Kinetics and regulation of the tyrosine phosphorylation of epidermal growth factor receptor in intact A431 cells

Phospho-seq: Integrated, multi-modal profiling of intracellular protein dynamics in single cells

Cell signaling plays a critical role in regulating cellular behavior and fate. While multimodal single-cell sequencing technologies are rapidly advancing, scalable and flexible profiling of cell signaling states alongside other molecular modalities remains challenging. Here we present Phospho-seq, an integrated approach that aims to quantify phosphorylated intracellular and intranuclear proteins, and to connect their activity with cis-regulatory elements and transcriptional targets. We utilize a simplified benchtop antibody conjugation method to create large custom antibody panels for simultaneous protein and scATAC-seq profiling on whole cells, and integrate this information with scRNA-seq datasets via bridge integration. We apply our workflow to cell lines, induced pluripotent stem cells, and 3-month-old brain organoids to demonstrate its broad applicability. We demonstrate that Phospho-seq can define cellular states and trajectories, reconstruct gene regulatory relationships, and characterize the causes and consequences of heterogeneous cell signaling in neurodevelopment.

A Split-Abl Kinase for Direct Activation in Cells

To dissect the cellular roles of individual kinases, it is useful to design tools for their selective activation. We describe the engineering of a split-cAbl kinase (sKin-Abl) that is rapidly activated in cells with rapamycin and allows temporal, dose, and compartmentalization control. Our design strategy involves an empirical screen in mammalian cells and identification of split site in the N lobe. This split site leads to complete loss of activity, which can be restored upon small-molecule-induced dimerization in cells. Remarkably, the split site is transportable to the related Src Tyr kinase and the distantly related Ser/Thr kinase, AKT, suggesting broader applications to kinases. To quantify the fold induction of phosphotyrosine (pTyr) modification, we employed quantitative proteomics, NeuCode SILAC. We identified a number of known Abl substrates, including autophosphorylation sites and novel pTyr targets, 432 pTyr sites in total. We believe that this split-kinase technology will be useful for direct activation of protein kinases in cells.

Dynamic recruitment of protein tyrosine phosphatase PTPD1 to EGF stimulation sites potentiates EGFR activation

Balanced activity of protein tyrosine kinases and phosphatases (PTPs) controls tyrosine phosphorylation levels and, consequently, is needed to prevent pathologies like cancer. Phosphatase activity is tightly regulated in space and time. Thus, in order to understand how phospho-tyrosine signalling is regulated, the intracellular dynamics of PTPs should be investigated. Here, we have studied the intracellular dynamics of PTPD1, a FERM (four-point-one, ezrin, radixin, moesin) domain-containing PTP that is over expressed in cancer cells and potentiates EGFR signalling. Whereas PTPD1 was excluded from E-cadherin rich cell-cell adhesions in epithelial cell monolayers, it diffused from the cytoplasm to those membranes in contact with the extracellular medium. Localisation of PTPD1 at the plasma membrane was mediated by its FERM domain and enabled the formation of EGFR/PTPD1-containing signalling complexes that pre-existed at the plasma membrane before EGF stimulation. PTPD1 and EGFR transiently co-localised at EGF stimulation sites until the formation of macropinosomes containing active species of EGFR. Interference of PTPD1 expression caused a decrease in EGFR phosphorylated species at the periphery of the cell. Presented data suggest that the transient formation of dynamic PTPD1/EGFR signalling complexes strengthens EGF signalling by promoting the spatial propagation of EGFR phosphorylated species.

Gastrointestinal hormones stimulate growth of Foregut Neuroendocrine Tumors by transactivating the EGF receptor

Foregut neuroendocrine tumors [NETs] usually pursuit a benign course, but some show aggressive behavior. The treatment of patients with advanced NETs is marginally effective and new approaches are needed. In other tumors, transactivation of the EGF receptor (EGFR) by growth factors, gastrointestinal (GI) hormones and lipids can stimulate growth, which has led to new treatments. Recent studies show a direct correlation between NET malignancy and EGFR expression, EGFR inhibition decreases basal NET growth and an autocrine growth effect exerted by GI hormones, for some NETs. To determine if GI hormones can stimulate NET growth by inducing transactivation of EGFR, we examined the ability of EGF, TGFα and various GI hormones to stimulate growth of the human foregut carcinoid,BON, the somatostatinoma QGP-1 and the rat islet tumor,Rin-14B-cell lines. The EGFR tyrosine-kinase inhibitor, AG1478 strongly inhibited EGF and the GI hormones stimulated cell growth, both in BON and QGP-1 cells. In all the three neuroendocrine cell lines studied, we found EGF, TGFα and the other growth-stimulating GI hormones increased Tyr(1068) EGFR phosphorylation. In BON cells, both the GI hormones neurotensin and a bombesin analogue caused a time- and dose-dependent increase in EGFR phosphorylation, which was strongly inhibited by AG1478. Moreover, we found this stimulated phosphorylation was dependent on Src kinases, PKCs, matrix metalloproteinase activation and the generation of reactive oxygen species. These results raise the possibility that disruption of this signaling cascade by either EGFR inhibition alone or combined with receptor antagonists may be a novel therapeutic approach for treatment of foregut NETs/PETs.

A versatile coiled-coil tethering system for the oriented display of ligands on nanocarriers for targeted gene delivery

Surface modification of non-viral gene delivery nanocarriers may provide advanced features such as receptor targeting, endosomal escape and nuclear import. We here report the design of a versatile and tunable immobilization protocol to functionalize nanocarriers for improved transient gene expression. Our strategy is based on specific interactions occurring between a coil-tagged ligand and a complementary coil-functionalized nanocarrier. As a proof of concept, targeting of DNA/polyethylenimine polyplexes to the epidermal growth factor receptor of A431 cells was investigated. Coiled-coil-mediated oriented tethering of epidermal growth factor triggered a drastic increase of the internalization rate of the targeted polyplexes. To explore the tunability of our platform, surface density of targeting ligand was varied; our results indicated that the internalization rate varied with the ligand-to-polyplex ratio in a "switch mode" fashion. This work prefigures possible avenues for our coiled-coil platform in multiplex functionalization to address transient gene expression bottlenecks in recombinant protein production.

ErbB1 dimerization is promoted by domain co-confinement and stabilized by ligand binding

The extent to which ligand occupancy and dimerization contribute to erbB1 signaling is controversial. To examine this, we utilized two-color Quantum Dot tracking for visualization of erbB1 homodimerization and quantification of the dimer off rate (k_off) on living cells. Kinetic parameters were extracted using a 3-state Hidden Markov Model to identify transition rates between free, co-confined, and dimerized states. We report that dimers composed of 2 ligand-bound receptors are long-lived and their k_off is independent of kinase activity. By comparison, unliganded dimers have >4-fold faster k_off. Transient co-confinement of receptors promotes repeated encounters and enhances dimer formation. Mobility decreases >6-fold when ligand-bound receptors dimerize. Blockade of erbB1 kinase activity or disruption of actin networks results in faster diffusion of receptor dimers. These results implicate both signal propagation and the cortical cytoskeleton in reduced mobility of signaling-competent erbB1 dimers.

Minimally invasive sinus augmentation

Sinus lift surgery has become more common as patients choose dental implants for tooth replacement. The recent development of a graft material that stimulates osteogenesis coupled with the application of tissue engineering principles has allowed for refinement of this surgical modality. A simple nontraumatic subantral sinus lift microsurgery is presented. This sinus lift microsurgery resulted in a 97% implant success rate.

Bovine colostrum promotes growth and migration of the human keratinocyte HaCaT cell line

Bovine colostrum represents a rich source of growth factors, which are known to play a central role in wound healing. The aim of our study was to investigate the possible mitogenic and motogenic effects induced by colostrum on human keratinocytes. Cell proliferation evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide test and 5-Bromo-2'-deoxyuridine incorporation revealed that colostrum exerts a growth promoting activity. Scratch assay and immunofluorescence of actin cytoskeleton showed its effectiveness also in inducing cell migration. Furthermore, colostrum treatment increases the levels of tyrosine phosphorylated proteins and the activated forms of the extracellular signal-regulated kinases 1 and 2 and such effects appear to be repressed by the tyrosine kinase inhibitor genistein. Our results indicate that the biological activities of colostrum are specifically mediated by the growth factor-induced activation of tyrosine kinase receptors and underline the relevance of the synergistic action exerted by the growth factors in stimulating keratinocyte proliferation and migration essential for tissue repair.

Phosphoproteomics identified Endofin, DCBLD2, and KIAA0582 as novel tyrosine phosphorylation targets of EGF signaling and Iressa in human cancer cells

With the completion of the human genome project, analysis of enriched phosphotyrosyl proteins from epidermal growth factor (EGF)-induced phosphotyrosine proteome permits the identification of novel downstream substrates of the EGF receptor (EGFR). Using cICAT-based LC-MS/MS method, we identified and relatively quantified the tyrosine phosphorylation levels of 21 proteins between control and EGF-treated A431 human cervical cancer cells. Of these, Endofin, DCBLD2, and KIAA0582 were validated to be novel tyrosine-phosphorylation targets of EGF signaling and Iressa, a highly selective inhibitor of EGFR. In addition, EGFR activity was shown to be necessary for EGF-induced localization of Endofin, an FYVE domain-containing protein regulated by phosphoinositol lipid and engaged in endosome-mediated receptor modulation. Although several groups have conducted phosphoproteomics of EGF signaling in recent years, our study is the first to identify and validate Endofin, DCBLD2, and KIAA0582 as part of a complex EGF phosphotyrosine signaling network. These novel data will provide new insights into the complex EGF signaling and may have implications on target-directed cancer therapeutics.

Modulation of EGF receptor autophosphorylation by alpha-hemolysin of Staphylococcus aureus via protein tyrosine phosphatase

In the presence of assembled alpha-hemolysin (alpha-HL) of Staphylococcus aureus, the epidermal growth factor receptor (EGFr) is rapidly dephosphorylated. Several obvious possibilities that otherwise would have contributed to the dephosphorylation were ruled out. Instead, an elevation in the activity of a protein tyrosine phosphatase appears to be responsible for the observed loss of phosphorylation signal of EGFr. For this dephosphorylation, the assembly of alpha-HL is necessary while lytic pore formation is not required. In summary, the EGFr is unable to retain its phosphorylation signal in the presence of alpha-HL and the process is irreversible.

Regulation of Tie Receptor Expression on Human Endothelial Cells by Protein Kinase C-Mediated Release of Soluble Tie

The expression and activity of receptor tyrosine kinases (RTK) at the cell surface can be modulated by several different pathways including the proteolytic release of the extracellular domain as a soluble receptor. We investigated the regulation of tie receptor expression, an orphan RTK restricted to cells of hematopoietic and endothelial lineages, on primary human endothelial cells and a stably transfected Chinese hamster ovary (CHO) cell line. Tie was expressed in cells as a doublet of 135 and 125 kD; the 135-kD band represented mature cell surface receptor containing sialic acid and N-linked oligosaccharide residues, whereas the 125-kD band represented an intracellular pool of immature receptor. Phorbol 12-myristate 13-acetate (PMA) had dramatic effects on tie expression at the cell surface. Within 15 minutes of PMA treatment, the 135-kD band disappeared from the cell surface and was accompanied by the appearance of a 100-kD band in cell supernatants. The 100-kD band continued to accumulate in the media throughout the duration of PMA treatment during which mature tie receptor was undetectable on the cell surface by fluorescence-activated cell sorting (FACS) or in cell lysates by immunoblot analysis. Using specific antibodies, this 100-kD species was shown to be a soluble form of the tie receptor containing the extracellular domain. PMA-dependent release of soluble tie was mediated through the activation of protein kinase C (PKC); soluble tie was not released in the presence of PKC inhibitors, an inactive PMA analog, or following the downregulation of PKC through chronic PMA treatment. These results indicate that tie receptor expression on endothelial cells is regulated by the release of a soluble extracellular fragment following activation of PKC. Parallel pathways regulating c-kit, tumor necrosis factor (TNF), and colony-stimulating factor (CSF) receptor expression suggest that the release of extracellular receptor fragments represents an alternative mechanism through which cells modulate responses to growth factors and cytokines.

Regulation of Tie Receptor Expression on Human Endothelial Cells by Protein Kinase C-Mediated Release of Soluble Tie

The expression and activity of receptor tyrosine kinases (RTK) at the cell surface can be modulated by several different pathways including the proteolytic release of the extracellular domain as a soluble receptor. We investigated the regulation of tie receptor expression, an orphan RTK restricted to cells of hematopoietic and endothelial lineages, on primary human endothelial cells and a stably transfected Chinese hamster ovary (CHO) cell line. Tie was expressed in cells as a doublet of 135 and 125 kD; the 135-kD band represented mature cell surface receptor containing sialic acid and N-linked oligosaccharide residues, whereas the 125-kD band represented an intracellular pool of immature receptor. Phorbol 12-myristate 13-acetate (PMA) had dramatic effects on tie expression at the cell surface. Within 15 minutes of PMA treatment, the 135-kD band disappeared from the cell surface and was accompanied by the appearance of a 100-kD band in cell supernatants. The 100-kD band continued to accumulate in the media throughout the duration of PMA treatment during which mature tie receptor was undetectable on the cell surface by fluorescence-activated cell sorting (FACS) or in cell lysates by immunoblot analysis. Using specific antibodies, this 100-kD species was shown to be a soluble form of the tie receptor containing the extracellular domain. PMA-dependent release of soluble tie was mediated through the activation of protein kinase C (PKC); soluble tie was not released in the presence of PKC inhibitors, an inactive PMA analog, or following the downregulation of PKC through chronic PMA treatment. These results indicate that tie receptor expression on endothelial cells is regulated by the release of a soluble extracellular fragment following activation of PKC. Parallel pathways regulating c-kit, tumor necrosis factor (TNF), and colony-stimulating factor (CSF) receptor expression suggest that the release of extracellular receptor fragments represents an alternative mechanism through which cells modulate responses to growth factors and cytokines.

A natural hepatocyte growth factor/scatter factor autocrine loop in myoblast cells and the effect of the constitutive Met kinase activation on myogenic differentiation

As a rule, hepatocyte growth factor/scatter factor (HGF/SF) is produced by mesenchymal cells, while its receptor, the tyrosine kinase encoded by the met proto-oncogene, is expressed by the neighboring epithelial cells in a canonical paracrine fashion. In the present work we show that both HGF/SF and met are coexpressed by undifferentiated C2 mouse myoblasts. In growing cells, the autocrine loop is active as the receptor exhibits a constitutive phosphorylation on tyrosine that can be abrogated by exogenously added anti-HGF/SF neutralizing antibodies. The transcription of HGF/SF and met genes is downregulated when myoblasts stop proliferating and differentiate. The coexpression of HGF/SF and met genes is not exclusive to C2 cells since it has been assessed also in other myogenic cell lines and in mouse primary satellite cells, suggesting that HGF/SF could play a role in muscle development through an autocrine way. To analyze the biological effects of HGF/SF receptor activation, we stably expressed the constitutively activated receptor catalytic domain (p65(tpr-met)) in C2 cells. This active kinase determined profound changes in cell shape and inhibited myogenesis at both morphological and biochemical levels. Notably, a complete absence of muscle regulatory markers such as MyoD and myogenin was observed in p65(tpr-met) highly expressing C2 clones. We also studied the effects of the ectopic expression of human isoforms of met receptor (h-met) and of HGF/SF (h-HGF/SF) in stable transfected C2 cells. Single constitutive expression of h-met or h-HGF/SF does not alter substantially the growth and differentiation properties of the myoblast cells, probably because of a species-specific ligand-receptor interaction. A C2 clone expressing simultaneously both h-met and h-HGF/SF is able to grow in soft agar and shows a decrease in myogenic potential comparable to that promoted by p65(tpr-met) kinase. These data indicate that a met kinase signal released from differentiation-dependent control provides a negative stimulus for the onset of myogenic differentiation.

Extracellular matrix is required for MAP kinase activation and proliferation of rat glomerular epithelial cells

This study examined the role of extracellular matrix (ECM) in the regulation of glomerular epithelial cell (GEC) proliferation. Epidermal growth factor (EGF) stimulated proliferation of GEC when the cells were adherent to collagen matrices, but not plastic substratum. Significant and prolonged EGF receptor (R) tyrosine autophosphorylation (which reflects EGF-R kinase activation) was induced by EGF only in GEC adherent to collagen. In addition, EGF stimulated the activity and tyrosine phosphorylation of p42 mitogen-activated protein (MAP) kinase (ERK2) in collagen-adherent GEC, but not in cells on plastic. An inhibitor of the p-42 MAP kinase pathway, PD98059, blocked EGF-induced MAP kinase activity and proliferation. Thus, adhesion to ECM enables EGF to induce proliferation of GEC, by facilitating activation of EGF-R and the p42 MAP kinase pathway. Signals from ECM to growth factor receptor tyrosine kinases may regulate cell turnover in the glomerulus under normal conditions and during immune glomerular injury.

Epidermal growth factor induces rapid and transient association of phospholipase C-gamma 1 with EGF-receptor and filamentous actin at membrane ruffles of A431 cells

Addition of epidermal growth factor to A431 cells results in dramatic changes in cell morphology. Initially the cells form membrane ruffles accompanied by increased actin polymerization, followed by cell rounding. Activation of the tyrosine kinase of the receptor by binding epidermal growth factor leads also to phosphorylation and activation of phospholipase C-gamma 1, a key enzyme in the phosphoinositide pathway. In this study we have investigated the localization of phospholipase C-gamma 1 during cell activation by epidermal growth factor. It is shown that addition of the growth factor to A431 cells leads to a translocation of phospholipase C-gamma 1 from the cytosol to the membrane fraction. Interestingly, this relocation is exclusively directed to the membrane ruffles. Most of the phospholipase C-gamma 1 associates to the membrane and a small fraction to the underlying skeleton. Immunocytochemical studies demonstrated that phospholipase C-gamma 1 co-localizes with the epidermal growth factor receptor and also filamentous actin at the membrane ruffles. Moreover, using anti-phosphotyrosine antibodies we found that the membrane ruffles are significantly enriched in phosphotyrosyl proteins. Between 5 and 10 minutes after stimulation the membrane ruffles disappear and also the co-localization of phospholipase C-gamma 1 with the epidermal growth factor receptor and filamentous actin. These results support the notion that activation of A431 cells by epidermal growth factor leads to the formation of a signalling complex of its receptor, phospholipase C-gamma 1 and filamentous actin which is primarily localized at membrane ruffles.

Adaptation of EGF receptor signal transduction to three-dimensional culture conditions: changes in surface receptor expression and protein tyrosine phosphorylation

A431 cells grown as three-dimensional spheroids show growth stimulation in response to nanomolar concentrations of EGF in contrast to monolayer cultures that show inhibition. In investigating the alterations in EGF signal transduction that underlie this modification of the proliferative response, we have compared the expression of EGF receptors on A431 cells under these conditions and related our findings to tyrosine phosphorylation and the growth response. EGF receptors were measured by 125I-EGF binding to trypsin-dispersed cells. Unexpectedly, dispersion of the monolayers caused an 80% decrease in surface EGF receptor, although, after dispersion, EGF receptor was digested by trypsin with a half-life of 69 +/- 32 min. No evidence for a comparable loss of cellular EGF receptor was seen on trypsin dispersion of spheroids. After allowing for this effect, we found that the receptor density on nondispersed monolayers (5 x 10(6) per cell) was twentyfold greater than that on spheroids (0.25 x 10(6) per cell). EGF-induced tyrosine phosphorylation was confined to the outermost cells of the spheroid, although the presence of surface-expressed EGF binding sites could be demonstrated throughout the structure and the number of EGF receptors/cell on dispersed spheroid cells showed a single distribution peak by flow cytometry, with no evidence for more than one population. Using RCM-lysozyme as a substrate, tyrosine phosphatase activity in spheroids lay within the range observed in monolayer cultures. Autophosphorylation of the EGF receptor following EGF stimulation in monolayer cultures of A431 cells rose rapidly in the first 10 seconds and then slowly increased for at least 3 h. In spheroids, it reached a maximum within 10 seconds and then declined over 3 h. Since the microenvironment within a tumor resembles that in a spheroid, a similar reduction in surface EGF receptor expression may be expected in tumors relative to monolayer cultures, together with corresponding growth stimulation in response to EGF.

Importance of tyrosine phosphatases in the effects of cell-cell contact and microenvironments on EGF-stimulated tyrosine phosphorylation

We have compared the EGF responses of A431 cells when grown as monolayers at a variety of cell densities or as multicellular spheroids in order to investigate the effects of cell contact and 3-dimensional structure on signal transduction. Proliferation of the A431 squamous carcinoma cell line grown in our laboratory was unaffected by EGF when grown in monolayer culture. As 3-dimensional, multicellular spheroids, however, growth was stimulated by EGF. The maximum volume attainable in the presence of EGF was more than 30 times that in its absence. EGF-dependent tyrosine phosphorylation was compared under these conditions by immunohistochemistry and Western blotting. In initial experiments using published procedures, tyrosine phosphorylation was density-dependent in monolayers and undetectable in spheroids. However, the density-dependence was abolished by the addition of high concentrations of protein tyrosine phosphatase inhibitors (1 mM Zn++ and VO4(3)-). The density dependence of EGF-stimulated tyrosine phosphorylation in monolayers was, therefore, largely the result of changes in phosphatase activity rather than kinase. Using high concentrations of phosphatase inhibitors, phosphotyrosine was clearly visible by immunohistochemistry in the outermost cells of spheroids, but it was still not visible in the spheroid center. The lack of response within the spheroid was not related to the presence of EGF receptor nor diffusion of EGF. In companion experiments, we showed that staining for EGF receptor was present homogeneously throughout the spheroid and that EGF penetrated to its center under the conditions of the experiment. Thus, although an increase in tyrosine phosphatase activity was a major factor affecting tyrosine phosphorylation in the outer cells, other factors were important in the inner cells. We concluded that an increase of tyrosine phosphatase activity was the most important component of the adaptation of the EGF signal transduction system to high cell density in monolayer cultures. In spheroids, tyrosine phosphatases are also enhanced, but other factors, such as autocrine synthesis of TGF-alpha and possibly the cellular distribution of EGF receptors and cell shape, play a role.

Aberrant elevation of tyrosine-specific phosphorylation in human gastric cancer cells

Phosphotyrosine-containing proteins in various human cancer cell lines were studied by immunoblotting with anti-phosphotyrosine antibody. Of 29 cell lines derived from oral epidermoid cancer, esophageal cancer, gastric cancer, colon cancer, pancreatic cancer, hepatocellular carcinoma and malignant melanoma, 3 of the 6 gastric cancer cells showed aberrant elevation of tyrosine-specific phosphorylation. On the other hand, both esophageal cancer cells and colon cancer cells, which were reported to have amplified epidermal growth factor receptor and activated p60v-src kinase, respectively, showed no apparent elevation of tyrosine-specific phosphorylation, and their profiles of phosphorylation were similar to that of normal human fibroblasts. Two gastric cancer cells, NUGC-4 and MKN-45, showed similar profiles of phosphorylation but their responses to growth factors differed from each other. Tyrosine phosphorylation in NUGC-4 was strongly activated by treatment with epidermal growth factor and quickly reduced by the acid treatment which is effective in removing growth factors from cellular surface receptors. On the contrary, phosphorylation in MKN-45 did not respond to either growth factor or acid treatment. These results suggest that NUGC-4 and MKN-45 have tyrosine kinases which are activated by different mechanisms but share similar substrates.

Ligand-independent tyrosine phosphorylation of the receptor encoded by the c-neu oncogene

Incubation of Swiss 3T3 murine fibroblasts at low temperatures induces phosphorylation on tyrosine of a transmembrane protein of 175 kDa. This phenomenon is time and temperature dependent and reaches a maximum after 2 hr at 4 degrees C. The 175 kDa protein phosphorylated in vivo at low temperatures can be immunoprecipitated by phosphotyrosine antibodies and displays auto-kinase activity in vitro in the presence of radiolabelled ATP. This molecule was found to react with anti-peptide antibodies directed against the product of the HER2/neu proto-oncogene only when immunoprecipitated with phosphotyrosine antibodies from cold-stimulated cells. Activation of protein kinase-C by treatment of the cells with phorbol esters, bombesin or PDGF inhibits the effect of the exposure to low temperatures. Phosphorylation of p175 is not induced by treatment of the cells with the phosphatases inhibitor sodium orthovanadate. These results suggest that, at low temperatures, the tyrosine kinase associated with the putative receptor encoded by c-neu is activated by physico-chemical modifications of the plasma membrane.

Kinetics of tyrosine phosphorylation and internalization of human EGF receptors overexpressed in NIH 3T3 fibroblasts

Binding of epidermal growth factor (EGF) to cells rapidly induces tyrosine phosphorylation of its receptor which is followed by its internalization and dephosphorylation. The kinetics of these processes differs widely in time from minutes to hours according to cell types. In this paper we analyzed EGF receptor phosphorylation and down-regulation in NIH 3T3 cells transfected with the recombinant hEGF-R cDNA which express 4 X 10(5) receptors/cell. In the presence of EGF receptor phosphorylation reached a maximum after 1 min and was then maintained for about 1 h, while during this time the number of EGF-binding sites was reduced to 40% of the initial number. Detailed analysis of the fate of a population of receptors previously activated and autophosphorylated at 4 degrees C, after warming to 37 degrees C in the absence of the ligand, showed that internalization of the cell surface-associated EGF and dephosphorylation of the receptor were rapid (t1/2 15 min) and followed a similar kinetics. Our data indicate that at any given time only a fraction of the total cell surface receptors is phosphorylated on tyrosine and that dephosphorylation occurs at the cell surface or very rapidly after internalization. In addition the data also suggest that a certain recycling of previously internalized receptors may occur in these cells during EGF treatment.

Epidermal growth factor induces tyrosine phosphorylation of epidermal growth factor receptors not occupied with ligand in intact A431 cells

The mechanism of epidermal growth factor receptor (EGF-R) autophosphorylation in intact A431 cells was studied. We detected epidermal growth factor (EGF) induced tyrosine phosphorylation of EGF-R not occupied with ligand. Cell monolayers were subjected to irradiation after incubation with photoreactive derivative of EGF and uncoupled EGF was extracted by acidic treatment. Subsequent immunoprecipitation with antiphosphotyrosine antibodies resulted in precipitation of both EGF-R complexes with EGF and EGF-R with unoccupied ligand-binding site. The fact of precipitation of EGF-R with unoccupied ligand-binding site in conjunction with our finding of rapid dephosphorylation of EGF-R after EGF extraction by acidic treatment, strongly supports the interpretation that cross-phosphorylation of EGF-R may take place in intact cells.

Effect of the different dimeric forms of the platelet-derived growth factor on cellular responses in mouse Swiss 3T3 fibroblasts

PDGF consists of two polypeptide chains, A and B, and all three possible dimers have been isolated from different sources. Human PDGF, essentially AB, porcine PDGF (BB) and recombinant PDGF-AA were tested on Swiss 3T3 fibroblasts for their ability to stimulate mitogenesis, phosphoinositide turnover and tyrosine phosphorylation of the PDGF receptor. When used in saturating amounts, the three isoforms were equally active in inducing mitogenesis. However, PDGF-AA was less active than AB and BB to induce the phosphorylation of the receptor and the turnover of phosphoinositides (30% and 50%, respectively). These findings suggest that, in Swiss 3T3 fibroblasts, PDGF receptors of the alpha-type are present in a slightly lower amount than beta-type. In addition, the two types of receptor appear to have similar physiological functions.

Kinetics and Regulation of Tyrosine Phosphorylation of the Platelet Derived Growth Factor Receptor

Platelet derived growth factor (PDGF) interaction with the cells induces rapid tyrosine phosphorylation of the PDGF receptor in a dose dependent manner. At 37 °C phosphorylation of the receptor is followed by its dephosphorylation and internalization. It is observed that the higher the ligand concentration, the more transient is the response, and the observed kinetics are explained by a simple kinetic model. At 4 °C the phosphorylated form of the receptor is more stable; however, if PDGF is dissociated from the cell surface-associated ligand-receptor complexes, the receptors are rapidly dephosphorylated, indicating that phosphatases specific for phosphotyrosine groups are very active within the cells. In fact, addition of orthovanadate stabilizes the phosphorylated form of the receptor and helps in recognizing possible physiological substrates of the PDGF receptor kinase. The expression of PDGF receptors on the cell surface has been investigated under different growth conditions: a positive correlation exists between the amount of PDGF receptors and the duplication times of exponentially growing cultures. Moreover, during exponential growth the PDGF receptors are scarcely expressed, and their number increases reaching a maximal value when the population enters the stationary phase.

Identification of a protein cross-reacting with anti-phosphotyrosine antibodies in yeast insoluble cytoplasmic matrices

Immunoblot analysis with anti-phosphotyrosine antibodies of total extracts from exponentially growing yeast cells reveals a unique cross-reactive polypeptide of about 75 Kd (p75). The specificity of the immunodecorations has been checked by experiments of competition with phosphoaminoacids. A common behaviour has been observed for the 75 kd band and the 170 kd band corresponding to the platelet-derived growth factor receptor from Swiss 3T3 cells, which it has been known to be autophosphorylated on tyrosine upon ligand binding and used as a control throughout this work. We have found that p75 is associated to detergent insoluble cytoplasmic matrices. The stability of p75 detection by antibodies following treatments that specifically hydrolyze phosphohistidine and its susceptibility to potato acid phosphatase treatment provide further evidences that the epitope recognized by these antibodies in the yeast p75 polypeptide is indeed phosphotyrosine.

Evidence for autocrine activation of a tyrosine kinase in a human gastric carcinoma cell line

Phosphotyrosine (P-Tyr) antibodies have been used to identify the phosphorylated forms of growth factor receptors and oncogene-coded tyrosine kinases. Western blot analysis of a gastric carcinoma cell line with P-Tyr antibodies revealed a tyrosine-phosphorylated protein of Mr 145,000 (P145). In addition, in vitro phosphorylation with (gamma-32P)ATP or P-Tyr immunoprecipitates of the same cells resulted in labelling of this protein on tyrosine. P145 appears to be a transmembrane glycoprotein, with features suggestive of a growth factor receptor. However, the in vivo or in vitro addition of known growth factors did not affect P145 tyrosine phosphorylation. We now report that P145 is rapidly dephosphorylated in vivo when cells are exposed to low pH, a condition known to dissociate ligands from their receptors. The addition of serum-free medium, conditioned by the gastric carcinoma cells, fully restores the tyrosine phosphorylation lost with acid treatment. These data suggest that the activity responsible for P145 phosphorylation on tyrosine, whether intrinsic to P145 itself or due to an associated kinase, is stimulated by a factor secreted by the tumor cells themselves.

Effect of the growth conditions on the expression of cell-surface-associated platelet-derived growth factor receptors in mouse fibroblasts

The conditions affecting the appearance and disappearance of platelet-derived growth factor (PDGF) receptors from the pool of active cell surface-associated receptors were studied. Receptor molecules were revealed in intact Swiss 3T3 fibroblasts by their ability to bind 125I-labeled PDGF and, due to their property to become phosphorylated in tyrosine following ligand binding, by antibodies to phosphotyrosine. PDGF receptor molecules were found to be quite scarce in exponentially growing fibroblasts as compared to quiescent cells. When growing cells were either shifted to a medium containing plasma or received suramin in the culture medium, cell surface-associated PDGF receptors largely increased. This process required about 12 h. Incubation of quiescent cells in serum, but not in plasma, induced a slow decrement of ligand-activatable receptors. In the presence of PDGF the rate of receptor removal from the cell surface was very rapid and was a function of the PDGF concentration. Quiescent cells deprived of cell-surface receptors by incubation with PDGF reexpressed PDGF receptors in about 14 h.

Dissociation of the ligand and dephosphorylation of the platelet-derived growth factor receptor

The ligand-induced phosphorylation of the platelet-derived growth factor (PDGF) receptor was followed at 37 degrees C by a rapid dephosphorylation which was roughly parallel to the down regulation of the 125I-PDGF binding sites. At 4 degrees C, when the ligand-receptor complexes remain associated with the cell surface, the phosphorylated form of the receptor was more stable. However if the ligand was dissociated from the receptor by means of a mild acid wash or a treatment with suramin, the dephosphorylation of the receptor also occurred at a low temperature. These data suggest that, due to the dissociation of the ligand, the kinase activity of the receptor is switched off so that the phosphotyrosine-containing receptors remain exposed to the action of phosphatases that rapidly dephosphorylate them.