mTORC1 signaling can regulate growth factor activation of p44/42 mitogen-activated protein kinases through protein phosphatase 2A.
The mTORC1 complex (mammalian target of rapamycin (mTOR)-raptor) is modulated by mitogen-activated protein (p44/42 MAP) kinases (p44/42) through phosphorylation and inactivation of the tuberous sclerosis complex. However, a role for mTORC1 signaling in modulating activation of p44/42 has not been reported. We show that in two cancer cell lines regulation of the p44/42 MAPKs is mTORC1-dependent. In Rh1 cells rapamycin inhibited insulin-like growth factor-I (IGF-I)-stimulated phosphorylation of Thr(202) but not Tyr(204) and suppressed activation of p44/42 kinase activity. Down-regulation of raptor, which inhibits mTORC1 signaling, had a similar effect to rapamycin in blocking IGF-I-stimulated Tyr(204) phosphorylation. Rapamycin did not block maximal phosphorylation of Tyr(204) but retarded the rate of dephosphorylation of Tyr(204) following IGF-I stimulation. IGF-I stimulation of MEK1 phosphorylation (Ser(217/221)) was not inhibited by rapamycin. Higher concentrations of rapamycin (> or =100 ng/ml) were required to inhibit epidermal growth factor (EGF)-induced phosphorylation of p44/42 (Thr(202)). Rapamycin-induced inhibition of p44/42 (Thr(202)) phosphorylation by IGF-I was reversed by low concentrations of okadaic acid, suggesting involvement of protein phosphatase 2A (PP2A). Both IGF-I and EGF caused dissociation of PP2A catalytic subunit (PP2Ac) from p42. Whereas low concentrations of rapamycin (1 ng/ml) inhibited dissociation of PP2Ac after IGF-I stimulation, it required higher concentrations (> or =100 ng/ml) to block EGF-induced dissociation, consistent with the ability for rapamycin to attenuate growth factor-induced activation of p44/42. The effect of rapamycin on IGF-I or insulin activation of p44/42 was recapitulated by amino acid deprivation. Rapamycin effects altering the kinetics of p44/42 phosphorylation were completely abrogated in Rh1mTORrr cells that express a rapamycin-resistant mTOR, whereas the effects of amino acid deprivation were similar in Rh1 and Rh1mTORrr cells. These results indicate complex regulation of p44/42 by phosphatases downstream of mTORC1. This suggests a model in which mTORC1 modulates the phosphorylation of Thr(202) on p44/42 MAPKs through direct or indirect regulation of PP2Ac.
Department of Molecular Pharmacology, St. Jude Children's Research Hospital, 332 N. Lauderdale Street, Memphis, TN 38105, USA.
Franklin C
FC
Harwood
Lili
L
Shu
Peter J
PJ
Houghton
D000595:Amino Acid Sequence; D001483:Base Sequence; D045744:Cell Line, Tumor; D004247:DNA / Q000235:genetics; D004815:Epidermal Growth Factor / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007700:Kinetics; D020935:MAP Kinase Signaling System / Q000187:drug effects; D000076222:Mechanistic Target of Rapamycin Complex 1; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism*; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes; D019319:Okadaic Acid / Q000494:pharmacology; D010766:Phosphorylation; D054648:Protein Phosphatase 2 / Q000378:metabolism*; D011506:Proteins; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
D007328:Insulin; D046912:Multiprotein Complexes; D011506:Proteins; D014157:Transcription Factors; D019319:Okadaic Acid; D004815:Epidermal Growth Factor; D007334:Insulin-Like Growth Factor I; D004247:DNA; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D054648:Protein Phosphatase 2; D020123:Sirolimus