title: SAG-UPS attenuates proapoptotic SARM and Noxa to confer survival advantage to early hepatocellular carcinoma.
abstract: Hepatocellular carcinoma (HCC) is a deadly cancer because of its commonly late diagnosis and limited treatment options. SAG (sensitive to apoptosis gene)-dependent UPS (ubiquitin-proteasome system) is a key switch between immune-mediated apoptosis and overactivation-mediated protumorigenesis, prompting us to hypothesize that SAG-UPS modulates chronic inflammation-induced tumorigenesis. Here, we investigated the molecular mechanism by which SAG-UPS regulates death/survival of liver cancer cells. By retrospective studies, we found reciprocal expressions of anti-/proapoptotic factors: SAG/SARM and SAG/Noxa in human primary HCC tissues - the antiapoptotic SAG was significantly upregulated whereas the proapoptotic SARM and Noxa were markedly downregulated, suggesting their involvement in hepatocarcinogenesis. Upregulated SAG-UPS effectively manipulates the levels of high-molecular-weight ubiquitinated SARM and Noxa in carcinoma tissues compared with corresponding normal tissues. SAG-overexpressing HCC cell lines display reduced SARM and Noxa (but not Bcl-2, Bax and Bcl-xL), suggesting that SARM and Noxa are specific substrates of SAG-dependent ubiquitination. SARM overexpression activated caspase-3 and caspase-9, reducing cell viability. SAG knockdown significantly elevated apoptosis with increased cytosolic cytochrome c, confirming SAG-mediated antiapoptosis in HCC. SAG overexpression stimulated protumorigenic cytokines, IL-1β, IL-6 and TNF, but not antitumorigenic IL-12p40 and anti-inflammatory IL-10. This is consistent with higher proinflammatory cytokines (IL-1β, IL-6 and TNF) in hepatoma compared with healthy tissues. Altogether, early stage-upregulated SAG-UPS exacerbates hepatocarcinogenesis progression, through: (1) ubiquitination-mediated degradation of proapoptotic SARM and Noxa; and (2) production of protumorigenic cytokines that induce a protumorigenic microenvironment, conferring survival advantage to HCC cells. Thus, we propose SAG-UPS to be an early diagnostic marker for HCC, and a potential target for therapeutics development.
authors.affiliation: Department of Biological Sciences, National University of Singapore , 117543, Singapore.
authors.forename: S C
authors.initials: SC
authors.lastname: Chang
authors.affiliation: Department of Biological Sciences, National University of Singapore, 117543, Singapore; NUS Graduate School for Integrative Science and Engineering, National University of Singapore, 117543, Singapore.
authors.forename: W Q W
authors.initials: WQ
authors.lastname: Choo
authors.affiliation: Division of Medical Oncology, National Cancer Centre Singapore , 169610, Singapore.
authors.forename: H C
authors.initials: HC
authors.lastname: Toh
authors.affiliation: Department of Biological Sciences, National University of Singapore , 117543, Singapore.
authors.forename: J L
authors.initials: JL
authors.lastname: Ding