Regulation of WNK1 kinase by extracellular potassium

Mutations of WNK kinase genes were identified as the cause of a hereditary hypertensive disease, pseudohypoaldosteronism type II; however, little is known about the regulation of WNK kinases. In the present study, we focused on anisosmotic conditions as the initial clues for clarifying a stimulating factor for WNK kinase activity. Endogenous WNK kinase activity in COS7 cells was monitored by the phosphorylation of its substrate, OSR1. Knockdown experiments revealed that WNK1 was a major WNK kinase in COS7 cells. In contrast to the transient increase in WNK1 activity caused by hypertonic medium, hypotonic medium increased the phosphorylation of OSR1 for 24 h, suggesting that the hypotonic medium included a signal for continuously stimulating WNK1 kinase activity. To identify the signal, ion substitution experiments were performed. Surprisingly, even isotonic media with low Cl- or low K+ was found to increase OSR1 phosphorylation as well as the hypotonic medium. Furthermore, WNK1 activation by the hypotonic medium was completely blocked by quinine (500 mu M) but not by 5-nitro-2-(3-phenylpropylamino) benzoic acid (100 mu M), and this inhibition was closely correlated with the inhibition of Rb-86(+) (=K+) efflux but not with the inhibition of I-125(-) (=Cl-) efflux. These results suggest that K+, rather than hypotonicity or low Cl-, may be an important regulator for WNK1 activation. Finally, we confirmed that high K+ and low K+ media under the physiological range decreased and increased WNK1 activity, respectively. Extracellular K+ is an important regulator of WNK1 kinase activity.