Cells adapted to high NaCl have many DNA breaks and impaired DNA repair both in cell culture and in vivo.

Acute exposure of cells in culture to high NaCl damages DNA and impairs its repair. However, after several hours of cell cycle arrest, cells multiply in the hypertonic medium. Here, we show that, although adapted cells proliferate rapidly and do not become apoptotic, they nevertheless contain numerous DNA breaks, which do not elicit a DNA damage response. Thus, in adapted cells, Mre11 exonuclease is mainly present in the cytoplasm, rather than nucleus, and histone H2AX and chk1 are not phosphorylated, as they normally would be in response to DNA damage. Also, the adapted cells are deficient in repair of luciferase reporter plasmids damaged by UV irradiation. On the other hand, the DNA damage response activates rapidly when the level of NaCl is reduced. Then, Mre11 moves into the nucleus, and H2AX and chk1 become phosphorylated. Renal inner medullary cells in vivo are normally exposed to a variable, but always high, level of NaCl. As with adapted cells in culture, inner medullary cells in normal mice exhibit numerous DNA breaks. These DNA breaks are rapidly repaired when the NaCl level is decreased by injection of the diuretic furosemide. Moreover, repair of DNA breaks induced by ionizing radiation is inhibited in the inner medulla. Histone H2AX does not become phosphorylated, and repair synthesis is not detectable in response to total body irradiation unless NaCl is lowered by furosemide. Thus, both in cell culture and in vivo, although cells adapt to high NaCl, their DNA is damaged and its repair is inhibited.