Furthermore, in maternal caruncle and fetal cotyledonary tissues, expression of VEGF and Flt1 and KDR is highly correlated positively to placental vascularization and uteroplacental and fetoplacental blood flows in pregnant ewes [128, 9], suggesting that the VEGF-VEGFR system is critically involved in placental angiogenesis. VEGF has been shown to regulate all steps of the angiogenesis process. It stimulates endothelial expression of proteases such as urokinase-type and tissue-type plasminogen activators and interstitial collagenase that break down extracellular

matrix and release endothelial cells from anchorage, allowing them to migrate and proliferate buy FK228 [94, 113]. In vitro, VEGF strongly stimulates placental endothelial cell proliferation and migration as well as the formation of tube-like structures on matrigel [75, 76]. VEGF can activate endothelial cells, generating various vascular active agents that themselves affect angiogenesis. For example, VEGF strongly stimulates placental artery endothelial production of NO [81, 130], which DMXAA chemical structure serves as a potent vasodilator and angiogenic factor in the placenta [129] as it does in other organs [45, 44]. VEGF can also recruit pericytes to the newly formed vessels [4] and participates in the continued survival [46] of nascent endothelial cells, both

of which promote the maturation and vessel stability of the newly formed vessels [53]. Interestingly, Bates et al. described a novel group of VEGF splice variants that were named VEGFXXXb, such as VEGF121b (-)-p-Bromotetramisole Oxalate and VEGF165b [6, 48]. They are also encoded by the VEGF gene but with alternative splicing at the distal site in the terminal

exon (called exon 9) that differs from the terminal exon 8 for the conventional VEGF isoforms, which encode their last six amino acids [6]. Thus, VEGFxxxb and the conventional sister VEGFxxx have different sequences but with the same size; however, they seem to possess opposite functions in angiogenesis. For example, VEGF165b inhibits VEGF165-mediated endothelial cell proliferation and migration in vitro and VEGF165-mediated vasodilation ex vivo [6] as well as angiogenesis in vivo [120]. In tumors such as renal cell carcinoma VEGF165b is significantly decreased [6]. Downregulation of VEGF165b leads to metastatic melanoma, while overexpression of VEGF165b prevents metastasis of malignant melanoma [97]. These observations support an anti-angiogenic role of VEGF165b. Apparently, the discovery of VEGFxxxb has raised a critical question as to whether the existing VEGF literature needs to be reevaluated with new reagents and methods that can differentiate the pro-angiogenic VEGFxxx from the anti-angiogenic VEGFxxxb isoforms.