To examine a possible role for IGF-II in the regulation of IGF-I receptors we measured 125I-IGF-I binding on IM-9 cells following pre-incubation with IGF-II/IGF-I mixtures, purified MSA (a rat IGF-II-like peptide), pure IGF-I, or insulin. Whereas all preparations tested induced down-regulation of IGF-I binding after 20 hours, distinct differences were noted after six hour pre-incubation: IGF-I (100 ng/ml) and insulin 1 μg/ml) both induced down-regulation of IGF-I binding (15 ± 2% and 19 ± 2% respectively). However, a mixture of IGF-II and IGF-I (100 ng/ml each) induced consistent up-regulation of IGF-I binding (16 ± 2%) (mean ± SE, n = 14), and a preparation enriched in IGF-II (250 ng/ml IGF-II and 75 ng/ml IGF-I) induced 20 ± 5% (n = 3) up-regulation at six hours. Purified MSA (200 ng/ml) induced 15% up-regulation of IGF-I binding at six hours. Scatchard analysis of displacement curves showed that increased binding was due to loss of low affinity binding, with enhancement of high affinity sites. The up-regulation of IGF-I binding was unaffected by treatment with 0.1 mM cycloheximide, but was blunted by 5 μM colchicine. It is concluded that 1. IGF-II induces up-regulation of IGF-I receptors on IM-9 cells following 6 hour pre-incubation; 2. This phenomenon is not mimicked by the structurally-related peptides IGF-I or insulin; the up-regulation is due to enhanced high affinity binding sites. It is independent of protein synthesis, but involves microtubular function; 4. IGF-II may indirectly modulate growth in some tissues by regulation of IGF-I receptors.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical