PHOSPHORYLATION OF RIBOSOMAL-PROTEIN S6 IS INHIBITORY FOR AUTOPHAGY IN ISOLATED RAT HEPATOCYTES

In rat hepatocytes, autophagy is known to be inhibited by amino acids, Insulin and cell swelling promote inhibition by amino acids, Each of the conditions leading to inhibition of autophagic proteolysis was fou nd to be associated with phosphorylation of a 31-kDa protein that we i dentified as ribosomal protein S6. A combination of leucine, tyrosine, and phenylalanine, which efficiently inhibits autophagic proteolysis, was particularly effective in stimulating S6 phosphorylation, The rel ationship between the percentage inhibition of proteolysis and the deg ree of Se phosphorylation was linear, Thus, inhibition of autophagy an d phosphorylation of S6 are under the control of the same signal trans duction pathway, Stimulation of S6 phosphorylation by the presence of amino acids was due to activation of S6 kinase and not to inhibition o f S6 phosphatase. The inhibition by amino acids of both autophagic pro teolysis and autophagic sequestration of electro-injected cytosolic [C -14]sucrose was partially prevented by rapamycin, a compound known to inhibit activation of p70 S6 kinase, In addition, rapamycin partially inhibited the rate of protein synthesis. We conclude that the fluxes t hrough the autophagic and protein synthetic pathways are regulated in an opposite manner by the degree to which S6 is phosphorylated. Possib le mechanisms by which S6 phosphorylation can cause inhibition of auto phagy are discussed.