TRAF2 and OTUD7B govern a ubiquitin-dependent switch that regulates mTORC2 signalling

Abstract

The mechanistic target of rapamycin (mTOR) has a key role in the integration of various physiological stimuli to regulate several cell growth and metabolic pathways1. mTOR primarily functions as a catalytic subunit in two structurally related but functionally distinct multi-component kinase complexes, mTOR complex 1 (mTORCl) and mTORC2 (refs 1, 2). Dysregulation of mTOR signalling is associated with a variety of human diseases, including metabolic disorders and cancer1. Thus, both mTORCl and mTORC2 kinase activity is tightly controlled in cells. mTORCl is activated by both nutrients3–6 and growth factors7, whereas mTORC2 responds primarily to extracellular cues such as growth-factor-triggered activation of PI3K signalling8–10. Although both mTOR and GβL (also known as MLST8) assemble into mTORC1 and mTORC2 (refs 11–15), it remains largely unclear what drives the dynamic assembly of these two functionally distinct complexes. Here we show, in humans and mice, that the K63-linked polyubiquitination status of GβL dictates the homeostasis of mTORC2 formation and activation. Mechanistically, the TRAF2 E3 ubiquitin ligase promotes K63-linked polyubiquitination of GβL, which disrupts its interaction with the unique mTORC2 component SIN1 (refs 12–14) to favour mTORC1 formation. By contrast, the OTUD7B deubiquitinase removes polyubiquitin chains from GβL to promote GβL interaction with SIN1, facilitating mTORC2 formation in response to various growth signals. Moreover, loss of critical ubiquitination residues in GβL, by either K305R/K313R mutations or a melanoma-associated GβL(ΔW297) truncation, leads to elevated mTORC2 formation, which facilitates tumorigenesis, in part by activating AKT oncogenic signalling. In support of a physiologically pivotal role for OTUD7B in the activation of mTORC2/AKT signalling, genetic deletion of Otud7b in mice suppresses Akt activation and Kras-driven lung tumorigenesis in vivo. Collectively, our study reveals a GβL-ubiquitination-dependent switch that fine-tunes the dynamic organization and activation of the mTORC2 kinase under both physiological and pathological conditions.

Ubiquitin ligases in oncogenic transformation and cancer therapy

“…95). TRAF2 mediates K63-linked ubiquitylation of GβL (also known as mLST8; a component of both mTORCs) and disrupts its binding to the mTORC2-specific component mSIN1 (also known as MAPKAP1), thereby restricting formation of mTORC2 complexes 95 .…”

“…95). TRAF2 mediates K63-linked ubiquitylation of GβL (also known as mLST8; a component of both mTORCs) and disrupts its binding to the mTORC2-specific component mSIN1 (also known as MAPKAP1), thereby restricting formation of mTORC2 complexes 95 . Deubiquitylation of GβL by OTU domain-containing protein 7B (OTUD7B) re-establishes the GβL–mSIN1 interaction, mTORC2 formation and AKT phosphorylation (S473) 95 .…”

“…TRAF2 mediates K63-linked ubiquitylation of GβL (also known as mLST8; a component of both mTORCs) and disrupts its binding to the mTORC2-specific component mSIN1 (also known as MAPKAP1), thereby restricting formation of mTORC2 complexes 95 . Deubiquitylation of GβL by OTU domain-containing protein 7B (OTUD7B) re-establishes the GβL–mSIN1 interaction, mTORC2 formation and AKT phosphorylation (S473) 95 . The physiological relevance of this pathway can be illustrated in melanoma, where GβL mutants that lack the ubiquitylation site were identified, and ectopic expression of these mutants in a melanoma cell line increased chemoresistance in vitro and tumour growth upon xenotransplantation in nude mice 95 .…”

“…Deubiquitylation of GβL by OTU domain-containing protein 7B (OTUD7B) re-establishes the GβL–mSIN1 interaction, mTORC2 formation and AKT phosphorylation (S473) 95 . The physiological relevance of this pathway can be illustrated in melanoma, where GβL mutants that lack the ubiquitylation site were identified, and ectopic expression of these mutants in a melanoma cell line increased chemoresistance in vitro and tumour growth upon xenotransplantation in nude mice 95 . Second, OTUD7B amplifications were identified in a variety of cancers, including lung cancer, and homozygous deletion of Otud7b in the Kras LA2 lung cancer mouse model inhibited KRAS-driven lung tumorigenesis 95 .…”

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

“…mTOR (mammalian target of rapamycin), a serine/threonine protein kinase that centrally regulates cell metabolism, constitutively serves as the catalytic module inside two intracellular protein complexes, namely mTORC1 (mTOR Complex 1) and mTORC2 (mTOR Complex 2) (Wang, Jie, Joo, Ordureau, & Liu, 2017; Xu, Liu & Wei, 2014). With respect to structure, mTORC1 consists of three components, including mTOR, Raptor, and mLST8 (mTOR‐associated protein, LST8 homolog).…”