In serum-free medium, insulin-like growth factor-I/somatomedin-C (IGF-I/SM-C) was weakly mitogenic for adult human fibroblasts in culture. However, in the presence of 0.5% human hypopituitary serum (HHS), which by itself had little effect, there was a marked dose-dependent response to IGF-I/SM-C with a 10- to 20-fold increase in [3H]thymidine incorporation at 25 ng/ml IFG-I/SM-C. With the further addition of dexamethasone or hydrocortisone to the combination of IGF-I/SM-C + 0.5% HHS, there was a dramatic synergistic effect resulting in a 60- to 70-fold increase in [3H]thymidine incorporation. This stimulation was two times greater than that seen with 20% FCS. In contrast, glucocorticoids had no effect in serum-free medium or with HHS alone. These [3H]thymidine incorporation results were clearly supported by cell replication studies. Dose-response curves for 125I IGF-I/SM-C binding and IGF-I/SM-C stimulation of [3H]thymidine incorporation were similar with 1/2 maximal effects for both at 5 ng/ml. However, the striking synergism seen with glucocorticoids occurred in the absence of any glucocorticoid-induced change in IGF-I/SM-C binding, indicating that the interaction of IGF-I/SM-C and glucocorticoids occurs at a postreceptor level. These data demonstrate that in the presence of a low concentration of HHS, IGF-I/SM-C and glucocorticoids stimulate complete cell cycle traverse and replication of human fibroblasts.

We have investigated the effects on carbohydrate metabolism of human GH produced by recombinant DNA technology (methionyl-hGH) compared with pituitary hGH. Twelve normal adult male subjects received four daily im injections of either methionyl-hGH or pituitary hGH in a double blind, crossover study. Oral glucose tolerance tests and assays of insulin binding to peripheral monocytes were performed before th initial administration and 12 h after the fourth injection of both hGH preparations. Both methionyl-hGH and pituitary hGH resulted in significant carbohydrate intolerance, with a rise in fasting plasma glucose from 96.6 +/- 2.9 to 105.9 +/- 3.0 mg/ml (mean +/- SEM) after pituitary hGH and from 96.2 +/- 1.5 to 107.5 +/- 3.3 mg/dl after methionyl-hGH (P less than 0.01). The area under the glucose tolerance curve increased by 34% after pituitary hGH and by 37% after methionyl-hGH. With both hGH preparations, carbohydrate intolerance was associated with marked hyperinsulinemia, with a rise in fasting plasma insulin levels from 9.4 +/- 1.2 to 33.2 +/- 7.8 microU/ml after pituitary hGH and from 7.4 +/- 1.1 to 45.8 +/- 11.1 microU/ml after methionyl-hGH (P less than 0.01). The integrated plasma insulin levels during the oral glucose tolerance test tripled after both hGH preparations. The pronounced insulin resistance could not be attributed to an alteration in insulin receptor concentrations. Both hGH preparations were associated with small reductions in insulin binding to monocytes at tracer concentrations, but the decline in binding was not statistically significant. The calculated binding sites per cell and Ke were not significantly altered by hGH administration. We conclude that methionyl-hGH and pituitary hGH are indistinguishable in their ability to induce insulin-resistant carbohydrate intolerance. This decrease in insulin sensitivity cannot be attributed to an alteration in insulin binding, and presumably represents a postreceptor defect in insulin action.

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