Regulation of production of a platelet‐derived growth factor‐like protein by cultured bovine aortic endothelial cells

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Abstract
Platelet‐derived growth factor (PDGF) is a potent mitogen for cultured cells of mesenchymal origin. Known sources of PDGF or PDGF‐like protein are blood platelets, several transformed cell lines, and cultured endothelial cells (EC). We have examined the regulation of production of a PDGF‐like protein in cultures of bovine aortic EC using a specific radioreceptor assay for PDGF. EC constitutively secreted PDGF‐like protein into serum‐containing or serum‐free medium. The rate of production of PDGF‐like protein was constant for at least 3 weeks and was not due to release of an internal store, since cell lysis by repeated freeze/thaw cycles did not relase significant amounts of the protein. Synthesis of PDGF‐like protein was sensitive to changes in the pH of the media and was maximal at pH 8.5. Production of PDGF‐like protein was independent of EC growth rate: rapidly dividing cells and confluent, quiescent cells produced equal amounts per cell. However, sparse, quiescent EC produced more PDGF‐like protein per cell than did confluent, quiescent cells. Several phorbol esters stimulated production of PDGF‐like protein. At a concentration of 10−6 M, a twofold stimulation was observed upon addition of the tumor promoter 12‐O‐tetradecanoylphorbol 13‐acetate (TPA) and nearly a fourfold stimulation upon addition of the nonpromoting analog, methyl TPA. Incubation of EC with endotoxin (10 μ/ml) resulted in a twofold stimulation of PDGF‐like protein production. In all experiments with endotoxin and phorbol esters, an increase in the production of PDGF‐like protein was accompanied by morphological changes in the EC cultures. The cells appeared elongated and fibroblastic and exhibited low viability. A mathematical model was developed in which PDGF‐like protein production was shown to consist of two separate components—production at a constant rate by healthy cells and a large burst of synthesis and secretion by dying cells. These results suggest that injurious agents may be capable of stimulating production of a growth factor by the endothelium.