Osmotic Response Element Enhancer Activity: REGULATION THROUGH p38 KINASE AND MITOGEN-ACTIVATED EXTRACELLULAR SIGNAL-REGULATED KINASE KINASE

Abstract

Hypertonicity induces a group of genes that are responsible for the intracellular accumulation of protective organic osmolytes such as sorbitol and betaine. 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 BGT1 mRNA in the renal epithelial Madin-Darby canine kidney cell line is inhibited by SB203580, a specific p38 kinase inhibitor. In these studies we report that the hypertonic induction of aldose reductase mRNA in HepG2 cells as well as the osmotic response element (ORE)-driven reporter gene expression in transfected HepG2 cells are both inhibited by SB203580, suggesting that p38 kinase mediates the activation and/or binding of the transcription factor(s) to the ORE. Electrophoretic gel mobility shift assays with cell extracts prepared from SB203580-treated, hypertonically stressed HepG2 cells further show that the binding of trans-acting factors to the ORE is prevented and is thus also dependent on the activity of p38 kinase. Similarly, treatment of hypertonically stressed cells with PD098059, a mitogen-activated extracellular regulated kinase kinase (MEK1) inhibitor, results in inhibition of the hypertonic induction of aldose reductase mRNA, ORE-driven reporter gene expression, and the binding of trans-acting factors to the ORE. ORE-driven reporter gene expression was not affected by p38 kinase inhibition or MEK1 inhibition in cells incubated in iso-osmotic media. These data indicate that p38 kinase and MEK1 are involved in the regulation of the hyperosmotic stress response.