Pro inflammatory and pro angiogenic factors are

Pro-inflammatory and pro-angiogenic factors are known to be activated in the diabetic retina. Prominent among these are the inflammatory cytokine, interleukin (IL)-1β, tumor necrosis factor α (TNFα) and IL-6; the adhesion molecules ICAM1 and VCAM1; and the growth factor, vascular endothelial growth factor A (VEGF-A). Elevated VEGF-A production results in increased retinal endothelial permeability and cell injury (Caldwell et al., 2005; Hu et al., 2013; Penn et al., 2008; Wang et al., 2015; Yang et al., 2013; Boyer et al., 2013; Harhaj et al., 2006; Simo et al., 2014). In addition to these key factors, we previously identified that Rigosertib manufacturer sphingomyelinase (ASM), the enzyme converting sphingomyelin into pro-inflammatory and pro-apoptotic ceramide, is highly activated by diabetes in the retina (Opreanu et al., 2011). Endothelial cells, which represent a major source of ASM, had the highest level of activation of ASM in diabetic retina. The mechanism(s) of this activation remains largely unknown. Retinal endothelial cell damage in diabetes (Joussen et al., 2001; Roy et al., 2015; Chronopoulos et al., 2011) is further confounded by inadequate vascular repair due, in part, to compromised function of the bone marrow (Busik et al., 2009; Caballero et al., 2007; Grant et al., 2002; Bhatwadekar et al., 2010; Chakravarthy et al., 2016). Bone marrow-derived circulating angiogenic cells (CACs) normally serve to mitigate endothelial injury, but are unable to participate in vascular repair in the retina of humans and rodents with chronic diabetes (Busik et al., 2009; Chakravarthy et al., 2016; Abu El-Asrar et al., 2011; Krady et al., 2005; Li Calzi et al., 2010; Tan et al., 2010; Liu et al., 2013; Sukmawati and Tanaka, 2015; Balaiya et al., 2014; Caballero et al., 2013). In human subjects we and others have used CD34+cells as a marker of vascular reparative populations. We used the term CAC, however in the literature the term circulating progenitor cells (CPCs) is also used to define the same population of immature bone marrow (BM)–derived cells, mostly of hematopoietic origin, which have been associated with several aspects of CVDs, from diagnosis to therapy. In clinical studies, CACs/CPCs are generally defined by flow cytometry based on the surface expression of the hematopoietic stem cell markers CD34 and CD133 (Ingram et al., 2005). CPCs include phenotypes with vascular endothelial specification, usually called endothelial progenitor cells (EPCs). EPCs account for ≤15% of CPCs and are characterized by the co-expression of endothelial markers (mostly the type 2 vascular endothelial growth factor receptor KDR) (Fadini et al., 2008; Fadini et al., 2012). We have used a single marker, CD34, as Fadini et al. demonstrated that it was as predictive as using multiple markers (Rigato et al., 2016) and with isolation of human cells, in particular cells from diabetic patients, it is difficult to obtain sufficient numbers of cells and with the selection of each additional maker the number of cells isolated decreases, limiting what is available for study. In animal models, BM-derived progenitor cells contribute to vascular repair and include linage negative cells which are then positively selected for Sca-1. In this study, we identified miR-15a as a miRNA that provides inhibition to both ASM and VEGF-A activation. MiR-15a was shown to be significantly downregulated in the blood of diabetic patients and T2D hyperglycemic Lepob mice (Zampetaki et al., 2010). Importantly, we provide an entirely new mechanism for the pathogenesis of diabetic retinopathy based on diabetes-induced downregulation of miR-15a expression leading to pro-inflammatory and pro-angiogenic changes in the diabetic retina due to unopposed activation of miR-15a target genes, ASM and VEGF-A.
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Discussion Diabetic retinopathy is a complex disorder that involves both systemic and retinal tissue-specific initiating factors and cell types. A number of hyperglycemia- and dyslipidemia-activated pathways leading to retinal endothelial cell and CAC dysfunction have been identified (Busik et al., 2009; Grant et al., 2002; Bhatwadekar et al., 2010; Chakravarthy et al., 2016; Abu El-Asrar et al., 2011; Li Calzi et al., 2010; Tan et al., 2010; Liu et al., 2013; Caballero et al., 2013). Prominent among these are pathways promoting the increase of pro-inflammatory cytokines, pro-inflammatory lipids and pro-angiogenic factors. Dysregulation of these pathways is hypothesized to involve miRNAs. These small non-coding RNAs anneal imperfectly to target genes and simultaneously control translation and transcription. Single species of miRNA can interact with a wide range of target transcripts. Several miRNA classes have been shown to contribute to diabetes and diabetic complications (Pandey et al., 2009; Zampetaki et al., 2010), including diabetic retinopathy (Suarez and Sessa, 2009).