Expression of hTERT mRNA in a mortal liver cell line during S phase without detectable telomerase activity

Abstract

Normal human liver cells have a limited capacity for proliferation due to telomere shortening, whereas immortalized cells prevent shortening of the 3' single strand telomeric repeat by expressing telomerases, including human telomerase reverse transcriptase (hTERT). The hTERT transcript contains three deletion sites that give rise to alternatively spliced variants (ASVs). Recently, hTERT expression was observed in cycling primary presenescent human fibroblasts, which were believed to lack hTERT expression and telomerase activity. hTERT mRNA was expressed in the synthesis (S) phase of the cell cycle. Although hTERT mRNA has eight isoforms, it is not known which of the hTERT ASVs are expressed in S phase. In order to determine the possible relationships between the cell cycle and ASV expressions, we measured the full-length isoform and ASVs of hTERT mRNA in a mortal liver cell line and immortal cell lines that were synchronized in S phase of the cell cycle. Using RT nested-PCR analysis, the full-length isoform and alpha-deletion ASV of hTERT were detected in the LI90 mortal liver cell line at points when cells in S phase represented >48% of the cell population without detectable telomerase activity. hTERT was always expressed in the HLE and Huh-7 hepatocellular carcinoma cell lines, regardless of the cell cycle. Our results suggest the possibility that telomerase is regulated in a cell cycle-dependent manner in normal liver cells.