PTG (Protein Targeting to Glycogen) regulates the osmoprotective transcription factor NFAT5 via SHP‐1 phosphatase

Abstract

Hypertonicity alters phosphorylation of NFAT5, which regulates its activity. PTG is a regulatory subunit of protein phosphatase1 (PP1). By screening a genome‐wide siRNA library, we previously found that PTG inhibits NFAT5 transcriptional activity. The present study addresses the mechanism involved. siRNA‐mediated knock‐down of PTG increases high NaCl‐induced NFAT5 transactivating activity and protein abundance, and over expression of PTG reduces high NaCl‐dependent NFAT5 nuclear localization. We examined the effect of PTG siRNAs on phosphorylation of regulatory sites in p38 and ERK kinases and SHP‐1 phosphatase, previously known to contribute to regulation of NFAT5. SHP‐1 was affected, but not p38 or ERK. PTG siRNAs increase high NaCl‐induced phosphorylation of SHP‐1‐S591, which inactivates it. Mutation of SHP‐1‐S591 to A increases the inhibitory effect of SHP‐1 on NFAT5 transcriptional activity. Further, siRNA‐mediated knock‐down of the catalytic subunit of PP1, PP1gamma, also increases NFAT5 transcriptional activity. High NaCl reduces PP1 activity. We conclude: 1) that reduction by high NaCl of the inhibitory effect of protein phosphatase 1 on NFAT5 activity contributes to high NaCl‐induced activation of NFAT5 by increasing NFAT5 transactivating activity, nuclear localization and protein abundance, and 2) this effect is mediated through SHP‐1.