Regulation of the bioavailability of thioredoxin in the lens by a specific thioredoxin-binding protein (TBP-2)

Abstract

Thioredoxin (TRx) is known to control redox homeostasis in cells. In recent years, a specific TRx binding protein called thioredoxin binding protein-2 (TBP-2) was found in other cell types and it appeared to negatively regulate TRx bioavailability and thereby control TRx biological function. In view of the sensitivity of lens transparency to redox status, proper regulation of TRx bioavailability is of the utmost importance. This study was conducted to examine the presence and function of TBP-2 in human lens epithelial cells (HLE B3). We cloned human lens TBP-2 from a human cDNA library (GenBank accession number AY 594328) and showed that it is fully homologous to the human brain TBP-2 gene. The recombinant TBP-2 protein was partially purified and mass spectrometric analysis confirmed its sequence homology to that of brain TBP-2. Immunoprecipitates obtained from HLE B3 cells using anti-TRx and anti-TBP-2 antibodies showed the presence of TRx and TBP-2 in immunoprecipitates indicating the formation of a TRx-TBP-2 complex in vivo. Furthermore, under H 2 O 2 -stress conditions, TRx gene expression was transiently up-regulated while TBP-2 gene expression was inversely down-regulated as seen in both HLE B3 cells and in the epithelial cell layers from cultured pig lenses. Cells with overexpressed TBP-2 showed lower TRx activity, grew slower and were more susceptible to oxidative stress-induced apoptosis. This is the first report of the presence of a TRx-specific binding protein in the lens. Our data suggest that TBP-2 is likely a negative regulator for the bioavailability, and therefore, the overall function of TRx in the lens.

The Emerging Role of the Thioredoxin System in Angiogenesis

“…Despite the demonstration that TXNIP inhibits TRX activity in multiple cell types in vitro including ECs, 10 vascular smooth muscle cells, 20 breast cancer cells, 21 lens epithelial cells 94 and mesangial cells, 95 and in aortic tissue ex vivo 91 this observation has not been consistently demonstrated in vivo . This is particularly true of the total and tissue-specific TXNIP knockout mice, 22, 23, 96, 97 and TXNIP-deficient mouse models available, 98 which demonstrate no significant changes in TRX activity.…”

Redox controls UPR to control redox

“…TXNIP fulfills many functions, including negative regulation of mammalian target of rapamycin (mTOR) complex 1 (Jin et al, 2011), activation of the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome (Zhou et al, 2010) and inhibition of the antioxidant enzyme TRX (Nishiyama et al, 1999). The latter function is associated with higher susceptibility to oxidative stress (Liyanage et al, 2007), which again links UPR signaling with oxidative challenge.…”

Overexpression of thioredoxin-binding protein 2increases oxidation sensitivity and apoptosis in human lens epithelialcells

“…Trx1 is present in the lens and has been demonstrated to be an effective oxidation defense enzyme [23]. TBP-2 is also present in the lens, and under oxidative stress it has been shown to regulate the bioavailability of Trx [24]. Furthermore, it was shown that human lens epithelial B3 (HLE B3) cells with over-expressed TBP-2 displayed lower Trx activity, slower cell growth and increased susceptibility to H 2 O 2 -induced apoptosis [24].…”

“…TBP-2 is also present in the lens, and under oxidative stress it has been shown to regulate the bioavailability of Trx [24]. Furthermore, it was shown that human lens epithelial B3 (HLE B3) cells with over-expressed TBP-2 displayed lower Trx activity, slower cell growth and increased susceptibility to H 2 O 2 -induced apoptosis [24]. However, little is known about the mechanism responsible for slow cell growth, and the accelerated H 2 O 2 -induced apoptosis in cells enriched with TBP-2.…”