Date of Graduation
Doctor of Philosophy (PhD)
Mong-Hong Lee, Ph.D.
Sai-Ching Yeung, M.D. Ph.D.
Randy Legerski, Ph.D.
Hui-Kuan Lin, Ph.D.
Zhimin Lu, Ph.D.
SCFFBXW7 is a tumor suppressor E3 ligase protein that targets numerous oncoproteins such as Cyclin E, c-Myc, c-Jun and MCL1. The deregulation of these proteins often leads to the proliferation of cancer cells. Thus, intracellular stability and functional activity of FBXW7 is critical for regulating cancer. However, there is a gap of knowledge about the intracellular signaling pathway or if there is another ubiquitin ligase that regulates FBXW7 stability. Here, I identify a novel mechanism of FBXW7 stability regulation which involves constitutive photomorphogenic 1 (COP1), AKT and CSN6 (i.e. the COP9 signalosome 6).
COP1 is an E3 ubiquitin ligase targeting important substrates such as, p53, 14-3-3-σ, and c-Jun. I found that COP1 binds to FBXW7 at the binding motif at V200 and P201. Interestingly, binding between COP1 and FBXW7 was dissociated by loss function of AKT. In addition, IGF-1 or EGF induced AKT1 directly regulates FBXW7 stability through phosphorylation of FBXW7 at T226 and S227. Moreover, phosphorylation of FBXW7 at T226 and S227 by AKT facilitates its interaction with COP1, and consequently induces ubiquitination of FBXW7 by COP1. Significantly, TSAA FBXW7 mutant reduced cell invasion, migration, proliferation and cell cycle progression. Thus, results define a novel signaling pathway for regulation of FBXW7 through the AKT-COP1 axis which can be applied for therapeutic intervention in cancers overexpressing AKT and COP1.
CSN functions as an adaptor between substrate proteins and the 26S proteasome to facilitate proteasomal degradation of ubiquitinated proteins. In this study, I also found that CSN6 associates with, and causes the degradation of, FBXW7. Moreover, CSN6 regulates stabilization of Cyclin E and c-Jun through its negative effect on FBXW7, which in turn reduces ubiquitin-mediated protein degradation of Cyclin E and c-Jun. Therefore, CSN6 knockdown results in reduced cell migration, transformational activity, and tumor growth.
Together, my findings indicate the novel signaling pathway for regulation of FBXW7 through COP1, AKT or CSN6 which can be applied for therapeutic intervention in cancers overexpressing AKT and COP1 or Cyclin E and c-Jun.
FBXW7, COP1, AKT, CSN6, Cyclin E, c-Jun