title: mTOR complex 2 is involved in regulation of Cbl-dependent c-FLIP degradation and sensitivity of TRAIL-induced apoptosis.
abstract: The mTOR positively regulates cell proliferation and survival through forming 2 complexes with raptor (mTOR complex 1; mTORC1) or rictor (mTOR complex 2; mTORC2). Compared with the mTORC1, relatively little is known about the biologic functions of mTORC2. This study focuses on addressing whether mTORC2 regulates apoptosis, particularly induced by TRAIL (TNFSF10). Using the mTOR kinase inhibitor, PP242, as a research tool, we found that it synergized with TRAIL to augment apoptosis of cancer cells. PP242 reduced the abundance of the short form of c-FLIP (FLIP(S), CFLAR(S)) and survivin (BIRC5). Enforced expression of ectopic FLIP(S), but not survivin, attenuated augmented apoptosis induced by PP242 plus TRAIL. Thus, it is FLIP(S) downregulation that contributes to synergistic induction of apoptosis by PP242 plus TRAIL. PP242 decreased FLIP(S) stability, increased FLIP(S) ubiquitination, and facilitated FLIP(S) degradation. Moreover, knockdown of the E3 ligase Cbl (CBL) abolished PP242-induced FLIP(S) reduction. Thus, PP242 induces Cbl-dependent degradation of FLIP(S), leading to FLIP(S) downregulation. Consistently, knockdown of rictor or mTOR, but not raptor, mimicked PP242 in decreasing FLIP(S) levels and sensitizing cells to TRAIL. Rictor knockdown decreased FLIP(S) stability, whereas enforced expression of rictor stabilized FLIP(S). Moreover, silencing of Cbl abrogated FLIP(S) reduction induced by rictor knockdown. Collectively we conclude that it is mTORC2 inhibition that results in FLIP(S) downregulation and subsequent sensitization of TRAIL-induced apoptosis. Our findings provide the first evidence showing that mTORC2 stabilizes FLIP(S), hence connecting mTORC2 signaling to the regulation of death receptor-mediated apoptosis.
authors: Department of Hematology and Medical Oncology, Emory University School of Medicine and Winship Cancer Institute, Atlanta, Georgia 30322, USA.
authors: Liqun
authors: L
authors: Zhao
authors: Ping
authors: P
authors: Yue
authors: Fadlo R
authors: FR
authors: Khuri
authors: Shi-Yong
authors: SY
authors: Sun
mesh_terms: D017209:Apoptosis / Q000502:physiology*; D001483:Base Sequence; D015153:Blotting, Western; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000378:metabolism*; D045744:Cell Line, Tumor; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D000076225:Mechanistic Target of Rapamycin Complex 2; D046912:Multiprotein Complexes / Q000037:antagonists & inhibitors / Q000502:physiology*; D059748:Proteolysis; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D050721:Proto-Oncogene Proteins c-cbl / Q000502:physiology*; D011687:Purines / Q000494:pharmacology; D034741:RNA, Small Interfering; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*
chemical_list: D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; C506386:CFLAR protein, human; D007211:Indoles; D046912:Multiprotein Complexes; D011687:Purines; D034741:RNA, Small Interfering; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; D050721:Proto-Oncogene Proteins c-cbl; D000076225:Mechanistic Target of Rapamycin Complex 2; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; C572919:PP242