MiR-210 and miR-155 are Upregulated in CFBE Cells and Involved in Fe-S Protein Assembly via ISCU Downregulation as well as HO-1 Expression via BACH1.

Cystic fibrosis (CF) is an inherited lung disorder associated with chronic endobronchial inflammation. Hemeoxygenase-1 (HMOX1), an enzyme involved in heme degradation, is upregulated in inflammation and negatively regulated via the transcription factor BACH1. Here we studied the role of microRNAs (miRs) in the heme metabolism of cystic fibrosis bronchial epithelial cells (CFBE41o-), applying qRT-PCR, luciferase promoter assays, immunoblotting, and enzyme activity assays. In comparison to normal human bronchial epithelial cells (16HBE14o-) miR-210 and miR-155 were 4- to 5-fold upregulated in CFBE41o- cells. MiR-210 has been implicated in the negative regulation of the mitochondrial iron-sulfur (Fe-S) cluster assembly scaffold ISCU, and consistently we found ISCU expression being decreased in CF cells. Accordingly, activities of the Fe-S enzymes aconitase, succinate dehydrogenase, and ferrochelatase (FECH) were also diminished. Despite this latter constraint total heme content was higher in CF than in control cells. Intriguingly, HMOX1 expression in CF cells was severely curtailed, implying that despite the low FECH activity heme formation was in favour of heme degradation. Moreover, we could link HMOX1 expression to TLR4 surface expression via miR-155 which in turn negatively regulates the expression of BACH1. Since TLR4 expression is impaired in CF we applied an TLR4 inhibitor in normal HBE cells and found levels of both miR-155 and BACH1 increased in a dose-dependent manner. Hence our results suggest that in CF miR-155 is ineffective in repressing BACH1, leading to low HMOX1 levels and altered heme metabolism, while miR-210 affects cellular Fe-S assembly with potential general impact on cellular iron homeostasis.

*Presenting author