Amino acids stimulate phosphorylation of p70S6k and organization of rat adipocytes into multicellular clusters

Stimulative effect of dietary protein on the phosphorylation of p70 S6 kinase in the skeletal muscle and liver of food-deprived rats

The effect of dietary protein on p70S6k phosphorylation was examined in rats starved for 18 h and then fed either a 20% casein diet (20C) or a protein-free diet (0C). Refeeding the 20C diet, but not the 0C diet, increased p70S6k phosphorylation in both the skeletal muscle and liver. The plasma insulin concentrations were the same after refeeding the 20C or 0C diet, suggesting that a combination of dietary protein and insulin may be required to stimulate p70S6k phosphorylation.

L-Asparaginase inhibits the rapamycin-targeted signaling pathway

L-Asparaginase is widely used in the treatment of acute lymphoblastic leukemia. L-Asparaginase preparation derived from E. coli converts asparagine (Asn) and glutamine (Gln) to aspartate (Asp) and glutamate (Glu), respectively, and causes rapid depletion of Asn and Gln. It thus suppresses growth of malignant cells that are more dependent on an exogenous source of Asn and Gln than are normal cells. It remains unclear, however, which signaling events in leukemic cells are affected by L-asparaginase. Recently, amino acid sufficiency has been demonstrated to selectively regulate p70 S6 kinase (p70(s6k)) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), both of which are targeted by the anti-proliferative drug rapamycin. Here we demonstrate that addition of L-asparaginase to human leukemic cells inhibits activity of p70(s6k) and phosphorylation of 4E-BP1, but not activities of other cell growth-related serine/threonine kinases. The rate and kinetics of p70(s6k) inhibition by L-asparaginase were comparable to those seen by deprivation of Asn and/or Gln from cell culture media, suggesting that the effect of L-asparaginase on p70(s6k) is explained by depletion of Asn and/or Gln. Moreover, L-Asparaginase as well as rapamycin selectively suppressed synthesis of ribosomal proteins at the level of mRNA translation. These data indicate that L-asparaginase and rapamycin target a common signaling pathway in leukemic cells.

Amino acid-dependent signal transduction and insulin sensitivity

Recent developments indicate that amino acids, in addition to their function as substrates for many metabolic pathways, can stimulate a signal transduction pathway that shares components with insulin-stimulated signalling cascades. Insulin sensitivity is dependent on the ambient amino acid concentration. Amino acid-dependent signal transduction is present in all insulin-sensitive tissues and in pancreatic beta cells. A defect in amino acid-dependent signal transduction may result in phenomena similar to those found in diabetes mellitus.

The PIK-related kinases intercept conventional signaling pathways

Early efforts to place the first cloned mammalian PIK-related kinase, FRAP, into a conventional membrane to nuclear pathway met with little success. More recent data suggest that members of the family of PIK-related kinases act as intracellular sensors that govern radial and horizontal pathways. These pathways can impinge upon classical membrane to nuclear pathways, as well as components of the cell-cycle machinery.

Structural requirement of leucine for activation of p70 S6 kinase

The addition of leucine induced activation of p70S6k in amino acid-depleted H4IIE cells. Whereas the activation of p70S6k by leucine was transient, the complete amino acid stimulated p70S6k more persistently. The effect of leucine on p70S6k was sensitive to rapamycin, but less sensitive to wortmannin. Using various amino acids and derivatives of leucine, we found that the chirality, the structure of the four branched hydrocarbons, and the primary amine are required for the ability of leucine to stimulate p70S6k, indicating that the structural requirement of leucine to induce p70S6k activation is very strict and precise. In addition, some leucine derivatives exhibited the ability to stimulate p70S6k and the other derivatives acted as inhibitors against the leucine-induced activation of p70S6k.

Amino acid-dependent control of p70(s6k). Involvement of tRNA aminoacylation in the regulation

In human T-lymphoblastoid cells, downstream signaling events of mammalian target of rapamycin (mTOR), including the activity of p70(s6k) and phosphorylation of eukaryotic initiation factor 4E-binding protein 1, were dependent on amino acid concentration in the culture media, whereas other growth-related protein kinases were not. Amino acid-induced p70(s6k) activation was completely inhibited by rapamycin but only partially inhibited by wortmannin. Moreover, amino acid concentration similarly affected the p70(s6k) activity, which was dependent on a rapamycin-resistant mutant (S2035I) of mTOR. These data indicate that mTOR is required for amino acid-dependent activation of p70(s6k). The mechanism by which amino acids regulate p70(s6k) activity was further explored: 1) amino acid alcohols, which inhibit aminoacylation of tRNA by their competitive binding to tRNA synthetases, suppressed p70(s6k) activity; 2) suppression of p70(s6k) by amino acid depletion was blocked by cycloheximide or puromycin, which inhibit utilization of aminoacylated tRNA in cells; and 3) in cells having a temperature-sensitive mutant of histidyl tRNA synthetase, p70(s6k) was suppressed by a transition of cells to a nonpermissible temperature, which was partially restored by addition of high concentrations of histidine. These results indicate that suppression of tRNA aminoacylation is able to inhibit p70(s6k) activity. Deacylated tRNA may be a factor negatively regulating p70(s6k).

Phosphorylation of p70(S6k) correlates with increased skeletal muscle mass following resistance exercise

High-resistance exercise training results in an increase in muscle wet mass and protein content. To begin to address the acute changes following a single bout of high-resistance exercise, a new model has been developed. Training rats twice a week for 6 wk resulted in 13.9 and 14.4% hypertrophy in the extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, respectively. Polysome profiles after high-resistance lengthening contractions suggest that the rate of initiation is increased. The activity of the 70-kDa S6 protein kinase (p70(S6k)), a regulator of translation initiation, is also increased following high-resistance lengthening contractions (TA, 363 +/- 29%; EDL, 353 +/- 39%). Furthermore, the increase in p70(S6k) activity 6 h after exercise correlates with the percent change in muscle mass after 6 wk of training (r = 0.998). The tight correlation between the activation of p70(S6k) and the long-term increase in muscle mass suggests that p70(S6k) phosphorylation may be a good marker for the phenotypic changes that characterize muscle hypertrophy and may play a role in load-induced skeletal muscle growth.

Amino acid effects on translational repressor 4E-BP1 are mediated primarily by L-leucine in isolated adipocytes

Previous studies indicated that amino acids may activate the protein kinase activity of the target of rapamycin (TOR) and thereby augment and/or mimic the effects of insulin on protein synthesis, p70(S6k) phosphorylation, and multicellular clustering in adipocytes. To identify the individual amino acids responsible for these effects, the present study focused on the TOR substrate and translational repressor 4E-BP1. A complete mixture of amino acids stimulated the phosphorylation of 4E-BP1, decreasing its association with eukaryotic initiation factor eIF-4E. Studies on subsets of amino acids and individual amino acids showed that L-leucine was the amino acid responsible for most of the effects on 4E-BP1 phosphorylation; however, the presence of other amino acids was required to observe a maximal effect. The stimulatory effect of leucine was stereospecific and not mimicked by other branched chain amino acids but was mimicked by the leucine metabolite alpha-ketoisocaproate (alpha-KIC). The effect of alpha-KIC, but not leucine, was attenuated by the transaminase inhibitor (aminooxy)acetate. The latter result indicates that the effects of alpha-KIC required its conversion to leucine. Half-maximal stimulation of 4E-BP1 phosphorylation occurred at approximately 430 microM; therefore, the response was linear within the range of circulating concentrations of leucine found in various nutritional states.

Amino acid availability regulates p70 S6 kinase and multiple translation factors

Incubation of Chinese hamster ovary cells without amino acids for up to 60 min caused a rapid marked decrease in p70 S6 kinase activity and increased binding of initiation factor eIF4E to its inhibitory regulator protein 4E-BP1. This was associated with dephosphorylation of 4E-BP1 and eIF4E and dissociation of eIF4E from eIF4G. All these effects were rapidly reversed by resupplying a mixture of amino acids and this was blocked by rapamycin and by inhibitors of phosphatidylinositol 3-kinase, implying a role for phosphatidylinositol 3-kinase in the signalling pathway linking amino acids with the control of p70 S6 kinase activity and the phosphorylation of these translation factors. Amino acid withdrawal also led to changes in the phosphorylation of other translation factors; phosphorylation of eIF4E decreased whereas elongation factor eEF2 became more heavily phosphorylated, each of these changes being associated with decreased activity of the factor in question. Earlier studies have suggested that protein kinase B (PKB) may act upstream of p70 S6 kinase. However, amino acids did not affect the activity of PKB, indicating that amino acids activate p70 S6 kinase through a pathway independent of this enzyme. Studies with individual amino acids suggested that the effects on p70 S6 kinase activity and translation-factor phosphorylation were independent of cell swelling. The data show that amino acid supply regulates multiple translation factors in mammalian cells.