Expression of the hTERT gene is regulated at the level of transcriptional initiation and repressed by mad1

Telomerase, an enzymatic activity responsible for the replication of chromosome end structures, is strongly upregulated in most human cancers. In contrast, most differentiated tissues are telomerase negative. The rate-limiting step for telomerase activity seems to be the expression of the catalytic subunit of the enzyme, encoded by the human telomerase reverse transcriptase (hTERT) gene. The precise mechanism of how hTERT is regulated has not been elucidated yet. We show here that the down-regulation of hTERT mRNA during 12-O-tetradecanoylphorbol-13-acetate-induced differentiation of human U937 cells is a consequence of a fast decrease in the rate of transcription rather than changes in its half-life. The only transcription factor that has so far been implicated in the regulation of hTERT expression is the c-Myc oncoprotein. Our analysis shows that another member of the myc/marx/mad network, mad1, encoding a transcriptional repressor that is significantly increased by 12-O-tetra-decanoylphorbol-13-acetate treatment, represses hTERT promoter-driven reporter gene activity in transient transfection assays. This effect is dependent on the NH2 terminal domain of Madl, which mediates the association with the transcriptional corepressor mSin3. Our findings suggest the involvement of an additional transcription factor in the regulation of hTERT expression and may provide a model for how hTERT activity is controlled during the differentiation process in human somatic tissues.