The Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT/HIF-1β) is Influenced by Hypoxia and Hypoxia-Mimetics

Abstract

Background: The Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT, HIF-1β) is a member of the basic-Helix-Loop-Helix PER/ARNT/SIM (bHLH/PAS) protein family and a vital transcriptional regulator regarding development and physiological adaptation processes. ARNT is discussed to be linked with cancer, and other diseases. ARNT is known to be translocated into the cell nucleus, where accumulation of the protein takes place. ARNT is a heterodimerisation partner of the xenobiotic ligand activated Aryl Hydrocarbon Receptor (AhR), the Single Minded proteins (SIM), the cardiovascular helix-loop-helix factor 1 and the Hypoxia Inducible Factor proteins (HIF-α). ARNT is obligatory for HIF-1, HIF-2 and HIF-3 binding to DNA. Whereas degradation of the HIF-α subunits is suppressed by hypoxia, ARNT is generally regarded as constitutively expressed in excess within the cell, and stabilisation is commonly thought to be oxygen-independent. However, we provide evidence that the regulation of ARNT is far more complex. The aim of our study was to reevaluate the regulation of ARNT expression. Methods: We examined cell lines of different origin like MCF-7 and T47D (human breast cancer), HeLa (human cervix carcinoma), Hep3B and HepG2 (human hepatoma), Kelly (human neuroblastoma), REPC (human kidney) and Cos7 (primary primate kidney) cells. We used immunoblot analysis, densitometry, RT-PCR and transient transfection. Results and Conclusion: Our results show that ARNT protein levels are influenced by hypoxia and hypoxia mimetics such as cobalt(II)-chloride (CoCl₂) and dimethyloxalylglycine (DMOG) in a cell line specific manner. We demonstrate that this effect might be triggered by HIF-1α which plays an important role in the process of stabilizing ARNT in hypoxia.