Tonicity-dependent regulation of osmoprotective genes in mammalian cells

Cells in the renal medulla are normally exposed to levels of NaCl that are extremely high and that vary with concentration of the urine. Such high levels of NaCl cause cellular perturbations, including increased DNA double-strand breaks, increased oxidation of DNA and proteins, and cytoskeletal alterations. Despite these perturbations the cells are able to survive and function because of osmoprotective responses that include accumulation of compatible organic osmolytes and increased abundance of heat shock proteins and water channels. Many of the responses are initiated by increased gene transcription, directed by the transcription factor TonEBP/OREBP. Here, we review the sensors of hypertonicity, the signaling pathways to TonEBP/OREBP, and the ways in which it is activated to increase transcription. Multiple signals are involved, including some that arise directly from the cellular perturbations caused by hypertonicity. Although the combination of these signals is necessary for full osmotic activation of TonEBP/OREBP, no one of them, alone, is sufficient. We conclude that hypertonicity profoundly alters the state of cells, providing numerous interrelated inputs to the osmoregulatory network.