Rapamycin-insensitive regulation of 4e-BP1 in regenerating rat liver.
In cultured cells, growth factor-induced phosphorylation of two translation modulators, p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), is blocked by nanomolar concentrations of the immunosuppressant rapamycin. Rapamycin also attenuates liver regeneration after partial hepatectomy, but it is not known if this growth-suppressive effect is due to dephosphorylation of p70 S6 kinase and/or 4E-BP1. We found that partial hepatectomy induced a transient increase in liver p70 S6 kinase activity and 4E-BP1 phosphorylation as compared with sham-operated rats. The amount of p70 S6 kinase protein in regenerating liver did not increase, but active kinase from partially hepatectomized animals was highly phosphorylated. Phosphorylated 4E-BP1 from regenerating liver was unable to form an inhibitory complex with initiation factor 4E. Rapamycin blocked the activation of p70 S6 kinase in response to partial hepatectomy in a dose-dependent manner, but 4E-BP1 phosphorylation was not inhibited. By contrast, functional phosphorylation of 4E-BP1 induced by injection of cycloheximide or growth factors was partially reversed by the drug. The mammalian target of rapamycin (mTOR) has been proposed to directly phosphorylate 4E-BP1. Western blot analysis using phospho-specific antibodies showed that phosphorylation of Thr-36/45 and Ser-64 increased in response to partial hepatectomy in a rapamycin-resistant manner. Thus, rapamycin inhibits p70 S6 kinase activation and liver regeneration, but not functional phosphorylation of 4E-BP1, in response to partial hepatectomy. These results indicate that the effect of rapamycin on 4E-BP1 function in vivo can be significantly different from its effect in cultured cells.
Departments of Pharmacology and Medicine, University of Texas Health Science Center and the Research Service, Audie L. Murphy Memorial Veterans Hospital, San Antonio, Texas 78229, USA.
Y P
YP
Jiang
L M
LM
Ballou
R Z
RZ
Lin
D000818:Animals; D002352:Carrier Proteins; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D047908:Intracellular Signaling Peptides and Proteins; D008115:Liver Regeneration; D008297:Male; D010750:Phosphoproteins / Q000502:physiology*; D010766:Phosphorylation; D051381:Rats; D017207:Rats, Sprague-Dawley; D019893:Ribosomal Protein S6 Kinases / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000494:pharmacology*
D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D007166:Immunosuppressive Agents; D047908:Intracellular Signaling Peptides and Proteins; D010750:Phosphoproteins; D019893:Ribosomal Protein S6 Kinases; D020123:Sirolimus