Abnormal Na+/H+ antiporter phenotype and turnover of immortalized lymphoblasts from type 1 diabetic patients with nephropathy

Gestational diabesity and foetoplacental vascular dysfunction

Gestational diabetes mellitus (GDM) shows a deficiency in the metabolism of D-glucose and other nutrients, thereby negatively affecting the foetoplacental vascular endothelium. Maternal hyperglycaemia and hyperinsulinemia play an important role in the aetiology of GDM. A combination of these and other factors predisposes women to developing GDM with pre-pregnancy normal weight, viz. classic GDM. However, women with GDM and prepregnancy obesity (gestational diabesity, GDty) or overweight (GDMow) show a different metabolic status than women with classic GDM. GDty and GDMow are associated with altered l-arginine/nitric oxide and insulin/adenosine axis signalling in the human foetoplacental microvascular and macrovascular endothelium. These alterations differ from those observed in classic GDM. Here, we have reviewed the consequences of GDty and GDMow in the modulation of foetoplacental endothelial cell function, highlighting studies describing the modulation of intracellular pH homeostasis and the potential implications of NO generation and adenosine signalling in GDty-associated foetal vascular insulin resistance. Moreover, with an increase in the rate of obesity in women of childbearing age worldwide, the prevalence of GDty is expected to increase in the next decades. Therefore, we emphasize that women with GDty and GDMow should be characterized with a different metabolic state from that of women with classic GDM to develop a more specific therapeutic approach for protecting the mother and foetus.

Oxidative Stress-Activated NHE1 Is Involved in High Glucose-Induced Apoptosis in Renal Tubular Epithelial Cells

PURPOSE

Diabetic nephropathy (DN) is a prevalent chronic microvascular complication of diabetes mellitus involving disturbances in electrolytes and the acid-base balance caused by a disorder of glucose metabolism. NHE1 is a Na⁺/H⁺ exchanger responsible for keeping intracellular pH (pHi) balance and cell growth. Our study aimed to investigate roles of NHE1 in high glucose (HG)-induced apoptosis in renal tubular epithelial cells.

MATERIALS AND METHODS

Renal epithelial tubular cell line HK-2 was cultured in medium containing 5 mM or 30 mM glucose. Then, cell apoptosis, oxidative stress, NHE1 expression, and pHi were evaluated. NHE1 siRNA and inhibitor were used to evaluate its role in cell apoptosis.

RESULTS

HG significantly increased cell apoptosis and the production of reactive oxygen species (ROS) and 8-OHdG (p<0.05). Meanwhile, we found that HG induced the expression of NHE1 and increased the pHi from 7.0 to 7.6 after 48 h of incubation. However, inhibiting NHE1 using its specific siRNA or antagonist DMA markedly reduced cell apoptosis stimulated by HG. In addition, suppressing cellular oxidative stress using antioxidants, such as glutathione and N-acetyl cysteine, significantly reduced the production of ROS, accompanied by a decrease in NHE1. We also found that activated cyclic GMP-Dependent Protein Kinase Type I (PKG) signaling promoted the production of ROS, which contributed to the regulation of NHE1 functions.

CONCLUSION

Our study indicated that HG activates PKG signaling and elevates the production of ROS, which was responsible for the induction of NHE1 expression and dysfunction, as well as subsequent cell apoptosis, in renal tubular epithelial cells.

Newly diagnosed diabetes mellitus patients presenting with proliferative diabetic retinopathy as an initial sign

AIM

To investigate the clinical features of newly diagnosed diabetes mellitus (NDM) patients showing proliferative diabetic retinopathy (PDR) as an initial sign.

METHODS

As a retrospective case series, the medical records of a total of four hundred and thirty-two patients who underwent a vitrectomy due to PDR were reviewed to find the subjects. Of 432 patients, six cases of NDM patients showing PDR as an initial sign were included and analyzed with their systemic and ocular features.

MAIN OUTCOME MEASURES

the systemic features and ocular features [preoperative and postoperative best corrected visual acuity (BCVA), intraoperative findings].

RESULTS

The mean onset age of visual symptoms was 36.3 years old. The mean serum insulin and C-peptide titer was below the normal range. The mean fasting plasma glucose was 178mg/dL and the mean postprandial 2h plasma glucose was 306mg/dL. The mean HbA1c at diagnosis was 11.02%. In all cases, an acute progressive fibrovascular proliferation was observed. Intraoperative retinal tears were found in three cases of six. The mean preoperative BCVA was +0.67±0.58 logMAR and the mean BCVA at postoperative 6 months was +0.20±0.30 logMAR.

CONCLUSION

All patients were considered to have latent autoimmune diabetes in adults (LADA). A rapid deterioration of kidney function as well as poor diabetic control status at diagnosis was observed in all six cases. The ocular features of the patients showed acute progressive fibrovascular proliferation and relatively favorable postoperative visual acuity.

Skin fibroblasts as a tool for identifying the risk of nephropathy in the type 1 diabetic population

Human fibroblasts in culture have been employed as an in vitro system to investigate some pathophysiological mechanisms of diabetes mellitus also associated with the development of diabetic nephropathy. In fact, there is increasing evidence that genetic factors either convey the risk of, or protect from, diabetic nephropathy and that the expression profiles and/or the behaviour of the cultured skin fibroblasts from type 1 diabetic patients could reflect these genetic influences. On the other hand, alterations could be attributable not only to changes in DNA sequence, but also to epigenetic factors. Our aim is to make a critical overview of the studies involving primary cultures of skin fibroblasts as tools to investigate the pathophysiology of diabetic nephropathy performed until now in this area. Cultured skin fibroblasts could be useful not only for the identification of patients at risk of developing diabetic renal disease, but also for a better understanding of the complex multifactorial mechanisms leading to the long-term complications in diabetes.

A model of Na+/H+ exchanger and its central role in regulation of pH and Na+ in cardiac myocytes

A new kinetic model of the Na(+)/H(+) exchanger (NHE) was developed by fitting a variety of major experimental findings, such as ion-dependencies, forward/reverse mode, and the turnover rate. The role of NHE in ion homeostasis was examined by implementing the NHE model in a minimum cell model including intracellular pH buffer, Na(+)/K(+) pump, background H(+), and Na(+) fluxes. This minimum cell model was validated by reconstructing recovery of pH(i) from acidification, accompanying transient increase in [Na(+)](i) due to NHE activity. Based on this cell model, steady-state relationships among pH(i), [Na(+)](I), and [Ca(2+)](i) were quantitatively determined, and thereby the critical level of acidosis for cell survival was predicted. The acidification reported during partial blockade of the Na(+)/K(+) pump was not attributed to a dissipation of the Na(+) gradient across the membrane, but to an increase in indirect H(+) production. This NHE model, though not adapted to the dimeric behavioral aspects of NHE, can provide a strong clue to quantitative prediction of degree of acidification and accompanying disturbance of ion homeostasis under various pathophysiological conditions.

Modeling the cardiac Na(+)/H (+) exchanger based on major experimental findings

Na(+)-H(+) exchanger (NHE) is the main acid extruder in cardiac myocytes. We review the experimental findings of ion-dependency of NHE activity, and the mathematical modeling developed so far. In spite of extensive investigation, many unsolved questions still remain. We consider that the precise description of NHE activity with mathematical models elucidates the roles of NHE in maintaining ionic homeostasis, especially under pathophysiological conditions.

Regulation of cell survival by Na+/H+ exchanger-1

Na(+)/H(+) exchanger-1 (NHE1) is a ubiquitous plasma membrane Na(+)/H(+) exchanger typically associated with maintenance of intracellular volume and pH. In addition to the NHE1 role in electroneutral Na(+)/H(+) transport, in renal tubular epithelial cells in vitro the polybasic, juxtamembrane NHE1 cytosolic tail domain acts as a scaffold, by binding with ezrin/radixin/moesin (ERM) proteins and phosphatidylinositol 4,5-bisphosphate, which initiates formation of a signaling complex that culminates in Akt activation and opposition to initial apoptotic stress. With robust apoptotic stimuli renal tubular epithelial cell NHE1 is a caspase substrate, and proteolytic cleavage may permit progression to apoptotic cell death. In vivo, genetic or pharmacological NHE1 loss of function causes renal tubule epithelial cell apoptosis and renal dysfunction following streptozotocin-induced diabetes, ureteral obstruction, and adriamycin-induced podocyte toxicity. Taken together, substantial in vivo and in vitro data demonstrate that NHE1 regulates tubular epithelial cell survival. In contrast to connotations of NHE1 as an unimportant "housekeeping" protein, this review highlights that NHE1 activity is critical for countering tubular atrophy and chronic renal disease progression.

Diabetic nephropathy is associated with gene expression levels of oxidative phosphorylation and related pathways

The in vitro behavior of skin fibroblasts from patients with or without diabetic nephropathy is associated with diabetic nephropathy risk. Here we compared skin fibroblast gene expression profiles from two groups of type 1 diabetic patients: 20 with very fast ("fast-track") versus 20 with very slow ("slow-track") rates of development of diabetic nephropathy lesions. Gene expression profiles of skin fibroblasts grown in 25 mmol/l glucose for 36 h were assessed by Affymetrix HG-U133A GeneChips to determine the proportion of genes in a given biological pathway that were directionally consistent in their group differences. Five pathways reached statistical significance. All had significantly greater proportions of genes with higher expression levels in the fast-track group. These pathways, the first four of which are closely related and have overlapping genes, included oxidative phosphorylation (P<0.001), electron transport system complex III (P=0.017), citrate cycle (P=0.037), propanoate metabolism (P=0.044), and transcription factors (P=0.046). These results support the concept that oxidative phosphorylation and related upstream pathways may be important in the pathogenesis of diabetic nephropathy. Whether these findings reflect inherent genetic cellular characteristics, "cell memory," or both requires further study.

Cellular basis of diabetic nephropathy: III. In vitro GLUT1 mRNA expression and risk of diabetic nephropathy in type 1 diabetic patients

AIMS/HYPOTHESIS

Altered glucose transporter expression has been implicated in the pathogenesis of diabetic nephropathy. There is increasing evidence that genetic factors convey risk of, or protection from, diabetic nephropathy and that the behaviour of cultured skin fibroblasts from type 1 diabetic patients may reflect these genetic influences. This study aimed to compare GLUT1 mRNA expression levels in skin fibroblasts from type 1 diabetic patients with either rapid ("fast-track", n=25) or slow ("slow-track", n=25) development of diabetic nephropathy and from non-diabetic normal control subjects (controls, n=25).

METHODS

Skin fibroblasts were cultured in Dulbecco's Modified Eagle's Medium with 25 mmol/l glucose for 36 h. Total RNA was isolated, and GLUT1 mRNA levels were estimated by microarray analysis and RT-PCR.

RESULTS

Levels of GLUT1 mRNA expression in skin fibroblasts from "slow-track" patients were greater than those from "fast-track" patients (p=0.02), as initially detected by microarray. GLUT1 mRNA expression levels were confirmed by RT-PCR to be higher in skin fibroblasts from "slow-track" patients (4.59+/-2.04) than in those from "fast-track" patients (3.34+/-1.2, p=0.02), and were also higher than in skin fibroblasts from control subjects (3.52+/-1.66, p=0.03). There was no statistically significant difference between levels of expression in the "fast-track" patients and the control subjects.

CONCLUSIONS/INTERPRETATION

This finding is consistent with the presence of cellular protection factors against diabetic nephropathy in the "slow-track" patients. These factors could be associated with the regulation of the GLUT1 pathway and may be genetically determined.

Diabetic nephropathy and retinopathy

Diabetic nephropathy and retinopathy are arguably the two most dreaded complications of diabetes. Together they contribute to serious morbidity and mortality. As they progress to end-stage renal disease and blindness, they impose enormous medical, economic,and social costs on both the patient and the health care system. Because nephropathy and retinopathy are frequently linked in patients,this article reviews their common and individual aspects of pathophysiology, clinical features, and management.

Ca2+/calmodulin-dependent modulation of cell cycle elements pRb and p27kip1 involved in the enhanced proliferation of lymphoblasts from patients with Alzheimer dementia

Failure of cell cycle regulation in neurons might be critically involved in the process of neurodegeneration in Alzheimer's disease (AD). We present here evidence to support the hypothesis that cell cycle alterations occur in cells other than neurons in AD sufferers. Lymphocytes from AD patients immortalized with Epstein-Barr virus showed an enhanced rate of proliferation and increased phosphorylation of the retinoblastoma protein (pRb) and other members of the family of pocket proteins compared with cell lines derived from normal age-matched controls. The calmodulin antagonist calmidazolium, as well as W-7 and W-13, abrogated the enhanced activity of AD cells without altering the normal basal rate of proliferation. The effect of calmidazolium was accompanied by partially dephosphorylation of pRb. No changes were found in the expression levels of the G1 cyclin/Cdks complexes. However, lymphoblasts derived from AD patients showed reduced levels of the Cdk inhibitor p27(kip1), which were restored after anti-calmodulin treatment of the cultures. These observations suggest that in AD cells the enhanced rates of cell proliferation and phosphorylation of pRb and the intracellular content of p27(kip1) may be interrelated events controlled by a mechanism dependent on the Ca(2+)/calmodulin signaling pathway. The distinct functional features of lymphoblastoid cells from AD patients offer an invaluable, noninvasive tool to investigate the etiopathogenesis, and eventually, for the early diagnosis and prognosis of this devastating disease.

Identification of a common risk haplotype for diabetic nephropathy at the protein kinase C-beta1 (PRKCB1) gene locus

Abnormal activation of protein kinase C-beta isoforms in the diabetic state has been implicated in the development of diabetic nephropathy. It is thus plausible that DNA sequence differences in the protein kinase C-beta1 gene (PRKCB1), which encodes both betaI and betaII isoforms, may influence susceptibility to nephropathy. Nine single-nucleotide polymorphisms (SNP) in PRKCB1 were tested for association with diabetic nephropathy in type I diabetes mellitus, by using both case-control and family-study designs. Allele and genotype distributions of two SNP in the promoter (--1504C/T and --546C/G) differed significantly between case patients and control patients (P < 0.05). These associations were particularly strong with diabetes mellitus duration of <24 yr (P = 0.002). The risk of diabetic nephropathy was higher among carriers of the T allele of the --1504C/T SNP, compared with noncarriers (odds ratio, 2.54; 95% confidence interval, 1.39 to 4.62), and among carriers of the G allele of the --546C/G SNP (odds ratio, 2.45; 95% confidence interval, 1.37 to 4.38). Among individuals with diabetes mellitus duration of >/==" BORDER="0">24 yr, these two SNP were not associated with diabetic nephropathy. These positive findings were confirmed by using the family-based transmission disequilibrium test. The T-G haplotype, with both risk alleles, was transmitted more frequently than expected from heterozygous parents to offspring who developed diabetic nephropathy during the first 24 yr of diabetes mellitus. It is concluded that DNA sequence differences in the promoter of PRKCB1 contribute to diabetic nephropathy susceptibility in type I diabetes mellitus.

Alternative splicing of NHE-1 mediates Na-Li countertransport and associates with activity rate

Sodium-lithium countertransport (SLC) is an ouabain-insensitive exchange of Na for Li found in the erythrocyte membrane of several mammalian species. Although increased SLC activity is presently the most consistent intermediate phenotype of essential hypertension and diabetic nephropathy in humans, the gene responsible for this membrane transport has not been identified. Because of functional similarities, SLC was suggested to represent an in vitro mode of operation of the Na-H exchanger (NHE). This hypothesis, however, has been long hampered by the total insensitivity of SLC to amiloride, which is an intrinsic inhibitor of the first isoform of NHE, the only NHE isoform detected in human erythrocytes. We describe here the identification in human reticulocytes and erythrocytes of an alternative splicing of NHE lacking the amiloride binding site. Transfection experiments with this spliced variant restore amiloride-insensitive, phloretin-sensitive SLC activity. Expression of both regular and spliced transcripts of NHE is increased in subjects with high SLC activity. Altogether, these findings, by extending to NHE the characteristics of inheritance and predictivity previously attributed to SLC, eventually restore the candidacy of NHE isoform 1 as a gene involved in the pathogenesis of essential hypertension and diabetic nephropathy.

In situ protein Kinase C activity is increased in cultured fibroblasts from Type 1 diabetic patients with nephropathy

AIMS/HYPOTHESIS

To verify whether individual susceptibility to diabetic nephropathy resides in an intrinsic difference in Protein Kinase C (PKC) activity.

METHODS

We compared the effect of different glucose concentrations on PKC activity, PKC isoform expression and diacylglycerol (DAG) content in cultured fibroblasts from 14 Type 1 diabetic patients who developed nephropathy with those in cells from 14 patients without nephropathy. We recruited 14 normal subjects as control patients. Forearm skin fibroblasts were cultured in either normal (5 mmol/l) or high (20 mmol/l) glucose concentrations.

RESULTS

In normal glucose, in situ PKC activity was higher in Type 1 patients with nephropathy (10.1+/-1.4 pmol/min/mg protein; p<0.01) than in those without (6.8+/-0.8) and the normal control subjects (6.3+/-0.5). This difference was due to increased concentrations of PKCalpha isoform in the membrane fraction of fibroblasts from patients with nephropathy. DAG content was also higher in cells from Type 1 patients with nephropathy. Incubation in high glucose concentration caused a further increase in PKC activity and DAG content in quiescent fibroblasts from patients with diabetic nephropathy, with no significant changes in cells from diabetic patients without nephropathy and normal control subjects.

CONCLUSION/INTERPRETATION

Differences in PKC activation could contribute to the individual susceptibility to renal damage in Type 1 diabetic patients.

Neutrophil activation and production of reactive oxygen species in pre-eclampsia

OBJECTIVE

To investigate neutrophil NADPH oxidase activation and subsequent production of reactive oxygen species (ROS) in pre-eclampsia.

DESIGN

Baseline values and the activated response of neutrophils upon stimulation of the NADPH oxidase with the agonists was measured. Neutrophils from 17 third-trimester pre-eclamptic and 17 age- and gestation-matched normal pregnant women were examined.

METHODS

Neutrophil ROS production was measured by both lucigenin- and luminol-derived chemiluminescence. The abundance of the various phox proteins was examined using Western blotting techniques. Lucigenin-derived ROS generation was significantly increased in neutrophils isolated from women with pre-eclampsia compared with normotensive controls in the case of both agonists [n-formyl-met-leu-phe (fMLP): pre-eclamptic 2.071 +/- 0.336 relative light units seconds (RLU.s) and normotensive 1.141 +/- 0.249 RLU.s, P = 0.035; phorbol-12-myristate-13-acetate (PMA): pre-eclamptic 34.954 +/- 2.634 RLU.s and normotensive 17.208 +/- 3.325 RLU.s, P = 0.0001]. Luminol-derived ROS generation was also significantly increased in the neutrophils isolated from the women with pre-eclampsia compared with the normotensive controls in the case of both agonists (fMLP: pre-eclamptic 1.955 +/- 0.316 RLU.s and normotensive 1.058 +/- 0.191 RLU.s, P = 0.023; PMA: pre-eclamptic 4.108 +/- 0.351 RLU.s and normotensive 3.073 +/- 0.332 RLU.s, P = 0.042). There were no differences between the relative abundance of the phox proteins in the two groups.

CONCLUSIONS

Neutrophils isolated from women with pre-eclampsia during the third trimester showed increased sensitivity to agonist stimulation and produced significantly more ROS than age-matched normotensive controls. This was not due to an increased abundance of any of the phox proteins. Increased ROS production in pre-eclampsia may highlight a role for neutrophils in the oxidative stress and associated endothelial dysfunction that are characteristic of the condition.

Low Ca(2+) pump activity in diabetic nephropathy

Elevated cell Na(+)-H(+) exchange (NHE) activity characterizes diabetic nephropathy (DN), but the mechanisms of this abnormality are unclear. Recent evidence suggests that NHE and the Ca(2+) pump share similar regulatory pathways, but whether abnormalities in Ca(2+) metabolism characterize DN is not known. We investigated Ca(2+) efflux rates, NHE activity, cytosolic Ca(2+) ([Ca(2+)](i)) concentrations, and intracellular pH (pH(i)) in human skin fibroblasts from 20 patients with type 1 (insulin-dependent) diabetes and nephropathy; 20 patients with diabetes with normoalbuminuria matched for age, sex, and duration of diabetes; and 10 individuals without diabetes. Ca(2+) pump-mediated Ca(2+) efflux was significantly lower in patients with nephropathy than in patients with normoalbuminuria and individuals without diabetes (0.074 +/- 0.01 versus 0.115 +/- 0.01 versus 0.131 +/- 0.02 nmol.mg(protein)(-1).min(-1); analysis of variance [ANOVA], P = 0.015). Elevated maximal velocity of the Na(+)-H(+) exchanger was confirmed in fibroblasts from patients with nephropathy (14.4 +/- 1.2 versus 7.1 +/- 0.7 versus 8.0 +/- 1.2 mmol H(+).l cell(-1).min(-1); ANOVA, P < 0.0001). A reverse correlation between Ca(2+) pump activity and NHE rates could be shown. Adjustment for glycated hemoglobin and plasma lipid levels did not affect these findings. Finally, [Ca(2+)](i) concentrations and pH(i) were normal in all patients. Low Ca(2+) pump activity is a concomitant event of elevated NHE rates in DN; the molecular dysfunction(s) underlying these abnormalities remains to be established.

Salt-loading elevates blood pressure and aggravates insulin resistance in Wistar fatty rats: a possible role for enhanced Na+ -H+ exchanger activity

OBJECTIVE

Increased Na+-H+ exchanger activity (NHE) has been reported as an intermediate phenotype in hypertensive subjects, particularly those with insulin resistance. To investigate whether NHE abnormality plays a role in hypertension, Wistar fatty rat (WFR) with overt obesity, hyperglycemia and marked hyperinsulinemia was examined.

METHODS

WFR and Wistar lean rats (WLR) as a control (n = 12, each) were fed either with normal (0.38%) or high sodium (4% NaCl) diet for 12 weeks and then sacrificed to examine platelets NHE activity.

RESULTS

Mean arterial pressure (MAP) was higher in WFR than in WLR (113 +/- 4 versus 96 +/- 7 mmHg, P < 0.05) under a normal chow. Vmax values of NHE activity were significantly higher in WFR than in WLR. WFR fed with a high sodium diet showed higher MAP than those with a normal chow (128 +/- 3 versus 113 +/- 4 mmHg, P < 0.05). Though Km values were not different between WFR and WLR under a normal chow, both maximal transport rate (Vmax) and half maximal transport (Km) values were significantly higher in WFR with a high salt diet than those with a control diet. Vmax showed significant correlation with MAP, whereas Km values correlated with immunoreactive insulin (IRI) levels. Significant interaction between dietary sodium intake and the strain differences was observed both on blood pressure and on IRI levels by two-way analysis of variance (ANOVA).

CONCLUSION

WFR presented salt-sensitive blood pressure elevation. NHE activity was enhanced in WFR in correlation with the blood pressure. These results suggest that augmented NHE activity contributes to the development of salt-sensitive blood pressure elevation in WFR.

Leucocyte intracellular pH and Na+/H+ exchanger isoform-1 activity in postpartum women with pre-eclampsia

OBJECTIVE

To investigate leucocyte Na+/H+ exchanger isoform 1 activity in postpartum pre-eclamptics.

DESIGN

Exchanger isoform-1 activity and intracellular resting pH were established in leucocytes isolated from two study groups.

SAMPLE

Leucocytes isolated from 10 women who had had pre-eclamptic pregnancies more than five months postpartum, and from 10 age-matched normotensive women who were more than five months postpartum.

SETTING

Hypertension Clinic, Antenatal Assessment Area, Leicester Royal Infirmary.

METHODS

A well validated technique involving flurometry using a pH sensitive dye (BCECF-AM) was performed to determine exchanger isoform-1 activity and intracellular pH. Determination of exchanger isoform-1 protein abundance was performed by western blotting. Exchanger isoform-3 protein abundance was examined to rule out the possibility of activity due to this particular isoform.

RESULTS

Intracellular pH was significantly lower in the postpartum pre-eclamptic group (7.11 +/- 0.02), compared with the postpartum normotensive controls (7.33 +/- 0.04; P < 0.001). Exchanger isoform-1 efflux rate (in mmol/L/minute) was significantly higher in the postpartum pre-eclamptic group (35.91 +/- 3.1), compared with the postpartum normotensives (23.94 +/- 2.0; P = 0.005). Exchanger isoform-1 protein density was established to be similar among the two subject groups. No exchanger isoform-3 protein was identified by western blotting.

CONCLUSION

Our results suggest that elevated exchanger isoform-1 activity is an important finding in women who have suffered from pre-eclampsia. This increased activity is not due to an increase in exchanger isoform-1 protein abundance or the presence of exchanger isoform-3.

Enhanced proliferation of lymphoblasts from patients with Alzheimer dementia associated with calmodulin-dependent activation of the na+/H+ exchanger

We have recently reported that lymphoblasts from late onset Alzheimer's disease (AD) patients show distinct intracellular pH homeostatic features than those obtained from age-matched healthy donors. Here we report that another distinct feature of AD lymphoblasts is their increased rate of proliferation in serum containing medium, suggesting a different responsiveness of AD cells to serum activators. The increased proliferation of AD cells was accompanied by intracellular alkalinization and was prevented by blockers of the plasma membrane Na+/H+ antiporter (NHE), indicating that the exchanger had to be activated to elicit the cellular responses. The activity of this exchanger can be controlled through several signaling pathways, but only the inhibition of calmodulin activity impeded the serum-induced intracellular alkalinization and enhanced proliferation of AD cells. In contrast, the inhibition of calmodulin did not alter the rate of proliferation of normal cells. Thus, it seems plausible to conclude that the enhanced proliferation of AD cells is the result of a surface receptor-mediated activation of the Ca(2+)-calmodulin signaling pathway. Our observations add further support in favor that AD may be considered a systemic disease which underlying etiopathogenic mechanism may be an altered responsiveness to cell activating agents. Thus, the use of lymphoblastoid cells from AD patients may be a useful model to investigate cell biochemical aspects of this disease.

G-protein beta(3)-subunit C825T genotype and nephropathy in diabetes mellitus

BACKGROUND

Recent studies have identified a novel polymorphism (C825T) of the gene encoding the beta(3) subunit of heterotrimeric G proteins (G:beta(3)) which is associated with enhanced activation of G-proteins and appears to be more common in hypertensive patients and possibly contributes to decreased kidney allograft survival.

METHODS

In the present study we examined the relationship between this genetic variant, type 1 and type 2 diabetes and renal complications of diabetes in 1008 Caucasian patients recruited from an outpatient diabetes clinic and four dialysis centres. We also studied 1940 healthy controls.

RESULTS

After multivariate adjustment and in univariate statistics, the G:beta(3) 825TT genotype was not associated with a significantly enhanced risk of diabetes or renal complications.

CONCLUSIONS

These findings indicate that the G:beta(3) 825T allele apparently does not contribute to the development of diabetes or associated renal complications in patients with type 1 or type 2 diabetes mellitus.

G protein beta 3 subunit 825T allele, hypertension, obesity, and diabetic nephropathy

The 825T allele of the gene GNB3 which encodes the beta 3 subunit of heterotrimeric G proteins is associated with enhanced signal transduction via G proteins through the generation of a splice variant termed Gbeta3s. It was detected following a classical candidate gene approach using cell lines from patients with enhanced signal transduction and essential hypertension. The high frequency of the 825T allele in 'old' ethnicities, e.g. bushmen and Australian aborigines as well as in black populations, together with its strong association with obesity suggests that the 825T allele is a true 'thrifty genotype'. Development of obesity associated with the 825T allele is strongly influenced by lifestyle, e.g. physical activity, and other exogenous influences like pregnancy. In hypertension the 825T allele is associated with low renin activity and appears to strongly predict the development of left ventricular hypertrophy. In type 2 diabetes the 825T allele was reported to be predispose for end-stage renal disease, whereas this effect has not yet been confirmed for patients with type 1 diabetes.

Increased sodium-proton antiporter activity in patients with obstructive sleep apnoea

Cytosolic pH (pH(i)) and the activity of the sodium-proton antiporter (Na(+)/H(+) antiporter) were measured in lymphocytes from 22 patients with obstructive sleep apnoea and from 24 age-matched healthy subjects (Controls). The cellular Na(+)/H(+) antiporter was measured spectrophotometrically using a pH-sensitive fluorescent dye after intracellular acidification using sodium propionate. Resting pHi was similar in lymphocytes from patients with obstructive sleep apnoea and from controls (7.36 +/- 0.20, n=22; vs. 7.35 +/- 0.19, n=24; mean +/- SD). The Na(+)/H(+) antiporter activity was significantly higher in patients with obstructive sleep apnoea than in controls (11.87 +/- 3.26 x 10(-3) pH(i)/s vs. 4.38 +/- 1.40 x 10(-3) pH(i)/s; P < 0. 0001). The apparent affinity of the Na+/H+ antiporter was not significantly different between the groups (6.90 +/- 0.23 vs. 6.87 +/- 0.20). In patients with obstructive sleep apnoea the activity of the Na(+)/H(+) antiporter remained stable during the night. The activity of the Na(+)/H(+) antiporter was 13.49 +/- 4.80 x 10(-3) pH(i)/s at 20.00 and 13.26 +/- 6.13 x 10(-3) pH(i)/s at 02.00. From the present results it is concluded that an increased cellular Na(+)/H(+) antiporter activity may be a genetic marker for patients who are predisposed to obstructive sleep apnoea.

Red cell Na+/Li+ countertransport and Na+/H+ exchanger isoforms in human proximal tubules

BACKGROUND

Increased activity of the Na+/Li+ countertransporter (SLC) is a well-recognized intermediate phenotype of hypertension and diabetic nephropathy and may indicate a predisposition to hypertension. Previous work has attempted to link this membrane transport marker to altered Na+ reabsorption in the proximal tubule. Since the Na+/H+ exchanger (NHE) isoforms 1 and 3 are expressed in the basolateral and apical membranes of the proximal tubule, respectively, we investigated the relationship between these transport proteins and red cell SLC to examine whether the peripheral blood transport phenotype is associated with altered levels of transport proteins in the proximal tubule.

METHODS

Proximal tubules were prepared from human nephrectomy specimens. NHE-1 and NHE-3 were detected on Western blots by specific antibodies. Red cell SLC was also measured.

RESULTS

Both NHE-1 and NHE-3 proteins were demonstrated, with molecular weights of 97 and 85 kD, respectively. SLC was very strongly correlated with the level of NHE-3 protein (r = 0.78, P < 0.001) and was negatively related to NHE-1 protein (r = -0.32). In multiple regression analysis, only NHE-3 and NHE-1 protein levels were significant predictors of red cell SLC, accounting for up to about 70% of the variance of this parameter.

CONCLUSIONS

We conclude that red cell SLC may be a marker of increased NHE-3 protein expression in the proximal tubule, which may account for the blunted pressure natriuresis and predisposition to hypertension.

Polymorphism of the 5' untranslated region of NHE1 gene associated with type-I diabetes

The ubiquitous form of the sodium-hydrogen exchanger, NHE1, is devoted to the regulation of intracellular pH and cell volume. In addition, NHE1 activity is stimulated by growth factors and increased NHE rates are found in both circulating and immortalized cells during diabetes or diabetic nephropathy. In this context, we searched for polymorphisms of the 5'-flanking regulatory region of NHE1 gene in subjects with type-I diabetes. We identified a C/T transition 696 bases upstream the translation initiation start site which disrupts a repeated palindromic GC sequence. The TT genotype was significantly more frequent in type-1 diabetics and may have functional importance. Genetic linkage between NHE1 and diabetes has been previously described in NOD mice strains with consequences on NHE rates. Hence, the polymorphism described hereby may act as a predisposition factor to type-I diabetes or to diabetic complications, and may be useful to investigate the genetic involvement of NHE1 in human pathophysiology.

Regulation of the Na+/H+ antiporter in patients with mild chronic renal failure: effect of glucose

BACKGROUND

The aim of this study was to determine the glucose-dependent regulation of the sodium-proton-antiporter (Na+/H+ antiporter) in patients with mild chronic renal failure (CRF).

METHODS

We measured plasma glucose concentrations, plasma insulin concentrations, plasma C peptide concentrations, arterial blood pressure, cytosolic pH (pHi), cellular Na+/H+ antiporter activity, and cytosolic sodium concentration ([Na+]i) in 19 patients with CRF and 41 age-matched healthy control subjects (control) during a standardized oral glucose tolerance test. Intracellular pHi, [Na+]i, and Na+/H+ antiporter activity was measured in lymphocytes using fluorescent dye techniques.

RESULTS

Under resting conditions, the pHi was significantly lower, whereas the Na+/H+ antiporter activity was significantly higher in CRF patients compared with controls (each P < 0.0001). The oral administration of 100 g glucose significantly increased the Na+/H+ antiporter activity in CRF patients from 13.35 +/- 1.26 x 10-3 pHi/second to 16.44 +/- 1.37 x 10-3 pHi/second after one hour and to 14.06 +/- 1.36 x 10-3 pHi/second after two hours (mean +/- SEM, P = 0.008 by Friedmans's two-way analysis of variance). In controls, the administration of 100 g glucose significantly increased the Na+/H+ antiporter activity from 4.23 +/- 0.20 x 10-3 pHi/second to 6.00 +/- 0.56 x 10-3 pHi/second after one hour and to 6.65 +/- 0.64 x 10-3 pHi/second after two hours (P = 0.0003). The glucose-induced enhancement of the Na+/H+ antiporter activity was more pronounced in CRF patients compared with controls (P = 0.011). Resting [Na+]i was not significantly different between the two groups.

CONCLUSIONS

CRF patients show an intracellular acidosis leading to an increased Na+/H+ antiporter activity. In addition, high glucose levels exaggerate the differences in Na+/H+ antiporter activity already present between cells from patients with mild CRF and those from control subjects.

Insulin-dependent diabetic sibling pairs are concordant for sodium-hydrogen antiport activity

Insulin-dependent diabetic sibling pairs are concordant for sodium-hydrogen antiport activity.

BACKGROUND

Recent findings of enhanced Na+/H+ antiport activity in cultured fibroblasts and immortalized lymphoblasts from type 1 diabetic patients with nephropathy support the view that a phenotypic or genotypic factor(s) underlies nephropathy risk. This study evaluated the kinetic properties of Na+/H+ antiporter in cultured fibroblasts from families with two siblings affected by type 1 (insulin-dependent) diabetes.

METHODS

Seventeen diabetic sibling pairs were studied. The age was 38 +/- 10 years (mean +/- SD) in probands, the first to develop diabetes, and 39 +/- 7 in siblings; the duration of diabetes was, by definition, longer in probands (24 +/- 12 vs. 17 +/- 8 years in siblings). Na+/H+ antiport activity was determined using a microfluorometric technique with the pH sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in skin fibroblasts cultured for at least six passages.

RESULTS

There were no significant differences between probands and siblings for the following parameters: glycated hemoglobin, 8.3 +/- 0.8% in probands and 8.6 +/- 1.4% in siblings; creatinine clearance, 103 +/- 24 ml/min/1.73 m2 in probands and 103 +/- 25 in siblings; albumin excretion rate, 6.8 (1 to 860) microgram/min (median and range) in probands and 4.9 (2 to 1334) in siblings. Intracellular pH and buffering capacity were superimposable in the sibling pairs. The Vmax for the antiport was 39.2 +/- 14.7 mmol/liter cell/min in probands and 40.3 +/- 17.6 in siblings. The internal pH for half-maximal activation (Km) and Hill coefficient was also similar in probands and siblings. There were correlations between probands and siblings in values for intracellular pH (r = 0.51, P < 0.04), Vmax (r = 0.84, P < 0.0001), and buffering capacity (r = 0.53, P < 0. 03). Glycated hemoglobin values over five years were not significantly correlated in the sibling pairs (r = 0.3, P > 0.1). Vmax was related with the albumin excretion rate (r = +0.49, P = 0. 005) and glycated hemoglobin (r = +0.41, P = 0.017) in the total cohort of sibling pairs. However, multiple regression analysis, using Vmax as the dependent variable, found no correlations between any of the subjects' clinical and demographic variables.

CONCLUSIONS

Familial concordance for Na+/H+ antiport activity in long-term cultured skin fibroblasts from type 1 diabetic siblings suggests that at least some of the in vitro phenotypical characteristics of these cells are likely to be genetically determined and to be, at least in part, independent of in vivo metabolic control.

Impaired glycolysis and protein catabolism induced by acid in L6 rat muscle cells

BACKGROUND

In skeletal muscle, metabolic acidosis stimulates protein degradation and oxidation of branched-chain amino acids. This could occur to compensate for impairment of glucose utilization induced by acid.

METHODS

To test this hypothesis, glycolysis and protein degradation (release of [14C]-phenylalanine) were measured in L6 skeletal muscle cells cultured in Eagle's minimum essential medium at pH 7.1 or 7.5 for up to 3 days.

RESULTS

No marked changes in total DNA or in cell viability were detected, nor was there any significant effect on intracellular pH or the water content of the cells (which is thought to be a key regulator of protein turnover, especially in liver). In spite of this, acid stimulated protein degradation, induced net protein loss from the cultures, inhibited glucose uptake and glycolysis (lactate output) and was associated with increased [1-14C]-leucine oxidation. Effects on protein degradation and glycolysis were gradual, reaching a maximum after 20-30 h. To investigate whether glycolytic flux itself can influence protein degradation, increased glycolysis was simulated by adding glucose (20 mmol L-1) or pyruvate (1 mmol L-1) to the medium. At pH 7.1, neither addition had any effect on protein degradation.

CONCLUSION

Although acid-induced protein wasting is associated with impaired glycolysis, no obligatory coupling exists between glycolytic flux and protein degradation.

Nephropathy and hypertension in diabetes

The treatment of the patient with diabetes, with or without hypertension, is complex and challenging. Hyperglycemic treatment should ideally not only control blood glucose, but also prevent the chronic complications and associated metabolic derangements that can lead to increased morbidity and mortality. Hypertensive treatment should not only decrease blood pressure, but also reduce the risk of macrovascular and microvascular disease. The use of antihypertensive agents that improve insulin resistance, dyslipidemia, glycemic control, and nephropathy is preferred whenever possible. The real key to success in the care of the hypertensive diabetic patient is adequate screening and appropriate, early treatment. Currently, there is ample evidence to support the use of intensive management with the goal of near-normalization of blood glucose levels in most patients with diabetes. Similarly, aggressive treatment of hypertension is the current standard. Accomplishing these goals helps to prevent the development of chronic diabetic complications, including nephropathy. ESRD need not be the inevitable outcome for individuals with early diabetic nephropathy. Interventions currently available that are targeted at the known modifiable risk factors underlying the development and progression of diabetic nephropathy offer the best hope for reducing the incidence and severity of this complication. Prevention of the complications of diabetes, including nephropathy, must be the goal for the future on behalf of all those who now have diabetes.

Quantitative contribution of NHE2 and NHE3 to rabbit ileal brush-border Na+/H+ exchange

Intestinal neutral NaCl absorption, which is made up of brush-border (BB) Na+/H+ exchange linked to BB Cl-/HCO3- exchange, is up- and downregulated as part of digestion and diarrheal diseases. Glucocorticoids stimulate ileal NaCl absorption and BB Na+/H+ exchange. Intestinal BB contains two Na+/H+ exchanger isoforms, NHE2 and NHE3, but their relative roles in rabbit ileal BB Na+/H+ exchange has not been determined. A technique to separate the contribution of NHE2 and NHE3 to ileal BB Na+/H+ exchange activity was standardized by using an amiloride-related compound, HOE-694. Under basal conditions, both NHE2 and NHE3 contribute approximately 50% to ileal Na+/H+ exchange. Glucocorticoids (methylprednisolone) increase BB Na+/H+ exchange (2.5 times) but increase only ileal NHE3 activity (4.1 times), without an effect on NHE2 activity. Thus ileal BB Na+/H+ exchange in animals treated with glucocorticoids is 69% via NHE3. A quantitative Western analysis for NHE3 was developed, using as an internal standard a fusion protein of the COOH-terminal 85 amino acids of NHE3 and maltose binding protein. Glucocorticoid treatment increased the amount of BB NHE3. The quantitative Western analysis showed that NHE3 makes up 0.018% of ileal BB protein in control rabbits and 0.042% (2.3 times as much) in methylprednisolone-treated rabbits. Methylprednisolone treatment did not alter the amount of ileal BB NHE2 protein. NHE3 turnover number was estimated to be 458 cycles/s under basal conditions and 708 cycles/s in glucocorticoid-treated ileum. Thus methylprednisolone stimulates ileal BB Na+/H+ exchange activity only by an effect on NHE3 and not on NHE2; it does so primarily by increasing the amount of BB NHE3, although it also increases the NHE3 turnover number.

Independence of dimethylamiloride-sensitive Li+ efflux pathways and Na+-Li+ countertransport in human erythrocytes

The in vivo function of the erythrocyte Na+-Li+ countertransport (SLC) is unknown. Whether SLC may reflect an operational mode of the widespread Na+-H+ exchanger (NHE) or may otherwise be expression of an independent membrane transport, remains presently unclear. We explored the presence of 5-(N,N-dimethyl)-amiloride (DMA)-sensitive Li+ pathways in human erythrocytes where the activity of the Na+ pump, Na+-K+ cotransport and anion exchange were suitably inhibited. A total of 0.02 mM DMA had no effect on SLC as expected, but gave a significant inhibition of Li+ efflux into both Na+ and Na+-free media. This DMA-sensitive Li+ pathway, but not SLC, was significantly enhanced by hyperosmolar cell shrinkage, which is a characteristic feature of NHE. In conclusion, DMA-sensitive Li+ pathways, possibly mediated by NHE, are present in erythrocytes and coexist with the DMA-insensitive, SLC. This finding supports the notion that SLC is independent of amiloride-sensitive NHE.

Hypertension and nephropathy in diabetes mellitus: what is inherited and what is acquired?

Prolonged duration of diabetes mellitus, poor long term glycemic control and raised blood pressure have all been clearly related to the development of diabetic nephropathy. Evidence exists to suggest that a subset of individuals with diabetes have a genetic predisposition to diabetic nephropathy. Cases of diabetic nephropathy cluster in families and a parental history of hypertension is more common in patients with diabetic nephropathy. Current evidence suggests an important role for hypertension in the genetic susceptibility to diabetic nephropathy but the extent of this is unknown. While cellular and animal studies have generated a plethora of data regarding mechanisms involved in the role of hypertension and diabetic nephropathy, these are not helpful for drawing conclusions in humans. In the following review, we examine the available clinical, epidemiologic and family studies to assess the relationship between the development of hypertension and diabetic nephropathy in IDDM and NIDDM. We will demonstrate the differences in the epidemiology of hypertension in diabetes depending on the type of diabetes and thus, move the emphasis of nephropathy susceptibility away from hypertension per se. We hope to emphasize instead the homogeneity of nephropathy risk in both IDDM and NIDDM and also the idea that a common genetic susceptibility exists for all types of diabetes and is conditional on cumulative exposure to hyperglycemia. Regarding the interaction of hypertension and nephropathy in diabetes mellitus, any conclusions at this time about what is inherited and what is acquired must be regarded as speculative. However we will discuss some potential mechanisms of hypertension in the evolution of nephropathy and we will allude to the role for novel genetic studies in the search for nephropathy susceptibility gene(s).

A 90-kD Na(+)-H+ exchanger kinase has increased activity in spontaneously hypertensive rat vascular smooth muscle cells

Increased activity of the Na(+)-H+ exchanger (NHE-1 isoform) has been observed in cells and tissues from hypertensive humans and animals, including the spontaneously hypertensive rat (SHR). No mutation in NHE-1 DNA sequence or alteration in NHE-1 mRNA and protein expression has been demonstrated in hypertension, indicating that alterations in proteins that regulate NHE-1 activity are responsible for increased activity. The recent finding that NHE-1 phosphorylation in SHR vascular smooth muscle cells (VSMCs) was greater than in Wistar-Kyoto rat (WKY) VSMCs suggested that NHE-1 kinases may represent an abnormal regulatory pathway present in hypertension. To define NHE-1 kinases altered in the hypertensive phenotype. We measured NHE-1 kinase activity by an in-gel-kinase assay using a recombinant glutathione S-transferase NHE-1 fusion protein as a substrate. At least 7 NHE-1 kinases (42 to 90 kD) were present in VSMCs. We studied a 90-kD kinase because it was the major NHE-1 kinase and exhibited differences between SHR and WKY. Comparison of 90-kD kinase activity revealed that SHR VSMCs had increased activity in growth-arrested cells and in cells stimulated by angiotensin II (100 nmol/L for 5 minutes). Activation of the 90-kD kinase by angiotensin II was Ca2+ dependent, PKC independent, and partially dependent on the mitogen-activated protein kinase pathway. These findings indicate that increased activity of a 90-kD NHE-1 kinase is a characteristic of SHR VSMCs in culture and suggest that alterations in the 90-kD NHE-1 kinase and/or proteins that regulate its activity may be a pathogenic component in hypertension in the SHR.

Two distinct uptake mechanisms for ascorbate and dehydroascorbate in human lymphoblasts and their interaction with glucose

In diabetes, a major cause of mortality is from cardiovascular causes, and low levels of antioxidants such as vitamin C have been associated with such complications. Leucocyte ascorbic acid status can reflect total body stores but the mechanisms that mediate the uptake of ascorbic acid (AA) or dehydroascorbic acid (DHA) in human lymphoid cells are undefined. We have investigated the uptake of AA and DHA with mass assays in human lymphoblasts by using HPLC, with precautions to prevent the oxidation of AA and to take into account the instability of DHA in buffers. Human lymphoblasts exhibit distinct uptake mechanisms for both AA and DHA, with Vmax values of 1.35+/-0.14 and 29.0+/-5.8 nmol/h per 10(6) cells and Km values of 23.5+/-6 and 104+/-84 microM respectively. The AA uptake was Na+-dependent and inhibitable with ouabain, whereas DHA uptake was independent of Na+ and ouabain-insensitive. Both uptake mechanisms were inhibited by phloretin or cytochalasin B. AA uptake was decreased significantly (by 13+/-2%) only at extracellular glucose concentrations of 20 mM (P<0.05). In contrast, glucose competitively inhibited DHA uptake with a Ki of 2.2 mM so that DHA uptake was decreased by glucose even in the physiological range. Phorbol esters stimulated AA but not DHA uptake; this was abolished in the presence of extracellular reductant, indicating that AA was converted to DHA before uptake occurred. Prolonged increased glucose levels (20 mM) led to a decrease in the Vmax of DHA uptake. At concentrations of plasma AA or DHA, the AA uptake mechanism might be nearly half-saturated but the DHA mechanism has enormous spare capacity. This allows for cellular uptake and regeneration of AA from DHA derived from oxidative stress. In diabetes, high glucose levels might impair DHA uptake acutely by competitive inhibition or by down-regulation of uptake with chronic glucose exposure, leading to an impaired ability to store and recycle oxidized AA.

Calcium-induced activation of the rat vascular myocyte Na+/H+ exchanger isoform-1

An established intermediate phenotype of human hypertension and diabetic nephropathy is an elevation of Na+/H+ exchanger (NHE) activity, but the mechanism for this is unclear. This phenotype is maintained in vascular myocytes from the spontaneously hypertensive rat (SHR) compared with the normotensive Wistar Kyoto rat (WKY). Since intracellular calcium levels ([Ca2+]i) following agonist stimulation were elevated in cells from both hypertensive humans and SHR, we have examined the role of calcium-calmodulin (CaM) in the mechanism of increased NHE activity in vascular myocytes of SHR by determining the activity and phosphorylation state of NHE isoform-1 (NHE-1) in cells from SHR and WKY when [Ca2+]i was elevated by the ionophores A23187 or ionomycin. NHE activity was measured using fluorometry and NHE-1 phosphorylation by immunoprecipitating the exchanger from 32P-orthophosphate-labeled cells with a polyclonal NHE-1-specific antibody. The ionophore A23187 increased [Ca2+]i in both cell types to approximately 700 to 800 nmol x L(-1), and led to stimulation of NHE-1 activity only in WKY myocytes, with no effect on SHR cells. An inhibitor of CaM kinase II (KN-62) failed to abolish stimulation of NHE-1 by A23187 in WKY cells, and had no effect on unstimulated NHE-1 activity in both cell types. Ionomycin also elevated [Ca2+]i in both cell types to approximately 1,000 nmol x L(-1) and activated NHE-1 activity in only WKY cells. Activation of NHE-1 in WKY cells by an increased [Ca2+]i was not mediated by an increase in NHE-1 phosphorylation, whether in the presence or absence of KN-62. The elevated NHE-1 phosphorylation in SHR cells was not affected by elevated [Ca2+]i or KN-62. Calmodulin-agarose beads bound NHE-1 extracted from SHR cells to a lesser extent than that from WKY cells. We conclude that calcium-induced NHE-1 activation in WKY cells was not mediated by CaM kinase II. The elevated NHE-1 activity and phosphorylation of SHR cells was not further modulated by increased [Ca2+]i, and was also independent of CaM kinase II. Non-phosphorylation-dependent mechanisms of activation of NHE-1 may therefore be responsible for alterations of NHE-1 activity in these cells, such as the direct binding of CaM to NHE-1. This direct binding of CaM to NHE-1 may be impaired in SHR compared with WKY cells.

Enhanced mitogen-activated protein kinase activity and phosphorylation of the Na+/H+ exchanger isoform-1 of human lymphoblasts in hypertension

Increased activity of the Na+/H+ exchanger isoform-1 (NHE-1) is recognized as an intermediate phenotype for hypertension, but the basis for this association is unclear. We have previously demonstrated an increased phosphorylation of NHE-1 in lymphoblasts from hypertensives that was associated with increased cell proliferation. Due to the central importance of mitogen-activated protein kinases (MAPKs) in signaling cascades transducing responses from extracellular growth factors and hormones, we examined the activity of this kinase in a specific peptide phosphorylation assay. Cells from hypertensives showed a significant twofold enhancement of MAPK activity (P < .001). This was not associated with any increase in p42mapk and p44mapk protein content. There was no significant increase in the level of tyrosine phosphorylation of MAPK in cells from hypertensives. MAPK activity was correlated with NHE-1 activity (r(s) = .55, P < .01) and phosphorylation (r(s) = .51, P < .02). These findings suggest that the increased cell proliferation rate, NHE-1 activity, and phosphorylation of lymphoblasts from hypertensives may be associated with enhanced MAPK activity, suggesting upregulation of this signaling pathway in hypertension.

Growth phenotype of cultured skin fibroblasts from IDDM patients with and without nephropathy and overactivity of the Na+/H+ antiporter

An increased activity of the Na+/H+ antiporter in cells from patients with insulin-dependent diabetes mellitus (IDDM) has been proposed as a potential marker of nephropathy. We evaluated Na+/H+ antiporter activity and its relationship to DNA and protein synthesis in cultured skin fibroblasts from patients with IDDM classified as having either overt nephropathy or absence of nephropathy on the basis of urinary albumin excretion and kidney biopsy findings. In IDDM patients with overt nephropathy, Na+/H+ antiporter activity in serum stimulated cells was increased as compared to cells from control subjects (9.62 +/- 0.89 vs. 5.67 +/- 0.97 mmol H+/min, P < 0.005, respectively) and cells from IDDM patients without nephropathy (7.22 +/- 0.67 mmol H+/min, P < 0.025). By contrast, in cells made quiescent by serum deprivation Na+/H+ antiporter activity was lower than in serum-stimulated cells and there were no significant differences between the three groups. DNA synthesis assessed by [3H] thymidine incorporation was increased in the IDDM group with nephropathy as compared to the group without nephropathy (138 +/- 14 vs. 105 +/- 13 cpm/1000 cells, respectively, P < 0.05) and as compared to control subjects (65 +/- 11 cpm/1000 cells, P < 0.001). By contrast, protein synthesis assessed by [14C] L-leucine incorporation was not increased in fibroblasts from IDDM patients with nephropathy, suggesting that cellular hypertrophy is not a feature of their altered growth phenotype. After chronic inhibition of the Na+/H+ antiporter using EIPA (25 microM), [3H] thymidine incorporation was reduced by about 20% both in cells from IDDM patients and controls. This parameter therefore remained higher in cells from IDDM patients with nephropathy than in those from controls (81 +/- 16 vs. 40 +/- 6 cpm/1000 cells, P < 0.05), while in cells from IDDM patients without nephropathy [3H] thymidine incorporation after EIPA (56 +/- 7.0 cpm/1000 cells) was intermediate between cells from controls and IDDM patients with nephropathy. We argue that cultured skin fibroblasts from IDDM patients, with nephropathy display an abnormal growth phenotype characterized by cell hyperplasia. This growth phenotype is associated with overactivity of the Na+/H+ antiporter during serum stimulation but not when cells are made quiescent and persists after inhibition of the Na+/H+ antiporter. Our data, therefore, further shows that overactivity of the Na+/H+ antiporter is not required for the expression of the altered growth phenotype of cultured skin fibroblasts from IDDM patients with nephropathy.

Increased lymphocytic Na+/H+ exchange activity after hemodialysis: evidence for an endogenous inhibitor of Na+/H+ exchange in patients with end-stage renal failure

The Na+/H+ exchange antiport activity was measured in lymphocytes from 16 patients with end-stage renal failure pre- and postdialysis. In addition the effect of the patients' plasma on lymphocytes from healthy subjects was tested. Resting pH (pHi) was not significantly different in lymphocytes pre- and postdialysis. On the other hand, the Na+/H+ exchange activity was significantly lower in lymphocytes before hemodialysis (6.22 +/- 0.73 x 10(-3) pHi/s) than after hemodialysis (9.32 +/- 1.58 x 10(-3) pHi/s; n = 16; p < 0.05). The buffer capacity was not significantly different before and after hemodialysis. The incubation of lymphocytes from healthy control subjects with plasma from patients with end-stage chronic renal failure significantly reduced the lymphocytic Na+/H+ exchange activity. The addition of ultrafiltrate also significantly reduced the Na+/H+ exchange activity in lymphocytes from healthy control subjects. The study indicates the existence of an endogenous inhibitor of the Na+/H+ exchange that is accumulated in plasma from patients with end-stage chronic renal failure.

Premature senescence of skin fibroblasts from insulin-dependent diabetic patients with kidney disease

Diabetic glomerulopathy develops in a subset only of patients with insulin-dependent diabetes (IDDM) and early, in its course, is characterized by cell hypertrophy and by excessive extracellular matrix production. These observations suggest that an alteration in the control of cell growth processes may contribute to its pathogenesis and be related to the susceptibility to kidney disease. We therefore investigated whether the development of diabetic nephropathy is associated with abnormalities of cell growth and morphology. Cultured skin fibroblasts from 14 IDDM patients with nephropathy (DN) were compared with those of 10 IDDM patients without nephropathy (D) and of 14 control non-diabetic subjects (C). Cell volume (in arbitrary units) and total protein content (microgram/10, 000 cells) were increased in serially passaged skin fibroblasts of IDDM patients with nephropathy (DN = 809.5 +/- 33.1 and 1.93 +/- 0.38 vs. D = 764.4 +/- 31.5 and 1.5 +/- 0.37, P = 0.005 and P = 0.03, respectively; vs. C = 756.2 +/- 36.3 and 1.5 +/- 0.38, P = 0.0006 and P = 0.03, respectively). These hypertrophic cells had a tendency to a slower duplication rate and exhibited a dissociation of the DNA and cytoplasmic cell-cycles, resulting in a higher proportion of tetraploid cells (DN = 25 +/- 15% vs. D = 6 +/- 4%, P = 0.005; and vs. C = 10 +/- 8%, P = 0.04). The frequency of terminally differentiated post-mitotic fibrocytes, cells specialized for extracellular matrix production, was higher in patients with nephropathy compared to that of patients without nephropathy and normal controls (DN = 34 +/- 14% vs. D = 21 +/- 10%, P = 0.02; and vs. C = 19 +/- 12%, P = 0.008). That early differentiation was a specific feature of cells derived from patients with diabetic nephropathy was confirmed by the study of cell life-span which demonstrated that these cells aged prematurely (log rank test, chi 2 = 10,012; P = 0.0067). We conclude that an acceleration of cell aging is a peculiar feature of diabetic kidney disease and may contribute to its pathological tissue changes.

Abnormal Na+/H+ antiport activity in cultured fibroblasts from NIDDM patients with hypertension and microalbuminuria

An increased activity of Na+/H+ antiport has been reported in leukocytes and fibroblasts from insulin-dependent diabetic (IDDM) patients with nephropathy. To test whether a similar abnormality is present in fibroblasts from non-insulin-dependent diabetic (NIDDM) patients with microalbuminuria and hypertension, we examined intracellular pHi and Na+/H+ antiport activity, using the pH sensitive dye 2', 7'-bis (2-carboxyethyl-5(6)-carboxyfluorescein (BCECF), in cultured skin fibroblasts obtained from 34 NIDDM patients, divided into four groups based upon whether they had microalbuminuria or hypertension, or both: Group 1, nine NIDDM patients with microalbuminuria and hypertension. Group 2, nine NIDDM patients with hypertension and normal albumin excretion rate. Group 3, seven NIDDM patients with microalbuminuria and normal blood pressure. Group 4, nine NIDDM patients with normal blood pressure and normal albumin excretion rate. Nine normal subjects served as control group. Resting pHi was more alkaline in fibroblasts from Group 1 (7.22 +/- 0.03; p < 0.05), Group 2 (7.21 +/- 0.02; p < 0.05) and Group 3 (7.19 +/- 0.02, p = 0.17) than in Group 4 and normal subjects. This was due to higher Vmax values of Na+/H+ antiport activity in cultured fibroblasts from Group 1 (52.1 +/- 5.3 mmol H+/min; p < 0.05), Group 2 (57.7 +/- 8.3; p < 0.05) and Group 3 (60.6 +/- 7.4, p < 0.05) than those from Group 4 (31.2 +/- 3.6) or control subjects (31.3 +/- 3.5). The intracellular pH for half-maximal activation, Hill coefficient and buffering power capacity was similar in all the groups. These data suggest that in vitro phenotypic abnormalities of long-term cultured fibroblasts from NIDDM patients with microalbuminuria and/ or hypertension are likely to be, at least in part, independent of the degree of metabolic control in vivo and to be an intrinsic feature of these cells.

Na+/H+ exchange in hypertension and in diabetes mellitus--facts and hypotheses

An enhancement of Na+/H+ exchange (NHE) in blood cells of selected patients with essential hypertension and with diabetic nephropathy has been described by various investigators. Recent studies have shown that enhanced NHE activity persists in immortalized lymphoblasts from these patients after prolonged cell culture and, thus, appears to be under genetic control. Available evidence strongly argues against a mutation in the encoding gene or an overexpression of the NHE. Immortalized cells from hypertensive patients with enhanced NHE activity display two-fold enhanced agonist-induced rises of the cytosolic free Ca2+ concentration and the underlying reason was identified as an increased activation of pertussis toxin (PTX)-sensitive G proteins. The molecular mechanism(s) of this phenomenon have not yet been elucidated. It appears likely that similar changes contribute to the enhanced NHE activity phenotype in diabetic nephropathy, although experimental evidence for this is still lacking. An enhanced activation of PTX-sensitive G proteins could explain many of the hitherto unexplained phenomena in essential hypertension, e.g. inheritance, increased vasoconstriction, hypertrophy of remodeling of arterial blood vessels and the heart, enhanced platelet aggregation etc. In diabetes the same defect could provide the basis for the susceptibility to nephropathy, e.g. by enhancing the deleterious effects of autocrine and paracrine growth factors. Thus, the experimental approach of immortalizing blood cells from patients with essential hypertension and diabetic nephropathy has opened new horizons in the identification of genetically fixed abnormalities in intracellular signal transduction which could contribute to both pathologies and which can now be studied without the confounding influences of the diabetic or hypertensive in vivo milieu.

Phorbol ester activation of the rat vascular myocyte Na(+)-H(+) exchanger isoform 1

Vascular myocytes from the spontaneously hypertensive rat (SHR) demonstrate elevated Na(+)-H(+) exchanger activity associated with increased cell proliferation and hyperresponsiveness to agonists such as phorbol esters. Since the Na(+)-H(+) exchanger isoform 1 (NHE-1) is stimulated by protein kinase C, we have investigated the effects of phorbol esters on NHE-1 activity and its phosphorylation in vascular myocytes of these rats. SHR cells demonstrated a larger alkalinization response to 12-O-tetradecanoylphorbol 13-acetate than Wistar-Kyoto rat (WKY) cells. Kinetic analyses indicated that whereas 12-O-tetradecanoylphorbol 13-acetate increased the maximal transport capacity of NHE-1 in both cell types, affinity for H+ was increased in WKY cells and cooperativity for H+ at the internal modifier site was reduced in SHR cells. In neither cell type was the subcellular distribution of NHE-1 altered by phorbol ester stimulation. NHE-1 phosphorylation was markedly reduced in WKY cells stimulated by the phorbol ester, an effect abolished by inhibition of protein kinase C. In contrast, NHE-1 phosphorylation in quiescent SHR cells was approximately double that of WKY cells and was reduced after phorbol ester treatment. Inhibition of protein kinase C in SHR cells led to a marked elevation of NHE-1 phosphorylation that was not associated with a change in the exchanger activity, but WKY cells exhibited a small, insignificant rise in NHE-1 phosphorylation. Thus, the kinetic responses of NHE-1 to phorbol esters in vascular myocytes of these rat strains are different, the changes in exchanger kinetics of SHR resembling those described in human hypertension. NHE-1 phosphorylation has an inverse relationship with protein kinase C activity. However, modulation of NHE-1 phosphorylation may not be associated with concurrent alterations in activity, indicating a role for non-phosphorylation-dependent mechanisms.

Glucose-induced changes in activity and phosphorylation of the Na+/H+ exchanger, NHE-1, in vascular myocytes from Wistar-Kyoto and spontaneously hypertensive rats

Increased Na+/H+ exchanger (NHE) activity has been demonstrated in cells from patients with hypertension and diabetic nephropathy. Vascular myocytes from the spontaneously hypertensive rat (SHR) also exhibit increased NHE activity as compared with cells from the normotensive Wistar Kyoto rat (WKY). The interaction of increased glucose concentrations with NHE activity is unclear. The effect of glucose on NHE activity, NHE-1 (isoform 1) protein expression, and phosphorylation of cultured vascular myocytes from these rat strains was thus investigated. NHE activity was determined fluorometrically with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). A rabbit NHE-1-specific polyclonal antibody was used (1) to measure NHE-1 abundance in Western blots of cell extracts and (2) for immunoprecipitating 32P-labeled NHE-1. Cells from SHR exhibited increased NHE activity and NHE-1 phosphorylation as compared with cells from WKY, with similar NHE-1 protein content per cell. Incubation in 25 mmol.L-1 glucose for 24 hours led to increased NHE activity only in WKY cultures, with no change in NHE-1 protein but a concomitantly reduced NHE-1 phosphorylation. Changes in NHE activity in WKY cells were reversed by inhibition of protein kinase C. Incubation of SHR cells with 25 mmol.L-1 glucose did not enhance the increased NHE activity or NHE-1 phosphorylation present in these cells. Thus, high glucose levels have disparate effects on NHE activity and NHE-1 phosphorylation in cells from different rat strains. The glucose-induced increase in NHE-1 turnover number in WKY cells is not mediated by an increase in its direct phosphorylation, but is dependent on protein kinase C.