Upstream stimulatory factor (USF) as a transcriptional suppressor of human telomerase reverse transcriptase (hTERT) in oral cancer cells.
Telomerase activity is suppressed in normal human somatic tissues but is activated in cancer cells and immortal cell lines. The reverse transcriptase (RT) subunit human telomerase reverse transcriptase (hTERT) is the key regulator of telomerase activity. The hTERT promoter contains E-box elements and may allow upstream stimulatory factor (USF), a basic helix-loop-helix (bHLH) leucine zipper family proteins, to bind and regulate the expression. In this study, we investigated whether and how USF effect on hTERT. Through luciferase reporter assays, we found that both USF1 and USF2 possess a comparable effect on the inhibition of hTERT expression. Immunoprecipitation (IP) and immunoblotting (IB) analysis reveal that the suppression of hTERT by USF was not through the interaction of USF with c-myc or mad, nor disturbed the cellular protein levels of those. In gel mobility shift and chromatin immunoprecipitation (CHIP) assays, we found that the USF suppression is through direct binding at the E-box site of hTERT promoter and rendering the effect actively. Analysis on clinical normal and tumor tissues reveal that the expression of USF1 and USF2 was lower in the tumor tissues, correlated with hTERT expression and telomerase activity. Taking together, our results demonstrate that USF is a negative transcriptional repressor for hTERT in oral cancer cells. It is possible that USF lose the inhibitory effect on hTERT expression leading to telomerase reactivation and oral carcinogenesis.
Chang
Joseph Tung-Chieh
JT
Department of Radiation Oncology, Chang Gung Memorial Hospital, Taoyuan, Taiwan.
Yang
Huei-Ting
HT
Wang
Tzu-Chien V
TC
Cheng
Ann-Joy
AJ
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors / Q000378:metabolism; D001665:Binding Sites; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D009062:Mouth Neoplasms / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism; D019098:Telomerase / Q000235:genetics* / Q000378:metabolism*; D014158:Transcription, Genetic / Q000235:genetics*; D051720:Upstream Stimulatory Factors / Q000235:genetics / Q000378:metabolism*
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C079439:MXD1 protein, human; D016271:Proto-Oncogene Proteins c-myc; D012097:Repressor Proteins; D051720:Upstream Stimulatory Factors; D019098:Telomerase