Emerging role of Akt substrate protein AS160 in the regulation of AQP2 translocation

Poly(ADP-ribose) polymerase-1 affects vasopressin-mediated AQP2 expression in collecting duct cells of the kidney

Poly(ADP-ribosyl)ation (PARylation), as a posttranslational modification mediated by poly(ADP-ribose) polymerases (PARPs) catalyzing the transfer of ADP-ribose from NAD+ molecules to acceptor proteins, involves a number of cellular processes. As mice lacking the PARP-1 gene (Parp1) produce more urine, we investigated the role of PARP-1, the most prevalent member of the PARP family, in the vasopressin-responsive expression of aquaporin-2 (AQP2). In biotin-conjugated nicotinamide adenine dinucleotide (biotin-NAD+) pulldown and immunoprecipitation assays of poly(ADP)-ribose in mpkCCDc14 cells, immunoblots demonstrated that 1-deamino-8-D-arginine vasopressin (dDAVP) induced the PARylation of total proteins, associated with an increase in the cleavage of PARP-1 and cleaved caspase-3 expression. By inhibiting PARP-1 with siRNA, the abundance of dDAVP-induced AQP2 mRNA and protein was significantly diminished. In contrast, despite a substantial decrease in PARylation, the PARP-1 inhibitor (PJ34) had no effect on the dDAVP-induced regulation of AQP2 expression. The findings suggest that PARP-1 protein expression itself, and not PARP-1-mediated PARylation, is necessary for dDAVP-regulated AQP2 expression. Bioinformatic analysis revealed that 408 proteins interact with PARP-1 in the collecting duct (CD) cells of the kidney. Among them, the signaling pathway of the vasopressin V2 receptor was identified for 49 proteins. In particular, β-catenin, which is phosphorylated at Ser552 by dDAVP, was identified as the PARP-1-interacting protein. A significant decrease of β-catenin phosphorylation (Ser552) in response to dDAVP was associated with siRNA-mediated PARP-1 knockdown. Taken together, PARP-1 is likely to play a role in vasopressin-induced AQP2 expression by interacting with β-catenin in renal CD cells.NEW & NOTEWORTHY The poly(ADP-ribose) polymerase (PARP) family catalyzes poly(ADP-ribosylation) (PARylation), which is one of the posttranslational modifications of largely undetermined physiological significance. This study investigated the role of PARP-1, the most prevalent member of the PARP family, in the vasopressin-responsive expression of aquaporin-2 (AQP2). The results demonstrated that PARP-1 protein expression itself, and not PARP-1-mediated PARylation, is necessary for dDAVP-regulated AQP2 expression. β-Catenin, which is phosphorylated at Ser552 by dDAVP, was identified as the PARP-1-interacting protein.

The Small GTPase Rab14 Regulates the Trafficking of Ceramide from Endoplasmic Reticulum to Golgi Apparatus and Facilitates Classical Swine Fever Virus Assembly

Classical swine fever virus (CSFV) is a highly pathogenic RNA virus belonging to the Flaviviridae family that can cause deadly classical swine fever (CSF) in pigs. However, the molecular details of virus replication in the host are still unclear. Our previous studies have reported that several Rab proteins mediate CSFV entry into host cells, but it is unknown whether CSFV hijacks other Rab proteins for effective viral infection. Here, we systematically studied the role of Rab14 protein in regulating lipid metabolism for promoting viral assembly. First, Rab14 knockdown and overexpression significantly affected CSFV replication, indicating the essential role of Rab14 in CSFV infection. Interestingly, Rab14 could significantly affect virus replication in the late stage of infection. Mechanistically, CSFV NS5A recruited Rab14 to the ER, followed by ceramide transportation to the Golgi apparatus, where sphingomyelin was synthesized. The experimental data of small molecule inhibitors, RNA interference, and replenishment assay showed that the phosphatidylinositol-3-kinase (PI3K)/AKT/AS160 signaling pathway regulated the function of Rab14 to affect the transport of ceramide. More importantly, sphingomyelin on the Golgi apparatus contributed to the assembly of viral particles. Blockage of the Rab14 regulatory pathway induced the reduction of the content of sphingomyelin on the Golgi apparatus, impairing the assembly of virus particles. Our study clarifies that Rab14 regulates lipid metabolism and promotes CSFV replication, which provides insight into a novel function of Rab14 in regulating vesicles to transport lipids to the viral assembly factory. IMPORTANCE The Rab protein family members participate in the viral replication of multiple viruses and play important roles in the virus infection cycle. Our previous research focused on Rab5/7/11, which regulated the trafficking of vesicles in the early stage of CSFV infection, especially in viral endocytosis. However, the role of other Rab proteins in CSFV replication is unclear and needs further clarification. Strikingly, we screened some Rabs and found the important role of Rab14 in CSFV infection. Virus infection mobilized Rab14 to regulate the vesicle to transport ceramide from the ER to the Golgi apparatus, further promoting the synthesis of sphingomyelin and facilitating virus assembly. The treatment of inhibitors showed that the lipid transport mediated by Rab14 was regulated by the PI3K/AKT/AS160 signaling pathway. Knockdown of Rab14 or the treatment with PI3K/AKT/AS160 inhibitors reduced the ceramide content in infected cells and hindered virus assembly. Our study is the first to explain that vesicular lipid transport regulated by Rab promotes CSFV assembly, which is conducive to the development of antiviral drugs.

Protein Structure and Modification of Aquaporins

Aquaporins (AQPs) allow water molecules and other small, neutral solutes to quickly pass through membrane. The protein structures of AQPs solved by crystallographic methods or cryo-electron microscopy technology show that AQP monomer consists of six membrane-spanning alpha-helices that form the central water-transporting pore. AQP monomers assemble to form tetramers, forming the functional units in the membrane, to transport water or other small molecules. The biological functions of AQPs are regulated by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation and protein interactions. Modifications of AQP combined with structural properties contribute to a better functional mechanism of AQPs. Insight into the molecular mechanisms responsible for AQP modifications as well as gating and transport properties proved to be fundamental to the development of new therapeutic targets or reliable diagnostic and prognostic biomarkers.

EPDR1 is a noncanonical effector of insulin-mediated angiogenesis regulated by an endothelial-specific TGF-β receptor complex

Insulin signaling in blood vessels primarily functions to stimulate angiogenesis and maintain vascular homeostasis through the canonical PI3K and MAPK signaling pathways. However, angiogenesis is a complex process coordinated by multiple other signaling events. Here, we report a distinct crosstalk between the insulin receptor and endoglin/activin receptor-like kinase 1 (ALK1), an endothelial cell-specific TGF-β receptor complex essential for angiogenesis. While the endoglin-ALK1 complex normally binds to TGF-β or bone morphogenetic protein 9 (BMP9) to promote gene regulation via transcription factors Smad1/5, we show that insulin drives insulin receptor oligomerization with endoglin-ALK1 at the cell surface to trigger rapid Smad1/5 activation. Through quantitative proteomic analysis, we identify ependymin-related protein 1 (EPDR1) as a major Smad1/5 gene target induced by insulin but not by TGF-β or BMP9. We found endothelial EPDR1 expression is minimal at the basal state but is markedly enhanced upon prolonged insulin treatment to promote cell migration and formation of capillary tubules. Conversely, we demonstrate EPDR1 depletion strongly abrogates these angiogenic effects, indicating that EPDR1 is a crucial mediator of insulin-induced angiogenesis. Taken together, these results suggest important therapeutic implications for EPDR1 and the TGF-β pathways in pathologic angiogenesis during hyperinsulinemia and insulin resistance.

Peptide Analogues of VPP and IPP with Improved Glucose Uptake Activity in L6 Myotubes can be Released from Cereal Proteins

VPP (Val-Pro-Pro) and IPP (Ile-Pro-Pro) are two famous antihypertensive peptides with possible benefits for type 2 diabetes mellitus (T2DM). The study was aimed to investigate the effect of peptide analogues of VPP and IPP on glucose uptake activity in L6 myotubes. The analogues were designed by replacing the N-terminal, middle, or C-terminal amino acid residues of VPP and IPP with one amino acid at a time from five amino acid groups (polar, nonpolar, basic, acidic, and aromatic amino acids). Among 26 tripeptides tested, IQP, IPQ, VPE, and VEP showed significantly higher glucose uptake activity than their parent peptides, and all were successfully released from rice proteins at the contents of 5415.82 ± 63.34, 1586.77 ± 14.94, 354.07 ± 6.56, and 596.10 ± 2.32 ng/mg dry basis, respectively, and quantified by liquid chromatography-mass spectrometry (MS)/MS using multiple reaction monitoring. All four peptides were shown to promote glucose uptake via the adenosine monophosphate-activated protein kinase pathway accompanied by glucose transporter type 4 (Glut4) translocation rather than the insulin signaling pathway.

Carvacrol Attenuates Diabetic Cardiomyopathy by Modulating the PI3K/AKT/GLUT4 Pathway in Diabetic Mice

Background: Diabetic cardiomyopathy (DCM), a common complication of diabetes mellitus, eventually leads to heart failure. Carvacrol is a food additive with diverse bioactivities. We aimed to study the protective effects and mechanisms of carvacrol in DCM.

Methods: We used a streptozotocin-induced and db/db mouse model of types 1 and 2 diabetes mellitus (T1DM and T2DM), respectively. Both study groups received daily intraperitoneal injections of carvacrol for 6 weeks. Cardiac remodeling was evaluated by histological analysis. We determined gene expression of cardiac remodeling markers (Nppa and Myh7) by quantitative real-time PCR and cardiac function by echocardiography. Changes of PI3K/AKT signaling were determined with Western blotting. GLUT4 translocation was evaluated by Western blotting and immunofluorescence staining.

Results: Compared with control mice, both T1DM and T2DM mice showed cardiac remodeling and left ventricular dysfunction. Carvacrol significantly reduced blood glucose levels and suppressed cardiac remodeling in mice with T1DM and T2DM. At the end of the treatment period, both T1DM and T2DM mice showed lesser cardiac hypertrophy, Nppa and Myh7 mRNA expressions, and cardiac fibrosis, compared to mice administered only the vehicle. Moreover, carvacrol significantly restored PI3K/AKT signaling, which was impaired in mice with T1DM and T2DM. Carvacrol increased levels of phosphorylated PI3K, PDK1, AKT, and AS160 and inhibited PTEN phosphorylation in mice with T1DM and T2DM. Carvacrol treatment promoted GLUT4 membrane translocation in mice with T1DM and T2DM. Metformin was used as the positive drug control in T2DM mice, and carvacrol showed comparable effects to that of metformin on cardiac remodeling and modulation of signaling pathways.

Conclusion: Carvacrol protected against DCM in mice with T1DM and T2DM by restoring PI3K/AKT signaling-mediated GLUT4 membrane translocation and is a potential treatment of DCM.

New insights into the transcriptional regulation of aquaporin-2 and the treatment of X-linked hereditary nephrogenic diabetes insipidus

The kidney collecting duct (CD) is a tubular segment of the kidney where the osmolality and final flow rate of urine are established, enabling urine concentration and body water homeostasis. Water reabsorption in the CD depends on the action of arginine vasopressin (AVP) and a transepithelial osmotic gradient between the luminal fluid and surrounding interstitium. AVP induces transcellular water reabsorption across CD principal cells through associated signaling pathways after binding to arginine vasopressin receptor 2 (AVPR2). This signaling cascade regulates the water channel protein aquaporin-2 (AQP2). AQP2 is exclusively localized in kidney connecting tubules and CDs. Specifically, AVP stimulates the intracellular translocation of AQP2-containing vesicles to the apical plasma membrane, increasing the osmotic water permeability of CD cells. Moreover, AVP induces transcription of the Aqp2 gene, increasing AQP2 protein abundance. This review provides new insights into the transcriptional regulation of the Aqp2 gene in the kidney CD with an overview of AVP and AQP2. It summarizes current therapeutic approaches for X-linked nephrogenic diabetes insipidus caused by AVPR2 gene mutations.

Hydrogen sulfide upregulates renal AQP-2 protein expression and promotes urine concentration

Increasing evidence supports the important role of H₂S in renal physiology and the pathogenesis of kidney injury. Whether H₂S regulates water metabolism in the kidney and the potential mechanism are still unknown. The present study was conducted to determine the role of H₂S in urine concentration. Inhibition of both cystathionine-γ-lyase (CSE) and cystathionine-β-synthase (CBS), 2 major enzymes for endogenous H₂S production, with propargylglycine (PPG) and amino-oxyacetate (AOAA), respectively, caused increased urine output and reduced urine osmolality in mice that was associated with decreased expression of aquaporin (AQP)-2 in the renal inner medulla. Mice treated with both PPG and AOAA developed a urine concentration defect in response to dehydration that was accompanied by reduced AQP-2 protein expression. Inhibition of CSE alone was associated with a mild decrease in AQP-2 protein level in the renal medulla of heterozygous CBS mice. GYY4137, a slow H₂S donor, markedly improved urine concentration and prevented the down-regulation of renal AQP-2 protein expression in mice with lithium-induced nephrogenic diabetes insipidus (NDI). GYY4137 significantly increased cAMP levels in cell lysates prepared from inner medullary collecting duct (IMCD) suspensions. AQP-2 protein expression was also upregulated, but was significantly inhibited by the adenyl cyclase inhibitor MDL12330A or the PKA inhibitor H89, but not the vasopressin 2 receptor (V₂R) antagonist tolvaptan. Inhibition of endogenous H₂S production impaired urine concentration in mice, whereas an exogenous H₂S donor improved urine concentration in lithium-induced NDI by increasing AQP-2 expression in the collecting duct principal cells. H₂S upregulated AQP-2 protein expression, probably via the cAMP-PKA pathway.-Luo, R., Hu, S., Liu, Q., Han, M., Wang, F., Qiu, M., Li, S., Li, X., Yang, T., Fu, X., Wang, W., Li, C. Hydrogen sulfide upregulates renal AQP-2 protein expression and promotes urine concentration.

Peripheral insulin resistance in ILK-depleted mice by reduction of GLUT4 expression

The development of insulin resistance is characterized by the impairment of glucose uptake mediated by glucose transporter 4 (GLUT4). Extracellular matrix changes are induced when the metabolic dysregulation is sustained. The present work was devoted to analyze the possible link between the extracellular-to-intracellular mediator integrin-linked kinase (ILK) and the peripheral tissue modification that leads to glucose homeostasis impairment. Mice with general depletion of ILK in adulthood (cKD-ILK) maintained in a chow diet exhibited increased glycemia and insulinemia concurrently with a reduction of the expression and membrane presence of GLUT4 in the insulin-sensitive peripheral tissues compared with their wild-type littermates (WT). Tolerance tests and insulin sensitivity indexes confirmed the insulin resistance in cKD-ILK, suggesting a similar stage to prediabetes in humans. Under randomly fed conditions, no differences between cKD-ILK and WT were observed in the expression of insulin receptor (IR-B) and its substrate IRS-1 expressions. The IR-B isoform phosphorylated at tyrosines 1150/1151 was increased, but the AKT phosphorylation in serine 473 was reduced in cKD-ILK tissues. Similarly, ILK-blocked myotubes reduced their GLUT4 promoter activity and GLUT4 expression levels. On the other hand, the glucose uptake capacity in response to exogenous insulin was impaired when ILK was blocked in vivo and in vitro, although IR/IRS/AKT phosphorylation states were increased but not different between groups. We conclude that ILK depletion modifies the transcription of GLUT4, which results in reduced peripheral insulin sensitivity and glucose uptake, suggesting ILK as a molecular target and a prognostic biomarker of insulin resistance.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

ILK and cytoskeletal architecture: an important determinant of AQP2 recycling and subsequent entry into the exocytotic pathway

Within the past decade tremendous efforts have been made to understand the mechanism behind aquaporin-2 (AQP2) water channel trafficking and recycling, to open a path toward effective diabetes insipidus therapeutics. A recent study has shown that integrin-linked kinase (ILK) conditional-knockdown mice developed polyuria along with decreased AQP2 expression. To understand whether ILK also regulates AQP2 trafficking in kidney tubular cells, we performed in vitro analysis using LLCPK1 cells stably expressing rat AQP2 (LLC-AQP2 cells). Upon treatment of LLC-AQP2 cells with ILK inhibitor cpd22 and ILK-siRNA, we observed increased accumulation of AQP2 in the perinuclear region, without any significant increase in the rate of endocytosis. This perinuclear accumulation did not occur in cells expressing a serine-256-aspartic acid mutation that retains AQP2 in the plasma membrane. We then examined clathrin-mediated endocytosis after ILK inhibition using rhodamine-conjugated transferrin. Despite no differences in overall transferrin endocytosis, the endocytosed transferrin also accumulated in the perinuclear region where it colocalized with AQP2. These accumulated vesicles also contained the recycling endosome marker Rab11. In parallel, the usual vasopressin-induced AQP2 membrane accumulation was prevented after ILK inhibition; however, ILK inhibition did not measurably affect AQP2 phosphorylation at serine-256 or its dephosphorylation at serine-261. Instead, we found that inhibition of ILK increased F-actin polymerization. When F-actin was depolymerized with latrunculin, the perinuclear located AQP2 dispersed. We conclude that ILK is important in orchestrating dynamic cytoskeletal architecture during recycling of AQP2, which is necessary for its subsequent entry into the exocytotic pathway.

Molecular mechanisms regulating aquaporin-2 in kidney collecting duct

The kidney collecting duct is an important renal tubular segment for regulation of body water homeostasis and urine concentration. Water reabsorption in the collecting duct principal cells is controlled by vasopressin, a peptide hormone that induces the osmotic water transport across the collecting duct epithelia through regulation of water channel proteins aquaporin-2 (AQP2) and aquaporin-3 (AQP3). In particular, vasopressin induces both intracellular translocation of AQP2-bearing vesicles to the apical plasma membrane and transcription of the Aqp2 gene to increase AQP2 protein abundance. The signaling pathways, including AQP2 phosphorylation, RhoA phosphorylation, intracellular calcium mobilization, and actin depolymerization, play a key role in the translocation of AQP2. This review summarizes recent data demonstrating the regulation of AQP2 as the underlying molecular mechanism for the homeostasis of water balance in the body.

4-PBA improves lithium-induced nephrogenic diabetes insipidus by attenuating ER stress

Endoplasmic reticulum (ER) stress has been implicated in some types of glomerular and tubular disorders. The objectives of this study were to elucidate the role of ER stress in lithium-induced nephrogenic diabetes insipidus (NDI) and to investigate whether attenuation of ER stress by 4-phenylbutyric acid (4-PBA) improves urinary concentrating defect in lithium-treated rats. Wistar rats received lithium (40 mmol/kg food), 4-PBA (320 mg/kg body wt by gavage every day), or no treatment (control) for 2 wk, and they were dehydrated for 24 h before euthanasia. Lithium treatment resulted in increased urine output and decreased urinary osmolality, which was significantly improved by 4-PBA. 4-PBA also prevented reduced protein expression of aquaporin-2 (AQP2), pS256-AQP2, and pS261-AQP2 in the inner medulla of kidneys from lithium-treated rats after 24-h dehydration. Lithium treatment resulted in increased expression of ER stress markers in the inner medulla, which was associated with dilated cisternae and expansion of ER in the inner medullary collecting duct (IMCD) principal cells. Confocal immunofluorescence studies showed colocalization of a molecular chaperone, binding IgG protein (BiP), with AQP2 in principal cells. Immunohistochemistry demonstrated increased intracellular expression of BiP and decreased AQP2 expression in IMCD principal cells of kidneys from lithium-treated rats. 4-PBA attenuated expression of ER stress markers and recovered ER morphology. In IMCD suspensions isolated from lithium-treated rats, 4-PBA incubation was also associated with increased AQP2 expression and ameliorated ER stress. In conclusion, in experimental lithium-induced NDI, 4-PBA improved the urinary concentrating defect and increased AQP2 expression, likely via attenuating ER stress in IMCD principal cells.

AS160 controls eukaryotic cell cycle and proliferation by regulating the CDK inhibitor p21

AS160 (TBC1D4) has been implicated in multiple biological processes. However, the role and the mechanism of action of AS160 in the regulation of cell proliferation remain unclear. In this study, we demonstrated that AS160 knockdown led to blunted cell proliferation in multiple cell types, including fibroblasts and cancer cells. The results of cell cycle analysis showed that these cells were arrested in the G1 phase. Intriguingly, this inhibition of cell proliferation and the cell cycle arrest caused by AS160 depletion were glucose independent. Moreover, AS160 silencing led to a marked upregulation of the expression of the cyclin-dependent kinase inhibitor p21. Furthermore, whereas AS160 overexpression resulted in p21 downregulation and rescued the arrested cell cycle in AS160-depeleted cells, p21 silencing rescued the inhibited cell cycle and proliferation in the cells. Thus, our results demonstrated that AS160 regulates glucose-independent eukaryotic cell proliferation through p21-dependent control of the cell cycle, and thereby revealed a molecular mechanism of AS160 modulation of cell cycle and proliferation that is of general physiological significance.

Increased aldosterone-dependent Kv1.5 recycling predisposes to pacing-induced atrial fibrillation in Kcne3-/- mice

Hyperaldosteronism is associated with an increased prevalence of atrial fibrillation (AF). Mutations in KCNE3 have been associated with AF, and Kcne3(-/-) mice exhibit hyperaldosteronism. In this study, we used recently developed Kcne3(-/-) mice to study atrial electrophysiology with respect to development of aldosterone-dependent AF. In invasive electrophysiology studies, Kcne3(-/-) mice displayed a reduced atrial effective refractory period (AERP) and inducible episodes of paroxysmal AF. The cellular arrhythmogenic correlate for AF predisposition was a significant increase in atrial Kv currents generated by the micromolar 4-aminopyridine-sensitive Kv current encoded by Kv1.5. Electrophysiological alterations in Kcne3(-/-) mice were aldosterone dependent and were associated with increased Rab4, -5, and -9-dependent recycling of Kv1.5 channels to the Z-disc/T-tubulus region and lateral membrane via activation of the Akt/AS160 pathway. Treatment with spironolactone inhibited Akt/AS160 phosphorylation, reduced Rab-dependent Kv1.5 recycling, normalized AERP and atrial Kv currents to the wild-type level, and reduced arrhythmia induction in Kcne3(-/-) mice. Kcne3 deletion in mice predisposes to AF by a heretofore unrecognized mechanism-namely, increased aldosterone-dependent Kv1.5 recycling via Rab GTPases. The findings uncover detailed molecular mechanisms underpinning a channelopathy-linked form of AF and emphasize the inevitability of considering extracardiac mechanisms in genetic arrhythmia syndromes.-Lisewski, U., Koehncke, C., Wilck, N., Buschmeyer, B., Pieske, B., Roepke, T. K. Increased aldosterone-dependent Kv1.5 recycling predisposes to pacing-induced atrial fibrillation in Kcne3(-/-) mice.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Akt Links Insulin Signaling to Albumin Endocytosis in Proximal Tubule Epithelial Cells

Diabetes mellitus (DM) has become an epidemic, causing a significant decline in quality of life of individuals due to its multisystem involvement. Kidney is an important target organ in DM accounting for the majority of patients requiring renal replacement therapy at dialysis units. Microalbuminuria (MA) has been a valuable tool to predict end-organ damage in DM but its low sensitivity has driven research efforts to seek other alternatives. Albumin is taken up by albumin receptors, megalin and cubilin in the proximal tubule epithelial cells. We demonstrated that insulin at physiological concentrations induce albumin endocytosis through activation of protein kinase B (Akt) in proximal tubule epithelial cells. Inhibition of Akt by a phosphorylation deficient construct abrogated insulin induced albumin endocytosis suggesting a role for Akt in insulin-induced albumin endocytosis. Furthermore we demonstrated a novel interaction between Akt substrate 160kDa (AS160) and cytoplasmic tail of megalin. Mice with type 1 DM (T1D) displayed decreased Akt, megalin, cubilin and AS160 expression in their kidneys in association with urinary cubilin shedding preceding significant MA. Patients with T1D who have developed MA in the EDC (The Pittsburgh Epidemiology of Diabetes Complications) study demonstrated urinary cubilin shedding prior to development of MA. We hypothesize that perturbed insulin-Akt cascade in DM leads to alterations in trafficking of megalin and cubilin, which results in urinary cubilin shedding as a prelude to MA in early diabetic nephropathy. We propose that utilization of urinary cubilin shedding, as a urinary biomarker, will allow us to detect and intervene in diabetic nephropathy (DN) at an earlier stage.

Rab-GAP TBC1D4 (AS160) is dispensable for the renal control of sodium and water homeostasis but regulates GLUT4 in mouse kidney

The Rab GTPase-activating protein TBC1D4 (AS160) controls trafficking of the glucose transporter GLUT4 in adipocytes and skeletal muscle cells. TBC1D4 is also highly abundant in the renal distal tubule, although its role in this tubule is so far unknown. In vitro studies suggest that it is involved in the regulation of renal transporters and channels such as the epithelial sodium channel (ENaC), aquaporin-2 (AQP2), and the Na+-K+-ATPase. To assess the physiological role of TBC1D4 in the kidney, wild-type (TBC1D4+/+) and TBC1D4-deficient (TBC1D4-/-) mice were studied. Unexpectedly, neither under standard nor under challenging conditions (low Na+/high K+, water restriction) did TBC1D4-/- mice show any difference in urinary Na+ and K+ excretion, urine osmolarity, plasma ion and aldosterone levels, and blood pressure compared with TBC1D4+/+ mice. Also, immunoblotting did not reveal any change in the abundance of major renal sodium- and water-transporting proteins [Na-K-2Cl cotransporter (NKCC2) NKCC2, NaCl cotransporter (NCC), ENaC, AQP2, and the Na+-K+-ATPase]. However, the abundance of GLUT4, which colocalizes with TBC1D4 along the distal nephron of TBC1D4+/+ mice, was lower in whole kidney lysates of TBC1D4-/- mice than in TBC1D4+/+ mice. Likewise, primary thick ascending limb (TAL) cells isolated from TBC1D4-/- mice showed an increased basal glucose uptake and an abrogated insulin response compared with TAL cells from TBC1D4+/+ mice. Thus, TBC1D4 is dispensable for the regulation of renal Na+ and water transport, but may play a role for GLUT4-mediated basolateral glucose uptake in distal tubules. The latter may contribute to the known anaerobic glycolytic capacity of distal tubules during renal ischemia.

PKB-Mediated Thr649 Phosphorylation of AS160/TBC1D4 Regulates the R-Wave Amplitude in the Heart

The Rab GTPase activating protein (RabGAP), AS160/TBC1D4, is an important substrate of protein kinase B (PKB), and regulates insulin-stimulated trafficking of glucose transporter 4. Besides, AS160/TBC1D4 has also been shown to regulate trafficking of many other membrane proteins including FA translocase/CD36 in cardiomyocytes. However, it is not clear whether it plays any role in regulating heart functions in vivo. Here, we found that PKB-mediated phosphorylation of Thr⁶⁴⁹ on AS160/TBC1D4 represented one of the major PAS-binding signals in the heart in response to insulin. Mutation of Thr⁶⁴⁹ to a non-phosphorylatable alanine increased the R-wave amplitude in the AS160^Thr649Ala knockin mice. However, this knockin mutation did not affect the heart functions under both normal and infarct conditions. Interestingly, myocardial infarction induced the expression of a related RabGAP, TBC1D1, in the infarct zone as well as in the border zone. Together, these data show that the Thr⁶⁴⁹ phosphorylation of AS160/TBC1D4 plays an important role in the heart’s electrical conduction system through regulating the R-wave amplitude.

Extracellular pH affects phosphorylation and intracellular trafficking of AQP2 in inner medullary collecting duct cells

Kidney collecting duct cells are continuously exposed to the changes of extracellular pH (pHe). We aimed to study the effects of altered pHe on desmopressin (dDAVP)-induced phosphorylation (Ser256, Ser261, Ser264, and Ser269) and apical targeting of aquaporin-2 (AQP2) in rat kidney inner medullary collecting duct (IMCD) cells. When freshly prepared IMCD tubule suspensions exposed to HEPES buffer with pH 5.4, 6.4, 7.4, or 8.4 for 1 h were stimulated with dDAVP (10−10 M, 3 min), AQP2 phosphorylation at Ser256, Ser264, and Ser269 was significantly attenuated under acidic conditions. Next, IMCD cells primary cultured in transwell chambers were exposed to a transepithelial pH gradient for 1 h (apical pH 6.4, 7.4, or 8.4 vs. basolateral pH 7.4 and vice versa). Immunocytochemistry and cell surface biotinylation assay revealed that exposure to either apical pH 6.4 or basolateral pH 6.4 for 1 h was associated with decreased dDAVP (10−9 M, 15 min, basolateral)-induced apical targeting of AQP2 and surface expression of AQP2. Fluorescence resonance energy transfer analysis revealed that the dDAVP (10−9 M)-induced increase of PKA activity was significantly attenuated when LLC-PK1 cells were exposed to pHe 6.4 compared with pHe 7.4 and 8.4. In contrast, forskolin (10−7 M)-induced PKA activation and dDAVP (10−9 M)-induced increases of intracellular Ca2+ were not affected. Taken together, dDAVP-induced phosphorylation and apical targeting of AQP2 are attenuated in IMCD cells under acidic pHe, likely via an inhibition of vasopressin V2 receptor-G protein-cAMP-PKA actions.

Tankyrase-mediated β-catenin activity regulates vasopressin-induced AQP2 expression in kidney collecting duct mpkCCDc14 cells

Aquaporin-2 (AQP2) mediates arginine vasopressin (AVP)-induced water reabsorption in the kidney collecting duct. AVP regulates AQP2 expression primarily via Gsα/cAMP/PKA signaling. Tankyrase, a member of the poly(ADP-ribose) polymerase family, is known to mediate Wnt/β-catenin signaling-induced gene expression. We examined whether tankyrase plays a role in AVP-induced AQP2 regulation via ADP-ribosylation of G protein-α (Gα) and/or β-catenin-mediated transcription of AQP2. RT-PCR and immunoblotting analysis revealed the mRNA and protein expression of tankyrase in mouse kidney and mouse collecting duct mpkCCDc14 cells. dDAVP-induced AQP2 upregulation was attenuated in mpkCCDc14 cells under the tankyrase inhibition by XAV939 treatment or small interfering (si) RNA knockdown. Fluorescence resonance energy transfer image analysis, however, revealed that XAV939 treatment did not affect dDAVP- or forskolin-induced PKA activation. Inhibition of tankyrase decreased dDAVP-induced phosphorylation of β-catenin (S552) and nuclear translocation of phospho-β-catenin. siRNA-mediated knockdown of β-catenin decreased forskolin-induced AQP2 transcription and dDAVP-induced AQP2 expression. Moreover, inhibition of phosphoinositide 3-kinase/Akt, which was associated with decreased nuclear translocation of β-catenin, diminished dDAVP-induced AQP2 upregulation, further indicating that β-catenin mediates AQP2 expression. Taken together, tankyrase plays a role in AVP-induced AQP2 regulation, which is likely via β-catenin-mediated transcription of AQP2, but not ADP-ribosylation of Gα. The results provide novel insights into vasopressin-mediated urine concentration and homeostasis of body water metabolism.

Effect of troxerutin on insulin signaling molecules in the gastrocnemius muscle of high fat and sucrose-induced type-2 diabetic adult male rat

Troxerutin is a trihydroxyethylated derivative of the flavonoid, rutin. It has been reported to possess the hepatoprotective, nephroprotective, antioxidant, anti-inflammatory, and antihyperlipidemic activities. Troxerutin treatment reduced the blood glucose and glycosylated hemoglobin levels in high-cholesterol-induced insulin-resistant mice and in type-2 diabetic patients. However, the mechanism by which it exhibits antidiabetic property was unknown. Therefore, the present study was designed to evaluate the effect of troxerutin on insulin signaling molecules in gastrocnemius muscle of high fat and sucrose-induced type-2 diabetic rats. Wistar male albino rats were selected and divided into five groups. Group I: Control. Group II: High fat and sucrose-induced type-2 diabetic rats. Group III: Type-2 diabetic rats treated with troxerutin (150 mg/kg body weight/day orally). Group IV: Type-2 diabetic rats treated with metformin (50 mg/kg body weight/day orally). Group V: Normal rats treated with troxerutin (150 mg/kg body weight/day orally). After 30 days of treatment, fasting blood glucose, oral glucose tolerance, serum lipid profile, and the levels of insulin signaling molecules, glycogen, glucose uptake, and oxidation in gastrocnemius muscle were assessed. Diabetic rats showed impairment in insulin signaling molecules (IR, p-IRS-1(Tyr632), p-Akt(Ser473), β-arrestin-2, c-Src, p-AS160(Thr642), and GLUT4 proteins), glycogen concentration, glucose uptake, and oxidation. Oral administration of troxerutin showed near normal levels of blood glucose, serum insulin, lipid profile, and insulin signaling molecules as well as GLUT4 proteins in type-2 diabetic rats. It is concluded from the present study that troxerutin may play a significant role in the management of type-2 diabetes mellitus, by improving the insulin signaling molecules and glucose utilization in the skeletal muscle.

Integrin-linked kinase regulates tubular aquaporin-2 content and intracellular location: a link between the extracellular matrix and water reabsorption

One of the clinical alterations observed in chronic renal disease (CRD) is the impaired urine concentration, known as diabetes insipidus (DI). Tubulointerstitial fibrosis of the kidney is also a pathological finding observed in CRD and involves composition of extracellular matrix (ECM). However, an association between these two events has not been elucidated. In this study, we showed that the extracellular-to-intracellular scaffold protein integrin-linked kinase (ILK) regulates expression of tubular water channel aquaporin-2 (AQP2) and its apical membrane presence in the renal tubule. Basally, polyuria and decreased urine osmolality were present in ILK conditional-knockdown (cKD-ILK) adult mice compared with nondepleted ILK littermates. No changes were observed in arginine-vasopressin (AVP) blood levels, renal receptor (V2R), or AQP3 expression. However, tubular AQP2 was decreased in expression and apical membrane presence in cKD-ILK mice, where the canonical V2R/cAMP axis activation is still functional, but independent of the absence of ILK. Thus, cKD-ILK constitutes a nephrogenic diabetes insipidus (NDI) model. AQP2 and ILK colocalize in cultured inner medullary collecting duct (mIMCD3) cells. Specific ILK siRNAs and collagen I (Col) decrease ILK and AQP2 levels and AQP2 presence on the membrane of tubular mIMCD3 cells, which impairs the capacity of the cells to transport water under hypotonic stress. The present work points to ILK as a therapeutic target in NDI.

Elucidation of the distal convoluted tubule transcriptome identifies new candidate genes involved in renal Mg2+ handling

The kidney plays a key role in the maintenance of Mg2+ homeostasis. Specifically, the distal convoluted tubule (DCT) is instrumental in the fine-tuning of renal Mg2+ handling. In recent years, hereditary Mg2+ transport disorders have helped to identify important players in DCT Mg2+ homeostasis. Nevertheless, several proteins involved in DCT-mediated Mg2+ reabsorption remain to be discovered, and a full expression profile of this complex nephron segment may facilitate the discovery of new Mg2+-related genes. Here, we report Mg2+-sensitive expression of the DCT transcriptome. To this end, transgenic mice expressing enhanced green fluorescent protein under a DCT-specific parvalbumin promoter were subjected to Mg2+-deficient or Mg2+-enriched diets. Subsequently, the Complex Object Parametric Analyzer and Sorter allowed, for the first time, isolation of enhanced green fluorescent protein-positive DCT cells. RNA extracts thereof were analyzed by DNA microarrays comparing high versus low Mg2+ to identify Mg2+ regulatory genes. Based on statistical significance and a fold change of at least 2, 46 genes showed differential expression. Several known magnesiotropic genes, such as transient receptor potential cation channel, subfamily M, member 6 ( Trpm6), and Parvalbumin, were upregulated under low dietary Mg2+. Moreover, new genes were identified that are potentially involved in renal Mg2+ handling. To confirm that the selected candidate genes were regulated by dietary Mg2+ availability, the expression levels of solute carrier family 41, member 3 ( Slc41a3), pterin-4 α-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor-1α ( Pcbd1), TBC1 domain family, member 4 ( Tbc1d4), and uromodulin ( Umod) were determined by RT-PCR analysis. Indeed, all four genes show significant upregulation in the DCT of mice fed a Mg2+-deficient diet. By elucidating the Mg2+-sensitive DCT transcriptome, new candidate genes in renal Mg2+ handling have been identified.

Regulation of aquaporin-2 in the kidney: A molecular mechanism of body-water homeostasis

The kidneys play a key role in the homeostasis of body water and electrolyte balance. Aquaporin-2 (AQP2) is the vasopressin-regulated water-channel protein expressed at the connecting tubule and collecting duct, and plays a key role in urine concentration and body-water homeostasis through short-term and long-term regulation of collecting duct water permeability. The signaling transduction pathways resulting in the AQP2 trafficking to the apical plasma membrane of the collecting duct principal cells, including AQP2 phosphorylation, RhoA phosphorylation, actin depolymerization, and calcium mobilization, and the changes of AQP2 abundance in water-balance disorders have been extensively studied. Dysregulation of AQP2 has been shown to be importantly associated with a number of clinical conditions characterized by body-water balance disturbances, including hereditary nephrogenic diabetes insipidus (NDI), lithium-induced NDI, electrolytes disturbance, acute and chronic renal failure, ureteral obstruction, nephrotic syndrome, congestive heart failure, and hepatic cirrhosis. Recent studies exploiting omics technology further demonstrated the comprehensive vasopressin signaling pathways in the collecting ducts. Taken together, these studies elucidate the underlying molecular mechanisms of body-water homeostasis and provide the basis for the treatment of body-water balance disorders.

The role of 70-kDa heat shock protein in dDAVP-induced AQP2 trafficking in kidney collecting duct cells

It has been reported that several proteins [heat shock protein 70 (Hsp70 and Hsc70), annexin II, and tropomyosin 5b] interact with the Ser256 residue on the COOH terminus of aquaporin-2 (AQP2), where vasopressin-induced phosphorylation occurs for mediating AQP2 trafficking. However, it remains unknown whether these proteins, particularly Hsp70, play a role in AQP2 trafficking. Semiquantitative immunoblotting revealed that renal expression of AQP2 and Hsp70 was significantly increased in water-restricted or dDAVP-infused rats. In silico analysis of the 5′-flanking regions of AQP2, Hsp70-1, and Hsp70-2 genes revealed that transcriptional regulator binding elements associated with cAMP response were identified at both the Hsp70-1 and Hsp70-2 promoter regions, in addition to AQP2. Luciferase reporter assay demonstrated the significant increase of luminescence after dDAVP stimulation (10−8 M, 6 h) in the LLC-PK1 cells transfected with luciferase vector containing 1 kb of the 5′-flanking region of Hsp70-2 gene. Hsp70-2 protein expression was also increased in mpkCCDc14 cells treated by dDAVP in a concentration-dependent manner. Cell surface biotinylation analysis demonstrated that forskolin (10−5 M, 15 min)-induced AQP2 targeting to the apical plasma membrane was significantly attenuated in the mpkCCDc14 cells with Hsp70-2 knockdown. Moreover, forskolin-induced AQP2 phosphorylation (Ser256) was not significantly induced in the mpkCCDc14 cells with Hsp70-2 knockdown. In contrast, Hsp70-2 knockdown did not affect the dDAVP-induced AQP2 abundance. In addition, siRNA-directed knockdown of Hsp70 significantly decreased cell viability. The results suggest that Hsp70 is likely to play a role in AQP2 trafficking to the apical plasma membrane, partly through affecting AQP2 phosphorylation at Ser256 and cell viability.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

Insulin and AMPK regulate FA translocase/CD36 plasma membrane recruitment in cardiomyocytes via Rab GAP AS160 and Rab8a Rab GTPase

The FA translocase cluster of differentiation 36 (CD36) facilitates FA uptake by the myocardium, and its surface recruitment in cardiomyocytes is induced by insulin, AMP-dependent protein kinase (AMPK), or contraction. Dysfunction of CD36 trafficking contributes to disordered cardiac FA utilization and promotes progression to disease. The Akt substrate 160 (AS160) Rab GTPase-activating protein (GAP) is a key regulator of vesicular trafficking, and its activity is modulated via phosphorylation. Our study documents that AS160 mediates insulin or AMPK-stimulated surface translocation of CD36 in cardiomyocytes. Knock-down of AS160 redistributes CD36 to the surface and abrogates its translocation by insulin or the AMPK agonist 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR). Conversely, overexpression of a phosphorylation-deficient AS160 mutant (AS160 4P) suppresses the stimulated membrane recruitment of CD36. The AS160 substrate Rab8a GTPase is shown via overexpression and knock-down studies to be specifically involved in insulin/AICAR-induced CD36 membrane recruitment. Our findings directly demonstrate AS160 regulation of CD36 trafficking. In myocytes, the AS160 pathway also mediates the effect of insulin, AMPK, or contraction on surface recruitment of the glucose transporter GLUT4. Thus, AS160 constitutes a point of convergence for coordinating physiological regulation of CD36 and GLUT4 membrane recruitment.

Aquaporin-5: a marker protein for proliferation and migration of human breast cancer cells

Aquaporin (AQP) is a family of transmembrane proteins for water transport. Recent studies revealed that AQPs are likely to play a role in tumor progression and invasion. We aimed to examine the potential role of AQP5 in the progression of human breast cancer cells. Expression of AQP5 mRNA and protein was seen in human breast cancer cell line (both MCF7 and MDA-MB-231) by RT-PCR and immunoblotting analysis. Immunoperoxidase labeling of AQP5 was observed at ductal epithelial cells of human breast tissues. In benign tumor, AQP5 labeling was mainly seen at the apical domains of ductal epithelial cells. In contrast, in invasive ductal carcinoma, prominent AQP5 labeling was associated with cancer cells, whereas some ducts were unlabeled and apical polarity of AQP5 in ducts was lost. Cell proliferation (BrdU incorporation assay) and migration of MCF7 cells were significantly attenuated by lentivirus-mediated AQP5-shRNA transduction. Hyperosmotic stress induced by sorbitol treatment (100 mM, 24 h) reduced AQP5 expression in MCF7 cells, which was also associated with a significant reduction in cell proliferation and migration. Taken together, prominent AQP5 expression in breast cancer cells with the loss of polarity of ductal epithelial cells was seen during the progression of breast carcinoma. shRNA- or hyperosmotic stress-induced reduction in AQP5 expression of MCF7 cells was associated with significantly reduced cell proliferation and migration. In conclusion, AQP5 overexpression is likely to play a role in cell growth and metastasis of human breast cancer and could be a novel target for anti-breast cancer treatment.

E3 ubiquitin-protein ligases in rat kidney collecting duct: response to vasopressin stimulation and withdrawal

The E3 ubiquitin (Ub)-protein ligases (E3s) play a role as regulators of protein trafficking and degradation. We aimed to integrate the profile of E3s in rat kidney and examine the changes in protein abundance of the selected E3s in response to 1-deamino-8-D-arginine vasopressin (dDAVP) stimulation/withdrawal. Sprague-Dawley rats were infused with vehicle (n = 13), dDAVP for 5 days (n = 13), or dDAVP was withdrawn for periods (15 min, 30 min, 1, 3, 6, 12, or 24 h) after 5-day infusion (n = 46). Total RNA was isolated from the inner medulla (IM) for transcriptome analysis. Plasma membrane (PM)- or intracellular vesicle (ICV)-enriched fractions of whole kidney were immunoisolated for liquid chromatography-tandem mass spectrometry analysis. dDAVP infusion for 5 days (D5d) significantly increased urine osmolality, which was maintained during 3-h withdrawal of dDAVP after 5-day infusion (D5d-3h). Consistent with this, aquaporin-2 (AQP2) expression in the PM fractions of D5d and D5d-3h increased, whereas AQP2 expression in the ICV fractions of D5d-3h was further increased, indicating internalization of AQP2. Transcriptome analysis revealed 86 genes of E3s and LC-MS/MS analysis demonstrated 16 proteins of E3s. Among these, seven E3s (BRCA1, UBR4, BRE1B, UHRF1, NEDD4, CUL5, and FBX6) were shared. RT-PCR demonstrated mRNA expressions of the seven identified E3s in the kidney, and immunoblotting demonstrated changes in protein abundance of the selected E3s (BRE1B, NEDD4, and CUL5) in response to dDAVP stimulation/withdrawal or lithium-induced nephrogenic diabetes insipidus. The rate of AQP2 degradation was retarded in mpkCCDc14 cells with small interfering RNA-mediated knockdown of NEDD4 or CUL5. Taken together, identified E3s could be involved in the degradation of proteins associated with vasopressin-induced urine concentration.