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

CD36 overexpression: a possible etiopathogenic mechanism of atherosclerosis in patients with prediabetes and diabetes

RATIONALE

CD36 is a scavenger receptor located on monocytes which is involved in foam cell transformation.

AIM

To evaluate CD36 expression under different glycemic states in both healthy subjects and in atherosclerotic patients.

SUBJECTS AND METHODS

In order to evaluate the possible effects of hyperglycemia on CD36 expression in healthy subjects, an in vitro experiment was carried out using monocyte in three different conditions: extreme hyperglycemia (HG), euglycemia (EG) and in the absence of glucose. On the other hand, three groups of atherosclerotic patients were evaluated according to their glycemic conditions: normoglycemic (NG), prediabetic (preDM) and diabetic (DM) patients. CD36 expression (mRNA, non-glycated and glycated protein) was analyzed in monocytes.

RESULTS

CD36 mRNA expression in the in vitro experiment peaked at 4 and 24 h under HG conditions. No differences in mRNA levels were found in the EG and control group. The level of non-glycated proteins was higher in HG and EG conditions compared with control group. Glycated protein expression was inhibited by glucose in a sustained manner. In atherosclerotic patients, a significant association was observed when comparing glycated CD36 protein expression in DM with NG patients (p = 0.03). No significant differences were found in mRNA and non-glycated CD36 expression in these patients. Moreover, BMI, insulin, weight and treatment were shown to be related to CD36 expression (mRNA, non-glycated and glycated protein levels, depending of the case) in atherosclerotic patients.

CONCLUSIONS

Hyperglycemia is an important modulator of CD36 mRNA and non-glycated protein expression in vitro, increasing de novo synthesis in healthy subjects. In atherosclerotic patients, there are progressive increases in CD36 receptors, which may be due to a post-translational stimulus.

Na+/H+-exchanger-1 inhibition counteracts diabetic cataract formation and retinal oxidative-nitrative stress and apoptosis

The Na⁺-H⁺-exchanger-1 (NHE-1) controls intracellular pH and glycolytic enzyme activities, and its expression and activity are increased by diabetes and high glucose. NHE-1-dependent upregulation of the upper part of glycolysis, under conditions of inhibition (lens) or insufficient activation (retina) of glyceraldehyde 3-phosphate dehydrogenase, underlies diversion of the excessive glycolytic flux towards several pathways contributing to oxidative stress, a causative factor in diabetic cataractogenesis and retinopathy. This study evaluated the role for NHE-1 in diabetic cataract formation and retinal oxidative stress and apoptosis. Control and streptozotocin-diabetic rats were maintained with or without treatment with the NHE-1 inhibitor cariporide (Sanofi-Aventis, 10 mgkg⁻¹d⁻¹) for 3.5 months. In in vitro studies, bovine retinal pericytes and endothelial cells were cultured in 5 or 30 mM glucose, with or without 10 μM cariporide, for 7 days. A several-fold increase of the by-product of glycolysis, α-glycerophosphate, indicative of activation of the upper part of glycolysis, was present in both rat lens and retina at an early (1-month) stage of streptozotocin-diabetes. Cariporide did not affect diabetic hyperglycemia and counteracted lens oxidative-nitrative stress and p38 MAPK activation, without affecting glucose or sorbitol pathway intermediate accumulation. Cataract formation (indirect ophthalmoscopy and slit-lamp examination) was delayed, but not prevented. The number of TUNEL-positive cells per flat-mounted retina was increased 4.4-fold in diabetic rats (101±17 vs. 23±8 in controls, P<0.01), and this increase was attenuated by cariporide (45±12, P<0.01). Nitrotyrosine and poly(ADP-ribose) fluorescence and percentage of TUNEL-positive cells were increased in pericytes and endothelial cells cultured in 30 mM glucose, and these changes were at least partially prevented by cariporide. In conclusion, NHE-1 contributes to diabetic cataract formation, and retinal oxidative-nitrative stress and apoptosis. The findings identify a new therapeutic target for diabetic ocular complications.

Involvement of signaling molecules on na/h exchanger-1 activity in human monocytes

Background:

Sodium/hydrogen exchanger-1 (NHE-1) contributes to maintaining intracellular pH (pHi). We assessed the effect of glucose, insulin, leptin and adrenaline on NHE-1 activity in human monocytes in vitro. These cells play a role in atherogenesis and disturbances in the hormones evaluated are associated with obesity and diabetes.

Methods and Results:

Monocytes were isolated from 16 healthy obese and 10 lean healthy subjects. NHE-1 activity was estimated by measuring pHi with a fluorescent dye. pHi was assessed pre- and post-incubation with glucose, insulin, leptin and adrenaline. Experiments were repeated after adding a NHE-1 inhibitor (cariporide) or an inhibitor of protein kinase C (PKC), nitric oxide synthase (NOS), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, phosphoinositide 3-kinases (PI3K) or actin polymerization. Within the whole study population, glucose enhanced NHE-1 activity by a processes involving PKC, NOS, PI3K and actin polymerization (p = 0.0006 to 0.01). Insulin-mediated activation of NHE-1 (p = <0.0001 to 0.02) required the classical isoforms of PKC, NOS, NADPH oxidase and PI3K. Leptin increased NHE-1 activity (p = 0.0004 to 0.04) through the involvement of PKC and actin polymerization. Adrenaline activated NHE-1 (p = <0.0001 to 0.01) by a process involving the classical isoforms of PKC, NOS and actin polymerization. There were also some differences in responses when lean and obese subjects were compared. Incubation with cariporide attenuated the observed increase in NHE-1 activity.

Conclusions:

Selective inhibition of NHE-1 in monocytes could become a target for drug action in atherosclerotic vascular disease.

Oxidized laminin-1 induces increased monocyte attachment and expression of ICAM-1 in endothelial cells

Atherosclerosis has been associated with increased oxidative stress and monocyte recruitment by endothelial cells. Sub-endothelial basement membrane proteins, such as laminins that play a central role in cell adhesion, are exposed to reactive oxygen species. In the present study monocyte attachment on human umbilical cord vein endothelial cells (HUVEC) that were preattached to oxidized or native laminin, was investigated. Intracellular cell adhesion molecule-1 (ICAM-1) expression by HUVEC was estimated by an enzyme-linked immunosorbent assay. HUVEC attachment to oxidized or native laminin-1 was examined using the Hemacolor kit. Anti-alphaL, anti-alphaM, anti-alpha2 and anti-beta2 integrin subunit antibodies were used in order to further investigate the above phenomena. HUVEC that were preattached to oxidized laminin expressed higher levels of ICAM-1 and monocytes attached at a higher degree to these cells as compared to HUVEC that were preattached to native laminin. Incubation of monocytes with monoclonal antibodies against the alphaM and beta2 integrin subunits equalized the above mentioned differences. Moreover, HUVEC attached to oxidized laminin at a higher degree as compared to native laminin. This difference was equalized after incubation with the antibody against the alpha2 integrin subunit. These results indicate a modified interaction between HUVEC and the basement membranes in cases where laminin is oxidatively modified. This modified interaction results in increased ICAM-1 expression by endothelial cells and consequently increased monocyte recruitment capacity.

The ambiguous role of the Na+-H+ exchanger isoform 1 (NHE1) in leptin-induced oxidative stress in human monocytes

Leptin, a 16-kDa cytokine produced mainly by the adipose tissue, is known to increase energy expenditure while at the same time lowering food intake by acting directly on the hypothalamus. ObRb, the leptin receptor mostly involved in intracellular signaling, is expressed in a wide range of tissues, thus allowing leptin to affect a much broader diversity of biological processes. High concentrations of leptin are encountered in patients with hyperleptinemia, a condition which very often accompanies obesity and which is a direct result of leptin resistance. In the present study, moderate and high concentrations of leptin (16 and 160 ng/ml) were mostly utilized in order to investigate the role of this cytokine in oxidative stress levels in human monocytes. Leptin was found to increase oxidative species production as measured with 2',7'-dichlorodihydrofluorescein diacetate (general marker of oxidative species, but not O2-*) and dihydroethidium (marker of O2-*). Surprisingly, it also augmented superoxide dismutase activity. Inhibition of the Na+-H+ exchanger isoform 1 (NHE1) also inhibited leptin-induced superoxide anion production but at the same time amplified leptin-induced production of other oxidative species. Signaling proteins such as phosphoinositide 3 kinase and conventional isoforms of protein kinase C (alpha-, beta(i)-, beta(ii)-), as well as NADPH oxidase, also participated in leptin signaling. Finally, leptin was found to increase glutathionylation levels of NHE1-bound heat shock protein 70 kDa (Hsp70) but not Hsp70 binding to NHE1.