Juan M. Capasso

The expression of the gamma subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms.

The α and β subunits of Na-K-ATPase are up-regulated by hypertonicity in inner-medullary collecting duct cells adapted to survive in hypertonic conditions. We examined the regulation of the γ subunit by hypertonicity. Although cultured inner-medullary collecting duct cells lacked the γ subunits, both variants γa and γb were expressed in cells adapted to 600 and 900 mosmol/KgH2O. This expression was reversible with a half-time of 17.2 ± 0.5 h. The message of the γ subunit was absent in isotonic conditions and increased with higher tonicity in adapted cells. In acute experiments the appearance of the γ subunit was found to be both time-dependent (≥24 h) and osmolality-dependent (≥500 mosmol/KgH2O). No induction was noted with urea and only minimal induction with mannitol. Increasing concentrations of the phosphatidylinositol 3-kinase inhibitor LY294002 resulted in a dose-dependent decrement in the expression of the γ subunit with total abolition at 10 μM. This was associated with a decrease in cell viability as <20% survived the treatment with 10 μM of LY294002. Neither inhibition of extracellular response kinase nor p38 mitogen-activated protein kinase inhibited osmotic induction of the γ subunit. In contrast, cells transfected with a dominant negative c-Jun N-terminal kinase 2-APF construct displayed complete inhibition of the γ subunit. Such cells have accelerated loss of viability in hypertonic conditions. This study describes the regulation of the γ subunit of Na-K-ATPase by hypertonicity. This regulation is transcriptionally regulated and involves signaling mediated by phosphatidylinositol 3-kinase and c-Jun N-terminal kinase 2 pathways.

The tight junction protein, MUPP1, is up-regulated by hypertonicity and is important in the osmotic stress response in kidney cells

Antibody array proteomics was used to detect differentially expressed proteins in inner medullary collecting duct 3 (IMCD3) cells grown under isotonic and chronic hypertonic conditions. Of 512 potential proteins, >90% were unchanged in expression. Noteworthy was the up-regulation of several tight junction-related proteins, including MUPP1 (multi-PDZ protein-1), ZO1 (zonula occludens 1), and Af6. The most robustly up-regulated protein under hypertonic conditions was MUPP1 (7.2×, P < 0.001). Changes in expression for MUPP1 were verified by quantitative PCR for message and Western blot for protein. In mouse kidney tissues, MUPP1 expression was substantial in the papilla and was absent in the cortex. Furthermore, MUPP1 expression increased 253% (P < 0.01) in the papilla upon 36 h of thirsting. Localization of MUPP1 protein expression was confirmed by immunocytochemical analysis demonstrating only minor staining under isotonic conditions and the substantial presence in chronically adapted cells at the basolateral membrane. Message and protein half-life in IMCD3 cells were 26.2 and 17.8 h, respectively. Osmotic initiators of MUPP1 expression included NaCl, sucrose, mannitol, sodium acetate, and choline chloride but not urea. Stable IMCD3 clones silenced for MUPP1 expression used the pSM2-MUPP1 vector. In cell viability experiments, clones silenced for MUPP1 demonstrated only a minor loss in cell survival under acute sublethal osmotic stress compared with empty vector control cells. In contrast, a 24% loss (P < 0.02) in transepithelial resistance for monolayers of MUPP1-silenced cells was determined as compared with controls. These results suggest that MUPP1 specifically, and potentially tight junction complexes in general, are important in the renal osmoadaptive response.

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

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 p38alpha 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.

Chloride, not sodium, stimulates expression of the γ subunit of Na/K-ATPase and activates JNK in response to hypertonicity in mouse IMCD3 cells

Hypertonicity induced by NaCl, but not by urea or mannitol, up-regulates expression of the γ subunit of Na/K-ATPase in cells of the murine inner medullary collecting duct line (IMCD3) by activation of the Jun kinase 2 (JNK2) pathways. We examined the ionic mediators of the osmosensitive response. An increase in osmolality to 550 milliosmoles per kg of water (mosmol/kgH2O) for 48 h by replacement of NaCl with choline chloride did not prevent the up-regulation of the γ subunit. Neither Na+ ionophores nor inhibitors of cellular Na+ uptake altered the up-regulation of the γ subunit or JNK activation. Changes in cell cation concentrations driven by incubation in low-K+ medium were effective in up-regulating the α1 subunit of Na/K-ATPase but did not have any effect on the γ subunit. The replacement of NaCl with choline chloride did not down-regulate γ-subunit expression in cells adapted to hypertonicity. In contrast, the replacement of NaCl with sodium acetate, or pretreatment of cells with the Cl- channel inhibitor 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB) completely blocked γ-subunit up-regulation, inhibited JNK activation, and caused a significant decrement in cell survival in hypertonic but not isotonic conditions. In adapted cells, replacement of 300 mosmol/kgH2O NaCl with sodium acetate resulted in down-regulation of the γ subunit. In conclusion, we describe a Na+-independent, Cl--dependent mechanism for hypertonicity-mediated activation of the JNK and the subsequent synthesis of the γ subunit of Na/K-ATPase, which are necessary for cellular survival in these anisotonic conditions.

Expression of the calcium-binding protein S100A4 is markedly up-regulated by osmotic stress and is involved in the renal osmoadaptive response.

Proteomic analysis of Inner Medullary Collecting Duct (IMCD3) cells adapted to increasing levels of tonicity (300, 600, and 900 mosmol/kg H2O) by two-dimensional difference gel electrophoresis and mass spectrometry revealed several proteins as yet unknown to be up-regulated in response to hypertonic stress. Of these proteins, one of the most robustly up-regulated (22-fold) was S100A4. The identity of the protein was verified by high pressure liquid chromatography-mass spectrometry. Western blot analysis confirmed increased expression with increased tonicity, both acute and chronic. S100A4 protein expression was further confirmed by immunocytochemical analysis. Cells grown in isotonic conditions showed complete absence of immunostaining, whereas chronically adapted IMCD3 cells had uniform cytoplasmic localization. The protein is also regulated in vivo as in mouse kidney tissues S100A4 expression was many -fold greater in the papilla as compared with the cortex and increased further in the papilla upon 36 h of thirsting. Increased expression of S100A4 was also observed in the medulla and papilla, but not the cortex of a human kidney. Data from Affymetrix gene chip analysis and quantitative PCR also revealed increased S100A4 message in IMCD3 cells adapted to hypertonicity. The initial expression of message increased at 8-10 h following exposure to acute sublethal hypertonic stress (550 mosmol/kg H2O). Protein and message half-life in IMCD3 cells were 85.5 and 6.8 h, respectively. Increasing medium tonicity with NaCl, sucrose, mannitol, and choline chloride stimulated S100A4 expression, whereas urea did not. Silencing of S100A4 expression using a stable siRNA vector (pSM2; Open Biosystems) resulted in a 48-h delay in adaptation of IMCD3 cells under sublethal osmotic stress, suggesting S100A4 is involved in the osmoadaptive response. In summary, we describe the heretofore unrecognized up-regulation of a small calcium-binding protein, both in vitro and in vivo, whose absence profoundly delays osmoadaptation and slows cellular growth under hypertonic conditions.

A novel method for phosphoprotein extraction from macroalgae

Abstract The extraction of proteins from macroalgae has always been difficult, primarily because of their high internal levels of polysaccharides and polyphenols. Several methods for protein extraction have been developed recently that allow the production of high quality extracts for 1-D SDS gel electrophoresis or 2-D SDS-polyacrylamide gel electrophoresis for proteomic analysis. However, none of the existing methods are applicable to the study of the phosphorylation state of kinases involved in stress responses of macroalgae, primarily because trichloroacetic acid and phenol are used for protein precipitation. In this work, we describe a novel method for protein extraction from macroalgae based on the counterintuitive use of commercial RNA purification kits; this method maintains the phosphorylation state of the kinases intact, as demonstrated by either polyclonal or monoclonal antibodies against the phosphorylated forms of mitogen-activated protein kinases using Western blot methodology. This method may be appropriate for wider use in the analysis of phosphoproteomes.