Stimulation by Somatomedin-C of Aminoisobutyric Acid Uptake in Human Fibroblasts: A Possible Test for Cellular Responsiveness to Somatomedin*

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Abstract
The effect of purified somatomedin-C (Sm-C)/ insulin-like growth factor I on the uptake of a-aminoisobutyric acid (AIB) by confluent cultures of human fibroblasts was studied. An increase in [3]AIB uptake was observed within 30 min of Sm-C addition, and a maximal effect was reached at 2.5 h (averaging 200% of control AIB uptake). Under the conditions employed, less Sm-C was required for maximal stimulation of [3H]AIB uptake (10 ng/ml in most cell lines) than for a maximal effect on [3H]thymidine incorporation (>30 ng/ml). In multiple experiments with different lines of foreskin-derived and nongenital fibroblasts, the concentration of Sm-C resulting in half-maximal stimulation of [3H]AIB uptake was reproducible and was between 1.7 and 4.8 ng/ml in all cell lines tested except one. No significant difference was observed in the Sm responsiveness of cells from newborns and that of thosefrom normal older children. Determination of binding of [125I] Sm-C to confluent monolayers of one fibroblast line revealed that the concentration of Sm-C resulting in half-maximal binding was nearly identical to that producing half-maximal [3H] AIB uptake stimulation. Stimulation of AIB uptake by an optimal concentration of epidermal growth factor (5 ng/ml) was also determined in several normal cell lines and was consistently close to 140% of the control value. Since epidermal growth factor and Sm interact with different receptors, this response may be a useful measure of cell integrity which is independent of the Sm receptor. The techniques for measurement of AIB uptake described in this report may prove useful in determining whether some children with growth failure of unknown etiology have target cell resistance to Sms.

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