An investigation of kinases responsible for site‐specific phosphorylation of p70S6K1 in skeletal muscle following chronic functional overloading (1163.13)

Abstract

The present project was designed to investigate the phosphorylation of p70S6K1 in an animal model of induced hypertrophic growth of skeletal muscle. Within 24 h of chronic functional overloading of the plantaris muscle by unilateral tenotomy in male Sprague Dawley rats, phosphorylation of p70S6K1 on Thr389 and Thr421/Ser424 was significantly increased. Treatment of rats with inhibitors of mTOR and c‐Jun N‐terminal Kinase (JNK) provided evidence of a role for the JNK signaling pathway in mediating regulation of p70S6K1. To investigate further a role for JNK in p70S6K1 phosphorylation, a cell culture model was employed. In HEK293E cells deprived of serum, treatment with IGF1 enhanced phosphorylation of Thr389 and Thr421/Ser424 sites. Inhibition of mTORC1 and/or JNK using selective inhibitors revealed a complex interaction suggesting that mTORC1 signaling pathway mediates the IGF1‐induced phosphorylation of both Thr389 and the Thr421/Ser424 sites, whereas JNK is responsible for basal phosphorylation (i.e. phosphorylation in serum‐deprived cells). Further support for this suggestion is provided by the finding that anisomycin, a JNK activator specifically stimulated phosphorylation of Thr421/Ser424 but not Thr389. Taken together, these results demonstrate that both the mTORC1 and JNK signaling pathways contribute to the stimulation of cell growth by mediating site specific phosphorylation of p70S6K1.Grant Funding Source: Supported by NIH Grant DK15658