P38 KINASE-ACTIVITY IS ESSENTIAL FOR OSMOTIC INDUCTION OF MESSENGER-RNAS FOR HSP70 AND TRANSPORTER FOR ORGANIC SOLUTE BETAINE IN MADIN-DARBY CANINE KIDNEY-CELLS

In renal cells, hypertonicity induces genes for heat shock proteins (H SP70, alpha B-crystallin), as well as enzymes and transporters directl y involved in the metabolism and transport of protective organic osmol ytes, While heat shock proteins are induced by many stresses including osmotic stress, the induction of the osmolytes genes appears to be sp ecific to osmotic stress. These two adaptive mechanisms allow kidney c ells 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 (extrace llular regulated kinase), JNK (Jun N-terminal kinase), and p38, ERK ac tivation by osmotic stress is a consistent finding in many cells, but it is not essential for transcriptional. regulation of mRNA for transp orter of organic osmolyte betaine, While the growth of yeast cells on NaCl-supplemented medium is dependent on HOG1 pathway, it is still unc lear which pathway mediates the adaptation to osmotic stress in mammal ian cells, Here, we show that inhibition of p38 kinase activity, using the specific inhibitor SB203580 rophenyl)-2-(4-methylsulfonylphenyl)- 5-(4-pyridyl) imidazole), abolishes the hypertonicity-mediated inducti on of mRNAs for HSP70 and betaine transporter in Madin-Darby canine ki dney cells, The inhibition is dose-dependent and correlates with the i n 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 nat ive p38 kinase activity correlates with up-regulation of native JNK-1 activity in osmotically stressed cells, p38 mRNA is induced by hyperto nic stress and is attenuated with p38 kinase inhibition, We also find that thermal induction of HSP70 mRNA is not affected by p38 kinase inh ibition, Such findings suggest that p38 kinase activity is essential f or the induction of genes involved in the adaptation of mammalian cell s to osmotic stress and that the increased activity of JNK-1 during p3 8 kinase inhibition is consistent with regulation of JNK-1 by p38 kina se in osmotically stressed cells, In addition, the transduction pathwa ys mediating HSP70 mRNA induction by different stresses appear to be d ivergent; osmotic induction of HSP70 is p38 kinase-dependent, while th ermal induction is not.