Pericyte adhesion is impaired on extracellular matrix produced by endothelial cells in high hexose concentrations

Biochemical mechanism underlying the pathogenesis of diabetic retinopathy and other diabetic complications in humans: the methanol-formaldehyde-formic acid hypothesis

Hyperglycemia in diabetic patients is associated with abnormally-elevated cellular glucose levels. It is hypothesized that increased cellular glucose will lead to increased formation of endogenous methanol and/or formaldehyde, both of which are then metabolically converted to formic acid. These one-carbon metabolites are known to be present naturally in humans, and their levels are increased under diabetic conditions. Mechanistically, while formaldehyde is a cross-linking agent capable of causing extensive cytotoxicity, formic acid is an inhibitor of mitochondrial cytochrome oxidase, capable of inducing histotoxic hypoxia, ATP deficiency and cytotoxicity. Chronic increase in the production and accumulation of these toxic one-carbon metabolites in diabetic patients can drive the pathogenesis of ocular as well as other diabetic complications. This hypothesis is supported by a large body of experimental and clinical observations scattered in the literature. For instance, methanol is known to have organ- and species-selective toxicities, including the characteristic ocular lesions commonly seen in humans and non-human primates, but not in rodents. Similarly, some of the diabetic complications (such as ocular lesions) also have a characteristic species-selective pattern, closely resembling methanol intoxication. Moreover, while alcohol consumption or combined use of folic acid plus vitamin B is beneficial for mitigating acute methanol toxicity in humans, their use also improves the outcomes of diabetic complications. In addition, there is also a large body of evidence from biochemical and cellular studies. Together, there is considerable experimental support for the proposed hypothesis that increased metabolic formation of toxic one-carbon metabolites in diabetic patients contributes importantly to the development of various clinical complications.

Thiamine and diabetes: back to the future?

The first reports of a link between thiamine and diabetes date back to the 1940s. Some years later, a role for thiamine deficiency in diabetic neuropathy became evident, and some pilot studies evaluated the putative effects of thiamine supplementation. However, the administration of thiamine and its lipophilic derivative benfotiamine for the treatment of this complication gained consensus only at the end of the '90 s. The first evidence of the beneficial effects of thiamine on microvascular cells involved in diabetic complications dates to 1996: from then on, several papers based on in vitro and animal models have addressed the potential use of this vitamin in counteracting diabetic microangiopathy. A few pilot studies in humans reported beneficial effects of thiamine administration on diabetic nephropathy, but, despite all promising proofs-of-concept, the possible role of thiamine in counteracting development or progression of retinopathy has not been addressed until now. Thiamine is a water-soluble vitamin, rapidly expelled from the body, with no issues of over-dosage or accumulation; unfortunately, it is non-patentable, and neither industry nor independent donors are interested in investing in large-scale randomized controlled clinical trials to investigate its potential in diabetes and its complications. Consequently, science will not be able to disprove a promising hypothesis and, more importantly, diabetic people remain deprived of a possible way to ameliorate their condition.

Retinal capillary basement membrane thickening: Role in the pathogenesis of diabetic retinopathy

Vascular basement membrane (BM) thickening has been hailed over half a century as the most prominent histological lesion in diabetic microangiopathy, and represents an early ultrastructural change in diabetic retinopathy (DR). Although vascular complications of DR have been clinically well established, specific cellular and molecular mechanisms underlying dysfunction of small vessels are not well understood. In DR, small vessels develop insidiously as BM thickening occurs. Studies examining high resolution imaging data have established BM thickening as one of the foremost structural abnormalities of retinal capillaries. This fundamental structural change develops, at least in part, from excess accumulation of BM components. Although BM thickening is closely associated with the development of DR, its contributory role in the pathogenesis of DR is coming to light recently. DR develops over several years before clinical manifestations appear, and it is during this clinically silent period that hyperglycemia induces excess synthesis of BM components, contributes to vascular BM thickening, and promotes structural and functional lesions including cell death and vascular leakage in the diabetic retina. Studies using animal models show promising results in preventing BM thickening with subsequent beneficial effects. Several gene regulatory approaches are being developed to prevent excess synthesis of vascular BM components in an effort to reduce BM thickening. This review highlights current understanding of capillary BM thickening development, role of BM thickening in retinal vascular lesions, and strategies for preventing vascular BM thickening as a potential therapeutic strategy in alleviating characteristic lesions associated with DR.

Beneficial effects of fenofibric acid on overexpression of extracellular matrix components, COX-2, and impairment of endothelial permeability associated with diabetic retinopathy

In the Fenofibric Acid (FA) Intervention and Event Lowering in Diabetes (FIELD) study, FA, a lipid-lowering drug, has been shown to significantly reduce macular edema in diabetic patients. In the present study, we investigated whether FA reduces vascular permeability by inhibiting cyclooxygenase-2 (COX-2), a critical mediator of inflammation, and reducing overexpression of fibronectin (FN) and collagen IV (Coll IV), two basement membrane (BM) components upregulated in diabetic retinopathy. Rat retinal endothelial cells (RRECs) were grown in normal (N:5 mM glucose) or high glucose (HG:30 mM glucose) medium with or without FA for 7 days. Total protein isolated from these cells was assessed for FN, Coll IV, COX-2, and zonula occludens-1 (ZO-1), a tight junction protein, using Western blot analysis. In addition, the distribution and localization of ZO-1 was determined by immunofluorescence microscopy, and cell monolayer permeability was studied by in vitro permeability (IVP) assay. RRECs grown in HG medium showed significant increase in FN, Coll IV, and COX-2 expression (179%, 144%, 139% of N respectively), and a decrease in ZO-1 expression (48% of N) compared to those of N cells. Cells grown in HG medium supplemented with FA significantly reduced FN, Coll IV, and COX-2 expression by 47%, 32%, and 34% respectively, with concomitant increase in ZO-1 expression by 42%. In parallel studies, IVP assays showed a significant increase (139% of N) in cell monolayer permeability in RRECs grown in HG medium, which was significantly reduced with FA treatment. Additionally, immunostaining results indicated FA prevents HG-induced downregulation of ZO-1. The findings indicate that the beneficial effect of FA in reducing excess permeability is mediated, at least in part, by downregulating abnormal overexpression of BM components and inflammatory factors and preventing compromised tight junctions associated with diabetic retinopathy.

The progress in understanding and treatment of diabetic retinopathy

Diabetic retinopathy is the most frequently occurring complication of diabetes mellitus and remains a leading cause of vision loss globally. Its aetiology and pathology have been extensively studied for half a century, yet there are disappointingly few therapeutic options. Although some new treatments have been introduced for diabetic macular oedema (DMO) (e.g. intravitreal vascular endothelial growth factor inhibitors ('anti-VEGFs') and new steroids), up to 50% of patients fail to respond. Furthermore, for people with proliferative diabetic retinopathy (PDR), laser photocoagulation remains a mainstay therapy, even though it is an inherently destructive procedure. This review summarises the clinical features of diabetic retinopathy and its risk factors. It describes details of retinal pathology and how advances in our understanding of pathogenesis have led to identification of new therapeutic targets. We emphasise that although there have been significant advances, there is still a pressing need for a better understanding basic mechanisms enable development of reliable and robust means to identify patients at highest risk, and to intervene effectively before vision loss occurs.

Differential effects of glyoxalase 1 overexpression on diabetic atherosclerosis and renal dysfunction in streptozotocin-treated, apolipoprotein E-deficient mice

The reactive dicarbonyls, glyoxal and methylglyoxal (MG), increase in diabetes and may participate in the development of diabetic complications. Glyoxal and MG are detoxified by the sequential activities of glyoxalase 1 (GLO1) and glyoxalase 2. To determine the contribution of these dicarbonyls to the etiology of complications, we have genetically manipulated GLO1 levels in apolipoprotein E‐null (Apoe^−/−) mice. Male Apoe^−/− mice, hemizygous for a human GLO1 transgene (GLO1TGApoe^−/− mice) or male nontransgenic Apoe^−/− litter mates were injected with streptozotocin or vehicle and 6 or 20 weeks later, aortic atherosclerosis was quantified. The GLO1 transgene lessened streptozotocin (STZ)‐induced increases in immunoreactive hydroimidazolone (MG‐H1). Compared to nondiabetic mice, STZ‐treated GLO1TGApoe^−/− and Apoe^−/− mice had increased serum cholesterol and triglycerides and increased atherosclerosis at both times after diabetes induction. While the increased GLO1 activity in the GLO1TGApoe^−/− mice failed to protect against diabetic atherosclerosis, it lessened glomerular mesangial expansion, prevented albuminuria and lowered renal levels of dicarbonyls and protein glycation adducts. Aortic atherosclerosis was also quantified in 22‐week‐old, male normoglycemic Glo1 knockdown mice on an Apoe^−/− background (Glo1KDApoe^−/− mice), an age at which Glo1KD mice exhibit albuminuria and renal pathology similar to that of diabetic mice. In spite of ~75% decrease in GLO1 activity and increased aortic MG‐H1, the Glo1KDApoe^−/− mice did not show increased atherosclerosis compared to age‐matched Apoe^−/− mice. Thus, manipulation of GLO1 activity does not affect the development of early aortic atherosclerosis in Apoe^−/− mice but can dictate the onset of kidney disease independently of blood glucose levels.

Increased levels of methylglyoxal and methylglyoxal‐derived advanced glycation end products may contribute to the development of diabetic complications. We show that overexpression of an enzyme that participates in the pathway of methylglyoxal detoxification, glyoxalase 1, protects streptozotocin‐treated, apolipoprotein E‐deficient mice from diabetic kidney disease but not from diabetes‐induced accelerated aortic atherosclerosis.

Connection of pericyte-angiopoietin-Tie-2 system in diabetic retinopathy: friend or foe?

Pericytes are distinctive regulators of vascular morphologenesis and function during vascular development and homeostasis. Pericytes have recently come into focus as implications of aberrant interactions between pericytes and endothelial cells in number of pathological angiogenesis conditions, including diabetic retinopathy and tumor angiogenesis. Pericyte dropout is a hallmark of early diabetic retinopathy. Abnormal angiopoietin (Ang)-Tie-2 signaling is one principal system participating in pericyte/endothelial cell dissociation during early stages of diabetic retinopathy. Angiopoietin 2 (Ang-2) is among the relevant growth factors induced by hypoxia and plays an important role in the initiation of retinal neovascularization and cause pericyte loss. Furthermore, high levels of VEGF synergize Ang-Tie-2 signaling during the development of diabetic retinopathy. An accelerated rate of clinical development Ang-Tie-2-manipulating drugs requests a better mechanistic understanding the connection between pericytes and Ang-Tie-2 systems both under normal and disease conditions. We summarize recent advances in pericyte study in conjunction with Ang-Tie-2 signaling and also discuss possible therapeutic strategies for diabetic retinopathy by targeting pericytes through manipulating Ang-Tie-2 signaling.

Advances in our understanding of diabetic retinopathy

Diabetic retinopathy remains the most common complication of diabetes mellitus and is a leading cause of visual loss in industrialized nations. The clinicopathology of the diabetic retina has been extensively studied, although the precise pathogenesis and cellular and molecular defects that lead to retinal vascular, neural and glial cell dysfunction remain somewhat elusive. This lack of understanding has seriously limited the therapeutic options available for the ophthalmologist and there is a need to identify the definitive pathways that initiate retinal cell damage and drive progression to overt retinopathy. The present review begins by outlining the natural history of diabetic retinopathy, the clinical features and risk factors. Reviewing the histopathological data from clinical specimens and animal models, the recent paradigm that neuroretinal dysfunction may play an important role in the early development of the disease is discussed. The review then focuses on the molecular pathogenesis of diabetic retinopathy with perspective provided on new advances that have furthered our understanding of the key mechanisms underlying early changes in the diabetic retina. Studies have also emerged in the past year suggesting that defective repair of injured retinal vessels by endothelial progenitor cells may contribute to the pathogenesis of diabetic retinopathy. We assess these findings and discuss how they could eventually lead to new therapeutic options for diabetic retinopathy.

Molecular basis of the inner blood-retinal barrier and its breakdown in diabetic macular edema and other pathological conditions

Breakdown of the inner endothelial blood-retinal barrier (BRB), as occurs in diabetic retinopathy, age-related macular degeneration, retinal vein occlusions, uveitis and other chronic retinal diseases, results in vasogenic edema and neural tissue damage, causing loss of vision. The central mechanism of altered BRB function is a change in the permeability characteristics of retinal endothelial cells caused by elevated levels of growth factors, cytokines, advanced glycation end products, inflammation, hyperglycemia and loss of pericytes. Subsequently, paracellular but also transcellular transport across the retinal vascular wall increases via opening of endothelial intercellular junctions and qualitative and quantitative changes in endothelial caveolar transcellular transport, respectively. Functional changes in pericytes and astrocytes, as well as structural changes in the composition of the endothelial glycocalyx and the basal lamina around BRB endothelium further facilitate BRB leakage. As Starling's rules apply, active transcellular transport of plasma proteins by the BRB endothelial cells causing increased interstitial osmotic pressure is probably the main factor in the formation of macular edema. The understanding of the complex cellular and molecular processes involved in BRB leakage has grown rapidly in recent years. Although appropriate animal models for human conditions like diabetic macular edema are lacking, these insights have provided tools for rational design of drugs aimed at restoring the BRB as well as for design of effective transport of drugs across the BRB, to treat the chronic retinal diseases such as diabetic macular edema that affect the quality-of-life of millions of patients.

The osteogenic differentiation of human bone marrow MSCs on HUVEC-derived ECM and β-TCP scaffold

Extracellular matrix (ECM) serves a key role in cell migration, attachment, and cell development. Here we report that ECM derived from human umbilical vein endothelial cells (HUVEC) promoted osteogenic differentiation of human bone marrow mesenchymal stem cells (hMSC). We first produced an HUVEC-derived ECM on a three-dimensional (3D) beta-tricalcium phosphate (β-TCP) scaffold by HUVEC seeding, incubation, and decellularization. The HUVEC-derived ECM was then characterized by SEM, FTIR, XPS, and immunofluorescence staining. The effect of HUVEC-derived ECM-containing β-TCP scaffold on hMSC osteogenic differentiation was subsequently examined. SEM images indicate a dense matrix layer deposited on the surface of struts and pore walls. FTIR and XPS measurements show the presence of new functional groups (amide and hydroxyl groups) and elements (C and N) in the ECM/β-TCP scaffold when compared to the β-TCP scaffold alone. Immunofluorescence images indicate that high levels of fibronectin and collagen IV and low level of laminin were present on the scaffold. ECM-containing β-TCP scaffolds significantly increased alkaline phosphatase (ALP) specific activity and up-regulated expression of osteogenesis-related genes such as runx2, alkaline phosphatase, osteopontin and osteocalcin in hMSC, compared to β-TCP scaffolds alone. This increased effect was due to the activation of MAPK/ERK signaling pathway since disruption of this pathway using an ERK inhibitor PD98059 results in down-regulation of these osteogenic genes. Cell-derived ECM-containing calcium phosphate scaffolds is a promising osteogenic-promoting bone void filler in bone tissue regeneration.

Vascular basement membrane thickening in diabetic retinopathy

Vascular basement membrane (BM) thickening is a fundamental structural alteration of small blood vessels in diabetes. Over two decades of research has established hyperglycemia as the primary causal factor mediating this alteration. Various high glucose-induced mechanisms have been investigated and excess synthesis of BM components has been identified as a major contributing factor to BM thickening. Although BM thickening has been long hailed as the histological hallmark of diabetic microangiopathy, the consequences of BM thickening on the functionality of target organs of diabetes remain elusive even today. This review presents an overview of our current understanding of the BM structure and function, and focuses on how capillary BM thickening develops, its effect on retinal vascular function, and potential strategies for preventing the development of BM thickening in diabetic retinopathy.

Effects of high glucose and thiamine on the balance between matrix metalloproteinases and their tissue inhibitors in vascular cells

Pericyte survival in diabetic retinopathy depends also on interactions with extracellular matrix (ECM) proteins, which are degraded by matrix metalloproteinases (MMP). Elevated glucose influences ECM turnover, through expression of MMP and their tissue inhibitors, TIMP. We reported on reduced pericyte adhesion to high glucose-conditioned ECM and correction by thiamine. We aimed at verifying the effects of thiamine and benfotiamine on MMP-2, MMP-9 and TIMP expression and activity in human vascular cells with high glucose. In HRP, MMP-2 activity, though not expression, increased with high glucose and decreased with thiamine and benfotiamine; TIMP-1 expression increased with high glucose plus thiamine and benfotiamine; MMP-9 was not expressed. In EC, MMP-9 and MMP-2 expression and activity increased with high glucose, but thiamine and benfotiamine had no effects; TIMP-1 expression was unchanged. Neither glucose nor thiamine modified TIMP-2 and TIMP-3 expression. TIMP-1 concentrations did not change in either HRP or EC. High glucose imbalances MMP/TIMP regulation, leading to increased ECM turnover. Thiamine and benfotiamine correct the increase in MMP-2 activity due to high glucose in HRP, while increasing TIMP-1.

Microaneurysm number and distribution in the macula of Chinese type 2 diabetics with early diabetic retinopathy: a population-based study in Kinmen, Taiwan

To explore the number and distribution of microaneurysms (MA) in the macula of type 2 diabetics in Kinmen, Taiwan and associated risks for early diabetic retinopathy (DR). A baseline mass screening extracts 971 type 2 diabetics between 1991 and 1993 and follow-up eye screening at 1999 was then done. Number of MA was counted and associated systemic factors collected from a centered 45 degrees color fundus photograph of dilated eyes of type 2 diabetics (Topcon-NW5). Thirty-three (6.2%, n = 527) had MA retinopathy. Those with one eye involved (n = 26) had two mean MA; those with bilateral involvement (n = 7) had four mean per eye. Most MA were located temporally (mean MA = 0.79), then superiorly (mean = 0.58), inferiorly (mean = 0.46) and nasal (mean = 0.28). MA occurred at the 6,000-microm outermost ring (mean = 1.51), then the 3,000-microm ring (mean = 0.35), the 1,500-microm ring (mean = 0.26) and center 500 microm circle (mean = 0.01). After adjusting for confounding factors, early DR with MA was associated with longer duration of type 2 diabetes [odds ratios (OR) = 1.10, 95% confidence interval (CI) 1.00-1.21], higher glycosylated hemoglobin (OR = 1.36, 95%CI 1.11-1.66) and wider waist circumference (OR = 1.04, 95%CI 1.01-1.07). Our photograph grading system suggests that MA number and distribution at the macula may reflect severity of early DR and is a valid tool for predicting the progression of DR. Identifying those factors associated with MA only DR is important in developing prevention strategies for this early microvascular complication of type 2 diabetes.

Hyperglycemia and glycation in diabetic complications

Diabetes mellitus is a multifactorial disease, classically influenced by genetic determinants of individual susceptibility and by environmental accelerating factors, such as lifestyle. It is considered a major health concern,as its incidence is increasing at an alarming rate, and the high invalidating effects of its long-term complications affect macro- and microvasculature, heart, kidney, eye, and nerves. Increasing evidence indicates that hyperglycemia is the initiating cause of the tissue damage occurring in diabetes, either through repeated acute changes in cellular glucose metabolism, or through the long-term accumulation of glycated biomolecules and advanced glycation end products (AGEs). AGEs represent a heterogeneous group of chemical products resulting from a nonenzymatic reaction between reducing sugars and proteins, lipids, nucleic acids, or a combination of these.The glycation process (glucose fixation) affects circulating proteins (serum albumin, lipoprotein, insulin, hemoglobin),whereas the formation of AGEs implicates reactive intermediates such as methylglyoxal. AGEs form cross-links on long-lived extracellular matrix proteins or react with their specific receptor RAGE, resulting inoxidative stress and proinflammatory signaling implicated in endothelium dysfunction, arterial stiffening, and microvascular complications. This review summarizes the mechanism of glycation and of AGEs formation and the role of hyperglycemia, AGEs, and oxidative stress in the pathophysiology of diabetic complications.

Thiamine and benfotiamine prevent apoptosis induced by high glucose-conditioned extracellular matrix in human retinal pericytes

BACKGROUND

Early and selective loss of pericytes and thickening of the basement membrane are hallmarks of diabetic retinopathy. We reported reduced adhesion, but no changes in apoptosis, of bovine retinal pericytes cultured on extracellular matrix (ECM) produced by endothelial cells in high glucose (HG). Since human and bovine pericytes may behave differently in conditions mimicking the diabetic milieu, we verified the behaviour of human retinal pericytes cultured on HG-conditioned ECM.

METHODS

Pericytes were cultured in physiological/HG on ECM produced by human umbilical vein endothelial cells in physiological/HG, alone or in the presence of thiamine and benfotiamine. Adhesion, proliferation, apoptosis, p53 and Bcl-2/Bax ratio (mRNA levels and protein concentrations) were measured in wild-type and immortalized human pericytes.

RESULTS

Both types of pericytes adhered less to HG-conditioned ECM and plastic than to physiological glucose-conditioned ECM. DNA synthesis was impaired in pericytes cultured in HG on the three different surfaces but there were no differences in proliferation. DNA fragmentation and Bcl-2/Bax ratio were greatly enhanced by HG-conditioned ECM in pericytes kept in both physiological and HG. Addition of thiamine and benfotiamine to HG during ECM production completely prevented these damaging effects.

CONCLUSIONS

Apoptosis is strongly increased in pericytes cultured on ECM produced by endothelium in HG, probably due to impairment of the Bcl-2/Bax ratio. Thiamine and benfotiamine completely revert this effect. This behaviour is therefore completely different from that of bovine pericytes, underlining the importance of establishing species-specific cell models to study the mechanisms of diabetic retinopathy.

Effects of extracellular matrix glycosylation on proliferation and apoptosis of human dermal fibroblasts via the receptor for advanced glycosylated end products

The balance between proliferation and apoptosis of skin cells is responsible for skin turnover and the success of the wound healing process. Recent reports have shown that advanced glycosylation end product (AGE) formation participates in dermatologic problems in diabetes. However, the effect on proliferation and apoptosis of dermal fibroblasts remains unclear. The aim of this study was to investigate the effects of dermal microenvironment glycosylation on the balance of cellular proliferation and apoptosis. Histology and immunohistochemical staining were performed on type II diabetic and nondiabetic skin tissue specimens to determine the distributions of proliferating cell nuclear antigen, apoptotic cells, AGEs, and receptors for AGEs (RAGEs). Matrix secreted by cultured human fibroblasts was glycosylated by 0.5 M D-ribose. RAGE-blocking antibodies were applied to inhibit the interaction of RAGE and AGEs in this system and then cell viability, cell cycle phase distribution, and apoptosis were measured. Diabetic skin has degenerative, loosely arranged collagen and increased apoptotic cells compared with normal skin. Expression of AGE and RAGE in diabetic skin tissue increased. Glycosylated matrix induced cell cycle arrest and apoptosis of cultured dermal fibroblasts, whereas application of RAGE-blocking antibodies redressed these changes. The accumulation of glycosylated extracellular matrix in diabetic skin tissue is a critical mediator of cellular function. Mediation of RAGE affects the balance of cellular proliferation and apoptosis, which confirms that diabetic wounds possess atypical origin in the repair process.

Effects of thiamine and benfotiamine on intracellular glucose metabolism and relevance in the prevention of diabetic complications

Thiamine (vitamin B1) is an essential cofactor in most organisms and is required at several stages of anabolic and catabolic intermediary metabolism, such as intracellular glucose metabolism, and is also a modulator of neuronal and neuro-muscular transmission. Lack of thiamine or defects in its intracellular transport can cause a number of severe disorders. Thiamine acts as a coenzyme for transketolase (TK) and for the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes, enzymes which play a fundamental role for intracellular glucose metabolism. In particular, TK is able to shift excess fructose-6-phosphate and glycerhaldeyde-3-phosphate from glycolysis into the pentose-phosphate shunt, thus eliminating these potentially damaging metabolites from the cytosol. Diabetes might be considered a thiamine-deficient state, if not in absolute terms at least relative to the increased requirements deriving from accelerated and amplified glucose metabolism in non-insulin dependent tissues that, like the vessel wall, are prone to complications. A thiamine/TK activity deficiency has been described in diabetic patients, the correction of which by thiamine and/or its lipophilic derivative, benfotiamine, has been demonstrated in vitro to counteract the damaging effects of hyperglycaemia on vascular cells. Little is known, however, on the positive effects of thiamine/benfotiamine administration in diabetic patients, apart from the possible amelioration of neuropathic symptoms. Clinical trials on diabetic patients would be necessary to test this vitamin as a potential and inexpensive approach to the prevention and/or treatment of diabetic vascular complications.

Matrix metalloproteinase-2 expression and apoptogenic activity in retinal pericytes: implications in diabetic retinopathy

Diabetic retinopathy (DR) commences as a basement membrane disorder with a dramatic loss of the innate retinal vascular autoregulation. In this process, retinal pericytes, which regulate endothelial cell proliferation and survival, undergo morphometric changes consistent with apoptosis. The ability of retinal pericytes to survive is dependent on their interaction with extracellular matrix (ECM) proteins, which are susceptible to rapid degradation by matrix metalloproteinases (MMPs). Here, we examined the potential involvement of MMPs and a membrane-type MMP in retinal pericyte death in experimental diabetes and in cultured retinal pericytes. Our data showed that chemically induced diabetes of 6 months' duration significantly increased the expression and activity of both MMP-2 and its physiological activator MT1-MMP. TdT-mediated dUTP nick end labeling (TUNEL)-positive pericytes and endothelial cells were concomitantly detected within the retinal capillaries of diabetic animals. In situ zymography showed a weak MMP activity in control retinas but an intense perivascular MMP activity in retinas from diabetic animals. In vitro studies showed that hyperglycemia-induced retinal pericyte apoptosis in vitro was attenuated by a specific MMP inhibitor. Incubation of pericytes with purified MMP-2 significantly increased the number of apoptotic cells. Our data suggest that increased MMP-2 activity compromises retinal pericyte survival possibly through MMP-2 action on ECM proteins and/or direct association of MMP-2 with integrins, which promotes apoptosis/anoikis by loss of cell contact with an appropriate ECM.

Regulation of intracellular glucose and polyol pathway by thiamine and benfotiamine in vascular cells cultured in high glucose

Hyperglycemia is a causal factor in the development of the vascular complications of diabetes. One of the biochemical mechanisms activated by excess glucose is the polyol pathway, the key enzyme of which, aldose reductase, transforms d-glucose into d-sorbitol, leading to imbalances of intracellular homeostasis. We aimed at verifying the effects of thiamine and benfotiamine on the polyol pathway, transketolase activity, and intracellular glucose in endothelial cells and pericytes under high ambient glucose. Human umbilical vein endothelial cells and bovine retinal pericytes were cultured in normal (5.6 mmol/liter) or high (28 mmol/liter) glucose, with or without thiamine or benfotiamine 50 or 100 mumol/liter. Transketolase and aldose reductase mRNA expression was determined by reverse transcription-PCR, and their activity was measured spectrophotometrically; sorbitol concentrations were quantified by gas chromatography-mass spectrometry and intracellular glucose concentrations by fluorescent enzyme-linked immunosorbent assay method. Thiamine and benfotiamine reduce aldose reductase mRNA expression, activity, sorbitol concentrations, and intracellular glucose while increasing the expression and activity of transketolase, for which it is a coenzyme, in human endothelial cells and bovine retinal pericytes cultured in high glucose. Thiamine and benfotiamine correct polyol pathway activation induced by high glucose in vascular cells. Activation of transketolase may shift excess glycolytic metabolites into the pentose phosphate cycle, accelerate the glycolytic flux, and reduce intracellular free glucose, thereby preventing its conversion to sorbitol. This effect on the polyol pathway, together with other beneficial effects reported for thiamine in high glucose, could justify testing thiamine as a potential approach to the prevention and/or treatment of diabetic complications.

Expression of placenta growth factor is regulated by both VEGF and hyperglycaemia via VEGFR-2

Placenta growth factor (PlGF) has been implicated in both physiological and pathological angiogenesis; however, little is known about what regulates its expression. In this study, retinal microvascular endothelial cells and pericytes were exposed to varying concentrations of VEGF and glucose and PlGF expression measured by RT-PCR and Western blotting. Both PlGF mRNA and protein were observed in unstimulated microvascular endothelial cells with only weak expression in pericytes. In endothelial cells, VEGF (100 ng/ml) and glucose (15 mM) induced an increase in expression of PlGF at both the mRNA and protein level while no effect was observed for pericytes. The increase in PlGF expression could be totally abolished by blocking VEGFR-2, and in the case of glucose by neutralising VEGF. VEGF-stimulated PlGF expression was largely inhibited by PD 98059, an inhibitor of mitogen-activated protein kinase (MAPK) and partially by GF 109203X, an inhibitor of protein kinase C (PKC), indicating that VEGF up-regulates PlGF expression via the MAPK signalling pathway and partially through PKC. Taken together, our findings suggest that VEGF orchestrates the contribution of PlGF in angiogenesis via more than one intracellular pathway and that hyperglycaemia, as occurs in diabetes, is an important regulator of PlGF expression via VEGF up-regulation.

Substrates modified by advanced glycation end-products cause dysfunction and death in retinal pericytes by reducing survival signals mediated by platelet-derived growth factor

AIMS/HYPOTHESIS

Premature death of retinal pericytes is a pathophysiological hallmark of diabetic retinopathy. Among the mechanisms proposed for pericyte death is exposure to AGE, which accumulate during diabetes. The current study used an in vitro model, whereby retinal pericytes were exposed to AGE-modified substrate and the mechanisms underlying pericyte death explored.

METHODS

Pericytes were isolated from bovine retinal capillaries and propagated on AGE-modified basement membrane (BM) extract or non-modified native BM. The extent of AGE modification was analysed. Proliferative responses of retinal pericytes propagated on AGE-modified BM were investigated using a 5-bromo-2-deoxy-uridine-based assay. The effect of extrinsically added platelet-derived growth factor (PDGF) isoforms on these proliferative responses was also analysed alongside mRNA expression of the PDGF receptors. Apoptotic death of retinal pericytes grown on AGE-modified BM was investigated using terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling labelling, mitochondrial membrane depolarisation and by morphological assessment. We also measured both the ability of PDGF to reverse Akt dephosphorylation that was mediated by AGE-modified BM, and increased pericyte apoptosis.

RESULTS

Retinal pericytes exposed to AGE-modified BM showed reduced proliferative responses in comparison to controls (p<0.05-0.01), although this effect was reversed at low-AGE modifications. PDGF mRNA levels were differentially altered by exposure to low and high AGE levels, and AGE-modified BM caused significantly increased apoptosis in retinal pericytes. Pre-treatment of AGE-modified BM with PDGF-AA and -BB reversed the apoptosis (p<0.05-0.001) and restored Akt phosphorylation in retinal pericytes.

CONCLUSIONS/INTERPRETATION

Evidence suggests that substrate-derived AGE such as those that occur during diabetes could have a major influence on retinal pericyte survival. During diabetic retinopathy, AGE modification of vascular BM may reduce bioavailability of pro-survival factors for retinal pericytes.

Aspects of human fetoplacental vasculogenesis and angiogenesis. III. Changes in complicated pregnancies

Patterns of fetoplacental angiogenesis vary not only during the course of a normal pregnancy but also in certain pregnancy pathologies. Here, we review some of the molecular and morphological events which occur in complicated pregnancies. The pregnancy complications are chosen in an attempt to represent the possible different origins (preplacental, uteroplacental, postplacental) of fetal hypoxia. Molecular events focus on reported changes in hypoxia-inducible factors, angiopoietins and the vascular endothelial, basic fibroblast and placenta growth factors and their receptors. Morphological changes focus on patterns of angiogenesis (branching and non-branching) and a consistent set of morphometric descriptors (covering measures of total capillary growth, villous capillarization and capillary size and shape in transverse section). Apart from some uncertainties due to lack of information, or failure to resolve fully the effects of intrauterine growth restriction and pre-eclampsia, alterations in the angiogenic growth factors and morphologies of capillaries and villi in different complicated pregnancies seem to conform reasonably well to those predicted by the fetal hypoxia paradigm. However, it is clear that future studies on the effects of different origins of fetal hypoxia should exercise more care in the choice and interpretation of relevant descriptors and take more account of the parallel effects of possible confounders. In addition, rather than comparing uncomplicated and complicated pregnancies only at term, more information about molecular and morphological events that occur throughout gestation would be extremely valuable. This includes further studies on changes in growth factor receptors, the less-well-documented angiogenic factors (e.g. angiogenin, angiostatin, endostatin) and the associations between endothelial cells and pericytes. A more integrated approach involving also parallel analysis of the effects of erythropoietin and other potential vasoactive factors on the behaviour and morphology of fetal vessels would be beneficial.

Intervention against the Maillard reaction in vivo

The field of Maillard/glycation reactions in vivo has grown enormously during the past 20 years, going from 25 to 500 publications per year. It is now well recognized that many of the "advanced" products form oxidatively or anaerobically and can have deleterious effects on macromolecular and biological function. The feasibility of developing pharmacological agents with beneficial in vivo properties, based on in vitro inhibition of glycation, has been surprisingly successful. This Editorial sets the stage for a series of articles by experts in the field, who have made key contributions to our understanding of the Maillard reaction in vivo.

Microvascular basement membranes in diabetes mellitus

The alterations in the microvascular system of diabetes mellitus patients are responsible for the most devastating complications of this widespread disease. In the kidney, the microangiopathy leads to thickening of the glomerular capillary basement membrane but also to the expansion of the mesangial matrix and thickening of the tubular basement membrane. Several mechanisms are implicated in the pathogenesis of diabetic renal microangiopathy. These include increased synthesis of type IV collagen following hyperglycaemia-induced alteration of the pattern of podocyte-integrin expression, decreased expression of matrix metalloproteinases (MMP-2 and 3), and increased expression of tissue inhibitor of metalloproteinase (TIMP). An altered morphology of podocytes accompanies these basement membrane alterations. Other factors which may contribute to renal matrix accumulation include vascular endothelial growth factor (VEGF), since treatment with anti-VEGF antibodies attenuates glomerular basement membrane thickening, platelet-derived growth factor (PDGF) (B chain) and its receptor, which appear to be highly expressed in mesangial and visceral epithelial cells and might play a role in the development of diabetic nephropathy. Also oxygen radicals/oxidative stress may play a role in matrix accumulation in diabetic nephropathy as aminoguanidine, an inhibitor of the formation of advanced glycation end-products but with antioxidant properties, attenuates diabetic nephropathy. Retinal diabetic microangiopathy follows much the same principles, be it that microvascular proliferation is a distinctive element in the retina. Nephropathy and retinopathy occur frequently but not always together, indicating that in their multifactorial pathogenesis much remains to be clarified.

Effects of protein kinase C inhibition and activation on proliferation and apoptosis of bovine retinal pericytes

AIMS/HYPOTHESIS

Drop-out of capillary pericytes occurs early and selectively in diabetic retinopathy. High glucose concentrations decrease replication and increase apoptosis of cultured pericytes. Since glucose activates protein kinase C, we investigated the effects of modulating this intracellular mediator on replication, cell cycle and apoptosis of cultured bovine retinal pericytes.

METHODS

Pericytes cultured in 5.6 or 28 mmol/l glucose were exposed to a protein kinase C activator (phorbol 12-myristate 13-acetate) and/or a selective inhibitor of its beta2 isoform (LY379196). Cells were counted after 7 days. Proliferation by the tetrazolium to formazan assay and DNA synthesis by 5-bromo-2'-deoxyuridine incorporation were measured at day 4. Cell cycle by flow cytometry and apoptosis by ELISA were assessed at day 2.

RESULTS

High glucose reduced pericyte replication and increased apoptosis. Protein kinase C activation increased proliferation, while inhibition of its beta2 isoform decreased it. Cell cycle was accelerated by protein kinase C activation and delayed by inhibition. Apoptosis was enhanced by protein kinase C inhibition and reduced by activation.

CONCLUSIONS/INTERPRETATION

Protein kinase C inhibition amplifies the anti-proliferative and pro-apoptotic effects of high glucose on cultured pericytes, whereas stimulation reduces apoptosis and promotes proliferation both in physiological glucose and high glucose. Protein kinase C inhibition, proposed for the treatment of diabetic macular edema and proliferative retinopathy, might accelerate pericyte dropout in earlier stages when these cells are still present in retinal capillaries.

A study of capillary pericyte viability on extracellular matrix produced by endothelial cells in high glucose

AIMS/HYPOTHESIS

Thickening of the basement membrane and selective loss of pericytes occur early in diabetic retinopathy. As we showed previously that pericyte adhesion is impaired on extracellular matrix produced by endothelial cells in high hexose concentrations, we aimed to verify if altered adhesion could influence pericyte viability and replication.

METHODS

Conditioned extracellular matrices were obtained by growing human umbilical vein endothelial cells in media containing 28 mmol/l D-glucose, with or without the inhibitors of protein glycation thiamine or aminoguanidine, and D-galactose or L-glucose up to 28 mmol/l. Having removed the endothelium, bovine retinal pericytes were grown on these matrices and, in separate experiments, on laminin, fibronectin or type IV collagen. Pericyte viability and replication were measured by cell counts and DNA synthesis after 7 days, cell cycle traversal after 2 days and apoptosis after 18 h, 2 days and 7 days.

RESULTS

Pericyte counts and DNA synthesis were reduced on matrices produced in high D-glucose and D-galactose, whilst matrix obtained in L-glucose reduced DNA synthesis but not counts. Both thiamine and aminoguanidine corrected reduced pericyte viability when added to high D-glucose. Cell cycle and apoptosis were not affected by growing pericytes on different conditioned matrices. Laminin, fibronectin and type IV collagen did not modify pericyte replication.

CONCLUSIONS/INTERPRETATIONS

Reduced pericyte counts could depend on impaired initial adhesion to the extracellular matrix produced by endothelium in high hexose concentrations, rather than impaired replication or viability. Altered cell-matrix interactions might facilitate pericyte dropout in diabetic retinopathy, independently of the effects of high glucose on pericyte replication.