G protein coupling to the thrombin receptor in Chinese hamster lung fibroblasts

Disruption of Calcium Signaling in Fibroblasts and Attenuation of Bleomycin-Induced Fibrosis by Nifedipine

Fibrotic lung disease afflicts millions of people; the central problem is progressive lung destruction and remodeling. We have shown that external growth factors regulate fibroblast function not only through canonical signaling pathways but also through propagation of periodic oscillations in Ca(2+). In this study, we characterized the pharmacological sensitivity of the Ca(2+)oscillations and determined whether a blocker of those oscillations can prevent the progression of fibrosis in vivo. We found Ca(2+) oscillations evoked by exogenously applied transforming growth factor β in normal human fibroblasts were substantially reduced by 1 μM nifedipine or 1 μM verapamil (both L-type blockers), by 2.7 μM mibefradil (a mixed L-/T-type blocker), by 40 μM NiCl2 (selective at this concentration against T-type current), by 30 mM KCl (which partially depolarizes the membrane and thereby fully inactivates T-type current but leaves L-type current intact), or by 1 mM NiCl2 (blocks both L- and T-type currents). In our in vivo study in mice, nifedipine prevented bleomycin-induced fibrotic changes (increased lung stiffness, overexpression of smooth muscle actin, increased extracellular matrix deposition, and increased soluble collagen and hydroxyproline content). Nifedipine had little or no effect on lung inflammation, suggesting its protective effect on lung fibrosis was not due to an antiinflammatory effect but rather was due to altering the profibrotic response to bleomycin. Collectively, these data show that nifedipine disrupts Ca(2+) oscillations in fibroblasts and prevents the impairment of lung function in the bleomycin model of pulmonary fibrosis. Our results provide compelling proof-of-principle that interfering with Ca(2+) signaling may be beneficial against pulmonary fibrosis.

Calcium Homeostasis and Ionic Mechanisms in Pulmonary Fibroblasts

Fibroblasts are key cellular mediators of many chronic interstitial lung diseases, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, drug-induced interstitial lung disease, and interstitial lung disease in connective tissue disease. A great deal of effort has been expended to understand the signaling mechanisms underlying the various cellular functions of fibroblasts. Recently, it has been shown that Ca(2+) oscillations play a central role in the regulation of gene expression in human pulmonary fibroblasts. However, the mechanisms whereby cytosolic [Ca(2+)] are regulated and [Ca(2+)] oscillations transduced are both poorly understood. In this review, we present the general concepts of [Ca(2+)] homeostasis, of ionic mechanisms responsible for various Ca(2+) fluxes, and of regulation of gene expression by [Ca(2+)]. In each case, we then also summarize the original findings that pertain specifically to pulmonary fibroblasts. From these data, we propose an overall signaling cascade by which excitation of the fibroblasts triggers pulsatile release of internally sequestered Ca(2+), which, in turn, activates membrane conductances, including voltage-dependent Ca(2+) influx pathways. Collectively, these events produce recurring Ca(2+) oscillations, the frequency of which is transduced by Ca(2+)-dependent transcription factors, which, in turn, orchestrate a variety of cellular events, including proliferation, synthesis/secretion of extracellular matrix proteins, autoactivation (production of transforming growth factor-β), and transformation into myofibroblasts. That unifying hypothesis, in turn, allows us to highlight several specific cellular targets and therapeutic intervention strategies aimed at controlling unwanted pulmonary fibrosis. The relationships between Ca(2+) signaling events and the unfolded protein response and apoptosis are also explored.

Regulation of protease-activated receptor signaling by post-translational modifications

Protease-activated receptors (PARs) are a unique family of G-protein-coupled receptors (GPCRs) that are irreversibly activated following proteolytic cleavage of their extracellular N-terminus. PARs play critical functions in hemostasis, thrombosis, inflammation, embryonic development, and cancer progression. Because of the irreversible proteolytic nature of PAR activation, signaling by the receptors is tightly regulated. Three distinct processes including desensitization, internalization, and lysosomal degradation, regulate the temporal and spatial aspects of activated PAR signaling. Post-translational modifications play a critical role in regulating each of these processes and here we review the nature of PAR post-translational modifications and their importance in signal regulation. The PARs are activated by numerous proteases, and some can elicit distinct cellular responses, how this biased agonism is determined is unknown. Further study of the function of post-translational modifications of the PARs will lead to a greater understanding of the physiological regulation of baised agonism and how PAR signaling is precisely controlled in different cellular contexts.

N-linked glycosylation of protease-activated receptor-1 second extracellular loop: a critical determinant for ligand-induced receptor activation and internalization

Protease-activated receptor-1 (PAR1) contains five N-linked glycosylation consensus sites as follows: three residing in the N terminus and two localized on the surface of the second extracellular loop (ECL2). To study the effect of N-linked glycosylation in the regulation of PAR1 signaling and trafficking, we generated mutants in which the critical asparagines of the consensus sites were mutated. Here, we report that both the PAR1 N terminus and ECL2 serve as sites for N-linked glycosylation but have different functions in the regulation of receptor signaling and trafficking. N-Linked glycosylation of the PAR1 N terminus is important for transport to the cell surface, whereas the PAR1 mutant lacking glycosylation at ECL2 (NA ECL2) trafficked to the cell surface like the wild-type receptor. However, activated PAR1 NA ECL2 mutant internalization was impaired compared with wild-type receptor, whereas constitutive internalization of unactivated receptor remained intact. Remarkably, thrombin-activated PAR1 NA ECL2 mutant displayed an enhanced maximal signaling response compared with wild-type receptor. The increased PAR1 NA ECL2 mutant signaling was not due to defects in the ability of thrombin to cleave the receptor or signal termination mechanisms. Rather, the PAR1 NA ECL2 mutant displayed a greater efficacy in thrombin-stimulated G protein signaling. Thus, N-linked glycosylation of the PAR1 extracellular surface likely influences ligand docking interactions and the stability of the active receptor conformation. Together, these studies strongly suggest that N-linked glycosylation of PAR1 at the N terminus versus the surface of ECL2 serves distinct functions critical for proper regulation of receptor trafficking and the fidelity of thrombin signaling.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.

Opposing effects of β-arrestin1 and β-arrestin2 on activation and degradation of Src induced by protease-activated receptor 1

Protease-activated receptor 1 (PAR1), a G protein-coupled receptor for thrombin, is irreversibly proteolytically activated. beta-Arrestin1 and beta-arrestin2 have been reported to have different effects on signal desensitization and transduction of PAR1. In this study, we investigated whether beta-arrestin1 and beta-arrestin2 regulate Src-dependent activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) induced by PAR1 in HEK 293 cells. Our results show that PAR1-mediated activation of Src and ERK1/2 in HEK 293 cells was increased with overexpression of beta-arrestin1 or depletion of beta-arrestin2. PAR1-mediated activation of Src and ERK1/2 in HEK 293 cells was decreased or eliminated with depletion of beta-arrestin1 or overexpression of beta-arrestin2. Furthermore, depletion of beta-arrestin2 blocked PAR1-induced degradation of Src. Thus, beta-arrestin1 and beta-arrestin2 have opposing roles in regulating the activation of Src induced by PAR1. beta-Arrestin2 also appears to promote PAR1-induced degradation of Src. This degradation of Src provides a possible mechanism for terminating PAR1 signaling.

Characterization of proteinase-activated receptor 2 signalling and expression in rat hippocampal neurons and astrocytes

Proteinase-activated receptors (PARs1-4) have recently been identified as the molecular entity underlying the cellular effects of serine proteinases. In the present study we have investigated PAR2 signalling, expression and desensitization using cultured and acute slice preparations. Trypsin, SLIGRL and 2f-LIGKV-OH, agonists for PAR2, induced a transient increase in intracellular Ca(2+) levels in both neurons and astrocytes, via activation of the phospholipase C/IP(3) pathway. Furthermore, a single application of trypsin, but not SLIGRL nor 2f-LIGKV-OH, leads to prolonged desensitization of PAR2 responses. PAR2 immunoreactivity was observed in neurons (glutamatergic and GABAergic) and astrocytes within cultures and acute slices, with prominent labelling in neuronal somata and proximal dendrites. Functionally, cultured neurons which exhibited the highest levels of PAR2 labelling, also exhibited the largest Ca(2+) signals upon PAR2 activation. Given the importance of Ca(2+) signalling in hippocampal synaptic plasticity and neurodegeneration, PAR2 may play a key modulatory role in these processes.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

Thrombin receptors activate G(o) proteins in endothelial cells to regulate intracellular calcium and cell shape changes

Thrombin receptors couple to G(i/o), G(q), and G(12/13) proteins to regulate a variety of signal transduction pathways that underlie the physiological role of endothelial cells in wound healing or inflammation. Whereas the involvement of G(i), G(q), G(12), or G(13) proteins in thrombin signaling has been investigated extensively, the role of G(o) proteins has largely been ignored. To determine whether G(o) proteins could contribute to thrombin-mediated signaling in endothelial cells, we have developed minigenes that encode an 11-amino acid C-terminal peptide of G(o1) proteins. Previously, we have shown that use of the C-terminal minigenes can specifically block receptor activation of G protein families (). In this study, we demonstrate that G(o) proteins are present in human microvascular endothelial cells (HMECs). Moreover, we show that thrombin receptors can stimulate [(35)S]guanosine-5'-O-(3-thio)triphosphate binding to G(o) proteins when co-expressed in Sf9 membranes. The potential coupling of thrombin receptors to G(o) proteins was substantiated by transfection of the G(o1) minigene into HMECs, which led to a blockade of thrombin-stimulated release of [Ca(2+)](i) from intracellular stores. Transfection of the beta-adrenergic kinase C terminus blocked the [Ca(2+)](i) response to the same extent as with G(o1) minigene peptide, suggesting that this G(o)-mediated [Ca(2+)](i) transient was caused by Gbetagamma stimulation of PLCbeta. Transfection of a G(i1/2) minigene had no effect on thrombin-stimulated [Ca(2+)](i) signaling in HMEC, suggesting that Gbetagamma derived from G(o) but not G(i) could activate PLCbeta. The involvement of G(o) proteins on events downstream from calcium signaling was further evidenced by investigating the effect of G(o1) minigenes on thrombin-stimulated stress fiber formation and endothelial barrier permeability. Both of these effects were sensitive to pertussis toxin treatment and could be blocked by transfection of G(o1) minigenes but not G(i1/2) minigenes. We conclude that the G(o) proteins play a role in thrombin signaling distinct from G(i1/2) proteins, which are mediated through their Gbetagamma subunits and involve coupling to calcium signaling and cytoskeletal rearrangements.

A novel role for Gq alpha in alpha-thrombin-mediated mitogenic signalling pathways

alpha-Thrombin activates several G-proteins including members of the Gq, Gi, and G12/13 families, although the physiological importance of these proteins is still not completely understood. We specifically investigated the role of Gq alpha in modulating alpha-thrombin-induced mitogenesis. In Gqa1 cells, a stable cell line expressing reduced amounts of Gq alpha, concentrations of alpha-thrombin (1 NIH unit/ml), which induce cell cycle reentry and progression into S phase in wild-type IIC9 cells, do not stimulate phosphatidylinositol (PI) hydrolysis, the rapid early phase of ERK activity, and transit through G1 into S phase as quantified by cyclin-dependent kinase (CDK)4-cyclin D activity and [3H]thymidine incorporation. Interestingly, high concentrations of alpha-thrombin restore these activities and cell cycle progression into S phase. While, it is well documented that alpha-thrombin-induced sustained ERK activity mediates important responses for transit through G1 into S phase, the importance of the rapid, Gq-dependent phase as a prerequisite for alpha-thrombin-mediated mitogenesis has not been appreciated.

alpha-Thrombin induces rapid and sustained Akt phosphorylation by beta-arrestin1-dependent and -independent mechanisms, and only the sustained Akt phosphorylation is essential for G1 phase progression

In Chinese hamster embryonic fibroblasts (IIC9 cells) alpha-thrombin activates the MAPK(ERK) and phosphatidylinositol 3-OH-kinase (PI 3-kinase)/Akt pathways, and both are essential for progression through the G(1) phase of the cell cycle. We investigated in IIC9 cells, the role of beta-arrestin1 in alpha-thrombin signaling to these pathways. alpha-Thrombin stimulates rapid and sustained PI 3-kinase and Akt activities. Expression of a dominant negative beta-arrestin1 (beta-arrestin1(V53D)) inhibits rapid but not sustained PI 3-kinase and Akt activities. Surprisingly, expression of beta-arrestin1(V53D) does not block activation of the MAPK(ERK) pathway. PI 3-kinase and Akt activities are also inhibited by expression of a beta-arrestin1 mutant, which impairs binding to c-Src (beta-arrestin1(P91G-P121E)), indicating the involvement of c-Src in the rapid stimulation of the PI 3-kinase/Akt pathway. Consistent with these results, PP1, a selective inhibitor of c-Src family kinases, prevents alpha-thrombin-stimulated Akt phosphorylation. Expression of beta- arrestin1(V53D) does not prevent G(1) progression, as its expression has no effect on [(3)H]thymidine incorporation into DNA. In agreement with the ineffectiveness of beta-arrestin1(V53D) to block G(1) progression, cyclin D1 protein amounts and CDK4-cyclin D1 activity is unaffected by expression of beta-arrestin1(V53D). Thus in IIC9 cells, alpha-thrombin activates rapid beta-arrestin1-dependent and sustained beta-arrestin1-independent Akt activity, suggesting that two mechanisms are involved. Furthermore, although blocking the beta-arrestin1-independent PI 3-kinase/Akt pathway prevents G(1) progression, inhibition of the beta-arrestin1-dependent pathway does not, indicating different roles for the rapid and sustained activities.

G alpha minigenes expressing C-terminal peptides serve as specific inhibitors of thrombin-mediated endothelial activation

The C termini of G protein alpha subunits are critical for binding to their cognate receptors, and peptides corresponding to the C terminus can serve as competitive inhibitors of G protein-coupled receptor-G protein interactions. This interface is quite specific as a single amino acid difference annuls the ability of a G alpha(i) peptide to bind the A(1) adenosine receptor (Gilchrist, A., Mazzoni, M., Dineen, B., Dice, A., Linden, J., Dunwiddie, T., and Hamm, H. E. (1998 ) J. Biol. Chem. 273, 14912--14919). Recently, we demonstrated that a plasmid minigene vector encoding the C-terminal sequence of G alpha(i) could specifically inhibit downstream responses to agonist stimulation of the muscarinic M(2) receptor (Gilchrist, A., Bunemann, M., Li, A., Hosey, M. M., and H. E. Hamm (1999) J. Biol. Chem. 274, 6610--6616). To selectively antagonize G protein signal transduction events and determine which G protein underlies a given thrombin-induced response, we generated minigene vectors that encode the C-terminal sequence for each family of G alpha subunits. Minigene vectors expressing G alpha C-terminal peptides (G alpha(i), G alpha(q), G alpha(12), and G alpha(13)) or the control minigene vector, which expresses the G alpha(i) peptide in random order (G(iR)), were systematically introduced into a human microvascular endothelial cell line. The C-terminal peptides serve as competitive inhibitors presumably by blocking the site on the G protein-coupled receptor that normally binds the G protein. Our results not only confirm that each G protein can control certain signaling events, they emphasize the specificity of the G protein-coupled receptor-G protein interface. In addition, the C-terminal G alpha minigenes appear to be a powerful tool for dissecting out the G protein that mediates a given physiological function following thrombin activation.

Differentiation of cultured brown adipocytes is associated with a selective increase in the short variant of g(s)alpha protein. Evidence for higher functional activity of g(s)alphaS

In order to examine whether the differentiation process in brown adipocytes cultivated in primary culture is associated with substantial alterations in the complement of G proteins, the levels of these proteins were investigated with immuno-electrophoretic techniques in membrane preparations from proliferating and differentiated cultured mouse brown adipocytes. We observed that differentiation was associated with a dramatic (more than threefold) increase in the short variant of G(s)alpha protein (G(s)alphaS). The long variant of G(s)alpha (G(s)alphaL), as well as G(i)1alpha, G(i)2alpha, G(q)alpha, G(11)alpha and Gbeta subunit proteins remained unchanged whereas G(i)3alpha protein was decreased. These changes were accompanied by marked increase in isoprenaline-, forskolin- as well as manganese-stimulated adenylyl cyclase. Thus, the marked increase in beta-adrenergic responsiveness of fully differentiated confluent brown adipocytes (day 8-9), as compared with that of proliferating undifferentiated cells of 'fibroblast phenotype' (day 3-4), is associated with a significant increase in the relative proportion between the short and long variants of G(s)alpha (the G(s)alphaS/G(s)alphaL ratio) along with a decrease in G(i)3alpha protein. These data also suggest that the short variant of G(s)alpha exhibits higher functional activity than the long variant of this G protein.

Activation of B-Raf and regulation of the mitogen-activated protein kinase pathway by the G(o) alpha chain

Many receptors coupled to the pertussis toxin-sensitive G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) pathway. The role of the alpha chains of these G proteins in MAPK activation is poorly understood. We investigated the ability of Galpha(o) to regulate MAPK activity by transient expression of the activated mutant Galpha(o)-Q205L in Chinese hamster ovary cells. Galpha(o)-Q205L was not sufficient to activate MAPK but greatly enhanced the response to the epidermal growth factor (EGF) receptor. This effect was not associated with changes in the state of tyrosine phosphorylation of the EGF receptor. Galpha(o)-Q205L also potentiated MAPK stimulation by activated Ras. In Chinese hamster ovary cells, EGF receptors activate B-Raf but not Raf-1 or A-Raf. We found that expression of activated Galpha(o) stimulated B-Raf activity independently of the activation of the EGF receptor or Ras. Inactivation of protein kinase C and inhibition of phosphatidylinositol-3 kinase abolished both B-Raf activation and EGF receptor-dependent MAPK stimulation by Galpha(o). Moreover, Galpha(o)-Q205L failed to affect MAPK activation by fibroblast growth factor receptors, which stimulate Raf-1 and A-Raf but not B-Raf activity. These results suggest that Galpha(o) can regulate the MAPK pathway by activating B-Raf through a mechanism that requires a concomitant signal from tyrosine kinase receptors or Ras to efficiently stimulate MAPK activity. Further experiments showed that receptor-mediated activation of Galpha(o) caused a B-Raf response similar to that observed after expression of the mutant subunit. The finding that Galpha(o) induces Ras-independent and protein kinase C- and phosphatidylinositol-3 kinase-dependent activation of B-Raf and conditionally stimulates MAPK activity provides direct evidence for intracellular signals connecting this G protein subunit to the MAPK pathway.

Trypsin stimulates integrin alpha(5)beta(1)-dependent adhesion to fibronectin and proliferation of human gastric carcinoma cells through activation of proteinase-activated receptor-2

Trypsin is widely expressed in various non-pancreatic tissues at low levels and overexpressed in some types of human cancers. In the present study, we found that trypsin stimulates integrin-dependent adhesion and growth of MKN-1 human gastric carcinoma cells. MKN-1 cells expressed both proteinase-activated receptor-1 (PAR-1) and PAR-2, which are activated by thrombin and trypsin, respectively. Both trypsin and the PAR-2 ligand SLIGKV promoted integrin alpha(5)beta(1)-mediated adhesion of MKN-1 cells to fibronectin, and less effectively integrin alpha(v)beta(3)-mediated cell adhesion to vitronectin, but not that to type IV collagen or laminin-1 at all. Thrombin and the PAR-1 ligand SFLLRN promoted the cell adhesion to vitronectin more strongly than trypsin or the PAR-2 ligand, but not the cell adhesion to fibronectin at all. The cell adhesion-stimulating effect of the PAR-2 ligand was significantly reduced by the pre-treatment of cells with trypsin, indicating that the effect of trypsin is mediated by PAR-2 activation. The trypsin-stimulated cell adhesion to vitronectin, but not to fibronectin, was effectively inhibited by the G(i) protein blocker pertussis toxin, and both cell adhesions were completely inhibited by the Src kinase inhibitor herbimycin A. Furthermore, trypsin and the PAR-2 ligand stimulated growth of MKN-1 cells more strongly than thrombin or the PAR-1 ligand. These results show that trypsin regulates cellular adhesion and proliferation by inducing PAR-2/G protein signalings, and that the integrin alpha(5)beta(1)- and integrin alpha(v)beta(3)-dependent cell adhesions are regulated by different PAR/G protein signalings.

Thrombin upregulates interleukin-8 in lung fibroblasts via cleavage of proteolytically activated receptor-I and protein kinase C-gamma activation

Acute and chronic interstitial lung diseases are accompanied by evidence of inflammation and vascular injury. Thrombin activity in bronchoalveolar lavage fluid from such conditions is often increased, as well as interleukin (IL)-8. We observed that conditioned medium from lung fibroblasts exposed to thrombin has chemotactic activity for polymorphonuclear cells, and that this activity can be abolished by antibody to IL-8. We report that thrombin stimulates expression of IL-8 in human lung fibroblasts on both the messenger RNA and protein levels in a time- and dose-dependent manner. Stimulation of IL-8 expression by thrombin is inhibited by specific thrombin inhibitors. Synthetic thrombin receptor agonist peptide-14 mimics thrombin's stimulation of IL-8 expression in a dose-dependent manner consistent with the idea that upregulation of IL-8 by thrombin in human lung fibroblasts requires cleavage of proteolytically activated receptor-I. We demonstrate further that thrombin-induced IL-8 synthesis is regulated by protein kinase (PK) C. PKC-gamma may be involved in the upregulation of lung fibroblast IL-8 by thrombin because stimulation of lung fibroblasts with thrombin caused significant upregulation of PKC-gamma and because PKC-gamma antisense oligonucleotides inhibited the accumulation of PKC-gamma protein and IL-8 protein. Our data suggest that the PKC-gamma isoform increase observed after thrombin stimulation is required for thrombin-induced IL-8 formation by human lung fibroblasts.

Evidence that thrombin present in lungs of patients with pulmonary metastasis may contribute to the development of the disease

Early cellular events in the lung which may lead to the development of pulmonary metastases (PM) are still poorly understood. Thrombin, a key component of the coagulation cascade, may be involved in the development of PM as it has been shown to be an enhancer of platelet-tumor interaction in vitro and metastasis in vivo, and because it has been found in high levels in lungs from patients with PM. In this study, we assessed the potential role of thrombin in promoting PM by inducing an enhancement of tumor cell adhesion to platelets and tumor cell chemoinvasion and proliferation. We used bronchoalveolar lavage fluid (BALF) from 20 patients with PM. Results were compared with those from healthy controls. We found an enhancement of adhesion of PM-BALF-treated tumor cells to untreated platelets. BALF from patients with PM significantly increased chemoinvasion and proliferation in three human tumor cell lines. These activities were attenuated significantly by a thrombin inhibitor: hirudin. These results indicate that the thrombin present in the lungs of patients with PM is, at least in part, responsible for their adhesive, invasive and mitogenic activity on three different tumor cell lines. They also suggest that thrombin may be involved in the development of PM.

Differential cytoskeletal changes during growth cone collapse in response to hSema III and thrombin

Growth cones are known as the site of action of many factors that influence neurite growth behavior. To assess how different collapsing agents influence the growth cone cytoskeleton, we used recombinant human Semaphorin III (hSema III) and the serine protease thrombin. Embryonic chick dorsal root ganglion neurons showed a dramatic depolymerization of actin filaments within 5 min upon hSema III exposure and virtually no influence on microtubules (MT). Only at later time points (20-30 min) was the polymerization/depolymerization rate of MT significantly affected. Thrombin induced a morphologically and kinetically similar growth cone collapse. Moreover, thrombin induced an early and selective depolymerization of dynamic MT, accompanied by the formation of loops of stable MT bundles. Selective changes in the phosphorylation pattern of tau were associated with microtubule assembly in thrombin-induced responses. Our data provide evidence that different signal transduction pathways lead to distinct changes of the growth cone cytoskeleton.

Thrombin peptide, TP508, induces differential gene expression in fibroblasts through a nonproteolytic activation pathway

Prior studies have shown that synthetic peptides representing the domain of thrombin responsible for high-affinity binding to fibroblasts stimulate chemotactic and cell proliferative signals through a nonproteolytic mechanism. One of these peptides, TP508, has recently been shown to be chemotactic for neutrophils, to enhance collagen accumulation in wounds, to enhance revascularization of wounds, and to accelerate the healing of incisional and open wounds in normal animals and in animals with impaired healing. To determine whether TP508 activates the proteolytically activated receptor for thrombin (PAR1), or the signals that are activated by PAR1, we treated human fibroblasts with TP508 and the PAR1-activating peptide, SFLLRNP, and analyzed the effects of these peptides on gene expression using differential display reverse transcriptase polymerase chain reaction. TP508 induces expression of a number of specific message fragments with short tyrosine kinase-like domains that are not induced by SFLLRNP. Sequencing full-length clones prepared by Marathon extension of TP508-induced fragments revealed that among the induced transcripts, there was a sequence with 88% homology to human annexin V. Northern analysis with authentic annexin V cDNA confirms that TP508, but not SFLLRNP, induces expression of annexin V in human fibroblasts. These results demonstrate that TP508 activates a cellular response separate from that activated through PAR1 and supports the hypothesis that TP508 acts through a separate nonproteolytically activated thrombin receptor that may be responsible for high-affinity thrombin binding and for nonproteolytic signals that are required for thrombin stimulation of cell proliferation.

Signaling from G-protein-coupled receptors to mitogen-activated protein (MAP)-kinase cascades

Heterotrimeric GTP-binding protein (G-protein)-coupled receptors are able to induce a variety of responses including cell proliferation, differentiation, and activation of several intracellular kinase cascades. Prominent among these kinases are the activation of mitogen-activated protein (MAP) kinase, including the extracellular signal-regulated kinases (ERKs), ERK1 and ERK2 (p44mapk and p42mapk, respectively); stress-activated protein kinases (SAPKs/JNKs); and p38 kinase. These receptors signal through G-proteins. Recent data have shown that the activation of mitogen-activated protein/ERK kinase induced by G-protein-coupled receptors is mediated by both Galpha and Gbetagamma subunits involving a common signaling pathway with receptor-tyrosine-kinases. Gbetagamma-mediated mitogen-activated protein kinase activation is mediated by activation of phosphoinositide 3-kinase, followed by a tyrosine phosphorylation event, and proceeds in a sequence of events that involve functional association among the adaptor proteins Shc, Grb2, and Sos. SAPKs/JNKs and p38 are able to be activated by Gbetagamma proteins in a pathway involving Rho family proteins including RhoA, Rac1, and Cdc42.

Coactivation of two different G protein-coupled receptors is essential for ADP-induced platelet aggregation

ADP is an important platelet agonist causing shape change and aggregation required for physiological hemostasis. We recently demonstrated the existence of two distinct G protein-coupled ADP receptors on platelets, one coupled to phospholipase C, P2Y1, and the other to inhibition of adenylyl cyclase, P2TAC. In this study, using specific antagonists for these two receptors, we demonstrated that concomitant intracellular signaling from both the P2TAC and P2Y1 receptors is essential for ADP-induced platelet aggregation. Inhibition of signaling through either receptor, by specific antagonists, is sufficient to block ADP-induced platelet aggregation. Furthermore, signaling through the P2TAC receptor could be replaced by activation of alpha2A-adrenergic receptors. On the other hand, activation of serotonin receptors supplements signaling through the P2Y1 receptor. Moreover, this mechanism of ADP-induced platelet aggregation could be mimicked by coactivation of two non-ADP receptors coupled to Gi and Gq, neither of which can cause platelet aggregation by itself. We propose that platelet aggregation results from concomitant signaling from both the Gi and Gq, a mechanism by which G protein-coupled receptors elicit a physiological response.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Thrombin aivation of the 9E3/CEF4 chemokine involves tyrosine kinases including c-src and the epidermal growth factor receptor

The 9E3/CEF4 gene codes for a chemokine that is highly homologous to human interleukin-8 and melanoma growth-stimulating activity/groalpha. These chemokines belong to a family of molecular mediators that are importantly involved in inflammation, wound healing, tumor development, and viral entry into cells. On the chorioallantoic membrane the 9E3 protein is chemotactic for monocyte/macrophages and lymphocytes and is angiogenic. In cultured chicken embryo fibroblasts, which have many of the properties of wound fibroblasts, the gene is stimulated by a variety of agents including oncogenes, growth factors, phorbol esters, and thrombin. The strong stimulation of 9E3 by thrombin in culture correlates well with the observation that in young chicks this gene is stimulated to very high levels in fibroblasts upon wounding and remains high throughout wound repair. Activation of 9E3 by thrombin: (i) occurs very rapidly, one minute exposure to thrombin is sufficient to initiate the signals necessary for gene activation; (ii) is independent of mitogenesis; (iii) operates through the proteolytically activated receptor for thrombin; (iv) is mediated by tyrosine kinases, including c-src and the epidermal growth factor (EGF) receptor, rather than Ser/Thr kinases such as protein kinase C and protein kinase A. Inhibition of either c-src or the EGF receptor tyrosine kinase inhibits the stimulation of 9E3 by thrombin. We show here for the first time that activation of the EGF receptor through a cell-surface receptor that does not have tyrosine kinase activity can lead to expression of an immediate early response gene which encodes for a secreted protein, a chemokine. This rapidly activated tyrosine kinase pathway may be a general stress response by which in vivo a localized cell population reacts to emergency situations such as viral infection, wounding, or tumor growth.

Calcium mobilization and protease-activated receptor cleavage after thrombin stimulation in motor neurons

Thrombin, the ultimate enzyme in the blood coagulation cascade, has prominent actions on various cells, including neurons. As in platelets, thrombin increases [Ca2+]i mobilization in neurons, and also retracts neurites. Both these effects are mediated through a G protein-coupled, proteolytically activated receptor for thrombin (PAR-1). Prolonged exposure to thrombin kills neurons via apoptosis, that may also involve PAR-1 activation. Increased [Ca2+]i has been a unifying mechanism proposed for cell death in several neurodegenerative diseases. Thrombin-elevated calcium levels may activate intracellular cascades in neurons leading to cell death. Since thrombin mediates its diverse effects on cells through both heterotrimeric and monomeric G proteins, we also explored what effect altering differential G protein coupling would have on the neuronal response to thrombin. We studied calcium mobilization by thrombin in a model motor neuronal cell line, NSC19, using fluorescence image analysis. Confirming effects in other neuronal types, thrombin caused dramatic increases in [Ca2+]i levels, both transiently and after prolonged exposure, which involved activation and cleavage of the PAR-1 receptor. Using enzyme linked immunosorbent assay (ELISA) and dot-blot analysis, we found that the N-terminal fragment of PAR-1 was released into the medium after exposure to thrombin. We confirmed that PAR-1 protein and mRNA expression occurred in motor neurons. We found that cholera toxin inhibited thrombin-mediated Ca2+ influx, pertussis toxin did not significantly alter thrombin action, and lovastatin, a small 21-kDa Ras GTPase (Rho) modulator, showed a tendency to reduce the thrombin effect. These data indicate that thrombin-increased [Ca2+]i, sufficient to trigger cell death in motor neurons, might be approached in vivo by modulating thrombin signaling through PAR-1.

Mechanism of the mitogenic effect of fluoride on osteoblast-like cells: evidences for a G protein-dependent tyrosine phosphorylation process

Recent results indicate that a fluoroalumino complex (AlFx) is probably the molecule responsible for the mitogenic effect of fluoride in MC3T3-E1 osteoblast-like cells. Initial analysis suggested that a tyrosine phosphorylation (tyr phos) process similar to that induced by thrombin and activation of the p42 MAP kinase (ERK 2) mediate this cellular response. In the present study, the signaling mechanism activated by AlFx was further investigated. The results indicated that AlFx dose-dependently enhanced the tyr phos of the cell adhesion proteins FAK and paxillin, as well as of the adaptor molecules p46shc, p52shc, and p66shc and their association with GRB2. Pretreatment of MC3T3-E1 cells with cytochalasin D completely prevented FAK and paxillin tyr phos without any alteration in the tyr phos of Shc proteins and activation of ERK2 induced by AlFx. This observation suggests that in confluent MC3T3-E1 cells, there is no link between the activation of FAK induced by AlFx and the stimulation of ERK2. Pretreatment of the cells with pertussis toxin inhibited Shc phosphorylation, activation of ERK2, and markedly reduced cell replication induced by AlFx. This toxin also significantly reduced the stimulation of Pi transport activity induced by AlFx in these cells. Alteration in tyr phos induced by AlFx was not associated with any detectable inhibition of tyrosine phosphatase activity in MC3T3-E1 cell homogenates, suggesting that enhanced tyr phos induced by AlFx probably resulted from activation of a tyrosine kinase. In conclusion, the results of this study suggest that the mitogenic effect of fluoride in MC3T3-E1 osteoblast-like cells is mediated by the activation of a pertussis toxin-sensitive Gi/o protein and suggest an important role for these heterotrimeric G proteins in controlling the growth and differentiation of bone-forming cells.

Inhibition of thrombin-stimulated cell proliferation by ceramide is not through inhibition of extracellular signal-regulated protein kinase

Activation of the thrombin receptor provides a strong mitogenic signal in CCL39 cells. Ceramide was found to inhibit thrombin-mediated mitogenesis in these cells while dihydroceramide had no effect. Many growth inhibitors exert their effect by inhibiting extracellular signal-regulated kinase (ERK) signaling pathway. However, neither ceramide nor dihydroceramide blocked the thrombin-induced activation of ERK. In contrast, both agents potentiated ERK activity. The expression of c-fos, c-jun and cyclin D1, which are downstream of ERK in the mitogenic pathway were stimulated by thrombin but this stimulation was not affected by ceramide or dihydroceramide. Therefore, the ceramide inhibition of thrombin-stimulated cell growth in CCL39 cells does not appear to be mediated by an effect on the activation of ERK. Furthermore, the data also suggest that the separate effects of ceramide on thrombin-stimulated cell growth and ERK activity are mediated by different mechanisms.

Platelet signal transduction defect with Galpha subunit dysfunction and diminished Galphaq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of alpha-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[gammaS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Galphas) and its inhibition (mediated by Galphai) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of Galpha subunits in the patient's platelet membranes showed a decrease in Galphaq (<50%) but not Galphai, Galphaz, Galpha12, and Galpha13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Galphaq in thrombin-induced responses.

An evaluation of strategies available for the identification of GTP-binding proteins required in intracellular signalling pathways

Strategies which can be used to elucidate the nature of a GTP-binding regulatory protein (G-protein) involved in an intracellular pathway of interest in the complex environment of the cell are described and evaluated. A desirable strategy is considered to be one in which the first stage indicates a requirement for one or more G-proteins, provides information on whether a monomeric, trimeric or other type of G-protein is involved, and gives some idea of the G-protein sub-class. In the second stage the specific G-protein involved is identified. Approaches available for investigations in the first stage include the use of analogues of GTP and GDP, AlF4-, inhibitors of G-protein isoprenylation, bacterial toxins which covalently modify G-proteins, and the introduction of a purified GDP dissociation inhibitor, GDP exchange and/or GTP-ase activating protein. Identification of the specific G-protein in the second stage can be achieved using anti G-protein antibodies, G-protein-or receptor-derived peptides, antisense G-protein RNA and over-expressed, constitutively-active or dominant-negative G-protein mutants. The correct interpretation of results obtained with GTP and GDP analogues and AlF4- in the first stage is complex and often difficult, and requires a thorough understanding of the functions and mechanisms of activation of G-proteins. Nevertheless, it is important to reach the correct conclusion at this stage since considerable time and expense are usually required for investigations in the second stage.

The thrombin receptor second cytoplasmic loop confers coupling to Gq-like G proteins in chimeric receptors. Additional evidence for a common transmembrane signaling and G protein coupling mechanism in G protein-coupled receptors

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor. Thrombin cleavage of this receptor's amino-terminal exodomain unmasks a new amino terminus. This then binds intramolecularly to the body of the receptor to trigger transmembrane signaling and activation of Gi- and Gq-like G proteins. Toward identifying the domains responsible for thrombin receptor-G protein interactions, we examined the signaling properties of chimeric receptors in which thrombin receptor cytoplasmic sequences replaced the cognate sequences in the Gs-coupled beta2-adrenergic receptor (beta2AR) or the Gi-coupled dopamine D2 receptor (D2R). In Xenopus oocytes, a chimeric beta2AR bearing the thrombin receptor second cytoplasmic (C2) loop gained the ability to trigger intracellular Ca2+ release in response to adrenergic agonist, whereas a beta2AR bearing the cognate C2 loop from the D2R did not. Similarly, in COS-7 cells, a chimeric D2R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic agonist, apparently by coupling to a Gq-like G protein. No detectable Gs coupling was seen. Thus, the thrombin receptor C2 loop was able to confer Gq-like coupling in several different receptor contexts. These observations suggest that the thrombin receptor C2 loop specifies Gq coupling by directly contacting Gq or by contributing to a structure required for Gq coupling. The ability of the thrombin receptor C2 loop to function in the context of the D2R and beta2AR strongly suggests that the transmembrane switching and G protein activation strategies used by the thrombin receptor must be very similar to those used by the D2R and beta2AR despite the thrombin receptor's strikingly different liganding mechanism.

Ras-dependent activation of fibroblast mitogen-activated protein kinase by 5-HT1A receptor via a G protein beta gamma-subunit-initiated pathway

Serotonin (5-HT) is a potent mitogen in many cells types, an action which is frequently mediated through pertussis toxin-sensitive G proteins. In the current study, we used pharmacological inhibitors and dominant negative signaling constructs to delineate elements which participate in the activation of MAPK, a growth-associated mitogen-activated protein kinase, by human G protein-coupled 5-HT1A receptor transfected into CHO-K1 cells in a stable manner. The activation pathway does not directly involve phorbol ester-sensitive protein kinase C types, but does require (i) pertussis toxin-sensitive G protein beta gamma-subunits, (ii) a staurosporine- and genistein-sensitive protein kinase, (iii) phosphoinositide-3'-kinase activity, (iv) activation of Sos in a multimolecular complex that contains p46Shc, and p52Shc, and Grb2, (v) the GTPase p21Ras, and (vi) the protein kinase p74Raf-1. These data demonstrate that the 5-HT1A receptor mediates MAPK activity by convergence upon a common activation pathway that is shared with receptor tyrosine kinases.

The cloned thrombin receptor is necessary and sufficient for activation of mitogen-activated protein kinase and mitogenesis in mouse lung fibroblasts. Loss of responses in fibroblasts from receptor knockout mice

The mitogenic activity of thrombin on fibroblasts and smooth muscle cells may contribute to embryonic development and normal wound healing, and it may also play a role in pathological responses to vascular injury. To examine the importance of thrombin signaling in vivo and to define the cloned thrombin receptor's role, we disrupted the thrombin receptor gene (tr) in mice. Platelets from tr-/- mice responded normally to thrombin, but tr-/- fibroblasts showed no thrombin-induced calcium mobilization or phosphoinositide hydrolysis. Thus distinct thrombin receptors act in different tissues. This study focuses on the role of the thrombin receptor in thrombin-induced mitogenesis and mitogen-activated protein (MAP) kinase activation in mesenchymal cells. Thrombin and thrombin receptor agonist peptide both stimulated DNA synthesis and MAP kinase activation in fibroblasts derived from wild-type mice. These responses were selectively lost in fibroblasts from tr-/- mice. Activation of the cloned thrombin receptor is therefore necessary and sufficient for thrombin-induced mitogenesis and MAP kinase activation in mouse lung fibroblasts. The tr-/- mouse thus provides a valuable model for defining the role of thrombin-induced proliferative events in vivo. Because thrombin-induced MAP kinase activation was attributable to a single receptor expressed at natural levels, mouse lung fibroblasts presented an opportunity to define the pathways that normally mediate activation of MAP kinase by the thrombin receptor. Elimination of phorbol-sensitive protein kinase C by prolonged exposure to phorbol ester only partially inhibited MAP kinase activation by thrombin but completely blocked c-Raf kinase activation. Pertussis toxin partially inhibited MAP kinase activation by thrombin but had no significant effect on c-Raf kinase activation. Thus in mouse lung fibroblasts, one thrombin receptor utilizes two pathways for MAP kinase activation: one is protein kinase C- and c-Raf dependent, and a second is Gi-dependent and c-Raf-independent.

The effect of glucocorticoids on proliferation of human cultured airway smooth muscle

1. Airway smooth muscle proliferation is a significant component of the airway wall remodelling that occurs in asthma. In this study, the effects of glucocorticoids on mitogenic responses of human airway smooth muscle have been examined. 2. Pretreatment of smooth muscle cells with dexamethasone (100 nM, 60 min) inhibited thrombin-induced increases in [3H]-thymidine incorporation (DNA synthesis) and cell number. 3. Inhibition of thrombin-induced [3H]-thymidine incorporation was also observed with hydrocortisone (0.01-1 microM) and methylprednisolone (0.001-0.1 microM) pretreatment. In contrast, pretreatment with either testosterone (0.001-1 microM) progesterone (0.001-1 microM), 17 beta-oestradiol (0.001-1 microM), or aldosterone (0.001-1 microM) had no effect on the response to thrombin. 4. Responses to a range of mitogens including thrombin (0.01-. 10 u ml-1), epidermal growth factor (EGF, 3-3000 pM), basic fibroblast growth factor (bFGF, 0.3-300 pM) and foetal calf serum (FCS, 0.1-10% v/v) were inhibited by dexamethasone (100 nM) pretreatment. However, the magnitude of the inhibitory effect was dependent on the mitogen, with EGF being the least, and thrombin being the most sensitive to the inhibitory effect. 5. The potency of hydrocortisone as an inhibitor of [3H]-thymidine incorporation was reduced when FCS (10% v/v, which caused a 40 fold increase in [3H]-thymidine incorporation) was used as the mitogen in place of thrombin (0.3 u ml-1, which caused a 10 fold increase in [3H]-thymidine incorporation). 6. The effect of post-treatment with dexamethasone (100 nM) indicated that addition of the glucocorticoid up to 17-19 h after thrombin (0.3 u ml-1) produced similar degrees of inhibition to those obtained when it was added as a pretreatment. Dexamethasone no longer produced an inhibitory effect if added 21 h or more after the addition of thrombin. 7. These results suggest that glucocorticoids regulate airway smooth muscle proliferation initiated by a range of stimuli. This effect may be of importance in the therapeutic actions of these compounds in asthma, particularly when they are used for prolonged periods of time.

Evidence that the pertussis toxin-sensitive trimeric GTP-binding protein Gi2 is required for agonist- and store-activated Ca2+ inflow in hepatocytes

The role of a trimeric GTP-binding protein (G-protein) in the mechanism of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-protein alpha subunits, and carboxyl-terminal alpha-subunit synthetic peptides. An anti-Gi1-2 alpha antibody and a Gi2 alpha peptide (Gi2 alpha) Ile345-Phe355), but not a Gi3 alpha peptide (Gi3 alpha Ile344-Phe354), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ from intracellular stores, and caused partial inhibition of thapsigargin-stimulated release of Ca2+. An anti-Gq alpha antibody also inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. Immunofluorescence measurements showed that Gi2 alpha is distributed throughout much of the interior of the hepatocyte as well as at the periphery of the cell. By contrast, Gq/11 alpha was found principally at the cell periphery. It is concluded that the trimeric G-protein, Gi2, is required for store-activated Ca2+ inflow in hepatocytes and acts between the release of Ca2+ from the endoplasmic reticulum (presumably adjacent to the plasma membrane) and the receptor-activated Ca2+ channel protein(s) in the plasma membrane.

G alpha 12 and G alpha 13 stimulate Rho-dependent stress fiber formation and focal adhesion assembly

Rho, a member of the Ras superfamily of GTP-binding proteins, regulates actin polymerization resulting in the formation of stress fibers and the assembly of focal adhesions. In Swiss 3T3 cells, heterotrimeric G protein-coupled receptors for lysophosphatidic acid and gastrin releasing peptide stimulate Rho-dependent stress fiber and focal adhesion formation. The specific heterotrimeric G protein subunits mediating Rho-dependent stress fiber and focal adhesion formation have not been defined previously. We have expressed GTPase-deficient, constitutively activated G protein alpha subunits and mixtures of beta and gamma subunits in Swiss 3T3 cells. Measurement of actin polymerization and focal adhesion formation indicated that GTPase-deficient alpha 12 and alpha 13, but not the activated forms of alpha 12 or alpha q stimulated stress fiber and focal adhesion assembly. Combinations of beta and gamma subunits were unable to stimulate stress fiber or focal adhesion formation. G alpha 12- and alpha 13-mediated stress fiber and focal adhesion assembly was inhibited by botulinum C3 exoenzyme, which ADP-ribosylates and inactivates Rho, indicating that alpha 12 and alpha 13, but not other G protein alpha subunits or beta gamma complexes, regulate Rho-dependent responses. The results define the integration of G12 and G13 with the regulation of the actin cytoskeleton.

G12 requirement for thrombin-stimulated gene expression and DNA synthesis in 1321N1 astrocytoma cells

Thrombin stimulation of 1321N1 astrocytoma cells leads to Ras-dependent AP-1-mediated transcriptional activation and to DNA replication. In contrast to what has been observed in most cell systems, in 1321N1 cells these responses are pertussis toxin-insensitive. The pertussis toxin-insensitive G-protein G12 has been implicated in cell growth and transformation in different cell systems. We have examined the potential role of this protein in AP-1-mediated transcriptional activation and DNA synthesis in 1321N1 cells. Transient expression of an activated (GTPase-deficient) mutant of G alpha 12 increased AP-1-dependent gene expression. This response was inhibited by co-expression of a dominant negative Ala-15 Ras protein. To determine whether the pertussis toxin-insensitive G12 protein is involved in the thrombin-stimulated DNA synthesis, an inhibitory antibody against the C-terminal sequence of G alpha 12 subunit was microinjected into 1321N1 cells. Microinjection of the anti-G alpha 12 resulted in a concentration-dependent inhibition of thrombin-stimulated DNA synthesis. In contrast, microinjection of nonimmune IgG or an antibody directed against the C terminus of G alpha o did not reduce the mitogenic response to thrombin. Furthermore, microinjection of the anti-G alpha 12 antibody had no effect on fibroblast growth factor-stimulated DNA synthesis. These results demonstrate a specific role for G alpha 12 in the mitogenic response to thrombin in human astroglial cells.

Cyclic GMP-dependent protein kinase blocks pertussis toxin-sensitive hormone receptor signaling pathways in Chinese hamster ovary cells

cGMP-dependent protein kinase (cGMP kinase) has been implicated in the regulation of the cytosolic calcium level ([Ca2+]i). In Chinese hamster ovary (CHO) cells stably transfected with the cGMP kinase I alpha (CHO-cGK cells), cGMP kinase suppressed the thrombin-induced increase in inositol 1,4,5-trisphosphate and [Ca2+]i (Ruth, P., Wang, G.-X., Boekhoff, I., May, B., Pfeifer, A., Penner, R., Korth, M., Breer, H., and Hofmann, F. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 2623-2627). Cholecystokinin activated intracellular calcium release via a pertussis toxin (PTX)-insensitive pathway in CHO-cGK cells. cGMP kinase did not attenuate the CCK-stimulated [Ca2+]i. In contrast, cGMP kinase suppressed calcium influx stimulated by insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) via PTX-sensitive pathways. The effects of PTX and cGMP kinase on [Ca2+]i were not additive. 8-Bromo-cGMP had no effect on [Ca2+]i stimulated by IGF-1 or IGF-2 in wild type CHO cells. These results suggested that cGMP kinase inhibited the different signaling pathways by the phosphorylation of a PTX-sensitive G protein. cGMP kinase phosphorylated the alpha subunits of Gi1, Gi2, and Gi3 in vitro. Phosphorylation stoichiometry was 0.4 mol of phosphate/mol of G alpha i1 after reconstitution of heterotrimeric Gi1 in phospholipid vesicles. The alpha subunit of Gi was also phosphorylated in vivo. These results show that cGMP kinase blocks transduction of distinct hormone pathways that signal via PTX-sensitive Gi proteins.

The thrombin receptor in human platelets is coupled to a GTP binding protein of the G alpha q family

The thrombin receptor is a G protein-coupled receptor, but the G proteins functionally coupled to this receptor in human platelets are not yet definitively identified. Thrombin stimulation of platelets leads to phospholipase C-mediated increases in intracellular calcium, and previous studies have suggested that the thrombin receptor is coupled to members of the Gq family. We now demonstrate direct GTPase activation by thrombin receptor activation peptide (TRAP) in human platelet membranes, and specific inhibition of TRAP-activated GTPase by antibodies to Gq. These data demonstrate functional coupling of the thrombin receptor to a member of the Gq family.

Characterization of thrombin receptor expression during vascular lesion formation

Blood vessels respond to injury by initiating cell proliferation and migration that result in vascular lesion formation. To determine the roles of thrombin and the thrombin receptor in this process, we characterized thrombin receptor expression in normal and injured arteries, thrombin receptor-mediated smooth muscle cell mitogenesis, and the regulation of thrombin receptor mRNA expression in vitro. Thrombin receptor mRNA was not detected in normal rat or baboon arteries by in situ hybridization. Immunohistochemistry using an antithrombin receptor antibody (TR-R9), directed against the thrombin cleavage site of the rat aortic smooth muscle cell thrombin receptor, revealed low-level staining for thrombin receptor protein in endothelial cells and smooth muscle cells of normal arteries. In contrast, balloon catheter injury increased thrombin mRNA expression in medial smooth muscle cells within 6 hours. This increased thrombin receptor expression continued within the media and in neointimal cells throughout vascular lesion formation, predominantly in areas of active cell proliferation. In vitro, alpha-thrombin stimulates rat aortic smooth muscle cell proliferation in a concentration-dependent manner. That thrombin receptor activation is required for the mitogenic response was confirmed by demonstrating that the polyclonal antibody TR-R9 inhibits thrombin-induced cell proliferation. Thrombin receptor mRNA synthesis was induced by both basic fibroblast growth factor (maximal stimulation of 1.8-fold at 1 hour) and platelet-derived growth factor (maximal stimulation of 2.4-fold at 8 and 24 hours) in quiesced cultured rat aortic smooth muscle cells. In summary, upregulation of smooth muscle cell thrombin receptor expression occurs very early after vascular injury and continues throughout neointimal development.(ABSTRACT TRUNCATED AT 250 WORDS)