Stimulation of phospholipase C-gamma 1 membrane association by epidermal growth factor

Targeting Protein Kinase C Downstream of Growth Factor and Adhesion Signalling

The signaling outputs of Receptor Tyrosine Kinases, G-protein coupled receptors and integrins converge to mediate key cell process such as cell adhesion, cell migration, cell invasion and cell proliferation. Once activated by their ligands, these cell surface proteins recruit and direct a diverse range of proteins to disseminate the appropriate response downstream of the specific environmental cues. One of the key groups of proteins required to regulate these activities is the family of serine/threonine intracellular kinases called Protein Kinase Cs. The activity and subcellular location of PKCs are mediated by a series of tightly regulated events and is dependent on several posttranslational modifications and the availability of second messengers. Protein Kinase Cs exhibit both pro- and anti-tumorigenic effects making them an interesting target for anti-cancer treatment.

Interplay between cell migration and neurite outgrowth determines SH2B1β-enhanced neurite regeneration of differentiated PC12 cells

The regulation of neurite outgrowth is crucial in developing strategies to promote neurite regeneration after nerve injury and in degenerative diseases. In this study, we demonstrate that overexpression of an adaptor/scaffolding protein SH2B1β promotes neurite re-growth of differentiated PC12 cells, an established neuronal model, using wound healing (scraping) assays. Cell migration and the subsequent remodeling are crucial determinants during neurite regeneration. We provide evidence suggesting that overexpressing SH2B1β enhances protein kinase C (PKC)-dependent cell migration and phosphatidylinositol 3-kinase (PI3K)-AKT-, mitogen activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) kinase (MEK)-ERK-dependent neurite re-growth. Our results further reveal a cross-talk between pathways involving PKC and ERK1/2 in regulating neurite re-growth and cell migration. We conclude that temporal regulation of cell migration and neurite outgrowth by SH2B1β contributes to the enhanced regeneration of differentiated PC12 cells.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Phospholipase C is a key enzyme regulating intracellular calcium and modulating the phosphoinositide balance

Spatial and temporal activation of phosphoinositide turnover enables eukaryotic cells to perform various functions such as cell proliferation/differentiation, fertilization, neuronal functions, and cell motility. In this system, phospholipase C (PLC) is a key enzyme, which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) into two second messengers, inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and diacylglycerol (DAG). Ins(1,4,5)P(3) triggers the release of calcium from intracellular stores, and DAG mediates the activation of protein kinase C (PKC). In parallel, PI(4,5)P(2) also directly regulates a variety of cellular functions, including cytoskeletal remodeling, cytokinesis, phagocytosis, membrane dynamics, and channel activity, in addition to its role as a substrate for PLC and phosphatidylinositol 3-kinase (PI3K), which generates PI(3,4,5)P(3). An imbalance of these phosphoinositides contributes to the pathogeneses of various human diseases. Therefore, strict regulation of the levels of PI(4,5)P(2) and PI(3,4,5)P(3) by PLC or other interconverting enzymes is necessary for cellular functions. In this review, we focus on the roles of PLC as a calcium-regulating enzyme and as a modulator of the phosphoinositide balance.

Phospholipase C-gamma1 is required for the epidermal growth factor receptor-induced squamous cell carcinoma cell mitogenesis

The epidermal growth factor receptor (EGFR) is a key driver in the process of squamous cell carcinoma (SCC) cell mitogenesis. Phospholipase C-gamma1 (PLC-gamma1) is a downstream target of EGFR signaling, but the role and necessity of PLC-gamma1 in EGFR-induced cell mitogenesis remain unclear. In the present study, we report an elevated expression of PLC-gamma1 in human SCC biopsies relative to adjacent normal epidermis, and in human SCC cell lines compared to normal human keratinocytes. EGFR-induced SCC cell mitogenesis was blocked by small interfering RNA knockdown of PLC-gamma1. However, inhibition of the catalytic activity of phospholipase C had no effect on EGFR-induced SCC cell mitogenesis. In response to the EGFR ligand epidermal growth factor (EGF), PLC-gamma1 was translocated not only to the plasma membrane but also to the nucleus. These data suggest that PLC-gamma1 is required for EGFR-induced SCC cell mitogenesis and the mitogenic function of PLC-gamma1 is independent of its lipase activity.

PLC-gamma1 regulates fibronectin assembly and cell aggregation

Phospholipase C-gamma1 (PLC-gamma1) mediates cell adhesion and migration through an undefined mechanism. Here, we examine the role of PLC-gamma1 in cell-matrix adhesion in a hanging drop assay of cell aggregation. Plcg1 Null (-/-) mouse embryonic fibroblasts formed aggregates that were larger and significantly more resistant to dissociation than cells in which PLC-gamma1 is re-expressed (Null+ cells). Aggregate formation could be disrupted by inhibition of fibronectin interaction with integrins, indicating that fibronectin assembly may mediate aggregate formation. Fibronectin assembly was mediated by integrin alpha5beta1 in both cell lines, while assays measuring fibronectin assembly revealed increased assembly in the Null cells. Null and Null+ cells exhibited equivalent fibronectin mRNA levels and equivalent levels of fibronectin protein in pulse-labeling experiments. However, levels of secreted fibronectin in the conditioned medium were increased in Null cells. The data implicates a negative regulatory role for PLC-gamma1 in cell aggregation by controlling the secretion of fibronectin into the media and its assembly into fibrils.

Intramolecular regulation of phospholipase C-gamma1 by its C-terminal Src homology 2 domain

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) is a key enzyme that governs cellular functions such as gene transcription, secretion, proliferation, motility, and development. Here, we show that PLC-gamma1 is regulated via a novel autoinhibitory mechanism involving its carboxy-terminal Src homology (SH2C) domain. Mutation of the SH2C domain tyrosine binding site led to constitutive PLC-gamma1 activation. The amino-terminal split pleckstrin homology (sPHN) domain was found to regulate the accessibility of the SH2C domain. PLC-gamma1 constructs with mutations in tyrosine 509 and phenylalanine 510 in the sPHN domain no longer required an intact amino-terminal Src homology (SH2N) domain or phosphorylation of tyrosine 775 or 783 for activation. These data are consistent with a model in which the SH2C domain is blocked by an intramolecular interaction(s) that is released upon cellular activation by occupancy of the SH2N domain.

PKC isozymes and diacylglycerol-regulated proteins as effectors of growth factor receptors

Growth factors exert their cellular effects through signal transduction pathways that are initiated by the ligation of growth factors to their cell surface receptors. One of the well-established effectors of growth factor receptors is protein kinase C (PKC), a family of serine-threonine kinases that have been known for years as the main target of the phorbol ester tumor promoters. While there is abundant information regarding downstream PKC effectors and partners, how individual PKC isozymes become activated by growth factors and the regulation of receptor function by PKCs is only partially understood. Moreover, the identification of novel "non-kinase" DAG-binding proteins has added a new level of complexity to the field of DAG signaling.

Re-distribution of phospholipase C gamma 2 in macrophage precursors is mediated by the actin cytoskeleton under the control of the Src kinases

Macrophage colony-stimulating factor (M-CSF) is a growth factor that is known to trigger several signalling pathways through receptor tyrosine kinase activation. We investigated the specific requirements for the activation of phospholipase C gamma 2 (PLC-gamma2) during the differentiation of mouse bone marrow-derived macrophage precursors. M-CSF stimulation induced rapid PLC-gamma2 translocation and phosphorylation from the cytosolic compartment to the cell periphery. Both events were dependent on cytoskeleton integrity and Src kinase activity, but only PLC-gamma2 phosphorylation did not require PI3-kinase activity. Biochemical experiments as well as confocal microscopy analyses indicate that the translocation of PLC-gamma2 is mediated by the direct association of this protein with the actin cytoskeleton. Using GST-fusion proteins containing various deletions of the PLC-gamma2 Src homology region, it was found that PLC-gamma2 binds to F-actin via its SH2 domains, a feature that has equally been found in a co-sedimentation assay. This association, which is increased during actin reorganisation and disrupted by cytoskeleton inhibitors, seems to be a primary means to recruit this enzyme to the cell periphery. These results indicate that, upon M-CSF stimulation, PLC-gamma2 cellular localisation and phosphorylation are strongly dependent on cytoskeleton architecture of the macrophage precursor as well as the PI3-kinase and the Src kinases.

Agonist-induced interactions between angiotensin AT1 and epidermal growth factor receptors

In rat hepatic C9 cells, angiotensin II (Ang II)-induced activation of angiotensin type 1 (AT(1)) receptors (AT(1)-Rs) stimulates extracellular signal-regulated kinase (ERK) 1/2 phosphorylation via transactivation of the endogenous epidermal growth factor (EGF) receptor (EGF-R) by a protein kinase C (PKC) delta/Src/Pyk2-dependent pathway. This leads to phosphorylation of the EGF-R as well as its subsequent internalization. On the other hand, EGF-induced activation of the EGF-R in C9 cells was found to cause phosphorylation of the AT(1)-R. This was prevented by selective inhibition of the intrinsic tyrosine kinase activity of the EGF-R by AG1478 [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline] and was reduced by inhibition of PKC and phosphoinositide 3-kinase. EGF-induced AT(1)-R phosphorylation was associated with a decrease in membrane-associated AT(1)-Rs and a reduced inositol phosphate response to Ang II. Agonist activation of endogenous AT(1)-Rs and EGF-Rs induced the formation of a multireceptor complex containing both the AT(1)-R and the transactivated EGF-R. The dependence of these responses on caveolin was indicated by the finding that cholesterol depletion of C9 cells abolished Ang II-induced inositol phosphate production, activation of Akt/PKB and ERK1/2, and AT(1)-R internalization. Confocal microscopy demonstrated that caveolin-1 was endogenously phosphorylated and was distributed on the plasma membrane in patches that undergo redistribution during Ang II stimulation. Agonist-induced phosphorylation and association of caveolin 1 with the AT(1)-R was observed, consistent with a scaffolding role of caveolin during transactivation of the EGF-R by Ang II. The EGF-induced AT(1)-R/caveolin association was abolished by AG1478, suggesting that activation of the EGF-R promotes the association of caveolin and the AT(1)-R.

Double feature at the signalplex

In this issue of Molecular Pharmacology, Oliveras-Reyes et al. (p. 356) describe the agonist-stimulated formation of a caveolin-dependent signalplex that includes both the angiotensin AT(1) receptor and the epidermal growth factor receptor, and probably also a number of other signal transduction intermediates. The signalplex is thought to facilitate the action of protein kinases that mediate angiotensin II-induced transactivation of the epidermal growth factor receptor and activation of extracellular signal-regulated kinase, and epidermal growth factor-induced inositol phosphate accumulation and phosphorylation/desensitization of the AT(1) receptor. This work contributes to an emerging view of the complexity and nonlinearity of signaling via G protein-coupled receptors and receptor tyrosine kinases, and of the importance of membrane compartmentalization to signal transduction.

Phospholipase C, protein kinase C, Ca2+/calmodulin-dependent protein kinase II, and redox state are involved in epigallocatechin gallate-induced phospholipase D activation in human astroglioma cells

We show that epigallocatechin-3 gallate (EGCG), a major component of green tea, stimulates phospholipase D (PLD) activity in U87 human astroglioma cells. EGCG-induced PLD activation was abolished by the phospholipase C (PLC) inhibitor and a lipase inactive PLC-gamma1 mutant, which is dependent on intracellular or extracellular Ca(2+), with the possible involvement of Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). EGCG induced translocation of PLC-gamma1 from the cytosol to the membrane and PLC-gamma1 interaction with PLD1. EGCG regulates the activity of PLD by modulating the redox state of the cells, and antioxidants reverse this effect. Moreover, EGCG-induced PLD activation was reduced by PKC inhibitors or down-regulation of PKC. Taken together, these results show that, in human astroglioma cells, EGCG regulates PLD activity via a signaling pathway involving changes in the redox state that stimulates a PLC-gamma1 [Ins(1,4,5)P(3)-Ca(2+)]-CaM kinase II-PLD pathway and a PLC-gamma1 (diacylglycerol)-PKC-PLD pathway.

Roles of PLC-γ2 and PKCα in TPA-induced apoptosis of gastric cancer cells

AIM: To investigate the roles of PLCγ2 and PKCα in TPA-induced apoptosis of gastric cancer cells.

METHODS: Human gastric cancer cell line MGC80-3 was used. Protein expression levels of PLCγ2 and PKCα were detected by Western blot. Protein localization of PLCγ2 and PKCα was shown by immunofluoscence analysis under laser-scanning confocal microscope. Apoptotic morphology was observed by DAPI fluorescence staining, and apoptotic index was counted among 1000 cells randomly.

RESULTS: Treatment of gastric cancer cells MGC80-3 with TPA not only up-regulated expression of PLC-γ2 protein, but also induced PLC-γ2 translocation from the cytoplasm to the nucleus. However, this process was not directly associated with apoptosis induction. Further investigation showed that PKCα translocation from the cytoplasm to the nucleus was correlated with initiation of apoptosis. To explore the inevitable linkage between PLC-γ2 and PKCα during apoptosis induction, PLC inhibitor U73122 was used to block PLC-γ2 translocation, in which neither stimulating PKCα translocation nor inducing apoptosis occurred in MGC80-3 cells. However, when U73122-treated cells were exposed to TPA, not only PLC-γ2, but also PKCα was redistributed. On the other hand, when cells were treated with PKC inhibitor alone, PLC-γ2 protein was still located in the cytoplasm. However, redistribution of PLC-γ2 protein occurred in the presence of TPA, no matter whether PKC inhibitor existed or not.

CONCLUSION: PLC-γ2 translocation is critical in transmitting TPA signal to its downstream molecule PKCα. As an effector, PKCα directly promotes apoptosis of MGC80-3 cells. Therefore, protein translocation of PLCγ2 and PKCα is critical event in the process of apoptosis induction.

Regulation of EGF-induced phospholipase C-gamma1 translocation and activation by its SH2 and PH domains

Translocation of phospholipase C-gamma1 is essential for its function in response to growth factors. However, in spite of recent progress, the phospholipase C-gamma1 translocation pattern and the molecular mechanism of the translocation are far from fully understood. Contradictory results were reported as to which domain, PH or SH2, controls the epidermal growth factor-induced translocation of phospholipase C-gamma1. In this communication, we studied epidermal growth factor-induced translocation of phospholipase C-gamma1 by using comprehensive approaches including biochemistry, indirect fluorescence and live fluorescence imaging. We provided original evidence demonstrating that: (i) endogenous phospholipase C-gamma1, similar to YFP-tagged phospholipase C-gamma1, translocated to endosomes following its initial translocation from cytosol to the plasma membrane in response to epidermal growth factor; (ii) phospholipase C-gamma1 remained phosphorylated in endosomes, but phospholipase C-gamma1 activity is not required for its translocation, which suggests a signaling role for phospholipase C-gamma1 in endosomes; (iii) the PH domain was not required for the initial translocation of phospholipase C-gamma1 from cytosol to the plasma membrane, but it stabilizes phospholipase C-gamma1 in the membrane at a later time; (iv) the function of the phospholipase C-gamma1 PH domain in stabilizing phospholipase C-gamma1 membrane association is very important in maintaining the activity of phospholipase C-gamma1; and (v) the role of the PH domain in phospholipase C-gamma1 membrane association and activation is dependent on PI3K activity. We conclude that the phospholipase C-gamma1 SH2 and PH domains coordinate to determine epidermal growth factor-induced translocation and activation of phospholipase C-gamma1.

Directional motility induced by epidermal growth factor requires Cdc42

Cell motility is actuated by a host of intracellular signaling cascades that result in movement of the cell in one direction, even without an external gradient. Phospholipase C-gamma (PLCgamma) has been shown to be important for growth factor-induced lamellipodial protrusion at the front of the cell while Cdc42 has been implicated in both filopodium formation at the leading edge and control of polarity of migrating cells. We asked whether these asymmetries in effector molecules may be linked. When we overexpressed either constitutively active, dominant negative, or GFP-tagged Cdc42, wild-type NR6 fibroblasts lost directionality, as expected. On epidermal growth factor (EGF) exposure these cells produced multiple, transient protrusions in every direction; these extensions failed to result in productive motility. GFP-tagged Cdc42 appeared transiently at edges of newly formed protrusions in EGF-stimulated cells while they moved haphazardly. While PLCgamma is distributed throughout the cell, the ratio of active, tyrosyl-phosphorylated PLCgamma was increased at the leading edge, where phosphatidylinositol (4,5)-bisphosphate (PIP(2)) hydrolysis is concentrated. This co-localization of activities may be due to Cdc42 directing PLCgamma to the cell front, as PLCgamma associated with Cdc42 in an EGF-dependent manner. We conclude that Cdc42 controls cell polarity, likely in part, through its binding to active PLCgamma.

Cell-specific and developmental expression of phospholipase C-gamma and diacylglycerol in fetal lung

Epidermal growth factor (EGF) receptor (EGFR) regulates development of cell-cell communication in fetal lung, but the signal transduction mechanisms involved are unknown. We hypothesized that, in late-gestation fetal rat lung, phospholipase C-gamma (PLC-gamma) expression and activation by EGF is cell specific and developmentally regulated. PLC-gamma immunolocalized to cuboidal epithelium and mesenchymal clusters underlying developing saccules. PLC-gamma protein increased from day 17 to day 19 and then decreased. In cultured fetal lung fibroblasts, EGF stimulated PLC-gamma phosphorylation 2.6-fold (day 17), 10.8-fold (day 19), and 4.2-fold (day 21). EGF stimulated (3)H-labeled diacylglycerol production in fibroblasts (beginning on day 18 in female and on day 19 in male rats), but not in type II cells at any time during gestation. EGFR blockade abrogated the observed stimulation of PLC-gamma phosphorylation by EGF. In conclusion, PLC-gamma expression and activation by EGF in fetal lung are cell specific, corresponding to the development of EGFR expression. EGF induces diacylglycerol production in a cell- and gestation-specific manner. PLC-gamma activation by EGFR in fetal lung fibroblasts may be involved in EGF control of lung development.

70Z/3 Cbl induces PLC gamma 1 activation in T lymphocytes via an alternate Lat- and Slp-76-independent signaling mechanism

The oncoprotein 70Z/3 Cbl signals in an autonomous fashion or through blockade of endogenous c-Cbl, a negative regulator of signaling. The mechanism of 70Z/3 Cbl-induced signaling was investigated by comparing the molecular requirements for 70Z/3 Cbl- and TCR-induced phospholipase C gamma 1 (PLC gamma 1) activation. 70Z/3 Cbl-induced PLC gamma 1 tyrosine phosphorylation required, in addition to the PLC gamma 1 N-terminal SH2 domain, the C-terminal SH2 and SH3 domains that were dispensable for TCR-induced phosphorylation. Deletion of the leucine zipper of 70Z/3 Cbl did not eliminate 70Z/3 Cbl-induced PLC gamma 1 phosphorylation, suggesting that blockage of c-Cbl via dimerization with 70Z/3 Cbl cannot fully explain 70Z/3 Cbl activating characteristics. The complete elimination of PLC gamma 1 phosphorylation required deleting the SH3 domain-binding region of 70Z/3 Cbl, consistent with 70Z/3 Cbl binding the PLC gamma 1 SH3 domain. 70Z/3 Cbl-induced PLC gamma 1 phosphorylation required Zap-70, as for the TCR, and the tyrosine kinase binding domain of 70Z/3 Cbl, which binds Zap-70, but did not require PLC gamma 1 binding to Lat, a crucial interaction in TCR-induced PLC gamma 1 phosphorylation. Furthermore, 70Z/3 Cbl-induced activation of NFAT, a PLC gamma 1/Ca(2+)-dependent transcriptional event, required Zap-70, but was independent of Slp-76, an adapter required for TCR-induced NFAT activation. These results suggest that 70Z/3 Cbl and PLC gamma 1 form a TCR-, Lat- and Slp-76-independent complex that leads to PLC gamma 1 phosphorylation and activation.

Overexpression of kidney phosphatidylinositol 4-kinasebeta and phospholipase C(gamma1) proteins in two rodent models of polycystic kidney disease

Our studies of renal phosphoinositide levels and metabolism in the pcy mouse with polycystic kidney disease (PKD) suggest that phosphatidylinositol kinase (PtdInsK) and phospholipase C (PLC) are elevated in this renal disorder. Therefore, the steady-state levels of select isoforms of these enzymes were examined in renal cytosolic and particulate (detergent-soluble) fractions in male and female normal and CD1-pcy/pcy (pcy) mice at 60, 120 and 180 days of age, and in male and female normal and diseased (Han:SPRD-cy) rats at 28 and 70 days of age. Disease-related increases in phosphatidylinositol 4-kinasebeta (PtdIns4Kbeta) and PLC(gamma1) levels were present in both models. PtdIns4Kbeta levels were higher by as much as 233% in pcy mice and by 95% in diseased Han:SPRD-cy rats compared to normals of the same age and gender. Steady-state levels of PLC(gamma1) were as much as 74% and 35% higher in pcy mice and diseased Han:SPRD-cy rats, respectively, compared to their controls. The consistency of these alterations in two accepted models of PKD indicates the importance of the phosphoinositide signalling pathway in the evolution of this disorder, and represents a potential site for therapeutic intervention.

Nongenomic action of 1 alpha,25(OH)(2)-vitamin D3. Activation of muscle cell PLC gamma through the tyrosine kinase c-Src and PtdIns 3-kinase

We have previously demonstrated that the steroid hormone 1 alpha,25(OH)(2)-vitamin D(3)[1 alpha,25(OH)(2)D(3)] stimulates the production of inositol trisphosphate (InsP(3)), the breakdown product of phosphatidylinositol 4,5-biphosphate (PtdInsP(2)) by phospholipase C (PtdIns-PLC), and activates the cytosolic tyrosine kinase c-Src in skeletal muscle cells. In the present study we examined whether 1 alpha,25(OH)(2)D(3) induces the phosphorylation and membrane translocation of PLC gamma and the mechanism involved in this isozyme activation. We found that the steroid hormone triggers a significant phosphorylation on tyrosine residues of PLC gamma and induces a rapid increase in membrane-associated PLC gamma immunoreactivity with a time course that correlates with that of phosphorylation in muscle cells. Genistein, a tyrosine kinase inhibitor, blocked the phosphorylation of PLC gamma. Inhibition of 1 alpha,25(OH)(2)D(3)-induced c-Src activity by its specific inhibitor PP1 or muscle cell transfection with an antisense oligodeoxynucleotide directed against c-Src mRNA, prevented hormone stimulation of PLC gamma tyrosine phosphorylation. The isozyme phosphorylation is also blocked by both wortmannin and LY294002, two structurally different inhibitors of phosphatidyl inositol 3-kinase (PtdIns3K), the enzyme that produces PtdInsP(3) known to activate PLC gamma isozymes specifically by interacting with their SH2 and pleckstrin homology domains. The hormone also increases the physical association of c-Src and PtdIns3K with PLC gamma and induces a c-Src-dependent tyrosine phosphorylation of the p85 regulatory subunit of PtdIns3K. The time course of hormone-dependent PLC gamma phosphorylation closely correlates with the time course of its redistribution to the membrane, suggesting that phosphorylation and redistribution to the membrane of PLC gamma are two interdependent events. 1 alpha,25(OH)(2)D(3)-induced membrane translocation of PLC gamma was prevented to a great extent by c-Src and PtdIns3K inhibitors, PP1 and LY294002. Taken together, the present data indicates that the cytosolic tyrosine kinase c-Src and PtdIns 3-kinase play indispensable roles in 1 alpha,25(OH)(2)D(3) signal transduction cascades leading to PLC gamma activation.

EGF-dependent translocation of green fluorescent protein-tagged PLC-gamma1 to the plasma membrane and endosomes

Growth factor-dependent translocation of phospholipase C-gamma1 (PLC-gamma1) was investigated using a green fluorescent protein-tagged PLC-gamma1 (PLC-gamma1-GFP) expressed in human epidermoid carcinoma A-431 cells. In the absence of growth factors, PLC-gamma1-GFP was present throughout the cytoplasm of A-431 cells. Treatment of the cells with epidermal growth factor (EGF) produced a very rapid redistribution of PLC-gamma1-GFP to the plasma membrane in a nonuniform manner. This translocation to the plasma membrane was insensitive to an inhibitor of phosphatidylinositol 3-kinase and was independent of cell adhesion. However, the translocation was disrupted by an agent which depolymerizes the actin cytoskeleton. At later times following the addition of EGF, PLC-gamma1-GFP appeared associated with intracellular vesicles. Stimulation of A-431 cells by Texas red-conjugated EGF for more than 10 min resulted in punctate intracellular PLC-gamma1-GFP distribution that colocalized with Texas red-conjugated EGF. This suggests that PLC-gamma1 is translocated to endosomes after EGF treatment, probably by associating with the internalized and autophosphorylated EGF receptor. Fractionation studies demonstrated that the EGF-induced plasma membrane-localized PLC-gamma1 is concentrated in caveolae microdomains. Disruption of caveolae with methyl-beta-cyclodextrin resulted in the ablation of EGF-induced, but not bradykinin-induced, mobilization of intracellular Ca(2+). This treatment, however, only partially decreased PLC-gamma1 membrane translocation.

Real time fluorescence imaging of PLC gamma translocation and its interaction with the epidermal growth factor receptor

The translocation of fluorescently tagged PLC gamma and requirements for this process in cells stimulated with EGF were analyzed using real time fluorescence microscopy applied for the first time to monitor growth factor receptor--effector interactions. The translocation of PLC gamma to the plasma membrane required the functional Src homology 2 domains and was not affected by mutations in the pleckstrin homology domain or inhibition of phosphatidylinositol (PI) 3-kinase. An array of domains specific for PLC gamma isoforms was sufficient for this translocation. The dynamics of translocation to the plasma membrane and redistribution of PLC gamma, relative to localization of the EGF receptor and PI 4,5-biphosphate (PI 4,5-P(2)), were shown. Colocalization with the receptor was observed in the plasma membrane and in membrane ruffles where PI 4,5-P(2) substrate could also be visualized. At later times, internalization of PLC gamma, which could lead to separation from the substrate, was observed. The data support a direct binding of PLC gamma to the receptor as the main site of the plasma membrane recruitment. The presence of PLC gamma in membrane structures and its access to the substrate appear to be transient and are followed by a rapid incorporation into intracellular vesicles, leading to downregulation of the PLC activity.

12-lipoxygenase is increased in glucose-stimulated mesangial cells and in experimental diabetic nephropathy

BACKGROUND

Arachidonic acid-derived 12-lipoxygenase (12-LO) products have potent growth and chemotactic properties. The present studies examined whether 12-LO and fibronectin are induced in cultured rat mesangial cells (MCs) exposed to high glucose and whether they are expressed in experimental diabetic nephropathy.

METHODS

To determine the effect of high glucose on MC 12-LO mRNA and protein expression, rat MCs were incubated with RPMI medium containing 100 (NG) or 450 mg/dL glucose (HG). For animal studies, rats were injected with diluent (control) or streptozotocin. The latter were left untreated (DM) or treated with insulin (DM + I). At sacrifice after four months, GAPDH, 12-LO, and fibronectin mRNA were measured by competitive reverse transcription-polymerase chain reaction (RT-PCR) in microdissected glomeruli (G). Renal sections were semiquantitatively scored (0 to 4+) for diabetic changes and for 12-LO and fibronectin by immunohistochemistry.

RESULTS

12-LO mRNA expression in MC exposed to HG (12.71 +/- 1.17 attm/microL) and DM G (1.78 +/- 0.65 x 10-3 attm/glomerulus) was significantly higher than those of MCs in NG media (6.71 +/- 0.78 attm/microL) and control G (0.34 +/- 0.12 x 10-3 attm/glomerulus, P < 0.005), respectively. Western blot revealed a 1.7- and a 2.8-fold increase in MC and G 12-LO protein expression, respectively (P < 0.05). The immunohistochemistry score for G 12-LO and diabetic nephropathy score was significantly greater in DM and DM + I than controls. MC and G GAPDH mRNA remained unchanged.

CONCLUSIONS

In MCs exposed to HG and in diabetic rat glomeruli, increments in 12-LO mRNA and protein are associated with changes modeling diabetic nephropathy. These findings suggest a role for the 12-LO pathway in the pathogenesis of diabetic nephropathy.

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.

Characterization of the tyrosine kinase Tnk1 and its binding with phospholipase C-gamma1

Tnk1 is a nonreceptor tyrosine kinase cloned from CD34+/Lin-/CD38- hematopoietic stem/progenitor cells. The cDNA predicts a 72-kDa protein containing an NH(2)-terminal kinase, a Src Homology 3 (SH3) domain, and a proline-rich (PR) tail. We generated rabbit antiserum to a GST-Tnk1(SH3) fusion protein. Affinity-purified anti-Tnk1 antibodies specifically recognized a 72-kDa protein in Tnk1-transfected COS-1 cells and cells which express Tnk1 mRNA. Western blot analysis indicated that Tnk1 is expressed in fetal blood cells, but not in any other hematopoietic tissues examined. Tnk1 immunoprecipitated from cell lysates possessed kinase activity and was tyrosine phosphorylated. In binding experiments with a panel of GST-fusion constructs, only GST-PLC-gamma1(SH3) interacted with in vitro translated Tnk1. GST-protein precipitations from cell lysates confirmed that GST-PLC-gamma1(SH3) associated with endogenously expressed Tnk1. Conversely, GST-Tnk1(PR) protein constructs complexed with endogenously expressed PLC-gamma1. The association of Tnk1 with PLC-gamma1 suggests a role for Tnk1 in phospholipid signal transduction.

Differential roles of the Src homology 2 domains of phospholipase C-gamma1 (PLC-gamma1) in platelet-derived growth factor-induced activation of PLC-gamma1 in intact cells

Upon stimulation of cells with platelet-derived growth factor (PDGF), phospholipase C-gamma1 (PLC-gamma1) binds to the tyrosine-phosphorylated PDGF receptor through one or both of its Src homology 2 (SH2) domains, is phosphorylated by the receptor kinase, and is thereby activated to hydrolyze phosphatidylinositol 4, 5-bisphosphate. Association of PLC-gamma1 with the insoluble subcellular fraction is also enhanced in PDGF-stimulated cells. The individual roles of the two SH2 domains of PLC-gamma1 in mediating the interaction between the enzyme and the PDGF receptor have now been investigated by functionally disabling each domain. A critical Arg residue in each SH2 domain was mutated to Ala. Both wild-type and mutant PLC-gamma1 proteins were transiently expressed in a PLC-gamma1-deficient fibroblast cell line, and these transfected cells were stimulated with PDGF. The mutant protein in which the COOH-terminal SH2 domain was disabled bound to the PDGF receptor. Accordingly, it was phosphorylated by the receptor, catalyzed the production of inositol phosphates, and mobilized intracellular calcium to extents similar to (but slightly less than) those observed with the wild-type enzyme. In contrast, the mutant in which the NH(2)-terminal SH2 domain was impaired did not bind to the PDGF receptor and consequently was neither phosphorylated nor activated. These results suggest that the NH(2)-terminal SH2 domain, but not the COOH-terminal SH2 domain, of PLC-gamma1 is required for PDGF-induced activation of PLC-gamma1. Functional impairment of the SH2 domains did not affect the PDGF-induced redistribution of PLC-gamma1, suggesting that recruitment of PLC-gamma1 to the particulate fraction does not involve the SH2 domains.

Phospholipase C-gamma as a signal-transducing element

A ubiquitous signaling event in hormonal responses is the phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4, 5-bisphosphate to produce the metabolite second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol. The former provokes a transient increase in intracellular free Ca(2+), while the latter serves as a direct activator of protein kinase C. In tyrosine kinase-dependent signaling pathways this reaction is mediated by the PLC-gamma isozymes. These are direct substrates of many tyrosine kinases in a wide variety of cell types. The mechanism of PLC-gamma activation involves its association with and phosphorylation by receptor and non-receptor tyrosine kinases, as well as interaction with specialized adaptor molecules and, perhaps, other second messenger molecules. However, the biochemistry of PLC-gamma is at a more advanced state than a clear understanding of exactly how this signaling element functions in the generation of a mitogenic response.

Functional independence and interdependence of the Src homology domains of phospholipase C-gamma1 in B-cell receptor signal transduction

B-cell receptor (BCR)-induced activation of phospholipase C-gamma1 (PLCgamma1) and PLCgamma2 is crucial for B-cell function. While several signaling molecules have been implicated in PLCgamma activation, the mechanism coupling PLCgamma to the BCR remains undefined. The role of PLCgamma1 SH2 and SH3 domains at different steps of BCR-induced PLCgamma1 activation was examined by reconstitution in a PLCgamma-negative B-cell line. PLCgamma1 membrane translocation required a functional SH2 N-terminal [SH2(N)] domain, was decreased by mutation of the SH3 domain, but was unaffected by mutation of the SH2(C) domain. Tyrosine phosphorylation did not require the SH2(C) or SH3 domains but depended exclusively on a functional SH2(N) domain, which mediated the association of PLCgamma1 with the adapter protein, BLNK. Forcing PLCgamma1 to the membrane via a myristoylation signal did not bypass the SH2(N) domain requirement for phosphorylation, indicating that the phosphorylation mediated by this domain is not due to membrane anchoring alone. Mutation of the SH2(N) or the SH2(C) domain abrogated BCR-stimulated phosphoinositide hydrolysis and signaling events, while mutation of the SH3 domain partially decreased signaling. PLCgamma1 SH domains, therefore, have interrelated but distinct roles in BCR-induced PLCgamma1 activation.

Involvement of protein serine and threonine phosphorylation in human sperm capacitation

The involvement of serine and threonine phosphorylation in human sperm capacitation was investigated. Anti-phosphoserine monoclonal antibody (mAb) recognized six protein bands in the 43-55-kDa, 94 +/- 2-kDa, 110-kDa, and 190-kDa molecular regions, in addition to a faint band each in the 18-kDa and 35-kDa regions. Anti-phosphothreonine mAb recognized protein bands in six similar regions, except that the 18-kDa, 35-kDa, and 94 +/- 2-kDa protein bands were sharper and thicker, and an additional band was observed in the 110-kDa molecular region. In the 43-55-kDa molecular region, there was a well-characterized glycoprotein, designated fertilization antigen, that showed a further increase in serine/threonine phosphorylation after exposure to solubilized human zona pellucida. In a cell-free in vitro kinase assay carried out on beads or in solution, four to eight proteins belonging to similar molecular regions, namely 20 +/- 2 kDa, 43-55 kDa, 94 +/- 2 kDa, and 110 +/- 10 kDa, as well as in 80 +/- 4 and 210 +/- 10 kDa regions, were phosphorylated at dual residues (serine/tyrosine and threonine/tyrosine). Capacitation increased the intensity of serine/threonine phosphorylation per sperm cell, increased the number of sperm cells that were phosphorylated, and induced a subcellular shift in the serine/threonine-specific fluorescence. These findings indicate that protein serine/threonine phosphorylation is involved and may have a physiological role in sperm capacitation.

Development of an assay for phospholipase C using column-reconstituted, extruded phospholipid vesicles

The reconstitution of heterotrimeric G proteins into phospholipid vesicles has been widely used for the measurement of PLC-beta activity in vitro. We have developed an improved and sensitive method for the assay of PLC-beta activity. This approach involves reconstitution of purified betagamma dimers into extruded phospholipid vesicles containing phosphatidylinositol 4, 5-bisphosphate and using a gel-filtration technique to separate the reconstituted vesicles from monodispersed betagamma dimers and the detergent used to solubilize G proteins. The method provides physical information about the partitioning of betagamma dimers into phospholipid vesicles and was used to examine the effect of different prenyl groups on the gamma subunits in the activation of PLC-beta. The beta1gamma1 dimer (containing the farnesyl group) and the beta1gamma2 dimer (containing the geranylgeranyl group) were purified from baculovirus-infected Sf9 insect cells and were found to partition equally into phospholipid vesicles. The beta1gamma2 dimer is more potent and effective in stimulating PLC-beta activity than the beta1gamma1 dimer. The EC50 values of betagamma dimers for the activation of PLC-beta determined with this method were lower than those determined by previous methodology, showing that betagamma subunits have a subnanomolar affinity for PLC-beta.

Real-time visualization of PH domain-dependent translocation of phospholipase C-delta1 in renal epithelial cells (MDCK): response to hypo-osmotic stress

Green fluorescent protein (GFP)-tagged phospholipase C (PLC)-delta1 and its mutants were expressed in Madin-Darby canine kidney (MDCK) cells. GFP-PLC-delta1 or the GFP-tagged pleckstrin homology (PH) domain of PLC-delta1 itself was found to be predominantly localized at the plasma membrane. The DeltaPH mutant or a site-directed mutant containing a PH domain which does not bind inositol 1,4, 5-trisphosphate and cannot hydrolyze phosphatidylinositol 4, 5-bisphosphate in vitro was seen only in the cytosol. In living MDCK cells hypo-osmotic stress caused a rapid dissociation of GFP-PLC-delta1 from the plasma membrane, which coincided with phosphoinositide breakdown. A PLC inhibitor, U73122, blocked this translocation, but depletion of extracellular Ca2+ had no effect. The translocation was reversed by replacement with an iso-osmotic buffer. Our results demonstrate that the PH domain plays a critical role in the membrane targeting of PLC-delta1 and that the intracellular distribution of the enzyme is regulated by osmotic stress-driven phosphoinositide turnover.

Novel Evidence of Expression and Activity of Ecto-Phospholipase C γ1 in Human T Lymphocytes

Although much is known about the intracellular phospholipase C (PLC) specific for inositol phospholipids, few data are available about the presence of a less common PLC at the external side of the membrane bilayer of some cell types. This ectoenzyme seems to play particular roles in cellular function by hydrolyzing inositol lipids located on the outer leaflet of the plasma membrane. Here, we provide the first evidence that peripheral T lymphocytes express a discrete level of a PLCγ1 at the outer leaflet of the plasma membrane. Flow cytometry showed that the PLCγ1-positive (PLCγ1+) cells (∼37%) were CD8+ and CD45RA+. Biochemical evidence indicated that (1) this ectoenzyme displays a mass similar to the cytoplasmic form, (2) it is phosphorylated on tyrosine residues, and (3) its activity is Ca2+-dependent. In addition, this enzyme appeared to be correlated with the proliferative state of the cell, since stimulation with phytohemagglutinin (PHA) downregulated both its expression and activity, which were restored by treatment with an antiproliferative agent like natural interferon beta. Moreover, the different kinetics of formation of its hydrolytic products, inositol 1 phosphate and inositol 1:2 cyclic phosphate (Ins(1)P and Ins(1:2 cycl)P), formed upon incubation of the lymphocytes with [3H]-lyso-phosphatidylinositol (PI), allow the hypothesis of a selective involvement of the two inositol phosphates in the mechanisms regulating the metabolism of particular T-lymphocyte subsets.

Novel Evidence of Expression and Activity of Ecto-Phospholipase C γ1 in Human T Lymphocytes

Although much is known about the intracellular phospholipase C (PLC) specific for inositol phospholipids, few data are available about the presence of a less common PLC at the external side of the membrane bilayer of some cell types. This ectoenzyme seems to play particular roles in cellular function by hydrolyzing inositol lipids located on the outer leaflet of the plasma membrane. Here, we provide the first evidence that peripheral T lymphocytes express a discrete level of a PLCγ1 at the outer leaflet of the plasma membrane. Flow cytometry showed that the PLCγ1-positive (PLCγ1+) cells (∼37%) were CD8+ and CD45RA+. Biochemical evidence indicated that (1) this ectoenzyme displays a mass similar to the cytoplasmic form, (2) it is phosphorylated on tyrosine residues, and (3) its activity is Ca2+-dependent. In addition, this enzyme appeared to be correlated with the proliferative state of the cell, since stimulation with phytohemagglutinin (PHA) downregulated both its expression and activity, which were restored by treatment with an antiproliferative agent like natural interferon beta. Moreover, the different kinetics of formation of its hydrolytic products, inositol 1 phosphate and inositol 1:2 cyclic phosphate (Ins(1)P and Ins(1:2 cycl)P), formed upon incubation of the lymphocytes with [3H]-lyso-phosphatidylinositol (PI), allow the hypothesis of a selective involvement of the two inositol phosphates in the mechanisms regulating the metabolism of particular T-lymphocyte subsets.

Effects of omega 3 fatty acids on receptor tyrosine kinase and PLC activities in EMT6 cells

The effects of omega 3 fatty acids and epidermal growth factor (EGF) on the activity of receptor tyrosine kinase (RTK) and phospholipase C (phosphatidylinositol (PI)-specific PLC) were examined in EMT6 cells. The non-omega 3 treated, non-EGF stimulated cells served as controls. Treatment of the EMT6 cells with omega 3 fatty acids resulted in a 62% increase in RTK activity and a 67% increase in PI-specific PLC activity. When EGF was added to incubations for RTK activity, it stimulated the RTK activity 40% in the control cells and 130% in the omega 3-treated cells. When EGF was added to incubations for PI-specific PLC activity, a 54% increase in PI-specific PLC activity was observed in control cells and a 94% increase in the omega 3-treated cells. Thus, treating EMT6 cells with omega 3 fatty acids seems to increase RTK activity and PI-specific PLC activity to a similar extent, but has differential effects on the ability of these enzyme activities to be stimulated by EGF.

Membrane-binding properties of phospholipase C-beta1 and phospholipaseC-beta2: role of the C-terminus and effects of polyphosphoinositides, G-proteins and Ca2+

We have studied the binding of two G-protein-regulated phospholipase C (PLC) enzymes, PLCs-beta1 and -beta2, to membrane surfaces using sucrose-loaded bilayer phospholipid vesicles of varying compositions. Neither enzyme binds appreciably to pure phosphatidylcholine vesicles at lipid concentrations up to 10(-3) M. PLC-beta1 and PLC-beta2 bind vesicles composed of phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine (molar ratio 1:1:1) with an approximate Kd of 10(-5) M. Inclusion of 2% PtdIns(4,5)P2 in these vesicles had no effect on the affinity of this interaction. As reported by others, removal of the C-terminus of PLC-beta1 and PLC-beta2 produces catalytically active fragments. The affinity of these truncated proteins for phospholipid vesicles is dramatically reduced suggesting that this region of the proteins contains residues important for membrane binding. Inclusion of G-protein alpha- and betagamma-subunit activators in the phospholipid vesicles does not increase the binding of PLC-beta1 or PLC-beta2, and the magnitude of G-protein-mediated PLC activation observed at low phospholipid concentrations (10(-6) M) is comparable to that observed at concentrations at which the enzymes are predominantly membrane-bound (10(-3) M). PLC-beta1 and -beta2 contain C2 domains but Ca2+ does not enhance binding to the vesicles. Our results indicate that binding of these enzymes to membranes involves the C-temini of the proteins and suggest that activation of these enzymes by G-proteins results from a regulated interaction between the membrane-bound proteins rather than G-protein-dependent recruitment of soluble enzymes to a substrate-containing phospholipid surface.

Impaired plasma membrane targeting of Grb2-murine son of sevenless (mSOS) complex and differential activation of the Fyn-T cell receptor (TCR)-zeta-Cbl pathway mediate T cell hyporesponsiveness in autoimmune nonobese diabetic mice

Nonobese diabetic (NOD) mouse thymocytes are hyporesponsive to T cell antigen receptor (TCR)-mediated stimulation of proliferation, and this T cell hyporesponsiveness may be causal to the onset of autoimmune diabetes in NOD mice. We previously showed that TCR-induced NOD T cell hyporesponsiveness is associated with a block in Ras activation and defective signaling along the PKC/Ras/MAPK pathway. Here, we report that several sequential changes in TCR-proximal signaling events may mediate this block in Ras activation. We demonstrate that NOD T cell hyporesponsiveness is associated with the (a) enhanced TCR-beta-associated Fyn kinase activity and the differential activation of the Fyn-TCR-zeta-Cbl pathway, which may account for the impaired recruitment of ZAP70 to membrane-bound TCR-zeta; (b) relative inability of the murine son of sevenless (mSOS) Ras GDP releasing factor activity to translocate from the cytoplasm to the plasma membrane; and (c) exclusion of mSOS and PLC-gamma1 from the TCR-zeta-associated Grb2/pp36-38/ZAP70 signaling complex. Our data suggest that altered tyrosine phosphorylation and targeting of the Grb2/pp36-38/ZAP70 complex to the plasma membrane and cytoskeleton and the deficient association of mSOS with this Grb2-containing complex may block the downstream activation of Ras and Ras-mediated amplification of TCR/CD3-mediated signals in hyporesponsive NOD T cells. These findings implicate mSOS as an important mediator of downregulation of Ras signaling in hyporesponsive NOD T cells.

A new function for phospholipase C-gamma1: coupling to the adaptor protein GRB2

Epidermal growth factor (EGF)-induced autophosphorylation of the EGF receptor results in high-affinity binding of the adaptor protein GRB2, which serves as a convergence point for multiple signaling pathways. Present studies demonstrate that EGF induces the co-immunoprecipitation of phospholipase C (PLC)-gamma1 with the adaptor protein GRB2 and the guanine nucleotide exchange factor Sos, but not with the adaptor protein SHC, in WB cells. Inhibition of PLC-gamma1 tyrosine phosphorylation by phenylarsine oxide reduces the co-immunoprecipitation of PLC-gamma1 with GRB2. Furthermore, angiotensin II, a G protein-coupled receptor agonist, also induces the tyrosine phosphorylation of PLC-gamma1 and its co-immunoprecipitation with GRB2 in WB cells. Interestingly, angiotensin II stimulation also causes tyrosine phosphorylation of the EGF receptor, suggesting that angiotensin II-induced PLC-gamma1 tyrosine phosphorylation in WB cells may be via EGF receptor tyrosine kinase activation. In addition, there is some level of association between PLC-gamma1 and GRB2 that is independent of the tyrosine phosphorylation of PLC-gamma1 in both in vivo and in vitro studies. In vitro studies further demonstrate that the Tyr771 and Tyr783 region of PLC-gamma1 and the SH2 domain of GRB2 are potentially involved in the tyrosine phosphorylation-dependent association between PLC-gamma1 and GRB2. The association of PLC-gamma1 with GRB2 and Sos suggests that PLC-gamma1 may be directly involved in the Ras signaling pathway and that GRB2 may be involved in the translocation of PLC-gamma1 from cytosol to the plasma membrane as a necessary step for its effect on inositol lipid hydrolysis.

Structural requirements of phospholipase C delta1 for regulation by spermine, sphingosine and sphingomyelin

We studied the relationship between sphingomyelin, calcium, spermine and sphingosine in regulation of phospholipase C (PLC) delta1 activity. Inhibition of PLC delta1 by sphingomyelin was promoted by spermine and Ca2+ and was partially abolished by sphingosine. The effect of sphingosine and spermine entirely depended on Ca2+. In the absence of Ca2+, no effect of these substances on PLC delta1 activity was observed. Using deletion mutants and active fragments of PLC delta1 generated by limited proteolysis, we have studied the structural requirements of the enzyme for regulation by these compounds. The deletion mutant of PLC delta1 lacking the first 58 amino acids and the mutant lacking the entire pleckstrin homology (PH) domain were fully active in the detergent assay, and their activities were affected by spermine, sphingosine, Ca2+ and sphingomyelin to the same extent as the native enzyme. The limited proteolysis of PLC delta1 generated two fragments of 40 kDa and 30 kDa, which formed a stable active complex. The relationship between Ca2+ concentration and enzymatic activity was almost identical for the native PLC delta1 and the proteolytic complex. The activity of the proteolytic complex formed by the 40 kDa and 30 kDa peptides was not affected by spermine and sphingosine. Sphingomyelin inhibited the complex slightly less than the native PLC delta1, and this inhibition was not promoted by spermine. These observations suggest that for activation of PLC delta1 by spermine and sphingosine, the region spanning domains of high conservation, named X and Y, must be intact. In contrast, the PH domain and the intact spanning region of the X and Y domains are not essential for inhibition of PLC delta1 by sphingomyelin.

Regulation of effectors by G-protein alpha- and beta gamma-subunits. Recent insights from studies of the phospholipase c-beta isoenzymes

Both the alpha- and beta gamma-subunits of heterotrimeric guanine nucleotide-dependent regulatory proteins (G-proteins) couple members of the heptahelical class of cell-surface receptors to a diverse range of signal-generating effectors including retinal cyclic GMP phosphodiesterase, ion channels, adenylylcyclases, phosphoinositide 3-kinase, and members of the beta-class of inositol lipid-specific phospholipases C. Although the molecular details of the G-protein-regulated phospholipase C system were elucidated comparatively recently, these enzymes have become an important model for investigations of the process of G-protein effector coupling. A combination of molecular biological, biochemical, and structural studies using the phospholipase C-beta enzymes has provided some important insights into the interplay between G-proteins and their effectors and promises to reveal the mechanisms by which G-protein alpha- and beta gamma-subunits selectively associate with and activate effectors.

Cytoskeletal association of epidermal growth factor receptor and associated signaling proteins is regulated by cell density in IEC-6 intestinal cells

Epidermal growth factor (EGF) mediates a variety of physiologic responses in rat intestine. EGF receptor (EGFR) responsiveness to EGF is mediated by the surface expression of high affinity EGFR, which is associated with the cytoskeleton (CSK). EGFR signal transduction appears to be mediated by the CSK association of EGFR and related signaling proteins. In the nontransformed intestinal cell line IEC-6, expression of EGFR, Src homology and collagen protein (SHC), phospholipase C gamma 1 (PLC gamma), and their tyrosine phosphorylation in response to EGF was assayed by immunoblot. The distribution of EGFR and tyrosine-phosphorylated EGFR was regulated by cell density. At confluence, EGFR and tyrosine-phosphorylated EGFR were predominantly associated with the Triton X-100-insoluble CSK at confluence, while predominantly Triton X-100-soluble at subconfluence. PLC gamma was predominantly soluble at both states of confluence. Confluent but not subconfluent IEC-6 cells demonstrated a cascade of EGF-mediated events consisting of a transient CSK association of PLC gamma with EGFR, a brief expression of tyrosine-phosphorylated PLC gamma, a brief increase in PLC gamma CSK association, and a prolonged soluble association of PLC gamma with the EGFR. EGF led to an increase in the CSK association of SHC at both states of confluence and was greater at confluence. EGFR association with SHC was primarily soluble at subconfluence, while at confluence EGFR association was markedly increased and predominantly in the CSK. Thus, cell density regulates the CSK association of the EGFR and its ability to associate and activate signaling pathways in intestinal cells.

Phospholipase C-gamma1: regulation of enzyme function and role in growth factor-dependent signal transduction

Phospholipase C(gamma)1 (PLC-gamma1), a tyrosine kinase substrate, is a multi-domain molecule that modulates the intracellular levels of the second messenger molecules: Ca2+ and diacylglycerol. Although a wide variety of growth factor receptor tyrosine kinases phosphorylate and activate PLC-gamma1, the biological role and necessity of this signal transduction element in mitogenesis has remained unclear. Recent results, however, point to a more essential role than was suggested by initial studies. Also, biochemical studies have indicated a putative means for the intramolecular repression of PLC-gamma1 activity and provide a means for interpreting activation signals through a derepression mechanism.

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.

Thrombin activation of human platelets dissociates a complex containing gelsolin and actin from phosphatidylinositide-specific phospholipase Cgamma1

We have examined the association of two cytoskeleton proteins, gelsolin and actin, with phosphatidylinositide-specific phospholipase Cgamma1 (PLCgamma1) in resting and thrombin-stimulated human platelets. In unstimulated platelets, gelsolin, actin and PLCgamma1 were immunoprecipitated as a complex by a polyclonal antibody to PLCgamma1. The association of gelsolin and actin was specific for PLCgamma1 because immunoprecipitates of PLCs beta2, beta3, gamma2 and delta1, which are also expressed in human platelets, did not contain detectable gelsolin or actin. Activation with thrombin resulted in platelet aggregation and the dissociation of gelsolin and actin from PLCgamma1. Inhibition of thrombin-induced platelet aggregation blocked the dissociation of gelsolin and actin from PLCgamma1. After stimulation with thrombin, PLCgamma1 activity in immunoprecipitates was increased 2-3-fold. This elevation in PLCgamma1 activity in response to thrombin activation was not observed when platelet aggregation was blocked. Although PLCgamma1 is tyrosine phosphorylated in response to many agonists, we could not detect, by Western analysis with anti-phosphotyrosine antibodies, tyrosine phosphorylation of PLCgamma1 immunoprecipitated from thrombin-stimulated platelets. These results demonstrate that PLCgamma1 is associated with gelsolin and actin in resting platelets, and that thrombin-induced platelet aggregation results in the dissociation of PLCgamma1 from gelsolin and actin, and the stimulation of PLCgamma1 activity.

Effect of sphingomyelin and its metabolites on the activity of human recombinant PLC delta 1

In an attempt to obtain sufficient quantities of pure phospholipase C delta 1 (PLC delta 1) necessary for structural and kinetic studies, human fibroblast PLC delta 1 was cloned in the pPROEX-1 vector, expressed in E. coli cells as a (6xHis) fusion protein and purified to homogeneity. From 11 of E. coli culture 21 mg of pure PLC delta 1 was obtained by a two-step purification procedure, which includes Ni(2+)-NAT agarose and Mono S cation exchange chromatography. Catalytic properties of recombinant PLC delta 1 with respect to activation by spermine and calcium ions and inhibition by sphingomyelin were similar to or identical to PLC delta 1 purified from rat liver. Calcium activation of PLC delta 1 was dependent on the presence of spermine. Half-maximal activity was attained at 250 and 170 nM of free Ca2+ in the presence and absence of spermine, respectively. Sphingomyelin and lysosphingomyelin were mixed type inhibitors with respect to PIP2. Ceramide inhibits PLC delta 1 very weakly. GM1, which is a ceramide bound glucosidically to the oligosaccharide moiety, was a strong non-competitive inhibitor of PLC delta 1. In the absence of spermine, sphingosine and phytosphingosine weakly activated PLC delta 1. The results indicate that the effect of sphingomyelin and its metabolites on PLC delta 1 activity depends on the presence of spermine. It is postulated that, among other factors, in vivo, activity of PLC delta 1 may depend on the turnover of sphingomyelin.

Regulation of phospholipase C delta1 by sphingosine

Sphingosine, which is on the pathway of sphingomyelin degradation, activates phospholipase C (PLC) delta1 moderately. In the liposome assay effect of sphingosine on PLC delta1 activity depends on KCl concentration. Stimulation of PLC delta1 by sphingosine increased as the KCl concentration is increased from 0 to 100 mM, and then diminished with the increasing KCl. In the liposome assay sphingosine diminishes inhibition of PLC delta1 by sphingomyelin. To determine the domain of PLC delta1 which interacts with sphingosine active proteolytic fragments of PLC delta1 were generated by trypsin digestion of the native enzyme. Sphingosine affects the activity of PLC delta1 fragment which lacked the amino-terminal domain (first 60 amino acids) but not the active fragment that has cleaved the domain spanning the X and Y region of PLC delta1. These observations indicate that for interaction of sphingosine with PLC delta1 intact domain that span regions of conservation, designated as X and Y is necessary. When the activity of PLC delta1 was assayed with PIP2 in the erythrocyte membrane as substrate, sphingosine strongly inhibited PLC delta1. The other homolog of sphingosine 4-hydroxysphinganine (phytosphingosine) inhibited PLC delta1 to much lesser extent. The activity of PLC delta1 was inhibited by 68% and 22% in the presence of 20 microM sphingosine and phytosphingosine, respectively. This inhibition was completely abolished by deoxycholate at a concentration of 1.5 mM. These observations suggest that sphingosine may regulate activity of PLC delta1 in the cell.

Signal transduction through epidermal growth factor receptor is altered in HeLa monolayer cells during mitosis

Epidermal growth factor (EGF)-induced signalling was studied separately in the mitosis and G2-phases of HeLa monolayer cells presynchronized (1) by amethopterin inhibition and thymidine release or (2) by nocodazole. For comparison, cells were treated with the phorbol ester phorbol 12-myristate 13-acetate (PMA). In contrast with the observed responses effected by PMA, which seem to be independent of cell cycle and synchronization conditions, those induced by EGF are greatly influenced by both criteria. Synchronization with nocodazole abolished the EGF-induced stimulation of phosphoinositide hydrolysis in G2 as well as in mitotic cells although tyrosine phosphorylation of the EGF receptor and phospholipase Cgamma1 could be shown to occur, especially in G2 cells. Synchronization with amethopterin/thymidine showed that, in contrast with G2 cells, mitotic cells were not able to react to EGF with an increase in phosphoinositide hydrolysis although a certain degree of EGF receptor dimerization and autophosphorylation as well as tyrosine phosphorylation of phospholipase Cgamma1 could still be shown to occur in mitosis. The results seem to indicate that the EGF pathway leading to a stimulation of phosphoinositide hydrolysis is attenuated at different levels and requires a cytoskeletal condition that is not present either after treatment (24 h) with nocodazole or during normal mitosis of a monolayer cell.

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.