In renal cells, hypertonicity induces genes for heat shock proteins (HSP70, ␣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-methylsulfonylphenyl)-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.During water deprivation, the extracellular osmolality in the mammalian renal medulla can exceed 3000 mosmol/kg of H 2 O. While such an osmolality is incompatible with the survival of cells from other organs, kidney cells tolerate it well. The survival of kidney medullary cells in hypertonic environment is essential for the generation of the concentrating gradient, which is key for maintenance of body solute and water homeostasis.Many organisms, including bacteria, yeast, plants, and animals, adapt to sustained hyperosmotic stress by accumulating osmotically active organic solutes (compatible organic osmolytes) (1). These compounds do not perturb cellula...

The response of the kidney to ischemic injury includes increased DNA synthesis, which is preceded by rapid and brief expression of the c-fos proto-oncogene. While the timing of these two events would suggest that c-Fos participates in an immediate-early gene program leading to proliferation, no direct test of this hypothesis exists. The purpose of these studies was (1) to determine whether c-fos is expressed as part of a typical immediate-early (IE) gene response, which would require co-expression of c-jun and sensitivity to cycloheximide, and (2) to determine whether the cells expressing c-Fos are the same as those undergoing DNA synthesis. Northern analysis was performed on renal mRNA at different times following release of a 50 minute period of renal hilar clamping. c-jun and c-fos mRNA were rapidly and briefly expressed following renal ischemia and their expression was superinduced by cycloheximide in a manner typical of an immediate-early gene response. 3H-thymidine autoradiography performed on semi-thin sections from intravascularly perfusion fixed kidneys 24 hours following induction of ischemia showed labeled nuclei in cells lining the damaged proximal tubules of the outer stripe of the outer medulla, as well as proximal tubules in the cortex and interstitial cells throughout the kidney. However, immunohistochemical localization of c-Fos and c-Jun protein occurred predominantly in nuclei of the thick ascending limb, distal tubule and collecting duct cells. The studies demonstrate that c-fos and c-jun are expressed following renal ischemia as a typical immediate-early gene response, but they are expressed in cells that do not enter the cell cycle. The failure of the cells to enter the cell cycle may depend on the co-expression of jun-B and jun-D, which suppress the mitogenic activity of c-Jun in other cells. The data suggest that the IE response following renal ischemia is part of the stress response, which is antiproliferative rather than proliferative. The role of the stress response during renal ischemia and the fate of the cells undergoing it are unknown.