Ligand-receptor interactions involved in the stimulation of Swiss 3T3 fibroblasts by insulin-like growth factors and insulin.
1. The binding of 125I-labelled insulin-like growth factor 1 (125I-IGF-1) to Swiss mouse 3T3 fibroblasts was time- and concentration-dependent. Unlabelled IGF-1 had a slightly higher potency than multiplication-stimulating activity (MSA) in inhibiting the binding of 125I-IGF-1, and insulin gave a parallel inhibition curve at 300-1000-fold lower potency. Chemical cross-linking of bound 125I-IGF-1 to its receptors, followed by polyacrylamide-gel electrophoresis under reducing conditions, revealed a major band of Mr 130,000, the labelling of which was inhibited by IGF-1 or high concentrations of insulin. 2. The binding of 125I-IGF-1 was not affected by either co-incubation or preincubation of the cells with a range of heterologous growth factors and mitogens. However, IGF-1 and MSA each induced down-regulation of 125I-IGF-1 binding sites. 3. The maximal stimulations of DNA synthesis induced by IGF-1, MSA and insulin, in the presence of a synergizing mitogen, were similar. The dose-response curve for insulin was not parallel to those for IGF-1 and MSA; in particular, low concentrations of insulin induced a greater stimulation than expected on the basis of its potency in the inhibition or down-regulation of 125I-IGF-1 binding. 4. The preincubation of 125I-IGF-1 with Swiss 3T3 cells at 37 degrees C decreased its ability to bind to a second batch of cells. This inactivation did not occur when the preincubation was performed at 4 degrees C or in the presence of cycloheximide. Chemical cross-linking revealed that the cells released an IGF-binding protein, giving a complex of Mr about 48,000. 5. It is concluded that type I IGF receptors mediate the stimulation of Swiss 3T3 cells by insulin-like mitogens, but that insulin probably stimulates the cells through insulin receptors. The cells can modulate the amount of ligand binding, both by down-regulation of the receptors and by the secretion of an IGF-binding protein.
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AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge, U.K.
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D000818:Animals; D002460:Cell Line; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008024:Ligands; D051379:Mice; D008934:Mitogens / Q000494:pharmacology; D011972:Receptor, Insulin / Q000187:drug effects / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000494:pharmacology*; D013268:Stimulation, Chemical
D007328:Insulin; D008024:Ligands; D008934:Mitogens; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D011972:Receptor, Insulin