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Transcription factor HMG-box protein 1 (HBP1) is a member of the HMG-box family of transcription factors and has been found to play an important role in the transcriptional repression of the p47phox gene. The promoter region of p40phox also has a HBP1 binding site, which makes p40phox a possible candidate for HBP1. In this study, we examined the role of HBP1 in the regulation of p47phox and p40phox in endothelial cells. Knockdown of p47phox in a mouse lymphoid endothelial cell line (SVEC4-10) resulted an ∼50% increase of HBP1 protein expression, and this was accompanied with a significant increase in p40phox protein expression as detected by Western blot. The levels of HBP1 expression were significantly higher (∼2.3-fold) in coronary microvascular endothelial cells isolated from p47phox knockout mice compared with cells isolated from wild-type mice. The role of HBP1 in the transcriptional regulation of p40phox and p47phox expression was further examined by transient in-vitro knockdown of HBP1 using shRNA in human microvascular endothelial cell (HMEC1). Knockdown of HBP1, as shown by Western blot, resulted in a significant increase in p47phox expression and this was accompanied with a significant reduction in p40phox expression. In conclusion, HBP1 plays dual roles in the regulation of NADPH oxidase: it represses p47phox expression and in the mean time promotes p40phox expression. HBP1 may represent an important transcriptional mechanism involved in the regulation of endothelial reactive oxygen species production by NADPH oxidase.
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