title: Ligand and protein kinase C downmodulate the colony-stimulating factor 1 receptor by independent mechanisms.
abstract: The turnover of the colony-stimulating factor 1 receptor (CSF-1R), the c-fms proto-oncogene product, is accelerated by ligand binding or by activators of protein kinase C (PKC), such as the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). The mechanisms of ligand- and TPA-induced downmodulation were shown to differ by the following criteria. First, in cells in which PKC was downmodulated, CSF-1R reexpressed at the cell surface remained sensitive to ligand but was refractory to TPA-induced degradation. Second, a kinase-defective receptor containing a methionine-for-lysine substitution at amino acid 616 at its ATP-binding site failed to undergo ligand-induced downmodulation but remained responsive to TPA. Following CSF-1 stimulation, no intermediates of receptor degradation could be immunoprecipitated with polyvalent antisera to CSF-1R. In contrast, TPA induced specific proteolytic cleavage of the receptor near its transmembrane segment, resulting in the release of the extracellular ligand-binding domain from the cell and the generation of an intracellular fragment containing the kinase domain. Two-dimensional phosphopeptide mapping demonstrated no new sites of phosphorylation in response to TPA in either the residual intact receptor or the intracellular proteolytic fragment. Therefore, PKC appears not to trigger downmodulation by directly phosphorylating the receptor but, rather, activates a protease which recognizes CSF-1R as a substrate.
authors.affiliation: Department of Pathology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105.
authors.forename: J R
authors.initials: JR
authors.lastname: Downing
authors.forename: M F
authors.initials: MF
authors.lastname: Roussel
authors.forename: C J
authors.initials: CJ
authors.lastname: Sherr
mesh_terms: D002460:Cell Line; D003001:Cloning, Molecular; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008024:Ligands / Q000378:metabolism*; D010449:Peptide Mapping; D010766:Phosphorylation; D011233:Precipitin Tests; D011493:Protein Kinase C / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D016186:Receptor, Macrophage Colony-Stimulating Factor; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
chemical_list: D008024:Ligands; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins; D011505:Protein-Tyrosine Kinases; D016186:Receptor, Macrophage Colony-Stimulating Factor; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate