title: Phosphorylation of insulin-like growth factor binding protein-3 by deoxyribonucleic acid-dependent protein kinase reduces ligand binding and enhances nuclear accumulation.
abstract: The IGF binding proteins (IGFBPs) regulate the mitogenic effects of IGFs in the extracellular environment. Several members of this family, including IGFBP-3, also appear to have IGF-independent effects on cell function. For IGFBP-3 and IGFBP-5, both of which are translocated to the cell nuclei, these effects may be related to their putative nuclear actions. Because reversible phosphorylation is an important mechanism for controlling nuclear protein import, we have examined the effect of phosphorylating IGFBP-3 with a number of serine/threonine protein kinases on its nuclear import. Phosphorylation of IGFBP-3 by the double-stranded DNA-dependent protein kinase (DNA-PK) increased both the nuclear import of IGFBP-3 and the binding of IGFBP-3 to components within the nucleus compared with nonphosphorylated IGFBP-3. However, there was no difference in the binding of the nuclear transport factor, importin beta, to nonphosphorylated and phosphorylated IGFBP-3. The ability of the DNA-PK phosphoform of IGFBP-3 to bind IGFs was severely attenuated, and in contrast to nonphosphorylated IGFBP-3, the DNA-PK phosphoform was unable to transport IGF-I to the nucleus. Furthermore, IGFBP-3 was phosphorylated by DNA-PK when complexed to IGF-I causing the phosphoform to release IGF-I. Together, these results suggest that when IGF-I is cotransported into the nucleus by IGFBP-3, phosphorylation of IGFBP-3 by nuclear DNA-PK provides a means for releasing bound IGF-I and creating a phosphoform of IGFBP-3 with increased affinity for nuclear components.
authors.affiliation: Kolling Institute of Medical Research, University of Sydney, Royal North Shore Hospital, Sydney, New South Wales 2065, Australia. lyns@med.usyd.edu.au
authors.forename: Lynette J
authors.initials: LJ
authors.lastname: Schedlich
authors.forename: Trine
authors.initials: T
authors.lastname: Nilsen
authors.forename: Anna P
authors.initials: AP
authors.lastname: John
authors.forename: David A
authors.initials: DA
authors.lastname: Jans
authors.forename: Robert C
authors.initials: RC
authors.lastname: Baxter
mesh_terms: D001667:Binding, Competitive; D002467:Cell Nucleus / Q000378:metabolism*; D051747:DNA-Activated Protein Kinase; D004268:DNA-Binding Proteins; D006367:HeLa Cells; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D008024:Ligands; D009687:Nuclear Proteins; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D028961:beta Karyopherins / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D018972:Insulin-Like Growth Factor Binding Protein 3; D008024:Ligands; D009687:Nuclear Proteins; D028961:beta Karyopherins; D007334:Insulin-Like Growth Factor I; D051747:DNA-Activated Protein Kinase; C497263:PRKDC protein, human; D017346:Protein Serine-Threonine Kinases