Insulin-like growth factor-I (IGF-I) has been known to be mitogenic to a variety of cell types, although a growth-regulatory role for IGF-I on bovine mammary epithelial cells has not been fully investigated. In the present study, we examined the receptor binding of IGF-I and its effect on growth in a bovine mammary epithelial cell line (MAC-T3). Specific receptors for IGF-I were detected on cultured bovine mammary epithelial cells. Competitive binding revealed that half-maximal inhibition of 125I-labelled IGF-I binding by IGF-I was approximately 3 μg/l. Dissociation rate constant of the IGF-I receptor was 3·10±0·06 nmol/l (s.e.m.) with a receptor site concentration of 366 ± 8 fmol/mg protein for the average of three experiments. IGF-I exerted a positive mitogenic effect on MAC-T3 cells according to both direct DNA assay and thymidine incorporation assay. Moreover, the mitogenic effect of IGF-I on MAC-T3 cells was enhanced by the addition of fetal calf serum in the culture media. The present results suggest that the bovine mammary epithelial cell line (MAC-T3) provides a useful model system with which to study the biological actions of insulin-like growth factors on the bovine mammary secretory tissue in vitro.
Journal of Endocrinology (1992) 134, 307–312
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