Abstract
Hepatocyte nuclear factor-1B (HNF-1B) is a transcription factor involved in embryonic development and tissue-specific gene expression in several organs, including the kidney. Recently heterozygous mutations in the HNF1B gene have been identified in patients with hypomagnesemia due to renal Mg(2+) wasting. Interestingly, ChIP-chip data revealed HNF-1B binding sites in the FXYD2 gene, encoding the gamma-subunit of the Na(+)/K(+)-ATPase. The gamma-subunit has been described as one of the molecular players in the renal Mg(2+) reabsorption in the distal convoluted tubule (DCT). Of note, the FXYD2 gene can be alternatively transcribed into two main variants, namely gammaa and gammab. In the present study, we demonstrated via two different reporter gene assays that HNF-1B specifically acts as an activator of the gammaa-subunit, whereas the gammab-subunit expression was not affected. Moreover, the HNF-1B mutations H69fsdelAC, H324S325fsdelCA, Y352finsA and K156E, previously identified in patients with hypomagnesemia, prevented transcription activation of gammaa-subunit via a dominant negative effect on wild type HNF1-B. By immunohistochemistry, it was shown that the gammaa- and gammab-subunits colocalize at the basolateral membrane of the DCT segment of mouse kidney. On the basis of these data, we suggest that abnormalities involving the HNF-1B gene may impair the relative abundance of gammaa and gammab, thus affecting the transcellular Mg(2+) reabsorption in the DCT.
