Cyclic adenosine monophosphate stimulates insulin-like growth factor binding protein-4 and its messenger ribonucleic...

Endothelial cells regulate the passage of insulin-like growth factors (IGFs) or IGFs complexed to IGF-binding proteins (IGFBPs) from plasma to the extravascular space, and in addition respond to plasma and tissue IGFs. The IGFBPs are thought to determine the availability and localization of IGFs to tissues, and to inhibit or potentiate their biological activity. Because of the potential importance of the IGF system in endothelial cell pathophysiology, and because IGFBPs modulate IGF action, we have characterized the IGFBPs synthesized by a clonal endothelial cell line (BPE-1) established from bovine parathyroid microvessels. The only IGFBPs identified by ligand blotting in media conditioned by BPE-1 cells were N-glycosylated 28 kilodalton and non-N-glycosylated 24 kilodalton IGFBP-4 species. Both forms were immunoprecipitated by antibodies to human IGFBP-4. Northern blot hybridization of BPE-1 RNA identified messenger RNAs for IGFBP-4 and IGFBP-6, but not for IGFBP-1, 2, 3, or 5. Agents that increase cAMP including forskolin, (Bu)2cAMP, isobutyl-methylxanthine, and histamine increased IGFBP-4 and IGFBP-4 messenger RNA in BPE-1 cells 2- to 5-fold and 2- to 3-fold, respectively, similar to results previously reported in human osteosarcoma cells and fibroblasts. Increased IGFBP-4 was detected in BPE-1 media 6 h after forskolin addition and was maximal after 24 h. Maximal stimulation (6- to 9-fold) was observed with 1-30 microM forskolin. IGFBP-4 also was the predominant IGFBP synthesized by two other endothelial cell lines, a clonal cell line established from bovine bone microvessels, and a polyclonal cell line established from calf pulmonary artery. Further study is required to determine the role of endothelial cell IGFBP-4 on endothelium and adjacent cells, and on the transport of IGFs from plasma to specific subendothelial sites.