Negative feedback for endothelial apoptosis: a potential physiological role for fibroblast growth factor

Although most physiologically important processes are regulated through negative feedback loops and numerous factors regulating endothelial apoptosis are identified, little is known about negative feedback mechanisms for endothelial apoptosis. Here we describe the release of soluble anti-apoptotic activity by endothelial cells undergoing apoptosis, and identify fibroblast growth factor-2 (FGF-2) as contributing to this. In brief, apoptosis of human umbilical vein endothelial cells was induced by serum deprivation and confirmed by transmission electron microscopy, DNA gel electrophoresis and a sub-diploid population seen by fluorescence-activated cell scanning analysis. Apoptosis was most rapid early during culture, and this was demonstrated to be due to the accumulation of soluble anti-apoptotic activity in experiments replacing culture medium with fresh medium, or alternatively returning conditioned medium to cells. FGF-2 antigen was detected in both conditioned medium and cell lysates, while neutralizing antibodies reduced the protective effect of medium conditioned by apoptotic endothelium. Experiments with the highly specific FGF-2 receptor tyrosine kinase inhibitor SU5402 indicated that FGF-2 accounted for the protective activity. We propose FGF-2 negative feedback as potentially important in microvascular remodelling. This is consistent with the absence of a secretory signal sequence in FGF-2, as well as canalicular fragmentation, which is unique to endothelial apoptosis and appears to result in minor cytoplasmic leakage.