Both transcriptional and posttranscriptional mechanisms regulate human telomerase template RNA levels

The human telomerase RNA component (hTR) is present in normal somatic cells at lower levels than in cancer-derived cell lines. To understand the mecha nisms regulating hTR levels in different cell types, we have compared the s teady-state hTR levels in three groups of cells: (i) normal telomerase nega tive human diploid cells; (ii) normal cells transfected with the human telo merase catalytic subunit, hTERT; and (iii) cells immortalized in vitro and cancer cells expressing their own endogenous hTERT, To account for the diff erences in steady-state hTR levels observed in these cell types, we compare d the transcription rate and half-life of hTR in a subset of these cells. T he half-life of hTR in telomerase-negative cells is about 5 days and is inc reased 1.6-fold in the presence of hTERT. The transcription rate of hTR is essentially unchanged in cells expressing exogenous hTERT, and the increase d steady-state hTR level appears to be due to the increased half-life. Howe ver, the transcription rate of hTR is greatly increased in cells expressing endogenous hTERT, suggesting some overlap in transcriptional regulatory co ntrol. We conclude that the higher hTR level in cells expressing an endogen ous telomerase can be a result of both increased transcription and a longer half-life and that the longer half-life might be partially a result of pro tection or stabilization by the telomerase catalytic subunit. The 4-week ha lf-life of hTR in H1299 tumor cells is the longest half-life yet reported f or any RNA.