Zinc stimulates the activity of the insulin- and nutrient-regulated protein kinase mTOR

Recent studies indicate that zinc activates p70 S6 kinase (p70(S6k)) by a m echanism involving phosphatidylinositol 3-kinase (PI 3-kinase) and Akt (pro tein kinase B). Here it is shown that phenanthroline, a zinc and heavy meta l chelator, inhibited both amino acid- and insulin-stimulated phosphorylati on of p70(S6k). Both amino acid and insulin activations of p70(S6k) involve a rapamycin-sensitive step that involves the mammalian target of rapamycin (mTOR, also known as FRAP and RAFT). However, in contrast to insulin, amin o acids activate p70(S6k) by an unknown PI 3-kinase- and Akt-independent me chanism. Thus the effects of chelator on amino acid activation of p70(S6k) were surprising. For this reason, we tested the hypothesis that zinc direct ly regulates mTOR activity, independently of PI 3-kinase activation. In sup port of this, basal and amino acid stimulation of p70(S6k) phosphorylation was increased by zinc addition to the incubation media. Furthermore, the pr otein kinase activities of mTOR immunoprecipitated from rat brain lysates w ere stimulated two- to fivefold by 10-300 muM Zn2+ in the presence of an ex cess of either Mn2+ or Mg2+, whereas incubation with 1,10-phenanthroline ha d no effect. These findings indicate that Zn2+ regulates, but is not absolu tely required for, mTOR protein kinase activity. Zinc also stimulated a rec ombinant human form of mTOR. The stimulatory effects of Zn2+ were maximal a t similar to 100 muM but decreased and became inhibitory at higher physiolo gically irrelevant concentrations. Micromolar concentrations of other dival ent cations, Ca2+, Fe2+, and Mn2+, had no effect on the protein kinase acti vity of mTOR in the presence of excess Mg2+. Our results and the results of others suggest that zinc acts at multiple steps in amino acid- and insulin cell-signaling pathways, including mTOR, and that the additive effects of Zn2+ on these steps may thereby promote insulin and nutritional signaling.