No abstract

The development and endocrine function of the ovarian corpus luteum (CL) are dependent on the growth of new capillary vessels. Although several molecules have been implicated as mediators of CL angiogenesis, at present there is no direct evidence for the involvement of any. Here we report the unexpected finding that treatment with truncated soluble Flt-1 receptors, which inhibit vascular endothelial growth factor (VEGF) bioactivity, resulted in virtually complete suppression of CL angiogenesis in a rat model of hormonally induced ovulation. This effect was associated with inhibition of CL development and progesterone release. Failure of maturation of the endometrium was also observed. Areas of ischemic necrosis were demonstrated in the corpora lutea (CLs) of treated animals. However, no effect on the preexisting ovarian vasculature was observed. These findings demonstrate that, in spite of the redundancy of potential mediators, VEGF is essential for CL angiogenesis. Furthermore, they have implications for the control of fertility and the treatment of ovarian disorders characterized by hypervascularity and hyperplasia.

No abstract

Vascular endothelial growth factor (VEGF) has two highly homologous tyrosine kinase receptors: Flt‐1 (VEGFR‐1) and KDR (VEGFR‐2). KDR is strongly phosphorylated on tyrosines and can transmit mitogenic and motogenic signals following VEGF binding, while Flt‐1 is markedly less effective in mediating such functions. To dissect the regions that account for the differences between the two receptors, we generated a series of chimeric Flt‐1–KDR molecules. We found that the juxtamembrane region of Flt‐1 prevents key signaling functions. When the juxtamembrane region of Flt‐1 is replaced by that of KDR, Flt‐1 becomes competent to mediate endothelial cell migration and phosphatidylinositol 3′‐kinase activation in response to VEGF. Further mutational analysis shows that a short divergent sequence is responsible for such repressor function. However, mutant Flt‐1 receptors lacking this sequence do not transmit effective proliferative signals, suggesting that this receptor function is regulated separately. These results define a novel functional domain that serves to repress Flt‐1 activity in endothelial cells.

Vascular endothelial growth factor (VEGF) is an angiogenic inducer that mediates its effects through two high affinity receptor tyrosine kinases, Flt‐1 and KDR. Flt‐1 is required for endothelial cell morphogenesis whereas KDR is involved primarily in mitogenesis. Flt‐1 has an alternative ligand, placenta growth factor (PlGF). Both Flt‐1 and KDR have seven immunoglobulin (Ig)‐like domains in the extracellular domain. The significance and function of these domains for ligand binding and receptor activation are unknown. Here we show that deletion of the second domain of Flt‐1 completely abolishes the binding of VEGF. Introduction of the second domain of KDR into an Flt‐1 mutant lacking the homologous domain restored VEGF binding. However, the ligand specificity was characteristic of the KDR receptor. We then created chimeric receptors where the first three or just the second Ig‐like domains of Flt‐1 replaced the corresponding domains in Flt‐4, a receptor that does not bind VEGF, and analyzed their ability to bind VEGF. Both swaps conferred upon Flt‐4 the ability to bind VEGF with an affinity nearly identical to that of wild‐type Flt‐1. Furthermore, transfected cells expressing these chimeric Flt‐4 receptors exhibited increased DNA synthesis in response to VEGF or PlGF. These results demonstrate that a single Ig‐like domain is the major determinant for VEGF‐PlGF interaction and that binding to this domain may initiate a signal transduction cascade.