Objective: Telomerase plays a major role in the control of replicative capacity, a property critical to successful angiogenesis and maintenance of endothelial integrity. We have previously demonstrated that in normal human endothelial cells telomerase is differentially regulated by fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF), two major pro-angiogenic growth factors. Here we examined the transcriptional control of the catalytic subunit of telomerase, telomerase reverse transcriptase (hTERT), by these endothelial mitogens and report the functional consequences of these differential effects on the onset of senescence.
Methods and Results: In cultured human umbilical vein endothelial cells (HUVEC) both telomerase activity and mRNA levels of hTERT decreased reversibly upon induction of quiescence. Treatment of quiescent HUVEC with FGF-2 restored telomerase activity in a time and dose-dependent manner, whereas VEGF had no such effect, though both factors induced comparable mitogenic responses. FGF-2, but not VEGF, up-regulated the mRNA levels for hTERT and for the hTERT gene transactivation factor Sp1. In contrast, mRNA levels for c-Myc, a second important hTERT transactivation factor, were equally up-regulated by FGF-2 and VEGF, whereas Max and Mad1 mRNA levels were not influenced by the proliferative status of endothelial cells. Finally, serial passage of HUVEC in the presence of individual growth factors accelerated the accumulation of senescent cells in VEGF-treated cultures compared to cultures treated with FGF-2.
Conclusions: In human endothelial cells FGF-2, but not VEGF, up-regulates telomerase activity, telomerase reverse transcriptase and its transactivation factor Sp1, while c-Myc is induced equally by both growth factors. These findings are consistent with a model of cooperation between Sp1 and c-Myc in the transcriptional control of hTERT. FGF-2 was more effective than VEGF in maintaining the replicative capacity of endothelial cells.