Phosphorylation at threonine-654 is not required for negative regulation of the epidermal growth factor receptor by non-phorbol tumor promoters

Annexin A6 is a scaffold for PKCα to promote EGFR inactivation

Protein kinase Cα (PKCα) can phosphorylate the epidermal growth factor receptor (EGFR) at threonine 654 (T654) to inhibit EGFR tyrosine phosphorylation (pY-EGFR) and the associated activation of downstream effectors. However, upregulation of PKCα in a large variety of cancers is not associated with EGFR inactivation, and factors determining the potential of PKCα to downregulate EGFR are yet unknown. Here, we show that ectopic expression of annexin A6 (AnxA6), a member of the Ca(2+) and phospholipid-binding annexins, strongly reduces pY-EGFR levels while augmenting EGFR T654 phosphorylation in EGFR overexpressing A431, head and neck and breast cancer cell lines. Reduced EGFR activation in AnxA6 expressing A431 cells is associated with reduced EGFR internalization and degradation. RNA interference (RNAi)-mediated PKCα knockdown in AnxA6 expressing A431 cells reduces T654-EGFR phosphorylation, but restores EGFR tyrosine phosphorylation, clonogenic growth and EGFR degradation. These findings correlate with AnxA6 interacting with EGFR, and elevated AnxA6 levels promoting PKCα membrane association and interaction with EGFR. Stable expression of the cytosolic N-terminal mutant AnxA6(1-175), which cannot promote PKCα membrane recruitment, does not increase T654-EGFR phosphorylation or the association of PKCα with EGFR. AnxA6 overexpression does not inhibit tyrosine phosphorylation of the T654A EGFR mutant, which cannot be phosphorylated by PKCα. Most strikingly, stable plasma membrane anchoring of AnxA6 is sufficient to recruit PKCα even in the absence of EGF or Ca(2+). In summary, AnxA6 is a new PKCα scaffold to promote PKCα-mediated EGFR inactivation through increased membrane targeting of PKCα and EGFR/PKCα complex formation.

Thermostable direct hemolysin diminishes tyrosine phosphorylation of epidermal growth factor receptor through protein kinase C dependent mechanism

BACKGROUND

Adequate evidence mounts to the fact that several bacteria and their toxins have protective or curative roles in colorectal cancers. Thermostable direct hemolysin (TDH), produced by Vibrio parahaemolyticus, down regulates cell proliferation in colon carcinoma cell lines. TDH induces Ca2+ influx from an extracellular environment accompanied by protein kinase C phosphorylation. Activated protein kinase C inhibits the tyrosine kinase activity of epidermal growth factor receptor (EGFR), the rational target of anti-colorectal cancer therapy.

METHODS

Immunoblotting analyses were performed to ascertain protein kinase C activation, EGFR status, EGFR phosphorylation and mitogen activated protein kinase (MAPK) activity. Flow cytometry analysis and ELISA reconfirmed tyrosine phosphorylation of EGFR and ERK activations, respectively. PKC-α siRNA knockdown was done to corroborate the involvement of PKC-α in the undertaken study.

RESULTS

Our study showed the translocation of PKC-α from cytosol to the membrane fraction in colon carcinoma cell lines on incubation with TDH. The EGFR tyrosine kinase activity exhibited a down regulation on TDH treatment which involved PKC-α, as confirmed by siRNA knockdown. Also ERK phosphorylation occurred on PKC-α activation.

CONCLUSION

TDH activated PKC-α down regulates EGFR tyrosine kinase activity by MEK dependent mechanism involving MAPK.

GENERAL SIGNIFICANCE

In this study we have seen that TDH has an implication in EGFR based therapeutic approach in colorectal cancer via PKC mediated mechanism.

The effects of extracellular nucleotides on [Ca2+]i signalling in a human-derived renal proximal tubular cell line (HKC-8)

HKC-8 cells are a human-derived renal proximal tubular cell line and provide a useful model system for the study of human renal cell function. In this study, we aimed to determine [Ca(2+)](i) signalling mediated by P2 receptor in HKC-8. Fura-2 and a ratio imaging method were employed to measure [Ca(2+)](i) in HKC-8 cells. Our results showed that activation of P2Y receptors by ATP induced a rise in [Ca(2+)](i) that was dependent on an intracellular source of Ca(2+), while prolonged activation of P2Y receptors induced a rise in [Ca(2+)](i) that was dependent on intra- and extracellular sources of Ca(2+). Pharmacological and molecular data in this study suggests that TRPC4 channels mediate Ca(2+) entry in coupling to activation of P2Y in HKC-8 cells. U73221, an inhibitor of PI-PLC, did not inhibit the initial ATP-induced response; whereas D609, an inhibitor of PC-PLC, caused a significant decrease in the initial ATP-induced response, suggesting that P2Y receptors are coupled to PC-PLC. Although P2X were present in HKC-8, The P2X agonist, alpha,beta me-ATP, failed to cause a rise in [Ca(2+)](i). However, PPADS at a concentration of 100 microM inhibits the ATP-induced rise in [Ca(2+)](i). Our results indicate the presence of functional P2Y receptors in HKC-8 cells. ATP-induced [Ca(2+)](i) elevation via P2Y is tightly associated with PC-PLC and TRP channel.

An electrostatic engine model for autoinhibition and activation of the epidermal growth factor receptor (EGFR/ErbB) family

We propose a new mechanism to explain autoinhibition of the epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases based on a structural model that postulates both their juxtamembrane and protein tyrosine kinase domains bind electrostatically to acidic lipids in the plasma membrane, restricting access of the kinase domain to substrate tyrosines. Ligand-induced dimerization promotes partial trans autophosphorylation of ErbB1, leading to a rapid rise in intracellular [Ca²⁺] that can activate calmodulin. We postulate the Ca²⁺/calmodulin complex binds rapidly to residues 645–660 of the juxtamembrane domain, reversing its net charge from +8 to −8 and repelling it from the negatively charged inner leaflet of the membrane. The repulsion has two consequences: it releases electrostatically sequestered phosphatidylinositol 4,5-bisphosphate (PIP₂), and it disengages the kinase domain from the membrane, allowing it to become fully active and phosphorylate an adjacent ErbB molecule or other substrate. We tested various aspects of the model by measuring ErbB juxtamembrane peptide binding to phospholipid vesicles using both a centrifugation assay and fluorescence correlation spectroscopy; analyzing the kinetics of interactions between ErbB peptides, membranes, and Ca²⁺/calmodulin using fluorescence stop flow; assessing ErbB1 activation in Cos1 cells; measuring fluorescence resonance energy transfer between ErbB peptides and PIP₂; and making theoretical electrostatic calculations on atomic models of membranes and ErbB juxtamembrane and kinase domains.

Heterodimerization of the epidermal-growth-factor (EGF) receptor and ErbB2 and the affinity of EGF binding are regulated by different mechanisms

When clathrin-dependent endocytosis is inhibited in HeLa cells by overexpression of a K44A (Lys(44)-->Ala) mutant of the GTPase dynamin, high-affinity binding of epidermal growth factor (EGF) to the EGF receptor (EGFR) is disrupted [Ringerike, Stang, Johannessen, Sandnes, Levy and Madshus (1998) J. Biol. Chem. 273, 16639-16642]. We now report that the effect of [K44A]dynamin on EGF binding was counteracted by incubation with the non-specific kinase inhibitor staurosporine (SSP), implying that a protein kinase is responsible for disrupted high-affinity binding of EGF upon overexpression of [K44A]dynamin. The effect of [K44A]dynamin on EGF binding was not due to altered phosphorylation of the EGFR, suggesting that the activated kinase is responsible for phosphorylation of a substrate other than EGFR. The number of EGFR molecules was increased in cells overexpressing [K44A]dynamin, while the number of proto-oncoprotein ErbB2 molecules was unaltered. EGF-induced receptor dimerization was not influenced by overexpression of [K44A]dynamin. ErbB2-EGFR heterodimer formation was found to be ligand-independent, and the number of heterodimers was not altered by overexpression of [K44A]dynamin. Neither SSP nor the phorbol ester PMA, which disrupts high-affinity EGF-EGFR interaction, had any effect on the EGFR homo- or hetero-dimerization. Furthermore, the EGF-induced tyrosine phosphorylation of ErbB2 was not affected by overexpression of [K44A]dynamin, implying that EGFR-ErbB2 dimers were fully functional. Our results strongly suggest that high-affinity binding of EGF and EGFR-ErbB2 heterodimerization are regulated by different mechanisms.

Long-term phenotypic correction of rodent hemiparkinsonism by gene therapy using genetically modified myoblasts

Rat myoblasts were genetically modified to express tyrosine hydroxylase (TH) and produce dopamine in culture. Implanting TH gene-transfected myoblasts into the denervated striatum of 6-OHDA-lesioned rats significantly decreased rotational asymmetry by 50 to approximately 60%. Improvement persisted for up to 13 months. Genetically modified cells could survive and express transgene in the striatum as demonstrated by RT-PCR and immunohistochemical stain-ing. The dopamine content in the striatum tissue of the gene therapy group recovered to 49% of the normal level and was 25-fold higher than that of a control group receiving parental cells. Neither tumor formation nor immunorejection was observed in this study. These results show that myoblasts may be useful as gene carriers for ex vivo gene therapy in the CNS. Gene Therapy (2000) 7, 445-449.

Ca2+/calmodulin-dependent kinase II phosphorylates the epidermal growth factor receptor on multiple sites in the cytoplasmic tail and serine 744 within the kinase domain to regulate signal generation

Down-regulation of receptor tyrosine kinase activity plays an essential role in coordinating and controlling cellular growth/differentiation. Ca2+/calmodulin-dependent kinase II (CaM kinase II)-mediated phosphorylation of threonine 1172 in the cytoplasmic tail of HER2/c-erbB2 can modulate tyrosine kinase activity and consensus phosphorylation sites are also found at serines 1046/1047 in the structurally related epidermal growth factor receptor (EGFR). We show that serines 1046/1047 are sites for CaM kinase II phosphorylation, although there is a preference for serine 1047, which resides within the consensus -R-X-X-S-. In addition, we have identified major phosphorylation sites at serine 1142 and serine 1057, which lie within a novel -S-X-D- consensus. Mutation of serines 1046/1047 in full-length EGFR enhanced both fibroblast transformation and tyrosine autokinase activity that was significantly potentiated by additional mutation of serines 1057 and 1142. A single CaM kinase II site was also identified at serine 744 within sub-kinase domain III, and autokinase activity was significantly affected by mutation of this serine to an aspartic acid making this site appear constitutively phosphorylated. We have addressed the mechanism by which CaM kinase II phosphorylation of the EGFR might regulate receptor autokinase activity and show that this modification can hinder association of the cytoplasmic tail with the kinase domain to prevent an enzyme-substrate interaction. We postulate that the location and greater number of CaM kinase II phosphorylation sites in the EGFR compared with HER-2/c-erbB2, leading to differential regulation of autokinase activity, contributes to differences in the strength of downstream signaling events and may explain the higher relative transforming potential of HER-2/cerbB2.

C-Met signalling in an HGF/SF-insensitive variant MDCK cell line with constitutive motile/invasive behaviour

The Met protein is a receptor tyrosine kinase for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional growth factor with mitogenic, motogenic and morphogenic properties. A morphologically altered variant of the MDCK cell line, MDCK-1, spontaneously exhibits a number of features associated with a partial HGF/SF-Met induced phenotype (less adhesive colonies in culture, enhanced invasion and motility, nascent tubule formation), but paradoxically does not respond to HGF/SF treatment. Although the overall cell surface expression and distribution of Met were found to be similar in parental MDCK cells and the MDCK-1 cell line, p145met autophosphorylation (+/ HGF/SF) was significantly reduced in MDCK-1 cells in vitro and in vivo when compared with parental MDCK cells. In contrast, EGF induced cell proliferation and EGF receptor autophosphorylation to similar levels in both cell lines. The basal levels of protein tyrosine phosphorylation were higher in MDCK-1 cells when compared with parental MDCK cells, including that of two prominent proteins with molecular masses of approximately 185 kDa and 220 kDa. Moreover, both p185 and p220 are present and tyrosine phosphorylated in Met immunoprecipitates from MDCK-1 cells (+/-HGF/SF), but not parental MDCK cells. In addition, Met immunocomplexes from MDCK-1 cells exhibited an approximately 3-fold increased tyrosine kinase activity in vitro when compared with MDCK cells, correlating with the higher basal levels of total phosphotyrosine. Treatment of MDCK-1 cells with the tyrosine kinase inhibitor herbimycin A reverted the cell phenotype to a more MDCK-like morphology in culture, with a concomitant reduction in the tyrosine phosphorylation predominantly of p220. Taken together these data suggest that aberrations in Met activity and associated signalling render MDCK-1 cells insensitive to HGF/SF, and may also mediate alterations in MDCK-1 cell behaviour.

Role of mitogen-activated protein kinase kinase in regulation of the epidermal growth factor receptor by protein kinase C

The epidermal growth factor receptor (EGFR) is regulated by at least two mechanisms involving protein kinase C (PKC), inhibition of EGF binding and inhibition of EGF-stimulated tyrosine kinase activity. In this study we investigated whether mitogen-activated protein kinase (MAPK) mediates the inhibitory effects of PKC on EGFR binding or kinase activity by pretreating NIH3T3 and Chinese hamster ovary cells expressing the EGFR with PD98059, an inhibitor of MAPK/extracellular signal-regulated kinase kinase (MEK). We also determined whether substitution of cysteine for threonine at residue 669, the site of MAPK phosphorylation of the EGFR, alters the inhibition of kinase activity by PKC. The results indicate that 1) PKC down-regulates EGFR tyrosine kinase activity by an MEK-dependent mechanism presumably involving MAPK; 2) the inhibition by PKC is not a direct result of phosphorylation of the EGFR by PKC or MAPK; 3) activation of MAPK is not sufficient to regulate EGFR kinase activity; and 4) PKC-mediated down-regulation of EGF binding and EGFR kinase activity occur by different mechanisms. These data are consistent with a model for regulation of the EGFR by other receptors whereby their activation of PKC, in conjunction with MAPK, results in the phosphorylation of a protein(s) that modulates EGFR kinase activity.

Effect of tyrphostin on cell growth and tyrosine kinase activity of epidermal growth factor receptor in human gliomas

The effects of tyrphostin, a selective protein tyrosine kinase inhibitor, on epidermal growth factor (EGF)-stimulated cell growth and EGF-receptor tyrosine kinase activity were studied in four human glioma cell lines. Stimulation by EGF induced variable enhancements of cell growth as well as tyrosine phosphorylation of EGF receptor and intracellular target proteins in all glioma cell lines. The level of immunoreactive EGF receptor detected with antibodies against extra- and intracellular domains was moderate in all four glioma cell lines, but markedly decreased with the latter antibody in two glioma cell lines. This variation was associated with considerable reduction of the EGF-stimulated tyrosine autophosphorylation level. Tyrphostin inhibited dose-dependently the EGF-stimulated cell growth and tyrosine autophosphorylation in all glioma cell lines, and the optimum time for the maximum inhibitory effect on tyrosine autophosphorylation was 12 to 18 hours after treatment with tyrphostin. The antiproliferative activity of tyrphostin nearly correlated quantitatively with its potency as an inhibitor of the EGF-stimulated EGF receptor tyrosine kinase activity. Tyrphostin had no significant effect on the immunoreactive EGF receptor levels, on the affinity constants and numbers of EGF receptor, or on the down-regulation and specific internalization of EGF receptor in any glioma cell line, suggesting that the effects of tyrphostin are not likely to be the results of reduction in EGF receptor and EGF binding capacity. In addition, the serum-stimulated cell growth was also inhibited dose-dependently by higher concentrations of tyrphostin in all glioma cell lines. It might be suggested, therefore, that tyrphostin inhibits EGF-stimulated cell growth by a specific suppression of EGF receptor tyrosine kinase activity, and at higher concentrations there appears to be some degree of either nonspecific inhibition or inhibition of serum-stimulated protein tyrosine kinase activity to induce the cell growth inhibition of gliomas.

Effect of epidermal growth factor on mouse sperm acrosome reaction induced by zona pellucida

PROBLEM

The effect of epidermal growth factor (EGF) on the acid-solubilized zona pellucida (ZP)-induced acrosome reaction was investigated in mouse sperm.

METHOD

Mouse epididymal sperm were capacitated in modified Krebs-Ringer bicarbonate buffer (m-KRB) for 120 min and further treated with acid-solubilized ZP(4 zona/microliters) for an additional 60 min to induce the acrosome reaction. The chlortetracycline fluorescence assay was used to monitor the acrosome reaction. The acrosome reacted sperm featured the acrosome reacted (AR) pattern, which demonstrates the lack of fluorescence on the head and bright midpiece.

RESULTS

EGF caused an early increase in the AR pattern in response to the acid-solubilized ZP in a dose-dependent manner. The EGF-dependent stimulation of the ZP-induced acrosome reaction was inhibited by an inhibitor of protein tyrosine kinases, genistein or activators of Ca++ and phospholipid-dependent protein kinase (protein kinase C). Furthermore, the stimulatory effect of EGF was not attenuated when sperm were capacitated in the presence of islet-activating protein, an inactivator of inhibitory guanine nucleotide-binding regulatory protein (Gi protein).

CONCLUSION

EGF stimulates the ZP-induced acrosome reaction in a manner that is independent of the Gi protein. The EGF action is regulated by protein tyrosine kinase and protein kinase C.

Calphostin-C stimulates epidermal growth factor receptor phosphorylation and internalization via light-dependent mechanism

Calphostin-C with perylenequinone structure is known to bind the regulatory domain of protein kinase C (PKC) and to inhibit kinase activity in vitro in a light-dependent fashion. We have found that calphostin-C induces substantial serine and threonine phosphorylation of the epidermal growth factor (EGF) receptor in a light-dependent fashion in the EGF receptor-hyperproducing squamous carcinoma cell line NA. Tryptic phospho-peptide mapping and phospho-amino acid analysis revealed that calphostin-C-enhanced phosphorylation was on threonine 669, serine 671, serine 1046/1047, and serine 1166. However, calphostin-C did not inhibit phosphorylation of the 80 K protein, a cytosolic major substrate of PKC (MARCKS). Staurosporine, a potent PKC inhibitor with affinity for the catalytic domain of PKC, inhibited phosphorylation of the 80 K protein and 12-O-tetradecanoyl-13-phorbol acetate induction of EGF receptor phosphorylation but did not inhibit the calphostin-C induction of the EGF receptor phosphorylation. These results suggest that the target of calphostin-C in vivo is different from that of staurosporine and thus calphostin-C in vivo does not inhibit PKC. Furthermore, calphostin-C enhanced the internalization of phosphorylated EGF receptor. Thus, calphostin-C apparently activates a novel signal transduction pathway which involves phosphorylation and internalization of the EGF receptor via light-dependent mechanism.

Effects of gravity on the cellular response to epidermal growth factor

EGF and related polypeptides are involved in the regulation of cell growth and differentiation of continuously regenerating tissues, in tissue repair processes and in placental and fetal development. Their initial mode of action generally constitutes binding to specific plasma membrane localized receptors, transduction of the signal across the plasma membrane, subsequent activation of signalling pathways in the cell, and the induction of early nuclear gene expression. EGF-induced signal transmission from the plasma membrane to the nucleus has been studied in microgravity in order to gain insight in the molecular mechanisms that constitute the effects of gravity on cell growth. Exposure of human A431 cells to microgravity strongly suppresses EGF- and PMA-induced c-fos and c-jun expression. In contrast, forskolin- and A23187-induced c-fos expression and constitutive beta-2 microglobulin expression remain unaffected. This suggests that microgravity differentially modulates EGF-induced signal transduction pathways. Since both EGF and PMA are known to be activators of PKC, which is not the case for forskolin and A23187, PKC-mediated signal transduction may be a cellular target for microgravity. Inhibition of EGF-induced c-fos expression by microgravity occurs downstream of the initiation of EGF-induced signal transduction, i.e., EGF binding and EGFR redistribution. In addition to PKC signaling, actin microfilament organization appears to be sensitive to microgravity. Therefore, the inhibition of signal transduction by microgravity may be related to alterations in actin microfilament organization. The fact that early gene expression is affected by agents that alter the organization of the actin microfilament system supports this hypothesis. The decrease in c-fos and c-jun expression in microgravity may result in the decreased formation of the FOS and JUN proteins. Consequently, a short-term reduction in gene expression in microgravity may have a more dramatic effect over the long term, since both the JUN and FOS protein families are required for normal cell cycle progression. However, since more than 20 years of manned spaceflight have shown that humans can survive in microgravity for prolonged periods, it appears that cells in the human body can partly or completely overcome gravitational stress. Although some insight in the molecular basis on human cells has been obtained, future studies will be needed for a better understanding of the grounds for alterations in the cellular biochemistry due to altered gravity conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenovirus and protein kinase C have distinct molecular requirements for regulating epidermal growth factor receptor trafficking

The ligand-activated tyrosine kinase receptor for epidermal growth factor (EGF) is down-regulated by an integral membrane protein coded for by the E3 early transcription unit of group C adenoviruses. The E3 protein appears to block recycling of constitutively internalized receptors, causing them instead to traffic to lysosomes where they are degraded. Expression of functional EGF receptors is also regulated by protein kinase C (PKC), which directly phosphorylates the EGF receptor at Thr-654. The goal of this study was to determine potential interactions between PKC and the E3 protein, since membrane-bound PKC activity is elevated by the adenovirus E1A protein. Our results show that although tumor promoters which activate PKC cause a coordinate induction of E3 protein synthesis and EGF receptor degradation, the E3 protein-induced pathway for receptor down-regulation functions independently of PKC and other kinases that are inhibited by staurosporine. This suggests that in contrast to other mechanisms that modulate receptor expression (i.e., ligand and PKC), the E3 protein is not regulated by phosphorylation but is constitutively active. We also report that adenovirus-mediated degradation is the preferred pathway in infected cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) to induce receptor recycling.

Effect of epidermal growth factor on human sperm capacitation

OBJECTIVE

To examine the effect of epidermal growth factor (EGF) on human sperm capacitation and the involvement of protein phosphorylation in the regulation of the EGF action.

DESIGN

Human sperm were capacitated in modified Krebs-Ringer's bicarbonate medium of Biggers, Whitten, and Whittingham in the presence of EGF and various agents known to phosphorylate the EGF receptor. The chlortetracycline fluorescence assay was used to monitor capacitated sperm. Capacitation was confirmed by the ability of sperm to undergo the acrosome reaction in response to solubilized mouse zonae pellucidae (ZP).

RESULTS

In 15 minutes, the appearance of the clear perimeter pattern increased significantly in the sperm treated with 100 ng/mL EGF compared with the controls. In EGF-treated sperm, the percent clear perimeter pattern remained stable for 3 hours without affecting the acrosome reaction pattern and the motility. Epidermal growth factor stimulated the appearance of the clear perimeter pattern at concentrations > 100 pg/mL. The stimulation by EGF was attenuated by the treatment with genistein, 12-O-tetradecanoylphorbol-13-acetate, or thapsigargin. In sperm that were incubated in the presence of 100 ng/mL EGF for 30 minutes and further induced the acrosome reaction by mouse ZP, the percent acrosome reaction pattern increased significantly compared with the controls.

CONCLUSION

Epidermal growth factor stimulates human sperm capacitation by activating the tyrosine kinase of the EGF receptor which is regulated by multisite phosphorylation.

Cross talk among tyrosine kinase receptors in PC12 cells: desensitization of mitogenic epidermal growth factor receptors by the neurotrophic factors, nerve growth factor and basic fibroblast growth factor

We have studied the effects of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) on epidermal growth factor (EGF) binding to PC12 cells. We show that NGF and bFGF rapidly induce a reduction in 125I-EGF binding to PC12 cells in a dose-dependent manner. This decrease amounts to 50% for NGF and 35% for bFGF. Both factors appear to act through a protein kinase C(PKC)-independent pathway, because their effect persists in PKC-downregulated PC12 cells. Scatchard analysis indicates that NGF and bFGF decrease the number of high affinity EGF binding sites. In addition to their effect on EGF binding, NGF and bFGF activate in intact PC12 cells one or several serine/threonine kinases leading to EGF receptor threonine phosphorylation. Using an in vitro phosphorylation system, we show that NGF- or bFGF-activated extracellular regulated kinase 1 (ERK1) is able to phosphorylate a kinase-deficient EGF receptor. Phosphoamino acid analysis indicates that this phosphorylation occurs mainly on threonine residues. Furthermore, two comparable phosphopeptides are observed in the EGF receptor, phosphorylated either in vivo after NGF treatment or in a cell-free system by NGF-activated ERK1. Finally, a good correlation was found between the time courses of ERK1 activation and 125I-EGF binding inhibition after NGF or bFGF treatment. In conclusion, in PC12 cells the NGF- and bFGF-stimulated ERK1 appears to be involved in the induction of the threonine phosphorylation of the EGF receptor and the decrease in the number of high affinity EGF binding sites.

Diacylglycerol kinase is phosphorylated in vivo upon stimulation of the epidermal growth factor receptor and serine/threonine kinases, including protein kinase C-epsilon

In signal transduction, diacylglycerol (DG) kinase attenuates levels of the second messenger DG by converting it to phosphatidic acid. A previously cloned full-length human 86 kDa DG kinase cDNA was expressed as fusion protein in Escherichia coli, to aid in the generation of DG-kinase-specific monoclonal antibodies suitable for immunoprecipitation experiments. To investigate whether phosphorylation of DG kinase is a possible mechanism for its regulation, COS-7 cells were transiently transfected with the DG kinase cDNA and phosphorylation of the expressed DG kinase was induced by various stimuli. Activation of both cyclic AMP-dependent protein kinase and protein kinase C (PKC) resulted in phosphorylation of DG kinase on serine residues in vivo, and both kinases induced this phosphorylation within the same tryptic phosphopeptide, suggesting that they may exert similar control over DG kinase. No phosphorylation was observed upon ionomycin treatment, intended to activate Ca2+/calmodulin-dependent kinases. Co-transfections of DG kinase with either PKC-alpha or PKC-epsilon cDNA revealed that both protein kinases, when stimulated, are able to phosphorylate DG kinase. For PKC-epsilon, DG kinase is the first in vivo substrate identified. Stimulation with epidermal growth factor (EGF) of COS-7 cells transfected with both DG kinase and EGF-receptor cDNA results mainly in phosphorylation of DG kinase on tyrosine. Since the EGF receptor has an intrinsic tyrosine kinase activity, this finding implies that DG kinase may be a direct substrate for the activated EGF receptor.

Point mutation of an FGF receptor abolishes phosphatidylinositol turnover and Ca2+ flux but not mitogenesis

Stimulation of certain receptor tyrosine kinases results in the tyrosine phosphorylation and activation of phospholipase C gamma (PLC gamma), an enzyme that catalyses the hydrolysis of phosphatidylinositol (PtdIns). This hydrolysis generates diacylglycerol and free inositol phosphate, which in turn activate protein kinase C and increase intracellular Ca2+, respectively. PLC gamma physically associates with activated receptor tyrosine kinases, suggesting that it is a substrate for direct phosphorylation by these kinases. Here we report that a fibroblast growth factor (FGF) receptor with a single point mutation at residue 766 replacing tyrosine with phenylalanine fails to associate with PLC gamma in response to FGF. This mutant receptor also failed to mediate PtdIns hydrolysis and Ca2+ mobilization after FGF stimulation. However, the mutant receptor phosphorylated itself and several other cellular proteins, and it mediated mitogenesis in response to FGF. These findings show that a point mutation in the FGF receptor selectively eliminates activation of PLC gamma and that neither Ca2+ mobilization nor PtdIns hydrolysis are required for FGF-induced mitogenesis.

Regulation of the transmodulated epidermal growth factor receptor by cholera toxin and the protein phosphatase inhibitor okadaic acid

Addition of tumor promoting phorbol esters, such as phorbol 12-myristate 13-acetate (PMA), to many cell lines results in a decrease of 125I-epidermal growth factor (EGF) binding and increased serine/threonine phosphorylation of the EGF receptor in a process termed transmodulation. It is, however, unclear whether or not receptor phosphorylation is causally related to the inhibition of high affinity EGF binding. We have investigated the significance of phosphorylation/dephosphorylation events in the mechanism of PMA-induced transmodulation using the adenylate cyclase activator cholera toxin and the serine/threonine protein phosphatase inhibitor okadaic acid. In Rat-1 fibroblasts treated at 37 degrees C, PMA induced a rapid decrease in EGF binding which persisted for 3 hours. In contrast, cells exposed to PMA in the presence of cholera toxin exhibited a marked recovery of binding within 60 minutes. The PMA-stimulated decrease in binding correlated with a rapid increase in the phosphorylation state of the EGF receptor. While phosphorylation of the receptor was sustained at an elevated level for at least three hours in cells receiving PMA alone, EGF receptor phosphorylation decreased between 1 and 3 hours in cells treated with PMA and cholera toxin. Furthermore, the cholera toxin-stimulated return of EGF binding was inhibited by treatment with the phosphatase inhibitor okadaic acid. These results suggest that a cholera toxin-activated phosphatase can increase binding capacity of the transmodulated EGF receptor in Rat-1 cells. Cholera toxin treatment elicited a qualitatively similar response in cells transmodulated by platelet-derived growth factor (PDGF). Okadaic acid antagonized the natural return of binding observed in cells stimulated with PDGF alone, indicating that a dephosphorylation event may be required for the recovery of normal EGF binding after receptor transmodulation.

Rapid uptake of tyrphostin into A431 human epidermoid cells is followed by delayed inhibition of epidermal growth factor (EGF)-stimulated EGF receptor tyrosine kinase activity

Treatment of A431 human epidermoid cells with epidermal growth factor (EGF; 20 nM) results in decreased proliferation. This is associated with blockage of the cells in the S and/or G2 phases of the cell cycle. We found that tyrphostin, a putative tyrosine kinase inhibitor, in the range of 50 to 100 microM, partially reversed the growth-inhibitory and cell cycle changes induced by EGF. By using high-pressure liquid chromatography with electrochemical detection, we found that tyrphostin was readily incorporated into A431 cells, reaching maximal levels within 1 h. Although tyrphostin (50 to 100 microM) had no effect on high-affinity binding of EGF to its receptor in A431 cells for up to 24 h, the compound partially inhibited EGF-stimulated EGF receptor tyrosine kinase activity. However, this effect was evident only after prolonged treatment of the cells (4 to 24 h) with the drug. When the peak intracellular concentration of tyrphostin occurred (1 h), no inhibition of tyrosine kinase activity was observed. After both 1 and 24 h, tyrphostin was a less effective inhibitor of tyrosine kinase activity than the potent tumor promoter 12-O-tetradecanoyl phorbol-13-acetate, which almost completely blocked EGF receptor autophosphorylation. On the basis of our data, we hypothesize that tyrphostin is not a competitive inhibitor of the EGF receptor tyrosine kinase in intact cells and that it functions by an indirect mechanism.

Rapid uptake of tyrphostin into A431 human epidermoid cells is followed by delayed inhibition of epidermal growth factor (EGF)-stimulated EGF receptor tyrosine kinase activity

Treatment of A431 human epidermoid cells with epidermal growth factor (EGF; 20 nM) results in decreased proliferation. This is associated with blockage of the cells in the S and/or G2 phases of the cell cycle. We found that tyrphostin, a putative tyrosine kinase inhibitor, in the range of 50 to 100 microM, partially reversed the growth-inhibitory and cell cycle changes induced by EGF. By using high-pressure liquid chromatography with electrochemical detection, we found that tyrphostin was readily incorporated into A431 cells, reaching maximal levels within 1 h. Although tyrphostin (50 to 100 microM) had no effect on high-affinity binding of EGF to its receptor in A431 cells for up to 24 h, the compound partially inhibited EGF-stimulated EGF receptor tyrosine kinase activity. However, this effect was evident only after prolonged treatment of the cells (4 to 24 h) with the drug. When the peak intracellular concentration of tyrphostin occurred (1 h), no inhibition of tyrosine kinase activity was observed. After both 1 and 24 h, tyrphostin was a less effective inhibitor of tyrosine kinase activity than the potent tumor promoter 12-O-tetradecanoyl phorbol-13-acetate, which almost completely blocked EGF receptor autophosphorylation. On the basis of our data, we hypothesize that tyrphostin is not a competitive inhibitor of the EGF receptor tyrosine kinase in intact cells and that it functions by an indirect mechanism.

Epidermal growth factor (EGF) receptor T669 peptide kinase from 3T3-L1 cells is an EGF-stimulated "MAP" kinase

The epidermal growth factor (EGF) receptor is both an activator and a target of growth factor-stimulated kinases involved in cellular signaling. Threonine-669 (T669) of the EGF receptor is phosphorylated in response to a wide variety of growth-modulating agents. MAP kinase is similarly phosphorylated as well as stimulated by growth activators, including EGF. To determine whether a MAP-type kinase is responsible for T669 kinase activity in EGF-stimulated 3T3-L1 cells, we partially purified and characterized the T669 peptide kinase. The results indicate that a MAP kinase phosphorylates the T669 peptide and raise the possibility that this enzyme may participate in a feedback loop, being activated by the EGF receptor and in turn phosphorylating the receptor.

Evidence for autocrine/paracrine growth stimulation by transforming growth factor-alpha during the process of skin tumor promotion

A single topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) to mouse skin decreased 125I-labeled epidermal growth factor (EGF) binding in epidermal membrane preparations within 1 h while 1,8-dihydroxy-3-methyl-9-anthrone (chrysarobin) gradually reduced binding with maximum inhibition at 15 h. Subsequently, 125I-EGF binding increased to approximately 200% of control in epidermal membrane preparations from both TPA- and chrysarobin-treated mice. A single application of TPA but not chrysarobin resulted in a rapid translocation of protein kinase C (PKC) to the membrane; however, treatment with both promoters ultimately led to a time-dependent loss of PKC activity in both membrane and cytosol fractions. The initial inhibition of 125I-EGF binding was sustained for at least 24 h after single and multiple treatments with both promoting agents. Acid washing restored EGF binding to control levels in membrane preparations obtained 24 h after a single application, whereas acid washing of membrane preparations obtained 24 h after a second application of TPA or chrysarobin increased binding (2.5-fold and 1.5-fold that of the control, respectively). The presence of increased amounts of ligands for the EGF receptor in tumor promoter-treated epidermis was initially confirmed in 125I-EGF binding competition experiments using NRK-49F cells. A single topical application of TPA or chrysarobin induced elevated levels of transforming growth factor-alpha (TGF-alpha) mRNA at 6 h or 15-24 h, respectively. Elevated levels of a TGF-alpha precursor (21 kDa) were subsequently observed in cytosol and membrane preparations after single and multiple applications of TPA or chrysarobin. These results suggest that repeated topical application of tumor promoters may lead to sustained loss of a negative-feedback mechanism involving phosphorylation at Thr-654 of the EGF receptor by PKC. The concomitant elevation of ligands, such as TGF-alpha, may provide a mechanism for sustained cell proliferation essential for skin tumor promotion.

Second messenger modulation of epidermal growth factor receptor function does not occur at the level of receptor dimerization

Epidermal growth factor (EGF)-induced receptor dimerization may provide a mechanism for activation of the receptor protein tyrosine kinase and for initiation of post-receptor signalling pathways. We have examined whether second messengers and agents that modulate EGF receptor function act at the level of receptor dimerization. Both the Ca2+ ionophore ionomycin and the tumour promotor tetradecanoylphorbol acetate (TPA), added shortly before EGF, inhibit EGF receptor protein tyrosine kinase activity in intact cells. In permeabilized cells, elevation of Ca2+ similarly inhibits EGF receptor function. The inhibitory effect of Ca2+, unlike that of TPA, appears not to be dependent on protein kinase C activity. Neither ionomycin nor phorbol ester affects EGF-induced receptor dimerization, as shown by cross-linking and immunoblotting techniques, although the phosphotyrosine content of both monomeric and dimeric receptors is strongly decreased. Furthermore, we show that EGF receptor dimerization is not affected by increases in cyclic AMP or intracellular pH, nor by changes in transmembrane potential, medium osmolarity or the glycosylation state of the receptor. These result suggest that modulation of EGF receptor function occurs at a step other than receptor dimerization.