Abstract

In renal cells, hypertonicity induces genes for heat shock proteins (HSP70, alpha B-crystallin), as well as enzymes and transporters directly involved in the metabolism and transport of protective organic osmolytes. While heat shock proteins are induced by many stresses including osmotic stress, the induction of the osmolytes genes appears to be specific to osmotic stress. These two adaptive mechanisms allow kidney cells to survive and function in the hypertonic environment that exists on routine basis in kidney medulla. In mammalian cells, hypertonicity induces three mitogen-activated protein kinase pathways: ERK (extracellular regulated kinase), JNK (Jun N-terminal kinase), and p38. ERK activation by osmotic stress is a consistent finding in many cells, but it is not essential for transcriptional regulation of mRNA for transporter of organic osmolyte betaine. While the growth of yeast cells on NaCl-supplemented medium is dependent on HOG1 pathway, it is still unclear which pathway mediates the adaptation to osmotic stress in mammalian cells. Here, we show that inhibition of p38 kinase activity, using the specific inhibitor SB203580 (4-(fluorophenyl)-2-(4-methylsulfonyl-phenyl)-5-(4-pyridyl) imidazole), abolishes the hypertonicity-mediated induction of mRNAs for HSP70 and betaine transporter in Madin-Darby canine kidney cells. The inhibition is dose-dependent and correlates with the in situ activity of native p38 kinase, determined as MAPKAPK-2 activity in cell extracts. As reported previously, the activities of ERK-1 and -2 were not affected by SB203580, but surprisingly, inhibition of native p38 kinase activity correlates with up-regulation of native JNK-1 activity in osmotically stressed cells. p38 mRNA is induced by hypertonic stress and is attenuated with p38 kinase inhibition. We also find that thermal induction of HSP70 mRNA is not affected by p38 kinase inhibition. Such findings suggest that p38 kinase activity is essential for the induction of genes involved in the adaptation of mammalian cells to osmotic stress and that the increased activity of JNK-1 during p38 kinase inhibition is consistent with regulation of JNK-1 by p38 kinase in osmotically stressed cells. In addition, the transduction pathways mediating HSP70 mRNA induction by different stresses appear to be divergent; osmotic induction of HSP70 is p38 kinase-dependent, while thermal induction is not.

Highlights

  • During water deprivation, the extracellular osmolality in the mammalian renal medulla can exceed 3000 mosmol/kg of H2O

  • We conclude that p38 kinase activity is essential for hypertonic induction of BGT1 mRNA, a representative gene involved in the accumulation of compatible organic solute betaine in Madin-Darby canine kidney (MDCK) cells

  • These findings suggest that p38 kinase is essential for the adaptation of kidney cells to osmotic stress, either by induction of heat shock proteins or of osmoprotective genes

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Summary

Introduction

The extracellular osmolality in the mammalian renal medulla can exceed 3000 mosmol/kg of H2O. Protein was visualized using ECL detection system p38 Kinase and Osmotic Induction of Betaine Transporter in MDCK Cells To investigate the involvement of p38 kinase in mediating osmotically driven transcription of these genes, MDCK cells were exposed for 16 h to hypertonic medium, in the absence or presence of increasing concentrations of SB203580, a potent inhibitor of p38 kinase, and the abundance of BGT1 mRNA was measured.

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