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

Down-regulated miR-15a mediates the epithelial–mesenchymal transition in renal tubular epithelial cells promoted by high glucose

High glucose (HG) has been reported to be associated with renal dysfunction. And one potential mechanism underlining the dysfunction is the epithelial–mesenchymal transition (EMT) of renal tubular epithelial cells. Present study showed that EMT was induced in the HG-treated renal tubular epithelial cells by promoting the expression of mesenchymal phenotype molecules, such as α-SMA and collagen I, and down-regulating the expression of epithelial phenotype molecule E-cadherin. Moreover, we have identified the down-regulation of miR-15a which was accompanied with the HG-induced EMT. And the miR-15a overexpression inhibited the α-SMA, collagen I expression, and the promotion of E-cadherin expression by targeting and down-regulating AP4 which was also significantly promoted by the HG in the renal tubular epithelial cells. Thus, this study revealed that the weakening regulation on the AP4 expression by miR-15a might contribute to the HG-induced EMT in the renal tubular epithelial cells.

13-cis-Retinoic acid specific down-regulation of angiotensin type 1 receptor in rat liver epithelial and aortic smooth muscle cells

Transcriptional repression through cis- and trans-acting factors enabling an alternate approach to control angiotensin type 1 receptor (AT1 or AGTR1 as listed in the MGI database) expression has not been studied. In previous investigations, treatment with retinoic acid was found to be associated with enhanced insulin sensitivity. In our previous study, expression of AT1 was found to be inversely correlated with intracellular glucose concentrations. Therefore, we hypothesized that 13-cis-retinoic acid (13cRA), an antioxidant, enhances insulin-sensitive glucose-mediated down-regulation of the AT1. In this study, we used continuously passaged rat liver epithelial cells. Our study shows that cells exposed to 13cRA specifically down-regulated the AT1 protein in a dose- and time-dependent manner, independently of any change in receptor affinity. Down-regulation of the AT1 expression leads to reduced AngII-mediated intracellular calcium release, a hallmark of receptor-mediated intracellular signaling. Similarly with receptor down-regulation, we observed a significant reduction in AT1 mRNA; however, the AT1 down-regulation was independent of insulin-sensitive glucose uptake and retinoic acid receptor activation (RAR/RXR). Treatment with 13cRA resulted in phosphorylation of p42/p44 MAP kinases in these cells. Subsequent studies using MEK inhibitor PD98059 prevented 13cRA-mediated AT1 down-regulation and restored AngII-mediated intracellular calcium response. Furthermore, 13cRA-mediated inhibitory effects on AT1 were validated in primary rat aortic smooth muscle cells. In summary, our results demonstrate for the first time that 13cRA has a glucose- and RAR/RXR-independent mechanism for transcriptional inhibition of AT1, suggesting its therapeutic potential in systems in which AT1 expression is deregulated in insulin-sensitive and -insensitive tissues.

Pharmacologic effects of 2-methoxyestradiol on angiotensin type 1 receptor down-regulation in rat liver epithelial and aortic smooth muscle cells

BACKGROUND

Delayed onset of cardiovascular disease (CVD) in female patients is not well understood, but could be due in part to the protective effect of estrogen before menopause. Experimental studies have identified the angiotensin type 1 receptor (AT1R) as a key factor in the progression of CVD.

OBJECTIVE

We examined the effects of the estrogen metabolite 2-methoxyestradiol (2ME2) on AT1R expression.

METHODS

Rat liver cells were exposed to 2ME2 for 24 hours, and angiotensin II (AngII) binding and AT1R mRNA expressions were assessed.

RESULTS

In the presence of 2ME2, cells exhibited significant down-regulation of AngII binding that was both dose and time dependent, independent of estrogen receptors (ERα/ERβ). Down-regulation of AngII binding was AT1R specific, with no change in receptor affinity. Under similar conditions, we observed lower expression of AT1R mRNA, significant inhibition of AngII-mediated increase in intracellular Ca(2+), and increased phosphorylation of ERK1/2. Pretreatment of cells with the MEK inhibitor PD98059 prevented 2ME2-induced ERK1/2 phosphorylation and down-regulation of AT1R expression, which suggests that the observed inhibitory effect is mediated through ERK1/2 signaling intermediates. Similar analyses in stably transfected CHO (Chinese hamster ovary) cell lines with a constitutively active cytomegalovirus promoter showed no change in AT1R expression, which suggests that 2ME2-mediated effects are through transcriptional regulation. The effects of 2ME2 on AT1R down-regulation through ERK1/2 were consistently reproduced in primary rat aortic smooth muscle cells.

CONCLUSIONS

Because AT1R has a critical role in the control of CVD, 2ME2-induced changes in receptor expression may provide beneficial effects to the cardiovascular and other systems.

Tannic acid down-regulates the angiotensin type 1 receptor through a MAPK-dependent mechanism

In the present study, we investigated the effects of tannic acid (TA), a hydrolysable polyphenol, on angiotensin type 1 receptor (AT1R) expression in continuously passaged rat liver epithelial cells. Under normal conditions, exposure of cells to TA resulted in the down-regulation of AT1R-specific binding in concentrations ranging from 12.5-100 μg/ml (7.34-58.78 μm) over a time period of 2-24 h with no change in receptor affinity to angiotensin II (AngII). The inhibitory effect of TA on AT1R was specific and reversible. In TA-treated cells, we observed a significant reduction in AngII-mediated intracellular calcium signaling, a finding consistent with receptor down-regulation. Under similar conditions, TA down-regulated AT1R mRNA expression without changing the rate of mRNA degradation, suggesting that TA's effect is mediated through transcriptional inhibition. Cells expressing recombinant AT1R without the native promoter show no change in receptor expression, whereas a pCAT reporter construct possessing the rat AT1R promoter was significantly reduced in activity. Furthermore, TA induced the phosphorylation of MAPK p42/p44. Pretreatment of the cells with a MAPK kinase (MEK)-specific inhibitor PD98059 prevented TA-induced MAPK phosphorylation and down-regulation of the AT1R. Moreover, there was no reduction in AngII-mediated intracellular calcium release upon MEK inhibition, suggesting that TA's observed inhibitory effect is mediated through MEK/MAPK signaling. Our findings demonstrate, for the first time, that TA inhibits AT1R gene expression and cellular response, suggesting the observed protective effects of dietary polyphenols on cardiovascular conditions may be, in part, through inhibition of AT1R expression.

Cannabinoid receptor 1 mediates palmitic acid-induced apoptosis via endoplasmic reticulum stress in human renal proximal tubular cells

The endocannabinoid system (ECS) is activated at the onset of obesity and diverse metabolic diseases. Endocannabinoids mediate their physiological and behavioral effects by activating specific cannabinoid receptors, mainly cannabinoid receptor 1 (CB(1)R). Diabetic nephropathy (DN) is induced by hyperlipidemia, and renal proximal tubule cells are an important site for the onset of DN. However, the pathophysiology of CB(1)R, especially in the hyperlipidemia of DN, has not been elucidated. Therefore, we examined the effect of palmitic acid (PA) on CB(1)R expression and its related signal pathways in human renal proximal tubular cells (HK-2 cells). PA significantly increased CB(1)R mRNA and protein levels and induced CB(1)R internalization. PA-induced activation of CB(1)R is prevented by the treatment of AACOCF(3) (a cPLA(2) inhibitor), indomethacin and NS398 (a COX 2 inhibitors). Indeed, PA increased cPLA(2), and COX-2 but not COX-1. We also investigated whether the PA-induced activation of CB(1)R is linked to apoptosis. As a result, AM251 (a CB(1)R antagonist) attenuated PA-mediated apoptosis in a concentration-dependent manner. Furthermore, PA decreased GRP78 expression and induced increases in the endoplasmic reticulum (ER) stress signaling pathways p-PERK, p-eIF2α, p-ATF4, and CHOP, which were blocked by AM251 treatment. Moreover, PA increased the Bax/Bcl-2 ratio, cleaved PARP, and caspase-3 levels. The PA-induced apoptotic effects were decreased with CB(1)R-specific antagonist (AM251) treatment and CB1 si-RNA transfection. In conclusion, PA induced apoptosis through ER stress via CB(1)R expression in human proximal tubule cells. Our results provide evidence that CB(1)R blockade may be a potential anti-diabetic therapy for the treatment of DN.

Local RAS

The concept of a circulating RAS is well established and known to play an endocrine role in the regulation of fluid homeostasis (see Section 4.1, Chapter 4). However, it is more appropriate to view the RAS in the contemporary notion as an “angiotensin-generating system”, which consists of angiotensinogen, angiotensin-generating enzymes, and angiotensins, as well as their receptors. Some RASs can be termed as “complete”, having renin and ACE involved in the biosynthesis of angiotensin II peptide, i.e. in a renin and/or ACE-dependent manner which is exemplified in the circulating RAS. On the other hand, some RAS can be termed as “partial”, having alternate enzymes to renin and ACE, such as chymase and ACE2 (see Section 4.3, Chapter 4) available for the generation of angiotensin II and other bioactive angiotensin peptides in the biosynthetic cascade, i.e. in a renin and/or ACE-independent manner. Complete vs. partial RASs can be exemplified in the so-called intrinsic angiotensin-generating system or local RAS; for example, a local and functional RAS with renin and ACE-dependent but a renin-independent pathway have been indentified in the pancreas and carotid body, respectively. In the past two decades, local RASs have gained increasing recognition especially with regards to their clinical importance. Distinct from the circulating RAS, these functional local RASs exist in such diverse tissues and organs as the pancreas, liver, intestine, heart, kidney, vasculature, carotid body, and adipose, as well as the nervous, reproductive, and digestive systems. Taken into previous findings from our laboratory and others together, Table 5.1 is a summary of some recently identified local RASs in various levels of tissues and organs.

Troglitazone ameliorates high glucose-induced EMT and dysfunction of SGLTs through PI3K/Akt, GSK-3β, Snail1, and β-catenin in renal proximal tubule cells

Peroxisome proliferator-activated receptor-γ (PPARγ) agonists ameliorate renal fibrotic lesions in diabetic nephropathy. However, the effects of the agonists on the epithelial-mesenchymal transition (EMT) linked to membrane transport dysfunction are unknown. The present study aimed to verify the effects of the PPARγ agonist troglitazone on high glucose (HG)-induced EMT in primary cultured renal proximal tubular epithelial cells (PTCs). HG (25 mM) as well as hydrogen peroxide (H2O2) and transforming growth factor-β1 (TGF-β1) decreased expression of epithelial cell marker E-cadherin and increased the expression of the mesenchymal markers vimentin and α-smooth muscle actin (α-SMA). HG, H2O2, and TGF-β1 decreased Na+/H+ exchangers (NHEs) or Na+-glucose cotransporters (SGLTs) and glucose uptake, showing membrane transport dysfunction. HG stimulated the production of cellular reactive oxygen species (ROS), and antioxidants blocked the HG-induced increase in phosphatidylinositol 3-kinase (PI3K)/Akt activation. Antioxidants and inhibitors of PI3K/Akt reversed HG-induced EMT protein expression. Inhibition of PI3K/Akt also blocked HG-induced glycogen synthase kinase-3β (GSK-3β) phosphorylation. HG and lithium chloride (GSK-3β inhibitor) blocked Snail1 and β-catenin activation. Moreover, transfection with Snail1 or β-catenin small interfering RNA (siRNA) reversed HG-induced EMT protein expression. Importantly, HG decreased PPARγ activation and troglitazone reversed HG-induced expression of PI3K/Akt, GSK-3β, Snail1, and β-catenin as well as EMT proteins. Finally, inhibitors of PI3K/Akt, Snail1/β-catenin siRNA, and troglitazone blocking the HG-induced EMT restored glucose uptake in PTCs. In conclusion, HG induces EMT through ROS, PI3K/Akt, GSK-3β, Snail, and β-catenin. Subsequently, HG-induced EMT may result in SGLT dysfunction that is restored by the PPARγ agonist troglitazone in primary cultured PTCs.

Diabetes mellitus and expression of the enterocyte renin-angiotensin system: implications for control of glucose transport across the brush border membrane

Streptozotocin-induced (Type 1) diabetes mellitus (T1DM) in rats promotes jejunal glucose transport, but the trigger for this response remains unclear. Our recent work using euglycemic rats has implicated the enterocyte renin-angiotensin system (RAS) in control of sodium-dependent glucose transporter (SGLT1)-mediated glucose uptake across the jejunal brush border membrane (BBM). The aim of the present study was to examine whether expression of enterocyte RAS components is influenced by T1DM. The effects of mucosal addition of angiotensin II (AII) on [(14)C]-D-glucose uptake by everted diabetic jejunum was also determined. Two-week diabetes caused a fivefold increase in blood glucose level and reduced mRNA and protein expression of AII type 1 (AT(1)) and AT(2) receptors and angiotensin-converting enzyme in isolated jejunal enterocytes. Angiotensinogen expression was, however, stimulated by diabetes while renin was not detected in either control or diabetic enterocytes. Diabetes stimulated glucose uptake into everted jejunum by 58% and increased the BBM expression of SGLT1 and facilitated glucose transporter 2 (GLUT2) proteins, determined by Western blotting by 25% and 135%, respectively. Immunohistochemistry confirmed an enhanced BBM expression of GLUT2 in diabetes and also showed that this was due to translocation of the transporter from the basolateral membrane to BBM. AII (5 microM) or L-162313 (1 microM), a nonpeptide AII analog, decreased glucose uptake by 18% and 24%, respectively, in diabetic jejunum. This inhibitory action was fully accountable by an action on SGLT1-mediated transport and was abolished by the AT(1) receptor antagonist losartan (1 microM). The decreased inhibitory action of AII on in vitro jejunal glucose uptake in diabetes compared with that noted previously in jejunum from normal animals is likely to be due to reduced RAS expression in diabetic enterocytes, together with a disproportionate increase in GLUT2, compared with SGLT1 expression at the BBM.

Regulatory mechanisms of Na(+)/glucose cotransporters in renal proximal tubule cells

Glucose is a key fuel and an important metabolic substrate in mammals. Renal proximal tubular cells (PTCs) not only reabsorb filtered glucose but are also believed to play a role in the glucotoxicity associated with renal pathogenesis, such as in diabetes. The proximal tubule environment is where 90% of the filtered glucose is reabsorbed by the low-affinity/high-capacity Na(+)/glucose cotransporter 2 (SGLT2) and facilitated diffusion glucose transporter 2 (GLUT2). Both active and facilitative glucose transporters have distinct distribution profiles along the proximal tubule related to their particular kinetic characteristics. A number of mechanisms contribute to the changes in the cellular functions, which occur in response to exposure to various endogenous factors. Hyperglycemia was reported to regulate the renal SGLT activities through the reactive oxygen species-nuclear factor-kappaB pathways, which suggests that the transcellular glucose uptake within the PTCs contribute to the development of diabetic-like nephropathy. Angiotensin II (ANG II) plays an important role in its development through epidermal growth factor receptor (EGFR) transactivation. Therefore, a combination of high glucose, ANG II, and EGF are involved in diabetic-like nephropathy by regulating the SGLT activity. In addition, endogenously enhanced SGLTs have a cytoprotective function. The renal proximal tubules play a major role in regulating the plasma glucose levels, and there is increasing interest in the renal glucose transporters on account of their potential implications in the treatment of various conditions including diabetes mellitus.

Targeting glucagon receptor signalling in treating metabolic syndrome and renal injury in Type 2 diabetes: theory versus promise

Pancreatic bi-hormones insulin and glucagon are the Yin and Yang in the regulation of glucose metabolism and homoeostasis. Insulin is synthesized primarily by pancreatic beta-cells and is released in response to an increase in blood glucose levels (hyperglycaemia). By contrast, glucagon is synthesized by pancreatic alpha-cells and is released in response to a decrease in blood glucose (hypoglycaemia). The principal role of glucagon is to counter the actions of insulin on blood glucose homoeostasis, but it also has diverse non-hyperglycaemic actions. Although Type 1 diabetes is caused by insulin deficiency (insulin-dependent) and can be corrected by insulin replacement, Type 2 diabetes is a multifactorial disease and its treatment is not dependent on insulin therapy alone. Type 2 diabetes in humans is characterized by increased insulin resistance, increased fasting blood glucose, impaired glucose tolerance and the development of glomerular hyperfiltration and microalbuminuria, ultimately leading to diabetic nephropathy and end-stage renal disease. Clinical studies have suggested that an inappropriate increase in hyperglycaemic glucagon (hyperglucagonaemia) over hypoglycaemic insulin (not insulin deficiency until advanced stages) plays an important role in the pathogenesis of Type 2 diabetes. However, for decades, research efforts and resources have been devoted overwhelmingly to studying the role of insulin and insulin-replacement therapy. By contrast, the implication of glucagon and its receptor signalling in the development of Type 2 diabetic metabolic syndromes and end-organ injury has received little attention. The aim of this review is to examine the evidence as to whether glucagon and its receptor signalling play any role(s) in the pathogenesis of Type 2 diabetic renal injury, and to explore whether targeting glucagon receptor signalling remains only a theoretical antidiabetic strategy in Type 2 diabetes or may realize its promise in the future.

Alteration of the gene and protein levels of insulin-like growth factors in streptozotocin-induced diabetic male rats

Insulin-like growth factor (IGFs: IGF-I and IGF-II) systems have been reported to be associated with the onset of diabetic mellitus. Therefore, we investigated the effect of diabetes on regulation of the IGF system in the liver, kidneys and heart, which are important organs in the pathogenesis of diabetes. The experimental groups were subdivided into three groups: 1) controls, 2) streptozotocin (STZ)-induced untreated diabetic group, and 3) an insulin-treated group (plus diabetic rats). In the present study, starting on the second day after STZ treatment, the diabetic group exhibited hyperglycemia, polyuria, and polydipsia, which are characteristic of diabetes melittus. Serum levels of IGF-I were decreased, but those of IGF-II were increased in the diabetic group compared with the controls. The expression levels of IGF-I and IGF-II protein in the livers of the diabetic group had a similar pattern to the serum. In addition, the expression levels of liver IGF-I mRNA and IGF-II mRNA were decreased in the diabetic groups. In the heart, IGF-I levels were decreased, but IGF-II levels were increased in the untreated diabetic groups, which was consistent with the expression levels of their mRNA. However, both the IGF-I and IGF-II levels in the kidneys were increased in the untreated diabetic groups, but the mRNA levels were decreased. Insulin treatment ameliorated the changes of IGF system in the serum, liver, kidneys, and heart. In conclusion, diabetes induced alteration of the IGF system tissue-specifically, and this was blocked by insulin treatment.

Acute effect of high glucose on long-term cell growth: a role for transient glucose increase in proximal tubule cell injury

Although chronic exposure of renal cells to high glucose has been shown to cause cell injury, the effect of acute exposure has not been elucidated. In this study, we demonstrate that acute (10 min) exposure of human proximal tubule epithelial cells (hPTEC) to high glucose (25 mM) induces a time-dependent dual effect consisting of an early proliferation and a late apoptosis. Acute exposure of hPTEC to high glucose induced a twofold increase in DNA synthesis and cell number at 12 h. However, after 36 h, a significant decrease in cell growth is observed, followed by apoptosis. On glucose treatment, both p42/p44 mitogen-activated protein (MAP) kinases and the downstream signaling intermediate NF-κB were phosphorylated and translocated to the nucleus. Pretreatment of cells with MAP kinase and NF-κB-specific inhibitors abolished glucose-induced proliferation. However, these inhibitors were ineffective in preventing glucose-induced apoptosis. Interestingly, conditioned medium from cells exposed to high-glucose concentrations inhibited proliferation and concomitantly induced apoptosis in normal cells, suggesting that the inhibitory effect of glucose occurs through secretion of a secondary factor(s). In parallel to apoptosis, we observed an increased production of reactive oxygen species (ROS). Pretreatment of cells with the antioxidant N-acetyl cysteine reversed glucose-mediated ROS production and apoptosis, suggesting that ROS is involved in apoptosis. Our study demonstrates for the first time that a single high-glucose exposure for 10 min alone is sufficient to elicit proliferation and apoptosis in hPTEC and suggests that episodes of transient increase in glucose may contribute to cell damage leading to epithelial cell dysfunction.

Interactions between Angiotensin ll and Atrial Natriuretic Peptide in Renomedullary Interstitial Cells: The Role of Neutral Endopeptidase

BACKGROUND/AIMS

Neutral endopeptidase (NEP) inhibition attenuates renal damage in the diabetic kidney, but little is known about the mechanisms of this renoprotective effect.

METHODS

We examined the interaction between angiotensin II (Ang II) and atrial natriuretic peptide (ANP) under low (5 mM) and high (30 mM) glucose conditions, on cell proliferation and extracellular matrix (ECM) synthesis in renomedullary interstitial cells (RMICs) derived from wild-type (WT) and NEP-deficient (NEP-) mice.

RESULTS

Under high glucose conditions, Ang II (10(-6)M) increased cell proliferation (control, 174.3 +/- 16.9; Ang II, 846.3 +/- 91.0 cpm/well) and ECM synthesis (control, 22.3 +/- 3.1; Ang II, 79.0 +/- 9.6 cpm/cell) in RMICs derived from WT and NEP- mice to a similar extent. ANP (10(-7)M) reduced Ang II-induced cell proliferation and ECM synthesis in RMICs derived from both strains, but more efficiently in RMICs derived from NEP- mice. The Ang II-induced cell proliferation and ECM synthesis was attenuated with AT1 receptor blockade, but more efficiently in RMICs-derived NEP- mice.

CONCLUSIONS

This data shows that ANP and AT1 receptor blockade attenuate Ang II-induced RMIC proliferation and ECM synthesis more efficiently in the absence of NEP. These results support the concept that NEP inhibition is beneficial in attenuating abnormal cell growth and ECM metabolism associated with diabetic nephropathy.

Cross-talk between angiotensin II and glucagon receptor signaling mediates phosphorylation of mitogen-activated protein kinases ERK 1/2 in rat glomerular mesangial cells

We have recently shown that the pancreatic hormone glucagon-induced phosphorylation of mitogen-activated protein (MAP) kinase ERK 1/2 as well as growth and proliferation of rat glomerular mesangial cells (MCs) via activation of cAMP-dependent protein kinase A (PKA)- and phospholipase C (PLC)/Ca2+-mediated signaling pathways. Since circulating glucagon and tissue angiotensin II (Ang II) levels are inappropriately elevated in type 2 diabetes, we tested the hypothesis that glucagon induces phosphorylation of ERK 1/2 in MCs by interacting with Ang II receptor signaling. Stimulation of MCs by glucagon (10 nM) induced a marked increase in intracellular [Ca2+]i that was abolished by [Des-His1, Glu9]-glucagon (1 microM), a selective glucagon receptor antagonist. Both glucagon and Ang II-induced ERK 1/2 phosphorylation (glucagon: 214+/-14%; Ang II: 174+/-16%; p<0.001 versus control), and these responses were inhibited by the AT1 receptor blocker losartan (glucagon + losartan: 77+/-14%; Ang II + losartan: 84+/-18%; p<0.01 versus glucagon or Ang II) and the AT2 receptor blocker PD 123319 (glucagon + PD: 78+/-7%; Ang II + PD: 87+/-7%; p<0.01 versus glucagon or Ang II). Inhibition of cAMP-dependent PKA with H89 (1 microM) or PLC with U73122 (1 microM) also markedly attenuated the phosphorylation of ERK 1/2 induced by glucagon (glucagon + U73122: 109+/-15%; glucagon + H89: 113+/-16%; p<0.01 versus glucagon) or Ang II (Ang II + U73122: 111+/-13%; Ang II + H89: 86+/-10%; p<0.01 versus Ang II). Wortmannin (1 microM), a selective PI 3-kinase inhibitor, also blocked glucagon- or Ang II-induced ERK 1/2 phosphorylation. These results suggest that AT1 receptor-activated cAMP-dependent PKA, PLC and PI 3-kinase signaling is involved in glucagon-induced MAP kinase ERK 1/2 phosphorylation in MCs. The inhibitory effect of PD 123319 on glucagon-induced ERK 1/2 phosphorylation further suggests that AT2 receptors also play a similar role in this response.

Glucagon receptor-mediated extracellular signal-regulated kinase 1/2 phosphorylation in rat mesangial cells: role of protein kinase A and phospholipase C

Glucagon, a major insulin counterregulatory hormone, binds to specific Gs protein-coupled receptors to activate glycogenolytic and gluconeogenic pathways, causing blood glucose levels to increase. Inappropriate increases in serum glucagon play a critical role in the development of insulin resistance and target organ damage in type 2 diabetes. We tested the hypotheses that: (1) glucagon induces proliferation of rat glomerular mesangial cells through glucagon receptor-activated phosphorylation of mitogen-activated protein kinase extracellular signal-regulated kinase 1/2 (p-ERK 1/2); and (2) this phosphorylation involves activation of cAMP-dependent protein kinase A (PKA) and phospholipase C (PLC)/[Ca2+]i signaling pathways. In rat mesangial cells, glucagon (1 nM) stimulated [3H]-thymidine incorporation by 96% (P<0.01). This proliferative effect was blocked by the specific glucagon receptor antagonist [Des-His1-Glu9] glucagon (1 micromol/L; P<0.01), a mitogen-activated protein kinase/ERK kinase inhibitor PD98059 (10 micromol/L; P<0.01), a PLC inhibitor U73122 (1 micromol/L; P<0.01), or a PKA inhibitor H-89 (1 micromol/L; P<0.01). The proliferation was associated with a 2-fold increase in p-ERK 1/2 that peaked 5 minutes after glucagon stimulation (P<0.01) and also was blocked by [Des-His1-Glu9] glucagon. Total ERK 1/2 was not affected by glucagon. Pretreating of mesangial cells with U73122 or H89 significantly attenuated ERK 1/2 phosphorylation induced by glucagon. We believe that these are the first data showing that glucagon activates specific receptors to induce ERK 1/2 phosphorylation and thereby increase mesangial cell proliferation and that this effect of glucagon involves both PLC/[Ca2+]i- and cAMP-dependent PKA-activated signaling cascades.

Caffeic acid phenethyl ester ameliorates changes in IGFs secretion and gene expression in streptozotocin-induced diabetic rats

The protective effect of caffeic acid phenethyl ester (CAPE) against diabetes-induced alteration of IGFs protein and gene expression was investigated in serum, liver, heart, and kidney. In the present study, diabetic rats exhibited the decrease of IGF-I content in serum, liver and heart but the increase of that in kidney and CAPE blocked them. Diabetic rats also manifested the increase of IGF-II content in serum, liver, heart, and kidney and CAPE prevented them. CAPE prevented the diabetes-induced decrease of liver IGF-I mRNA and IGF-II mRNA, which is similar to pattern of IGFs mRNA in kidney. Moreover, diabetic rats exhibited the decrease of heart IGF-I mRNA but the increase of IGF-II mRNA and CAPE blocked them. In conclusion, CAPE, in part, prevented diabetes-induced alteration of IGF-I and IGF-II protein and gene expression in liver, heart, and kidney in rats.

Cariporide counteracts atherosclerosis-related functions in monocytes from obese and normal individuals

OBJECTIVE

This study aimed to show the effect of high glucose concentrations in combination with a pharmaceutical analog of the Na+/H+ antiport inhibitor, cariporide, on scavenger receptor CD36 expression, cell adhesion, and cell migration of human monocytes derived from obese and normal individuals.

RESEARCH METHODS AND PROCEDURES

Monocytes were isolated from six healthy obese individuals and six healthy age- and sex-matched controls by use of whole blood Percoll sedimentation and plastic surface monocyte binding. The density of CD36 scavenger receptors on the surface of monocytes was assessed by the use of a fluorescent fluorescein isothiocyanate (FITC)-linked monoclonal antibody. Transmigration of monocytes through laminin-1-coated filters was performed on 5-microm pore Transwell culture inserts. Monocyte attachment to laminin was estimated by a solid phase assay.

RESULTS

High glucose concentrations caused an increase in monocytes from normal and obese individuals in the expression of CD36 receptors and positively influenced monocyte migration and adhesion to laminin. Cariporide together with glucose counteracted these effects. The effects of migration and adhesion of monocytes to laminin were specific to glucose, because the effect was significantly higher when monocytes were incubated in the presence of 20 mM of glucose than in the presence of 20 mM of fructose. Monocytes from obese subjects showed greater response than in normal to all of the studied effects, with the highest response in laminin attachment.

DISCUSSION

The data of this study suggest that cariporide counteracts atherosclerosis-related functions through Na+/H+ antiport inhibition in monocytes from both normal and obese individuals.

High glucose concentrations stimulate human monocyte sodium/hydrogen exchanger activity and modulate atherosclerosis-related functions

In the present study, the effect of high (20 mM) glucose concentrations on human monocyte sodium/hydrogen exchanger (NHE1) activity, scavenger receptor CD36 expression, cell adhesion, and cell migration have been investigated. Incubation with high glucose concentrations caused an increase in NHE1 activity, as estimated by internal pH and sodium-uptake measurements. This effect was specific for glucose, since it was not observed when monocytes were incubated in the presence of 20 mM of galactose, fructose, or mannitol. In addition, the activation of sodium uptake was inhibited by ethylisopropyl amiloride (EIPA), phloretine and cytochalasine B, and calphostin C. High glucose concentrations also increased the expression of CD36 receptors on the surface of monocytes and positively influenced monocyte migration and adhesion to laminin. EIPA added together with glucose counteracted these effects. The data of the present study suggest that a high glucose concentration can influence atherosclerosis-related monocyte functions via NHE1 activation.

Rac and protein kinase C-delta regulate ERKs and cytosolic phospholipase A2 in FcepsilonRI signaling to cysteinyl leukotriene synthesis in mast cells

Although cysteinyl leukotrienes (cysLTs) are known to be principal inflammatory lipid mediators released from IgE-stimulated mast cells, the signaling mechanisms involved in the synthesis of cysLTs remain largely unknown. In the present study, therefore, we investigated the signaling pathway by which IgE induces cysLTs synthesis after binding to its high affinity receptor (FcepsilonRI) in RBL-2H3 mast cells. We found that IgE-induced cysLT synthesis is completely abolished in RBL-2H3(Rac-N17) cells, a stable cell line expressing Rac(N17), a dominant negative Rac1 mutant; conversely, synthesis was enhanced in cells expressing Rac(V12), a constitutively active Rac1 mutant, suggesting that Rac1 is a key mediator of IgE signaling to cysLT synthesis. Further analysis aimed at identifying mediators downstream of Rac1 revealed that pretreating cells with a protein kinase C-delta (PKC-delta) inhibitor or infection with an adenoviral vector harboring a dominant negative PKC-delta mutant significantly attenuates IgE-induced ERKs phosphorylation, cytosolic phospholipase A(2) phosphorylation/translocation, and cysLT synthesis. In addition, the expression of Rac(N17) blocked PKC-delta translocation and impaired the phosphorylation of ERKs and cytosolic phospholipase A(2) otherwise elicited by IgE stimulation. Taken together these results suggest that PKC-delta also plays a critical mediatory role in the IgE signaling pathway leading to cysLT synthesis, acting downstream of Rac1. Finally, the physiological significance of PKC-delta in the IgE signaling pathway was demonstrated in an Ag (OVA)-challenged in vivo mouse model, in which induced levels of cysLTs and airway responsiveness in lung airways were significantly diminished by prior i.p. injection of a PKC-delta inhibitor.

Signaling cascade of ANG II-induced inhibition of alpha-MG uptake in renal proximal tubule cells

ANG II and Na+-glucose cotransporter have been reported to be associated with the onset of diverse renal diseases. However, the effect of ANG II on Na+-glucose cotransporter activity was not elucidated. The effects of ANG II on alpha-methyl-D-[14C]glucopyranoside (alpha-MG) uptake and its related signal pathways were examined in the primary cultured rabbit renal proximal tubule cells (PTCs). ANG II (>2 h; >10(-9) M) inhibited alpha-MG uptake in a time- and concentration-dependent manner and decreased the protein level of Na+-glucose cotransporters, the expression of which was abrogated by both actinomycin D and cycloheximide exposure. ANG II-induced inhibition of alpha-MG uptake was blocked by losartan, an ANG II type 1 (AT1) receptor blocker, but not by PD-123319, an ANG II type 2 receptor blocker. ANG II-induced inhibition of alpha-MG uptake was blocked by genistein, herbimycin A [tyrosine kinase (TK) inhibitors], mepacrine, and AACOCF3 (phospholipase A2 inhibitors), suggesting the role of TK phosphorylation and arachidonic acid (AA). Indeed, ANG II increased AA release, which was blocked by losartan or TK inhibitors. The effects of ANG II on AA release and alpha-MG uptake also were abolished by staurosporine and bisindolylmaleimide I (protein kinase C inhibitors) or PD-98059 (p44/42 MAPK inhibitor), but not SB-203580 (p38 MAPK inhibitor), respectively. Indeed, ANG II increased p44/42 MAPK activity. ANG II-induced activation of p44/42 MAPK was blocked by staurosporine. In conclusion, ANG II inhibited alpha-MG uptake via PKC-MAPK-cPLA2 signal cascade through the AT1 receptor in the PTCs.

Diabetes and nephropathy

PURPOSE OF REVIEW

Diabetic nephropathy is the single most common disorder leading to renal failure. Its annual incidence has more than doubled in the past decade to reach 44% of all end-stage renal disease, despite recent therapeutic advances. Thus, research into diabetic nephropathy pathophysiology that could lead to new treatment approaches is urgently needed and this review aims to summarize the work performed in this area in the past year.

RECENT FINDINGS

There have been advances in the understanding of diabetic nephropathy pathology. Clearly, structural changes may be advanced before any clinical findings are apparent. Not all functional consequences of the condition are explained by current structural analyses. Genetic studies have connected the disorder risk to multiple candidate genes and a few genetic loci, but the exact genetic predisposition or protectors are not fully described. Perturbations in multiple metabolic pathways are associated with diabetic nephropathy in animals and humans, but their relative importance requires further work. Glycemia and blood pressure control are crucial for diabetic nephropathy prevention and treatment, but new modalities are needed.

SUMMARY

Recent advances in molecular biology and genetics will bring new insights to the mechanisms involved in diabetic nephropathy development. This will allow early identification of patients at risk of, or safe from, diabetic nephropathy and will hopefully lead to preventive strategies, based on the understanding of the pathophysiology of the disorder. Meanwhile, aggressive implementation of proven therapies to prevent (glycemic control) and slow (antihypertensive therapy, especially with renin-angiotensin system blockers) the progression of diabetic nephropathy are strongly recommended.

Methylglyoxal-bovine serum albumin stimulates tumor necrosis factor alpha secretion in RAW 264.7 cells through activation of mitogen-activating protein kinase, nuclear factor kappaB and intracellular reactive oxygen species formation

Accumulating evidence suggests that the pathophysiology of diabetes is analogous to chronic inflammatory states. Circulating levels of inflammatory cytokines such as IL-6 and tumor necrosis factor alpha (TNFalpha) are increased in both type 1 and type 2 diabetes. TNFalpha plays an important role in the pathogenesis of insulin resistance in type 2 diabetes. However, the reason for this increase remains unclear. Levels of the dicarbonyl methylglyoxal (MGO) are elevated in diabetic plasma and MGO-modified bovine serum albumin (MGO-BSA) can trigger cellular uptake of TNF. Therefore we tested the hypothesis that MGO-modified proteins may cause TNFalpha secretion in macrophage-like RAW 264.7 cells. Treatment of cells with MGO-BSA induced TNFalpha release in a dose-dependent manner. MGO-modified ribonuclease A and chicken egg ovalbumin had similar effects. Cotreatment of cells with antioxidant reagent N-acetylcysteine (NAC) inhibited MGO-BSA-induced TNFalpha secretion. MGO-BSA stimulated the simultaneous activation of p44/42 and p38 mitogen-activated protein kinase. PD98059, a selective MEK inhibitor, inhibited MGO-BSA-induced TNFalpha release as well as ERK phosphorylation. Pretreatment of cells with NAC also resulted in inhibition of MGO-BSA-induced ERK phosphorylation. MGO-BSA induced dose-dependent NFkappaB activation as shown by electrophoresis mobility shift assay. The MGO-BSA-induced NFkappaB activation was prevented in the presence of PD98059, NAC, and parthenolide, a selective inhibitor of NFkappaB. Furthermore, the NFkappaB inhibitor parthenolide suppressed MGO-BSA-induced TNFalpha secretion. Confocal microscopy using dichlorofluorescein to demonstrate intracellular reactive oxygen species (ROS) showed that MGO-BSA produced more ROS compared with native BSA. MGO-BSA could also stimulate protein kinase C (PKC) translocation to the cell membrane, considered a key signaling pathway in diabetes. However, there was no evidence that PKC was involved in TNFalpha release based on inhibition by calphostin C and staurosporine. Our findings suggest that the presence of chronically elevated levels of MGO-modified bovine serum albumin may contribute to elevated levels of TNFalpha in diabetes.