Effects of epidermal growth factor and 12-O-tetradecanoylphorbol-13-acetate on metabolism of the epidermal growth factor receptor in normal human fibroblasts

Nerve growth factor promotes the survival of sympathetic neurons through the cooperative function of the protein kinase C and phosphatidylinositol 3-kinase pathways

The signaling pathways activated by nerve growth factor (NGF) that account for its ability to promote the survival of neurons are not completely understood. Phosphatidylinositol 3-kinase (PI3K) is critical for the survival of several cell types, including neurons. To determine whether additional signaling pathways cooperate with PI3K to promote survival, we examined other pathways known to be activated by NGF. NGF activated protein kinases C (PKCs) in sympathetic neurons, and pharmacologic PKC activation rescued neurons from apoptosis induced by the withdrawal of NGF. Inhibition of PKCs did not inhibit the survival of NGF-maintained neurons. Similarly, inhibition of PI3K caused only a modest attrition of neurons in the presence of NGF. In contrast, the simultaneous inhibition of both PKCs and PI3K induced the apoptotic death of NGF-maintained sympathetic neurons. Inhibition of both PI3K and PKCs promoted the expression and phosphorylation of the proapoptotic transcription factor c-Jun, indicating that these pathways inhibit programmed cell death at the stage of proapoptotic gene expression. In culture conditions under which PI3K inhibition alone kills NGF-maintained neurons, PKC inhibition also led to a significant loss of viability, indicating that both pathways are required. Therefore, PKC and PI3K, regardless of the culture conditions, cooperate to promote the NGF-dependent survival of sympathetic neurons.

Protein kinase Calpha activation by RET: evidence for a negative feedback mechanism controlling RET tyrosine kinase

We have studied the role of protein kinase C (PKC) in signaling of the RET tyrosine kinase receptor. By using a chimeric receptor (E/R) in which RET kinase can be tightly controlled by the addition of epidermal growth factor (EGF), we have found that RET triggering induces a strong increase of PKCalpha, PKCdelta and PKCzeta activity and that PKCalpha, not PKCdelta and PKCzeta, forms a ligand-dependent protein complex with E/R. We have identified tyrosine 1062 in the RET carboxyl-terminal tail as the docking site for PKCalpha. Block of PKC activity by bisindolylmaleimide or chronic phorbol esters treatment decreased EGF-induced serine/threonine phosphorylation of E/R, while it caused a similarly sized increase of EGF-induced E/R tyrosine kinase activity and mitogenic signaling. Conversely, acute phorbol esters treatment, which promotes PKC activity, increased the levels of E/R serine/threonine phosphorylation and significantly decreased its phosphotyrosine content. A threefold reduction of tyrosine phosphorylation levels of the constitutively active RET/MEN2A oncoprotein was observed upon coexpression with PKCalpha. We conclude that RET binds to and activates PKCalpha. PKCalpha, in turn, causes RET phosphorylation and downregulates RET tyrosine kinase and downstream signaling, thus functioning as a negative feedback loop to modulate RET activity.

Interactions between the juxtamembrane domain of the EGFR and calmodulin measured by surface plasmon resonance

One early response to epidermal growth factor receptor (EGFR) activation is an increase in intracellular calcium. We have used surface plasmon resonance (SPR) to study real-time interactions between the intracellular juxtamembrane (JM) region of EGFR and calmodulin. The EGFR-JM (Met(644)-Phe(688)) was expressed as a GST fusion protein and immobilised on a sensor chip surface. Calmodulin specifically interacts with EGFR-JM in a calcium-dependent manner with a high on and high off rate. Chemical modification of EGFR-JM by using arginine-selective phenylglyoxal or deletion of the basic segment Arg(645)-Arg(657) inhibits the interaction. Phosphorylation of EGFR-JM by protein kinase C (PKC) or glutamate substitution of Thr(654) inhibits the interaction, suggesting that PKC phosphorylation electrostatically interferes with calmodulin binding to basic arginine residues. Calmodulin binding was also inhibited by suramin. Our results suggest that EGFR-JM is essential for epidermal growth factor (EGF)-mediated calcium-calmodulin signalling and for signal integration between other signalling pathways.

Spatial organization of EGF receptor transmodulation by PDGF

Even though the modulation of EGF receptors by PDGF is well documented, it is not known where on the cell surface cross-talk between the two receptor systems takes place. The recent finding that both populations of receptors are concentrated in cell surface caveolae suggestes that the confinement of the two receptors to this space might facilitate their interaction. Here we show that stimulation of PDGF receptors in caveolae with PDGF causes a subpopulation of EGF receptors in the same membrane fraction to become phosphorylated on tyrosine. Coincident with tyrosine phosphorylation, the binding of EGF to its receptor markedly declines. Loss of EGF binding is partially blocked by tyrosine kinase inhibitors. Despite the close proximity of the two receptors in caveolae, we saw no evidence that EGF could stimulated PDGFR tyrosine phosphorylation. These results suggest that these two receptor systems are highly organized in caveolae.

Intensification of growth factor receptor signalling by phorbol treatment of ligand-primed cells implies a dimer-stabilizing effect of protein kinase C-dependent juxtamembrane domain phosphorylation

Protein kinase C (PKC) phosphorylates the juxtamembrane domain of many growth factor receptors, but the physiologic effect of this modification on ligand signalling and desensitisation is unclear. Here we show that PKC-dependent transmodulation of EGFR and ErbB2 signalling is schedule-specific: prolonged pre-treatment of A431 cells with the PKC agonist phorbol dibutyrate potently inhibits subsequent ligand-induced EGFR signalling as expected, but EGF pre-treatment reverses the inhibitory effect of phorbol. The agonist activity of PKC on receptor signalling is even more apparent when cells are treated with phorbol in the presence of a tyrosine phosphatase inhibitor. Because these findings suggested a synergistic interaction between tyrosine- and PKC-dependent phosphorylation events, we sought to define the interactions of tyrosine-phosphorylated and PKC-modified ErbB2 subsets within EGF-inducible hetero-oligomers. Growth factor-dependent PKC transphosphorylation takes place exclusively within endocytosed tyrosine-phosphorylated receptor oligomers. Moreover, phorbol differentially affects two ErbB2 C-terminal autophosphorylation sites: whereas phosphorylation of Tyr1222 is reduced, phosphorylation of Tyr1139 is increased. These results suggest that PKC-dependent phosphorylation of the juxtamembrane domain may contribute positively to both internalisation and signalling of ligand-activated receptors, simultaneously accelerating termination of growth factor action. We propose that transient PKC-dependent signal amplification results from enhanced stability of liganded receptor oligomers due to phosphorylation-dependent juxtamembrane domain interactions, analogous to the protein-protein binding now known to be induced by serine-threonine phosphorylation of CREB and SMAD.

small wing encodes a phospholipase C-(gamma) that acts as a negative regulator of R7 development in Drosophila

Phospholipase C-(gamma) (PLC-(gamma)) is activated in many cell types following growth factor stimulation. Our understanding of the role of PLC-(gamma) in cell growth and differentiation has been severely limited by the dearth of mutations in any organism. In this study, we show that the Drosophila gene small wing (sl), identified by Bridges in 1915, encodes a PLC-(gamma). Mutations of sl result in extra R7 photoreceptors in the compound eye, consistent with overactivation of the receptor tyrosine kinase pathways that control R7 development. The data presented here provide the first genetic evidence that PLC-(gamma) is involved in Ras-mediated signaling and indicate that PLC-(gamma) acts as a negative regulator in such pathways in Drosophila.

Human cancer cells exhibit protein kinase C-dependent c-erbB-2 transmodulation that correlates with phosphatase sensitivity and kinase activity

The c-erbB-2 receptor tyrosine kinase is often overexpressed in human tumors, but the functional implications of this phenotype remain unclear. We previously used phosphorylation-specific antibodies to define major differences in c-erbB-2 tyrosine kinase activity between overexpressing human tumor cell lines (Epstein, R. J., Druker, B. J., Roberts, T. M., and Stiles, C. D. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 10435-10439). Here we extend this approach to define the relationship between c-erbB-2 tyrosine phosphorylation and protein kinase C (PKC)-dependent transmodulation. Phosphorylation-specific antibodies to the juxtamembrane PKC site Thr686 recognize tyrosine-dephosphorylated wild-type c-erbB-2 following G8/DHFR 3T3 cell treatment with PKC agonists. B104-1-1 cells transformed by activated c-erbB-2 express a subset of tyrosine-phosphorylated receptors that are homologously phosphorylated on Thr686, indicating that Thr686 phosphorylation alone is insufficient to abrogate receptor tyrosine phosphorylation. Similarly, the c-erbB-2-overexpressing human cancer cell lines SK-Ov-3 and BT-474 express constitutively Thr686-phosphorylated receptors. SK-Ov-3 cells express predominantly kinase-inactive c-erbB-2 that is heavily Thr686-phosphorylated, indicating that Thr686 phosphorylation in this line is heterologous in origin. In contrast, BT-474 cells express constitutively autophosphorylated c-erbB-2 despite Thr686 phosphorylation. These results indicate that Thr686 phosphorylation does not directly abolish c-erbB-2 activity and suggest that such phosphorylation reflects constitutive PKC activity induced by either receptor-activating mutations or heterologous growth factors. The latter possibility suggests in turn that c-erbB-2 interacts in an as yet undefined way with heterologous growth factor receptors in human tumor cells.

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.

Signal transduction mechanisms in mesenchymal cells

Mesenchymal cells are continually stimulated by a wide spectrum of biological mediators. These mediators bind to receptors on the cell surface and initiate a cascade of signaling events. The initial signal transduction pathways known to be stimulated in mesenchymal cells included phospholipase C, phospholipase D, phospholipase A2, adenylate cyclase, receptor tyrosine kinases, and receptor serine/threonine kinases. These pathways are reviewed and specific applications for therapeutic intervention in wound healing and regenerative therapy in the periodontium are discussed.

The calpain cleavage sites in the epidermal growth factor receptor kinase domain

The proteolysis of the human epidermal growth factor receptor cytoplasmic domain by calpain has been studied in vitro using purified recombinant cytoplasmic domain expressed in insect cells. Limited proteolysis produced kinase that was truncated at either N- or C-termini, as well as in the hinge region. We identified seven sites of calpain proteolysis by N-terminal sequencing of purified fragments. Calpain cleaved between the catalytic and autophosphorylation domains at two sites in the sequence Gln996-Asp1059, in the hinge region. Three new sites were also found in the autophosphorylation domain, preceding each of the major autophosphorylation sites. A fourth new site was located in the juxta-membrane domain, C-terminal to the regulatory Thr654. We purified an active 42-kDa fragment generated by calpain proteolysis between Leu659-Gln660 in the juxta-membrane domain, and in the hinge region. A fifth new site of calpain cleavage was found between the nucleotide binding motif Gly-Xaa-Gly-Xaa-Xaa-Gly and the essential Lys721 in the catalytic core of the kinase. Since both of these features are required for catalysis, calpain cleavage at this site may potentially provide a mechanism for down-regulation of kinase activity in vivo, under conditions of calpain activation. Thus the distribution of calpain cleavage sites along the kinase domain is consistent with a role for calpain both as a processing and as a degradative protease in epidermal growth factor receptor signalling.

Tumor necrosis factor alpha stimulates amnion prostaglandin biosynthesis primarily via an action on fatty acid cyclooxygenase

The purpose of this study was to determine how tumor necrosis factor alpha (TNF alpha) stimulates prostaglandin E2 production in human amnion. Amnion cells were isolated from term placentae and grown to confluence in culture. Incubations were conducted in quadruplicate wells for 16 hours with TNF alpha and protein synthesis inhibitors cycloheximide and actinomycin D, or arachidonic acid, acetylsalicylic acid (ASA), or staurosporine or H7 which inhibit protein kinase C activity. Prostaglandin E2 (PGE2) was measured by radioimmunoassay and cellular protein determined. The stimulatory action of TNF alpha on amnion PGE2 production was blocked by protein synthesis inhibitors, and the addition of arachidonic acid always enhanced the stimulatory properties of TNF alpha. TNF alpha consistently induced more rapid recovery from ASA treatment, and protein kinase C inhibition attenuated the stimulatory effects of TNF alpha. These results suggest that the stimulatory action of TNF alpha on amnion PGE2 production is likely at the level of induction of fatty acid cyclooxygenase activity and is partially dependent upon activation of protein kinase C.

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.

Modulation of the epidermal growth factor receptor by basic fibroblast growth factor

Treatment of Swiss 3T3 fibroblasts with basic fibroblast growth factor (bFGF) lead to a rapid reduction in epidermal growth factor (EGF) binding and a slower inhibition of EGF receptor autophosphorylation. The reduction in binding was due to a complete loss of the highest affinity EGF binding sites and a reduction in the lower affinity binding sites. Neither the inhibition of EGF binding nor the inhibition of EGF receptor autophosphorylation required protein kinase C. Treatment of cells with bFGF stimulated the phosphorylation of the EGF receptor, which persisted for several hours. The inhibition of EGF receptor autophosphorylation by bFGF was reduced in the presence of cycloheximide. However, cycloheximide had no effect on the reduction of EGF binding by bFGF. In contrast to these results with Swiss 3T3 fibroblasts, treatment of PC12 cells with bFGF lead to a reduction in EGF binding but no inhibition of EGF receptor autophosphorylation. Thus inhibition of EGF receptor autophosphorylation and inhibition of EGF binding can be uncoupled.

Characterization of the epidermal growth factor receptor in preimplantation pig conceptuses

Embryos recovered from sows on Days 9-13 of pregnancy (Day 0 = first day of estrus) exhibited saturable and time-dependent specific binding of 125I-epidermal growth factor (EGF). The specific binding (pg/mg protein) was greater (P less than 0.001) for Day 13 elongated conceptuses than for conceptuses of earlier stages. Scatchard analyses showed two classes of binding sites (Kd = 7.0 +/- 2.6 x 10(-11) M, Bmax = 6.2 +/- 1.4 fmol/mg protein and Kd = 3.4 +/- 0.2 x 10(-8) M, Bmax = 420 +/- 80 fmol/mg protein). The EGF receptor in Day 13 conceptus membranes is a 170-kDa protein and was phosphorylated in the presence of EGF and adenosine triphosphate. EGF stimulated protein tyrosine kinase activity about 1.6-fold over basal levels. The results show that the preimplantation pig conceptus possesses EGF-binding sites with the properties of functional EGF-receptors.

Evidence of a role for protein kinase C in epidermal growth factor-induced prostaglandin E2 synthesis in amnion cells

Human amnion cells synthesize and release prostaglandin E2 in response to epidermal growth factor. The protein kinase C activator, phorbol 12-myristate, 13-acetate also stimulates amnion cell prostaglandin E2 synthesis. We used a human amnion cell line (WISH) to conduct in vitro experiments to investigate a potential role of protein kinase C in the signal transduction pathway leading to epidermal growth factor-induced prostaglandin E2 production. Pretreatment of cultured amnion cells with a low, nonstimulating dose of phorbol 12-myristate, 13-acetate potentiated the action of epidermal growth factor in causing prostaglandin E2 production as measured by radioimmunoassay. The protein kinase C-selective inhibitor staurosporine inhibited epidermal growth factor-induced prostaglandin E2 production, further suggesting a role for protein kinase C in epidermal growth factor action. Experiments were conducted in which amnion cells were rendered protein kinase C-deficient by chronic exposure to phorbol ester, which has been shown to down-regulate the enzyme. In these cells, epidermal growth factor caused prostaglandin E2 synthesis at levels comparable to native (non-protein kinase C-deficient) cells. We conclude that protein kinase C plays a more modulatory than direct role in the epidermal growth factor signal transduction cascade that leads to prostaglandin E2 production by amnion cells. We propose a bifurcating transduction scheme in which, under conditions of protein kinase C inactivation, epidermal growth factor alone causes prostaglandin E2 synthesis. When protein kinase C is activated by as yet unknown endogenous substances, the epidermal growth factor responsiveness of the amnion cells is greatly enhanced. This pathway could have important implications in a feed-forward mechanism regulating the level of prostaglandin E2 production during the onset of labor.

Density-induced down regulation of epidermal growth factor receptors

Previous studies have shown that cell density can regulate the binding of several growth factors. To determine whether cell density exerts a uniform effect on the expression of epidermal growth factor (EGF) receptors, seven cell lines were examined in detail. For each cell line, EGF binding was found to decrease as cell density increases. Scatchard analysis of the binding data reveals that decreases in EGF binding are due to reductions in the number of cell surface EGF receptors. The only apparent exception is the effect of cell density on the binding of EGF to A-431 cells. For these cells, increases in cell density lead to two effects: decreases in the number of high affinity EGF receptors and increases in the total number of EGF receptors. In addition to the effects of cell density on EGF receptors, it was determined that increases in cell density can coordinately down-regulate receptors for as many as four different growth factors. Overall, the findings described in this report for EGF and those previously described for transforming growth factor type-beta (TGF-beta) and fibroblast growth factor (FGF) demonstrate the existence of a common mechanism for down-regulating growth factor receptors.

Reversal of inhibition of rat glomerular epithelial cell growth by growth factors

The ability of several growth factors to reverse heparin-induced inhibition of rat glomerular epithelial cell (GEC) growth and the mechanism of growth inhibition were explored in vitro. Insulin-like growth factor-1, rat multiplication-stimulating activity, and platelet-derived growth factor had no effect on proliferation of cultured GEC exposed to heparin (100 micrograms/ml). Epidermal growth factor (EGF) partially reversed heparin-induced growth inhibition in a dose-dependent fashion with a maximum effect seen at 1 ng/ml. No additive effect was seen with combinations of EGF and the other growth factors assayed. A decrease in EGF-stimulated incorporation of 3H-thymidine by GEC was seen with as little as 2 hours of heparin exposure and persisted for up to 48 hours. Heparin consistently increased binding of 125I-EGF to GEC with a significant increase apparent after 2 hours of exposure and a further increase with a 24-hour exposure. Increased EGF binding to heparin-treated cells was due to a significant increase in the association constant of EGF and its receptor with no effect on receptor number. Interactions between GEC and heparinlike glycosaminoglycans in the glomerular basement membrane may play a role in the regulation of GEC proliferation in normal and diseased states.

Differential regulation of oncogenic and cellular p185 by serine/threonine kinases

185c-neu is a member of a family of growth factor receptors with tyrosine kinase activity. A point mutation in the transmembrane region leads to activation of the enzymatic domain. We demonstrate that TPA (phorbol-12-myristate-13-acetate) stimulates the phosphorylation of p185c-neu on serine and threonine residues coincident with the inhibition of its intrinsic tyrosine kinase and the proliferation of cells that express it. The tyrosine kinase activity as well as the phosphorylation pattern of serine and threonine residues of oncogenic p185 (p185neu) and the growth of p185neu-expressing cells are not influenced by TPA. These observations indicate that the functional activity of p185c-neu can be regulated through protein kinase C (PKC) but the transmembrane point mutation present in p185neu renders it refractory to serine/threonine kinase regulation.

Bradykinin blocks the action of EGF, but not PDGF, on fibroblast division

Quiescent fibroblasts derived from human fetal lung can be stimulated to reinitiate DNA synthesis by sequential addition of 3 nM IGF-1 and a low concentration (8 pM) of EGF or by continuous exposure to 10% fetal calf serum or 10 ng/ml PDGF. Bradykinin blocks the IGF-1 and EGF-dependent signals without affecting the response to serum or PDGF. It activates protein kinase C and its anti-mitogenic effect is abolished after this kinase has been down-regulated. Bradykinin has no effect on the binding affinity of the EGF receptor whereas phorbol ester induces its 'transmodulation' to low affinity.

Regulation of osteosarcoma EGF receptor affinity by phorbol ester and cyclic AMP

We studied the binding and degradation of 125I-labeled epidermal growth factor (EGF) by UMR-106 osteosarcoma cells and the regulation of EGF receptor affinity for EGF by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and by treatments that raise intracellular levels of cyclic AMP. Cell surface binding of [125I]EGF to A431 cells reached a plateau after a 30 minute incubation at 37 degrees C but was undetectable in UMR-106 cells. Degradation of [125I]EGF proceeded at a 50-fold higher rate in A431 cells on a per cell basis, but receptor-bound [125I]EGF was internalized and degraded at a 3.5-fold higher rate by UMR-106 cells on a per receptor basis. At 4 degrees C, [125I]EGF labeled a single class of surface binding sites in the UMR-106 cell. Treatment with TPA at 37 degrees C reduced subsequent cell surface binding of [125I]EGF at 4 degrees C a maximum of 80% with an IC50 of 1.25 ng/ml. Maximal TPA reduction of [125I]EGF binding was observed within 5-15 minutes and was due to a reduction in the affinity of cell surface receptors of [125I]EGF without a change in receptor density. Pretreatment of the cells for 4 h with 30 microM forskolin, 1 mM isobutylmethylxanthine (IBMX) plus 30 microM forskolin, or 1 mM IBMX plus 100 ng/ml parathyroid hormone (PTH) attenuated the loss in [125I]EGF binding caused by a subsequent dose of 10 ng/ml of TPA by 17% (p less than 0.0005), 39% (p less than 0.0002), and 35% (p less than 0.002), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

HeLa cell mutants resistant to epidermal growth factor ricin A-chain conjugate

Epidermal growth factor (EGF) was linked to the toxic A chain of ricin toxin (RTA) to produce an EGF-receptor-specific cytotoxic agent, EGF-RTA. Three EGF-RTA-resistant mutants of the human HeLa cell line were selected. These mutant cell lines are 10-fold to more than 100-fold more resistant to EGF-RTA when compared to HeLa cells. The EGF-RTA-resistant mutants have at least as many EGF receptors as parent cells; the basis for the EGF-RTA-resistant phenotype must be distal to EGF binding. The EGF-RTA-resistant cells are not cross-ressitant to ricin or to diphtheria toxin; their mutant phenotype appears to be EGF specific. The EGF-RTA-resistant mutants are able to internalize and degrade EGF. However, the mutants have altered EGF receptor down-regulation and phorbol 12-tetradecanoate 13-acetate modulation properties. EGF-RTA/ammonium chloride and EGF-RTA/adenovirus co-treatment data suggest that the mutant defect(s) which confers EGF-RTA resistance is either in the endosome or at a step(s) in the intracellular EGF processing pathway between the endosome and the lysosome.

Structural basis for epidermal growth factor receptor function

The receptor for epidermal growth factor (EGF) has been the subject of intense study primarily as a consequence of the pioneering studies of Cohen on growth factors and also because of its homology to the transforming protein encoded by the avian oncogene v-erbB, which is a truncated receptor and its consequent role in cancer. Although similar structural mutation of the EGF receptor has not yet been found in human tumours, aberrant overexpression of both EGF receptors and c-erbB2, a closely related putative receptor, have been found to occur in squamous cell carcinomas and glial tumours, and mammary carcinomas respectively. In addition to EGF, the related polypeptides transforming growth factor alpha (TGF alpha) and vaccinia virus growth factor are also ligands for the EGF receptor. Expression of TGF alpha occurs during embryonal development and in specific adult tissues; it may also play a role in cellular transformation These important properties, as well as the potential roles of both TGF alpha and EGF in wound repair, have emphasized the need to understand EGF receptor structure, function and regulation. This review discusses the structural properties of the EGF receptor and how these can be related to receptor function and regulation.

Epidermal growth factor induces internalization but not degradation of the epidermal growth factor receptor in a human breast cancer cell line

Metabolism of the epidermal growth factor (EGF) receptor was studied in the MDA-MB-231 human breast cancer cell line. As in normal fibroblasts the EGF receptor from MDA-MB-231 cells was synthesized from a Mr = 160,000 precursor and tunicamycin treatment of cells resulted in accumulation of a Mr = 130,000 polypeptide. Unlike normal fibroblasts in which a Mr = 170,000 mature form of the EGF receptor was found, MDA-MB-231 cells contained a Mr = 172,000 mature form. Addition of EGF to MDA-MB-231 cells led to rapid internalization of EGF receptors, however, internalization did not affect receptor half-life and receptors did not recycle to the cell surface. EGF receptors could be visualized by immunofluorescence and remained sequestered in intracellular membranous structures following internalization. EGF was degraded slowly by MDA-MB-231 cells relative to degradation of EGF by normal cells. A high endogenous level of in vivo phosphorylation of threonine 654 of the EGF receptor was found in MDA-MB-231 cells and treatment of cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) further stimulated phosphorylation of this residue. EGF induced receptor internalization resulted in dephosphorylation of threonine 654. The significance of these unusual properties of EGF receptor metabolism in MDA-MB-231 cells is discussed.

Improved clonal and nonclonal growth of human, rat and bovine adrenocortical cells in culture

This report describes the development of a culture system for long-term growth and cloning of human fetal adrenocortical cells. Optimal conditions for stimulating clonal growth were determined by testing the efficacy of horse serum (HS), fetal bovine serum (FBS), fibroblast growth factor (FGF), epidermal growth factor (EGF), fibronectin, and a combination of growth factors, UltroSer G, in stimulating growth from low density. Optimal conditions for clonal growth were achieved using fibronectin-coated dishes and DME/F12 medium with 10% FBS, 10% HS, 2% UltroSer G, and 100 ng/ml FGF or 100 pM EGF. Conditions for growth at clonal density were found to be optimal for growth of early passage, nonclonal cultures at higher densities. The improved growth conditions used for cloning were shown to allow continued long-term growth of nonclonal human adrenocortical cells without fibroblast overgrowth. All cells in cultures grown in HS, FBS, and UltroSer G had morphologic characteristics of adrenocortical cells, whereas cells grown in FBS only rapidly became overgrown with fibroblasts. Clonal and nonclonal early passage human adrenocortical cells had similar mitogenic responses to FGF and EGF. Whereas FGF, EGF, and UltroSer G showed similar stimulation of DNA synthesis and clonal growth in human adrenocortical cells and human adrenal gland fibroblasts, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate stimulated growth only in adrenocortical cells and was strongly inhibitory to growth in fibroblasts. In both cell types, forskolin inhibited DNA synthesis. Human adrenocortical cell cultures were functional and synthesized cortisol, dehydroepiandrosterone, and dehydroepiandrosterone sulfate. The improved growth conditions for clonal growth of human adrenocortical cells also provided optimal conditions for long-term growth of cultured rat adrenocortical cells and increased the cloning efficiency of cultured bovine adrenocortical cells.

Growth factors modify the epidermal growth factor receptor through multiple pathways

Previous results have shown that tumor promoters modify the properties of the epidermal growth factor (EGF) receptor through the activation of protein kinase C. Diacylglycerol-generating factors such as platelet-derived growth factor (PDGF) and p28sis should activate protein kinase C and alter EGF receptor properties in a similar manner. To test directly the involvement of protein kinase C in the action of media from v-sis-transformed cells on the EGF receptor, Swiss 3T3 cells were first extensively treated with various concentrations of the tumor-promoter phorbol dibutyrate (PDBu) This treatment reduced levels of active protein kinase C in the cells, making them less responsive to subsequent rechallenge with the tumor promoter. The results demonstrate that there are at least two components to the action of media from v-sis transformed cells on EGF binding: a labile factor that confers protein kinase C independence and a stable factor that appears to be dependent on protein kinase C. The action of the first factor cannot be mimicked by transforming growth factor-beta or EGF in either the presence or absence of PDGF. The action of the second factor is similar to that of PDGF. These findings indicate that heterologous regulation of the EGF receptor can occur through both protein kinase C-dependent and -independent pathways.

Regulatory features of the epidermal growth factor receptor

The role of EGF receptor concentration in tumor growth was investigated in athymic mice by measuring the rate of growth of clonal human epidermoid carcinoma A431 cells containing different extents of EGF receptor gene amplification and protein expression. A direct correlation-between the rate of tumor growth and EFG receptor concentration was found, supporting previous cell culture studies that quantitated the relationship between activated EGF receptors and cell proliferation. Holo EGF receptor is activated by ligand binding to the extracellular domain to activate cytoplasmic tyrosine protein kinase activity. A model of single molecule transmembrane signaling is proposed. The function of two phosphorylation sites on the EGF receptor has been analyzed by use of site-directed mutagenesis. Comparison of normal and mutant hEGF receptors expressed in rodent cells lacking endogenous EGF receptors indicates that: 1) Thr654, located 10 amino acids carboxyl terminal to the inner membrane boundary, is a major site of heterologous regulation via protein kinase C catalyzed phosphorylation, and 2) Tyr1173, the major site of self-phosphorylation, located at the carboxyl terminus, provides a secondary level of regulation of receptor function by acting as a competitive inhibitor with exogenous substrates.