Endothelial cells exhibiting angiogenesis in vitro proliferate in response to TGF‐β1

Abstract

Transforming growth factor-β1 (TGF-β1) has been implicated in the positive regulation of angiogenesis in vivo, whereas it inhibits the proliferation of endothelial cells in vitro. To reconcile these apparently contradictory effects, we have investigated the effect of TGF-β1 on bovine aortic endothelial cells that exhibit spontaneous angiogenesis in vitro. We show that concentrations of TGF-β1 which stimulate proliferation of cells that form endothelial cords and/or tubes inhibit proliferation of the same cells grown at subconfluent densities. An increase in cell number of 35% over control cultures was achieved with 0.5 ng TGF-β1/ml. The proliferative effect was blocked by antibodies against TGF-β. Immunological detection of BrdU-labeled nuclei revealed an increase greater than 220% in cells treated with TGF-β1. Moreover, a population of cells within the cords appeared to be a selective target for this cytokine. The stimulatory effect was not restricted to bovine aortic endothelial cells, as similar results were obtained with endothelial cells derived from rat microvessels. Significant levels of active TGF-β1 were detected in cultures containing cords/tubes, whereas only latent TGF-β1 was detected in subconfluent cultures. We show further that endothelial cells exhibiting angiogenesis in vitro secrete plasminogen activator, an enzyme that regulates activation of TGF-β. The major increases in mRNA transcripts for extracellular matrix proteins that are typically associated with TGF-β1 were not seen in cells exhibiting angiogenesis in vitro. Since the formation of tubular networks requires both invasion and proliferation, we propose that TGF-β1 is a major morphoregulatory factor in angiogenesis that specifically controls endothelial cell proliferation and extracellular matrix turnover.