| Summary: | 博士 === 國立陽明大學 === 生化暨分子生物研究所 === 95 === Nijmegen breakage syndrome (NBS) is an autosomal recessive hereditary disorder characterized by microcephaly, growth retardation, immunodeficiency, radiosensitivity, chromosomal instability, and predisposition to tumor formation. The NBS1 gene product, NBS1 (p95) or nibrin, is a part of the hMre11 complex, a central player associated with DNA double strand break (DSB) repair. Our previous report demonstrated that c-Myc directly activates NBS1 expression. Here I have shown that constitutive expression of NBS1 in Rat1a and HeLa cells induces/enhances their transformation. Repression of endogenous NBS1 levels using short interference RNA (siRNA) reduces the transformation activity in HeLa and A594 cells. Increased NBS1 expression is observed in 40-52% of non-small cell lung cancer, hepatoma, and esophageal cancer samples. I have demonstrated that NBS1 overexpression stimulates phosphatidylinositol (PI) 3-kinase activity, leading to increased phosphorylation levels of Akt and its downstream targets such as glycogen synthase kinase 3b (GSK3b) and mammalian target of rapamycin (mTOR) in different cell lines and tumor samples. Transformation induced by NBS1 overexpression can be inhibited by a PI 3-kinase inhibitor (LY294002). Repression of endogenous Akt expression by short interference RNA decreases the transformation activity of Rat1a cells overexpressing NBS1.
Furthermore, I have demonstrated that NBS1 directly interacts, through a highly conserved C-terminal domain (a.a. 653-669) of NBS1, with the N-terminal domain (a.a. 1-108) of the p110a catalytic subunit of PI 3-kinase, and stimulates PI 3-kinase activity. Mutations of different regions of the 17 a.a. conserved motif abolish the ability of NBS1 to activate PI 3-kinase. Finally, expression of p110a is significantly associated with NBS1 expression in the head and neck squamous cell carcinoma (HNSCC) patient samples and co-expression of both markers predicts the worst prognosis.
In conclusion, our study demonstrates that overexpression of NBS1 is an oncogenic event that contributes to transformation through the activation of PI 3-kinase/Akt and NBS1 can function as an adaptor/activator of p110a PI 3-kinase through a novel activation motif (a.a. 653-669).
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