Contribution of SHP‐1 protein tyrosine phosphatase to osmotic regulation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor TonEBP/OREBP resulting in increased expression of osmoprotective genes, including those responsible for accumulation of organic osmolytes and heat shock proteins. Phosphorylation of TonEBP/OREBP contributes to its activation. Several of the kinases that are involved have been identified, but the phosphatases have not. By screening a genome‐wide human phosphatase siRNA library in HEK293 cells, we found that siRNAs against the protein tyrosine phosphatase SHP‐1 increases tonicity‐dependent TonEBP/OREBP activity, implying that SHP‐1 normally reduces that activity. We further studied in detail SHP‐1. We confirmed that SHP‐1 is inhibitory by over expressing it, which reduces TonEBP/OREBP transcriptional activity at 500 mosmol/kg. SHP‐1 dephosphorylates TonEBP/OREBP at a known regulatory site, Y143, both in vivo and in vitro. It inhibits TonEBP/OREBP by both reducing TonEBP/OREBP nuclear localization, which is TonEBP‐Y143‐dependent, and by lowering high NaCl‐induced TonEBP/OREBP transactivating activity. SHP‐1 coimmunoprecipitates with TonEBP/OREBP and vice versa, suggesting that they are physically associated in the cell. High NaCl inhibits the effect of SHP‐1 on TonEBP/OREBP by increasing phosphorylation of SHP‐1 on Ser591, which reduces its phosphatase activity and localization to the nucleus. Thus, SHP‐1, whose inhibition by high NaCl increases phosphorylation of TonEBP/OREBP at Y143, contributes to the nuclear localization and activation of TonEBP/OREBP.