Expression of catalytically active telomerase does not prevent premature senescence caused by overexpression of oncogenic Ha-Ras in normal human fibroblasts

All normal cells in culture display a limited capacity to divide and eventu ally undergo an irreversible growth arrest known as replicative cellular se nescence, The development of cellular immortality has been implicated as an important factor in the progression of human cancers. Expression of telome rase has been shown to elicit a bypass of senescence and the acquirement of an extended life span. Although oncogenic pas induces malignant transforma tion in most immortal cells, it has been shown to cause a senescence-like c ell cycle arrest in presenescent human fibroblasts. To test the relationshi p between the senescence-inducing effect of pas and the senescence-bypassin g activity of telomerase, we used retroviral vector infection to introduce the catalytic subunit of human telomerase into normal human lung fibroblast s, Cell clones displaying in vitro telomerase catalytic activity and life s pan extension were obtained. However, these cells still became senescent af ter infection with a retrovirus vector expressing oncogenic Ha-Ras. No diff erences in premature senescence phenotypes between normal and telomerase-ex pressing cells were observed, A small number of colonies were recovered aft er the introduction of Ha-pas into either normal or telomerase-expressing c ells, but in all eases, these clones failed to express the exogenously intr oduced pas, We propose that even in the presence of active telomerase, the cellular senescence machinery remains intact and can be activated by approp riate signals, Consequently, interventions aimed at the activation of the l atent senescence program may be a fruitful approach in cancer therapy.