Tyrosine kinase inhibitors and immunosuppressants perturb the myo-inositol but not the betaine cotransporter in isotonic and hypertonic MDCK cells

Effects of Genistein on Common Kidney Diseases

Genistein is a naturally occurring phytoestrogen (soy or soybean products) that is classified as an isoflavone, and its structure is similar to that of endogenous estrogens; therefore, genistein can exert an estrogen-like effect via estrogen receptors. Additionally, genistein is a tyrosine kinase inhibitor, which enables it to block abnormal cell growth and proliferation signals through the inhibition of tyrosine kinase. Genistein is also an angiogenesis inhibitor and an antioxidant. Genistein has effects on kidney cells, some of the kidney’s physiological functions, and a variety of kidney diseases. First, genistein exerts a protective effect on normal cells by reducing the inflammatory response, inhibiting apoptosis, inhibiting oxidative stress, inhibiting remodeling, etc., but after cell injury, the protective effect of genistein decreases or even has the opposite effect. Second, genistein can regulate renin intake to maintain blood pressure balance, regulate calcium uptake to regulate Ca²⁺ and Pi balances, and reduce vasodilation to promote diuresis. Third, genistein has beneficial effects on a variety of kidney diseases (including acute kidney disease, kidney cancer, and different chronic kidney diseases), such as reducing symptoms, delaying disease progression, and improving prognosis. Therefore, this paper reviews animal and human studies on the protective effects of genistein on the kidney in vivo and in vitro to provide a reference for clinical research in the future.

Different effects of CsA and FK506 on aquaporin-2 abundance in rat primary cultured collecting duct cells

Calcineurin (Cn) inhibitors (CnI) such as cyclosporine A (CsA) and FK506 are nephrotoxic immunosuppressant drugs, which decrease tubular function. Here, we examined the direct effect of CnI on aquaporin-2 (AQP2) expression in rat primary cultured inner medullary collecting duct cells. CsA (0.5-5 μM) but not FK 506 (0.01-1 μM) decreased expression of AQP2 protein and messenger RNA (mRNA) in a concentration and time dependent manner, without affecting mRNA stability. This effect was observed despite similar inhibition of Cn activity by both CnI, thereby suggesting that the CsA-dependent decrease in AQP2 expression was Cn independent. Another inhibitor of cyclophilin A, the primary intracellular target of CsA, had no effect on AQP2 expression. In order to investigate the mechanism of decreased AQP2 transcription, we studied activation status of two suggested transcriptional regulators of AQP2, cAMP-responsive element binding protein (CREB), and tonicity enhancer binding protein (TonEBP). Localization of TonEBP, as well as TonEBP-mediated gene transcription, was not affected by CsA. Phosphorylation of CREB at an activating phosphorylation site (S133) was decreased by CsA, but not by FK506. However, both CnI did not affect cellular cAMP levels. We show that CsA decreases transcription of AQP2, a process that is in part independent of Cn or cyclophilin A and suggests dependence on decreased activity of CREB.

Hippocampal betaine/GABA transporter mRNA expression is not regulated by inflammation or dehydration post-status epilepticus

Seizure activity can alter GABA transporter and osmoprotective gene expression, which may be involved in the pathogenesis of epilepsy. However, the response of the betaine/GABA transporter (BGT1) is unknown. The goal of the present study was to compare the expression of BGT1 mRNA to that of other osmoprotective genes and GABA transporters following status epilepticus (SE). The possible contributory role of dehydration and inflammation was also investigated because both have been shown to be involved in the regulation of GABA transporter and/or osmoprotective gene expression. BGT1 mRNA was increased 24 h post-SE, as were osmoprotective genes. BGT1 was decreased 72 h and 4 weeks post-SE, as were the GABA transporter mRNAs. The mRNA values for osmoprotective genes following 24-h water withdrawal were significantly lower than the values obtained 24 h post-SE despite similarities in their plasma osmolality values. BGT1 mRNA was not altered by lipopolysaccharide-induced inflammation while the transcription factor tonicity-responsive enhancer binding protein and the GABA transporters 1 and 3 were. These results suggest that neither plasma osmolality nor inflammation fully account for the changes seen in BGT1 mRNA expression post-SE. However, it is evident that BGT1 mRNA expression is altered by SE and displays a temporal pattern with similarities to both GABA and osmolyte transporters. Further investigation of BGT1 regulation in the brain is warranted.

Cyclosporin-A induced toxicity in rat renal collecting duct cells: interference with enhanced hypertonicity induced apoptosis

BACKGROUND/AIMS

Rat renal inner medullary collecting duct (IMCD) cells are physiologically exposed to a wide range of ambient tonicity. To maintain their function upon changes in osmolality, IMCD cells induce expression of osmoprotective and antiapoptotic genes, mainly mediated by the transcription factor Tonicity Enhancer Binding Protein (TonEBP). Some drugs like Cyclosporin-A (CsA) are discussed to interfere with the activity of TonEBP and thereby mediate their nephrotoxic effects. The aim of our study was to further understand CsA toxicity during elevation of ambient osmolality.

METHODS

First we examined cytotoxicity of CsA in IMCD exposed to elevated tonicity. Employing microarray analysis of gene expression, real-time PCR and immunoassays, we scrutinized pathways contributing to this effect.

RESULTS

We show that in IMCD cells CsA but not FK506 increases apoptosis upon an increase in tonicity. This effect is independent of cellular TonEBP localization or activity and reactive oxygen species. Microarray studies revealed marked quantitative differences in gene expression. Functional analysis showed overrepresentation of genes associated with cell death in presence of CsA. This correlated with increased mRNA expression of genes associated with the death receptor pathway and detection of TNFα in culture medium of cells treated with CsA.

CONCLUSION

Our results show that CsA cytotoxicity is induced under elevated ambient osmolality and that death receptor signaling probably contributes to CsA cytotoxicity.

Effects of hyperosmolarity on the Na+-myo-inositol cotransporter SMIT2 stably transfected in the Madin-Darby canine kidney cell line

Myo-inositol (MI) is a compatible osmolyte used by cells to compensate for changes in the osmolarity of their surrounding milieu. In kidney, the basolateral Na+-MI cotransporter (SMIT1) and apical SMIT2 proteins are homologous cotransporters responsible for cellular uptake of MI. It has been shown in the Madin-Darby canine kidney (MDCK) cell line that SMIT1 expression was under the control of the tonicity-sensitive transcription factor, tonicity-responsive enhancer binding protein (TonEBP). We used an MDCK cell line stably transfected with SMIT2 to determine whether variations in external osmolarity could also affect SMIT2 function. Hyperosmotic conditions (+200 mosM raffinose or NaCl but not urea) generated an increase in SMIT2-specific MI uptake by three- to ninefold in a process that required protein synthesis. Using quantitative RT-PCR, we have determined that hyperosmotic conditions augment both the endogenous SMIT1 and the transfected SMIT2 mRNAs. Transport activities for both SMIT1 and SMIT2 exhibited differences in their respective induction profiles for both their sensitivities to raffinose, as well as in their time course of induction. Application of MG-132, which inhibits nuclear translocation of TonEBP, showed that the effect of osmolarity on transfected SMIT2 was unrelated to TonEBP, unlike the effect observed with SMIT1. Inhibition studies involving the hyperosmolarity-related MAPK suggested that p38 and JNK play a role in the induction of SMIT2. Further studies have shown that hyperosmolarity also upregulates another transfected transporter (Na+-glucose), as well as several endogenously expressed transport systems. This study shows that hyperosmolarity can stimulate transport in a TonEBP-independent manner by increasing the amount of mRNA derived from an exogenous DNA segment.

Prostaglandin E2 stimulates expression of osmoprotective genes in MDCK cells and promotes survival under hypertonic conditions

The cells of the renal medulla produce large amounts of prostaglandin E2 (PGE2) via cyclooxygenases (COX)-1 and -2. PGE2 is well known to play a critical role in salt and water balance and maintenance of medullary blood flow. Since renal medullary PGE2 production increases in antidiuresis, and since COX inhibition is associated with damage to the renal medulla during water deprivation, PGE2 may promote the adaptation of renal papillary cells to high interstitial solute concentrations. To address this question, MDCK cells were exposed to a gradual tonicity increase in the presence or absence of 20 microM PGE2 prior to analysis of (i) cell survival, (ii) expression of osmoprotective genes (AR, BGT1, SMIT, HSP70 and COX-2), (iii) subcellular TonEBP/NFAT5 abundance, (iv) TonEBP/NFAT5 transcriptional activity and (v) aldose reductase promoter activity. Cell survival and apoptotic indices after raising the medium tonicity improved markedly in the presence of PGE2. PGE2 significantly increased tonicity-mediated up-regulation of AR, SMIT and HSP70 mRNAs. However, neither nuclear abundance nor TonEBP/NFAT5-driven reporter activity were elevated by PGE2, but aldose reductase promoter activity was significantly increased by PGE2. Interestingly, tonicity-induced COX-2 expression and activity was also stimulated by PGE2, suggesting the existence of a positive feedback loop. These results demonstrate that the major medullary prostanoid, PGE2, stimulates the expression of osmoprotective genes and favours the adaptation of medullary cells to increasing interstitial tonicities, an effect that is not explained directly by the presence of TonEs in the promoter region of the respective target genes. These findings may be relevant in the pathophysiology of medullary damage associated with analgesic drugs.

How tonicity regulates genes: story of TonEBP transcriptional activator

TonEBP stimulates genes whose products drive cellular accumulation of organic osmolytes and HSP70, which protect cells from the deleterious effects of hypertonicity and urea, respectively. Mice deficient in the TonEBP gene display severe atrophy of the renal medulla because cells failed to adapt to the hyperosmolality. Emerging data suggest that TonEBP plays a key role in the urinary concentrating mechanism by stimulating the UT-A urea transporters and possibly AQP2 water channel. Thus, TonEBP is an essential regulator in the urinary concentrating mechanism. Studies on structural basis of TonEBP function have revealed the structure of the DNA binding domain, and defined the transactivation domains. Molecular mechanisms underlying the nucleocytoplasmic trafficking, transactivation, and phosphorylation in response to changes in tonicity need to be understood in molecular detail. Such knowledge is needed for the identification of the sensor that detects changes in ambient tonicity and signals to TonEBP.

Hypertonic upregulation of betaine transport in renal cells is blocked by a proteasome inhibitor

The renal betaine transporter (BGT1) protects cells in the hypertonic medulla by mediating uptake and accumulation of the osmolyte betaine. Transcription plays an essential role in upregulating BGT1 transport in MDCK cells subjected to hypertonic stress. During hypertonic stress, the abundance of the transcription factor TonEBP increases and it shifts from the cytoplasm to the nucleus where it activates transcription of the BGT1 gene. Little is known about post-transcriptional regulation of BGT1 protein. In the presence of the proteasome inhibitor MG-132, which blocked nuclear translocation of TonEBP, the hypertonic upregulation of BGT1 protein and transport was prevented and cell viability in hypertonic medium was impaired over 24 h. Urea also prevented the hypertonic upregulation of BGT1 protein and transport, but did not interfere with TonEBP translocation and cell viability. Shorter treatments of hypertonic cells with MG-132 avoided viability problems and produced dose-dependent inhibition of translocation and transport. When stably transfected MDCK cells that over-expressed BGT1 were treated for 6 h with hypertonic medium containing 3 microM MG-132, there was 43% inhibition of nuclear translocation, 83% inhibition of BGT1 transport, and no change in viability. While other proteasome functions may be involved, these data are consistent with a critical role for nuclear translocation of TonEBP in upregulation and membrane insertion of BGT1 protein.

Effect of ammonium on the expression of osmosensitive genes in Madin-Darby canine kidney cells

The cells of the kidney medulla are exposed routinely to high extracellular concentrations of various solutes including NaCl, urea and ammonium (NH4+). Although it is well established that the expression of a variety of osmosensitive genes and proteins, which confer cytoprotection on renal medullary cells, is induced by high NaCl concentrations, the role of NH4+ in these cellular responses is unclear. This study thus addressed the effect of NH4+ on the expression of the betaine/GABA transporter (BGT-1), the sodium/myo-inositol cotransporter (SMIT), aldose reductase (AR), and heat shock protein 70 (HSP70) in Madin-Darby canine kidney (MDCK) cells, using Northern and Western blot analyses and enzyme-linked immunosorbent assay (ELISA). The incidence of apoptosis was monitored by determining caspase-3 activity and annexin V binding. Addition of NH4Cl (50 mM; total osmolality 400 mosmol (kg H2O)(-1) to the medium was more effective than equiosmolar NaCl in increasing BGT-1 and HSP70 mRNA abundance, but less effective in enhancing BGT-1 and HSP70 expression at the protein level. Qualitatively similar results were obtained for SMIT and AR mRNAs. Exposure to both isotonic and hypertonic, NH4Cl-containing medium enhanced apoptosis compared with equiosmolar, NaCl-containing media. These results suggest that, in addition to NaCl, NH4Cl may play a role in regulating the intracellular accumulation of organic osmolytes, the abundance of HSP70 and cell turnover in the renal medulla in vivo.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Characterization of the expression and regulation of genes necessary for myo-inositol biosynthesis and transport in the seminiferous epithelium

In many mammals, the concentration of myo-inositol in the fluid of the seminiferous tubules is dramatically higher than levels found in serum. Two enzymes involved in myo-inositol synthesis: myo-inositol-1-phosphate synthase (ISYNA1) and myo-inositol monophosphatase-1 (IMPA1), are known to have high activity in the testes. ISYNA1 is an isomerase that catalyzes the conversion of glucose-6-phoshate to myo-inositol-1-phosphate. IMPA1 then hydrolyzes the phosphate group to produce myo-inositol. Although no physiological role for the high concentration of myo-inositol has yet to be elucidated, it has been suggested that it could be involved in osmoregulation. Previous research on these enzymes in the testis has focused on enzyme activity. The objective of this study was to evaluate the expression of these genes and the myo-inositol transporter, Slc5a3, within the testis. Using Northern blot analyses, we found that all three genes, Impa1, Isyna1, and Slc5a3 are expressed in Sertoli cells. Isyna1 is highly expressed in two types of germ cells, pachytene spermatocytes and round spermatids. IMPA1 was expressed in round spermatids. Slc5a3 expression is upregulated when Sertoli cells are treated with 0.1 mM dibutyryl cAMP. When Sertoli cells were cultured in a hypertonic medium, there was an increase in the expression of Isyna1 and Slc5a3. We postulate that this upregulation is a result of the capability of the Sertoli cell to sense and then react to a change in osmolarity by increasing the transport and production of the osmolyte myo-inositol.

Subcellular redistribution of the renal betaine transporter during hypertonic stress

The betaine transporter (BGT1) protects cells in the hypertonic renal inner medulla by mediating uptake and accumulation of the osmolyte betaine. Transcriptional regulation plays an essential role in upregulation of BGT1 transport when renal cells are exposed to hypertonic medium for 24 h. Posttranscriptional regulation of the BGT1 protein is largely unexplored. We have investigated the distribution of BGT1 protein in live cells after transfection with BGT1 tagged with enhanced green fluorescent protein (EGFP). Fusion of EGFP to the NH2 terminus of BGT1 produced a fusion protein (EGFP-BGT) with transport properties identical to normal BGT1, as determined by ion dependence, inhibitor sensitivity, and apparent Km for GABA. Confocal microscopy of EGFP-BGT fluorescence in transfected Madin-Darby canine kidney (MDCK) cells showed that hypertonic stress for 24 h induced a shift in subcellular distribution from cytoplasm to plasma membrane. This was confirmed by colocalization with anti-BGT1 antibody staining. In fibroblasts, transfected EGFP-BGT caused increased transport in response to hypertonic stress. The activation of transport was not accompanied by increased expression of EGFP-BGT, as determined by Western blotting. Membrane insertion of EGFP-BGT protein in MDCK cells began within 2-3 h after onset of hypertonic stress and was blocked by cycloheximide. We conclude that posttranscriptional regulation of BGT1 is essential for adaptation to hypertonic stress and that insertion of BGT1 protein to the plasma membrane may require accessory proteins.

Transcriptional regulation by changes in tonicity

Most organisms respond to a hypertonic environment by accumulating small organic solutes. In contrast to high concentrations of electrolytes, the small organic solutes do not perturb the activity of enzymes and other macromolecules within the cell. When the renal medulla becomes hypertonic during antidiuresis, multiple signaling pathways are activated. Here, we review the role of tonicity responsive enhancers (TonE) binding protein (TonEBP), a transcription factor activated in hypertonic cells. The activation of TonEBP by hypertonicity results from its translocation to the nucleus as well as an increase in TonEBP mRNA and protein. TonEBP may have a role beyond the response to tonicity since it is highly expressed in activated lymphocytes and in developing tissues.

Hypertonic activation of the renal betaine/GABA transporter is microtubule dependent

BACKGROUND

Epithelial cells in the renal inner medulla accumulate osmolytes such as betaine to maintain normal cell volume during prolonged extracellular hypertonic stress. Betaine accumulation is the result of activation of transcription of the BGT1 transporter gene followed by increased betaine transport.

METHODS

We studied the possible role of microtubules in this adaptive mechanism using renal cells in culture. RESULTS.: In cultured renal cell lines [Madin-Darby canine kidney (MDCK) and mouse inner medullary collecting duct (mIMCD-3)], up-regulation of BGT1 activity was maximal after 24 to 30 hours in growth medium made hypertonic (510 mOsm/kg) by the addition of sucrose or NaCl. Up-regulation was reversed within 24 to 36 hours after returning cells to isotonic medium. Both cycloheximide (20 micromol/L) and nocodazole (20 micromol/L) blocked the hypertonic up-regulation of BGT1. Nocodazole was partially effective even when added 16 to 20 hours after the switch to hypertonic medium. Recovery from nocodazole action was rapid, and there was full activation of BGT1 transport within three to six hours after nocodazole removal, suggesting rapid trafficking to the cell surface once microtubules repolymerized. Hypertonic activation of BGT1 transport was detected in an isolated membrane fraction and was blocked by cycloheximide but not by nocodazole. Confocal microscopy confirmed the increased abundance of BGT1 proteins in the plasma membrane of hypertonic cells and showed that BGT1 remained intracellular during nocodazole treatment.

CONCLUSIONS

Hypertonic activation of BGT1 in renal cells requires de novo protein synthesis and microtubule-dependent trafficking of additional transporters to the cell surface. The apparent resistance of membrane BGT1 to nocodazole blockade is likely due to the presence in the membrane fraction of an increased intracellular pool of active BGT1 transporters.

The NFAT-related protein NFATL1 (TonEBP/NFAT5) is induced upon T cell activation in a calcineurin-dependent manner

NFAT DNA binding complexes regulate programs of cellular activation and differentiation by translating receptor-dependent signaling events into specific transcriptional responses. NFAT proteins, originally defined as calcium/calcineurin-dependent regulators of cytokine gene transcription in T lymphocytes, are expressed in many different cell types and represent critical signaling intermediates that mediate an increasingly wide spectrum of biologic responses. Recent studies have identified a novel protein containing a region of similarity to the NFAT DNA binding domain. Here we demonstrate that this protein, designated NFATL1 (also known as tonicity enhancer binding protein and NFAT5) is expressed at high levels in the thymus but is undetectable in mature lymphocytes. However, NFATL1 can be induced in both primary quiescent T lymphocytes and differentiated Th1 and Th2 cell populations upon mitogen- or Ag receptor-dependent activation. The induction of NFATL1 protein, as well as NFATL1-dependent transcription, is inhibited by cyclosporin A and FK506, and expression of constitutively active calcineurin induces NFATL1-dependent transcription. Overexpression of NFATc1 and inhibition of NFATc activity through the use of a dominant negative NFATc1 protein have no affect on NFATL1-dependent transcription, indicating that NFATc proteins do not play a role in the calcineurin-dependent induction of NFATL1. Interestingly, induction of NFATL1 by a hyperosmotic stimulus is not blocked by the inhibition of calcineurin. Moreover, osmotic stress response genes such as aldose reductase are not induced upon T cell activation. Thus inducible expression of NFATL1 represents a mechanism by which receptor-dependent signals as well as osmotic stress signals are translated into transcriptional responses that regulate cell function.

Bidirectional regulation of tonicity-responsive enhancer binding protein in response to changes in tonicity

Tonicity-responsive enhancer binding protein (TonEBP) regulates transcription of tonicity responsive genes such as the sodium-myo-inositol cotransporter (SMIT), the sodium-chloride-betaine cotransporter (BGT1), and aldose reductase (AR). To characterize signals that activate TonEBP in Madin-Darby canine kidney (MDCK) cells, the abundance and nuclear distribution of TonEBP were studied after the osmolality of the culture medium was changed. Hypertonicity but not hyperosmolality is effective in activation of TonEBP as expected. Surprisingly, exposure to hypotonic medium leads to a dramatic downregulation of TonEBP both in abundance and nuclear distribution, indicating that under isotonic conditions, TonEBP is at a low-level activated state and can respond to both increase and decrease in tonicity. Additional experiments suggest that cellular ionic strength is the signal that initiates regulation of TonEBP. The increase in abundance of TonEBP is mediated by an increase in mRNA abundance and a parallel increase in synthesis of TonEBP. The stability of TonEBP mRNA is not affected by hypertonicity indicating that transcription plays a major role in the induction of TonEBP by hypertonicity.