Hydrogen peroxide induces dimerization of protein kinase G type Iα subunits and increases albumin permeability in cultured rat podocytes

ADAM10 as a major activator of reactive oxygen species production and klotho shedding in podocytes under diabetic conditions

Early stages of diabetes are characterized by elevations of insulin and glucose concentrations. Both factors stimulate reactive oxygen species (ROS) production, leading to impairments in podocyte function and disruption of the glomerular filtration barrier. Podocytes were recently shown to be an important source of αKlotho (αKL) expression. Low blood Klotho concentrations are also associated with an increase in albuminuria, especially in patients with diabetes. We investigated whether ADAM10, which is known to cleave αKL, is activated in glomeruli and podocytes under diabetic conditions and the potential mechanisms by which ADAM10 mediates ROS production and disturbances of the glomerular filtration barrier. In cultured human podocytes, high glucose increased ADAM10 expression, shedding, and activity, NADPH oxidase activity, ROS production, and albumin permeability. These effects of glucose were inhibited when cells were pretreated with an ADAM10 inhibitor or transfected with short-hairpin ADAM10 (shADAM10) or after the addition soluble Klotho. We also observed increases in ADAM10 activity, NOX4 expression, NADPH oxidase activity, and ROS production in αKL-depleted podocytes. This was accompanied by an increase in albumin permeability in shKL-expressing podocytes. The protein expression and activity of ADAM10 also increased in isolated glomeruli and urine samples from diabetic rats. Altogether, these results reveal a new mechanism by which hyperglycemia in diabetes increases albumin permeability through ADAM10 activation and an increase in oxidative stress via NOX4 enzyme activation. Moreover, αKlotho downregulates ADAM10 activity and supports redox balance, consequently protecting the slit diaphragm of podocyteσ under hyperglycemic conditions.

Inhibition of phosphodiesterase 5A by tadalafil improves SIRT1 expression and activity in insulin-resistant podocytes

A decrease in intracellular levels of 3',5'-cyclic guanosine monophosphate (cGMP) has been implicated in the progression of diabetic nephropathy. Hyperglycemia significantly inhibits cGMP-dependent pathway activity in the kidney, leading to glomerular damage and proteinuria. The enhancement of activity of this pathway that is associated with an elevation of cGMP levels may be achieved by inhibition of the cGMP specific phosphodiesterase 5A (PDE5A) using selective inhibitors, such as tadalafil. Hyperglycemia decreased the insulin responsiveness of podocytes and impaired podocyte function. These effects were associated with lower protein amounts and activity of the protein deacetylase sirtuin 1 (SIRT1) and a decrease in the phosphorylation of adenosine monophosphate-dependent protein kinase (AMPK). We found that PDE5A protein levels increased in hyperglycemia, and PDE5A downregulation improved the insulin responsiveness of podocytes with reestablished SIRT1 expression and activity. PDE5A inhibitors potentiate nitric oxide (NO)/cGMP signaling, and NO modulates the activity and expression of SIRT1. Therefore, we investigated the effects of tadalafil on SIRT1 and AMPK in the context of improving the insulin sensitivity in podocytes and podocyte function in hyperglycemia. Our study revealed that tadalafil restored SIRT1 expression and activity and activated AMPK by increasing its phosphorylation. Tadalafil also restored stimulating effect of insulin on glucose transport in podocytes with high glucose-induced insulin resistance. Additionally, tadalafil improved the function of podocytes that were exposed to high glucose concentrations. Our results display novel mechanisms involved in the pathogenesis of glomerulopathies in diabetes, which may contribute to the development of more effective treatment strategies for diabetic nephropathy.

The Role of PKGIα and AMPK Signaling Interplay in the Regulation of Albumin Permeability in Cultured Rat Podocytes

The permeability of the glomerular filtration barrier (GFB) is mainly regulated by podocytes and their foot processes. Protein kinase G type Iα (PKGIα) and adenosine monophosphate-dependent kinase (AMPK) affect the contractile apparatus of podocytes and influence the permeability of the GFB. Therefore, we studied the interplay between PKGIα and AMPK in cultured rat podocytes. The glomerular permeability to albumin and transmembrane FITC-albumin flux decreased in the presence of AMPK activators and increased in the presence of PKG activators. The knockdown of PKGIα or AMPK with small-interfering RNA (siRNA) revealed a mutual interaction between PKGIα and AMPK and influenced podocyte permeability to albumin. Moreover, PKGIα siRNA activated the AMPK-dependent signaling pathway. AMPKα2 siRNA increased basal levels of phosphorylated myosin phosphate target subunit 1 and decreased the phosphorylation of myosin light chain 2. Podocytes that were treated with AMPK or PKG activators were characterized by the different organization of actin filaments within the cell. Our findings suggest that mutual interactions between PKGIα and AMPKα2 regulate the contractile apparatus and permeability of the podocyte monolayer to albumin. Understanding this newly identified molecular mechanism in podocytes provides further insights into the pathogenesis of glomerular disease and novel therapeutic targets for glomerulopathies.

Enhancement of cGMP-dependent pathway activity ameliorates hyperglycemia-induced decrease in SIRT1-AMPK activity in podocytes: Impact on glucose uptake and podocyte function

Hyperglycemia significantly decreases 3',5'-cyclic guanosine monophosphate (cGMP)-dependent pathway activity in the kidney. A well-characterized downstream signaling effector of cGMP is cGMP-dependent protein kinase G (PKG), exerting a wide range of downstream effects, including vasodilation and vascular smooth muscle cells relaxation. In podocytes that are exposed to high glucose concentrations, crosstalk between the protein deacetylase sirtuin 1 (SIRT1) and adenosine monophosphate-dependent protein kinase (AMPK) decreased, attenuating insulin responsiveness and impairing podocyte function. The present study examined the effect of enhancing cGMP-dependent pathway activity on SIRT1-AMPK crosstalk in podocytes under hyperglycemic conditions. We found that enhancing cGMP-dependent pathway activity using a cGMP analog was associated with increases in SIRT1 protein levels and activity, with a concomitant increase in the degree of AMPK phosphorylation. The beneficial effects of enhancing cGMP-dependent pathway activity on SIRT1-AMPK crosstalk also included improvements in podocyte function. Based on our findings, we postulate an important role for SIRT1-AMPK crosstalk in the regulation of albumin permeability in hyperglycemia that is strongly associated with activity of the cGMP-dependent pathway.

Insulin controls cytoskeleton reorganization and filtration barrier permeability via the PKGIα-Rac1-RhoA crosstalk in cultured rat podocytes

Podocyte foot processes are an important cellular layer of the glomerular barrier that regulates glomerular permeability. Insulin via the protein kinase G type Iα (PKGIα) signaling pathway regulates the balance between contractility and relaxation (permeability) of the podocyte barrier by regulation of the actin cytoskeleton. This mechanism was shown to be disrupted in diabetes. Rho family guanosine-5'-triphosphates (GTPases) are dynamic modulators of the actin cytoskeleton and expressed in cells that form the glomerular filtration barrier. Thus, changes in Rho GTPase activity may affect glomerular permeability to albumin. The present study showed that Rho family GTPases control podocyte migration and permeability. Moreover these processes are regulated by insulin in PKGIα-dependent manner. Modulation of the PKGI-dependent activity of Rac1 and RhoA GTPases with inhibitors or small-interfering RNA impair glomerular permeability to albumin. We also demonstrated this mechanism in obese, insulin-resistant Zucker rats. We propose that PKGIα-Rac1-RhoA crosstalk is necessary in proper organization of the podocyte cytoskeleton and consequently the stabilization of glomerular architecture and regulation of filtration barrier permeability.

Insulin resistance in glomerular podocytes: Potential mechanisms of induction

Glomerular podocytes are a target for the actions of insulin. Accumulating evidence indicates that exposure to nutrient overload induces insulin resistance in these cells, manifested by abolition of the stimulatory effect of insulin on glucose uptake. Numerous recent studies have investigated potential mechanisms of the induction of insulin resistance in podocytes. High glucose concentrations stimulated reactive oxygen species production through NADPH oxidase activation, decreased adenosine monophosphate-activated protein kinase (AMPK) phosphorylation, and reduced deacetylase sirtuin 1 (SIRT1) protein levels and activity. Calcium signaling involving transient receptor potential cation channel C, member 6 (TRPC6) also was demonstrated to play an essential role in the regulation of insulin-dependent signaling and glucose uptake in podocytes. Furthermore, podocytes exposed to diabetic environment, with elevated insulin levels become insulin resistant as a result of degradation of insulin receptor (IR), resulting in attenuation of insulin signaling responsiveness. Also elevated levels of palmitic acid appear to be an important factor and contributor to podocytes insulin resistance. This review summarizes cellular and molecular alterations that contribute to the development of insulin resistance in glomerular podocytes.

Role of Klotho in Hyperglycemia: Its Levels and Effects on Fibroblast Growth Factor Receptors, Glycolysis, and Glomerular Filtration

Hyperglycemic conditions (HG), at early stages of diabetic nephropathy (DN), cause a decrease in podocyte numbers and an aberration of their function as key cells for glomerular plasma filtration. Klotho protein was shown to overcome some negative effects of hyperglycemia. Klotho is also a coreceptor for fibroblast growth factor receptors (FGFRs), the signaling of which, together with a proper rate of glycolysis in podocytes, is needed for a proper function of the glomerular filtration barrier. Therefore, we measured levels of Klotho in renal tissue, serum, and urine shortly after DN induction. We investigated whether it influences levels of FGFRs, rates of glycolysis in podocytes, and albumin permeability. During hyperglycemia, the level of membrane-bound Klotho in renal tissue decreased, with an increase in the shedding of soluble Klotho, its higher presence in serum, and lower urinary excretion. The addition of Klotho increased FGFR levels, especially FGFR1/FGFR2, after their HG-induced decrease. Klotho also increased levels of glycolytic parameters of podocytes, and decreased podocytic and glomerular albumin permeability in HG. Thus, we found that the decrease in the urinary excretion of Klotho might be an early biomarker of DN and that Klotho administration may have several beneficial effects on renal function in DN.

The PKGIα-Rac1 pathway is a novel regulator of insulin-dependent glucose uptake in cultured rat podocytes

Insulin plays a major role in regulating glucose homeostasis in podocytes. Protein kinase G type Iα (PKGIα) plays an important role in regulating glucose uptake in these cells. Rac1 signaling plays an essential role in the reorganization of the actin cytoskeleton and is also essential for insulin-stimulated glucose transport. The experiments were conducted using primary rat podocytes. We performed western blot analysis, evaluated small GTPases activity assays, measured radioactive glucose uptake, and performed immunofluorescence imaging to analyze the role of PKGIα-Rac1 signaling in regulating podocyte function. We also utilized a small-interfering RNA-mediated approach to determine the role of PKGIα and Rac1 in regulating glucose uptake in podocytes. The present study investigated the influence of the PKGI pathway on the insulin-dependent regulation of activity and cellular localization of small guanosine triphosphatases in podocytes. We found that the PKGIα-dependent activation of Rac1 signaling induced activation of the PAK/cofilin pathway and increased insulin-mediated glucose uptake in podocytes. The downregulation of PKGIα or Rac1 expression abolished this effect. Rac1 silencing prevented actin remodeling and GLUT4 translocation close to the cell membrane. These data provide evidence that PKGIα-dependent activation of the Rac1 signaling pathways is a novel regulator of insulin-mediated glucose uptake in cultured rat podocytes.

The PKGIα/VASP pathway is involved in insulin- and high glucose-dependent regulation of albumin permeability in cultured rat podocytes

Podocytes, the principal component of the glomerular filtration barrier, regulate glomerular permeability to albumin via their contractile properties. Both insulin- and high glucose (HG)-dependent activation of protein kinase G type Iα (PKGIα) cause reorganization of the actin cytoskeleton and podocyte disruption. Vasodilator-stimulated phosphoprotein (VASP) is a substrate for PKGIα and involved in the regulation of actin cytoskeleton dynamics. We investigated the role of the PKGIα/VASP pathway in the regulation of podocyte permeability to albumin. We evaluated changes in high insulin- and/or HG-induced transepithelial albumin flux in cultured rat podocyte monolayers. Expression of PKGIα and downstream proteins was confirmed by western blot and immunofluorescence. We demonstrate that insulin and HG induce changes in the podocyte contractile apparatus via PKGIα-dependent regulation of the VASP phosphorylation state, increase VASP colocalization with PKGIα, and alter the subcellular localization of these proteins in podocytes. Moreover, VASP was implicated in the insulin- and HG-dependent dynamic remodelling of the actin cytoskeleton and, consequently, increased podocyte permeability to albumin under hyperinsulinaemic and hyperglycaemic conditions. These results indicate that insulin- and HG-dependent regulation of albumin permeability is mediated by the PKGIα/VASP pathway in cultured rat podocytes. This molecular mechanism may explain podocytopathy and albuminuria in diabetes.

Extracellular ATP modulates podocyte function through P2Y purinergic receptors and pleiotropic effects on AMPK and cAMP/PKA signaling pathways

Podocytes and their foot processes interlinked by slit diaphragms, constitute a continuous outermost layer of the glomerular capillary and seem to be crucial for maintaining the integrity of the glomerular filtration barrier. Purinergic signaling is involved in a wide range of physiological processes in the renal system, including regulating glomerular filtration. We evaluated the role of nucleotide receptors in cultured rat podocytes using non-selective P2 receptor agonists and agonists specific for the P2Y₁, P2Y₂, and P2Y₄ receptors. The results showed that extracellular ATP evokes cAMP-dependent pathways through P2 receptors and influences remodeling of the podocyte cytoskeleton and podocyte permeability to albumin via coupling with RhoA signaling. Our findings highlight the relevance of the P2Y₄ receptor in protein kinase A-mediated signal transduction to the actin cytoskeleton. We observed increased cAMP concentration and decreased RhoA activity after treatment with a P2Y₄ agonist. Moreover, protein kinase A inhibitors reversed P2Y₄-induced changes in RhoA activity and intracellular F-actin staining. P2Y₄ stimulation resulted in enhanced AMPK phosphorylation and reduced reactive oxygen species generation. Our findings identify P2Y-PKA-RhoA signaling as the regulatory mechanism of the podocyte contractile apparatus and glomerular filtration. We describe a protection mechanism for the glomerular barrier linked to reduced oxidative stress and reestablished energy balance.

Cathepsin C is a novel mediator of podocyte and renal injury induced by hyperglycemia

A growing body of evidence suggests a role of proteolytic enzymes in the development of diabetic nephropathy. Cathepsin C (CatC) is a well-known regulator of inflammatory responses, but its involvement in podocyte and renal injury remains obscure. We used Zucker rats, a genetic model of metabolic syndrome and insulin resistance, to determine the presence, quantity, and activity of CatC in the urine. In addition to the animal study, we used two cellular models, immortalized human podocytes and primary rat podocytes, to determine mRNA and protein expression levels via RT-PCR, Western blot, and confocal microscopy, and to evaluate CatC activity. The role of CatC was analyzed in CatC-depleted podocytes using siRNA and glycolytic flux parameters were obtained from extracellular acidification rate (ECAR) measurements. In functional analyses, podocyte and glomerular permeability to albumin was determined. We found that podocytes express and secrete CatC, and a hyperglycemic environment increases CatC levels and activity. Both high glucose and non-specific activator of CatC phorbol 12-myristate 13-acetate (PMA) diminished nephrin, cofilin, and GLUT4 levels and induced cytoskeletal rearrangements, increasing albumin permeability in podocytes. These negative effects were completely reversed in CatC-depleted podocytes. Moreover, PMA, but not high glucose, increased glycolytic flux in podocytes. Finally, we demonstrated that CatC expression and activity are increased in the urine of diabetic Zucker rats. We propose a novel mechanism of podocyte injury in diabetes, providing deeper insight into the role of CatC in podocyte biology.

Cysteine-Based Redox Sensing and Its Role in Signaling by Cyclic Nucleotide-Dependent Kinases in the Cardiovascular System

Oxidant molecules are produced in biological systems and historically have been considered causal mediators of damage and disease. While oxidants may contribute to the pathogenesis of disease, evidence continues to emerge that shows these species also play important regulatory roles in health. A major mechanism of oxidant sensing and signaling involves their reaction with reactive cysteine thiols within proteins, inducing oxidative posttranslational modifications that can couple to altered function to enable homeostatic regulation. Protein kinase A and protein kinase G are regulated by oxidants in this way, and this review focuses on our molecular-level understanding of these events and their role in regulating cardiovascular physiology during health and disease.

Augmentation of glucagon-like peptide-1 receptor signalling by neprilysin inhibition: potential implications for patients with heart failure

Augmentation of glucagon-like peptide-1 (GLP-1) receptor signalling is an established approach to the treatment of type 2 diabetes. However, endogenous GLP-1 and long-acting GLP-1 receptor analogues are degraded not only by dipeptidyl peptidase-4, but also by neprilysin. This observation raises the possibilities that endogenous GLP-1 contributes to the clinical effects of neprilysin inhibition and that patients concurrently treated with sacubitril/valsartan and incretin-based drugs may experience important drug-drug interactions. Specifically, potentiation of GLP-1 receptor signalling may underlie the antihyperglycaemic actions of sacubitril/valsartan. Neprilysin inhibitors may also be able to augment the effects of long-acting GLP-1 analogues to increase heart rate and myocardial cyclic AMP, and thus, potentiate these deleterious actions; if so, concomitant treatment with GLP-1 receptor agonists may limit the efficacy of neprilysin inhibitors in patients with both heart failure and diabetes. For patients not concurrently treated with GLP-1 analogues, the action of neprilysin to enhance the effects of GLP-1 may be particularly relevant in the brain, where augmentation of GLP-1 and other endogenous peptides may act to inhibit amyloid-induced neuroinflammation and cytotoxicity and improve memory formation and executive functioning. Experimentally, neprilysin inhibitors may also potentiate the effects of endogenous GLP-1 and GLP-1 receptor agonists on blood vessels and the kidney. The role of neprilysin in the metabolism of endogenous GLP-1 and long-acting GLP-1 analogues points to a range of potential pathophysiological effects that may be clinically relevant to patients with heart failure, with or without diabetes.

Glucose Transporters in Diabetic Kidney Disease-Friends or Foes?

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes and a common cause of end-stage renal disease worldwide. DKD manifests as an increased urinary protein excretion (albuminuria). Multiple studies have shown that insulin resistance correlates with the development of albuminuria in non-diabetic and diabetic patients. There is also accumulating evidence that glomerular epithelial cells or podocytes are insulin sensitive and that insulin signaling in podocytes is essential for maintaining normal kidney function. At the cellular level, the mechanisms leading to the development of insulin resistance include mutations in the insulin receptor gene, impairments in the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway, or perturbations in the trafficking of glucose transporters (GLUTs), which mediate the uptake of glucose into cells. Podocytes express several GLUTs, including GLUT1, GLUT2, GLUT3, GLUT4, and GLUT8. Of these, the most studied ones are GLUT1 and GLUT4, both shown to be insulin responsive in podocytes. In the basal state, GLUT4 is preferentially located in perinuclear and cytosolic vesicular structures and to a lesser extent at the plasma membrane. After insulin stimulation, GLUT4 is sorted into GLUT4-containing vesicles (GCVs) that translocate to the plasma membrane. GCV trafficking consists of several steps, including approaching of the GCVs to the plasma membrane, tethering, and docking, after which the lipid bilayers of the GCVs and the plasma membrane fuse, delivering GLUT4 to the cell surface for glucose uptake into the cell. Studies have revealed novel molecular regulators of the GLUT trafficking in podocytes and unraveled unexpected roles for GLUT1 and GLUT4 in the development of DKD, summarized in this review. These findings pave the way for better understanding of the mechanistic pathways associated with the development and progression of DKD and aid in the development of new treatments for this devastating disease.

Multiple myeloma cells' capacity to decompose H2O2 determines lenalidomide sensitivity

Lenalidomide is an immunomodulatory drug (IMiDs) with clinical efficacy in multiple myeloma (MM) and other late B-cell neoplasms. Although cereblon (CRBN) is an essential requirement for IMiD action, the complete molecular and biochemical mechanisms responsible for lenalidomide-mediated sensitivity or resistance remain unknown. Here, we report that IMiDs work primarily via inhibition of peroxidase-mediated intracellular H₂O₂ decomposition in MM cells. MM cells with lower H₂O₂-decomposition capacity were more vulnerable to lenalidomide-induced H₂O₂ accumulation and associated cytotoxicity. CRBN-dependent degradation of IKZF1 and IKZF3 was a consequence of H₂O₂-mediated oxidative stress. Lenalidomide increased intracellular H₂O₂ levels by inhibiting thioredoxin reductase (TrxR) in cells expressing CRBN, causing accumulation of immunoglobulin light-chain dimers, significantly increasing endoplasmic reticulum stress and inducing cytotoxicity by activation of BH3-only protein Bim in MM. Other direct inhibitors of TrxR and thioredoxin (Trx) caused similar cytotoxicity, but in a CRBN-independent fashion. Our findings could help identify patients most likely to benefit from IMiDs and suggest direct TrxR or Trx inhibitors for MM therapy.

Role of Protein Kinase G and Reactive Oxygen Species in the Regulation of Podocyte Function in Health and Disease

Podocytes and their foot processes form an important cellular layer of the glomerular barrier involved in the regulation of glomerular permeability. Disturbing the function of podocytes plays a central role in the development of proteinuria in diabetic nephropathy. Retraction of the podocyte foot processes that form slit diaphragms is a common feature of proteinuria; although, the correlation between these events in not well understood. Notably, it is unclear whether podocyte foot processes are able to regulate slit diaphragm permeability and glomerular ultrafiltration. The occurrence of reactive oxygen species generation, insulin resistance, and hyperglycemia characterizes early stages of type 2 diabetes. Protein kinase G type I alpha (PKGIα) is an intracellular target for vasorelaxant factors. It is activated in both cGMP-dependent and cGMP-independent manners. Recently, we demonstrated a relationship between oxidative stress, PKGIα activation, actin reorganization, and changes in the permeability of the filtration barrier. This review discusses how redox imbalance affects both the activity of PKGIα and PKGI-dependent signaling pathways in podocytes. J. Cell. Physiol. 232: 691-697, 2017. © 2016 Wiley Periodicals, Inc.

Extracellular purines' action on glomerular albumin permeability in isolated rat glomeruli: insights into the pathogenesis of albuminuria

Purinoceptors (adrengeric receptors and P2 receptors) are expressed on the cellular components of the glomerular filtration barrier, and their activation may affect glomerular permeability to albumin, which may ultimately lead to albuminuria, a well-established risk factor for the progression of chronic kidney disease and development of cardiovascular diseases. We investigated the mechanisms underlying the in vitro and in vivo purinergic actions on glomerular filter permeability to albumin by measuring convectional albumin permeability (Palb) in a single isolated rat glomerulus based on the video microscopy method. Primary cultured rat podocytes were used for the analysis of Palb, cGMP accumulation, PKG-Iα dimerization, and immunofluorescence. In vitro, natural nucleotides (ATP, ADP, UTP, and UDP) and nonmetabolized ATP analogs (2-meSATP and ATP-γ-S) increased Palb in a time- and concentration-dependent manner. The effects were dependent on P2 receptor activation, nitric oxide synthase, and cytoplasmic guanylate cyclase. ATP analogs significantly increased Palb, cGMP accumulation, and subcortical actin reorganization in a PKG-dependent but nondimer-mediated route in cultured podocytes. In vivo, 2-meSATP and ATP-γ-S increased Palb but did not significantly affect urinary albumin excretion. Both agonists enhanced the clathrin-mediated endocytosis of albumin in podocytes. A product of adenine nucleotides hydrolysis, adenosine, increased the permeability of the glomerular barrier via adrenergic receptors in a dependent and independent manner. Our results suggest that the extracellular nucleotides that stimulate an increase of glomerular Palb involve nitric oxide synthase and cytoplasmic guanylate cyclase with actin reorganization in podocytes.

Urinary polycyclic aromatic hydrocarbons and measures of oxidative stress, inflammation and renal function in adolescents: NHANES 2003-2008

OBJECTIVE

Recent evidence has suggested that polycyclic aromatic hydrocarbons (PAHs) may contribute to cardiometabolic and kidney dysfunction by increasing oxidative stress, but little is known about impacts in childhood.

STUDY DESIGN

We performed cross-sectional analyses of 660 adolescents aged 12-19 years in the 2003-2008 National Health and Nutrition Examination Survey (NHANES), using levels of 10 monohydroxylated urinary PAH metabolites as our exposure. Our primary outcomes of interest were biomarkers of oxidative stress and renal function, including estimated glomerular filtration rate (eGFR), urinary albumin to creatinine ratio (ACR), insulin resistance, and serum uric acid, gamma glutamyl transferase (GGT) and C-reactive protein (CRP).

RESULTS

We observed statistically significant associations between PAH metabolites and levels of serum GGT, CRP, uric acid and eGFR. Each 100% increase in 2-hydroxyphenanthrene was related to a 3.36% increase in uric acid (95% CI: 0.338-6.372; p=0.032), a 3.86% increase in GGT (95% CI: 1.361-6.362; p=0.005) and a 16.78% increase in CRP (95% CI: 1.848-31.689; p=0.029). Each 100% increase in 4-hydroxyphenanthrene was associated with a 6.18% increase in GGT (95% CI: 4.064-8.301; p<0.001) and a 13.66% increase in CRP (95% CI: 2.764-24.564; p=0.017). Each 100% increase in 9-hydroxyfluorene was associated with a 2.58% increase in GGT (95% CI: 0.389-4776; p=0.024). Each 100% increase in 3-hydroxyphenanthrene was associated with a 2.66% decrease in eGFR (95% CI: -4.979 to -0.331; p=0.028).

CONCLUSIONS

Urinary PAH metabolites were associated with serum uric acid, GGT and CRP, suggesting possible impacts on cardiometabolic and kidney function in adolescents. Prospective work is needed to investigate the potential long-term health consequences of these findings.

Dexamethasone-dependent modulation of cyclic GMP synthesis in podocytes

Podocytes may be direct target for glucocorticoid therapy in glomerular proteinuric disease. Permeability of podocytes largely depends on their capacity to migrate which involves the contractile apparatus in their foot processes. In this study, we examined the effect of synthetic glucocorticoid dexamethasone (DEX) on the ability of podocytes to produce cyclic guanosine monophosphate (cGMP) in the presence of vasoactive factors, atrial natriuretic peptide (ANP), nitric oxide (NO), and angiotensin II (Ang II). We investigated also the effects of cGMP and DEX on podocyte motility. Primary rat podocytes and immortalized mouse podocytes were pretreated with 1 µM DEX for 4 or 24 h. Glomerular hypertension was mimicked by subjecting the cells to mechanical stress. Total and subcellular cGMP levels were determined in podocytes incubated with 0.1 µM ANP, 1 µM S-nitroso-N-acetyl penicillamine (SNAP), and 1 µM Ang II. Cell motility was estimated by a wound-healing assay. The ANP-dependent production of cGMP increased after 4 h exposition to DEX, but was attenuated after 24 h. Adversely, a 24-h pretreatment with DEX augmented the NO-dependent cGMP synthesis. Ang II suppressed the ANP-dependent cGMP production and the effect was enhanced by DEX in mechanical stress conditions. Mechanical stress reduced total cGMP production in the presence of all stimulators, whereas extracellular to total cGMP ratio increased. 8-Br cGMP enhanced podocyte migration which was accompanied by F-actin disassembly. In the presence of DEX these effects were prevented. We conclude that DEX modulates the production of cGMP in podocytes stimulated with vasoactive factors such as Ang II, ANP, and NO, and the effect is time-dependent. cGMP increases podocyte motility, which is prevented by DEX. This mechanism may account for the antiproteinuric effect of glucocorticoids.

Redox regulation of cGMP-dependent protein kinase Iα in the cardiovascular system

Elevated levels of oxidants in biological systems have been historically referred to as “oxidative stress,” a choice of words that perhaps conveys an imbalanced view of reactive oxygen species in cells and tissues. The term stress suggests a harmful role, whereas a contemporary view is that oxidants are also crucial for the maintenance of homeostasis or adaptive signaling that can actually limit injury. This regulatory role for oxidants is achieved in part by them inducing oxidative post-translational modifications of proteins which may alter their function or interactions. Such mechanisms allow changes in cell oxidant levels to be coupled to regulated alterations in enzymatic function (i.e., signal transduction), which enables “redox signaling.” In this review we focus on the role of cGMP-dependent protein kinase (PKG) Ia disulfide dimerisation, an oxidative modification that is induced by oxidants that directly activates the enzyme, discussing how this impacts on the cardiovascular system. Additionally, how this oxidative activation of PKG may coordinate with or differ from classical activation of this kinase by cGMP is also considered.

The effects of environmental chemicals on renal function

The global incidence of chronic kidney disease (CKD) is increasing among individuals of all ages. Despite advances in proteomics, genomics and metabolomics, there remains a lack of safe and effective drugs to reverse or stabilize renal function in patients with glomerular or tubulointerstitial causes of CKD. Consequently, modifiable risk factors that are associated with a progressive decline in kidney function need to be identified. Numerous reports have documented the adverse effects that occur in response to graded exposure to a wide range of environmental chemicals. This Review summarizes the effects of such chemicals on four aspects of cardiorenal function: albuminuria, glomerular filtration rate, blood pressure and serum uric acid concentration. We focus on compounds that individuals are likely to be exposed to as a consequence of normal consumer activities or medical treatment, namely phthalates, bisphenol A, polyfluorinated alkyl acids, dioxins and furans, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Environmental exposure to these chemicals during everyday life could have adverse consequences on renal function and might contribute to progressive cumulative renal injury over a lifetime. Regulatory efforts should be made to limit individual exposure to environmental chemicals in an attempt to reduce the incidence of cardiorenal disease.

Combined effect of insulin and high glucose concentration on albumin permeability in cultured rat podocytes

Podocytes play a fundamental role in regulating glomerular permeability to albumin. This mechanism is disrupted in the course of diabetes. Both insulin and high glucose concentrations enhance the permeability of podocytes to albumin by stimulating oxygen free radical production, primarily by NAD(P)H oxidase-4 (NOX4), and by activating protein kinase G, isoform Iα (PKGIα). However, no study has investigated the combined effects of insulin and high glucose concentration. Here, we investigated the effects of applying insulin (INS, 300 nM) and high glucose (HG, 30 mM), both separately and combined, for 5 days, on cultured rat podocyte permeability to albumin. We measured podocyte permeability with a transmembrane albumin flux assay. We measured NOX4 and PKGIα mRNA expression with real-time PCR. We used Western blots to evaluate protein expression levels of NOX4, PKGIα, the myosin-binding subunit of myosin phosphatase 1, and myosin light chain. We found that INS and HG had a synergistic effect on podocyte permeability to albumin, and this synergy was not dependent on NOX4 or PKGIα. These results suggested that the combined action of INS and HG may exacerbate glomerular dysfunction in diabetes.

Insulin stimulates glucose transport via protein kinase G type I alpha-dependent pathway in podocytes

Podocyte resistance to the actions of insulin on glucose transport could contribute to the pathogenesis of diabetic podocytopathy (DP) via disturbances in cyclic-dependent protein kinase signaling. To determine whether cGMP-dependent protein kinase (PKG) is involved in the insulin regulation of glucose transport, we measured insulin-dependent glucose uptake into cultured rat podocytes under conditions of modified PKG activity using pharmacological (PKG activator or inhibitor) and biochemical (siRNA PKGIα, siRNA insulin receptor β) means. Our findings indicate the participation of PKG in insulin-stimulated transport and provide new insights into how PKG may trigger the resistance of glucose transport to insulin in DP.

High glucose increases glomerular filtration barrier permeability by activating protein kinase G type Iα subunits in a Nox4-dependent manner

Hyperglycemia is a primary factor that disturbs podocyte function in the glomerular filtration process; this disturbance leads to the development of diabetic nephropathy, and ultimately, renal failure. Podocyte function may also be altered by biological agents that modify protein kinase activity, including the cGMP-activated protein kinase type Iα (PKGIα). We hypothesized that hyperglycemia-induced podocyte protein hyperpermeability was dependent on PKGIα activation, and that PKGIα was activated via dimerization induced by reactive oxygen species. This hypothesis was investigated in rat podocytes cultured in high glucose (HG, 30 mM). Protein expression was measured with Western blot and immunofluorescence. Podocyte permeability was measured with a transmembrane albumin flux assay. We found that HG increased podocyte permeability in long-term incubations (1, 3, and 5 days); permeability was increased by 66% on day 5. This effect was abolished with apocynin, a NAD(P)H inhibitor, and Rp-8-Br-cGMPS, a PKG inhibitor. It was also abolished by introducing small interfering RNAs (siRNAs) against Nox4 and PKGIα into cultured podocytes. Furthermore, HG increased PKGIα dimerization by 138% (0.23 ± 0.04 vs. 0.54 ± 0.09; P<0.05); this effect was abolished with a siRNA against Nox4. Our observations suggested that HG could increase albumin permeability across the podocyte filtration barrier via Nox4-dependent PKGIα dimerization.

Protein kinase G oxidation is a major cause of injury during sepsis

Sepsis is a common life-threatening clinical syndrome involving complications as a result of severe infection. A cardinal feature of sepsis is inflammation that results in oxidative stress. Sepsis in wild-type mice induced oxidative activation of cGMP-dependent protein kinase 1 alpha (PKG Iα), which increased blood vessel dilation and permeability, and also lowered cardiac output. These responses are typical features of sepsis and their combined effect is a lowering of blood pressure. This hypotension, a hallmark of sepsis, resulted in underperfusion of end organs, resulting in their damage. A central role for PKG Iα oxidative activation in injury is supported by oxidation-resistant Cys42Ser PKG Iα knock-in mice being markedly protected from these clinical indices of injury during sepsis. We conclude that oxidative activation of PKG Iα is a key mediator of hypotension and consequential organ injury during sepsis.

H₂O₂ lowers the cytosolic Ca²⁺ concentration via activation of cGMP-dependent protein kinase Iα

The cGMP-dependent protein kinase I (cGKI) is a key mediator of cGMP signaling, but the specific functions of its two isoforms, cGKIα and cGKIβ, are poorly understood. Recent studies indicated a novel cGMP-independent role for cGKIα in redox sensing. To dissect the effects of oxidative stress on the cGKI isoforms, we used mouse embryonic fibroblasts and vascular smooth muscle cells (VSMCs) expressing both, one, or none of them. In cGKIα-expressing cells, but not in cells expressing only cGKIβ, incubation with H₂O₂ induced the formation of a disulfide bond between the two identical subunits of the dimeric enzyme. Oxidation of cGKIα was associated with increased phosphorylation of its substrate, vasodilator-stimulated phosphoprotein. H₂O₂ did not stimulate cGMP production, indicating that it activates cGKIα directly via oxidation. Interestingly, there was a mutual influence of H₂O₂ and cGMP on cGKI activity and disulfide bond formation, respectively; preoxidation of the kinase with H₂O₂ slightly impaired its activation by cGMP, whereas preactivation of the enzyme with cGMP attenuated its oxidation by H₂O₂. To evaluate the functional relevance of the noncanonical H₂O₂-cGKIα pathway, we studied the regulation of the cytosolic Ca²⁺ concentration ([Ca²⁺](i)). H₂O₂ suppressed norepinephrine-induced Ca²⁺ transients in cGKIα-expressing VSMCs and, to a lower extent, in VSMCs expressing only cGKIβ or none of the isoforms. Thus, H₂O₂ lowers [Ca²⁺](i) mainly via a cGKIα-dependent pathway. These results indicate that oxidative stress selectively targets the cGKIα isoform, which then modulates cellular processes in a cGMP-independent manner. A decrease in [Ca²⁺](i) in VSMCs via activation of cGKIα might be a major mechanism of H₂O₂-induced vasodilation.