The effect of phosphorylation by casein kinase 2 on the activity of insulin-like growth factor-binding protein-3.
Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) is known to be secreted as a phosphoprotein, constitutively phosphorylated at casein kinase 2 (CK2) sites. To examine the effect of phosphorylation by CK2 on the properties of glycosylated human IGFBP-3, we phosphorylated plasma-derived IGFBP-3, containing less than 1 mol/mol phosphoserine, in vitro. As judged by incorporated 32P, enzymatic deglycosylation did not decrease the phosphate content of phospho-IGFBP-3. Phosphorylation had no effect on IGF-I or IGF-II binding, but was inhibitory to acid-labile subunit binding in the presence of either IGF. Determined in simian virus 40-transformed human fibroblasts, cell association by phospho-IGFBP-3 was inhibited approximately 50% compared with that of the nonphosphorylated preparation. Phospho-IGFBP-3 showed significant resistance to proteolysis by plasmin and a cysteine protease secreted by MCF-7 cells. However, no difference was seen between the two preparations in their inhibition of IGF-I-stimulated DNA synthesis when coincubated with IGF-I in neonatal skin fibroblasts or MCF-7 breast cancer cells, and little difference was found in their ability to potentiate IGF-I-stimulated DNA synthesis when preincubated with fibroblasts. These results indicate that IGFBP-3 interaction with acid-labile subunit and with the cell surface, both of which involve basic carboxyl-terminal residues, may be modulated by phosphorylation. Relative resistance to proteolysis and poor binding to cells suggest that CK2-phospho-IGFBP-3 may be a significant inhibitor of IGF activity in the extracellular environment.
Coverley
J A
JA
Kolling Institute of Medical Research, University of Sydney, Royal North Shore Hospital, St. Leonards, New South Wales, Australia.
Martin
J L
JL
Baxter
R C
RC
D047390:Casein Kinase II; D002478:Cells, Cultured; D005347:Fibroblasts / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000097:blood / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D007700:Kinetics; D010766:Phosphorylation; D010768:Phosphoserine / Q000032:analysis; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*
D018972:Insulin-Like Growth Factor Binding Protein 3; D010768:Phosphoserine; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases