Increased reactive oxygen species contribute to high NaCl-induced activation of the osmoregulatory transcription factor TonEBP/OREBP

Proteomic identification of proteins associated with the osmoregulatory transcription factor TonEBP/OREBP: functional effects of Hsp90 and PARP-1

Hypertonicity (e.g., high NaCl) activates the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP), increasing transcription of protective genes. In the present studies, by stably expressing amino acids 1-547 of TonEBP/OREBP in HEK 293 cells and immunoprecipitating it plus associated proteins from the nuclei of cells exposed to high NaCl, we identify 14 proteins that are physically associated with TonEBP/OREBP. The associated proteins fall into several classes: 1) DNA-dependent protein kinase, both its catalytic subunit and regulatory subunit, Ku86; 2) RNA helicases, namely RNA helicase A, nucleolar RNA helicase II/Gu, and DEAD-box RNA helicase p72; 3) small or heterogeneous nuclear ribonucleoproteins (snRNPs or hnRNPs), namely U5 snRNP-specific 116 kDa protein, U5 snRNP-specific 200 kDa protein, hnRNP U, hnRNP M, hnRNP K, and hnRNP F; 4) heat shock proteins, namely Hsp90beta and Hsc70; and 5) poly(ADP-ribose) polymerase-1 (PARP-1). We confirm identification of most of the proteins by Western analysis and also demonstrate by electrophoretic mobility-shift assay that they are present in the large complex that binds specifically along with TonEBP/OREBP to its cognate DNA element. In addition, we find that PARP-1 and Hsp90 modulate TonEBP/OREBP activity. PARP-1 expression reduces TonEBP/OREBP transcriptional activity and the activity of its transactivating domain. Hsp90 enhances those activities and sustains the increased abundance of TonEBP/OREBP protein in cells exposed to high NaCl.

Mitochondrial reactive oxygen species contribute to high NaCl-induced activation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), resulting in increased expression of genes involved in osmoprotective accumulation of organic osmolytes, including glycine betaine, and in increased expression of osmoprotective heat shock proteins. Our previous studies showed that high NaCl increases reactive oxygen species (ROS), which contribute to activation of TonEBP/OREBP. Mitochondria are a major source of ROS. The purpose of the present study was to examine whether mitochondria produce the ROS that contribute to activation of TonEBP/OREBP. We inhibited mitochondrial ROS production in HEK293 cells with rotenone and myxothiazol, which inhibit mitochondrial complexes I and III, respectively. Rotenone (250 nM) and myxothiazol (12 nM) reduce high NaCl-induced ROS over 40%, whereas apocynin (100 microM), an inhibitor of NADPH oxidase, and allopurinol (100 microM), an inhibitor of xanthine oxidase, have no significant effect. Rotenone and myxothiazol reduce high NaCl-induced increases in TonEBP/OREBP transcriptional activity (ORE/TonE reporter assay) and BGT1 (betaine transporter) mRNA abundance ranging from 53 to 69%. They inhibit high NaCl-induced TonEBP/OREBP transactivating activity, but not its nuclear translocation. Release of ATP into the medium on hypertonic stress has been proposed to be a signal that triggers cellular osmotic responses. However, we do not detect release of ATP into the medium or inhibition of high NaCl-induced ORE/TonE reporter activity by an ATPase, apyrase (20 U/ml), indicating that high NaCl-induced activation of TonEBP/OREBP is not mediated by release of ATP. We conclude that high NaCl increases mitochondrial ROS production, which contributes to the activation of TonEBP/OREBP by increasing its transactivating activity.