Bombesin, vasopressin, and endothelin rapidly stimulate tyrosine phosphorylation in intact Swiss 3T3 cells

Protein kinase D1 (PKD1) phosphorylation on Ser203 by type I p21-activated kinase (PAK) regulates PKD1 localization

Although PKC-mediated phosphorylation of protein kinase D1 (PKD1) has been extensively characterized, little is known about PKD1 regulation by other upstream kinases. Here we report that stimulation of epithelial or fibroblastic cells with G protein-coupled receptor agonists, including angiotensin II or bombesin, induced rapid and persistent PKD1 phosphorylation at Ser²⁰³, a highly conserved residue located within the PKD1 N-terminal domain. Exposure to PKD or PKC family inhibitors did not prevent PKD1 phosphorylation at Ser²⁰³, indicating that it is not mediated by autophosphorylation. In contrast, several lines of evidence indicated that the phosphorylation of PKD1 at Ser²⁰³ is mediated by kinases of the class I PAK subfamily, specifically 1) exposing cells to four structurally unrelated PAK inhibitors (PF-3758309, FRAX486, FRAX597, and IPA-3) that act via different mechanisms abrogated PKD1 phosphorylation at Ser²⁰³, 2) siRNA-mediated knockdown of PAK1 and PAK2 in IEC-18 and Swiss 3T3 cells blunted PKD1 phosphorylation at Ser²⁰³, 3) phosphorylation of Ser²⁰³ markedly increased in vitro when recombinant PKD1 was incubated with either PAK1 or PAK2 in the presence of ATP. PAK inhibitors did not interfere with G protein-coupled receptor activation-induced rapid translocation of PKD1 to the plasma membrane but strikingly prevented the dissociation of PKD1 from the plasma membrane and blunted the phosphorylation of nuclear targets, including class IIa histone deacetylases. We conclude that PAK-mediated phosphorylation of PKD1 at Ser²⁰³ triggers its membrane dissociation and subsequent entry into the nucleus, thereby regulating the phosphorylation of PKD1 nuclear targets, including class IIa histone deacetylases.

Gastrin-releasing peptide, immune responses, and lung disease

Gastrin-releasing peptide (GRP) is produced by pulmonary neuroendocrine cells (PNECs), with highest numbers of GRP-positive cells present in fetal lung. Normally GRP-positive PNECs are relatively infrequent after birth, but PNEC hyperplasia is frequently associated with chronic lung diseases. To address the hypothesis that GRP mediates chronic lung injury, we present the cumulative evidence implicating GRP in bronchopulmonary dysplasia (BPD), the chronic lung disease of premature infants who survive acute respiratory distress syndrome. The availability of well-characterized animal models of BPD was a critical tool for demonstrating that GRP plays a direct role in the early pathogenesis of this disease. Potential mechanisms by which GRP contributes to injury are analyzed, with the main focus on innate immunity. Autoreactive T cells may contribute to lung injury late in the course of disease. A working model is proposed with GRP triggering multiple cell types in both the innate and adaptive immune systems, promoting cascades culminating in chronic lung disease. These observations represent a paradigm shift in the understanding of the early pathogenesis of BPD, and suggest that GRP blockade could be a novel treatment to prevent this lung disease in premature infants.

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

Receptor-operated Ca2+ influx and its association with the Src family in secretagogue-stimulated pancreatic acini

We investigated signal transduction between receptor-operated Ca(2+) influx (ROCI) and Src-related nonreceptor protein tyrosine kinase (PTK) in rat pancreatic acini. CCK and the Ca(2+) ionophore enhanced the Src-related PTK activity, whereas the high-affinity CCK-A receptor agonists, fibroblast growth factor (FGF), and the protein kinase C (PKC) activator had no or little effect. This increase was abolished by eliminating [Ca(2+)](o), loading of the intracellular Ca(2+) chelator, and administering the PTK inhibitor genistein. While genistein inhibited extracellular Ca(2+) or Mn(2+) entry induced by CCK and carbachol, it did not affect intracellular Ca(2+) release and oscillations. CCK dose-dependently increased the Src phosphotransferase activity, which was abolished by inhibitors of G(q) protein, phospholipase C (PLC), and Src, but not by the calmodulin kinase (CaMK) inhibitor. Intensities of the Src band and amounts of tyrosine phosphorylated Src were enhanced by CCK stimulation. Thus, Src cascades appear to be coupled to the low-affinity CCK-A receptor and utilize G(q)-PLC pathways for their activation, independent of PKC and CaMK cascades. The low-affinity CCK-A receptor regulates ROCI via mediation of Src-related PTK and activates Src pathways to cause [Ca(2+)](o)-dependent pancreatic exocytosis.

EGF receptor transactivation is obligatory for protein synthesis stimulation by G protein-coupled receptors

The epidermal growth factor receptor (EGFR) was recently identified as a signal transducer of G protein-coupled receptors (GPCRs). In this study, we have examined the contribution of EGFR transactivation to the growth-promoting effect of GPCRs on vascular smooth muscle cells. Activation of the G(q)-coupled ANG II receptor or G(i)-coupled lysophosphatidic acid receptor resulted in increased tyrosine phosphorylation and activation of EGFR. Specific inhibition of EGFR kinase activity by tyrphostin AG-1478 or expression of a dominant-negative EGFR mutant abolished this response. Importantly, inhibition of EGFR function strongly attenuated the global stimulation of protein synthesis by GPCR agonists in vitro in cultured aortic smooth muscle cells and in vivo in the rat aorta and in small resistance arteries. The growth inhibition was associated with a marked reduction of extracellular signal-regulated kinase and phosphoinositide 3-kinase pathway activity and the resulting suppression of eukaryotic translation initiation factor 4E and 4E binding protein 1 phosphorylation. Our results demonstrate that EGFR transactivation is a physiologically relevant action of GPCRs linked to translational control and protein synthesis.

Src-family tyrosine kinases, phosphoinositide 3-kinase and Gab1 regulate extracellular signal-regulated kinase 1 activation induced by the type A endothelin-1 G-protein-coupled receptor

The multisubstrate docking protein, growth-factor-receptor-bound protein 2-associated binder 1 (Gab1), which is phosphorylated on tyrosine residues following activation of receptor tyrosine kinases and cytokine receptors, regulates cell proliferation, survival and epithelial morphogenesis. Gab1 is also tyrosine phosphorylated following activation of G-protein-coupled receptors (GPCRs) where its function is poorly understood. To elucidate the role of Gab1 in GPCR signalling, we investigated the mechanism by which the type A endothelin-1 (ET-1) GPCR induced tyrosine phosphorylation of Gab1. Tyrosine phosphorylation of Gab1 induced by endothelin-1 was inhibited by PP1, a pharmacological inhibitor of Src-family tyrosine kinases. ET-1-induced Gab1 tyrosine phosphorylation was also inhibited by LY294002, which inhibits phosphoinositide 3-kinase (PI 3-kinase) enzymes. Inhibition of Src-family tyrosine kinases or PI 3-kinase also inhibited ET-1-induced activation of the mitogen activated protein kinase family member, extracellular signal-regulated kinase (ERK) 1. Thus we determined whether Gab1 regulated ET-1-induced ERK1 activation. Overexpression of wild-type Gab1 potentiated ET-1-induced activation of ERK1. Structure-function analyses of Gab1 indicated that mutant forms of Gab1 that do not bind the Src homology (SH) 2 domains of the p85 adapter subunit of PI 3-kinase or the SH2-domain-containing protein tyrosine phosphatase 2 (SHP-2) were impaired in their ability to potentiate ET-1-induced ERK1 activation. Taken together, our data indicate that PI 3-kinase and Src-family tyrosine kinases regulate ET-1-induced Gab1 tyrosine phosphorylation, which, in turn, induces ERK1 activation via PI 3-kinase- and SHP-2-dependent pathways.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Functional cross-talk between the cyclic AMP and Jak/STAT signaling pathways in vascular smooth muscle cells

Angiotensin II (Ang II), the primary effector of the renin-angiotensin system, is a multifunctional hormone that plays an important role in vascular function. In addition to its classical vasoconstrictor action, more recent studies demonstrated that Ang II stimulates the growth of a number of cell types, including vascular smooth muscle cells (SMC) (reviewed in [1-3]). In vivo studies have shown that chronic infusion of Ang II leads to the development of vascular hypertrophy in rats, whereas administration of angiotensin-converting enzyme (ACE) inhibitors or Ang II receptor antagonists prevents or regresses vascular hypertrophy in models of genetic and experimental hypertension [4]. Consistent with in vivo data, several laboratories have shown that Ang II stimulates protein synthesis and induces cellular hypertrophy, but not cell proliferation, in cultured aortic SMC [5-9]. Ang II also induces directed migration (chemotaxis) of vascular SMC [10, 11], although its effect is less prominent than that of platelet-derived growth factor (PDGF). The cellular mechanisms underlying these diverse actions of Ang II are not clearly understood but are likely to involve the activation of distinct signaling pathways.

Signaling mechanisms regulating bombesin-mediated AP-1 gene induction in the human gastric cancer SIIA

The hormone bombesin (BBS) and its mammalian equivalent gastrin-releasing peptide (GRP) act through specific GRP receptors (GRP-R) to affect multiple cellular functions in the gastrointestinal tract; the intracellular signaling pathways leading to these effects are not clearly defined. Previously, we demonstrated that the human gastric cancer SIIA possesses GRP-R and that BBS stimulates activator protein-1 (AP-1) gene expression. The purpose of our present study was to determine the signaling pathways leading to AP-1 induction in SIIA cells. A rapid induction of c-jun and jun-B gene expression was noted after BBS treatment; this effect was blocked by specific GRP-R antagonists, indicating that BBS is acting through the GRP-R. The signaling pathways leading to increased AP-1 gene expression were delineated using phorbol 12-myristate 13-acetate (PMA), which stimulates protein kinase C (PKC)-dependent pathways, by forskolin (FSK), which stimulates protein kinase A (PKA)-dependent pathways, and by the use of various protein kinase inhibitors. Treatment with PMA stimulated AP-1 gene expression and DNA binding activity similar to the effects noted with BBS; FSK stimulated jun-B expression but produced only minimal increases of c-jun mRNA and AP-1 binding activity. Pretreatment of SIIA cells with either H-7 or H-8 (primarily PKC inhibitors) inhibited the induction of c-jun and jun-B mRNAs in response to BBS, whereas H-89 (PKA inhibitor) exhibited only minimal effects. Pretreatment with tyrphostin-25, a protein tyrosine kinase (PTK) inhibitor, attenuated the BBS-mediated induction of c-jun and jun-B, but the effect was not as pronounced as with H-7. Collectively, our results demonstrate that BBS acts through its receptor to produce a rapid induction of both c-jun and jun-B mRNA and AP-1 DNA binding activity in the SIIA human gastric cancer. Moreover, this induction of AP-1, in response to BBS, is mediated through both PKC- and PTK-dependent signal transduction pathways with only minimal involvement of PKA.

The CXC Chemokine Stromal Cell-Derived Factor Activates a Gi-Coupled Phosphoinositide 3-Kinase in T Lymphocytes

The cellular effects of stromal cell-derived factor-1 (SDF-1) are mediated primarily by binding to the CXC chemokine receptor-4. We report in this study that SDF-1 and its peptide analogues induce a concentration- and time-dependent accumulation of phosphatidylinositol-(3,4,5)-trisphosphate (PtdIns(3,4,5)P3) in Jurkat cells. This SDF-1-stimulated generation of D-3 phosphoinositide lipids was inhibited by pretreatment of the cells with an SDF-1 peptide antagonist or an anti-CXCR4 Ab. In addition, the phosphoinositide 3 (PI 3)-kinase inhibitors wortmannin and LY294002, as well as the Gi protein inhibitor pertussis toxin, also inhibited the SDF-1-stimulated accumulation of PtdIns(3,4,5)P3. The effects of SDF-1 on D-3 phosphoinositide lipid accumulation correlated well with activation of the known PI 3-kinase effector protein kinase B, which was also inhibited by wortmannin and pertussis toxin. Concentrations of PI 3-kinase inhibitors, sufficient to inhibit PtdIns(3,4,5)P3 accumulation, also inhibited chemotaxis of Jurkat and peripheral blood-derived T lymphocytes in response to SDF-1. In contrast, SDF-1-stimulated actin polymerization was only partially inhibited by PI 3-kinase inhibitors, suggesting that while chemotaxis is fully dependent on PI 3-kinase activation, actin polymerization requires additional biochemical inputs. Finally, SDF-1-stimulated extracellular signal-related kinase (ERK)-1/2 mitogen-activated protein kinase activation was inhibited by PI 3-kinase inhibitors. In addition, the mitogen-activated protein/ERK kinase inhibitor PD098059 partially attenuated chemotaxis in response to SDF-1. Hence, it appears that ERK1/2 activation is dependent on PI 3-kinase activation, and both biochemical events are involved in the regulation of SDF-1-stimulated chemotaxis.

Bombesin and platelet-derived growth factor induce association of endogenous focal adhesion kinase with Src in intact Swiss 3T3 cells

Stimulation of quiescent Swiss 3T3 cells with bombesin induces a rapid increase in the formation of complexes between focal adhesion kinase (FAK) and Src family members, which can be extracted with a buffer containing Triton, deoxycholate, and SDS but not with a buffer containing Triton alone. An increase in complex formation between FAK and Src in response to bombesin could be detected within 1 min, reached a maximum after 10 min, and declined toward base-line levels after 60 min of bombesin treatment. Bradykinin, endothelin, and lysophosphatidic acid also stimulated FAK-Src complex formation. Bombesin stimulated FAK/Src association through a Ca(2+) and phosphatidylinositol 3'-kinase-independent pathway that requires the integrity of the actin filament network and is partly dependent on functional protein kinase C. Treatment with the selective Src kinase inhibitor PP-2 inhibited both FAK activation and phosphorylation of FAK at Tyr(577) induced by bombesin in intact cells. Platelet-derived growth factor at low concentrations (1-10 ng/ml) also induced FAK-Src complex formation via a pathway that depended on the integrity of the actin cytoskeleton and phosphatidylinositol 3'-kinase. Thus, G protein-coupled receptor agonists and platelet-derived growth factor promote complex formation between endogenous FAK and Src in attached cells through different signal transduction pathways.

Bombesin promotes synergistic stimulation of DNA synthesis by ethanol and insulin in fibroblasts

In NIH 3T3 fibroblasts and several other cellular systems, ethanol (50-80 mM) was previously shown to greatly enhance the mitogenic effects of insulin particularly in the presence of zinc. Here we report that in NIH 3T3 fibroblasts the combined stimulatory effects of ethanol and insulin on DNA synthesis can be further increased by bombesin both in the absence and presence of zinc. Bombesin also enhanced insulin-plus-ethanol-induced DNA synthesis in mouse Swiss 3T3 and Balb/c 3T3 fibroblasts, but in these cells bombesin was effective only in the presence of zinc. In NIH 3T3 fibroblasts, the potentiating effects of ethanol on insulin-induced DNA synthesis by the zinc-dependent and bombesin-dependent mechanisms were additive. Wortmannin, an inhibitor of phosphatidylinositol 3'-kinase (PI3K), prevented the comitogenic effect of ethanol in the presence of bombesin but not in the presence of zinc. Furthermore, bombesin, but not ethanol, was found to enhance the stimulatory effect of insulin on PI3K activity. Rapamycin, an indirect inhibitor of p70 S6 kinase actions, inhibited the comitogenic effects of ethanol in the presence of both zinc and bombesin. However, only ethanol, but not bombesin, enhanced the stimulatory effect of insulin on p70 S6 kinase activity; this effect of ethanol was zinc-dependent. Neither ethanol nor bombesin enhanced the stimulatory effects of insulin on the phosphorylation (activation) of p38/p42/p44 mitogen-activated protein kinases. The results suggest that in mouse fibroblasts maximal stimulation of DNA synthesis by physiologically relevant concentrations of ethanol occurs if both PI3K and p70 S6 kinase are activated. These data suggest a mechanism by which ethanol may affect growth in affected human tissues during its tumor promoting actions.

Cholecystokinin-induced redistribution of paxillin in rat pancreatic acinar cells

Cholecystokinin (CCK) dose-dependently stimulates enzyme secretion or loss of cell integrity in the exocrine pancreas. Signaling mechanims include tyrosine phosphorylation of p125(FAK) and paxillin. Here, we examine their potential function. Maximum phosphorylation of both proteins was observed after stimulation of freshly isolated rat pancreatic acini with 10 nM CCK, a concentration known to initiate breakdown of the terminal actin web and cell damage. Under these conditions, CCK initiated transient redistribution of paxillin from the apical cytosol to the apical and lateral plasma membrane within 2 min, where it colocalized with the terminal actin web. Relocation of paxillin was confirmed in subcellular fractions by western blotting and coincided with maximum phosphorylation of membrane-bound p125(FAK) and paxillin. Subsequently, paxillin was redistributed to the basolateral cytosol and was degraded. p125(FAK) remained membrane-bound. We conclude that phosphorylation and redistribution of paxillin and phosphorylation of p125(FAK) may participate in the CCK-induced disassembly of acinar cell actin.

The CC chemokine monocyte chemotactic peptide-1 activates both the class I p85/p110 phosphatidylinositol 3-kinase and the class II PI3K-C2alpha

The cellular effects of MCP-1 are mediated primarily by binding to CC chemokine receptor-2. We report here that MCP-1 stimulates the formation of the lipid products of phosphatidylinositol (PI) 3-kinase, namely phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate (PI 3,4,5-P3) in THP-1 cells that can be inhibited by pertussis toxin but not wortmannin. MCP-1 also stimulates an increase in the in vitro lipid kinase activity present in immunoprecipitates of the class 1A p85/p110 heterodimeric PI 3-kinase, although the kinetics of activation were much slower than observed for the accumulation of PI 3,4,5-P3. In addition, this in vitro lipid kinase activity was inhibited by wortmannin (IC50 = 4.47 +/- 1.88 nM, n = 4), and comparable concentrations of wortmannin also inhibited MCP-stimulated chemotaxis of THP-1 cells (IC50 = 11.8 +/- 4.2 nM, n = 4), indicating that p85/p110 PI 3-kinase activity is functionally relevant. MCP-1 also induced tyrosine phosphorylation of three proteins in these cells, and a fourth tyrosine-phosphorylated protein co-precipitates with the p85 subunit upon MCP-1 stimulation. In addition, MCP-1 stimulated lipid kinase activity present in immunoprecipitates of a class II PI 3-kinase (PI3K-C2alpha) with kinetics that closely resembled the accumulation of PI 3,4,5-P3. Moreover, this MCP-1-induced increase in PI3K-C2alpha activity was insensitive to wortmannin but was inhibited by pertussis toxin pretreatment. Since this mirrored the effects of these inhibitors on MCP-1-stimulated increases in D-3 phosphatidylinositol lipid accumulation in vivo, these results suggest that activation of PI3K-C2alpha rather than the p85/p110 heterodimer is responsible for mediating the in vivo formation of D-3 phosphatidylinositol lipids. These data demonstrate that MCP-1 stimulates protein tyrosine kinases as well as at least two separate PI 3-kinase isoforms, namely the p85/p110 PI 3-kinase and PI3K-C2alpha. This is the first demonstration that MCP-1 can stimulate PI 3-kinase activation and is also the first indication of an agonist-induced activation of the PI3K-C2alpha enzyme. These two events may play important roles in MCP-1-stimulated signal transduction and biological consequences.

Chemokines: understanding their role in T-lymphocyte biology

The chemokines are a complex superfamily of small, secreted proteins that were initially characterized through their chemotactic effects on a variety of leucocytes. The superfamily is divided into families based on structural and genetic considerations and have been termed the CXC, CC, C and CX3C families. Chemokines from these families have a key role in the recruitment and function of T lymphocytes. Moreover, T lymphocytes have also been identified as a source of a number of chemokines. T lymphocytes also express most of the known CXC and CC chemokine receptors to an extent that depends on their state of activation/differentiation and/or the activating stimuli. The expression of two chemokine receptors, namely CXCR4 and CCR5, together with the regulated production of their respective ligands, appears to be extremely important in determining sensitivity of T cells to HIV-1 infection. The intracellular events which mediate the effects of chemokines, particularly those elicited by the CC chemokine RANTES, include activation of both G-protein- and protein tyrosine kinase-coupled signalling pathways. The present review describes our current understanding of the structure and expression of chemokines and their receptors, the effects of chemokines on T-cell function(s), the intracellular signalling pathways activated by chemokines and the role of certain chemokines and chemokine receptors in determining sensitivity to HIV-1 infection.

Bombesin, vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine induce focal adhesion kinase activation in intact Swiss 3T3 cells

Treatment of quiescent Swiss 3T3 cells with bombesin rapidly increased focal adhesion kinase (FAK)-associated tyrosine kinase activity in immune complexes. The effect was rapid (maximum at 2.5 min) and dose dependent (half-maximum response at 0.05 nM). Addition of vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine also elicited a rapid increase in FAK-associated tyrosine kinase activity. Addition of the selective Src inhibitor pyrazolopyrimidine directly to the in vitro kinase assay potently inhibited Src kinase activity induced by bombesin but did not affect the kinase activity of FAK measured by autophosphorylation or by synthetic substrate phosphorylation in paralell assays. In addition, Src activity was not detected in FAK immunoprecipitates using an optimal Src peptide substrate. Thus, agonist-induced tyrosine kinase activity measured in FAK immunoprecipitates is mediated by FAK activation rather than by co-immunoprecipitating Src. Bombesin-induced FAK activation is not dependent either on protein kinase C or Ca2+ mobilization but was completely blocked by treatment with cytochalasin D or by placing the cells in suspension. These findings indicate that FAK activation requires an intact actin cytoskeleton. Our results demonstrate that agonists that act via 7-transmembrane domain receptors stimulate FAK kinase activation.

Receptor-operated calcium influx mediated by protein tyrosine kinase pathways

Calcium influx from the extracellular space elicited by activation of heterotrimeric G protein-coupled and heptahelical receptors plays a critical role in transmembrane signal transduction in a wide variety of cell systems. In nonexcitable cells, the precise voltage-independent mechanism by which calcium enters the cell remains unknown. Multiple mechanisms appear to be operating in different cell types (1-3): 1. G protein-operated calcium influx, 2. Second messenger-operated calcium influx, 3. Capacitative calcium influx, and 4. Phosphorylation of calcium channels. Receptor-operated calcium channels have a fundamental role in stimulus-secretion coupling in many different cells, but these channels remain to be purified and cloned. This review proposes that receptor-operated calcium influx is mediated by protein tyrosine kinase pathways. The function of protein tyrosine kinase pathways and their interactions with other receptor-operated calcium influx mechanisms are described.

Signal pathways that transduce growth factor-stimulated mitogenesis in bone cells

This investigation examined which signal pathways are of relevance in growth factor-stimulated bone cell mitogenesis. Platelet-derived growth factor (PDGF) and insulin-like growth factor-II (IGF-II) were potent mitogens for both the MG-63 osteoblast cell line and for primary cultures of human osteoblasts (HObs). The mitogenic action of both IGF-II and PDGF was attenuated by pertussis toxin (Ptx), by indomethacin, and by the lipoxygenase inhibitors BW755C74 and BW4AC. A combination of Ptx and indomethacin caused much greater inhibition but failed to abolish mitogenesis completely. PDGF significantly elevated inositol phosphates levels in both cell types; IGF-II had no effect on this pathway. In MG-63 cells, we demonstrated tyrosine phosphorylation of high-molecular-weight substrates elicited by both PDGF and IGF-II. Genistein inhibited the phosphorylation and mitogenic response to PDGF, but had no effect on IGF-II-induced tyrosine phosphorylation or mitogenesis. Another inhibitor of tyrosine kinases, methyl 2,5-dihydroxycinnamate, (MDHC), inhibited PDGF-stimulated mitogenesis effectively in both cell types but only blocked IGF-II-induced mitogenesis in MG-63 cells. The specificity of these inhibitors suggests that particular tyrosine kinases may regulate growth factor-induced stimulation of bone cells.

Galpha12 and Galpha13 stimulate Rho-dependent tyrosine phosphorylation of focal adhesion kinase, paxillin, and p130 Crk-associated substrate

We examined whether constitutively active mutants of the Galpha proteins Galpha12 and Galpha13, which together comprise the G12 subfamily of Galpha proteins, induce Rho-dependent tyrosine phosphorylation of the focal adhesion proteins p125 focal adhesion kinase, paxillin, and p130 Crk-associated substrate. We report that transient expression of the constitutively active mutants of Galpha12 or of Galpha13 in human embryonic kidney 293 cells stimulates tyrosine phosphorylation of a set of proteins of Mr of 110,000-130,000, 97,000, and 60,000-70,000. We identified p125 focal adhesion kinase, paxillin, and p130 Crk-associated substrate as prominent tyrosine-phosphorylated proteins in human embryonic kidney 293 cells expressing constitutively active Galpha12 and Galpha13. In common with the increased tyrosine phosphorylation of these proteins mediated by mitogens acting through heptahelical receptors, the Galpha12- and Galpha13-mediated increase in tyrosine phosphorylation is blocked by cytochalasin D, which specifically disrupts the actin cytoskeleton, and by the Clostridium botulinum C3 exoenzyme, which ADP-ribosylates and inactivates Rho. Our results support the hypothesis that Galpha12 and Galpha13 activate Rho and suggest that Galpha12 and Galpha13 may mediate the tyrosine phosphorylation of p125 focal adhesion kinase, paxillin, and p130 Crk-associated substrate.

Ability of various bombesin receptor agonists and antagonists to alter intracellular signaling of the human orphan receptor BRS-3

Bombesin (Bn) receptor subtype 3 (BRS-3) is an orphan receptor that is a predicted member of the heptahelical G-protein receptor family and so named because it shares a 50% amino acid homology with receptors for the mammalian bombesin-like peptides neuromedin B (NMB) and gastrin-releasing peptide. In a recent targeted disruption study, in which BRS-3-deficient mice were generated, the mice developed obesity, diabetes, and hypertension. To date, BRS-3's natural ligand remains unknown, its pharmacology unclear, and cellular basis of action undetermined. Furthermore, there are few tissues or cell lines found that express sufficient levels of BRS-3 protein for study. To define the intracellular signaling properties of BRS-3, we examined the ability of [D-Phe6,beta-Ala11,Phe13, Nle14]Bn-(6-14), a newly discovered peptide with high affinity for BRS-3, and various Bn receptor agonists and antagonists to alter cellular function in hBRS-3-transfected BALB 3T3 cells and hBRS-3-transfected NCI-H1299 non-small cell lung cancer cells, which natively express very low levels of hBRS-3. This ligand stimulated a 4-9-fold increase in [3H]inositol phosphate formation in both cell lines under conditions where it caused no stimulation in untransfected cells and also stimulated an increase in [3H]IP1, [3H]IP2, and 3H]IP3. The elevation of [3H]IP was concentration-dependent, with an EC50 of 20-35 nM in both cell lines. [D-Phe6,beta-Ala11,Phe13,Nle14]Bn-(6-14) stimulated a 2-3-fold increase in [Ca2+]i, a 3-fold increase in tyrosine phosphorylation of p125(FAK) with an EC50 of 0.2-0.7 nM, but failed to either stimulate increases in cyclic AMP or inhibit forskolin-stimulated increases. None of nine naturally occurring Bn peptides or three synthetic Bn analogues reported to activate hBRS-3 did so with high affinity. No high affinity Bn receptor antagonists had high affinity for the hBRS-3 receptor, although two low affinity antagonists for gastrin-releasing peptide and NMB receptors, [D-Arg1,D-Trp7,9, Leu11]substance P and [D-Pro4,D-Trp7,9,10]substance P-(4-11), inhibited hBRS-3 receptor activation. The NMB receptor-specific antagonist D-Nal,Cys,Tyr,D-Trp,Lys,Val, Cys,Nal-NH2 inhibited hBRS-3 receptor activation in a competitive fashion (Ki = 0.5 microM). Stimulation of p125(FAK) tyrosine phosphorylation by hBRS-3 activation was not inhibited by the protein kinase C inhibitor, GF109203X, or thapsigargin, alone or in combination. These results show that hBRS-3 receptor activation increases phospholipase C activity, which causes generation of inositol phosphates and changes in [Ca2+]i and is also coupled to tyrosine kinase activation, but is not coupled to adenylate cyclase activation or inhibition. hBRS-3 receptor activation results in tyrosine phosphorylation of p125(FAK), and it is not dependent on activation of either limb of the phospholipase C cascade. Although the natural ligand is not a known bombesin-related peptide, the availability of [D-Phe6,beta-Ala11, Phe13,Nle14]Bn-(6-14), which functions as a high affinity agonist in conjunction with hBRS-3-transfected cell lines and the recognition of three classes of receptor antagonists including one with affinity of 0.5 microM, should provide important tools to assist in the identification of its natural ligand, the development of more potent selective receptor antagonists and agonists, and further exploration of the signaling properties of the hBRS-3 receptor.

Prostaglandin F2 alpha (PGF2 alpha) triggers protein kinase C (PKC) and tyrosine kinase activity in cultured mammalian cells

Prostaglandin F2 alpha (PGF2 alpha) added to confluent resting Swiss 3T3 cells triggers tyrosine kinase (PTK) activation characterized by the phosphorylation of a set of 75, 86, 110 and 140 kD proteins. PGF2 alpha induces this event independently of PKC activation. However, both PKC and PTK activities appear to act concertedly to cause mitogenesis. Here we discuss their relevance in the control of mammalian cell division.

Mammalian bombesin receptors are coupled to multiple signal transduction pathways in pancreatic acini

We investigated the structural requirements for bombesin (BB)-like peptides to stimulate amylase secretion in rat pancreatic acini and examined the responsible intracellular signal transduction pathways. The tetradecapeptide BB-(1-14) was a full agonist, whereas the heptapeptide BB-(8-14) did not evoke amylase secretion. The mammalian BB analog neuromedin C decapeptide [NMC-(5-14)] was as potent as BB-(1-14) in stimulating amylase secretion, suggesting that Gly5-Asn6-His7 (or Gln7) of the COOH-terminal decapeptide are essential amino acids for full biological activity. BB and NMC equipotently stimulated D-myo-inositol 1,4,5-trisphosphate production, which was inhibited by the phospholipase C (PLC) inhibitor U-73122. BB and NMC also stimulated protein tyrosine kinase (PTK) activities. The half-maximal effective concentration (EC50) for NMC-activated PTK was 2 log units less than the EC50 for BB-activated PTK. NMC was 10-34 times more potent than BB in increasing leukotriene C4 (an index of arachidonic acid production). The production of leukotriene C4 was inhibited by the phospholipase A2 (PLA2) inhibitor ONO-RS-082. NMC is structurally homologous to BB-(5-14) except that Gln7 in BB is replaced by His7 in NMC. Therefore, substitution of Gln7 for His7 may alter the signal transduction systems to include the PTK and PLA2 pathways. U-73122 inhibited Ca2+ spiking and amylase secretion induced by NMC and BB. However, the PTK inhibitor genistein and the PLA2 inhibitor ONO-RS-082 inhibited secretion induced by NMC but not that induced by BB. In contrast to nonmammalian BB receptors, which primarily use the PLC pathway, the rat BB receptor is linked to three different signal transduction systems: PLC, PTK, and PLA2 pathways.

Protein tyrosine phosphorylation in pancreatic acini: differential effects of VIP and CCK

Cholecystokinin (CCK) and vasoactive intestinal peptide (VIP) stimulate enzyme secretion from pancreatic acini by binding to heptahelical receptors without intrinsic tyrosine kinase activity. Signal transduction by the CCK receptor involves activation of phospholipase C by Gq proteins and activation of tyrosine kinases, whereas occupation of VIP receptors stimulates adenylyl cyclase through binding to Gs proteins. Here, we use electrophoretic separation of cellular proteins and antiphosphotyrosine immunoblotting to demonstrate a VIP-stimulated rapid and dose-dependent increase in tyrosine phosphorylation of proteins migrating at 130, 115, and 93 kDa in freshly isolated rat pancreatic acini. Phosphorylation of these proteins was increased after direct stimulation of adenylyl cyclase or the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase with forskolin or dibutyryl cAMP and was inhibited by the tyrosine kinase inhibitors genistein or tyrphostin 23. Compared with VIP, CCK stimulated tyrosine phosphorylation of additional proteins migrating at 60, 66, and 72/78 kDa. Using two-dimensional electrophoretic separation or immunoprecipitation, the 72/78-kDa phosphoprotein was identified as paxillin. We propose that paxillin might be involved in CCK-but not in VIP-induced exocytosis.

Cyclic AMP-mediated inhibition of angiotensin II-induced protein synthesis is associated with suppression of tyrosine phosphorylation signaling in vascular smooth muscle cells

In the present study, we have examined the effect of increased cyclic AMP (cAMP) levels on the stimulatory action of angiotensin II (Ang II) on protein synthesis. Treatment with cAMP-elevating agents potently inhibited Ang II-induced protein synthesis in rat aortic smooth muscle cells and in rat fibroblasts expressing the human AT1 receptor. The inhibition was dose-dependent and was observed at all concentrations of the peptide. To explore the mechanism of cAMP action, we have analyzed the effects of forskolin and 3-isobutyl-1-methylxanthine on various receptor-mediated responses. Elevation of cAMP did not alter the binding properties of the AT1 receptor and did not interfere with the activation of phospholipase C or the induction of early growth response genes by Ang II. Likewise, Ang II-dependent activation of the mitogen-activated protein kinases ERK1/ERK2 and p70 S6 kinase was unaffected by cAMP. In contrast, we found that increased concentration of cAMP strongly inhibited the stimulatory effect of Ang II on protein tyrosine phosphorylation. Specifically, cAMP abolished Ang II-induced tyrosine phosphorylation of the focal adhesion-associated protein paxillin and of the tyrosine kinase Tyk2. These results identify a novel mechanism by which the cAMP signaling system may exert growth-inhibitory effects in specific cell types.

Inhibitory effect of heparin on serotonin-induced hyperplasia and hypertrophy of smooth muscle cells

Serotonin (5-HT) produces both hyperplastic and hypertrophic effects on smooth muscle cell (SMC) in culture. Heparin is known to inhibit serum-induced hyperplasia of SMC but has not been previously tested on the stimulatory effect of 5-HT on SMC. Our present data show that at 24 h heparin inhibited by 50% the stimulation of 3H-thymidine incorporation into bovine pulmonary artery SMC and at 7 days totally reversed both cellular proliferation and enlargement of SMC produced by 1 microM 5-HT. Heparin failed to alter 5-HT uptake by SMC, but inhibited the stimulation of tyrosine phosphorylation of GTPase-activating protein, a proposed intermediate in the 5-HT stimulatory process. Thus heparin inhibits both hyperplastic and hypertrophic effects of 5-HT on SMC, perhaps through the inhibition of a phosphorylated intermediate protein.

Vascular endothelial growth factor stimulates tyrosine phosphorylation and recruitment to new focal adhesions of focal adhesion kinase and paxillin in endothelial cells

Vascular endothelial growth factor (VEGF) stimulated the tyrosine phosphorylation of multiple components in confluent human umbilical vein endothelial cells (HUVECs) including bands of Mr 205,000, corresponding to the VEGF receptors Flt-1 and KDR, and Mr 145,000, 120,000, 97,000, and 65,000-70,000. VEGF caused a striking and transient increase in mitogen-activated protein (MAP) kinase activity and stimulated phospholipase C-gamma tyrosine phosphorylation, but it had no effect on phosphatidylinositol 3'-kinase activity. VEGF caused a marked increase in tyrosine phosphorylation of p125 focal adhesion kinase (p125(FAK)), which was both rapid and concentration-dependent. VEGF produced similar effects on p125(FAK) in the endothelial cell line ECV.304. VEGF stimulated tyrosine phosphorylation of the 68-kDa focal adhesion-associated component, paxillin, with similar kinetics and concentration dependence to that for p125(FAK). Thrombin and the phorbol ester, phorbol 12-myristate 13-acetate, also increased p125(FAK) tyrosine phosphorylation in HUVECs. The effect of VEGF on p125(FAK) tyrosine phosphorylation was completely inhibited by the actin filament-disrupting agent cytochalasin D and was partially inhibited by the protein kinase C inhibitor GF109203X. Inhibition of the MAP kinase pathway using a specific inhibitor of MAP kinase kinase had no effect on p125(FAK) tyrosine phosphorylation. VEGF stimulated migration and actin stress fiber formation in confluent HUVEC, and VEGF-induced p125(FAK)/paxillin tyrosine phosphorylation was accompanied by increased immunofluorescent staining of p125(FAK), paxillin, and phosphotyrosine in focal adhesions in confluent cultures of HUVECs. These findings identify p125(FAK) and paxillin as components in a VEGF-stimulated signaling pathway and suggest a novel mechanism for VEGF regulation of endothelial cell functions.

Tyrosine phosphorylation of p130(cas) by bombesin, lysophosphatidic acid, phorbol esters, and platelet-derived growth factor. Signaling pathways and formation of a p130(cas)-Crk complex

Treatment of Swiss 3T3 cells with bombesin rapidly induced tyrosine phosphorylation of the p130 Crk-associated substrate (p130(cas)). Vasopressin, endothelin, bradykinin, lysophosphatidic acid, sphingosylphosphorylcholine, and phorbol 12,13-dibutyrate also stimulated p130(cas) tyrosine phosphorylation. Bombesin-induced p130(cas) tyrosine phosphorylation could be dissociated from both protein kinase C activation and Ca2+ mobilization from intracellular stores. In contrast, cytochalasin D, which disrupts the network of actin microfilaments, completely prevented tyrosine phosphorylation of p130(cas) by bombesin. Platelet-derived growth factor, at low concentrations (1-5 ng/ml), also induced tyrosine phosphorylation of p130(cas) via a pathway that depended on the integrity of the actin cytoskeleton. The phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 prevented tyrosine phosphorylation of p130(cas) in response to platelet-derived growth factor but not in response to neuropeptides, lysophosphatidic acid, sphingosylphosphorylcholine, or phorbol 12,13-dibutyrate. All agonists that induced p130(cas) tyrosine phosphorylation also promoted the formation of a p130(cas).Crk complex in intact Swiss 3T3 cells. Thus, our results identified distinct signal transduction pathways that lead to tyrosine phosphorylation of p130(cas) in the same cells and suggest that p130(cas) could play a role in mitogen-mediated signal transduction.

Association of Tyr phosphorylation of GTPase-activating protein with mitogenic action of serotonin

We have previously shown that serotonin (5-HT) induces both hyperplasia and hypertrophy of pulmonary artery smooth muscle cells (SMC) but not of endothelial cells (EC) through its high-affinity uptake. The present studies demonstrate rapid enhancement by 5-HT of Tyr phosphorylation of proteins, including p120, which also occurs in SMC but not in EC. The p120 protein was identified as GTPase-activating protein (GAP) by immunoprecipitation. Its phosphorylation occurred within minutes and preceded other events associated with 5-HT-induced mitogenesis. Tyr kinase (TK) and 5-HT uptake inhibitors and 8-bromoadenosine 3',5'-cyclic monophosphate blocked both the 5-HT-induced DNA synthesis and Tyr phosphorylation of GAP. Vanadate elevated DNA synthesis and Tyr phosphorylation of GAP of both control and 5-HT-treated cells. 5-HT failed to alter Tyr phosphorylation of GAP in cellular homogenates, as opposed to intact cells. In the presence of 3-isobutyl-1-methylxanthine, 5-HT inhibited cellular growth, presumably through its action on 5-HT1A or 5-HT4 receptors and elevation of adenosine 3',5'-cyclic monophosphate, but this was not associated with an alteration of Tyr phosphorylation of GAP. Similarly, a 5-HT1 or 5-HT2 receptor agonist failed to stimulate Tyr phosphorylation or DNA synthesis of SMC. Stimulation of cellular proliferation and enlargement produced by 1 microM 5-HT were totally abolished by TK inhibitors that did not affect 5-HT uptake. These data indicate that Tyr phosphorylation of GAP may act as an intermediate signal in 5-HT-induced mitogenesis of SMC which requires cellular internalization of 5-HT rather than its action on a membrane receptor.

[D-Arg1,D-Trp5,7,9,Leu11]Substance P coordinately and reversibly inhibits bombesin- and vasopressin-induced signal transduction pathways in Swiss 3T3 cells

The novel substance P (SP) analogue, [D-Arg1,D-Trp5,7,9,Leu11]SP like [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP inhibited DNA synthesis induced by bombesin, vasopressin, and bradykinin, but did not interfere with the mitogenic response induced by other growth factors or pharmacological agents in Swiss 3T3 cells. [D-Arg1,D-Trp5, 7,9,Leu11]SP reversibly inhibited bombesin-induced DNA synthesis, causing a 6-fold greater rightward shift in the bombesin dose response than [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP at identical concentrations (10 microM). We found that the new, more potent, SP analogue coordinately and reversibly inhibited bombesin-induced Ca2+ mobilization and protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation. The dose-response curves for bombesin-induced Ca2+ mobilization and MAP kinase activation were similarly displaced (51- and 40-fold, respectively) by [D-Arg1, D-Trp5,7,9,Leu11]SP. In addition, [D-Arg1,D-Trp5,7,9,Leu11]SP reversibly inhibited bombesin-induced tyrosine phosphorylation of Mr 110,000-130,000 and 70,000-80,000 bands as well as p125 focal adhesion kinase. [D-Arg1,D-Trp5,7,9,Leu11]SP also reversibly and coordinately inhibited vasopressin-induced Ca2+ mobilization, PKC stimulation, MAP kinase activation, tyrosine phosphorylation, and DNA synthesis in Swiss 3T3 cells. Surprisingly, deletion of the terminal Leu of [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP to yield [D-Arg1, D-Phe5,D-Trp7,9]SP1-10 resulted in a selective loss of inhibitory activity of this analogue against bombesin- but not vasopressin-stimulated DNA synthesis, Ca2+ mobilization, and MAP kinase activation. Collectively, these results suggest that SP analogues act at the receptor level to coordinately and reversibly antagonize bombesin- or vasopressin-induced signal transduction in Swiss 3T3 cells.

Bombesin, bradykinin, vasopressin, and phorbol esters rapidly and transiently activate Src family tyrosine kinases in Swiss 3T3 cells. Dissociation from tyrosine phosphorylation of p125 focal adhesion kinase

Treatment of quiescent Swiss 3T3 cells with bombesin induces a rapid (</=40 s) and transient increase in the kinase activity of the Src family of tyrosine kinases, as determined by autophosphorylation in immune complex kinase assays (4.6 +/- 0.2-fold stimulation, n = 44) and phosphorylation of exogenous substrates. Phorbol 12, 13-dibutyrate increased the activity of Src family kinases with similar kinetics but was less effective than bombesin. However, Src family kinase activation by bombesin is not dependent either on protein kinase C or Ca2+. Bombesin stimulation of Src family kinase activity could also be dissociated from p125 focal adhesion kinase tyrosine phosphorylation. Neither treatment with cytochalasin D nor placement of the cells in suspension prevented the stimulation of Src family kinase activity induced by bombesin, but both abolished bombesin-induced tyrosine phosphorylation of p125 focal adhesion kinase. The stimulation of the Src family kinase activity by bombesin was completely prevented by treatment with vanadate, a potent inhibitor of protein-tyrosine phosphatases. Bradykinin and vasopressin also stimulated Src family kinase activity transiently, and this stimulation was also inhibited by vanadate. Our results dissect two separate pathways that lead to protein tyrosine phosphorylation in neuropeptide-stimulated Swiss 3T3 cells.

G-protein coupling of muscarinic receptors in adult and neonatal rat submandibular cells

The submandibular glands of neonatal and adult rats express muscarinic cholinergic receptors. Receptor occupancy initiates signaling through activation of phospholipase C, hydrolysis of inositol phospholipids, and calcium mobilization. The increased cytoplasmic [Ca2+] activates ion transport pathways, resulting in secretion of primary saliva. We have previously shown that muscarinic receptors are present in the gland of neonates and that they couple effectively to inositol trisphosphate production and Ca2+ mobilization but that the monovalent ion transport paths are poorly activated. To characterize age-related differences in signal transduction further, we examined the coupling of muscarinic receptors to G-proteins by determining the effect of GTP on the IC50 for competition by the muscarinic agonist carbachol with the radiolabeled antagonist, 3H-quinuclidinyl benzylate. Data were fit to one-site and two-site models, and in all cases the two-site model provided the better fit. Using the two-site model, a substantial GTP-induced shift from high affinity to low affinity binding was observed in membranes from adults, whereas more of the receptors were already in the low affinity form in the membranes from neonates, and little additional shift was induced by GTP. These results suggest differences in the G-protein coupling of muscarinic receptors in submandibular cells of adult and early postnatal rats that may be associated with differences in the content, affinity, or properties (i.e., posttranslational modifications) of G-proteins as the cells mature.

Tyrphostin inhibition of ATP-stimulated DNA synthesis, cell proliferation and fos-protein expression in vascular smooth muscle cells

1. We and others have shown that extracellular ATP (adenosine triphosphate), released from sympathetic nerves and platelets, stimulates growth of vascular smooth muscle cells (SMC). To study the importance of tyrosine kinases for ATP-mediated proliferation in vascular smooth muscle cells we used tyrphostins, a recently developed group of highly specific inhibitors of tyrosine kinases. 2. ATP induced a powerful concentration-dependent increase in DNA synthesis measured by [3H]-thymidine incorporation in rat aorta SMC (RASMC) and an increase in total cell number after 72 h of incubation as measured by an enzymatic cell proliferation assay. Tyrphostin 25 (10(-5) M) had no effect per se on basal DNA synthesis but reduced ATP-stimulated DNA synthesis and increase in cell number in a dose-dependent manner. Higher concentrations of ATP could not reverse the inhibitory effect of tyrphostin 25. The potency of several (six) other tyrphostins was also examined and found to be slightly greater than tyrphostin 25 with equal efficacy. 3. When RASMC were incubated with 10(-5) M ATP for 2 h, nearly all of the cells (87 +/- 5%) were intensely stained with an antibody to the Fos protein while in the controls only 1 +/- 2% of the cells were weakly stained. Tyrphostin 25 greatly reduced the Fos-protein staining (14 +/- 2%). 4. ATP induced a concentration-dependent increase in 45Ca(2+)-influx and formation of inositol phosphates (IPtotal) in RASMC. These effects were not inhibited by tyrphostin 25. 5. Tyrphostin 25 did not alter ATP-induced contraction in ring segments of rat aorta. 6. In conclusion, tyrphostin 25 inhibited ATP-induced DNA synthesis, cell proliferation and Fos-protein expression, but not ATP-induced 45Ca(2+)-influx, inositolphosphate-production or vasoconstriction. This indicates that the mitogenic effect of ATP on vascular smooth muscle cells is dependent on tyrosine kinases in contrast to the contractile effect of ATP in blood vessels.

Signalling pathways activated by endothelin stimulation of renal cells

1. Endothelin mediates its effects in a variety of renal cells via a multiplicity of intracellular signalling pathways. 2. Stimulation of phosphatidylinositol-specific phospholipase C (PI-PLC), resulting in the activation of inositol trisphosphate and diacylglycerol, can be detected even at picomolar concentrations of peptide. 3. Endothelin activation of cPLA2 is sensitive to ambient [Ca2+]i, is not contingent upon protein kinase C activation and is independent of PI-PLC stimulation, being coupled to the endothelin receptor in a yet to be determined manner. 4. Activation by endothelin of phosphatidylcholine-specific phospholipase D is under the dual regulation of protein kinase C and [Ca2+]i, with protein kinase C being the major regulator and [Ca2+]i playing a secondary, modulatory role. 5. Phosphatidylcholine-specific phospholipase C (PC-PLC) is stimulated by endothelin and accounts for the prolonged activation of diacylglycerol by this peptide. PC-PLC activity is critically dependent upon [Ca2+]i, whereas protein kinase C plays no role in modulating the activity of this enzyme. 6. Endothelin enhances the phosphorylation of protein tyrosine kinases, with evidence that phosphorylation of pp60 Src may be an important early event.

Possible involvement of a tyrosine kinase-dependent pathway in the regulation of phosphoinositide metabolism by vanadate in normal mouse islets

The potential roles of protein tyrosine kinases (TKs) and of phosphotyrosine phosphatases (PTPs) in pancreatic islet function are not known. In this study, we investigated whether vanadate, a potent PTP inhibitor, affects phosphoinositide (PI) metabolism by a TK-dependent pathway in isolated mouse islets. To avoid the confounding effects of changes in Ca2+ influx, all experiments were performed in the absence of Ca2+. In the presence of 15mM glucose, vanadate, acetylcholine (ACh) or [Arg]vasopressin (AVP) strongly stimulated InsP production. Vanadate also increased PtdInsP levels in membranes. The TK inhibitor genistein (not its inactive analogues genistin and daidzein) significantly reduced vanadate effects, but was without effect in the absence of stimulation or in the presence of ACh or AVP. Islet proteins resolved by SDS/PAGE were analysed by immunobloting with anti-phosphotyrosine antibody. Under control conditions, several phosphotyrosyl-proteins (PYPs) were present. Vanadate increased phosphotyrosine residues on several PYPs, notably two proteins of 145 and 85 kDa. This effect was prevented by genistein, p145 and p85 could correspond to phospholipate Cgamma(PLCgamma) and the regulatory subunit of PtdIns-3-kinase (PtdIns-3K) respectively. Both proteins are expressed in islets, as revealed by immunoblots with specific antibodies. Tungstate, another PTP inhibitor, reproduced vanadate effects, but inhibition of PtdIns-3K by wortmannin failed to affect vanadate-increased PtdInsP levels. Incubation of the islets in the presence of 10% (v/v) fetal calf serum instead of BSA increased InsP production and this effect was prevented by genistein. These results suggest that inhibition of PTP increases InsP production in mouse islets by a TK-dependent pathway. They also provide evidence for a potential role of TK and PTP in pancreatic B-cell function.

Requirement for phosphatidylinositol 3'-kinase activity in platelet-derived growth factor-stimulated tyrosine phosphorylation of p125 focal adhesion kinase and paxillin

The role of phosphatidylinositol 3'-kinase (PI 3'-kinase) activity in platelet-derived growth factor (PDGF)-stimulated tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin has been examined. The tyrosine phosphorylation of p125FAK and paxillin in response to PDGF was markedly inhibited by wortmannin in a dose-dependent manner. PDGF-stimulated PI 3'-kinase activity, membrane ruffle formation, and tyrosine phosphorylation of p125FAK and paxillin were all inhibited by the same low concentrations of wortmannin (>90% inhibition at 40nM). In contrast, tyrosine phosphorylation of p125FAK and paxillin in response to bombesin, endothelin, and phorbol 12,13-dibutyrate was not inhibited by wortmannin in these cells. Furthermore, LY294002, an inhibitor of PI 3'-kinase structurally unrelated to wortmannin, also inhibited PDGF-stimulated p125FAK tyrosine phosphorylation. PDGF was shown to stimulate the tyrosine phosphorylation of p125FAK in porcine aortic endothelial (PAE) cells transfected with the wild type PDGF-beta receptors, but not in PAE cells transfected with PDGF-beta receptors in which the PI 3'-kinase binding sites (Tyr-740/751) were replaced by phenylalanine. PDGF-stimulated, PI 3'-kinase-dependent tyrosine phosphorylation of p125FAK was not inhibited by rapamycin, and thus it was dissociated from the activation of p70 S6 kinase, previously identified as a molecular downstream target of PI 3'-kinase. Thus, we have identified a PI 3'-kinase-dependent signal transduction pathway in the action of PDGF, which leads to the phosphorylation of p125FAK and paxillin.

Thrombin receptor activation and integrin engagement stimulate tyrosine phosphorylation of the proto-oncogene product, p95vav, in platelets

The vav proto-oncogene product, p95vav or Vav, is primarily expressed in hematopoietic cells and has been shown to be a substrate for tyrosine kinases. Although its function is unknown, Vav shares a region of homology with DBL, an exchange factor for the Rho family of GTP-binding proteins. The presence of this domain and the observation that cells transformed with Vav display prominent stress fibers and focal adhesions similar to those that are observed in RhoA transformed cells suggests that Vav may play a role in regulating the actin cytoskeleton. We have, therefore, examined Vav phosphorylation in platelets, which undergo dramatic cytoskeletal reorganization in response to agonists. Two potent platelet agonists, thrombin (via its G protein-coupled receptor) and collagen (via its interaction with the alpha2beta1 integrin), caused Vav to become phosphorylated on tyrosine. Weaker platelet agonists, including ADP, epinephrine and the thromboxane A2 analog, U46619, did not. The phosphorylation of Vav in response to thrombin was maximal within 15 s and was unaffected by aspirin, inhibitors of aggregation, or the presence of the ADP scavenger, apyrase. Vav phosphorylation was also observed when platelets became adherent to immobilized collagen (via integrin alpha2beta1), fibronectin (via integrin alpha5beta1), and fibrinogen (via integrin alphaIIbbeta3). These results show that Vav phosphorylation by tyrosine kinases 1) occurs during platelet activation by potent agonists, 2) also occurs when platelets adhere to biologically relevant matrix proteins, 3) requires neither platelet aggregation nor the release of secondary agonists such as ADP and TxA2, and 4) can be initiated by at least some members of two additional classes of receptors, G protein-coupled receptors and integrins, providing further evidence that both of these can couple to tyrosine kinases.

The association of focal adhesion kinase with a 200-kDa protein that is tyrosine phosphorylated in response to platelet-derived growth factor

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase implicated in the signal transduction pathways initiated by integrins. However, we have previously found that platelet-derived growth factor (PDGF) could stimulate the association of FAK with phosphatidylinositol 3-kinase in NIH 3T3 cells [Chen, H.-C. & Guan, J.-L. (1994) J. Biol. Chem 269, 31229-31223], suggesting that FAK might participate in some of the cellular effects of the growth factors in modulating cell morphology and migration. In this report, we describe the association of FAK with a 200-kDa protein (pp200) that is tyrosine phosphorylated in response to PDGF stimulation in NIH 3T3 cells. Although the identity of pp200 is unknown at present, we have excluded the possibilities that it is the PDGF receptor beta, tension, talin, myosin or the guanosine-triophosphatase-activating-protein-associated p190 protein. Furthermore, we found that the tyrosine phosphorylation of FAK-associated pp200 upon PDGF stimulation is largely independent of cell adhesion or the integrity of the cytoskeleton. Therefore, pp200 and its interaction with FAK may also be involved in growth-factor-induced cellular effects such as the modulation of cell adhesion or cell migration via cytoskeletal reorganization or disruption of focal adhesions.

Aluminum potentiates the effect of fluoride on tyrosine phosphorylation and osteoblast replication in vitro and bone mass in vivo

Osteosclerosis in workers exposed to fluoride (F) and aluminum (Al) (industrial fluorosis) led to the use of F as a treatment to increase bone mass in osteoporosis patients. Because the influence of traces of Al on the effects of F on bone formation is heretofore unknown, we have investigated this issue both in vitro and in vivo. We have found that minute amounts of Al (< or = 10(-5) M) potentiate the effects of F in vitro such that osteoblast proliferation increased by 15 +/- 2.7% at 50 microM (p < 0.001) and by 117.6 +/- 5.1% at 750 microM (p < 0.001), concentrations of F with no mitogenic effect alone. F + Al time-dependently modulated a growth factor signaling pathway(s) associated with enhanced tyrosine phosphorylation (TyrP) of several proteins (p90 [2.9x], p77 [4.9x], p68 [9.6x], and mitogen activated protein kinases [3x]). TyrP was only slightly or not at all changed by F and Al alone, respectively. The effects of F + Al on TyrP and cell proliferation were markedly reduced by 100 microM tyrphostin-51, a tyrosine kinase inhibitor. Protein kinase A (PKA) and protein kinase C (PKC) pathways were not involved in this response. In vivo, F + Al administered for 8 months, at doses that had no effect when the minerals were administered individually, significantly enhanced proximal tibia bone mineral density (BMD) by 6.3 +/- 1% compared with initial values and by 2-fold compared with control ovariectomized rats (p < 0.0001). These effects are consistent with a crucial role of Al in osteosclerosis observed in industrial fluorosis. The results suggest that the combination of F + Al modulates a growth factor-dependent TyrP pathway enhancing mitogen-activated protein kinase and osteoblastic proliferation and bone mass.

Interaction between G proteins and tyrosine kinases upon T cell receptor.CD3-mediated signaling

Engagement of the T cell receptor (TCR).CD3 complex results in the induction of multiple intracellular events, with protein tyrosine kinases playing a pivotal role in their initiation. Biochemical studies also exist suggesting the involvement of heterotrimeric GTP-binding proteins (G proteins); however, the functional consequence of this participation in TCR.CD3-mediated signaling is unresolved. Here, we report TCR.CD3-mediated guanine nucleotide exchange among the 42-kDa G protein alpha subunits of the G alpha q/11 family, their physical association with CD3 epsilon, and the G alpha 11-dependent activation of phospholipase C beta. Protein tyrosine kinase inhibitors, however, abrogate TCR.CD3-mediated G protein activation. Quite interesting is the observation that cells transfected with a function-deficient mutant of G alpha 11 display diminished tyrosine phosphorylation of TCR.CD3 zeta and epsilon chains, as well as ZAP-70, upon anti-CD3 antibody triggering. These data indicate the involvement of the G alpha q/11 family in TCR.CD3 signaling at a step proximal to the receptor and suggest a reciprocal regulation between tyrosine kinases and G proteins in T cells.

Angiotensin II activates at least two tyrosine kinases in rat liver epithelial cells. Separation of the major calcium-regulated tyrosine kinase from p125FAK

In rat liver epithelial cell lines (WB or GN4), angiotensin II (Ang II) stimulates cytosolic tyrosine kinase activity, in part, through a calcium-dependent mechanism. In other cell types, selected hormones that activate Gi- or Gq-coupled receptors stimulate the soluble tyrosine kinase, p125FAK. Immunoprecipitation of p125FAK from Ang II-activated GN4 cells demonstrated a doubling of p125FAK kinase activity. However, an additional Ang II-activated tyrosine kinase (or kinases) representing the majority of the total activity was detected when the remaining cell lysate, immunodepleted of p125FAK, was reimmunoprecipitated with an anti-phosphotyrosine antibody. Cytochalasin D pretreatment blocks G-protein receptor-dependent tyrosine phosphorylation in Swiss 3T3 cells. While cytochalasin D decreased the Tyr(P) content of 65-75-kDa substrates in Ang II-treated GN4 cells, it did not diminish tyrosine phosphorylation of 115-130-kDa substrates, again suggesting activation of at least two tyrosine kinase pathways in GN4 cells. To search for additional Ang II-activated enzymes, we used molecular techniques to identify 20 tyrosine kinase sequences in these cell lines. None was the major cytosolic enzyme activated by Ang II. Specifically, JAK2, which had been shown by others to be stimulated by Ang II in smooth muscle cells, was not activated by Ang II in GN4 cells. Finally, we purified Tyr(P)-containing tyrosine kinases from Ang II-treated cells, using anti-Tyr(P) and ATP affinity resins; 80% of the tyrosine kinase activity migrated as a single 115-120-kDa tyrosine-phosphorylated protein immunologically distinct from p125FAK. In summary, Ang II activates at least two separate tyrosine kinases in rat liver epithelial cells; p125FAK and a presumably novel, cytosolic 115-120-kDa protein referred to as the calcium-dependent tyrosine kinase.

Angiotensin II stimulates calcium-dependent activation of c-Jun N-terminal kinase

In GN4 rat liver epithelial cells, angiotensin II (Ang II) and other agonists which activate phospholipase C stimulate tyrosine kinase activity in a calcium-dependent, protein kinase C (PKC)-independent manner. Since Ang II also produces a proliferative response in these cells, we investigated downstream signaling elements traditionally linked to growth control by tyrosine kinases. First, Ang II, like epidermal growth factor (EGF), stimulated AP-1 binding activity in a PKC-independent manner. Because increases in AP-1 can reflect induction of c-Jun and c-Fos, we examined the activity of the mitogen-activated protein (MAP) kinase family members Erk-1 and -2 and the c-Jun N-terminal kinase (JNK), which are known to influence c-Jun and c-Fos transcription. Ang II stimulated MAP kinase (MAPK) activity but only approximately 50% as effectively as EGF; again, these effects were independent of PKC. Ang II also produced a 50- to 200-fold activation of JNK in a PKC-independent manner. Unlike its smaller effect on MAPK, Ang II was approximately four- to sixfold more potent in activating JNK than EGF was. Although others had reported a lack of calcium ionophore-stimulated JNK activity in lymphocytes and several other cell lines, we examined the role of calcium in GN4 cells. The following results suggest that JNK activation in rat liver epithelial cells is at least partially Ca(2+) dependent: (i) norepinephrine and vasopressin hormones that increase inositol 1,4,5-triphosphate stimulated JNK; (ii) both thapsigargin, a compound that produces an intracellular Ca(2+) signal, and Ca(2+) ionophores stimulated a dramatic increase in JNK activity (up to 200-fold); (iii) extracellular Ca(2+) chelation with ethylene glycol tetraacetic acid (EGTA) inhibited JNK activation by ionophore and intracellular chelation with 1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl-ester (BAPTA-AM) partially inhibited JNK activation by Ang II or thapsigargin; and (iv) JNK activation by Ang II was inhibited by pretreatment of cells with thapsigargin and EGTA, a procedure which depletes intracellular Ca(2+) stores. JNK activation following Ang II stimulation did not involve calmodulin; either W-7 nor calmidizolium, in concentrations sufficient to inhibit Ca(2+)/calmodulin-dependent kinase II, blocked JNK activation by Ang II. In contrast, genistein, in concentrations sufficient to inhibit Ca(2+)-dependent tyrosine phosphorylation, prevented Ang II and thapsigargin-induced JNK activation. In summary, in GN4 rat liver epithelial cells, Ang II stimulates JNK via a novel Ca(2+)-dependent pathway. The inhibition by genistein suggest that Ca(2+)-dependent tyrosine phosphorylation may modulate the JNK pathway in a cell type-specific manner, particularly in cells with a readily detectable Ca(2+)-regulated tyrosine kinase.

Inositol lipid-mediated signalling in response to endothelin and ATP in the mammalian testis

The testis is a complex organ in which local control is achieved by signalling between its constituent cells. Herein we describe the responses of cultured rat testicular cells and a mouse Sertoli cell-line to stimulation by endothelin and ATP, and elsewhere we have shown that rat peritubular myoid cells possess phosphoinositidase C-coupled V1a-vasopressin receptors identical to those of liver (Howl, J. et al, 1995, Endocrinology 136: 2206-2213). 1. Peritubular myoid cells from pre-pubertal rats responded through ETA receptors with PtdIns(4,5)P2 hydrolysis [EC50 for endothelin-1 (ET-1) approximately 0.4 nM], elevation of intracellular [Ca2+], and tyrosine phosphorylation of a variety of cellular proteins. They also showed enhanced adenylate cyclase activity, with an EC50 for ET-1 of approximately 3 nM, also through ETA receptors. Pharmacological elevation of [cAMP] did not immediately change the ET-1-stimulated formation of inositol phosphates, but attenuated the response after several hours. 2. Pre-pubertal rat Sertoli cells showed no detectable responses to ET-1, but responded to FSH with elevated [cAMP] and to ATP with PtdIns(4,5)P2 hydrolysis. PtdIns(4,5)P2 hydrolysis was equally responsive to ATP and UTP, and so appears to be activated by P2U-purinergic receptors. This response was enhanced by protein kinase C inhibition and attenuated by PKC activation. 3. Despite its lack of effect on rat Sertoli cells in primary culture, ET-1 provoked PtdIns(4,5)P2 hydrolysis in the TM4 murine Sertoli cell line (EC50 approximately 0.6 nM), and this response was negatively regulated by protein kinase C activation. 5. No receptor-stimulated activation of phosphoinositase C was detected in 'germ cell' populations, but the non-specific G protein activator A1F4-provoked inositol phosphate accumulation in these cells, so demonstrating their potential to respond through yet to be identified G protein-coupled receptors with phosphoinositidase C activation. 6. Immunoblotting studies showed the presence in rat testis of phosphoinositidase C-beta 1 and the alpha-subunits(s) of the G-protein(s) Gq and/or G11. These studies show that testicular myoid and Sertoli cells use at least three G protein-coupled receptors (V1a-vasopressins, ETA-endothelin and P2U-purinergic) to signal through phosphoinositidase C activation, that ET-1 can activate multiple signalling pathways in myoid cells, and that the ET-1-stimulated phosphoinositidase C responses of myoid and Sertoli cells have different regulatory characteristics.

Tyrosine kinase-mediated signal transduction pathways and the actions of polypeptide growth factors and G-protein-coupled agonists in smooth muscle

This synopsis focuses on the role that tyrosine kinase pathways may play in the acute regulation of smooth muscle contractility by receptor-kinase-activating growth factors, such as epidermal growth factor-urogastrone (EGF-URO) and by G-protein-coupled agonists, such as angiotensin-II. Growth factor-activated response paradigms that modulate smooth muscle contractility are summarized and the parallels between the actions of G-protein-coupled agonists and growth factors in these response systems are pointed out. A possible dynamic interplay between tyrosine kinase and tyrosine phosphatase activities to modulate tissue tension is also hypothesized. Finally, a model is proposed, wherein an intermediary tyrosine kinase pathway is suggested as a point of convergence for the regulation of smooth muscle contractility by agonists as diverse as EGF-URO and angiotensin-II.

Interaction of focal adhesion kinase with cytoskeletal protein talin

The interaction of cells with extracellular matrix proteins plays a critical role in a variety of biological processes. Recent studies suggest that cell-matrix interactions mediated by integrins can transduce biochemical signals to the cell interior that regulate cell proliferation and differentiation. These studies have placed the focal adhesion kinase (FAK), an intracellular protein tyrosine kinase, in a central position in integrin-initiated signal transduction pathways (Zachary, I., and Rozengurt, E. (1992) Cell 71, 891-894; Schaller, M., and Parsons, J. T. (1993) Trends Cell Biol. 3, 258-262). Here, we report data suggesting a possible association of FAK with the cytoskeletal protein talin in NIH 3T3 cells. We have identified a 48-amino acid sequence in the carboxyl-terminal domain of FAK necessary for talin binding in vitro. Furthermore, we have correlated the ability of integrin to induce FAK phosphorylation with its ability to bind talin using a mutant integrin lacking the carboxyl-terminal 13 amino acids. These studies suggest talin may be a mediator for FAK activation in signaling initiated by integrins and may provide an explanation for the dependence on the integrity of actin-cytoskeleton of multiple intracellular signaling pathways converging to FAK activation and autophosphorylation.