Activation of Endoplasmic Reticulum Stress by Hyperglycemia Is Essential for Müller Cell–Derived Inflammatory Cytokine Production in Diabetes

Abstract

Inflammation plays an important role in diabetes-induced retinal vascular leakage. The purpose of this study is to examine the role of endoplasmic reticulum (ER) stress and the signaling pathway of ER stress–induced activating transcription factor 4 (ATF4) in the regulation of Müller cell–derived inflammatory mediators in diabetic retinopathy. In diabetic animals, elevated ER stress markers, ATF4, and vascular endothelial growth factor (VEGF) expression were partially localized to Müller cells in the retina. In cultured Müller cells, high glucose induced a time-dependent increase of ER stress, ATF4 expression, and inflammatory factor production. Inducing ER stress or overexpressing ATF4 resulted in elevated intracellular adhesion molecule 1 and VEGF proteins in Müller cells. In contrast, alleviation of ER stress or blockade of ATF4 activity attenuated inflammatory gene expression induced by high glucose or hypoxia. Furthermore, we found that ATF4 regulated the c-Jun NH2-terminal kinase pathway resulting in VEGF upregulation. ATF4 was also required for ER stress–induced and hypoxia-inducible factor-1α activation. Finally, we showed that administration of chemical chaperone 4-phenylbutyrate or genetic inhibition of ATF4 successfully attenuated retinal VEGF expression and reduced vascular leakage in mice with STZ-induced diabetes. Taken together, our data indicate that ER stress and ATF4 play a critical role in retinal inflammatory signaling and Müller cell–derived inflammatory cytokine production in diabetes.

Impact of ER stress-regulated ATF4/p16 signaling on the premature senescence of renal tubular epithelial cells in diabetic nephropathy

“…Although studies have found that ATF4 expression in kidney tissues is not obviously altered with aging (15), the expression of ATF4 is involved in cell cycle by regulation of the gene expression of p21, p27, and p16 in the progression of age-related diseases (3,10,11,13,21). A recent study showed that ATF4 plays a critical role in retinal inflammatory signaling and Muller cell-derived inflammatory cytokine production in diabetes (38). Here, our results show that GRP78 and ATF4 or p16 were colocalized in the same SAHF-positive cells exposed to ER stress inducers, and suppressing endogenous ATF4 or p16 expression successfully prevented premature senescence induced by ER stress inducers, suggesting that the ATF4/p16 pathway is implicated in ER stress-mediated premature senescence.…”

Angiogenic Factors and Cytokines in Diabetic Retinopathy

“…Thus, ATF4 protein level control provides a complementary mechanism to increase VEGF expression in response to oxygen deprivation, and upregulation of VEGF expression in response the UPR provides a means to recruit additional blood supply in response to several stresses associated with ischemia or oxidative stress. Importantly, Zong et al found that treatment of STZ-induced diabetic mice with a chemical protein chaperone to alleviate ER stress resulted in inhibition of both retinal VEGF expression and vascular permeability [81]. This group also demonstrated that retinal vascular permeability during STZ-induced diabetes was abrogated in ATF4 gene knockout mice [83].…”

“…Roybal and co-workers demonstrated that upregulation of VEGF expression in response to oxidative stress and ER stresses are dependent on ATF4 function and that the human VEGFA gene contains an ATF4 binding site [79,80]. Zong et al demonstrated that ATF4 is necessary for VEGF expression by Müller cells exposed to high glucose levels and that blocking ATF4 activity inhibited expression of VEGF by Müller cells exposed to hypoxia [81]. Furthermore, two other transcription factors that contribute to the UPR, ATF6 and XBP-1, also stimulate VEGF expression [82].…”

“…Reactive glial cells act as cytokine producers [109,110]. In addition, several studies have documented the inflammatory response of Müller cells to in vitro high glucose conditions, with the hypothesis that such a direct response may alter the balance between inflammatory and anti-inflammatory cytokine expression observed in DR [81,111,112]. Furthermore, in the CNS, astrocytes contribute greatly to the control of leukocyte infiltration due to both their role in the maintenance of the endothelial blood brain barrier, and by virtue of limiting leukocyte migration across the perivascular barrier formed by astrocyte end feet, the glia limitans [113].…”

Anti-VEGF agents in the treatment of diabetic macular edema

“…Мощным триггером повышения синтеза VEGF и его рецепторов при ДР является гипоксия или ишемия сетчатки. Кроме того, продукцию VEGF в ре-тинальных клетках запускают гипергликемия и связанные с ней биохимические аномалии: накопление поздних про-дуктов гликирования [22], стресс эндоплазматического ре-тикулума [23], окислительный стресс [24].…”