MKP‐1 inhibits high NaCl‐induced phosphorylation of p38, but does not inhibit high NaCl‐induced activation of TonEBP: opposite roles of p38alpha and p38delta in activation of TonEBP.

High NaCl acutely activates p38 MAP kinase by phosphorylating it, the phosphorylation presumably being regulated by a balance of kinases and phosphatases. The kinases are known, but identity of the phosphatases is uncertain. Our initial purpose was to identify the phosphatases. We find that in HEK293 cells transient over expression of MKP‐1, a dual specificity phosphatase, inhibits high NaCl‐induced phosphorylation of p38, and that over expression of a dominant negative mutant of MKP‐1 does the opposite. High NaCl regulates MKP‐1 activity through reactive oxygen species, which inhibit MKP‐1 and by reduction of MKP‐1 binding to p38. Since inhibition of p38alpha (p38a) is known to reduce hypertonicity‐induced activation of the osmoprotective transcription factor, TonEBP, we anticipated that MKP‐1 expression might also. However, over expression of MKP‐1 has no significant effect on this activity. The paradox is explained by opposing effects of p38a and p38delta (p38d), both of which are activated by high NaCl. Thus, we find that over expression of p38a increases high NaCl‐induced TonEBP activity, but over expression of p38d has the opposite effect. Also, siRNA‐mediated knock‐down of p38d enhances high NaCl‐induced activation of TonEBP. We conclude that high NaCl inhibits MKP‐1, and that this contributes to activation of p38. However, opposing actions of p38a and p38d result in no net effect on TonEBP activity. Thus, 1) contrary to previous interpretations, activation of p38 isoforms by hypertonicity may not contribute to activation of TonEBP because of opposing effects of p38a and p38d and 2) effects of inhibitors of p38 depend on which isoform is affected, which can be misleading.