Signals from the extracellular environment cooperate with ras to immortalize vHMEC and upregulate telomerase activity. (a) Growth curves of HMEC (red line), vHMEC expressing control vector (blue line), and vHMEC expressing Ha-rasV12 (orange line). Arrow indicates time at which vHMEC-ras cells were exposed to serum (day 220). These cells demonstrated increased population doublings within two passages (day 243) in serum-containing media, and are referred to as vHMEC-ras0.5 cells. Their growth curve is depicted in green. After 560 days, the vHMEC-ras0.5 cells were cultured in the absence of serum and continued to proliferate. These cells are referred to as vHMEC-ras0.5- >0 and their growth curve is depicted in black. (b) Telomerase activity assay depicting the amount of telomerase activity in lysates prepared from parental vHMEC (par), vHMEC-vector (vec), vHMEC-ras (ras), and vHMEC-ras0.5 (ras0.5) cells, as well as two clones isolated from the ras0.5 cell population (cl-1 and cl-2).