Regulation of JNK signaling by GSTp

Studies of low basal Jun N-terminal kinase (JNK) activity in non-stressed c ells led us to identify a JNK inhibitor that was purified and identified as glutathione S-transferase Pi (GSTp) and was characterized as a JNK-associa ted protein. UV irradiation or H2O2 treatment caused GSTp oligomerization a nd dissociation of the GSTp-JNK complex, indicating that it is the monomeri c form of GSTp that elicits JNK inhibition. Addition of purified GSTp to th e Jun-JNK complex caused a dose-dependent inhibition of JNK activity. Conve rsely, immunodepleting GSTp from protein extracts attenuated JNK inhibition . Furthermore, JNK activity was increased in the presence of specific GSTp inhibitors and a GSTp-derived peptide. Forced expression of GSTp decreased MKK4 and JNK phosphorylation which coincided with decreased JNK activity, i ncreased c-Jun ubiquitination and decreased c-Jun-mediated transcription. C o-transfection of MEKK1 and GSTp restored MKK4 phosphorylation but did not affect GSTp inhibition of JNK activity, suggesting that the effect of GSTp on JNK is independent of the MEKK1-MKK4 module. Mouse embryo fibroblasts fr om GSTp-null mice exhibited a high basal level of JNK activity that could b e reduced by forced expression of GSTp cDNA, In demonstrating the relations hips between GSTp expression and its association with JNK, our findings pro vide new insight into the regulation of stress kinases.