Serum, bradykinin and vasopressin stimulate release of inositol phosphates from human fibroblasts

Kinin receptors in cultured rat microglia

Kinins are produced and act at the site of injury and inflammation in various tissues. They are likely to initiate a particular cascade of inflammatory events, which evokes physiological and pathophysiological responses including an increase in blood flow and plasma leakage. In the central nervous system (CNS), kinins are potent stimulators of the production and release of pro-inflammatory mediators represented by prostanoids and cytotoxins. They are known to induce neural tissue damage. Many of the cytotoxins such as cytokines and free radicals and prostanoids are released from glial cells. Among glial cells, astrocytes and oligodendrocytes are known to possess bradykinin (BK) B(2) receptors that phosphoinositide (PI) turnover and raise intracellular Ca(2+) concentration. The presence of bradykinin receptors in microglia has been of great significance. We recently showed that rat primary microglia express kinin receptors. In resting microglia, B(2) receptors but not B(1) receptors are expressed. When the microglia are activated by bradykinin, B(1) receptors are up-regulated, while B(2) receptors are down-regulated. As observed in other glial cells, electrophysiological measurements suggest that B(2) receptors in phosphoinositide turnover and intracellular Ca(2+) concentration in microglia. Release of cytotoxins is likely consequent upon the activation of BK receptors. Our study provides the first evidence that microglia express functional kinin receptors and suggests that microglia play an important role in CNS inflammatory responses.

Mechanisms regulating the expression, self-maintenance, and signaling-function of the bradykinin B2 and B1 receptors

Bradykinin (BK) is a potent short-lived effector belonging to a class of peptides known as kinins. It participates in inflammatory and vascular regulation and processes including angioedema, tissue permeability, vascular dilation, and smooth muscle contraction. BK exerts its biological effects through the activation of the bradykinin B2 receptor (BKB2R) which is G-protein-coupled and is generally constitutively expressed. Upon binding, the receptor is activated and transduces signal cascades which have become paradigms for the actions of the Galphai and Galphaq G-protein subunits. Following activation the receptor is then desensitized, endocytosed, and resensitized. The bradykinin B1 (BKB1R) is a closely related receptor. It is activated by desArg(10)-kallidin or desArg(9)-BK, metabolites of kallidin and BK, respectively. This receptor is induced following tissue injury or after treatment with bacterial endotoxins such as lipopolysacharide or cytokines such as interleukin-1 or tumor necrosis factor-alpha. In this review we will summarize the BKB2R and BKB1R mediated signal transduction pathways. We will then emphasize the relevance of key residues and domains of the intracellular regions of the BKB2R as they relate to modulating its function (signal transduction) and self-maintenance (desensitization, endocytosis, and resensitization). We will examine the features of the BKB1R gene promoter and its mRNA as these operate in the expression and self-maintenance of this inducible receptor. This communication will not cover areas discussed in earlier reviews pertaining to the actions of peptide analogs. For these we refer you to earlier reviews (Regoli and Barabé, 1980, Pharmacol Rev 32:1-46; Regoli et al., 1990, J Cardiovasc Pharmacol 15(Suppl 6):S30-S38; Regoli et al., 1993, Can J Physiol Pharmacol 71:556-557; Marceau, 1995, Immunopharmacology 30:1-26; Regoli et al., 1998, Eur J Pharmacol 348:1-10).

Vascular smooth muscle cells preloaded with eicosapentaenoic acid and docosahexaenoic acid fail to respond to serotonin stimulation

Epidemiological, animal and clinical studies indicate that n-3 fatty acids may benefit individuals with known history of cardiovascular disease or at risk of developing it. Though there is indirect evidence to suggest that the beneficial effects of n-3 fatty acids may be because of their ability to inhibit smooth muscle cell (SMC) proliferation, there are no studies that have examined this hypothesis. In this study, the mitogenic effect of serotonin (5HT) and platelet derived growth factor (PDGF), known mitogens for vascular SMC, on aortic SMCs preloaded with eicosapentaenoic acid (EPA) and/or docosahexaenoic acid (DHA) is examined. 5HT and PDGF could only partially stimulate proliferation of SMC that were preloaded with EPA or DHA as compared to the control cells. gamma-Linolenic acid (LA) and oleic acid (OA) did not block the 5HT or PDGF induced 3[H]thymidine incorporation suggesting that the anti-proliferative effect was specific to n-3 fatty acids only. Further, when EPA and DHA were combined in the ratio they are present in fishoils, there was a synergistic interaction in inhibiting the proliferation of SMC. Further, SMC grown in the presence of EPA or DHA, when stimulated with 5HT, failed to show an increase in 5HT(2) receptor mRNA. One of the potential mechanism by which fish oils may prevent the development of atherosclerosis or restenosis could be inhibition of the mitogen induced SMC proliferation. Combination of EPA with DHA is likely to be more beneficial.

Hormone-defined cell system for studying G-protein coupled receptor agonist-activated growth modulation: delta-opioid and serotonin-5HT2C receptor activation show opposite mitogenic effects

G-protein-coupled receptor (GPCR) agonist-activated transformation of NIH/3T3 fibroblast cells has been documented by many workers. Our present interest is in the growth control exerted by these agonists. The mechanisms involved in GPCR agonist-activated growth regulation are not known and investigations using existing cell lines are complicated by the endogenous expression of numerous different GPCRs as well as by the fact that these cell lines are cultured in serum that contains naturally occurring agonists for these receptors. To study the agonist induced growth response of cells transfected with either delta-opioid or serotonin-5HT2C neurotransmitter receptor genes, we have developed new clonal cell lines derived from NIH/3T3 mouse fibroblast cells. These new cell lines, designated with the suffix 3T3DA, can be cultured stably in serum-free, hormone-defined medium: insulin is the only exogenous growth factor added to the culture medium of proliferating 3T3DA cell lines, and their proliferation can be stopped and started by the respective removal or addition of insulin. Micromolar concentrations of agonists were used to activate the corresponding opioid and serotonin receptors over periods extending to 6 days. We observed distinct patterns of GPCR-specific, agonist-activated growth regulation in serum-free cultures, but not in serum-supplemented cultures. At concentrations > 10 microM, morphine inhibits growth of delta-opioid receptor-expressing cells by 40% with respect to normal 3T3DA cells. Opioid agonist induced inhibition of cyclic AMP (cAMP) production as well as growth down-regulation are pertussis toxin sensitive indicating that the exogenously expressed delta-opioid receptors demonstrate classical opioid receptor signaling. The presence of 1 microM serotonin stimulates growth of serotonin-5HT2C receptor- expressing cells by approximately 100% with respect to normal 3T3DA cells. Neither the untreated nor the agonist-treated cells form colonies in soft agar, indicating that they retain anchorage-dependent growth control. These cell lines provide a simple system that could be used as a tool for probing the complex molecular mechanisms associated with GPCR agonist-activated growth control.

Bradykinin stimulates fructose 2,6-bisphosphate metabolism in human fibroblasts

Bradykinin (BK), a peptide released during inflammatory response, has been investigated for its ability to regulate glucose metabolism in human fibroblasts. The peptide is able to significantly increase glycolytic flux in these cells. The strict relationship between the glycolytic rate and the levels of fructose 2,6-bisphosphate (Fru-2,6-P2) strongly suggests that the metabolite plays a key role in the regulation of glucose metabolism by bradykinin. The mechanism by which bradykinin increases Fru-2,6-P2 content involves the activation of 6-phosphofructo-2-kinase (PFK-2), the enzyme responsible for the synthesis of the metabolite. The study of the multiple signalling systems triggered by bradykinin demonstrates the involvement of the rise in intracellular Ca2+ concentration and of protein kinase C mediated pathway in the mechanism by which bradykinin increases Fru-2,6-P2 content and PFK-2 activity.

Decreased bradykinin binding sites in fibroblasts from progressive systemic scleroderma

The numbers of bradykinin receptors (BK-R) in cultured dermal fibroblasts from patients with progressive systemic sclerosis (PSS) and from healthy controls were measured using a receptor binding assay. The numbers of BK-R were significantly fewer in PSS fibroblasts than in control fibroblasts (P < 0.02). However, no differences in affinity were observed in BK-R between PSS and control fibroblasts. The BK-R mRNA levels were determined in PSS and control fibroblasts by Northern blot hybridization using BK-R cDNA, but no significant differences were found. These findings suggest that the decrease in BK-R in PSS fibroblasts might occur during a posttranslational step.

Effects of insulin-like growth factor II on the generation of inositol trisphosphate, diacylglycerol and cAMP in human fibroblasts

In human fibroblasts, exogenous insulin-like growth factor-II (IGF-II) induce a rapid redistribution of mannose 6-phosphate/IGF-II receptors. To analyze the mechanism transducing the IGF-II signal the phosphoinositide hydrolysis, 1,2-diacyglycerol and cAMP formation were studied after incubation with IGFs. While IGF-I (10 nM, 30 s) increased the inositol trisphosphate formation IGF-II (10 nM, up to 10 min) failed to affect phosphoinositide hydrolysis and had neither an effect on basal concentrations of diacylglycerol containing arachidonic acid or myristic acid nor on intracellular cAMP. On the contrary, pretreatment with IGF-II for 10 min enhanced the cAMP production stimulated by bradykinin (10 nM, 3 min) by 2.5-fold whereas no additive effects of IGF-II on the increased ligand binding to the mannose 6-phosphate/IGF-II receptor in response to bradykinin were observed. These results indicate that in fibroblasts the rapid IGF-II-induced redistribution of mannose 6-phosphate/IGF-II receptors is not mediated by inositol trisphosphate, diacylglycerol or cAMP, but that IGF-II may modulate permissively other agonist-generated signals.

Regulation of angiotensin I-converting enzyme in cultured bovine bronchial epithelial cells

The purpose of this study was to determine whether angiotensin I-converting enzyme (ACE) is present in cultured bovine bronchial epithelial cells (BBECs) and whether its activity can be modulated. We found that extracts of confluent monolayers of cultured BBECs degraded [glycine-1-14C]hippuryl-L-histidyl-L-leucine at a rate of 843 +/- 66 pmol/hr/mg protein (mean +/- SEM, n = 5). In addition, we found that the enzyme was shed into the culture medium. ACE activity in BBECs was inhibited by three selective, but structurally different, ACE inhibitors (captopril, quinapril, and cisalaprilat) with an IC50 of approximately 2 nM. Increasing chloride concentration in the assay buffer resulted in an increase in BBECs ACE activity of 63%. Enzyme activity was also modulated by the presence of zinc cation in the assay buffer. Addition of dexamethasone to the culture medium was associated with a significant increase in BBECs ACE activity (P < 0.05), which was inhibited by the steroid receptor antagonist RU 38486. Western blot analysis of BBECs, tracheal and bronchial mucosal strips utilizing a cross-reacting rabbit anti-mouse ACE antibody, showed a faint 175 kDa band and additional strong 52 kDa and 47 kDa band. The mechanism of generation of the low M.W. bands is unknown. Our data indicate the presence of ACE in cultured BBECs and that enzyme activity can be modulated.

Mediation of growth factor induced DNA synthesis and calcium mobilization by Gq and Gi2

A newly identified subclass of the heterotrimeric GTP binding regulatory protein family, Gq, has been found to be expressed in a diverse range of cell types. We investigated the potential role of this protein in growth factor signal transduction pathways and its potential relationship to the function of other G alpha subclasses. Recent biochemical studies have suggested that Gq regulates the beta 1 isozyme of phospholipase C (PLC beta 1), an effector for some growth factors. By microinjection of inhibitory antibodies specific to distinct G alpha subunits into living cells, we have determined that G alpha q transduces bradykinin- and thrombin-stimulated intracellular calcium transients which are likely to be mediated by PLC beta 1. Moreover, we found that G alpha q function is required for the mitogenic action of both of these growth factors. These results indicate that both thrombin and bradykinin utilize Gq to couple to increases in intracellular calcium, and that Gq is a necessary component of the mitogenic action of these factors. While microinjection of antibodies against G alpha i2 did not abolish calcium transients stimulated by either of these factors, such microinjection prevented DNA synthesis in response to thrombin but not to bradykinin. These data suggest that thrombin-induced mitogenesis requires both Gq and Gi2, whereas bradykinin needs only the former. Thus, different growth factors operating upon the same cell type use overlapping yet distinct sets of G alpha subtypes in mitogenic signal transduction pathways. The direct identification of the coupling of both a pertussis toxin sensitive and insensitive G protein subtype in the mitogenic pathways utilized by thrombin offers an in vivo biochemical clarification of previous results obtained by pharmacologic studies.

Control of Ca2+ entry into HL60 and U937 human leukaemia cells by the filling state of the intracellular Ca2+ stores

Differentiation of HL60 cells by treatment with dimethyl sulphoxide induces the expression of membrane receptors for N-formylmethionyl-leucyl-phenylalanine (fMLP) and for platelet-activating factor (PAF). In these cells both agonists produced an increase in the cytosolic Ca2+ concentration ([Ca2+]i) by release of Ca2+ from the intracellular stores, followed shortly by an acceleration of the entry of Ca2+ or Mn2+, used here as a Ca2+ surrogate for Ca2+ channels. Cytochrome P-450 inhibitors blocked the agonist-induced entry of Ca2+ or Mn2+ with no modification of Ca2+ release from the stores. Emptying the intracellular Ca2+ stores either by treatments inducing no inositol phosphate production, such as prolonged incubation in Ca(2+)-free medium or treatment with the Ca2+ ionophore ionomycin, increased the plasma-membrane permeability to Ca2+ and Mn2+. This Ca(2+)-store-regulated Mn2+ entry was inhibited by Ni2+ and by cytochrome P-450 inhibitors. Refilling of the Ca2+ stores by incubation in Ca(2+)-containing medium restored low Mn2+ permeability. The same mechanism is present and functional in non-differentiated cells, before expression of membrane receptors for fMLP and PAF. These results suggest that agonist-induced Ca2+ (Mn2+) entry is secondary to the emptying of the intracellular Ca2+ stores, which in turn activates plasma-membrane channels by a mechanism involving cytochrome P-450.

Differential effects of platelet-derived growth factor, serum and bombesin on phospholipase D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In previous studies, activators of protein kinase C, sphingosine, ATP and various oncogenes were each found to enhance phospholipase D-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Here I examined possible stimulation of PtdEtn hydrolysis by various growth-stimulatory agents, including serum, bombesin, platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and insulin. Treatment of NIH 3T3 fibroblasts, prelabelled with [14C]Etn or [32P]PtdEtn, with PDGF-BB resulted in enhanced formation of [14C]Etn or [32P]phosphatidic acid from the respective labelled cellular pools of PtdEtn. A maximal effect (approximately 3-fold stimulation) on PtdEtn hydrolysis was obtained with 50 ng of PDGF/ml after 5 min of treatment. Phosphatidylcholine (PtdCho) was also hydrolysed, although less extensively than PtdEtn, in PDGF-stimulated cells. PDGF-stimulate hydrolysis of both PtdEtn and PtdCho was prevented by prolonged (30 h) treatment of cells with 400 nM-phorbol 12-myristate 13-acetate (PMA). Similar to PDGF, fetal calf serum (1-10%) also stimulated PtdEtn hydrolysis. However, in contrast to PDGF, the effect of serum on PtdEtn hydrolysis (i) was not diminished by pretreatment with PMA, and (ii) was synergistic with that of PMA after a 1 h incubation. Compared with PDGF and serum, bombesin had less effect on PtdEtn hydrolysis, while FGF and insulin had no effects at all. In contrast to PDGF or serum, bombesin inhibited the effect of PMA on PtdEtn hydrolysis.

Molecular link between membrane cholesterol and Na+/H+ exchange within human platelets

Incubation of human platelets with cholesterol-poor, cholesterol-normal and cholesterol-rich liposomes revealed that: (i) acquisition or depletion of platelet membrane cholesterol was highly selective; (ii) variation in membrane cholesterol was highly selective. Variation in membrane cholesterol content (cholesterol-to-phospholipid molar ratio from 0.15-1.2) with respect to values found in unmodified normal platelets, was paralleled by the observed changes in amiloride-sensitive cytoplasmic pH, as well as phospholipase A2 activity. However, a decrease in cytoplasmic pH was accompanied by an increase in phospholipase A2 activity; (iii) membrane cholesterol-modulated changes in intra-platelet pH, as well as phospholipase A2 activity, was completely inhibited when platelets were pretreated with quinacrine (a specific phospholipase A2 inhibitor) before exposure to various types of liposomes. Although exposure of platelets (pretreated with amiloride) with various types of liposomes resulted in the inhibition of Na+/H+ exchange it had no noticeable effect upon the observed phospholipase A2 activity. Based upon these results we suggest that membrane cholesterol-modulated phospholipase A2 activity may be the basic mechanism responsible for the nature of Na+/H+ exchanger activity observed in cholesterol-enriched platelets, leading these platelets to a hypersensitized state.

Alterations in intracellular calcium transients of fibroblasts from progressive systemic sclerotic patients: a digital imaging microscopic study

Intracellular Ca2+ concentrations ([Ca2+]i) in cultured skin fibroblasts from normal subjects and progressive systemic sclerosis (PSS) patients were determined by using Fura-2 and fluorescent videomicroscopy. With the exception of fibroblasts from one PSS patient showing a higher [Ca2+]i, no significant difference was observed in resting [Ca2+]i between the two groups of fibroblasts. Bradykinin (BK) (10 microM) induced a transient [Ca2+]i increase in normal fibroblasts, whereas the BK-induced [Ca2+]i increase was reduced or not detectable in PSS fibroblasts. Removal of extracellular Ca2+ did not eliminate the BK-induced [Ca2+]i increase in normal fibroblasts. These findings suggest that BK stimulates Ca2+ release from intracellular stores in human fibroblasts, and also that the BK-mediated Ca2+ release is impaired in PSS fibroblasts.

Ca2+ responses to interleukin 1 and tumor necrosis factor in cultured human skin fibroblasts. Possible implications for Reye syndrome

Elevated concentrations of cytokines were found in the plasma of patients acutely ill with Reye syndrome (RS) but not in control subjects or recovered RS patients. To determine whether this disorder involves a genetically determined abnormal response to cytokines, the effects of tumor necrosis factor (TNF) and IL-1 on intracellular free Ca2+ were compared in cultured skin fibroblasts from control subjects and patients with RS. IL-1 and TNF caused rapid, transient, and concentration-dependent increases in cytosolic free Ca2+. The peak cytosolic free Ca2+ was greater and occurred at higher concentrations of IL-1 and TNF in patient cells than in cells from age-matched controls. In control cells, the Ca2+ transient diminished sharply with increasing amounts of IL-1 or TNF above the maximum stimulatory concentration. In contrast, in patient fibroblast this bell-shaped curve of concentration dependency was much less apparent. Bradykinin-stimulated Ca2+ transients were similar in the two groups and did not exhibit the bell-shaped concentration dependency. Thus, plasma cytokine levels are elevated in RS patients and the Ca2+ response to cytokines is increased in cells derived from these patients. We propose that the increased response reflects a genetic defect in cytokine receptor-modulated signal transduction.

Interactions between inositol phosphates and cytosolic free calcium following bradykinin stimulation in cultured human skin fibroblasts

The inositol triphosphate (IP3) that results from hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) is generally accepted to be responsible for the mobilization of intracellular calcium. However, some studies suggest that low concentrations of agonists elevate cytosolic free calcium concentration ([Ca2+]i) without IP3 formation. Thus, in the present studies, a comparison of the temporal response of inositol phosphates (IP3, IP2 and IP) and [Ca2+]i to a wide range of bradykinin concentrations was used to examine the relation of these two signal transduction events in cultured human skin fibroblasts (GM3652). In addition, the effects of alterations in internal or external calcium on the response of these second messengers to bradykinin were determined. Bradykinin stimulated accumulation of inositol phosphates and a rise of [Ca2+]i in a time- and dose-dependent manner. Decreasing the bradykinin concentration from 1 microM to 0.1 microM increased the time until the IP3 peak, and when the bradykinin concentration was reduced to 0.01 microM IP3 was not detected. [Ca2+]i was examined under parallel conditions. As the bradykinin concentration was reduced from 1 microM to 0.01 microM, the time to reach the peak of [Ca2+]i increased progressively, but the magnitude of the peak was unaltered. These two second messengers were variably dependent on external calcium. Although the bradykinin-stimulated initial spike of [Ca2+]i did not depend on extracellular calcium, the subsequent sustained levels of [Ca2+]i were abolished in calcium free medium. The bradykinin-stimulated inositol phosphate formation was not dependent on the extracellular calcium nor on the elevation of [Ca2+]i that was produced with Br-A23187. These results demonstrate that bradykinin-induced IP3 formation can be independent of [Ca2+]i and of external calcium, whereas changes in [Ca2+]i are partially dependent on external calcium.

Alterations of G-protein coupling function in phosphoinositide signaling pathways of cells transformed by ras and other membrane-associated and cytoplasmic oncogenes

We showed previously that transformation by cytoplasmic and membrane-associated oncogenes including ras results in uncoupling between surface stimulation by platelet-derived growth factor, bombesin, and serum and activation of intracellular phospholipase C (PLC); this uncoupling does not involve alterations at the receptor or effector enzyme levels (T. Alonso, R. O. Morgan, J. C. Marvizon, H. Zarbl, and E. Santos, Proc. Natl. Acad. Sci. USA 85:4271-4275, 1988). In this study, we stimulated normal and oncogene-transformed NIH 3T3 cells with fluoroaluminate (AIF4-), thus directly activating PLC-associated G protein(s) and bypassing the receptor step. A1F4(-)-elicited PLC responses were significantly impaired in transformed cells when compared with those in their normal counterparts, suggesting that the uncoupling of PLC is the result, at least in part, of functional impairment at the G-protein level. Transformation by ras oncogenes has also been reported to result in enhanced PLC response to bradykinin resulting from increased receptor numbers (G. Parries, R. Hoebel, and E. Racker, Proc. Natl. Acad. Sci. USA 84:2648-2652, 1987; J. Downward, J. de Gunzburg, R. Riehl, and R. Weinberg, Proc. Natl. Acad. Sci. USA 85:5774-5778, 1988). We demonstrate here that transformation by other membrane-associated and cytoplasmic oncogenes also results in increased responsiveness to bradykinin ("supercoupling") and enhanced receptor numbers. However, there is no direct correlation between the number of receptors and the enhancement in responsiveness, suggesting that other factors besides receptor number are also involved in the enhanced responses. We propose that a common effect of transformation by cytoplasmic and membrane-associated oncogenes is functional alteration of coupling G proteins and that a similar modification of different kinds of G proteins may account for the pleiotropic alterations of signal transduction (uncoupling and supercoupling) observed.

Alterations of G-protein coupling function in phosphoinositide signaling pathways of cells transformed by ras and other membrane-associated and cytoplasmic oncogenes

We showed previously that transformation by cytoplasmic and membrane-associated oncogenes including ras results in uncoupling between surface stimulation by platelet-derived growth factor, bombesin, and serum and activation of intracellular phospholipase C (PLC); this uncoupling does not involve alterations at the receptor or effector enzyme levels (T. Alonso, R. O. Morgan, J. C. Marvizon, H. Zarbl, and E. Santos, Proc. Natl. Acad. Sci. USA 85:4271-4275, 1988). In this study, we stimulated normal and oncogene-transformed NIH 3T3 cells with fluoroaluminate (AIF4-), thus directly activating PLC-associated G protein(s) and bypassing the receptor step. A1F4(-)-elicited PLC responses were significantly impaired in transformed cells when compared with those in their normal counterparts, suggesting that the uncoupling of PLC is the result, at least in part, of functional impairment at the G-protein level. Transformation by ras oncogenes has also been reported to result in enhanced PLC response to bradykinin resulting from increased receptor numbers (G. Parries, R. Hoebel, and E. Racker, Proc. Natl. Acad. Sci. USA 84:2648-2652, 1987; J. Downward, J. de Gunzburg, R. Riehl, and R. Weinberg, Proc. Natl. Acad. Sci. USA 85:5774-5778, 1988). We demonstrate here that transformation by other membrane-associated and cytoplasmic oncogenes also results in increased responsiveness to bradykinin ("supercoupling") and enhanced receptor numbers. However, there is no direct correlation between the number of receptors and the enhancement in responsiveness, suggesting that other factors besides receptor number are also involved in the enhanced responses. We propose that a common effect of transformation by cytoplasmic and membrane-associated oncogenes is functional alteration of coupling G proteins and that a similar modification of different kinds of G proteins may account for the pleiotropic alterations of signal transduction (uncoupling and supercoupling) observed.

Pertussis toxin inhibits EGF-, phorbol ester- and insulin-stimulated DNA synthesis in BALB/c3T3 cells: evidence for post-receptor activation of Gi alpha

The contribution of the GTP-binding protein, Gi, to EGF, phorbol dibutyrate (PdBu)-, and insulin-stimulated DNA synthesis was examined in BALB/c3T3 cells. Pertussis toxin inhibited DNA synthesis by each agonist, particularly at suboptimal agonist concentrations, but the inhibition could be partially overcome with higher agonist concentrations and combinations of these agonists. This suggested that (1) some, but not all, of the mitogenic signals for all three agonists were transduced by Gi (2) Gi may be activated by post-receptor mechanisms involving protein kinase C. Gi alpha-specific antibodies and ADP-ribosylation by pertussis toxin using 32P-NAD each labelled a single protein band, representing one or more species of Gi alpha. Pertussis toxin treatment increased the synthesis of Gi alpha. These results are discussed in relation to possible direct effects of Gi alpha on nuclear control during division.

[Hyp3]-bradykinin and [Hyp3]-Lys-bradykinin interact with B2-bradykinin receptors and stimulate inositol phosphate production in cultured human fibroblasts

The recently isolated, naturally occurring peptide hormones [Hyp3]-bradykinin and [Hyp3]-Lys-bradykinin were investigated for their agonist activity on solubilized binding sites from human fibroblasts. Both ligands competed with [3H]bradykinin binding in a dose-dependent fashion with potencies similar to bradykinin (BK) and Lys-BK. Biological activity was assessed by determination of inositol phosphate accumulation and cyclic 3',5'-adenosine monophosphate synthesis in intact cultured cells. Stimulation by the hydroxylated peptides resulted in a pronounced accumulation of both parameters with similar effectiveness as BK and Lys-BK. These results indicate that [Hyp3]-BK and [Hyp3]-Lys-BK are agonists at the bradykinin receptor system with properties comparable to their non-hydroxylated analogues. This suggests that hydroxylation of kinins does not alter receptor interaction or signal transduction in cultured human fibroblasts.

Lysophosphatidate-induced cell proliferation: identification and dissection of signaling pathways mediated by G proteins

Lysophosphatidate (LPA), the simplest natural phospholipid, is highly mitogenic for quiescent fibroblasts. LPA-induced cell proliferation is not dependent on other mitogens and is blocked by pertussis toxin. LPA initiates at least three separate signaling cascades: activation of a pertussis toxin-insensitive G protein mediating phosphoinositide hydrolysis with subsequent Ca2+ mobilization and stimulation of protein kinase C; release of arachidonic acid in a GTP-dependent manner, but independent of prior phosphoinositide hydrolysis; and activation of a pertussis toxin-sensitive Gi protein mediating inhibition of adenylate cyclase. The peptide bradykinin mimics LPA in inducing the first two responses but fails to activate Gi and to stimulate DNA synthesis. Our data suggest that the mitogenic action of LPA occurs through Gi or a related pertussis toxin substrate and that the phosphoinositide hydrolysis-protein kinase C pathway is neither required nor sufficient, by itself, for mitogenesis. The results further suggest that LPA or LPA-like phospholipids may have a novel role in G protein-mediated signal transduction.

Differential activation of human skin cells by platelet activating factor: stimulation of phosphoinositide turnover and arachidonic acid mobilization in keratinocytes but not in fibroblasts

Treatment of cultured adult human keratinocytes with platelet activating factor (PAF) resulted in a rapid, dose dependent accumulation of inositol phosphates. Inositol trisphosphate (IP3), inositol bisphosphate (IP2) and inositol phosphate (IP) were elevated within 15 seconds of exposure to PAF (1 microM). Lyso-PAF, phosphatidylcholine (PC) and lyso-PC had no effect on levels of inositol phosphates, indicating that the effect of PAF was specific. PAF also raised cellular 1,2-diacylglycerol content (2-fold) within two minutes of addition and stimulated mobilization of arachidonic acid (AA) and release of prostaglandin E2. In contrast, PAF did not stimulate phosphoinositide turnover or AA release in cultured dermal fibroblasts. These results suggest that the inflammatory effects of PAF in human skin result, at least in part, from its ability to directly activate keratinocytes and stimulate release of pro-inflammatory eicosanoids.

Coupling of bradykinin receptors to phospholipase C in cultured fibroblasts is mediated by a G-protein

In cultured foreskin fibroblasts, bradykinin stimulates inositol phosphate generation, arachidonic acid release, and Na+/H+ exchange, with doses of 1-3 nM yielding half-maximal stimulation. Binding of 3H-bradykinin to these cells demonstrates a single receptor site with a Kd of 2.0 nM and a Bmax of 91 fmoles/mg protein. Bradykinin analogs of the B2 type inhibit this binding. GTP synergizes with bradykinin to stimulate phosphatidylinositol turnover in permeabilized fibroblasts and GTP-gamma-S decreases the Bmax of bradykinin binding to fibroblast membranes, indicating that a G-protein couples the receptor to phospholipase C. Pretreatment of fibroblasts with either cholera or pertussis toxin enhances bradykinin stimulation of inositol phosphate accumulation.

Presence of T-kininogen and kinins in sponge-induced exudates in rats

1. The content of kinins and T-kininogen (the third kininogen) in exudates induced by the subcutaneous implantation of saline-soaked sponges have been measured by radioimmunoassay in normal Wistar rats and in Brown Norway rats from a strain which is deficient in high and low molecular weight kininogens. 2. In both strains, sponge implantation induced a rise of T-kininogen in plasma with subsequent accumulation in the sponge exudate. This accumulation correlated with the extravasation of plasma proteins during the first 6 h. Bioassays showed that the T-kinin moiety was retained in T-kininogen. 3. In Wistar rats, a large release of immunoreactive kinins up to a mean value of 6.4 ng ml-1 was observed during the first 6 h and on the second day after the implantation. In Brown Norway rats, the kinin level in the exudates did not exceed 0.53 ng ml-1. 4. Of the kinins present during the first 6 h in the exudates withdrawn from Wistar rats, 60% were identified by high performance liquid chromatography as bradykinin. 5. The volume of the exudate induced by the implantation of dry sponges was smaller in Brown Norway rats than in Wistar rats. 6. We conclude that the role of T-kininogen in this kind of exudate was mainly the inhibition of thiol proteinases and not the release of T-kinin. In Wistar rats, bradykinin acts as a pro-inflammatory factor during the first hours and may play a role during the healing process.

Modulation of arachidonic acid metabolism by Rous sarcoma virus

Arachidonic acid (C20:4) metabolites were released constitutively from wild-type Rous sarcoma virus-transformed chicken embryo fibroblasts (CEF). 3H-labeled C20:4 and its metabolites were released from unstimulated and uninfected CEF only in response to stimuli such as serum, phorbol ester, or the calcium ionophore A23187. High-pressure liquid chromatography analysis showed that the radioactivity released from [3H]arachidonate-labeled transformed cells was contained in free arachidonate and in the cyclooxygenase products prostaglandin E2 and prostaglandin F2 alpha; no lipoxygenase products were identified. The release of C20:4 and its metabolites from CEF infected with pp60src deletion mutants was correlated with serum-independent DNA synthesis and with the expression of the mRNA for 9E3, a gene expressed in Rous sarcoma virus-transformed cells which has homology with several mitogenic and inflammatory peptides. 3H-labeled C20:4 release was not correlated with p36 phosphorylation, which argues against a role for this protein as a phospholipase A2 inhibitor. CEF infected with other oncogenic viruses encoding a tyrosine kinase also released C20:4, as did CEF infected with viruses that contained mos and ras; however, infection with a crk-containing virus did not result in stimulation of 3H-labeled C20:4 release, suggesting that utilization of this signaling pathway is specific for particular transformation stimuli.

Separation and identification of two phosphatidylinositol 4-kinase activities in bovine uterus

Growth factor-activated second messenger pathways are mediated in part via breakdown products of phosphoinositides. We have separated two phosphatidylinositol (PtdIns) 4-Kinases from bovine uteri which appear to be regulated independently. The predominant type II enzyme previously was purified to apparent homogeneity; the type I enzyme has been purified approximately 1000 fold (specific activity, approximately 30 nmoles/mg/min). The type I and type II enzymes differ sharply in apparent Km for ATP and response to divalent cations. In contrast to type II enzyme, type I PtdIns kinase was resistant to inhibition by adenosine, inhibited by increasing concentrations of Triton X-100, and less stable to storage than type II enzyme at pH values below 6.5 and above 8.5. Type I PtdIns 4-kinase has an apparent molecular mass of approximately 200 kD and type II enzyme of approximately 80 kD. Using both enzymatic and chemical criteria, both enzymes specifically phosphorylated the fourth hydroxyl group of PtdIns. The results thus establish the presence of two distinct and separate enzymes catalyzing PtdIns 4-kinase activity with different physical, kinetic, and regulatory properties, suggesting an important site for the regulation of second messenger signals transducing the responsiveness of cells to growth factors.

Platelet-derived growth factor induces multisite phosphorylation of pp60c-src and increases its protein-tyrosine kinase activity

We have shown previously that pp60c-src is a substrate for protein kinase C in vivo and that the target of protein kinase C phosphorylation in mammalian pp60c-src is serine 12. We now demonstrate that in addition to tumor promoters, all activators of phosphatidylinositol turnover that we have tested in fibroblasts (platelet-derived growth factor, fibroblast growth factor, serum, vasopressin, sodium orthovanadate, and prostaglandin F2 alpha) lead to the phosphorylation of pp60c-src at serine 12. In addition to stimulating serine 12 phosphorylation in pp60c-src, platelet-derived growth factor treatment of quiescent fibroblasts induces phosphorylation of one or two additional serine residues and one tyrosine residue within the N-terminal 16 kilodaltons of the enzyme and activates its immune complex protein-tyrosine kinase activity.

1-oleoyl-2-acetylglycerol and A23187 potentiate axolemma- and myelin-induced Schwann cell proliferation

The effects of 1-oleoyl-2-acetylglycerol (OAG) and the calcium ionophore A23187 on the proliferation of Schwann cells stimulated with either a myelin-enriched membrane fraction (MEF) or an axolemma-enriched membrane fraction (AEF) have been examined. Using incorporation of [3H]thymidine as an index of proliferation, 16% of the cells became labeled after incubation with MEF (20 micrograms protein/ml) and AEF (40 micrograms protein/ml) for 72 h. Only 0.5% of the cells became labeled in cultures which were not exposed to the membrane fractions. Addition of OAG (10-500 microM) or A23187 (1.9-190 nM) in the absence of the membrane mitogens had no effect on the proliferative response of quiescent cultures of Schwann cells. When added simultaneously, however, OAG and A23187 were able to induce proliferation of the cells, although the response was only 30% of the response achieved with maximal doses of either AEF or MEF. Both OAG and A23187 were able to potentiate the mitogenicity of AEF or MEF, but only when AEF and MEF were added at submaximal concentrations. When Schwann cells were prelabeled with [3H]glycerol and then stimulated to proliferate with AEF or MEF, the amount of [3H]diacylglycerol was increased two- to threefold above that in control cultures for time periods up to 1 h. These results suggest that the proliferation of Schwann cells induced by either AEF or MEF is partially mediated through the combined effects of diacylglycerol and an increase in intracellular calcium.

Protein kinase C is required for responses to T cell receptor ligands but not to interleukin-2 in T cells

We have tested the role of protein kinase C in mRNA expression and T cell proliferation mediated through the T cell receptor and through the interleukin-2 (IL-2) receptor. Chronic treatment of a mouse T cell clone with phorbol esters caused a complete loss of protein kinase C activity and a concomitant loss of proliferation to T cell receptor ligands (antigen, lectins, antireceptor antibodies). In contrast, kinase C-depleted T cells retained the ability to proliferate to IL-2. Loss of the T cell receptor response was not due to decreased cell surface expression of receptor or impairment of early receptor function (phosphatidylinositol turnover, calcium mobilization). Kinase C-depleted T cells showed no induction of mRNAs for activation-associated genes on exposure to the T cell receptor ligand Concanavalin A; expression of a subset of the same mRNAs in response to IL-2 was unaffected. We conclude that kinase C is required for mRNA expression and subsequent proliferation mediated through the T cell receptor pathway but is not involved in mRNA expression and proliferation in response to IL-2.

Serotonin stimulates DNA synthesis in fibroblasts acting through 5-HT1B receptors coupled to a Gi-protein

Growth factors can be divided into two classes which act through distinct signal transduction pathways. One class including epidermal growth factor, platelet derived growth factor and fibroblast growth factor activates receptor tyrosine kinases, and the second class, including thrombin, bombesin, bradykinin and vasopressin activates a phosphoinositide-specific phospholipase C through GTP-binding proteins which can be inactivated by pertussis toxin. In Chinese hamster lung fibroblasts, thrombin-induced mitogenicity seems to correlate well with phospholipase C activation and both events are sensitive to pertussis toxin. Thrombin, like the other mitogens in this class, simultaneously inhibits adenylate cyclase. This involves an inhibitory G protein (Gi), a well established pertussis toxin substrate. The relative contributions of the two signalling pathways to mitogenicity has not been evaluated so far. We report here that the neurotransmitter serotonin (5-hydroxytryptamine), a contracting agent and mitogen for smooth muscle cells, activates phospholipase C, inhibits adenylate cyclase and stimulates DNA synthesis in fibroblasts. These events are sensitive to pertussis toxin. We show that the mitogenicity of 5-hydroxytryptamine can be uncoupled from phospholipase C activation that is mediated by 5-HT2 receptors, but correlates perfectly with inhibition of adenylate cyclase through 5-HT1B receptor. We propose that inhibition of adenylate cyclase or activation of an undefined effector system by Gi is important in 5-hydroxytryptamine induced DNA synthesis and contributes to the strong mitogenicity of the other members of this family of growth factors.

Simultaneous addition of EGF prolongs the increase in cytosolic free calcium seen in response to bradykinin in NRK-49F cells

The calcium-sensitive fluorescent indicator fura-2 and a microscope equipped for rapidly changing excitation wavelengths were used to look at the effects of growth factors on cytosolic free calcium ([Ca2+]i) in NRK-49F cells. In these cells bradykinin induced a rapid increase in [Ca2+]i, which generally decayed to near basal [Ca2+]i within 3 minutes. The initial rise in [Ca2+]i in response to bradykinin was relatively independent of extracellular calcium; however, the decay to basal [Ca2+]i was more rapid in the absence of extracellular calcium. Measurements made on individual cells showed a heterogeneity in the response to bradykinin. Epidermal growth factor (EGF) had no effect on [Ca2+]i in NRK-49F cells when added alone in the presence of extracellular calcium. Simultaneous addition of bradykinin and EGF produced a more prolonged increase in [Ca2+]i than bradykinin alone. The prolongation was dependent on the presence of extracellular calcium and did not occur in its absence. Transient increases in [Ca2+]i occurring after the initial peak were occasionally seen in these cells. Our results indicate that there is rapid interaction between the signaling mechanisms for bradykinin and EGF. When this occurs, one effect is the transport of calcium into the cell from the extracellular environment, causing a more prolonged rise in [Ca2+]i. This effect occurs within 1 minute after combined addition of bradykinin and EGF.

Hydrolysis of inositol lipids: an early signal of human cytomegalovirus infection

The production of inositol 1,4,5-triphosphate (IP 3) and 1,2-diacylglycerol (DG) as an early cellular response to human cytomegalovirus (HCMV) infection was investigated in human embryo lung (LU) cells. The results obtained show transient 85 and 30% increases in IP3 and DG, respectively, 20 min post-exposure to HCMV. Following heat inactivation or serum neutralization of HCMV stock or in the presence of papaverine we failed to detect increased levels of IP3 and DG relative to mock-infected LU cultures.

Platelet-derived growth factor induces multisite phosphorylation of pp60c-src and increases its protein-tyrosine kinase activity

We have shown previously that pp60c-src is a substrate for protein kinase C in vivo and that the target of protein kinase C phosphorylation in mammalian pp60c-src is serine 12. We now demonstrate that in addition to tumor promoters, all activators of phosphatidylinositol turnover that we have tested in fibroblasts (platelet-derived growth factor, fibroblast growth factor, serum, vasopressin, sodium orthovanadate, and prostaglandin F2 alpha) lead to the phosphorylation of pp60c-src at serine 12. In addition to stimulating serine 12 phosphorylation in pp60c-src, platelet-derived growth factor treatment of quiescent fibroblasts induces phosphorylation of one or two additional serine residues and one tyrosine residue within the N-terminal 16 kilodaltons of the enzyme and activates its immune complex protein-tyrosine kinase activity.

Studies on protein kinases involved in regulation of nucleocytoplasmic mRNA transport

The rate of energy-dependent nucleoside triphosphatase (NTPase)-mediated nucleocytoplasmic translocation of poly(A)-containing mRNA [poly(A)+mRNA] across the nuclear envelope is thought to be regulated by poly(A)-sensitive phosphorylation and dephosphorylation of nuclear-envelope protein. Studying the phosphorylation-related inhibition of the NTPase, we found that phosphorylation of one polypeptide of rat liver envelopes by endogenous NI- and NII-like protein kinase was particularly sensitive to poly(A). This polypeptide (106 kDa) was also phosphorylated by nuclear-envelope-bound Ca2+-activated and phospholipid-dependent protein kinase (protein kinase C). Activation of kinase C by tumour-promoting phorbol esters resulted in inhibition of nuclear-envelope NTPase activity and in a concomitant decrease of mRNA (actin) efflux rate from isolated rat liver nuclei. Protein kinase C, but not nuclear envelope NI-like or NII-like protein kinase, was found to be solubilized from the envelope by Triton X-100, whereas the presumable poly(A)-binding site [the 106 kDa polypeptide, representing the putative carrier for poly(A)+mRNA transport] remained bound to this structure. RNA efflux from detergent-treated nuclei lost its susceptibility to phorbol esters. Addition of purified protein kinase C to these nuclei restored the effect of the tumour promoters. Protein kinase C was found to bind also to isolated rat liver nuclear matrices in the absence but not in the presence of ATP. The NII-like nuclear-envelope protein kinase co-purified together with the 106 kDa polypeptide which specifically binds to poly(A) in an ATP-labile linkage.

Protein kinase C activation aggravates hypoxic myocardial injury by stimulating Na+/H+ exchange

The physiological and pathophysiological roles of protein kinase C activation were investigated in cultured mouse myocardial cells. First, effects of 12-O-tetra-decanoyl-phorbol-13-acetate (TPA), a potent activator of protein kinase C, on the intracellular pH (pHi) and cytosolic free Ca2+ level [( Ca2+]i) were studied, using 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) and quin-2, respectively. In the presence of the Ca ionophore A23187, TPA induced a rise in pHi by activating amiloride-sensitive Na+/H+ exchange and also produced a rise in [Ca2+]i above that seen with A23187 alone. These effects were totally inhibited by amiloride. Second, the effect of TPA on hypoxia-induced myocardial cell injury was evaluated. The addition of TPA to the culture medium enhanced creatine kinase release from hypoxic myocardial cells (95% N2 + 5% CO2). This effect was markedly suppressed by the addition of amiloride. These data suggests that protein kinase C activation aggravates hypoxic myocardial injury, presumably by inducing Ca2+ overload. This event is secondary to activation of Na+/Ca2+ exchange through accelerated influx of Na+ into the cells as a result of Na+/H+ exchange stimulation by protein kinase C.

A phosphoinositide-linked peptide response in astrocytes: evidence for regional heterogeneity

A phosphoinositide-linked peptide response in cultured rat astrocytes was studied by measuring the accumulation of [3H]inositol phosphates in the presence of lithium. Cultures derived from cortex, cerebellum and spinal cord each showed a unique pattern or degree of stimulation to a panel of neuropeptides. Cortical and cerebellar astrocytes were similar, responding to bradykinin, oxytocin, vasopressin, eledoisin and neurokinin beta, whereas spinal cord astrocytes were stimulated by substance P, bradykinin, eledoisin, and neurokinins alpha and beta. These observations are evidence in favour of regional specialisations of astrocytes which may respond uniquely to peptides released by particular populations of neurons.

Mitogenesis in response to PDGF and bombesin abolished by microinjection of antibody to PIP2

The turnover of phosphatidylinositol 4,5-bisphosphate (PIP2) is believed to constitute a crucial step in the signaling pathways for stimulation of cells by a variety of bioactive substances, including mitogens, but decisive evidence for the idea has not been obtained. In the present study, a monoclonal antibody to PIP2 was microinjected into the cytoplasm of NIH 3T3 cells before or after exposure to mitogens. The antibody completely abolished nuclear labeling with [3H]thymidine induced by platelet-derived growth factor and bombesin, but not by fibroblast growth factor, epidermal growth factor, insulin, or serum. The findings strongly suggest that PIP2 breakdown is crucial in the elicitation and sustaining of cell proliferation induced by some types of mitogens such as platelet-derived growth factor and bombesin.

The role of ion transport in the regulation of cell proliferation

The stimulation of growth in a variety of cell types is followed by rapid changes in ion transport across the plasma membrane and in the intracellular concentration of various ions. The addition of various growth factors to fibroblasts, for example, causes stimulation of Na+ entry through the Na(+)-H+ antiport. This results in the alkalinization of the cytosol and an increase in intracellular Na+ concentration. The increased intracellular Na+ in turn stimulates the Na+/K+ pump, raising the concentration of K+ and lowering the Na+ toward normal. These changes in monovalent ion transport appear to be a necessary part of the proliferative response. In addition to the changes in cytosolic Na+, K+, and pH, a number of growth factors also cause a rapid increase in the cytosolic concentration of Ca2+. The additional Ca2+ appears to come from intracellular organelles, since the effect does not require Ca2+ in the extracellular medium. The change in intracellular Ca2+ concentration persists for only a few minutes. Changes in ion transport have been observed after the addition of mitogens to a variety of cell types, including epithelial cells. For example, we have found that stimulation of proliferation of MDCK (dog kidney epithelial) cells by either serum or vasopressin is followed by stimulation of the activity of the Na+/K+ pump. The manner in which these rapid changes in ion transport may play a role in signalling the onset of the mitogenic response will be discussed.

Inhibition by pertussis toxin of fibroblast growth factor-stimulated hexose transport in Swiss 3T3 cells

Addition of fibroblast growth factor to quiescent cultures of Swiss 3T3 cells stimulated the membrane transport of 2-deoxyglucose. Treatment of the cells with pertussis toxin (islet-activating protein) inhibited fibroblast growth factor-stimulated hexose transport. 5'-Guanylyl imidodiphosphate (p[NH]ppG), a non hydrolyzable analogue of GTP, increased the number of hexose carriers in the plasma membrane of saponin-permeabilized cells. These results suggest that guanine nucleotide binding protein may be involved in the regulation of hexose transport system by fibroblast growth factor in Swiss 3T3 cells.

Adenylate cyclase stimulating agents and mitogens raise fructose 2,6-bisphosphate levels in human fibroblasts. Evidence for a dual control of the metabolite

Fructose 2,6-bisphosphate, the most potent activator of 6-phosphofructo-1-kinase, has been demonstrated to mediate the increase of glycolytic flux induced by mitogens human fibroblasts. In the present work the molecular basis of transmembrane control of fructose 2,6-bisphosphate has been investigated. Prostacyclin and isoprenaline, known to activate adenylate cyclase, are able to increase fructose 2,6-bisphosphate levels, indicating that in human fibroblasts cyclic AMP plays a positive role in the control of the metabolite concentration, opposite to that exerted in hepatocytes. Substances known to activate protein kinase C such as phorbol 12-myristate 13-acetate, or to stimulate phosphoinositide turnover such as thrombin and bradykinin are also effective in raising fructose 2,6-bisphosphate. Therefore, we conclude that cyclic AMP and protein kinase C are likely involved in the control of fructose 2,6-bisphosphate levels in human fibroblasts.