Soluble Flt-1 gene therapy ameliorates albuminuria but accelerates tubulointerstitial injury in diabetic mice

sFlt-1 in Chronic Kidney Disease: Friend or Foe?

Placental growth factor (PlGF) and its receptor, fms-like tyrosine kinase-1 (Flt-1), are important regulators involved in angiogenesis, atherogenesis, and inflammation. This review article focuses on the function of PlGF/Flt-1 signaling and its regulation by soluble Flt-1 (sFlt-1) in chronic kidney disease (CKD). Elevation of circulating sFlt-1 and downregulation of sFlt-1 in the vascular endothelium by uremic toxins and oxidative stress both exacerbate heart failure and atherosclerosis. Circulating sFlt-1 is inconsistent with sFlt-1 synthesis, because levels of matrix-bound sFlt-1 are much higher than those of circulating sFlt-1, as verified by a heparin loading test, and are drastically reduced in CKD.

Circulating Soluble Fms-like Tyrosine Kinase in Renal Diseases Other than Preeclampsia

Soluble Fms-like tyrosine kinase (sFlt-1/sVEGFR1) is a naturally occurring antagonist of vascular endothelial growth factor (VEGF). Despite being a secreted, soluble protein lacking cytoplasmic and transmembrane domains, sFlt-1 can act locally and be protective against excessive microenvironmental VEGF concentration or exert autocrine functions independently of VEGF. Circulating sFlt-1 may indiscriminately affect endothelial function and the microvasculature of distant target organs. The clinical significance of excess sFlt-1 in kidney disease was first shown in preeclampsia, a major renal complication of pregnancy. However, circulating sFlt-1 levels appear to be increased in various diseases with varying degrees of renal impairment. Relevant clinical associations between circulating sFlt-1 and severe outcomes (e.g., endothelial dysfunction, renal impairment, cardiovascular disease, and all-cause mortality) have been observed in patients with CKD and after kidney transplantation. However, sFlt-1 appears to be protective against renal dysfunction-associated aggravation of atherosclerosis and diabetic nephropathy. Therefore, in this study, we provide an update on sFlt-1 in several kidney diseases other than preeclampsia, discuss clinical findings and experimental studies, and briefly consider its use in clinical practice.

Antiangiogenic therapy in diabetic nephropathy: A double‑edged sword (Review)

Diabetes and the associated complications are becoming a serious global threat and an increasing burden to human health and the healthcare systems. Diabetic nephropathy (DN) is the primary cause of end-stage kidney disease. Abnormal angiogenesis is well established to be implicated in the morphology and pathophysiology of DN. Factors that promote or inhibit angiogenesis serve an important role in DN. In the present review, the current issues associated with the vascular disease in DN are highlighted, and the challenges in the development of treatments are discussed.

The role of the PlGF/Flt-1 signaling pathway in the cardiorenal connection

Although the concept of the cardiorenal connection is widely accepted, athe underlying molecular mechanism has not been clearly defined. Nevertheless, accumulating evidence indicates that the nervous system and both the humoral and cellular immune systems are all involved. This review article focuses on the roles of the signaling pathway of placental growth factor (PlGF) and its receptor, fms-like tyrosine kinase-1 (Flt-1), in the development of the cardiorenal connection. PlGF, a member of the vascular endothelial cell growth factor family, is a specific ligand for Flt-1 and plays roles in the development of atherosclerosis, wound healing after ischemia injury, and angiogenesis through Flt-1 signaling. Flt-1, a tyrosine-kinase type receptor with a single transmembrane domain, has a soluble isoform (sFlt-1) consisting of only extracellular domains, and is an intrinsic antagonist of PlGF. In renal dysfunction, PlGF is upregulated and sFlt-1 is downregulated by oxidative stress or uremic toxins, leading to activation of the PlGF/Flt-1 signaling pathway, which in turn plays a role in the worsening of atherosclerosis and heart failure, both of which are frequently associated with renal dysfunction. Monocyte chemotactic protein-1 (MCP-1) is involved in the process downstream of the Flt-1 signaling pathway. Plasma levels of sFlt-1 correlate with the severity of renal dysfunction in patients with heart failure or myocardial infarction, and are associated with the incidence of cardiovascular events. This is inconsistent with the concept of relative activation of the PlGF/Flt-1 pathway in renal dysfunction. However, the level of circulating sFlt-1 does not always parallel sFlt-1 synthesis, probably because sFlt-1 is stored on cell surfaces through its heparin-binding domains and its quantity is regulated differently in renal dysfunction. This review summarizes a novel concept wherein noninfectious inflammation via PlGF/Flt-1 signaling is involved in the development of renal dysfunction-related cardiovascular complications.

Chymase inhibition retards albuminuria in type 2 diabetes

Chymase released from mast cells produces pro‐fibrotic, inflammatory, and vasoconstrictor agents. Studies were performed to test the hypothesis that chronic chymase inhibition provides a renal protective effect in type 2 diabetes. Diabetic (db/db) and control mice (db/m) were chronically infused with a chymase‐specific inhibitor or vehicle for 8 weeks. Baseline urinary albumin excretion (UalbV) averaged 42 ± 3 and 442 ± 32 microg/d in control (n = 22) and diabetic mice (n = 27), respectively (p < .05). After administration of chymase inhibitor to diabetic mice, the change in UalbV was significantly lower (459 ± 57 microg/d) than in vehicle‐treated diabetic mice (645 ± 108 microg/d). U_NGALV was not different at baseline between diabetic mice that would receive the chymase inhibitor (349 ± 56 ng/d, n = 6) and vehicle (373 ± 99 ng/d, n = 6) infusions, but increased significantly only in the vehicle‐treated diabetic mice (p < .05). Glomeruli of diabetic kidneys treated chronically with chymase inhibition demonstrated reduced mesangial matrix expansion compared to glomeruli from untreated diabetic mice. Plasma angiotensin II levels were not altered by chymase inhibitor treatment. In summary, chronic chymase inhibition slowed the progression of urinary albumin excretion in diabetic mice. In conclusion, renal chymase may contribute to the progression of albuminuria in type 2 diabetes renal disease.

Mechanism and Consequences of The Impaired Hif-1α Response to Hypoxia in Human Proximal Tubular HK-2 Cells Exposed to High Glucose

Renal hypoxia and loss of proximal tubular cells (PTC) are relevant in diabetic nephropathy. Hypoxia inhibits hypoxia-inducible factor-1α (HIF-1α) degradation, which leads to cellular adaptive responses through HIF-1-dependent activation of gene hypoxia-responsive elements (HRE). However, the diabetic microenvironment represses the HIF-1/HRE response in PTC. Here we studied the mechanism and consequences of impaired HIF-1α regulation in human proximal tubular HK-2 cells incubated in hyperglycemia. Inhibition at different levels of the canonical pathway of HIF-1α degradation did not activate the HIF-1/HRE response under hyperglycemia, except when proteasome was inhibited. Further studies suggested that hyperglycemia disrupts the interaction of HIF-1α with Hsp90, a known cause of proteasomal degradation of HIF-1α. Impaired HIF-1α regulation in cells exposed to hyperglycemic, hypoxic diabetic-like milieu led to diminished production of vascular endothelial growth factor-A and inhibition of cell migration (responses respectively involved in tubular protection and repair). These effects, as well as impaired HIF-1α regulation, were reproduced in normoglycemia in HK-2 cells incubated with microparticles released by HK-2 cells exposed to diabetic-like milieu. In summary, these results highlight the role of proteasome-dependent mechanisms of HIF-1α degradation on diabetes-induced HK-2 cells dysfunction and suggest that cell-derived microparticles may mediate negative effects of the diabetic milieu on PTC.

Increased soluble fms-like tyrosine kinase 1 after ischemia reperfusion contributes to adverse clinical outcomes following kidney transplantation

Renal ischemia reperfusion injury (IRI) adversely affects clinical outcomes following kidney transplantation. Understanding the cellular mechanisms and the changes in gene/protein expression following IRI may help to improve these outcomes. Serum soluble fms-like tyrosine kinase 1 (sFlt-1), a circulating antiangiogenic protein, is increased in the first week following kidney transplantation. We evaluated the casual relationship of elevated sFlt-1 levels with renal microvascular dysfunction following IRI in a longitudinal study of 93 kidney transplant recipients and in several animal models. Transplant recipients with higher sFlt-1 levels had higher odds of delayed graft function, graft rejection, impaired graft function, and death. In a subgroup of 25 participants who underwent kidney biopsy within 4 months of kidney transplantation, peritubular capillary area was lower in those with elevated serum sFtl-1 levels. The administration of recombinant sFlt-1 into rodents resulted in significant structural and functional changes of the renal microvasculature, including reduced peritubular capillary density and intracapillary blood volume, and lead to increased expression of inflammatory genes and increased fibrosis. In a murine model of IRI, the kidney was a site of sFlt-1 production, and systemic neutralization of sFlt-1 preserved peritubular capillary density and alleviated renal fibrosis. Our data indicate that high sFlt-1 levels after IRI play an important role in the pathogenesis of microvascular dysfunction, thereby contributing to adverse clinical outcomes following kidney transplantation.

The relationship of serum VEGF and sVEGFR-1 levels with vascular involvement in patients with Behçet's disease

Behçet's disease (BD) is a rare, chronic, inflammatory disorder characterized by multisystemic vasculitis including mucocutaneous, neurologic, and ophthalmic involvement. Our aim is to compare vascular endothelial growth factor (VEGF) and soluble vascular endothelial growth factor receptor-1 (sVEGFR-1) levels in BD, among the patients having or not having organ involvement, disease activation and especially vascular involvement. Fifty-five patients with BD, 25 of which were accompanied by vascular involvement, and 31 control subjects were included in the study. Disease activity was assessed with the Turkish version of Behçet Disease Current Activity Form (BDCAF) and active vasculitis lesions at the time of study were recorded. Age at diagnosis was 32.2 ± 4.6, while the mean duration of BD was 96.3 (72.3) months. The median for BDCAF score was 2.0 (range 0, 3.0), and 29 (52%) of patients had active BD. The serum VEGF and sVEGFR-1 levels in patients with BD were significantly higher than that in controls [(298 (338.5) pg/mL; 93 (93.5) pg/mL in patients and 136.2 (73) pg/mL; 56.5 (48.5) pg/mL in controls, respectively, p < .001 for both values] while difference in VEGF/sVEGFR-1 ratio was obtained close to borderline of significance (p = .03). Our study is the first report indicating elevated serum VEGF, sVEGFR-1, and more importantly VEGF/sVEGFR-1 ratio could play an important role in the development of trombosis in BD. VEGF and/or sVEGFR-1 should not be evaluated independently in the same patient group and the ratio of these two parameters is a more important indicator, especially in the evaluation of BD especially with vascular involvement together with the duration of disease.

Endogenous Antiangiogenic Factors in Chronic Kidney Disease: Potential Biomarkers of Progression

Chronic kidney disease (CKD) is a major global health problem. Unless intensive intervention is initiated, some patients can rapidly progress to end-stage kidney disease. However, it is often difficult to predict renal outcomes using conventional laboratory tests in individuals with CKD. Therefore, many researchers have been searching for novel biomarkers to predict the progression of CKD. Angiogenesis is involved in physiological and pathological processes in the kidney and is regulated by the balance between a proangiogenic factor, vascular endothelial growth factor (VEGF)-A, and various endogenous antiangiogenic factors. In recent reports using genetically engineered mice, the roles of these antiangiogenic factors in the pathogenesis of kidney disease have become increasingly clear. In addition, recent clinical studies have demonstrated associations between circulating levels of antiangiogenic factors and renal dysfunction in CKD patients. In this review, we summarize recent advances in the study of representative endogenous antiangiogenic factors, including soluble fms-related tyrosine kinase 1, soluble endoglin, pigment epithelium-derived factor, VEGF-A₁₆₅b, endostatin, and vasohibin-1, in associations with kidney diseases and discuss their predictive potentials as biomarkers of progression of CKD.

The VEGF-A inhibitor sFLT-1 improves renal function by reducing endothelial activation and inflammation in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

Animal models of diabetic nephropathy show increased levels of glomerular vascular endothelial growth factor (VEGF)-A, and several studies have shown that inhibiting VEGF-A in animal models of diabetes can prevent albuminuria and glomerular hypertrophy. However, in those studies, treatment was initiated before the onset of kidney damage. Therefore, the aim of this study was to investigate whether transfecting mice with the VEGF-A inhibitor sFlt-1 (encoding soluble fms-related tyrosine kinase 1) can reverse pre-existing kidney damage in a mouse model of type 1 diabetes. In addition, we investigated whether transfection with sFlt-1 can reduce endothelial activation and inflammation in these mice.

METHODS

Subgroups of untreated 8-week-old female C57BL/6J control (n = 5) and diabetic mice (n = 7) were euthanised 5 weeks after the start of the experiment in order to determine the degree of kidney damage prior to treatment with sFLT-1. Diabetes was induced with three i.p. injections of streptozotocin (75 mg/kg) administered at 2 day intervals. Diabetic nephropathy was then investigated in diabetic mice transfected with sFlt-1 (n = 6); non-diabetic, non-transfected control mice (n = 5); non-diabetic control mice transfected with sFlt-1(n = 10); and non-transfected diabetic mice (n = 6). These mice were euthanised at the end of week 15. Transfection with sFlt-1 was performed in week 6.

RESULTS

We found that transfection with sFlt-1 significantly reduced kidney damage by normalising albuminuria, glomerular hypertrophy and mesangial matrix content (i.e. glomerular collagen type IV protein levels) (p < 0.001). We also found that transfection with sFlt-1 reduced endothelial activation (p < 0.001), glomerular macrophage infiltration (p < 0.001) and glomerular TNF-α protein levels (p < 0.001). Finally, sFLT-1 decreased VEGF-A-induced endothelial activation in vitro (p < 0.001).

CONCLUSIONS/INTERPRETATION

These results suggest that sFLT-1 might be beneficial in treating diabetic nephropathy by inhibiting VEGF-A, thereby reducing endothelial activation and glomerular inflammation, and ultimately reversing kidney damage.

Antiangiogenic Therapy for Diabetic Nephropathy

Angiogenesis has been shown to be a potential therapeutic target for early stages of diabetic nephropathy in a number of animal experiments. Vascular endothelial growth factor (VEGF) is the main mediator for abnormal angiogenesis in diabetic glomeruli. Although beneficial effects of anti-VEGF antibodies have previously been demonstrated in diabetic animal experiments, recent basic and clinical evidence has revealed that the blockade of VEGF signaling resulted in proteinuria and renal thrombotic microangiopathy, suggesting the importance of maintaining normal levels of VEGF in the kidneys. Therefore, antiangiogenic therapy for diabetic nephropathy should eliminate excessive glomerular angiogenic response without accelerating endothelial injury. Some endogenous antiangiogenic factors such as endostatin and tumstatin inhibit overactivation of endothelial cells but do not specifically block VEGF signaling. In addition, the novel endothelium-derived antiangiogenic factor vasohibin-1 enhances stress tolerance and survival of the endothelial cells, while inhibiting excess angiogenesis. These factors have been demonstrated to suppress albuminuria and glomerular alterations in a diabetic mouse model. Thus, antiangiogenic therapy with promising candidates will possibly improve renal prognosis in patients with early stages of diabetic nephropathy.

VEGF and the diabetic kidney: More than too much of a good thing

Over a decade and a half has passed since the publication of early reports hinting at a pathogenetic role for vascular endothelial growth factor ("VEGF") in the development of diabetic kidney disease. In diabetic rats, renal mRNA levels of the VEGF-A isoform were upregulated and administration of a VEGF-A neutralizing antibody attenuated albuminuria: VEGF was "bad" in diabetic nephropathy. Since that time, our understanding of the complexity of the renal VEGF system has advanced. Unlike its experimental counterpart, human diabetic nephropathy is associated with diminished VEGF-A levels and experience in the oncological setting has taught us that VEGF blocking therapy can cause adverse renal effects in patients. Correspondingly, investigational studies in cultured cells and rodent models have demonstrated that the biological effects of the VEGF system are dependent not only on the amount of VEGF, but also the type of VEGF, its sites of action and the prevailing milieu. Here we reflect back on the discoveries that have been made since those initial reports that shone the spotlight on the importance of the VEGF system in the diabetic kidney and we consider that the role of VEGF in diabetic nephropathy extends well beyond being "too much of a good thing".

Endothelial Progenitor Cells in Diabetic Microvascular Complications: Friends or Foes?

Despite being featured as metabolic disorder, diabetic patients are largely affected by hyperglycemia-induced vascular abnormality. Accumulated evidence has confirmed the beneficial effect of endothelial progenitor cells (EPCs) in coronary heart disease. However, antivascular endothelial growth factor (anti-VEGF) treatment is the main therapy for diabetic retinopathy and nephropathy, indicating the uncertain role of EPCs in the pathogenesis of diabetic microvascular disease. In this review, we first illustrate how hyperglycemia induces metabolic and epigenetic changes in EPCs, which exerts deleterious impact on their number and function. We then discuss how abnormal angiogenesis develops in eyes and kidneys under diabetes condition, focusing on “VEGF uncoupling with nitric oxide” and “competitive angiopoietin 1/angiopoietin 2” mechanisms that are shared in both organs. Next, we dissect the nature of EPCs in diabetic microvascular complications. After we overview the current EPCs-related strategies, we point out new EPCs-associated options for future exploration. Ultimately, we hope that this review would uncover the mysterious nature of EPCs in diabetic microvascular disease for therapeutics.

Diabetic nephropathy - complications and treatment

Diabetic nephropathy is a significant cause of chronic kidney disease and end-stage renal failure globally. Much research has been conducted in both basic science and clinical therapeutics, which has enhanced understanding of the pathophysiology of diabetic nephropathy and expanded the potential therapies available. This review will examine the current concepts of diabetic nephropathy management in the context of some of the basic science and pathophysiology aspects relevant to the approaches taken in novel, investigative treatment strategies.

Cell biology of diabetic nephropathy: Roles of endothelial cells, tubulointerstitial cells and podocytes

Diabetic nephropathy is the major cause of end-stage renal failure throughout the world in both developed and developing countries. Diabetes affects all cell types of the kidney, including endothelial cells, tubulointerstitial cells, podocytes and mesangial cells. During the past decade, the importance of podocyte injury in the formation and progression of diabetic nephropathy has been established and emphasized. However, recent findings provide additional perspectives on pathogenesis of diabetic nephropathy. Glomerular endothelial damage is already present in the normoalbuminuric stage of the disease when podocyte injury starts. Genetic targeting of mice that cause endothelial injury leads to accelerated diabetic nephropathy. Tubulointerstitial damage, previously considered to be a secondary effect of glomerular protein leakage, was shown to have a primary significance in the progression of diabetic nephropathy. Emerging evidence suggests that the glomerular filtration barrier and tubulointerstitial compartment is a composite, dynamic entity where any injury of one cell type spreads to other cell types, and leads to the dysfunction of the whole apparatus. Accumulation of novel knowledge would provide a better understanding of the pathogenesis of diabetic nephropathy, and might lead to a development of a new therapeutic strategy for the disease.

Vascular endothelial growth factor-receptor 1 inhibition aggravates diabetic nephropathy through eNOS signaling pathway in db/db mice

The manipulation of vascular endothelial growth factor (VEGF)-receptors (VEGFRs) in diabetic nephropathy is as controversial as issue as ever. It is known to be VEGF-A and VEGFR2 that regulate most of the cellular actions of VEGF in experimental diabetic nephropathy. On the other hand, such factors as VEGF-A, -B and placenta growth factor bind to VEGFR1 with high affinity. Such notion instigated us to investigate on whether selective VEGFR1 inhibition with GNQWFI hexamer aggravates the progression of diabetic nephropathy in db/db mice. While diabetes suppressed VEGFR1, it did increase VEGFR2 expressions in the glomerulus. Db/db mice with VEGFR1 inhibition showed more prominent features with respect to, albuminuria, mesangial matrix expansion, inflammatory cell infiltration and greater numbers of apoptotic cells in the glomerulus, and oxidative stress than that of control db/db mice. All these changes were related to the suppression of diabetes-induced increases in PI3K activity and Akt phosphorylation as well as the aggravation of endothelial dysfunction associated with the inactivation of FoxO3a and eNOS-NOx. In cultured human glomerular endothelial cells (HGECs), high-glucose media with VEGFR1 inhibition induced more apoptotic cells and oxidative stress than did high-glucose media alone, which were associated with the suppression of PI3K-Akt phosphorylation, independently of the activation of AMP-activated protein kinase, and inactivation of FoxO3a and eNOS-NOx pathway. In addition, transfection with VEGFR1 siRNA in HGECs also suppressed PI3K-Akt-eNOS signaling. In conclusion, the specific blockade of VEGFR1 with GNQWFI caused severe renal injury related to profound suppression of the PI3K-Akt, FoxO3a and eNOS-NOx pathway, giving rise to the oxidative stress-induced apoptosis of glomerular cells in type 2 diabetic nephropathy.

Targeted glomerular angiopoietin-1 therapy for early diabetic kidney disease

Vascular growth factors play an important role in maintaining the structure and integrity of the glomerular filtration barrier. In healthy adult glomeruli, the proendothelial survival factors vascular endothelial growth factor-A (VEGF-A) and angiopoietin-1 are constitutively expressed in glomerular podocyte epithelia. We demonstrate that this milieu of vascular growth factors is altered in streptozotocin-induced type 1 diabetic mice, with decreased angiopoietin-1 levels, VEGF-A upregulation, decreased soluble VEGF receptor-1 (VEGFR1), and increased VEGFR2 phosphorylation. This was accompanied by marked albuminuria, nephromegaly, hyperfiltration, glomerular ultrastructural alterations, and aberrant angiogenesis. We subsequently hypothesized that restoration of angiopoietin-1 expression within glomeruli might ameliorate manifestations of early diabetic glomerulopathy. Podocyte-specific inducible repletion of angiopoietin-1 in diabetic mice caused a 70% reduction of albuminuria and prevented diabetes-induced glomerular endothelial cell proliferation; hyperfiltration and renal morphology were unchanged. Furthermore, angiopoietin-1 repletion in diabetic mice increased Tie-2 phosphorylation, elevated soluble VEGFR1, and was paralleled by a decrease in VEGFR2 phosphorylation and increased endothelial nitric oxide synthase Ser(1177) phosphorylation. Diabetes-induced nephrin phosphorylation was also reduced in mice with angiopoietin-1 repletion. In conclusion, targeted angiopoietin-1 therapy shows promise as a renoprotective tool in the early stages of diabetic kidney disease.

Role of the eNOS-NO system in regulating the antiproteinuric effects of VEGF receptor 2 inhibition in diabetes

Subtle perturbations in intraglomerular VEGF/VEGFR-2 signaling or in the influencing microenvironment can profoundly affect renal function, resulting in the apparently paradoxical observation that VEGF blockade attenuates proteinuria development in experimental diabetes despite exerting the opposite effect under other circumstances. In the present study, we sought to explore the role of eNOS-NO activity in regulating the differential response to VEGF blockade in the diabetic and nondiabetic settings. In a rodent model of accelerated renal injury, the transgenic (mRen-2)27 (Ren-2) rat, VEGFR-2 inhibition with the small molecule vandetanib resulted in an increase in urine protein excretion preceding a subsequent rise in systolic blood pressure. When compared to their normoglycaemic counterparts, diabetic Ren-2 rats exhibited an increase in the renal expression of eNOS and in urinary excretion of nitric oxide (NO) metabolites. In contrast to the heavy proteinuria observed with vandetanib in nondiabetic TGR(mRen-2)27 rats, VEGFR-2 inhibition reduced urine protein excretion in diabetic animals, despite a comparable magnitude of histological injury. However, proteinuria was markedly increased by concomitant treatment of diabetic Ren-2 rats with vandetanib and the nitric oxide synthase inhibitor L-NAME. These observations highlight the pivotal role that the eNOS-NO system plays in regulating the biologic response to VEGF within the glomerulus.

Contrast ultrasound and targeted microbubbles: diagnostic and therapeutic applications in progressive diabetic nephropathy

Diabetic nephropathy remains one of the most common causes for end-stage renal disease worldwide. Although therapies aimed at optimizing glycemic control and systemic blood pressure have benefit, the reduction in progressive nephropathy remains modest at best. Thus, research continues to focus on newer therapies to address the unmet needs for additional renal protective strategies. The ability to noninvasively image the molecular and cellular processes that underlie diabetic nephropathy would be useful in risk stratifying patients with diabetes, and more importantly would aid in the evaluation of novel therapies to prevent and treat nephropathy. In addition, the development of ultrasound technologies that allow targeted gene delivery using high-power ultrasound and DNA-bearing microbubbles may have applicability for gene therapy to prevent diabetic nephropathy. This review highlights contrast-enhanced ultrasound imaging techniques for the evaluation of renal pathologies, including perfusion and molecular imaging techniques, and ultrasound-mediated gene delivery for therapeutic applications in diabetic nephropathy, that have potential for translation to clinical practice.

Vegfa protects the glomerular microvasculature in diabetes

Vascular endothelial growth factor A (VEGFA) expression is increased in glomeruli in the context of diabetes. Here, we tested the hypothesis that this upregulation of VEGFA protects the glomerular microvasculature in diabetes and that therefore inhibition of VEGFA will accelerate nephropathy. To determine the role of glomerular Vegfa in the development and progression of diabetic nephropathy, we used an inducible Cre-loxP gene-targeting system that enabled genetic deletion of Vegfa selectively from glomerular podocytes of wild-type or diabetic mice. Type 1 diabetes was induced in mice using streptozotocin (STZ). We then assessed the extent of glomerular dysfunction by measuring proteinuria, glomerular pathology, and glomerular cell apoptosis. Vegfa expression increased in podocytes in the STZ model of diabetes. After 7 weeks of diabetes, diabetic mice lacking Vegfa in podocytes exhibited significantly greater proteinuria with profound glomerular scarring and increased apoptosis compared with control mice with diabetes or Vegfa deletion without diabetes. Reduced local production of glomerular Vegfa in a mouse model of type 1 diabetes promotes endothelial injury accelerating the progression of glomerular injury. These results suggest that upregulation of VEGFA in diabetic kidneys protects the microvasculature from injury and that reduction of VEGFA in diabetes may be harmful.

Diabetic angiopathy and angiogenic defects

Diabetes is one of the most serious health problems in the world. A major complication of diabetes is blood vessel disease, termed angiopathy, which is characterized by abnormal angiogenesis. In this review, we focus on angiogenesis abnormalities in diabetic complications and discuss its benefits and drawbacks as a therapeutic target for diabetic vascular complications. Additionally, we discuss glucose metabolism defects that are associated with abnormal angiogenesis in atypical diabetic complications such as cancer.

Fluorescent microangiography is a novel and widely applicable technique for delineating the renal microvasculature

Rarefaction of the renal microvasculature correlates with declining kidney function. However, current technologies commonly used for its evaluation are limited by their reliance on endothelial cell antigen expression and assessment in two dimensions. We set out to establish a widely applicable and unbiased optical sectioning method to enable three dimensional imaging and reconstruction of the renal microvessels based on their luminal filling. The kidneys of subtotally nephrectomized (SNx) rats and their sham-operated counterparts were subjected to either routine two-dimensional immunohistochemistry or the novel technique of fluorescent microangiography (FMA). The latter was achieved by perfusion of the kidney with an agarose suspension of fluorescent polystyrene microspheres followed by optical sectioning of 200 µm thick cross-sections using a confocal microscope. The fluorescent microangiography method enabled the three-dimensional reconstruction of virtual microvascular casts and confirmed a reduction in both glomerular and peritubular capillary density in the kidneys of SNx rats, despite an overall increase in glomerular volume. FMA is an uncomplicated technique for evaluating the renal microvasculature that circumvents many of the limitations imposed by conventional analysis of two-dimensional tissue sections.

Podocyte vascular endothelial growth factor (Vegf₁₆₄) overexpression causes severe nodular glomerulosclerosis in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

The pathogenic role of excessive vascular endothelial growth factor (VEGF)-A in diabetic nephropathy has not been defined. We sought to test whether increased podocyte VEGF-A signalling determines the severity of diabetic glomerulopathy.

METHODS

Podocyte-specific, doxycycline-inducible Vegf₁₆₄ (the most abundant Vegfa isoform) overexpressing adult transgenic mice were made diabetic with low doses of streptozotocin and examined 12 weeks after onset of diabetes. We studied diabetic and non-diabetic transgenic mice fed a standard or doxycycline-containing diet. VEGF-A and albuminuria were measured by ELISA, creatinine was measured by HPLC, renal morphology was examined by light and electron microscopy, and gene expression was assessed by quantitative PCR, immunoblotting and immunohistochemistry.

RESULTS

Podocyte Vegf₁₆₄ overexpression in our mouse model of diabetes resulted in advanced diabetic glomerulopathy, characterised by Kimmelstiel-Wilson-like nodular glomerulosclerosis, microaneurysms, mesangiolysis, glomerular basement membrane thickening, podocyte effacement and massive proteinuria associated with hyperfiltration. It also led to increased VEGF receptor 2 and semaphorin3a levels, as well as nephrin and matrix metalloproteinase-2 downregulation, whereas circulating VEGF-A levels were similar to those in control diabetic mice.

CONCLUSIONS/INTERPRETATION

Collectively, these data demonstrate that increased podocyte Vegf₁₆₄ signalling dramatically worsens diabetic nephropathy in a streptozotocin-induced mouse model of diabetes, resulting in nodular glomerulosclerosis and massive proteinuria. This suggests that local rather than systemic VEGF-A levels determine the severity of diabetic nephropathy and that semaphorin3a signalling and matrix metalloproteinase-2 dysregulation are mechanistically involved in severe diabetic glomerulopathy.

Amelioration of renal alterations in obese type 2 diabetic mice by vasohibin-1, a negative feedback regulator of angiogenesis

The involvement of VEGF-A as well as the therapeutic efficacy of angiogenesis inhibitors in diabetic nephropathy have been reported. We recently reported the therapeutic effects of vasohibin-1 (VASH-1), an endogenous angiogenesis inhibitor, in a type 1 diabetic nephropathy model (Nasu T, Maeshima Y, Kinomura M, Hirokoshi-Kawahara K, Tanabe K, Sugiyama H, Sonoda H, Sato Y, Makino H. Diabetes 58: 2365-2375, 2009). In this study, we investigated the therapeutic efficacy of VASH-1 on renal alterations in obese mice with type 2 diabetes. Diabetic db/db mice received intravenous injections of adenoviral vectors encoding human VASH-1 (AdhVASH-1) and were euthanized 8 wk later. AdhVASH-1 treatment resulted in significant suppression of glomerular hypertrophy, glomerular hyperfiltration, albuminuria, increase in the CD31(+) glomerular endothelial area, F4/80(+) monocyte/macrophage infiltration, the accumulation of type IV collagen, and mesangial matrix. An increase in the renal levels of VEGF-A, VEGFR-2, transforming growth factor (TGF)-β1, and monocyte chemoattractant protein-1 in diabetic animals was significantly suppressed by AdhVASH-1 (immunoblotting). AdhVASH-1 treatment significantly recovered the loss and altered the distribution patterns of nephrin and zonula occludens (ZO)-1 and suppressed the increase in the number of fibroblast-specific protein-1 (FSP-1(+)) and desmin(+) podocytes in diabetic mice. In vitro, recombinant human VASH-1 (rhVASH-1) dose dependently suppressed the upregulation of VEGF induced by high ambient glucose (25 mM) in cultured mouse podocytes. In addition, rhVASH-1 significantly recovered the mRNA levels of nephrin and the protein levels of ZO-1 and P-cadherin and suppressed the increase in protein levels of desmin, FSP-1, Snail, and Slug in podocytes under high-glucose condition. Taken together, these results suggest the potential use of VASH-1 as a novel therapeutic agent in type 2 diabetic nephropathy mediated via antiangiogenic effects and maintenance of podocyte phenotype in association with antiproteinuric effects.

Recent insights into diabetic renal injury from the db/db mouse model of type 2 diabetic nephropathy

The db/db mouse is the most widely used animal model of type 2 diabetic nephropathy. Recent studies have utilized genetic backcrossing with transgenic mouse strains to create novel db/db strains that either lack or overexpress specific genes. These novel strains [ICAM-1-/-, CCL2-/-, MKK3-/-, osteopontin-/-, plasminogen activator inhibitor-1 (PAI-1)-/-, endothelial nitric oxide synthase-/-, SOD-Tg, rCAT-Tg] have provided valuable insights into the molecular mechanisms which promote diabetic renal injury. In addition, surgical removal of one kidney has been shown to accelerate injury in the remaining kidney of diabetic db/db mice. A number of novel therapeutic agents have also been tested in db/db mice, including inhibitors of inflammation (chemokine receptor antagonists, anti-CCL2 RNA aptamer, anti-c-fms antibody); oxidative stress (oxykine, biliverdin); the renin-angiotensin-aldosterone system (aliskiren, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, eplerenone); advanced glycation end products (AGE; pyridoxamine, alagebrium, soluble AGE receptor); angiogenesis (NM-3, anti-CXCL12 RNA aptamer, soluble Flt-1); lipid accumulation (statins, farnesoid X receptor agonists, Omacor); intracellular signaling pathways (PKC-β or JNK inhibitors); and fibrosis [transforming growth factor (TGF)-β antibody, TGF-βR kinase inhibitor, soluble betaglycan, SMP-534, CTGF-antisense oligonucleotide, mutant PAI-1, pirfenidone], which have identified potential therapeutic targets for clinical translation. This review summarizes the advances in knowledge gained from studies in genetically modified db/db mice and treatment of db/db mice with novel therapeutic agents.

Angiogenesis and chronic kidney disease

The number of patients requiring renal replacement therapy due to end-stage renal disease (ESRD) is increasing worldwide. The prevalence of chronic kidney disease (CKD), and the importance of CKD as a risk factor in development of ESRD and in complicating cardiovascular disease (CVD) have been confirmed. In recent years, the involvement of angiogenesis-related factors in the progression of CKD has been studied, and the potential therapeutic effects on CKD of modulating these factors have been identified. Vascular endothelial growth factor (VEGF)-A, a potent pro-angiogenic factor, is involved in the development of the kidney, in maintenance of the glomerular capillary structure and filtration barrier, and in the renal repair process after injury. VEGF-A is also involved in the development of early diabetic nephropathy, demonstrated by the therapeutic effects of anti-VEGF-A antibody. Angiopoietin (Ang)-1 induces the maturation of newly formed blood vessels, and the therapeutic effects of Ang-1 in diabetic nephropathy have been described. In experimental models of diabetic nephropathy, the therapeutic effects of angiogenesis inhibitors, including angiostatin, endostatin and tumstatin peptides, the isocoumarin NM-3, and vasohibin-1, have been reported.

Further analysis of the involvement of angiogenesis-related factors in the development of CKD is required. Determining the disease stage at which therapy is most effective and developing an effective drug delivery system targeting the kidney will be essential for pro-or anti-angiogenic strategies for patients with CKD.

Podocyte-specific overexpression of GLUT1 surprisingly reduces mesangial matrix expansion in diabetic nephropathy in mice

Increased expression of the facilitative glucose transporter, GLUT1, leads to glomerulopathy that resembles diabetic nephropathy, whereas prevention of enhanced GLUT1 expression retards nephropathy. While many of the GLUT1-mediated effects are likely due to mesangial cell effects, we hypothesized that increased GLUT1 expression in podocytes also contributes to the progression of diabetic nephropathy. Therefore, we generated two podocyte-specific GLUT1 transgenic mouse lines (driven by a podocin promoter) on a db/m C57BLKS background. Progeny of the two founders were used to generate diabetic db/db and control db/m littermate mice. Immunoblots of glomerular lysates showed that transgenic mice had a 3.5-fold (line 1) and 2.1-fold (line 2) increase in GLUT1 content compared with wild-type mice. Both lines showed similar increases in fasting blood glucose and body weights at 24 wk of age compared with wild-type mice. Mesangial index (percent PAS-positive material in the mesangial tuft) increased 88% (line 1) and 75% (line 2) in the wild-type diabetic mice but only 48% (line 1) and 39% (line 2) in the diabetic transgenic mice (P < 0.05, transgenic vs. wild-type mice). This reduction in mesangial expansion was accompanied by a reduction in fibronectin accumulation, and vascular endothelial growth factor (VEGF) levels increased only half as much in the transgenic diabetic mice as in wild-type diabetic mice. Levels of nephrin, neph1, CD2AP, podocin, and GLUT4 were not significantly different in transgenic compared with wild-type mice. Taken together, increased podocyte GLUT1 expression in diabetic mice does not contribute to early diabetic nephropathy; surprisingly, it protects against mesangial expansion and fibronectin accumulation possibly by blunting podocyte VEGF increases.