Bcl-2 modulates telomerase activity

Apoptosis is a physiological mechanism of cell death that plays an important role in the regulation of tissue homeostasis. The regulation of apoptosis is a complex process and involves a number of gene products including the survival factor Bcl-2, which has been found to be frequently deregulated in human cancers. In addition to deregulation of apoptosis, the process of neoplasia is also believed to be driven by the activation of telomerase, a ribonucleoprotein complex that adds telomeric repeats (hexanucleotide 5'-TTAGGG-3') to the ends of replicating chromosomes. Activation of telomerase has been detected in a vast majority of human cancer cells. Although recent studies have demonstrated the wide occurrence of telomerase activation and Bcl-2 deregulation in human cancer cells, it remains unclear whether there is any linkage between the deregulation of Bcl-2 and telomerase activity in cancer cells. In the studies presented here, we report that the stable overexpression of Bcl-2 in human cancer cells with low Bcl-2 expression was accompanied by increased levels of telomerase activity. In addition, using an IL-2-dependent cytotoxic T-cell line, CTLL-2, we demonstrated that IL-2 deprivation (8 h), which is known to down-regulate Bcl-2 expression, also resulted in concurrent inhibition of telomerase activity in the absence of any detectable apoptosis and accumulation of cells in the G0/G1 phase of the cell cycle. Re-exposure of IL-2-deprived CTLL-2 cells to the recombinant IL-2 led to the up-regulation of both Bcl-2 expression and telomerase activity. Taken together, these findings establish a close linkage between the modulation of telomerase activity by survival factor Bcl-2, and provide a model to study regulation of telomerase activity by an anti-apoptotic pathway that is widely deregulated in cancer cells.