[The phosphorylation and tyrosine kinase activity of an internalized complex of epidermal growth factor and its receptor].
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A study was made of the functional state of the epidermal growth factor (EGF)--receptor complexes in A-431 cells. Conditions of surface bound EGF extraction were selected which allow to consider the intracellular EGF--receptor complexes only. A procedure of high efficient and specific immunoprecipitation of tyrosyl-phosphorylated EGF receptors was developed. It is shown that the dissociation of EGF--receptor complexes leads to receptor dephosphorylation due to a rapid and reversible inactivation of EGF receptor tyrosine kinase. The internalized receptor is found to be tyrosyl-phosphorylated and to retain tyrosine kinase for at least an hour after the internalization. The dynamics of dissociation, degradation and dephosphorylation of EGF--receptor complexes has been estimated. The rates of these processes prove to be almost negligible for the first 2.5 hours after internalization.Keywords:
Internalization
Dephosphorylation
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Phosphorylation cascade
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Dephosphorylation
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To determine whether GH receptor (GHR) cytoplasmic tyrosine residue(s) and tyrosine phosphorylation are required for signal transduction, we have substituted the eight porcine (p) GHR cytoplasmic tyrosines with phenylalanine individually or in a stepwise manner from the C terminus. Conversely, the eight tyrosines were individually regenerated in a non-tyrosine-containing pGHR analog. Mutated pGHR cDNAs were transfected into mouse L cells (MLCs) and cell lines were established. Each individual tyrosine-substituted pGHR analogs was able to activate STAT5 (signal transducer and activator of transcription 5; previously termed pp95) at levels comparable to those of wild type pGHR. Analyses of these pGHR analogs revealed that a single tyrosine residue at position 487, 534, 566, or 627 is sufficient for STAT5 phosphorylation. This result suggested that a redundancy in tyrosine residue requirement may be employed in GH-mediated signal transduction. Also, we found that the requirement of tyrosine residues for STAT5 phosphorylation directly correlated with their phosphorylation status. Combining both STAT5 and GHR tyrosine phosphorylation results, we have deduced that Y332, Y487, Y534, Y566, and Y627 are pGHR tyrosine phosphorylation sites. Additionally, Janus kinase 2 was activated by GH in all pGHR tyrosine-substituted analogs, including one containing no intracellular tyrosines, which agrees with a previous report that Janus kinase 2 activation is independent of GHR tyrosine phosphorylation.
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A study was made of the functional state of the epidermal growth factor (EGF)--receptor complexes in A-431 cells. Conditions of surface bound EGF extraction were selected which allow to consider the intracellular EGF--receptor complexes only. A procedure of high efficient and specific immunoprecipitation of tyrosyl-phosphorylated EGF receptors was developed. It is shown that the dissociation of EGF--receptor complexes leads to receptor dephosphorylation due to a rapid and reversible inactivation of EGF receptor tyrosine kinase. The internalized receptor is found to be tyrosyl-phosphorylated and to retain tyrosine kinase for at least an hour after the internalization. The dynamics of dissociation, degradation and dephosphorylation of EGF--receptor complexes has been estimated. The rates of these processes prove to be almost negligible for the first 2.5 hours after internalization.
Internalization
Dephosphorylation
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Kv1.3, a voltage-dependent potassium channel cloned from mammalian brain and T lymphocytes, contains multiple tyrosine residues that are putative targets for tyrosine kinases. We have examined the tyrosine phosphorylation of Kv1.3, expressed transiently in human embryonic kidney (or HEK) 293 cells, by endogenous and coexpressed tyrosine kinases. Tyrosine phosphorylation is measured by a strategy of immunoprecipitation followed by. Western blot analysis, using antibodies that specifically recognize Kv1.3 and phosphotyrosine. Coexpression of the constitutively active tyrosine kinase v-src, together with Kv1.3, causes a large increase in the tyrosine phosphorylation of the channel protein. This phosphorylation of Kv1.3 can be reversed by treatment with alkaline phosphatase before Western blot analysis. Coexpression with a receptor tyrosine kinase, the human epidermal growth factor receptor, also causes an increase in tyrosine phosphorylation of Kv1.3. The effects of endogenous tyrosine kinases were examined by treating Kv1.3-transfected cells with the specific membrane-permeant tyrosine phosphatase inhibitor pervanadate. Pervanadate treatment causes a time- and concentration-dependent increase in the tyrosine phosphorylation of Kv1.3. This increased tyrosine phosphorylation of Kv1.3 is accompanied by a time-dependent decrease in Kv1.3 current, measured by patch-clamp analysis with cell-attached membrane patches. The pervanadate-induced suppression of current and much of the channel tyrosine phosphorylation are eliminated by mutation of a specific tyrosine residue, at position 449 of Kv1.3, to phenylalanine. Thus, there is a continual phosphorylation and dephosphorylation of Kv1.3 by endogenous kinases and phosphatases, and perturbation of this constitutive phosphorylation/dephosphorylation cycle can profoundly influence channel activity.
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The relationship between epidermal growth factor receptor (EGF-R) protein tyrosine kinase activation and ligand-induced receptor dimerization was investigated using several bivalent anti-EGF-R antibodies directed against various receptor epitopes. In A431 membrane preparations and permeabilized cells, all antibodies were able to activate the EGF-R tyrosine kinase, as measured by EGF-R autophosphorylation and phosphorylation of other substrates on tyrosine residues. EGF-R tyrosine kinase activation correlated strongly with the induction of EGF-R dimerization. (i) Both processes specifically occurred in a narrow antibody concentration range; (ii) both processes required the presence of detergent; and (iii) both processes depended on antibody bivalence since monovalent Fab fragments were inactive yet regained full activity after cross-linking by a second bivalent antibody. These data demonstrate that antibody bivalence is essential and sufficient for EGF-R activation and that activation occurs regardless of the EGF-R epitope recognized. Finally, EGF-R dimerization was shown not to depend on receptor autophosphorylation since it still occurred in the absence of ATP. Also, partial inhibition of the tyrosine kinase activity by the specific EGF-R tyrosine kinase inhibitor tyrphostin AG 213 did not affect formation of EGF-R dimers. Taken together these results demonstrate that induction of EGF-R dimerization is sufficient and in case of antibody action, essential, for activation of the EGF-R tyrosine kinase and thus provide strong support for an intermolecular mechanism of EGF-R tyrosine kinase activation.
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We have utilized a broad approach to address whether tyrosine kinases and the growth pathways they regulate might be functionally aberrant in squamous cell carcinomas (SCC) of the upper aerodigestive tract. This strategy involved assaying for evidence of tyrosine kinase action in lysates of cell lines representing SCC. Our findings revealed a spectrum of elevated tyrosine phosphorylation in SCC lines ranging from less than 2-fold to more than 10-fold above that of control human epidermal keratinocytes. Thus the ability to regulate growth and other pathways controlled by tyrosine phosphorylation was impaired in all the 19 lines examined. Assessment of the receptor for epidermal growth factor (EGF) revealed that its activity was elevated above normal in 14 of the 19 cell lines examined, suggesting that at least a portion of the increased tyrosine phosphorylation observed could be attributed to excessive EGF receptor activity. Our findings provide functional evidence that growth pathways are aberrantly regulated in cell lines representing SCC of the upper aerodigestive tract.
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