Hypoxia is a potent stimulus to angiogenesis. Expression of the angiogenic growth factor vascular endothelial growth factor (VEGF) and its receptors (VEGFR-1 and VEGFR-2) is up-regulated by hypoxia in a variety of organs and cell lines. We have previously reported that VEGF expression is not increased in renal ischemia-reperfusion injury, although tubular cells concentrate VEGF at their basolateral surface. In this study we assess whether altered VEGF receptor expression compensates for the lack of VEGF regulation during renal ischemia-reperfusion injury.
VEGFR-1 mRNA expression was assessed by Northern blotting and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). VEGFR-2 mRNA expression was analyzed by Northern blotting and in situ hybridization (ISH), while VEGFR-2 protein expression was studied using immunohistochemistry. VEGF mRNA expression was assessed by ISH.
VEGFR-2 mRNA and protein expression were up-regulated without an increase in VEGF or VEGFR-1 expression. Normal kidneys showed low-level VEGFR-2 mRNA and protein expression in glomerular and peritubular endothelium. Following ischemia and ischemia-reperfusion, a marked increase in VEGFR-2 mRNA and protein expression was seen (2- to 4-fold). Most prominent was VEGFR-2 mRNA up-regulation in the glomerulus although, surprisingly, increased protein was not demonstrated here. ISH showed that VEGF mRNA was not up-regulated in this model, confirming our previous findings for VEGF.
VEGF and VEGFR-1 expression are not increased by renal ischemia and ischemia-reperfusion injury. Instead, endothelial expression of VEGFR-2 is increased. VEGFR-2 up-regulation in renal ischemia-reperfusion may be important in mediating the mitogenic and anti-apoptotic actions of VEGF on endothelial cells, thereby preserving the integrity of the endothelium and the potential for blood supply to ischemic tissues.