TAK1 represses transcription of the human telomerase reverse transcriptase gene

In human cells, telomerase activity is tightly regulated by the expression of its catalytic subunit, namely, the human telomerase reverse transcriptase (hTERT). However, the molecular mechanisms involved in the regulation of hTERT expression have not been completely clarified. We have previously reported that transforming growth factor beta (TGF-beta) represses the expression of the hTERT gene. In the present study, we demonstrated that TGF-beta-activated kinase 1 (TAK1), originally identified as a mitogen-activated kinase kinase kinase, represses the hTERT core promoter activity in an E-box-independent manner, and it also represses the transcription of the hTERT gene in human lung adenocarcinoma cell line, A549 cells. This TAK1-induced repression was found to be caused by the recruitment of histone deacetylase to Sp1 at the hTERT promoter and a consequent reduction in the amount of acetylated histone H4 at the hTERT promoter. Finally, we demonstrated that TAK1 induces cellular senescence programs in normal human diploid cells. Thus, we assume that TAK1 triggers the repression mechanisms of the hTERT gene as a result of evoking cellular senescence programs. Considered together, TAK1 is thought to play a causative role in the determination of a finite replicative lifespan of normal and cancer cells.