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Mre11-Rad50-Nbs1 complex is activated by hypertonicity

¹Division of Nephrology, ⁴Departmnet of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland; ²Department of Veterinary Medicine, Chungnam National University, Daejeon, Korea; and ³Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan

Submitted 2 May 2006 ; accepted in final form 12 June 2006

When exposed to hypertonic conditions, cells accumulate double-strand DNA breaks (DSBs) like they are exposed to ionizing radiation. It has been proposed that inactivation of the Mre11-Rad50-Nbs1 (MRN) complex due to nuclear exit is responsible for the accumulation of DSBs as cells fail to repair DSBs produced during normal cellular activity. In this study, we examined the MRN complex in cells switched to hypertonicity. Surprisingly, we found that the MRN complex stayed in the nucleus and remained intact in response to hypertonicity. In fact, the MRN complex was dramatically activated after 4 h of switch to hypertonicity in a dose-dependent manner as shown by formation of foci. Activation of ATM and the MRN complex by hypertonicity and bleomycin was additive as was activation of their downstream targets including H2AX and Chk2 indicating that the cellular response to DSB was intact in hypertonic conditions. Activation of Chk2 in response to hypertonicity was not observed in mutant cells with functionally impaired MRN complex confirming that they are in the same pathway. After 20 h of a switch to hypertonicity, MRN foci and H2AX returned to a control level, suggesting that cells adapted to hypertonicity by repairing DNA. We conclude that cells respond normally to DSB and repair the DNA damages induced by hypertonicity.

ATM; double-strand DNA breaks; renal medulla; cell cycle checkpoint; DNA repair; histone H2AX; Chk2

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