Rat C6 Glial Cells Synthesize Insulin-Like Growth Factor I (IGF-I) and Express IGF-I Receptors and IGF-II/ Mannose 6-Phosphate Receptors*

Abstract

We have used the rat C6 glial cell line as a model system to study the role of insulin-like growth factors (IGF) in neuroglial cells of the central nervous system (CNS). Northern blot analysis of C6 RNA demonstrated the presence of IGF-I mRNA and undetectable IGF-II mRNA. IGF-I and IGF-binding protein(s), but not IGF-II, were detected in C6 glial cell-conditioned medium. The level of IGF-I was 1-4 ng/ml in conditioned medium based on a human IGF-I standard. The immunoreactive IGF-I inhibited [125I]IGF-I binding to the IGF-I receptor on chick embryo fibroblasts and stimulated [3H]thymidine incorporation into chick embryo fibroblast DNA. Competitive binding and affinity cross-linking experiments using [125]IGF-I and [125I]IGF-II demonstrated the presence of IGF-I receptors (type I) and IGF-II/mannose 6-phosphate receptors (type II) on C6 glial cell membranes. An immunoglobulin (no. 3637) directed against the rat IGF-II receptor blocked the degradation of [125I]IGF-II added to C6 glial cells, presumably by blocking receptor-mediated internalization. We were unable to demonstrate an autocrine role for IGF in the C6 glial cell line, since [3H]thymidine incorporation into DNA was stimulated equally well by IGF-I-deficient rat serum and normal serum, and added IGF did not stimulate [3H]thymidine incorporation into DNA when tested alone or when added to IGF-I-deficient serum. We propose that neuroglial cell-derived IGF-I may serve as a paracrine growth stimulus in the central nervous system.

Loss of tumorigenicity of rat glioblastoma directed by episome-based antisense cDNA transcription of insulin-like growth factor I.

“…2 depicts an autoradiogram from transfer blots of 10 ,g of total RNA of transfected and untransfected C6 and B-104 cells exposed to serum-free medium. The pattern of IGF-I transcripts in parental and transfected cells matches that published by us and others (6) showing prominent bands at 7.5, 1.8, and 1 kilobase (kb).…”

“…We (5) and others (6) have reported that the rat glioma cell line C6 expresses high levels of insulin-like growth factor I (IGF-I), which is enhanced when grown in serum-free medium. Therefore, IGF-I expression may be coupled to cellular proliferation and tumorigenicity.…”

“…Therefore, IGF-I expression may be coupled to cellular proliferation and tumorigenicity. Since C6 cells express IGF-I and IGF-II receptors (6), an autocrine loop involving IGF-I may exist and drive the tumorigenic phenotype. Until now, the autocrine hypothesis of malignant transformation has derived experimental support primarily from antibody blocking (7) and sense gene transfer studies.…”

Expression of IGF-I and IGF-II Receptors in Rat C6 Glioma Cells as a Function of the Growth Phase

“…C6 cells are capable of producing IGF-I and releasing it into the culture medium [35]. Possibly, the autocrine secretion of IGF-I peptides by the cells may cause this densitydependent change in receptor number.…”

Insulin‐Like Growth Factor‐II is a Substrate for Dipeptidylpeptidase I (Cathepsin C)

“…Serum-starved cells were incubated with MCDB 104 medium containing 0.1% BSA, 0.1 uM dexamethasone and 1 pg/ml transferrin (Furlanetto, 1988) to which was added serial dilutions of IGF-I, IGF-11, or DAP-1-treated IGF-11. After 22 hours, the cells were pulsed for one hour with [3H]thymidine and the monolayers were processed to determine the incorporation of radioactivity into trichloroaceticacid-precipitable material (Kiess et al, 1989).…”