A POTENTIAL ROLE FOR ENDOTHELIN-1 IN HUMAN PLACENTAL GROWTH - INTERACTIONS WITH THE INSULIN-LIKE GROWTH-FACTOR FAMILY OF PEPTIDES

Endothelin-I (Et-1) stimulated DNA synthesis in placental fibroblasts in a dose-dependent manner, as measured by [H-3]thymidine incorporation (ED50, 0.2-0.3 ng/mL). Insulin-like growth factor-I (IGF-I) interacted synergistically with Et-I to potentiate the stimulation of DNA synthesis. Additionally, Et-1 stimulated the turnover of phosphoinositides in a time- and concentration-dependent manner (ED50, 1 ng/mL), as measured by a 2- to 3-fold increase in the total accumulation of [H-3]inositol phosphates. This was accompanied by a 2- to 3-fold rise in intracellular calcium, as measured by fura-2 fluorescence. IGF-1, however, had no potentiating effect on Et-1-stimulated phosphoinositide turnover or increase in cytosolic Ca2+. The ability of Et-1 to stimulate the production of IGF-11 and IGFBPs by placental fibroblasts was then studied. Western ligand blot analysis using an [I-125]IGF-II probe revealed the presence of six major binding proteins corresponding to 42, 38, 35, 32, 31, and 24 kilodaltons. Et-1 (50 ng/mL) stimulated all binding fractions concordantly. This was accompanied by a similar increase in immunoreactive IGF-11 secretion, as assessed by a specific RIA. No increase in immunoreactive IGF-I was observed. The ability of the placenta to produce Et-1 was examined by Northern blot analysis. Placentae at 14 and 17 weeks gestation expressed small amounts Et-1 mRNA, whereas significantly higher levels of mRNA were expressed at term. These data suggest that the human placenta produces Et-1 in a developmentally regulated manner that may act via paracrine and/or autocrine mechanisms to regulate placental growth.