Functional evidence that vascular endothelial growth factor may act as an autocrine factor on human podocytes

Abstract

Vascular endothelial growth factor (VEGF) is expressed by renal glomerular epithelial cells (podocytes) and is thought to be protective against nephrotoxic agents. VEGF has been shown to be an autocrine survival factor in neuropilin-1-positive, VEGF receptor-negative breast carcinoma cells. Normal human podocytes are also known to express neuropilin-1, VEGF, and are VEGFR2 negative. Here, we investigated whether a similar VEGF autocrine loop may exist in podocytes. Podocyte cytosolic calcium concentration ([Ca2+]i) was analyzed in primary cultured and conditionally immortalized podocytes using ratiometric fluorescence measurement. Cytotoxicity was determined by lactate dehydrogenase assay, proliferation by [3H]-thymidine incorporation, and cell counts by hemocytometric assay. VEGF decreased [Ca2+]i in primary podocytes (from 179 ± 36 to 121 ± 25 nM, P < 0.05) and conditionally immortalized podocytes (from 95 ± 10 to 66 ± 8 nM, P < 0.02) in the absence of extracellular calcium. The type III receptor tyrosine-kinase inhibitor PTK787/ ZK222584 abolished this reduction. VEGF increased podocyte [3H]-thymidine incorporation (3,349 ± 283 cpm, control 2,364 ± 301 cpm, P < 0.05) and cell number (4.5 ± 0.7 × 104/ml, control 2.6 ± 0.5 × 104/ml, P < 0.05) and decreased cytotoxicity (5.9 ± 0.7%, control 12 ± 3%, P < 0.05), whereas a monoclonal antibody to VEGF increased cytotoxicity. Electron microscopy of normal human glomeruli demonstrated that the glomerular VEGF is mostly podocyte cell membrane associated. These results indicate that one of the functions of VEGF secreted from podocytes may be to act as an autocrine factor on calcium homeostasis and cell survival.