Tyrosine Nitration of Prostacyclin Synthase Is Associated with Enhanced Retinal Cell Apoptosis in Diabetes

Abstract

The risk of diabetic retinopathy is associated with the presence of both oxidative stress and toxic eicosanoids. Whether oxidative stress actually causes diabetic retinopathy via the generation of toxic eicosanoids, however, remains unknown. The aim of the present study was to determine whether tyrosine nitration of prostacyclin synthase (PGIS) contributes to retinal cell death in vitro and in vivo. Exposure of human retinal pericytes to heavily oxidized and glycated LDL (HOG-LDL), but not native forms of LDL (N-LDL), for 24 hours significantly increased pericyte apoptosis, accompanied by increased tyrosine nitration of PGIS and decreased PGIS activity. Inhibition of the thromboxane receptor or cyclooxygenase-2 dramatically attenuated HOG-LDL-induced apoptosis without restoring PGIS activity. Administration of superoxide dismutase (to scavenge superoxide anions) or L-N G -nitroarginine methyl ester (L-NAME, a nonselective nitric oxide synthase inhibitor) restored PGIS activity and attenuated pericyte apoptosis. In Akita mouse retinas, diabetes increased intraretinal levels of oxidized LDL and glycated LDL, induced PGIS nitration, enhanced apoptotic cell death, and impaired blood-retinal barrier function. Chronic administration of tempol, a superoxide scavenger, reduced intraretinal oxidized LDL and glycated LDL levels, PGIS nitration, and retina cell apoptosis, thereby preserving the integrity of blood-retinal barriers. In conclusion, oxidized LDL-mediated PGIS nitration and associated thromboxane receptor stimulation might be important in the initiation and progression of dia- Loss of pericytes from retinal microvessels is an important event in the early stage of diabetic retinopathy, which is associated with increased pericyte apoptosis. Although the mechanisms responsible for pericyte apoptosis remain unclear, several studies have shown associations between dyslipidemia and the severity of diabetic retinopathy.1,2 Specifically, elevated levels of triglycerides, low density lipoprotein (LDL), and apolipoprotein B, a principal lipoprotein component of LDL, have been associated with diabetic retinopathy.1 However, these associations are relatively weak, leading us to hypothesize that the key contribution of lipoproteins to the propagation of diabetic retinopathy is not through changes in their plasma levels but, rather, through extravasation via a disrupted blood-retinal barrier and subsequent modification by glycation and oxidation. In support of this model, we have demonstrated that oxidized LDL is present in the retina in diabetic humans in proportion to the severity of diabetic retinopathy, 3 and in cell culture studies we have shown that oxidized LDL has injurious effects on retinal pericytes. 4,5

Diabetes-Induced Superoxide Anion and Breakdown of the Blood-Retinal Barrier: Role of the VEGF/uPAR Pathway

“…These protective effects were associated with significant decreases in lipid peroxidation and tyrosine nitration in the diabetic retina. These results are consistent with a recent report showing that administration of tempol prevented retinal tyrosine nitration and pericyte cell death and preserved BRB in Akita diabetic mice [31]. The latter study did not examine the direct effects of superoxide anion production within endothelial cells.…”

Targeting nitrative stress for attenuating cisplatin-induced downregulation of cochlear LIM domain only 4 and ototoxicity

“…Nitrated proteins play an important role in mediating cell death responses, and their signaling has been effectively targeted to inhibit cell death in several experimental models [42], [43], [44]. However, the nitrative stress mechanism underlying cisplatin-induced ototoxicity remains only partially understood.…”

Oxidative and Endoplasmic Reticulum Stresses Mediate Apoptosis Induced by Modified LDL in Human Retinal Müller Cells

“…10 Further studies revealed that upregulation of receptors for advanced glycation end-products (AGEs) causes proinflammatory responses in Müller cells. 11 In previous work, we proposed that in addition to hyperglycemia, extravasation of plasma lipoproteins through leaking blood retinal barriers (BRB) and their subsequent modification (glycation, oxidation) are important in the propagation of DR. [12][13][14][15][16][17][18] Several lines of evidence support this concept. Clinical studies indicate that dyslipidemia is associated with the severity of DR In particular, DR is positively associated with serum levels of LDL, apolipoprotein B (ApoB), and LDL particle concentration in type 1 diabetic patients.…”

“…Previous studies from our own and other laboratories have shown that, in addition to hyperglycemia, qualitative and quantitative lipoprotein abnormalities are implicated in mediating vascular injury in diabetes, and specifically, are associated with the development of DR. [13][14][15][16][17][18][19][20][21] Our previous studies indicated that modified (oxidized and glycated) LDL induced apoptosis in human retinal capillary pericytes in vitro. 12,14,17 In addition, large quantities of extravasated apoB100 and oxidized LDL were identified in retinal tissues from diabetic human retinas.…”

“…The current study extends our knowledge of the effects of modified LDL to Müller glial cells, and expands on our previous results showing toxicity of modified LDL on retinal endothelial cells and pericytes. 12,[14][15][16][17][18] Due to their critical functions in maintaining the healthy retina, any damage to Müller cells is likely to promote both retinal neuronal and vascular injury. The resulting enhancement of vascular leakage means that this effect may contribute to the initiation and progression of DR.…”