Differential decrease in soluble and DNA-bound telomerase in senescent human fibroblasts

The role of telomere shortening in the induction of replicative cellular senescence (CS) is well known and as a result, the involvement of telomerase and in particular its catalytic subunit, the telomerase reverse transcriptase (TERT) in CS has also been investigated. However, the majority of studies were conducted on cells that generally express high levels of TERT (cancer and immortalized cells) while the role of telomerase in CS in normal cells has been investigated to a much lesser extent. In particular, it was reported that active TERT is expressed in early passages of cultured human keratinocytes but rapidly diminished towards entry to CS, without telomere shortening. With the putative importance of TERT/telomerase in CS and the aging process in mind, we investigated the expression of TERT and telomerase activity in primary cultures of adult human dermal fibroblasts (HDFs) in the in vitro model of replicative CS. We found that (i) HDFs expressed active TERT; (ii) TERT protein levels and telomerase activity were markedly decreased in senescent HDFs; and (iii) the reduction of TERT in the soluble fraction was more pronounced than in the DNA-bound one. The results suggest the importance of the non-canonical (telomere-unrelated) functions of TERT in cellular senescence.