Identification of two distinct elements mediating activation of telomerase (hTERT) gene expression in association with cell growth in human T cells.

Lymphocytes are exceptional among normal somatic cells in that they express high telomerase activity like germline and malignant cells. We investigated the induction of telomerase in human T cells in association with cell growth. IL-2 significantly augmented the expression of mRNA for human telomerase reverse transcriptase (hTERT), a rate-limiting component of telomerase, in PHA-activated human peripheral blood leukocytes. An isolated 5'-flanking sequence (-3927-+51) of the hTERT gene was examined for its promoter activity in an IL-2-dependent human T cell line Kit 225. Addition of IL-2 into quiescent Kit 225 cells induced activation of the hTERT promoter. Reporter assays with mutant fragments of the hTERT promoter further revealed that IL-2-dependent activation was independently mediated by two elements within the +9-+51 regions. The two elements showed similar kinetics of activation in response to IL-2, which coincided with the G1 to S phase transition of the cell cycle. Interestingly, introduction of mutation in the elements increased background promoter activities in resting T cells in the absence of IL-2. Our results demonstrate that the hTERT promoter may be suppressed by the elements and IL-2 may signal for de-suppression in association with promotion of cell growth. IL-2-dependent activation of the hTERT promoter may be necessary for prevention from senescence induced by extraordinary cell division during immune reactions.