Annexin A6 is a scaffold for PKCα to promote EGFR inactivation

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

Annexin A6 inhibits Ras signalling in breast cancer cells

Overexpression of epidermal growth factor receptor (EGFR) is associated with enhanced activation of wild-type (hyperactive) Ras in breast cancer. Little is known about the regulation of Ras inactivation and GTPase-activating proteins (GAPs), such as p120GAP, in cells with hyperactive Ras. Recently, we showed that in EGFR-overexpressing A431 cells, which lack endogenous Annexin A6 (AnxA6), ectopic expression of AnxA6 stimulates membrane recruitment of p120GAP to modulate Ras signalling. We now demonstrate that, AnxA6 is downregulated in a number of EGFR-overexpressing and estrogen receptor (ER)-negative breast cancer cells. In these cells, AnxA6 overexpression promotes Ca(2+)- and EGF-inducible membrane targeting of p120GAP. In ER-negative MDA-MB-436 cells, overexpression of p120GAP, but not CAPRI or a p120GAP mutant lacking the AnxA6-binding domain inhibits Ras/MAPK activity. AnxA6 knockdown in MDA-MB-436 increases Ras activity and cell proliferation in anchorage-independent growth assays. Furthermore, AnxA6 co-immunoprecipitates with H-Ras in a Ca(2+)- and EGF-inducible manner and fluorescence resonance energy transfer (FRET) microscopy confirmed that AnxA6 is in close proximity of active (G12V), but not inactive (S17N) H-Ras. Thus, association of AnxA6 with H-Ras-containing protein complexes may contribute to regulate p120GAP/Ras assembly in EGFR-overexpressing and ER-negative breast cancer cells.

Activation of Raf-1 is defective in annexin 6 overexpressing Chinese hamster ovary cells

Annexin 6 is a Ca2+-dependent phospholipid-binding protein involved in membrane trafficking. In this study we demonstrate the association of Raf-1 with recombinant rat annexin 6. Raf-annexin 6 interaction was shown to be independent of cell activation by epidermal growth factor (EGF) or phorbol esters (12-O-tetradecanoyl-phorbol-13-acetate (TPA)). A stable Chinese hamster ovary (CHO)-anx6 cell line overexpressing annexin 6 was established to examine the function of annexin 6. In these cells, no increase of Ras-GTP levels, induced by EGF or TPA, was detected. In addition, the activity of Raf was completely inhibited, whereas the mitogen-activated protein kinase-P was unaffected.

Morphologic and functional characterization of caveolae in rat liver hepatocytes

Caveolae are small pits on the plasma membrane found in several, if not all, differentiated cells. They are involved in potocytosis, endocytosis, transcytosis, membrane trafficking, and signal transduction. Although caveolin has recently been identified in subcellular fractions from rat liver there is no clear-cut morphologic evidence for the presence of prototypical caveolae on the surface of hepatocytes. In this study the presence of caveolae at the cell surface of hepatocytes was directly shown by rapid-freeze, deep-etching electron microscopy. Moreover, combined deep-etching and immunogold techniques revealed caveolin in caveolae of the dorsal membrane of primary culture hepatocytes. Using reagents that perturb membrane cholesterol and interfere with endocytosis through the caveolae, a caveolae-dependent internalization of cholera toxin B and retinol-binding protein by hepatocytes in primary culture was shown. Finally, immunocytochemical analysis of caveolin in nonparenchymal cells of the rat liver showed its presence in Kupffer and stellate cells, however no caveolin could be detected in endothelial cells.

Epidermal growth factor secreted from submandibular salivary glands interferes with the lipolytic effect of adrenaline in mice

We had described that epidermal growth factor (EGF) interfered with the lipolytic effect of catecholamines in isolated adipocytes. Since catecholamines stimulate the release of EGF from submandibular salivary glands to blood plasma in male mice, we studied whether EGF affected also the lipolytic response to adrenaline in whole animals. We studied the effect of adrenaline in sialoadenectomized and sham-operated mice receiving or not a high dose of EGF following adrenaline injection. There was no difference in plasma EGF concentration between sham-operated and sialoadenectomized animals receiving saline. After adrenaline administration plasma EGF increased by 20-fold in sham-operated but did not increase in sialoadenectomized mice. Indeed, the increase was much higher (more than 100-fold) in mice receiving exogenous EGF. The effect of adrenaline on plasma concentration of both glycerol and nonesterified fatty acids was higher as lower was plasma EGF concentration. Isolated adipocytes obtained from sham-operated or sialoadenectomized mice had identical lipolytic response to adrenaline. The lipolytic response of adipocytes to isoproterenol was decreased by addition of EGF. To study whether the interference with the in vivo lipolytic effect of adrenaline had further metabolic consequences, we measured plasma beta-hydroxybutyrate concentration in plasma. There was no difference in the response to adrenaline between sham-operated and sialoadenectomized mice in spite of the difference in plasma nonsterified fatty acid concentration. Studies in isolated hepatocytes indicated that ketogenesis run at near maximal rate in this range of substrate concentration. These results suggest that EGF in the physiological range decreases the lipolytic effect of adrenaline but does not compromise further metabolic events like the enhancement of ketogenesis.

Clathrin assembly lymphoid myeloid leukemia (CALM) protein: localization in endocytic-coated pits, interactions with clathrin, and the impact of overexpression on clathrin-mediated traffic

The clathrin assembly lymphoid myeloid leukemia (CALM) gene encodes a putative homologue of the clathrin assembly synaptic protein AP180. Hence the biochemical properties, the subcellular localization, and the role in endocytosis of a CALM protein were studied. In vitro binding and coimmunoprecipitation demonstrated that the clathrin heavy chain is the major binding partner of CALM. The bulk of cellular CALM was associated with the membrane fractions of the cell and localized to clathrin-coated areas of the plasma membrane. In the membrane fraction, CALM was present at near stoichiometric amounts relative to clathrin. To perform structure-function analysis of CALM, we engineered chimeric fusion proteins of CALM and its fragments with the green fluorescent protein (GFP). GFP-CALM was targeted to the plasma membrane-coated pits and also found colocalized with clathrin in the Golgi area. High levels of expression of GFP-CALM or its fragments with clathrin-binding activity inhibited the endocytosis of transferrin and epidermal growth factor receptors and altered the steady-state distribution of the mannose-6-phosphate receptor in the cell. In addition, GFP-CALM overexpression caused the loss of clathrin accumulation in the trans-Golgi network area, whereas the localization of the clathrin adaptor protein complex 1 in the trans-Golgi network remained unaffected. The ability of the GFP-tagged fragments of CALM to affect clathrin-mediated processes correlated with the targeting of the fragments to clathrin-coated areas and their clathrin-binding capacities. Clathrin-CALM interaction seems to be regulated by multiple contact interfaces. The C-terminal part of CALM binds clathrin heavy chain, although the full-length protein exhibited maximal ability for interaction. Altogether, the data suggest that CALM is an important component of coated pit internalization machinery, possibly involved in the regulation of clathrin recruitment to the membrane and/or the formation of the coated pit.

Clathrin, adaptors and eps15 in endosomes containing activated epidermal growth factor receptors

Activation of the epidermal growth factor receptor (EGFR) by EGF results in binding of clathrin adaptor protein complex AP-2 to the receptor cytoplasmic tail. The transient interaction with AP-2 is thought to be responsible for the selective recruitment of the EGFR into coated pits during endocytosis. In this study we found that EGF-induced EGFR/AP-2 association, measured by co-immunoprecipitation, persists after receptor internalization. Double-label immunofluorescence of EGF-treated A-431 and COS-1 cells revealed the presence of AP-2, clathrin and eps15, another component of the plasma membrane coated pits, in the large perinuclear endosomes loaded with EGFRs. By optical sectioning and image deconvolution, the immunoreactivities were seen to be distributed within vesicular and tubular elements of these endosomes. In addition, these compartments contained the transferrin receptors and a EEA.1 protein, markers of early endosomes. Furthermore, Golgi clathrin adaptor complex AP-1 was found in EGFR-containing endosomes and EGFR immunoprecipitates in A-431 cells. The direct interaction of the EGFR with micro1 as well as micro2 subunits of AP-1 and AP-2, correspondingly, was shown using the yeast two-hybrid assay. Brefeldin A, a drug that releases AP-1 from the trans-Golgi membranes, had no effect on AP-1 association with endosomes and its co-precipitation with EGFR. Taken together, the data suggest that endosomal EGFR-AP complexes make up a significant portion of the total amount of these complexes detectable by co-immunoprecipitation. It can be proposed that APs are capable of binding to the endosomal membrane via a mechanism that requires AP interaction with the intracellular tails of multimeric receptors like activated EGFR, which in turn allows recruitment of clathrin and eps15. The hypothesis that the competition between adaptor complexes for binding to the receptor tails in endosomes may regulate of the sorting of receptors is discussed.

Eps15 is constitutively oligomerized due to homophilic interaction of its coiled-coil region

Eps15 is a member of an emerging family of proteins containing a novel protein/protein interaction domain, the EH domain, of as yet unknown function. Recent findings of Eps15 association with clathrin adaptor complex AP-2 and its localization in clathrin-coated pits have implicated Eps15 in the regulation of vesicle trafficking. Here we show that Eps15 exists in several multimeric states in vivo. When purified recombinant Eps15 or lysates of NIH 3T3 cells were treated with cross-linking reagents, covalent dimers of Eps15 and larger covalent multimers were detected in high yield. Large Eps15 oligomers co-immunoprecipitated with AP-2 at an efficiency higher than that of Eps15 dimers. Furthermore, cross-linking of the membrane-bound fraction of Eps15 in mildly permeabilized cells was as efficient as that of the cytosolic fraction. Size-exclusion column chromatography of recombinantly produced Eps15 and of total cell lysates was performed to examine the equilibrium ratio of the monomers versus the aggregated forms of Eps15. These experiments showed that essentially all the Eps15 was aggregated, whereas monomers of Eps15 could be obtained only under strong denaturing conditions. To map the region of Eps15 responsible for dimerization, fusion proteins corresponding to the three structural domains of Eps15 were prepared. Cross-linking analysis revealed that the central portion of Eps15, which possesses a coiled-coil region (residues 321-520), serves as the interacting interface. The possibility that hetero-oligomeric complexes of Eps15 dimers and AP-2 function during the recruitment of proteins into coated pits is discussed.

Eps15 is a component of clathrin-coated pits and vesicles and is located at the rim of coated pits

Eps15, a phosphorylation substrate of the epidermal growth factor (EGF) receptor kinase, has been shown to bind to the alpha-subunit of the clathrin-associated protein complex AP-2. Here we report that in cells, virtually all Eps15 interacts with the cytosol and membrane-bound forms of AP-2. This association is not affected by the treatment of cells with EGF. Immunofluorescence microscopy reveals nearly absolute co-localization of Eps15 with AP-2 and clathrin, and analysis by immunoelectron microscopy shows that the localization of membrane-associated Eps15 is restricted to the profiles corresponding to endocytic coated pits and vesicles. Unexpectedly, Eps15 was found at the edge of forming coated pits and at the rim of budding coated vesicles. This asymmetric distribution is in sharp contrast to the localization of AP-2 that shows an even distribution along the same types of clathrin-coated structures. These findings suggest several possible regulatory roles of Eps15 during the formation of coated pits.

The antilipolytic effects of insulin and epidermal growth factor in rat adipocytes are mediated by different mechanisms

Epidermal growth factor (EGF) and insulin induced similar effects in isolated rat adipocytes. To determine whether EGF and insulin produced similar effects through the same mechanisms, we focused on lipolysis. Insulin inhibited the lipolysis stimulated by isoproterenol, glucagon (either alone or in combination with adenosine deaminase), adenosine deaminase itself, or forskolin. In contrast, EGF did not inhibit the lipolysis stimulated by forskolin or by hormones when the cells were also incubated with adenosine deaminase. The effect of insulin, but not that of EGF, on isoproterenol-stimulated lipolysis disappeared when adipocytes were incubated with 1 microM wortmannin. These results indicate that EGF and insulin affected lipolysis through different mechanisms. We observed that EGF, but not insulin, increased cytosolic Ca2+. The effect of EGF, but not that of insulin, disappeared when the cells were incubated in a Ca2+-free medium. We suggest that EGF, but not insulin, mediate its antilipolytic effect through a Ca2+-dependent mechanism which, however, do not involve Ca2+-activated protein kinase C isoforms. This is based on the following: 1) phorbol 12-myristate 13-acetate affected lipolysis in an opposite way to that of EGF; and 2) the protein kinase C inhibitor bisindolylmaleimide GF 109203X did not affect the antilipolytic action of EGF. Our results indicate that the antilipolytic effect of EGF resembles more that of vasopressin than that of insulin.

Epidermal growth factor administration decreases liver glycogen and causes mild hyperglycaemia in mice

Several laboratories report different effects of epidermal growth factor (EGF) on glycogen metabolism in hepatocytes. The discrepancies may be attributed to differences in the experimental conditions. It is therefore important to establish the actual effect of EGF in vivo. Because large physiological variations of EGF concentration in plasma occur in mice, we used this species to address this question. In freshly isolated mouse hepatocytes, EGF increased glycogen degradation in a dose-dependent manner. The maximal effect (36% increase over basal glycogenolysis) was smaller than maximal effects of classical glycogenolytic hormones like adrenaline or glucagon (more than 150% increase over basal). This is in keeping with the smaller effect of EGF on phosphorylase a activity. In contrast with these hormones, EGF did not inhibit glycolysis. Thus these effects of EGF in mouse hepatocytes are similar to those recently described by us in rat hepatocytes [Quintana, Grau, Moreno, Soler, Ramirez and Soley (1995) Biochem J 308, 889-894]. When administered to whole animals, EGF increased phosphorylase a activity, decreased the glycogen content in the liver and caused mild hyperglycaemia. Taking together the results obtained for isolated cells and for whole animals, we suggest that the glucosyl residues released from glycogen are used mostly by the liver rather than released to the circulation. This would be different from the action of the classical glycogenolytic hormones, adrenaline and glucagon.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Epidermal growth factor modulates the lipolytic action of catecholamines in rat adipocytes. Involvement of a Gi protein

In isolated adipocytes, epidermal growth factor (EGF) did not affect basal (nonstimulated) lipolysis, but interfered with the lipolytic action of isoproterenol (ISO) or glucagon. Similarly, EGF did not affect basal levels of cyclic AMP but interfered with the signal generated by ISO. However, EGF did not affect lipolysis stimulated by forskolin or cyclic AMP analogues. These results suggest that EGF interfered with the signal transduction between lipolytic hormone receptors and adenylate cyclase. To determine whether EGF was activating a Gi protein, adenosine deaminase (ADA) was added to degrade endogenously released adenosine. While the nonmetabolizable adenosine analogue N6-(phenylisopropyl)adenosine (PIA) inhibited ADA-stimulated lipolysis, EGF affected neither ADA-stimulated lipolysis nor the dose-response curve for PIA. However, EGF did not affect ISO-stimulated lipolysis in pertussis toxin-treated cells. Similarly, in the presence of ADA, the effects of ISO on lipolysis and on cyclic AMP levels were not affected by EGF. The addition of PIA restored the effect of EGF on both lipolysis and cyclic AMP. Since EGF decreased the IC50 for the inhibitory effect of PIA on (ISO+ADA)-stimulated lipolysis, we suggest that EGF modulates the interaction between GS and Gi in the control of adenylate cyclase.