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 chromo some end structures, is strongly upregulated in most human cancers. In cont rast, most differentiated tissues are telomerase negative. The rate-limitin g step for telomerase activity seems to be the expression of the catalytic subunit of the enzyme, encoded by the human telomerase reverse transcriptas e (hTERT) gene. The precise mechanism of how hTERT is regulated has not bee n elucidated vet. We show here that the down-regulation of hTERT mRNA durin g 12-O-tetradecanoylphorbol-13-acetate-induced differentiation of human U93 7 cells is a consequence of a fast decrease in the rate of transcription ra ther 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 o ncoprotein, Our analysis shows that another member of the myc/max/mad netwo rk, mad1 encoding a transcriptional repressor that is significantly increas ed by 12-O-tetradecanoylphorbol-13-acetate treatment, represses hTERT promo ter-driven reporter gene activity in transient transfection assays. This ef fect is dependent on the NH2 terminal domain of Mad1, which mediates the as sociation 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 controll ed during the differentiation process in human somatic tissues.