Chronic hyperosmolarity mediates constitutive expression of molecular chaperones and resistance to injury

Renal medullary cells are exposed to elevated and variable osmolarities and low oxygen tension. Despite the harsh environment, these cells are resistant to the effects of many harmful events. To test the hypothesis that this resistance is a consequence of these cells developing a stress tolerance phenotype to survive in this milieu, we created osmotically tolerant cells [hypertonic (HT) cells] by gradually adapting murine inner medullary collecting duct 3 cells to hyperosmotic medium containing NaCl and urea. HT cells have a reduced DNA synthesis rate, with the majority of cells arrested in the G₀/G₁ phase of the cell cycle, and show constitutive expression of heat shock protein 70 that is proportional to the degree of hyperosmolarity. Unlike acute hyperosmolarity, chronic hyperosmolarity failed to activate MAPKs. Moreover, HT cells acquired protein translational tolerance to further stress treatment, suggesting that HT cells have an osmotolerant phenotype that is analogous to thermotolerance but is a permanent condition. In addition to osmotic shock, HT cells were more resistant to heat, H₂O₂, cyclosporin, and apoptotic inducers, compared with isotonic murine inner medullary duct 3 cells, but less resistant to amphotericin B and cadmium. HT cells demonstrate that in renal medullary cells, hyperosmotic stress activates biological processes that confer cross-tolerance to other stressful conditions.