Prostaglandin E₂ regulation of amnion cell vascular endothelial growth factor expression: Relationship with intramembranous absorption rate in fetal sheep

We hypothesized that prostaglandin E₂ (PGE₂) stimulates amniotic fluid transport across the amnion by upregulating vascular endothelial growth factor (VEGF) expression in amnion cells and that amniotic PGE₂ concentration correlates positively with intramembranous (IM) absorption rate in fetal sheep. The effects of PGE₂ at a range of concentrations on VEGF₁₆₄ and caveolin-1 gene expressions were analyzed in cultured ovine amnion cells. IM absorption rate, amniotic fluid (AF) volume, and PGE₂ concentration in AF were determined in late-gestation fetal sheep during control conditions, isovolumic fetal urine replacement (low IM absorption rate), or intra-amniotic fluid infusion (high IM absorption rate). In ovine amnion cells, PGE₂ induced dose- and time-dependent increases in VEGF₁₆₄ mRNA levels and reduced caveolin-1 mRNA and protein levels. VEGF receptor blockade abolished the caveolin-1 response, while minimally affecting the VEGF response to PGE₂. In sheep fetuses, urine replacement reduced amniotic PGE₂ concentration by 58%, decreased IM absorption rate by half, and doubled AF volume (P < 0.01). Intra-amniotic fluid infusion increased IM absorption rate and AF volume (P < 0.01), while amniotic PGE₂ concentration was unchanged. Neither IM absorption rate nor AF volume correlated with amniotic PGE₂ concentration under each experimental condition. Although PGE₂ at micromolar concentrations induced dose-dependent responses in VEGF and caveolin-1 gene expression in cultured amnion cells consistent with a role of PGE₂ in activating VEGF to mediate AF transport across the amnion, amniotic PGE₂ at physiological nanomolar concentrations does not appear to regulate IM absorption rate or AF volume.