Reduction/oxidation (redox) regulation mediates numerous cellular responses and contributes to several physiological diseases. The transcription factor nuclear factor kappaB (NF-kappaB) is known to be a redox-sensitive factor. NF-kappaB plays a central role in immune responses and inflammation, through regulation of the gene expression of a large number of cytokines and other immune response genes. NF-kappaB is trapped in the cytoplasm in stimulated cells and translocates into the nucleus in response to several stimuli, including oxidative stress. Reactive oxygen species enhance the signal transduction pathways for NF-kappaB activation in the cytoplasm and translocation into the nucleus. In contrast, the DNA binding activity of oxidized NF-kappaB is significantly diminished, and that activity is restored by reducing enzymes, such as thioredoxin or redox factor 1. This review describes the signal transduction pathways for NF-kappaB activation and redox regulation of NF-kB activation in the cytoplasm and nucleus.

APE1/Ref-1 is thought to be a multifunctional protein involved in reduction–oxidation (redox) regulation and base excision DNA repair, and is required for early embryonic development in mice. APE1/Ref-1 has redox activity and AP endonuclease activity, and is able to enhance DNA-binding activity of several transcription factors, including NF-κB, AP-1 and p53, through reduction of their critical cysteine residues. However, it remains elusive exactly how APE1/Ref-1 carries out its essential functions in vivo. Here, we show that APE1/Ref-1 not only reduces target transcription factors directly but also facilitates their reduction by other reducing molecules such as glutathione or thioredoxin. The new activity of APE1/Ref-1, termed redox chaperone activity, is exerted at concentration significantly lower than that required for its redox activity and is neither dependent on its redox activity nor on its AP endonuclease activity. We also show evidence that redox chaperone activity of APE1/Ref-1 is critical to NF-κB-mediated gene expression in human cells and is mediated through its physical association with target transcription factors. Thus, APE1/Ref-1 may play multiple roles in an antioxidative stress response pathway through its different biochemical activities. These findings also provide new insight into the mechanism of intracellular redox regulation.

A basic-leucine zipper transcription factor, MafA, was recently identified as one of the most important transactivators of insulin gene expression. This protein controls the glucose-regulated and pancreatic -cell-specific expression of the insulin gene through a cis-regulatory element called RIPE3b/MARE (Maf-recognition element). Here, we show that MafA expression is restricted to -cells of pancreatic islets in vivo and in insulinoma cell lines. We also demonstrate that c-Maf, another member of the Maf family of transcription factors, is expressed in islet -cells and in a glucagonoma cell line ( TC1), but not in -and -cells. An insulinoma cell line, TC6, also expressed c-Maf, albeit at a low level. Chromatin immunoprecipitation assays demonstrated that Maf proteins associate with insulin and glucagon promoters in -and -cell lines, respectively. c-Maf protein stimulated glucagon promoter activity in a transient luciferase assay, and activation of the glucagon promoter by c-Maf was more efficient than by the other -cell-enriched transcription factors, Cdx2, Pax6, and Isl-1. Furthermore, inhibition of c-Maf expression in TC1 cells by specific short hairpin RNA resulted in marked reduction of the glucagon promoter activity. Thus, c-Maf and MafA are differentially expressed in -and -cells where they regulate glucagon and insulin gene expression, respectively.

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The LIM homeodomain protein Islet-1 (Isl1), one of the earliest markers for motor neuron differentiation, is also expressed in all classes of islet cells in the pancreas. Isl1 is known to bind and regulate the promoters of the insulin, glucagon and somatostatin genes. In this study, we describe isolation of a novel isl1 cDNA species from the mouse islet cell line TC6, which arose from the utilization of an alternative splicing acceptor site in the fifth exon. This shorter cDNA encodes an Isl1 isoform (Isl1-) lacking the carboxy-terminal 23 amino acids of the previously reported product Isl1-. Although the level of isl1-mRNA is much lower than that of isl1-, isl1-is preferentially expressed in murine insulinoma cell lines but not in glucagonoma cell line. Upon transient transfection, both Isl1-and Isl1-accumulate in the nuclei of murine insulinoma cells. We found that Isl1-is a relatively more potent transcriptional activator of the insulin promoter than Isl1-and that the Isl1-isoform undergoes phosphorylation. Therefore, the transcriptional activity of Isl1 is potentially regulated by the alternative splicing of its mRNA and by phosphorylation.