sirolimus and naringin

Several protein phosphatase-inhibitory toxins (okadaic acid, microcystin, calyculin A, cantharidin, tautomycin) administered to isolated rat hepatocytes were found to induce phosphorylation in the tail region of S6 kinase (S6K; p70S6K1) as detected with a phosphospecific antibody against doubly phosphorylated Thr-421/Ser424. 5-Aminoimidazole-4-carboxamide riboside (AICAR), an adenosine analogue that elicits activation of the hepatocellular AMP-activated protein kinase (AMPK), similarly stimulated S6K tail phosphorylation. The flavonoid naringin prevented the effects of AICAR, okadaic acid, and microcystin on AMPK activation as well as on S6K tail phosphorylation, suggesting AMPK as a mediator of the latter. The effects of AICAR and the toxins were rapamycin resistant; in contrast, amino acids induced an S6K tail phosphorylation that was rapamycin sensitive, suggesting mediation by the protein kinase mammalian target of rapamycin (mTOR). Amino acids activated S6K by phosphorylation at Thr-389, but the toxins did not, and AICAR in fact suppressed the activating phosphorylation induced by the amino acids. The possibility thus must be considered that the phosphorylated S6K tail may transmit a toxin-induced signal independently of S6K enzymatic activity. Despite their inability to activate S6K, the toxins (but not AICAR) stimulated phosphorylation of the ribosomal protein S6, presumably by activating some other S6-phosphorylating protein kinase.

Topics: Amino Acids; Aminoimidazole Carboxamide; Androstadienes; Animals; Anti-Bacterial Agents; Antioxidants; Blotting, Western; Cell Separation; Cyclic AMP-Dependent Protein Kinases; Electrophoresis, Polyacrylamide Gel; Flavanones; Gene Expression Regulation, Enzymologic; Hepatocytes; Immunosuppressive Agents; In Vitro Techniques; Indicators and Reagents; Male; Phosphorylation; Rats; Rats, Wistar; Ribonucleotides; Ribosomal Protein S6 Kinases; Sirolimus; Toxins, Biological; Wortmannin