Abstract
Purpose: :
Oxidative stress is recognized to play some role in diabetic complications, but the source(s) of such stress is not understood. In the mammalian system, thioredoxin (Trx) controls the homeostasis of redox in the cells, but the bioavailability of thioredoxin is regulated by its intrinsic binding protein, thioredoxin-binding protein-2 (TBP-2). Our purpose is to investigate if diabetic condition alters TBP-2 expression and thus compromises Trx activity, which may contribute to oxidative stress in the diabetic lenses.
Methods: :
Rats (100g) were induced into diabetes by tail vein injection with streptozotocin (STZ, 75 mg/kg) and divided into 2 groups of untreated (6 in regular chow) and treated with an aldose reductase inhibitor (6 in chow containing 0.0125% AL1576). Rats without STZ injection were used as controls. Blood glucose was determined with a glucose dipstick. Lenses from the three groups were surgically dissected out of the eye and collected at 3 wks and 8 wks post STZ injection. All lenses were weighed and homogenized in 1 ml of lysis buffer with a glass-to-glass homogenizer. The supernatant was collected and used for free GSH measurement, Trx activity assay, and Western blotting of Trx and TBP-2.
Results: :
All the rats in the diabetic treated or untreated group had over 400 mg/dl blood glucose level. The lenses in the diabetic untreated group showed slight haziness in 3 wks, and moderate to severe opacity in 8 wks. Lenses in the ARI-treated group remained clear as did the lenses of the control animals. The untreated diabetic group also showed severe loss in GSH (< 20% of control in both 3 and 8 wks) and Trx activity (< 35% of the control in both 3 and 8 wks). However, Western blot analysis showed that TBP-2 expression was unchanged at 3 wks but elevated ~4-fold over the control in 8 wks while Trx expression remained constant at both time points. ARI treatment protected the GSH level, Trx activity, and TBP-2 expression from any alteration during the entire experimental period.
Conclusions: :
Induced diabetic condition in vivo up-regulated TBP-2 with concurrent suppression of Trx activity, confirming our previous results of in vitro hyperglycemic studies. The results suggest that the loss of the bioavailability of TRx, in conjunction with a severe GSH depletion, is likely to contribute to oxidative stress observed in diabetic conditions. Since ARI can normalize these changes, it also suggests that osmotic stress may induce the observed oxidative stress and accelerate lens opacification.
Keywords: diabetes • oxidation/oxidative or free radical damage • enzymes/enzyme inhibitors