Diabetes reduces basal retinal insulin receptor signaling: reversal with systemic and local insulin

Deletion of the p85alpha regulatory subunit of phosphoinositide 3-kinase in cone photoreceptor cells results in cone photoreceptor degeneration

PURPOSE

Downregulation of the retinal insulin/mTOR pathway in mouse models of retinitis pigmentosa is linked to cone cell death, which can be delayed by systemic administration of insulin. A classic survival kinase linking extracellular trophic/growth factors with intracellular antiapoptotic pathways is phosphoinositide 3-kinase (PI3K), which the authors have shown to protect rod photoreceptors from stress-induced cell death. The role of PI3K in cones was studied by conditional deletion of its p85α regulatory subunit.

METHODS

Mice expressing Cre recombinase in cones were bred to mice with a floxed pi3k gene encoding the p85α regulatory subunit of the PI3K and were back-crossed to ultimately generate offspring with cone-specific p85α knockout (cKO). Cre expression and cone-specific localization were confirmed by Western blot analysis and immunohistochemistry (IHC), respectively. Cone structural integrity was determined by IHC using peanut agglutinin and an M-opsin-specific antibody. Electroretinography (ERG) was used to assess rod and cone photoreceptor function. Retinal structure was examined by light and electron microscopy.

RESULTS

An age-related cone degeneration was found in cKO mice, evidenced by a reduction in photopic ERG amplitudes and loss of cone cells. By 12 months of age, approximately 78% of cones had died, and progressive disorganization of synaptic ultrastructure was noted in surviving cone terminals in cKO retinas. Rod viability was unaffected in p85α cKO mice.

CONCLUSIONS

The present study suggests that PI3K signaling pathway is essential for cone survival in the mouse retina.

Insulin signaling in retinal neurons is regulated within cholesterol-enriched membrane microdomains

Neuronal cell death is an early pathological feature of diabetic retinopathy. We showed previously that insulin receptor signaling is diminished in retinas of animal models of diabetes and that downstream Akt signaling is involved in insulin-mediated retinal neuronal survival. Therefore, further understanding of the mechanisms by which retinal insulin receptor signaling is regulated could have therapeutic implications for neuronal cell death in diabetes. Here, we investigate the role of cholesterol-enriched membrane microdomains to regulate PKC-mediated inhibition of Akt-dependent insulin signaling in R28 retinal neurons. We demonstrate that PKC activation with either a phorbol ester or exogenous application of diacylglycerides impairs insulin-induced Akt activation, whereas PKC inhibition augments insulin-induced Akt activation. To investigate the mechanism by which PKC impairs insulin-stimulated Akt activity, we assessed various upstream mediators of Akt signaling. PKC activation did not alter the tyrosine phosphorylation of the insulin receptor or IRS-2. Additionally, PKC activation did not impair phosphatidylinositol 3-kinase activity, phosphoinositide-dependent kinase phosphorylation, lipid phosphatase (PTEN), or protein phosphatase 2A activities. Thus, we next investigated a biophysical mechanism by which insulin signaling could be disrupted and found that disruption of lipid microdomains via cholesterol depletion blocks insulin-induced Akt activation and reduces insulin receptor tyrosine phosphorylation. We also demonstrated that insulin localizes phosphorylated Akt to lipid microdomains and that PMA reduces phosphorylated Akt. In addition, PMA localizes and recruits PKC isotypes to these cholesterol-enriched microdomains. Taken together, these results demonstrate that both insulin-stimulated Akt signaling and PKC-induced inhibition of Akt signaling depend on cholesterol-enriched membrane microdomains, thus suggesting a putative biophysical mechanism underlying insulin resistance in diabetic retinopathy.

The significance of vascular and neural apoptosis to the pathology of diabetic retinopathy

The most striking features of diabetic retinopathy are the vascular abnormalities that are apparent by fundus examination. There is also strong evidence that diabetes causes apoptosis of neural and vascular cells in the retina. Thus, there is good reason to define diabetic retinopathy as a form of chronic neurovascular degeneration. In keeping with the gradual onset of retinopathy in humans, the rate of cell loss in the animal models is insidious, even in uncontrolled diabetes. This is not surprising given that a sustained high rate of cell loss without regeneration would soon lead to catastrophic tissue destruction. The consequences of ongoing cell death are difficult to detect, and even the quantification of cumulative cell loss requires painstaking histology and microscopy. This subtle cell loss raises the issue of the relevance of the phenomenon to the progression of diabetic retinopathy and the ultimate loss of vision. Neuronal function may be compromised in advance of apoptosis, contributing to an early deterioration of vision. Here we review some of the evidence supporting apoptotic cell death as a contributing mechanism of diabetic retinopathy, explore some of the potential causes, and discuss the potential links between apoptosis and loss of visual function in diabetic retinopathy.

An integrated approach to diabetic retinopathy research

This review discusses the pathophysiology of diabetic retinopathy related to direct effects of loss of insulin receptor action and metabolic dysregulation on the retina. The resulting sensory neuropathy can be diagnosed by structural and functional tests in patients with mild nonproliferative diabetic retinopathy. Research teams can collaborate to integrate ocular and systemic factors that impair vision and to design strategies to maintain retinal function in persons with diabetes mellitus. Evolving concepts may lead to inclusion of tests of retinal function in the detection of diabetic retinopathy and neuroprotective strategies to preserve vision for persons with diabetes.

Rhodopsin-regulated insulin receptor signaling pathway in rod photoreceptor neurons

The retina is an integral part of the central nervous system and retinal cells are known to express insulin receptors (IR), although their function is not known. This article describes recent studies that link the photoactivation of rhodopsin to tyrosine phosphorylation of the IR and subsequent activation of phosphoinositide 3-kinase, a neuron survival factor. Our studies suggest that the physiological role of this process is to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. We focus mainly on our recently identified regulation of the IR pathway through the G-protein-coupled receptor rhodopsin. Various mutant and knockout proteins of phototransduction cascade have been used to study the light-induced activation of the retinal IR. Our studies suggest that rhodopsin may have additional previously uncharacterized signaling functions in photoreceptors.

Application of isoproterenol inhibits diabetic-like changes in the rat retina

Diabetic retinopathy is the leading cause of blindness to working-age adults. We have recently shown that surgical removal or genetic manipulations to eliminate sympathetic neurotransmission produces many of the retinal changes similar to rodent diabetic retinopathy with normal glucose levels. We hypothesized that application of a beta-adrenergic receptor agonist, isoproterenol, could reach the retina to elicit normal cellular signaling and inhibit the functional and morphological markers of early stage diabetic retinopathy in the rat. Rats were made diabetic by injection of 60 mg/kg streptozotocin. Within 3 days of diabetes-induction, rats were placed into 1 of 3 groups (control, diabetes, or diabetic + isoproterenol). Dose and time course studies were done for isoproterenol using a PKA ELISA and CREB analyses. Once the optimal dose and time course were established, electrical activity of the retina was analyzed by electroretinogram each month for the 8-month study. Western blotting was done for insulin receptor signaling and Akt and ELISA analyses for TNFalpha concentration and cleavage of caspase 3 at 2- and 8-months of diabetes. Diabetes-induced degeneration of neural cells and retinal thickness were assessed at 2 months, while degenerate capillaries were quantitated at 8 months of treatment. Daily application of 50 mM isoproterenol was effective in inhibiting the diabetes-induced loss of a- and b-wave and oscillatory potential amplitudes in the electroretinogram. Isoproterenol blocked the increase in TNFalpha and apoptosis in the diabetic retina. The numbers of degenerate capillaries were also reduced in the treated + diabetes retina. These data strongly suggest that loss of beta-adrenergic receptor signaling may be a key factors in early stage diabetic retinopathy. Resolution of this loss of adrenergic receptor signaling can inhibit some of the hallmarks of diabetic retinopathy in the retina.

Prediction of diabetic retinopathy: role of oxidative stress and relevance of apoptotic biomarkers

Diabetic retinopathy (DR) is the foremost cause of blindness in working-aged worldwide; it is characterized by vascular and neuronal degeneration. Features of DR include leukocyte adhesion, increased vascular permeability, neovascularization and neuronal cell death. Early diagnosis and intervention are important to prevent or at least ameliorate the development of DR. Recent reports indicate that pathophysiological mechanisms leading to diabetic retinopathy include oxidative stress and retinal cell death cascades. Circulating biomarkers of oxidative stress such as malondialdehyde (MDA), thiobarbituric acid reacting substances (TBARS), conjugated diene (CD), advanced oxidation protein products (AOPP), protein carbonyl, 8-hydroxydeoxyguanosin (8-OHdG), nitrotyrosine, and F(2) isoprostanes and pro-apoptosis molecules (caspase-3, Fas, and Bax) are associated with increased susceptibility to develop DR in diabetic subjects. Thus, identification of oxidative stress and cell death biomarkers in diabetic patients could be in favor of predicting, diagnosis, and prevention of DR, and to target for novel therapeutic interventions.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.

Therapeutic strategies for diabetes and complications: a role for sphingolipids?

Diabetes is a debilitating chronic disease that has no cure and can only be managed by pharmaceutical or nutritional interventions. Worldwide, the incidence of diabetes and diabetic complications is dramatically increasing. This may reflect the incomplete knowledge base underlying the role of inflammatory or nutritional stresses to exacerbate diabetic complications. Despite the knowledge that hyperlipidemia is a cardinal feature of both Type 1 and 2 diabetes, the actual lipid species that contribute to complications such as diabetic nephropathy, retinopathy, neuropathy and cardiovascular disease have not been well defined, or have not elucidated new treatment strategies. Sphingolipids comprise only a fraction of total lipids but a body of evidence has now identified dysfunctional sphingolipid metabolism and/or generation of specific sphingolipid metabolites as contributors to diabetic complications. This review suggests that pharmacological therapies that target dysfunctional sphingolipid metabolism and/or signaling may prove beneficial in decreasing the chronic pathology of hyperglycemia and hyperlipidemia. Moreover, the review suggests that these treatment options may also prove beneficial to ameliorate or delay pancreatic beta cell failure.

A multistep validation process of biomarkers for preclinical drug development

Biomarkers that can be measured in preclinical models in a high-throughput, reproducible manner offer the potential to increase the speed and efficacy of drug development. Development of therapeutic agents for many conditions is hampered by the limited number of validated preclinical biomarkers available to gauge pharmacoefficacy and disease progression, but the validation process for preclinical biomarkers has received limited attention. This report defines a five-step preclinical biomarker validation process and applies the process to a case study of diabetic retinopathy. By showing that a gene expression panel is highly reproducible, coincides with disease manifestation, accurately classifies individual animals and identifies animals treated with a known therapeutic agent, a biomarker panel can be considered validated. This particular biomarker panel consisting of 14 genes (C1inh, C1s, Carhsp1, Chi3l1, Gat3, Gbp2, Hspb1, Icam1, Jak3, Kcne2, Lama5, Lgals3, Nppa, Timp1) can be used in diabetic retinopathy pharmacotherapeutic research, and the biomarker development process outlined here is applicable to drug development efforts for other diseases.

Role of altered insulin signaling pathways in the pathogenesis of podocyte malfunction and microalbuminuria

PURPOSE OF REVIEW

In diabetic nephropathy, insulin resistance and hyperinsulinemia correlate with the development of albuminuria. The possibility that altered insulin signaling in glomerular cells and particularly podocytes contributes to the development of diabetic nephropathy will be discussed.

RECENT FINDINGS

Whereas normal podocytes take up glucose in response to insulin, diabetic podocytes become insulin resistant in experimental diabetic nephropathy prior to the development of significant albuminuria. Both clinical and experimental data suggest that insulin sensitizers may be renoprotective independent of their systemic effects on the metabolic control of diabetes.

SUMMARY

We will review the clinical and experimental evidence that altered insulin signaling correlates with the development of diabetic nephropathy in both type 1 and type 2 diabetes, and that insulin sensitizers may be superior to other hypoglycemic agents in the prevention of diabetic nephropathy. We will then review potential mechanisms by which altered podocyte insulin signaling may contribute to the development of diabetic nephropathy. Understanding the role of podocytes in glucose metabolism is important because it may lead to the discovery of novel pathogenetic mechanisms of diabetic nephropathy, it may affect current strategies for prevention and treatment of diabetic nephropathy, and it may allow the identification of novel therapeutic targets.

Insulin receptor signaling regulates actin cytoskeletal organization in developing photoreceptors

The insulin receptor (IR) and IR signaling proteins are widely distributed throughout the CNS. IR signaling provides a trophic signal for transformed retinal neurons in culture and we recently reported that deletion of IR in rod photoreceptors by Cre/lox system resulted in stress-induced photoreceptor degeneration. These studies suggest a neuroprotective role of IR in rod photoreceptor cell function. However, there are no studies available on the role of insulin-induced IR signaling in the development of normal photoreceptors. To examine the role of insulin-induced IR signaling, we analyzed cultured neuronal cells isolated from newborn rodent retinas. In insulin-lacking cultures, photoreceptors from wild-type rat retinas exhibited an abnormal morphology with a wide axon cone and disorganization of the actin and tubulin cytoskeleton. Photoreceptors from IR knockout mouse retinas also exhibited a similar abnormal morphology. A novel finding in this study was that addition of docosahexaenoic acid, a photoreceptor trophic factor, restored normal axonal outgrowth in insulin-lacking cultures. These data suggest that IR signaling pathways regulate actin and tubulin cytoskeletal organization in photoreceptors; they also imply that insulin and docosahexaenoic acid activate at least partially overlapping signaling pathways that are essential for the development of normal photoreceptors.

Subconjunctivally implantable hydrogels with degradable and thermoresponsive properties for sustained release of insulin to the retina

The objective of this work is to develop subconjunctivally implantable, biodegradable hydrogels for sustained release of intact insulin to the retina to prevent and treat retinal neurovascular degeneration such as diabetic retinopathy. The hydrogels are synthesized by UV photopolymerization of N-isopropylacrylamide (NIPAAm) monomer and a dextran macromer containing multiple hydrolytically degradable oligolactate-(2-hydroxyetheyl methacrylate) units (Dex-lactateHEMA) in 25:75 (v:v) ethanol:water mixture solvent. Insulin is loaded into the hydrogels during the synthesis process with loading efficiency up to 98%. The hydrogels can release biologically active insulin in vitro for at least one week and the release kinetics can be modulated by varying the ratio between NIPAAm and Dex-lactateHEMA and altering the physical size of the hydrogels. The hydrogels are not toxic to R28 retinal neuron cells in culture medium with 100% cell viability. The hydrogels can be implanted under the conjunctiva without causing adverse effects to the retina based on hematoxylin and eosin stain, immunostaining for microglial cell activation, and electroretinography. These subconjunctivally implantable hydrogels have potential for long-term periocular delivery of insulin or other drugs to treat diabetic retinopathy and other retinal diseases.

Cytoskeletal components enhance the autophosphorylation of retinal insulin receptor

Insulin receptor (IR) signaling provides a trophic signal for transformed retinal neurons in culture, and we recently reported that deletion of IR from rod photoreceptors resulted in stress-induced photoreceptor degeneration. Retinal insulin receptor has a high basal level autophosphorylation compared to liver and the reasons for higher autophosphorylation are not known. In the current study we report a novel finding that cytoplasmic actin associates with and activates the retinal IR in vivo. Similar to insulin, actin also induced autophosphorylation at tyrosines 1158, 1162 and 1163 in the catalytic loop of IR. Our studies also suggest that globular actin activates the retinal IR more effectively than does filamentous actin. Retinal IR kinase activity has been shown to decrease in hyperglycemia and we found a decreased binding of actin to the IR under hyperglycemia. This is the first study which demonstrates that cytoplasmic actin regulates autophosphorylation of the retinal IR.

FOXO1 plays an important role in enhanced microvascular cell apoptosis and microvascular cell loss in type 1 and type 2 diabetic rats

OBJECTIVE

To investigate early events leading to microvascular cell loss in diabetic retinopathy.

RESEARCH DESIGN AND METHODS

FOXO1 was tested in vivo by DNA binding activity and by nuclear translocation in microvascular cells in retinal trypsin digests. In vivo studies were undertaken in STZ-induced diabetic rats and Zucker diabetic fatty rats using the tumor necrosis factor (TNF)-specific blocker, pegsunercept, or by inhibiting FOXO1 with RNAi. Microvascular cell apoptosis, formation of pericyte ghosts, and acellular capillaries were measured. Upstream and downstream effects of high-glucose-induced FOXO1 were tested on rat microvascular endothelial cells (RMECs) by small-interfering RNA (siRNA) in vitro.

RESULTS

DNA binding or nuclear translocation of FOXO1, which was reduced by TNF inhibition, was elevated in type 1 and type 2 diabetic retinas. Diabetes stimulated microvascular cell apoptosis; pericyte ghost and acellular capillary development was inhibited by FOXO1 siRNA. High glucose in vitro decreased FOXO1 phosphorylation and DNA binding activity and decreased Akt phosphorylation in RMECs. High-glucose-stimulated FOXO1 DNA binding activity was mediated through TNF-alpha and formation of reactive oxygen species (ROS), while inhibitors of TNF and ROS and FOXO1 siRNA reduced high-glucose-enhanced RMEC apoptosis. The caspase-3/7 activity and capacity of high glucose to increase mRNA levels of several genes that regulate RMEC activation and apoptosis were knocked down by FOXO1 siRNA.

CONCLUSIONS

FOXO1 plays an important role in rat retinal microvascular cell loss in type 1 and type 2 diabetic rats and can be linked to the effect of high glucose on FOXO1 activation.

Diabetes reduces autophosphorylation of retinal insulin receptor and increases protein-tyrosine phosphatase-1B activity

PURPOSE

Protein-tyrosine phosphatase-1B (PTP1B) has been implicated in the negative regulation of insulin signaling. The expression, activity, and functional role of PTP1B in the retina are unknown. In this study, the authors examined the relationship between the retinal insulin receptor (IR) and PTP1B in normal and diabetic mouse retinas.

METHODS

IR and PTP1B localization was examined by immunohistochemistry. The activation of IR was analyzed using specific antibodies against phosphotyrosine. PTP1B activity was determined in anti-PTP1B immunoprecipitates. Glutathione-S-transferase fusion proteins containing wild-type and catalytically inactive mutant PTP1B was used to study the interaction between IR and PTP1B. Anti-IR immunoprecipitates and the cytoplasmic domain of purified IR were incubated in the presence of ATP, and the autophosphorylation of IR with antiphosphotyrosine antibody was analyzed.

RESULTS

Immunohistochemical analysis of PTP1B shows that it is predominantly expressed in nonphotoreceptor layers of the retina, though it is clearly expressed in the inner segments of the rod photoreceptors. The IR is predominately expressed in rod inner segments. Biochemical analysis of rod outer segments indicates the presence of IR and PTP1B. Retinal IR exhibits a high level of basal autophosphorylation, and this autophosphorylation is reduced in diabetic mouse retinas. In vitro, PTP1B is able to dephosphorylate the autophosphorylated IR. Substrate mutant-trap results indicate a stable interaction between IR and PTP1B. Further, PTP1B activity was increased in diabetic mouse retinas.

CONCLUSIONS

These studies indicate that diabetes reduces the autophosphorylation of retinal IR and increased PTP1B activity. Further, PTP1B regulates the state of IR phosphorylation in the retina.

Phosphorylation site mapping of endogenous proteins: a combined MS and bioinformatics approach

We present a novel approach that combines MALDI-TOF profile analysis and bioinformatics-based inclusion criteria to comprehensively predict phosphorylation sites on a single protein of interest from limiting sample. It is technologically difficult to unambiguously identify phosphorylated residues, as many physiologically important phosphorylation sites are of too low abundance in vivo to be unambiguously assigned by mass spectrometry. Conversely, phosphorylation site prediction algorithms, while increasingly accurate, nevertheless overestimate the number of phosphorylation sites. In this study, we show that MODICAS, an MS data management and analysis tool, can be effectively merged with the bioinformatics attributes of residue conservation and phosphosite prediction to generate a short list of putative phosphorylation sites that can be subsequently verified by additional methodologies such as phosphospecific antibodies or mutational analysis. Therefore, the combination of MODICAS driven MS data analysis with bioinformatics-based filtering represents a substantial increase in the ability to putatively identify physiologically relevant phosphosites from limited starting material.

Retinal insulin receptor signaling in hyperosmotic stress

In the diabetic eye, the increased accumulation of sorbitol in the retina has been implicated in the pathogenesis of diabetic retinopathy (DR). Neurodegeneration is an important component of DR as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Insulin receptor (IR) activation has been shown to rescue retinal neurons from apoptosis through a phosphoinositide 3-kinase and protein kinase B (Akt) survival cascade. In this study, we examined the IR signaling in sorbitol-induced hyperosmotic stressed retinas.

Selective regulation of heme oxygenase-1 expression and function by insulin through IRS1/phosphoinositide 3-kinase/Akt-2 pathway

Heme oxygenase 1 (HO-1) is a representative mediator of antioxidants and cytoprotectants against various stress stimuli including oxidants in vascular cells. Intensive insulin treatment can delay the onset and progression of diabetic retinopathy and other vascularopathies, yet little is known about insulin regulation of anti-apoptotic and antioxidant molecules such as HO-1 in vascular cells. Intravitreous injection or in vitro addition of insulin increased HO-1 protein expression in rat retina and in cultured bovine retinal pericytes, retinal endothelial cells, and retinal pigment epithelial cells. In bovine retinal pericytes, insulin induced mRNA and protein expression of HO-1 in a time- and concentration-dependent manner. Using HO-1 promoter analysis, the luciferase reporter assay showed that induction of HO-1 expression by insulin is mediated by additional response elements in the ho-1 promoter gene, which was not responsive to antioxidants. Insulin-induced HO-1 mRNA expression through activation of PI3-kinase/Akt pathway without affecting ERK and p38 MAPK. Overexpression of an adenoviral vector of native IRS1, IRS2, and Akt dominant negative or small interfering RNA transfection of Akt1 and Akt2 targeted gene demonstrated that insulin regulated HO-1 expression via IRS1 and Akt2 pathway, selectively. Further, insulin treatment prevented H(2)O(2)-induced NF-kappaB and caspase-8 activation and apoptosis via the IRS1/PI3K/Akt2/HO-1 pathway in the pericytes. In conclusion, we suggest that the anti-apoptotic properties of insulin are mediated partly by increasing HO-1 expression at transcriptional level via IRS1/PI3K/Akt2 activation, a potential explanation for how insulin is retarding the progression of microvascular complications induced by diabetes.

The retinal proteome in experimental diabetic retinopathy: up-regulation of crystallins and reversal by systemic and periocular insulin

Diabetic retinopathy is the leading cause of blindness in working age persons. Targeted studies have uncovered several components of the pathophysiology of the disease without unveiling the basic mechanisms. This study describes the use of complementary proteomic and genomic discovery methods that revealed that the proteins of the crystallin superfamily are increased dramatically in early diabetic retinopathy. Orthogonal methods confirmed that the amplitude of the up-regulation is greater than other changes described so far in diabetic retinopathy. A detailed time course study during diabetes showed differential up-regulation of the different isoforms of the crystallins superfamily. alpha- and beta-crystallins were regulated primarily at the translation level, whereas gamma-crystallins were also regulated transcriptionally. We also demonstrated cell-specific patterns of expression of the different crystallins in normal and diabetic rat retinas. In addition, systemic and periocular insulin treatments restored retinal crystallin protein expression during diabetes, indicating effects of phosphoinositide 3-kinase/Akt activity. Altogether this work shows the importance of proteomics discovery methods coupled with targeted approaches to unveil new disease mechanistic details and therapeutic targets.

Inhibition of C-jun N-terminal kinase improves insulin sensitivity but worsens albuminuria in experimental diabetes

C-jun N-terminal kinase (JNK) regulates both the development of insulin resistance and inflammation. Podocytes of the widely used db/db mouse model of diabetic nephropathy lose their ability to respond to insulin as albuminuria develops, in comparison to control db/+ mice. Here we tested whether JNK inhibition or its gene deletion would prevent albuminuria in experimental diabetes. Phosphorylated/total JNK was significantly increased in vivo in glomeruli of db/db compared to db/+ mice. Treatment of podocytes isolated from these two strains of mice with tumor necrosis factor-alpha caused greater phosphorylation of JNK in those obtained from diabetic animals. When db/db mice were treated with a cell-permeable TAT-JNK inhibitor peptide, their insulin sensitivity and glycemia significantly improved compared to controls. We induced diabetes in JNK1 knockout mice with streptozotocin and found that they had significantly better insulin sensitivity compared to diabetic wild-type or JNK2 knockout mice. Albuminuria was, however, worse in all mice treated with the JNK inhibitor and in diabetic JNK2 knockout mice compared to controls. Nephrin expression was also reduced in JNK inhibitor-treated mice compared to controls. A similar degree of mesangial expansion was found in all diabetic mice. Our study shows that targeting JNK to improve systemic insulin sensitivity does not necessarily prevent diabetic nephropathy.

Phospho-Site-Specific Antibody Microarray to Study the State of Protein Phosphorylation in the Retina

Neurodegeneration is an important component of diabetic retinopathy as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Accumulation of sorbitol and fructose and the generation or enhancement of oxidative stress has been reported in the whole retina of diabetic animals. Aldose reductase (AR), the first and the rate limiting enzyme in the pathway reduces glucose to sorbitol and the diabetic complications are prevented by drugs that inhibit AR. In this study we examined the phosphorylation state of various retinal proteins in response to sorbitol-treatment by phosphor-site-specific antibody microarray. Our results suggest that various retinal protein kinases and cytoskeletal proteins either activated or down regulated in response to sorbitol treatment. Further, our study also indicates the activation of retinal insulin- and insulin growth factor 1 receptor and their downstream signaling proteins such as phosphoinositide 3-kinanse and protein kinase B (Akt). Understanding the regulation of retinal proteins involved in polyol (sorbitol) pathway would help to design therapeutic agents for the treatment of diabetic retinopathy.

Novel potential mechanisms for diabetic macular edema: leveraging new investigational approaches

This article evaluates the current knowledge of the molecular mechanisms by which diabetes ocular and systemic inflammation induce breakdown of the blood-retinal barrier resulting in macular edema. We also summarize the relationship between molecular targets and the use of therapeutic inhibitors in preclinical studies and clinical trials. Further studies are needed to understand the regulation of normal blood-retinal barrier physiology and the relationship between events in animal models of diabetic retinopathy and humans with diabetes.

Suppressors of cytokine-signaling proteins induce insulin resistance in the retina and promote survival of retinal cells

OBJECTIVE

Suppressors of cytokine signaling (SOCS) are implicated in the etiology of diabetes, obesity, and metabolic syndrome. Here, we show that some SOCS members are induced, while others are constitutively expressed, in retina and examine whether persistent elevation of SOCS levels in retina by chronic inflammation or cellular stress predisposes to developing insulin resistance in retina, a condition implicated in diabetic retinopathy.

RESEARCH DESIGN AND METHODS

SOCS-mediated insulin resistance and neuroprotection in retina were investigated in 1) an experimental uveitis model, 2) SOCS1 transgenic rats, 3) insulin-deficient diabetic rats, 4) retinal cells depleted of SOCS6 or overexpressing SOCS1/SOCS3, and 5) oxidative stress and light-induced retinal degeneration models.

RESULTS

We show that constitutive expression of SOCS6 protein in retinal neurons may improve glucose metabolism, while elevated SOCS1/SOCS3 expression during uveitis induces insulin resistance in neuroretina. SOCS-mediated insulin resistance, as indicated by its inhibition of basally active phosphoinositide 3-kinase/AKT signaling in retina, is validated in retina-specific SOCS1 transgenic rats and retinal cells overexpressing SOCS1/SOCS3. We further show that the SOCS3 level is elevated in retina by oxidative stress, metabolic stress of insulin-deficient diabetes, or light-induced retinal damage and protects ganglion cells from apoptosis, suggesting that upregulation of SOCS3 may be a common physiologic response of neuroretinal cells to cellular stress.

CONCLUSIONS

Our data suggest two-sided roles of SOCS proteins in retina. Whereas SOCS proteins may improve glucose metabolism, mitigate deleterious effects of inflammation, and promote neuroprotection, persistent SOCS3 expression caused by chronic inflammation or cellular stress can induce insulin resistance and inhibit neurotrophic factors, such as ciliary neurotrophic factor, leukemia inhibitory factor, and insulin, that are essential for retinal cell survival.

Loss of neuroprotective survival signal in mice lacking insulin receptor gene in rod photoreceptor cells

Insulin receptor (IR) signaling provides a trophic signal for transformed retinal neurons in culture, but the role of IR activity in vivo is unknown. We previously reported that light causes increased tyrosine phosphorylation of the IR in vivo, which leads to the downstream activation of the phosphoinositide 3-kinase and Akt pathway in rod photoreceptor cells. The functional role of IR in rod photoreceptor cells is not known. We observed that light stress induced tyrosine phosphorylation of the IR in rod photoreceptor cells, and we hypothesized that IR activation is neuroprotective. To determine whether IR has a neuroprotective role on rod photoreceptor cells, we used the Cre/lox system to specifically inactivate the IR gene in rod photoreceptors. Rod-specific IR knock-out mice have reduced the phosphoinositide 3-kinase and Akt survival signal in rod photoreceptors. The resultant mice exhibited no detectable phenotype when they were raised in dim cyclic light. However, reduced IR expression in rod photoreceptors significantly decreased retinal function and caused the loss of photoreceptors in mice exposed to bright light stress. These results indicate that reduced expression of IR in rod photoreceptor cells increases their susceptibility to light-induced photoreceptor degeneration. These data suggest that the IR pathway is important for photoreceptor survival and that activation of the IR may be an essential element of photoreceptor neuroprotection.

Failure to phosphorylate AKT in podocytes from mice with early diabetic nephropathy promotes cell death

Loss of podocytes by apoptosis characterizes the early stages of diabetic nephropathy. To examine its mechanism we studied glomeruli and podocytes isolated from db/db mice with early diabetic nephropathy and albuminuria. Phosphorylation of AKT (protein kinase B, a key survival protein) was found to be lower in the glomeruli of 12 week old db/db compared to db/+ mice. In vitro, insulin phosphorylated AKT solely in podocytes from db/+ mice. Serum deprivation and exposure to tumor necrosis factor-alpha significantly compromised cell viability in podocytes from db/db but not from db/+ mice, and this was associated with a significant decrease in AKT phosphorylation. Inhibition of AKT was necessary to achieve the same degree of cell death in db/+ podocytes. Our study shows that podocyte inability to respond to insulin and susceptibility to cell death may partially account for the decreased podocyte number seen in early diabetic nephropathy.

Alanine variant of the Pro12Ala polymorphism of the PPARgamma gene might be associated with decreased risk of diabetic retinopathy in type 2 diabetes

OBJECTIVE

Molecular background of diabetic retinopathy (DR) remains unknown. An interesting group of candidate genes encode proteins involved in insulin resistance.

AIM

To search for association between the PPARgamma, calpain 10, PTPN1 genes and DR in type 2 diabetes mellitus (T2DM).

METHODS

We examined 238 T2DM subjects without DR (NDR) and 121 with DR (mean diabetes duration: 9.1+/-6.8 and 15.1+/-7.7, respectively). The subjects were genotyped for four markers: Pro12Ala of PPARgamma, SNP43 of calpain 10, rs3787345 and rs754118 of PTPN1. The distributions of the genotypes were compared using the chi(2)-test and Fisher exact test.

RESULTS

The alleles and genotypes were not associated with DR in non-stratified analysis. To investigate the impact of T2DM duration, we performed analysis that excluded short duration NDR subjects and long-duration DR subjects. It allowed obtaining groups with similar T2DM duration but different DR status (DR: 88 individuals, 11.4+/-5.3 years; NDR: 136 individuals, 13.2 years+/-6.2, respectively). This analysis suggested that the alanine variant of Pro12Ala might be associated with decreased risk of DR (p=0.026 for alleles, p=0.038 and p=0.014 for genotypes in additive and dominant models, respectively). In multivariable logistic regression that included non-genetic parameters, Pro12Ala was not an independent risk factor (p=0.28). Further analysis showed, however, that Pro12Ala remained significant when urea level was excluded from the model.

CONCLUSION

The alanine variant of the Pro12Ala polymorphism of PPARgamma might be associated with decreased risk of DR in T2DM. This effect may be indirect, at least in part, due to diabetic kidney disease.

Insulin, insulin analogues and diabetic retinopathy

Insulin is absolutely vital for living beings. It is not only involved in metabolism, but also in the regulation of growth factors, e.g. IGF-1. In this review we address the role insulin has in the natural evolution of diabetic retinopathy. On the one hand, chronic deficiency of insulin and IGF-1 at the retina is thought to cause capillary degeneration, with subsequent ischaemia. On the other hand, acute abundance of (exogenously administered) insulin and IGF-1 enhances ischaemia-induced VEGF expression. A critical ratio of tissue VEGF-susceptibility: VEGF-availability triggers vascular proliferation (i.e. of micro-aneurysms and/or abnormal vessels). The patent-protected insulin analogues Lispro, Glulisine, Aspart, Glargine and Detemir are artificial insulin derivatives with altered biological responses compared to natural insulin (e.g. divergent insulin and /or IGF-1 receptor-binding characteristics, signalling patterns, and mitogenicity). Their safety profiles concerning diabetic retinopathy remain to be established by randomised controlled trials. Anecdotal reports and circumstantial evidence suggest that Lispro and Glargine might worsen diabetic retinopathy.

Early changes in insulin receptor signaling and pain sensation in streptozotocin-induced diabetic neuropathy in rats

The objective of the present study was to evaluate the time course of changes in peripheral nerve insulin receptor (IR) signaling and compare observed findings with behavioral responses to noxious mechanical and thermal stimuli in streptozotocin (STZ)-diabetic rats over 12 weeks of diabetes. Diabetic rats developed mechanical hyperalgesia, as indicated by decreased paw withdrawal thresholds to mechanical stimuli that were detectable after 2 weeks of diabetes; they also developed thermal hypoalgesia, as indicated by increased tail flick latencies to thermal stimuli that were detectable at 1 week of diabetes. Western blot analysis revealed decreased phosphorylated: total IR protein ratio that was detectable as early as 2 weeks of diabetes, whereas phosphorylated:total Akt protein ratio was decreased at 2 weeks and increased at 12 weeks of diabetes with unchanged PI-3K protein levels. To our knowledge, the present study is the first to demonstrate that impaired peripheral nerve IR signaling, as indicated by decreased phosphorylated:total IR protein ratio, coincides with early mechanical hyperalgesia and thermal hypoalgesia in STZ-diabetic rats. This finding may improve understanding of how altered pain sensation develops rapidly in this model.

PERSPECTIVE

This study examined peripheral nerve IR signaling during the early course of altered nociception in STZ-diabetic rats. In diabetic rats, impaired peripheral nerve IR signaling is observed shortly after STZ injection, as is altered nociception. This finding suggests a possible role of impaired IR signaling in diabetic sensory neuropathy.

G-protein-coupled receptor rhodopsin regulates the phosphorylation of retinal insulin receptor

We have shown previously that phosphoinositide 3-kinase in the retina is activated in vivo through light-induced tyrosine phosphorylation of the insulin receptor (IR). The light effect is localized to photoreceptor neurons and is independent of insulin secretion (Rajala, R. V., McClellan, M. E., Ash, J. D., and Anderson, R. E. (2002) J. Biol. Chem. 277, 43319-43326). These results suggest that there exists a cross-talk between phototransduction and other signal transduction pathways. In this study, we examined the stage of phototransduction that is coupled to the activation of the IR. We studied IR phosphorylation in mice lacking the rod-specific alpha-subunit of transducin to determine if phototransduction events are required for IR activation. To confirm that light-induced tyrosine phosphorylation of the IR is signaled through bleachable rhodopsin, we examined IR activation in retinas from RPE65(-/-) mice that are deficient in opsin chromophore. We observed that IR phosphorylation requires the photobleaching of rhodopsin but not transducin signaling. To determine whether the light-dependent activation of IR is mediated through the rod or cone transduction pathway, we studied the IR activation in mice lacking opsin, a mouse model of pure cone function. No light-dependent activation of the IR was found in the retinas of these mice. We provide evidence for the existence of a light-mediated IR pathway in the retina that is different from the known insulin-mediated pathway in nonneuronal tissues. These results suggest that IR phosphorylation in rod photoreceptors is signaled through the G-protein-coupled receptor rhodopsin. This is the first study demonstrating that rhodopsin can initiate signaling pathway(s) in addition to its classical phototransduction.

Diabetes alters sphingolipid metabolism in the retina: a potential mechanism of cell death in diabetic retinopathy

Dysregulated sphingolipid metabolism causes neuronal cell death and is associated with insulin resistance and diseases. Thus, we hypothesized that diabetes-induced changes in retinal sphingolipid metabolism may contribute to neuronal pathologies in diabetic retinopathy. ESI-MS/MS was used to measure ceramide content and ceramide metabolites in whole retinas after 2, 4, and 8 weeks of streptozotocin-induced diabetes. After 4 and 8 weeks of diabetes, a approximately 30% decrease in total ceramide content was observed, concomitant with a significant approximately 30% increase in glucosylceramide levels in fed diabetic rats compared with their age-matched controls. Acute insulin therapy as well as a short-term lowering of glucose via fasting did not affect the increase in glucosylceramide composition. To assess the putative biological consequences of the increase in glucosylceramide composition, R28 retinal neurons were treated with glucosylceramide synthase inhibitors. Inhibiting glycosphingolipid metabolism increased insulin sensitivity in retinal neurons. Glycosphingolipid inhibitors augmented insulin-stimulated p70 S6kinase activity in the presence of inhibitory concentrations of high glucose or glucosamine. Inhibition of glycosphingolipid synthesis also suppressed glucosamine- and interleukin-1beta-induced death. Consistent with these inhibitor studies, pharmacological accumulation of glycosphingolipids increased activation of the endoplasmic reticulum stress response, a putative modulator of insulin resistance and neuronal apoptosis. It is speculated that an increase in glucosylceramide, and possibly higher-order glycosphingolipids, could contribute to the pathogenesis of diabetic retinopathy by contributing to local insulin resistance, resulting in neuronal cell death. Thus, dysfunctional glycosphingolipid metabolism may contribute to metabolic stress in diabetes, and therapeutic strategies to restore normal sphingolipid metabolism may be a viable approach for treatment of diabetic retinopathy.

Diabetic retinopathy: seeing beyond glucose-induced microvascular disease

Diabetic retinopathy remains a frightening prospect to patients and frustrates physicians. Destruction of damaged retina by photocoagulation remains the primary treatment nearly 50 years after its introduction. The diabetes pandemic requires new approaches to understand the pathophysiology and improve the detection, prevention, and treatment of retinopathy. This perspective considers how the unique anatomy and physiology of the retina may predispose it to the metabolic stresses of diabetes. The roles of neural retinal alterations and impaired retinal insulin action in the pathogenesis of early retinopathy and the mechanisms of vision loss are emphasized. Potential means to overcome limitations of current animal models and diagnostic testing are also presented with the goal of accelerating therapies to manage retinopathy in the face of ongoing diabetes.