Growth factor-induced release of placental alkaline phosphatase from human syncytiotrophoblast membranes

The ability of epidermal growth factor, insulin or guanosine thiotriphosphate to induce the release of two glycosyl-phosphatidylinositol-linked proteins from isolated human placental syncytiotrophoblast plasma membrane vesicles was investigated. Epidermal growth factor induced the ATP-dependent release of a fraction of syncytiotrophoblast plasma membrane placental alkaline phosphatase, whereas no release was detected following insulin treatment. This effect of epidermal growth factor was apparent at 30 min but not at 5 min. Guanosine thiotriphosphate stimulated the release of a small amount of syncytiotrophoblast plasma membrane placental alkaline phosphatase and appeared to have an additive effect when applied together with epidermal growth factor. Guanosine thiodiphosphate did not induce phosphatase release, but partially inhibited the epidermal growth factor response. 28.7% of syncytiotrophoblast plasma membrane 5'-nucleotidase was solubilized using glycosyl-phosphatidylinositol-specific phospholipase C. However, unlike placental alkaline phosphatase, no detectable release of 5'-nucleotidase was observed following treatment of syncytiotrophoblast plasma membrane vesicles with epidermal growth factor or guanosine thiotriphosphate. These results indicate (i) the presence of at least two placental alkaline phosphatase-releasing pathways in syncytiotrophoblast plasma membrane vesicles, and (ii) the presence of subpopulations of glycosyl-phosphatidylinositol-linked proteins sensitive to growth factor-induced release.