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.
Jiang
Y P
YP
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.
Ballou
L M
LM
Lin
R Z
RZ
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