Vascular endothelial growth factor (VEGF) stimulates new blood vessel growth and may also be involved in recovery of muscle following periods of ischemia. Zaccagnini et al. provide evidence in vivo and in vitro that telomerase is involved in mediating these effects of VEGF. Rats were subjected to hind limb ischemia and then in some animals VEGF was delivered through gene therapy. In the VEGF animals, there was increased capillary density. The VEGF-treated ischemic tissue exhibited increased mRNA for the catalytic subunit of telomerase (TERT) and increased activity of telomerase compared with tissue from animals not treated with VEGF or tissue not subjected to ischemia. Adenovirus-mediated gene transfer of human TERT also produced increased capillary density in ischemic tissue compared with control tissue, and a dominant-negative version of hTERT inhibited the capillary-stimulating effects of VEGF. Although overexpression of hTERT alone did not increase perfusion, both hTERT and VEGF inhibited apoptosis in ischemic tissue, measured as the number of TUNEL-positive cells. Switching to cultured cells, the authors investigated the relation between VEGF and telomerase in C2C12 myotubes and human umbilical vein endothelial cells (HUVECs). Endogenous TERT was negligible in differentiated C2C12 myotubes or proliferating HUVECs; however, VEGF treatment stimulated both the mRNA for TERT and telomerase activity in these cells. If telomerase activity was inhibited pharmacologically with 3'-azido-3'-deoxythymidine (AZT) or by transfection of dominant-negative TERT, then HUVECs did not form capillary-like structures in response to VEGF. VEGF stimulates the phosphoinositide 3-kinase (PI3K) pathway through nitric oxide (NO), and this pathway was important for the induction of TERT mRNA and the increased telomerase activity observed in HUVECs, because the PI3K inhibitor LY294002 or the NO inhibitor 7-nitroindazole (7N) blocked these effects. Furthermore, transfection of dominant-negative Akt, which is downstream of PI3K, also blocked the formation of capillary-like structures and the increase in telomerase activity observed in HUVECs in response to VEGF. In addition, transfection of an activated endothelial cell nitric oxide synthase (eNOS) also stimulated a hTERT reporter gene. Thus, telomerase is not only important for keeping cells young but also appears to be important in tissue regeneration.
G. Zaccagnini, C. Gaetano, L. Della Pietra, S. Nanni, A. Grasselli, A. Mangoni, R. Benvenuto, M. Fabrizi, S. Truffa, A. Germani, F. Moretti, A. Pontecorvi, A. Sacchi, S. Bacchetti, M. C. Capogrossi, A. Farsetti, Telomerase mediates vascular endothelial growth factor-dependent responsiveness in a rat model of hind limb ischemia. J. Biol. Chem. 280, 14790-14798 (2005). [Abstract] [Full Text]