Aldehyde dehydrogenase 3 gene regulation: studies on constitutive and hypoxia-modulated expression

We have previously shown that expression of the Class 3 aldehyde dehydrogenase gene (ALDH3) is abrogated by hypoxia. This phenomenon occurs in rat hepatoma systems in which ALDH3 expression is xenobiotic-inducible as well as in rat primary corneal epithelial cells that exhibit high constitutive ALDH3 expression. We have begun to test various segments of the ALDH3 5' flanking region for elements that may mediate this effect using CAT reporter gene constructs. In addition, although the involvement of the Ah receptor nuclear translocator (ARNT) in xenobiotic induction of ALDH3 is well established, the role of ARNT in constitutive ALDH3 expression is not clear. Moreover, ARNT is also a component of the hypoxia inducible factor-1 (HIF-1) bipartite transcription factor complex that mediates hypoxic induction of a variety of genes. Concomitant activation of the xenobiotic and hypoxia pathways results in cross-talk and functional interference. It has been hypothesized that this interference is due to limiting levels of ARNT. To examine if ARNT levels are limiting during hypoxic and xenobiotic induction in the context of ALDH3 expression and to examine possible roles of ARNT in constitutive expression of ALDH3 in corneal epithelial cells we co-transfected rat corneal epithelial cells and H4-II-EC3 rat hepatoma cells with ALDH3 5' UTR-CAT reporter genes and expression vectors containing either wild type or dominant negative forms of ARNT. Our results indicate that during hypoxia and xenobiotic induction of ALDH3 in H4-II-EC3 cells ARNT is not the limiting transcription factor. Further, neither wild type nor dominant negative ARNT had effects on constitutive ALDH3 expression in corneal epithelial cells.