The SRC-associated protein CUB Domain-Containing Protein-1 regulates adhesion and motility.

Multiple SRC-family kinases (SFKs) are commonly activated in carcinoma and appear to have a role in metastasis through incompletely understood mechanisms. Recent studies have shown that CDCP1 (CUB (complement C1r/C1s, Uegf, Bmp1) Domain-Containing Protein-1) is a transmembrane protein and an SRC substrate potentially involved in metastasis. Here we show that increased SFK and CDCP1 tyrosine phosphorylation is, surprisingly, associated with a decrease in FAK phosphorylation. This appears to be true in human tumors as shown by our correlation analysis of a mass spectrometric data set of affinity-purified phosphotyrosine peptides obtained from normal and cancer lung tissue samples. Induction of tyrosine phosphorylation of CDCP1 in cell culture, including by a mAb that binds to its extracellular domain, promoted changes in SFK and FAK tyrosine phosphorylation, as well as in PKC(TM), a protein known to associate with CDCP1, and these changes are accompanied by increases in adhesion and motility. Thus, signaling events that accompany the CDCP1 tyrosine phosphorylation observed in cell lines and human lung tumors may explain how the CDCP1/SFK complex regulates motility and adhesion.

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.

Phosphatidylinositol 3-kinase-dependent activation of trypsinogen modulates the severity of acute pancreatitis.

Intra-acinar cell activation of digestive enzyme zymogens including trypsinogen is generally believed to be an early and critical event in acute pancreatitis. We have found that the phosphatidylinositol 3-kinase inhibitor wortmannin can reduce the intrapancreatic activation of trypsinogen that occurs during two dissimilar experimental models of rodent acute pancreatitis, secretagogue- and duct injection-induced pancreatitis. The severity of both models was also reduced by wortmannin administration. In contrast, the NF-kappa B activation that occurs during the early stages of secretagogue-induced pancreatitis is not altered by administration of wortmannin. Ex vivo, caerulein-induced trypsinogen activation is inhibited by wortmannin and LY294002. However, the cytoskeletal changes induced by caerulein were not affected by wortmannin. Concentrations of caerulein that induced ex vivo trypsinogen activation do not significantly increase phosphatidylinositol-3,4-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate levels or induce phosphorylation of Akt/PKB, suggesting that class I phosphatidylinositol 3-kinases are not involved. The concentration of wortmannin that inhibits trypsinogen activation causes a 75% decrease in phosphatidylinositol 3-phosphate, which is implicated in vesicle trafficking and fusion. We conclude that a wortmannin-inhibitable phosphatidylinositol 3-kinase is necessary for intrapancreatic activation of trypsinogen and regulating the severity of acute pancreatitis. Our observations suggest that phosphatidylinositol 3-kinase inhibition might be of benefit in preventing acute pancreatitis.

Rottlerin is a mitochondrial uncoupler that decreases cellular ATP levels and indirectly blocks protein kinase Cdelta tyrosine phosphorylation.

Protein kinase Cdelta (PKCdelta) is activated by stimuli that increase its tyrosine phosphorylation, including neurotransmitters that initiate fluid secretion in salivary gland (parotid) epithelial cells. Rottlerin, a compound reported to be a PKCdelta-selective inhibitor, rapidly increased the rate of oxygen consumption (QO2) of parotid acinar cells and PC12 cells. In parotid cells, this was distinct from the effects of the muscarinic receptor ligand carbachol, which promoted a sodium pump-dependent increase in respiration. Rottlerin increased the QO2 of isolated rat liver mitochondria to a level similar to that produced when oxidative phosphorylation was initiated by ADP or when mitochondria were uncoupled by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The effects of rottlerin on mitochondrial QO2 were neither mimicked nor blocked by the PKC inhibitor GF109203X. Rottlerin was not effective in blocking PKCdelta activity in vitro. Exposure of freshly isolated parotid acinar cells to rottlerin and FCCP reduced cellular ATP levels and reduced stimuli-dependent increases in tyrosine phosphorylation of PKCdelta. Neither rottlerin nor FCCP reduced stimuli-dependent PKCdelta tyrosine phosphorylation in RPG1 cells (a salivary ductal line) or PC12 cells, consistent with their dependence on glycolysis rather than oxidative phosphorylation for energy-dependent processes. These results demonstrate that rottlerin directly uncouples mitochondrial respiration from oxidative phosphorylation. Previous studies using rottlerin should be evaluated cautiously.

P2Y(6) nucleotide receptor mediates monocyte interleukin-8 production in response to UDP or lipopolysaccharide.

Extracellular nucleotides are autocrine and paracrine cellular mediators that signal through P2 nucleotide receptors. Monocytic cells express several P2Y receptors but the role of these G protein-coupled receptors in monocytes is not known. Here, we present evidence that P2Y(6) regulates chemokine production and release in monocytes. We find that UDP, a selective P2Y(6) agonist, stimulates interleukin (IL)-8 release in human THP-1 monocytic cells whereas other nucleotides are relatively inactive. P2 receptor antagonists or P2Y(6) antisense oligonucleotides inhibit IL-8 release induced by UDP. Furthermore, UDP specifically activated IL-8 production in astrocytoma 1321N1 cells transfected with human P2Y(6). Since lipopolysaccharide has been suggested to activate P2 receptors via nucleotide release, we tested whether IL-8 production stimulated by lipopolysaccharide might result from P2Y(6) activation. P2 antagonists or apyrase, an enzyme which hydrolyzes nucleotides including UDP, inhibit IL-8 production induced by lipopolysaccharide but not by other stimuli. Furthermore, IL-8 gene expression activated by lipopolysaccharide is enhanced by P2Y(6) overexpression and inhibited by P2Y(6) antisense oligonucleotides. Thus, UDP activates IL-8 production via P2Y(6) in monocytic cells. Furthermore, lipopolysaccharide mediates IL-8 production at least in part by autocrine P2Y(6) activation. These findings indicate a novel role for P2Y(6) in innate immune defenses.

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

Protein kinase C (PKC) delta 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 PKCdelta tyrosine phosphorylation and effects of phosphorylation on PKCdelta activity. Carbachol- and substance P-promoted increases in PKCdelta 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 PKCdelta 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 PKCdelta activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKCdelta activation. Two PKCdelta regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKCdelta tyrosine phosphorylation. These results demonstrate that PKCdelta 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.

Activation of PKCdelta by tyrosine phosphorylation in rat parotid acinar cells.

Receptor- and nonreceptor-mediated stimuli produce increases in both PKCdelta tyrosine phosphorylation and activity in rat parotid acinar cells and other cells. In vivo and in vitro increases and decreases in tyrosine phosphorylation resulted in increases and decreases, respectively, of PKCdelta activity. These studies demonstrated that increases in PKCdelta activity by G protein-coupled receptors and other stimuli were controlled by alterations in tyrosine phosphorylation.

PI3K signaling in the murine kidney inner medullary cell response to urea.

Growth factors and other stimuli increase the activity of phosphatidylinositol-3 kinase (PI3K), an SH2 domain-containing lipid kinase. In the murine kidney inner medullary mIMCD3 cell line, urea (200 mM) increased PI3K activity in a time-dependent fashion as measured by immune complex kinase assay. The PI3K effector, Akt, was also activated by urea as measured by anti-phospho-Akt immunoblotting. In addition, the Akt (and PI3K) effector, p70 S6 kinase, was activated by urea treatment in a PI3K-dependent fashion. PI3K inhibition potentiated the proapoptotic effect of hypertonic and urea stress. Urea treatment also induced the tyrosine phosphorylation of Shc and the recruitment to Shc of Grb2. Coexistence of activated Shc and PI3K in a macromolecular complex was suggested by the increase in PI3K activity evident in anti-Shc immunoprecipitates prepared from urea-treated cells. Taken together, these data suggest that PI3K may regulate physiological events in the renal medullary cell response to urea stress and that an upstream tyrosine kinase conferring activation of both PI3K and Shc may govern urea signaling in these cells.

Multiple roles for Src in a PDGF-stimulated cell.

The observation that platelet-derived growth factor (PDGF) increases the catalytic activity of Src family members (Src) suggests that they contribute to PDGF-dependent responses. The role of Src in PDGF-dependent cell cycle progression, phosphorylation of proteins, and chemotaxis has been tested by investigators using a variety of cell types and approaches, and it appears that the contribution of Src is highly variable. This idea is perhaps best illustrated by the finding that Src plays radically different roles downstream of the PDGF alpha- and beta-receptor subunits. Hence, Src is a versatile signal relay enzyme, whose contribution to a signaling cascade depends on variables such as the nature of the receptor via which the cell is activated, as well as the cell type itself.

Involvement of protein kinases and phosphatases in tyrosine phosphorylation of PKCdelta in rat parotid acinar cells exposed to secretory stimuli.

The aim of these studies was to examine the involvement of tyrosine phosphorylation in the signal transduction pathways and secretory events that are promoted by receptor agonists acting on rat parotid acinar cells. Fluid secretion by parotid acinar cells is initiated by the binding of neurotransmitters to GTP(G)-protein-coupled receptors that are linked to phospholipase C, which hydrolyzes phosphatidlyinositol 4,5-bisphosphate to diacylglycerol and inositol 1,4,5-trisphosphate. Although growth factors produce large changes in tyrosine phosphorylation of multiple proteins involved in proliferation and other cellular processes, tyrosine phosphorylation is not considered to be a general phenomenon of G-protein-coupled receptor activation. However, our results shown that carbachol (a muscarinic acetylcholine receptor agonist), and ligands to other phospholipase C-linked receptors, promoted a rapid increase in the tyrosine phosphorylation of protein kinase Cdelta (PKCdelta), a member of the PKC family of proteins. Phorbol 12-myristate 13-acetate, which binds to the site on PKCdelta to which the endogenous activator sn-1,2-diacylglycerol binds, also increased the tyrosine phosphorylation of PKCdelta. Genistein and staurosporine, two protein kinase inhibitors, blocked the tyrosine phosphorylation of this protein. Thus, PKCdelta becomes tyrosine phosphorylated in response to receptor activation, and this event appears to involve both diacylglycerol production and protein tyrosine kinase activity. This may contribute to early physiological events, including alterations in fluid secretion, that are initiated by neurotransmitters acting on the parotid salivary gland.

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.

The activation of growth factor receptors and receptors coupled to heterotrimeric guanine nucleotide-binding proteins (G-proteins) can increase mitogen-activated protein (MAP) kinase activity in many cells. Previously, we demonstrated that the activation of G-protein-coupled P2Y2 receptors by extracellular ATP and UTP stimulated MAP (p42 ERK2) kinase by a mechanism that was dependent on the elevation of [Ca2+]i and the activation of related adhesion focal tyrosine kinase (RAFTK) (also called PYK2, CAKbeta, and CADTK) and protein kinase C (PKC). Here, we examine further the signaling cascade between the P2Y2 receptor and MAP kinase. MAP kinase was transiently activated by exposure of PC12 cells to UTP. UTP, ionomycin, and phorbol ester (phorbol 12-myristate 13-acetate) increased MAP kinase activity and also promoted the tyrosine phosphorylation of RAFTK, the epidermal growth factor (EGF) receptor, SHC, and p120(cbl). Down-regulation of PKC and inhibition of the elevation of [Ca2+]i, conditions that block the activation of MAP kinase, also blocked the increases in the tyrosine phosphorylation of RAFTK and the EGF receptor. AG1478, a tyrphostin selective for the EGF receptor, reduced the activation of MAP kinase, the tyrosine phosphorylation of SHC, the association of Grb2 with SHC, and the tyrosine phosphorylation of the EGF receptor and p120(cbl) but did not block the tyrosine phosphorylation of RAFTK. The similar effects of UTP, ionomycin, and phorbol 12-myristate 13-acetate (PMA) on these signaling proteins demonstrate that the two signaling molecules from phosphatidylinositol 4,5-bisphosphate hydrolysis ([Ca2+]i, from inositol 1,4,5-trisphosphate production, and diacylglycerol) can individually initiate the activation of MAP kinase in an EGF receptor-dependent manner. These results demonstrate that the P2Y2 receptor-mediated transactivation of the EGF receptor occurs at a point downstream of RAFTK and indicate that the EGF receptor is required for P2Y2 receptor-mediated MAP kinase activation. Although P2Y2 and EGF receptors may both activate a similar multiprotein signaling cascade immediately upstream of MAP kinase, the P2Y2 receptor appears to uniquely utilize [Ca2+]i, PKC, and, subsequently, RAFTK.

Development of salivary gland cell lines for studies of signaling and physiology.

We developed and characterized an immortalized rat parotid cell line to use in salivary gland studies. The cells were immortalized by retroviral transduction of SV40 large T antigen into isolated parotid cells. Using immunocytochemical techniques, we found that the immortalized epithelial cells were ductal, rather than acinar, in nature. Cells were grown under coculture conditions with lethally irradiated NIH3T3 cells. One cell line, which was designated RPG1/SV40 cells (for rat parotid gland 1/SV40 transformant), was selected for characterization. These cells formed a sheet epithelium with tight junctions and a measurable transepithelial resistance. RPG1/SV40 cells responded to muscarinic receptor (carbachol) and/or P2 purinoceptor (ATP and UTP) stimuli with increases in the following: (1) intracellular free-calcium concentration ([Ca2+]i); (2) the short-circuit current (ISC) across the epithelium; (3) the tyrosine phosphorylation of PKC delta; and (4) MAP kinase activity. Thus, the cells appear to be useful for a wide range of studies involving physiology, biochemistry, and signal transduction approaches.

Activation of P2Y2 receptors by UTP and ATP stimulates mitogen-activated kinase activity through a pathway that involves related adhesion focal tyrosine kinase and protein kinase C.

We examined downstream signaling events that followed the exposure of PC12 cells to extracellular ATP and UTP, and we compared the effects of these P2 receptor agonists with those of growth factors and other stimuli. Based on early findings, we focused particular attention on the mitogen-activated protein (MAP) kinase pathway. ATP and/or UTP produced increases in tyrosine phosphorylation of multiple proteins, including p42 MAP (ERK2) kinase, related adhesion focal tyrosine kinase (RAFTK) (PYK2, CAKbeta), focal adhesion kinase (FAK), Shc, and protein kinase Cdelta (PKCdelta). MAP (ERK2) kinase activity (quantified by substrate phosphorylation) was increased by UTP, ATP, phorbol 12-myristate 13-acetate, ionomycin, and growth factors. UTP and ATP were equipotent (EC50 approximately 25 microM) in stimulating MAP kinase activity, suggesting that these effects were mediated via the Gi-linked P2Y2 (P2U) receptor. Consistent with this, the UTP- and ATP-promoted activation of MAP kinase was diminished in pertussis toxin-treated cells. Treatment of cells with pertussis toxin also reduced both the UTP-dependent increases in intracellular calcium ion concentration ([Ca2+]i) and the tyrosine phosphorylation of RAFTK. Similarly, when [Ca2+]i elevation was prevented using BAPTA and EGTA, the activation of MAP kinase by UTP and ionomycin was blocked, and the tyrosine phosphorylation of RAFTK was reduced. The UTP-promoted increase in MAP kinase activity was partially reduced in cells in which PKC was down-regulated, suggesting that both PKC-dependent and PKC-independent pathways were involved. PKCdelta, which increases MAP kinase activity in some systems, became tyrosine-phosphorylated within 15 s of exposure of cells to ATP or UTP; but epidermal growth factor, nerve growth factor, and insulin had little effect. UTP also promoted the association of Shc with Grb2. These results suggest that the P2Y2 receptor-initiated activation of MAP kinase was dependent on the elevation of [Ca2+]i, involved the recruitment of Shc and Grb2, and was mediated by RAFTK and PKC.

A dominant role of the juxtamembrane region of the TrkA nerve growth factor receptor during neuronal cell differentiation.

All receptor tyrosine kinases share a common intracellular signaling machinery, including ras activation, whereas cellular responses vary from mitogenesis to cell differentiation. To investigate the structural basis for receptor tyrosine kinase action for nerve growth factor, the juxtamembrane region of TrkA was transferred to a corresponding region of the epidermal growth factor (EGF) receptor. The resulting chimeric receptor contains an additional Shc site, Tyr490, in the juxtamembrane region. In transfected PC12 cell lines, neuronal differentiation was observed with EGF treatment, as evidenced by increased neurite extension. The action of the chimeric receptor was correlated with prolonged activation of MAP kinases and a 3-4-fold increase in phosphatidylinositol 3-kinase activity. The effect of the juxtamembrane chimera was dependent upon the Shc site at Tyr490, because expression of a chimeric receptor containing a Y490F mutation resulted in a complete loss of neuritogenesis by EGF treatment. These findings indicate that the juxtamembrane region of the TrkA receptor serves as a key functional domain that can confer a dominant effect upon neuronal differentiation.

Extracellular calcium and platelet-derived growth factor promote receptor-mediated chemotaxis in osteoblasts through different signaling pathways.

The discovery of a calcium receptor has stimulated interest in the signaling events underlying extracellular calcium ([Ca2+]o)-induced cell-specific responses. In osteoblasts, elevated levels of extracellular calcium mediate both mitogenesis and chemotaxis. Here we provide evidence that [Ca2+]o-stimulated chemotaxis of MC3T3-E1 osteoblast-like cells involves a G-protein-linked calcium-sensing receptor. [Ca2+]o promotes chemotaxis in a concentration-dependent manner. Pertussis toxin blocked almost all of [Ca2+]o-stimulated chemotaxis but had only a small effect on platelet-derived growth factor (PDGF)-stimulated chemotaxis. Consistent with the signaling model for PDGF-mediated chemotaxis, activation of phospholipase C played a critical role in [Ca2+]o-initiated chemotaxis: U-73122, an inhibitor of the activation of phospholipase C, blocked approximately 50% of PDGF-stimulated chemotaxis but blocked nearly all of the [Ca2+]o-stimulated chemotaxis. Down-regulation of protein kinase C also blocked about 50% of PDGF-stimulated chemotaxis but did not block [Ca2+]o-stimulated chemotaxis. Thus, unlike PDGF-mediated chemotaxis, chemotaxis stimulated by [Ca2+]o does not appear to require protein kinase C activation. This finding suggests events downstream of inositol 1,4,5-trisphosphate production rather than diacylglycerol production are critical to [Ca2+]o-promoted chemotaxis of MC3T3-E1 cells. The signal transduction mechanism underlying PDGF-induced chemotaxis involves the activation of phosphoinositide 3-kinase, as judged by the in vivo production of phosphatidylinositol 3,4-diphosphate and 3,4,5-trisphosphate and the partial sensitivity of chemotaxis to wortmannin, an inhibitor of phosphoinositide 3-kinase. In contrast, [Ca2+]o-stimulated chemotaxis was not blocked by wortmannin and elevations in [Ca2+]o did not increase the production of lipid products of phosphoinositide 3-kinase. Overall, [Ca2+]o-promoted chemotaxis of osteoblasts appears to utilize a unique signaling mechanism via a calcium-sensing receptor.

Insulin-like growth factor and potassium depolarization maintain neuronal survival by distinct pathways: possible involvement of PI 3-kinase in IGF-1 signaling.

Cultured cerebellar granule neurons die by apoptosis when switched from a medium containing an elevated level of potassium (K+) to one with lower K+ (5 mM). Death resulting from the lowering of K+ can be prevented by insulin-like growth factor (IGF-1). To understand how IGF-1 inhibits apoptosis and maintains neuronal survival, we examined the role of phosphoinositide 3-kinase (PI 3-kinase). Activation of PI 3-kinase has been shown previously to be required for NGF-mediated survival in the PC12 pheochromocytoma cell line. We find that in primary neurons, IGF-1 treatment leads to a robust activation of PI 3-kinase, as judged by lipid kinase assays and Western blot analysis. Activation of PI 3-kinase is likely to occur via tyrosine phosphorylation of the insulin receptor substrate protein. Treatment with two chemically distinct inhibitors of PI 3-kinase, wortmannin and LY294002, reduces PI 3-kinase activation by IGF-1 and inhibits its survival-promoting activity, suggesting that PI 3-kinase is necessary for IGF-1-mediated survival. Death resulting from PI 3-kinase blockade is accompanied by DNA fragmentation, a hallmark of apoptosis. Furthermore, neurons subjected to PI 3-kinase blockade can be rescued by transcriptional and translation inhibitors, suggesting that IGF-1-mediated activation of PI 3-kinase leads to a suppression of "killer gene" expression. In sharp contrast to IGF-1, elevated K+ does not activate PI 3-kinase and can maintain neuronal survival in the presence of PI 3-kinase inhibitors. Therefore, survival of granule neurons can be maintained by PI 3-kinase dependent (IGF-1-activated) and independent (elevated K+-activated) pathways.

p120cbl is a cytosolic adapter protein that associates with phosphoinositide 3-kinase in response to epidermal growth factor in PC12 and other cells.

Although epidermal growth factor (EGF) activates phosphoinositide (PI) 3-kinase activity in a number of types of cells or cell lines, in most cases that we have investigated the p85 regulatory subunit of PI 3-kinase does not appear to bind directly to the EGF receptor. Previously we demonstrated that EGF-dependent activation of PI 3-kinase activity in A431 cells is accompanied by the binding of p85 to ErbB3, an EGF receptor homologue. However, this mechanism did not explain the large activation of PI 3-kinase activity that was found in PC12 and A549 cells, which possess little or no ErbB3. Here we provide evidence that the p120cbl proto-oncoprotein is an intracellular adapter protein that associates with PI 3-kinase and thus is involved in the EGF-dependent activation of this enzyme in these two cell lines. Using an anti-p120cbl antibody, we immunoprecipitated the EGF receptor from PC12 cells and PI 3-kinase activity from PC12 and A549 cells in an EGF-dependent fashion. Treatment of PC12 cells with nerve growth factor or insulin stimulated large increases in PI 3-kinase activity that was immunoprecipitated using anti-Tyr(P) antibody but not using anti-p120cbl antibody. In EGF-treated PC12 cells, the tyrosine phosphorylation of p120cbl displayed similar kinetics to the activation of PI 3-kinase as measured by both in vivo lipid production and lipid kinase assays conducted using anti-p120cbl and anti-Tyr(P) immunoprecipitates. The use of glutathione S-transferase fusion proteins of various domains of p85 demonstrated that p120cbl associated with both the SH2 and SH3 domains of p85. p120cbl was also present in A431 cells and offers an additional pathway by which EGF can activate PI 3-kinase in these cells.

Carbachol, substance P, and phorbol ester promote the tyrosine phosphorylation of protein kinase C delta in salivary gland epithelial cells.

The initiation of saliva formation by parotid acinar cells, which comprise the majority of cells in this salivary gland, is initiated by the release of neurotransmitters (acetylcholine, substance P) from parasympathetic nerves. In response to substance P and the muscarinic agonist carbachol, two ligands that activate phospholipase C-linked receptors, which stimulate fluid secretion, PKC delta was phosphorylated on tyrosine residues. The maximal agonist-dependent tyrosine phosphorylation occurred within seconds of the addition of either agonist and then returned rapidly to a smaller increased level. Phorbol ester also caused a rapid increase in tyrosine phosphorylation, which reached a maximal level 5 min after the addition of phorbol 12-myristate 13-acetate. The increase in tyrosine phosphorylation of PKC delta was blocked by tyrosine kinase inhibitors genistein and staurosporine. Ionophore-mediated elevation of [Ca2+]i or activation of the beta-adrenergic receptor, epidermal growth factor receptor, or insulin receptor did not promote the tyrosine phosphorylation of PKC delta. These results indicate that tyrosine phosphorylation plays a role in early signal transduction events promoted by the activation of muscarinic and substance P receptors and suggests that the tyrosine phosphorylation of PKC delta has a role in the activation of fluid secretion by neurotransmitters binding to phospholipase C-linked receptors.

Purinoceptor P2U identification and function in human intrahepatic biliary epithelial cell lines.

The mechanisms that regulate ion and fluid transport by the human intrahepatic bile duct have not been well defined. Human intrahepatic biliary cell lines that we have developed were used to identify and characterize purinoceptors based on increases in intracellular calcium in response to ATP and other nucleotides. Intracellular free calcium was measured in cell suspensions using the fluorescent probe Fura-2 and a fluorescence spectrophotometer. Halide efflux was measured in single cells using fluorescence microscopy and the fluorescent probe SPQ. Intracellular calcium increases equivalently in response to ATP and UTP, peaking, then diminishing to a new, elevated baseline. The peak elevation of calcium is the result of both the release of intracellular stores of calcium and the influx of extracellular calcium. The purinoceptor P2U-subtype was identified based on the potency rank order of ATP-analogues. Halide efflux increases with P2U-purinoceptor stimulation which is consistent with the opening of a Ca(2+)-sensitive Cl- channel. The physiological significance of P2U-purinoceptor activation and its effect on the ionic content and flow rate of bile remains to be determined.

Heregulin stimulates mitogenesis and phosphatidylinositol 3-kinase in mouse fibroblasts transfected with erbB2/neu and erbB3.

Heregulin (HRG) is a pluripotent growth factor that can stimulate the growth of some human mammary tumor cells and the differentiation of others. Two members of the epidermal growth factor receptor family of receptor/tyrosine kinases, p180erbB3 and p180erbB4, serve as receptors for the HRG ligand. While HRG appears to be capable of stimulating the autophosphorylation activity of p180erbB4, the co-expression of p185erbB2/neu with p180erbB3 is necessary for the HRG-stimulated tyrosine phosphorylation of both of these receptors. On the basis of the sequences surrounding their putative tyrosine phosphorylation sites, we predict that the different HRG-responsive receptors couple to different intracellular SH2 domain-containing proteins. Hence, the different receptors may mediate different cellular responses to the HRG ligand. In the present study we show that HRG beta 1 is mitogenic for erbB3-transfected DHFR/G8 cells, an NIH3T3 mouse fibroblast derivative that over-expresses p185erbB2/neu. HRG stimulated the incorporation of [3H]thymidine into the DNA of these cells with an EC50 of 70 +/- 7 pM. HRG was not mitogenic for parental DHFR/G8 cells that do not express the ErbB3 protein. Phosphatidylinositol (PI) 3-kinase, an enzyme believed to be important in cellular growth regulation by growth factors and oncogenes, is predicted to couple to tyrosine-phosphorylated ErbB3. We observed that HRG stimulated the association of PI 3-kinase with both p185erbB2/neu and ErbB3 in transfected DHFR/G8 cells, but not in the parental cell line. We conclude that the ErbB3 protein is capable of mediating a proliferative response of fibroblasts to HRG, and that the activation of PI 3-kinase is an integral part of the growth signaling mechanism.