Mesangial cell proliferation and growth factor over-expression are characteristic features of several glomerular diseases. Vascular endothelial growth factor (VEGF), a potent mitogen, is expressed in podocytes in the glomerulus, and VEGF receptors (flt-1, KDR, and neuropilin-1) are present on endothelial cells and other cell types. This study examined whether human mesangial cells (HMC) express VEGF receptors in vitro and ex vivo and evaluated the effect of VEGF on HMC proliferation. All receptor types were detected in HMC in vitro by immunofluorescence and Western blotting. VEGF(165) induced a dose-responsive increase in (3)H-thymidine incorporation (25 ng/ml VEGF(165) : 2.3-fold increase; 50 ng/ml : 3.8-fold; 100 ng/ml : 4. 8-fold; 200 ng/ml : 3.4-fold; P = 0.016) and in cell number (50 ng/ml VEGF(165) : 1.2-fold increase; 100 ng/ml : 1.6-fold; 200 ng/ml : 1.4-fold; P = 0.005), effects prevented by an anti-VEGF(165) polyclonal neutralizing antibody (100 microg/ml). The proliferative effect was confirmed by a tetrazolium dye-based assay (100 ng/ml VEGF(165) : 1.4-fold increase). In ex vivo experiments, VEGF receptors in biopsy material from normal and diseased kidneys were detected by immunohistochemistry. No mesangial flt-1 receptor staining was seen in normal renal cortical tissue samples, and only weak mesangial KDR staining was detected. In contrast, mesangial flt-1 and KDR receptor staining were both clearly seen in biopsy samples from proliferative renal diseases. In conclusion, flt-1, KDR, and neuropilin-1 are present on cultured HMC, and VEGF(165) induces HMC proliferation. In addition, the flt-1 and KDR receptors are expressed in the mesangium in mesangioproliferative disease.