A site of tyrosine phosphorylation in the C terminus of the epidermal growth factor receptor is required to activate phospholipase C

Effect of acute acid-base disturbances on ErbB1/2 tyrosine phosphorylation in rabbit renal proximal tubules

The renal proximal tubule (PT) is a major site for maintaining whole body pH homeostasis and is responsible for reabsorbing ∼80% of filtered HCO3(-), the major plasma buffer, into the blood. The PT adapts its rate of HCO3(-) reabsorption (JHCO3(-)) in response to acute acid-base disturbances. Our laboratory previously showed that single isolated perfused PTs adapt JHCO3(-) in response to isolated changes in basolateral (i.e., blood side) CO2 and HCO3(-) concentrations but, surprisingly, not to pH. The response to CO2 concentration can be blocked by the ErbB family tyrosine kinase inhibitor PD-168393. In the present study, we exposed enriched rabbit PT suspensions to five acute acid-base disturbances for 5 and 20 min using a panel of phosphotyrosine (pY)-specific antibodies to determine the influence of each disturbance on pan-pY, ErbB1-specific pY (four sites), and ErbB2-specific pY (two sites). We found that each acid-base treatment generated a distinct temporal pY pattern. For example, the summated responses of the individual ErbB1/2-pY sites to each disturbance showed that metabolic acidosis (normal CO2 concentration and reduced HCO3(-) concentration) produced a transient summated pY decrease (5 vs. 20 min), whereas metabolic alkalosis produced a transient increase. Respiratory acidosis (normal HCO3(-) concentration and elevated CO2 concentration) had little effect on summated pY at 5 min but produced an elevation at 20 min, whereas respiratory alkalosis produced a reduction at 20 min. Our data show that ErbB1 and ErbB2 in the PT respond to acute acid-base disturbances, consistent with the hypothesis that they are part of the signaling cascade.

A kinase cascade leading to Rab11-FIP5 controls transcytosis of the polymeric immunoglobulin receptor

Polymeric immunoglobulin A (pIgA) transcytosis, mediated by the polymeric immunoglobulin receptor (pIgR), is a central component of mucosal immunity and a model for regulation of polarized epithelial membrane traffic. Binding of pIgA to pIgR stimulates transcytosis in a process requiring Yes, a Src family tyrosine kinase (SFK). We show that Yes directly phosphorylates EGF receptor (EGFR) on liver endosomes. Injection of pIgA into rats induced EGFR phosphorylation. Similarly, in MDCK cells, pIgA treatment significantly increased phosphorylation of EGFR on various sites, subsequently activating extracellular signal-regulated protein kinase (ERK). Furthermore, we find that the Rab11 effector Rab11-FIP5 is a substrate of ERK. Knocking down Yes or Rab11-FIP5, or inhibition of the Yes-EGFR-ERK cascade, decreased pIgA-pIgR transcytosis. Finally, we demonstrate that Rab11-FIP5 phosphorylation by ERK controls Rab11a endosome distribution and pIgA-pIgR transcytosis. Our results reveal a novel Yes-EGFR-ERK-FIP5 signalling network for regulation of pIgA-pIgR transcytosis.

Loss of protein tyrosine phosphatase N2 potentiates epidermal growth factor suppression of intestinal epithelial chloride secretion

The Crohn's disease candidate gene, protein tyrosine phosphatase nonreceptor type 2 (PTPN2), has been shown to regulate epidermal growth factor (EGF)-induced phosphatidylinositol 3-kinase (PI3K) activation in fibroblasts. In intestinal epithelial cells (IECs), EGF-induced EGF receptor (EGFR) activation and recruitment of PI3K play a key role in regulating many cellular functions including Ca(2+)-dependent Cl(-) secretion. Moreover, EGFR also serves as a conduit for signaling by other non-growth factor receptor ligands such as the proinflammatory cytokine, IFN-γ. Here we investigated a possible role for PTPN2 in the regulation of EGFR signaling and Ca(2+)-dependent Cl(-) secretion in IECs. PTPN2 knockdown enhanced EGF-induced EGFR tyrosine phosphorylation in T(84) cells. In particular, PTPN2 knockdown promoted EGF-induced phosphorylation of EGFR residues Tyr-992 and Tyr-1068 and led subsequently to increased association of the catalytic PI3K subunit, p110, with EGFR and elevated phosphorylation of the downstream marker, Akt. As a functional consequence, loss of PTPN2 potentiated EGF-induced inhibition of carbachol-stimulated Ca(2+)-dependent Cl(-) secretion. In contrast, PTPN2 knockdown affected neither IFN-γ-induced EGFR transactivation nor EGF- or IFN-γ-induced phosphorylation of ERK1/2. In summary, our data establish a role for PTPN2 in the regulation of EGFR signaling in IECs in response to EGF but not IFN-γ. Knockdown of PTPN2 directs EGFR signaling toward increased PI3K activation and increased suppression of epithelial chloride secretory responses. Moreover, our findings suggest that PTPN2 dysfunction in IECs leads to altered control of intestinal epithelial functions regulated by EGFR.

Epidermal growth factor receptor mutations in non-small cell lung cancer influence downstream Akt, MAPK and Stat3 signaling

PURPOSE

The efficacy of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors in non-small cell lung cancer (NSCLC) has been linked to activating mutations in the EGFR gene. So far these mutations have been extensively characterized in established cell lines. The aim of this study was to determine the effects of EGFR mutations on downstream signaling in human tumor specimens.

METHODS

We have looked for mutations of the EGFR gene in specimens of 67 patients with NSCLC and correlated these with EGFR phosphorylation and the activity of its three main downstream signaling cascades Akt, MAPK and Stat3 by immunohistochemistry.

RESULTS

We show that the phosphorylation of tyrosine residues 922 and 1173, but not 1068, are primarily affected by the activating EGFR mutations. Akt activity was significantly higher in patients with EGFR mutations but we found no difference in Stat3 or MAPK phosphorylation. Our results suggest that EGFR mutations not only increase receptor activity, but also alter responses of downstream signaling cascades in human NSCLCs and that these finding differ from results obtained in cell lines.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Consequences of Direct Versus Indirect Activation of Epidermal Growth Factor Receptor in Intestinal Epithelial Cells Are Dictated by Protein-tyrosine Phosphatase 1B

The epidermal growth factor receptor (EGFR) is an integral regulator of many cellular functions. EGFR also acts as a central conduit for extracellular signals involving direct activation of the receptor by EGFR ligands or indirect activation by G protein-coupled receptor (GPCR)-stimulated transactivation of the EGFR. We have previously shown that EGFR negatively regulates epithelial chloride secretion as a result of transforming growth factor-alpha-mediated EGFR transactivation in response to muscarinic GPCR activation. Here we show that direct activation of the EGFR by EGFR ligands produces a different pattern of EGFR tyrosine phosphorylation and downstream phosphatidylinositol 3-kinase recruitment than GPCR-stimulated transactivation of the EGFR occurring via paracrine EGFR ligand release. Moreover, we demonstrate that this differential signaling and its consequences depend on protein-tyrosine phosphatase 1B activity. Thus protein-tyrosine phosphatase 1B governs differential recruitment of signaling pathways involved in EGFR regulation of epithelial ion transport. Our findings furthermore establish how divergent signaling outcomes can arise from the activation of a single receptor.

Structure and dynamics of the epidermal growth factor receptor C-terminal phosphorylation domain

The C-terminal phosphorylation domain of the epidermal growth factor receptor is believed to regulate protein kinase activity as well as mediate the assembly of signal transduction complexes. The structure and dynamics of this proposed autoregulatory domain were examined by labeling the extreme C terminus of the EGFR intracellular domain (ICD) with an extrinsic fluorophore. Fluorescence anisotropy decay analysis of the nonphosphorylated EGFR-ICD yielded two rotational correlation times: a longer time, consistent with the global rotational motion of a 60- to 70-kDa protein with an elongated globular conformation, and a shorter time, presumably contributed by segmental motion near the fluorophore. A C-terminally truncated form of EGFR-ICD yielded a slow component consistent with the rotational motion of the 38-kDa kinase core. These findings suggested a structural arrangement of the EGFR-ICD in which the C-terminal phosphorylation domain interacts with the kinase core to move as an extended structure. A marked reduction in the larger correlation time of EGFR-ICD was observed upon its autophosphorylation. This dynamic component was faster than predicted for the globular motion of the 62-kDa EGFR-ICD, suggesting an increase in the mobility of the C-terminal domain and a likely displacement of this domain from the kinase core. The interaction between the SH2 domain of c-Src and the phosphorylated EGFR C-terminal domain was shown to impede its mobility. Circular dichroism spectroscopy indicated that the EGFR C-terminal domain possessed a significant level of secondary structure in the form of alpha-helices and beta-sheets, with a marginal change in beta-sheet content occurring upon phosphorylation.

Active EGF receptors have limited access to PtdIns(4,5)P2 in endosomes: implications for phospholipase C and PI 3-kinase signaling

Although prolonged cell signaling is attenuated by internalization and downregulation of active receptors, it is now appreciated that many receptors continue to signal in intracellular compartments. Employing enhanced green fluorescent protein fusion probes, we have investigated the hypothesis that multiple signaling pathways are affected by the differential trafficking of membrane substrates such as PtdIns(4,5)P2. A phosphotyrosine-specific probe, but not a PtdIns(4,5)P2-specific probe, colocalized with internalized EGF as well as transferrin in EGF-stimulated living cells expressing autophosphorylation-competent EGF receptors. Neither probe colocalized with transferrin in the absence of EGF, demonstrating that the reduced level of accessible PtdIns(4,5)P2 in endosomes is constitutive. Finally, a PtdIns(3,4,5)P3-specific probe, which monitors phosphorylation of PtdIns(4,5)P2 by phosphoinositide 3-kinases, was recruited to the plasma membrane but not to EGF- or transferrin-containing endosomes in response to EGF stimulation. These results suggest that while many internalized receptors continue to engage intracellular enzymes, the phospholipase C and phosphoinositide 3-kinase signaling pathways are abrogated by the constitutive lack of accessible PtdIns(4,5)P2 in endosomes.

Actin filament assembly and actin-myosin contractility are necessary for anchorage- and EGF-dependent activation of phospholipase Cgamma

Formation of actin stress fibers and the focal adhesion complex between cell and the substratum are crucial for nonmalignant cells to achieve anchorage-dependent growth. We show here that the adhesion complex formed in normal human mammary epithelial (HME) cells which adhered to type IV collagen, involved the EGF receptor (EGFR) and phospholipase Cgamma (PLCgamma) as signaling molecules, in addition to integrin beta1, alpha-actinin, and actin even before stimulation of the cells with EGF. Stimulation of cells with EGF induced tyrosine phosphorylation of EGFR and activation of PLCgamma, as assessed by the production of a second messenger diacylglycerol (DAG), without any significant increase in the amount of EGFR-bound PLCgamma. Disruption of either actin filaments by cytochalasin D (CD) or actin-myosin contractility by ML-7, an inhibitor of myosin light chain kinase (MLCK), altered the flattened morphology of quiescent cells to a retracted one, without affecting the association between EGFR and PLCgamma. Stimulation of CD- or ML-7-treated cells with EGF failed to inhibit tyrosine phosphorylation of EGFR and its association and colocalization with PLCgamma, but inhibited the PLCgamma activation. Phosphatidylinositol 4,5-bisphosphate (PtdInsP2), substrate of PLCgamma, was tightly associated with alpha-actinin and the content of alpha-actinin-bound PtdInsP2 was reduced by treatment of cells with ML-7 but not with CD. The amount of PtdInsP2 bound to alpha-actinin was increased by the addition of EGF and this EGF-induced increase was blocked by either CD or ML-7. The present results suggest that anchorage-dependent EGF signaling in HME cells may require both actin filament assembly and actin-myosin contractility for the PLCgamma activation.

Regulation of phosphoinositide-specific phospholipase C

Eleven distinct isoforms of phosphoinositide-specific phospholipase C (PLC), which are grouped into four subfamilies (beta, gamma, delta, and epsilon), have been identified in mammals. These isozymes catalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to inositol 1,4,5-trisphosphate and diacylglycerol in response to the activation of more than 100 different cell surface receptors. All PLC isoforms contain X and Y domains, which form the catalytic core, as well as various combinations of regulatory domains that are common to many other signaling proteins. These regulatory domains serve to target PLC isozymes to the vicinity of their substrate or activators through protein-protein or protein-lipid interactions. These domains (with their binding partners in parentheses or brackets) include the pleckstrin homology (PH) domain [PtdIns(3)P, beta gamma subunits of G proteins] and the COOH-terminal region including the C2 domain (GTP-bound alpha subunit of Gq) of PLC-beta; the PH domain [PtdIns(3,4,5)P3] and Src homology 2 domain [tyrosine-phosphorylated proteins, PtdIns(3,4,5)P3] of PLC-gamma; the PH domain [PtdIns(4,5)P2] and C2 domain (Ca2+) of PLC-delta; and the Ras binding domain (GTP-bound Ras) of PLC-epsilon. The presence of distinct regulatory domains in PLC isoforms renders them susceptible to different modes of activation. Given that the partners that interact with these regulatory domains of PLC isozymes are generated or eliminated in specific regions of the cell in response to changes in receptor status, the activation and deactivation of each PLC isoform are likely highly regulated processes.

Threonine phosphorylation of the beta 3 integrin cytoplasmic tail, at a site recognized by PDK1 and Akt/PKB in vitro, regulates Shc binding

The mechanism of outside-in signaling by integrins parallels that for growth factor receptors. In both pathways, phosphorylation of a cytoplasmic segment on tyrosine generates a docking site for proteins containing Src homology 2 (SH2) and phosphotyrosine binding domains. We recently observed that phosphorylation of a threonine (Thr-753), six amino acids proximal to tyrosine 759 in beta(3) of the platelet specific integrin alpha(IIb)beta(3), inhibits outside-in signaling through this receptor. We hypothesized that the presence of phosphothreonine 753 either renders beta(3) a poor substrate for tyrosine kinases or inhibits the docking capabilities of the tyrosyl-phosphorylated form of beta(3.) The first alternative was tested by comparing the phosphorylation of beta(3) model peptides by the tyrosine kinase pp60(c-src) and we found that the presence of a phosphate group on a residue corresponding to Thr-753 did not detectably alter the kinetics of tyrosine phosphorylation. However, the presence of phosphate on this threonine inhibited the binding of Shc to tyrosyl-phosphorylated beta(3) peptide. The inhibitory effect of the phosphate group could be mimicked by substituting an aspartic acid for Thr-753, suggesting that a negative charge at this position modulates the binding of Shc and possibly other phosphotyrosine binding domain- and SH2-containing proteins. A survey of several protein kinases revealed that Thr-753 was avidly phosphorylated by PDK1 and Akt/PKB in vitro. These observations suggest that activation of PDK1 and/or Akt/PKB in platelets may modulate the binding activity and/or specificity of beta(3) for signaling molecules.

Aging fibroblasts present reduced epidermal growth factor (EGF) responsiveness due to preferential loss of EGF receptors

Wound healing is compromised in aging adults in part due to decreased responsiveness of fibroblasts to extracellular signals. However, the cellular mechanisms underlying this phenomenon are not known. Aged dermal fibroblasts with reduced remaining replicative capacities demonstrated decreased epidermal growth factor (EGF)-induced cell migrative and cell proliferative capacities, as reported previously. Thus, as cells approach senescence, programmed in vivo or in vitro, EGF responsiveness is preferentially lost. To define the rate-limiting signaling event, we found that the activity of two different EGF receptor (EGFR)-signaling pathways to cell migration (phospholipase-C gamma) and/or mitogenesis (extracellular signal/regulated-mitogen-activated kinases) were decreased in near senescent cells despite unchanged levels of effector molecules. Aged cells presented decreased levels of EGFR, although insulin receptor and transferrin receptor levels were relatively unchanged. EGFR mRNA levels and production of new transcripts decreased during aging, suggesting that this preferential loss of EGFR was due to diminished production, which more than counteracts the reduced ligand-induced receptor loss. Since these data suggested that the decrement in EGF was rate-limiting, higher levels of EGFR were established in near senescent cells by electroporation of EGFR cDNA. These cells presented higher levels of EGFR and recovered their EGF-induced migration and proliferation responsiveness. Thus, the defect in EGF responsiveness of aged dermal fibroblasts is secondary to reduced EGFR message transcription. Our experimental model suggests that EGFR gene delivery might be an effective future therapy for compromised wound healing.

Epidermal growth factor receptor activation of calpain is required for fibroblast motility and occurs via an ERK/MAP kinase signaling pathway

To become migratory, cells must reorganize their connections to the substratum, and during locomotion they must break rear attachments. The molecular and biochemical mechanisms underlying these biophysical processes are unknown. Recent studies have implicated both extracellular signal-regulated kinase/mitogen-activated protein (ERK/MAP) kinase and calpain (EC 3.4.22.17) in these processes, but it is uncertain whether these are two distinct pathways acting on different modes of motility. We report that cell deadhesion involved in epidermal growth factor (EGF) receptor-mediated fibroblast motility requires activation of M-calpain downstream of ERK/MAP kinase signaling. NR6 fibroblasts expressing full-length wild type epidermal growth factor receptor required both calpain and ERK activation, as demonstrated by pharmacological inhibitors (calpeptin and calpain inhibitor I and PD98059, respectively) for EGF-induced deadhesion and motility. EGF induced rapid activation of calpain that was preventable by molecular inhibition of the Ras-Raf-MEK but not phospholipase Cgamma signaling pathway, and calpain was stimulated by transfection of constitutively active MEK. Enhanced calpain activity was not mirrored by increased calpain protein levels or decreased levels of its endogenous inhibitor calpastatin. The link between ERK/MAP kinase signaling and cell motility required the M-isoform of calpain (calpain II), as determined by specific antisense-mediated down-regulation. These data promote a previously undescribed signaling pathway of ERK/MAP kinases activating calpain to destabilize cell-substratum adhesions in response to EGF stimulation.

Activation of epidermal growth factor receptor promotes late terminal differentiation of cell-matrix interaction-disrupted keratinocytes

The biological effects of epidermal growth factor receptor (EGFR) activation may differ between epidermal suprabasal and basal keratinocytes, since growth factors are mitogenic in adherent cells only in the presence of cell-extracellular matrix (ECM) interaction. To investigate biological effects of EGFR activation on keratinocytes without cell-ECM interaction, we cultured normal human keratinocytes on polyhydroxyethylmethacrylate-coated plates, which disrupt cell-ECM but not cell-cell interaction. The cells initially expressed keratin 10 (K10) and then profilaggrin, mimicking sequential differentiation of epidermal suprabasal keratinocytes. The addition of EGF or transforming growth factor-alpha promoted late terminal differentiation (profilaggrin expression, type 1 transglutaminase expression and activity, and cornified envelope formation) of the suspended keratinocytes, while suppressing K10 expression, an early differentiation marker. These effects were attenuated by EGFR tyrosine kinase inhibitor PD153035 or an anti-EGFR monoclonal antibody, whereas protein kinase C inhibitors H7 and bisindolylmaleimide I or mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor PD98059 abolished profilaggrin up-regulation but not K10 suppression. Since the antidifferentiative role of EGFR on cell-ECM interaction-conserved keratinocytes has been well documented, our results indicate that the biological effects of EGFR on keratinocytes are influenced by cell-ECM interaction and suggest that EGFR activation promotes rather than inhibits the terminal differentiation of suprabasal epidermal keratinocytes.

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.

The insulin receptor tyrosine kinase domain in a chimaeric epidermal growth factor-insulin receptor generates Ca2+ signals through the PLC-gamma1 pathway

The receptors for insulin (IR) and epidermal growth factor (EGFR) are members of the tyrosine kinase receptor (TKR) family. Despite homology of their cytosolic TK domains, both receptors induce different cellular responses. Tyrosine phosphorylation of insulin receptor substrate (IRS) molecules is a specific IR post-receptor response. The EGFR specifically activates phospholipase C-gamma1 (PLC-gamma1). Recruitment of substrate molecules with Src homology 2 (SH2) domains or phosphotyrosine binding (PTB) domains to phosphotyrosines in the receptor is one of the factors creating substrate specificity. In addition, it has been shown that the TK domains of the IR and EGFR show preferences to phosphorylate distinct peptides in vitro, suggesting additional mechanisms of substrate recognition. We have examined to what extent the substrate preference of the TK domain contributes to the specificity of the receptor in vivo. For this purpose we determined whether the IR TK domain, in situ, is able to tyrosine-phosphorylate substrates normally used by the EGFR. A chimaeric receptor, consisting of an EGFR in which the juxtamembrane and tyrosine kinase domains were exchanged by their IR counterparts, was expressed in CHO-09 cells lacking endogenous EGFR. This receptor was found to activate PLC-gamma1, indicating that the IR TK domain, in situ, is able to tyrosine phosphorylate substrates normally used by the EGFR. These findings suggest that the IR TK domain, in situ, has a low specificity for selection and phosphorylation of non-cognate substrates.

Effect of epidermal growth factor receptor internalization on regulation of the phospholipase C-gamma1 signaling pathway

The epidermal growth factor receptor (EGFR) ligands, epidermal growth factor (EGF), and transforming growth factor-alpha (TGFalpha) elicit differential postendocytic processing of ligand and receptor molecules, which impacts long-term cell signaling outcomes. These differences arise from the higher affinity of the EGF-EGFR interaction versus that of TGFalpha-EGFR in the acidic conditions of sorting endosomes. To determine whether EGFR occupancy in endosomes might also affect short-term signaling events, we examined activation of the phospholipase C-gamma1 (PLC-gamma1) pathway, an event shown to be essential for growth factor-induced cell motility. We found that EGF continues to stimulate maximal tyrosine phosphorylation of EGFR following internalization, while, as expected, TGFalpha stimulates markedly less. The resulting higher level of receptor activation by EGF, however, did not yield higher levels of phosphatidylinositol (4,5)-bisphosphate (PIP2) hydrolysis over those stimulated by TGFalpha. By altering the ratio of activated receptors between the cell surface and the internalized compartment, we found that only cell surface receptors effectively participate in PLC function. In contrast to PIP2 hydrolysis, PLC-gamma1 tyrosine phosphorylation correlated linearly with the total level of Tyr(P)-EGFR stimulated by either ligand, indicating that the functional deficiency of internal EGFR cannot be attributed to an inability to interact with and phosphorylate signaling proteins. We conclude that EGFR signaling through the PLC pathway is spatially restricted at a point between PLC-gamma1 phosphorylation and PIP2 hydrolysis, perhaps because of limited access of EGFR-bound PLC-gamma1 to its substrate in endocytic trafficking organelles.

The influence of deletion mutations on phospholipase C-gamma 1 activity

Phospholipase C-gamma1, a substrate for many growth factor receptor and nonreceptor tyrosine kinases, produces second messenger molecules that are elements of signal transduction pathways related to cell proliferation. The influence of deletion mutations, which do not intrude on the domains required for catalytic function, on the basal activity of this enzyme is reported. Removal of the first 74 amino-terminal residues increases phospholipase C activity, while deletion of the carboxy-terminal 81 residues decreases enzyme activity. Deletion of the SH2-SH2-SH3 central region, which separates the two domains (X, Y) responsible for catalytic function, also increases enzymatic activity. Interestingly, addition of a recombinant SH2-SH2-SH3 fragment of phospholipase C-gamma1 to the holoenzyme inhibits its phospholipase activity at pH 7.0, but not at pH 5.0. However, addition of individual SH2 or SH3 domains does not influence activity of the holoenzyme. All three deletion mutants, in contrast to the holoenzyme, are relatively resistant to V8 proteolysis and activation induced by the epidermal growth factor receptor tyrosine kinase, which require, respectively, specific proteolysis and phosphorylation sites within the SH region. This suggests a conformational change is induced in the SH region by deletion at either the amino- or carboxy-terminus.

Adjacent carboxyterminal tyrosine phosphorylation events identify functionally distinct ErbB2 receptor subsets: implications for molecular diagnostics

Site-directed mutagenesis can define the effects of altering one or more amino acids within a protein, but this technique may lack sensitivity when used to characterize proteins which differ conformationally or posttranslationally at multiple sites. A novel alternative approach involves the direct characterization of wild-type protein isoforms identified by site-specific immunodetection. To this end we have developed antibodies which recognize ErbB2 subsets characterized by adjacent tyrosine phosphorylation events (Y1222 and Y1248) in the C-terminal tail of the oncoprotein. Here we use these phosphoantibodies to demonstrate the existence of tyrosine-phosphorylated ErbB2 subsets which differ in their patterns of heterooligomer formation, in vitro autophosphorylation, and recruitment of SH2-containing substrates. Furthermore, Y1222 and/or Y1248 phosphoantibody immunoreactivity is readily detectable in ErbB2-overexpressing human breast tumors, in which context these phosphorylation events exhibit significant discordance. These data confirm the value of site-specific immunodetection as a strategy for characterizing phosphoprotein function in vitro and in vivo and suggest that multisite phosphotyping of human tumors may contribute novel clinicopathologic insights into the significance of the ErbB2 overexpression phenotype.

A negative regulatory function for the protein tyrosine phosphatase PTP2C revealed by reconstruction of platelet-derived growth factor receptor signalling in Schizosaccharomyces pombe

We have exploited reconstitution in the fission yeast Schizosaccharomyces pombe to investigate how activation of phospholipase Cgamma (PLCgamma) by the platelet-derived growth factor-beta receptor (PDGFbetaR) is regulated by the SH2 domain-containing protein tyrosine phosphatase PTP2C (also known as SHP-2). When co-expressed in S. pombe, PTP2C abolished PDGFbetaR autophosphorylation as well as its ability to phosphorylate and activate PLCgamma. Inhibition of PDGFbetaR signalling by PTP2C appears specific insofar that PTPIC, a close homologue of PTP2C, does not suppress activation of either PDGFbetaR or PLCgamma. Surprisingly, an inactive PTP2C mutant (C459S), which dephosphorylates neither PDGFbetaR nor PLCgamma, remains fully effective as an inhibitor of [3H]inositol phosphate generation indicating that negative regulation is at least in part independent of catalytic activity. This contrasts with PLCgamma activation by c-Src which, although blocked by active PTP2C, is not inhibited by the mutant PTP2C C459S. These observations indicate that in addition to a reported positive role relaying trophic signals, PTP2C can also exert a negative effect on the PDGFbetaR and its signalling to PLCgamma.

Modulation of fructose-2,6-bisphosphate metabolism by components of the extracellular matrix in cultured cells. Interaction with epidermal growth factor

The use of NIH3T3 fibroblasts overexpressing different mutations of the EGF receptor shows that regulation of fructose-2,6-bisphosphate (Fru-2,6-P2) metabolism by EGF is mediated by the kinase activity of the EGF receptor and suggests a PLCgamma1-mediated mechanism. The effect of several extracellular matrix components on glucose metabolism was assessed by incubating A431 cells and NIH3T3 fibroblasts with heparin, laminin, fibronectin, collagen and PG-I and PG-II proteoglycans and measuring the levels of Fru-2,6-P2. Laminin increased the levels of Fru-2,6-P2 and heparin decreased the levels of the metabolite, whereas the other molecules did not have any effect. No effect of laminin or heparin in glucose uptake by the cell was observed. Laminin was able to modulate the effects of EGF on Fru-2,6-P2 concentration, suggesting cross-talk between these agents.

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.

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.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

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)

Src phosphorylation of the epidermal growth factor receptor at novel sites mediates receptor interaction with Src and P85 alpha

Following ligand binding, the epidermal growth factor receptor (EGF-R) autophosphorylates itself on tyrosine residues located in its carboxyl terminus; in vitro, three sites are highly phosphorylated, while two other sites are phosphorylated to lesser extents. In the presence of the Src protein-tyrosine kinase, in vitro phosphorylation of the minor autophosphorylation sites was increased, and four additional residues were phosphorylated. Following EGF stimulation, two (Tyr-891 and Tyr-920) were found to be phosphorylated in a colorectal cell line (DLD-1) and in a breast tumor cell line (MCF7). The remaining in vitro sites were not found to be highly phosphorylated in vivo. The sequences surrounding Tyr-891 and Tyr-920 match the reported consensus binding sequences for the SH2 domains of Src and the regulatory domain of phosphatidylinositol 3-kinase (p85 alpha), respectively. In vitro, both of these proteins were found to bind to Src-phosphorylated EGF-R with approximately 100-fold greater affinity than to autophosphorylated EGF-R, demonstrating that Src creates new sites for SH2 binding. Furthermore, Csk-inactivated Src was activated by interaction with Src-phosphorylated EGF-R but not by autophosphorylated EGF-R. Upon EGF treatment of MCF7 or three colorectal carcinoma cell lines (WiDr, DLD-1, and LS174T), the EGF-R coimmunoprecipitated with both p85 alpha and Src. Evidence is also presented that suggests that an EGF-R-related protein, ErbB2, may be involved in similar Src-mediated interactions. These data demonstrate that EGF-R is phosphorylated in vivo at non-autophosphorylation sites and that these novel sites can act as docking sites for Src, P85 alpha, and potentially other SH2-containing proteins. In addition, the data suggest a tyrosine phosphatase-independent mechanism for the elevation of Src activity in cells exposed to growth factors. Overexpression of Src, EGF-R, and/or ErbB2 in breast and colorectal tumor cells suggests the potential that such interactions may contribute to the transformed phenotype of these carcinomas.

HIV-1 glycoprotein gp120 disrupts CD4-p56lck/CD3-T cell receptor interactions and inhibits CD3 signaling

Using the CD4+ human T cell clone P28, we demonstrated that the HIV-1 glycoprotein gp120 inhibited CD3-induced inositol trisphosphate production, calcium influx and T cell proliferation. Additionally, gp120 was shown to dissociate the tyrosine kinase p56lck from CD4 in CEM cells, with a concommittant inhibition of CD4-linked kinase activity. We have addressed the question whether disruption of CD4/p56lck or CD4/CD3-T cell receptor interactions, or both, could account for the inhibitory effect of gp120 in P28 cells. By comparing the effects of various anti-CD4 monoclonal antibodies (mAb) with those of gp120, we show that gp120 and IOT4a modulate CD4 expression, and decrease CD4-associated p56lck and CD4-linked kinase activity at the plasma membrane. In contrast, OKT4A and OKT4 anti-CD4 mAb have no inhibitory effect. Interestingly, gp120 also inhibits CD3-induced Lck activation and cellular tyrosine phosphorylation, particularly of phosphoinositide-specific phospholipase C-gamma-1. Kinetic experiments reveal that the inhibitory effect of gp120 on CD3-induced tyrosine phosphorylation appears as early as 30 min, but culminate when CD4-p56lck complexes disappear from the cell surface after 4 h. These results suggest that a negative signal is triggered by gp120 that results, after a few hours, in down-modulation of CD4-p56lck complexes and the impairment of CD3 signaling. Supporting this hypothesis, gp120 inhibits CD3-linked kinase activity as shown by the inhibition of the phosphorylation of CD3 chains, leading to the inhibition of subsequent signal transduction.

Significance of PIP2 hydrolysis and regulation of phospholipase C isozymes

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important component of several intracellular signaling pathways. It serves as a substrate for phospholipase C, which produces the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. It is also a substrate for a phosphatidylinositol 3-kinase, and regulates the function of a number of actin-binding proteins. PIP2 has been shown recently to serve as a cofactor for a phosphatidylcholine-specific phospholipase D and as a membrane-attachment site for many signaling proteins containing pleckstrin homology domains. The need to stringently regulate the cellular concentration of PIP2 is reflected in part by the fact that there are at least ten distinct mammalian phospholipase C isozymes and multiple mechanisms linking these isozymes to various receptors.

Epidermal growth factor stimulates hydrolysis of phosphatidylinositol 4,5-bisphosphate, generation of diacylglycerol and exocytosis in mouse spermatozoa

Mouse spermatozoa stimulated with epidermal growth factor (EGF) or zona pellucida (ZP) experienced phosphatidylinositol 4,5-bisphosphate hydrolysis, diacylglycerol (DAG) generation and acrosomal exocytosis. The agonists showed additive effects but the action of EGF is likely to be mediated by a distinct receptor because maximal stimulation achieved with EGF was enhanced further by ZP. Generation of DAG and exocytosis stimulated by EGF were inhibited by tyrphostin A48, indicating that tyrosine kinase activity mediates EGF action. On the other hand, pertussis toxin did not affect the EGF-induced formation of DAG or exocytosis, ruling out the involvement of sperm Gi-like proteins. These results indicate that EGF could be an important co-factor in the initiation of exocytosis in spermatozoa.

An SH3 domain is required for the mitogenic activity of microinjected phospholipase C-gamma 1

Phospholipase activity is elevated in dividing cells. In response to growth factor stimulation, phospholipase C-gamma (PLC-gamma) binds to activated tyrosine kinase receptors via SH2 binding domains, resulting in phosphorylation of PLC-gamma and activation of its enzyme activity. These observations suggest that PLC-gamma participates in the signal transduction pathway employed by growth factors to promote mitogenesis. Consistent with this hypothesis, microinjection of purified bovine PLC-gamma into quiescent fibroblasts has been previously reported to initiate a mitogenic response [Smith et al. (1989) Proc. Natl. Acad. Sci. 86, 3659]. We have reproduced this result using recombinant rat PLC-gamma protein. Surprisingly, however, a catalytically inactive mutant of PLC-gamma, H335Q, also elicited a full mitogenic response. The capacity to induce mitogenesis by microinjection of PLC-gamma was mapped to the 'Z' domain of the protein, which contains PLC-gamma's SH2 and SH3 motifs. Inactivation of the phosphorylated tyrosine binding properties of both SH2 domains had no effect on the mitogenic activity of the Z-domain peptide. However, deletion of the SH3 domain resulted in a complete loss of activity. These results suggest that PLC-gamma's mitogenic properties do not require the enzyme's phospholipase activity, but are instead mediated by a novel pathway for mitogenic stimulation which is dependent upon an intact SH3 domain.

SH2 domain proteins as high-affinity receptor tyrosine kinase substrates

Activation of a growth factor receptor tyrosine kinase (RTK) is accompanied by a rapid autophosphorylation of the receptor on tyrosine residues. Receptor activation has been shown to promote the association of signal-transducing proteins containing SH2 domains (second domain of src homology). These receptor-associated proteins can, in turn, be phosphorylated by the RTK, an event which presumably regulates their activities. It has been suggested that SH2 domains in signal-transducing proteins target these proteins as substrates of the activated RTK. To test this hypothesis, recombinant proteins were generated that contained tyrosine phosphorylation sites of the erbB3 receptor and/or the SH2 domain of c-src. Incorporation of the SH2 domain led to a decrease in KM and an increase in Vmax for the substrate. The KM determined for one chimeric SH2/erbB3 substrate was among the lowest reported for epidermal growth factor RTK substrates. Experiments with a truncated kinase lacking C-terminal autophosphorylation sites indicated that the reduction in KM for these substrates was mediated by interactions between the substrate SH2 domain and phosphotyrosine residues of the RTK. These interactions could also inhibit RTK activity. These results demonstrate that the SH2 domain can effectively target substrates to a RTK and that SH2 domain proteins can regulate RTK activity.

Intracellular signalling mediated by protein-tyrosine kinases: networking through phospholipid metabolism

In recent years, it has become apparent that receptor-mediated intracellular signals are not linear cascades beginning at the plasma membrane and terminating with the production of a needed metabolite or the induction of gene expression. Instead, complex networks of interactive intracellular signals are activated in response to extracellular stimuli. Many responses to extracellular stimuli are mediated by protein-tyrosine kinases (PTKs). Activating PTKs leads to the recruitment of a variety of intracellular signalling molecules that execute a complex set of instructions. The response to PTK activity is dependent upon which PTK is activated and the cellular context in which the PTK exists. Several signalling molecules recruited by PTKs are involved in the metabolism of phospholipids. In this Mini Review, intracellular signalling networks activated by PTKs are discussed with an emphasis on the potential for generating highly specific and sophisticated responses to PTK activity through phospholipid metabolism.

The assembly of signalling complexes by receptor tyrosine kinases

Cell proliferation in response to growth factors is mediated by specific high affinity receptors. Ligand-binding by receptors of the protein tyrosine kinase family results in the stimulation of several intracellular signal transduction pathways. Key signalling enzymes are recruited to the plasma membrane through the formation of stable complexes with activated receptors. These interactions are mediated by the conserved, non-catalytic SH2 domains present in the signalling molecules, which bind with high affinity and specificity to tyrosine-phosphorylated sequences on the receptors. The assembly of enzyme complexes is emerging as a major mechanism of signal transduction and may regulate the pleiotropic effects of growth factors.

Growth factor second messenger systems: oncogenes and the heterotrimeric GTP-binding protein connection

We feel that there is now compelling evidence that the GTP-binding proteins play more than just a coordinating role in the actions of both tyrosine kinase and nontyrosine kinase receptor signal transduction. These similarities appear to represent just a small component of the convergence in the signaling pathways for structurally dissimilar receptor subsets. Future years will see further understanding of the intricacies of these G-protein-proto-oncogene interactions, and the extension into the potential role in growth factor action played by the expanding number of known members of this G-protein family.

Gi alpha and Gi beta are part of a signalling complex in Balb/c3T3 cells: phosphorylation of Gi beta in growth-factor-activated fibroblasts

Stimulation of division of Balb/c3T3 cells by epidermal growth factor (EGF) and/or insulin is inhibited by pertussis toxin. The G-protein involvement in this response includes the growth factor receptor-induced translocation of the alpha-subunit of Gi (Gi alpha) to the nucleus, where Gi alpha binds specifically to chromatin of dividing cells. This paper reports the first data of studies on the mode of interaction of tyrosine kinase growth factor receptors with Gi alpha, and the mechanism by which Gi affects cell proliferation. When Gi alpha was immunoprecipitated from Triton X-100 extracts of Balb/c3T3 cells, several other proteins were co-precipitated. The major proteins, of 110,000, 60,000 and 36,000 M(r), were not directly recognized by the Gi alpha antibody, showing that Gi alpha was in a complex with these proteins. The 36,000 M(r) protein was recognized by G beta-common antiserum, so confirming its identity as Gi beta. The 36,000 M(r) protein was phosphorylated in cells activated for 20 h with platelet-derived growth factor, epidermal growth factor and insulin, but not after 3 min or 1 h of stimulation. Both Gi alpha and G beta-common antibodies precipitated the phosphorylated 36,000 protein. Gi beta phosphorylation was similarly observed in response to activation by EGF alone for 20 h, but to a lesser extent. Phosphotyrosine antibodies also precipitated a 36,000 M(r) phosphorylated protein from growth factor-activated cells, suggesting that Gi beta may be phosphorylated on tyrosine. Therefore, Gi beta phosphorylation appears to represent a late event after activation of cells by tyrosine kinase growth factor receptors. We are currently examining the role of this event in signal transduction, particularly in relation to control of nuclear responses.