p38 MAP kinase modulates liver cell volume through inhibition of membrane Na+ permeability
Andrew P. Feranchak, … , Jiahuai Han, J. Gregory Fitz
Andrew P. Feranchak, … , Jiahuai Han, J. Gregory Fitz
Published November 15, 2001
Citation Information: J Clin Invest. 2001;108(10):1495-1504. https://doi.org/10.1172/JCI12190.
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Article

p38 MAP kinase modulates liver cell volume through inhibition of membrane Na+ permeability

Abstract

In hepatocytes, Na+ influx through nonselective cation (NSC) channels represents a key point for regulation of cell volume. Under basal conditions, channels are closed, but both physiologic and pathologic stimuli lead to a large increase in Na+ and water influx. Since osmotic stimuli also activate mitogen-activated protein (MAP) kinase pathways, we have examined regulation of Na+ permeability and cell volume by MAP kinases in an HTC liver cell model. Under isotonic conditions, there was constitutive activity of p38 MAP kinase that was selectively inhibited by SB203580. Decreases in cell volume caused by hypertonic exposure had no effect on p38, but increases in cell volume caused by hypotonic exposure increased p38 activity tenfold. Na+ currents were small when cells were in isotonic media but could be increased by inhibiting constitutive p38 MAP kinase, thereby increasing cell volume. To evaluate the potential inhibitory role of p38 more directly, cells were dialyzed with recombinant p38α and its upstream activator, MEK-6, which substantially inhibited volume-sensitive currents. These findings indicate that constitutive p38 activity contributes to the low Na+ permeability necessary for maintenance of cell volume, and that recombinant p38 negatively modulates the set point for volume-sensitive channel opening. Thus, functional interactions between p38 MAP kinase and ion channels may represent an important target for modifying volume-sensitive liver functions.

Authors

Andrew P. Feranchak, Tomas Berl, Juan Capasso, Paul A. Wojtaszek, Jiahuai Han, J. Gregory Fitz

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Osmolarity-sensitive changes in MAP kinase activity. HTC cells were expo...
Osmolarity-sensitive changes in MAP kinase activity. HTC cells were exposed to isotonic (∼300 mOsm), hypotonic (∼100 mOsm), or hypertonic (∼500 mOsm) buffer solutions for 5 minutes, then immediately homogenized in lysis buffer. (a) Cell lysates were subjected to SDS-PAGE and Western blot analysis with either an antibody specific for the activated (phosphorylated at Thr-180 and Tyr-182) form of p38 (phospho-p38) or a control antibody that does not distinguish between phospho- and dephosphorylated p38 MAP kinase (total p38). Constitutive phospho-p38 activity was observed under isotonic conditions; values increased with hypotonic exposure. Activity was inhibited by the p38 inhibitor SB203580 (1 μM), but not by the tyrosine kinase inhibitor genistein (10 μM). None of the various exposures resulted in changes in total p38. (b) Average MAP kinase activity in response to osmotic changes performed as described in Methods. Under isotonic conditions, there was constitutive activity of p38, JNK, and ERK kinases. Hypertonic exposure stimulated a large increase in both ERK (fivefold) and JNK (fourfold) activity, but had little effect on p38 MAP kinase activity. In contrast, hypotonic exposure resulted in a large increase in p38 MAP kinase activity (tenfold), but had little effect on ERK or JNK activity compared with isotonic conditions. The putative p38 MAP kinase inhibitor SB203580 did not affect ERK or JNK activity, but completely inhibited constitutive p38 MAP kinase activity.