SAG/ROC2/Rbx2 is a novel activator protein-1 target that promotes c-Jun degradation and inhibits 12-O-tetradecanoylphorbol-13-acetate-induced neoplastic transformation.
SAG (sensitive to apoptosis gene) was first identified as a stress-responsive protein that, when overexpressed, inhibited apoptosis both in vitro and in vivo. SAG was later found to be the second family member of ROC1 or Rbx1, a RING component of SCF and DCX E3 ubiquitin ligases. We report here that SAG/ROC2/Rbx2 is a novel transcriptional target of activator protein-1 (AP-1). AP-1 bound both in vitro and in vivo to two consensus binding sites in a 1.3-kb region of the mouse SAG promoter. The SAG promoter activity, as measured by luciferase reporter assay, was dependent on these sites. Consistently, endogenous SAG is induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) with an induction time course following the c-Jun induction in both mouse epidermal JB6-Cl.41 and human 293 cells. TPA-mediated SAG induction was significantly reduced in JB6-Cl.41 cells overexpressing a dominant-negative c-Jun, indicating a requirement of c-Jun/AP-1. On the other hand, SAG seemed to modulate the c-Jun levels. When overexpressed, SAG remarkably reduced both basal and TPA-induced c-Jun levels, whereas SAG small interfering RNA (siRNA) silencing increased substantially the levels of both basal and TPA-induced c-Jun. Consistently, SAG siRNA silencing reduced c-Jun polyubiquitination and blocked c-Jun degradation induced by Fbw7, an F-box protein of SCF E3 ubiquitin ligase. Finally, SAG overexpression inhibited, whereas SAG siRNA silencing enhanced, respectively, the TPA-induced neoplastic transformation in JB6-Cl.41 preneoplastic model. Thus, AP-1/SAG establishes an autofeedback loop, in which on induction by AP-1, SAG promotes c-Jun ubiquitination and degradation, thus inhibiting tumor-promoting activity of AP-1.
Gu
Qingyang
Q
Department of Radiation Oncology, University of Michigan Comprehensive Cancer Center, 1500 East Medical Center Drive, Ann Arbor, MI 48109, USA.
Tan
Mingjia
M
Sun
Yi
Y
D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism*; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D003001:Cloning, Molecular; D016384:Consensus Sequence; D000089662:Doublecortin Protein; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D006367:HeLa Cells; D006801:Humans; D011401:Promoter Regions, Genetic; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000037:antagonists & inhibitors / Q000494:pharmacology; D018808:Transcription Factor AP-1 / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism
D002352:Carrier Proteins; C000714847:Dcx protein, mouse; D000089662:Doublecortin Protein; D016755:Proto-Oncogene Proteins c-jun; D016601:RNA-Binding Proteins; D018808:Transcription Factor AP-1; D014452:Ubiquitins; C118307:RNF7 protein, human; D044767:Ubiquitin-Protein Ligases; D013755:Tetradecanoylphorbol Acetate