[Expression and regulation of renal sodium-cotransporters and -antiporters, and related-transport proteins].

The authors' researches have been focused on pathogenic, physiological and biochemical mechanisms in hypertension and diabetes. Studies on hypertension were performed using salt-sensitive hypertensive Dahl rats as compared with the corresponding normotensive rats. Especially, implication with mobilization of electrolytes such as sodium, potassium, calcium and magnesium in hypertension gave rise to provocative to the author. Furthermore, complications of diabetes with hypertension were themes for the authors' researches. Thus, sodium-dependent glucose transport has been studied on sodium-dependent glucose transporters such as SGLT1 and SGLT2 using cell lines of porcelain renal cell, LLC-PK(1), and murine renal cell, NRK-52E. Relationship between magnesium mobilization and NO in hypertension has been explored using renal epithelial cell-lines and salt-sensitive hypertensive Dahl rats in the latter half of the author's research life.

Reorganization of ZO-1 by sodium-dependent glucose transporter activation after heat stress in LLC-PK1 cells.

Heat stress (HS) induces activation of high-affinity sodium-dependent glucose transporter (SGLT1) in porcine renal LLC-PK(1) cells. In this study, we investigated the roles of SGLT1 activation in reorganization of zonula occludens-1 (ZO-1), a cytosolic tight junction (TJ) protein, after HS. HS (42 degrees C, 3 h) caused decrease in transepithelial electrical resistance (TER). Subsequent incubation at 37 degrees C for 12 h increased TER above pre-HS level. The treatment of phloridzin, a potent SGLT1 inhibitor, or the replacement of glucose with a nonmetabolizable glucose analog blocked the recovery of TER and increased the transepithelial flux of FITC-dextran (4,000 Da). Immunofluorescent staining of ZO-1 showed that HS diffused ZO-1 from cell contact to cytosolic sites. Furthermore, the fraction of ZO-1 was distributed from the Triton X-100 insoluble to the Triton X-100 soluble pool. After incubation at 37 degrees C for 12 h, cell contact and ZO-1 extractability with Triton X-100 returned to pre-HS conditions, but the recovery was completely prevented by phloridzin. Tyrosine kinases activity was increased by HS that was inhibited by phloridzin. Genistein and CGP77675, tyrosine kinases inhibitors, blocked the recovery of TER and increased the transepithelial flux of FITC-dextran. Furthermore, these inhibitors prevented the recovery of cell contact and ZO-1 extractability with Triton X-100 as same as phloridzin. These findings suggested that the activation of SGLT1 reorganized ZO-1 mediated by elevation of tyrosine kinases activity after heat injury.

Arachidonic acid-activated Na+-dependent Mg2+ efflux in rat renal epithelial cells.

Arachidonic acid (AA), a metabolite of membrane phospholipids, and its metabolites are increased in Mg2+ deficiency. We examined whether the extracellular Mg2+ concentration affects AA production and whether AA regulates a putative Na+-dependent Mg2+ efflux pathway in renal epithelial NRK-52E cells. We used the cells cultured in 5 mM Mg2+-containing medium for 2 days because they enable us to detect Na+-stimulated Mg2+ efflux that was not observed in normal culture medium. Removal of extracellular Mg2+ increased AA release both in the absence and presence of extracellular Na+. This was inhibited by methyl arachidonyl fluorophosphonate (MAFP, 10 microM), an inhibitor of cytosolic phospholipase A) (cPLA2) and Ca2+-independent phospholipase A2 (iPLA2), and bromoenol lactone (BEL, 10 microM), an inhibitor of iPLA2. However, LY-311727 (10 microM), a secretory phospholipase A2 (sPLA2) inhibitor, had no inhibitory effect. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed that NRK-52E cells express cPLA2 and iPLA2 mRNAs, but not sPLA2. In the mag-fura 2 fluorescence measurements, extracellular Mg2+ removal caused slight decrease in the intracellular free Mg2+ concentration ([Mg2+]i) in the Na+-free condition. The addition of Na+ caused a rapid decrease in [Mg2+]i, indicating the presence of a Na+-dependent Mg2+ efflux pathway. The Na+-dependent [Mg2+]i decrease was suppressed by MAFP and BEL. On the other hand, AA metabolite inhibitors, nordihydroguaiaretic acid (NDGA) (50 microM), indomethacin (10 microM) and 17-octadecynoic acid (ODYA) (10 microM), enhanced the Na+-dependent [Mg2+]i decrease. Furthermore, the addition of exogenous AA (30 microM) enhanced the Na+-dependent [Mg2+]i decrease, which was significantly inhibited by imipramine (0.1 mM), a putative Na+/Mg2+-exchanger inhibitor. These results suggest that extracellular Mg2+ removal elevates AA release mediated mainly by iPLA2 and that AA upregulates the Na+-dependent Mg2+ efflux in NRK-52E cells.

Recovery from heat shock injury by activation of Na+-glucose cotransporter in renal epithelial cells.

Exposure of cells or organs to sublethal physical or chemical stresses induces disruption of cellular structures and functions. Here, we examined whether Na(+)-glucose cotransporter (SGLT1) is involved in the recovery from heat shock (HS) injury in porcine renal epithelial LLC-PK(1) cells. Recovery from HS (42 degrees C for 3 h, then 37 degrees C for 12 h) increased SGLT1 activity, assessed by [14C]alpha-methyl glucopyranoside uptake, and a maximal transport rate (V(max)) from 2.4 to 5.9 nmol/mg protein/30 min, but did not alter an apparent affinity constant (K(m)). Protein distribution of SGLT1 in apical membrane fraction was also increased after recovery from HS without changing in total membrane fraction. Membrane integrity assessed by calcein accumulation was decreased by HS, and then returned to basal level. This recovery was inhibited by phloridzin, a potent SGLT1 inhibitor, and nonmetabolizable glucose analogues. Anti-transforming growth factor-beta 1 (TGF-beta 1) antibody inhibited both elevation of SGLT1 distribution in apical membrane and recovery of calcein accumulation induced by HS. Taken together, HS increases in the number of SGLT1 protein in apical membrane mediated via TGF-beta 1 signaling pathway. The increase of glucose uptake is necessary to repair plasma membrane integrity.

Chicken DT40 cells stably transfected with the rat P2X7 receptor ion channel: a system suitable for the study of purine receptor-mediated cell death.

We have generated and characterised a clone of chicken DT40 lymphocytes stably transfected with the rat P2X(7) receptor (rP2X(7)). Successful transfection was confirmed by Western blotting. Under voltage clamp, P2X(7)-expressing cells responded to ATP and dibenzoyl-ATP (Bz-ATP) (a more potent P2X(7) receptor agonist) with a rapidly activating and sustained inward current. The EC(50) values for these agonists were 305 and 15 microM, respectively. Bz-ATP evoked Ca(2+) and Mn(2+) influx into transfected cells as determined by Fura-2 spectrofluorimetry. Responses to Bz-ATP were inhibited by pre-treatment of cells with oxidised ATP. Treatment of cells with Bz-ATP for up to 24hr produced time- and concentration-dependent cell death. This was associated with an increase in caspase-3-like activity, exposure of phosphatidylserine on the outside of cell membrane and DNA cleavage, indicating death by apoptosis. Pre-treatment with Z-VAD-fmk, a pan-caspase inhibitor, reduced the DNA fragmentation and phosphatidylserine externalisation, but did not affect overall rates of cell death at 24hr, implicating caspase-independent mechanisms. The properties of rP2X(7) receptors expressed in DT40 cells are similar to those described for other expression systems. Because DT40 cells lack functionally detectable endogenous P2 receptors and are highly amenable to genetic manipulation, stably transfected DT40 cells provide a novel and potentially useful model system in which to investigate the intracellular signal transduction pathways associated with P2X(7) receptor stimulation, in particular those involved in induction of cell death.

Up-regulation of Na+-dependent Mg2+ transport by nitric oxide and cyclic GMP pathway in renal epithelial cells.

A putative, Na(+)-dependent Mg(2+) transport pathway controls the intracellular free Mg(2+) concentration ([Mg(2+)](i)) in various mammalian cells. The characteristics of this Mg(2+) transport pathway have not been clarified. Herein, we examined the regulatory mechanism of Na(+)-dependent Mg(2+) efflux in renal epithelial NRK-52E cells. Mg(2+) removal from the extracellular bathing solution induced an Na(+)-dependent [Mg(2+)](i) decrease in Mg(2+) (5 mM)-loaded cells but not in control cells. Amiloride inhibited the [Mg(2+)](i) decrease in a dose-dependent manner (IC(50) = 3 microM). Similarly, atomic absorption spectrophotometry showed that Mg(2+) removal decreased intracellular Mg(2+) content, while it increased Na(+) content. Calphostin C (1 microM), a protein kinase C inhibitor, and genistein, a tyrosine kinase inhibitor (10 microM), blocked the [Mg(2+)](i) decrease. The [Mg(2+)](i) decrease was accompanied by an increase in intracellular nitric oxide (NO) and cyclic GMP contents. (E)-4-methyl-2-[(E)-hydoxyimino]-5-nitro-6-methoxy-3-hexenamide (0.1 mM), an NO donor, and 8-bromo-cyclic GMP (0.1 mM), a membrane-permeable cyclic GMP analogue, accelerated the [Mg(2+)](i) decrease. In contrast, N(G)-monomethyl-L-arginine (L-NMMA, 0.1 mM), an NO competitive inhibitor, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM), an NO-sensitive guanylate cyclase inhibitor, significantly blocked the [Mg(2+)](i) decrease. These results indicate that a decrease in extracellular Mg(2+) concentration induces the production of NO and cyclic GMP, which leads to the up-regulation of Na(+)-dependent Mg(2+) efflux.

[Fundamental and applied studies on transport and metabolism of electrolytes and glucose--aim to contact with molecular biology].

The authors' research focuses on polyuria, natriuresis, glucosuria, glycemia, and renal calcification in occupational lead poisoning and endemic fluorosis. Changes in electrolyte mobilization and in glucose metabolism and transport following the administration of lead compounds or fluoride were examined to elucidate these mechanisms. The results suggest fundamental approaches to the mechanism of aging and life style diseases. Our results show that: 1) Natriuresis and polyuria in lead poisoning and fluorosis are due to a decrease in renal Na/K-ATPase activity; 2) Renal calcification in fluorosis is due to stimulation of parathyroid function and activation of the renal phosphatidylinositol cascade; 3) Glycemia in fluorosis is due to elevation of renal and hepatic glucose-6-phosphatase activities; 4) Glusosuria in fluorosis is due to decreased renal Na/K-ATPase activity (but fluoride administered directly did not damage the renal Na/glucose cotransporter (SGLT); 5) Renal calcification in fluorosis is due to stimulation of parathyroid function; and 6) The decrease in renal Na/K-ATPase and SGLT activities with aging and hypertension is due to a decrease in phosphorylation activity by protein kinase C (PKC) etc. (decrease in PKC productivity with aging and hypertension).

Magnesium influx enhanced by nitric oxide in hypertensive rat proximal tubule cells.

An abnormal handling of renal magnesium has been suggested to cause salt-sensitive hypertension. The filtered magnesium is first reabsorbed in the proximal tubule. Amiloride has been shown to enhance renal magnesium conservation, but the regulatory mechanisms are unknown yet. High-salt (8% NaCl) diet decreased serum magnesium concentration, while increased urinary magnesium in Dahl salt-sensitive (DS) rat. Furthermore, the expression of nitric oxide synthase type 3 and nitric oxide (NO) content were decreased in high-salt loaded DS rat. In isolated proximal tubule cells, amiloride (0.1 mM) increased intracellular free magnesium concentration ([Mg(2+)](i)). However, the net [Mg(2+)](i) increase in the high-salt loaded DS rat was smaller than other groups. NOR1 (0.1 mM), a NO donor, restored the increase of [Mg(2+)](i) to the same level of other groups. On the contrary, L-NMMA (0.1 mM), an inhibitor of NO production, inhibited the increase of [Mg(2+)](i) in all groups. These results suggest that intracellular NO has an important role to up-regulate amiloride-elicited magnesium influx.

Up-regulation of sodium-dependent glucose transporter by interaction with heat shock protein 70.

Heat shock stress induces some heat shock proteins, including Hsp70, and activates sodium-dependent glucose transport in porcine renal LLC-PK(1) cells, but its mechanisms have not been described in detail. We investigated whether sodium-dependent glucose transporter (SGLT1) interacts with Hsp70 to increase SGLT1 activity. Heat shock stress increased SGLT1 activity without changing SGLT1 expression. The increase of SGLT1 activity was completely inhibited by an anti-transforming growth factor-beta1 (TGF-beta1) antibody. Instead of heat shock stress, TGF-beta1 increased SGLT1 activity dose- and time-dependently without changing SGLT1 expression. We found that the amount of Hsp70 immunoprecipitated from TGF-beta1-treated cells with an anti-SGLT1 antibody was higher than that of the control cells. Transfection of an anti-Hsp70 antibody into the cells inhibited the increase of SGLT1 activity. With confocal laser microscopy, both SGLT1 and Hsp70 was localized near the apical membrane in the TGF-beta1-treated cells, and an anti-Hsp70 antibody disturbed this localization. Furthermore, we clarified that an anti-Hsp70 antibody inhibited interaction of SGLT1 with Hsp70 in vitro. These results suggest that Hsp70 forms a complex with SGLT1 and increases the expression level of SGLT1 in the apical membrane, resulting in up-regulation of glucose uptake.

Phosphatidylinositol 3-kinase mediates inhibitory effect of angiotensin II on sodium/glucose cotransporter in renal epithelial cells.

Effects of angiotensin II (ANGII) on regulation of sodium/glucose cotransporter (SGLT1) activity were investigated in LLC-PK(1) cells, renal proximal epithelial cell line. ANGII inhibited alpha-[14C] methyl-D-glucopyranoside (AMG) uptake into LLC-PK(1) cells in a dose-dependent manner. This inhibition was based on a decrease in maximal transport rate (Vmax) of AMG from 2.20 nmol/mg protein/15 min to 1.19 nmol/mg protein/15 min, although apparent affinity constant (Km) did not alter. In western blot analysis, protein level of SGLT1 in brush border membrane (BBM) was decreased by ANGII, although total SGLT1 was not altered. In the aspect of intracellular signal transduction, ANGII blocked the formation of cAMP. Pertussis toxin, an inactivator of Gi protein that control intracellular cAMP level, completely prevented the decrease of AMG uptake caused by ANGII. 8-Br-cAMP, a cell membrane permeable cAMP analogue, increased AMG uptake and protein level of SGLT1 in BBM. Both wortmannin and LY294002 that are phosphatidylinositol (PI) 3-kinase inhibitors, inhibited the SGLT1 activity, and also attenuated the effect of 8-Br-cAMP on SGLT1 activity. Those inhibitors prevented the 8-Br-cAMP-induced expression of SGLT1 in plasma membrane. We conclude that ANGII plays an important role in post-translational regulation in SGLT1. Inhibition of SGLT1 translocation is suggested to be caused by inactivation of protein kinase A and decrease of PI 3-kinase activity.

Estimation of endotoxin-like substances in deep seawater by using bioassay.

Deep seawater has recently been under trial as a fundamental material for mineral water, food, face lotion and an efficacious reagent for the cure of atopic dermatitis in Japan. However, little is known about the biologically effective substances, including toxic compounds in deep seawater. In this study, we investigated the effects of deep seawater on the function of murine macrophages in vitro, and examined the endotoxin-like substances in seawater. Mitochondrial activity and NO production in macrophage cells cultured with stimulants were enhanced in a depth dependent manner by pretreatment with deep seawater. In addition, fractions from deep seawater, enriched by hydrophobic column chromatography, activated the macrophage cells much more than the corresponding fractions from surface seawater. Furthermore, the effects of the fractions on macrophage cells remained significant, even with the addition of polymyxin B. which is a specific inhibitor of endotoxins. These results indicate that endotoxins and unknown substances, which affect macrophage functions, exist in a depth dependent manner in seawater.

Expression of GFP-tagged low affinity Na+-dependent glucose transporter in Xenopus oocytes and CHO cells.

A low-affinity and high-capacity Na(+)-dependent glucose transporter (SGLT2) was inserted into the expression vector tagging of green fluorescent protein (EGFP). The protein expression and glucose transport activity were examined in Xenopus oocytes and Chinese hamster ovary (CHO) cells. In Western blotting analysis, EGFP-tagged SGLT2 protein expressed in both Xenopus oocytes and CHO cells. We also observed the EGFP fluorescence in both cells with a confocal laser microscope. To determine the function of EGFP-tagged SGLT2, we measured the uptake of [(14)C]-alpha-methyl glucopyranoside (AMG), a specific substrate for SGLT. The AMG uptake was time-dependently increased and inhibited by phloridzin in the EGFP-tagged SGLT2-expressing cells. The K(m) value of 1.7 mM for AMG and the IC(50) of 2 microM for phloridzin consist with the renal low affinity Na(+)-dependent glucose transporter. These results indicate that EGFP-tagged SGLT2 protein functionally expressed both in Xenopus oocytes and CHO cells, and these models are useful for studying the regulatory mechanisms of glucose reabsorption.