Hormonal Regulation of Insulin-Like Growth Factor Binding Protein-1 Expression in the Rat

Abstract

The insulin-like growth factors (IGF) are present in the serum, other biological fluids and tissue extracts in association with high affinity binding proteins1. When human or adult rat serum is analyzed by gel permeation chromatography two groups of IGF binding proteins are identified. These have apparent molecular mass of 150-200 kDa and 30–45 kDa1–3. However, more detailed analyses of sera using SDS polyacrylamide gel electrophoresis and ligand blotting with either 1251-IGF-I or 125I-IGF-II, identifies at least 5 bands. The approximate size of these binding proteins in rat serum when analyzed on reduced gels is 42, 40, 39, 30, 29 and 24 kDa 4–6. In some case where antibody probes are available it has been possible to identify the various binding proteins detected by ligand blotting however since a number of IGF binding proteins appear to be of similar molecular weight, it is possible that a band identified by ligand blotting may consist of more than one binding protein. Although the exact number of these binding proteins present in serum remains to be determined, evidence for the existence of at least five distinct binding proteins has been presented in the recent literature. Four of these binding proteins have been extensively characterized and cDNAs encoding these binding proteins have been cloned7–11.