Amino acid depletion activates TonEBP and sodium-coupled inositol transport

The expression of the osmosensitive sodium/myo-inositol cotransporter (SMIT ) is regulated by multiple tonicity-responsive enhancers (TonEs) in the 5'- flanking region of the gene. In response to hypertonicity, the nuclear abun dance of the transcription factor TonE-binding protein (TonEBP) is increase d, and the transcription of the SMIT gene is induced. Transport system A fo r neutral amino acids, another osmosensitive mechanism, is progressively st imulated if amino acid substrates are not present in the extracellular comp artment. Under this condition, as in hypertonicity, cells shrink and mitoge n-activated protein kinases are activated. We demonstrate here that a clear -cut nuclear redistribution of TonEBP, followed by SMIT expression increase and inositol transport activation, is observed after incubation of culture d human fibroblasts in Earle's balanced salts (EBSS), an isotonic, amino ac id-free saline. EBSS-induced SMIT stimulation is prevented by substrates of system A, although these compounds do not compete with inositol for transp ort through SMIT. We conclude that the incubation in isotonic, amino acid-f ree saline triggers an osmotic stimulus and elicits TonEBP-dependent respon ses like hypertonic treatment.