Hypoxia down-regulates placenta growth factor, whereas fetal growth restriction up-regulates placenta growth factor expression: Molecular evidence for "placental hyperoxia" in intrauterine growth restriction

Early placental development occurs in an environment of relative hypoxia. H ypoxia promotes angiogenesis and up-regulates vascular endothelial growth f actor (VEGF) expression while it dawn-regulates placenta growth factor (PIG F) that possess 53% homology with VEGF. Morphological studies show poor pla cental vascular development and an increase in the mitotic index of cytotro phoblasts in intrauterine growth restriction (IUGR). We hypothesized that t he reported relatively high oxygen level in the intervillous space in conta ct with IUGR placental villi will limit angiogenesis by changes in VEGF and PIGF expression and function. Western immunoblot analysis demonstrates a d iametric expression of PIGF and VEGF proteins throughout pregnancy with PIG F levels increasing and VEGF levels decreasing, consistent with placental o xygenation. In IUGR placentae, the ratio of PIGF/GAPDH mRNA was increased b y 2.3-fold (p < 0.03) and PIGF protein levels were also increased, Go < 0.0 5) as compared with gestationally-matched normal placentae. PIGF mRNA and p rotein were localized to the trophoblast bilayer and villous mesenchyme of the human placenta throughout gestation. In vitro studies demonstrated that increasing oxygen tension (hyperoxia) up-regulated PIGF protein in term pl acental villous explants, whereas hypoxic culture of a term trophoblast cho riocarcinoma cell line (BeWo) down-regulated PIGF mRNA and protein and VEGF R-1 (Flt-1) autophosphorylation. The addition of PIGF-1 to a spontaneously transformed first trimester cytotrophoblast cell line stimulated DNA synthe sis while PIGF-2 had little effect. VEGF and PIGF exert their biological ac tions by means of a common receptor VEGFR-1. In the first trimester trophob last cells, PIGF-1 increased the association of phosphorylated extracellula r signal-related kinase (ERK) with VEGFR-1 immunoprecipitates while both PI GF-1 and PIGF-2 also potentiated endogenous VEGF mediated association of ph osphorylated extracellular related kinase (ERK) with VEGFR-2 (KDR). More im portantly, the addition of PIGF-1 had little effect while PIGF-2 inhibited cell growth in cultured endothelial cells derived from human umbilical vein . Nitric oxide (NO) is reported to promote angiogenesis and PIGF-2 inhibite d the basal release of NO from the first trimester trophoblast. The tissue expression and functional studies support the hypothesis of "placental hype roxia" in early-onset IUGR because hypoxia down-regulates trophoblast PIGF levels, PIGF expression is increased in IUGR, and PIGF-2 inhibits endotheli al cell growth. Taken together, these changes provide a cellular explanatio n for the observed poor angiogenesis in the pathogenesis of IUGR and show t hat the two PIGF isoforms may modulate trophoblast and endothelial cell fun ction differently, possibly through potentiation of VEGF mediated activatio n of VEGF-2.