title: ULK1 inhibits the kinase activity of mTORC1 and cell proliferation.
abstract: ULK1 (Unc51-like kinase, hATG1) is a Ser/Thr kinase that plays a key role in inducing autophagy in response to starvation. ULK1 is phosphorylated and negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1). Previous studies have shown that ULK1 is not only a downstream effector of mTORC1 but also a negative regulator of mTORC1 signaling. ( 1-3)  Here, we investigated how ULK1 regulates mTORC1 signaling, and found that ULK1 inhibits the kinase activity of mTORC1 and cell proliferation. Deficiency or knockdown of ULK1 or its homolog ULK2 enhanced mTORC1 signaling, cell proliferation rates and accumulation of cell mass, whereas overexpression of ULK1 had the opposite effect. Knockdown of Atg13, the binding partner of ULK1 and ULK2, mimicked the effects of ULK1 or ULK2 deficiency or knockdown. Both insulin and leucine stimulated mTORC1 signaling to a greater extent when ULK1 or ULK2 was deficient or knocked down. In contrast, Atg5 deficiency did not have a significant effect on mTORC1 signaling and cell proliferation. The stimulatory effect of ULK1 knockdown on mTORC1 signaling occurred even in the absence of tuberous sclerosis complex 2 (TSC2), the negative regulator of mTORC1 signaling. In addition, ULK1 was found to bind raptor, induce its phosphorylation, and inhibit the kinase activity of mTORC1. These results demonstrate that ULK1 negatively regulates the kinase activity of mTORC1 and cell proliferation in a manner independent of Atg5 and TSC2. The  inhibition of mTORC1 by ULK1 may be important to coordinately regulate cell growth and autophagy with optimized utilization of cellular energy.
authors.affiliation: Department of Biochemistry, Molecular Biology and Biophysics; University of Minnesota, Minneapolis, MN, USA.
authors.forename: Chang Hwa
authors.initials: CH
authors.lastname: Jung
authors.forename: Minchul
authors.initials: M
authors.lastname: Seo
authors.forename: Neil Michael
authors.initials: NM
authors.lastname: Otto
authors.forename: Do-Hyung
authors.initials: DH
authors.lastname: Kim
mesh_terms: D000818:Animals; D000071187:Autophagy-Related Protein 5; D000071189:Autophagy-Related Protein-1 Homolog; D002460:Cell Line; D049109:Cell Proliferation; D005786:Gene Expression Regulation; D015972:Gene Expression Regulation, Neoplastic; D057809:HEK293 Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D046912:Multiprotein Complexes; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: C502778:ATG5 protein, human; C495773:Atg5 protein, mouse; D000071187:Autophagy-Related Protein 5; D047908:Intracellular Signaling Peptides and Proteins; D008869:Microtubule-Associated Proteins; D046912:Multiprotein Complexes; D011506:Proteins; C000624653:TSC2 protein, human; C000624654:Tsc2 protein, mouse; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins; D000071189:Autophagy-Related Protein-1 Homolog; D000076222:Mechanistic Target of Rapamycin Complex 1; D017346:Protein Serine-Threonine Kinases; D058570:TOR Serine-Threonine Kinases; C484754:ULK1 protein, human; C112444:Ulk1 protein, mouse