Sequestration and secretion of insulin‐like growth factor‐I by bovine aortic endothelial cells

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Abstract
Endothelial cells elaborate growth promoting activities in culture medium that support limited smooth muscle cell and fibroblast growth in vitro in the absence of serum. We investigated whether insulin-like growth factor-I (IGF-I) was synthesized and secreted by bovine aortic endothelial cells in vitro. Subconfluent endothelial cell cultures in serum-free medium secreted severalfold higher IGF-I levels than confluent cultures by acid-sizing chromatography and IGF-I radioimmunoassay. The IGF-I secretory level was not sustained during a second serum-free incubation. In contrast, secretion of IGF binding proteins persisted and was maintained at constant levels throughout the same observation periods. Analysis of poly (A+)RNA by northern blots revealed hybridization of an IGF-I cDNA to a 7.5- to 7.0-kb transcript and superinduction of the 7.5–7.0-kb mRNA by the translational inhibitor, cyclohexamide. However, no endogenously labeled IGF-I was detected in conditioned media after incubation of cultures with [35S]cysteine or [3H]leucine. When cultures were incubated in the presence of serum supplemented with IGF-I, subconfluent cultures sequestered and released more IGF-I than confluent cultures. We concluded that the majority of IGF-I secreted in vitro was sequestered from serum.