S6 kinase inactivation impairs growth and translational target phosphorylation in muscle cells maintaining proper regulation of protein turnover

Am J Physiol Cell Physiol. 2007 Aug;293(2):C712-22. doi: 10.1152/ajpcell.00499.2006. Epub 2007 May 9.

Abstract

A defect in protein turnover underlies multiple forms of cell atrophy. Since S6 kinase (S6K)-deficient cells are small and display a blunted response to nutrient and growth factor availability, we have hypothesized that mutant cell atrophy may be triggered by a change in global protein synthesis. By using mouse genetics and pharmacological inhibitors targeting the mammalian target of rapamycin (mTOR)/S6K pathway, here we evaluate the control of translational target phosphorylation and protein turnover by the mTOR/S6K pathway in skeletal muscle and liver tissues. The phosphorylation of ribosomal protein S6 (rpS6), eukaryotic initiation factor-4B (eIF4B), and eukaryotic elongation factor-2 (eEF2) is predominantly regulated by mTOR in muscle cells. Conversely, in liver, the MAPK and phosphatidylinositol 3-kinase pathways also play an important role, suggesting a tissue-specific control. S6K deletion in muscle mimics the effect of the mTOR inhibitor rapamycin on rpS6 and eIF4B phosphorylation without affecting eEF2 phosphorylation. To gain insight on the functional consequences of these modifications, methionine incorporation and polysomal distribution were assessed in muscle cells. Rates and rapamycin sensitivity of global translation initiation are not altered in S6K-deficient muscle cells. In addition, two major pathways of protein degradation, autophagy and expression of the muscle-specific atrophy-related E3 ubiquitin ligases, are not affected by S6K deletion. Our results do not support a role for global translational control in the growth defect due to S6K deletion, suggesting specific modes of growth control and translational target regulation downstream of mTOR.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Autophagy
  • Calcium-Calmodulin-Dependent Protein Kinases / metabolism
  • Cells, Cultured
  • Elongation Factor 2 Kinase
  • Eukaryotic Initiation Factors / metabolism
  • Hepatocytes / enzymology
  • Hepatocytes / metabolism
  • Hepatocytes / pathology
  • Insulin / metabolism
  • Leucine / metabolism
  • Liver / drug effects
  • Liver / enzymology
  • Liver / growth & development
  • Liver / metabolism*
  • Liver / pathology
  • Male
  • Mice
  • Mice, Knockout
  • Mitogen-Activated Protein Kinases / metabolism
  • Muscle Development
  • Muscle Fibers, Skeletal / enzymology
  • Muscle Fibers, Skeletal / metabolism*
  • Muscle Fibers, Skeletal / pathology
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / enzymology
  • Muscle, Skeletal / growth & development
  • Muscle, Skeletal / metabolism*
  • Muscle, Skeletal / pathology
  • Muscular Atrophy / enzymology
  • Muscular Atrophy / genetics
  • Muscular Atrophy / metabolism*
  • Muscular Atrophy / pathology
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphorylation
  • Protein Biosynthesis* / drug effects
  • Protein Kinases / metabolism*
  • Ribosomal Protein S6 / metabolism
  • Ribosomal Protein S6 Kinases, 90-kDa / deficiency
  • Ribosomal Protein S6 Kinases, 90-kDa / genetics
  • Ribosomal Protein S6 Kinases, 90-kDa / metabolism*
  • Signal Transduction
  • Sirolimus / pharmacology
  • TOR Serine-Threonine Kinases
  • Ubiquitin-Protein Ligases / metabolism

Substances

  • Eukaryotic Initiation Factors
  • Insulin
  • Ribosomal Protein S6
  • eIF-4B
  • ribosomal protein S6, mouse
  • Ubiquitin-Protein Ligases
  • Protein Kinases
  • mTOR protein, mouse
  • Ribosomal Protein S6 Kinases, 90-kDa
  • Rps6ka1 protein, mouse
  • TOR Serine-Threonine Kinases
  • ribosomal protein S6 kinase, 90kDa, polypeptide 3
  • Calcium-Calmodulin-Dependent Protein Kinases
  • Eef2k protein, mouse
  • Elongation Factor 2 Kinase
  • Mitogen-Activated Protein Kinases
  • Leucine
  • Sirolimus