Tyrosine kinase activity is essential for the association of phospholipase C-gamma with the epidermal growth factor receptor

Comparative effects of hepatocyte growth factor and epidermal growth factor on motility, morphology, mitogenesis, and signal transduction of primary rat hepatocytes

Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are major hepatocyte mitogens, but HGF, also known as scatter factor (SF), has also been shown as a potent motogen for epithelial and endothelial cells. The mechanisms by which HGF is a stronger motogen compared to other mitogens are not understood. Here we report a comparative study of the effect of the two growth factors on cultured primary rat hepatocytes regarding their differential effects on morphology, mitogenicity, and motility as well as the phosphorylation of cytoskeletal-associated proteins. Using three different motility assays, both HGF and EGF increased the motility of hepatocytes, but HGF consistently elicited a significantly greater motility response than EGF. Additionally, HGF induced a more flattened, highly spread morphology compared to EGF. To examine if HGF and EGF phosphorylated different cytoskeletal elements as signal transduction targets in view of the observed variation in morphology and motility, primary cultures of 32P-loaded rat hepatocytes were stimulated by either HGF or EGF for up to 60 min. Both mitogens rapidly stimulated four isoforms of MAP kinase with similar kinetics and also rapidly facilitated the phosphorylation of cytoskeletal-associated F-actin. Two cytoskeletal-associated proteins, however, were observed to undergo rapid phosphorylation by HGF and not EGF during the time points described. One protein of 28 kDa was observed to become phosphorylated fivefold over controls, while the EGF-stimulated cells showed only a slight increase in the phosphorylation of this protein. Another protein with an apparent mwt of 42 kDa was phosphorylated 20-fold at 1 min and remained phosphorylated over 50-fold over control up to the 60 min time point. This protein was observed to become phosphorylated by EGF only after 10 min, and to a lesser extent (20-fold). Taken together, the data suggest that HGF and EGF stimulate divergent as well as redundant signal transduction pathways in the hepatocyte cytoskeleton, and this may result in unique HGF- or EGF-specific motility, morphology, and mitogenicity in hepatocytes.

Receptor tyrosine kinases and their targets

One of the ways in which higher eukaryotes receive messages from the environment is via cell surface receptor tyrosine kinases. These are transmembrane proteins with an extracellular binding domain that specifies the growth factor with which it will interact, and an intracellular domain that encodes the tyrosine kinase. The mechanism by which receptor tyrosine kinases direct intracellular signal relay appears to involve receptor autophosphorylation that permits the stable binding of SH2 domain containing signal transduction enzymes. Some of the more recent advances are summarized in this review.

Platelet-derived growth factor AB heterodimer interchain interactions influence secretion as well as receptor binding and activation

Platelet-derived growth factor (PDGF) is a disulfide-linked dimer comprised of two related polypeptide chains. To investigate the effects of an inactivating lesion introduced into one chain of the nascent PDGF dimer, approaches were developed to optimize synthesis, assembly, secretion, and purification of heterodimers between normal PDGF A and wild-type or mutant PDGF B. PDGF AB heterodimers were released into culture fluids less efficiently than PDGF AA, but to a greater degree than the cell-associated PDGF BB. These results suggest that interactions between two chains influence PDGF secretion. Analysis of heterodimers between PDGF A and disabled PDGF B mutants on cells that express either alpha or beta PDGFRs demonstrated that the impaired biologic activity of the mutant PDGF B chain was ameliorated with respect to binding and triggering of alpha PDGFRs. In cells that expressed both receptor types, heterodimers of mutant PDGF B and wild-type PDGF A gained substantially in their ability to recruit and trigger alpha, but not beta, PDGFRs. Partial rescue of impaired PDGF B mutant chain function by dimer formation with a wild-type PDGF A chain implies that interchain interactions markedly affect PDGFR binding and activation.

Thrombin activation of human platelets causes tyrosine phosphorylation of PLC-gamma 2

Human platelets contain phospholipase C (PLC)-gamma 2, a distinct isoform closely related to PLC-gamma 1. Both inositol phospholipid-specific phospholipases C contain the src-related SH2 regions. Stimulation of platelets with the potent agonist, thrombin, led to a rapid and transient phosphorylation of PLC-gamma 2 on tyrosine residues. Activated platelets lysed in the absence of sodium orthovanadate had levels of tyrosine-phosphorylated PLC-gamma 2 paralleling those seen in unstimulated platelets. Previously, it had been shown that PLC-gamma 1 was phosphorylated on tyrosine residues by the agonist-occupied platelet-derived growth factor (PDGF) receptor and epidermal growth factor (EGF) receptor in cells other than platelets. In addition, more recent data have indicated that PLC-gamma 2 is also capable of being tyrosine-phosphorylated in cells of hematopoietic origin, such as B cells and natural killer (NK) cells. Here we report that PLC-gamma 2 expressed in a terminally-differentiated hematopoietic cell is also tyrosine-phosphorylated in response to an agonist.

Therapy with central active catechol-O-methyltransferase (COMT)-inhibitors: is addition of monoamine oxidase (MAO)-inhibitors necessary to slow progress of neurodegenerative disorders?

Neurotrophic factors, like e.g. nerve growth factor (NGF), neurotrophin 3 (NT-3) or brain-derived neurotrophic factor (BDNF) promote the survival and function of neurones in the peripheral and central nervous system. Dopamine or other biogenic amines induce the biosynthesis of neurotrophic factors in glial and neuronal cells. Therefore inhibition of enzymes, like the extraneuronal and neuronal located MAO or the predominantly glial situated COMT, which both metabolize catecholamines, may induce an increased biosynthesis of neurotrophic factors. Due to clinical studies especially MAO-B-inhibitors appear to slow the progression of neurological deficits in Parkinson's disease and the cognitive decline in Alzheimer's disease. On the one hand inhibition of COMT alone may also slow the metabolisation of biogenic amines in glial cells and may consequently induce synthesis of neurotrophic factors in glial cells. But on the other hand in vivo and in vitro studies show, that COMT-inhibitors may intensify the metabolisation of catecholamines in neurones by MAO, what may cause an enhanced generation of free radicals. This increase of free radicals may induce lipid peroxidation of membranes and therefore cause accelerated neuronal and glial cell death. For that reason we conclude, that centrally active COMT-inhibitors may only be used together with MAO-inhibitors in the neuroprotective treatment of neurodegenerative disorders. Medical treatment with both inhibitors will have to be performed very carefully due to cytotoxic effects of high catecholamine levels on neuronal and glial cells and due to possible prolongation or potentiation of the activity of several noradrenergic drugs in the periphery.

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.

Modulating effects of the extracellular sequence of the human insulinlike growth factor I receptor on its transforming and tumorigenic potential

We reported previously that an N-terminally truncated insulinlike growth factor I receptor (IGFR) fused to avian sarcoma virus UR2 gag p19 had a greater transforming potential than did the native IGFR, but it failed to cause tumors in vivo. To investigate whether the 36 amino acids (aa) of the IGFR extracellular (EC) sequence in the gag-IGFR fusion protein encoded by the retrovirus UIGFR have a modulatory effect on the biological and biochemical properties of the protein, four mutants, NM1, NM2, NM3, and NM4 of the EC sequence were constructed. NM1 lacks the entire 36 aa residues; NM2 lacks the N-terminal 16 aa residues (aa 870 to 885), including two potential N-linked glycosylation sites of the EC sequence; NM3 contains a deletion of the C-terminal 20 aa residues (aa 886 to 905) of the EC sequence; and NM4 contains N-to-Q substitutions at both N-linked glycosylation sites. NM1 was the strongest of the four mutants in promoting anchorage-independent growth of transfected chicken embryo fibroblasts, while NM2 and NM4 had weaker transforming potential than did the original UIGFR virus. Only NM1 and NM3 were able to induce sarcomas in chickens. The four NM mutant-transformed cells expressed the expected proteins with comparable steady-state levels. The in vitro tyrosine kinase activity of P53NM1 was about fourfold higher than that of the parental P57-75UIGFR, whereas NM2 and NM4 proteins exhibited four- to fivefold-lower kinase activities. Despite lacking the IGFR EC sequence, P53NM1 formed covalent dimers similar to those formed by the parental P57-75UIGFR. Increased phosphatidylinositol (PI) 3-kinase activity was found to be associated with the mutant IGFR proteins. Among NM4 proteins. Elevated tyrosine phosphorylation of cellular proteins of 35, 120, 140, 160, and 170 kDa was detected in all mutant IGFR-transformed cells. We conclude that the EC 36-aa sequence of IGFR in the gag-IGFR fusion protein exerts intricate modulatory effects on the protein's transforming and tumorigenic potential. The 20 aa residues immediately upstream of the transmembrane domain have an inhibitory effect on the tumorigenic potential of gag-IGFR, whereas N-linked glycosylation within the EC sequence appears to have a positive effect on the transforming potential of UIGFR. Increased in vitro kinase activity and, to a lesser extent, in vivo tyrosine phosphorylation as well as the elevated association of PI 3-kinase activity with IGFR proteins seem to be correlated with the transforming potential of IGFR mutant proteins.

Activation of phosphatidylinositol-3 kinase by nerve growth factor involves indirect coupling of the trk proto-oncogene with src homology 2 domains

Growth factor receptor tyrosine kinases can form stable associations with intracellular proteins that contain src homology (SH) 2 domains, including the p85 regulatory subunit of phosphatidylinositol (PI)-3 kinase. The activation of this enzyme by growth factors is evaluated in PC12 pheochromocytoma cells and NIH 3T3 fibroblasts expressing the pp140c-trk nerve growth factor (NGF) receptor (3T3-c-trk). NGF causes the rapid stimulation of PI-3 kinase activity detected in anti-phosphotyrosine, but not in anti-trk, immunoprecipitates. This effect coincides with the tyrosine phosphorylation of two proteins, with molecular masses of of 100 kd and 110 kd, that coimmunoprecipitate with p85. Similar phosphorylation patterns are induced when an immobilized fusion protein containing the amino-terminal SH2 domain of p85 is used to precipitate tyrosine-phosphorylated proteins. Thus, although NGF produces the rapid activation of PI-3 kinase through a mechanism that involves tyrosine phosphorylation, there is no evidence for tyrosine phosphorylation of p85, or for its ligand-dependent association with the NGF receptor. Perhaps another phosphoprotein may link the NGF receptor to this enzyme.

The SH2 and SH3 domain-containing protein GRB2 links receptor tyrosine kinases to ras signaling

A cDNA clone encoding a novel, widely expressed protein (called growth factor receptor-bound protein 2 or GRB2) containing one src homology 2 (SH2) domain and two SH3 domains was isolated. Immunoblotting experiments indicate that GRB2 associates with tyrosine-phosphorylated epidermal growth factor receptors (EGFRs) and platelet-derived growth factor receptors (PDGFRs) via its SH2 domain. Interestingly, GRB2 exhibits striking structural and functional homology to the C. elegans protein sem-5. It has been shown that sem-5 and two other genes called let-23 (EGFR like) and let-60 (ras like) lie along the same signal transduction pathway controlling C. elegans vulval induction. To examine whether GRB2 is also a component of ras signaling in mammalian cells, microinjection studies were performed. While injection of GRB2 or H-ras proteins alone into quiescent rat fibroblasts did not have mitogenic effect, microinjection of GRB2 together with H-ras protein stimulated DNA synthesis. These results suggest that GRB2/sem-5 plays a crucial role in a highly conserved mechanism for growth factor control of ras signaling.

Intracellular signalling pathways involved in mesoderm induction by FGF

We have examined the possible role of two signal transducing mechanisms, tyrosine phosphorylation and activation of protein kinase C (PKC), during fibroblast growth factor (FGF)-induced mesoderm induction in Xenopus. Tyrosine phosphorylation was examined through the use of a monoclonal anti-phosphotyrosine antibody. This antibody was shown to recognize the FGF receptor crosslinked to radioiodinated FGF. We also studied the response of Xenopus ectodermal explants to sodium orthovanadate, a compound that has been shown to elevate intracellular phosphotyrosine levels. Thirty percent of explants cultured in 100 microM vanadate were induced. In addition, vanadate synergized with FGF to give inductions that were more dorsal in nature than either vanadate or FGF alone. The role of PKC was evaluated by measuring PKC activity during mesoderm induction by FGF and by examining the effect of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) on explants. TPA did not induce mesoderm, however, activation of PKC was detected in FGF-treated explants. Therefore, activation of the PKC pathway alone is not sufficient for mesoderm induction. Simultaneous treatment with TPA and FGF resulted in a significant inhibition of mesoderm induction by FGF, suggesting that activation of PKC could be part of a negative feedback mechanism. In contrast, TPA had no effect on induction by activin A.

Regulation of phospholipase C isozymes

Phosphatidylinositol bisphosphate hydrolysis is an immediate response to many hormones, including growth factors. The hydrolysis of phosphatidylinositol bisphosphate is catalyzed by phosphatidylinositol-specific phospholipase C. A number of phospholipase C isozymes have been identified. Different isozymes are activated by different receptor classes. This review will summarize the different isozymes of phospholipase C, and the current knowledge of the mechanisms by which phospholipase C activity is modulated by growth factors.

Highly conserved eight amino acid sequence in SH2 is important for recognition of phosphotyrosine site

Src homology region 2(SH2) has been demonstrated to recognize phosphotyrosine site. To clarify the precise mechanism of the recognition, we developed in vitro binding assay system using EGF receptor and SH2/SH3 region of phospholipase C(PLC) gamma 1. Phosphorylated EGF receptor bound to immobilized SH2/SH3 of PLC gamma 1 in Sepharose beads, while nonphosphorylated EGF receptor did not bind. In SH2 domain of PLC gamma 1, there are several highly conserved amino acid sequences that are common in a variety of SH2-containing proteins. Especially the eight amino acid sequence, G(S/T)FLVR(E/D)S is highly conserved in these proteins. We synthesized several peptides related to these sequences and examined the effect of peptides on the binding of EGF receptor to SH2 of PLC gamma 1. P1, GSFLVRES was the most effective inhibitor to suppress the binding. P2, GSFLVAES in which one amino acid, arginine of P1 is substituted by alanine is still effective. But a peptide, P3, SFLVRE in which two amino acids are deleted from P1 did not inhibit markedly. Moreover, P1 peptide immobilized in Sepharose beads also bound phosphorylated EGF receptor. These data suggest that highly conserved amino acid sequence GSFLVRES is the minimum essential unit to recognize tyrosine phosphorylated site.

Nerve growth factor binds to the 140 kd trk proto-oncogene product and stimulates its association with the src homology domain of phospholipase C gamma 1

The cellular actions of nerve growth factor (NGF) involve regulation of protein phosphorylation. In PC-12 pheochromocytoma cells, exposure of [125I]NGF followed by crosslinking indicates that the ligand binds to two discreet receptors, the previously described 75 kd protein, as well as the trk proto-oncogene product pp140c-trk. Competition experiments reveal that of the two, pp 140c-trk binds to NGF with higher affinity. Following exposure to NGF, pp140c-trk undergoes a rapid autophosphorylation on tyrosine residues, and concomitantly phosphorylates and associates with phospholipase C gamma 1 (PLC gamma 1), through interaction with its src homology domains. The binding of NGF to pp140c-trk with high affinity, the NGF-dependent homology domains. The binding of NGF to pp140c-trk with high affinity, the NGF-dependent activation of its tyrosine kinase activity and the specific association with the effector molecule, PLC gamma 1, suggests that this is the biologically relevant signaling receptor for NGF.

Interaction of epidermal growth factor with vasoactive hormones in the regulation of phospholipase A2

The interaction of growth factors with their receptors initiates a series of intracellular events that are of critical importance in the control of normal cell proliferation. In this regard considerable attention has focused on the coupling of phospholipase C-gamma to growth factor receptor tyrosine kinases. In contrast, the interaction of growth factors with phospholipase A2 has received less attention, most likely because the arachidonic acid release response has been considered to be an accompaniment of phospholipase C activation. Work from our laboratory using a well defined model system demonstrated a distinct coupling relationship of epidermal growth factor to phospholipase A2. This review focuses on the interaction of the epidermal growth factor receptor with phospholipases involved in both mitogenic and non-mitogenic responses and discusses their possible relation with vasoactive hormones.

PDGF stimulation of inositol phospholipid hydrolysis requires PLC-gamma 1 phosphorylation on tyrosine residues 783 and 1254

PDGF binding to its receptor promotes the association with and stimulates the phosphorylation of PLC-gamma 1 at tyrosine and serine residues. Also, PDGF induces an increase in the hydrolysis of inositol phospholipids by PLC. How PDGF activates PLC was investigated by substituting phenylalanine for tyrosine at PLC-gamma 1 phosphorylation sites 771, 783, and 1254 and expressing the mutant enzymes in NIH 3T3 cells. Phenylalanine substitution at Tyr-783 completely blocked the activation of PLC by PDGF, whereas mutation at Try-1254 inhibited and mutation at Tyr-771 enhanced the response. Like the wild type, PLC-gamma 1 substituted with phenylalanine at Tyr-783 became associated with the PDGF receptor and underwent phosphorylation at serine residues in response to PDGF. These results suggest that PLC-gamma 1 is the PLC isozyme that mediates PDGF-induced inositol phospholipid hydrolysis, that phosphorylation on Tyr-783 is essential for PLC-gamma 1 activation. These results provide direct evidence that growth factor receptors activate the function of intracellular protein by tyrosine phosphorylation.

Cloning of PI3 kinase-associated p85 utilizing a novel method for expression/cloning of target proteins for receptor tyrosine kinases

A novel method has been developed to allow cloning of protein targets for receptors with tyrosine kinase activity. By utilizing the carboxy-terminal tail of EGF receptor (EGFR) as a probe to screen lambda gt11 expression libraries, several EGFR-binding proteins have been cloned; two have been analyzed and contain unique SH2 and SH3 domains. One gene (GRB-1) has been fully sequenced, is expressed in various tissues and cell lines, and has a molecular mass of 85 kd. Interestingly, GRB-1 encodes the human counterpart of the PI3 kinase-associated protein p85. Advantages of this technique include the ease of cloning tyrosine kinase receptor targets present at low levels and the ability to identify proteins that are related in their capacity to bind activated receptors but contain no significant DNA sequence homology. This method, termed CORT (for cloning of receptor targets), offers a general approach for the identification and cloning of various receptor targets.

Characterization of two 85 kd proteins that associate with receptor tyrosine kinases, middle-T/pp60c-src complexes, and PI3-kinase

Affinity-purified bovine brain phosphatidylinositol 3-kinase (PI3-kinase) contains two major proteins of 85 and 110 kd. Amino acid sequence analysis and cDNA cloning reveals two related 85 kd proteins (p85 alpha and p85 beta), which both contain one SH3 and two SH2 regions (src homology regions). When expressed, these 85 kd proteins bind to and are substrates for tyrosine-phosphorylated receptor kinases and the polyoma virus middle-T antigen/pp60c-src complex, but lack PI3-kinase activity. However, an antiserum raised against p85 beta immunoprecipitates PI3-kinase activity. The active PI3-kinase complex containing p85 alpha or p85 beta and the 110 kd protein binds to PDGF but not EGF receptors. p85 alpha and p85 beta may mediate specific PI3-kinase interactions with a subset of tyrosine kinases.

Tyrphostins inhibit PDGF-induced DNA synthesis and associated early events in smooth muscle cells

Tyrphostins are low-molecular-weight synthetic inhibitors of protein tyrosine kinase, which block cell proliferation. Since platelet-derived growth factor (PDGF) is thought to figure prominently in disorders of vascular smooth muscle cells (VSMC), such as atherosclerosis, hypertension, and restenosis, we examined whether tyrphostins would inhibit PDGF-induced mitogenesis in VSMC. In this communication, we demonstrate that tyrphostins with the benzenemalononitrile nucleus inhibited PDGF-dependent growth of VSMC as well as PDGF-dependent DNA synthesis in these cells, with the concentrations for 50% inhibition ranging from 0.04 to 9 microM. Up to 30-fold higher tyrphostin concentrations were required to inhibit serum-stimulated DNA synthesis of VSMC. The effect of the tyrphostins is reversible, since on their removal a normal proliferative response to PDGF was resumed. Tyrphostins also inhibited PDGF-receptor autophosphorylation and PDGF-induced phosphorylation of intracellular substrates, including the phosphorylation of phospholipase C-gamma, with a potency ratio similar to their antimitogenic activity. The expression of c-fos mRNA, a mitogenic nuclear signal, was also reduced in PDGF-stimulated VSMC treated with tyrphostins at concentrations which inhibit PDGF-induced mitogenesis. It is concluded that tyrphostins are potent reversible inhibitors of PDGF-induced mitogenesis which act by inhibiting the tyrosine kinase activity of the PDGF receptor and the subsequent signaling cascade. Tyrphostins may be useful in the study and treatment of VSMC proliferation disorders.