title: S6K1 regulates GSK3 under conditions of mTOR-dependent feedback inhibition of Akt.
abstract: Feedback inhibition of the PI3K-Akt pathway by the mammalian target of rapamycin complex 1 (mTORC1) has emerged as an important signaling event in tumor syndromes, cancer, and insulin resistance. Cells lacking the tuberous sclerosis complex (TSC) gene products are a model for this feedback regulation. We find that, despite Akt attenuation, the Akt substrate GSK3 is constitutively phosphorylated in cells and tumors lacking TSC1 or TSC2. In these settings, GSK3 phosphorylation is sensitive to mTORC1 inhibition by rapamycin or amino acid withdrawal, and GSK3 becomes a direct target of S6K1. This aberrant phosphorylation leads to decreased GSK3 activity and phosphorylation of downstream substrates and contributes to the growth-factor-independent proliferation of TSC-deficient cells. We find that GSK3 can also be regulated downstream of mTORC1 in a HepG2 model of cellular insulin resistance. Therefore, we define conditions in which S6K1, rather than Akt, is the predominant GSK3 regulatory kinase.
authors.affiliation: Department of Genetics and Complex Diseases, Harvard School of Public Health, Harvard Medical School, Boston, Massachusetts 02115, USA.
authors.forename: Hui H
authors.initials: HH
authors.lastname: Zhang
authors.forename: Alex I
authors.initials: AI
authors.lastname: Lipovsky
authors.forename: Christian C
authors.initials: CC
authors.lastname: Dibble
authors.forename: Mustafa
authors.initials: M
authors.lastname: Sahin
authors.forename: Brendan D
authors.initials: BD
authors.lastname: Manning
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D005347:Fibroblasts / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D008954:Models, Biological; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000378:metabolism
chemical_list: C500984:CRTC1 protein, human; D014157:Transcription Factors; D011494:Protein Kinases; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D038362:Glycogen Synthase Kinase 3