Ku80 functions as a tumor suppressor in hepatocellular carcinoma by inducing S-phase arrest through a p53-dependent pathway.
ABSTRACT Ku80 is a component of the protein complex called DNA-dependent protein kinase, which is involved in DNA double-strand break repair and multiple other functions. Previous studies revealed that Ku80 haplo-insufficient and poly (adenosine diphosphate-ribose) polymerase-null transgenic mice developed hepatocellular carcinoma (HCC) at a high frequency. The role of Ku80 has never been investigated in human HCC. Ku80 expressions in HCC and adjacent liver tissue were investigated by using immunohistochemical staining and western blot. Ku80 was transfected into a Ku80-deficient HCC cell line SMMC7721 cells, and the growth features of the Ku80-expressing cells and vector-transfected cells were studied both in vitro and in vivo. Cell cycle analysis and RNA interference were employed to investigate the mechanisms underlying the growth regulation associated with Ku80 expression. Ku80 was found frequently downregulated in HCC compared with adjacent liver tissue. Ku80 downregulation was significantly correlated with elevated hepatitis B virus-DNA load and severity of liver cirrhosis. Overexpression of Ku80 in SMMC7721 cells significantly suppressed cell proliferation in vitro and in vivo. Ku80 overexpression caused S-phase cell cycle arrest and was associated with upregulation of p53 and p21(CIP1/WAF1), and the inhibition of p53 or p21(CIP1/WAF1) expression by RNA interference overcame the growth suppression and S-phase arrest in the Ku80-expressing cells. A novel mechanism was revealed that Ku80 functions as a tumor suppressor in HCC by inducing S-phase arrest through a p53-dependent pathway.
- SourceAvailable from: Narendra Tuteja[show abstract] [hide abstract]
ABSTRACT: Ku is a heterodimeric protein composed of approximately 70- and approximately 80-kDa subunits (Ku70 and Ku80) originally identified as an autoantigen recognized by the sera of patients with autoimmune diseases. Ku has high binding affinity for DNA ends and that is why originally it was known as a DNA end binding protein, but now it is known to also bind the DNA structure at nicks, gaps, hairpins, as well as the ends of telomeres. It has been reported also to bind with sequence specificity to DNA and with weak affinity to RNA. Ku is an abundant nuclear protein and is present in vertebrates, insects, yeast, and worms. Ku contains ssDNA-dependent ATPase and ATP-dependent DNA helicase activities. It is the regulatory subunit of the DNA-dependent protein kinase that phosphorylates many proteins, including SV-40 large T antigen, p53, RNA-polymerase II, RP-A, topoisomerases, hsp90, and many transcription factors such as c-Jun, c-Fos, oct-1, sp-1, c-Myc, TFIID, and many more. It seems to be a multifunctional protein that has been implicated to be involved directly or indirectly in many important cellular metabolic processes such as DNA double-strand break repair, V(D)J recombination of immunoglobulins and T-cell receptor genes, immunoglobulin isotype switching, DNA replication, transcription regulation, regulation of heat shock-induced responses, regulation of the precise structure of telomeric termini, and it also plays a novel role in G2 and M phases of the cell cycle. The mechanism underlying the regulation of all the diverse functions of Ku is still obscure.Critical Reviews in Biochemistry and Molecular Biology 02/2000; 35(1):1-33. · 5.58 Impact Factor
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ABSTRACT: Hepatitis B virus (HBV) is a major cause of chronic liver inflammation worldwide. Recent knowledge of the virological and immunological events secondary to HBV infection has increased our understanding of the mechanisms involved in viral clearance and persistence. In this review, how the early virological and immunological events might influence the development of a coordinate activation of adaptive immunity necessary to control HBV infection is analysed. The mechanism(s) by which high levels of viral antigens, liver immunological features, regulatory cells and dendritic cell defects might maintain the HBV-specific immunological collapse, typical of chronic hepatitis B patients, is also examined.Journal of General Virology 07/2006; 87(Pt 6):1439-49. · 3.13 Impact Factor
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ABSTRACT: The Ku70/80 heterodimer is the regulatory subunit of the DNA-dependent protein kinase (DNA-PK) and its DNA-binding activity mediates DNA double-strand breaks repair. Although Ku80 was recently proposed as a caretaker gene involved in the control of genome integrity, no data are available on Ku70/80 DNA-binding activity in human tumors. Heterodimer DNA-binding activity and protein expression were assayed by electrophoretic-mobility-shift-assay (EMSA) and Western blot analysis, in nuclear and cytoplasmic extracts from eight breast, seven bladder primary tumors and three metastatic nodes from breast cancers. Corresponding normal tissues of the same patients were used as controls. Ten out of 15 tumors showed nuclear Ku-binding activity 3-10 times higher than in the normal tissues, irrespective of bladder or breast origin. Conversely, in 5/15 primary tumors and in all the metastatic nodes analysed, nuclear Ku-activity was 1.5-4.5-fold lower than in the corresponding normal tissues. Cytoplasmic heterodimer activity significantly differed between tumor and normal tissues, displaying a 2-10-fold increase in neoplastic tissues. Three different patterns combining both Ku expression and activity with tumor characteristics were identified. In low aggressive breast tumors p70/p80 proteins were expressed in tumor but not in normal tissues. The heterodimer binding-activity matched the protein levels. In non-invasive bladder carcinomas no significant differences in protein expression between tumor and the corresponding normal tissues were found, however heterodimer binding-activity was increased in tumor samples. In breast and bladder tumors, at the advanced stage and in node metastases, the binding activity was strongly reduced in tumor biopsies, however no differences were demonstrated between normal and tumor protein levels. Our results suggest a different modulation of Ku70/80 DNA-binding activity in human neoplastic tissues, possibly related to tumor progression. Findings provide further data on tissue-specific protein expression and post-translational regulation of heterodimer activity.Oncogene 03/2001; 20(6):739-47. · 7.36 Impact Factor