AbstractTransforming growth factor-beta 1 (TGFβ1) and basic fibroblast growth factor (bFGF) are known to be potent inhibitors and stimulators, respectively, of endothelial cell growth in vitro. In the present study we examined the effect of bFGF on endothelial cell growth inhibitory activity of TGFβ1 and on the binding of (l25I)-TGFβl to these cells. The concentration of TGFβ1 required for half-maximal inhibition of endothelial cell growth was increased in a dose-dependent manner by bFGF (a 20–100 fold increase at 1 ng/ml of bFGF). A 24 h-pretreatment of cells with bFGF resulted in abolition of the TGFβ1 inhibitory effect on DNA synthesis. Moreover, the binding of (l25I)-TGFβl to the endothelial cell surface was decreased in a concentration-dependent and time-dependent manner after a preincubation of these cells with bFGF. Analysis of the binding parameters showed that bFGF decreased by two-fold the number of TGFβreceptors (to approximately 6000 receptors per cell). Cross-linking experiments with disuccinimidyl suberate demonstrated the presence of two TGFβreceptor subtypes, a predominant 85 kDa form and a minor 65 kDa form. Basic FGF decreased selectively the labeling of the 85 kDa TGFβreceptor subtype. These findings suggest that the growth stimulator bFGF can attenuate the cell's response to the growth inhibitor TGFβ1.