Effects of diabetes on retinal pigment epithelial cell proliferation and mitogen-activated protein kinase signaling in dark Agouti rats

Modulating monocyte-derived macrophage polarization in cerebral ischemic injury with hyperglycemia

Ischemic stroke (IS), characterized by high mortality rate, occurs owing to diminished or blocked blood flow to the brain. Hyperglycemia (HG) is a major contributor to the risk of IS. HG induces augmented oxidative stress and Blood-Brain Barrier breakdown, which increases the influx of blood-derived myeloid cells into the brain parenchyma. In cerebral ischemia, infiltrating monocytes undergo differentiation into pro-inflammatory or anti-inflammatory macrophages, having a large effect on outcomes of ischemic stroke. In addition, interleukin-4 (IL-4) and interleukin-13 (IL-13) engage in post-ischemia repair by polarizing the infiltrating monocytes into an anti-inflammatory phenotype. In this study, we aimed to determine the effect of phenotypic polarization of monocyte-derived macrophages on the prognosis of IS with HG (HG-IS). We first established a hyperglycemic mouse model using streptozotocin (150 mg/kg) and induced transient middle cerebral artery occlusion. We observed that blood-brain barrier permeability increased in HG-IS mice, as per two-photon live imaging and Evans blue staining. We also confirmed the increased infiltration of monocyte-derived macrophages and the downregulation of anti-inflammatory macrophages related to tissue remodeling after inflammation in HG-IS mice through immunohistochemistry, western blotting, and flow cytometry. We observed phenotypic changes in monocyte-derived macrophages, alleviated infarct volume, and improved motor function in HG-IS mice treated with IL-4 and IL-13. These findings suggest that the modulation of phenotypic changes in monocyte-derived macrophages following IS in hyperglycemic mice may influence ischemic recovery.

Effect of bone marrow mesenchymal stem cells-derived exosomes on diabetes-induced retinal injury: Implication of Wnt/ b-catenin signaling pathway

BACKGROUND

Diabetic retinopathy (DR) is a serious microvascular complication of diabetes mellitus. Mesenchymal stem cells are currently studied as therapeutic strategy for management of DR. Exosomes, considered as a promising cell-free therapy option, display biological functions similar to those of their parent cells. In retinal development, Wnt/b-catenin signaling provides key cues for functional progression. The present study aimed to evaluate the potential efficacy of bone marrow-derived mesenchymal stem cell-derived exosomes (BM-MSCs-Ex) in diabetes-induced retinal injury via modulation of the Wnt/ b-catenin signaling pathway.

METHODS

Eighty-one rats were allocated into 6 groups (control, DR, DR + DKK1, DR + exosomes, DR + Wnt3a and DR + exosomes+Wnt3a). Evaluation of each group was via histopathological examination, assessment of gene and/or protein expression concerned with oxidative stress (SOD1, SOD2, Nox2, Nox4, iNOS), inflammation (TNF-α, ICAM-1, NF-κB) and angiogenesis (VEGF, VE-cadherin).

RESULTS

Results demonstrated that exosomes blocked the wnt/b-catenin pathway in diabetic retina concomitant with significant reduction of features of DR as shown by downregulation of retinal oxidants, upregulation of antioxidant enzymes, suppression of retinal inflammatory and angiogenic markers. These results were further confirmed by histopathological results, fundus examination and optical coherence tomography. Additionally, exosomes ameliorative effects abrogated wnt3a-triggered retinal injury in DR.

CONCLUSION

Collectively, these data demonstrated that exosomes ameliorated diabetes-induced retinal injury via suppressing Wnt/ b-catenin signaling with subsequent reduction of oxidative stress, inflammation and angiogenesis.

Kir4.2 Potassium Channels in Retinal Pigment Epithelial Cells In Vitro: Contribution to Cell Viability and Proliferation, and Down-Regulation by Vascular Endothelial Growth Factor

Dedifferentiation and proliferation of retinal pigment epithelial (RPE) cells are characteristics of retinal diseases. Dedifferentiation is likely associated with changes of inwardly rectifying potassium (Kir) channels. The roles of Kir4.2 channels in viability, and proliferation of cultured RPE cells were investigated. Gene expression levels were determined using qRT-PCR. RPE cells expressed Kir2.1, 2.2, 2.4, 3.2, 4.1, 4.2, 6.1, and 7.1 mRNA. Kir4.2 protein was verified by immunocytochemistry and Western blotting. Kir4.2 mRNA in cultured cells was upregulated by hypoxia (hypoxia mimetic CoCl₂ or 0.2% O₂) and extracellular hyperosmolarity (addition of high NaCl or sucrose). Kir4.2 mRNA was suppressed by vascular endothelial growth factor (VEGF), blood serum, and thrombin whereas platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and transforming growth factor-β1 (TGF-β1) increased it. Hyperosmotic Kir4.2 gene expression was mediated by TGF-β1 receptor signaling while hypoxic gene transcription was dependent on PDGF receptor signaling. VEGF receptor-2 blockade increased Kir4.2 mRNA level under control, hyperosmotic, and hypoxic conditions. SiRNA-mediated knockdown of Kir4.2 decreased the cell viability and proliferation under control and hyperosmotic conditions. Kir4.2 channels play functional roles in maintaining the viability and proliferation of RPE cells. Downregulation of Kir4.2 by VEGF, via activation of VEGF receptor-2 and induction of blood-retinal barrier breakdown, may contribute to decreased viability of RPE cells under pathological conditions.

Hypoxic and osmotic expression of Kir2.1 potassium channels in retinal pigment epithelial cells: Contribution to vascular endothelial growth factor expression

Retinal pigment epithelial (RPE) cells express different subtypes of inwardly rectifying potassium (Kir) channels. We investigated whether human and rat RPE cells express genes of strongly rectifying Kir2 channels. We also determined the hypoxic and hyperosmotic regulation of Kir2.1 gene expression in cultured human RPE cells and the effects of siRNA-mediated knockdown of Kir2.1 on VEGFA expression, VEGF secretion, proliferation, and viability of the cells. Extracellular hyperosmolarity was induced by addition of NaCl or sucrose. Hypoxia and chemical hypoxia were produced by cell culture in 0.25% O₂ and addition of CoCl₂, respectively. Gene expression levels were evaluated by real-time RT-PCR. Rat RPE cells contained Kir2.1, Kir2.2, Kir2.3, and Kir2.4 gene transcripts while human RPE cells contained Kir2.1, Kir2.2, and Kir2.4 transcripts. Immunocytochemical data may suggest that Kir2.1 protein in cultured human cells is expressed in both perinuclear and plasma membranes. Kir2.1 gene expression and Kir2.1 protein level in human cells increased under hypoxic and hyperosmotic conditions. The expression of the Kir2.1 gene was mediated in part by diverse intracellular signal transduction pathways and transcription factor activities under both conditions; the hyperosmotic, but not the CoCl₂-induced Kir2.1 gene expression was dependent on intracellular calcium signaling. Autocrine/paracrine activation of purinergic receptors contributed to Kir2.1 gene expression under hyperosmotic (P2Y₁, P2Y₂, P2X₇) and CoCl₂-induced conditions (P2Y₂, P2X₇). Exogenous VEGF, TGF-β1, and blood serum decreased Kir2.1 gene expression. Inhibition of VEGF receptor-2 increased the Kir2.1 gene expression under control conditions and in CoCl₂-simulated hypoxia, and decreased it under high NaCl conditions. Knockdown of Kir2.1 by siRNA inhibited the CoCl₂-induced and hyperosmotic transcription of the VEGFA gene and caused a delayed decrease of the constitutive VEGFA gene expression while VEGF protein secretion was not altered. Kir2.1 knockdown stimulated RPE cell proliferation under control and hyperosmotic conditions without affecting cell viability. The data indicate that Kir2.1 channel activity is required for the expression of the VEGFA gene and inhibits the proliferation of RPE cells. Under control and hypoxic conditions, the extracellular VEGF level may regulate the production of VEGF via its inhibitory effect on the Kir2.1 gene transcription; this feedback loop may prevent overproduction of VEGF.

Effects of trans-resveratrol on type 1 diabetes-induced up-regulation of apoptosis and mitogen-activated protein kinase signaling in retinal pigment epithelium of Dark Agouti rats

Microvascular changes and retinal degeneration precede diabetic retinopathy. Oxidative stress alters several intracellular signaling pathways, which form the basis of diabetic retinopathy. Many antioxidants have been investigated as possible preventive and therapeutic remedies for diabetic retinopathy. The current study investigated the modulatory effects of trans-resveratrol on streptozotocin-induced type 1 diabetes mediated changes in the transcription and levels of apoptosis-related proteins and mitogen-activated protein kinases (MAPKs) in the retinal pigment epithelium (RPE) of adult male dark Agouti rats. In control rats, 5 mg/kg/d trans-resveratrol administration for 30 days increased gene expressions of tumor suppressor protein 53, Bcl2-associated X protein, B-cell lymphoma-2 (Bcl2), Caspase-3 (CASP3), CASP8 and CASP9, p38αMAPK, c-Jun N-terminal kinase-1 (JNK1), and extracellular signal-regulated kinase-1 (ERK1). On the other hand, diabetes decreased gene expressions of CASP3, CASP8, p38αMAPK, JNK, and ERK1. Trans-resveratrol reversed the inhibited gene expressions of CASP8, p38αMAPK, JNK, and ERK1 to normal control levels in diabetic rats. Trans-resveratrol normalized diabetes-induced upregulation of CASP3 and -9, cytochrome-c, Bcl-2, and ERK1 proteins. In conclusion, Trans-resveratrol-induced alterations in gene expressions do not seem to affect RPE functions as they do not reflect as altered protein functions. Trans-resveratrol imparts its protective effects by normalizing apoptosis-related proteins and ERK1 but does not affect JNK proteins. Trans-resveratrol causes cytostasis in RPE of normal rats by upregulating Bcl2 protein and apoptotic proteins.

The anti-angiogenesis role of FBXW7 in diabetic retinopathy by facilitating the ubiquitination degradation of c-Myc to orchestrate the HDAC2

Diabetic retinopathy (DR) is the most prevalently occurring microvascular complication in diabetic patients that triggers severe visual impairments. The anti-angiogenesis role of FBXW7 has been identified in breast cancer. Therefore, this study intends to decipher the mechanism of FBXW7 in angiogenesis of DR. DR model was induced on mice using high-glucose (HG) and high-fat diet, and retinal microvascular endothelial cells (RMECs) isolated from normal mice were induced with HG, followed by evaluation of FBXW7, Ki67, HIF-1α and VEGF expression by immunofluorescence, immunohistochemistry or Western blot analysis. After gain- and loss-of-function assays in normal and DR mice, angiogenesis was assessed by CD31 fluorescence staining and Western blot analysis. After ectopic expression and silencing experiments in HG-induced RMECs, RMEC proliferation, migration and angiogenesis were, respectively, determined by EdU, Transwell and in vitro angiogenesis assays. The impact of FBXW7 on the ubiquitination of c-Myc was studied by cycloheximide chase assay and proteasome inhibition, and the binding of c-Myc to HDAC2 promoter by dual-luciferase reporter gene experiment. DR mice and HG-induced RMECs possessed down-regulated FBXW7 and up-regulated Ki67, HIF-1α and VEGF. Silencing FBXW7 enhanced angiogenesis in normal mouse retinal tissue, but overexpressing FBXW7 or silencing c-Myc diminished angiogenesis in DR mouse retinal tissue. Overexpressing FBXW7 or silencing c-Myc depressed proliferation, migration and angiogenesis in HG-induced RMECs. FBXW7 induced c-Myc ubiquitination degradation, and c-Myc augmented HDAC2 expression by binding to HDAC2 promoter. Conclusively, our data provided a novel sight of anti-angiogenesis role of FBXW7 in DR by modulating the c-Myc/HDAC2 axis.

Therapeutic Potential of Tpl2 (Tumor Progression Locus 2) Inhibition on Diabetic Vasculopathy Through the Blockage of the Inflammasome Complex

OBJECTIVE

Diabetic retinopathy, one of retinal vasculopathy, is characterized by retinal inflammation, vascular leakage, blood-retinal barrier breakdown, and neovascularization. However, the molecular mechanisms that contribute to diabetic retinopathy progression remain unclear. Approach and Results: Tpl2 (tumor progression locus 2) is a protein kinase implicated in inflammation and pathological vascular angiogenesis. Nε-carboxymethyllysine (CML) and inflammatory cytokines levels in human sera and in several diabetic murine models were detected by ELISA, whereas liquid chromatography-tandem mass spectrometry analysis was used for whole eye tissues. The CML and p-Tpl2 expressions on the human retinal pigment epithelium (RPE) cells were determined by immunofluorescence. Intravitreal injection of pharmacological inhibitor or NA (neutralizing antibody) was used in a diabetic rat model. Retinal leukostasis, optical coherence tomography, and H&E staining were used to observe pathological features. Sera of diabetic retinopathy patients had significantly increased CML levels that positively correlated with diabetic retinopathy severity and foveal thickness. CML and p-Tpl2 expressions also significantly increased in the RPE of both T1DM and T2DM diabetes animal models. Mechanistic studies on RPE revealed that CML-induced Tpl2 activation and NADPH oxidase, and inflammasome complex activation were all effectively attenuated by Tpl2 inhibition. Tpl2 inhibition by NA also effectively reduced inflammatory/angiogenic factors, retinal leukostasis in streptozotocin-induced diabetic rats, and RPE secretion of inflammatory cytokines. The attenuated release of angiogenic factors led to inhibited vascular abnormalities in the diabetic animal model.

CONCLUSIONS

The inhibition of Tpl2 can block the inflammasome signaling pathway in RPE and has potential clinical and therapeutic implications in diabetes-associated retinal microvascular dysfunction.

The Role of Microglia in Diabetic Retinopathy: Inflammation, Microvasculature Defects and Neurodegeneration

Diabetic retinopathy is a common complication of diabetes mellitus, which appears in one third of all diabetic patients and is a prominent cause of vision loss. First discovered as a microvascular disease, intensive research in the field identified inflammation and neurodegeneration to be part of diabetic retinopathy. Microglia, the resident monocytes of the retina, are activated due to a complex interplay between the different cell types of the retina and diverse pathological pathways. The trigger for developing diabetic retinopathy is diabetes-induced hyperglycemia, accompanied by leukostasis and vascular leakages. Transcriptional changes in activated microglia, mediated via the nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) and extracellular signal–regulated kinase (ERK) signaling pathways, results in release of various pro-inflammatory mediators, including cytokines, chemokines, caspases and glutamate. Activated microglia additionally increased proliferation and migration. Among other consequences, these changes in microglia severely affected retinal neurons, causing increased apoptosis and subsequent thinning of the nerve fiber layer, resulting in visual loss. New potential therapeutics need to interfere with these diabetic complications even before changes in the retina are diagnosed, to prevent neuronal apoptosis and blindness in patients.