The effect of phosphorylation by casein kinase 2 on the activity of insulin-like growth factor-binding protein-3

Endocrinology. 2000 Feb;141(2):564-70. doi: 10.1210/endo.141.2.7306.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Casein Kinase II
  • Cells, Cultured
  • Fibroblasts / metabolism
  • Glycosylation
  • Humans
  • Insulin-Like Growth Factor Binding Protein 3 / blood
  • Insulin-Like Growth Factor Binding Protein 3 / metabolism*
  • Insulin-Like Growth Factor I / metabolism*
  • Insulin-Like Growth Factor II / metabolism*
  • Kinetics
  • Phosphorylation
  • Phosphoserine / analysis
  • Protein-Serine-Threonine Kinases / metabolism*

Substances

  • Insulin-Like Growth Factor Binding Protein 3
  • Phosphoserine
  • Insulin-Like Growth Factor I
  • Insulin-Like Growth Factor II
  • Casein Kinase II
  • Protein-Serine-Threonine Kinases