A factor contained in plasma is required for IGF binding protein‐1 to potentiate the effect of IGF‐I on smooth muscle cell DNA synthesis

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Abstract
One of the forms of the insulin‐like growth factor (IGF) binding proteins present in human amniotic fluid has been shown to potentiate the growth‐promoting effect of IGF‐I markedly. This study was undertaken to determine the cellular and hormonal factors that modulate this potentiation and to determine whether this protein would potentiate the effects of other mitogens. Although the combination of the IGFBP‐1 (20 ng/ml) and IGF‐I (10 ng/ml) induced a five‐ to sixfold increase in DNA synthesis compared with IGF‐I alone, this response required the simultaneous addition of IGF‐I with 0.1% platelet‐poor plasma (PPP). If PPP was omitted from the incubation medium, no increase above the effect that was obtained with IGF‐I alone was noted. Substitution of cerebrospinal fluid (CSF) for PPP permitted a full mitogenic response, although substitution with amniotic fluid resulted in no enhancement. The factor contained in PPP was heat and acid stable. If the binding protein was co‐incubated with fibroblast growth factor (FGF), platelet‐derived growth factor (PDGF), or epidermal growth factor (EGF), a slight inhibition of the cellular response to each of these factors was detected. Co‐incubation of IGF‐I with the IGF‐binding protein plus these other peptide growth factors resulted in no further enhancement of DNA synthesis above the level observed with IGF‐I and the binding protein alone. Likewise, addition of plasma proteins such as transferrin or albumin did not result in a further enhancement of the DNA synthesis response to IGF‐I plus binding protein, and these proteins could not substitute for PPP or IGFBP‐1. Transient exposure of the cultures (2 hr) to the binding protein plus IGF‐I resulted in a submaximal DNA synthesis response, and the binding protein had to be present continuously to achieve a maximal effect. These studies indicate that a factor contained in plasma and CSF is required for a maximal cellular response to IGFBP‐1 plus IGF‐I, and this factor does not appear to be a well‐defined mitogen.

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