Independent mechanisms account for the regulation by protein kinase C of the epidermal growth factor receptor affinity and tyrosine-protein kinase activity

Impact of epidermal growth factor and/or β-mercaptoethanol supplementations on the in vitro produced buffaloes' embryos

The present study investigated the effects of epidermal growth factors (EGF) and/or β-Mercaptoethanol (βME) supplementations to oocyte maturation, fertilization, and culture media on the buffalo in vitro embryo production. The ovaries were collected and transferred within 2 h to the laboratory. The cumulus oocytes complexes were aspirated from 3 to 8 mm diameter follicles. Firstly, EGF; 0, 10, 20, or 50 ng/mL or βME; 0, 25, 50, 100, or 200 μM were supplemented to the in vitro maturation (TCM-199), fertilization (IVF-TALP), or culture (IVC: SOF) media. Our results revealed that supplementing EGF (20 ng/mL) to the TCM-199, IVF-TALP, or SOF media could efficiently improve the growth rates and development of buffalos' embryos, while EGF (50 ng/mL) could stimulate the embryo production only after treatment of the IVF-TALP /or SOF media, but not the IVM medium. However, βME was less efficient than EGF; it stimulated the growth rates of buffalo embryos when supplemented with the maturation and fertilization (IVF-TALP) media in a 50 μM concentration. Secondly, combined EGF (20 ng/mL) and βME (50 μM) were supplemented to the maturation media as effective concentration. The combined treatment of EGF (20 ng/mL) and βME (50 μM) showed no significant enhancing effect on the buffalo embryos compared to each alone. For future perspectives, further study is required to examine the effects of combined EGF and βME on the maturation and fertilization of buffalo oocytes at different categories of age and seasonal localities.

EGF RECEPTOR DYNAMICS IN EGF-RESPONDING CELLS REVEALED BY FUNCTIONAL IMAGING DURING SINGLE PARTICLE TRACKING

The epidermal growth factor (EGF) receptor transduces the extracellular EGF signal into the cells. The distribution of these EGF receptors in the plasma membrane is heterogeneous and dynamic, which is proposed to be important for the regulation of cell signaling. The response of the cells to a physiological concentration of EGF is not homogeneous, which makes it difficult to analyze the dynamics related to the response. Here we developed a system to perform functional imaging during single particle tracking (SPT) analysis. This system made it possible to observe the cytosolic Ca 2+ concentration to monitor the cell response while tracking individual EGF molecules and found that about half of the cells responded to the stimulation with 1.6 nM EGF. In the responding cells, the EGF receptor showed 3 modes of movement: fast (the diffusion coefficient of 0.081 ± 0.009 μm2/sec, 29 ± 9%), slow (0.020 ± 0.005 μm2/sec, 22 ± 6%), and stationary (49 ± 13%). The diffusion coefficient of the fast mode movement in the responding cells was significantly larger than that in the nonresponding cells (0.069 ± 0.009 μm2/sec, p < 0.05). The diffusion coefficient of the fast mode movement is thought to reflect the monomer–dimer equilibrium of the EGF receptor. We assumed that the feedback regulation via the Ca 2+ signaling pathway slightly shifts the equilibrium from dimer to monomer in the responding cells.

[Formula: see text]Special Issue Comment: This research paper is about the diffusion of EGF receptors in the membrane. It is therefore related with various projects in this Special Issue: the reviews about FRET41 and enzymes,42 and the projects about solving single molecules trajectories.43

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Simultaneous profiling of 194 distinct receptor transcripts in human cells

Many signal transduction cascades are initiated by transmembrane receptors with the presence or absence and abundance of receptors dictating cellular responsiveness. We provide a validated array of quantitative reverse transcription polymerase chain reaction (qRT-PCR) reagents for high-throughput profiling of the presence and relative abundance of transcripts for 194 transmembrane receptors in the human genome. We found that the qRT-PCR array had greater sensitivity and specificity for the detected receptor transcript profiles compared to conventional oligonucleotide microarrays or exon microarrays. The qRT-PCR array also distinguished functional receptor presence versus absence more accurately than deep sequencing of adenylated RNA species by RNA sequencing (RNA-seq). By applying qRT-PCR-based receptor transcript profiling to 40 human cell lines representing four main tissues (pancreas, skin, breast, and colon), we identified clusters of cell lines with enhanced signaling capabilities and revealed a role for receptor silencing in defining tissue lineage. Ectopic expression of the interleukin-10 (IL-10) receptor-encoding gene IL10RA in melanoma cells engaged an IL-10 autocrine loop not otherwise present in this cell type, which altered signaling, gene expression, and cellular responses to proinflammatory stimuli. Our array provides a rapid, inexpensive, and convenient means for assigning a receptor signature to any human cell or tissue type.

Protein phosphorylation profiling using an in situ proximity ligation assay: phosphorylation of AURKA-elicited EGFR-Thr654 and EGFR-Ser1046 in lung cancer cells

The epidermal growth factor receptor (EGFR), which is up-regulated in lung cancer, involves the activation of mitogenic signals and triggers multiple signaling cascades. To dissect these EGFR cascades, we used 14 different phospho-EGFR antibodies to quantify protein phosphorylation using an in situ proximity ligation assay (in situ PLA). Phosphorylation at EGFR-Thr654 and -Ser1046 was EGF-dependent in the wild-type (WT) receptor but EGF-independent in a cell line carrying the EGFR-L858R mutation. Using a ProtoAarray™ containing ∼5000 recombinant proteins on the protein chip, we found that AURKA interacted with the EGFR-L861Q mutant. Moreover, overexpression of EGFR could form a complex with AURKA, and the inhibitors of AURKA and EGFR decreased EGFR-Thr654 and -Ser1046 phosphorylation. Immunohistochemical staining of stage I lung adenocarcinoma tissues demonstrated a positive correlation between AURKA expression and phosphorylation of EGFR at Thr654 and Ser1046 in EGFR-mutant specimens, but not in EGFR-WT specimens. The interplay between EGFR and AURKA provides an explanation for the difference in EGF dependency between EGFR-WT and EGFR-mutant cells and may provide a new therapeutic strategy for lung cancer patients carrying EGFR mutations.

Temporal profiling of lapatinib-suppressed phosphorylation signals in EGFR/HER2 pathways

Lapatinib is a clinically potent kinase inhibitor for breast cancer patients because of its outstanding selectivity for epidermal growth factor receptor (EGFR) and EGFR2 (also known as HER2). However, there is only limited information about the in vivo effects of lapatinib on EGFR/HER2 and downstream signaling targets. Here, we profiled the lapatinib-induced time- and dose-dependent phosphorylation dynamics in SKBR3 breast cancer cells by means of quantitative phosphoproteomics. Among 4953 identified phosphopeptides from 1548 proteins, a small proportion (5-7%) was regulated at least twofold by 1-10 μm lapatinib. We obtained a comprehensive phosphorylation map of 21 sites on EGFR/HER2, including nine novel sites on HER2. Among them, serine/threonine phosphosites located in a small region of HER2 (amino acid residues 1049-1083) were up-regulated by the drug, whereas all other sites were down-regulated. We show that cAMP-dependent protein kinase is involved in phosphorylation of this particular region of HER2 and regulates HER2 tyrosine kinase activity. Computational analyses of quantitative phosphoproteome data indicated for the first time that protein-protein networks related to cytoskeletal organization and transcriptional/translational regulation, such as RNP complexes (i.e. hnRNP, snRNP, telomerase, ribosome), are linked to EGFR/HER2 signaling networks. To our knowledge, this is the first report to profile the temporal response of phosphorylation dynamics to a kinase inhibitor. The results provide new insights into EGFR/HER2 regulation through region-specific phosphorylation, as well as a global view of the cellular signaling networks associated with the anti-breast cancer action of lapatinib.

Impaired degradation followed by enhanced recycling of epidermal growth factor receptor caused by hypo-phosphorylation of tyrosine 1045 in RBE cells

Background

Since cholangiocarcinoma has a poor prognosis, several epidermal growth factor receptor (EGFR)-targeted therapies with antibody or small molecule inhibitor treatment have been proposed. However, their effect remains limited. The present study sought to understand the molecular genetic characteristics of cholangiocarcinoma related to EGFR, with emphasis on its degradation and recycling.

Methods

We evaluated EGFR expression and colocalization by immunoblotting and immunofluorescence, cell surface EGFR expression by fluorescence-activated cell sorting (FACS), and EGFR ubiquitination and protein binding by immunoprecipitation in the human cholangiocarcinoma RBE and immortalized cholangiocyte MMNK-1 cell lines. Monensin treatment and Rab11a depletion by siRNA were adopted for inhibition of EGFR recycling.

Results

Upon stimulation with EGF, ligand-induced EGFR degradation was impaired and the expression of phospho-tyrosine 1068 and phospho-p44/42 MAPK was sustained in RBE cells as compared with MMNK-1 cells. In RBE cells, the process of EGFR sorting for lysosomal degradation was blocked at the early endosome stage, and non-degradated EGFR was recycled to the cell surface. A disrupted association between EGFR and the E3 ubiquitin ligase c-Cbl, as well as hypo-phosphorylation of EGFR at tyrosine 1045 (Tyr1045), were also observed in RBE cells.

Conclusion

In RBE cells, up-regulation of EGFR Tyr1045 phosphorylation is a potentially useful molecular alteration in EGFR-targeted therapy. The combination of molecular-targeted therapy determined by the characteristics of individual EGFR phosphorylation events and EGFR recycling inhibition show promise in future treatments of cholangiocarcinoma.

Human epidermal growth factor receptor (EGFR) aligned on the plasma membrane adopts key features of Drosophila EGFR asymmetry

The ability of epidermal growth factor receptor (EGFR) to control cell fate is defined by its affinity for ligand. Current models suggest that ligand-binding heterogeneity arises from negative cooperativity in signaling receptor dimers, for which the asymmetry of the extracellular region of the Drosophila EGFR has recently provided a structural basis. However, no asymmetry is apparent in the isolated extracellular region of the human EGFR. Human EGFR also differs from the Drosophila EGFR in that negative cooperativity is found only in full-length receptors in cells. To gain structural insights into the human EGFR in situ, we developed an approach based on quantitative Förster resonance energy transfer (FRET) imaging, combined with Monte Carlo and molecular dynamics simulations, to probe receptor conformation in epithelial cells. We experimentally demonstrate a high-affinity ligand-binding human EGFR conformation consistent with the extracellular region aligned flat on the plasma membrane. We explored the relevance of this conformation to ligand-binding heterogeneity and found that the asymmetry of this structure shares key features with that of the Drosophila EGFR, suggesting that the structural basis for negative cooperativity is conserved from invertebrates to humans but that in human EGFR the extracellular region asymmetry requires interactions with the plasma membrane.

Calmodulin-mediated regulation of the epidermal growth factor receptor

In this review, we first describe the mechanisms by which the epidermal growth factor receptor generates a Ca(2+) signal and, subsequently, we compile the available experimental evidence regarding the role that the Ca(2+)/calmodulin complex, formed after the rise in cytosolic free Ca(2+) concentration, exerts on the receptor. We focus not only on the indirect action that Ca(2+)/calmodulin exerts on the epidermal growth factor receptor, as a result of the activation of distinct calmodulin-dependent kinases, but also, and more extensively, on the direct interaction of Ca(2+)/calmodulin with the receptor. We also describe several mechanistic models that could account for the Ca(2+)/calmodulin-mediated regulation of epidermal growth factor receptor activity. The control exerted by calmodulin on distinct epidermal growth factor receptor-mediated cellular functions is also discussed. Finally, the phosphorylation of this Ca(2+) sensor by the epidermal growth factor receptor is highlighted.

Protein kinase C decreases the apparent affinity of the inositol 1,4,5-trisphosphate receptor type 3 in RINm5F cells

In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ channel which plays a major role in Ca2+ signalling. Three isoforms of IP3R have been identified (IP3R-1, IP3R-2 and IP3R-3) and most cell types express different proportions of each isoform. The differences between the pharmacological and functional properties of the various isoforms of IP3R are poorly known. RINm5F cells who express almost exclusively (approximately 90%) the IP3R-3, represent an interesting model to study this particular isoform. Here, we investigated a regulatory mechanism by which protein kinase C (PKC) may influence IP3R-3-mediated Ca2+ release. With an immunoprecipitation approach we confirmed that RINm5F cells express almost exclusively the IP3R-3 isoform. With an in vitro phosphorylation approach, we showed that the immunopurified IP3R-3 was efficiently phosphorylated by exogenous PKC. With a direct in cellulo approach and an indirect in cellulo back-phosphorylation approach we showed that phorbol-12-myristate-13-acetate (PMA) causes the phosphorylation of IP3R-3 in intact RINm5F cells. In saponin-permeabilized RINm5F cells, 3-induced Ca2+ release was reduced after a pre-treatment with PMA. PMA also reduced the Ca2+ response of intact RINm5F cells stimulated with carbachol and EGF, two agonists that use different receptor types to activate phospholipase C. These results suggest the existence of a negative feedback mechanism involving two components of the Ca2+ signalling cascade, whereby activated PKC dampens IP3R-3 activity.

Membrane-permeable calmodulin inhibitors (e.g. W-7/W-13) bind to membranes, changing the electrostatic surface potential: dual effect of W-13 on epidermal growth factor receptor activation

Membrane-permeable calmodulin inhibitors, such as the napthalenesulfonamide derivatives W-7/W-13, trifluoperazine, and calmidazolium, are used widely to investigate the role of calcium/calmodulin (Ca2+/CaM) in living cells. If two chemically different inhibitors (e.g. W-7 and trifluoperazine) produce similar effects, investigators often assume the effects are due to CaM inhibition. Zeta potential measurements, however, show that these amphipathic weak bases bind to phospholipid vesicles at the same concentrations as they inhibit Ca2+/CaM; this suggests that they also bind to the inner leaflet of the plasma membrane, reducing its negative electrostatic surface potential. This change will cause electrostatically bound clusters of basic residues on peripheral (e.g. Src and K-Ras4B) and integral (e.g. epidermal growth factor receptor (EGFR)) proteins to translocate from the membrane to the cytoplasm. We measured inhibitor-mediated translocation of a simple basic peptide corresponding to the calmodulin-binding juxtamembrane region of the EGFR on model membranes; W-7/W-13 causes translocation of this peptide from membrane to solution, suggesting that caution must be exercised when interpreting the results obtained with these inhibitors in living cells. We present evidence that they exert dual effects on autophosphorylation of EGFR; W-13 inhibits epidermal growth factor-dependent EGFR autophosphorylation under different experimental conditions, but in the absence of epidermal growth factor, W-13 stimulates autophosphorylation of the receptor in four different cell types. Our interpretation is that the former effect is due to W-13 inhibition of Ca2+/CaM, but the latter results could be due to binding of W-13 to the plasma membrane.

Protein kinase C alpha but not PKCzeta suppresses intestinal tumor formation in ApcMin/+ mice

Members of the protein kinase C (PKC) family of serine/threonine kinases play key regulatory roles in numerous cellular processes, including differentiation and proliferation. Of the 11 mammalian PKC isoforms known, several have been implicated in tumor development and progression. However, in most cases, isotype specificity is poorly defined, and even contrary functions for a single PKC have been reported mostly because appropriate molecular and genetic tools were missing to specifically assess the contribution of single PKC isoforms in vivo. In this report, we therefore used PKC genetic targeting to study the role of PKCalpha and PKCzeta in colorectal cancer. Both isoforms were found to be strongly down-regulated in intestinal tumors of ApcMin/+ mice. A deletion of PKCzeta did not affect tumorigenesis in this animal model. In contrast, PKCalpha-deficient ApcMin/+ mice developed more aggressive tumors and died significantly earlier than their PKCalpha-proficient littermates. Even without an additional Apc mutation, PKCalpha knockout mice showed an elevated tendency to develop spontaneous intestinal tumors. Transcriptional profiling revealed a role for this kinase in regulating epidermal growth factor receptor (EGFR) signaling and proposed a synergistic mechanism for EGFR/activator protein and WNT/APC pathways in mediating intestinal tumor development.

Retinoids arrest breast cancer cell proliferation: retinoic acid selectively reduces the duration of receptor tyrosine kinase signaling

Retinoic acid (RA) induces cell cycle arrest of hormone-dependent human breast cancer (HBC) cells. Previously, we demonstrated that RA-induced growth arrest of T-47D HBC cells required the activity of the RA-induced protein kinase, protein kinase Calpha (PKCalpha) [J. Cell Physiol. 172 (1997) 306]. Here, we demonstrate that RA treatment of T-47D cells interfered with growth factor signaling to downstream, cytoplasmic and nuclear targets. RA treatment did not inhibit epidermal growth factor (EGF) receptor activation but resulted in rapid inactivation. The lack of sustained EGFR activation was associated with transient rather than sustained association of the EGFR with the Shc adaptor proteins and activation of Erk 1/2 and with compromised induction of expression of immediate early response genes. Inhibiting the activity of PKCalpha, a retinoic acid-induced target gene, prevented the effects of RA on cell proliferation and EGF signaling. Constitutive expression of PKCalpha, in the absence of RA, decreased cell proliferation and decreased EGF signaling. RA treatment increased steady-state levels of the protein tyrosine phosphatase PTP-1C and all measured effects of RA on EGF receptor function were reversed by the tyrosine phosphate inhibitor orthovanadate. These results indicate that RA-induced target genes, particularly PKCalpha, prevent sustained growth factor signaling, uncoupling activated receptor tyrosine kinases and nuclear targets that are required for cell cycle progression.

The receptor tyrosine kinase Ror2 associates with and is activated by casein kinase Iepsilon

Ror2, a member of the mammalian Ror family of receptor tyrosine kinases, plays important roles in developmental morphogenesis, although the mechanism underlying activation of Ror2 remains largely elusive. We show that when expressed in mammalian cells, Ror2 associates with casein kinase Iepsilon (CKIepsilon), a crucial regulator of Wnt signaling. This association occurs primarily via the cytoplasmic C-terminal proline-rich domain of Ror2. We also show that Ror2 is phosphorylated by CKIepsilon on serine/threonine residues, in its C-terminal serine/threonine-rich 2 domain, resulting in autophosphorylation of Ror2 on tyrosine residues. Furthermore, it was found that association of Ror2 with CKIepsilon is required for its serine/threonine phosphorylation by CKIepsilon. Site-directed mutagenesis of tyrosine residues in Ror2 reveals that the sites of phosphorylation are contained among the five tyrosine residues in the proline-rich domain but not among the four tyrosine residues in the tyrosine kinase domain. Moreover, we show that in mammalian cells, CKIepsilon-mediated phosphorylation of Ror2 on serine/threonine and tyrosine residues is followed by the tyrosine phosphorylation of G protein-coupled receptor kinase 2, a kinase with a developmental expression pattern that is remarkably similar to that of Ror2. Intriguingly, a mutant of Ror2 lacking five tyrosine residues, including the autophosphorylation sites, fails to tyrosine phosphorylate G protein-coupled receptor kinase 2. This indicates that autophosphorylation of Ror2 is required for full activation of its tyrosine kinase activity. These findings demonstrate a novel role for CKIepsilon in the regulation of Ror2 tyrosine kinase.

Endogenous calmodulin interacts with the epidermal growth factor receptor in living cells

We have previously shown that exogenous calmodulin (CaM) binds to the epidermal growth factor receptor (EGFR) at its cytosolic juxtamembrane region inhibiting its tyrosine kinase activity. We demonstrate in this report that endogenous CaM binds to EGFR in intact cells as CaM co-immunoprecipitates with EGF-activated and non-activated receptors. We also show in living cells that cell-permeable CaM inhibitors prevent the full transphosphorylation of wild type EGFR but not the transphosphorylation of an insertional EGFR mutant in which the CaM-binding domain was divided into two parts. Overall these results suggest that CaM interacts with EGFR in vivo.

Promising developments in targeted therapies for non-small-cell lung cancer

Despite advances in chemotherapy, radiation therapy, and surgery, the overall survival for patients with lung cancer remains poor. Thus, novel therapeutic approaches are warranted. As knowledge of the molecular abnormalities and dysregulated cellular processes contributing to the pathogenesis and progression of lung cancer has been acquired, intense interest has been directed at developing agents that target these abnormalities. New agents targeting aberrant receptor tyrosine kinases, the Ras oncoprotein, mediators of metastases and angiogenesis, and the tumor suppressor gene p53 have, among other agents, shown promise in preclinical studies. Early clinical trials with these agents in patients with advanced malignancies suggest preliminary evidence of clinical activity and possible applications in non-small-cell lung cancer (NSCLC). Ongoing clinical trials will help clarify the settings in which these agents are of greatest therapeutic value, the optimal schedule of administration, toxicities associated with chronic administration, and hopefully, provide additional insight into the biology of lung cancer. Selected clinical trials will be presented to highlight the use of rationally designed, targeted therapies for patients with NSCLC.

A ligand-inducible epidermal growth factor receptor/anaplastic lymphoma kinase chimera promotes mitogenesis and transforming properties in 3T3 cells

Oncogenic rearrangements of the anaplastic lymphoma kinase (ALK) gene, encoding a receptor type tyrosine kinase, are frequently associated with anaplastic large cell lymphomas. Such rearrangements juxtapose the intracellular domain of ALK to 5'-end sequences belonging to different genes and create transforming fusion proteins. To understand how the oncogenic versions of ALK contribute to lymphomagenesis, it is important to analyze the biological effects and the biochemical properties of this receptor under controlled conditions of activation. To this aim, we constructed chimeric receptor molecules in which the extracellular domain of the ALK kinase is replaced by the extracellular, ligand-binding domain of the epidermal growth factor receptor (EGFR). Upon transfection in NIH 3T3 fibroblasts, the EGFR/ALK chimera was correctly synthesized and transported to the cell surface, where it was fully functional in forming high versus low affinity EGF-binding sites and transducing an EGF-dependent signal intracellularly. Overexpression of the EGFR/ALK chimera in NIH 3T3 was sufficient to induce the malignant phenotype; the appearance of the transformed phenotype was, however, conditionally dependent on the administration of EGF. Moreover, the EGFR/ALK chimera was significantly more active in inducing transformation and DNA synthesis than the wild type EGFR when either was expressed at similar levels in NIH 3T3 cells. Comparative analysis of the biochemical pathways implicated in the transduction of mitogenic signals did not show any increased ability of the EGFR/ALK to phosphorylate PLC-gamma and MAPK compared with the EGFR. On the contrary, EGFR/ALK showed to have a consistently greater effect on phosphatidylinositol 3-kinase activity compared with the EGFR, indicating that this enzyme plays a major role in mediating the mitogenic effects of ALK in NIH 3T3 cells.

Role of calmodulin in the modulation of the MAPK signalling pathway and the transactivation of epidermal growth factor receptor mediated by PKC

We have recently shown that calmodulin (CaM) regulates the trafficking of epidermal growth factor receptor (EGFR) as well as the mitogen-activated protein kinase (MAPK) signalling pathway. However, the overall regulation of the MAPK pathway is achieved through a complex interplay of other several upstream effectors including G-proteins, EGF, EGFR, protein kinase C (PKC), phosphatidylinositol-3-kinase and CaM. In order to understand the role of CaM in the PKC-mediated transactivation of EGFR we have analysed the effect of a CaM antagonist, N-(4-aminobutyl)-5-chloro-2-naphthalenesulfonamide, on the 12-O-tetradecanoylphorbol-13-acetate-mediated activation of EGFR and the subsequent MAPK activation. The results show that CaM interferes with MAPK activation and the transactivation of EGFR mediated by PKC.

Controls of EGF-induced morphological transformation of human bronchial epithelial cells

Human bronchial epithelial cells, both normal primary (NHBE) and the BEAS-2B line, respond to epidermal growth factor (EGF) by extruding lengthy filaments, or filapodia. The morphological transformation of BEAS-2B cells maximized at 48 h using 1-10 nM EGF. EGF-induced filapodia extension was inhibited by co-exposure to transforming growth factor beta, which did not affect tyrosine phosphorylation of the EGF receptor (EGFR). Inhibition was also effected by phorbol myristoyl acetate (PMA), which reduced the rate of EGFR tyrosine phosphorylation. Dibutyryl-cAMP had no effect, whereas the protein kinase inhibitor H-89 stimulated the EGF response. The ability to regulate cellular responses to EGF by hormonal and chemical approaches has implications for current investigations into the roles of EGF in lung growth, differentiation, and wound repair.

G Protein-coupled Receptors Desensitize and Down-regulate Epidermal Growth Factor Receptors in Renal Mesangial Cells

Different types of plasma membrane receptors engage in various forms of cross-talk. We used cultures of rat renal mesangial cells to study the regulation of EGF receptors (EGFRs) by various endogenous G protein-coupled receptors (GPCRs). GPCRs (5-hydroxytryptamine(2A), lysophosphatidic acid, angiotensin AT(1), bradykinin B(2)) were shown to transactivate EGFRs through a protein kinase C-dependent pathway. This transactivation resulted in the initiation of multiple cellular signals (phosphorylation of the EGFRs and ERK and activation of cAMP-responsive element-binding protein (CREB), NF-kappaB, and E2F), as well as subsequent rapid down-regulation of cell-surface EGFRs and internalization and desensitization of the EGFRs without change in the total cellular complement of EGFRs. Internalization of the EGFRs and the down-regulation of cell-surface receptors in mesangial cells were blocked by pharmacological inhibitors of clathrin-mediated endocytosis and in HEK293 cells by transfection of cDNA constructs that encode dominant negative beta-arrestin-1 or dynamin. Whereas all of the effects of GPCRs on EGFRs were dependent to a great extent on protein kinase C, those initiated by EGF were not. These studies demonstrate that GPCRs can induce multiple signals through protein kinase C-dependent transactivation of EGFRs. Moreover, GPCRs induce profound desensitization of EGFRs by a process associated with the loss of cell-surface EGFRs through clathrin-mediated endocytosis.

Activation of protein kinase C stimulates the dephosphorylation of natriuretic peptide receptor-B at a single serine residue: a possible mechanism of heterologous desensitization

The binding of atrial natriuretic peptide and C-type natriuretic peptide (CNP) to the guanylyl cyclase-linked natriuretic peptide receptors A and B (NPR-A and -B), respectively, stimulates increases in intracellular cGMP concentrations. The vasoactive peptides vasopressin, angiotensin II, and endothelin inhibit natriuretic peptide-dependent cGMP elevations by activating protein kinase C (PKC). Recently, we identified six in vivo phosphorylation sites for NPR-A and five sites for NPR-B and demonstrated that the phosphorylation of these sites is required for ligand-dependent receptor activation. Here, we show that phorbol 12-myristate 13-acetate, a direct activator of PKC, causes the dephosphorylation and desensitization of NPR-B. In contrast to the CNP-dependent desensitization process, which results in coordinate dephosphorylation of all five sites in the receptor, phorbol 12-myristate 13-acetate treatment causes the dephosphorylation of only one site, which we have identified as Ser(523). The conversion of this residue to alanine or glutamate did not reduce the amount of mature receptor protein as indicated by detergent-dependent guanylyl cyclase activities or Western blot analysis but completely blocked the ability of PKC to induce the dephosphorylation and desensitization of NPR-B. Thus, in contrast to previous reports suggesting that PKC directly phosphorylates and inhibits guanylyl cyclase-linked natriuretic peptide receptors, we show that PKC-dependent dephosphorylation of NPR-B at Ser(523) provides a possible molecular explanation for how pressor hormones inhibit CNP signaling.

Mechanisms of proliferation synergy by receptor tyrosine kinase and G protein-coupled receptor activation in human airway smooth muscle

Despite recent studies depicting the capacity of G protein-coupled receptors (GPCRs) to activate mitogenic signaling pathways more commonly associated with receptor tyrosine kinases (RTKs), little is known regarding the interactive effects of GPCR and RTK activation on cell growth and signal transduction. Such interactions likely mediate the physiologic growth in most cells in vivo as well as the aberrant, non-neoplastic growth that occurs in diseases such as asthma, where disruptions of the local hormonal or inflammatory state can contribute to significant GPCR activation. In this study, we show that numerous inflammatory or contractile agents, including thrombin, histamine, and carbachol, potentiate epidermal growth factor (EGF)-stimulated proliferation of human airway smooth muscle (ASM), thus demonstrating a clear synergy between RTK and GPCR activation. Alterations in promitogenic nuclear signaling were evidenced by additive or synergistic increases in Elk-1 and activator protein-1 activation, and by increases in cyclin D1 expression. Interestingly, GPCR activation did not cause EGF receptor tyrosine phosphorylation nor did it increase EGF-stimulated autophosphorylation. In the presence of EGF, histamine or carbachol did not alter the time-dependent phosphorylation of p42/p44, whereas thrombin was capable of increasing phospho-p42/p44 levels at selected time points in some, but not all, cultures. In contrast to their relative inability to alter EGF receptor-linked p42/p44 activation, thrombin, histamine, and carbachol consistently increased the late phase (> 1 h) activity of p70 S6 kinase. Collectively, these findings suggest that inflammatory and contractile agents that activate GPCRs can significantly modulate RTK-mediated ASM growth through a p70 S6 kinase-dependent, p42/p44-independent mechanism.

Replacing two conserved tyrosines of the EphB2 receptor with glutamic acid prevents binding of SH2 domains without abrogating kinase activity and biological responses

Eph receptor tyrosine kinases play key roles in pattern formation during embryonic development, but little is known about the mechanisms by which they elicit specific biological responses in cells. Here, we investigate the role of tyrosines 605 and 611 in the juxtamembrane region of EphB2, because they are conserved Eph receptor autophosphorylation sites and demonstrated binding sites for the SH2 domains of multiple signaling proteins. Mutation of tyrosines 605 and 611 to phenylalanine impaired EphB2 kinase activity, complicating analysis of their function as SH2 domain binding sites and their contribution to EphB2-mediated signaling. In contrast, mutation to the negatively charged glutamic acid disrupted SH2 domain binding without reducing EphB2 kinase activity. By using a panel of EphB2 mutants, we found that kinase activity is required for the changes in cell-matrix and cell - cell adhesion, cytoskeletal organization, and activation of mitogen-activated protein (MAP) kinases elicited by EphB2 in transiently transfected cells. Instead, the two juxtamembrane SH2 domain binding sites were dispensable for these effects. These results suggest that phosphorylation of tyrosines 605 and 611 is critical for EphB2-mediated cellular responses because it regulates EphB2 kinase activity.

Role of epidermal growth factor receptor in basal and stimulated colonic epithelial cell migration in vitro

Colonic mucosal wounds are repaired, in part, by epithelial migration. Signaling mechanisms regulating this migration are poorly characterized. This study aimed to examine the role that the epidermal growth factor (EGF) receptor (EGF-R) and its ligands, EGF and transforming growth factor-alpha (TGF-alpha), play in migration in wounded in vitro models of colonic epithelium. Migration was assessed over 24 h in circular wounds made in confluent monolayers of LIM1215 human colon cancer cells. EGF and TGF-alpha stimulated migration twofold from 4 h after wounding. Basal migration and the motogenic effects of short chain fatty acids and hepatocyte growth factor were mediated through enhanced binding of TGF-alpha to EGF-R, while trefoil peptide-mediated motogenesis required EGF-R activation independently of TGF-alpha binding. Activation of protein kinase C (PKC) stimulated migration, an effect more potent than, and independent of, EGF-R activation. However, neither inhibition of PKC by Ro 31-8220 nor depletion of PKC by pretreatement with phorbol myristate acetate attenuated EGF-R-mediated motogenesis. In conclusion, EGF-R activation via TGF-alpha binding, or intracellularly, mediates basal LIM1215 migration and the effects of several motogens, with the exception of PKC activators. Since EGF-R and PKC have physiological activators in vivo, they may control colonic mucosal repair processes following injury.

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.

Growth factor-induced signal transduction in adherent mammalian cells is sensitive to gravity

Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in a wide variety of cells. This activation leads to increased cell proliferation in most cell types. Among the early effects evoked by EGF are receptor clustering, cell rounding, and early gene expression. The influence of gravity on EGF-induced EGF receptor clustering and gene expression as well as on actin polymerization and cell rounding have been investigated in adherent A431 epithelial cells with the use of sounding rockets to create microgravity conditions. EGF-induced c-fos and c-jun expression decreased in microgravity. This was caused by alteration of the EGF receptor and protein kinase C-mediated signal transduction pathways. In contrast, neither the binding of EGF to the receptor nor the receptor clustering were changed under microgravity conditions. Because cell morphology was also modulated under microgravity conditions, and the growth factor-induced signal transduction cascades have been demonstrated to be linked to the actin microfilament system, it is tempting to suggest that the actin microfilament system constitutes the gravity-sensitive cell component.

Transmodulation of epidermal growth factor receptor function by cyclic AMP-dependent protein kinase

Binding of epidermal growth factor (EGF) to its receptor (EGFR) augments the tyrosine kinase activity of the receptor and autophosphorylation. Exposure of some tissues and cells to EGF also stimulates adenylyl cyclase activity and results in an increase in cyclic AMP (cAMP) levels. Because cAMP activates the cAMP-dependent protein kinase A (PKA), we investigated the effect of PKA on the EGFR. The purified catalytic subunit of PKA (PKAc) stoichiometrically phosphorylated the purified full-length wild type (WT) and kinase negative (K721M) forms of the EGFR. PKAc phosphorylated both WT-EGFR as well as a mutant truncated form of EGFR (Delta1022-1186) exclusively on serine residues. Moreover, PKAc also phosphorylated the cytosolic domain of the EGFR (EGFRKD). Phosphorylation of the purified WT as well as EGFRDelta1022-1186 and EGFRKD was accompanied by decreased autophosphorylation and diminished tyrosine kinase activity. Pretreatment of REF-52 cells with the nonhydrolyzable cAMP analog, 8-(4-chlorophenylthio)-cAMP, decreased EGF-induced tyrosine phosphorylation of cellular proteins as well as activation of the WT-EGFR. Similar effects were also observed in B82L cells transfected to express the Delta1022-1186 form of EGFR. Furthermore, activation of PKAc in intact cells resulted in serine phosphorylation of the EGFR. The decreased phosphorylation of cellular proteins and diminished activation of the EGFR in cells treated with the cAMP analog was not the result of altered binding of EGF to its receptors or changes in receptor internalization. Therefore, we conclude that PKA phosphorylates the EGFR on Ser residues and decreases its tyrosine kinase activity and signal transduction both in vitro and in vivo.

Actin polymerization is required for negative feedback regulation of epidermal growth factor-induced signal transduction

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of a variety of cells. To investigate the significance of this process for signal transduction, actin polymerization is inhibited by dihydrocytochalasin B (CB). CB almost completely abolishes EGF-induced actin polymerization, as assessed by quantitative confocal laser scanning microscopy. Under these conditions, EGF induces enhanced EGF receptor (EGFR) tyrosine kinase activity, as well as superinduction of the c-fos proto-oncogene. These data suggest that EGF-induced actin polymerization may be important for negative feedback regulation of signal transduction by the EGFR. The phosphorylation of Thr654 by protein kinase C (PKC) is a well-characterized negative feedback control mechanism for signal transduction by the EGFR tyrosine kinase. A synthetic peptide, corresponding to the regions flanking Thr654 of the EGFR, is used to analyze EGF stimulated PKC activity by incorporation of 32P into the peptide. Cotreatment of cells with CB and EGF results in a complete loss of EGF-induced phosphorylation of the peptide. These data suggest that actin polymerization is obligatory for negative feedback regulation of the EGFR tyrosine kinase through the C-kinase pathway.

Role of epidermal growth factor (EGF) in oesophageal mucosal integrity

Oesophageal mucosa has well established protective mechanisms, which operate within pre-epithelial, epithelial and post-epithelial compartments. Since refluxed acid and pepsin always act from the luminal side of the mucosa, protective factors like EGF, operating as a part of pre-epithelial defence, are thought to be pivotal in the maintenance of the integrity of the oesophageal mucosa. The significant contribution of salivary EGF to the quality of the oesophageal mucosal barrier has been demonstrated in an experimental setting and in a clinical scenario. Patients with low salivary EGF levels are predisposed to severe oesophageal damage if they develop gastro-oesophageal reflux and are a high-risk group for development of Barrett's oesophagus. Not only the salivary glands but also the human oesophagus has a profound ability to elaborate and release EGF. Some changes in luminal release of EGF during oesophageal mucosal exposure to intraluminal damaging factors imply its role in the oesophageal protective mechanisms. To exert biological effects within the oesophageal mucosal compartment, EGF requires binding to the ligand-binding domain of its receptor. This process results in receptor dimerisation, autophosphorylation and activation of intracellular signal transduction pathways. EGF receptors are localised on the basolateral and luminal aspect of the mucosal cells playing an important role in fast regeneration of oesophageal epithelium through the high mitotic activity of its proliferative zone. An increase in the rate of salivary EGF secretion during masticatory stimulation suggests its potential therapeutic benefit in the treatment of patients with damaged oesophageal mucosa.

Down-regulation of epidermal growth factor receptors by nerve growth factor in PC12 cells is p140(trk)-, Ras-, and Src-dependent

Nerve growth factor (NGF) treatment causes a profound down-regulation of epidermal growth factor receptors during the differentiation of PC12 cells. This process is characterized by a progressive decrease in epidermal growth factor (EGF) receptor level measured by 125I-EGF binding, tyrosine phosphorylation, and Western blotting. Treatment of the cells with NGF for 5 days produces a 95% reduction in the amount of [35S]methionine-labeled EGF receptors. This down-regulation does not occur in PC12nnr5 cells, which lack the p140(trk) NGF receptor. However, in PC12nnr5 cells stably transfected with p140(trk), the NGF-induced heterologous down-regulation of EGF receptors is reconstituted in part. NGF-induced heterologous down-regulation, but not EGF-induced homologous down-regulation of EGF receptors, is blocked in Ras- and Src-dominant-negative PC12 cells. Treatment with either pituitary adenylate cyclase-activating peptide (PACAP) or staurosporine stimulates neurite outgrowth in PC12 cell variants, but neither induces down-regulation of EGF receptors. NGF treatment of PC12 cells in suspension induces down-regulation of EGF receptors in the absence of neurite outgrowth. These results strongly suggest a p140(trk)-, Ras- and Src-dependent mechanism of NGF-induced down-regulation of EGF receptors and separate this process from NGF-induced neurite outgrowth in PC12 cells.

Binding of zinc finger protein ZPR1 to the epidermal growth factor receptor

ZPR1 is a zinc finger protein that binds to the cytoplasmic tyrosine kinase domain of the epidermal growth factor receptor (EGFR). Deletion analysis demonstrated that this binding interaction is mediated by the zinc fingers of ZPR1 and subdomains X and XI of the EGFR tyrosine kinase. Treatment of mammalian cells with EGF caused decreased binding of ZPR1 to the EGFR and the accumulation of ZPR1 in the nucleus. The effect of EGF to regulate ZPR1 binding is dependent on tyrosine phosphorylation of the EGFR. ZPR1 therefore represents a prototype for a class of molecule that binds to the EGFR and is released from the receptor after activation.

Rapid and long-term effects on protein kinase C on receptor tyrosine kinase phosphorylation and degradation

Rapid and long term effects of protein kinase C alpha activation on receptor tyrosine kinase signaling parameters were investigated in human 293 embryonic fibroblasts and mouse NIH 3T3 cells. Within minutes of phorbol 12-myristate 13-acetate treatment, epidermal growth factor receptor and HER2 tyrosine phosphorylation was decreased, while platelet-derived growth factor receptor and insulin receptor autophosphorylation was upregulated. These effects are not mediated by protein kinase C-dependent receptor tyrosine kinase phosphorylation but apparently by activation or inactivation of receptor tyrosine kinase-specific phosphatases, as indicated by neutralization of these phenomena upon treatment of cells with sodium orthovanadate. In contrast to these short term effects, sustained activation of protein kinase C alpha by phorbol 12-myristate 13-acetate results in translocation of protein kinase C from the cytosol to the membrane fraction where it forms stable complexes with all receptor tyrosine kinases investigated. Ligand-induced receptor tyrosine kinase/protein kinase C association in NIH 3T3 fibroblasts is accompanied by a mobility shift of the receptor, indicating phosphorylation by activated protein kinase C. This phenomenon correlates with the disappearance of receptor tyrosine kinases from the cell surface, implying that this interaction plays a role in the process of receptor internalization and degradation. Interestingly, ligand-stimulated receptor down-regulation is also enhanced by overexpression of phospholipase C gamma, which strongly indicates a role for this common receptor tyrosine kinase substrate in negative regulation of growth factor signals.

Pro-inflammatory cytokines and environmental stress cause p38 mitogen-activated protein kinase activation by dual phosphorylation on tyrosine and threonine

Protein kinases activated by dual phosphorylation on Tyr and Thr (MAP kinases) can be grouped into two major classes: ERK and JNK. The ERK group regulates multiple targets in response to growth factors via a Ras-dependent mechanism. In contrast, JNK activates the transcription factor c-Jun in response to pro-inflammatory cytokines and exposure of cells to several forms of environmental stress. Recently, a novel mammalian protein kinase (p38) that shares sequence similarity with mitogen-activated protein (MAP) kinases was identified. Here, we demonstrate that p38, like JNK, is activated by treatment of cells with pro-inflammatory cytokines and environmental stress. The mechanism of p38 activation is mediated by dual phosphorylation on Thr-180 and Tyr-182. Immunofluorescence microscopy demonstrated that p38 MAP kinase is present in both the nucleus and cytoplasm of activated cells. Together, these data establish that p38 is a member of the mammalian MAP kinase group.

Stimulation of phospholipase D by epidermal growth factor requires protein kinase C activation in Swiss 3T3 cells

The proposal that epidermal growth factor (EGF) activates phospholipase D (PLD) by a mechanism(s) not involving phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis was examined in Swiss 3T3 fibroblasts. EGF, basic fibroblast growth factor (bFGF), bombesin, and platelet-derived growth factor (PDGF) activated PLD as measured by transphosphatidylation of butanol to phosphatidylbutanol. The increase in inositol phosphates induced by bFGF, EGF, or bombesin was significantly enhanced by Ro-31-8220, an inhibitor of protein kinase C (PKC), suggesting that PtdIns(4,5)P2-hydrolyzing phospholipase is coupled to the receptors for these agonists but that the response is down-regulated by PKC. Activation of PLD by EGF was inhibited dose dependently by the PKC inhibitors bis-indolylmaleimide and Ro-31-8220, which also inhibited the effects of bFGF, bombesin, and PDGF. Down-regulation of PKC by prolonged treatment with 4 beta-phorbol 12-myristate 13-acetate also abolished EGF- and PDGF-stimulated phosphatidylbutanol formation. EGF and bombesin induced biphasic translocations of PKC delta and epsilon to the membrane that were detectable at 15 s. In the presence of Ro-31-8220, translocation of PKC alpha became evident, and membrane association of the delta- and epsilon-isozymes was enhanced and/or sustained in response to the two agonists. The inhibitor also enhanced EGF-stimulated [3H]diacylglycerol formation in cells preincubated with [3H]arachidonic acid, which labeled predominantly phosphatidylinositol, but inhibited [3H]diacylglycerol production in cells preincubated with [3H]myristic acid, which labeled mainly phosphatidylcholine. These data support the conclusion that EGF can stimulate diacylglycerol formation from PtdIns(4,5)P2 and that PKC performs the dual role of down-regulating this response as well as mediating phosphatidylcholine hydrolysis. In summary, all of the results of the study indicate that PLD activation by EGF is downstream of PtdIns(4,5)P2-hydrolyzing phospholipase and is dependent upon subsequent PKC activation.

Regulation of protein kinase C and role in cancer biology

Protein kinase C (PKC) is a family of closely related lipid-dependent and diacyglycerol-activated isoenzymes known to play an important role in the signal transduction pathways involved in hormone release, mitogenesis and tumor promotion. Reversible activation of PKC by the second messengers diacylglycerol and calcium is an established model for the short term regulation of PKC in the immediate events of signal transduction. PKC can also be modulated long term by changes in the levels of activators or inhibitors for a prolonged period or by changes in the levels of functional PKC isoenzymes in the cell during development or in response to hormones and/or differentiation factors. Indeed, studies have indicated that the sustained activation or inhibition of PKC activity in vivo may play a critical role in regulation of long term cellular events such as proliferation, differentiation and tumorigenesis. In addition, these regulatory events are important in colon cancer, where a decrease in PKC activators and activity suggests PKC acts as an anti-oncogene, in breast cancer, where an increase in PKC activity suggests an oncogenic role for PKC, and in multidrug resistance (MDR) and metastasis where an increase in PKC activity correlates with increased resistance and metastatic potential. These studies highlight the importance and significance of regulation of PKC activity in vivo.

The role of protein kinase C in arachidonic acid release and prostaglandin E production from CHO cells transfected with EGF receptors

Arachidonic acid release and prostaglandin production are stimulated by both phorbol esters and growth factors in various cell types. Whereas phorbol esters activate and transmit a signal via protein kinase C, this pathway is not necessarily involved in growth factor signal transduction. We investigated the involvement of protein kinase C in the pathway of arachidonic acid metabolism by CHO cells transfected with full-length EGF receptor (CHOwt). Two isoforms of protein kinase C were identified in CHOwt cells, alpha and zeta. On downregulation, the parallel loss of phorbol ester-stimulated arachidonic acid release and the alpha-isoform suggests a possible involvement of this isoform in phospholipase A2 activation in these cells. In addition, we propose that the zeta-isoform may be separately involved in prostaglandin production as residual phorbol ester-stimulation of PGE production occurs in downregulated cells where PKC zeta is the sole remaining isoform. EGF stimulation of arachidonic acid release, as a measure of phospholipase A2 activation, and subsequent prostaglandin production are unaffected by inhibition of protein kinase C in CHOwt cells. Indeed one such inhibitor, staurosporine, augmented the EGF effect. These results suggest that PKC is not required for EGF activation of phospholipase A2 in these cells.

Induction of hepatocyte growth factor in human skin fibroblasts by epidermal growth factor, platelet-derived growth factor and fibroblast growth factor

Hepatocyte growth factor (HGF) is a potent mitogen for rat and human hepatocytes in primary culture and appears to be the physiological hepatotrophic factor that triggers or modulates liver regeneration. Regulation of HGF gene expression and the protein production in human skin fibroblasts was examined. Addition of epidermal growth factor (EGF), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), acidic fibroblast growth factor (aFGF) and transforming growth factor-alpha (TGF-alpha) to confluent cultures of the cells markedly stimulated HGF secretion from the cells. The stimulating effect of EGF, PDGF and bFGF was further investigated. The effect of all three growth factors was maximal at 3-30 ng/ml and was accompanied by an increase in HGF mRNA levels. The mRNA levels were not elevated at 5 h but were at 10 h or more after addition of EGF. The levels of HGF mRNA in fibroblasts treated with the optimal doses of EGF, PDGF, bFGF, aFGF and TGF-alpha for 24 h were 6, 4, 5, 4 and 5 times that of control cultures incubated in medium only, respectively. The growth factor-induced HGF mRNA expression and HGF secretion was inhibited by addition of TGF-beta 1 or dexamethasone. Pretreatment with a high dose of phorbol 12-myristate 13-acetate (PMA), which causes down-regulation in protein kinase C (PKC) activity and PMA-induced HGF secretion, did not reduce the effects of the growth factors on HGF mRNA expression and HGF secretion, but rather enhanced them.(ABSTRACT TRUNCATED AT 250 WORDS)

The epidermal growth factor mitogenic signal is modulated by protein kinase C in T51B rat liver cells

The regulation of cell proliferation involves a complex interplay between several signal transduction pathways. The effect of EGF on DNA synthesis in serum starved quiescent, synchronized T51B cells was investigated by [3H]thymidine incorporation and flow cytometry. 1 nM EGF or readdition of serum initiated G1 progression and entry into S phase by 18 h and DNA synthesis reached a maximum by 28 h. Low concentrations of EGF markedly stimulated DNA synthesis, but EGF was not as potent as readdition of serum. The effect of EGF on DNA synthesis was only partially blocked by the tyrosine inhibitors genistein and tyrphostin, suggesting that other signalling pathways play a role in EGF-stimulated mitogenesis. 1 nM EGF caused a rapid, transient increase in the activity of membrane-associated protein kinase C (PKC) followed by a longer sustained increase that continued into S phase. TPA (12-O-tetradecanoyl-phorbol-13-acetate) did not mimic EGF, rather it caused a slight stimulation of membrane-associated PKC activity within 1 h followed by a dramatic downregulation of PKC within 4 h. TPA was without effect on DNA synthesis alone, but when added along with EGF or serum TPA caused a significant enhancement of DNA synthesis. Pretreatment of quiescent, serum-deprived T51B cells with TPA reduced the basal level of DNA synthesis; however, under these conditions EGF became as potent a mitogen as serum. We hypothesize that EGF via activation of PKC regulates the activity of its receptor by switching from high affinity to low affinity states. Downregulation of PKC by long term treatment with TPA removes this regulation thus rendering T15B cells more sensitive to exogenous EGF.

Melittin inhibits epidermal growth factor-induced protein tyrosine phosphorylation: comparison with phorbol myristate acetate and calcium ionophore A23187

In HER14 cells, epidermal growth factor (EGF) induces tyrosine phosphorylation of several proteins, including its own receptor. The bee venom peptide, melittin, impaired EGF-dependent protein tyrosine phosphorylation in a calcium-dependent manner. The melittin effect was similarly reproduced by calcium ionophore A23187. The effect of melittin and calcium ionophore A23187 on EGF-dependent protein tyrosine phosphorylation was abolished by treatment of cells with the calcium chelator EGTA. Phorbol-myristate acetate (PMA) inhibited EGF-dependent protein tyrosine phosphorylation, and when compared to melittin or calcium ionophore A23187, only PMA potentiated the EGF-induced tyrosine phosphorylation of two proteins immunologically related to mitogen activated protein (MAP) kinases of 40 kDa and 44 kDa molecular mass. Unlike PMA, the effect of melittin and calcium ionophore A23187 on inhibition of EGF-dependent protein tyrosine phosphorylation was lost neither in protein kinase C-depleted cells nor in cells treated with the protein tyrosine phosphatase inhibitors NaF and Na3VO4. Melittin inhibited high affinity binding of EGF to its receptor in intact cells, but this effect was not prevented by EGTA. It is concluded that melittin and calcium ionophore A23187 differ from PMA in their inhibition of EGF-dependent protein tyrosine phosphorylation in vivo, by acting via a Ca(2+)-dependent pathway, that is independent of protein kinase C, protein tyrosine phosphatase activity and high affinity binding of EGF to its receptor.

The expression of PDGF-alpha but not PDGF-beta receptors is suppressed in Swiss/3T3 fibroblasts over-expressing protein kinase C-alpha

The generation and characterization of Swiss/3T3 cells which stably over-express protein kinase C (PKC)-alpha were previously described by us. In these cells over-expression of PKC-alpha reduced the expression of epidermal growth factor (EGF) receptor molecules [(1990) J. Biol. Chem. 265, 13290-13296]. Here we show that the expression of PDGF-alpha receptors, but not PDGF-beta receptors, was specifically decreased in these cells. Not only were the levels of PDGF-alpha receptor mRNA transcript and protein significantly diminished in the PKC-alpha over-producing cells, but their ability to respond to short- and long-term growth factor signals was appropriately compromised. This was reflected in a reduced tyrosine autophosphorylation signal in response to PDGF-AA, as well as in decreased growth rates of PKC-alpha over-expressing cells when supplied with external PDGF-AA. A similar decrease in PDGF-alpha receptors was also demonstrated in parental Swiss/3T3 cells treated with phorbol esters. Our studies imply that PKC-alpha is involved in a cellular mechanism suppressing the expression of PDGF-alpha receptors in Swiss/3T3 cells. Hence, activation of PKC-alpha or alterations in its cellular levels may affect, in turn, the expression of a specific set of cell surface receptors and their responses to external growth factors.

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.

Inhibition of epidermal growth factor-dependent protein tyrosine phosphorylation by phorbol myristate acetate is mediated by protein tyrosine phosphatase activity

Incubation of HER14 cells with phorbol myristate acetate (PMA) decreases epidermal growth factor (EGF)-dependent protein tyrosine phosphorylation, except for a 40-kDa MAP kinase II-like protein, whose tyrosine phosphorylation is further enhanced. The inhibitory effect of PMA on EGF-dependent protein tyrosine phosphorylation is reversed if cell are pre-incubated with a combination of Na3VO4 and NaF, two known inhibitors of protein tyrosine phosphatase activity. Protein tyrosine phosphatase activity of cell homogenate was measured on immunopurified EGF receptor, and was found to be enhanced in PMA-treated cells. These data suggest that the inhibitory effect of PMA on EGF-dependent protein tyrosine phosphorylation in HER14 cells may be mediated by protein tyrosine phosphatase activity.

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.

Activation of protein kinase C inhibits human keratinocyte migration

The involvement of protein kinase C (PKC) in epidermal growth factor (EGF)-induced human keratinocyte migration was studied with the phagokinetic assay. It was concluded that PKC activation does not mediate, but rather inhibits, EGF-induced keratinocyte migration. The following experimental observations support these conclusions: 1) The PKC inhibitor H-7 did not inhibit EGF-induced migration but instead led to a modest enhancement. 2) PKC activators such as phorbol-12-myristate-13-acetate (PMA), phorbol-12,13-dibutyrate (PDBu), and 1,2-dioctanoly-sn-glycerol inhibited migration, but biologically inactive 4 alpha-PMA had no effect. 3) PMA did not inhibit keratinocyte attachment and spreading but blocked migration almost immediately after addition. 4) Migration of PKC-depleted cells, which were produced by prolonged treatment with PDBu, was enhanced similarly to normal cells by EGF. 5) PKC-depleted cells were not susceptible to the inhibitory effects of phorbol esters on migration. Additional experiments, in which cells were preactivated with EGF, suggested that PKC inhibits the EGF effect at a post-receptor level. The inhibitory effect of PKC on keratinocyte migration was not restricted to EGF-induced migration; PKC activation also inhibited keratinocyte migration induced by bovine pituitary extract, insulin, insulin-like growth factor-1, and keratinocyte growth factor.