Receptors for Insulin-Like Growth Factors and Growth Effects of Multiplication-Stimulating Activity (Rat Insulin-Like Growth Factor II) in Rat Embryo Fibroblasts*

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Abstract
Levels of multiplication-stimulating activity (MSA) in fetal rat serum are high (2–4 μg/ml), suggesting that MSA may have a role in fetal growth. We now demonstrate that fibroblasts derived from rat embryos (REFs) have specific MSA receptors and respond to MSA with increased DNA synthesis. Two types of insulin-like growth factor (IGF) receptors were demonstrated by competitive binding of radioiodinated MSA, GF-I, and IGF-II and by chemical cross-linking of [125I]iodo- MSA and [125I]iodo-IGF-I to REFs. One type of receptor (mol wt, 260,000 under reducing conditions) did not interact with insulin, and another type of receptor (mol wt, 130,000, under reducing conditions) was recognized by insulin. Scatchard analysis of [125I]iodo-MSA binding data was consistent with one class of noninteracting binding sites. A biological response of MSA, with increased DNA synthesis, was demonstrated with autoradiography in REFs. During a 16-hr incubation, DNA synthesis was stimu ated by normal rat serum, and platelet-poor plasma plus platelet-derived growth factor (PDGF), but not by serum from hypophysectomized (hypox) rats or hypophysectomized (hypox) platelet-poor plasma plus PDGF. However, when MSA was added to hypox serum or to hypox platelet-poor plasma plus PDGF, DNA synthesis was stimulated to the level achieved by normal rat serum. By contrast, during a longer cell multiplication experiment, REFs grew equally well in normal or hypox rat serum, raising the possibility that REFs may produce a MSAlike factor.

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