Expression of aldose reductase, sorbitol dehydrogenase and Na+/myo-inositol and Na+/Cl-/betaine transporter mRNAs in individual cells of the kidney during changes in the diuretic state

Protective role of fructokinase blockade in the pathogenesis of acute kidney injury in mice

Acute kidney injury is associated with high mortality, especially in intensive care unit patients. The polyol pathway is a metabolic route able to convert glucose into fructose. Here we show the detrimental role of endogenous fructose production by the polyol pathway and its metabolism through fructokinase in the pathogenesis of ischaemic acute kidney injury (iAKI). Consistent with elevated urinary fructose in AKI patients, mice undergoing iAKI show significant polyol pathway activation in the kidney cortex characterized by high levels of aldose reductase, sorbitol and endogenous fructose. Wild type but not fructokinase knockout animals demonstrate severe kidney injury associated with ATP depletion, elevated uric acid, oxidative stress and inflammation. Interestingly, both the renal injury and dysfunction in wild-type mice undergoing iAKI is significantly ameliorated when exposed to luteolin, a recently discovered fructokinase inhibitor. This study demonstrates a role for fructokinase and endogenous fructose as mediators of acute renal disease.

The polyol pathway, which converts glucose into sorbitol and fructose, is active in chronic conditions like hepatic steatosis and chronic kidney disease. Here, Andres-Hernando et al. show that fructose production promotes renal injury and fructokinase inhibition protects against kidney damage during ischaemic acute kidney disease.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

Osmoregulation of ceroid neuronal lipofuscinosis type 3 in the renal medulla

Recessive inheritance of mutations in ceroid neuronal lipofuscinosis type 3 (CLN3) results in juvenile neuronal ceroid lipofuscinosis (JNCL), a childhood neurodegenerative disease with symptoms including loss of vision, seizures, and motor and mental decline. CLN3p is a transmembrane protein with undefined function. Using a Cln3 reporter mouse harboring a nuclear-localized bacterial beta-galactosidase (beta-Gal) gene driven by the native Cln3 promoter, we detected beta-Gal most prominently in epithelial cells of skin, colon, lung, and kidney. In the kidney, beta-Gal-positive nuclei were predominant in medullary collecting duct principal cells, with increased expression along the medullary osmotic gradient. Quantification of Cln3 transcript levels from kidneys of wild-type (Cln3(+/+)) mice corroborated this expression gradient. Reporter mouse-derived renal epithelial cultures demonstrated a tonicity-dependent increase in beta-Gal expression. RT-quantitative PCR determination of Cln3 transcript levels further supported osmoregulation at the Cln3 locus. In vivo, osmoresponsiveness of Cln3 was demonstrated by reduction of medullary Cln3 transcript abundance after furosemide administration. Primary cultures of epithelial cells of the inner medulla from Cln3(lacZ/lacZ) (CLN3p-null) mice showed no defect in osmolyte accumulation or taurine flux, arguing against a requirement for CLN3p in osmolyte import or synthesis. CLN3p-deficient mice with free access to water showed a mild urine-concentrating defect but, upon water deprivation, were able to concentrate their urine normally. Unexpectedly, we found that CLN3p-deficient mice were hyperkalemic and had a low fractional excretion of K(+). Together, these findings suggest an osmoregulated role for CLN3p in renal control of water and K(+) balance.

Ketohexokinase: expression and localization of the principal fructose-metabolizing enzyme

Ketohexokinase (KHK, also known as fructokinase) initiates the pathway through which most dietary fructose is metabolized. Very little is known about the cellular localization of this enzyme. Alternatively spliced KHK-C and KHK-A mRNAs are known, but the existence of the KHK-A protein isoform has not been demonstrated in vivo. Using antibodies to KHK for immunohistochemistry and Western blotting of rodent tissues, including those from mouse knockouts, coupled with RT-PCR assays, we determined the distribution of the splice variants. The highly expressed KHK-C isoform localized to hepatocytes in the liver and to the straight segment of the proximal renal tubule. In both tissues, cytoplasmic and nuclear staining was observed. The KHK-A mRNA isoform was observed exclusively in a range of other tissues, and by Western blotting, the presence of endogenous immunoreactive KHK-A protein was shown for the first time, proving that the KHK-A mRNA is translated into KHK-A protein in vivo, and supporting the suggestion that this evolutionarily conserved isoform is physiologically functional. However, the low levels of KHK-A expression prevented its immunohistochemical localization within these tissues. Our results highlight that the use of in vivo biological controls (tissues from knockout animals) is required to distinguish genuine KHK immunoreactivity from experimental artifact.

Transcription factor AP-1 regulates TGF-beta(1)-induced expression of aldose reductase in cultured human mesangial cells

AIM

The previous studies demonstrated that transforming growth factor-beta(1) (TGF-beta(1)) could upregulate the expression of aldose reductase (AR). The aim of this study is to clarify (investigate) the mechanism of TGF-beta(1)-induced AR expression.

METHODS

Real-time polymerase chain reaction and western blot were used to analyse the AR expression in mRNA and protein levels in human mesangial cells, and reporter assay was used to analyse the function of various sites within 5'-flanking region of AR gene in AR expression.

RESULTS

TGF-beta(1) (4 ng/mL) stimulation could upregulate AR expression. The cells pretreated with pharmacological inhibitors, U0126 and PD98059 for blocking extracellular signal-related kinase (ERK) signalling pathway or SP6000125 for blocking c-Jun N-terminal kinase (JNK) signalling pathway, respectively, showed reduced expression of AR after TGF-beta(1) stimulation. Similarly, the cells transiently transfected with pCMVTAM67, which is an expression plasmid for DN-c-Jun showed decreasing AR expression. Reporter assay revealed that the 5'-promoter region of AR consisting of an AP-1 site and two putative antioxidant response elements (ARE) was responsible for TGF-beta(1) stimulation. Mutation of either ARE did not affect the promoter activity in the reporter assay while mutation of AP-1 site caused a significant decrease in the responsiveness to TGF-beta(1).

CONCLUSION

TGF-beta(1) upregulate AR expression in both mRNA and protein levels. The results demonstrated that ERK and JNK are involved in the downstream signalling pathways and transcription factor AP-1 plays an important role in the regulation of TGF-beta(1)-induced AR expression in mesangial cells.

Novel insights in the treatment of diabetic nephropathy

Diabetes is currently one of the leading causes of end-stage renal failure requiring renal replacement therapy in the Western World. About 15% to 20% of type 1 diabetic patients and 30% to 40% of type 2 diabetic patients will eventually develop end-stage renal failure. To prevent the development or progression of diabetic kidney disease, good glycaemic control remains the cornerstone in the management of diabetic patients. Beyond glycaemic control, other metabolic factors have been shown to be involved in the development of diabetic kidney disease, i.e. advanced glycation endproducts (AGEs) and the aldose reductase pathway. Furthermore, an adequate control of high blood pressure and treatment of microalbuminuria are major therapeutic targes. To achieve adequate blood pressure control, a combination therapy with different classes of antihypertensive agents is often necessary, especially including angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. Other vasoactive factors involved in diabetic nephropathy such as endothelin and nitric oxide will be covered briefly. Besides hyperglycaemia and high blood pressure, other risk factors have been identified in the development or progression of diabetic kidney disease: smoking, hyperlipidaemia, obesity and high protein intake. Their impact on renal function will be highlighted. Finally, recent research has also identified intracellular pathways such as the diacylglycerol-protein kinase C pathway and several growth factors, such as growth hormone, insulin-like growth factor, transforming growth factor-beta, vascular endothelial growth factor, and platelet derived growth factor as players in diabetic kidney disease.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

From hyperglycemia to diabetic kidney disease: the role of metabolic, hemodynamic, intracellular factors and growth factors/cytokines

At present, diabetic kidney disease affects about 15-25% of type 1 and 30-40% of type 2 diabetic patients. Several decades of extensive research has elucidated various pathways to be implicated in the development of diabetic kidney disease. This review focuses on the metabolic factors beyond blood glucose that are involved in the pathogenesis of diabetic kidney disease, i.e., advanced glycation end-products and the aldose reductase system. Furthermore, the contribution of hemodynamic factors, the renin-angiotensin system, the endothelin system, and the nitric oxide system, as well as the prominent role of the intracellular signaling molecule protein kinase C are discussed. Finally, the respective roles of TGF-beta, GH and IGFs, vascular endothelial growth factor, and platelet-derived growth factor are covered. The complex interplay between these different pathways will be highlighted. A brief introduction to each system and description of its expression in the normal kidney is followed by in vitro, experimental, and clinical evidence addressing the role of the system in diabetic kidney disease. Finally, well-known and potential therapeutic strategies targeting each system are discussed, ending with an overall conclusion.

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.

Expression of aldose reductase in developing rat kidney

Newborn rats are not capable of producing concentrated urine. With development of the concentrating system and a hypertonic medullary interstitium, intracellular osmolytes, such as sorbitol, accumulate in the renal medulla. Sorbitol is produced from glucose in a reaction catalyzed by aldose reductase (AR). The purpose of this study was to establish the time of expression and distribution of AR in the developing rat kidney. Kidneys from 16-, 18-, and 20-day-old fetuses and 1-, 3-, 4-, 5-, 7-, 14-, and 21-day-old pups were processed for immunohistochemistry and immunoblot analysis. In adult animals, AR was expressed only in the inner medulla, in which it was localized in ascending thin limbs (ATLs), inner medullary collecting ducts (IMCDs), and interstitial cells. AR immunoreactivity was not detected in fetal kidneys but was observed in the terminal part of the descending thin limb and IMCD in the renal papilla of 1-day-old pups. At birth, all of the loops of Henle are configured as short loops and there are no ATLs. After birth, papillary thick ascending limbs are gradually transformed into ATLs by a process that involves apoptotic deletion of cells from the thick ascending limb. During this time, AR immunoreactivity appeared in the cells undergoing transformation in the ascending limb, beginning at the papillary tip and ascending to the border between the outer medulla and the inner medulla. However, there was no labeling of apoptotic cells. The expression of AR in both the ATL and the IMCD gradually increased during kidney development. We conclude that AR expression in the inner medulla coincides with the increase in medullary tonicity that is known to occur during the first 3 wk after birth. On the basis of the observation that only AR-negative cells were deleted by apoptosis in the differentiating ATL, we propose that AR may protect ATL cells against apoptosis.

Relationship between intracellular ionic strength and expression of tonicity-responsive genes in rat papillary collecting duct cells

Intracellular ionic strength may play an important role in regulating the expression of genes encoding osmolyte-accumulating molecules. To establish whether a strict relation exists between these variables, intracellular ionic strength (sum of Na+, Cl- and K+ concentrations) and the relative abundance of mRNA derived from various tonicity-sensitive genes was examined using electron microprobe analysis and Northern blots on primary cultures of rat papillary collecting duct (PCD) cells following acute or long-term alterations in medium tonicity. Hypertonic medium (450 mosmol kg(-1)) evoked an initial rise in intracellular ionic strength (269 +/- 5 vs. 194 +/- 7 mmol (kg wet weight (wt))(-1) in isotonic controls; means +/- S.E.M.), which subsequently declined gradually, and a significantly higher abundance of bgt1 (Na+- and Cl- -dependent betaine transporter), smit (Na+/myo-inositol cotransporter), ar (aldose reductase) and osp94 (osmotic stress protein 94) mRNAs. Conversely, exposure to hypotonic medium (200 mosmol kg(-1)) for 12 h was associated with significantly reduced intracellular ionic strength (153 +/- 4 mmol (kg wet wt)(-1)) and significantly reduced the abundance of smit and ar mRNAs. PCD cells preconditioned in hypotonic medium and re-exposed to isotonic medium showed significantly higher abundance of these mRNAs than isotonic controls, although the intracellular ionic strength did not differ. Two further tonicity-sensitive genes responded differently to medium tonicity: while the abundance of hsp70 (heat shock protein 70) mRNA increased significantly following both hypo- and hypertonic stress, inos (inducible nitric oxide synthase) mRNA abundance correlated inversely with medium tonicity. These findings support the view that the effect of intracellular ionic strength on the expression of bgt1, smit, ar and osp94 is modulated by additional factors such as cell volume, and that its effect on the pathways regulating hsp70 and inos is even more complex.

Reabsorption of betaine in Henle's loops of rat kidney in vivo

This study was designed 1) to localize and 2) to characterize betaine reabsorption from the tubular lumen in rat kidney in vivo, and 3) to test whether reabsorption is modulated by the diuretic state. [(14)C]betaine (+ [(3)H]inulin) was microperfused through the proximal convoluted tubule (PCT) and microinfused into late proximal (LP) and early distal (ED) tubules, long loops of Henle (LLH), and vasa recta of the rat in vivo et situ, and the fractional recovery of the (14)C label was determined end proximally (PCT) and in the final urine, respectively. [(14)C]betaine was not reabsorbed during ED microinfusion, whereas fractional reabsorption during LP microinfusion was 82% at 0.06 mM betaine and decreased gradually to 4.8% at 60 mM. L-Proline had lower Michaelis-Menten constant (K(m)) and sarcosine a higher K(m) than betaine. Chronic, but not acute, diuresis inhibited betaine reabsorption in Henle's loops. Fractional [(14)C]betaine reabsorption in PCT was much smaller than that during LP microinfusion. [(14)C]betaine (7.28 mM) microinfused 1) into LLH was reabsorbed to 30% and 2) into vasa recta appeared in the ipsilateral urine to a much higher extent than contralaterally. In both cases, no saturation was detected at 70 mM. We conclude that betaine is reabsorbed by mediated transport from descending limbs of short Henle's loops by a proline-preferring carrier in a diuresis-modulated manner. In the deep medulla, bidirectional blood/urine betaine transport exists.

Hypertonicity-induced accumulation of organic osmolytes in papillary interstitial cells

BACKGROUND

Medullary cells of the concentrating kidney are exposed to high extracellular solute concentrations. It is well established that epithelial cells in this kidney region adapt osmotically to hypertonic stress by accumulating organic osmolytes. Little is known, however, of the adaptive mechanisms of a further medullary cell type, the papillary interstitial cell [renal papillary fibroblast (RPF)]. We therefore compared the responses of primary cultures of RPFs and papillary collecting duct (PCD) cells exposed to hypertonic medium.

METHODS

In RPFs and PCD cells, organic osmolytes were determined by high-performance liquid chromatography; mRNA expression for organic osmolyte transporters [Na+/Cl(-)-dependent betaine transporter (BGT), Na(+)-dependent myo-inositol transporter (SMIT)], and the sorbitol synthetic and degrading enzymes [aldose reductase (AR) and sorbitol dehydrogenase (SDH), respectively] was determined by Northern blot analysis.

RESULTS

Exposure to hypertonic medium (600 mOsm/kg by NaCl addition) caused intracellular contents of glycerophosphorylcholine, betaine, myo-inositol, and sorbitol, but not free amino acids, to increase significantly in both RPFs and PCD cells. The rise in intracellular contents of these organic osmolytes was accompanied by enhanced expression of mRNAs coding for BGT, SMIT, and AR in both RPFs and PCD cells. SDH mRNA abundance, however, was unchanged. Nonradioactive in situ hybridization studies on sections from formalin-fixed and paraffin-embedded, normally concentrating kidneys showed strong expression of BGT, SMIT, and AR mRNAs in interstitial and collecting duct cells of the papilla, whereas expression of SDH mRNA was much weaker in both cell types.

CONCLUSIONS

These results suggest that both RPFs and PCD cells use similar strategies to adapt osmotically to the high interstitial NaCl concentrations characteristic for the inner medulla and papilla of the concentrating kidney.