Hyperglycemia Induces Toll-Like Receptor-2 and -4 Expression and Activity in Human Microvascular Retinal Endothelial Cells: Implications for Diabetic Retinopathy

Abstract

Diabetic retinopathy (DR) causes visual impairment in working age adults and hyperglycemia-mediated inflammation is central in DR. Toll-like receptors (TLRs) play a key role in innate immune responses and inflammation. However, scanty data is available on their role in DR. Hence, in this study, we examined TLR2 and TLR4 mRNA and protein expression and activity in hyperglycemic human retinal endothelial cells (HMVRECs). HMVRECs were treated with hyperglycemia (HG) or euglycemia and mRNA and protein levels of TLR-2, TLR-4, MyD88, IRF3, and TRIF as well as NF-κB p65 activation were measured. IL-8, IL-1β, TNF-α and MCP-1, ICAM-1, and VCAM-1 as well as monocyte adhesion to HMVRECs were also assayed. HG (25 mM) significantly induced TLR2 and TLR4 mRNA and protein in HMVRECs. It also increased both MyD88 and non-MyD88 pathways, nuclear factor-κB (NF-κB), biomediators, and monocyte adhesion. This inflammation was attenuated by TLR-4 or TLR-2 inhibition, and dual inhibition by a TLR inhibitory peptide as well as TLR2 and 4 siRNA. Additionally, antioxidant treatment reduced TLR-2 and TLR4 expression and downstream inflammatory markers. Collectively, our novel data suggest that hyperglycemia induces TLR-2 and TLR-4 activation and downstream signaling mediating increased inflammation possibly via reactive oxygen species (ROS) and could contribute to DR.

Diabetic microangiopathy: Pathogenetic insights and novel therapeutic approaches

“…In addition, antioxidant treatments reduce TLR-2 and TLR-4 expression and downstream inflammatory events. Collectively, these data suggest that hyperglycemia induces TLR-2 and TLR-4 activation and downstream inflammatory signaling, possibly through ROS [65].…”

“…In addition, TLR-2 and -4 signaling pathways appear to be the main triggers of these inflammatory responses. Recent in vitro studies have shown that high concentrations of glucose significantly increase TLR-2 and TLR-4 mRNA and protein expression in human microvascular ECs, as well as the activation of NF-κB p65, the expression of inflammatory markers such as interleukin (IL)-8, IL-1β, TNF-α, MCP-1, and vascular cell adhesion molecule (VCAM)-1 and intercellular adhesion molecule (ICAM)-1 [64,65]. All these events were reversed by TLR-4 or TLR-2 inhibition, or dual inhibition of these pathways by TLR4/2 inhibitory peptide.…”

TLR4 polymorphisms seem not to be associated with prediabetes and type 2 diabetes but predispose to diabetic retinopathy; TLR4 polymorphisms in glucose continuum

“…TLR4 actively contributes to the infl ammatory response in development of microvascular complications, including diabetic retinopathy (Rajamani and Jialal 2014). Th e NFκB pathway is stimulated by hyperglycemia, reactive oxygen species, AGEs -all implicated in the pathogenesis of diabetes-associated vascular damage (Forbes and Cooper 2013).…”

Innate immunity in diabetes and diabetic nephropathy

“…High glucose conditions induce the overexpression of TLRs, such as TLR2 and TLR4 in endothelial cells in adipose tissue 72,133 , the retina 134 , coronary arteries 40 , and kidney tissues 135 . Endothelial cells exposed to fluctuating glucose concentrations induce an upregulation of TLR4 and TLR2 expression and this process has been associated with increased NFκB, IL8, MCP-1, ICAM-1, and VCAM-1 (Figure 3).…”