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DNA-PK is a nuclear protein with serine/threonine kinase activity and forms a complex consisting of the DNA-PKcs and a heterodimer of Ku70 and Ku80 proteins. Recent laboratory experiments have demonstrated that the DNA-PK complex formation is one of the major pathways by which mammalian cells respond to DNA double-strand breaks induced by ionizing radiation. In this study, we evaluated the relationship between expression levels of DNA-PKcs, Ku70 and Ku80 proteins and radiation sensitivity in oral squamous cell carcinoma (OSCC) cell lines and in OSCC patients treated with preoperative radiation therapy. The OSCC cell lines greatly differed in their response to irradiation, as assessed by a standard colony formation assay. However, the expression levels of the DNA-PK complex proteins were all similar, and there was no association between the magnitude of their expression and the tumor radiation sensitivity. Expression of DNA-PK complex proteins increased after radiation treatment, and the in- he DNA-PK complex is one of the major pathways by which cells respond to DNA double-strand breaks (DSBs). The DNA-PK complex consists of a heterodimer comprising 70-and 80-kDa proteins termed Ku and a 465-kDa serine/ threonine protein kinase catalytic subunit termed DNA-PKcs. 1)The Ku (p70/p80) component functions as an activator of the catalytic subunit, and also represents the major double-stranded DNA binding protein.1, 2) DNA-PK plays an important role in the repair of DSBs and in V(D)J recombination.3) Tumor cell lines defective in the expression of either Ku or DNA-PKcs exhibit marked radiation sensitivity. Cells lacking DNA-PK activity because of defects in DNA-PK components, such as human malignant glioma M059J cells and cells derived from scid mice, show hypersensitivity to ionizing radiation.3-7) These previous laboratory findings suggested that DNA-PK is a candidate as a predictor of cellular radiation sensitivity. There is, however, little information on the expression of DNA-PK in primary human tumors and the correlation, if any, with radiation sensitivity, though the results are not definitive. [8][9][10][11] Therefore, the aim of this study is to evaluate the relationship between expression levels of DNA-PK complex proteins and radiation sensitivity. Materials and MethodsCell culture. All SCC cell lines were grown in Ham/F12: DMEM (1:1) supplemented with 10% fetal bovine serum, 24 µg/ml adenine, 0.4 µg/ml hydrocortisone and 50 units/ml penicillin and streptomycin. The HSC2, HSC3 and HSC 4 cell lines were provided by Japanese Collection Research Bioresources. The SCC15, SCC25, SCC66 and SCC111 cell lines were provided by Dr.

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Previous studies have shown that low levels of p27^KIP1, an inhibitor of G1 cyclin-dependent kinases (CDK), are associated with high aggressiveness and poor prognosis in a variety of cancers. Decreased levels of p27^KIP1 are caused, at least in part, by an acceleration of degradation with Skp2 (S-phase kinase-associated protein 2) and Jab1 (Jun activation domain-binding protein 1). This investigation was undertaken to examine whether the Skp2 and Jab1 expression is correlated with p27^KIP1 protein levels, and how it is clinically relevant in oral squamous cell carcinoma (OSCC). The correlations between p27^KIP1 and Skp2, and p27^KIP1 and Jab1 expression were evaluated by Western blot analysis. Immunohistochemical analysis was done in 75 cases of OSCC. A strongly significant inverse correlation was found between levels of p27^KIP1 and Skp2, and p27^KIP1 and Jab1 (p < 0.0001). Thus, decreased levels of p27^KIP1 were associated with strongly increased levels of Skp2 and Jab1, whereas high levels of p27^KIP1 coincided with low levels of Skp2 and Jab1. Reductions of p27^KIP1 expression and overexpression of Skp2 and Jab1 were significantly associated with cervical lymph node metastasis and poor prognosis. Overexpression of Skp2 and Jab1 is associated with the reduction of p27^KIP1 expression, and may have a role in the progression of OSCC.