Akt2 and Akt3 seem not to play a major role in placental angiogenesis because Akt2-null mice display a type-II diabetes-like syndrome and mild growth retardation and age-dependent loss of adipose tissue [121] and Akt3 has been shown to be important in postnatal brain development [31]. The potent vasodilator NO is generated during the conversion of l-arginine to l-citrulline by a family of NO synthases (NOS), including eNOS, inducible NOS (iNOS) and neuronal NOS (nNOS) [106]. Placental
NO production increases during pregnancy, which selleck products is highly correlated with eNOS, but neither iNOS nor nNOS expression [127, 88], suggesting that eNOS is the major NOS isoform responsible for the increased NO in the placenta. During normal sheep pregnancy placental NO production increases [127, 69] in association with elevated local expression of VEGF and FGF2, vascular density, and blood flow to the placentas [128, 9], suggesting that eNOS-derived NO is important in placental angiogenesis. Indeed, the eNOS-derived NO is critical for the VEGF and FGF2- stimulated angiogenesis in vitro [76, 24] and in vivo [44]. The eNOS-derived
NO is also a potent vasodilator in the perfused human muscularized fetoplacental vessels [87], which might be critical for the maintenance of low vascular resistance in the fetoplacental circulation in pregnant sheep in vivo [18]. Early studies have shown that pharmacological NOS inhibition by l-NG-nitroarginine methyl ester results in preeclampsia-like symptoms and reduced litter size in rats [11]. This has been confirmed in eNOS-null mice whose dams develop proteinuria
[68] and fetuses PI3K inhibitor are growth restricted [68, 67, 66]. In eNOS-null pregnant mice, uteroplacental remodeling is impaired and their vascular adaptations to pregnancy are dysregulated [66, 114], resulting in decreased uterine and placental blood flows and greatly reduced vascularization in the placenta [67, 66]. These Lonafarnib concentration studies suggest that eNOS is critical for both vasodilation and angiogenesis, that is, the two rate-limiting mechanisms for blood flow regulation at the maternal–fetal interface. Numerous studies have shown that activation of the MAPK (ERK1/2, JNK1/2, and p38MAPK), PI3K/Akt1, and eNOS/NO pathways is critical for VEGF- and FGF2-stimulated angiogenesis in various endothelial cells. In placental endothelial cells, we have shown that activation of the MAPK pathways are important for the differential regulation of placental endothelial cell proliferation, migration, and tube formation (i.e., in vitro angiogenesis) in response to VEGF and FGF2 stimulation in vitro [130, 82, 35, 36]. Inhibition of the ERK1/2 pathway partially attenuates the FGF2-stimulated cell proliferation, whereas it completely blocks the VEGF-stimulated cell proliferation as well as the VEGF- and FGF2-stimulated cell migration [75, 76, 130, 35, 36].