Reactive oxygen species stimulate p44/42 mitogen-activated protein kinase and induce p27(Kip1): role in angiotensin II-mediated hypertrophy of proximal tubular cells

Why is chronic kidney disease progressive? Evolutionary adaptations and maladaptations

Despite significant advances in renal physiology, the global prevalence of chronic kidney disease (CKD) continues to increase. The emergence of multicellular organisms gave rise to increasing complexity of life resulting in trade-offs reflecting ancestral adaptations to changing environments. Three evolutionary traits shape CKD over the lifespan: 1) variation in nephron number at birth, 2) progressive nephron loss with aging, and 3) adaptive kidney growth in response to decreased nephron number. Although providing plasticity in adaptation to changing environments, the cell cycle must function within constraints dictated by available energy. Prioritized allocation of energy available through the placenta can restrict fetal nephrogenesis, a risk factor for CKD. Moreover, nephron loss with aging is a consequence of cell senescence, a pathway accelerated by adaptive nephron hypertrophy that maintains metabolic homeostasis at the expense of increased vulnerability to stressors. Driven by reproductive fitness, natural selection operates in early life but diminishes thereafter, leading to an exponential increase in CKD with aging, a product of antagonistic pleiotropy. A deeper understanding of the evolutionary constraints on the cell cycle may lead to manipulation of the balance between progenitor cell renewal and differentiation, regulation of cell senescence, and modulation of the balance between cell proliferation and hypertrophy. Application of an evolutionary perspective may enhance understanding of adaptation and maladaptation by nephrons in the progression of CKD, leading to new therapeutic advances.

Kidney fibrosis: from mechanisms to therapeutic medicines

Chronic kidney disease (CKD) is estimated to affect 10-14% of global population. Kidney fibrosis, characterized by excessive extracellular matrix deposition leading to scarring, is a hallmark manifestation in different progressive CKD; However, at present no antifibrotic therapies against CKD exist. Kidney fibrosis is identified by tubule atrophy, interstitial chronic inflammation and fibrogenesis, glomerulosclerosis, and vascular rarefaction. Fibrotic niche, where organ fibrosis initiates, is a complex interplay between injured parenchyma (like tubular cells) and multiple non-parenchymal cell lineages (immune and mesenchymal cells) located spatially within scarring areas. Although the mechanisms of kidney fibrosis are complicated due to the kinds of cells involved, with the help of single-cell technology, many key questions have been explored, such as what kind of renal tubules are profibrotic, where myofibroblasts originate, which immune cells are involved, and how cells communicate with each other. In addition, genetics and epigenetics are deeper mechanisms that regulate kidney fibrosis. And the reversible nature of epigenetic changes including DNA methylation, RNA interference, and chromatin remodeling, gives an opportunity to stop or reverse kidney fibrosis by therapeutic strategies. More marketed (e.g., RAS blockage, SGLT2 inhibitors) have been developed to delay CKD progression in recent years. Furthermore, a better understanding of renal fibrosis is also favored to discover biomarkers of fibrotic injury. In the review, we update recent advances in the mechanism of renal fibrosis and summarize novel biomarkers and antifibrotic treatment for CKD.

Effects of caffeic acid phenethyl ester use and inhibition of p42/44 MAP kinase signal pathway on caveolin 1 gene expression and antioxidant system in chronic renal failure model of rats

Effects of Caffeic acid phenethyl ester (CAPE) and/or PD98059 (PD) on the gene expression of Caveolin-1 (CAV1) and reduced glutathione (GSH), malondialdehyde (MDA), copper-zinc superoxide dismutase (CuZn-SOD), and catalase (CAT) enzyme activities were investigated in an experimental chronic renal failure model in rats. Eighty Wistar rats were divided into eight groups for a 28-day study: Control, CsA (Cyclosporine A), CsA-V (CsA solvent), CsA + PD (CsA + PD98059), CsA + PD + CAPE, CsA + CAPE, CAPE-V (CAPE solvent), and PD-V (PD98059 solvent). Serum blood urea nitrogen and creatinine levels, as well as histopathological findings indicated the development of renal failure in the CsA group. Kidney GSH levels decreased while MDA levels, CuZn-SOD, and CAT activities increased significantly in the CsA group compared to control indicating oxidative stress. CAV1 gene expression significantly decreased in the CsA group compared to the control. PD98059 and CAPE applications made positive improvements in the levels of the parameters investigated. PD98059 and CAPE applications in CsA given animals increased GSH and CAV1 gene expressions and decreased CuZn-SOD and CAT levels compared to the CsA group. In conclusion, it was shown that PD98059 and CAPE could attenuate the effects of chronic renal failure, and CAV1 is suggested as a therapeutic target and the inhibition of the p44/42 MAPK pathway may be a new approach for the treatment of renal degenerations.

Depletion of cyclic-GMP levels and inhibition of cGMP-dependent protein kinase activate p21Cip1 /p27Kip1 pathways and lead to renal fibrosis and dysfunction

Cell-cycle regulatory proteins (p21^Cip1 /p27^Kip1 ) inhibit cyclin and cyclin-dependent kinase (CDK) complex that promotes fibrosis and hypertrophy. The present study examined the role of CDK blockers, p21^Cip1 /p27^Kip1 in the progression of renal fibrosis and dysfunction using Npr1 (encoding guanylyl cyclase/natriuretic peptide receptor-A, GC-A/NPRA) gene-knockout (0-copy; Npr1^-/- ), 2-copy (Npr1^+/+ ), and 4-copy (Npr1^++/++ ) mice treated with GC inhibitor, A71915 and cGMP-dependent protein kinase (cGK) inhibitor, (Rp-8-Br-cGMPS). A significant decrease in renal cGMP levels and cGK activity was observed in 0-copy mice and A71915- and Rp-treated 2-copy and 4-copy mice compared with controls. An increased phosphorylation of Erk1/2, p38, p21^Cip1 , and p27^Kip1 occurred in 0-copy and A71915-treated 2-copy and 4-copy mice, while Rp treatment caused minimal changes than controls. Pro-inflammatory (TNF-α, IL-6) and pro-fibrotic (TGF-β1) cytokines were significantly increased in plasma and kidneys of 0-copy and A71915-treated 2-copy mice, but to lesser extent in 4-copy mice. Progressive renal pathologies, including fibrosis, mesangial matrix expansion, and tubular hypertrophy were observed in 0-copy and A71915-treated 2-copy and 4-copy mice, but minimally occurred in Rp-treated mice compared with controls. These results indicate that Npr1 has pivotal roles in inhibiting renal fibrosis and hypertrophy and exerts protective effects involving cGMP/cGK axis by repressing CDK blockers p21^Cip1 and p27^Kip1 .

Circulating bilirubin and defense against kidney disease and cardiovascular mortality: mechanisms contributing to protection in clinical investigations

Unconjugated bilirubin is an endogenous circulating antioxidant, bound to albumin, and therefore is retained in the vascular compartment. Bilirubin has well-documented neurotoxic effects in infants; however, current evidence indicates mildly elevated bilirubin is associated with protection from cardiovascular disease and all-cause mortality in adults. Recent clinical studies show mildly elevated bilirubin is associated with protection from kidney damage and dysfunction, in addition to cardiovascular events and all-cause mortality in patients undergoing hemodialysis. This is the first review to examine the clinical evidence and summarize the potential mechanisms of action that link bilirubin to protection from kidney damage, subsequent kidney failure, and dialysis-related mortality. With this understanding, it is hoped that new therapies will be developed to prevent renal dysfunction and mortality from cardiovascular disease in at-risk individuals.

Angiotensin 1–7 mediates renoprotection against diabetic nephropathy by reducing oxidative stress, inflammation, and lipotoxicity

The renin-angiotensin system, especially angiotensin II (ANG II), plays a key role in the development and progression of diabetic nephropathy. ANG 1–7 has counteracting effects on ANG II and is known to exert beneficial effects on diabetic nephropathy. We studied the mechanism of ANG 1–7-induced beneficial effects on diabetic nephropathy in db/db mice. We administered ANG 1–7 (0.5 mg·kg−1·day−1) or saline to 5-mo-old db/db mice for 28 days via implanted micro-osmotic pumps. ANG 1–7 treatment reduced kidney weight and ameliorated mesangial expansion and increased urinary albumin excretion, characteristic features of diabetic nephropathy, in db/db mice. ANG 1–7 decreased renal fibrosis in db/db mice, which correlated with dephosphorylation of the signal transducer and activator of transcription 3 (STAT3) pathway. ANG 1–7 treatment also suppressed the production of reactive oxygen species via attenuation of NADPH oxidase activity and reduced inflammation in perirenal adipose tissue. Furthermore, ANG 1–7 treatment decreased lipid accumulation in db/db kidneys, accompanied by increased expressions of renal adipose triglyceride lipase (ATGL). Alterations in ATGL expression correlated with increased SIRT1 expression and deacetylation of FOXO1. The upregulation of angiotensin-converting enzyme 2 levels in diabetic nephropathy was normalized by ANG 1–7. ANG 1–7 treatment exerts renoprotective effects on diabetic nephropathy, associated with reduction of oxidative stress, inflammation, fibrosis, and lipotoxicity. ANG 1–7 can represent a promising therapy for diabetic nephropathy.

Renin inhibition reverses renal disease in transgenic mice by shifting the balance between profibrotic and antifibrotic agents

Aliskiren, a direct renin inhibitor, is a novel antihypertensive drug. To study whether aliskiren can reverse chronic kidney disease, we administered it to renin transgenic mice, a strain characterized by elevated blood pressure and a slow decline of renal function, mimicking well the progression of hypertensive chronic kidney disease. Ten-month-old transgenic mice were treated either with aliskiren or placebo for 28 days. Age-matched wild-type mice treated or not with aliskiren were considered as normotensive controls. Aliskiren reduced blood pressure to wild-type levels from as early as day 14. Proteinuria and cardiac hypertrophy and fibrosis were also normalized. Renal interstitial fibrosis and inflammation were significantly ameliorated in aliskiren-treated mice (shown by the decrease of proinflammatory and profibrotic markers), and the phenotypes of tubular epithelial cells and podocytes were restored as evidenced by the reappearance of cellular proteins characteristic of normal phenotype of these cells. Profibrotic p38 and Erk mitogen-activated protein kinases were highly activated in placebo-treated transgenic animals. Aliskiren treatment cancelled this activation. This nephroprotection was not attributed to the antihypertensive activity of aliskiren, because blood pressure normalization after treatment with hydralazine failed to induce the regression of renal fibrosis. Direct inhibition of renin can restore renal function and structure in aged hypertensive animals with existing proteinuria. This finding suggests that, in addition to antihypertensive action, aliskiren can be also used to treat chronic kidney disease.

Reactive oxygen species regulate FSH-induced expression of vascular endothelial growth factor via Nrf2 and HIF1α signaling in human epithelial ovarian cancer

Follicle-stimulating hormone (FSH) and the FSH receptor contribute to tumor angiogenesis and are acknowledged risk factors for ovarian epithelial cancer (OEC). Accumulating evidence suggests that FSH can induce vascular endothelial growth factor (VEGF) and hypoxia inducible factor 1α (HIF1α) expression. We previously demonstrated that FSH induces reactive oxygen species (ROS) production and activates Nrf2 signaling. This study was performed to investigate whether FSH induces VEGF expression via a ROS-mediated Nrf2 signaling pathway. In the current study, OET cells were treated with FSH; dichlorofluorescein staining was used to determine ROS generation, western blotting was used to quantify Nrf2 expression and VEGF expression was measured using an ELISA. Nrf2 and HIF1α were knocked down using siRNAs to investigate the role of the Nrf2 and HIF1α signaling pathways in FSH-induced VEGF expression. The chromatin immunoprecipitation assay (ChIP) was used to determine HIF1α binding to the VEGF promoter. Finally, it was found that FSH induced ROS production and activated Nrf2 signaling; elimination of ROS or knockdown of Nrf2 blocked FSH-induced VEGF expression. Knockdown of Nrf2 impaired HIF1α signaling activation. Blockage of the FSH-ROS-Nrf2-HIF1α signaling pathway attenuated FSH-induced binding of HIF1α to the VEGF promoter. Collectively, this study indicates that ROS and aberrant expression of Nrf2 play an important role in FSH-induced angiogenesis in OEC, and provides insight into the mechanisms of FSH-induced VEGF expression. Elimination of ROS or inhibition of Nrf2 may represent potential therapeutic targets for the treatment of ovarian cancer.

Angiotensins inhibit cell growth in GH3 lactosomatotroph pituitary tumor cell culture: a possible involvement of the p44/42 and p38 MAPK pathways

The local renin-angiotensin system is present in the pituitary. We investigated the effects of angiotensins on GH3 lactosomatotroph cells proliferation in vitro and the involvement of p44/42 and p38 MAPK inhibitors in the growth-regulatory effects of angiotensins. Materials and Methods. Cell viability using the Mosmann method and proliferation by the measurement of BrdU incorporation during DNA synthesis were estimated. Results. Ang II and ang IV decreased the viability and proliferation of GH3 cells. Inhibitor of p44/42 MAPK attenuated the effects of ang II on cell viability and proliferation but did not affect the ang 5-8-dependent actions. Inhibitor of p38 MAPK prevented the decrease in the number of GH3 cells in ang-II- and ang-IV-treated groups. Conclusions. The growth-inhibitory effect of ang II is possibly mediated by the p44/42 MAPK. The p38 MAPK appears to mediate the inhibitory effects of both ang II and ang 5-8 upon cell survival.

Oxidative stress and inflammation: Implications in uremia and hemodialysis

Oxidative response and inflammation constitute a major defense against infections, but if not properly regulated they could also lead to a number of deleterious effects. Patients affected by different stages of acute and chronic kidney disease, particularly patients on hemodialysis, present a marked activation of oxidative and inflammatory processes. This condition exposes these patients to an elevated risk of morbidity and mortality. This Review is up to date and it analyses the newest notions about pathophysiological mechanisms of oxidative stress and inflammation in patients with renal diseases, also considering the different strategies studied to counterbalance this high risk state.

Attenuating effect of angiotensin-(1-7) on angiotensin II-mediated NAD(P)H oxidase activation in type 2 diabetic nephropathy of KK-A(y)/Ta mice

ANG-(1-7) is associated with vasodilation and nitric oxide synthase stimulation. However, the role of ANG-(1-7) in type 2 diabetes mellitus is unknown. In this study, we examined the hypothesis that ANG-(1-7) attenuates ANG II-induced reactive oxygen species stress (ROS)-mediated injury in type 2 diabetic nephropathy of KK-A(y)/Ta mice. KK-A(y)/Ta mice were divided into four groups: 1) a control group; 2) ANG II infusion group; 3) ANG II+ANG-(1-7) coinfusion group; and 4) ANG II+ANG-(1-7)+d-Ala(7)-ANG-(1-7) (A779) coinfusion group. In addition, primary mesangial cells were cultured and then stimulated with 25 mM glucose with or without ANG II, ANG-(1-7), and A779. The ANG II+ANG-(1-7) coinfusion group showed a lower urinary albumin/creatinine ratio increase than the ANG II group. ANG-(1-7) attenuated ANG II-mediated NAD(P)H oxidase activation and ROS production in diabetic glomeruli and mesangial cells. ANG II-induced NF-κB and MAPK signaling activation was also attenuated by ANG-(1-7) in the mesangial cells. These findings were related to improved mesangial expansion and to fibronectin and transforming growth factor-β1 production in response to ANG II and suggest that ANG-(1-7) may attenuate ANG II-stimulated ROS-mediated injury in type 2 diabetic nephropathy. The ACE2-ANG-(1-7)-Mas receptor axis should be investigated as a novel target for treatment of type 2 diabetic nephropathy.

Basal bioenergetic abnormalities in skeletal muscle from ryanodine receptor malignant hyperthermia-susceptible R163C knock-in mice

Malignant hyperthermia (MH) and central core disease in humans have been associated with mutations in the skeletal ryanodine receptor (RyR1). Heterozygous mice expressing the human MH/central core disease RyR1 R163C mutation exhibit MH when exposed to halothane or heat stress. Considering that many MH symptoms resemble those that could ensue from a mitochondrial dysfunction (e.g. metabolic acidosis and hyperthermia) and that MH-susceptible mice or humans have a higher than normal cytoplasmic Ca(2+) concentration at rest, we evaluated the role of mitochondria in skeletal muscle from R163C compared with wild type mice under basal (untriggered) conditions. R163C skeletal muscle exhibited a significant increase in matrix Ca(2+), increased reactive oxygen species production, lower expression of mitochondrial proteins, and higher mtDNA copy number. These changes, in conjunction with lower myoglobin and glycogen contents, Myh4 and GAPDH transcript levels, GAPDH activity, and lower glucose utilization suggested a switch to a compromised bioenergetic state characterized by both low oxidative phosphorylation and glycolysis. The shift in bioenergetic state was accompanied by a dysregulation of Ca(2+)-responsive signaling pathways regulated by calcineurin and ERK1/2. Chronically elevated resting Ca(2+) in R163C skeletal muscle elicited the maintenance of a fast-twitch fiber program and the development of insulin resistance-like phenotype as part of a metabolic adaptation to the R163C RyR1 mutation.

Arachidonic acid metabolites inhibit the stimulatory effect of angiotensin II in renal proximal tubules

Angiotensin II (Ang II) regulates renal proximal transport in a biphasic way via Ang II type 1 receptor (AT1). Whereas extracellular signal-regulated kinase (ERK) activation mediates the stimulatory effect, cytosolic phospholipase A2 (cPLA2) mediates the inhibitory effect independently of ERK. In this study, we tested the hypothesis that the cPLA2/P450 epoxygenase pathway might work to suppress the Ang II-mediated ERK activation. In the presence of arachidonic acid or 5,6-epoxyeicosatrienoic acid (EET), Ang II failed to stimulate the Na-HCO3 cotransporter activity in renal proximal tubules isolated from wild-type, AT1A-deficient, and cPLA2-alpha-deficient mice. In addition, Ang II failed to induce a significant ERK phosphorylation in the presence of arachidonic acid or 5,6-EET. Arachidonic acid or 5,6-EET also suppressed the stimulatory effect of Ang II on net proximal tubule bicarbonate absorption without changing cell Ca2+ concentrations. These results indicate that the cPLA2-alpha/P450/EET pathway blocks the stimulatory effect of Ang II by suppressing the ERK activation. Thus, the cPLA2-alpha/P450/EET pathway may operate as a unique negative feedback mechanism to attenuate excessive Ang II activity in the renal proximal tubules, where extremely high concentrations of Ang II are found.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Benazepril, an angiotensin-converting enzyme inhibitor, alleviates renal injury in spontaneously hypertensive rats by inhibiting advanced glycation end-product-mediated pathways

1. Advanced glycation end-products (AGE) and their receptors (RAGE) have been implicated in renal damage in diabetes. The aim of the present study was to investigate the effects of benazepril, an angiotensin-converting enzyme inhibitor (ACEI), on the formation of AGE, the expression RAGE and other associated components in the oxidative stress pathway in spontaneously hypertensive rats (SHR). 2. Groups of SHR were treated with or without 10 mg/kg per day benazepril for 12 weeks. Systolic blood pressure (SBP) and angiotensin (Ang) II levels were evaluated in SHR and control Wistar-Kyoto (WKY) rats. Renal function was investigated by determining levels of proteinuria and glomerulosclerosis. Furthermore, reactive oxygen species (ROS) in the rat renal cortex were analysed using an H(2)O(2)-based hydroxyl radical-detection assay and the renal content of AGE, RAGE, NADPH oxidase p47phox, nuclear factor (NF)-kappaB p65, phosphorylated (p-) NF-kappaB p65, vascular cell adhesion molecule (VCAM)-1 and transforming growth factor (TGF)-beta1 was determined by immunohistochemistry, quantitative real-time polymerase chain reaction and western blot analysis. 3. Treatment with benazepril inhibited the formation of AngII, reduced SBP and alleviated renal lesions in SHR compared with both untreated SHR and control WKY rats. Benazepril treatment significantly suppressed the accumulation of AGE and expression of RAGE in the kidney of SHR. In addition, benazepril treatment reduced the upregulation of NADPH oxidase p47phox, ROS generation and NF-kappaB p65, p-NF-kappaB p65, VCAM-1 and TGF-beta1 expression in the kidney of SHR compared with both untreated SHR and control WKY rats. 4. The results of the present study provide new insights into the regulation by the renin-angiotensin system of AGE-RAGE, oxidative stress and nephropathy, increasing our understanding of the role of the RAS in nephropathy.

p21(Cip1) expression is increased in ambient oxygen, compared to estimated physiological (5%) levels in rat muscle precursor cell culture

OBJECTIVE

While it is common practice to culture cells in the presence of ambient oxygen (approximately 21% O2), O2 level observed in the physiological environment is often much lower. Previous efforts to culture a variety of different stem cells, including muscle precursor cells (MPC), under O2 conditions that better mimic in vivo conditions have resulted in enhanced proliferation. In the present study, we hypothesized that 20% O2 in culture represents a sufficient stimulus to cause increased expression of two key negative regulators of the cell-cycle Cip/Kip family of cyclin-dependent kinase inhibitors, p21(Cip1) and p27(Kip1), in MPCs.

MATERIALS AND METHODS

MPCs were isolated from Fischer 344 x Brown Norway F(1) hybrid male rats and O2 was adjusted in culture using a tri-gas incubator.

RESULTS

5-Bromo-2'-deoxyuridine incorporation, cell number and nuclear proliferating cell nuclear antigen expression were all decreased after 48 h culture in 20% O2, compared to 5% O2. Twenty per cent O2 had no effect on either p27(Kip1) promoter activity or protein expression. Although p21(Cip1) promoter activity remained unchanged between 5% and 20% O2, there were significant increases in both p21(Cip1) mRNA and protein expression. Furthermore, 20% O2 caused an increase in p21(Cip1) mRNA stability and p53 transcription factor activity.

CONCLUSION

These findings are considered important because they reveal p21(Cip1) as a critical regulatory protein that needs to be considered when interpreting proliferation data from MPCs studied in culture. In addition, O2-dependent regulation of MPC proliferation is relevant to conditions, including sarcopenia, heart failure, cancer and muscular dystrophy, where increased oxidative stress exists.

Deletion of transient receptor potential vanilloid type 1 receptors exaggerates renal damage in deoxycorticosterone acetate-salt hypertension

To determine whether the transient receptor potential vanilloid type 1 (TRPV1) channel provides protection against hypertension-induced renal damage, hypertension was induced by uninephrectomy and by giving deoxycorticosterone acetate (DOCA)-salt in wild-type (WT) and TRPV1-null mutant (TRPV1-/-) mice. Mean arterial pressure, as determined by radiotelemetry, increased significantly and reached the peak 7 days after DOCA-salt treatment in both WT and TRPV1-/- mice. There was no difference in mean arterial pressure between the 2 strains at the baseline or at the peak that lasted for 4 treatment weeks. DOCA-salt treatment in both WT and TRPV1-/- mice led to increased urinary excretion of albumin and 8-isoprostane, glomerulosclerosis, renal cortical tubulointerstitial injury, tubulointerstitial fibrosis, increased number of tubular proliferating cell nuclear antigen-positive cells, and renal monocyte/macrophage infiltration, all of which were much more severe in DOCA-salt-treated TRPV1-/- compared with DOCA-salt-treated WT mice. Renal TRPV1 protein expression, but not the renal anandamide content, was elevated in DOCA-salt-treated WT compared with vehicle-treated WT mice. Renal anandamide levels were markedly elevated in DOCA-salt-treated TRPV1-/- but not in vehicle-treated TRPV1-/- mice. Thus, our data show that ablation of the TRPV1 gene exacerbates renal damage induced by DOCA-salt hypertension, indicating that TRPV1 may constitute a protective mechanism against end-organ damage induced by hypertension.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Angiotensin II-induced reactive oxygen species and the kidney

Angiotensin II (AngII) is an important mediator in renal injury. Accumulating evidence suggests that AngII stimulates intracellular formation of reactive oxygen species (ROS) such as the superoxide anion and hydrogen peroxide. AngII activates several subunits of the membrane-bound multicomponent NAD(P)H oxidase and also increases ROS formation in the mitochondria. Some of these effects may be induced by aldosterone and not directly by AngII. The superoxide anion and hydrogen peroxide influence other downstream signaling pathways, such as transcription factors, tyrosine kinases/phosphatases, ion channels, and mitogen-activated protein kinases. Through these signaling pathways, ROS have distinct functional effects on renal cells. They are transducers of cell growth, apoptosis, and cell migration and affect expression of inflammatory and extracellular matrix genes. For example, AngII-mediated expression of p27(Kip1), a cell-cycle regulatory protein, and induction of tubular hypertrophy depend on the generation of ROS. The effects of ROS generated within different renal cells ultimately depend on the locally generated concentrations and the balance of pro- and antioxidant pathways. Although the concept that AngII mediates oxidative stress in the kidney has been validated in experimental models, the exact role is still incompletely understood in human renal diseases.

Diabetic nephropathy and proximal tubule ROS: challenging our glomerulocentricity

Diabetic nephropathy is currently viewed as a predominantly glomerular process with glomerular injury driving secondary tubular loss. Brezniceanu and colleagues apply transgenic methods to support a prominent role for reactive oxygen species as mediators and for the proximal tubule as a major site of early disease activity in diabetes. Results support evidence for early tubular apoptosis and atrophy in human diabetic nephropathy.

Differential role of reactive oxygen species in the activation of mitogen-activated protein kinases and Akt by key receptors on B-lymphocytes: CD40, the B cell antigen receptor, and CXCR4

BACKGROUND

Antibodies produced by B-lymphocytes play a key role in the host defense against infection. The development, survival, and activation of B cell is regulated by multiple receptors including the B cell antigen receptor (BCR), which detects the presence of pathogens, CD40, which binds co-stimulatory molecules on activated T cells, and chemokines such as SDF-1 (CXCL12) that play key roles in B cell development and trafficking. Signaling by many receptors results in the generation of reactive oxygen species (ROS) that function as second messengers by regulating the activity of redox-sensitive kinases and phosphatases. We investigated the role of ROS in signaling by the BCR, CD40, and CXCR4, the receptor for SDF-1. We focused on activation of ERK, JNK, p38, and Akt, kinases that regulate multiple processes including cell survival, proliferation, and migration.

RESULTS

Using the anti-oxidants N-acetyl L-cysteine (NAC) and ebselen to deplete intracellular ROS, we identified a differential requirement for ROS in the activation of ERK, JNK, p38, and Akt by these receptors. We found that CD40 activated JNK, p38, and Akt via redox-dependent pathways that were sensitive to ROS depletion by NAC and ebselen. In contrast, BCR-induced activation of ERK, JNK, p38, and Akt was not affected by ROS depletion. We also found that CXCR4-induced Akt activation was ROS-dependent even though activation of the ERK, JNK, and p38 MAP kinases by CXCR4 occurred via ROS-independent pathways.

CONCLUSION

The differential requirement for ROS in the activation of ERK, JNK, p38, and Akt by the BCR, CD40, and CXCR4 likely reflects the multiplicity of upstream activators for each of these kinases, only some of which may be regulated in a redox-dependent manner. These findings support the idea that ROS are important second messengers in B cells and suggest that oxidants or anti-oxidants could be used to modulate B cell activation.

Catalase overexpression attenuates angiotensinogen expression and apoptosis in diabetic mice

Increased generation of reactive oxygen species (ROS) leads to oxidative stress in diabetes. Catalase is a highly conserved heme-containing protein that reduces hydrogen peroxide to water and oxygen and is an important factor decreasing cellular injury owing to oxidative stress. Hyperglycemic conditions increase oxidative stress and angiotensinogen gene expression. Angiotensinogen conversion to angiotensin II leads to a furtherance in oxidative stress through increased generation of reactive oxygen species. In this study, we utilized mice transgenically overexpressing rat catalase in a kidney-specific manner to determine the impact on ROS, angiotensinogen and apoptotic gene expression in proximal tubule cells of diabetic animals. Proximal tubules isolated from wild-type and transgenic animals without or with streptozotocin-induced diabetes were incubated in low glucose media in the absence or presence of angiotensin II or in a high-glucose media. Our results show that the overexpression of catalase prevents the stimulation of ROS and angiotensinogen mRNA in tubules owing to elevated glucose or angiotensin II in vitro. Additionally, overexpression of catalase attenuated ROS generation, angiotensinogen and proapoptotic gene expression and apoptosis in the kidneys of diabetic mice in vivo. Our studies point to an important role of ROS in the pathophysiology of diabetic nephropathy.

Oxidative stress contributes to accelerated development of the senescent phenotype in human peritoneal mesothelial cells exposed to high glucose

Increasing evidence indicates that cells exposed to high glucose exhibit shortened proliferative lifespan and enter the state of senescence earlier. However, the contribution of hyperglycemia-induced oxidative stress to premature cell senescence is not entirely clear. In the current study we have examined the role of oxidative stress in cellular senescence of human peritoneal mesothelial cells (HPMC) exposed to high glucose. The experiments were performed on primary omental-derived HPMC grown into senescence in the presence of normal (5 mM) and high (30 mM) glucose. Senescence of HPMC was associated with increased generation of reactive oxygen species (ROS) and decreased cellular glutathione (GSH). Exposure to high glucose significantly exacerbated these effects and increased the level of senescence-associated beta-galactosidase (SA-beta-Gal) and 8-hydroxy-2'-deoxyguanosine (8-OH-dG) expression. Furthermore, high glucose markedly increased senescence-related HPMC hypertrophy. The addition of L-2-oxothiazolidine-4-carboxylic acid, a GSH precursor, restored partially GSH levels and decreased ROS release. This effect was associated with reduced levels of SA-beta-Gal and 8-OH-dG, diminished TGF-beta1 and fibronectin release, and less pronounced hypertrophy of aged HPMC. These results indicate that the accelerated senescence response in HPMC exposed to high glucose is strongly related to oxidative stress.

Shigatoxin-induced endothelin-1 expression in cultured podocytes autocrinally mediates actin remodeling

Shigatoxin (Stx) is the offending agent of post-diarrheal hemolytic uremic syndrome, characterized by glomerular ischemic changes preceding microvascular thrombosis. Because podocytes are highly sensitive to Stx cytotoxicity and represent a source of vasoactive molecules, we studied whether Stx-2 modulated the production of endothelin-1 (ET-1), taken as candidate mediator of podocyte dysfunction. Stx-2 enhanced ET-1 mRNA and protein expression via activation of nuclear factor kappaB (NF-kappaB) and activator protein-1 (Ap-1) to the extent that transfection with the dominant-negative mutant of IkappaB-kinase 2 or with Ap-1 decoy oligodeoxynucleotides reduced ET-1 mRNA levels. We propose a role for p38 and p42/44 mitogen-activated protein kinases (MAPKs) in mediating NF-kappaB-dependent gene transcription induced by Stx-2, based on data that Stx-2 phosphorylated p38 and p42/44 MAPKs and that MAPK inhibitors reduced transcription of NF-kappaB promoter/luciferase reporter gene construct induced by Stx-2. Stx-2 caused F-actin redistribution and intercellular gaps via production of ET-1 acting on ETA receptor, because cytoskeleton changes were prevented by ETA receptor blockade. Exogenous ET-1 induced cytoskeleton rearrangement and intercellular gaps via phosphatidylinositol-3 kinase and Rho-kinase pathway and increased protein permeability across the podocyte monolayer. These data suggest that the podocyte is a target of Stx, a novel stimulus for the synthesis of ET-1, which may control cytoskeleton remodeling and glomerular permeability in an autocrine fashion.

Advanced Oxidation Protein Products Accelerate Renal Fibrosis in a Remnant Kidney Model

Accumulation of plasma advanced oxidation protein products (AOPP) has been found in patients with chronic kidney disease. However, the biologic consequences of AOPP consumption on progression of renal disease still are unclear. For testing of the hypothesis that AOPP accelerate progression of chronic kidney disease, Sprague-Dawley rats were subjected to five-sixths nephrectomy (5/6 Nx) or to sham operation. Rats in each group were randomly assigned in three subgroups (n = 30 in each group) and treated with repeated intravenous injections of AOPP-modified rat serum albumin (RSA), unmodified RSA, or vehicle for indicated period. Compared with RSA- or vehicle-treated 5/6 Nx rats, AOPP RSA-treated 5/6 Nx rats displayed greater proteinuria, higher serum creatinine, and lower creatinine clearance. AOPP challenge resulted in more renal hypertrophy, higher macrophage influx, and greater renal fibrosis in the remnant kidney. Chronic administration of AOPP in sham-operated rats increased urinary protein excretion and renal macrophage infiltration, but histologic renal fibrosis was not observed during the study period. AOPP treatment enhanced AOPP level in renal tissue. This was associated with marked increase of thiobarbituric acid reactive substances, decrease of glutathione peroxidase activity, and upregulated expression of monocyte chemoattractant protein-1 and TGF-beta1 in renal cortex. These data indicate that AOPP might be a new and potentially important mediator of renal fibrosis in the remnant kidney. Chronic accumulation of AOPP promotes renal fibrosis probably via a redox-sensitive inflammatory pathway.

Oxidative signaling in renal epithelium: Critical role of cytosolic phospholipase A2 and p38(SAPK)

Previous studies from this laboratory have demonstrated a critical role of cytosolic phospholipase A2 (cPLA2) and arachidonic acid in angiotensin II (Ang II) AT2 receptor-mediated signal transduction in renal epithelium. In primary proximal tubular epithelial cells exposed to hydrogen peroxide (H2O2), both the selective cPLA2 inhibitors and the cPLA2 antisense oligonucleotides significantly attenuated H2O2-induced arachidonic acid liberation and activation of p38(SAPK), ERK1/2, and Akt1. This H2O2-induced kinase activation was significantly attenuated by a Src kinase inhibitor PP2, or by transient transfection of carboxyl-terminal Src kinase (CSK) that maintained Src in the dormant form. Under basal conditions, Src coimmunoprecipitated with epidermal growth factor receptor (EGFR), while H2O2 increased EGFR phosphorylation in the complex. We observed that inhibition of EGFR kinase activity with AG1478 significantly attenuated H2O2-induced p38(SAPK) and ERK1/2 activation, but did not inhibit Akt1 activation. Furthermore, it seems that p38(SAPK) is upstream of ERK1/2 and Akt1, since a p38(SAPK) inhibitor SB203580 significantly blocked H2O2-induced activation of ERK1/2 and Akt1. Interestingly, overexpression of the dominant-negative p38(SAPK) isoform alpha inhibited ERK1/2 but not Akt1 activation. Our observations demonstrate that in these nontransformed cells, activation of cPLA2 is a converging point for oxidative stress and Ang II, which share common downstream signaling mechanisms including Src and EGFR. In addition, p38(SAPK) provides a positive input to both growth and antiapoptotic signaling pathways induced by acute oxidative stress.

Ras modulation of superoxide activates ERK-dependent fibronectin expression in diabetes-induced renal injuries

Although previous studies have demonstrated that diabetic nephropathy is attributable to early extracellular matrix accumulation in glomerular mesangial cells, the molecular mechanism by which high glucose induces matrix protein deposition remains not fully elucidated. Rat mesangial cells pretreated with or without inhibitors were cultured in high-glucose or advanced glycation end product (AGE) conditions. Streptozotocin-induced diabetic rats were given superoxide dismutase (SOD)-conjugated propylene glycol to scavenge superoxide. Transforming growth factor (TGF)-beta1, fibronectin expression, Ras, ERK, p38, and c-Jun activation of glomerular mesangial cells or urinary albumin secretion were assessed. Superoxide, not nitric oxide or hydrogen peroxide, mediated high glucose- and AGE-induced TGF-beta1 and fibronectin expression. Pretreatment with diphenyliodonium, not allopurinol or rotenone, reduced high-glucose and AGE augmentation of superoxide synthesis and fibronection expression. High glucose and AGEs rapidly enhanced Ras activation and progressively increased cytosolic ERK and nuclear c-Jun activation. Inhibiting Ras by manumycin A reduced the stimulatory effects of high glucose and AGEs on superoxide and fibronectin expression. SOD or PD98059 pretreatment reduced high-glucose and AGE promotion of ERK and c-Jun activation. Exogenous SOD treatment in diabetic rats significantly attenuated diabetes induction of superoxide, urinary albumin excretion, 8-hydroxy-2'-deoxyguanosine, TGF-beta1, and fibronectin immunoreactivities in renal glomerular mesangial cells. Ras induction of superoxide activated ERK-dependent fibrosis-stimulatory factor and extracellular matrix gene transcription of mesangial cells. Reduction of oxidative stress by scavenging superoxide may provide an alternative strategy for controlling diabetes-induced early renal injury.

Morphine modulates monocyte-macrophage conversion phase

Monocyte migration and their activation into the macrophage phenotype play a role in the modulation of tissue injury. We studied the effect of morphine on the monocyte-macrophage conversion phase (MMCP). Phorbol 12-myristate 13-acetate (PMA) activated THP-1 cells and promoted their adhesion to the substrate. Morphine inhibited PMA-induced MMCP. However, opiate receptor antagonists attenuated this effect of morphine. Interestingly, PMA as well as morphine-stimulated superoxide production by monocytes. Superoxide dismutase (SOD) not only inhibited PMA-mediated MMCP but also attenuated the inhibitory effect of morphine. PMA not only enhanced adhesion of monocytes to a filter but also promoted their migration. These findings suggest that the PMA-induced macrophage phenotype conversion may be accelerating their migration; whereas, morphine may be preventing the migration of monocytes by inhibiting MMCP.

Angiotensin-(1-7) inhibits angiotensin II-stimulated phosphorylation of MAP kinases in proximal tubular cells

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE, which is not blocked by ACE inhibitors. High amounts of ACE2 are present in the proximal tubule, and ACE2 catalyzes generation of angiotensin 1-7 (Ang-(1-7)) by this segment. Ang-(1-7) binds to a receptor distinct from the AT1 or AT2 Ang II receptor, identified as the mas receptor. We studied the effects of Ang-(1-7) on Ang II-mediated cell signaling pathways in proximal tubule. In primary cultures of rat proximal tubular cells, activation of mitogen-activated protein kinases (MAPK) was detected by immunoblotting, in the presence or absence of agonists/antagonists. Transforming growth factor-beta1 (TGF-beta1) was measured by enzyme-linked immunosorbent assay. Ang II (5 min, 10(-7) M) stimulated phosphorylation of the three MAPK (p38, extracellular signal-related kinase (ERK 1/2), and c-Jun N-terminal kinase (JNK)). While incubation of proximal tubular cells with Ang-(1-7) alone did not significantly affect MAPK phosphorylation, Ang-(1-7) (10(-7) M) completely inhibited Ang II-stimulated phosphorylation of p38, ERK 1/2, and JNK. This inhibitory effect was reversed by the Ang-(1-7) receptor antagonist, D-Ala7-Ang-(1-7). Ang II significantly increased production of TGF-beta1 in proximal tubular cells, an effect that was partly inhibited by Ang-(1-7). Ang-(1-7) had no significant effect on cyclic 3',5'-adenosine monophosphate production in these cells. In summary, Ang-(1-7) inhibits Ang II-stimulated MAPK phosphorylation in proximal tubular cells. Generation of Ang-(1-7) by proximal tubular ACE2 could thereby serve a protective role by counteracting the effects of locally generated Ang II.

Inhibition of platelet-derived growth factor-induced mesangial cell proliferation by cyclooxygenase-2 overexpression is abolished through reactive oxygen species

Proliferation of mesangial cells (MC) is an early event in many forms of glomerulonephritis. We have previously shown that platelet-derived growth factor (PDGF)-induced proliferation of MC was inhibited by the overexpression of cyclooxygenase-2 (COX-2). Since reactive oxygen species (ROS) are important mediators of mitogenic signaling, we evaluated the role of ROS in the COX-2 induced growth arrest in MC. We demonstrate that ROS are reduced in COX-2 overexpressing MC. Intracellular elevation of ROS restored PDGF-induced proliferation, while the expression of the cyclin-dependent kinase inhibitors p21(cip1) and p27(kip1) were decreased in these cells. The data suggest that COX-2 decreases ROS formation which consequently leads to the PDGF-induced inhibition of MC proliferation.

Role of reactive oxygen species in angiotensin II-mediated renal growth, differentiation, and apoptosis

Angiotensin II (ANG II) induces cell-cycle arrest of cultured proximal tubular cells, resulting in cellular hypertrophy. This ANG II-mediated hypertrophy is associated with the induction of p27(Kip1), an inhibitor of G1 phase cyclin-dependent kinase cyclin complexes. We have recently demonstrated that ANG II-mediated expression of p27(Kip1) and induction of cellular hypertrophy depend on the generation of reactive oxygen species (ROS). The effects of ROS are mediated by stimulation of mitogen-activated protein (MAP) kinases. p44/42 MAP kinase directly phosphorylates p27(Kip1) at serine-threonine residues and increases thereby its half-life time. AT2-receptor activation has been implicated in apoptosis and/or cell differentiation. Recent studies, however, revealed a more indirect role of hypoxia in the antiproliferative effects of ANG II transduced through AT2 receptors. We found that SM-20 is down-regulated in ANG II-stimulated PC12 cells that express only AT2 receptors. It turned out that SM20 is the rat homologue of a dioxygenase that regulates hypoxia-inducible factor 1 (HIF-1). ANG II induces HIF-1alpha by a posttranscriptional mechanism suggesting that SM20 down-regulation leads to stabilization of HIF-1. Thus, ANG II-induced ROS generation plays a pivotal role in several pathophysiological situations, leading to renal growth regulation and remodeling after injury.

N-Acetylcysteine Decreases Angiotensin II Receptor Binding in Vascular Smooth Muscle Cells

Antioxidants seem to inhibit angiotensin II (Ang II) actions by consuming stimulated reactive oxygen species. An alternative hypothesis was investigated: Antioxidants that are also strong reducers of disulfide bonds inhibit the binding of Ang II to its surface receptors with consequent attenuation of signal transduction and cell action. Incubation of cultured vascular smooth muscle cells, which possess Ang II type 1a receptors, with the reducing agent n-acetylcysteine (NAC) for 1 h at 37 degrees C resulted in decreased Ang II radioligand binding in a concentration-dependent pattern. NAC removal restored Ang II binding within 30 min. Incubation with n-acetylserine, a nonreducing analogue of NAC, did not lower Ang II binding, and oxidized NAC was less effective than reduced NAC in lowering Ang II binding. NAC did not decrease Ang II type 1a receptor protein content. Other antioxidants regulated Ang II receptors differently: alpha-Lipoic acid lowered Ang II binding after 24 h, and vitamin E did not lower Ang II binding at all. NAC inhibited Ang II binding in cell membranes at 21 or 37 but not 4 degrees C. Dihydrolipoic acid (the reduced form of alpha-lipoic acid), which contains free sulfhydryl groups as NAC does, decreased Ang II receptor binding in cell membranes, whereas alpha-lipoic acid, which does not contain free sulfhydryl groups, did not. Ang II-stimulated inositol phosphate formation was decreased by preincubation with NAC (1 h) or alpha-lipoic acid (24 h) but not vitamin E. In conclusion, certain antioxidants that are reducing agents lower Ang II receptor binding, and Ang II-stimulated signal transduction is decreased in proportion to decreased receptor binding.

Systemic infusion of angiotensin II exacerbates liver fibrosis in bile duct-ligated rats

Recent evidence indicates that the renin-angiotensin system (RAS) plays a major role in liver fibrosis. Here, we investigate whether the circulatory RAS, which is frequently activated in patients with chronic liver disease, contributes to fibrosis progression. To test this hypothesis, we increased circulatory angiotensin II (Ang II) levels in rats undergoing biliary fibrosis. Saline or Ang II (25 ng/kg/h) were infused into bile duct-ligated rats for 2 weeks through a subcutaneous pump. Ang II infusion increased serum levels of Ang II and augmented bile duct ligation-induced liver injury, as assessed by elevated liver serum enzymes. Moreover, it increased the hepatic concentration of inflammatory proteins (tumor necrosis factor alpha and interleukin 1beta) and the infiltration of CD43-positive inflammatory cells. Ang II infusion also favored the development of vascular thrombosis and increased the procoagulant activity of tissue factor in the liver. Livers from bile duct-ligated rats infused with Ang II showed increased transforming growth factor beta1 content, collagen deposition, accumulation of smooth muscle alpha-actin-positive cells, and lipid peroxidation products. Moreover, Ang II infusion stimulated phosphorylation of c-Jun and p42/44 mitogen-activated protein kinase and increased proliferation of bile duct cells. In cultured rat hepatic stellate cells (HSCs), Ang II (10(-8) mol/L) increased intracellular calcium and stimulated reactive oxygen species formation, cellular proliferation and secretion of proinflammatory cytokines. Moreover, Ang II stimulated the procoagulant activity of HSCs, a newly described biological function for these cells. In conclusion, increased systemic Ang II augments hepatic fibrosis and promotes inflammation, oxidative stress, and thrombogenic events.

Induction of metallothionein in proximal tubular cells by zinc and its potential as an endogenous antioxidant

BACKGROUND

This study was undertaken to gain further insights into the expression of metallothionein (MT) in kidney, to define the necessary dosage of a metal (zinc) to achieve induction of MT and to evaluate the antioxidative potential of MT in comparison to other more common antioxidative therapeutics, like N-acetyl-L-cysteine (NAC), and endogenous molecules, like glutathione.

METHODS

MT was measured in renal specimens from cadaver kidneys from patients with chronic diseases (n = 76) and controls (n = 21) by immunohistochemistry. In addition, induction experiments were performed in cell cultures of proximal tubular cells (LCC-PK1) and MT measured on the RNA and protein level (immunohistochemistry, Western and dot blotting). Antioxidative potential of MT was compared to NAC and glutathione.

RESULTS

MT was restricted to tubular cells with no differences between controls and patients. Zn caused a dose-dependent increase of MT on the RNA as well as on the protein level (RNA (ratio MT/histone 3.3): control 0.34 +/- 0.12; Zn 17 microM 0.65 +/- 0.26; Zn 35 microM 1.25 +/- 0.43 (p < 0.05), Zn 52 microM 1.35 +/- 0.46 (p < 0.05), and protein: 5.8-fold increase from 47 +/- 13 mg/g total protein (n = 6) to 272 +/- 140 mg/g total protein (n = 6)). The antioxidative effect of MT was equal to NAC and glutathione.

CONCLUSIONS

Induction of renal MT by zinc is easily achievable and might be an interesting therapeutic and preventive tool against oxidative stress.

Macroarray analysis reveals a strain-induced oxidant response in pulmonary epithelial cells

Mechanical strain initiates a variety of responses in pulmonary epithelial cells. The signaling pathways and molecular alterations leading to these responses remain unclear To identify novel signal transduction pathways activated by strain, macroarray analysis was performed on strained pulmonary epithelial cells. Glutathione S-transferase (GST) pi, GST mu, and heat shock protein (HSP)-27 were increased by strain. Western blotting verified that increases in cDNA of these redox-related molecules resulted in an increase in protein. Phosphorylation of HSP-27 was increased after strain, further supporting the role of HSP-27 in strain-induced signal transduction. Strain-induced oxidative stress was verified with the oxidant-sensitive dye dichlorodihydrofluorescein diacetate.

Angiotensin II receptor blocker inhibits p27Kip1 expression in glucose-stimulated podocytes and in diabetic glomeruli

BACKGROUND

Diabetic nephropathy is characterized by glomerular and tubular hypertrophy, and angiotensin II receptor blockers (ARBs) are known to prevent renal hypertrophy in diabetic patients.

METHODS

To determine the effect of ARB on podocyte p27(Kip1) mRNA and protein expression, podocytes were exposed to 5.6 mmol/L normal glucose or 25 mmol/L high glucose with or without ARB, 10(-7) mol/L L-158,809. For animal studies, streptozotocin-induced diabetic rats were left untreated or were treated with 1 mg/kg/day L-158,809 for 3 months (diabetes mellitus + ARB). Competitive reverse transcription-polymerase chain reaction (RT-PCR), Western blot, immunohistochemistry, and morphometric analyses were performed.

RESULTS

p27(Kip1) mRNA and protein expression in podocytes exposed to high glucose and in 3-month diabetic glomeruli were significantly increased (P < 0.01). High glucose significantly increased angiotensin II levels both in cell lysates and in media compared with normal glucose (P < 0.05) and exogenous angiotensin II also increased p27(Kip1) mRNA and protein expression in podocytes. L-158,809 treatment in podocytes inhibited the increase in p27(Kip1) mRNA expression by 84%, and protein expression by 89% (P < 0.05). p27(Kip1) mRNA and protein expression in diabetic + ARB glomeruli were also significantly reduced by 78% and 85%, respectively, compared with diabetic glomeruli (P < 0.01). ARB treatment also significantly ameliorated increased glomerular p27(Kip1) expression in diabetes mellitus as assessed by immunohistochemistry (P < 0.01). The increase in glomerular volume in diabetes mellitus was also inhibited by 81% with ARB treatment (P < 0.05).

CONCLUSION

p27(Kip1) mRNA and protein expression were increased in diabetic glomeruli as well as in high glucose-stimulated podocytes, and this increment in p27(Kip1) expression was ameliorated by ARB treatment. These findings indicate that ARB treatment has an additional effect on preventing renal hypertrophy in diabetes mellitus.

Oxidative stress in hypertension and chronic kidney disease: role of angiotensin II

Angiotensin II, via the type 1 (AT1) receptor, stimulates oxidative stress. The vasculature, interstitium, juxtaglomerular apparatus, and the distal nephron in the kidney express nicotinamide adenine dinucleotide phosphate (NADPH) oxidase that generates superoxide anion, which is an important component of angiotensin II-induced oxidative stress. The angiotensinogen gene is stimulated by NF-kappaB activation, which is sensitive to the redox ratio, providing a positive feedback loop that can upregulate angiotensin II production. Oxidative stress can accompany hypertension in many models, including the spontaneously hypertensive rat (SHR), angiotensin II-infused rats, renovascular hypertension, and the deoxycorticosterone acetate (DOCA) salt model of hypertension. AT1 receptor antagonists can abrogate the effects of angiotensin II on oxidative stress, thus providing an important mechanistic insight onto the renal protective effects of these agents in conditions associated with angiotensin II excess.

Oxidative stress activates FUS1 and RLM1 transcription in the yeast Saccharomyces cerevisiae in an oxidant-dependent Manner

Mating in haploid Saccharomyces cerevisiae occurs after activation of the pheromone response pathway. Biochemical components of this pathway are involved in other yeast signal transduction networks. To understand more about the coordination between signaling pathways, we used a "chemical genetic" approach, searching for compounds that would activate the pheromone-responsive gene FUS1 and RLM1, a reporter for the cell integrity pathway. We found that catecholamines (l-3,4-hydroxyphenylalanine [l-dopa], dopamine, adrenaline, and noradrenaline) elevate FUS1 and RLM1 transcription. N-Acetyl-cysteine, a powerful antioxidant in yeast, completely reversed this effect, suggesting that FUS1 and RLM1 activation in response to catecholamines is a result of oxidative stress. The oxidant hydrogen peroxide also was found to activate transcription of an RLM1 reporter. Further genetic analysis combined with immunoblotting revealed that Kss1, one of the mating mitogen-activated protein kinases (MAPKs), and Mpk1, an MAPK of the cell integrity pathway, participated in l-dopa-induced stimulation of FUS1 and RLM1 transcription. We also report that Mpk1 and Hog1, the high osmolarity MAPK, were phosphorylated upon induction by hydrogen peroxide. Together, our results demonstrate that cells respond to oxidative stress via different signal transduction machinery dependent upon the nature of the oxidant.

Angiotensin II-induced ERK1/ERK2 activation and protein synthesis are redox-dependent in glomerular mesangial cells

Angiotensin II (Ang II) stimulates hypertrophy of glomerular mesangial cells. The signalling mechanism by which Ang II exerts this effect is not precisely known. Downstream potential targets of Ang II are the extracellular-signal-regulated kinases 1 and 2 (ERK1/ERK2). We demonstrate that Ang II activates ERK1/ERK2 via the AT1 receptor. Arachidonic acid (AA) mimics the action of Ang II on ERK1/ERK2 and phospholipase A2 inhibitors blocked Ang II-induced ERK1/ERK2 activation. The antioxidant N-acetylcysteine as well as the NAD(P)H oxidase inhibitors diphenylene iodonium and phenylarsine oxide abolished both Ang II- and AA-induced ERK1/ERK2 activation. Moreover, dominant-negative Rac1 (N17Rac1) blocks activation of ERK1/ERK2 in response to Ang II and AA, whereas constitutively active Rac1 resulted in an increase in ERK1/ERK2 activity. Antisense oligonucleotides for Nox4 NAD(P)H oxidase significantly reduce activation of ERK1/ERK2 by Ang II and AA. We also show that protein synthesis in response to Ang II and AA is inhibited by N17Rac1 or MEK (mitogen-activated protein kinase/ERK kinase) inhibitor. These results demonstrate that Ang II stimulates ERK1/ERK2 by AA and Nox4-derived reactive oxygen species, suggesting that these molecules act as downstream signal transducers of Ang II in the signalling pathway linking the Ang II receptor AT1 to ERK1/ERK2 activation. This pathway involving AA, Rac1, Nox4, reactive oxygen species and ERK1/ERK2 may play an important role in Ang II-induced mesangial cell hypertrophy.

Albumin up-regulates the type II transforming growth factor-beta receptor in cultured proximal tubular cells1

BACKGROUND

Clinical and experimental observations suggest that proteinuria is not merely a marker of chronic nephropathies, but may also be involved in the progression to end-stage renal failure. Filtered proteins are taken up by tubular cells, and overwhelming this system may lead to tubular synthesis of various proinflammatory and profibrogenic cytokines, including transforming growth factor-beta (TGF-beta). TGF-beta acts by first binding to specific receptors. We studied in an in vitro system using a well-defined mouse proximal tubular cell line (MCT cells) whether fatty acid-free bovine albumin modulates expression of specific receptors for TGF-beta.

METHODS

MCT (and LLC-PK1) cells were challenged in serum-free medium with different concentrations of albumin. Activation of a local renin-angiotensin system was tested by real-time polymerase chain reaction (PCR) for renin and angiotensinogen transcripts and determination of secreted angiotensin II (Ang II) by enzyme-linked immunosorbent assay (ELISA). Some cells were also treated with the AT1 receptor antagonist losartan. TGF-beta receptor types I and II mRNA levels were determined by Northern analysis whereas protein abundance was measured by Western blots. To test for a functional consequence of up-regulated TGF-beta receptors, MCT cells were preincubated with albumin and subsequently treated with low-dose TGF-beta that normally does not induce collagen type IV expression by itself. Downstream signaling events were detected by Western blots for phosphorylated Smad2. Scatchard assays with [125I]TGF-beta1 were performed to estimate affinity and number of specific binding sites. Different length TGF-beta type II promoter constructs linked to CAT reporter were transiently transected into MCT cells to determine transcriptional activity.

RESULTS

Incubation of MCT cells with 0.5 to 10 mg/mL albumin leads to an increase in type II TGF-beta receptor mRNA and protein expression without influencing type I receptors. An increase in type II TGF-beta receptor protein expression was detected after 12 hours of albumin incubation and was still detectable after 48 hours. The albumin-mediated increase in type II TGF-beta receptor mRNA was attenuated in the presence of 1 micromol/L losartan, suggesting involvement of a local renin-angiotensin system. MCT cells treated with albumin significantly increased expression of angiotensinogen and renin transcripts and also secreted more Ang II into the culture supernatant. Analysis of transcriptional activity showed that promoter segments containing activating protein (AP-1)-binding sites are necessary for albumin-induced transcription of the TGF-beta type II receptor. Binding assays revealed that albumin treatment significantly increased the overall binding sites as well as the affinity for TGF-beta. This effect had functional consequences because MCT cells pretreated with albumin reacted with a stronger TGF-beta-mediated phosphorylation of down-stream Smad2 and also increased collagen IV expression compared with control cells.

CONCLUSION

Our findings indicate that albumin up-regulates ligand-binding TGF-beta receptors on cultured proximal tubular cells. Albumin-induced activation of local Ang II production appears to be responsible for this effect. This may amplify the matrix-stimulatory actions of TGF-beta on tubular cells and could be a novel mechanism for how proteinuria exhibits pathophysiologic effects on tubular cells ultimately leading to tubulointerstitial fibrosis.

Multiple oxidative stress-response members of the Adapt78 family

Adapt78 is an oxidative and calcium stress-response gene. Its protein product is a potent natural inhibitor of the intracellular calcium signaling protein calcineurin. Much of what is known about Adapt78 protein is based on cell-transfection studies. Toward understanding natural endogenous Adapt78, we used an antibody raised against cellular Adapt78 and recently determined that endogenous Adapt78 protein, like its mRNA, is oxidative and calcium stress responsive. Here we report the identification of a second endogenous form of this protein family of 41 kDa. Subcellular fractionation of human HeLa cells revealed that in contrast to results of previous transfection studies, most endogenous Adapt78, characterized as 29 and 41 kDa electrophoretic doublets, resides in the cellular cytosol. The 41 kDa form of Adapt78 was abundant and found to exhibit many characteristics in common with the previously reported oxidative stress-responsive 29 kDa form, including hypo- and hyperphosphorylation variants, rapid loss of the hypophosphorylated form following oxidative stress, response to various kinase and phosphatase inhibitors, and localization. However, it also exhibited some unique characteristics, most notably the lack of calcium inducibility. Finally, the 29 kDa form exhibited a much shorter half-life and strong stabilization following oxidant exposure compared with the 41 kDa Adapt78 form. These data reveal the presence of a novel oxidative stress-responsive 41 kDa Adapt78 species, lend further insight into the Adapt78 family of proteins and their distribution, and challenge previous conclusions obtained using transfection protocols.

Angiotensin II and Cell Cycle Regulation

Angiotensin II has emerged as an important growth factor for vascular, cardiac, and renal cells. Depending on the specific cell type and presence of other growth factors, angiotensin II induces proliferation (replication of DNA with subsequent successful division of cells), hypertrophy (increase in cell size, cell protein, and mRNA content without DNA replication), apoptosis (programmed cell death), or differentiation. Such angiotensin II-mediated modulation of growth process may underlie various pathophysiological processes such as atherosclerosis, vascular and cardiac remodeling, and progression of chronic renal disease. Clearly, angiotensin II-induced proliferation requires complete cell progression through the various steps of the cell cycle. In contrast, cells undergoing angiotensin II-mediated hypertrophy are arrested in the G1-phase. Upregulation of cell cycle-dependent kinase inhibitors (eg, p27Kip1) plays an important role in this process. Although accumulating evidence suggests that apoptosis is cell cycle-dependent, only few data are currently available concerning the interaction of angiotensin II with the cell cycle machinery in apoptosis. We review the various angiotensin II-mediated growth processes and their relationship to events governing cell cycle regulation.

Oxidative stress in the pathogenesis of experimental mesangial proliferative glomerulonephritis

Reactive oxygen species (ROS) are increasingly believed to be important intracellular signaling molecules in mitogenic pathways involved in the pathogenesis of glomerulonephritis (GN). We explored the effects of the antioxidants α-lipoic acid and N-acetyl-l-cysteine on ERK activation in cultured mesangial cells and the role of ERK activation in the severity of glomerular injury in a rat model of anti-Thy 1 GN. In cultured mesangial cells, growth factors stimulated ERK phosphorylation by 150–450%. Antioxidants reduced this increase by 50–60%. Induction of anti-Thy 1 nephritis in rats led to a 210% increase in glomerular ERK phosphorylation. This increase in phosphorylated ERK was reduced by 50% in animals treated with α-lipoic acid. Treatment with α-lipoic acid resulted in significant improvement of glomerular injury. Cellular proliferation was reduced by 100%, and the number of proliferating cell nuclear antigen-positive cells was reduced by 64%. The increased expression of glomerular transforming growth factor-β1 protein and mRNA in rats with anti-Thy 1 nephritis was significantly attenuated and mesangial cell transformation into myofibroblasts was completely prevented by treatment with α-lipoic acid. The effects of α-lipoic acid were at least partially due to inhibition of oxidative stress. In rats with anti-Thy 1 nephritis, ROS production was increased 400–500%, and this increase was inhibited by 55% by treatment with α-lipoic acid. We suggest that ROS may mediate glomerular injury by inducing ERK phosphorylation. α-Lipoic acid should be considered a potential therapeutic agent in certain types of human GN.

Ceramide Is Involved inR(+)-Methanandamide-Induced Cyclooxygenase-2 Expression in Human Neuroglioma Cells

Cannabinoids have recently been shown to induce the expression of the cyclooxygenase-2 (COX-2) isoenzyme in H4 human neuroglioma cells. Using this cell line, the present study investigates the contribution of the second messenger ceramide to this signaling pathway. Incubation of cells with the endocannabinoid analog R(+)-methanandamide (R(+)-MA) was associated with an increase of intracellular ceramide levels. Enhancement of ceramide formation by R(+)-MA was abolished by fumonisin B1, a ceramide synthase inhibitor, whereas inhibitors of neutral sphingomyelinase (spiroepoxide, glutathione) and serine palmitoyltransferase (l-cycloserine, ISP-1) were inactive in this respect. R(+)-MA caused a biphasic activation of the p38 and p42/44 mitogen-activated protein kinases (MAPKs), with phosphorylation peaks occurring after 15-min and 4- to 8-h treatments, respectively. Inhibition of ceramide synthesis with fumonisin B1 was associated with a suppression of R(+)-MA-induced delayed phosphorylations of p38 and p42/44 MAPKs and subsequent COX-2 expression. The involvement of ceramide in COX-2 expression was corroborated by findings showing that C2-ceramide and neutral sphingomyelinase from Bacillus cereus caused concentration-dependent increases of COX-2 expression that were suppressed in the presence of 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)imidazol (SB203580, a p38 MAPK inhibitor) or 2'-amino-3'-methoxyflavone (PD98059, a p42/44 MAPK activation inhibitor). In contrast, dihydro-C2-ceramide being used as a negative control did not induce MAPK phosphorylation and COX-2 expression. Collectively, our results demonstrate that R(+)-MA induces COX-2 expression in human neuroglioma cells via synthesis of ceramide and subsequent activation of p38 and p42/44 MAPK pathways. Induction of COX-2 expression via ceramide represents a hitherto unknown mechanism by which cannabinoids mediate biological effects within the central nervous system.

High glucose stimulates angiotensinogen gene expression and cell hypertrophy via activation of the hexosamine biosynthesis pathway in rat kidney proximal tubular cells

The present study investigated whether activation of the hexosamine biosynthesis pathway might mediate at least in part the high glucose effect on angiotensinogen (ANG) gene expression and immortalized renal proximal tubular cell (IRPTC) hypertrophy. IRPTC were cultured in monolayer. ANG, renin, and beta-actin mRNA expression were determined by specific RT-PCR assays. Phosphorylation of p38 MAPK, activating transcription factor-2 (ATF-2), and cAMP-responsive element-binding protein (CREB) was determined by Western blot analysis. Cell hypertrophy was assessed by flow cytometry, intracellular p27kip1 protein levels, and [3H]leucine incorporation into proteins. Glucosamine stimulated ANG and renin mRNA expression and enhanced p38 MAPK, ATF-2, and CREB phosphorylation in normal glucose (5 mm) medium. Azaserine and 6-diazo-5-oxo-l-norleucine (inhibitors of glutamine: fructose-6-phosphate amino transferase enzyme) blocked the stimulatory effect of high glucose, but not that of glucosamine, on ANG gene expression in IRPTCs. SB 203580 (a specific p38 MAPK inhibitor) attenuated glucosamine action on ANG gene expression as well as p38 MAPK and ATF-2 phosphorylation, but not that of CREB. GF 109203X and calphostin C (inhibitors of protein kinase C) blocked the effect of glucosamine on ANG gene expression and CREB phosphorylation, but had no impact on p38 MAPK and ATF-2 phosphorylation. Finally, both glucosamine and high glucose induced IRPTC hypertrophy. The hypertrophic effect of glucosamine was blocked in the presence of GF 109203X, but not azaserine and SB 203580. In contrast, the hypertrophic effect of high glucose was blocked in the presence of azaserine and GF 109203X, but not SB203580. Our studies demonstrate that the stimulatory effect of high glucose on ANG gene expression and IRPTC hypertrophy may be mediated at least in part via activation of hexosamine biosynthesis pathway signaling.

Oxidative and calcium stress regulate DSCR1 (Adapt78/MCIP1) protein

DSCR1 (adapt78) is a stress-inducible gene and cytoprotectant. Its protein product, DSCR1 (Adapt78), also referred to as MCIP1, inhibits intracellular calcineurin, a phosphatase that mediates many cellular responses to calcium. Exposure of human U251 and HeLa cells to hydrogen peroxide led to a rapid hyperphosphorylation of DSCR1 (Adapt78). Inhibitor and agonist studies revealed that a broad range of kinases were not responsible for DSCR1 (Adapt78) hyperphosphorylation, including ERK1/2, although parallel activation of the latter was observed. Phosphorylation of both DSCR1 (Adapt78) and ERK1/2 was attenuated by inhibitors of tyrosine phosphatase, suggesting the common upstream involvement of tyrosine dephosphorylation. The hyperphosphorylation electrophoretic shift in DSCR1 (Adapt78) mobility was also observed with other oxidizing agents (peroxynitrite and menadione) but not nonoxidants. Calcium ionophores strongly induced the levels of both hypo- and hyper-phosphorylated DSCR1 (Adapt78) but did not alter phosphorylation status. Calcium-dependent growth factor- and angiotensin II-stimulation also induced both DSCR1 (Adapt78) species. Phosphorylation of either or both serines in a 13-amino acid peptide made to a calcineurin-interacting conserved region of DSCR1 (Adapt78) attenuated inhibition of calcineurin. These data indicate that DSCR1 (Adapt78) protein is a novel, early stage oxidative stress-activated phosphorylation target and newly identified calcium-inducible protein, and suggest that these response mechanisms may contribute to the known cytoprotective and calcineurin-inhibitory activities of DSCR1 (Adapt78).