Volume 24 Issue 5 - July 19, 2013 PDF
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Involvement of aryl hydrocarbon receptor nuclear translocator in EGF-induced c-Jun/Sp1 mediated gene expression
Wan-Chen Huang1, Shu-Ting Chen1, Wei-Chiao Chang2, Kwang-Yu Chang3, Wen-Chang Chang1,4,5 and Ben-Kuen Chen1,4,5,*
1 Department of Pharmacology, College of Medicine, National Cheng Kung University, Tainan 701, Taiwan, ROC
2 Graduate Institute of Medical Genetics, Kaohsiung Medical University, 100 Tz-You First Road, Kaohsiung 807, Taiwan, ROC
3 National Institute of Cancer Research, National Health Research Institutes, Tainan 701, Taiwan, ROC
4 Center for Gene Regulation and Signal Transduction Research, and
5 Institute of Biosignal Transduction, College of Bioscience and Biotechnology, National Cheng Kung University, Tainan 701, Taiwan, ROC
 
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Background
EGFR activation is not only important in the regulation of cell proliferation and tumorigenesis through mediating genes expression but is expressed at high levels in a variety of human tumors and has been associated with poor prognosis and lower survival rate. Thus, to identify the factors which participate in the EGF-induced gene expression, resulting in tumorigenesis could improve our knowledge and application in the cancer therapy. One of the factors we first identified in EGF-activated signal pathways is aryl hydrocarbon receptor nuclear translocator (ARNT). ARNT has long been deemed as a general partner to HIF-1α in hypoxia in mediating tumor growth. In addition to its function which has been largely discussed under hypoxia and the stimulation of xenobiotics, less is known about its role in growth factor action. In this study, we clarify the role of ARNT in EGF-induced gene expression which may participate in the process of tumorigenesis.

Results
In order to study whether EGF produces effects on the activation of ARNT, we treated cervical cancer cells with EGF and then the expression of ARNT was detected. We found that EGF induced the phosphorylation and nuclear accumulation of ARNT. EGF also enhanced the association of ARNT with c-Jun/Sp1 complex. Hence, the c-Jun/ARNT/Sp1 complex bound to Sp1 site of genes, such as 12(S)-lipoxygenase and p21WAF1/CIP1 to induce gene expression. EGF induced promoter activity and the mRNA level of 12(S)-lipoxygenase and p21WAF1/CIP1 as well as the association between c-Jun and Sp1 were reduced by ARNT knockdown. Our results reveal a novel mechanism by which ARNT acts as a modulator to bridge the c-Jun/Sp1 interaction and plays a role in EGF-mediated c-Jun/Sp1-dependent gene expression under normoxic conditions. These results are also consistent with our previous report that ARNT/c-Jun complex also mediates EGF-induced cyclooxygenase-2 gene expression. Consistently, cyclooxygenase-2 and ARNT were cohorts present distinctively in clinical speciments of human cervical squamous cell carcinoma.

Conclusion
As shown in Fig. a, ARNT mediates gene expression by recruiting transcription factors c-Jun and Sp1 upon growth factor stimulation and thus activate specific gene expression. In our study we revealed that ARNT, in addition to response to hypoxia and xenobiotics, mediates the activation of growth factor signaling in normoxia (Fig. b). This broadens the role that ARNT plays in physiological and pathophysiological functions.
Fig. a. Putative transcriptional mechanism of ARNT involved in EGF induced gene expression. After the stimulation of EGF in normoxia, ARNT accumulates in the nucleus where it interacts with c-Jun and Sp1 to form c-Jun/ARNT and c-Jun/ARNT/Sp1 complex. These complexes then bind to CRE or Sp1 sites of gene promoter through Sp1, c-Jun protein and turn on relative gene expression. 12-LOX and COX-2 denoted 12(S)-lipoxygenase and cyclooxygenase-2, respectively.
Fig. b. ARNT binds to differential transcription factors in response to various stimuli to drive specific cellular functions.
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