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

Alternatively Spliced Variants of Murine CD247 Influence T Cell Development and Activation, Revealing the Importance of the CD3ζ C-Terminal Region

CD247, also known as CD3ζ, is a crucial signaling molecule that transduces signals delivered by TCR through its three ITAMs. CD3ζ is required for successful thymocyte development. Three additional alternatively spliced variants of murine CD247 have been described, that is, CD3ι, CD3θ, and CD3η, that differ from CD3ζ in the C terminus such that the third ITAM is lost. Previous studies demonstrated defects in T cell development in mice expressing CD3η, but the TCR signaling pathways affected by CD3η and the impacts of the CD3ι and CD3θ on T cell development were not explored. In this study, we used a retrovirus-mediated gene transfer technique to express these three isoforms individually and examined the roles of them on T cell development and activation. Rag1-/- mice reconstituted with CD3θ-expressing bone marrow failed to develop mature T cells. CD3ι-expressing T cells exhibited similar development and activation as cells expressing CD3ζ. In contrast, thymic development was severely impaired in CD3η-reconstituted mice. Single-positive but not double-positive CD3η-expressing thymocytes had reduced TCR expression, and CD5 expression was decreased at the double-positive stage, suggesting a defect in positive selection. Peripheral CD3η-expressing T cells had expanded CD44hi populations and upregulation of exhaustion markers seen by flow cytometry and RNA sequencing analysis. Analysis of early signaling events demonstrated significantly reduced activation of both the PLCγ1 and Akt/mTOR signaling pathways. There was also a reduction in the frequency of activation of CD3η-expressing T cells. These studies reveal the importance of the CD3ζ C-terminal region in T cell development and activation.

Phospholipase C: underrated players in microbial infections

During bacterial infections, one or more virulence factors are required to support the survival, growth, and colonization of the pathogen within the host, leading to the symptomatic characteristic of the disease. The outcome of bacterial infections is determined by several factors from both host as well as pathogen origin. Proteins and enzymes involved in cellular signaling are important players in determining the outcome of host-pathogen interactions. phospholipase C (PLCs) participate in cellular signaling and regulation by virtue of their ability to hydrolyze membrane phospholipids into di-acyl-glycerol (DAG) and inositol triphosphate (IP3), which further causes the activation of other signaling pathways involved in various processes, including immune response. A total of 13 PLC isoforms are known so far, differing in their structure, regulation, and tissue-specific distribution. Different PLC isoforms have been implicated in various diseases, including cancer and infectious diseases; however, their roles in infectious diseases are not clearly understood. Many studies have suggested the prominent roles of both host and pathogen-derived PLCs during infections. PLCs have also been shown to contribute towards disease pathogenesis and the onset of disease symptoms. In this review, we have discussed the contribution of PLCs as a determinant of the outcome of host-pathogen interaction and pathogenesis during bacterial infections of human importance.

Effects of Long-Term Endurance Exercise and Lithium Treatment on Neuroprotective Factors in Hippocampus of Obese Rats

To investigate the effects of long-term lithium treatment and low intensity endurance exercise on brain-derived neurotrophic factor (BDNF) expression and glycogen synthase kinase 3 beta (GSK3β) activity in the hippocampus of obese rats. Fifty 10-week-old male Sprague-Dawley rats were selected. There was a control group of 10 rats (chow control group) while the other forty rats were fed on a high-fat diet for eight weeks to induce obesity. Rats were then assigned into four random groups. The rats were given 10 mg/kg lithium chloride (LiCl) dissolved in 1 mL sterile distilled water once a day, 5 times a week. The rats did 20 min of treadmill walking with an exercise intensity of 40% maximal oxygen uptake (VO₂ max) (12 m/min, slope 0%). This was performed for 20 min a day, 3 days a week. Twelve weeks of lithium treatment or endurance exercise significantly reduced body weight and body fat mass in obese rats, without showing additive effects when the treatments were given in parallel or significant toxic responses in alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels in blood and kidney and liver tissues. BDNF expression in the hippocampus was significantly increased both in exercise and lithium groups with synergistic effects found in the group where both exercise and lithium treatments were given in parallel. On the other hand, the decrease in GSK3β activity was shown only in the lithium treatment group, without showing additive effects when the treatments were given in parallel. Lithium and low-intensity endurance exercise for 12 weeks increased the expression of BDNF, a neuroprotective factor in the hippocampus of obese mice. Lithium treatment alone inhibited the activity of GSK3β. This can be interpreted as a positive indication of applicability of the two factors in the prevention of neurodegenerative diseases.

Comprehensive review of targeted therapy for colorectal cancer

Colorectal cancer (CRC) is among the most lethal and prevalent malignancies in the world and was responsible for nearly 881,000 cancer-related deaths in 2018. Surgery and chemotherapy have long been the first choices for cancer patients. However, the prognosis of CRC has never been satisfying, especially for patients with metastatic lesions. Targeted therapy is a new optional approach that has successfully prolonged overall survival for CRC patients. Following successes with the anti-EGFR (epidermal growth factor receptor) agent cetuximab and the anti-angiogenesis agent bevacizumab, new agents blocking different critical pathways as well as immune checkpoints are emerging at an unprecedented rate. Guidelines worldwide are currently updating the recommended targeted drugs on the basis of the increasing number of high-quality clinical trials. This review provides an overview of existing CRC-targeted agents and their underlying mechanisms, as well as a discussion of their limitations and future trends.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

PDGF signaling pathway in hepatic fibrosis pathogenesis and therapeutics (Review)

The platelet‑derived growth factor (PDFG) signaling pathway exerts persistent activation in response to a variety of stimuli and facilitates the progression of hepatic fibrosis. Since this pathway modulates a broad spectrum of cellular processes, including cell growth, differentiation, inflammation and carcinogenesis, it has emerged as a therapeutic target for hepatic fibrosis and liver‑associated disorders. The present review exhibits the current knowledge of the role of the PDGF signaling pathway and its pathological profiles in hepatic fibrosis, and assesses the potential of inhibitors which have been investigated in the experimental hepatic fibrosis model, in addition to the clinical challenges associated with these inhibitors.

ErbB Receptors and Cancer

The ErbB receptor family, also known as the EGF receptor family or type I receptor family, includes the epidermal growth factor (EGF) receptor (EGFR) or ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. Among all RTKs, EGFR was the first RTK identified and the first one linked to cancer. Thus, EGFR has also been the most intensively studied among all RTKs. ErbB receptors are activated after homodimerization or heterodimerization. The ErbB family is unique among the various groups of receptor tyrosine kinases (RTKs) in that ErbB3 has impaired kinase activity, while ErbB2 does not have a direct ligand. Therefore, heterodimerization is an important mechanism that allows the activation of all ErbB receptors in response to ligand stimulation. The activated ErbB receptors bind to many signaling proteins and stimulate the activation of many signaling pathways. The specificity and potency of intracellular signaling pathways are determined by positive and negative regulators, the specific composition of activating ligand(s), receptor dimer components, and the diverse range of proteins that associate with the tyrosine phosphorylated C-terminal domain of the ErbB receptors. ErbB receptors are overexpressed or mutated in many cancers, especially in breast cancer, ovarian cancer, and non-small cell lung cancer. The overexpression and overactivation of ErbB receptors are correlated with poor prognosis, drug resistance, cancer metastasis, and lower survival rate. ErbB receptors, especially EGFR and ErbB2 have been the primary choices as targets for developing cancer therapies.

Magnesium-Dependent Ecto-ATP Diphosphohydrolase Activity in Leishmania donovani

In this work, we have described the expression of ecto-ATPDase on the external surface of Leishmania donovani. This enzyme has the ability to hydrolyze extracellular ATP. There is a low level of ATP hydrolysis in the absence of divalent cation 2.5 ± 0.51 nM Pi 10⁷ cells/h which shows the divalent cation-dependent activity of this enzyme in the intact parasite. However, MgCl₂ stimulated the ATP hydrolysis to a greater extent compared with CaCl₂ and ZnCl₂. This activity was also observed when replaced by MnCl₂. The Mg-dependent ecto-ATPase activity was 46.58 ± 6.248 nM Pi 10⁷ cells/h. The apparent K _m for ATP was 5.76 mM. Since Leishmania also possesses acid phosphatase activity and to discard the possibility that the observed ATP hydrolysis was due to acid phosphatase, the effect of pH was examined. In the pH range 6.0-9.0, in which the cells were viable, the phosphatase activity decreased while ATPase activity increased. To show that the observed ATP hydrolysis was not due to phosphatase or nucleotidase activity, certain inhibitors for these enzymes were tested. Vandate and NaF inhibited the phosphatase activity; Ammonium molybdate inhibited 5'-nucleotidase activity, but these inhibitors did not inhibit the observed ATP hydrolysis. However, when ADP was used as a substrate, there was no inhibition of ATP hydrolysis showing the possibility of ATP diphosphohydrolase activity. To confirm that this Mg-dependent ATPase activity is an ecto-ATPase activity, we used an impermeable inhibitor, 4,4'-diisothiocyanostilbene 2,-2'-disulfonic acid, as well as suramin, an antagonist of P2-purinoceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg²⁺-dependent ATPase activity in a dose-dependent manner. The presence of L. donovani E-NTPDase activity was demonstrated using antibodies against NTPDase by Western blotting and flow cytometry. The presence of Mg²⁺-dependent ATP diphosphohydrolase activity on the surface of L. donovani modulates the nucleotide concentration and protects the parasite from the lytic effects of the nucleotides mainly ATP. Ecto-ATPDase from L. donovani may be further characterized as a good antigen and as a target for immunodiagnosis and drug development, respectively.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

PLC-gamma1 and Rac1 coregulate EGF-induced cytoskeleton remodeling and cell migration

It is well established that epidermal growth factor (EGF) induces the cytoskeleton reorganization and cell migration through two major signaling cascades: phospholipase C-gamma1 (PLC-gamma1) and Rho GTPases. However, little is known about the cross talk between PLC-gamma1 and Rho GTPases. Here we showed that PLC-gamma1 forms a complex with Rac1 in response to EGF. This interaction is direct and mediated by PLC-gamma1 Src homology 3 (SH3) domain and Rac1 (106)PNTP(109) motif. This interaction is critical for EGF-induced Rac1 activation in vivo, and PLC-gamma1 SH3 domain is actually a potent and specific Rac1 guanine nucleotide exchange factor in vitro. We have also demonstrated that the interaction between PLC-gamma1 SH3 domain and Rac1 play a significant role in EGF-induced F-actin formation and cell migration. We conclude that PLC-gamma1 and Rac1 coregulate EGF-induced cell cytoskeleton remodeling and cell migration by a direct functional interaction.

Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding

Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H(+) efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics.

Trypanosoma brucei brucei: biochemical characterization of ecto-nucleoside triphosphate diphosphohydrolase activities

In this work we describe the ability of living cells of Trypanosoma brucei brucei to hydrolyze extracellular ATP. In these intact parasites there was a low level of ATP hydrolysis in the absence of any divalent metal (4.72+/-0.51 nmol Pi x 10(-7) cells x h(-1)). The ATP hydrolysis was stimulated by MgCl(2) and the Mg-dependent ecto-ATPase activity was 27.15+/-2.91 nmol Pi x 10(-7) cells x h(-1). This stimulatory activity was also observed when MgCl(2) was replaced by MnCl(2). CaCl(2) and ZnCl(2) were also able to stimulate the ATPase activity, although less than MgCl(2). The apparent K(m) for ATP was 0.61 mM. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities. To confirm that this Mg-dependent ATPase activity is an ecto-ATPase activity, we used an impermeable inhibitor, DIDS (4, 4'-diisothiocyanostylbene 2'-2'-disulfonic acid), as well as suramin, an antagonist of P(2) purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. Living cells sequentially hydrolyzed the ATP molecule generating ADP, AMP and adenosine, and supplementation of the culture medium with ATP was able to sustain the proliferation of T. brucei brucei as well as adenosine supplementation. Furthermore, the E-NTPDase activity of T. brucei brucei is modulated by the availability of purines in the medium. These results indicate that this surface enzyme may play a role in the salvage of purines from the extracellular medium in T. brucei brucei.

Akt binds to and phosphorylates phospholipase C-gamma1 in response to epidermal growth factor

Both phospholipase (PL) C-gamma1 and Akt (protein kinase B; PKB) are signaling proteins that play significant roles in the intracellular signaling mechanism used by receptor tyrosine kinases, including epidermal growth factor (EGF) receptor (EGFR). EGFR activates PLC-gamma1 directly and activates Akt indirectly through phosphatidylinositol 3-kinase (PI3K). Many studies have shown that the PLC-gamma1 pathway and PI3K-Akt pathway interact with each other. However, it is not known whether PLC-gamma1 binds to Akt directly. In this communication, we identified a novel interaction between PLC-gamma1 and Akt. We demonstrated that the interaction is mediated by the binding of PLC-gamma1 Src homology (SH) 3 domain to Akt proline-rich motifs. We also provide a novel model to depict how the interaction between PLC-gamma1 SH3 domain and Akt proline-rich motifs is dependent on EGF stimulation. In this model, phosphorylation of PLC-gamma1 Y783 by EGF causes the conformational change of PLC-gamma1 to allow the interaction of its SH3 domain with Akt proline-rich motifs. Furthermore, we showed that the interaction between PLC-gamma1 and Akt resulted in the phosphorylation of PLC-gamma1 S1248 by Akt. Finally, we showed that the interaction between PLC-gamma1 and Akt enhanced EGF-stimulated cell motility.

Recruitment and activation of phospholipase Cgamma1 by vascular endothelial growth factor receptor-2 are required for tubulogenesis and differentiation of endothelial cells

Vascular endothelial growth factor-mediated angiogenic signal transduction relay is achieved by coordinated induction of endothelial cell proliferation, migration, and differentiation. These complex cellular processes are most likely controlled by activation of both cooperative and antagonistic signals by vascular endothelial growth factor receptors (VEGFRs). Here, we investigated the contribution of tyrosine-phosphorylated residues of VEGFR-2/fetal liver kinase-1 to endothelial cell proliferation and differentiation and activation of signaling proteins. Mutation of tyrosine 1006 of VEGFR-2 to phenylalanine severely impaired the ability of this receptor to stimulate endothelial cell differentiation and tubulogenesis. Paradoxically, the mutant receptor stimulated endothelial cell proliferation far better than the wild-type receptor. Further analysis showed that tyrosine 1006 is responsible for phospholipase Cgamma1 (PLCgamma1) activation and intracellular calcium release in endothelial cells. Activation of PLCgamma1 was selectively mediated by tyrosine 1006. Mutation of tyrosines 799, 820, 949, 994, 1080, 1173, and 1221 had no measurable effect on the ability of VEGFR-2 to stimulate PLCgamma1 activation. Association of VEGFR-2 with PLCgamma1 was mainly established between tyrosine 1006 and the C-terminal SH2 domain of PLCgamma1 in vitro and in vivo. Taken together, the results indicate that phosphorylation of tyrosine 1006 is essential for VEGFR-2-mediated PLCgamma1 activation, calcium flux, and cell differentiation. More importantly, VEGFR-2-mediated endothelial cell proliferation is inversely correlated with the ability of VEGFR-2 to associate with and activate PLCgamma1.

AP180 binds to the C-terminal SH2 domain of phospholipase C-gamma1 and inhibits its enzymatic activity

The role of phospholipase Cgamma1 (PLCgamma1) in signal transduction was investigated by characterizing its SH domain-binding proteins that may represent components of a novel signaling pathway. A 180-kDa protein that binds to the SH2 domain of PLCgamma1 was purified from rat brain. The amino acid sequence of peptide derived from the purified protein is now identified as AP180, a clathrin assembly protein that has been implicated in clathrin-mediated synaptic vesicle recycling in synapses. In this report, we demonstrate the stable association of PLCgamma1 with AP180 in a clathrin-coated vesicle complex, which not only binds to the carboxyl-terminal SH2 domain of PLCgamma1, but also inhibits its enzymatic activity in a dose-dependent manner.

Brain-derived neurotrophic factor triggers a rapid glutamate release through increase of intracellular Ca(2+) and Na(+) in cultured cerebellar neurons

We reported previously that BDNF induced glutamate release was dependent on intracellular Ca(2+) but not extracellular Ca(2+) in cerebellar neurons (Numakawa et al., 1999). It was revealed that the release was through a non-exocytotic pathway (Takei et al., 1998; Numakawa et al., 1999). In the present study, we monitored the dynamics of intracellular Ca(2+) and Na(+) in cerebellar neurons, and investigated the possibility of reverse transport of glutamate mediated by BDNF. As reported, BDNF increased the intracellular Ca(2+) level. We found that the Ca(2+) increase induced by BDNF was completely blocked by xestospongin C, an IP(3) receptor antagonist, and U-73122, a PLC-gamma inhibitor. Xestospongin C and U-73122 also blocked the BDNF-dependent glutamate release, suggesting that the BDNF-induced transient increase of Ca(2+) through the activation of the PLC-gamma/ IP(3) pathway was essential for the glutamate release. We found that BDNF induced a Na(+) influx. This was blocked by treatment with TTX. U-73122 and xestospongin C blocked the BDNF-induced Na(+) influx, suggesting that the Na(+)influx required the BDNF-induced Ca(2+) increase. Next, we examined the possibility that a co-transporter of Na(+) and glutamate was involved in the BDNF-induced glutamate release. BDNF-induced glutamate release was blocked by L-trans-pyrollidine-2,4-dicalboxylic acid (t-PDC), a glutamate transporter inhibitor, whereas neither the 4-aminopyridine (4AP)- nor high potassium (HK(+))-induced release was blocked by t-PDC. In addition, DL-threo-beta-benzyloxyaspartate (DL-TBOA) also blocked the BDNF-mediated glutamate release, suggesting that reverse transport of glutamate may be involved. All the results therefore suggest that Na(+)-dependent reverse transport contributes to BDNF-mediated transmitter release through the PLC-gamma/IP(3)-mediated Ca(2+) signaling.

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.

Enhanced phosphoinositide hydrolysis via overexpression of phospholipase C beta1 or delta1 inhibits stimulus-induced insulin release in insulinoma MIN6 cells

To study the effects of enhanced phosphoinositide hydrolysis on insulin secretion, phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) or PLCdelta1 was overexpressed in insulinoma MIN6 cells via adenoviral vectors. Inositol phosphate production stimulated by NaF (with AlCl3) in PLCbeta1-overexpressing cells and that stimulated by KCl or glucose in both PLCbeta1- and PLCdelta1-overexpressing cells were greater than that in control cells. In addition, reduced phosphatidylinositol-4,5-bisphosphate levels were observed in these cells stimulated by NaF or KCl. The greater phosphoinositide hydrolysis was accompanied by 25-45% inhibition of insulin secretion. These data suggest that excessive phosphoinositide hydrolysis inhibits secretagogue-induced insulin release in MIN6 cells.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Interaction of phospholipase C gamma 1 via its COOH-terminal SRC homology 2 domain with synaptojanin

The role of the phospholipase C gamma 1 (PLC gamma 1) in signal transduction was investigated by characterizing its interactions with proteins that may represent components of a novel signalling pathway. A 145-kDa protein that binds SH2 domain of PLC gamma 1 was purified from rat brain. The sequence of peptide derived from the purified binding protein now identify it as synaptojanin, a nerve terminal protein that has been implicated in the endocytosis of fused synaptic vesicles and shown to be a member of the inositol polyphosphate 5-phosphatase family. We demonstrate here stable association of PLC gamma 1 with synaptojanin, a protein that not only binds carboxyl terminal SH2 domain of PLC gamma 1, but also inhibits PLC gamma 1 activity.

Mutations at residues Tyr771 and Tyr783 of phospholipase C-gamma1 have different effects on cell actin-cytoskeleton organization and cell proliferation in CCL-39 cells

Currently a central question that remains unresolved is the potential importance of phospholipase C (PLC)-gamma1 in cell mitogenesis. In this study, we introduced wild-type PLC-gamma1 plasmid and mutants Y771F and Y783F into CCL-39 fibroblasts and investigated their effect on host cell functions. To our surprise, Y771F and Y783F plasmids appeared to have opposite effects on CCL-39 cell actin-cytoskeleton organization and cell proliferation. Y771F transfectants increased cell proliferation by two-fold. Y783F transfectants showed much thicker actin filaments and decreased cell growth rate by 50%. These results suggest that PLC-gamma1 mutations have an essential impact on cell mitogenesis.

Requirement for phospholipase C-gamma1 enzymatic activity in growth factor-induced mitogenesis

The cytoplasmic regions of the receptors for epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) bind and activate phospholipase C-gamma1 (PLC-gamma1) and other signaling proteins in response to ligand binding outside the cell. Receptor binding by PLC-gamma1 is a function of its SH2 domains and is required for growth factor-induced cell cycle progression into the S phase. Microinjection into MDCK epithelial cells and NIH 3T3 fibroblasts of a polypeptide corresponding to the noncatalytic SH2-SH2-SH3 domains of PLC-gamma1 (PLC-gamma1 SH2-SH2-SH3) blocked growth factor-induced S-phase entry. Treatment of cells with diacylglycerol (DAG) or DAG and microinjected inositol-1,4,5-triphosphate (IP3), the products of activated PLC-gamma1, did not stimulate cellular DNA synthesis by themselves but did suppress the inhibitory effects of the PLC-gamma1 SH2-SH2-SH3 polypeptide but not the cell cycle block imposed by inhibition of the adapter protein Grb2 or p21 Ras. Two c-fos serum response element (SRE)-chloramphenicol acetyltransferase (CAT) reporter plasmids, a wild-type version, wtSRE-CAT, and a mutant, pm18, were used to investigate the function of PLC-gamma1 in EGF- and PDGF-induced mitogenesis. wtSRE-CAT responds to both protein kinase C (PKC)-dependent and -independent signals, while the mutant, pm18, responds only to PKC-independent signals. Microinjection of the dominant-negative PLC-gamma1 SH2-SH2-SH3 polypeptide greatly reduced the responses of wtSRE-CAT to EGF stimulation in MDCK cells and to PDGF stimulation in NIH 3T3 cells but had no effect on the responses of mutant pm18. These results indicate that in addition to Grb2-mediated activation of Ras, PLC-gamma1-mediated DAG production is required for EGF- and PDGF-induced S-phase entry and gene expression, possibly through activation of PKC.

Dissociation of tyrosine phosphorylation and activation of phosphoinositide phospholipase C induced by the protein kinase C inhibitor Ro-31-8220 in Swiss 3T3 cells treated with platelet-derived growth factor

Platelet-derived growth factor (PDGF) stimulates the hydrolysis of phosphatidylinositol 4,5-bisphosphate (Ptd InsP2) via phospholipase C-gamma1 (PLC-gamma1) in Swiss 3T3 cells. Treatment of cells with the protein kinase C (PKC) inhibitor Ro-31-8220 greatly decreased PDGF-induced tyrosine phosphorylation of PLC-gamma1, but paradoxically enhanced the production of inositol phosphates (InsPs). The inhibitor also caused an increase of PDGF receptor tyrosine phosphorylation at later times. The changes in phosphorylation of the receptor were correlated with alterations in PLC-gamma1 translocation to the particulate fraction. Thus, although activation of PLC-gamma1 was associated with phosphorylation of the receptor and translocation of the enzyme to the particulate fraction, it was dissociated from its tyrosine phosphorylation. A non-receptor-associated, cytosolic tyrosine kinase also was found to phosphorylate PLC-gamma1 in a PDGF-dependent manner, but was not inhibited by Ro-31-8220 in vitro. PKC depletion by phorbol ester treatment decreased the tyrosine phosphorylation of PLC-gamma1 induced by PDGF and slowed the translocation of PLC-gamma1, but Ro-31-8220 produced further effects. The effect of Ro-31-8220 to enhance the production of InsPs could not be attributed to inhibition of PKC since InsPs production with PDGF was decreased in PKC-depleted cells and a stimulatory effect of the inhibitor was still evident. Interestingly, Ro-31-8220 decreased the radioactivity in phosphatidylinositol and increased that in phosphatidylinositol 4-phosphate and PtdInsP2 in cells labeled with myo[3H]inositol. The increased synthesis of PtdInsP2 could contribute to the increased production of InsPs induced by Ro-31-8220. In summary, these results support the conclusion that the activation of PLC-gamma1 in response to PDGF requires autophosphorylation of the receptor and membrane association of PLC-gamma1, but not phosphorylation of the enzyme. Furthermore, the effects of Ro-31-8220 on the tyrosine phosphorylation and activity of PLC-gamma1, and on PtdInsP2 synthesis cannot be attributed to inhibition of PKC.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Calcineurin is essential for DNA synthesis in Swiss 3T3 fibroblasts

DNA synthesis was measured 16 h after stimulation of Swiss 3T3 fibroblasts in the resting phase with various growth factors (platelet-derived growth factor, fibroblast growth factor, lysophosphatidic acid and thrombin). When extracellular Ca2+ was chelated by EGTA, or when the influx of Ca2+ from outside to inside the cell was blocked by cobalt, DNA synthesis was completely inhibited. As there was no effect whatsoever on DNA synthesis when Ca2+ was chelated, or when the influx of Ca2+ was blocked up to the first 4 h after growth stimulation, it was concluded that, at an early stage, Ca2+ influx from outside to inside the cell is not related to the transition from the G1 to the S phase. A Ca2+/calmodulin-dependent protein kinase II inhibitor (KN-62) had no effect on DNA synthesis. However, cyclosporin A and FK-506, which are inhibitors of Ca2+/calmodulin-dependent protein phosphatase 2B (calcineurin), markedly inhibited DNA synthesis stimulated by all of the growth factors. These results indicate that calcineurin plays a role, not only in activation of T-cells of the immune system in the initial phase, but also in DNA synthesis in fibroblasts. It was concluded that Ca2+ influx from outside to inside the cell during the mid-to-late G1 phase, followed by calcineurin activation, is essential as a mechanism of growth signal transduction.

Mitogenic signaling from the egf receptor is attenuated by a phospholipase C-gamma/protein kinase C feedback mechanism

We recently demonstrated that epidermal growth factor receptor (EGFR)-mediated signaling of cell motility and mitogenesis diverge at the immediate post-receptor level. How these two mutually exclusive cell responses cross-communicate is not known. We investigated a possible role for a phospholipase C (PLC)-dependent feedback mechanism that attenuates EGF-induced mitogenesis. Inhibition of PLC gamma activation by U73122 (1 microM) augmented the EGF-induced [3H]thymidine incorporation by 23-55% in two transduced NR6 fibroblast lines expressing motility-responsive EGFR; increased cell division and mitosis was observed in parallel. The time dependence of this increase revealed that it was due to an increase in maximal incorporation and not a foreshortened cell cycle. Motility-responsive cell lines expressing a dominant-negative PLC gamma fragment (PLCz) also demonstrated augmented mitogenic responses by 25-68% when compared with control cells. PLCz- or U73122-augmented mitogenesis was not observed in three non-PLC gamma activating, nonmotility-responsive EGFR-expressing cell lines. Protein kinase C (PKC), which may be activated by PLC-generated second messengers, has been proposed as mediating feedback attenuation due to its capacity to phosphorylate EGFR and inhibit the receptor's tyrosine kinase activity. Inhibition of PKC by Calphostin C (0.05 microM) resulted in a 57% augmentation in the fold of EGF-induced thymidine incorporation. To further establish PKC's role in this feedback attenuation mechanism, an EGFR point mutation, in which the PKC target threonine654 was replaced by alanine, was expressed. Cells expressing these PKC-resistant EGFR constructs demonstrated EGF-induced motility comparable to cells expressing the threonine-containing EGFR. However, when these cells were treated with U73122 or Calphostin C, the mitogenic responses are not enhanced. These findings suggest a model in which PKC activation subsequent to triggering of motility-associated PLC gamma activity attenuates the EGFR mitogenic response.

Diversity of roles of protein kinase C alpha in the proliferation of Swiss 3T3 cells

We examined the role of protein kinase C alpha (PKC alpha ) in the stimulation of DNA synthesis of Swiss 3T3 cells induced by bombesin, platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA). We found that cells in which this kinase had been down-regulated showed a partially abrogated mitogenic response to bombesin. The response to PDGF was unaltered; however, the response to PMA was completely suppressed. The mitogenic effect of maximal doses of bombesin and PMA combined was greater than that of either agent alone, suggesting that bombesin does not fully activate the PKC pathway. Accordingly, bombesin-induced PKC alpha translocation from cytosol to membranes was partial, while that observed with PMA was essentially complete. Moreover, exposure to Ro-31-8220, a PKC inhibitor, had significantly greater effects on the response to PMA than on that to bombesin. Our findings point out different roles that PKC alpha may play in diversely activated cells: while, in the case of PMA, stimulation of this kinase may be necessary and sufficient to induce proliferation, it appears to be necessary only for a full response to bombesin, and redundant among the mechanisms triggered by PDGF.

Identification of six novel autophosphorylation sites on fibroblast growth factor receptor 1 and elucidation of their importance in receptor activation and signal transduction

Fibroblast growth factor receptor (FGFR) activation leads to receptor autophosphorylation and increased tyrosine phosphorylation of several intra cellular proteins. We have previously shown that autophosphorylated tyrosine 766 in FGFR1 serves as a binding site for one of the SH2 domains of phospholipase Cy and couples FGFR1 to phosphatidylinositol hydrolysis in several cell types. In this report, we describe the identification of six additional autophosphorylation sites (Y-463, Y-583, Y-585, Y-653, Y-654 and Y-730) on FGFR1. We demonstrate that autophosphorylation on tyrosines 653 and 654 is important for activation of tyrosine kinase activity of FGFR1 and is therefore essential for FGFR1-mediated biological responses. In contrast, autophosphorylation of the remaining four tyrosines is dispensable for FGFR1-mediated mitogen-activated protein kinase activation and mitogenic signaling in L-6 cells as well as neuronal differentiation of PC12 cells. Interestingly, both the wild-type and a mutant FGFR1 (FGFR1-4F) are able to phosphorylate Shc and an unidentified Grb2-associated phosphoprotein of 90 kDa (pp90). Binding of the Grb2/Sos complex to phosphorylated Shc and pp90 may therefore be the key link between FGFR1 and the Ras signaling pathway, mito-genesis, and neuronal differentiation.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the components of a nuclear inositide cycle during differentiation in murine erythroleukaemia cells

Differentiation of murine erythroleukaemia cells with the chemical agent DMSO leads to a cessation of proliferation and the production of a number of erythrocyte markers such as haemoglobin. We have previously demonstrated that activation of proliferation leads to an increase in the production of nuclear diacylglycerol (DAG). Here we demonstrate that differentiation leads to a decrease in the levels of nuclear DAG and the activity of the nuclear-associated phosphoinositidase C (PIC). The change in activity appears to be due to a decrease in the mass levels of the beta 1 isoform, as demonstrated by the use of isoform-specific antibodies. Moreover, the changes correlate with the cessation of proliferation and an increase in the number of cells in G1 phase of the cell cycle, rather than with the number of cells which have differentiated. Indeed, although treatment of the cells with phorbol 12-myristate 13-acetate (PMA) inhibits the differentiation programme as assessed by haemoglobin staining, it does not inhibit the number of cells blocking in G1 of the cell cycle or the changes in nuclear DAG or PIC activity. The possible involvement of this nuclear inositide cycle during progression through the cell cycle is discussed.

Electroporation of pp60c-src antibodies inhibits the angiotensin II activation of phospholipase C-gamma 1 in rat aortic smooth muscle cells

Our previous study has shown that angiotensin II induces the rapid tyrosine phosphorylation and activation of phospholipase C-gamma 1 in cultured rat aortic smooth muscle (RASM) cells (Marrero, M.B., Paxton, W.G., Duff, J. L., Berk, B. C., and Bernstein, K. E. (1994) J. Biol. Chem, 269, 10935-10939). This signaling pathway is initiated by ligand binding to the AT1 receptor, a cell surface G protein-coupled receptor. Antibodies to pp60c-src were introduced into RASM cells by electroporation. Angiotensin II-stimulated tyrosine phosphorylation of phospholipase C-gamma 1 was eliminated by the anti-pp60c-src antibodies but not by anti-mouse IgG or bovine serum albumin. Angiotensin II also induced the rapid tyrosine phosphorylation of pp120, a known pp60c-src kinase substrate, and this phosphorylation was also specifically inhibited by anti-pp60c-src antibodies. Electroporation of RASM cells with anti-pp60c-src antibodies had no effect on platelet-derived growth factor-stimulated tyrosine phosphorylation of PLC-gamma 1. Anti-pp60c-src also reduced the angiotensin II-stimulated inositol 1,4,5-trisphosphate production by 78%, while it had no effect on the platelet-derived growth factor-stimulated inositol 1,4,5-trisphosphate production. These data provide the first evidence for a direct involvement of pp60c-src kinase in angiotensin II-mediated PLC-gamma 1 phosphorylation and activation. Furthermore, it also describes a pathway in which a seven-transmembrane receptor can stimulate an intracellular tyrosine kinase.

Effects of CapG overexpression on agonist-induced motility and second messenger generation

Actin modulating proteins that bind polyphosphoinositides, such as phosphatidylinositol 4, 5-bisphosphate (PIP2), can potentially participate in receptor signaling by restructuring the membrane cytoskeleton and modulating second messenger generation through the phosphoinositide cycle. We examined these possibilities by overexpressing CapG, an actin filament end capping, Ca(2+)- and polyphosphoinositide-binding protein of the gelsolin family. High level transient overexpression decreased actin filament staining in the center of the cells but not in the cell periphery. Moderate overexpression in clonally selected cell lines did not have a detectible effect on actin filament content or organization. Nevertheless, it promoted a dose-dependent increase in rates of wound healing and chemotaxis. The motile phenotype was similar to that observed with gelsolin overexpression, which in addition to capping, also severs and nucleates actin filaments. CapG overexpressing clones are more responsive to platelet-derived growth factor than control-transfected clones. They form more circular dorsal membrane ruffles, have higher phosphoinositide turnover, inositol 1,4,5-trisphosphate generation and Ca2+ signaling. These responses are consistent with enhanced PLC gamma activity. Direct measurements of PIP2 mass showed that the CapG effect on PLC gamma was not due primarily to an increase in the PIP2 substrate concentration. The observed changes in cell motility and membrane signaling are consistent with the hypothesis that PIP(2)-binding actin regulatory proteins modulate phosphoinositide turnover and second messenger generation in vivo. We infer that CapG and related proteins are poised to coordinate membrane signaling with actin filament dynamics following cell stimulation.

Activation of phospholipase C gamma in Schizosaccharomyces pombe by coexpression of receptor or nonreceptor tyrosine kinases

The fission yeast Schizosaccharomyces pombe has no detectable endogenous receptor tyrosine kinases or associated signalling apparatus, and we have used this cell system to reconstitute mammalian platelet-derived growth factor beta (PDGF beta) receptor-linked activation of phospholipase C gamma 2 (PLC gamma 2). The PDGF beta receptor migrates as a glycosylated protein of 165 kDa associated exclusively with membrane fractions. No tyrosine autophosphorylation was detected when PDGF beta was expressed alone. PLC gamma 2 appears as a 140-kDa protein distributed between particulate and soluble fractions which exhibits characteristic selectivity for phosphatidylinositol 4,5-bisphosphate and is sensitive to powerful activation by Ca2+. When coexpressed, both PDGF beta and PLC gamma 2 undergo tyrosine phosphorylation, and this is accompanied by a > 26-fold increase in [3H]inositol 4,5-biphosphate ([3H]IP2) and [3H]inositol 1,4,5-triphosphate [3H]IP3 production. Treatment with the tyrosine phosphatase inhibitor pervanadate further increased PLC gamma 2 tyrosine phosphorylation as well as [3H]IP2 and [3H]IP3 generation. Phosphorylated PLC gamma 2 was found predominantly in membrane fractions. To test a nonreceptor tyrosine kinase, we then expressed the human proto-oncogene c-src together with its negative regulator Csk. These were immunodetectable as bands at 60 kDa (c-Src) and 50 kDa (Csk) and distributed between membrane and cytosolic fractions. When yeast coexpressing c-Src, Csk, and PLC gamma 2 was incubated with pervanadate, PLC gamma 2 was tyrosine phosphorylated and [3H]IP2 and [3H]IP3 production increased 11.0- and 7.0-fold, respectively. Csk expressed alone with PLC gamma 2 was ineffective. Similar PLC gamma 2 activation was observed upon in vitro mixing with the extracts expressing either c-Src or the PDGF beta receptor. In summary, this is the first report of a reconstitution of mammalian tyrosine kinase-linked effector activation in yeast cells and also the first demonstration of direct PLC gamma 2 activation by the proto-oncogene c-src. These observations indicate that S. pombe provides a powerful cell system in which to study critical molecular interactions and activities underlying receptor and nonreceptor tyrosine kinase-dependent cell signaling.

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.

PDGF-BB triggered cytoplasmic calcium responses in cells with endogenous or stably transfected PDGF beta-receptors

Platelet-derived growth factor-BB (PDGF-BB) triggered signal transduction was investigated in human foreskin fibroblasts with endogenous PDGF beta-receptors, and porcine aortic endothelial (PAE) cells with stably transfected PDGF beta-receptors. Immunoprecipitation and immunoblotting showed that PDGF induced dose-dependent autophosphorylation of PDGF beta-receptor, and the PLC-gamma associates with autophosphorylated PDGF beta-receptors and becomes phosphorylated. Activation of PLC-gamma is known to induce fluctuations of the concentration of cytoplasmic calcium ([Ca2+]i). Microfluorometry and digital imaging were employed for measurements of the concentration of [Ca2+]i. In both cell types the growth factor induced four types of [Ca2+]i responses; no rise, a small and sluggish monophasic rise, a biphasic rise with an initial transient peak followed by a sustain elevation, and finally regular oscillations. The frequencies and amplitudes of the oscillatory responses were independent of agonist concentration after stimulation with PDGF-BB. Latency, the period from application of stimulus to the first [Ca2+]i peak, was reduced at higher concentrations of agonist. Also, the proportion of responding cells increased with higher concentrations of ligand. Oscillations of [Ca2+]i were elicited at submaximal concentrations of agonist. In PAE cells PDGF-BB triggered a single [Ca2+]i peak in absence of external Ca2+. Ligand-induced oscillations and sustained increases of [Ca2+]i were counteracted by the inorganic Ca2+ channel blocker Ce3+. These results show that similar types of [Ca2+]i responses occur in different cell types independently of whether the PDGF beta-receptors are expressed endogeneously or after transfection. Potentially, the different [Ca2+]i responses have distinct physiological consequences.

EGF receptor-mediated signals are differentially modulated by concanavalin A

NIH 3T3 cells expressing high levels of the human epidermal growth factor (EGF) receptor were used to examine the effects of the lectin concanavalin A (Con A) on EGF-mediated signaling events. Proliferation of NIH 3T3 cells expressing high levels of the human EGF receptor was inhibited in a dose-dependent manner by Con A. At the same time, Con A also inhibited both dimerization and tyrosine phosphorylation of the EGF receptor. Tyrosine phosphorylation of the enzyme phospholipase C-gamma, a substrate of the phosphorylated EGF receptor kinase, was also inhibited. In contrast, EGF-stimulated changes in pH, calcium, and levels of inositol phosphates were unaffected by the presence of Con A. These results indicate that certain signals (changes in the levels of intracellular calcium, pH, and inositol phosphates) mediated by EGF binding to its receptor still occur when receptor dimerization and phosphorylation are dramatically decreased, suggesting that multiple independent signals are transmitted by the binding of EGF to its receptor.

Tyrosine kinase inhibition prevents deformation-stimulated vascular smooth muscle growth

The goal of this study was to determine the role of tyrosine phosphorylation in transducing deformation-stimulated vascular smooth muscle growth. Rat aorta-derived vascular smooth muscle cells were cultured on flexible silicone elastomer membranes and subjected to cyclic deformation (15 cycles per minute, deformed 2 seconds, relaxed 2 seconds). Deformation significantly increased proto-oncogene expression, [3H]thymidine incorporation, [3H]leucine incorporation, and cell number. Time course studies showed an 8-hour lag between initiation of cell deformation and onset of [3H]thymidine incorporation, with peak levels achieved after 18 to 24 hours. Western analysis of protein blots from deformed cells (10 minutes) demonstrated increased levels of phosphotyrosine-containing proteins having molecular weights of 110 to 130 and 70 to 80 kD. Deformation-stimulated tyrosine phosphorylation was prevented by the tyrosine kinase inhibitor Herbimycin A. Tyrosine kinase inhibition also prevented deformation-stimulated vascular smooth muscle cell growth as measured by [3H]thymidine incorporation. Cyclic deformation stimulates vascular smooth muscle proliferation through activation of tyrosine kinases. Inhibition of tyrosine phosphorylation is an effective means of preventing deformation-induced vascular smooth muscle growth in vitro.

A rapid bioassay for platelet-derived growth factor beta-receptor tyrosine kinase function

We have extended a melanophore-based bioassay for G-protein coupled receptors to include the functional expression of the murine platelet-derived growth factor (PDGF) beta-receptor. The homodimeric ligand PDGF-BB induced activation of the transiently expressed receptor in melanophore cells. This led to dose dependent pigment dispersion whereas it did not induce pigment dispersion in wild type cells. The effective concentration of PDGF-BB giving half-maximal pigment dispersion (EC50) was 1nM after 30 minutes exposure. PDGF-AA had no ability to induce pigment dispersion in melanophore cells transiently expressing the beta-PDGF receptor. PDGF-BB-induced pigment dispersion could be blocked by the bis-indolylmaleimide Ro 31-8220 which is an inhibitor of protein kinase C isoenzymes. Functional expression of the PDGF beta-receptor extends the use of the pigment translocation assay to include transmembrane signaling receptor tyrosine kinases. It opens the opportunity for the discovery of potent agonists and antagonists through massive drug screening and investigations of functional ligand-receptor interactions for single transmembrane domain receptors.

A protein that is highly related to GTPase-activating protein-associated p62 complexes with phospholipase C gamma

p62 is a highly tyrosyl phosphorylated protein that was first identified in immunoprecipitates of the GTPase-activating protein (GAP) of p21ras from cells transformed by oncogenic nonreceptor tyrosine kinases or stimulated through tyrosine kinase receptors (C. Ellis, M. Moran, F. McCormick, and T. Pawson, Nature 343:377-381, 1991). In this article we describe a highly related 62-kDa protein that becomes tyrosyl phosphorylated and associated with phospholipase C gamma (PLC gamma) in C3H10T1/2 cells stimulated with epidermal growth factor (EGF) or transformed by v-src. GAP-associated and PLC gamma-associated p62 comigrated in one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exhibited nearly identical phosphotryptic peptide patterns. That the association of p62 with PLC gamma was direct and not mediated through binding of GAP-p62 to PLC gamma or to the EGF receptor (and coprecipitation of the receptor with PLC gamma) was demonstrated by (i) the inability to detect GAP in PLC gamma immunocomplexes or PLC gamma in GAP immunocomplexes, (ii) the association of p62 with PLC gamma in v-src-transformed cells in the absence of EGF stimulation, and (iii) in vitro solution binding and direct blotting of p62 with a glutathione S-transferase fusion protein containing the Src homology 2 (SH2) domains of PLC gamma. Unlike GAP, whose N-terminal SH2 mediates the interaction between GAP and p62, PLC gamma was found to require both its N- and C-terminal SH2 regions for p62 binding. These studies demonstrate that a protein identical to or highly related to GAP-associated p62 binds PLC gamma and suggest a means by which "cross-talk" between PLC gamma- and GAP-mediated signalling may occur.

A protein that is highly related to GTPase-activating protein-associated p62 complexes with phospholipase C gamma

p62 is a highly tyrosyl phosphorylated protein that was first identified in immunoprecipitates of the GTPase-activating protein (GAP) of p21ras from cells transformed by oncogenic nonreceptor tyrosine kinases or stimulated through tyrosine kinase receptors (C. Ellis, M. Moran, F. McCormick, and T. Pawson, Nature 343:377-381, 1991). In this article we describe a highly related 62-kDa protein that becomes tyrosyl phosphorylated and associated with phospholipase C gamma (PLC gamma) in C3H10T1/2 cells stimulated with epidermal growth factor (EGF) or transformed by v-src. GAP-associated and PLC gamma-associated p62 comigrated in one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exhibited nearly identical phosphotryptic peptide patterns. That the association of p62 with PLC gamma was direct and not mediated through binding of GAP-p62 to PLC gamma or to the EGF receptor (and coprecipitation of the receptor with PLC gamma) was demonstrated by (i) the inability to detect GAP in PLC gamma immunocomplexes or PLC gamma in GAP immunocomplexes, (ii) the association of p62 with PLC gamma in v-src-transformed cells in the absence of EGF stimulation, and (iii) in vitro solution binding and direct blotting of p62 with a glutathione S-transferase fusion protein containing the Src homology 2 (SH2) domains of PLC gamma. Unlike GAP, whose N-terminal SH2 mediates the interaction between GAP and p62, PLC gamma was found to require both its N- and C-terminal SH2 regions for p62 binding. These studies demonstrate that a protein identical to or highly related to GAP-associated p62 binds PLC gamma and suggest a means by which "cross-talk" between PLC gamma- and GAP-mediated signalling may occur.