Phosphorylation of Rat Insulin-like Growth Factor Binding Protein-1 Does Not Affect its Biological Properties

• Published in: Archives of biochemistry and biophysics Vol. 357; no. 1; pp. 101 - 110
• Main Authors: Peterkofsky, Beverly, Gosiewska, Anna, Wilson, Shirley, Kim, Yeon-Ran
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.09.1998
• Subjects: 3T3 Cells; Amino Acid Sequence; Animals; Binding Sites; Carcinoma, Hepatocellular; Chromatography, Ion Exchange; DNA - antagonists & inhibitors; DNA - biosynthesis; DNA synthesis; insulin-like growth factor; Insulin-Like Growth Factor Binding Protein 1 - isolation & purification; Insulin-Like Growth Factor Binding Protein 1 - metabolism; Insulin-Like Growth Factor Binding Protein 1 - physiology; insulin-like growth factor binding proteins; Insulin-Like Growth Factor I - metabolism; ligand affinity; Mice; Molecular Sequence Data; Peptide Mapping; Phosphopeptides - metabolism; Phosphorylation; Protein Binding; Rats; Subcellular Fractions - metabolism; Substrate Specificity; Tumor Cells, Cultured; insulin-like growth factor binding proteins; DNA synthesis; ligand affinity; insulin-like growth factor; phosphorylation
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1006/abbi.1998.0797

Insulin-like growth factors (IGFs) I and II stimulate growth and expression of specific genes through binding to cell membrane receptors. IGF binding proteins also bind IGF-I with higher affinity than the receptor. They are found in the circulation and tissues and can modulate IGF actions. Human IGFBP-1 is phosphorylated on serine residues, which increases its affinity for IGF-I. An acidic, presumably phosphorylated, form of human IGFBP-1 inhibits IGF-I-stimulated DNA synthesis in cultured cells, while a less acidic, unphosphorylated form potentiates this function. Phosphorylation of human IGFBP-3, however, does not affect its affinity for IGF-I. Previously we found that multiple forms of rat IGFBP-1 are obtained by anion-exchange chromatography, raising the possibility that it also is phosphorylated, which led us to examine its properties. Phosphopeptide analysis of32P-labeled, immunoprecipitated rat IGFBP-1 synthesized by H-4-II-EC3 rat hepatoma cells indicated that it is phosphorylated on two sites that were deduced to be ser107 and ser132 in the central nonconserved domain. Dephosphorylation of purified phosphorylated rat IGFBP-1 did not affect its affinity for IGF-I or its specific binding activity, and the dephosphorylated form inhibited DNA synthesis in 3T3 cells. Incubation of cells labeled with radioactive proline in the presence of monensin and brefeldin A, which inhibit secretion at different sites, led to intracellular accumulation of the least phosphorylated form of rat IGFBP-1, but prevented further phosphorylation. The results suggested that phosphorylation occurs at two sites in cells, thecis-Golgi and thetrans-Golgi network. In summary, these studies have shown that rat IGFBP-1 is phosphorylated on two sites by reactions that occur in different secretory organelles and that similar to human IGFBP-3, but unlike human IGFBP-1, phosphorylation does not affect its affinity for IGF-I.