Angiotensin II stimulates protein-tyrosine phosphorylation in a calcium-dependent manner

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.

A role for protein tyrosine kinase in the steroidogenic pathway of angiotensin II in bovine zona glomerulosa cells

Stimulation of aldosterone synthesis in bovine adrenal zona glomerulosa (ZGB) cells by angiotensin II (AngII) is believed to be mediated by the phospholipase C (PLC) pathway that results in the increase of cytosolic free calcium concentration and in the activation of protein kinase C (PKC). However, the cell proliferation and contraction associated with AngII action are known to be mediated in part by protein tyrosine kinases (PTK). To assess the potential role of PTK in the stimulatory effect of AngII on adrenal steroidogenesis, the actions of a series of PTK inhibitors on this metabolic pathway were examined in isolated ZGB cells. Tyrphostin 23 (TP23) caused a dose-dependent inhibition of AngII-stimulated aldosterone production with an IC50 of 15 microM and reached complete inhibition at 100 microM. Genistein (GS) was more potent with an IC50 of 35 nM and complete inhibition at 10 microM. The stimulation of aldosterone production by the calcium-mobilizing agent thapsigargin (Thaps) was also dose-dependently inhibited by TP and GS with the same potency. A specific PKC inhibitor, calphostin C (0.1 microM) caused only a 51.7% inhibition of AngII-stimulated aldosterone production. In the same way, a specific Ca2+/calmodulin-dependent protein kinase inhibitor, KN-62 (1 microM), reduced aldosterone production stimulated by AngII by 64%. As expected, thapsigargin-stimulated aldosterone biosynthesis was not affected by calphostin C, but was completely inhibited by KN-62. These results demonstrate for the first time that protein tyrosine kinase activity is part of the angiotensin II signalling pathway in bovine zona glomerulosa cells. The activation of this PTK occurs subsequently to the mobilization of intracellular calcium. This calcium-dependent protein tyrosine kinase pathway is essential for the steroidogenic response to AngII in bovine zona glomerulosa cells.

Calcium-inducible transmodulation of receptor tyrosine kinase activity

Calcium is a potent mitogen and transmodulator of growth factor receptor activity, but does not activate tyrosine kinases in ligand-deprived cells (Epstein et al. 1992) Cell Growth Different. 3, 157-164). In this study the mitogenic and transcriptional effects of increased extracellular calcium and ionophore are shown to be identical in 3T3 cells, consistent with mediation of these effects via increased intracellular calcium availability. Near-maximal mitogenic and transcriptional effects are seen after brief exposure to increased extracellular calcium or ionophore, while additive effects occur with co-administration of calcium and platelet-derived growth factor (PDGF). Exposure of PDGF-primed cells to calcium or ionophore is associated with a substantial enhancement of receptor tyrosine autophosphorylation which is abrogated by calcium channel blockade or intracellular calcium chelation. In contrast, pretreatment of quiescent cells with calcium or ionophore significantly diminishes subsequent PDGF-inducible receptor autophosphorylation. Intracellular calcium thus appears to potentiate the kinase activity of ligand-stimulated PDGF receptors while inhibiting ligand-inducible activation of unstimulated receptors. These findings suggest a model of receptor tyrosine kinase regulation involving calcium-dependent positive and negative feedback loops which vary with the activation state of the receptor.

Role of p70 S6 protein kinase in angiotensin II-induced protein synthesis in vascular smooth muscle cells

Angiotensin II (AII) is a growth factor which induces cellular hypertrophy in cultured vascular smooth muscle cells (SMC). To understand the molecular basis of this action, we have examined the role of the 70-kDa S6 kinases (p70S6K) in the hypertrophic response to AII in aortic SMC. AII potently stimulated the phosphotransferase activity of p70S6K, which reached a maximal value at 15 min and persisted for at least 4 h. This response was completely abolished when the cells were incubated in the presence of the AT1-selective receptor antagonist losartan. The enzymatic activation of p70S6K was associated with increased phosphorylation of the enzyme on serine and threonine residues. The immunosuppressant drug rapamycin was found to selectively inhibit the activation of p70S6K by AII, but not the activation of mitogen-activated protein kinase or the induction of c-fos mRNA expression. Treatment of aortic SMC with rapamycin also potently inhibited AII-stimulated protein synthesis with a half-maximal concentration similar to that required for inhibition of p70S6K. These results provide strong evidence that p70S6K plays a critical role in the signaling pathways by which AII induces hypertrophy of vascular SMC.

Angiotensin II stimulates tyrosine phosphorylation of the focal adhesion-associated protein paxillin in aortic smooth muscle cells

Treatment of vascular smooth muscle cells (SMC) with angiotensin II (AII) leads to an increase in the tyrosine phosphorylation of multiple cellular substrates. Here, we have demonstrated that AII stimulates tyrosine phosphorylation of the focal adhesion-associated protein paxillin in rat aortic SMC. AII-induced phosphorylation of paxillin was detectable within 1 min and was sustained up to 60 min. Preincubation with the AT1-selective antagonist losartan abolished this response. The stimulatory effect of AII on paxillin tyrosine phosphorylation was observed only in aortic SMC and not in other target cells such as adrenal zona glomerulosa cells, chromaffin cells, or hepatocytes. The effect of AII was dependent on the activation of phospholipase C. Chelation of intracellular calcium completely inhibited the ability of AII to stimulate paxillin tyrosine phosphorylation, while selective inhibition of protein kinase C partially attenuated the response. In contrast, treatment of the cells with pertussis toxin had no effect on AII-induced paxillin tyrosine phosphorylation. These findings identify paxillin as a new substrate for AII-stimulated tyrosine phosphorylation and suggest a role for cytoskeleton-associated proteins in the growth response of aortic SMC.

Angiotensin II stimulation of rapid paxillin tyrosine phosphorylation correlates with the formation of focal adhesions in rat aortic smooth muscle cells

Angiotensin II is a potent vasoconstrictor that has been also implicated in vascular hyperproliferative diseases, including atherosclerosis and restenosis following angioplasty. Treatment of cultured, serum-starved rat aortic smooth muscle cells with angiotensin II causes rapid protein tyrosine phosphorylation that precedes cell mitogenesis. We have identified two of the phosphoproteins as paxillin (75 kilodaltons) and the tyrosine kinase pp125Fak, both components of actin-associated focal adhesion sites. Angiotensin II stimulated a 5-fold increase in the tyrosine phosphorylation of paxillin and a smaller (1.5-fold) increase in pp125Fak tyrosine phosphorylation. Paxillin tyrosine phosphorylation was evident within 1 minute, and was maximal after 10 minutes. Similar elevated protein tyrosine phosphorylation levels of paxillin were obtained with exposure of the rat aortic smooth muscle cells to peptides endothelin-1 and alpha-thrombin that function, as angiotensin II, through binding to members of the seven transmembrane domain G protein coupled receptors. Angiotensin II treatment also stimulated the production of a well-ordered actin-containing stress fiber network and prominent paxillin-containing focal adhesions. The focal adhesions stained intensely with anti-phosphotyrosine antibody suggesting the tyrosine phosphorylation of paxillin and cytoskeletal reorganization were tightly coupled. Angiotensin II receptor occupancy has been shown previously to lead to protein kinase C activation. However, compared to angiotensin II stimulation, a smaller, delayed increase in paxillin tyrosine phosphorylation was observed following direct protein kinase C activation by the phorbol ester phorbol 12-myristate-13-acetate. Paxillin tyrosine phosphorylation was selective for certain agonists since no increase in tyrosine phosphorylation of this protein was observed following exposure to the potent mitogen PDGF. Thus, actin-based cytoskeletal changes involving sites of cell adhesion to the extracellular matrix may play an important role in normal and pathophysiologic smooth muscle cell growth regulation in response to certain angiotensin II-type vasoactive agonists.

Angiotensin II and other hypertrophic stimuli mediated by G protein-coupled receptors activate tyrosine kinase, mitogen-activated protein kinase, and 90-kD S6 kinase in cardiac myocytes. The critical role of Ca(2+)-dependent signaling

Many hypertrophic stimuli such as angiotensin II (Ang II) activate phospholipases through G protein-coupled receptors in cardiac myocytes. However, it is not known whether these stimuli also activate the tyrosine phosphorylation-dependent signaling pathway, which plays an essential role in growth factor-induced mitogenic responses in other cell types. Serine/threonine kinases such as mitogen-activated protein (MAP) kinases and 90-kD S6 kinase (RSK) are activated in response to many growth stimuli and are important downstream signaling pathways of tyrosine kinases. Therefore, we examined whether Ang II activates these protein kinases in primary cultures of cardiac myocytes and fibroblasts from neonatal rats. Ang II rapidly induced tyrosine phosphorylation of multiple proteins, including 42-, 44-, 75- to 80-, and 120- to 130-kD proteins, in both cardiac myocytes and fibroblasts. This was accompanied by an increase in tyrosine kinase activity. The 42- and 44-kD proteins were immunologically related to an extracellular signal-regulated kinase family (MAP kinases). Ang II rapidly increased kinase activity of MAP kinases and their downstream kinase, RSK. The Ang II-induced tyrosine phosphorylation and activation of MAP kinases and RSK were AT1 receptor-mediated. Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate or an increase in intracellular Ca2+ by the Ca2+ ionophore A23187 was sufficient to cause tyrosine phosphorylation of multiple proteins and activation of MAP kinase and RSK. Although downregulation of PKC did not suppress Ang II-induced activation of MAP kinase and RSK, chelating intracellular Ca2+ by BAPTA-AM completely abolished Ang II-induced activation of these kinases. Activation of MAP kinases and RSK was also observed in myocytes stimulated with other agonists for Gq protein-coupled receptors, such as phenylephrine, norepinephrine, and endothelin 1, but not with agonists to Gs protein-coupled receptors, such as isoproterenol. These results suggest that Ang II and other hypertrophic stimuli, known to act through Gq protein-coupled receptors, rapidly cause tyrosine phosphorylation of several intracellular substrates through activation of tyrosine kinase and activate MAP kinases and RSK in cardiac myocytes as well as in cardiac fibroblasts. Furthermore, intracellular Ca2+, rather than PKC, seems to be critical for Ang II-induced activation of these protein kinases in cardiac myocytes.

Protein kinases and phosphatases modulate c-fos expression in rat hepatocytes. Effects of angiotensin II and phorbol myristate acetate

In isolated rat hepatocytes angiotensin II and phorbol 12-myristate 13-acetate (PMA) induce the expression of c-fos. We studied the possible transduction pathway(s) involved in this effect using inhibitors of serine-threonine and tyrosine protein kinases. Calphostin and staurosporine, inhibitors of protein kinase C and other serine-threonine protein kinases, block in a dose-dependent manner the effect of angiotensin II and PMA. Interestingly, genistein also blocks the induction of this proto-oncogene, suggesting a role for tyrosine protein kinases. Inhibitors of serine-threonine protein phosphatases, such as okadaic acid, microcystin LR and calyculin also induce c-fos expression. These data suggest that protein phosphatases exert a tonic inhibitory control of c-fos expression. The effect of these phosphatase inhibitors were not blocked by staurosporine, calphostin or genistein. Our results suggest that the expression of c-fos in rat hepatocytes is regulated by complex phosphorylation-dephosphorylation cascade(s) probably involving serine/threonine and tyrosine protein kinase and protein phosphatase activities.

Ca(2+)-mobilizing hormones potentiate hypotonicity-induced activation of ionic conductances in Intestine 407 cells

Human Intestine 407 cells respond to hyposmotic stimulation by activating the conductive efflux of both Cl- and K+ (regulatory volume decrease) through pathways involving protein tyrosine phosphorylation (Tilly, B. C., N. van den Berghe, L. G. J. Tertoolen, M. J. Edixhoven, and H. R. de Jonge. J. Biol. Chem. 268: 19919-19922, 1993). Stimulation of the cells with hormones linked to the phospholipase C signaling cascade (e.g., bradykinin, histamine, or thrombin) or with the phosphotyrosine phosphatase inhibitor vanadate, potentiated the osmosensitive anion efflux by two- to threefold but did not affect anion efflux under isotonic conditions. No substantial increase in intracellular Ca2+ concentration ([Ca2+]i) was observed on mild hypotonicity-induced cell swelling. In addition, loading the cells with the intracellular Ca2+ chelator 1,2-bis(2-amino-phenoxy)ethane- N,N,N',N',-tetraacetic acid acetoxymethyl ester (BAPTA-AM) caused a partial reduction of the osmoshock-induced 125I- efflux but did not affect its potentiation by vanadate. In contrast, bradykinin transiently elevated [Ca2+]i, and its potentiation of the osmosensitive anion efflux was completely inhibited after BAPTA-AM loading. Both the Ca(2+)-mobilizing hormones as well as osmotic cell swelling rapidly triggered the phosphorylation of several proteins on tyrosine residues. However, the effects of the hormones, but not the effect of hypotonicity, on protein tyrosine phosphorylation was largely abolished in BAPTA-loaded cells. Taken together the results indicate a novel role for Ca(2+)-mobilizing hormones, although elevation of [Ca2+]i, in potentiating volume-sensitive ionic efflux even in cell types lacking the expression of Ca(2+)-activated Cl- channels in their plasma membrane.

Involvement of protein kianse C and Ca2+ in angiotensin II-induced mitogenesis of cardiac fibroblasts

Angiotensin (ANG) II has been previously shown to stimulate proliferation of neonatal rat cardiac fibroblasts via AT1 receptors. Here we conducted studies to assess involvement in this process of two second messengers linked to AT1 receptors, protein kinase C (PKC) and Ca2+. Several findings argue against a dominant role for PKC in ANG II-induced mitogenesis: 1) [Sar1]ANG II, which produced a modest, transient increase in PKC activity, was equally effective in inducing thymidine incorporation into DNA in PKC-depleted cells, whereas the effect of platelet-derived growth factor (PDGF)-BB on thymidine incorporation was reduced to the level observed with [Sar1]ANG II; 2) phorbol 12-myristate 13-acetate (PMA), a potent PKC stimulator, was ineffective in stimulating thymidine incorporation; and 3) PKC downregulation or the highly specific PKC inhibitor, compound 3, eliminated PMA-induced mitogen-activated protein (MAP) kinase activity but did not affect comparable increases induced by [Sar1]ANG II or PDGF-BB. Increased intracellular Ca2+ may be sufficient to account for [Sar1]ANG II-induced MAP kinase activity because ionomycin also increased MAP kinase activity and chelation of intracellular Ca2+ eliminated [Sar1]ANG II-induced activity in PKC-depleted fibroblasts. However, Ca2+ chelation did not prevent [Sar1]ANG II-induced MAP kinase activity in non-PKC-depleted fibroblasts. Thus ANG II can activate MAP kinase in cardiac fibroblasts by either Ca(2+)- or PKC-dependent pathways, and whereas the full effect of PDGF-BB on thymidine incorporation and cell proliferation requires a phorbol ester-sensitive PKC, the hyperplastic growth effect of ANG II does not.

Stimulation of rat vascular smooth muscle cell glycosaminoglycan production by angiotensin II

Matrix production by smooth muscle cells (SMC) appears to play a major role in the intimal thickening process. Proteoglycans (PG) are the predominant extracellular matrix component of early restenotic lesions. As angiotensin II (A II) has been proposed as a mediator of restenotic process, we hypothesized that A II may directly affect PG synthesis by SMC. SMC were cultured in the presence of [35S]sulfate and angiotensin II, and both the secreted and membrane-bound proteoglycans were analyzed. A II (1 to 100 nM) evoked a dose- and time-dependent increase in both cell- and media-associated PG production, an effect abrogated by the A II receptor antagonist, saralasin. SMC constitutively synthesize small amounts of PG with a molecular mass of 170-250 kDa. After treatment with A II, the abundance of PG is increased, as well as its molecular mass (230-300 kDa). Selective degradation by chondroitinases and heparinase identified chondroitin and dermatan sulfate PG as the predominant form being induced. These results demonstrate that the effect of A II is not general and is specific to certain classes of PGs. In order to further examine the specificity of the A II effect, we compared the synthesis of PG induced by A II with that induced by platelet-derived growth factors AA and BB (PDGF-AA and -BB), insulin-like growth factor I (IGF-I), and tumor necrosis factor alpha (TNF alpha). This comparison demonstrated that the profile of PG induced by A II is different from the other factors examined. Taken together, these data indicate that A II may not only function as a hypertrophic factor for SMC, but in addition may also be a potent modulator of specific PG synthesis by these same cells, which could significantly contribute to the formation of atherosclerotic and restenotic lesions.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine kinase pathways and the regulation of smooth muscle contractility

Epidermal growth factor (EGF) mediates three tyrosine kinase-dependent smooth muscle response paradigms, two of which comprise a rapid increase in muscle tension and one of which is characterized by an agonist-mediated reduction in sensitivity to other agents. The three types of response are mediated via distinct signal transduction pathways, and marked tissue and species variation have been observed, even for a single growth factor agonist. Vasoactive agents, such as angiotensin II and vasopressin, that act via G protein-coupled receptors can also work via tyrosine kinase pathways to cause contraction in some of the same intact smooth muscle preparations that contract in response to EGF. In this review, Morley Hollenberg discusses the tyrosine kinase-modulated signal transduction pathways for EGF and also agonists that act via G protein-coupled receptors, and hypothesizes that there may be an intermediary non-receptor tyrosine kinase that may serve as a point of convergence for the contractile actions of these agents in smooth muscle.

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

Calcium as a permissive factor but not an initiation factor in DNA synthesis induction in cultured rat hepatocytes by the peroxisome proliferator ciprofibrate

The non-genotoxic hepatocarcinogen and peroxisome proliferating agent, ciprofibrate, is a liver mitogen both in vivo and in cultured adult rat hepatocytes, but the mechanisms of its mitogenicity have not been elucidated. We previously observed that ciprofibrate rapidly increased hepatocyte free intracellular Ca2+ concentration ([Ca2+]i), suggesting that this effect may play a role in the initiation of DNA synthesis. In the present study, we have identified a relationship between Ca2+ and the stimulation of hepatocyte DNA synthesis by ciprofibrate. Exposure of cultured adult rat hepatocytes to ciprofibrate (200 microM) for 48 hr increased DNA synthesis by approximately 2-fold, and this response was attenuated in a Ca(2+)-deficient medium and by the Ca2+ channel blockers nicardipine and verapamil. To examine the relationship between the stimulation of hepatocyte DNA synthesis and increases in [Ca2+]i by ciprofibrate, the intracellular Ca2+ chelator 5,5'-dimethyl-1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid (dimethyl-BAPTA) was employed. Pretreatment of hepatocytes with dimethyl-BAPTA blocked ciprofibrate-induced [Ca2+]i increase, but did not block ciprofibrate-induced hepatocyte DNA synthesis. Dimethyl-BAPTA was only effective in reducing ciprofibrate-induced DNA synthesis when present during the latter 24 hr of a 48-hr culture period. These data suggest that the early mobilization of hepatocyte [Ca2+]i by ciprofibrate does not play an initiating role in the induction of hepatocyte DNA synthesis but rather may operate as a permissive factor for the entry of ciprofibrate-treated adult rat hepatocytes into S-phase.

Angiotensin II is mitogenic in neonatal rat cardiac fibroblasts

Angiotensin II has been reported to be a hormonal stimulus of cardiac growth, a response that may involve myocyte hypertrophy as well as growth of nonmyocytes. This study was designed to determine whether neonatal rat cardiac fibroblasts have an angiotensin II receptor that is coupled with hypertrophic and/or proliferative growth. Competitive radioligand binding studies showed that cardiac fibroblasts have a single class of high-affinity (IC50, 1.0 nM) angiotensin II binding sites (Bmax, 778 fmol/mg protein) that are sensitive to the competitive nonpeptide AT1 receptor antagonist losartan (IC50, 13 nM). Other angiotensin peptides competed for [125I]angiotensin II binding in the following rank order: angiotensin II > angiotensin III > angiotensin I > > [des-Asp1-des-Arg2]angiotensin II. A nonhydrolyzable analogue of guanosine triphosphate increased the dissociation rate of bound [125I]angiotensin II and decreased hormone binding to the receptor at equilibrium. The angiotensin II receptor was coupled with increases in intracellular calcium. Incorporation of precursors into protein, DNA, and RNA in response to angiotensin II was determined. In serum-deprived cultures, a 24-hour exposure to 1 microM [Sar1]angiotensin II increased rates of phenylalanine, thymidine, and uridine incorporation by 58%, 103%, and 118%, respectively. These increases were blocked by the noncompetitive AT1 receptor antagonist EXP3174. After 48 hours, [Sar1]angiotensin II increased total protein and DNA of cardiac fibroblasts by 23% and 15%, respectively, with no change in the protein/DNA ratio. [Sar1]Angiotensin II increased cell number by 138% after a 24-hour exposure, without affecting cell area. In summary, cardiac fibroblasts have G protein-linked AT1 receptors that are coupled with proliferative growth. These results suggest that angiotensin II-induced cardiac hypertrophy is, in part, secondary to stimulated increases in nonmyocyte cellular growth.

Involvement of G proteins, cytoplasmic calcium, phospholipases, phospholipid-derived second messengers, and protein kinases in signal transduction from mitogenic cell surface receptors

Some putative mitogenic signal transduction mechanisms involving G proteins, calcium, phospholipases, and protein kinases have been discussed. Several elements in this signal transduction scheme are not yet well understood and require further experimental investigation. With regard to the heptahelix receptors, exactly how do they activate PLA2? Is PLA2 activation linked to mitogenic pathways? Is this via stimulation of protein kinase C or perhaps another mechanism? How do heptahelix receptors activate tyrosine phosphorylation, and is it important in their ability to stimulate cell growth? With regard to the various phospholipases that are thought to be regulated by receptor-mediated stimuli, only PI-PLC beta and PI-PLC gamma are well characterized. PLA2, PC-PLD, and PC-PLC require further study in regard to determination of molecular structure and elucidation of mechanisms of phospholipase activation (e.g., what are the molecular mechanisms whereby tyrosine kinases and Ras affect PC-PLC?). The protein kinase C dependent and protein kinase C independent mechanisms that enable mitogenic stimuli to activate the Erk/MAP kinase are enigmatic at this time. How Raf-1 activates SRE-containing gene promoters (such as the fos promoter) is also not known. However, given the current rapid rate of progress in this field, it is likely that a much more complete understanding of the mitogenic signal transduction process will soon be obtained.

Vasopressin elevation of Na+/H+ exchange is inhibited by genistein in human blood platelets

The regulation of intracellular Na+ and pHi in human blood platelets is known to be controlled by the function of the Na+/H+ exchanger. The phosphorylation state of the Na+/H+ exchanger which determines the exchanger activity in human blood platelets is regulated by the activities of protein kinases and protein phosphatases. Observations in this study indicate that arginine vasopressin (AVP) that interacts with a V1 receptor, activates the Na+/H+ exchange in human blood platelets through a genistein-inhibited mechanism. The AVP-activated Na+/H+ exchange is probably not regulated by protein kinase C (PKC), since this activation is not inhibited by staurosporine. The multiple ways in which platelet Na+/H+ exchange can be modulated may indicate the critical role played by this exchanger in the homeostasis control of pHi in human blood platelets.

Activation of transforming G protein-coupled receptors induces rapid tyrosine phosphorylation of cellular proteins, including p125FAK and the p130 v-src substrate

We have used the family of human muscarinic receptors (mAChRs) as a model for receptors coupled to G proteins and have shown that genes for certain mAChR subtypes can behave as potent agonist-dependent oncogenes. Furthermore, transforming mAChRs can transduce mitogenic signals in transfected NIH 3T3 cells. In this study, we show that in cells expressing ml mAChRs, the cholinergic agonist carbachol, induces a rapid and dose-dependent increase in tyrosine phosphorylation of cellular proteins which are different from those induced by PDGF. Interestingly, carbachol, but not PDGF, induces an increase in tyrosine phosphorylation of the p125FAK and p130 v-src substrates. Thus, growth promoting pathways activated by receptors coupled to G proteins might involve tyrosine phosphorylation of a small set of cellular proteins previously identified as substrates for oncogene-encoded tyrosine kinases.

Vasoconstrictor-induced protein-tyrosine phosphorylation in cultured vascular smooth muscle cells

In cultured rat aortic smooth muscle cells, angiotensin II induced tyrosine phosphorylation of at least 9 proteins with molecular masses of 190, 117, 105, 82, 79, 77, 73, 45 and 40 kDa in time- and dose-dependent manners. Other vasoconstrictors such as [Arg]vasopressin, 5-hydroxytryptamine and norepinephrine induced the tyrosine phosphorylation of the same set of proteins as angiotensin II. The tyrosine phosphorylation of these proteins was mimicked by the protein kinase C-activating phorbol ester, phorbol 12 myristate 13-acetate, and the Ca2+ ionophore, ionomycin. These results demonstrate that the vasoconstrictors stimulate the tyrosine phosphorylation of several proteins in vascular smooth muscle cells and suggest that the tyrosine phosphorylation reactions are the events distal to the activation of protein kinase C and Ca2+ mobilization in the intracellular signalling pathways of the vasoconstrictors.