title: Deficient tyrosine phosphorylation of c-Cbl and associated proteins in phorbol ester-resistant EL4 mouse thymoma cells.
abstract: Two tyrosine phosphoproteins in phorbol ester-sensitive EL4 (S-EL4) mouse thymoma cells have been identified as the p120 c-Cbl protooncogene product and the p85 subunit of phosphatidylinositol 3-kinase. Tyrosine phosphorylation of p120 and p85 increased rapidly after phorbol ester stimulation. Phorbol ester-resistant EL4 (R-EL4) cells expressed comparable amounts of c-Cbl and phosphatidylinositol 3-kinase protein but greatly diminished tyrosine phosphorylation. Co-immunoprecipitation experiments revealed complexes of c-Cbl with p85, and of p85 with the tyrosine kinase Lck in phorbol ester-stimulated S-EL4 but not in unstimulated S-EL4 or in R-EL4 cells. In vitro binding of c-Cbl with Lck SH2 or SH3 domains was detected in both S-EL4 and R-EL4 cells, suggesting that c-Cbl, p85, and Lck may form a ternary complex. In vitro kinase assays revealed phosphorylation of p85 by Lck only in phorbol ester-stimulated S-EL4 cells. Collectively, these results suggest that Cbl-p85 and Lck-p85 complexes may form in unstimulated S-EL4 and R-EL4 cells but were not detected due to absence of tyrosine phosphorylation of p85. Greatly decreased tyrosine phosphorylation of c-Cbl and p85 in the complexes may contribute to the failure of R-EL4 cells to respond to phorbol ester.
authors.affiliation: Department of Pharmacology and Cancer Center, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: X
authors.initials: X
authors.lastname: Luo
authors.forename: J J
authors.initials: JJ
authors.lastname: Sando
mesh_terms: D000818:Animals; D002460:Cell Line; D019008:Drug Resistance, Neoplasm; D051379:Mice; D015240:Phorbol 12,13-Dibutyrate / Q000633:toxicity*; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins / Q000302:isolation & purification / Q000378:metabolism; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D019000:Phosphotyrosine; D011505:Protein-Tyrosine Kinases / Q000302:isolation & purification / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000302:isolation & purification / Q000378:metabolism*; D050721:Proto-Oncogene Proteins c-cbl; D011993:Recombinant Fusion Proteins / Q000302:isolation & purification / Q000378:metabolism; D013945:Thymoma; D013953:Thymus Neoplasms; D014407:Tumor Cells, Cultured; D044767:Ubiquitin-Protein Ligases
chemical_list: D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D011993:Recombinant Fusion Proteins; D019000:Phosphotyrosine; D015240:Phorbol 12,13-Dibutyrate; D050721:Proto-Oncogene Proteins c-cbl; D044767:Ubiquitin-Protein Ligases; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D011505:Protein-Tyrosine Kinases; C493849:Cbl protein, mouse