Sensitive to Apoptosis Gene (SAG, human;Sag, mouse) also known as RBX2/ROC2, is an essential RING component of SCF (SKP1, Cullins, F-box proteins) E3 ubiquitin ligase which, by promoting ubiquitination and degradation of various key regulatory proteins, controls several important biological processes including cell cycle progression, signal transduction, transcription, and DNA replication. Our published and unpublished data showed a direct association of SAG with pancreatic cancer as follows: a) SAG is overexpressed in pancreatic cancer tissues, and SAG nuclear expression is associated with poor prognosis of pancreatic cancer patients;b) SAG siRNA knockdown caused accumulation of tumor suppressive proteins and inhibited survival and anchorage-independent growth of pancreatic cancer cells, and c) orthotopic in vivo growth and matrigel invasion of pancreatic cancer cells were significantly inhibited, respectively, by SAG siRNA knockdown. All these studies suggested that SAG overexpression could be oncogenic during pancreatic tumorigenesis. The objectives of this application are to use pancreatic specific Sag transgenic or KO mouse models to mechanistically study the role of Sag in pancreatic tumorigenesis triggered by Kras activation (KrasG12D) and p53 mutation (p53R172H). The central hypothesis is that SAG promotes tumorigenesis by promoting the degradation of tumor suppressive substrates such as DEPTOR, I?B, and p21/p27, leading to activation of the mTOR and NF?B pathways. Thus, SAG-pancreatic transgenic expression would promote pancreatic tumorigenesis, initiated by KrasG12D, whereas Sag pancreatic deletion would abrogate oncogenic signals by accumulation of tumor suppressive proteins, thus suppressing pancreatic tumorigenesis triggered by KrasG12D and p53R172H.
Two specific aims are proposed to 1) determine the role of Sag in KrasG12D/p53R172H-induced pancreatic tumorigenesis and 2) elucidate mechanism(s) by which Sag regulates pancreatic tumorigenesis. IMPACT: Our study uses mouse models that recapitulate the development of pancreatic ductal adenocarcinoma (PDAC) to investigate the role of Sag E3 ligase in the initiation and progression of PDAC and to elucidate its mechanism of action. Our study will mechanistically validate SAG E3 ubiquitin ligase as a novel anti-pancreatic cancer target and provide proof-of-concept evidence for future discovery of specific inhibitor of SAG E3 ligase as a novel class of anti-pancreatic cancer drugs. Our work is, therefore, highly innovative and of significant impact with translational value.

Public Health Relevance

Targeted cancer therapy relies on thorough validation of cancer targets. We found that SAG E3 ubiquitin ligase is overexpressed in pancreatic cancer, which is associated with poor prognosis of patients. In this study, we will mechanistically elucidate the role of Sag in pancreatic tumorigenesis triggered by Kras and p53 mutations using mouse KO models. Thus, this study is of highly translational value by mechanistically validating SAG E3 ubiquitin ligase as an attractive anti-pancreatic cancer target.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Exploratory/Developmental Grants (R21)
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Basic Mechanisms of Cancer Therapeutics Study Section (BMCT)
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Hildesheim, Jeffrey
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University of Michigan Ann Arbor
Schools of Medicine
Ann Arbor
United States
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Li, Hua; Sun, Grace Y; Zhao, Yongchao et al. (2014) DEPTOR has growth suppression activity against pancreatic cancer cells. Oncotarget 5:12811-9
Zhao, Yongchao; Morgan, Meredith A; Sun, Yi (2014) Targeting Neddylation pathways to inactivate cullin-RING ligases for anticancer therapy. Antioxid Redox Signal 21:2383-400
Tan, M; Li, H; Sun, Y (2014) Endothelial deletion of Sag/Rbx2/Roc2 E3 ubiquitin ligase causes embryonic lethality and blocks tumor angiogenesis. Oncogene 33:5211-20
Xiong, Xiufang; Zhao, Yongchao; Tang, Fei et al. (2014) Ribosomal protein S27-like is a physiological regulator of p53 that suppresses genomic instability and tumorigenesis. Elife 3:e02236
Li, Hua; Tan, Mingjia; Jia, Lijun et al. (2014) Inactivation of SAG/RBX2 E3 ubiquitin ligase suppresses KrasG12D-driven lung tumorigenesis. J Clin Invest 124:835-46
Zhou, Weihua; Wei, Wenyi; Sun, Yi (2013) Genetically engineered mouse models for functional studies of SKP1-CUL1-F-box-protein (SCF) E3 ubiquitin ligases. Cell Res 23:599-619
Zhao, Yongchao; Sun, Yi (2013) Cullin-RING Ligases as attractive anti-cancer targets. Curr Pharm Des 19:3215-25
Xie, Chuan-Ming; Wei, Wenyi; Sun, Yi (2013) Role of SKP1-CUL1-F-box-protein (SCF) E3 ubiquitin ligases in skin cancer. J Genet Genomics 40:97-106
Sun, Yi; Li, Hua (2013) Functional characterization of SAG/RBX2/ROC2/RNF7, an antioxidant protein and an E3 ubiquitin ligase. Protein Cell 4:103-16