may facilitate focal remodeling and address the paradox of why systemic regulators like PTH exert effects at discreet sites.

P2Y(2) nucleotide receptor signaling in human monocytic cells: activation, desensitization and coupling to mitogen-activated protein kinases

Activation of P2Y(2) receptors by extracellular nucleotides has been shown to induce phenotypic differentiation of human promonocytic U937 cells that is associated with the inflammatory response. The P2Y(2) receptor agonist, UTP, induced the phosphorylation of the MAP kinases MEK1/2 and ERK1/2 in a sequential manner, since ERK1/2 phosphorylation was abolished by the MEK1/2 inhibitor PD 098059. Other results indicated that P2Y(2) receptors can couple to MAP kinases via phosphatidylinositol 3-kinase (PI3K) and c-src. Accordingly, ERK1/2 phosphorylation induced by UTP was inhibited by the PI3K inhibitors, wortmannin and LY294002, and the c-src inhibitors, radicicol and PP2, but not by inhibitors of protein kinase C (PKC). The phosphorylation of ERK1/2 was independent of the ability of P2Y(2) receptors to increase the concentration of intracellular free calcium, since chelation of intracellular calcium by BAPTA did not diminish the phosphorylation of ERK1/2 induced by UTP. A 5-minute treatment with UTP reduced U937 cell responsiveness to a subsequent UTP challenge. UTP-induced desensitization was characterized by an increase in the EC(50) for receptor activation (from 0.44 to 9.3 microM) and a dramatic ( approximately 75%) decrease in the maximal calcium mobilization induced by a supramaximal dose of UTP. Phorbol ester treatment also caused P2Y(2) receptor desensitization (EC(50) = 12.3 microM UTP and maximal calcium mobilization reduced by approximately 33%). The protein kinase C inhibitor GF 109203X failed to significantly inhibit the UTP-induced desensitization of the P2Y(2) receptor, whereas the protein phosphatase inhibitor okadaic acid blocked receptor resensitization. Recovery of receptor activity after UTP-induced desensitization was evident in cells treated with agonist for 5 or 30 min. However, P2Y(2) receptor activity remained partially desensitized 30 min after pretreatment of cells with UTP for 1 h or longer. This sustained desensitized state correlated with a decrease in P2Y(2) receptor mRNA levels. Desensitization of ERK1/2 phosphorylation was induced by a 5-minute pretreatment with UTP, and cell responsiveness did not return even after a 30-minute incubation of cells in the absence of an agonist. Results suggest that desensitization of the P2Y(2) receptor may involve covalent modifications (i.e., receptor phosphorylation) that functionally uncouple the receptor from the calcium signaling pathway, and that transcriptional regulation may play a role in long-term desensitization. Our results indicate that calcium mobilization and ERK1/2 phosphorylation induced by P2Y(2) receptor activation are independent events in U937 monocytes.

An RGD sequence in the P2Y(2) receptor interacts with alpha(V)beta(3) integrins and is required for G(o)-mediated signal transduction

The P2Y(2) nucleotide receptor (P2Y(2)R) contains the integrin-binding domain arginine-glycine-aspartic acid (RGD) in its first extracellular loop, raising the possibility that this G protein-coupled receptor interacts directly with an integrin. Binding of a peptide corresponding to the first extracellular loop of the P2Y(2)R to K562 erythroleukemia cells was inhibited by antibodies against alpha(V)beta(3)/beta(5) integrins and the integrin-associated thrombospondin receptor, CD47. Immunofluorescence of cells transfected with epitope-tagged P2Y(2)Rs indicated that alpha(V) integrins colocalized 10-fold better with the wild-type P2Y(2)R than with a mutant P2Y(2)R in which the RGD sequence was replaced with RGE. Compared with the wild-type P2Y(2)R, the RGE mutant required 1,000-fold higher agonist concentrations to phosphorylate focal adhesion kinase, activate extracellular signal-regulated kinases, and initiate the PLC-dependent mobilization of intracellular Ca(2+). Furthermore, an anti-alpha(V) integrin antibody partially inhibited these signaling events mediated by the wild-type P2Y(2)R. Pertussis toxin, an inhibitor of G(i/o) proteins, partially inhibited Ca(2+) mobilization mediated by the wild-type P2Y(2)R, but not by the RGE mutant, suggesting that the RGD sequence is required for P2Y(2)R-mediated activation of G(o), but not G(q). Since CD47 has been shown to associate directly with G(i/o) family proteins, these results suggest that interactions between P2Y(2)Rs, integrins, and CD47 may be important for coupling the P2Y(2)R to G(o).

Enhancement of arachidonic acid release and prostaglandin F(2alpha) formation by Na3VO4 in PC12 cells and GH3 cells

Both activation of phospholipase A2 causing arachidonic acid release and tyrosine phosphorylation have been proposed to be involved in neuronal functions. Previously, we reported that orthovanadate (Na3VO4), an inhibitor of tyrosine phosphatases, stimulated tyrosine phosphorylation in proteins and enhanced Ca2+-induced noradrenaline release in rat pheochromocytoma PC12 cells. However, the role of tyrosine phosphorylation on phospholipase A2 activity and/or arachidonic acid release in neuronal cells has not been well established. The effects of Na3VO4 on arachidonic acid release and prostaglandin F(2alpha) formation were investigated in two types of neuronal cell lines. In PC12 cells, addition of Na3VO4 stimulated [3H]arachidonic acid release and prostaglandin F(2alpha) formation in a concentration-dependent manner. Co-addition of 5 mM Na3VO4 enhanced ionomycin-stimulated [3H]arachidonic acid release. Na3VO4 also enhanced ionomycin-stimulated [3H]arachidonic acid release from GH3 cells, a clonal strain from rat anterior pituitary. These findings suggest that the tyrosine phosphorylation pathway regulates arachidonic acid release by phospholipase A2 and prostaglandin F(2alpha) formation in neuronal cells.

Intracellular signals that control cell proliferation in mammalian balance epithelia: key roles for phosphatidylinositol-3 kinase, mammalian target of rapamycin, and S6 kinases in preference to calcium, protein kinase C, and mitogen-activated protein kinase

In fish, amphibians, and birds, the loss of hair cells can evoke S-phase entry in supporting cells and the production of new cells that differentiate as replacement hair cells and supporting cells. Recent investigations have shown that supporting cells from mammalian vestibular epithelia will proliferate in limited numbers after hair cells have been killed. Exogenous growth factors such as glial growth factor 2 enhance this proliferation most potently when tested on vestibular epithelia from neonates. In this study, the intracellular signaling pathways that underlie the S-phase entry were surveyed by culturing epithelia in the presence of pharmacological inhibitors and activators. The results demonstrate that phosphatidylinositol 3-kinase is a key element in the signaling cascades that lead to the proliferation of cells in mammalian balance epithelia in vitro. Protein kinase C, mammalian target of rapamycin, mitogen-activated protein kinase, and calcium were also identified as elements in the signaling pathways that trigger supporting cell proliferation.

Cell communication networks: epidermal growth factor receptor transactivation as the paradigm for interreceptor signal transmission

Communication between different cellular signaling systems has emerged as a common principle that enables cells to integrate a multitude of signals from its environment. Transactivation of the epidermal growth factor receptor (EGFR) represents the paradigm for cross-talk between G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). The recent identification of Zn2+-dependent metalloproteinases and transmembrane growth factor precursors as critical elements in GPCR-induced EGFR transactivation pathways has defined new components of a cellular communication network of rapidly increasing complexity. Further elucidation of the molecular details of the EGFR transactivation mechanism will provide new understanding of its relevance for normal physiological processes and their pathophysiological deviations.

Phosphorylation of the Gq/11-coupled m3-muscarinic receptor is involved in receptor activation of the ERK-1/2 mitogen-activated protein kinase pathway

We investigated the role played by agonist-mediated phosphorylation of the G(q/11)-coupled M(3)-muscarinic receptor in the mechanism of activation of the mitogen-activated protein kinase pathway, ERK-1/2, in transfected Chinese hamster ovary cells. A mutant of the M(3)-muscarinic receptor, where residues Lys(370)-Ser(425) of the third intracellular loop had been deleted, showed a reduced ability to activate the ERK-1/2 pathway. This reduction was evident despite the fact that the receptor was able to couple efficiently to the phospholipase C second messenger pathway. Importantly, the ERK-1/2 responses to both the wild-type M(3)-muscarinic receptor and DeltaLys(370)-Ser(425) receptor mutant were dependent on the activity of protein kinase C. Our results, therefore, indicate the existence of two mechanistic components to the ERK-1/2 response, which appear to act in concert. First, the activation of protein kinase C through the diacylglycerol arm of the phospholipase C signaling pathway and a second component, absent in the DeltaLys(370)-Ser(425) receptor mutant, that is independent of the phospholipase C signaling pathway. The reduced ability of the DeltaLys(370)-Ser(425) receptor mutant to activate the ERK-1/2 pathway correlated with an approximately 80% decrease in the ability of the receptor to undergo agonist-mediated phosphorylation. Furthermore, we have previously shown that M(3)-muscarinic receptor phosphorylation can be inhibited by a dominant negative mutant of casein kinase 1alpha and by expression of a peptide corresponding to the third intracellular loop of the M(3)-muscarinic receptor. Expression of these inhibitors of receptor phosphorylation reduced the wild-type M(3)-muscarinic receptor ERK-1/2 response. We conclude that phosphorylation of the M(3)-muscarinic receptor on sites in the third intracellular loop by casein kinase 1alpha contributes to the mechanism of receptor activation of ERK-1/2 by working in concert with the diacylglycerol/PKC arm of the phospholipase C signaling pathway.

Molecular mechanisms of ATP and insulin synergistic stimulation of coronary artery smooth muscle growth

Coronary artery disease (CAD) is the major cause of death in diabetics. Abnormal proliferation of coronary artery smooth muscle cells (CASMC) leads to intimal thickening in CAD. We examined signaling mechanisms involved in the mitogenic effect of ATP and insulin on CASMC. ATP and insulin individually stimulated DNA synthesis by 4- and 2-fold, respectively; however, they acted synergistically to stimulate an increase of 17-fold over basal. A similar synergistic stimulation of extracellular signal-regulated kinase (ERK) and mitogen-activated protein or ERK kinase activities was observed (ATP, 7-fold; insulin, 2-fold; and ATP + insulin, 16-fold over basal). However, the combination of ATP and insulin stimulated only an additive activation of Raf (ATP, 5-fold; insulin, <2-fold; and ATP + insulin, 8-fold over basal) and Ras (ATP, 5-fold; insulin, 2-fold; and ATP + insulin, 8-fold over basal). Thus convergence of ATP and insulin signals appears to be at the level of Ras and Raf. In addition, insulin stimulated activation of Akt (also known as protein kinase B) (10-fold over basal), whereas ATP had little effect. However, when ATP and insulin were added in combination, ATP dramatically reduced the insulin-stimulated Akt activation (2-fold above basal). Thus these results are consistent with ATP relieving an insulin-induced Akt-dependent inhibitory effect on the ERK signaling pathway, leading to synergistic stimulation of CASMC proliferation.

Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Traugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotrophs (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on PKC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH and EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activation of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 binding was predominant throughout. TRH and EGF markedly decreased IRP binding in MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP binding in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.

Involvement of Src and epidermal growth factor receptor in the signal-transducing function of Na+/K+-ATPase

Nontoxic concentrations of ouabain, causing partial inhibition of the cardiac myocyte Na(+)/K(+)-ATPase, induce hypertrophy and several growth-related genes through signal pathways that include the activation of Ras and p42/44 mitogen-activated protein kinase (MAPK). The aim of this work was to examine the ouabain-induced events upstream of the Ras/MAPK cascade. Treatment of myocytes with genistein antagonized ouabain-induced activation of the MAPK, suggesting that protein tyrosine phosphorylation has a role. Tyrosine phosphorylation of several myocyte proteins was increased rapidly upon cell exposure to ouabain. Lowering of extracellular K(+) had a similar ouabain-like effect. Ouabain also increased protein tyrosine phosphorylation in A7r5, HeLa, and L929 cells. In cardiac myocytes and A7r5 cells, herbimycin A antagonized the ouabain-induced increase in protein tyrosine phosphorylation and MAPK activation. In both cell types, ouabain stimulated Src kinase activity, Src translocation to the Triton-insoluble fraction, Src association with the epidermal growth factor receptor, and the tyrosine phosphorylation of this receptor on site(s) other than its major autophosphorylation site, Tyr(1173). The findings suggest that (a) the ouabain-induced activation of Src and the Src-induced phosphorylation of the growth factor receptor provide the scaffolding for the recruitment of adaptor proteins and Ras and the activation of Ras/MAPK cascade; and (b) the activation of such pathways may be a common feature of the signal-transducing function of Na(+)/K(+)-ATPase in most cells.

The galanin receptor type 2 initiates multiple signaling pathways in small cell lung cancer cells by coupling to G(q), G(i) and G(12) proteins

Neuropeptides like galanin produced and released by small cell lung cancer (SCLC) cells are considered principal mitogens in these tumors. We identified the galanin receptor type 2 (GALR2) as the only galanin receptor expressed in H69 and H510 cells. Photoaffinity labeling of G proteins in H69 cell membranes revealed that GALR2 activates G proteins of three subfamilies: G(q), G(i), and G(12). In H69 cells, galanin-induced Ca2+ mobilization was pertussis toxin-insensitive. While phorbol ester-induced extracellular signal-regulated kinase (ERK) activation required protein kinase C (PKC) activity, preincubation of H69 cells with the PKC-inhibitor GF109203X had no effect on galanin-dependent ERK activity. A rise of the intracellular calcium concentration was necessary and sufficient to mediate galanin-induced ERK activation. In support of G(i) coupling, stimulation of GALR2 expressed in HEK293 cells inhibited isoproterenol-induced cAMP accumulation and raised cAMP levels in COS-7 cells when coexpressed with a chimeric G alpha(S)-G alpha(i) protein In H69 cells, galanin activated the monomeric GTPase RhoA and induced stress fiber formation in Swiss 3T3 cells expressing GALR2. Thus, we provide the first direct evidence that in SCLC the mitogenic neuropeptide galanin, interacting with GALR2, simultaneously activates multiple classes of G proteins and signals through the G(q) phospholipase C/calcium sequence and a G(12)/Rho pathway. Oncogene (2000) 19, 4199 - 4209

Integrins regulate the linkage between upstream and downstream events in G protein-coupled receptor signaling to mitogen-activated protein kinase

Receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) can both activate mitogen-activated protein kinase (MAPK), a critical intermediate in the transduction of proliferative signals. Numerous observations have demonstrated that integrin-mediated cell anchorage can regulate the efficiency of signaling from RTKs to MAPK. Recently, a relationship between integrins and GPCR signaling has also emerged; however, little is understood concerning the mechanisms involved. Here, we investigate integrin regulation of GPCR signaling to MAPK, focusing on the P2Y class of GPCRs that function through activation of phospholipase Cbeta. P2Y receptor signaling to the downstream components mitogen-activated protein kinase kinase and MAPK is highly dependent on integrin-mediated cell anchorage. However, activation of upstream events, including inositol phosphate production and generation of calcium transients, is completely independent of cell anchorage. This indicates that integrins regulate the linkage between upstream and downstream events in this GPCR pathway, just as they do in some aspects of RTK signaling. However, the P2Y pathway does not involve cross-activation of a RTK, nor a role for Shc or c-Raf; thus, it is quite distinct from the classical RTK-Ras-Raf-MAPK cascade. Rather, integrin-modulated P2Y receptor stimulation of MAPK depends on calcium and on the activation of protein kinase C.

Carbachol-stimulated transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T(84) cells is mediated by intracellular Ca2+, PYK-2, and p60(src)

Ca(2+)-dependent agonists, such as carbachol (CCh), stimulate epidermal growth factor receptor (EGFR) transactivation and mitogen-activated protein kinase activation in T(84) intestinal epithelial cells. This pathway constitutes an antisecretory mechanism by which CCh-stimulated chloride secretion is limited. Here, we investigated mechanisms underlying CCh-stimulated epidermal growth factor receptor (EGFR) transactivation. Thapsigargin (TG, 2 microM) stimulated EGFR and extracellular signal-regulated kinase (ERK) phosphorylation in T(84) cells. Inhibition of either EGFR or ERK activation, with tyrphostin AG1478 (1 microM) and PD 98059 (20 microM), respectively, potentiated chloride secretory responses to TG, as measured by changes in short-circuit current (I(sc)) across T(84) cells. CCh (100 microM) stimulated tyrosine phosphorylation and association of the Ca(2+)-dependent tyrosine kinase, PYK-2, with the EGFR, which was inhibited by the Ca(2+) chelator, BAPTA (20 microM). The calmodulin inhibitor, fluphenazine (50 microM) inhibited CCh-stimulated PYK-2 association with the EGFR and phosphorylation of EGFR and ERK. CCh also induced tyrosine phosphorylation of p60(src) and association of p60(src) with both PYK-2 and the EGFR. The Src family kinase inhibitor, PP2 (20 nM-20 microM) attenuated CCh-stimulated EGFR and ERK phosphorylation and potentiated chloride secretory responses to CCh. We conclude that CCh-stimulated transactivation of the EGFR is mediated by a pathway involving elevations in intracellular Ca(2+), calmodulin, PYK-2, and p60(src). This pathway represents a mechanism that limits CCh-stimulated chloride secretion across intestinal epithelia.

Epidermal growth factor receptor tyrosine kinase mediates Ras activation by gonadotropin-releasing hormone

Gonadotropin releasing hormone (GnRH) contributes to the maintenance of gonadotrope function by increasing extracellular signal-regulated kinase (ERK) activity subsequent to binding to its cognate G-protein-coupled receptor. As the GnRH receptor exclusively interacts with G(q/11) proteins and as receptor expression is regulated in a beta-arrestin-independent fashion, it represents a good model to systematically dissect underlying signaling pathways. In alphaT3-1 gonadotropes endogenously expressing the GnRH receptor, GnRH challenge resulted in a rapid increase in ERK activity which was attenuated by the epidermal growth factor receptor (EGFR)-specific tyrosine kinase inhibitor AG1478. In COS-7 cells transiently expressing the human GnRH receptor, agonist-induced ERK activation was independent of free Gbetagamma subunits but could be mimicked by short-term phorbol ester treatment. Most notably, G(q/11)-induced ERK activation was sensitive to N17-Ras and to expression of the C-terminal Src kinase but also to other dominant negative mutants of signaling components localized upstream of Ras, like Shc and the EGFR. GnRH as well as phorbol esters led to Ras activation in COS-7 and alphaT3-1 cells, which was dependent on Src and EGFR tyrosine kinases, indicating that both tyrosine kinases act downstream of protein kinase C (PKC) and upstream of Ras. However, Src did not contribute to Shc tyrosine phosphorylation. GnRH or phorbol ester challenge resulted in PKC-dependent EGFR autophosphorylation. Furthermore, a 5-min phorbol ester treatment was sufficient to trigger tyrosine phosphorylation of the platelet-derived growth factor-beta receptor in L cells. Thus, in several cell systems PKC is able to stimulate Ras via activation of receptor tyrosine kinases.

Antagonists of P2 receptor prevent NGF-dependent neuritogenesis in PC12 cells

The pheochromocytoma PC12 cell line that develops neuronal characteristics of sympathetic cells after treatment with nerve growth factor (NGF) represents a well-established cellular model system for studying NGF signalling. Interesting information on the different mechanistic pathways of NGF can be obtained by adopting the pharmacological approach of inhibiting P2 receptors, expressed in naive PC12 cells and recognised as important biological mediators of neurotransmitters and growth factors. We show here that Basilen Blue, an antagonist of P2 receptor, reversibly prevents NGF-dependent neurite outgrowth with an IC(50) in the 5-10 microM range. Suramin, oxidised-ATP and diisothiocyanatostilbene-disulfonic acid, differently from other purinoceptor ligands, are also effective in this regard. NGF-dependent regeneration and stability of neurites, selected NGF-dependent extracellular and intracellular protein phosphorylations, binding of [(3)H] ATP to PC12 cell membranes are also modulated by Basilen Blue. On the contrary, cell adhesion, cellular duplication, 5'-nucleotidase activity, NGF-induced tyrosine autophosphorylation of TrkA receptors are not affected. NGF furthermore directly modulates the extracellular release of ATP and especially the levels of P2X(2) receptor protein in PC12 cells. In addition, extracellular ATP improves the neuritogenic effect of sub-optimal concentrations of NGF. Our study identifies P2 receptor ligands, particularly Basilen Blue, as useful tools to dissect different NGF-evoked functions, suggesting a mechanistic role for P2 receptors in the signalling pathways of NGF.

Chemokine receptors CXCR-1/2 activate mitogen-activated protein kinase via the epidermal growth factor receptor in ovarian cancer cells

Ovarian cancer typically disseminates widely in the abdomen, a characteristic that limits curative therapy. The mechanisms that promote ovarian cancer cell migration are incompletely understood. We studied model SK-OV-3 ovarian cancer cells and observed robust expression of the alpha chemokine receptors CXCR-1 and CXCR-2. Interleukin-8 (IL-8) treatment caused shape changes in the cells, with membrane ruffling and formation/retraction of thin actin-like projections, as detected by time-lapse microscopy. Stimulation of the CXCR-1/2 receptors by human interleukin 8 (IL-8) rapidly activated the p44/42 mitogen-activated protein (extracellular signal-regulated kinase (Erk1/2)) kinase pathway. Treatment of SK-OV-3 cells with the inhibitors genestein and herbimycin A indicated that tyrosine kinases were involved in the IL-8 activation of Erk1 and Erk2. Of note, IL-8 induced transient phosphorylation of the epidermal growth factor (EGF) receptor and its association with the adaptor molecules Shc and Grb2. This transactivation of the EGF receptor was dependent on intracellular Ca(2+) mobilization. Furthermore AG1478, a specific inhibitor of the EGF receptor kinase, blocked Erk1 and Erk2 activation. c-Src kinase was not involved in the IL-8-mediated phosphorylation of the EGF receptor, but was critical for Shc phosphorylation and downstream Erk1/2 kinase activation. These results suggest important "cross-talk" between chemokine and growth factor pathways that may link signals of cell migration and proliferation in ovarian cancer.

P2Y receptor regulation of PAI-1 expression in vascular smooth muscle cells

P2Y-type purine and pyrimidine nucleotide receptors play important roles in the regulation of vascular hemostasis. In this article, the regulation of plasminogen activator inhibitor-1 (PAI-1) expression in rat aortic smooth muscle cells (RASMCs) by adenine and uridine nucleotides was examined and compared. Northern analysis revealed that RASMCs express multiple P2Y receptor subtypes, including P2Y(1), P2Y(2), and P2Y(6). Treatment of RASMCs with UTP increased PAI-1 mRNA expression and extracellular PAI-1 protein levels by 21-fold (P<0.001) and 7-fold (P<0.001), respectively. The ED(50) for the effect of UTP on PAI-1 expression was approximately 1 micromol/L, and its maximal effect occurred at 3 hours. UDP stimulated a 5-fold increase (P<0.005) in PAI-1 expression. In contrast to these potent stimulatory effects of uridine nucleotides, ATP and 2-methylthioadenosine triphosphate (2-MeSATP) caused a small and transient increase in PAI-1 mRNA at 1 hour, followed by a rapid decrease to baseline levels. ADP produced only an inhibitory effect, reducing PAI-1 mRNA levels by 63% (P<0.05) at 3 hours. The relative nucleotide potency in stimulating PAI-1 expression is UTP>UDP>ATP=2-MeSATP, consistent with a predominant role of the P2Y(6) receptor. Further studies revealed that exposure of RASMCs to either ATP or ADP for 3 hours inhibited both UTP- and angiotensin II-stimulated PAI-1 expression by up to 90% (P<0.001). Thus, ATP induced a small and transient upregulation of PAI-1 that was followed by a strong inhibition of PAI-1 expression. These results show that extracellular adenine and uridine nucleotides exert potent and opposing effects on vascular PAI-1 expression.

Nonsteroidal anti-inflammatory drugs attenuate proliferation of colonic carcinoma cells by blocking epidermal growth factor-induced Ca++ mobilization

Numerous studies suggest that nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit colorectal carcinogenesis. We have previously reported that NSAIDs, in human colonic carcinoma cells (Caco-2), attenuate epidermal growth factor (EGF)-induced cellular proliferation through a process independent of their inhibitory effects on prostaglandin synthesis. Furthermore, separate studies have also suggested that NSAIDs inhibit EGF-induced store-operated Ca++ influx. Thus we developed the hypothesis that NSAIDs may limit the activity of EGF by altering intracellular Ca++ ([Ca++]i) mobilization. Serum-deprived Caco-2 cells were employed for all experimentation. [Ca++]i was measured with Fluo-3 and extracellular Ca++ influx was monitored by quenching Fluo-3 fluorescence with Mn++. Proliferation was quantitated with two assays: cellular nucleic acid and total protein content. Caco-2 cells exposed to EGF demonstrated an initial increase in [Ca++]i which was blocked by neomycin, an inhibitor of IPsubscript 3 generation, and the phospholipase C inhibitor U73122 but not U73343 (inactive control). This was followed by sustained extracellular Ca++ influx, which was attenuated with calcium-free buffer (-Ca++), the store- operated Ca++ channel blocker lanthanum, indomethacin, ibuprofen, and aspirin. In subsequent studies, cells were treated with either serum-free media or EGF +/- the aforementioned inhibitors, and again serum starved. Cells exposed to EGF +/- the inactive phospholipase C inhibitor U73343 demonstrated a significant increase in nucleic acid and protein. However, proliferation induced by EGF was not observed when [Ca++]i elevation was prevented by blocking either internal Ca++ store release via phospholipase C/IPsubscript 3 or sustained Ca++ influx through store-operated Ca++ channels. Sustained [Ca++]i elevation, as induced by EGF, appears to be required for mitogenesis. These data support our premise that one mechanism whereby NSAIDs may attenuate colonic neoplasia is by blocking EGF-induced Ca++ mobilization.