Effect of high glucose on mesangial cell protein kinase C-delta and -epsilon is polyol pathway-dependent

AGE-RAGE axis culminates into multiple pathogenic processes: a central road to neurodegeneration

Advanced glycation end-products (AGEs; e.g., glyoxal, methylglyoxal or carboxymethyl-lysine) are heterogenous group of toxic compounds synthesized in the body through both exogenous and endogenous pathways. AGEs are known to covalently modify proteins bringing about loss of functional alteration in the proteins. AGEs also interact with their receptor, receptor for AGE (RAGE) and such interactions influence different biological processes including oxidative stress and apoptosis. Previously, AGE-RAGE axis has long been considered to be the maligning factor for various human diseases including, diabetes, obesity, cardiovascular, aging, etc. Recent developments have revealed the involvement of AGE-RAGE axis in different pathological consequences associated with the onset of neurodegeneration including, disruption of blood brain barrier, neuroinflammation, remodeling of extracellular matrix, dysregulation of polyol pathway and antioxidant enzymes, etc. In the present article, we attempted to describe a new avenue that AGE-RAGE axis culminates to different pathological consequences in brain and therefore, is a central instigating component to several neurodegenerative diseases (NGDs). We also invoke that specific inhibitors of TIR domains of TLR or RAGE receptors are crucial molecules for the therapeutic intervention of NGDs. Clinical perspectives have also been appropriately discussed.

High Glucose Stimulates Mineralocorticoid Receptor Transcriptional Activity Through the Protein Kinase C β Signaling

Activation of mineralocorticoid receptor (MR) is shown in resistant hypertension including diabetes mellitus. Although protein kinase C (PKC) signaling is involved in the pathogenesis of diabetic complications, an association between PKC and MR is not known. Activation of PKCα and PKCβ by TPA (12-O-Tetradecanoylphorbol 13-acetate) increased MR proteins and its transcriptional activities in HEK293-MR cells. In contrast, a high glucose condition resulted in PKCβ but not PKCα activation, which is associated with elevation of MR protein levels and MR transcriptional activities. Reduction of endogenous PKCβ by siRNA decreased those levels. Interestingly, high glucose did not affect MR mRNA levels, but rather decreased ubiquitination of MR proteins. In db/db mice kidneys, levels of phosphorylated PKCβ2, MR and Sgk-1 proteins were elevated, and the administration of PKC inhibitor reversed these changes compared to db/+ mice. These data suggest that high glucose stimulates PKCβ signaling, which leads to MR stabilization and its transcriptional activities.

Hydrophobic motif site-phosphorylated protein kinase CβII between mTORC2 and Akt regulates high glucose-induced mesangial cell hypertrophy

PKCβII controls the pathologic features of diabetic nephropathy, including glomerular mesangial cell hypertrophy. PKCβII contains the COOH-terminal hydrophobic motif site Ser-660. Whether this hydrophobic motif phosphorylation contributes to high glucose-induced mesangial cell hypertrophy has not been determined. Here we show that, in mesangial cells, high glucose increased phosphorylation of PKCβII at Ser-660 in a phosphatidylinositol 3-kinase (PI3-kinase)-dependent manner. Using siRNAs to downregulate PKCβII, dominant negative PKCβII, and PKCβII hydrophobic motif phosphorylation-deficient mutant, we found that PKCβII regulates activation of mechanistic target of rapamycin complex 1 (mTORC1) and mesangial cell hypertrophy by high glucose. PKCβII via its phosphorylation at Ser-660 regulated phosphorylation of Akt at both catalytic loop and hydrophobic motif sites, resulting in phosphorylation and inactivation of its substrate PRAS40. Specific inhibition of mTORC2 increased mTORC1 activity and induced mesangial cell hypertrophy. In contrast, inhibition of mTORC2 decreased the phosphorylation of PKCβII and Akt, leading to inhibition of PRAS40 phosphorylation and mTORC1 activity and prevented mesangial cell hypertrophy in response to high glucose; expression of constitutively active Akt or mTORC1 restored mesangial cell hypertrophy. Moreover, constitutively active PKCβII reversed the inhibition of high glucose-stimulated Akt phosphorylation and mesangial cell hypertrophy induced by suppression of mTORC2. Finally, using renal cortexes from type 1 diabetic mice, we found that increased phosphorylation of PKCβII at Ser-660 was associated with enhanced Akt phosphorylation and mTORC1 activation. Collectively, our findings identify a signaling route connecting PI3-kinase to mTORC2 to phosphorylate PKCβII at the hydrophobic motif site necessary for Akt phosphorylation and mTORC1 activation, leading to mesangial cell hypertrophy.

Akt and RhoA activation in response to high glucose require caveolin-1 phosphorylation in mesangial cells

Glomerular matrix accumulation is a hallmark of diabetic renal disease. Serine/threonine kinase PKC-β1 mediates glucose-induced Akt S473 phosphorylation, RhoA activation, and transforming growth factor (TGF)-β1 upregulation and finally leads to matrix upregulation in mesangial cells (MCs). It has been reported that glucose-induced PKC-β1 activation is dependent on caveolin-1 and the presence of intact caveolae in MCs; however, whether activated PKC-β1 regulates caveolin-1 expression and phosphorylation are unknown. Here, we showed that, although the caveolin-1 protein level had no significant change, the PKC-β-specific inhibitor LY-333531 blocked caveolin-1 Y14 phosphorylation in high glucose (HG)-treated MCs and in the renal cortex of diabetic rats. The Src-specific inhibitor SU-6656 prevented the HG-induced association between PKC-β1 and caveolin-1 and PKC-β1 membrane translocation, whereas PKC-β1 small interfering RNA failed to block Src activation, indicating that Src kinase is upstream of PKC-β1 activation. Although LY-333531 blocked PKC-β1 membrane translocation, it had no effect on the PKC-β1/caveolin-1 association, suggesting that PKC-β1 activation requires the interaction of caveolin-1 and PKC-β1. PKC-β1-mediated Akt S473 phosphorylation, RhoA activation, and fibronectin upregulation in response to HG were prevented by SU-6656 and nonphosphorylatable mutant caveolin-1 Y14A. In conclusion, Src activation by HG mediates the PKC-β1/caveolin-1 association and PKC-β1 activation, which assists in caveolin-1 Y14 phosphorylation by Src kinase. The downstream effects, including Akt S473 phosphorylation, RhoA activation, and fibronectin upregulation, require caveolin-1 Y14 phosphorylation. Caveolin-1 is thus an important mediator of the profibrogenic process in diabetic renal disease.

Impact of high glucose and transforming growth factor-β on bioenergetic profiles in podocytes

Diabetic nephropathy is the most common cause of chronic renal failure in industrialized countries. Depletion of podocytes plays an important role in the progression of diabetic glomerulopathy. Various factors in the diabetic milieu lead to serious podocyte stress driving the cells toward cell cycle arrest (p27(Kip1)), hypertrophy, detachment, and apoptosis. Mitochondria are responsible for oxidative phosphorylation and energy supply in podocytes. Recent studies indicated that mitochondrial dysfunction is a key factor in diabetic nephropathy. In the present study, we investigated metabolic profiles of podocytes under diabetic conditions. We examined oxygen consumption rates (OCRs) and oxidative phosphorylation complex activities in murine podocytes. Cells were exposed to high glucose for 48 hours, cultured for 10 passages under high-glucose conditions (30 mmol/L), or incubated with transforming growth factor-β (5 ng/mL) for 24 hours. After prolonged exposure to high glucose, podocytes showed a significantly increased OCR at baseline and also a higher OCR after addition of oligomycin, indicating significant changes in mitochondrial energy metabolism. Higher OCRs after inhibition of respiration by rotenone also indicated changes in nonmitochondrial respiration. Podocytes stimulated with a proapoptotic concentration of transforming growth factor-β displayed similar bioenergetic profiles, even with decreased citrate synthase activity. In all tested conditions, we found a higher cellular nicotinamide adenine dinucleotide content and changes in activities of respiratory chain complexes. In summary, we provide for the first time evidence that key factors of the diabetic milieu induce changes in glucose metabolism and mitochondrial function in podocytes.

Aldose reductase, oxidative stress, and diabetic mellitus

Diabetes mellitus (DM) is a complex metabolic disorder arising from lack of insulin production or insulin resistance (Diagnosis and classification of diabetes mellitus, 2007). DM is a leading cause of morbidity and mortality in the developed world, particularly from vascular complications such as atherothrombosis in the coronary vessels. Aldose reductase (AR; ALR2; EC 1.1.1.21), a key enzyme in the polyol pathway, catalyzes nicotinamide adenosine dinucleotide phosphate-dependent reduction of glucose to sorbitol, leading to excessive accumulation of intracellular reactive oxygen species (ROS) in various tissues of DM including the heart, vasculature, neurons, eyes, and kidneys. As an example, hyperglycemia through such polyol pathway induced oxidative stress, may have dual heart actions, on coronary blood vessel (atherothrombosis) and myocardium (heart failure) leading to severe morbidity and mortality (reviewed in Heather and Clarke, 2011). In cells cultured under high glucose conditions, many studies have demonstrated similar AR-dependent increases in ROS production, confirming AR as an important factor for the pathogenesis of many diabetic complications. Moreover, recent studies have shown that AR inhibitors may be able to prevent or delay the onset of cardiovascular complications such as ischemia/reperfusion injury, atherosclerosis, and atherothrombosis. In this review, we will focus on describing pivotal roles of AR in the pathogenesis of cardiovascular diseases as well as other diabetic complications, and the potential use of AR inhibitors as an emerging therapeutic strategy in preventing DM complications.

High glucose-induced RhoA activation requires caveolae and PKCβ1-mediated ROS generation

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. We previously showed that RhoA activation by high glucose in mesangial cells (MC) leads to matrix upregulation (Peng F, Wu D, Gao B, Ingram AJ, Zhang B, Chorneyko K, McKenzie R, Krepinsky JC. Diabetes 57: 1683-1692, 2008). Here, we study the mechanism whereby RhoA is activated. In primary rat MC, RhoA activation required glucose entry and metabolism. Broad PKC inhibitors (PMA, bisindolylmaleimide, Gö6976), as well as specific PKCβ blockade with an inhibitor and small interfering RNA (siRNA), prevented RhoA activation by glucose. PKCβ inhibition also abrogated reactive oxygen species (ROS) generation by glucose. The ROS scavenger N-acetylcysteine (NAC) or NADPH oxidase inhibitors apocynin and DPI prevented glucose-induced RhoA activation. RhoA and some PKC isoforms localize to caveolae. Chemical disruption of these microdomains prevented RhoA and PKCβ1 activation by glucose. In caveolin-1 knockout cells, glucose did not induce RhoA and PKCβ1 activation; these responses were rescued by caveolin-1 reexpression. Furthermore, glucose-induced ROS generation was significantly attenuated by chemical disruption of caveolae and in knockout cells. Downstream of RhoA signaling, activator protein-1 (AP-1) activation was also inhibited by disrupting caveolae, was absent in caveolin-1 knockout MC and rescued by caveolin-1 reexpression. Finally, transforming growth factor (TGF)-β1 upregulation, mediated by AP-1, was prevented by RhoA signaling inhibition and by disruption or absence of caveolae. In conclusion, RhoA activation by glucose is dependent on PKCβ1-induced ROS generation, most likely through NADPH oxidase. The activation of PKCβ1 and its downstream effects, including upregulation of TGF-β1, requires caveolae. These microdomains are thus important mediators of the profibrogenic process associated with diabetic nephropathy.

Genetic deficiency of aldose reductase counteracts the development of diabetic nephropathy in C57BL/6 mice

AIMS/HYPOTHESIS

The aim of the study was to investigate the effects of genetic deficiency of aldose reductase in mice on the development of key endpoints of diabetic nephropathy.

METHODS

A line of Ar (also known as Akr1b3)-knockout (KO) mice, a line of Ar-bitransgenic mice and control C57BL/6 mice were used in the study. The KO and bitransgenic mice were deficient for Ar in the renal glomeruli and all other tissues, with the exception of, in the bitransgenic mice, a human AR cDNA knockin-transgene that directed collecting-tubule epithelial-cell-specific AR expression. Diabetes was induced in 8-week-old male mice with streptozotocin. Mice were further maintained for 17 weeks then killed. A number of serum and urinary variables were determined for these 25-week-old mice. Periodic acid-Schiff staining, western blots, immunohistochemistry and protein kinase C (PKC) activity assays were performed for histological analyses, and to determine the levels of collagen IV and TGF-β1 and PKC activities in renal cortical tissues.

RESULTS

Diabetes-induced extracellular matrix accumulation and collagen IV overproduction were completely prevented in diabetic Ar-KO and bitransgenic mice. Ar deficiency also completely or partially prevented diabetes-induced activation of renal cortical PKC, TGF-β1 and glomerular hypertrophy. Loss of Ar results in a 43% reduction in urine albumin excretion in the diabetic Ar-KO mice and a 48% reduction in the diabetic bitransgenic mice (p < 0.01).

CONCLUSIONS/INTERPRETATION

Genetic deficiency of Ar significantly ameliorated development of key endpoints linked with early diabetic nephropathy in vivo. Robust and specific inhibition of aldose reductase might be an effective strategy for the prevention and treatment of diabetic nephropathy.

All-trans retinoic acid and a novel synthetic retinoid tamibarotene (Am80) differentially regulate CD38 expression in human leukemia HL-60 cells: possible involvement of protein kinase C-delta

ATRA and a synthetic RAR agonist tamibarotene (Am80) induce granulocytic differentiation of human acute leukemia HL-60 cells and have been used in antineoplastic therapy. ATRA induces CD38 antigen during HL-60 cell differentiation, which interacts with CD31 antigen on the vascular EC surface and may induce disadvantages in the therapy. We here examined the mechanisms of the ATRA-mediated CD38 induction and compared the difference between ATRA- and tamibarotene-mediated induction. Tamibarotene-induced HL-60 cell adhesion to ECs was 38% lower than ATRA, and NB4 cell adhesion to ECs by tamibarotene was equivalent to ATRA, which induced CD38 gene transcription biphasically in HL-60 cells, the early-phase induction via DR-RARE containing intron 1, and the delayed-phase induction via RARE lacking the 5'-flanking region. In contrast to ATRA, tamibarotene induced only the early-phase induction, resulting in its lower CD38 induction than ATRA. A PKCδ inhibitor, rottlerin, and siRNA-mediated PKCδ knockdown suppressed the ATRA-induced CD38 promoter activity of the 5'-flanking region, whereas a RAR antagonist, LE540, or RAR knockdown did not affect it. Cycloheximide and rottlerin suppressed the delayed-phase induction of CD38 expression by ATRA but did not affect the early-phase induction. Moreover, ATRA, but not tamibarotene, induced PKCδ expression without affecting its mRNA stability. The diminished effect of tamibarotene on CD38-mediated HL-60 cell adhesion to ECs compared with ATRA is likely a result of the lack of its delayed-phase induction of CD38 expression, which may be advantageous in antineoplastic therapy.

Activation of protein kinase C isoforms and its impact on diabetic complications

Both cardio- and microvascular complications adversely affect the life quality of patients with diabetes and have been the leading cause of mortality and morbidity in this population. Cardiovascular pathologies of diabetes have an effect on microvenules, arteries, and myocardium. It is believed that hyperglycemia is one of the most important metabolic factors in the development of both micro- and macrovascular complications in diabetic patients. Several prominent hypotheses exist to explain the adverse effect of hyperglycemia. One of them is the chronic activation by hyperglycemia of protein kinase (PK)C, a family of enzymes that are involved in controlling the function of other proteins. PKC has been associated with vascular alterations such as increases in permeability, contractility, extracellular matrix synthesis, cell growth and apoptosis, angiogenesis, leukocyte adhesion, and cytokine activation and inhibition. These perturbations in vascular cell homeostasis caused by different PKC isoforms (PKC-alpha, -beta1/2, and PKC-delta) are linked to the development of pathologies affecting large vessel (atherosclerosis, cardiomyopathy) and small vessel (retinopathy, nephropathy and neuropathy) complications. Clinical trials using a PKC-beta isoform inhibitor have been conducted, with some positive results for diabetic nonproliferative retinopathy, nephropathy, and endothelial dysfunction. This article reviews present understanding of how PKC isoforms cause vascular dysfunctions and pathologies in diabetes.

Rosiglitazone prevents high glucose-induced vascular endothelial growth factor and collagen IV expression in cultured mesangial cells

Peroxisome proliferator-activated receptor (PPARγ), a ligand-dependent transcription factor, negatively modulates high glucose effects. We postulated that rosiglitazone (RSG), an activator of PPARγ prevents the upregulation of vascular endothelial growth factor (VEGF) and collagen IV by mesangial cells exposed to high glucose. Primary cultured rat mesangial cells were growth-arrested in 5.6 mM (NG) or 25 mM D-glucose (HG) for up to 48 hours. In HG, PPARγ mRNA and protein were reduced within 3 h, and enhanced ROS generation, expression of p22^phox, VEGF and collagen IV, and PKC-ζ membrane association were prevented by RSG. In NG, inhibition of PPARγ caused ROS generation and VEGF expression that were unchanged by RSG. Reduced AMP-activated protein kinase (AMPK) phosphorylation in HG was unchanged with RSG, and VEGF expression was unaffected by AMPK inhibition. Hence, PPARγ is a negative modulator of HG-induced signaling that acts through PKC-ζ but not AMPK and regulates VEGF and collagen IV expression by mesangial cells.

PKC-beta1 mediates glucose-induced Akt activation and TGF-beta1 upregulation in mesangial cells

Accumulation of glomerular matrix is a hallmark of diabetic nephropathy. The serine/threonine kinase Akt mediates glucose-induced upregulation of collagen I in mesangial cells through transactivation of the EGF receptor (EGFR). In addition, in renal tubular cells, glucose-induced secretion of TGF-beta requires phosphoinositide-3-OH kinase, suggesting a possible role for Akt in the modulation of TGF-beta expression, but the mechanisms of Akt activation and its involvement in TGF-beta regulation are unknown. Here, in primary mesangial cells, high glucose induced AktS473 phosphorylation, which correlates with its activation, in a protein kinase C beta (PKC-beta)-dependent manner. Glucose led to PKC-beta1 membrane translocation and association with Akt, and PKC-beta1 immunoprecipitated from glucose-treated cells phosphorylated recombinant Akt on S473. PKC is known to mediate glucose-induced TGF-beta1 upregulation through the transcription factor AP-1; here, inhibitors of phosphoinositide-3-OH kinase, PKC-beta and Akt, and dominant-negative Akt all prevented glucose-induced activation of AP-1 and upregulation of TGF-beta1. Finally, pharmacologic and dominant negative inhibition of EGFR blocked glucose-induced activation of PKC-beta1, phosphorylation of AktS473, activation of AP-1, and upregulation of TGF-beta1. In vivo, the PKC-beta inhibitor ruboxistaurin prevented Akt activation in the renal cortex of diabetic rats. In conclusion, PKC-beta1 is an Akt S473 kinase in glucose-treated mesangial cells, and TGF-beta1 transcriptional upregulation requires EGFR/PKC-beta1/Akt signaling. New therapeutic approaches for diabetic nephropathy may result from targeting components of this pathway, particularly the initial EGFR transactivation.

Protein kinase C-beta inhibition attenuates the progression of nephropathy in non-diabetic kidney disease

BACKGROUND

Activation of protein kinase C (PKC) has been implicated in the pathogenesis of diabetic nephropathy where therapy targeting the beta isoform of this enzyme is in advanced clinical development. However, PKC-beta is also increased in various forms of human glomerulonephritis with several potentially nephrotoxic factors, other than high glucose, resulting in PKC-beta activation. Accordingly, we sought to examine the effects of PKC-beta inhibition in a non-diabetic model of progressive kidney disease.

METHODS

Subtotally nephrectomized (STNx) rats were randomly assigned to receive either the selective PKC-beta inhibitor, ruboxistaurin or vehicle. In addition to functional and structural parameters, gene expression of the podocyte slit-pore diaphragm protein, nephrin, was also assessed.

RESULTS

STNx animals developed hypertension, proteinuria and reduced glomerular filtration rate (GFR) in association with marked glomerulosclerosis and tubulointerstitial fibrosis. Glomerular nephrin expression was also reduced. Without affecting blood pressure, ruboxistaurin treatment attenuated the impairment in GFR and reduced the extent of both glomerulosclerosis and tubulointerstitial fibrosis in STNx rats. In contrast, neither proteinuria nor the reduction in nephrin expression was improved by ruboxistaurin.

CONCLUSIONS

These findings indicate firstly that PKC-beta inhibition may provide a new therapeutic strategy in non-diabetic kidney disease and secondly that improvement in GFR is not inextricably linked to reduction in proteinuria.

Aldose reductase genotypes and cardiorenal complications: an 8-year prospective analysis of 1,074 type 2 diabetic patients

OBJECTIVE

We report the independent risk association of type 2 diabetic nephropathy with the z-2 allele of the 5'-(CA)(n) microsatellite and C-106T promoter polymorphisms of the aldose reductase gene (ALR2) using a case-control design. In this expanded cohort, we examined their predictive roles on new onset of cardiorenal complications using a prospective design.

RESEARCH DESIGN AND METHODS

In this 8-year prospective cohort of 1,074 type 2 diabetic patients (59% male, median age 61 years; disease duration 7 years) with an observation period of 8,592 person-years, none had clinical evidence of coronary heart disease (CHD) or chronic kidney disease at recruitment. The renal end point was defined as new onset of estimated glomerular filtration rate <60 ml/min per 1.72 m(2) or hospitalizations with dialysis or death due to renal disease, and CHD was defined as hospitalizations with myocardial infarction, ischemic heart disease, or related deaths.

RESULTS

After controlling for baseline risk factors and use of medications, we found that the ALR2 z-2 allele of (CA)(n) microsatellite carriers had increased risk of renal (hazard ratio 1.53 [95% CI 1.14-2.05], P = 0.005) or combined cardiorenal (1.31 [1.01-1.72], P = 0.047) end points. Carriers of the ALR2 C-106T polymorphism also had increased risk of renal (1.54 [1.15-2.07], P = 0.004) and cardiorenal (1.49 [1.14-1.95], P = 0.004) end points. Compared with noncarriers, patients with two risk-conferring genotypes had a twofold increased risk of renal (2.41 [1.57-3.70], P < 0.001) and cardiorenal (1.94 [1.29-2.91], P = 0.002) end points.

CONCLUSIONS

In Chinese type 2 diabetic patients, genetic polymorphisms of ALR2 independently predicted new onset of renal and cardiorenal end points, with the latter being largely mediated through renal disease.

Glucose in the dialysate: historical perspective and possible implications?

Hemodialysate solutions often contain high concentrations of glucose (up to 200 mg/dL). The historical reasons for the addition of glucose to the dialysate included: (1) aid in performance of ultrafiltration and (2) minimization of nutritional (caloric) losses during dialysis. However, recent experimental evidence supports the fact that exposure to high levels of glucose may be pro-inflammatory. Given the high morbidity and mortality associated with dialysis and its linkage to chronic inflammation, the routine use of glucose in the dialysate may warrant reexamination. This review examines the utility of glucose in the dialysate and discusses the potential implications on chronic inflammation in patients with end-stage renal disease. While there is currently no evidence for a casual relationship between dialysate glucose concentration and the chronic inflammation seen in ESRD, this possibility is explored.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

High glucose increases phosphocofilin via phosphorylation of LIM kinase due to Rho/Rho kinase activation in cultured pig proximal tubular epithelial cells

In proximal tubular epithelial cells (PTECs), depolymerization of actin by cofilin plays a crucial role in maintaining polarity and function. Cofilin is inactivated when phosphorylated by p-Lin-11/Isl-1/Mec-3 kinase (LIMK) to give p-cofilin. LIMK is phosphorylated by phosphorylated p21-activated kinase (PAK), a downstream signal of phosphoinositide 3-kinase (PI3K), or by Rho kinase (ROCK), and is dephosphorylated by slingshot (SSH). However, in PTECs the signaling pathways regulating phosphorylation and dephosphorylation of cofilin, and the influence of high glucose (HG) on these pathways remain to be elucidated. Here, we show that HG in cultured porcine PTECs (LLC-PK1) increases p-cofilin and p-LIMK1 beyond 6h and that the simultaneous presence of phlorizin reverses the increase. HG did not influence the levels of PI3K-p85, downstream signals to SSH1 and p-PAK1, and mRNA of cofilin, LIMK1 and SSH1. On the other hand, wortmannin and LY294002 markedly increased p-cofilin and p-LIMK1 without influencing on the level of SSH1 protein. HG-activated RhoA and ROCK2 beyond 3h, and phlorizin attenuated this activation. GF109203X inhibited HG-induced increase in membranous RhoA and ROCK2, and phorbol ester increased these proteins. Y27632 (a ROCK inhibitor) reversed HG-induced increases of p-cofilin and p-LIMK1. We conclude that HG increases p-cofilin by phosphorylating LIMK1 through activation of Rho/Rho kinase, probably due to diacylglycerol-sensitive PKC activation resulting from increased glucose influx. HG did not alter PI3K or its downstream signals, even though PI3K has a physiological role in maintaining the cofilin level by activating SSH1.

Increased renal gene transcription of protein kinase C-beta in human diabetic nephropathy: relationship to long-term glycaemic control

AIMS/HYPOTHESIS

Activation of protein kinase C (PKC) isoforms has been implicated as a central mediator in the pathogenesis of diabetic nephropathy. Although high glucose levels stimulate catalytic activity of PKC, the effects of high glucose levels on the expression of genes encoding PKC isoforms are unknown. We sought to determine whether in addition to activation, diabetes may lead to increased transcription of two PKC isoforms that have been implicated in the pathogenesis of diabetic nephropathy, PKC-alpha and PKC-beta.

METHODS

Recent advances in molecular biological techniques now permit quantitative analysis of mRNA from archival, formalin-fixed, paraffin-embedded tissue sections. RNA was extracted from scraped 6 microm sections of biopsy tissue, and PRKC-alpha and PRKC-beta (also known as PRKCA and PRKCB) mRNA measured using real-time PCR. Expression of genes encoding PKC isoforms was examined in renal biopsies (n=25) with classical histological features of diabetic nephropathy and compared with that in normal control tissue (n=6). Peptide localisation of PKC-alpha, PKC-beta and the activated forms phosphorylated PKC-alpha and -beta was also performed on matched paraffin-embedded sections of renal biopsies using immunohistochemistry. The effects of high glucose on PRKC-beta expression and peptide production in cultured human proximal tubular epithelial cells were assessed.

RESULTS

Quantitative real-time PCR demonstrated a 9.9-fold increase in PRKC-beta mRNA in kidney biopsies of diabetic patients relative to control (p<0.001). No increase in PRKC-alpha expression was seen. In addition, a correlation between renal PRKC-beta mRNA and HbA(1c) was observed in diabetic patients (r=0.63, p<0.05). There was co-localisation of PKC-beta and phospho-PKC-beta predominantly to proximal tubules. A 60% increase in PRKC-beta mRNA and peptide in cultured human proximal tubular epithelial cells exposed to high glucose (p<0.05) was seen in vitro.

CONCLUSIONS/INTERPRETATION

PKC-beta is upregulated at the gene expression level in human diabetic nephropathy. PRKC-beta mRNA correlates closely with serum HbA(1c), possibly partially explaining the relationship between glycaemic control and progression of diabetic nephropathy. Archival human tissue provides a valuable resource for molecular analyses.

Reactive oxygen species, PKC-beta1, and PKC-zeta mediate high-glucose-induced vascular endothelial growth factor expression in mesangial cells

Vascular endothelial growth factor (VEGF) is implicated in the development of proteinuria in diabetic nephropathy. High ambient glucose present in diabetes stimulates VEGF expression in several cell types, but the molecular mechanisms are incompletely understood. Here primary cultured rat mesangial cells served as a model to investigate the signal transduction pathways involved in high-glucose-induced VEGF expression. Exposure to high glucose (25 mM) significantly increased VEGF mRNA evaluated by real-time PCR by 3 h, VEGF cellular protein content assessed by immunoblotting or immunofluorescence within 24 h, and VEGF secretion by 24 h. High-glucose-induced VEGF expression was blocked by an antioxidant, Tempol, and antisense oligonucleotides directed against p22(phox), a NADPH oxidase subunit. Inhibition of protein kinase C (PKC)-beta(1) with the specific pharmacological inhibitor LY-333531 or inhibition of PKC-zeta with a cell permeable specific pseudosubstrate peptide also prevented enhanced VEGF expression in high glucose. Enhanced VEGF secretion in high glucose was prevented by Tempol, PKC-beta(1), or PKC-zeta inhibition. In normal glucose (5.6 mM), overexpression of p22(phox) or constitutively active PKC-zeta enhanced VEGF expression. Hypoxia inducible factor-1alpha protein was significantly increased in high glucose only by 24 h, suggesting a possible contribution to high-glucose-stimulated VEGF expression at later time points. Thus reactive oxygen species generated by NADPH oxidase, and both PKC-beta(1) and -zeta, play important roles in high-glucose-stimulated VEGF expression and secretion by mesangial cells.

Effects of long-term elevated glucose on collagen formation by mesangial cells

Glomerulosclerosis is one of the complications of diabetes that occurs after many years of uncontrolled hyperglycemia. Mesangial cells (MCs) exposed to high glucose (HG) for short periods have shown that transforming growth factor-beta (TGF-beta) and activated diacylglycerol-dependent protein kinase C (PKC) mediate increased collagen formation. Our study examined collagen formation by MCs exposed to HG for 8 weeks. Exposure to HG in overnight culture resulted in the activation of all PKC isoforms. In contrast, 8-week exposure to HG resulted in the persistent activation of PKC-delta, did not change PKC-alpha or -beta activity, and decreased PKC-epsilon activity while increasing collagen I and IV gene and protein expression. Collagen IV accumulation was reversed by specific PKC-delta inhibition. Collagen IV gene expression was completely normalized by TGF-beta neutralization; however, this was associated with plasminogen activator inhibitor-1 (PAI-1) overexpression and a modest reduction in collagen protein. Our studies suggest that prolonged exposure to HG results in PKC-delta-driven collagen accumulation by MCs mediated by PAI-1 but independent of TGF-beta.

Deletion of Protein Kinase C-ε Signaling Pathway Induces Glomerulosclerosis and Tubulointerstitial FibrosisIn Vivo

Protein kinase C (PKC), a family of 12 distinct serine-threonine kinases, is an important intracellular signaling pathway involved in various cellular functions, such as proliferation, hypertrophy, apoptosis, and adhesion. PKC-epsilon, a novel PKC isoform that is activated in the diabetic kidney, has been demonstrated to have a central role in the underlying signaling infrastructure of myocardial ischemia and hypertrophy. The renal phenotype of PKC-epsilon(-/-) mice was studied with regard to renal hypertrophy and fibrosis. PKC-epsilon(-/-) deficient knockout mice were generated and then killed after 6, 16, and 26 wk of life. Kidney/body weight ratio did not show any significant group difference compared with appropriate wild-type controls. Urinary albumin/creatinine ratio remained normal in wild-type mice, whereas PKC-epsilon(-/-) mice after 6 and 16 wk showed elevated albuminuria. Masson-Goldner staining revealed that tubulointerstitial fibrosis and mesangial expansion were significantly increased in PKC-epsilon(-/-) mice. However, this profibrotic phenotype was not observed in other organs, such as liver and lung. Immunohistochemistry of the kidneys from PKC-epsilon(-/-) mice showed increased renal fibronectin and collagen IV expression that was further aggravated in the streptozotocin-induced diabetic stress model. Furthermore, TGF-beta(1), phospho-Smad2, and phospho-p38 mitogen-activate protein kinase expression was increased in PKC-epsilon(-/-) mice, suggesting a regulatory role of PKC-epsilon in TGF-beta(1) and its signaling pathway in the kidney. These results indicate that deletion of PKC-epsilon mediates renal fibrosis and that TGF-beta1 and its signaling pathway might be involved. Furthermore, these data suggest that activation of PKC-epsilon in the diabetic state may rather represent a protective response to injury than be a mediator of renal injury.

Dysregulated intracellular signaling impairs CTGF-stimulated responses in human mesangial cells exposed to high extracellular glucose

High ambient glucose activates intracellular signaling pathways to induce the expression of extracellular matrix and cytokines such as connective tissue growth factor (CTGF). Cell responses to CTGF in already glucose-stressed cells may act to transform the mesangial cell phenotype leading to the development of glomerulosclerosis. We analyzed cell signaling downstream of CTGF in high glucose-stressed mesangial cells to model signaling in the diabetic milieu. The addition of CTGF to primary human mesangial cells activates cell migration which is associated with a PKC-zeta-GSK3beta signaling axis. In high ambient glucose basal PKC-zeta and GSK3beta phosphorylation levels are selectively increased and CTGF-stimulated PKC-zeta and GSK3beta phosphorylation was impaired. These effects were not induced by osmotic changes. CTGF-driven profibrotic cell signaling as determined by p42/44 MAPK and Akt phosphorylation was unaffected by high glucose. Nonresponsiveness of the PKC-zeta-GSK3beta signaling axis suppressed effective remodeling of the microtubule network necessary to support cell migration. However, interestingly the cells remain plastic: modulation of glucose-induced PKC-beta activity in human mesangial cells reversed some of the pathological effects of glucose damage in these cells. We show that inhibition of PKC-beta with LY379196 and PKC-beta siRNA reduced basal PKC-zeta and GSK3beta phosphorylation in human mesangial cells exposed to high glucose. CTGF stimulation under these conditions again resulted in PKC-zeta phosphorylation and human mesangial cell migration. Regulation of PKC-zeta by PKC-beta in this instance may establish PKC-zeta as a target for constraining the progression of mesangial cell dysfunction in the pathogenesis of diabetic nephropathy.

Deletion of protein kinase C-beta isoform in vivo reduces renal hypertrophy but not albuminuria in the streptozotocin-induced diabetic mouse model

The protein kinase C (PKC)-beta isoform has been implicated to play a pivotal role in the development of diabetic kidney disease. We tested this hypothesis by inducing diabetic nephropathy in PKC-beta-deficient (PKC-beta(-/-)) mice. We studied nondiabetic and streptozotocin-induced diabetic PKC-beta(-/-) mice compared with appropriate 129/SV wild-type mice. After 8 weeks of diabetes, the high-glucose-induced renal and glomerular hypertrophy, as well as the increased expression of extracellular matrix proteins such as collagen and fibronectin, was reduced in PKC-beta(-/-) mice. Furthermore, the high-glucose-induced expression of the profibrotic cytokine transforming growth factor (TGF)-beta1 and connective tissue growth factor were significantly diminished in the diabetic PKC-beta(-/-) mice compared with diabetic wild-type mice, suggesting a role of the PKC-beta isoform in the regulation of renal hypertrophy. Notably, increased urinary albumin-to-creatinine ratio persisted in the diabetic PKC-beta(-/-) mice. The loss of the basement membrane proteoglycan perlecan and the podocyte protein nephrin in the diabetic state was not prevented in the PKC-beta(-/-) mice as previously demonstrated in the nonalbuminuric diabetic PKC-alpha(-/-) mice. In summary, the differential effects of PKC-beta deficiency on diabetes-induced renal hypertrophy and albuminuria suggest that PKC-beta contributes to high-glucose-induced TGF-beta1 expression and renal fibrosis, whereas perlecan, as well as nephrin, expression and albuminuria is regulated by other signaling pathways.

Nephrin loss in experimental diabetic nephropathy is prevented by deletion of protein kinase C alpha signaling in-vivo

Albuminuria in diabetic nephropathy is due to endothelial dysfunction, a loss of negative charges in the basement membrane, and changes a of the slit-membrane diaphragm composition. We have recently shown that protein kinase C alpha (PKCalpha)-deficient mice are protected against the development of albuminuria under diabetic conditions. We here tested the hypothesis that PKCalpha mediates the hyperglycemia-induced downregulation of the slit-diaphragm protein nephrin. After 8 weeks of streptozotocin (STZ)-induced hyperglycemia the expression of glomerular nephrin was significantly reduced. In contrast, other slit-diaphragm proteins such as podocin and CD2AP were unaltered in diabetic state. In PKCalpha-/- mice, hyperglycemia-induced downregulation of nephrin was prevented. Podocin and CD2AP remained unchanged. In addition, the nephrin messenger RNA expression was also reduced in hyperglycemic wild-type mice but remained unaltered in PKCalpha-/- mice. We postulate that the underlying mechanism of the hyperglycemia-induced regulation of various proteins of the glomerular filtration barrier is a PKCalpha-dependent regulation of the Wilms' Tumor Suppressor (WT1) which previously has been shown to act as a direct transcription factor on the nephrin promoter. Our data suggest that PKCalpha activation may be an important intracellular signaling pathway in the regulation of nephrin expression and glomerular albumin permeability in the diabetic state.

Role of the JAK/STAT signaling pathway in diabetic nephropathy

Excessive cellular growth is a major contributor to pathological changes associated with diabetic nephropathy. In particular, high glucose-induced growth of glomerular mesangial cells is a characteristic feature of diabetes-induced renal complications. Glomerular mesangial cells respond to traditional growth factors, although in diabetes this occurs in the context of an environment enriched in both circulating vasoactive mediators and high glucose. For example, the vasoactive peptide ANG II has been implicated in the pathogenesis of diabetic renal disease, and recent findings suggest that high glucose and ANG II activate intracellular signaling processes, including the polyol pathway and generation of reactive oxygen species. These pathways activate the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling cascades in glomerular mesangial cells. Activation of the JAK/STAT signaling cascade can stimulate excessive proliferation and growth of glomerular mesangial cells, contributing to diabetic nephropathy. This review focuses on some of the key elements in the diabetic microenvironment, especially high glucose and the accumulation of advanced glycoxidation end products and considers their impact on ANG II and other vasoactive peptide-mediated signaling events in vitro and in vivo.

Glomerular expression of thrombospondin-1, transforming growth factor beta and connective tissue growth factor at different stages of diabetic nephropathy and their interdependent roles in mesangial response to diabetic stimuli

AIMS/HYPOTHESIS

We quantified the glomerular expression of thrombospondin-1 (THBS1, also known as TSP-1), transforming growth factor beta 1 (TGFB1, also known as TGF-beta1) and connective tissue growth factor (CTGF) at each stage of diabetic nephropathy. We also examined the roles of THBS1 and CTGF in mediating high-glucose- and glycated-albumin-induced synthesis of the matrix protein, fibronectin, by mesangial cells.

METHODS

THBS1, latent and active TGFB1, and CTGF, were detected by immunohistochemistry and in situ hybridisation in biopsies from 19 insulin-dependent diabetic patients with incipient, manifest and advanced diabetic nephropathy, and in 11 control kidneys. Findings were quantified by image analysis. Human mesangial cells were cultured with normal or high glucose, albumin or glycated albumin (Amadori product), +/-THBS1 or CTGF antisense oligonucleotides, or with peptide W, an inhibitor of TGFB1 bioactivation by THBS1. Proteins were measured by western blot analysis or ELISA.

RESULTS

In glomeruli of normal kidneys, mRNA and protein levels for THBS1, latent-TGFB1 and CTGF were low. They were increased in the incipient stage of diabetic nephropathy, predominantly in mesangial areas, with further increases at later stages of the disease. Little or no active TGFB1 immunostaining was detected prior to manifest diabetic nephropathy. In contrast to high-glucose conditions, increases in fibronectin synthesis that were stimulated by glycated albumin were not dependent on THBS1 activation of latent TGFB1. However, increased fibronectin synthesis in both conditions required CTGF.

CONCLUSIONS/INTERPRETATION

Increased glomerular expression of all three factors occurs from the earliest stage of diabetic nephropathy. In contrast to THBS1, CTGF is required for mesangial synthesis of fibronectin stimulated by high glucose or glycated albumin, and is thus a potential therapeutic target.

Mesangial cell-reduced Ca2+signaling in high glucose is due to inactivation of phospholipase C-β3by protein kinase C

In high glucose, glomerular mesangial cells (MCs) demonstrate impaired Ca2+signaling in response to seven-transmembrane receptor stimulation. To identify the mechanism, we first postulated decreased release from intracellular stores. Intracellular Ca2+was measured in fluo-3-loaded primary cultured rat MCs using confocal fluorescence microscopy. In high glucose (HG) 30 mM for 48 h, the 25 nM ionomycin-stimulated intracellular Ca2+response was reduced to 82% of that observed in normal glucose (NG). In NG 5.6 mM, Ca2+responses to endothelin (ET)-1 and platelet-derived growth factor (PDGF) were unchanged in cells cultured in 50 nM Ca2+vs. 1.8 mM Ca2+. Depletion of intracellular Ca2+stores with thapsigargin eliminated ET-1-stimulated Ca2+responses. Incubation in 30 mM glucose (HG) for 48 h or stimulation with phorbol myristate acetate (PMA) for 10 min eliminated the Ca2+response to ET-1 but had no effect on the PDGF response. Downregulation of protein kinase C (PKC) with 24-h PMA or inhibition with Gö6976 in HG normalized the Ca2+response to ET-1. Because ET-1 and PDGF stimulate Ca2+signaling through different phospholipase C pathways, we hypothesized that, in HG, PKC selectively phosphorylates and inhibits PLC-β3. Using confocal immunofluorescence imaging, in NG, a 1.6- to 1.7-fold increase in PLC-β3Ser1105phosphorylation was observed following PMA or ET-1 stimulation for 10 min. In HG, immunofluorescent imaging and immunoblotting showed increased PLC-β3phosphorylation, without change in total PLC-β3, which was reversed with 24-h PMA or Gö6976. We conclude that reduced Ca2+signaling in HG cannot be explained by reduced Ca2+stores but is due to conventional PKC-dependent phosphorylation and inactivation of PLC-β3.

Vascular endothelial growth factor expression and secretion by retinal pigment epithelial cells in high glucose and hypoxia is protein kinase C-dependent

Retinal pigment epithelial (RPE) cells express vascular endothelial growth factor (VEGF) in response to high glucose or hypoxia. We hypothesised that VEGF expression and secretion by RPE cells in high glucose and hypoxia are regulated by protein kinase C (PKC). Primary cultured RPE cells from Sprague-Dawley rats were growth-arrested for 48 hr in 0.5% FBS in 5.6 or 30 mm D-glucose. Cells were exposed to hypoxic conditions (<1% O(2), 5% CO(2)) for the last 15-18 hr of growth-arrest. PKC -alpha, -beta(1), -delta, -epsilon, and -zeta were expressed by RPE cells and exposure to high glucose for 48 hr had no effect on expression as demonstrated by Western immunoblotting. High glucose, hypoxia or VEGF stimulated translocation of a number of the PKC isozymes to the membrane or particulate fractions implying activation. In response to high glucose or acute phorbol myristate acetate (PMA) stimulation, VEGF mRNA analysed by RT-PCR was increased. Intracellular VEGF protein identified by immunoblotting and confocal immunofluorescence imaging was significantly increased by high glucose, hypoxia or acute PMA stimulation. Calphostin C or a specific inhibitor of PKC-zeta prevented high glucose-stimulated VEGF expression in high glucose. VEGF secretion, as measured by ELISA in the culture medium, was enhanced in hypoxia but not in high glucose. Following exposure of RPE cells to PMA for 24 hr, PKC-delta was significantly down regulated, whereas PKC-alpha, -beta, -epsilon and -zeta remained unchanged. Secretion of VEGF in normal or high glucose, or hypoxia was significantly reduced following treatment with PMA for 24 hr but not with the PKC-zeta inhibitor. We conclude that in high glucose and hypoxia PKC isozymes are activated and are necessary for VEGF expression. Secretion of VEGF is enhanced in hypoxia and appears to be regulated by PKC-delta. RPE cells may contribute to the pathogenesis of retinopathy caused by high glucose and hypoxia through the expression and secretion of VEGF that are regulated by PKC isozymes.

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

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

High glucose-induced membrane translocation of PKC betaI is associated with Arf6 in glomerular mesangial cells

Protein kinase C (PKC)-induced changes in glomerular mesangial cell (MC) phenotypic behavior has been implicated in diabetes. The activity of diacylglycerol-sensitive PKC isoforms in MCs is altered by ambient changes in glucose, but the regulation of PKC activity and subsequent intracellular signaling events are not yet clearly defined. Small GTP-binding proteins of the ADP-ribosylation factor (Arfs) family, may regulate protein kinase membrane recruitment and hence its activity in signaling events of non-polarized cells. Members of the ARF family may coordinate membrane dynamics and other cellular functions through their interaction with PKC. We studied the activation of Arf, PKC betaI and phospholipase D (PLD) in MCs cultured under normal or high glucose conditions. MCs cultured in high glucose medium exhibited predominantly cytosolic localization of PKC betaI, Arf3 and Arf6. However, phorbol ester (PMA) stimulation of cells cultured in high glucose significantly enhanced membrane association of PKC betaI and Arf6, but not Arf3. Using [3H]choline chloride to prelabel MCs and measuring [3H]choline-containing metabolite release as PLD activity, PMA stimulated a significant increase of PLD activity under high glucose condition. Our data suggest that Arf6 plays a specific role in activation of PKC betaI and PLD under high glucose condition, and may be a significant intracellular event in the change of the mesangial cell phenotype associated with diabetic nephropathy.

Interaction between the Polyol Pathway and Non-Enzymatic Glycation on Mesangial Cell Gene Expression

BACKGROUND/AIMS

Both activation of the polyol pathway and enhanced non-enzymatic glycation have been implicated in the pathogenesis of diabetic glomerulopathy. We investigated the interaction between these two pathways using normal mesangial cells (MCs) and transgenic (TG) MCs with elevated aldose reductase (AR) activity.

METHODS

TG mice with expression of the human AR (hAR) gene in kidney MCs were established. Mouse glomeruli and primary cultures of MCs from hAR TG and wild-type (WT) mice were studied regarding the changes in AR activity, transforming growth factor-beta1 (TGF-beta1) and type IV collagen mRNA and protein levels, in response to BSA modified by advanced glycation end-products (AGE-BSA).

RESULTS

Ex vivo addition of AGE-BSA increased AR activity, TGF-beta1 and type IV collagen mRNA levels in both WT and TG glomeruli, with greater rise in TG glomeruli. These increments were attenuated by zopolrestat, an AR inhibitor. In cultured MCs, AGE-BSA enhanced AR activity, TGF-beta(1) and type IV collagen mRNA and protein levels both in WT and TG MCs, again with greater increases in TG MCs. The AGE-induced enhancement in TGF-beta1 and type IV collagen expression were suppressed by either zopolrestat or transfection with an AR antisense oligonucleotide.

CONCLUSION

These data suggest that the activation of the polyol pathway by AGEs, more marked in genetic conditions with increased AR activity, may contribute to the pathogenesis of diabetic glomerulopathy, through enhancing mesangial cell expression of TGF-beta1 and type IV collagen.

Attenuation of mouse mesangial cell contractility by high glucose and mannitol: involvement of protein kinase C and focal adhesion kinase

Hyperglycemia and mannitol activate protein kinase C (PKC) and induce mesangial cell hypocontractility that subsequently may modulate renal function. Since focal adhesion kinase (FAK) activation is known to be linked with PKC activity, FAK may also be involved in mesangial cell contraction. To facilitate our understanding of the PKC- and FAK-modulating mechanism, we developed an in vitro model of mouse mesangial cell hypocontractility induced by hyperglycemia or mannitol. Mouse mesangial cells (CRL-1927) were exposed to: normal D-glucose (group N), high D-glucose (group H), and control groups at the same osmolality as H plus L-glucose (group L) and mannitol (group M). Changes in the planar surface area of cells in response to 1 microM phorbol 12-myristate 13-acetate (PMA) were determined. Western blot analyses for PKC, phosphorylated (p)-PKC, tyrosine phosphorylation, FAK, and p-FAK were done on each of these four groups. The effects of mannitol in various doses on cell contraction and activation of PKC and FAK were also assayed. The planar surface areas of groups H and M both showed an attenuated change in response to PMA stimulation. Before PMA stimulation, the baseline PKC expression of groups H and M showed a higher expression of p-PKC alpha and delta than that seen in group N (p < 0.05). Results of tyrosine phosphorylation and immunoprecipitation showed that FAK may be involved in this contraction process. The total amount of FAK showed no significant difference among the four experimental groups; however, p-FAK was found to have significantly increased in group M (p < 0.05). The use of PKC and tyrosine kinase inhibitors reduced PMA-induced mesangial cell contraction in all four groups. Activation of PKC alpha, delta, and FAK with the resultant inhibition of mesangial cell contraction by mannitol was found to be dose-dependent. These results may provide a correlation between increased expression of several PKC isoforms and, in particular, increased phosphorylation levels of PKC alpha and delta and hypocontractility induced by high glucose and mannitol treatment. Furthermore, the mannitol-induced hypocontractility involving PKC and FAK occurred in a dose-dependent manner.

Isoform-specific induction of PKC-epsilon by high glucose protects heart-derived H9c2 cells against hypoxic injury

We investigated which PKC isoforms are involved in high glucose-induced protection against hypoxic injury. Treatment for 48 h with high glucose (22 mM) markedly increased the expression of PKC- epsilon in the particulate fraction (213+/-22.1% of the control) but had no effect on other types of PKC isoforms, suggesting that the high glucose-induced increase in PKC expression is isoform-specific. The mRNA level for PKC- epsilon was also substantially increased, reaching its peak after 4h of high glucose treatment. The high glucose increased PKC-epsilon activity in the particulate fraction up to 183+/-32.2% of the control. During hypoxia, the amount of PKC-epsilon in the particulate fraction was remarkably diminished in the low glucose-treated cells, but remained at a higher level in high glucose-treated cells. The treatment with epsilon V1-2 (10 microM), a specific inhibitor of PKC epsilon, abolished the protective effect of high glucose against hypoxia. These results suggest that isoform-specific induction of PKC-epsilon is involved in high glucose-induced protection against hypoxic injury in heart-derived H9c2 cells.

High glucose-suppressed endothelin-1 Ca2+ signaling via NADPH oxidase and diacylglycerol-sensitive protein kinase C isozymes in mesangial cells

High glucose (HG) is the underlying factor contributing to long term complications of diabetes mellitus. The molecular mechanisms transforming the glomerular mesangial cell phenotype to cause nephropathy including diacylglycerol-sensitive protein kinase C (PKC) are still being defined. Reactive oxygen species (ROS) have been postulated as a unifying mechanism for HG-induced complications. We hypothesized that in HG an interaction between ROS generation, from NADPH oxidase, and PKC suppresses mesangial Ca2+ signaling in response to endothelin-1 (ET-1). In primary rat mesangial cells, growth-arrested (48 h) in 5.6 mM (NG) or 30 mm (HG) glucose, the total cell peak [Ca2+]i response to ET-1 (50 nM) was 630 +/- 102 nM in NG and was reduced to 159 +/- 15 nM in HG, measured by confocal imaging. Inhibition of PKC with phorbol ester down-regulation in HG normalized the ET-1-stimulated [Ca2+]i response to 541 +/- 74 nM. Conversely, an inhibitory peptide specific for PKC-zeta did not alter Ca2+ signaling in HG. Furthermore, overexpression of conventional PKC-beta or novel PKC-delta in NG diminished the [Ca2+]i response to ET-1, reflecting the condition observed in HG. Likewise, catalase or p47phox antisense oligonucleotide normalized the [Ca2+]i response to ET-1 in HG to 521 +/- 58 nM and 514 +/- 48 nM, respectively. Pretreatment with carbonyl cyanide m-chlorophenylhydrazone or rotenone did not restore Ca2+ signaling in HG. Detection of increased intracellular ROS in HG by dichlorofluorescein was inhibited by catalase, diphenyleneiodonium, or p47phox antisense oligonucleotide. HG increased p47phox mRNA by 1.7 +/- 0.1-fold as measured by reverse transcriptase-PCR. In NG, H2O2 increased membrane-enriched PKC-beta and -delta, suggesting activation of these isozymes. HG-enhanced immunoreactivity of PKC-delta visualized by confocal imaging was attenuated by diphenyleneiodium chloride. Thus, mesangial cell [Ca2+]i signaling in response to ET-1 in HG is attenuated through an interaction mechanism between NADPH oxidase ROS production and diacylglycerol-sensitive PKC.

High glucose augments the angiotensin II-induced activation of JAK2 in vascular smooth muscle cells via the polyol pathway

Angiotensin II (Ang II), protein kinase C (PKC), reactive oxygen species (ROS) generated by NADPH oxidase, the activation of Janus kinase 2 (JAK2), and the polyol pathway play important parts in the hyperproliferation of vascular smooth muscle cells (VSMC), a characteristic feature of diabetic macroangiopathy. The precise mechanism, however, remains unclear. This study investigated the relation between the polyol pathway, PKC-beta, ROS, JAK2, and Ang II in the development of diabetic macroangiopathy. VSMC cultured in high glucose (HG; 25 mm) showed significant increases in the tyrosine phosphorylation of JAK2, production of ROS, and proliferation activities when compared with VSMC cultured in normal glucose (5.5 mm (NG)). Both the aldose reductase specific inhibitor (zopolrestat) or transfection with aldose reductase antisense oligonucleotide blocked the phosphorylation of JAK2, the production of ROS, and proliferation of VSMC induced by HG, but it had no effect on the Ang II-induced activation of these parameters in both NG and HG. However, transfection with PKC-beta antisense oligonucleotide, preincubation with a PKC-beta-specific inhibitor (LY379196) or apocynin (NADPH oxidase-specific inhibitor), or electroporation of NADPH oxidase antibodies blocked the Ang II-induced JAK2 phosphorylation, production of ROS, and proliferation of VSMC in both NG and HG. These observations suggest that the polyol pathway hyperactivity induced by HG contributes to the development of diabetic macroangiopathy through a PKC-beta-ROS activation of JAK2.

Increased glucose uptake promotes oxidative stress and PKC-delta activation in adipocytes of obese, insulin-resistant mice

Increased oxidative stress is believed to be one of the mechanisms responsible for hyperglycemia-induced tissue damage and diabetic complications. In these studies, we undertook to characterize glucose uptake and oxidative stress in adipocytes of type 2 diabetic animals and to determine whether these promote the activation of PKC-delta. The adipocytes used were isolated either from C57Bl/6J mice that were raised on a high-fat diet (HF) and developed obesity and insulin resistance or from control animals. Basal glucose uptake significantly increased (8-fold) in HF adipocytes, and this was accompanied with upregulation of GLUT1 expression levels. Insulin-induced glucose uptake was inhibited in HF adipocytes and GLUT4 content reduced by 20% in these adipocytes. Reactive oxygen species (ROS) increased twofold in HF adipocytes compared with control adipocytes and were largely reduced with decreased glucose concentrations. At zero glucose, ROS levels were reduced to the normal levels seen in control adipocytes. The activity of PKC-delta increased twofold in HF adipocytes compared with control adipocytes and was further activated by H2O2. Moreover, PKC-delta activity was inhibited in HF adipocytes either by glucose deprivation or by treatment with the antioxidant N-acetyl-l-cysteine. In summary, we propose that increased glucose intake in HF adipocytes increases oxidative stress, which in turn promotes the activation of PKC-delta. These consequential events may be responsible, at least in part, for development of HF diet-induced insulin resistance in the fat tissue.

Glucose-mediated induction of TGF-beta 1 and MCP-1 in mesothelial cells in vitro is osmolality and polyol pathway dependent

BACKGROUND

Glucose is converted to sorbitol and then to fructose via the polyol pathway that has been implicated in the pathogenesis of organ damage. The contribution of the polyol pathway to mesothelial cell activation has, however, not been fully determined.

METHODS

The effect of increasing glucose concentrations on transforming growth factor-beta 1 (TGF-beta 1) and monocyte chemoattractant protein-1 (MCP-1) secretion by human peritoneal mesothelial cells (HPMC) was examined. The importance of the polyol pathway was identified by its specific inhibition with an aldose reductase inhibitor.

RESULTS

Incubation of HPMC with 5 to 100 mmol/L glucose resulted in an induction of aldose reductase mRNA and intracellular sorbitol accumulation accompanied by the induction of TGF-beta 1 and MCP-1 mRNA expression and protein secretion. Mannitol at the same concentrations also induced aldose reductase, TGF-beta 1 and MCP-1 mRNA and protein expression but at a lower level than glucose. Sorbinil dose-dependently reduced both intracellular sorbitol levels (79.8% reduction of 60 mmol/L D-glucose induced intracellular sorbitol with 100 micromol/L sorbinil (N = 3, P < 0.01) and glucose-induced TGF-beta 1 and MCP-1 secretion. Mannitol induced TGF-beta 1 and MCP-1 secretion was not reduced by sorbinil. The addition of 15 to 40 mmol/L sodium lactate, either alone or in the presence of D-glucose enhanced TGF-beta 1 and MCP-1 secretion, which was inhibited by sorbinil. In contrast, sodium pyruvate appeared to antagonize D-glucose-induced TGF-beta 1 and MCP-1 secretion.

CONCLUSION

These data suggest that the polyol pathway and osmolality contribute to the regulation of HPMC function by glucose. Control of polyol pathway activation might reduce glucose-mediated damage to the peritoneal membrane and promote its long-term survival.

Delayed treatment with lithospermate B attenuates experimental diabetic renal injury

Extracellular matrix (ECM) accumulation in the glomerular mesangium is a characteristic feature of diabetic nephropathy. While transforming growth factor-beta1 (TGF-beta1) is the final mediator of ECM accumulation, reactive oxygen species (ROS) and protein kinase C (PKC) are the upstream signaling molecules that mediate hyperglycemia-induced ECM expansion. Magnesium lithospermate B (LAB) is an active component isolated from Salvia miltiorrhizae with known renoprotective properties due to its antioxidative effects. Thus, the present study examined the effects of LAB on renal injury in streptozotocin-induced diabetic rats (STZR) and on the activation of mesangial cells cultured under high glucose conditions. Ten micrtograms of LAB/kg per day was started 8 wk after streptozotocin injection and continued for a period of 8 wk. It significantly suppressed renal malondialdehyde (MDA), microalbuminuria, glomerular hypertrophy, mesangial expansion, and the upregulation of renal TGF-beta1, fibronectin, and collagen in STZR without significantly affecting plasma glucose. Both 30 mM of glucose and 100 uM of H(2)O(2) significantly increased TGF-beta1 and fibronectin protein secretion by mesangial cells. LAB at 10 micro g/ml inhibited high glucose- and H(2)O(2)-induced TGF-beta1 and fibronectin secretion. LAB also inhibited glucose-induced intracellular ROS generation and PKC activation in mesangial cells, but it did not directly inhibit PKC activity at dosages that inhibited ROS generation. The in vitro data of this study show that LAB inhibits ROS generation leading to PKC activation and TGF-beta1 and fibronectin upregulation in mesangial cells cultured under high glucose conditions. Moreover, delayed treatment with LAB was found to significantly suppress the progression of renal injury in STZR. LAB may become a new therapeutic agent for the treatment of diabetic nephropathy.

Protein kinase C beta inhibition attenuates the progression of experimental diabetic nephropathy in the presence of continued hypertension

In addition to hyperglycemia, hypertension and the renin-angiotensin system have been consistently implicated in the pathogenesis of diabetic nephropathy. Each of these pathogenetic factors may induce changes in cellular function by a common intracellular signaling pathway, the activation of protein kinase C (PKC) beta. The present study thus sought to determine the in vivo effect of PKC beta inhibition in experimental diabetic nephropathy in the setting of continued hyperglycemia, hypertension, and activation of the RAS. Studies were conducted in the (mRen-2)27 rat, a rodent that is transgenic for the entire mouse renin gene (Ren-2) and develops many of the structural, functional, and molecular characteristics of human diabetic nephropathy when experimental diabetes is induced with streptozotocin (STZ). Six-week-old female Ren-2 rats received an injection of STZ or vehicle and were maintained for 6 months. Within 24 h, diabetic rats were further randomized to receive treatment with the specific PKC beta inhibitor, LY333531, admixed in diet (10 mg x kg(-1) x d(-1)) or no treatment (n = 8/group). Diabetic rats developed albuminuria, glomerulosclerosis, and tubulointerstitial fibrosis with a concomitant increase in transforming growth factor-beta (TGF-beta). Western blot analysis demonstrated increased PKC beta in diabetic animals, localized by immunofluorescence to the glomerular mesangium. In vivo inhibition of PKC beta with LY333531 led to a reduction in albuminuria, structural injury, and TGF-beta expression, despite continued hypertension and hyperglycemia.

Effects of high glucose on cytokine-induced nerve growth factor (NGF) expression in rat renal mesangial cells

Nerve growth factor (NGF) accumulates at sites of inflammation and modulates local immune reactions. To characterize the mechanisms of cytokine-induced NGF expression under physiological and pathophysiological conditions, we have used cultured glomerular mesangial cells, which play a key role in glomerular inflammatory diseases such as diabetic nephropathy. To study the effects of high glucose on cytokine-induced NGF expression, rat mesangial cells were treated with the cytokines interleukin-1beta and tumor necrosis factor alpha under normal (1.0 g/L) and high (4.5 g/L) glucose concentrations. In the presence of high glucose concentrations, the cytokines drastically potentiated NGF protein but not mRNA expression when compared to physiological glucose levels. The specific protein kinase C inhibitors Ro31-8220 and CGP41251 suppressed cytokine-induced NGF expression. Moreover, blocking the oxidative activation of the protein kinase C pathway by N-acetylcysteine inhibited glucose effects on NGF synthesis. Neutralizing antibodies against transforming growth factor-beta inhibited cytokine-induced NGF expression under normal glucose concentrations but not under high glucose conditions. Enhanced expression of NGF under high glucose conditions may contribute to kidney diseases such as diabetic nephropathy.

Inhibition of the Jak/STAT signaling pathway prevents the high glucose-induced increase in tgf-beta and fibronectin synthesis in mesangial cells

High glucose (HG) causes glomerular mesangial cell (GMC) growth, production of transforming growth factor (TGF)-beta, and increased synthesis of matrix proteins such as fibronectin, contributing to diabetic nephropathy. We recently found that exposure of cells to HG also activates the growth-promoting enzyme janus kinase 2 (JAK2) and its latent signal transducers and activators of transcription (STAT) transcription factors (STAT1, STAT3, and STAT5). Our purpose was to determine the effect that inhibition of JAK2 and these STAT transcription factors has on the HG-induced increase in TGF-beta and fibronectin synthesis in GMC. Exposure of GMC to 25 mmol/l glucose caused the activation of JAK2, STAT1, STAT3, and STAT5 plus an increase in TGF-beta and fibronectin synthesis, as compared with 5.5 mmol/l glucose. This HG-induced increase in synthesis of TGF-beta and fibronectin was prevented by concomitant incubation with AG-490, a specific JAK2 inhibitor. The HG-induced JAK2, STAT1, and STAT3 tyrosine phosphorylations in GMC were also abolished by AG-490. Preincubation of GMC cultured in 25 mmol/l glucose with a specific JAK2 or STAT1 antisense oligonucleotide also prevented both TGF-beta and fibronectin synthesis. These results provide direct evidence for linkages between JAK2, STAT1, and the glucose-induced overproduction of TGF-beta and fibronectin in GMC.

CTGF expression in mesangial cells: involvement of SMADs, MAP kinase, and PKC

BACKGROUND

The induction of excess matrix in renal fibrosis seems to be mediated, at least in part, by the transforming growth factor-beta (TGF-beta)-mediated induction of connective tissue growth factor (CTGF) in mesangial cells.

METHODS

By examining CTGF protein and mRNA expression and promoter activity in the presence or absence of TGF-beta or inhibitors, the signaling pathways controlling basal and TGF-beta-induced CTGF expression in mesangial cells were investigated.

RESULTS

TGF-beta enhances CTGF mRNA and protein expression in mesangial cells. Mutation of a consensus SMAD binding element in the CTGF promoter completely abolished TGF-beta-induced CTGF expression and reduced basal CTGF expression. The previously identified basal control element-1 (BCE-1) site, but not Sp1 contributes to basal CTGF promoter activity. Ras/MEK/ERK, protein kinase C (PKC) and tyrosine kinase activity also contribute to basal and TGF-beta-induced CTGF promoter activity in cultured mesangial cells.

CONCLUSIONS

The TGF-beta-induction of CTGF in mesangial cells requires SMADs and PKC/ras/MEK/ERK pathways. SMADs are involved in basal CTGF expression, which presumably reflects the fact that mesangial cells express TGF-beta endogenously. TGF-beta also induces CTGF through ras/MEK/ERK. Inhibiting ras/MEK/ERK seems not to reduce phosphorylation (that is, activation) of SMADs, suggesting that SMADs, although necessary, are insufficient for the TGF-beta-stimulation of the CTGF promoter through ras/MEK/ERK. Thus, maximal TGF-beta induction of CTGF requires synergy between SMAD and ras/MEK/ERK signaling.

Activation of PI 3-kinase by the hexosamine biosynthesis pathway

It has been shown that hyperglycaemia-induced defects in glucose transport and insulin action are mediated by increased flux of excess glucose through the hexosamine biosynthesis pathway (HBP). We have previously demonstrated that in rat adipocytes, increased flux through the HBP activates protein kinase C (PKC). The aim of the present study was to explore the mechanism for HBP-mediated activation of PKC. We show that activation of the HBP by either high glucose or glucosamine causes the translocation of PKC-zeta/lambda and PKC-epsilon but not other PKC isoforms tested (alpha, beta, delta). This translocation was inhibited by wortmannin, a PI 3-kinase inhibitor. Both high glucose and glucosamine caused widespread cellular activation of PI 3-kinase. We demonstrate that HBP-mediated activation of PI 3-kinase has an insulin-like effect to translocate GLUT4. We conclude that an acute increase of glucose flux through the HBP activates PI 3-kinase.

Mesangial cell protein kinase C isozyme activation in the diabetic milieu

High-glucose-induced activation of mesangial cell protein kinase C (PKC) contributes significantly to the pathogenesis of diabetic nephropathy. Excess glucose metabolism through the polyol pathway leads to de novo synthesis of both diacylglyerol (DAG) and phosphatidic acid, which may account for increased mesangial cell PKC-alpha, -beta, -delta, -epsilon, and -zeta activation/translocation observed within 48-h exposure to high glucose. Raised intracellular glucose causes generation of reactive oxygen species that may directly activate PKC isozymes and enhance their reactivity to vasoactive peptide signaling. In both diabetic rodent models of diabetes and cultured mesangial cells, PKC-beta appears to be the key isozyme required for the enhanced expression of transforming growth factor-beta(1), initiation of early accumulation of mesangial matrix protein, and increased microalbuminuria. Enhanced collagen IV expression by mesangial cells in response to vasoactive peptide hormone stimulation, e.g., endothelin-1, requires PKC-beta, -delta, -epsilon and -zeta. Loss of mesangial cell contractility to potent vasoactive peptides and coincident F-actin disassembly are due to high-glucose-activation of PKC-zeta. Inhibition of mesangial cell PKC isozyme activation in high glucose may prove to be the next important treatment for diabetic nephropathy.

High glucose down-regulates angiotensin II binding via the PKC-MAPK-cPLA2 signal cascade in renal proximal tubule cells

BACKGROUND

It has been reported that renal renin-angiotensin system contributes to the development of diabetic nephropathy. However, the mechanism of angiotensin II receptor regulation in diabetic condition has not been elucidated.

METHODS

The effects of high glucose on [(3)H]-arachidonic acid (AA) release and angiotensin II (Ang II) binding and its related signal pathway were examined in primary cultured rabbit renal proximal tubule cells (PTCs).

RESULTS

High glucose down-regulated (125)I-Ang II binding from 12 hours and this response was sustained over 48 hours. Thus, the treatment of 25 mmol/L glucose for 48 hours was used for this study. High glucose-induced down-regulation of (125)I-Ang II binding was reversed by the removal of extracellular glucose, suggesting a role for glucose specificity. The high glucose-induced down-regulation of (125)I-Ang II binding was blocked by mepacrine, AACOCF3, phospholipase A2 inhibitors, indomethacin, ibuprofen, and cyclooxygenase inhibitors. Indeed, high glucose significantly increased prostaglandin E2 synthesis. In addition, the high glucose-induced AA release was blocked by PD 98059, a p44/42 mitogen-activated protein kinase (MAPK) inhibitor. PD 98059 also prevented the down-regulation of (125)I-Ang II binding by high glucose, suggesting a role for p44/42 MAPK. Indeed, high glucose significantly increased p44/42 MAPK activity after the 15-minute time point. Protein kinase C (PKC) inhibitor blocked high glucose-induced activation of p44/42 MAPK, increase of the [(3)H]-AA release, and down-regulation of 125I-Ang II binding. W-7 and KN-62 also blocked the high glucose-induced increase of [(3)H]-AA release and down-regulation of (125)I-Ang II binding. However, phospholipase A2 inhibitor did not block high glucose-induced activation of p44/42 MAPK.

CONCLUSION

High glucose down-regulates (125)I-Ang II binding via the PKC-MAPK-cPLA2 signal pathway.

Methylglyoxal enhances cisplatin-induced cytotoxicity by activating protein kinase Cdelta

The cytotoxic side effects of anti-neoplastic drugs are increased in patients with either type 1 or type 2 diabetes mellitus by a mechanism that is not clearly defined. We report that the circulating glucose metabolite, methylglyoxal (MGO), enhances cisplatin-induced apoptosis by activating protein kinase Cdelta (PKCdelta). We found that treatment of myeloma cells with the antioxidant N-acetylcysteine completely blocked cisplatin-dependent intracellular GSH oxidation, reactive oxygen species (ROS) generation, poly(ADP-ribose) polymerase cleavage, and apoptosis. Importantly, co-treatment of cells with the reactive carbonyl MGO and cisplatin increased apoptosis by 90% over the expected additive effect of combined MGO and cisplatin treatment. This same synergism was also observed when ROS generation was examined. MGO and cisplatin increased PKCdelta activity by 4-fold, and this effect was blocked by the PKCdelta inhibitor rottlerin but not by NAC. Furthermore, rottlerin blocked combined MGO and cisplatin-induced ROS generation and apoptosis. Finally, MGO and cisplatin induced c-Abl activation and c-Abl:PKCdelta association. Rottlerin blocked c-Abl activation, but the c-Abl inhibitor STI-571 increased MGO and cisplatin-induced apoptosis by 50%. Taken together these data indicate that MGO synergistically enhances cisplatin-induced apoptosis through activation of PKCdelta and that PKCdelta is critical to both cell death and cell survival pathways. These findings suggest that in the patient with diabetes mellitus heightened oxidative stress can enhance the cytotoxicity of agents that induce DNA damage.

High glucose-enhanced activation of mesangial cell p38 MAPK by ET-1, ANG II, and platelet-derived growth factor

Mitogen-activated protein kinase (MAPK) p38 is activated in response to stress stimuli and growth factors relevant to the pathogenesis of diabetic nephropathy. We postulated that mesangial cells exposed to high glucose and to endothelin-1 (ET-1), angiotensin II (ANG II), and platelet-derived growth factor (PDGF) demonstrate enhanced p38 activity and subsequent activation of the cAMP responsive element binding (CREB) transcription factor. Primary rat mesangial cells exposed to 5.6 (NG) or 30 mM glucose (HG) or NG plus 24.4 mM sorbitol (osmotic control) for < or = 4 days were acutely stimulated with ET-1, ANG II, or PDGF. After 3 days of HG, p38 phosphorylation and kinase activity increased twofold (P < 0.05 vs. NG, n = 5). No change in p38 activity was observed with sorbitol. In HG, activation of p38 by ET-1, ANG II, or PDGF was enhanced compared with NG and was protein kinase C (PKC) independent. In HG, CREB phosphorylation in response to ET-1, ANG II, and PDGF stimulation was enhanced compared with NG and was abolished by p38 inhibition with SB202190. To conclude, in HG, mesangial cell p38 is activated, which in turn stimulates CREB phosphorylation. Furthermore, in HG, mesangial cell p38 responsiveness to ET-1, ANG II, and PDGF and consequent CREB phosphorylation are enhanced through a PKC-independent pathway, which may contribute to the pathogenesis of diabetic nephropathy.