Mechanism of cell cycle checkpoint in the cells transfected with sense or antisense gadd45 gene

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Gadd (growth arrest and DNA damage-inducible) 45 gene is inducible with ionizing radiation and suggested to be involved in the p53 dependent G1 checkpoint pathway. Ataxia telangiectasia (AT) cells are characterized by hypersensitivity to ionizing radiation-induced cell killing and chromosome aberrations. Recently, it has been shown that AT cells have a defect in p53 dependent G1 checkpoint pathway. To understand the defect of G1 checkpoint in AT cells, we studied the relationship between AT phenotype and abnormality of the expression of the gadd45 gene product (GADD45). We examined the time dependent expression of GADD45 after X-irradiation and confirmed that X-ray-induced expression of the GADD45 was severely delayd in AT5BIVA cells compared to the control RKO cells. The basal expression level also diminished in the AT cells. To know the function of the gadd45 gene in the AT cells, we transfected the inducible human gadd45 gene into the AT5BIVA cells and established the AT cells whose GADD45 expression was inducible by isopropyl thiogalactoside (IPTG) treatment. Using this AT cells, we investigated the effect of GADD45 expression on colony forming ability, cell growth, cell cycle arrest and radiation sensitivity. We found that the increased expression of GADD45 reduced plating efficiency to 60% of control and delayd cell growth, confirming that the gadd45 gene functions as a negative growth regulator. The excess GADD45 expression, however, did not cause G1 or G2 arrest. The hypersensitivity to X-rays in the AT cells was not changed after induction of GADD45. The results indicate that the defect in GADD45 expression is not responsible for the abnormality in cell cycle checkpoint and the radiation hypersensitivity in AT cells.