Pathway of phospholipase C activation initiated with platelet-derived growth factor is different from that initiated with vasopressin and bombesin

Protein-tyrosine kinase CAKbeta/PYK2 is activated by binding Ca2+/calmodulin to FERM F2 alpha2 helix and thus forming its dimer

CAKbeta (cell adhesion kinase beta)/PYK2 (proline-rich tyrosine kinase 2) is the second protein-tyrosine kinase of the FAK (focal adhesion kinase) subfamily. It is different from FAK in that it is activated following an increase in cytoplasmic free Ca2+. In the present study we have investigated how Ca2+ activates CAKbeta/PYK2. Calmodulin-agarose bound CAKbeta/PYK2, but not FAK, in the presence of CaCl2. An alpha-helix (F2-alpha2) present in the FERM (band four-point-one, ezrin, radixin, moesin homology) F2 subdomain of CAKbeta/PYK2 was the binding site of Ca2+/calmodulin; a mutant of this region, L176A/Q177A (LQ/AA) CAKbeta/PYK2, bound to Ca2+/calmodulin much less than the wild-type. CAKbeta/PYK2 is known to be prominently tyrosine phosphorylated when overexpressed from cDNA. The enhanced tyrosine phosphorylation was inhibited by W7, an inhibitor of calmodulin, and by a cell-permeable Ca2+ chelator and was almost defective in the LQ/AA-mutant CAKbeta/PYK2. CAKbeta/PYK2 formed a homodimer on binding of Ca2+/calmodulin, which might then induce a conformational change of the kinase, resulting in transphosphorylation within the dimer. The dimer was formed at a free-Ca2+ concentration of 8-12 muM and was stable at 500 nM Ca2+, but dissociated to a monomer in a Ca2+-free buffer. The dimer formation of CAKbeta/PYK2 FERM domain was partially defective in the LQ/AA-mutant FERM domain and was blocked by W7 and by a synthetic peptide with amino acids 168-188 of CAKbeta/PYK2, but not by a peptide with its LQ/AA-mutant sequence. It is known that the F2-alpha2 helix is found immediately adjacent to a hydrophobic pocket in the FERM F2 lobe, which locks, in the autoinhibited FAK, the C-lobe of the kinase domain. Our results indicate that Ca2+/calmodulin binding to the FERM F2-alpha2 helix of CAKbeta/PYK2 releases its kinase domain from autoinhibition by forming a dimer.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Calcium channel antagonist effect on in vitro meningioma signal transduction pathways after growth factor stimulation

OBJECTIVE

We have previously demonstrated that calcium channel antagonists inhibit the growth of human meningiomas in culture after stimulation with growth factors. This study examined the effects of these drugs on signaling transduction pathways in an attempt to elucidate potential mechanisms by which this growth inhibition is mediated.

METHODS

Primary cell cultures from patients with intracranial meningiomas were established. Cell growth studies were performed with inhibitors and stimulators of tyrosine kinase signal transduction. Intracellular calcium changes and inositol phosphate production were measured after growth factor exposure, with or without pretreatment by calcium channel antagonists.

RESULTS

The growth of meningiomas in culture can be inhibited by tyrosine kinase receptor inhibitors. Inhibitors and stimulators of phospholipase C can stimulate or inhibit the growth of in vitro meningiomas, respectively. Calcium channel antagonists inhibit intracellular calcium changes induced by serum and epidermal growth factor. Inositol phosphate production is increased after growth factor stimulation, and calcium channel antagonists potentiate this effect.

CONCLUSION

Calcium channel antagonists interfere with intracellular signaling pathways of cultured meningioma cells. This inhibition is unrelated to voltage-sensitive calcium channels. The findings of this project may aid in the understanding of the signal transduction mechanisms involved in growth factor-mediated meningioma proliferation and may lead to clinically relevant strategies for growth inhibition.

Changes of nuclear PI-PLC gamma1 during rat liver regeneration

We have previously demonstrated that rat liver nuclei contain PI-PLC beta1 and gamma1 in the inner nuclear matrix and lamina associated with specific phosphodiesterase activity (Bertagnolo et al., 1995, Cell Signall. 7, 669-678). Since compensatory hepatic growth is an informative and well characterized model for natural cell proliferation, the presence of specific PI-PLC isoforms and their activity as well as PIP2 recovery were studied at various regenerating times, ranging from 3 to 22 h after partial hepatectomy. Three PI-PLC isoforms (beta1, gamma1, delta1) were examined in control and regenerating liver cells by using specific antibodies. By means of in situ immunocytochemistry and confocal microscopy, PI-PLC beta1 was found mainly in the nucleoplasm and this pattern was not modified after hepatectomy. On the contrary, the nuclear gamma1 isoform showed a marked decrease at 3 and 16 h after hepatectomy, but a clear increase at 22 h covering with bright intensity the whole nucleus. The PI-PLC delta1 isoform, which is exclusively cytoplasmic, was not altered during rat liver regeneration. By western blotting analysis on whole cell homogenates, none of the PI-PLC isozymes under study showed proliferation-linked modification. However, analyses of isolated nuclei identified changes in the nucleus associated PI-PLC gamma1 that paralleled the in situ observation whereas the beta1 isoform was unmodified at all the times examined. Nuclear phosphodiesterase activity on PIP2 was lower at 3 and 16 h, in comparison with sham operated rats, increased at 6 h and reached the highest value after 22 h. Consistently, the recovery of PIP2, obtained in conditions that optimise PIP-kinase activity, showed a marked decrease at 3 h and an increase up to 16 h of liver regeneration, followed by a further decrease at 22 h. These data are consistent with a close relationship between cell proliferation and the nuclear inositide cycle, depending, in rat liver, predominantly on the modulation of the gamma1 isoform of PI-PLC.

Intracellular signaling by growth factors

Growth factors are involved in a variety of cellular responses such as growth, differentiation, migration, metabolism, and transformation. Binding of the growth factor to its corresponding cell surface receptor results in activation of the receptor's intrinsic tyrosine kinase activity, and subsequently in activation of complex multistep signal transduction cascades. Activation of these interconnected signaling pathways eventually leads to a biological response, which involves changes in gene expression and protein synthesis. The biological response has been shown to be receptor-specific and also cell-type (tissue)-specific, indicating that various receptors activate distinct signal transduction pathways in one tissue and that one receptor activates different pathways in various tissues. What determines receptor specificity and tissue specificity? In this context, this article will focus on certain receptors with intrinsic tyrosine kinase activity, including receptors for platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin, and nerve growth factor (NGF).

Cloning and characterization of cell adhesion kinase beta, a novel protein-tyrosine kinase of the focal adhesion kinase subfamily

A second protein-tyrosine kinase (PTK) of the focal adhesion kinase (FAK) subfamily, cell adhesion kinase beta (CAK beta), was identified by cDNA cloning. The rat CAK beta is a 115.7-kDa PTK that contains N- and C-terminal domains of 418 and 330 amino acid residues besides the central kinase domain. The rat CAK beta has a homology with mouse FAK over their entire lengths except for the extreme N-terminal 88 residues and shares 45% overall sequence identity (60% identical in the catalytic domain), which indicates that CAK beta is a protein structurally related to but different from FAK. The CAK beta gene is less evenly expressed in a variety of rat organs than the FAK gene. Anti-CAK beta antibody immunoprecipitated a 113-kDa protein from rat brain, 3Y1 fibroblasts, and COS-7 cells transfected with CAK beta cDNA. The tyrosine-phosphorylated state of CAK beta was not reduced on trypsinization, nor enhanced in response to plating 3Y1 cells onto fibronectin. CAK beta localized to sites of cell-to-cell contact in COS-7 transfected with CAK beta cDNA, in which FAK was found at the bottom of the cells. Thus, CAK beta is a PTK possibly participating in the signal transduction regulated by cell-to-cell contacts.

Rapid and long-term effects on protein kinase C on receptor tyrosine kinase phosphorylation and degradation

Rapid and long term effects of protein kinase C alpha activation on receptor tyrosine kinase signaling parameters were investigated in human 293 embryonic fibroblasts and mouse NIH 3T3 cells. Within minutes of phorbol 12-myristate 13-acetate treatment, epidermal growth factor receptor and HER2 tyrosine phosphorylation was decreased, while platelet-derived growth factor receptor and insulin receptor autophosphorylation was upregulated. These effects are not mediated by protein kinase C-dependent receptor tyrosine kinase phosphorylation but apparently by activation or inactivation of receptor tyrosine kinase-specific phosphatases, as indicated by neutralization of these phenomena upon treatment of cells with sodium orthovanadate. In contrast to these short term effects, sustained activation of protein kinase C alpha by phorbol 12-myristate 13-acetate results in translocation of protein kinase C from the cytosol to the membrane fraction where it forms stable complexes with all receptor tyrosine kinases investigated. Ligand-induced receptor tyrosine kinase/protein kinase C association in NIH 3T3 fibroblasts is accompanied by a mobility shift of the receptor, indicating phosphorylation by activated protein kinase C. This phenomenon correlates with the disappearance of receptor tyrosine kinases from the cell surface, implying that this interaction plays a role in the process of receptor internalization and degradation. Interestingly, ligand-stimulated receptor down-regulation is also enhanced by overexpression of phospholipase C gamma, which strongly indicates a role for this common receptor tyrosine kinase substrate in negative regulation of growth factor signals.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

In vitro growth inhibition of growth factor-stimulated meningioma cells by calcium channel antagonists

Studies have shown that a majority of meningiomas contain receptors for platelet-derived growth factor and epidermal growth factor and that these growth factors promote the proliferation of meningioma cells in culture. Although the mechanism of action has not been elucidated, intracellular calcium appears to be part of the signal transduction mechanism. Because alterations in intracellular calcium could interrupt this pathway and decrease cellular proliferation, we investigated the effects of calcium channel-blocking agents on the growth of meningioma cells in vitro. Primary meningioma cell cultures were established, and the cells were characterized by light and electron microscopy and by immunohistochemical studies. Then, the cultures were given growth factors and/or various calcium channel antagonists, and growth rates were measured. A dose-response decrease in cell growth was seen when verapamil, nifedipine, or diltiazem (voltage-dependent calcium channel-blocking agents) was added to serum-containing media. Also, these drugs blocked the growth stimulation of epidermal growth factor and platelet-derived growth factor in a similar fashion. Dantrolene, which inhibits the release of sequestered intracellular calcium, was also an effective blocker of the mitogenic stimulation of these growth factors.

A pleiotropic effect of fluoride on signal transduction in macrophages: is it mediated by GPT-binding proteins?

The activation of GTP-binding proteins (G-proteins) by sodium fluoride + aluminum (AlF4-) was shown in several cell free systems. In the intact cell, NaF +/- aluminum was shown to activate various signal transduction pathways and indirect evidence is in line with effector mechanisms involving regulation of G-protein activity. We have explored the effect of NaF on several components of signal transduction pathways in macrophages. NaF was shown to reduce intracellular ATP levels and to suppress agonist-induced protein tyrosine phosphorylation and reactive oxygen species formation. NaF led to in situ activation of nitrogen activated protein kinase, phospholipase A2 and PtdIns-phospholipase C. Addition of AlCl(3) or deferoxamine, a chelator of aluminum, had little or no effect on NaF mediated enzyme activation. The results suggest that at least some of the pleiotropic effects of NaF in intact cells may not be mediated by G-protein activation but rather by depletion of ATP which is essential for protein phosphorylation reactions.

The immunohistochemical distribution of protein kinase C isozymes is altered in the canine brain and basilar artery after subarachnoid hemorrhage

The effects of subarachnoid hemorrhage on immunoreactivities of protein kinase C isozymes (alpha, beta, gamma) were studied in canine brain and basilar artery. Mild and severe constriction of the basilar artery was shown 2 days and 7 days after hemorrhage, respectively. In control brain tissues, the isozymes showed distinct distributions and following hemorrhage, reactive astrocytes with protein kinase C alpha staining emerged in the surface of the pons and hippocampus. The control basilar artery showed protein kinase C alpha staining and its staining was decreased on day 7, consistent with the result of immunoblot. Our results demonstrated that subarachnoid hemorrhage induces gliosis with heavy protein kinase C alpha staining and down-regulation of protein kinase C in the artery.

Membrane association of phospholipase C encoded by the norpA gene of Drosophila melanogaster

Severe mutations within the norpA gene of Drosophila abolish the photoreceptor potential and render the fly blind by deleting phospholipase C, an essential component of the phototransduction pathway. To study the membrane association of phospholipase C, we have utilized biochemical assays of phospholipase C activity, which predominant measurable phospholipase C activity in head homogenates has been shown to be encoded by norpA, as well as antisera generated against the major gene product of norpA to examine its subcellular distribution before and during phototransduction. We find that both phospholipase C activity and the norpA protein are predominantly associated with membrane fractions in heads of both light- and dark-adapted flies. Moreover, phospholipase C activity as well as norpA protein can be easily extracted from membrane preparations of light- or dark-adapted flies using high salt, indicating that the norpA protein is peripherally localized on the membrane. These data suggest that the norpA encoded phospholipase C of Drosophila is a permanent peripheral membrane protein. If this is indeed the case, then it would mean that the reversible redistribution of phospholipase C from the cytosol to the membrane, as observed in epidermal growth factor receptor stimulation of mammalian phospholipase C gamma, is not a universal mechanism utilized by all types of phosphatidylinositol-specific phospholipase C.

Isoenzymes of carbohydrate metabolism in primary cultures of hepatocytes from thioacetamide-induced rat liver necrosis: responses to growth factors

Hepatocytes isolated from the liver of rats after a necrotizing dose of thioacetamide (6.6 mmol/kg) were used to study the postnecrotic process of liver regeneration. Flow cytometry analysis revealed populations of dedifferentiated hepatocytes exhibiting physical properties (size and fluorescence emission at 530 nm) similar to those found in fetal (22 days old) liver cells. The percentage of these cells increased progressively from 24 to 48 and 72 hr after thioacetamide administration. In primary cultures of hepatocytes the effects of phorbol 12-myristate 13-acetate, bombesin and insulin were investigated on the 6-phosphofructo 2-kinase/fructose 2,6 bisphosphate system. Bombesin and insulin stimulated 6-phosphofructo 2-kinase activity and fructose 2,6-bisphosphate content both in control and in thioacetamide-treated hepatocytes. However, phorbol 12-myristate 13-acetate stimulated 6-phosphofructo 2-kinase activity and increased fructose 2,6-bisphosphate concentration in thioacetamide-treated liver cells, whereas no similar response was found in hepatocytes from control rats. The response of postnecrotic thioacetamide-treated hepatocytes to phorbol 12-myristate 13-acetate was similar to that obtained from 22-day-old fetal liver cells, which reveals that different methods might control fructose 2,6-bisphosphate content and therefore the mechanisms of glycolysis and gluconeogenesis at this regulatory step. The lack of response to glucagon of glycogen phosphorylase a and 6-phosphofructo 2-kinase from thioacetamide-treated hepatocytes may indicate that the expression of specific enzymes of carbohydrate metabolism undergoes transitions to less-differentiated isoenzymatic forms. Moreover, the isoenzyme pattern of hexokinases elicits a complete disturbance in glucokinase and hexokinases activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters, bombesin and insulin elicit differential responses on the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase system in primary cultures of foetal and adult rat hepatocytes

The effects of 4 beta-phorbol 12-myristate 13-acetate (PMA), bombesin and insulin on 6-phosphofructo-2-kinase (PFK-2) activity, on fructose 2,6-bisphosphate concentration and on the phosphorylation state of PFK-2 were investigated in primary cultures of hepatocytes from foetal and adult rats. Bombesin stimulated PFK-2 activity and increased hexose phosphate (glucose 6-phosphate and fructose 6-phosphate) and fructose 2,6-bisphosphate content in hepatocytes both in the foetal and adult state. However, PMA-treated foetal cells exhibited a marked stimulation in fructose 2,6-bisphosphate concentration and in PFK-2 activity as well as in the content of hexose phosphates, while no response was found in the case of adult hepatocytes. Moreover, the effect of PMA on foetal hepatocytes was suppressed when cells were incubated with cycloheximide, but not when this effect was elicited by bombesin or insulin. These results, and those obtained on the phosphorylation state of PFK-2, suggest that there are different pathways that modulate fructose 2,6-bisphosphate content and, therefore, the control mechanisms of glycolysis and gluconeogenesis at this regulatory step, both in adult and foetal rat liver.

Phosphorylation of the inhibitory guanine-nucleotide-binding protein as a possible mechanism of inhibition by protein kinase C of agonist-induced Ca2+ mobilization in human platelet

Increases in the intracellular Ca2+ concentration of human platelets caused by receptor agonists, such as thrombin, 9,11-epithio-11,12-methanothromboxane A2 (STA2), platelet-activating factor (PAF) and arginine-vasopressin, were inhibited by prior addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) in time-dependent and concentration-dependent manners. The inhibitions were mostly reversed by staurosporine, and inhibitor of protein kinase C, added 1 min before TPA. Prior treatment of platelets with thrombin or STA2, the efficacious Ca2+ mobilizer, suppressed the increase in the intracellular Ca2+ concentration of the cells to other agonists, but treatment with less efficacious PAF or vasopressin did not. The heterologous receptor desensitizations were also reversed by staurosporine. The antibody, directed against the carboxy-terminal region of the alpha subunits 1 and 2 of the inhibitory guanine-nucleotide-binding proteins (Gi1 alpha and Gi2 alpha), was raised in rabbit and was used to immunoprecipitate Gi alpha in 32P-labeled platelets. The radioactivity was detected in Gi alpha after incubation of 32P-labeled platelets with TPA, thrombin or STA2, but not in the cells incubated with PAF or vasopressin. The time-dependency or concentration-dependency of TPA-induced phosphorylation of Gi alpha was similar to the dependency of its inhibitory action on agonist-induced Ca2+ mobilization. Thus, strong activation of Ca2+/phospholipid-dependent protein kinase C by phorbol ester or agonists of certain Ca(2+)-mobilizing receptors leads to phosphorylation of the alpha subunit of guanine-nucleotide-binding protein, thereby impairing the coupling of the G protein to receptors as a feedback regulatory component of the receptor-triggered intracellular Ca(2+)-mobilizing system.

Suppression by staurosporine of Ca(2+)-mobilization triggered by ligation of antigen-specific receptors on t and B lymphocytes. An essential role of protein tyrosine kinase in the signal transduction

It is known that the receptor for platelet-derived growth factor (PDGF) activates phospholipase C (PLC) by phosphorylating the gamma 1 isoform of PLC with the receptor protein-tyrosine kinase (PTK), whereas a guanine nucleotide-binding protein participates as a transducer in the PLC activation through the receptors for vasopressin, bombesin and prostaglandin F2 alpha (PGF2 alpha). We have shown in a rat fibroblast line that staurosporine is a potent PTK inhibitor capable of clearly discriminating the two types of receptor-stimulated Ca2+ mobilization and, by inference, PLC activations the response triggered by PDGF was completely inhibited, whereas the responses triggered by vasopressin, bombesin and PGF2 alpha were not affected at all. The Ca2+ mobilization in human T and B cell lines induced by anti-CD3 and anti-immunoglobulins (Ig) was completely suppressed by staurosporine. The results indicate that the PTK activity plays an essential role in the PLC activation through the T cell receptor/CD3 complex and through membrane Ig.

Calcium channels in PDGF-stimulated A172 cells open after intracellular calcium release and are not voltage-dependent

Using laser image cytometry and Indo-1 fluorescence, we investigated the intracellular free Ca2+ concentration ([Ca2+]i) of confluent A172 human glioblastoma cells stimulated by the BB homodimer of platelet-derived growth factor (PDGF-BB). The shape of the calcium transients and the delay time between stimulation and the beginning of the transient varied considerably. The percentage of responsive cells, the peak [Ca2+]i and the duration of the response were directly related to PDGF-BB dose, while the delay time was inversely related; the maximal response occurred at a PDGF-BB concentration of 20 ng/ml. Studies with EGTA and inorganic calcium-channel blockers (Ni2+, La3+) showed that the increase of [Ca2+]i resulted from initial release of intracellular stores and subsequent calcium influx across the plasma membrane. Opening of calcium channels in the plasma membrane, monitored directly by studying Mn2+ quenching of Indo-1 fluorescence, was stimulated by PDGF-BB and blocked by La3+; the opening occurred 55 +/- 10 s after the initial increase in [Ca2+]i. Therefore, in these tumor cells, intracellular release always occurs before channel opening in the plasma membrane. Depolarization of cells with high extracellular [K+] did not generally induce calcium transients but did decrease calcium influx. L-type calcium-channel blockers (verapamil, nifedipine, and diltiazem) had little or no effect on the calcium influx induced by PDGF-BB. These results indicate that PDGF-BB induces calcium influx by a mechanism independent of voltage-sensitive calcium channels in A172 human glioblastoma cells.

Stimulation by epidermal growth factor of inositol phosphate production in plasma membranes from A431 cells

Plasma membranes were isolated from A431 cells previously labelled with myo-[3H]inositol during exponential growth, using a rapid procedure on Percoll gradients. They displayed a significant phospholipase (PLC) activity against phosphoinositides, which was stimulated by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), epidermal growth factor (EGF) and fetal calf serum (FCS) (24%, 11% and 97% over controls, respectively). The effect of EGF was not significantly increased by GTP gamma S. Upon addition of cytosol, EGF promoted an almost 100% stimulation of inositol 1,4,5-trisphosphate and inositol bisphosphate generation, which displayed an absolute requirement for GTP gamma S. This dose-dependent effect of cytosol was linear until 60 micrograms/ml of cytosolic protein and decreased afterwards; it was abolished by heat treatment and trypsin hydrolysis, and it was not reproduced by an identical amount of bovine serum albumin. The same biphasic stimulation was observed with phosphotyrosyl proteins immunopurified from cytosol of A431 cells previously stimulated by EGF. Since phosphotyrosyl proteins displayed PLC activity, our data suggest that soluble protein substrates of EGF receptor tyrosine kinase, including PLC, could be involved in the regulation of phosphoinositide hydrolysis in response to EGF. Using phosphatidyl[3H]inositol 4,5-bisphosphate (PIP2) dispersed with unlabelled phosphatidylethanolamine and phosphatidylserine as an exogenous substrate, no stimulation of PLC activity by EGF could be detected, either with membranes or with membranes plus cytosol. It is concluded that EGF might stimulate hydrolysis of phosphoinositides by PLC through complex interactions between plasma membrane and cytosolic factors which still remain to be identified.

Ligand-independent tyrosine phosphorylation of the receptor encoded by the c-neu oncogene

Incubation of Swiss 3T3 murine fibroblasts at low temperatures induces phosphorylation on tyrosine of a transmembrane protein of 175 kDa. This phenomenon is time and temperature dependent and reaches a maximum after 2 hr at 4 degrees C. The 175 kDa protein phosphorylated in vivo at low temperatures can be immunoprecipitated by phosphotyrosine antibodies and displays auto-kinase activity in vitro in the presence of radiolabelled ATP. This molecule was found to react with anti-peptide antibodies directed against the product of the HER2/neu proto-oncogene only when immunoprecipitated with phosphotyrosine antibodies from cold-stimulated cells. Activation of protein kinase-C by treatment of the cells with phorbol esters, bombesin or PDGF inhibits the effect of the exposure to low temperatures. Phosphorylation of p175 is not induced by treatment of the cells with the phosphatases inhibitor sodium orthovanadate. These results suggest that, at low temperatures, the tyrosine kinase associated with the putative receptor encoded by c-neu is activated by physico-chemical modifications of the plasma membrane.

Receptor regulation of phosphoinositidase C

Numerous hormones, neurotransmitters and growth factors regulate intracellular events by acting at cell surface receptors which are coupled to the generation of inositol phospholipid-derived intracellular messengers. Receptors trigger the hydrolysis of inositol phospholipids by activating phosphoinositidase C (PIC) enzymes. At least four families of genes encode structurally distinct PIC enzymes and it is likely that distinct PIC isoenzymes participate in different pathways of signal transduction. Two different modes of receptor regulation have been identified and these involve distinct PIC isoenzymes. In the first of these, PIC-gamma is a substrate for growth factor receptor protein-tyrosine kinases. The second of these pathways involves PIC-beta plus other isoenzymes whose activities are regulated by G proteins in response to agonist binding to G protein-linked receptors. At least two types of G proteins regulate PIC activity and each may control the activity of different PIC isoenzymes.

Attachment of A172 human glioblastoma cells affects calcium signalling: a comparison of image cytometry, flow cytometry, and spectrofluorometry

The intracellular free calcium concentration ([Ca2+]i) of indo-1 loaded A172 human glioblastoma cells stimulated by platelet-derived growth factor (PDGF) was studied in cell suspensions by flow cytometry and spectrofluorometry and in confluent monolayers by laser image cytometry and spectrofluorometry. With all three techniques, the percentage of responsive cells, peak [Ca2+]i, and the duration of response were directly related, and the delay time was inversely related to PDGF dose. The maximum response occurred at a PDGF concentration of about 20 ng/ml. Basal and peak [Ca2+]i did not differ significantly from method to method even though different calibration procedures were used. Cells in suspension monitored by both spectrofluorometry and flow cytometry displayed significantly shorter calcium responses than attached cells. This did not appear to be a direct effect of trypsinization. Spectral analysis of indo-1 in cytoplasm, 40% glycerol, and aqueous solutions showed significant differences in the isosbestic point and quantum efficiency. Calibration of [Ca2+]i with spectrofluorometry is more accurate using the ratio of fluorescence intensities than the fluorescence intensities measured at either 405 or 485 nm.

Growth factor-induced release of a glycosyl-phosphatidylinositol (GPI)-linked protein from the HEp-2 human carcinoma cell line

The release of a GPI-linked oncofetal protein (PLAP) from HEp-2 cells has been studied as an indicator of endogenous PI-specific PLC activation. A non-hydrolysable GTP analogue, EGF and insulin all produced a dose-dependent release of PLAP from these cells. This indicates that an increase in PLC activity following either growth factor binding or G protein activation may cause cellular release of PLAP.

Characterization of the detergent solubilized receptor for gastrin-releasing peptide

Properties of detergent solubilized gastrin-releasing peptide receptor were investigated. Swiss 3T3 membranes were covalently labeled with [125I]GRP and homobifunctional cross-linkers. A major labeled protein of 75 kDa was resolved using SDS-polyacrylamide gel electrophoresis. When the same preparation was solubilized with zwitterionic detergent and analyzed under nondenaturing conditions the protein bound radioactivity was resolved in two different peaks, a major one of apparent molecular weight 220,000 (peak 1) and a minor one of 80,000 (peak 2) both containing the 75 kDa protein. Specific ligand binding activity also eluted with peak 1. These results indicate that the active form of bombesin/GRP receptor is a large complex containing the 75 kDa ligand binding domain.

Effect of phospholipase C-gamma overexpression on PDGF-induced second messengers and mitogenesis

Platelet-derived growth factor (PDGF) stimulates phospholipase C (PLC) activity and the phosphorylation of the gamma isozyme of PLC (PLC-gamma) in vitro and in living cells. The role of PLC-gamma in the phosphoinositide signaling pathway was addressed by examining the effect of overexpression of PLC-gamma on cellular responses to PDGF. Overexpression of PLC-gamma correlated with PDGF-induced tyrosine phosphorylation of PLC-gamma and with PDGF-induced breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2). However, neither bradykinin- nor lysophosphatidic acid-induced phosphoinositide metabolism was enhanced in the transfected cells, suggesting that the G protein-coupled phosphoinositide responses to these ligands are mediated by other PLC isozymes. The enhanced PDGF-induced generation of inositol trisphosphate (IP3) did not enhance intracellular calcium signaling or influence PDGF-induced DNA synthesis. Thus, enzymes other than PLC-gamma may limit PDGF-induced calcium signaling and DNA synthesis. Alternatively, PDGF-induced calcium signaling and DNA synthesis may use biochemical pathways other than phosphoinositide metabolism for signal transduction.

Evidence for coupling of different receptors for gonadotropin-releasing hormone to phospholipases C and A2 in cultured rat luteal cells

Effects of [D-Ala6,Des-Gly10]gonadotropin-releasing hormone (GnRH), ethylamide (GnRHa), and prostaglandin F2 alpha (PGF2 alpha) on inositol phosphate (IPs) formation and arachidonic acid (AA) release were studied in rat luteal cells of primary culture. In the cells obtained from one-day-old corpora lutea, PGF2 alpha (100 nM) and GnRHa (100 nM) significantly increased the IPs formation and the AA release. Antagonists of GnRH added solely or with GnRHa did not stimulate the IPs formation but did stimulate the AA release. In the cells obtained from 5-day-old corpora lutea, GnRHa failed to stimulate the IPs formation but significantly stimulated the AA release. The stimulation of both IPs formation and AA release by PGF2 alpha was consistently found in cells of two different luteal ages. These results suggest that GnRH receptor independently couples to both phospholipases C and A2 through different classes of GnRH receptors.

Vasopressin rapidly stimulates protein kinase C in digitonin-permeabilized Swiss 3T3 cells: involvement of a pertussis toxin-insensitive guanine nucleotide binding protein

Guanine nucleotides and pertussis toxin were used to test for the involvement of a guanine nucleotide binding protein in the vasopressin V1 receptor-mediated stimulation of protein kinase C activity in Swiss 3T3 cells. Addition of vasopressin in the presence of [gamma-32P]ATP and digitonin caused a marked and rapid increase (8 +/- 1-fold after 1 min) in the phosphorylation of an Mr = 80,000 cellular protein (80K), a specific marker for protein kinase C activation. This phosphorylation was selectively blocked by the V1 receptor antagonist Pmp1-0-Me-Tyr2 [Arg8] vasopressin, indicating that the effect was mediated through the vasopressin V1 receptor. Down regulation of protein kinase C by prior prolonged pretreatment of intact cells with phorbol 12,13-dibutyrate (PBt2) blocked the ability of vasopressin to stimulate the phosphorylation of 80K in digitonin-permeabilized cells. Addition of a submaximal concentration of vasopressin together with the GTP analogue GTP-gamma-S caused a synergistic stimulation of 80K phosphorylation. The GDP analogue GDP-beta-S caused a 50% inhibition of the phosphorylation of 80K induced by a saturating concentration of vasopressin and shifted the vasopressin dose-response curve to the right. GDP-beta-S had no effect on the dose-response for the stimulation of 80K phosphorylation induced by PBt2. Prior incubation of intact quiescent cultures of Swiss 3T3 cells with pertussis toxin did not impair either vasopressin-induced increase in cytosolic [Ca2+] or activation of protein kinase C. These findings provide functional evidence for the involvement of a pertussis toxin-insesitive G protein in the vasopressin V1 receptor-mediated stimulation of protein kinase C in Swiss 3T3 cells.

Diacylglycerol, but not inositol 1,4,5-trisphosphate, accounts for platelet-derived growth factor-stimulated proliferation of BALB 3T3 cells

Recently we found that an intracellular event related to phosphatidylinositol 4,5-bisphosphate (PIP2) is crucial for platelet-derived growth factor (PDGF)-induced mitogenesis in fibroblastic cells (Matuoka, K., et al.: Science 239:640-643, 1988). In the present study we examined the mitogenic effects of PIP2 and its hydrolysis products introduced into the cytoplasm of BALB 3T3 cells by micro-injection to confirm the role of PIP2 hydrolysis in PDGF stimulation of cell proliferation. Injection of 1,2-dioleylglycerol (diolein) into serum-deprived quiescent cells induced DNA synthesis with the same time course as that induced by exposure of the cells to PDGF and, in the presence of PDGF, caused no additional increase in the cell population entering S phase. The injection of PIP2, inositol 1,4,5-trisphosphate, or 1,2-dioleylphosphatidic acid into the cells did not induce mitogenesis. Consistent results were obtained in experiments in which the cells were exposed to 1-oleyl-2-acetylglycerol (OAG) and ionomycin; namely, OAG stimulated proliferation of BALB 3T3 cells, but ionomycin did not induce any mitogenesis. Desensitization of the protein kinase C pathway by prolonged exposure of the cells to phorbol ester abolished the induction of cell proliferation by subsequent injection of diolein or exposure to phorbol ester or OAG as well as by PDGF challenge. These findings strongly suggest that activation of the protein kinase C system following formation of diacylglycerol by PIP2 hydrolysis is mainly responsible for the mitogenic action of PDGF on BALB 3T3 cells.

Hormone and growth factor receptor-mediated regulation of phospholipase C activity

The broad importance of receptor-activated phosphoinositide hydrolysis in the physiological action of hormones, neurotransmitters and growth factors has sparked interest in the study of transmembrane signalling events responsible for activation of phospholipase C. As with receptors involved in regulation of adenylyl cyclase, ion channels and phototransmission, it is clear that a guanine nucleotide regulatory protein (G protein) is a necessary component of the hormone- and neurotransmitter-regulated phosphoinositide signalling mechanism. Recent evidence to support a possible second mode of regulation of phospholipase C by growth factor receptors is emerging in the form of realization that at least one isozyme of phospholipase C serves as a substrate for the tyrosine kinase activity of growth factor receptors known to stimulate phosphoinositide hydrolysis. In this review, José Boyer and colleagues summarize progress towards delineating the properties and identity of the G protein(s) involved in this pathway, recent advances in purification and molecular cloning of phospholipase C isozymes, and the current understanding of growth factor receptor-mediated regulation of phosphoinositide hydrolysis.

Kinetics and Regulation of Tyrosine Phosphorylation of the Platelet Derived Growth Factor Receptor

Platelet derived growth factor (PDGF) interaction with the cells induces rapid tyrosine phosphorylation of the PDGF receptor in a dose dependent manner. At 37 °C phosphorylation of the receptor is followed by its dephosphorylation and internalization. It is observed that the higher the ligand concentration, the more transient is the response, and the observed kinetics are explained by a simple kinetic model. At 4 °C the phosphorylated form of the receptor is more stable; however, if PDGF is dissociated from the cell surface-associated ligand-receptor complexes, the receptors are rapidly dephosphorylated, indicating that phosphatases specific for phosphotyrosine groups are very active within the cells. In fact, addition of orthovanadate stabilizes the phosphorylated form of the receptor and helps in recognizing possible physiological substrates of the PDGF receptor kinase. The expression of PDGF receptors on the cell surface has been investigated under different growth conditions: a positive correlation exists between the amount of PDGF receptors and the duplication times of exponentially growing cultures. Moreover, during exponential growth the PDGF receptors are scarcely expressed, and their number increases reaching a maximal value when the population enters the stationary phase.