Hypoxia-inducible factor 1 (HIF-1) is a master regulator of oxygen homeostasis that controls angiogenesis, erythropoiesis, and glycolysis via transcriptional activation of target genes under hypoxic conditions. O 2 -dependent binding of the von Hippel-Lindau (VHL) tumor suppressor protein targets the HIF-1␣ subunit for ubiquitination and proteasomal degradation. The activity of the HIF-1␣ transactivation domains is also O 2 regulated by a previously undefined mechanism. Here, we report the identification of factor inhibiting HIF-1 (FIH-1), a protein that binds to HIF-1␣ and inhibits its transactivation function. In addition, we demonstrate that FIH-1 binds to VHL and that VHL also functions as a transcriptional corepressor that inhibits HIF-1␣ transactivation function by recruiting histone deacetylases. Involvement of VHL in association with FIH-1 provides a unifying mechanism for the modulation of HIF-1␣ protein stabilization and transcriptional activation in response to changes in cellular O 2 concentration.

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Recent works have shown the importance of reduction/oxidation (redox) regulation in various biological phenomena. Thioredoxin (TRX) is one of the major components of the thiol reducing system and plays multiple roles in cellular processes such as proliferation, apoptosis, and gene expression. To investigate the molecular mechanism of TRX action, we used a yeast two-hybrid system to identify TRX-binding proteins. One of the candidates, designated as thioredoxin-binding protein-2 (TBP-2), was identical to vitamin D 3 up-regulated protein 1 (VDUP1). The association of TRX with TBP-2/ VDUP1 was observed in vitro and in vivo. TBP-2/VDUP1 bound to reduced TRX but not to oxidized TRX nor to mutant TRX, in which two redox active cysteine residues are substituted by serine. Thus, the catalytic center of TRX seems to be important for the interaction. Insulin reducing activity of TRX was inhibited by the addition of recombinant TBP-2/VDUP1 protein in vitro.In COS-7 and HEK293 cells transiently transfected with TBP-2/VDUP1 expression vector, decrease of insulin reducing activity of TRX and diminishment of TRX expression was observed. These results suggested that TBP-2/ VDUP1 serves as a negative regulator of the biological function and expression of TRX. Treatment of HL-60 cells with 1␣,25-dihydroxyvitamin D 3 caused an increase of TBP-2/VDUP1 expression and down-regulation of the expression and the reducing activity of TRX. Therefore, the TRX-TBP-2/VDUP1 interaction may be an important redox regulatory mechanism in cellular processes, including differentiation of myeloid and macrophage lineages.

Thioredoxin (TRX) is a pleiotropic cellular factor that has thiol-mediated redox activity and is important in regulation of cellular processes, including proliferation, apoptosis, and gene expression. The activity of several transcription factors is posttranslationally altered by redox modification(s) of specific cysteine residue(s). One such factor is nuclear factor (