Insulin induces bioenergetic changes and alters mitochondrial dynamics in podocytes

Diabetic nephropathy (DN) is one of the most frequent complications of diabetes. Early stages of DN are associated with hyperinsulinemia and progressive insulin resistance in insulin-sensitive cells, including podocytes. The diabetic environment induces pathological changes, especially in podocyte bioenergetics, which is tightly linked with mitochondrial dynamics. The regulatory role of insulin in mitochondrial morphology in podocytes has not been fully elucidated. Therefore, the main goal of the present study was to investigate effects of insulin on the regulation of mitochondrial dynamics and bioenergetics in human podocytes. Biochemical analyses were performed to assess oxidative phosphorylation efficiency by measuring the oxygen consumption rate (OCR) and glycolysis by measuring the extracellular acidification rate (ECAR). mRNA and protein expression were determined by real-time polymerase chain reaction and Western blot. The intracellular mitochondrial network was visualized by MitoTracker staining. All calculations were conducted using CellProfiler software. Short-term insulin exposure exerted inhibitory effects on various parameters of oxidative respiration and adenosine triphosphate production, and glycolysis flux was elevated. After a longer time of treating cells with insulin, an increase in mitochondrial size was observed, accompanied by a reduction of expression of the mitochondrial fission markers DRP1 and FIS1 and an increase in mitophagy. Overall, we identified a previously unknown role for insulin in the regulation of oxidative respiration and glycolysis and elucidated mitochondrial dynamics in human podocytes. The present results emphasize the importance of the duration of insulin stimulation for its metabolic and molecular effects, which should be considered in clinical and experimental studies of DN.

Role of lysosomes in insulin signaling and glucose uptake in cultured rat podocytes

Podocytes are sensitive to insulin, which governs the functional and structural integrity of podocytes that are essential for proper function of the glomerular filtration barrier. Lysosomes are acidic organelles that are implicated in regulation of the insulin signaling pathway. Cathepsin D (CTPD) and lysosome-associated membrane protein 1 (LAMP1) are major lysosomal proteins that reflect the functional state of lysosomes. However, the effect of insulin on lysosome activity and role of lysosomes in the regulation of insulin-dependent glucose uptake in podocytes are unknown. Our studies showed that the short-term incubation of podocytes with insulin decreased LAMP1 and CTPD mRNA levels. Insulin and bafilomycin A1 reduced both the amounts of LAMP1 and CTPD proteins and activity of CTPD, which were associated with a decrease in the fluorescence intensity of lysosomes that were labeled with LysoTracker. Bafilomycin A1 inhibited insulin-dependent endocytosis of the insulin receptor and increased the amounts of the insulin receptor and glucose transporter 4 on the cell surface of podocytes. Bafilomycin A1 also inhibited insulin-dependent glucose uptake despite an increase in the amount of glucose transporter 4 in the plasma membrane of podocytes. These results suggest that lysosomes are signaling hubs that may be involved in the coupling of insulin signaling with the regulation of glucose uptake in podocytes. The dysregulation of this mechanism can lead to the dysfunction of podocytes and development of insulin resistance.

Pit 1 transporter (SLC20A1) as a key factor in the NPP1-mediated inhibition of insulin signaling in human podocytes

Podocytes are crucially involved in blood filtration in the glomerulus. Their proper function relies on efficient insulin responsiveness. The insulin resistance of podocytes, defined as a reduction of cell sensitivity to this hormone, is the earliest pathomechanism of microalbuminuria that is observed in metabolic syndrome and diabetic nephropathy. In many tissues, this alteration is mediated by the phosphate homeostasis-controlling enzyme nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). By binding to the insulin receptor (IR), NPP1 inhibits downstream cellular signaling. Our previous research found that hyperglycemic conditions affect another protein that is involved in phosphate balance, type III sodium-dependent phosphate transporter 1 (Pit 1). In the present study, we evaluated the insulin resistance of podocytes after 24 h of incubation under hyperinsulinemic conditions. Thereafter, insulin signaling was inhibited. The formation of NPP1/IR complexes was observed at that time. A novel finding in the present study was our observation of an interaction between NPP1 and Pit 1 after the 24 h stimulation of podocytes with insulin. After downregulation of the SLC20A1 gene, which encodes Pit 1, we established insulin resistance in podocytes that were cultured under native conditions, manifested as a lack of intracellular insulin signaling and the inhibition of glucose uptake via the glucose transporter type 4. These findings suggest that Pit 1 might be a major factor that participates in the NPP1-mediated inhibition of insulin signaling.

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.

Role of L-lactate as an energy substrate in primary rat podocytes under physiological and glucose deprivation conditions

Lactate has long been acknowledged to be a metabolic waste product, but it has more recently been found as a fuel energy source in mammalian cells. Podocytes are an important component of the glomerular filter, and their role in maintaining the structural integrity of this structure was established. These cells rely on a constant energy supply and reservoir. The utilization of alternative energy substrates to preserve energetic homeostasis is a subject of extensive research, and lactate appears to be one such candidate. Therefore, we investigated the role of lactate as an energy substrate and characterize the lactate transport system in cultured rat podocytes during sufficient and insufficient glucose supplies. The present study, for the first time, demonstrated the presence of lactate transporters in podocytes. Moreover, we observed modified the amount of these transporters in response to limited glucose availability and after l-lactate supplementation. Simultaneously, exposure to l-lactate preserved cell survival during insufficient glucose supply. Interestingly, during glucose deprivation, lactate exposure allowed the steady flow of glycolysis and prevented glycogen reserves depletion. Summarizing, podocytes utilize lactate as an energy substrate and possess a developed system that controls lactate homeostasis, suggesting that it plays an essential role in podocyte metabolism, especially during fluctuations of energy availability.

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.

Hyperglycemic environment disrupts phosphate transporter function and promotes calcification processes in podocytes and isolated glomeruli

Soft tissue calcification is a pathological phenomenon that often occurs in end-stage chronic kidney disease (CKD), which is caused by diabetic nephropathy, among other factors. Hyperphosphatemia present during course of CKD contributes to impairments in kidney function, particularly damages in the glomerular filtration barrier (GFB). Essential elements of the GFB include glomerular epithelial cells, called podocytes. In the present study, we found that human immortalized podocytes express messenger RNA and protein of phosphate transporters, including NaPi 2c (SLC34A3), Pit 1 (SLC20A1), and Pit 2 (SLC20A2), which are sodium-dependent and mediate intracellular phosphate (Pi) transport, and XPR1, which is responsible for extracellular Pi transport. We found that cells that were grown in a medium with a high glucose (HG) concentration (30 mM) expressed less Pit 1 and Pit 2 protein than podocytes that were cultured in a standard glucose medium (11 mM). We found that exposure of the analyzed transporters in the cell membrane of the podocyte is altered by HG conditions. We also found that the activity of tissue nonspecific alkaline phosphatase increased in HG, causing a rise in Pi generation. Additionally, HG led to a reduction of the amount of ectonucleotide pyrophosphatase/phosphodiesterase 1 in the cell membrane of podocytes. The extracellular concentration of pyrophosphate also decreased under HG conditions. These data suggest that a hyperglycemic environment enhances the production of Pi in podocytes and its retention in the extracellular space, which may induce glomerular calcification.

Analysis of urinary kallikrein-related peptidase 13 for monitoring bladder cancer

BACKGROUND

Bladder cancer (BC) is one of the 10 most common types of cancer worldwide, with approximately 550,000 new cases each year. Early detection and appropriate diagnosis are important factors in successful treatment of the disease.

MATERIAL AND METHODS

We used specific fluorogenic substrate for the quantitative determination of urine kallikrein 13 (KLK13) activity in healthy (n = 15) and BC (n = 54) patients. The proteolytic activity in individual urine samples was determined by fluorescence measurements. Then, immunoenzymatic analyses (ELISA, Western blot) were performed to confirm the presence of KLK13 in the tested samples.

RESULTS

Urine samples from patients with G2 and G3 grade BC contained proteolytically active KLK13, as confirmed by kinetic analysis and immunochemical detection. KLK13 was not detected in the urine of patients with G1 grade BC.

DISCUSSION

Previous clinical study reveals the KLK13 significance for BC prognosis as increased KLK13 expression was highlighted in bladder tumours compared to normal adjacent tissues. Our findings correlate to the report. KLK13 activity was confirmed in BC patients with G2 and G3 stage of disease development.

CONCLUSIONS

Using specific chromogenic/fluorogenic peptides could be useful for the non-invasive disease monitoring of BC progress.

Hyperglycemia alters mitochondrial respiration efficiency and mitophagy in human podocytes

Podocytes constitute the outer layer of the renal glomerular filtration barrier. Their energy requirements strongly depend on efficient oxidative respiration, which is tightly connected with mitochondrial dynamics. We hypothesized that hyperglycemia modulates energy metabolism in glomeruli and podocytes and contributes to the development of diabetic kidney disease. We found that oxygen consumption rates were severely reduced in glomeruli from diabetic rats and in human podocytes that were cultured in high glucose concentration (30 mM; HG). In these models, all of the mitochondrial respiratory parameters, including basal and maximal respiration, ATP production, and spare respiratory capacity, were significantly decreased. Podocytes that were treated with HG showed a fragmented mitochondrial network, together with a decrease in expression of the mitochondrial fusion markers MFN1, MFN2, and OPA1, and an increase in the activity of the fission marker DRP1. We showed that markers of mitochondrial biogenesis, such as PGC-1α and TFAM, decreased in HG-treated podocytes. Moreover, PINK1/parkin-dependent mitophagy was inhibited in these cells. These results provide evidence that hyperglycemia impairs mitochondrial dynamics and turnover, which may underlie the remarkable deterioration of mitochondrial respiration parameters in glomeruli and 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.

Involvement of nitric oxide synthase/nitric oxide pathway in the regulation of SIRT1-AMPK crosstalk in podocytes: Impact on glucose uptake

The protein deacetylase sirtuin 1 (SIRT1) and adenosine monophosphate-dependent protein kinase (AMPK) play important roles in the development of insulin resistance. In glomerular podocytes, crosstalk between these two enzymes may be altered under hyperglycemic conditions. SIRT1 protein levels and activity and AMPK phosphorylation decrease under hyperglycemic conditions, with concomitant inhibition of the effect of insulin on glucose uptake into these cells. Nitric oxide (NO)-dependent regulatory signaling pathways have been shown to be downregulated under diabetic conditions. The present study examined the involvement of the NO synthase (NOS)/NO pathway in the regulation of SIRT1-AMPK signaling and glucose uptake in podocytes. We examined the effects of NOS/NO pathway alterations on SIRT1/AMPK signaling and glucose uptake using pharmacological tools and a small-interfering transfection approach. We also examined the ability of the NOS/NO pathway to protect podocytes against high glucose-induced alterations of SIRT1/AMPK signaling and insulin-dependent glucose uptake. Inhibition of the NOS/NO pathway reduced SIRT1 protein levels and activity, leading to a decrease in AMPK phosphorylation and blockade of the effect of insulin on glucose uptake. Treatment with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) prevented high glucose-induced decreases in SIRT1 and AMPK activity and increased GLUT4 protein expression, thereby improving glucose uptake in podocytes. These findings suggest that inhibition of the NOS/NO pathway may result in alterations of the effects of insulin on glucose uptake in podocytes. In turn, the enhancement of NOS/NO pathway activity may prevent these deleterious effects of high glucose concentrations, thus bidirectionally stimulating the SIRT1-AMPK reciprocal activation loop.

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.

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.

The TRPC6-AMPK Pathway is Involved in Insulin-Dependent Cytoskeleton Reorganization and Glucose Uptake in Cultured Rat Podocytes

Background/Aims: Podocytes are dynamic polarized cells on the surface of glomerular capillaries that are an essential part of the glomerular filtration barrier. AMP-activated protein kinase (AMPK), a key regulator of glucose and fatty acid metabolism, plays a major role in obesity and type 2 diabetes. Accumulating evidence suggests that TRPC6 channels are crucial mediators of calcium transport in podocytes and are involved in regulating glomerular filtration. Here we investigated whether the AMPK-TRPC6 pathway is involved in insulin-dependent cytoskeleton reorganization and glucose uptake in cultured rat podocytes. Methods: Western blot and immunofluorescence analysis confirmed AMPKα and TRPC6 expression, the phosphorylation of proteins associated with actin cytoskeleton reorganization (PAK, rac1, and cofilin), and the expression of insulin signaling proteins (Akt, Insulin receptor). Coimmunoprecipitation and immunofluorescence results demonstrated AMPKα/TRPC6 interaction. To ask whether TRPC6 is involved in the insulin regulation of glucose transport, we measured insulin-dependent (1, 2-3H)-deoxy-D-glucose uptake into podocytes after reducing TRPC6 activity pharmacologically and biochemically (TRPC6 siRNA). Results: The results suggested a key role for the TRPC6 channel in the mediation of insulin-dependent activation of AMPKα2 and glucose uptake. Moreover, AMPK and TRPC6 activation were required to stimulate the Rac1 signaling pathway. Conclusion: These results suggest a potentially important new mechanism that regulates glucose transport in podocytes and that could be injurious during diabetes.