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Toxin-Induced Tail Phosphorylation of Hepatocellular S6 Kinase: Evidence for a Dual Involvement of the AMP-Activated Protein Kinase in S6 Kinase Regulation
Toxin causes tail modification of liver S6 kinase, suggesting two roles for energy-sensing enzyme in controlling S6 kinase
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Abstract
Several protein phosphatase-inhibitory toxins induced phosphorylation in the tail region of S6 kinase (S6K) in isolated rat hepatocytes.
- Phosphorylation of S6K was detected using a specific antibody against doubly phosphorylated Thr-421/Ser424.
- The adenosine analogue AICAR also stimulated S6K tail phosphorylation and activated AMP-activated protein kinase (AMPK).
- Naringin prevented the activation of AMPK and S6K tail phosphorylation caused by AICAR and the toxins, indicating AMPK may mediate these effects.
- S6K tail phosphorylation induced by AICAR and toxins was resistant to rapamycin, while amino acids showed rapamycin-sensitive activation.
- Amino acids activated S6K by phosphorylating Thr-389, but toxins did not have this effect and AICAR suppressed it.
- The phosphorylated S6K tail may signal independently of S6K enzymatic activity, while toxins stimulated phosphorylation of the ribosomal protein S6 through a different kinase.
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