Liver X receptor agonists decrease ENaC-mediated sodium transport in collecting duct cells

Activation of farnesoid X receptor retards expansion of renal collecting duct cell-derived cysts via inhibition of CFTR-mediated Cl- secretion

The transcription factor farnesoid X receptor (FXR) regulates energy metabolism. Specifically, FXR functions to regulate cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl- secretion in intestinal epithelial cells. Therefore, this study aimed to investigate the role of FXR in CFTR-mediated Cl- secretion in renal tubular cells and to further elucidate its effects on renal cyst formation and growth. CFTR-mediated Cl- transport was evaluated via short-circuit current (ISC) measurements in Madin-Darby canine kidney (MDCK) cell monolayers and primary rat inner medullary collecting duct cells. The role of FXR in renal cyst formation and growth was determined by the MDCK cell-derived cyst model. Incubation with synthesized (GW4064) and endogenous (CDCA) FXR ligands reduced CFTR-mediated Cl- secretion in a concentration- and time-dependent manner. The inhibitory effect of FXR ligands was not due to the result of reduced cell viability and was attenuated by cotreatment with an FXR antagonist. FXR activation significantly decreased CFTR protein but not its mRNA. In addition, FXR activation inhibited CFTR-mediated Cl- secretion in primary renal collecting duct cells. FXR activation decreased ouabain-sensitive ISC without altering Na+-K+-ATPase mRNA and protein levels. Furthermore, FXR activation significantly reduced the number of cysts and renal cyst expansion. These inhibitory effects were correlated with a decrease in the expression of protein synthesis regulators mammalian target of rapamycin/S6 kinase. This study shows that FXR activation inhibits Cl- secretion in renal cells via inhibition of CFTR expression and retards renal cyst formation and growth. The discoveries point to a physiological role of FXR in the regulation of CFTR and a potential therapeutic application in polycystic kidney disease treatment.NEW & NOTEWORTHY The present study reveals that farnesoid X receptor (FXR) activation reduces microcyst formation and enlargement. This inhibitory effect of FXR activation is involved with decreased cell proliferation and cystic fibrosis transmembrane conductance regulator-mediated Cl- secretion in renal collecting duct cells. FXR might represent a novel target for the treatment of autosomal dominant polycystic kidney disease.

Nuclear receptors in renal health and disease

As a major social and economic burden for the healthcare system, kidney diseases contribute to the constant increase of worldwide deaths. A deeper understanding of the underlying mechanisms governing the etiology, development and progression of kidney diseases may help to identify potential therapeutic targets. As a superfamily of ligand-dependent transcription factors, nuclear receptors (NRs) are critical for the maintenance of normal renal function and their dysfunction is associated with a variety of kidney diseases. Increasing evidence suggests that ligands for NRs protect patients from renal ischemia/reperfusion (I/R) injury, drug-induced acute kidney injury (AKI), diabetic nephropathy (DN), renal fibrosis and kidney cancers. In the past decade, some breakthroughs have been made for the translation of NR ligands into clinical use. This review summarizes the current understanding of several important NRs in renal physiology and pathophysiology and discusses recent findings and applications of NR ligands in the management of kidney diseases.

New Pandemic: Obesity and Associated Nephropathy

Incidence of obesity related renal disorders have increased 10-folds in recent years. One of the consequences of obesity is an increased glomerular filtration rate (GFR) that leads to the enlargement of the renal glomerulus, i.e., glomerulomegaly. This heightened hyper-filtration in the setting of type 2 diabetes irreparably damages the kidney and leads to progression of end stage renal disease (ESRD). The patients suffering from type 2 diabetes have progressive proteinuria, and eventually one third of them develop chronic kidney disease (CKD) and ESRD. For ameliorating the progression of CKD, inhibitors of renin angiotensin aldosterone system (RAAS) seemed to be effective, but on a short-term basis only. Long term and stable treatment strategies like weight loss via restricted or hypo-caloric diet or bariatric surgery have yielded better promising results in terms of amelioration of proteinuria and maintenance of normal GFR. Body mass index (BMI) is considered as a traditional marker for the onset of obesity, but apparently, it is not a reliable indicator, and thus there is a need for more precise evaluation of regional fat distribution and amount of muscle mass. With respect to the pathogenesis, recent investigations have suggested perturbation in fatty acid and cholesterol metabolism as the critical mediators in ectopic renal lipid accumulation associated with inflammation, increased generation of ROS, RAAS activation and consequential tubulo-interstitial injury. This review summarizes the renewed approaches for the obesity assessment and evaluation of the pathogenesis of CKD, altered renal hemodynamics and potential therapeutic targets.

Mammalian urine concentration: a review of renal medullary architecture and membrane transporters

Mammalian kidneys play an essential role in balancing internal water and salt concentrations. When water needs to be conserved, the renal medulla produces concentrated urine. Central to this process of urine concentration is an osmotic gradient that increases from the corticomedullary boundary to the inner medullary tip. How this gradient is generated and maintained has been the subject of study since the 1940s. While it is generally accepted that the outer medulla contributes to the gradient by means of an active process involving countercurrent multiplication, the source of the gradient in the inner medulla is unclear. The last two decades have witnessed advances in our understanding of the urine-concentrating mechanism. Details of medullary architecture and permeability properties of the tubules and vessels suggest that the functional and anatomic relationships of these structures may contribute to the osmotic gradient necessary to concentrate urine. Additionally, we are learning more about the membrane transporters involved and their regulatory mechanisms. The role of medullary architecture and membrane transporters in the mammalian urine-concentrating mechanism are the focus of this review.

Ouabagenin is a naturally occurring LXR ligand without causing hepatic steatosis as a side effect

Ouabagenin (OBG) is an aglycone of the cardiotonic steroid ouabain and until now was considered a biologically inactive biosynthetic precursor. Herein, we revealed that OBG functions as a novel class of ligand for the liver X receptor (LXR). Luciferase reporter assays and in silico docking studies suggested that OBG has LXR-selective agonistic activity. In addition, OBG repressed the expression of epithelial sodium channel (ENaC), a LXR target gene, without causing hepatic steatosis, a typical side effect of conventional LXR ligands. This remarkable biological activity can be attributed to a unique mode of action; the LXR agonist activity mainly proceeds through the LXRβ subtype without affecting LXRα, unlike conventional LXR ligands. Thus, OBG is a novel class of LXR ligand that does not cause severe side effects, with potential for use as an antihypertensive diuretic or a tool compound for exploring LXR subtype-specific biological functions.

Liver X receptor activation inhibits SGLT2-mediated glucose transport in human renal proximal tubular cells

NEW FINDINGS

What is the central question of this study? The liver X receptor (LXR) has been reported to regulate several membrane transporters. It is imperative to investigate whether LXR activation regulates SGLT2-mediated glucose transport in human renal proximal tubular cells. What is the main finding and its importance? Liver X receptor activation inhibits SGLT2 transport function in normal and high-glucose conditions via reduction of SGLT2 protein expression. Liver X receptors (LXRs) are members of a nuclear receptor family consisting of two isoforms, LXRα and LXRβ. They play a major role in energy metabolism, including lipid and glucose metabolism. Recent studies reported that LXRs regulate plasma glucose, although the mechanism is still uncertain. The present study investigated whether LXR activation regulates sodium glucose cotransporter2 (SGLT2) in human renal proximal tubular cells. LXR agonists, T0901317 and GW3965, inhibited SGLT2-mediated glucose uptake in a concentration-dependent manner. The effect of T0901317 and GW3965 was attenuated by a LXR antagonist, fenofibrate. Activation of the retinoid X receptor (RXR) agonist, bexarotene, potentiates the inhibitory effect of these ligands. Thus, the inhibitory effect of LXR agonists on SGLT2 was mediated and facilitated by LXR and RXR activation, respectively. In addition, the inhibitory effect of LXR agonists was not mediated by cytotoxicity. Exposing HK-2 cells, a renal proximal tubular cell line, to LXR agonists significantly reduced the maximal transport rate of SGLT2 without any effect on transporter affinity. Western blot analysis revealed that LXR activation significantly decreased protein expression of SGLT2 with no change in mRNA level. In addition, LXR activation inhibited canagliflozin-sensitive short-circuit current, which represents SGLT2-mediated glucose transport in a polarized human renal proximal tubular cell monolayer. Furthermore, LXR activation inhibited the transport function of SGLT2 in hyperglycaemic conditions. As such, this study represents evidence for the inhibitory effect of LXR activation on glucose transport in human renal proximal tubular cells.

Activation of liver X receptor inhibits OCT2-mediated organic cation transport in renal proximal tubular cells

Liver X receptor (LXR) is transcriptional factor that plays an important role in the regulation of energy metabolism such as cholesterol, lipid, and glucose metabolism as well as membrane transporters and channels. Using both in vitro and in vivo models, LXR regulation of the expression and function of renal organic cation transporter 2 (OCT2) was observed. Synthetic LXR agonist (GW3965) and endogenous LXR agonist (22R-hydroxycholesterol) significantly reduced the uptake of ³H-MPP⁺, a prototypic substrate of OCT2, in both OCT2- Chinese hamster ovary K1 and human renal proximal tubular cells (RPTEC/TERT1). GW3965 decreased transport activity of OCT2 via a reduction of the maximal transport rate of MPP⁺ without affecting transporter affinity. The inhibitory effect of GW3965 was attenuated by co-treatment with LXR antagonist (fenofibrate) indicating the inhibition was LXR-dependent mechanism. In addition, co-treatment with a retinoic X receptor (RXR) ligand, 9-cis retinoic acid enhanced the inhibitory effect of GW3965, indicating negative regulation of OCT2 transport activity by the LXR/RXR complex. Treatment RPTEC/TERT1 cells with GW3965 significantly reduced OCT2 protein expression without changing mRNA expression. In parallel, the effect of LXR activation on OCT2 function was investigated in intact mouse kidney. Treating mice with 50 mg/kg BW T0901317 for 14 days significantly decreased ³H-MPP⁺ uptake into renal cortical slices, correlating with decreased OCT2 protein expression in renal cortex without changes in mRNA expression levels. Taken together, LXR/RXR activation downregulates the protein expression and function of OCT2 in renal proximal tubule, suggesting LXR might affect the total profile of renal excretion of cationic compounds.

Liver X receptor β increases aquaporin 2 protein level via a posttranscriptional mechanism in renal collecting ducts

Liver X receptors (LXRs) including LXRα and LXRβ are nuclear receptor transcription factors and play an important role in lipid and glucose metabolism. It has been previously reported that mice lacking LXRβ but not LXRα develop a severe urine concentrating defect, likely via a central mechanism. Here we provide evidence that LXRβ regulates water homeostasis through increasing aquaporin 2 (AQP2) protein levels in renal collecting ducts. LXRβ^-/- mice exhibited a reduced response to desmopressin (dDAVP) stimulation, suggesting that the diabetes insipidus phenotype is of both central and nephrogenic origin. AQP2 protein abundance in the renal inner medulla was significantly reduced in LXRβ^-/- mice but with little change in AQP2 mRNA levels. In vitro studies showed that AQP2 protein levels were elevated upon LXR agonist treatment in both primary cultured mouse inner medullary duct cells (mIMCD) and the mIMCD3 cell line with stably expressed AQP2. In addition, LXR agonists including TO901317 and GW3965 failed to induce AQP2 gene transcription but diminished its protein ubiquitination in primary cultured mIMCD cells, thereby inhibiting its degradation. Moreover, LXR activation-induced AQP2 protein expression was abolished by the protease inhibitor MG132 and the ubiquitination-deficient AQP2 (K270R). Taken together, the present study demonstrates that activation of LXRβ increases AQP2 protein levels in the renal collecting ducts via a posttranscriptional mechanism. As such, LXRβ represents a key regulator of body water homeostasis.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Soluble (pro)renin receptor via β-catenin enhances urine concentration capability as a target of liver X receptor

The extracellular domain of the (pro)renin receptor (PRR) is cleaved to produce a soluble (pro)renin receptor (sPRR) that is detected in biological fluid and elevated under certain pathological conditions. The present study was performed to define the antidiuretic action of sPRR and its potential interaction with liver X receptors (LXRs), which are known regulators of urine-concentrating capability. Water deprivation consistently elevated urinary sPRR excretion in mice and humans. A template-based algorithm for protein-protein interaction predicted the interaction between sPRR and frizzled-8 (FZD8), which subsequently was confirmed by coimmunoprecipitation. A recombinant histidine-tagged sPRR (sPRR-His) in the nanomolar range induced a remarkable increase in the abundance of renal aquaporin 2 (AQP2) protein in primary rat inner medullary collecting duct cells. The AQP2 up-regulation relied on sequential activation of FZD8-dependent β-catenin signaling and cAMP-PKA pathways. Inhibition of FZD8 or tankyrase in rats induced polyuria, polydipsia, and hyperosmotic urine. Administration of sPRR-His alleviated the symptoms of diabetes insipidus induced in mice by vasopressin 2 receptor antagonism. Administration of the LXR agonist TO901317 to C57/BL6 mice induced polyuria and suppressed renal AQP2 expression associated with reduced renal PRR expression and urinary sPRR excretion. Administration of sPRR-His reversed most of the effects of TO901317. In cultured collecting duct cells, TO901317 suppressed PRR protein expression, sPRR release, and PRR transcriptional activity. Overall we demonstrate, for the first time to our knowledge, that sPRR exerts antidiuretic action via FZD8-dependent stimulation of AQP2 expression and that inhibition of this pathway contributes to the pathogenesis of diabetes insipidus induced by LXR agonism.

Regulation of αENaC transcription

Aldosterone is a major regulator of Na(+) absorption and acts primarily by controlling the epithelial Na(+) channel (ENaC) function at multiple levels including transcription. ENaC consists of α, β, and γ subunits. In the classical model, aldosterone enhances transcription primarily by activating mineralocorticoid receptor (MR). However, how aldosterone induces chromatin alternation and thus leads to gene activation or repression remains largely unknown. Emerging evidence suggests that Dot1a-Af9 complex plays an important role in repression of αENaC by directly binding and modulating targeted histone H3 K79 hypermethylation at the specific subregions of αENaC promoter. Aldosterone impairs Dot1a-Af9 formation by decreasing expression of Dot1a and Af9 and by inducing Sgk1, which, in turn, phosphorylates Af9 at S435 to weaken Dot1a-Af9 interaction. MR counterbalances Dot1a-Af9 action by competing with Dot1a for binding Af9. Af17 derepresses αENaC by competitively interacting with Dot1a and facilitating Dot1a nuclear export. Consistently, MR(-/-) mice have impaired ENaC expression at day 5 after birth, which may contribute to progressive development of pseudohypoaldosteronism type 1 in a later stage. Af17(-/-) mice have decreased ENaC expression, renal Na(+) retention, and blood pressure. In contrast, Dot1l(AC) mice have increased αENaC expression, despite a 20% reduction of the principal cells. This chapter reviews these findings linking aldosterone action to ENaC transcription through chromatin modification. Future direction toward the understanding the role of Dot1a-Af9 complex beyond ENaC regulation, in particular, in renal fibrosis is also briefly discussed.

Mineralocorticoid receptor antagonizes Dot1a-Af9 complex to increase αENaC transcription

Aldosterone is a major regulator of Na(+) absorption and acts by activating the mineralocorticoid receptor (MR) to stimulate the epithelial Na(+) channel (ENaC). MR(-/-) mice exhibited pseudohypoaldosteronism type 1 (hyponatremia, hyperkalemia, salt wasting, and high levels of aldosterone) and died around postnatal day 10. However, if and how MR regulates ENaC transcription remain incompletely understood. Our earlier work demonstrated that aldosterone activates αENaC transcription by reducing expression of Dot1a and Af9 and by impairing Dot1a-Af9 interaction. Most recently, we reported identification of a major Af9 binding site in the αENaC promoter and upregulation of αENaC mRNA expression in mouse kidneys lacking Dot1a. Despite these findings, the putative antagonism between the MR/aldosterone and Dot1a-Af9 complexes has never been addressed. The molecular defects leading to PHA-1 in MR(-/-) mice remain elusive. Here, we report that MR competes with Dot1a to bind Af9. MR/aldosterone and Dot1a-Af9 complexes mutually counterbalance ENaC mRNA expression in inner medullary collecting duct 3 (IMCD3) cells. Real-time RT-quantitative PCR revealed that 5-day-old MR(-/-) vs. MR(+/+) mice had significantly lower αENaC mRNA levels. This change was associated with an increased Af9 binding and H3 K79 hypermethylation in the αENaC promoter. Therefore, this study identified MR as a novel binding partner and regulator of Af9 and a novel mechanism coupling MR-mediated activation with relief of Dot1a-Af9-mediated repression via MR-Af9 interaction. Impaired ENaC expression due to failure to inhibit Dot1a-Af9 may play an important role in the early stages of PHA-1 (before postnatal day 8) in MR(-/-) mice.

Activation of liver X receptors reduces CFTR-mediated Cl(-) transport in kidney collecting duct cells

Liver X receptors (LXRs) are transcription factors belonging to the nuclear receptor super family, which act as regulators of lipid and glucose metabolism. However, LXRs have been shown to regulate the function of transporters in the kidney, including the Na-Pi cotransporter, organic anion transporter, and epithelial Na(+) channel. In this report, we demonstrated the ability of LXR ligands, both endogenous [22 (R)-hydroxycholesterol] and synthetic (T0901317 and GW3965), to reduce CFTR-mediated Cl(-) secretion in a type I Madin-Darby canine kidney (MDCK) cell line and in murine primary inner medullary collecting duct (IMCD) cells, based on measurements of [Arg(8)]-vasopressin-induced Cl(-) current. However, treatment of MDCK cell monolayers with 5 μM T0901317 for 24 h did not alter ouabain-senstive current or Na(+)-K(+)-ATPase-α protein content. Furthermore, basolateral membranes permeabilization of MDCK cell monolayers still resulted in a decrease in apical Cl(-) current stimulated by both [Arg(8)]-vasopressin and 8-cholorophenyl-thio-cAMP, indicating that the factor(s) encoded by the target gene(s) of agonist-activated LXRs might be located at the apical membrane. Western blot analysis revealed that inhibition of Cl(-) secretion occurred via a decrease in CFTR protein, which was not due to downregulation of its mRNA expression. In addition, both synthetic LXR agonists significantly retarded the growth of forskolin-induced cysts formed in MDCK cells cultured in collagen gel. This is the first evidence showing that ligand-activated LXRs are capable of downregulating CFTR-mediated Cl(-) secretion of kidney cells and of retarding cyst growth in a MDCK cell model.