Osmotic response element enhancer activity - Regulation through p38 kinaseand mitogen-activated extracellular signal-regulated kinase kinase

Hypertonicity induces a group of genes that are responsible for the intrace llular accumulation of protective organic osmolytes such as sorbitol and be taine. Two representative genes are the aldose reductase enzyme (AR, EC 1.1 .1.21), which is responsible for the conversion of glucose to sorbitol, and the betaine transporter (BGT1), which mediates Na+-coupled betaine uptake in response to osmotic stress. We recently reported that the induction of B GT1 mRNA in the renal epithelial Madin-Darby canine kidney cell line is inh ibited by SB203580, a specific p38 kinase inhibitor. In these studies we re port that the hypertonic induction of aldose reductase mRNA in HepG2 cells as well as the osmotic response element (ORE) driven reporter gene expressi on in transfected HepG2 cells are both inhibited by SB203580, suggesting th at p38 kinase mediates the activation and/or binding of the transcription f actor(s) to the ORE. Electrophoretic gel mobility shift assays with cell ex tracts prepared from SB203580-treated, hypertonically stressed HepG2 cells further show that the binding of trans-acting factors to the ORE is prevent ed and is thus also dependent on the activity of p38 kinase. Similarly, tre atment of hypertonically stressed cells with PD098059, a mitogen-activated extracellular regulated kinase kinase (MEK1) inhibitor, results in inhibiti on of the hypertonic induction of aldose reductase mRNA ORE- driven reporte r gene expression, and the binding of trans-acting factors to the ORE. ORE- driven reporter gene expression was not affected by p38 kinase inhibition o r MEK1 inhibition in cells incubated in isoosmotic media, These data indica te that p38 kinase and MEK1 are involved in the regulation of the hyperosmo tic stress response.