The transcription factor hypoxia-inducible factor-1 (HIF-1) functions as a master regulator of hypoxic response by inducing the transcription of various genes responsible for cellular adaptation to hypoxia. In this study, we investigated the effects of protein inhibitor of activated STAT3 (PIAS3), a small ubiquitin-related modifier (SUMO) E3 ligase, on HIF-1-mediated transcriptional activation. We found that PIAS3 physically associated with HIF-1 alpha. Moreover, PIAS3 overexpression enhanced the transcriptional activity of HIF-1 alpha independently of its SUMO E3 ligase activity. Conversely, quantitative RT-PCR analysis showed that RNAi-mediated PIAS3 knockdown reduced the expression of HIF-1 target genes under hypoxia. In addition, PIAS3 knockdown induced the destabilization of HIF-1 alpha protein, and the destabilization was reversed by the proteasome inhibitor MG132. Taken together, these results suggest that PIAS3 functions as a positive regulator of HIF-1 alpha-mediated transcription by increasing its protein stability. (C) 2015 Elsevier Inc. All rights reserved.