Homologous posttranscriptional regulation of insulin-like growth factor-I receptor level via glycogen synthase kinase-3beta and mammalian target of rapamycin in adrenal chromaffin cells: effect on tau phosphorylation.
In cultured bovine adrenal chromaffin cells, approximately 24 h-treatment with insulin-like growth factor-I (IGF-I) decreased cell surface (125)I-IGF-I binding capacity and IGF-I receptor protein level by approximately 64% (EC(50) = 5.0 nM; t(1/2) = approximately 7 h). IGF-I-induced IGF-I receptor decrease was abolished by LY294002 (phosphoinositide 3-kinase inhibitor) and partially attenuated by rapamycin (an inhibitor of mammalian target of rapamycin [mTOR]). SB216763 (an inhibitor of glycogen synthase kinase-3 [GSK-3]) down-regulated IGF-I receptor, which was further decreased by IGF-I. IGF-I increased inhibitory Ser(9)-phosphorylation of GSK-3beta and stimulatory Ser(2448)-phosphorylation of mTOR. l-leucine increased phosphorylation of mTOR (but not GSK-3beta), and down-regulated IGF-I receptor, both events being abolished by rapamycin. IGF-I-induced IGF-I receptor decrease was not prevented by proteolysis inhibitors. Pulse-label with [(35)S]methionine/cysteine followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that SB216763 or L-leucine retarded synthesis of IGF-I receptor and its precursor molecule. SB216763 (but not l-leucine) destabilized IGF-I receptor mRNA and decreased its level, without changing IGF-I receptor gene transcription. In SB216763-treated cells, IGF-I-induced Tyr-autophosphorylation of IGF-I receptor was decreased by 36%, compared to nontreated cells. IGF-I attenuated constitutive Ser(396)-phosphorylation of tau by 30% in nontreated cells, but not in SB216763-treated cells. IGF-I-induced down-regulations of (125)I-IGF-I binding and IGF-I receptor, as well as IGF-I-induced phosphorylations of GSK-3beta and mTOR were restored to the control levels of nontreated cells after washout of IGF-I (10 nM for 12 h)-treated cells. Thus, IGF-I down-regulated functional IGF-I receptor via GSK-3beta inhibition and mTOR activation; constitutive activity of GSK-3beta maintained IGF-I receptor level in nonstimulated cells.
Department of Pharmacology, Miyazaki Medical College, University of Miyazaki, Miyazaki, Japan.
Takayuki
T
Nemoto
Shinya
S
Satoh
Toyoaki
T
Maruta
Tasuku
T
Kanai
Norie
N
Yoshikawa
Satoshi
S
Miyazaki
Toshihiko
T
Yanagita
Akihiko
A
Wada
D000311:Adrenal Glands / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000187:drug effects / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D038362:Glycogen Synthase Kinase 3 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D007930:Leucine / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases; D016875:tau Proteins / Q000378:metabolism*
D038362:Glycogen Synthase Kinase 3; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I; D047908:Intracellular Signaling Peptides and Proteins; D007930:Leucine; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D012333:RNA, Messenger; D017526:Receptor, IGF Type 1; D058570:TOR Serine-Threonine Kinases; D016875:tau Proteins