Characterization of a Subset of the Basic-Helix-Loop-Helix-PAS Superfamily That Interacts with Components of the Dioxin Signaling Pathway

Abstract

In an effort to better understand the mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin, we employed an iterative search of human expressed sequence tags to identify novel basic-helix-loop-helix-PAS (bHLH-PAS) proteins that interact with either the Ah receptor (AHR) or the Ah receptor nuclear translocator (ARNT). We characterized five new "members of the PAS superfamily," or MOPs 1-5, that are similar in size and structural organization to the AHR and ARNT. MOPs 1-4 have N-terminal bHLH and PAS domains and C-terminal variable regions. MOP5 contained the characteristic PAS domain and a variable C terminus; it is possible that the cDNA contains a bHLH domain, but the entire open reading frame has yet to be completed. Coimmunoprecipitation studies, yeast two-hybrid analysis, and transient transfection experiments demonstrated that MOP1 and MOP2 dimerize with ARNT and that these complexes are transcriptionally active at defined DNA enhancer sequences in vivo. MOP3 was found to associate with the AHR in vitro but not in vivo. This observation, coupled with the fact that MOP3 formed tighter associations with the 90-kDa heat shock protein than the human AHR, suggests that MOP3 may be a conditionally active bHLH-PAS protein that requires activation by an unknown ligand. The expression profiles of the AHR, MOP1, and MOP2 mRNAs, coupled with the observation that they all share ARNT as a common dimeric partner, suggests that the cellular pathways mediated by MOP1 and MOP2 may influence or respond to the dioxin signaling pathway.

Coexpression of endothelial PAS protein 1 with essential angiogenic factors suggests its involvement in human vascular development

“…HIFs are heterodimeric factors composed of an ␣ subunit that is specifically induced in hypoxic conditions and a ␤ subunit (the aryl hydrocarbon receptor nuclear translocator [ARNT]) that is constitutively and ubiquitously produced. Three types of HIF␣ subunit have been identified in mammals: (1) HIF1␣ (Wang et al, 1995); (2) EPAS1 (Tian et al, 1997), also called HIF2␣, MOP2 (Hogenesch et al, 1997), HLF (Ema et al, 1997), and HRF (Flamme et al, 1997); and (3) the recently identified HIF3␣ . HIF1␣ is widely produced in the embryo (Jain et al, 1998) and activates a variety of genes involved in several hypoxia-responsive pathways such as angiogenesis, erythropoiesis, and glycolysis (for review see Semenza, 2000).…”

Biology of hypoxia-inducible factor-2α in development and disease

“…[11][12][13][14] While it was initially detected in endothelial cells, HIF-2a is expressed in the parenchyma and interstitial cells of multiple organs. 11,15,16 HIF-2a stabilization can be observed by immunohistochemistry in renal interstitial cells, hepatocytes, epithelial cells of the duodenum, cardiomyocytes, and astrocytes in rodents exposed to systemic hypoxia.…”

Selection and Analysis of a Mutant Cell Line Defective in the Hypoxia-inducible Factor-1 α-Subunit (HIF-1α)

“…Since it possesses substantial sequence similarly to HIF-1␣, dimerizes with ARNT, and binds similar DNA sequences (15,22,23), we examined EPAS-1 mRNA levels in wild type and mutant cells by RNase protection. No signal was detected in either cell type in assays of up to 100 g of RNA, although EPAS-1 mRNA was clearly detect- able by this assay in RNA prepared from Chinese hamster liver (Fig.…”