Regulatory expression of bone morphogenetic protein 6 by 2,2′-dipyridyl

Background: Expression of hepcidin, a hormone produced by hepatocytes which negatively regulates the circulating iron levels, is known to be positively regulated by BMP6, a member of transforming growth factor (TGF)-beta family. Previous studies have shown that iron status is sensed by sinusoidal endothelial cells of hepatic lamina, leading to the modulation of BMP6 expression. Methods: ISOS-1, HUVEC, F-2, and SK-HEP1 endothelial cells were treated with either iron or 2,2'-dipyridyl (2DP), a cell-permeable iron-chelator, and expression level of Bmp6 was examined. To identify factors affecting Bmp6 transcription, stimulus screening for regulator of transcription (SSRT) was developed. Results: Treatment with iron slightly increased the expression levels of Bmp6, while 2DP unexpectedly increased Bmp6 expression in a dose-dependent manner. 2DP-induced Bmp6 expression was resistant to co-treatment with iron. 2DP-induced Bmp6 expression was also detected in HUVEC, F-2 cells, and SK-HEP1 cells. Luciferase-based reporter assays indicated that forced expression of JunB increased the transcription of Bmp6. 2DP induced phosphorylation of JunB; co-treatment with SP600125 blocked the 2DP-induced Bmp6 expression partially. JunB-induced Bmp6 transcription was not affected by mutations of putative JunB-responsive elements. Some endoplasmic reticulum stress inducers increased the expression of Bmp6. SSRT revealed pathways regulating Bmp6 transcription positively and negatively. Hepa1-6 liver cells and C2C12 myogenic cells were prone to 2DP induced Bmp6 expression. Conclusions: The present study reveals non-iron-regulated Bmp6 expression in endothelial cells. General significance: Regulatory expression of Bmp6 may be important as a key step for fine tuning of BMP activity.