Nuclear redistribution of tonicity-responsive enhancer binding protein requires proteasome activity

Tonicity-responsive enhancer binding protein (To-nEBP) is the transcription factor that regulates tonicity-responsive expression of the genes for the sodium-myoinositol cotransporter (SMIT) and the sodium-chloride-betaine cot ransporter (BGT1). Hypertonicity stimulates the activity of TonEBP due to a combination of increased protein abundance and increased nuclear distribut ion (proportion of TonEBP that is in the nucleus). We found that inhibitors of proteasome activity markedly reduce the induction of SMIT and BGT1 mRNA in response to hypertonicity. These inhibitors also reduce hypertonicity-i nduced stimulation of expression of a reporter gene controlled by the tonic ity-responsive enhancer. Western and immunohistochemical analyses revealed that the proteasome inhibitors reduce the hypertonicity-induced increase of TonEBP in the nucleus by inhibiting its nuclear redistribution without aff ecting its abundance. Although the nuclear distribution of TonEBP is sensit ive to inhibition of proteasome activity as is that of nuclear factor (NF)- kappa B, the signaling pathways appear to be different in that hypertonicit y does not affect the nuclear distribution of NF-kappa B. Conversely, treat ment with tumor necrosis factor-alpha increases the nuclear distribution of NF-kappa B but not TonEBP.