Abstract
We have recently shown that thioredoxin interacting protein (TXNIP) is required for VEGF-mediated VEGFR2 receptor activation and angiogenic signal. Retinas from TXNIP knockout mice (TKO) exhibited higher cellular antioxidant defense compared to wild type (WT). This study aimed to examine the impact of TXNIP deletion on hyperoxia-induced vaso-obliteration in ischemic retinopathy. TKO and WT pups were subjected to oxygen-induced retinopathy model. Retinal central capillary dropout was measured at p12. Retinal redox and nitrative state were assessed by reduced-glutathione (GSH), thioredoxin reductase activity and nitrotyrosine formation. Western blot and QT-PCR were used to assess VEGF, VEGFR-2, Akt, iNOS and eNOS, thioredoxin expression, ASK-1 activation and downstream cleaved caspase-3 and PARP in retinal lysates. Retinas from TKO mice exposed to hyperoxia showed significant increases (1.5-fold) in vaso-obliteration as indicated by central capillary drop out area compared to WT. Retinas from TKO showed minimal nitrotyrosine levels (10% of WT) with no change in eNOS or iNOS mRNA expression. There was no change in levels of VEGF or activation of VEGFR2 and its downstream Akt in retinas from TKO and WT. In comparison to WT, retinas from TKO showed significantly higher level of GSH and thioredoxin reductase activity in normoxia but comparable levels under hyperoxia. Exposure of TKO to hyperoxia significantly decreased the anti-apoptotic thioredoxin protein (∼50%) level compared with WT. This effect was associated with a significant increase in activation of the apoptotic ASK-1, PARP and caspase-3 pathway. Our results showed that despite comparable VEGF level and signal in TKO, exposure to hyperoxia significantly decreased Trx expression compared to WT. This effect resulted in liberation and activation of the apoptotic ASK-1 signal. These findings suggest that TXNIP is required for endothelial cell survival and homeostasis especially under stress conditions including hyperoxia.
Highlights
Imbalance in cellular redox system has been linked to be several cardiovascular disorders such as ischemic heart disease, inflammation, atherosclerosis and aberrant angiogenesis
The main finding of the present study is: Despite increased antioxidant defense and decreased nitrative stress, TXNIP knockout mice (TKO) mice were more vulnerable to hyperoxia and had aggravated retinal vascular cell death (Fig. 1,2,5)
Hyperoxia caused significant decrease in thioredoxin expression that was associated with activation Apoptosis signal-regulating kinase-1 (ASK-1) apoptotic signal in TKO mice compared to wild type (WT) (Fig. 6,7)
Summary
Imbalance in cellular redox system has been linked to be several cardiovascular disorders such as ischemic heart disease, inflammation, atherosclerosis and aberrant angiogenesis (reviewed in [13]). The thioredoxin (Trx) system, major regulator of antioxidant defense represents an attractive target for oxidative stressassociated disorders. Trx is a multifunctional protein that acts as a protein disulfide reductase and participates in redox-dependent processes [3], including protein folding, regulation of apoptosis and antioxidant protection from oxidative stress. Overexpression of Trx in transgenic mice increases the resistance to various oxidative stresses and attenuates ischemic damage in the brain [4] and myocardial infarction [5]. TXNIP regulates Trx-dependent cellular redox state and increases reactive oxygen species and oxidative stress [7]
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