Constitutive Secretion of an Endogenous Insulin-like Peptide Binding Protein with High Affinity for Insulin in Spodoptera frugiperda (Sf9) Cell Cultures

Abstract

Serum-free medium from batch cultures of Sf9 insect cells was examined for the occurrence of proteins related to the insulin-like growth factor family. We found that the Sf9 cell line constitutively produced and secreted a soluble protein with a MW of 27 kDa that exerted specific binding to human insulin-like growth factor-I (IGF-I) and -II. Moreover, the secreted protein bound human insulin and human proinsulin with higher affinity than IGF-I and -II. The order of affinity to the insulin peptides, determined by competitive inhibition of ligand binding, was: insulin > proinsulin > IGF-I ⪢ IGF-II. The dissociation constant (kd) for IGF-II was 28.5 ± 1.7 nM and for insulin 7.2 ± 1.3 nM, as determined by Scatchard plot analysis. The results suggest that the Sf9 cells produce an insulin binding protein similar to the human insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1).