A PDGF receptor domain essential for mitogenesis but not for many other responses to PDGF

KIDs rule: regulatory phosphorylation of RTKs

Receptor tyrosine kinases (RTKs) are mediators of multiple cell signaling networks linked to cell growth and differentiation. In general, they exhibit similar overall structure with a ligand-binding extracellular domain and a conserved intracellular tyrosine kinase domain. In many RTKs, the kinase domain is interrupted by a sequence known as the kinase insert domain (KID). In addition to phosphorylation sites within the kinase domain, regulatory phosphorylation also occurs within the KID of several RTKs important in human health and disease. Phosphorylation of specific Tyr or Ser residues within the KID of some RTKs triggers distinct cellular signaling outcomes. Here, we review the functionality of KIDs throughout all RTK families, and provide justification for further study of this often-overlooked domain.

The mas oncogene as a neural peptide receptor: expression, regulation and mechanism of action

The human mas oncogene, which renders transfected NIH/3T3 cells tumorigenic, was identified as a subtype of angiotensin receptor by transient expression in Xenopus oocytes and stable expression in the mammalian neuronal cell line, NG115-401L. The mas receptor preferentially recognizes angiotensin III, and is expressed at high levels in brain. The mas/angiotensin receptor functions through the breakdown of inositol lipids and can drive DNA synthesis, unlike another inositol-linked peptide receptor, that for bradykinin. Comparative analysis of several early biochemical events elicited by either angiotensin or bradykinin stimulation of mas-transfected cells has not indicated a specific difference correlated with mitogenic activity. In particular, the inositol lipid kinase, phosphatidylinositol-3-kinase, thought to be involved in the mitogenic mechanism of platelet-derived growth factor receptors, is unaffected by activation of mas. These results have shown that a proto-oncogene encodes a neural peptide receptor, indicating that peptide receptors may be involved in differentiation and proliferation processes, as are other identified proto-oncogenes.

Structural and functional aspects of platelet-derived growth factor and its receptors

Platelet-derived growth factor (PDGF) is a dimeric molecule that exists as homodimers or heterodimers of related polypeptide chains (A and B). Two types of PDGF receptor have been identified. The PDGF alpha-receptor binds all three isoforms with high affinity whereas the beta-receptor binds only PDGF-BB with high affinity, PDGF-AB with low affinity and does not appear to bind PDGF-AA. The alpha- and beta-receptors are structurally related, each having an intracellular protein tyrosine kinase domain. Ligand-induced functional activation of the receptors appears to involve receptor dimerization. Binding of PDGF to its receptor is followed by internalization and degradation of the ligand-receptor complex. Experiments with mutant receptors have shown that ligand-induced internalization is not absolutely dependent on the kinase activity of the beta-receptor. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral version of the PDGF B chain gene and SSV-transformation is mediated by an autocrine PDGF-like growth factor. Formal evidence that the expression of the PDGF beta-receptor is sufficient to confer susceptibility to SSV-transformation has been obtained using porcine endothelial cells expressing a recombinant human beta-receptor. PDGF is a chemotactic agent for several cell types. Recent experiments have shown that the PDGF beta-receptor mediates a chemotactic response and that this effect requires an intact protein tyrosine kinase activity.

The priming/completion paradigm to explain growth factor-dependent cell cycle progression

Approximately 50 years ago, researchers established conditions to maintain cells in tissue culture: Likely et al. (1952), Scherer et al. (1953), Eagle (1955). This simple model system set the stage for discovery of growth factors and the signaling systems that they engage to mediate cellular responses such as proliferation. The purpose of this review is to present the original view of how growth factors regulate cell cycle progression and an updated (priming/completion) version of how growth factors advance resting cells through the cell cycle.

PDGF C is a selective alpha platelet-derived growth factor receptor agonist that is highly expressed in platelet alpha granules and vascular smooth muscle

OBJECTIVE

The platelet-derived growth factor (PDGF) family consists of four members, PDGF A, PDGF B, and 2 new members, PDGF C and PDGF D, which signal through the alpha and beta PDGF receptor (PDGFR) tyrosine kinases. This study was performed to determine the receptor specificity and cellular expression profile of PDGF C.

METHODS AND RESULTS

PDGF C growth factor domain (GFD) was shown to preferentially bind and activate alpha PDGFR and activate beta PDGFR when it is co-expressed with alpha PDGFR through heterodimer formation. An investigation of PDGF C mRNA and protein expression revealed that during mouse fetal development, PDGF C was expressed in the mesonephric mesenchyme, prefusion skeletal muscle, cardiac myoblasts, and in visceral and vascular smooth muscle, whereas in adult human tissues expression was largely restricted to smooth muscle. Microarray analysis of various cell types showed PDGF C expression in vascular smooth muscle cells, renal mesangial cells, and platelets. PDGF C mRNA expression in platelets was confirmed by real-time polymerase chain reaction, and PDGF C protein was localized in alpha granules by immuno-gold electron microscopy. Western blot analysis of platelets identified 55-kDa and 80-kDa PDGF C isoforms that were secreted on platelet activation.

CONCLUSIONS

Taken together, our results demonstrated for the first time to our knowledge that like PDGF A and B, PDGF C is likely to play a role in platelet biology.

Dissociation between the Ca(2+) signal and tube formation induced by vascular endothelial growth factor in bovine aortic endothelial cells

The correlation between the intracellular Ca(2+) signal and the tube formation in collagen gels induced by vascular endothelial cell growth factor (VEGF) was investigated using cultured bovine aortic endothelial cells. The VEGF-induced sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](i)) was similarly inhibited by 10 microM 1-¿beta-[3-(4-methoxyphenyl)propyl]-4-methoxyphenethyl¿-1H-imidazole hydrochloride (SKF 96365) and 10 microM troglitazone. However, 10 microM diltiazem had no effect. The basal tube formation obtained with 1% serum was augmented twofold by 100 ng/ml VEGF. SKF 96365 (0. 1-10 microM) inhibited the VEGF-induced and basal tube formation, while 10 microM troglitazone or 10 microM diltiazem had no effect. The proliferation of endothelial cells was markedly inhibited by SKF 96365 but only slightly by troglitazone and diltiazem. The inhibition of tube formation by three Ca(2+) entry blockers thus correlated with the inhibition of cell proliferation. The [Ca(2+)](i) elevation is thus not a prerequisite for VEGF to induce tube formation.

PDGF induces an early and a late wave of PI 3-kinase activity, and only the late wave is required for progression through G1

BACKGROUND

Platelet-derived growth factor (PDGF) triggers cytoskeletal rearrangements and chemotaxis within minutes. These events are at least in part due to the activation of phosphoinositide (PI) 3-kinase; there is good temporal correlation between these events and the accumulation of 3-phosphorylated products of the kinase. Prolonged and continuous PDGF exposure results in S-phase entry many hours after the initial burst of activity. Although early signals appear responsible for the early responses, they may not fully account for later responses, such as cell-cycle progression.

RESULTS

We assessed when PI 3-kinase products accumulate in PDGF-stimulated cells. In addition to the previously identified early accumulation of products, we detected a second, prolonged wave of accumulation 3-7 hours after stimulation. To determine the relative contribution of each phase to PDGF-dependent DNA synthesis, we first developed an assay in which synthetic 3-phosphorylated lipids were used to rescue DNA synthesis in cells expressing a PDGF-receptor mutant. The lipids rescued DNA synthesis only when added 2-6 hours after PDGF. In addition, PI 3-kinase inhibitors failed to block PDGF-dependent DNA synthesis if added during the first wave of PI 3-kinase activity, but adding them later, in G1 phase, prevented PDGF-dependent cell-cycle progression.

CONCLUSIONS

PDGF induces distinct waves of PI 3-kinase activity. The second wave is required for PDGF-dependent DNA synthesis, whereas the initial wave is not. One of the ways in which cells use PI 3-kinase to mediate distinct cellular responses seems to be by regulating when its products accumulate.

Platelet-derived growth factor (PDGF) BB acts as a chemoattractant for human malignant mesothelioma cells via PDGF receptor beta-integrin alpha3beta1 interaction

Platelet-derived growth factor BB (PDGF BB) and the PDGF receptor beta are expressed on mesothelioma cells, but their biological function has not yet been defined. In the present study we used Boyden chambers fitted with filters coated with the adhesive matrix proteins fibronectin, laminin, collagen type IV or the nonmatrix adhesive molecule poly-L-lysine (PLL). Mesothelioma cells migrated towards PDGF BB at concentrations ranging from 0.78 to 12.5 ng/ml if matrix proteins were present as adhesive substrates. This migration was integrin dependent since the same cells failed to migrate if the adhesive interactions necessary for migration were provided by molecules other than integrins. Migration of mesothelioma cells on fibronectin, laminin or collagen-type IV in response to PDGF BB was inhibited if the cells were pretreated with blocking antibodies to alpha3beta1 integrin. These findings describe for the first time PDGF BB as a chemoattractant for malignant mesothelioma cells and that collaboration between PDGF receptor beta and integrin alpha3beta1 is necessary for the motile response of these cells to PDGF BB.

Indolocarbazoles: potent and selective inhibitors of platelet-derived growth factor receptor autophosphorylation

A quantitative assay for measuring the autophosphorylation of platelet-derived growth factor (PDGF) receptors in intact vascular smooth muscle cells has been developed and used to screen for novel tyrosine kinase (TK) inhibitors. Several novel inhibitors of PDGF receptor autophosphorylation have been identified from the indolocarbazole series, including the 3,9 dimethoxy derivative, 3744W (IC50 = 14.5+/-2 nM). Tested against a panel of tyrosine and serine/threonine kinases, 3744W is at least 1,000 fold selective for the PDGF receptor tyrosine kinase and was found to inhibit autophosphorylation of both the alpha and beta isoforms of the PDGF receptor in human smooth muscle cells. PDGF-BB-stimulated DNA synthesis in quiescent cultures of human smooth muscle cells was blocked in a concentration-dependent manner by 3744W, IC50 = 10 nM. Binding studies showed that 3744W did not block the binding of PDGF-BB to cell surface receptors on human airway smooth muscle cells. Furthermore, inhibition of bone marrow stem cell proliferation by 3744W was only observed at concentrations 100-1,000 times greater than those needed to block PDGF-driven DNA synthesis in human smooth muscle cells. 3744W represents a novel, potent and selective inhibitor of PDGF receptor autophosphorylation and a powerful biochemical probe for investigating PDGF-dependent responses in vitro.

Non-capacitative calcium entry in Chinese hamster ovary cells expressing the platelet-derived growth factor receptor

Platelet-derived growth factor (PDGF) is believed to produce intracellular calcium (Ca2+i) transients by inositol trisphosphate (InsP3)-mediated release of intracellular Ca2+ stores followed by "capacitative" Ca2+ entry due to emptying of these stores. We examined the roles for the phospholipase Cgamma-InsP3 pathway and the emptying of InsP3-dependent intracellular Ca2+ stores in PDGF-mediated Ca2+ entry. Intracellular Ca2+ release and Ca2+ entry were measured with fluorometric methods in Chinese hamster ovary cells expressing wild type or mutant PDGF receptors. Activation of the wild type PDGF receptor caused both intracellular "Ca2+ release, " measured in nominally 0 Ca2+ extracellular medium, and "Ca2+ entry, " measured upon addition of 2 mM Ca2+ medium. Both phases were absent in Chinese hamster ovary cells expressing a PDGF receptor mutant (Y977F,Y989F) that fails to bind phospholipase Cgamma. Blockade of the InsP3 receptor, by microinjection of single cells with low molecular weight heparin (5-50 mg/ml), blocked only Ca2+i release (following PDGF or flash photolysis of caged InsP3) and had no effect on PDGF-induced Ca2+ entry. In whole cell patch-clamp experiments, intracellular heparin also failed to block PDGF-evoked ion currents. Release of InsP3-dependent intracellular Ca2+ stores, by flash photolysis of caged InsP3, was apparently not sufficient to maximally activate Ca2+ entry. Intracellular InsP3 caused significantly less Ca2+ entry than PDGF alone. These data suggest that InsP3 alone is not sufficient to maximally activate Ca2+ entry by the capacitative pathway and that products of phosphatidylinositol 4,5-bisphosphate breakdown other than InsP3 probably play a role in PDGF-mediated Ca2+ entry.

Differential effects of lovastatin on mitogen induced calcium influx in human cultured vascular smooth muscle cells

1. In this study the effect of lovastatin, an inhibitor of cholesterol and isoprenoid synthesis, on the rises in intracellular calcium concentration ([Ca2+]i) induced by platelet derived growth factor BB (PDGF-BB), angiotensin II (AII), low density lipoproteins (LDL) and foetal calf serum (FCS) was examined in human cultured vascular smooth muscle cells (VSMC) from saphenous vein. Changes in [Ca2+]i were measured in cell suspensions by the Ca2+ sensitive probe, fura 2. 2. Incubation with lovastatin for 24-26 h markedly reduced the peak rise and sustained phase of [Ca2+]i elevation in response to PDGF-BB but the responses to AII, LDL and FCS were unaffected. Further experiments showed that lovastatin pretreatment inhibited PDGF-BB induced Ca2+ influx but not intracellular Ca2+ release. This inhibition could be overcome by co-incubation with mevalonic acid. 3. Pretreatment of cells with the heterotrimeric G protein inhibitor pertussis toxin for up to 24 h completely abolished AII-induced [Ca2+]i rises but the response to PDGF-BB was unaffected. 4. The tyrosine kinase inhibitor genistein largely abolished PDGF-BB-induced [Ca2+]i elevation but had no significant effect on AII-induced responses. 5. Pre-incubation with lovastatin had no effect on the level of tyrosine phosphorylation of PDGF-beta receptors (as measured by Western blot) in response to the PDGF-BB ligand. 6. PDGF-BB elicits Ca2+ influx via a tyrosine kinase-dependent mechanism distinct from the heterotrimeric G protein coupled pathway utilized by AII. Lovastatin most likely acts by inhibition of isoprenylation (via blockade of isoprenoid synthesis) of an intermediate molecule involved in PDGF-BB-induced Ca2+ influx.

Genistein selectively inhibits platelet-derived growth factor-stimulated versican biosynthesis in monkey arterial smooth muscle cells

Platelet-derived growth factor (PDGF) stimulates not only the proliferation and migration of arterial smooth muscle cells (ASMCs), but also the transcription, translation, and posttranslational processing of versican, a large chondroitin sulfate proteoglycan present in the extracellular matrix of blood vessels. PDGF receptor tyrosine kinase activity is required for signaling events associated with mitogenic and motogenic stimulation of cells by PDGF. Therefore, we have asked if inhibiton of tyrosine kinase activity by genistein also blocks the stimulation of both versican core protein synthesis and glycosaminoglycan (GAG) chain modifications induced by PDGF in ASMCs. The tyrosine kinase inhibitor, genistein, in a dose-dependent manner, reversibly inhibits PDGF-stimulated ASMC cell proliferation and RNA and core protein expression of versican, without affecting the expression of decorin and biglycan. In contrast, genistein does not affect the increase in GAG chain elongation that is induced by PDGF. This suggests that different aspects of the biosynthesis of versican are differentially regulated. To determine if such differential regulation involves downstream activation of protein kinase C, ASMCs were treated with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to directly activate this kinase. In comparison to PDGF stimulation, TPA has little effect on expression of versican mRNA expression, nor does TPA stimulate ASMC cell proliferation. However, like PDGF, TPA increases [35S]sulfate incorporation into proteoglycans and GAG chain elongation. These results indicate that PDGF-induced GAG chain elongation, which is not inhibited by genistein treatment and is stimulated by protein kinase C activation, involves signaling pathways different from those that regulate PDGF-stimulated versican mRNA and protein expression.

Cytoplasmic and nuclear localization sites of phosphatidylinositol 3-kinase in human osteosarcoma sensitive and multidrug-resistant Saos-2 cells

The intracellular localization of phosphatidyl-inositol 3-kinase (PI 3-kinase) has been analyzed by western blotting, confocal, and electron microscopy immunocytochemistry in human osteosarcoma Saos-2 cells. By western blotting, the enzyme appears to be present in both the cytoplasmic and nuclear subfractions. By confocal microscope immunocytochemistry, the cytoplasmic fluorescence is localized in the perinuclear region and on a network of filaments, while a diffused signal is present in the nucleus, except for the nucleolar areas. Ultrastructural analyses on whole cells and on in situ matrix preparations reveal that nuclear PI 3-kinase is localized in interchromatin domains, in stable association with inner nuclear matrix components, while the enzyme diffused in the cytosol is partly associated with the cytoskeletal filaments. Quantitative evaluations indicate that, in a multidrug-resistant variant obtained by continuous exposure of Saos-2 cells to doxorubicin, the amount of nuclear and cytoplasmic PI 3-kinase is significantly lower than in the sensitive parental cell line. The nuclear localization of PI 3-kinase and its variation in multidrug-resistant cells, characterized by a reduced mitotic index, are consistent with the data on the existence of a nuclear inositol lipid cycle, which could also utilize 3-phosphorylated inositides to modulate signal transduction for the control of some key functional activities.

Binding to the platelet-derived growth factor receptor transiently activates the p85alpha-p110alpha phosphoinositide 3-kinase complex in vivo

Ligand stimulation of the platelet-derived growth factor (PDGF) receptor results in its association with phosphoinositide 3-kinase activity and a corresponding synthesis of 3'-phosphorylated lipids. Early studies that examined this interaction in vivo employed anti-phosphotyrosine antiserum or antiserum against the PDGF receptor. The recent identification of multiple isoforms of both the regulatory and the catalytic subunit of the enzyme have led us to utilize antisera against p85alpha and p110alpha to characterize the association of this particular phosphoinositide 3-kinase complex with the PDGF receptor following ligand stimulation of murine fibroblasts. Both the p85alpha and p110alpha subunits rapidly associated with the ligand-activated receptor resulting in a transient, 2-fold increase in the total pool of p110alpha lipid kinase activity. This association was stable for 15 min after initial stimulation. Subsequently, both subunits began to dissociate from the receptor with similar kinetics. By 60 min this process was complete, demonstrating that p85alpha and p110alpha both associate with the receptor and dissociate from the receptor as a dimeric complex. At this time, marked PDGF receptor down-regulation was observed. Immunoprecipitation from metabolically labeled cells revealed that p85alpha is constitutively phosphorylated on serine residues in quiescent cultures. Upon PDGF stimulation, this phosphorylation upon serine residues was maintained in addition to tyrosine phosphorylation of this subunit. No phosphorylation of the p110alpha subunit was detected in either quiescent or PDGF-stimulated cells. Quantitation of Western blot analysis demonstrated that only 5% of the total pool of p85alpha associated with the PDGF receptor upon ligand stimulation. The 2-fold increase in the lipid kinase activity measured in immunoprecipitates using either anti-p85alpha or anti-p110alpha antiserum therefore reflects a far greater increase in the specific activity of the enzyme upon its association with the PDGF receptor.

SH2 domain protein interaction and possibilities for pharmacological intervention

SH2 domain containing proteins play a key role in the process of intracellular transmission of signalling events initiated at the cell surface. As a pre-requisite in the fulfillment of this function, these proteins bind to a variety of phospho-tyrosine (pY) containing target molecules. Delineation of these binding sites as essentially short linear peptides (both structurally and functionally) has led to the suggestion that the activity of these signalling complexes may be manipulated by the development of relatively simple peptide reagents. This review examines the range of possibilities open on this approach and the extent to which positive results have already been realised.

Functional platelet-derived growth factor-beta (PDGF-beta) receptor expressed on early B-lineage precursor cells

Growth and maturation of B lymphocytes from stem cells require a series of complex processes that are dependent at least in part on growth factors. Uncontrolled expression of receptors from these early growth factors may contribute to a leukaemogenesis of such early B cell progenitors. We show here that early pre-pre-B cells, but not mature B cells, express the PDGF receptor-beta (PDGFR-beta). These receptors contain a protein tyrosine kinase domain which is activated upon ligation with PDGF in pre-pre-B cells. Further, pre-pre-B leukaemia cells seem to express more PDGFR-beta compared with their normal counterparts, suggesting a role for these receptors in growth promotion of leukaemia cells.

Tyrosine 807 of the v-Fms oncogene product controls cell morphology and association with p120RasGAP

Expression of the v-fms oncogene of feline sarcoma virus in fibroblasts causes surface exposure of an activated receptor tyrosine kinase, v-Fms, that is autophosphorylated at multiple sites within its cytoplasmic domain. Cellular proteins interacting with this part of v-Fms modulate the mitogenic activity and morphology of the cells. We show here that the tyrosine residue in position 807 (Y-807) of the v-Fms molecule constitutes a major autophosphorylation site. The replacement of this residue by phenylalanine (Y807F mutation) allowed us to functionally dissect v-Fms-specific mitogenic and morphogenic cascades. Cells expressing the mutant v-Fms molecule resembled wild-type (wt) v-Fms-transformed (wt-v-Fms) cells in terms of [3H]thymidine uptake rates and activation of the Ras/Raf-1 mitogenic cascade. Such cells showed, however, a flat morphology and contained intact actin cables and fibronectin network. Our studies indicate that the v-Fms molecule controls cell morphology by a cascade that involves a direct interaction with p120RasGAP and p190RhoGAP: (i) in contrast to wt v-Fms molecules, the Y807F v-Fms protein failed to associate with and phosphorylate p120RasGAP; (ii) tight complexes between p120RasGAP and p190RhoGAP as well as detectable RhoGAP activity were present exclusively in wt-v-Fms cells; and (iii) p190RhoGAP was dispersed throughout the cytoplasm of wt-v-Fms cells, whereas its distribution was restricted to perinuclear regions of cells expressing the mutant v-Fms gene.

Identification of a putative Syp substrate, the PDGF beta receptor

Because the protein-tyrosine phosphatase (PTP) Syp associates with the tyrosine-phosphorylated platelet-derived growth factor beta receptor (beta PDGFR), the beta PDGFR is a likely Syp substrate. We tested this hypothesis by determining whether recombinant Syp (rSyp) and a control PTP, recombinant PTP1B (rPTP1B), were able to dephosphorylate the beta PDGFR. The beta PDGFR was phosphorylated at multiple tyrosine residues in an in vitro kinase assay and then incubated with increasing concentrations of rSyp or rPTP1B. While the receptor was nearly completely dephosphorylated by high concentrations of rPTP1B, receptor dephosphorylation by rSyp plateaued at approximately 50%. Two-dimensional phosphopeptide maps of the beta PDGFR demonstrated that rSyp displayed a clear preference for certain receptor phosphorylation sites; the most efficiently dephosphorylated sites were phosphotyrosines (Tyr(P)-771 and -751, followed by Tyr(P)740, while Tyr(P)-1021 and Tyr(P)-1009 were very poor substrates. In contrast, rPTP1B displayed no selectivity for the various rPTP1B displayed no selectivity for the various beta PDGFR tyrosine phosphorylation sites and dephosphorylated all of them with comparable efficiency. A Syp construct that lacked the SH2 domains was still able to discriminate between the various receptor phosphorylation sites, although less effectively than full-length Syp. These in vitro studies predicted that Syp can dephosphorylate the receptor in vivo. Indeed, we found that a beta PDGFR mutant (F1009) that associates poorly with Syp, had a much slower in vivo rate of receptor dephosphorylation than the wild type receptor. In addition, the GTPase-activating protein of Ras (GAP) and phosphatidylinositol 3-kinase were less stably associated with the wild type beta PDGFR than with the F1009 receptor. These findings are consistent with the in vitro experiments showign that Syp prefers to dephosphorylate sites on the beta PDGFR, that are important for binding phosphatidylinositol 3-kinase (Tyr(P)-740 and Tyr(P)-751) and GAP (Tyr(P)-771). These studies reveal that Syp is a substrate-selective PTP and that both the catalytic domain and the SH2 domains contribute to Syp's ability to choose substrates. Furthermore, it appears that Syp plays a role in PDGF-dependent intracellular signal relay by selectively dephosphorylating the beta PDGFR and thereby regulating the binding of a distinct group of receptor-associated signal relay enzymes.

Activation of mesangial cells by the phosphatase inhibitor vanadate. Potential implications for diabetic nephropathy

The metalion vanadate has insulin-like effects and has been advocated for use in humans as a therapeutic modality for diabetes mellitus. However, since vanadate is a tyrosine phosphatase inhibitor, it may result in undesirable activation of target cells. We studied the effect of vanadate on human mesangial cells, an important target in diabetic nephropathy. Vanadate stimulated DNA synthesis and PDGF B chain gene expression. Vanadate also inhibited total tyrosine phosphatase activity and stimulated tyrosine phosphorylation of a set of cellular proteins. Two chemically and mechanistically dissimilar tyrosine kinase inhibitors, genistein and herbimycin A, blocked DNA synthesis induced by vanadate. Vanadate also stimulated phospholipase C and protein kinase C. Downregulation of protein kinase C abolished vanadate-induced DNA synthesis. Thus, vanadate-induced mitogenesis is dependent on tyrosine kinases and protein kinase C activation. The most likely mechanism for the effect of vanadate on these diverse processes involves the inhibition of cellular phosphotyrosine phosphatases. These studies demonstrating that vanadate activates mesangial cells may have major implications for the therapeutic potential of vanadate administration in diabetes. Although vanadate exerts beneficial insulin-like effects and potentiates the effect of insulin in sensitive tissue, it may result in undesirable activation of other target cells, such as mesangial cells.

Inhibitors of phospholipid intracellular signaling as antiproliferative agents

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a rational approach of specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as drug resistance, metastasis, neo-vascularization (angiogenesis), and apoptosis. These are all important targets in their own right and the development of drugs against specific "upstream" targets in oncogenic or growth factor signal transduction cascades it may be possible to inhibit multiple "downstream" targets. Ultimately, to test the hypothesis that signaling pathways offer good targets for anticancer drug development will take several years of careful clinical study and we cannot say at this time whether the approach will work. There are a small number of compounds in the early stages of clinical development as anticancer agents that may act by inhibiting growth factor signaling pathways. In all cases the activity of the compounds on intracellular signaling pathways was discovered after their identification as antiproliferative agents. There are also compounds in preclinical development that have been specifically developed as inhibitors of growth factor signaling, although their selectivity for tumor cells compared to normal tissue remains to be investigated fully in appropriate animal tumor models. It is possible that a single antisignaling drug by itself may not have the power to completely inhibit tumor growth and a combination of drugs may be needed. It may also take a combination of drugs to prevent the emergence of resistance. Clearly there are several challenges to developing this new class of anticancer drugs, and there will undoubtedly be others that must be faced.

Beta PDGF receptor mutants defective for mitogenesis promote neurite outgrowth in PC12 cells

BACKGROUND

Platelet-derived growth factor (PDGF) promotes mitogenesis in fibroblast cell lines but stimulates neurite outgrowth in PC12 cells that ectopically express the beta PDGF receptor. To determine which substrates must associate with this receptor protein-tyrosine kinase in order to promote neurite outgrowth, we introduced into PC12 pheochromocytoma cells three mutant forms of the beta PDGF receptor that no longer associate with specific substrate proteins. We then assayed the ability of these receptor mutants to affect neurite extension.

RESULTS

Receptors lacking the kinase-insert domain did not associate with either phosphatidylinositol 3-kinase (PI 3-kinase) or Ras GTPase-activating protein (Ras-GAP) in PC12 cells. A carboxy-terminal truncation of the beta PDGF receptor eliminated the association of phospholipase C-gamma 1 (PLC-gamma 1) with the receptor and prevented phosphorylation of PLC-gamma 1 in PC12 cells. Finally, beta PDGF receptors that have tyrosine-to-phenylalanine point mutations at positions 708, 719, 977 and 989 did not associate with either PI 3-kinase or PLC-gamma 1. All three mutant forms of the beta PDGF receptor promoted PDGF-dependent neurite outgrowth in PC12 cells and elicited activation of mitogen-activated protein (MAP) kinases.

CONCLUSIONS

PC12 cells expressing the beta PDGF receptor extend neurites in response to PDGF in the absence of signalling through PI 3-kinase, RasGAP, and PLC-gamma 1. This contrasts with the requirements for mitogenesis for epithelial and fibroblast cell lines, in which the association of PI 3-kinase with the beta PDGF receptor is essential. This receptor protein-tyrosine kinase therefore phosphorylates and activates a similar set of intracellular signalling molecules in the context of both mitogenesis and differentiation, but the importance of particular pathways for each phenotypic response is distinct.

Differentiation of peptide molecular recognition by phospholipase C gamma-1 Src homology-2 domain and a mutant Tyr phosphatase PTP1bC215S

Activated epidermal growth factor receptor (EGFR) undergoes autophosphorylation on several cytoplasmic tyrosine residues, which may then associate with the src homology-2 (SH2) domains of effector proteins such as phospholipase C gamma-1 (PLC gamma-1). Specific phosphotyrosine (pTyr)-modified EGFR fragment peptides can inhibit this intermolecular binding between activated EGFR and a tandem amino- and carboxy-terminal (N/C) SH2 protein construct derived from PLC gamma-1. In this study, we further explored the molecular recognition of phosphorylated EGFR988-998 (Asp-Ala-Asp-Glu-pTyr-Leu-Ile-Pro-Gln-Gln-Gly, I) by PLC gamma-1 N/C SH2 in terms of singular Ala substitutions for amino acid residues N- and C-terminal to the pTyr (P site) of phosphopeptide I. Comparison of the extent to which these phosphopeptides inhibited binding of PLC gamma-1 N/C SH2 to activated EGFR showed the critical importance of amino acid side chains at positions P+2 (Ile994), P+3 (Pro995), and P+4 (Gln996). Relative to phosphopeptide I, multiple Ala substitution throughout the N-terminal sequence, N-terminal sequence, N-terminal truncation, or dephosphorylation of pTyr each resulted in significantly decreased binding to PLC gamma-1 N/C SH2. These structure-activity results were analyzed by molecular modeling studies of the predicted binding of phosphopeptide I to each the N- and C-terminal SH2 domains of PLC gamma-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine 508 of the 85-kilodalton subunit of phosphatidylinositol 3-kinase is phosphorylated by the platelet-derived growth factor receptor

The mechanisms by which growth factors and oncogenic agents activate phosphatidylinositol 3-kinase (PI3 kinase) are unknown. Previously, we reported that the 85-kDa regulatory subunit of PI3 kinase is tyrosine-phosphorylated both in vitro by the platelet-derived growth factor beta-receptor (PDGFR) tyrosine kinase and in fibroblasts in response to PDGF. As a first step in determining the role of tyrosine phosphorylation in PDGF signaling through PI3 kinase, we investigated which tyrosines on p85 are phosphorylated by the PDGFR. Recombinant p85 was phosphorylated with recombinant PDGF receptors, and tryptic phosphopeptides were purified by HPLC and analyzed by Edman degradation. By this approach and by mutational analysis, Y508 was identified as the major in vitro phosphorylation site. Tryptic phosphopeptide mapping demonstrated Y508 to also be phosphorylated in vivo in COS cells. Comparison of these data with a previous report [Hayashi, H., Nishioka, Y., Kamohara, S., Kanai, F., Ishii, K., Fukui, Y., Shibasaki, F., Takenawa, T., Kido, H., Katsunuma, N., & Ebina, Y. (1993) J. Biol. Chem. 268, 7107-7117] suggests that p85 is phosphorylated differently by the PDGF and insulin receptor tyrosine kinases. Therefore, p85 may be regulated differently by PDGF and insulin. Mapping of phosphorylation sites on p85 may lead to new insights into the regulation of signal transduction through PI3 kinase.

PDGF-mediated activation of phosphatidylinositol 3 kinase in human mesangial cells

Platelet-derived growth factor (PDGF) stimulates mitogenesis and exerts other biologic activities in glomerular mesangial cells. The precise mechanism of PDGF-induced mitogenesis in these cells is not clear. The activation of a signal transducing enzyme, phosphatidylinositol 3 kinase (PI 3 kinase) is associated with mitogenesis. Activation of PI 3 kinase results from stimulation of tyrosine kinase and G-protein-coupled classes of receptors. The synthesis of D3 phosphorylated inositides, the products of this enzymatic reaction, in non-nucleated cells such as blood platelets is dependent upon protein kinase C activation and G-proteins. We studied the activation of PI 3 kinase in response to PDGF in human glomerular mesangial cells. Using a PI 3 kinase 85 kD subunit specific antibody, we detected mesangial cell PI 3 kinase protein as 110 and 85 kD heterodimer. PDGF stimulated PI 3 kinase activity in antiphosphotyrosine immunoprecipitates in a dose-dependent manner showing maximum activation at 12 ng/ml. The antiphosphotyrosine associated PI 3 kinase activity showed biphasic kinetics with a fast peak within two minutes followed by a second peak at 10 minutes. Antiphosphotyrosine and PI 3 kinase immunoprecipitation studies indicated the association of the 85 kD PI 3 kinase subunit with PDGFR. Direct immunoprecipitation with PDGFR beta antibody showed the association of PI 3 kinase activity with the PDGF-receptor. The isoquinoline sulfonyl piperazine compound H7 at concentrations that inhibit PDGF-stimulated PKC activity had no effect on PDGF-stimulated PI 3 kinase activity in antiphospotyrosine immunoprecipitates. These data indicate that PI3 kinase activation is insensitive to PKC. Treatment of mesangial cells with pertussis toxin at concentrations that partially inhibited PDGF-induced DNA synthesis in human mesangial cells did not inhibit PDGF-induced PI 3 kinase activation. These data indicate that PDGF activates PI 3 kinase in mesangial cells and that pertussis toxin-sensitive G-proteins are not involved in PI 3 kinase activation. The data further dissociate activation of PI 3 kinase from mitogenesis in human mesangial cells.

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.

Inhibitors of phosphatidylinositol signalling as antiproliferative agents

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. Phosphatidylinositol (PtdIns) is a key component of two growth factor signalling pathways. It acts as a substrate for PtdIns specific phospholipase C (PtdInsPLC) and for PtdIns-3-kinase. In this review the antiproliferative properties of some inhibitors of PtdInsPLC and PtdIns-3-kinase are considered. There are some compounds already in clinical trial as anticancer drugs that may act by inhibiting PtdIns signalling, as well as several compounds in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Chemotaxis is an important component of wound healing, development, immunity and metastasis, yet the signalling pathways that mediate chemotaxis are poorly understood. Platelet-derived growth factor (PDGF) acts both as a mitogen and a chemoattractant. Upon stimulation, the tyrosine kinase PDGF receptor-beta (PDGFR-beta) autophosphorylates and forms a complex that includes SII2(Src homology 2)-domain-containing proteins such as the phosphatidylinositol-specific phospholipase C-gamma, Ras-GTPase-activating protein (GAP), and phosphatidylinositol-3-OH kinase. Specific tyrosine-to-phenylalanine substitutions in the PDGFR-beta can prevent binding of one SH2-domain-containing protein without affecting binding of other receptor-associated proteins. Here we use phospholipase C-gamma and PDGFR-beta mutants to map specific tyrosines involved in both positive and negative regulation of chemotaxis towards the PDGF-BB homodimer. Our results indicate that a delicate balance of migration-promoting (phospholipase C-gamma and phosphatidylinositol-3-OH kinase) and migration-suppressing (GAP) activities are recruited by the PDGFR-beta to drive chemotaxis towards PDGF-BB.

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

Signal transduction by the PDGF receptors

The three isoforms of PDGF bind with different affinities to two related tyrosine kinase receptors, denoted the PDGF alpha- and beta-receptors. Ligand binding induces receptor dimerization, creating receptor homo- or heterodimers. Dimerization is accompanied by, and might be a prerequisite for, receptor autophosphorylation and kinase activation. Receptor autophosphorylation serves to regulate the kinase activity and to create binding sites on the receptor molecule for downstream signalling components. The activities of the signalling components are ultimately manifested as specific biological responses. All the currently described PDGF receptor-binding components, e.g. phospholipase C-gamma, members of the src family of cytoplasmic tyrosine kinases, the rasGT-Pase activating protein and p85, the regulatory subunit of phosphatidylinositol 3' kinase, contain a conserved src homology 2-domain, through which the association with the receptor takes place. The receptor-binding components appear to either possess an intrinsic enzymatic activity, or they function as adaptors, which may complex with catalytically active components. For most receptor-binding components, there is insufficient understanding of how binding to the receptor affects the catalytic function. Certain of these components become tyrosine-phosphorylated, i.e. they are substrates for the receptor tyrosine kinase. Moreover, the change in subcellular localization, which most of the receptor binding components undergo in conjunction with receptor binding, could play a critical role. The current efforts of many laboratories are aimed at delineating different PDGF receptor signal transduction pathways and what roles the different receptor-binding components play in the establishment of these pathways.

Phospholipase C gamma activation, phosphotidylinositol hydrolysis, and calcium mobilization are not required for FGF receptor-mediated chemotaxis

Basic fibroblast growth factor (FGF) is a potent angiogenic factor that stimulates several cell types to migrate along a chemotactic gradient. Most chemoattractant receptors appear to share a common mechanism that involves activation of phospholipase C (PLC), hydrolysis of phosphotidylinositol, and mobilization of intracellular calcium. We transfected two different cell lines with either human FGF receptor-1 cDNA or chimeric FGF receptor cDNA. Ligand stimulation induced chemotaxis, activation of PLC gamma, phosphotidylinositol hydrolysis, and calcium mobilization in both wild-type receptor cell lines. No such response was elicited in control cells. Mutation of the two fibroblast growth factor receptors at residue 766, replacing tyrosine with phenylalanine, made the receptors incapable of associating with and activating PLC gamma following ligand stimulation. These mutant receptors also failed to mediate phosphotidylinositol hydrolysis and calcium mobilization. However, cells transfected with the mutant fibroblast growth factor receptors were as chemotactically responsive to the appropriate ligand as were cells transfected with the wild-type receptors. These findings demonstrate that the ability of the fibroblast growth factor receptor to promote chemotaxis is not dependent on increased activation of PLC gamma, increased hydrolysis of phosphotidylinositol, or increased global mobilization of calcium.

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

Two signaling molecules share a phosphotyrosine-containing binding site in the platelet-derived growth factor receptor

Autophosphorylation sites of growth factor receptors with tyrosine kinase activity function as specific binding sites for Src homology 2 (SH2) domains of signaling molecules. This interaction appears to be a crucial step in a mechanism by which receptor tyrosine kinases relay signals to downstream signaling pathways. Nck is a widely expressed protein consisting exclusively of SH2 and SH3 domains, the overexpression of which causes cell transformation. It has been shown that various growth factors stimulate the phosphorylation of Nck and its association with autophosphorylated growth factor receptors. A panel of platelet-derived growth factor (PDGF) receptor mutations at tyrosine residues has been used to identify the Nck binding site. Here we show that mutation at Tyr-751 of the PDGF beta-receptor eliminates Nck binding both in vitro and in living cells. Moreover, the Y751F PDGF receptor mutant failed to mediate PDGF-stimulated phosphorylation of Nck in intact cells. A phosphorylated Tyr-751 is also required for binding of phosphatidylinositol-3 kinase to the PDGF receptor. Hence, the SH2 domains of p85 and Nck share a binding site in the PDGF receptor. Competition experiments with different phosphopeptides derived from the PDGF receptor suggest that binding of Nck and p85 is influenced by different residues around Tyr-751. Thus, a single tyrosine autophosphorylation site is able to link the PDGF receptor to two distinct SH2 domain-containing signaling molecules.

Two signaling molecules share a phosphotyrosine-containing binding site in the platelet-derived growth factor receptor

Autophosphorylation sites of growth factor receptors with tyrosine kinase activity function as specific binding sites for Src homology 2 (SH2) domains of signaling molecules. This interaction appears to be a crucial step in a mechanism by which receptor tyrosine kinases relay signals to downstream signaling pathways. Nck is a widely expressed protein consisting exclusively of SH2 and SH3 domains, the overexpression of which causes cell transformation. It has been shown that various growth factors stimulate the phosphorylation of Nck and its association with autophosphorylated growth factor receptors. A panel of platelet-derived growth factor (PDGF) receptor mutations at tyrosine residues has been used to identify the Nck binding site. Here we show that mutation at Tyr-751 of the PDGF beta-receptor eliminates Nck binding both in vitro and in living cells. Moreover, the Y751F PDGF receptor mutant failed to mediate PDGF-stimulated phosphorylation of Nck in intact cells. A phosphorylated Tyr-751 is also required for binding of phosphatidylinositol-3 kinase to the PDGF receptor. Hence, the SH2 domains of p85 and Nck share a binding site in the PDGF receptor. Competition experiments with different phosphopeptides derived from the PDGF receptor suggest that binding of Nck and p85 is influenced by different residues around Tyr-751. Thus, a single tyrosine autophosphorylation site is able to link the PDGF receptor to two distinct SH2 domain-containing signaling molecules.

A region of the 85-kilodalton (kDa) subunit of phosphatidylinositol 3-kinase binds the 110-kDa catalytic subunit in vivo

Phosphatidylinositol (PI) 3-kinase is a heterodimer consisting of an 85-kDa subunit (p85) and 110-kDa subunit (p110). The 85-kDa noncatalytic subunit, which contains two Src homology 2 (SH2) domains, one SH3 domain, and a domain homologous to the carboxy terminus of the breakpoint cluster region gene product, is known to mediate the association of the PI 3-kinase complex with activated growth factor receptors. We previously demonstrated that the C-terminal SH2 domain of p85 is responsible for the interaction of PI 3-kinase with phosphorylated platelet-derived growth factor receptor. To define the region in p85 that directs the complex formation with the PI 3-kinase catalytic subunit, a series of truncated p85 mutants was analyzed for association with p110 in vivo. We found that a fragment of p85 containing the region between the two SH2 domains was sufficient to promote the interaction with p110 in vivo. The complex between the fragment of p85 and p110 had PI 3-kinase activity that was comparable in magnitude to the activity of p110 associated with full-length p85. The binding with p110 was abolished when this domain in p85 was disrupted. These results identify a novel structural and functional element that is responsible for localizing the catalytic subunit of PI 3-kinase.

Differential induction of phosphatidylcholine hydrolysis, diacylglycerol formation and protein kinase C activation by epidermal growth factor and transforming growth factor-alpha in normal human skin fibroblasts and keratinocytes

We have investigated coupling between the epidermal growth factor (EGF) receptor and the phospholipase C (PLC)/protein kinase C (PKC) signal-transduction system in normal skin fibroblasts and keratinocytes, for which EGF and transforming growth factor alpha (TGF-alpha) are mitogenic. EGF and TGF-alpha induced a rapid increase in tyrosine phosphorylation of the EGF receptor, in both fibroblasts and keratinocytes, but failed to induce tyrosine phosphorylation of PLC-gamma 1 or detectable phosphoinositide hydrolysis, as measured by two sensitive assays. In fibroblasts, EGF induced phosphatidylcholine (PC) hydrolysis, resulting in increased diacylglycerol (DAG). In contrast, in keratinocytes, there was no detectable PC hydrolysis or elevation of DAG in response to EGF or TGF-alpha. EGF and TGF-alpha activated PKC in fibroblasts, as evidenced by increased phosphorylation of a specific cellular PKC substrate (myristoylated alanine-rich C-kinase substrate, 'MARCKS'). In keratinocytes, TGF-alpha and EGF induced only a modest increase in MARCKS protein phosphorylation. This apparent modest activation of PKC, in the absence of detectable DAG formation, may have been mediated by arachidonic acid, which was released from keratinocytes in response to TGF-alpha, and has been shown to stimulate PKC activity in vitro. These data demonstrate that (1) in dermal fibroblasts and keratinocytes, which express normal levels of EGF receptors, EGF receptor activation is not coupled to tyrosine phosphorylation of PLC-gamma 1 or PtdIns hydrolysis, suggesting that these events are not required for the mitogenic activity of EGF or TGF-alpha in these cells, (2) coupling of EGF receptor to PC hydrolysis is cell-type specific, and (3) in skin fibroblasts, DAG, formed through EGF-induced PC hydrolysis, is capable of activating PKC.

A region of the 85-kilodalton (kDa) subunit of phosphatidylinositol 3-kinase binds the 110-kDa catalytic subunit in vivo

Phosphatidylinositol (PI) 3-kinase is a heterodimer consisting of an 85-kDa subunit (p85) and 110-kDa subunit (p110). The 85-kDa noncatalytic subunit, which contains two Src homology 2 (SH2) domains, one SH3 domain, and a domain homologous to the carboxy terminus of the breakpoint cluster region gene product, is known to mediate the association of the PI 3-kinase complex with activated growth factor receptors. We previously demonstrated that the C-terminal SH2 domain of p85 is responsible for the interaction of PI 3-kinase with phosphorylated platelet-derived growth factor receptor. To define the region in p85 that directs the complex formation with the PI 3-kinase catalytic subunit, a series of truncated p85 mutants was analyzed for association with p110 in vivo. We found that a fragment of p85 containing the region between the two SH2 domains was sufficient to promote the interaction with p110 in vivo. The complex between the fragment of p85 and p110 had PI 3-kinase activity that was comparable in magnitude to the activity of p110 associated with full-length p85. The binding with p110 was abolished when this domain in p85 was disrupted. These results identify a novel structural and functional element that is responsible for localizing the catalytic subunit of PI 3-kinase.