 expression [62].
Preclinical therapies targeting other factors have been reported. A single intravitreal injection of a vector expressing insulin-like growth factor binding protein-3 (IGFBP-3) into diabetic rat retina after 2 months of diabetes restores normal insulin signal transduction via regulation of the insulin receptor/TNF-α (tumor necrosis factor-alpha) pathway and leads to the reduction of proapoptotic markers or increases of antiapoptotic markers and the restoration of retinal function [63]. Blockage of TNF-α by intravitreal and intraperitoneal delivery of anti-TNF-α antibody in STZ-induced mice and Akita mice resulted in a dose-dependent prevention of increased retinal leukostasis, acellular capillary, BRB breakdown, and cell death [64]. Intraperitoneal injection of anti-VEGFR1 antibody (MF1) prevents vascular leakage and inhibits inflammation associated gene expression and abnormal distribution of tight-junction proteins in STZ-induced mice and Akita mice [65].
Fenofibrate is a peroxisome proliferator-activated receptor-α (PPAR-) agonist and is known for clinical treatment for dyslipidemia. Recently, it was shown to significantly ameliorate retinal vascular leakage and leukostasis in DR of STZ-induced diabetic rats and Akita mice through downregulation of ICAM-1, MCP-1 (monocyte chemoattractant protein-1), and NF-B (nuclear factor-kappa B) signaling [66]. Clinical studies demonstrated that Fenofibrate has protective effects on progression of proliferative DR in type 2 diabetic patients [67, 68]. Now, the use of this medication for DR is approved [69].
Omega-3 polyunsaturated fatty acid (-3PUFA) has been shown to be decreased in STZ-induced diabetic rat retina [70]. -3PUFA rich diets enhanced glucose homeostasis and preserved retinal function in db/db/mice, but the effect is independent of preservation of retinal vasculature integrity, inflammatory modulation, and retinal neuroprotection [71].
Because of the complicated etiology of DR, drugs such as inhibitors for signaling pathways and growth factors have been shown to be effective for the treatment of DR but have limitations. Currently, intravitreal injection of anti-VEGF and corticosteroids are popular therapeutics, but a high proportion of patients (~40%) do not respond to these therapies [58, 72]. This implies that other factors or pathways, independent of VEGF, are involved in the development of microvasculature and neovascularization. Therefore, there is an urgent need for finding potential target candidates and for the development of new treatment strategies for DR therapy.
Epigenetic chromatin modifications (DNA methylation, histone posttranslational modifications, and regulation by noncoding RNAs), acting on both cis- and trans-chromatin structural elements, can be regulated by TXNIP [25]. Aberrant epigenetic modifications have been identified in DR and implicated in the progression of DR [25, 26]. MicroRNAs (miRNAs) are a group of noncoding RNA sequences which are short and highly conservative and can posttranscriptionally control gene expression by degradation or repression of target mRNAs. They are implicated in a variety of biological activities including modulation of glucose, angiogenesis, and inflammatory responses, as well as pathogenesis of diabetes and related complications such as DR [10]. However, conflicting data were seen with different miRNAs. It has been shown that retinal miRNA expression was altered in early DR rats induced by STZ, in which miRNAs were differentially regulated compared to the controls without DR [73]. Downregulation of miR-200b has been shown to increase VEGF expression, and polycomb repressive complex 2 (PRC2) (histone methyltransferase complex) represses miR-200b through its histone H3 lysine-27 trimethylation. Thus inhibition of PRC2 through histone methylation of miR-200b increases miR-200b and reduces VEGF in STZ-induced diabetic rats [74]. The 3′-untranslated region (3′-UTR) of mRNA sequence contains regulatory regions including binding sites for miRNAs to repress translation and degrade mRNA transcripts. In DR rats, miRNA-195 was significantly upregulated after one month of diabetes, and the antioxidant enzyme MnSOD level was reduced. In situ hybridization indicated that miR-195 was overexpressed in the cells of INL and ONL and ganglion cell layers, but sirtuin 1 (SIRT1) was downregulated. SIRT1 is involved in many biological processes including cell survival and metabolism and miR-195 binds to the 3′-UTR of SIRT1 to regulate its expression. Intravitreal injection of miR-195 antagomir leads to downregulation of SIRT1, thus preventing DR damage caused by SIRT1-mediated downregulation of MnSOD [75]. Collectively, increasing amounts of data demonstrate the active involvement and critical role of miRNAs in development of DR, although the exact mechanisms by which miRNA or miRNAs act are not known. Increased knowledge of how miRNAs function as therapeutic agents will lead to their effective use in the treatment of DR.
Reactive oxygen species (ROS), the primary causative factor for a variety of diseases, have been shown to play an important role in promoting DR [12, 58, 76]. As a treatment target, evidence from preclinical and clinical studies indicates that antioxidant therapies which directly target ROS-producing enzymes are beneficial, although the outcome of large clinical trials has been less promising [76]. However, nuclear factor erythroid 2-related factor 2 (Nrf2), the regulator of phase II enzymes system and the network of cytoprotective genes [77, 78], is still attractive. Its activators have been proven effective in prevention of the development and progression of DR [79]. Here, we specifically point out that nanomedicine attracts more attention in the past several years because it has been beneficial in a variety of medical applications including its promising effects on disease therapy [80, 81]. We have been using cerium oxide nanoparticles (nanoceria) to treat several animal models for ocular diseases and demonstrated their nontoxic and long-lasting effectiveness in delaying retinal degeneration in tubby mice [82] and inhibiting retinal and choroidal neovascularization [83]. Due to their unique physicochemical features, nanoceria themselves exhibit superoxide dismutase and catalase activities under redox conditions and can upregulate phase II enzymes [84] and regulate the common antioxidant gene network downstream of Trx [85]. Nanoceria have an atom-comparable size which enables them to freely cross the cellular and nuclear membrane barriers. In addition, they do not need repeat dosing as is required by other antioxidants. Thus one single dose produces sustained protective effects [82–84] which suggests their great potential to be excellent agents for the treatment of DR.
Stem cells emerged as a regenerative therapeutic strategy for treatment of a variety of diseases because they are undifferentiated and retain their stem cell characteristics and possess the potential to differentiate into many different cell types under certain biological conditions [86, 87]. Stem cells have been obtained from multiple sources and have been shown to have a great potential for tissue repair and ocular disease treatment [87, 88]. Human embryonic stem cells (hESCs) can differentiate into more than 99% pure RPE cells and integrate into the host RPE layer and become matured. Phase I/II clinical trials for assessing the tolerability and safety of subretinal transplantation of hESC-derived RPE cells in patients with Stargardt’s macular dystrophy (ClinicalTrials.gov number: NCT01345006) and advanced dry AMD (ClinicalTrials.gov number: NCT01344993) have shown that hESCs improve visual acuity [89]. Assessment of their medium- and long-term safety, graft, and survival in patients is ongoing [90]. Mesenchymal stromal cells (MSCs) have been shown to have multiple effects including tissue repair, secretion of neuroprotective growth factors, suppression of host immune response, and lowering glucose levels [91, 92]. Bone marrow derived MSCs have been reported to be differentiated into retinal cells and rescue retinal degeneration in several animal models [91]. Clinical trial phase I assessing their effects on visual acuity in patients with retinitis pigmentosa (RP) (ClinicalTrials.gov number: NCT01068561) has been completed and phase I/II in patients with AMD and Stargardt (ClinicalTrials.gov number: NCT01518127) will be completed in December 2015 (also see review [92]). However, no clinical study of therapeutic effects of MSCs in DR has been reported. Progress has also been made in using several classes of stem cells (EPCs, endothelial progenitor cells; ASCs, adipose stromal cells; PSCs, pluripotent stem cells) to stimulate both neuroregeneration and vascular regeneration in the diabetic retina [92]. EPCs are circulating cells and can be recruited to the sites of vessel damage and tissue ischemia and promote vascular healing and reperfusion [93]. Clinical studies have shown that altered numbers of EPCs were found in patients of type I and type II diabetes with NPDR and PDR, suggesting that EPCs are potential biomarkers for DME and PDR and may be used as therapeutic modalities to treat DR [72]. Preclinical study of STZ-induced diabetic rats receiving a single intravitreal injection of human derived ASCs at two months after diabetes onset showed significant decreases in vascular leakage and apoptotic cells and downregulation of inflammatory gene expression and improved rod b-wave amplitude within one week after injection [94]. Furthermore, mouse ASCs (mASCs) were intravitreally injected into 5-week-old Akimba mice, and the mASCs integrated and associated with retinal microvasculature. Injection of TGF-β1-preconditioned mASCs into P9 Akimba pups resulted in a great decrease in capillary dropout areas and avascular areas [95]. These results suggest that regenerative medicine could be a permanent solution for fighting diabetes and associated complications.
Nevertheless, as we previously mentioned, DR has a complicated etiology and involves many factors.