SPAK and OSR1 Sensitive Kir2.1 K+ Channels

Background/Aims: Lithium, a widely used drug for the treatment of mood disorders, has previously been shown to stimulate the release of fibroblast growth factor FGF23, a powerful regulator of 1,25(OH)₂D₃ formation and mineral metabolism. The cellular mechanisms involved have remained elusive. Lithium has been shown to modify Ca²⁺ signaling. In a wide variety of cells, Ca²⁺ entry is accomplished by the pore-forming Ca²⁺ channel subunit Orai1 and its regulator STIM, which stimulates Orai following Ca²⁺ depletion of intracellular stores. Transcription factors promoting Orai1 expression include NF-κB. The present study thus explored whether the effect of lithium on FGF23 involves and requires Ca²⁺ entry. Methods: Experiments were performed in UMR106 osteoblastic cells and immortalized primary osteoblasts (IPO). FGF23 and Orai1 transcript levels were estimated from qRT-PCR, cytosolic Ca²⁺ concentration ([Ca²⁺]_i) from Fura2 fluorescence and store-operated Ca²⁺ entry (SOCE) from an increase in [Ca²⁺]_i following store depletion by inhibition of the sarcoendoplasmatic Ca²⁺ ATPase (SERCA) with thapsigargin (1 μM). Results: SOCE in UMR106 cells was enhanced by lithium treatment, an effect abrogated by Orai1 inhibitor 2-APB (50 μM). FGF23 transcript levels were increased by lithium and inhibited by Orai1 inhibitors 2-APB (50 μM) and YM58483 (100 nM) as well as NF-κB inhibitors wogonin (100 μM) and withaferin A (500 nM). Moreover, Orai1 transcript levels were up-regulated by lithium, an effect attenuated by wogonin and withaferin A. Conclusion: Lithium stimulates FGF23 release at least in part by NF-κB dependent up-regulation of Orai1 transcription and store operated Ca²⁺ entry.