RAT C6 GLIAL CELLS EXPRESS INSULIN-LIKE GROWTH FACTOR (IGF) RECEPTORS AND SYNTHESIZE AND SECRETE IGF-I

Abstract

The IGFs have been proposed to play a role in growth and maintenance of cells in the central nervous system (CNS). In the present studywe use the C6 rat glial cell line as a model system to study the role of IGFs in the CNS. Compotitive binding and affinity crosslinking experiments using 125I-IGF-I and -II demonstrated type I and type [I IGF receptors on C6 cells; insulin receptors were not detected. [3H- thymidine incorporation into C6 cell DNA was stimulated equally well by serum from normal and hypox (IGF-deficient) rats. Furthermore, exogenous IGFs did not stimulate DNA synthesis further when added to hypox rat serum. These results suggested that C6 cells might synthesize an IGF-like peptide. Therefore,serum-free conditioned medium from C6 cultures was gel filtered on a Sephadex G-75 column in IM acetic acid. IGF-I and -II were measured using specific radioimmuno-assays, IGF binding proteins determined using a charcoal-separation assay. IGF-I and IGF carrier protein, but not IGF-II were detected. Northern blot hybridization of C6 mRNA confirmed the presence of IGF-I mRNA and the absence of IGF-II mRNA. The level of IGF-I was 0.4-4 ng/ml based on a human IGF-I standard. The IGF-I like material inhibited 125I-IGF-I binding to the type I receptor of chick embryo fibroblasts and stimulated [3H]-thymidine incorporation into fibroblast DNA. We conclude that C6 glial cells possess IGF receptors, and synthesize and secrete IGF-I that is receptor-active and bioactive. We propose that IGF-I may serve as an autocrine growth stimulus in C6 glial cell cultures.