p70S6 kinase signals cell survival as well as growth, inactivating the pro-apoptotic molecule BAD

Cytokines often deliver simultaneous, yet distinct, cell growth and cell survival signals. The 70-kDa ribosomal protein S6 kinase (p70S6K) is known to regulate cell growth by inducing protein synthesis components. We purified membrane-based p70S6K as a kinase responsible for site-specific phosphorylation of BAD, which inactivates this proapoptotic molecule. Rapamycin inhibited mitochondrial-based p70S6K, which prevented phosphorylation of Ser-136 on BAD and blocked cell survival induced by insulin-like growth factor 1 (IGF-1). Moreover, IGF-1-induced phosphorylation of BAD Ser-136 was abolished in p70S6K-deficient cells. Thus, p70S6K is itself a dual pathway kinase, signaling cell survival as well as growth through differential substrates which include mitochondrial BAD and the ribosomal subunit S6, respectively

Amino Acid-Induced Translation of TOP mRNAs Is Fully Dependent on Phosphatidylinositol 3-Kinase-Mediated Signaling, Is Partially Inhibited by Rapamycin, and Is Independent of S6K1 and rpS6 Phosphorylation

Vertebrate TOP mRNAs contain an oligopyrimidine tract at their 5′ termini (5′TOP) and encode components of the translational machinery. Previously it has been shown that they are subject to selective translational repression upon growth arrest and that their translational behavior correlates with the activity of S6K1. We now show that the translation of TOP mRNAs is rapidly repressed by amino acid withdrawal and that this nutritional control depends strictly on the integrity of the 5′TOP motif. However, neither phosphorylation of ribosomal protein (rp) S6 nor activation of S6K1 per se is sufficient to relieve the translational repression of TOP mRNAs in amino acid-starved cells. Likewise, inhibition of S6K1 activity and rpS6 phosphorylation by overexpression of dominant-negative S6K1 mutants failed to suppress the translational activation of TOP mRNAs in amino acid-refed cells. Furthermore, TOP mRNAs were translationally regulated by amino acid sufficiency in embryonic stem cells lacking both alleles of the S6K1 gene. Inhibition of mTOR by rapamycin led to fast and complete repression of S6K1, as judged by rpS6 phosphorylation, but to only partial and delayed repression of translational activation of TOP mRNAs. In contrast, interference in the phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathway by chemical or genetic manipulations blocked rapidly and completely the translational activation of TOP mRNAs. It appears, therefore, that translational regulation of TOP mRNAs, at least by amino acids, (i) is fully dependent on PI3-kinase, (ii) is partially sensitive to rapamycin, and (iii) requires neither S6K1 activity nor rpS6 phosphorylation

Glucocorticoids and TNFα Interact Cooperatively to Mediate Sepsis-Induced Leucine Resistance in Skeletal Muscle

Sepsis blunts the ability of nutrient signaling by leucine to stimulate skeletal muscle protein synthesis by impairing translation initiation. The present study tested the hypothesis that overproduction of either tumor necrosis factor (TNF)-α or glucocorticoids mediate the sepsis-induced leucine resistance. Prior to producing peritonitis, rats received either vehicle, TNF binding protein (TNF(BP)) to inhibit endogenous TNFα action, and/or the glucocorticoid receptor antagonist RU486. Leucine was orally administered to all rats 24 h thereafter and the gastrocnemius removed 20 min later to assess protein synthesis and signaling components important in controlling peptide-chain initiation. Muscle protein synthesis was 65% lower in septic rats administered leucine than in leucine-treated control animals. This reduction was not prevented by either TNF(BP) or RU486 alone, but was completely reversed by the combination. This sepsis-induced leucine resistance was associated with an 80% reduction in the amount of active eIF4E·eIF4G complex, a 5-fold increase in the formation of the inactive eIF4E·4E-BP1 complex as well as markedly reduced (at least 70%) phosphorylation of 4E-BP1, eIF4G, S6K1, S6, and mTOR. Pretreatment of septic rats with either TNF(BP) or RU486 individually only nominally improved the leucine action as assessed by the above-mentioned endpoints. In contrast, when TNF(BP) and RU486 were co-administered, the ability of sepsis to impair the leucine-stimulated phosphorylation of 4E-BP1, eIF4G, S6K1, and S6 as well as the redistribution of eIF4E was essentially prevented. No differences in the total amount or phosphorylation of eIF2α and eIF2Bɛ were detected between the different groups, and changes could not be attributed to differences in the prevailing plasma concentration of insulin or leucine. Our data demonstrate the sepsis-induced leucine resistance in skeletal muscle results from the cooperative interaction of both TNFα and glucocorticoids

The response of intracellular signaling and muscle-protein metabolism to nutrition and exercise

In recent years, a body of literature examining the response of muscle-protein metabolism to exercise and nutrition has arisen. Developments in methods, especially stable isotopic tracer methodology, have allowed much information to be gathered in vivo in humans. The metabolic mechanism behind increased muscle mass requires that muscle-protein synthesis exceeds breakdown, i.e., net muscle-protein synthesis. Increased net muscle-protein balance may occur due to exercise, but net synthesis may occur only with the addition of nutrients, particularly a source of amino acids. The major impact of increased amino acid availability on net muscle-protein balance is due to stimulation of muscle-protein synthesis and less to inhibition of muscle-protein breakdown. Amino acids seem to stimulate muscle-protein synthesis, not only by mass action, i.e., providing substrate, but also as signals for initiation of protein synthesis. Stimulation of muscle-protein synthesis by amino acid ingestion may be linked to increased intracellular amino acid levels and/or to changing amino acid levels in the blood. Carbohydrate ingestion, most likely through the action of insulin, also may play a role in the response of muscle to exercise and nutrition. There is very little research in humans in vivo on the intracellular signaling that is linked to muscle-protein synthesis. It is clear that intracellular signaling responds to both insulin and amino acids, but the interactions with exercise are not well known; however, the details of the pathways have only just begun to be investigated, especially in humans. Delineation of these pathways is complicated, and there is little doubt that multiple intracellular signaling pathways with several levels of communication are involved in the hypertrophy process in response to nutrition and exercise. A systematic investigation of the relationship of the signaling to insulin and amino acids combined with exercise will provide important information, especially for populations vulnerable to muscle loss