Role for c-Jun and APC/β-Catenin pathway in the transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene during tumorigenesis

Abstract

Activation of telomerase is pivotal step for cells to gain immortality. In human cells, telomerase activity is tightly regulated by the expression of its catalytic subunit, human telomerase reverse transcriptase (hTERT). In most human normal somatic cells, the hTERT gene expression is completely repressed at transcription level and this repression acts as an important gatekeeping mechanism against tumorigenesis. Here I demonstrate that the hTERT gene is a target of oncoprotein c-Jun and β-catenin/Tcf-4.First, I show that the hTERT gene is the target of c-Jun. Transient over-expression of c-Jun in human normal lung fibroblast cells (IMR90 and WI38) activated the hTERT promoter. Interestingly, TSA treatment completely abrogated this activation, indicating possible functional competition between c-Jun and HDAC. Furthermore, mutant c-Jun, which is specifically defective in DNA binding, did activate the hTERT gene expression, suggesting the possibility that c-Jun is associated with the hTERT promoter via other factors occupying the hTERT promoter. To further elucidate the detailed molecular mechanism of this activation, C-terminal serial deletion analysis of c-Jun was performed in human normal fibroblast, IMR90. Through this trial, leucine zipper domain is identified as critical region for this activation. Using GAL4 system, it was shown that c-Jun can act as a superactivator of Sp1 in human normal somatic cell, IMR90. In this system, mutant c-Jun, which could not activate the hTERT promoter, did not act as a superactivator of Sp1. Notably, plasmid immunoprecipitation (Plamid ChIP) showed that c-Jun was associated with the hTERT promoter via Sp binding sites. Taken together, these results show that c-Jun can act as a molecular switch turning Sp1 from repressor to activator in the course of tumorigenesis.Second, I demonstrate that the hTERT gene is the target of β-catenin/Tcf-4. Through expression cloning approach with cDNA library prepared from human normal kidney c...