Phosphorylation and degradation of S6K1 (p70S6K1) in response to persistent JNK1 Activation

S6K (ribosomal S6 kinase p70, p70S6K) activation requires phosphorylation at two stages. The first phosphorylation is independent of insulin stimulation and mediated by an unknown kinase. The second phosphorylation is mediated by mTOR in insulin dependent manner. In this study, we identified JNIC1 (c-Jun N-terminal kinase 1) as a kinase in the first phosphotylation. S6K protein was phosphorylated by JNIC1 at S411 and S424 in the carboxyl terminal autoinhibitory domain. The phosphotylation was observed in kinase assay with purified S6K as a substrate, and in cells after JNK1 activation by TNF-alpha or MEKK1 expression. The phosphorylation was detected in JNK2 null cells, but not in JNK1 null cells after TNF-a treatment. When JNK1 activation was inhibited by MKK7 knockdown, the phosphorylation was blocked in cells. The phosphorylation led to S6K protein degradation in NF-kappa B deficient cells. The degradation was blocked by inhibition of proteasome activity with MG132. In wide type cells, the phosphorylation did not promote S6K degradation when IKK2 (IKK beta, I kappa B kinase beta) was activated. Instead, the phosphorylation allowed S6K activation by mTOR, which stabilizes S6K protein. In IKK2 null cells or cells treated by IKK2 inhibitor, the phosphorylation led to S6K degradation. These data suggest that S6K is phosphorylated by JNKI and the phosphorylation makes S6K protein unstable in the absence of IKK2(1(2 activation. This study provides a mechanism for regulation of S6K protein stability. (C) 2013 Elsevier B.V. All rights reserved.