Increased transendothelial permeation of albumin by high glucose concentration

Diametrical Effects of Glucose Levels on Microvascular Permeability of Peripheral Nerves in Patients With Type 2 Diabetes With and Without Diabetic Neuropathy

Clinical studies investigating the benefit of glucose control on the progression of diabetic neuropathy (DN) have come to controversial results in patients with type 2 diabetes (T2D). This study aimed to assess associations of HbA1c levels with parameters of nerve perfusion in patients with T2D with and without DN using dynamic contrast-enhanced magnetic resonance neurography (DCE-MRN) at 3 Tesla. A total of 58 patients with T2D (20 with DN and 38 without DN) took part in this cross-sectional single-center study. Groups were matched for age, BMI, HbA1c, duration of T2D, and renal function. All patients underwent DCE-MRN with subsequent electrophysiologic and serologic testing. The extended Tofts model was used to quantify the sciatic nerve's microvascular permeability (Ktrans), volume fraction of the extracapillary extracellular space, and volume fraction of the plasma space. As a main result, we found that Ktrans correlated positively with HbA1c in patients with DN, while a negative correlation between the two parameters was found in patients without DN. Our results indicate that the effect of glucose control on the capillary permeability of peripheral nerves differs between patients with T2D with and without DN.

Photodynamic Opening of the Blood–Brain Barrier Using Different Photosensitizers in Mice

In a series of previous studies, we demonstrated that the photodynamic therapy (PDT), as a widely used tool for treatment of glioblastoma multiforme (GBM), also site-specifically opens the blood–brain barrier (BBB) in PDT-dose and age-related manner via reversible disorganization of the tight junction machinery. To develop the effective protocol of PDT-opening of the BBB, here we answer the question of what kind of photosensitizer (PS) is the most effective for the BBB opening. We studied the PDT-opening of the BBB in healthy mice using commercial photosensitizers (PSs) such as 5-aminolevulenic acid (5-ALA), aluminum phthalocyanine disulfonate (AlPcS), zinc phthalocyanine (ZnPc) and new synthetized PSs such as galactose functionalized ZnPc (GalZnPc). The spectrofluorimetric assay of Evans Blue albumin complex (EBAC) leakage and 3-D confocal imaging of FITC-dextran 70 kDa (FITCD) extravasation clearly shows a revisable and dose depended PDT-opening of the BBB to EBAC and FITCD associated with a decrease in presence of tight junction (TJ) in the vascular endothelium. The PDT effects on the BBB permeability, TJ expression and the fluorescent signal from the brain tissues are more pronounced in PDT-GalZnPc vs. PDT-5-ALA/AlPcS/ZnPc. These pre-clinical data are the first important informative platform for an optimization of the PDT protocol in the light of new knowledge about PDT-opening of the BBB for drug brain delivery and for the therapy of brain diseases.

The locally activated renin-angiotensin system is involved in albumin permeability in glomerular endothelial cells under high glucose conditions

Background

Although the diabetic milieu per se , hemodynamic changes, oxidative stress and local growth factors such as angiotensin II (AII) are considered to be mediators in the pathogenesis of diabetic nephropathy, the underlying pathways mediating the changes in glomerular endothelial cells (GECs) are not well understood. Therefore, we investigated changes in the renin-angiotensin system (RAS) components in high glucose (HG)-stimulated GECs and the role of the local RAS in morphological and functional changes in GECs under diabetic conditions.

Methods

We stimulated GECs with 5.6 mM glucose or 30 mM glucose with or without an angiotensin II type I receptor blocker (ARB) in vitro and also performed experiments with Sprague-Dawley rats injected with diluent ( n = 16) or streptozotocin [ n = 16, diabetes (DM)]. Eight rats from each group were treated with ARB for 3 months in vivo . Real-time polymerase chain reaction, western blot analysis, enzyme-linked immunosorbent assay and immunofluorescent staining using cultured GECs were performed. The permeability of GECs to macromolecules was assessed by measuring the passage of fluorescein isothiocyanate-labeled bovine serum albumin. Morphological changes were also evaluated by scanning and transmission electron microscopy.

Results

There were significant increases in angiotensinogen expression in HG-stimulated GECs along with significant increases in AI and AII levels. Moreover, the expression of heparan sulfate glycosaminoglycans (HS-GAG) assessed by immunofluorescent staining was significantly lower and the permeability to albumin was significantly higher in GECs exposed to HG medium, and ARB treatment significantly abrogated these changes. Upon electron microscopy examination, the mean size of the GEC fenestrae was significantly greater in HG-stimulated GECs and DM rats, and these increases were significantly ameliorated by ARB.

Conclusions

The local RAS within GECs was activated under HG conditions, and this activation may be associated with both an alteration in GEC fenestration and a decrease in HS-GAG, resulting in the development of albuminuria in diabetic nephropathy.

First-Trimester Nuchal Translucency Screening in Pregnant Women Who Subsequently Developed Gestational Diabetes

OBJECTIVE

To compare the nuchal translucency (NT) thickness at 11-14 weeks of gestation in women who will later develop gestational diabetes mellitus (GDM) with that of women with normal glucose tolerance (NGT) in the second trimester of pregnancy.

METHODS

Four hundred sixty-four women underwent NT screening at 11-14 weeks of gestation according to the standards of the Fetal Medicine Foundation. They all underwent an oral glucose tolerance test (OGTT) between 24 and 28 weeks.

RESULTS

Women with GDM (n = 135) were significantly older (34.0 years [+/-4.8] vs 29.4 years [+/-5.2]; P < .001), had a significantly higher body mass index (BMI) (28.2 kg/m2 [+/-6.0] vs 24.4 [+/-5.0]; P < .001), and had a significantly higher serum concentration of hemoglobin A1c (HbA1c) at the time of the NT measurement (5.38% [+/-0.6] vs 4.95% [+/-0.4]; P < .001] than women with NGT (n = 329). There is no significant difference in the NT measurement at 11 to 14 weeks of gestation between women with GDM and NGT after correction for crown rump length (CRL) (r = -.08, P = .45). We performed multiple linear regression analysis with NT as the dependent variable, and BMI, maternal age, CRL, and GDM/NGT as independent variables. CRL (P < .001) was significantly related to the NT measurement, whereas GDM/NGT, BMI, and maternal age were not.

CONCLUSION

As no significant difference in the NT measurement between women with GDM and NGT was observed, the risk estimation for chromosomal abnormality derived from the maternal age and NT measurement can also be used in women with glucose disorders.

Ascorbic acid efflux from human brain microvascular pericytes: role of re-uptake

Microvascular pericytes take up ascorbic acid on the ascorbate transporter SVCT2. Intracellular ascorbate then protects the cells against apoptosis induced by culture at diabetic glucose concentrations. To investigate whether pericytes might also provide ascorbate to the underlying endothelial cells, we studied ascorbate efflux from human pericytes. When loaded with ascorbate to intracellular concentrations of 0.8-1.0 mM, almost two-thirds of intracellular ascorbate effluxed from the cells over 2 H. This efflux was opposed by ascorbate re-uptake from the medium, since preventing re-uptake by destroying extracellular ascorbate with ascorbate oxidase increased ascorbate loss even further. Ascorbate re-uptake occurred on the SVCT2, since its blockade by replacing medium sodium with choline, by the SVCT2 inhibitor sulfinpyrazone, or by extracellular ascorbate accelerated ascorbate loss from the cells. This was supported by finding that net efflux of radiolabeled ascorbate was increased by unlabeled extracellular ascorbate with a half-maximal effect in the range of the high affinity Km of the SVCT2. Intracellular ascorbate did not inhibit its efflux. To assess the mechanism of ascorbate efflux, known inhibitors of volume-regulated anion channels (VRACs) were tested. These potently inhibited ascorbate transport into cells on the SVCT2, but not its efflux. An exception was the anion transport inhibitor DIDS, which, despite inhibition of ascorbate uptake, also inhibited net efflux at 25-50 µM. These results suggest that ascorbate efflux from vascular pericytes occurs on a DIDS-inhibitable transporter or channel different from VRACs. Further, ascorbate efflux is opposed by re-uptake of ascorbate on the SVCT2, providing a potential regulatory mechanism.

Ascorbic acid transport in brain microvascular pericytes

Intracellular vitamin C, or ascorbic acid, has been shown to prevent the apoptosis of cultured vascular pericytes under simulated diabetic conditions. We sought to determine the mechanism by which ascorbate is transported into pericytes prior to exerting this protective effect. Measuring intracellular ascorbate, we found that pericytes display a linear uptake over 30 min and an apparent transport Km of 21 μM, both of which are consistent with activity of the Sodium-dependent Vitamin C Transporter 2 (SVCT2). Uptake of both radiolabeled and unlabeled ascorbate was prevented by inhibiting SVCT2 activity, but not by inhibiting the activity of GLUT-type glucose transporters, which import dehydroascorbate to also generate intracellular ascorbate. Likewise, uptake of dehydroascorbate was prevented with the inhibition of GLUTs, but not by inhibiting the SVCT2, indicating substrate specificity of both transporters. Finally, presence of the SVCT2 in pericytes was confirmed by western blot analysis, and immunocytochemistry was used to localize it to the plasma membrane and intracellular sites. Together, these data clarify previous inconsistencies in the literature, implicate SVCT2 as the pericyte ascorbate transporter, and show that pericytes are capable of concentrating intracellular ascorbate against a gradient in an energy- and sodium-dependent fashion.

Ascorbate reverses high glucose- and RAGE-induced leak of the endothelial permeability barrier

High glucose concentrations due to diabetes increase leakage of plasma constituents across the endothelial permeability barrier. We sought to determine whether vitamin C, or ascorbic acid (ascorbate), could reverse such high glucose-induced increases in endothelial barrier permeability. Human umbilical vein endothelial cells and two brain endothelial cell lines cultured at 25 mM glucose showed increases in endothelial barrier permeability to radiolabeled inulin compared to cells cultured at 5mM glucose. Acute loading of the cells for 30-60 min with ascorbate before the permeability assay prevented the high glucose-induced increase in permeability and decreased basal permeability at 5mM glucose. High glucose-induced barrier leakage was mediated largely by activation of the receptor for advanced glycation end products (RAGE), since it was prevented by RAGE blockade and mimicked by RAGE ligands. Intracellular ascorbate completely prevented RAGE ligand-induced increases in barrier permeability. The high glucose-induced increase in endothelial barrier permeability was also acutely decreased by several cell-penetrant antioxidants, suggesting that at least part of the ascorbate effect could be due to its ability to act as an antioxidant.

Acute and sustained actions of hyperglycaemia on endothelial and glomerular barrier permeability

Microalbuminuria is an established marker of systemic endothelial dysfunction, which for patients with diabetes signals an increased risk of both diabetic nephropathy and cardiovascular complications. A better understanding of the pathogenesis of microalbuminuria is important in the quest of finding new approaches to treat patients with diabetes. Direct acute effects of episodes of hyperglycaemia (HG) could have implications for the microalbuminuria seen in early diabetes before renal structural alterations have started, especially in those patients with poor glycaemic control. This review summarizes the literature evidence that acute or sustained HG may lead to an increased vascular or glomerular permeability. Special focus is on glomerular barrier permeability. There is evidence in the literature that HG increases systemic capillary and glomerular barrier permeability within 20-30 min in vivo in rats and mice. Furthermore, exposure of monolayers of cultured endothelial cells to HG has been shown to increase monolayer permeability rapidly and transiently (during 60-100 min). Instant cellular changes following F-actin cytoskeleton rearrangements, which could be abrogated by Rho-kinase (ROCK) inhibition, are implicated. Data in this review also suggest that activation of protein kinase C, the polyol pathway, and an increased release of reactive oxygen species (ROS) and cytokines could contribute to the increase in barrier permeability induced by HG. Recent in vitro data from cultured podocyte monolayers also designates a role of insulin in acute podocyte F-actin remodelling, underpinning the complexity of the mechanisms leading to glomerular and endothelial barrier alterations in diabetes mellitus.

Acute hyperglycemia induces rapid, reversible increases in glomerular permeability in nondiabetic rats

This study was performed to investigate the impact of acute hyperglycemia (HG) on the permeability of the normal glomerular filtration barrier in vivo. In anesthetized Wistar rats (250-280 g), the left ureter was catheterized for urine collection, while simultaneously blood access was achieved. Rats received an intravenous (iv) infusion of either 1) hypertonic glucose to maintain blood glucose at 20-25 mM (G; n = 8); 2) hypertonic glucose as in 1) and a RhoA-kinase inhibitor (Y-27632; Rho-G; n = 8); 3) 20% mannitol (MANN; n = 7) or 4) hypertonic (12%) NaCl to maintain plasma crystalloid osmotic pressure (pi(cry)) at approximately 320-325 mosmol/l (NaCl; n = 8) or 5) physiological saline (SHAM; n = 8). FITC-Ficoll 70/400 was infused iv for at least 20 min before termination of the experiments, and plasma and urine were collected to determine the glomerular sieving coefficients (theta) for polydisperse Ficoll (molecular radius 15-80 A) by high-performance size-exclusion chromatography. In G there was a marked increase in for Ficoll(55-80A) at 20 min, which was completely reversible within 60 min and abrogated by a Rho-kinase (ROCK) inhibitor, while glomerular permeability remained unchanged in MANN and NaCl. In conclusion, acute HG caused rapid, reversible increases in for large Ficolls, not related to the concomitant hyperosmolarity, but sensitive to ROCK inhibition. The changes observed were consistent with the formation of an increased number of large pores in the glomerular filter. The sensitivity of the permeability changes to ROCK inhibition strongly indicates that the cytoskeleton of the cells in the glomerular barrier may be involved in these alterations.

Protein kinase C-β inhibition and diabetic microangiopathy: effects on endothelial permeability responses in vitro

Protein kinase C (PKC)-beta and other PKC isozymes have been implicated in the loss of endothelial barrier function in diabetic microangiopathy. The effects of a PKC-beta-specific inhibitor, LY379196, on hyperpermeability responses to high-glucose, angiotensin II, alpha-thrombin and endothelin-1 were evaluated using an in vitro model of human pulmonary artery endothelial cell monolayers. LY379196 attenuated the increase in transendothelial albumin flux induced by glucose 40 mM (e.g. 411+/-160% [high-glucose] vs. 167+37% [high-glucose+LY379196], P<0.001) and angiotensin II 10 microM (e.g. 121+/-12% vs. 246+/-35%, P<0.01); endothelin-1 had no significant effect on monolayer permeability. LY379196 had no significant effect on the marked hyperpermeability response to alpha-thrombin 1 microM. Thus, two major pathways involved in vascular leakage in diabetic microangiopathy are amenable to therapeutic blockade by PKC-beta inhibition.

Identification and purification from the plasma of Type 1 diabetic subjects of a proteolytically active Grp94Evidence that Grp94 is entirely responsible for plasma proteolytic activity

AIMS/HYPOTHESIS

The overall increase in proteolytic activity in diabetes is known to be associated with the development and progression of vascular complications. Our aim was to investigate in detail the molecular nature of this activity in the plasma of Type 1 diabetic subjects.

METHODS

Plasma of both diabetic and control subjects was subjected to various purification procedures (ion exchange and affinity chromatography, HPLC, immunoprecipitation, electrophoresis, immunoblot and mass analyses) to identify the proteins of interest. Biological activities were measured on specific substrates.

RESULTS

In diabetic but not normal plasma we identified the presence of two heat shock proteins, Grp94 (Glucose-regulated protein94) and HSP70. The higher-than-normal proteolytic activity of Grp94 was: (i) directed against casein, but not against endogenous plasma proteins; (ii) fully and specifically inhibited only by anti-Grp94 polyclonal antibodies; and (iii) coupled with low-level ATPase activity. In addition, ATP binding to Grp94 was able to modulate proteolytic activity. We found that Grp94 in plasma circulates only as high molecular mass homo- and hetero-complexes, the latter mostly formed with IgG to which Grp94 is also linked by tenacious binding. Proteolytically-active Grp94 was purified by immunoprecipitation, which co-immunoprecipitated alpha(1)antitrypsin.

CONCLUSION/INTERPRETATION

Our results show the unexpected extracellular location and characteristic biological function of Grp94 even at a late stage of disease. These findings have physiopathological relevance for predicting activation of both autoimmune and inflammatory processes potentially associated with vascular complications.

Inhibitory effect of a novel bradykinin B1 receptor antagonist, R-954, on enhanced vascular permeability in type 1 diabetic mice

The morbidity and mortality associated with type 1 diabetes are essentially related to the micro- and macrovascular complications that develop over time and lead to several diabetic complications, including hypertension, atherosclerosis, and retinopathy, as well as coronary and renal failure. Normally absent in physiological conditions, the bradykinin B1 receptor (BKB1-R) was recently found to be overexpressed in pathological conditions, including type 1 diabetes. In the present study, we evaluated the effect of the new BKB1-R antagonist, R-954 (Ac-Orn-[Oic2, alpha-MePhe5, D-betaNal7, Ile8]desArg9-bradykinin, on the increase in vascular permeability in streptozotocin (STZ)-diabetic mice. The capillary permeability to albumin was measured by quantifying the extravasation of albumin-bound Evans blue dye in selected target tissues (liver, pancreas, duodenum, ileum, spleen, heart, kidney, stomach, skin, muscle, and thyroid gland). Acute single administration of R-954 (300 microg/kg, i.v.) to type 1 diabetic mice 4 weeks after STZ significantly inhibited the enhanced vascular permeability in most tissues. These data provide further experimental evidence for the implication of BKB1-R in the enhanced vascular permeability associated with type 1 diabetes.

Catalase and alpha-tocopherol attenuate blood-brain barrier breakdown in pentylenetetrazole-induced epileptic seizures in acute hyperglycaemic rats

Experimental data indicate that acute hyperglycaemia can aggravate the consequences of epileptic seizures on the permeability of the blood-brain barrier (BBB). The purpose of this study was to examine the effects of chronic administration of alpha -tocopherol (vitamin E) and acute catalase administration on the disrupted BBB during experimentally pentylenetetrazole-induced status epilepticus in acute hyperglycaemic rats. The integrity of the BBB was tested using the Evans Blue (EB) dye extravasation. The concentration of EB dye was measured in four regions of the brain. Epileptic seizures induced a significant increase in EB dye extravasation in the brain regions compared with that of the groups of rats treated with saline, glucose, catalase and alpha -tocopherol (P< 0.01). The content of EB dye in the brain regions of animals in the acute hyperglycaemia plus epileptic group was higher than that of the saline, glucose, catalase, alpha -tocopherol and epileptic groups (P< 0.01). The increased EB dye transfer from blood to the brain in status epilepticus and acute hyperglycaemia plus status epilepticus was attenuated by the treatment with catalase and alpha -tocopherol. These data suggest that a partial reduction in the production of reactive oxygen species by catalase and alpha -tocopherol contributes to decreases in the content of EB dye across the BBB during pentylenetetrazole-induced status epilepticus in acute hyperglycaemic rats.

Increased retinal endothelial cell monolayer permeability induced by the diabetic milieu: role of advanced non-enzymatic glycation and polyol pathway activation

BACKGROUND

Increased vascular permeability could be involved in the pathogenesis of diabetic retinopathy. The present study was aimed at assessing whether high glucose concentrations can impair retinal endothelial cell barrier function directly, irrespective of changes in other determinants of permeability, and the role of non-enzymatic glycation and polyol pathway activation in these alterations.

METHODS

Bovine retinal endothelial cells (BREC) were exposed for various periods to high glucose vs iso-osmolar mannitol and normal glucose containing media+/-agents mimicking or inhibiting advanced glycation end product (AGE) formation and polyol pathway activation. Monolayer permeability was assessed by measuring the transendothelial passage of (125)I-labeled proteins.

RESULTS

Permeability increased significantly (up to +70%) in BREC exposed to high glucose, but not to mannitol, for 1-30 days, vs normal glucose control cells. Exposure to AGE-modified bovine serum albumin (BSA) (> or = 90%) and, to a lesser extent, sorbitol (+28%) mimicked the high glucose effect. The AGE formation and nitric oxide synthase (NOS) inhibitor aminoguanidine significantly reduced (by 60%) changes induced by 30-day exposure to high glucose, whereas methylguanidine, which inhibits only NOS activity, did not affect permeability. Aldose reductase or sorbitol dehydrogenase inhibitors decreased (by approximately 40%) the enhanced leakage produced by 1-day, but not 30-day, incubation in high glucose.

CONCLUSIONS

The present results indicate that high glucose is capable of impairing retinal endothelial cell barrier function directly and that non-enzymatic glycation and polyol pathway activation may mediate these changes, with AGEs participating in the long-term alterations and increased flux through the sorbitol pathway in the short-term effect.

Protein kinase C activation contributes to microvascular barrier dysfunction in the heart at early stages of diabetes

The functional disturbance of microvasculature is recognized as an initiating mechanism that underlies the development of various diabetic complications. Although a causal relationship between microvascular leakage and tissue damage has been well documented in diabetic kidneys and eyes, there is a lack of information regarding the barrier function of coronary exchange vessels in the disease state. The aim of the present study was to evaluate the permeability property of coronary microvessels during the early development of experimental diabetes with a focus on the protein kinase C (PKC)-dependent signaling mechanism. The apparent permeability coefficient of albumin (Pa) was measured in isolated and perfused porcine coronary venules. The administration of high concentrations of D-glucose induced a dose-dependent increase in the Pa value, which was prevented by blockage of PKC with its selective inhibitors bisindolylmaleimide and Goe 6976. More importantly, an elevated basal permeability to albumin was observed in coronary venules at the early onset of streptozotocin-induced diabetes. The hyperpermeability was corrected with bisindolylmaleimide and the selective PKCbeta inhibitor hispidin. Concomitantly, protein kinase assay showed a high PKC activity in isolated diabetic venules. Immunoblot analysis of the diabetic heart revealed a significant subcellular translocation of PKCbetaII and PKCepsilon from the cytosol to the membrane, indicating that the specific activity of these isoforms was preferentially elevated. The results suggest that endothelial barrier dysfunction attributed to the activation of PKC occurs at the coronary exchange vessels in early diabetes.

Increased capillary filtration of albumin in diabetic patients--relation with gender, hypertension, microangiopathy, and neuropathy

The aim of this study was to investigate the factors associated with an increase in capillary filtration of albumin (CFA) in a large series of diabetic patients and its relationship with gender, hypertension, microangiopathy, and neuropathy. One hundred sixty-three unselected diabetic patients, 74 type I and 89 type II, were included. An isotopic test of CFA was performed with 99m technetium-labeled albumin injected intravenously. Radioactivity was counted externally at the forearm with a gamma camera before, during, and after venous compression. After removal of venous compression, interstitial albumin retention (AR) was calculated and the radioactivity disappearance curve was analyzed by the Fast Fourier transform, which provides an index for lymphatic uptake of interstitial albumin (low-frequency to high-frequency amplitude peak ratio [LF/HF]). An increase in AR and LF/HF was found in 65 (39.9%) and 117 (71.7%) patients, respectively. Increased AR was significantly more frequent in women than in men (P=.018) and in patients without microangiopathic complications than in those with them (P=.028). In men, it was significantly more frequent in type I versus type II diabetic patients (P=.004), and AR was significantly higher in patients with peripheral neuropathy than in those without (P=.004). The LF/HF was also significantly higher in men with peripheral neuropathy (P=.045). In women, the AR level correlated negatively with postprandial glycemia (P=.006) and was significantly higher in patients without microangiopathic complications (P=.003). These data suggest the role of hormonal factors, both sex steroids and insulin, and the major role of peripheral neuropathy in the increase in CFA. The highly prevalent increase in CFA before the onset of microangiopathic complications is consistent with the presence of a functional microcirculatory disorder that might contribute to the occurrence of microangiopathic lesions.

High glucose concentrations increase endothelial cell permeability via activation of protein kinase C alpha

Endothelial cell permeability is impaired in diabetes mellitus and may be increased by high extracellular glucose concentrations. High glucose activates protein kinase C (PKC), a family of kinases vital to intracellular signaling. We tested the hypothesis that high glucose concentration activates PKC in endothelial cells and leads to an increase in endothelial cell permeability via distinct PKC isoforms. Porcine aortic endothelial cells were used, and the PKC isoforms alpha, delta, epsilon, zeta, and theta were identified in these cells. Glucose caused a rapid dose-dependent increase in endothelial cell permeability, with an EC50 of 17.5 mmol/L. Phorbol 12-myristate 13-acetate (TPA) induced an increase in permeability very similar to that elicited by glucose. The effect of glucose and TPA was totally reversed by preincubating the cells with the PKC inhibitors staurosporine (10(-8) mol/L) and Goe 6976 (10(-8) mol/L). Downregulation of PKC by preincubation with TPA for 24 hours also abolished the effect of glucose and TPA on endothelial cell permeability. High glucose (20 mmol/L) caused an increase in PKC activity at 2, 10, and 30 minutes. Cell fractionation and Western blot analysis showed a glucose-induced translocation of PKC alpha and PKC epsilon. Confocal microscopy confirmed the translocation and showed an association of PKC alpha and PKC epsilon with nuclear structures and the cell membrane. Specific antisense oligodesoxynucleotides (ODNs) against PKC alpha reduced the expression of the isoform, abolished the effects of glucose on endothelial cell permeability completely, and reduced the TPA effect significantly. In contrast, specific antisense ODNs against PKC epsilon had no effect on glucose-induced permeability and only a minor effect on the TPA-induced increase in permeability. We conclude that an increase in extracellular glucose leads to a rapid dose-dependent increase in endothelial cell permeability via the activiation of PKC and that this effect is mediated by the PKC isoform alpha.

Heparin-induced structural modifications and oxidative cleavage of human serum albumin in the absence and presence of glucose--implications for transcapillary leakage of albumin in hyperglycaemia

Both unfractionated and fractionated, low-molecular-mass heparins were tested on human serum albumin in the absence and presence of glucose at concentrations similar to those frequently found in diabetic hyperglycaemic patients, to ascertain whether heparin and glucose interfered with each other in affecting the conformation of albumin. Reproducible results were obtained with both heparins when used at equal masses, but not when used at equal molar concentrations, suggesting a crucial role of the amount of the saccharide units in determining the observed effects. Spectroscopic studies showed that the binding sites of glucose and heparin on albumin do not overlap and that changes in protein structure depend on complex and mutual interference of glucose and heparin with the protein, although the effects of heparin in modifying the chromophore environment and increasing the ordered structure of the protein also prevailed in the presence of glucose. Heparin binding to albumin rapidly gave rise to oxidative reactions, which were responsible for the increase in the carbonyl content of the protein together with its higher susceptibility to tryptic digestion. Glucose enhanced and prolonged the production of heparin-induced oxidants. Oxidation caused peptide bond cleavage at Lys323 in the primary structure of albumin, yielding two large fragments of 27.5 kDa and 35 kDa which aggregated to form disulphide-linked homodimers visible in SDS/PAGE as two new bands of 54 kDa and 74 kDa, respectively. This was accompanied with a reduction in Val, Glu, and Gly residues, only partially counterbalanced by an increase in Thr and Ser residues. While only a small percentage of albumin molecules underwent fragmentation in the presence of heparin with glucose, albumin turned out to display in an even higher proportion structural modifications consistent with a higher degree of ordered structure. The mechanism(s) underlying this heparin-driven effect and possible physiopathological implications in vivo are discussed.