Thioredoxin genes in lens: regulation by oxidative stress

Hallmarks of lens aging and cataractogenesis

Lens homeostasis and transparency are dependent on the function and intercellular communication of its epithelia. While the lens epithelium is uniquely equipped with functional repair systems to withstand reactive oxygen species (ROS)-mediated oxidative insult, ROS are not necessarily detrimental to lens cells. Lens aging, and the onset of pathogenesis leading to cataract share an underlying theme; a progressive breakdown of oxidative stress repair systems driving a pro-oxidant shift in the intracellular environment, with cumulative ROS-induced damage to lens cell biomolecules leading to cellular dysfunction and pathology. Here we provide an overview of our current understanding of the sources and essential functions of lens ROS, antioxidative defenses, and changes in the major regulatory systems that serve to maintain the finely tuned balance of oxidative signaling vs. oxidative stress in lens cells. Age-related breakdown of these redox homeostasis systems in the lens leads to the onset of cataractogenesis. We propose eight candidate hallmarks that represent common denominators of aging and cataractogenesis in the mammalian lens: oxidative stress, altered cell signaling, loss of proteostasis, mitochondrial dysfunction, dysregulated ion homeostasis, cell senescence, genomic instability and intrinsic apoptotic cell death.

Current Trends in the Pharmacotherapy of Cataracts

Cataracts, one of the leading causes of preventable blindness worldwide, refers to lens degradation that is characterized by clouding, with consequent blurry vision. As life expectancies improve, the number of people affected with cataracts is predicted to increase worldwide, especially in low-income nations with limited access to surgery. Although cataract surgery is considered safe, it is associated with some complications such as retinal detachment, warranting a search for cheap, pharmacological alternatives to the management of this ocular disease. The lens is richly endowed with a complex system of non-enzymatic and enzymatic antioxidants which scavenge reactive oxygen species to preserve lens proteins. Depletion and/or failure in this primary antioxidant defense system contributes to the damage observed in lenticular molecules and their repair mechanisms, ultimately causing cataracts. Several attempts have been made to counteract experimentally induced cataract using in vitro, ex vivo, and in vivo techniques. The majority of the anti-cataract compounds tested, including plant extracts and naturally-occurring compounds, lies in their antioxidant and/or free radical scavenging and/or anti-inflammatory propensity. In addition to providing an overview of the pathophysiology of cataracts, this review focuses on the role of various categories of natural and synthetic compounds on experimentally-induced cataracts.

Thioredoxin reductase activity may be more important than GSH level in protecting human lens epithelial cells against UVA light

This study compares the abilities of the glutathione (GSH) and thioredoxin (Trx) antioxidant systems in defending cultured human lens epithelial cells (LECs) against UVA light. Levels of GSH were depleted with either L-buthionine-(S,R)-sulfoximine (BSO) or 1-chloro-2,4-dinitrobenzene (CDNB). CDNB treatment also inhibited the activity of thioredoxin reductase (TrxR). Two levels of O2 , 3% and 20%, were employed during a 1 h exposure of the cells to 25 J cm(-2) of UVA radiation (338-400 nm wavelength, peak at 365 nm). Inhibition of TrxR activity by CDNB, combined with exposure to UVA light, produced a substantial loss of LECs and cell damage, with the effects being considerably more severe at 20% O2 compared to 3%. In contrast, depletion of GSH by BSO, combined with exposure to UVA light, produced only a slight cell loss, with no apparent morphological effects. Catalase was highly sensitive to UVA-induced inactivation, but was not essential for protection. Although UVA light presented a challenge for the lens epithelium, it was well tolerated under normal conditions. The results demonstrate an important role for TrxR activity in defending the lens epithelium against UVA light, possibly related to the ability of the Trx system to assist DNA synthesis following UVA-induced cell damage.

Loss of thiol repair systems in human cataractous lenses

PURPOSE

The purpose of this study was to investigate the thiol repair systems of thioltransferase (TTase) and thioredoxin (Trx) and oxidation-damaged proteins in human cataractous lenses.

METHODS

Cataractous lenses in humans (57-85 years of age) were classified into cortical, nuclear, mixed, mature, and hypermature cataract types by using a lens opacity classification system, and were obtained by extracapsular cataract extraction (ECCE) procedure. Cortical and nuclear cataracts were grouped by decreasing order of visual acuity into optical chart reading (R), counting fingers (CF), hand motion (HM), and light perception (LP). ECCE lens homogenate was analyzed for glutathione (GSH) level and enzyme activities of TTase, glutathione reductase (GR), Trx, and thioredoxin reductase (TR). Cortical and nuclear cataractous lenses (8 of each) with visual acuity better than HM were each dissected into cortical and nuclear portions for measurement of glyceraldehyde 3-phosphate dehydrogenase (G3PD) activity. Clear lenses (in humans 49-71 years of age) were used as control.

RESULTS

Compared with control, all cataractous lenses lost more than 80% GSH and 70% GR; TR and Trx activity; and 40% to 70% TTase activity, corroborated with the loss in visual acuity. Among cataracts with R and CF visual acuity, cortical cataract lost more cortical G3PD activity (18% of control) than that of nuclear cataract (50% of control), whereas GSH depletion and TTase inactivation were similar in both cataracts.

CONCLUSIONS

Thiol repair systems were damaged in all types of cataracts. Cortical and nuclear cataracts showed differential G3PD inactivation in the cortex, implying those 2 type of cataracts might be formed through different mechanisms.

Effect of age on the thioltransferase (glutaredoxin) and thioredoxin systems in the human lens

PURPOSE

To investigate the effect of age on the key oxidation repair enzymes of the thioltransferase (TTase) and thioredoxin (TRx) systems in the human lens.

METHODS

Twenty-three normal human lenses (donor ages, 19-77 years) were grouped into second, third, fifth, sixth, and seventh decades and analyzed for TTase, TRx, glutathione reductase (GR), thioredoxin reductase (TR), and glyceraldehyde-3-phosphate dehydrogenase (G3PD) activities, as well as the glutathione (GSH) pool. Additionally, 19 contralateral lenses of the donor eyes were each divided into cortex and nucleus for enzyme distribution studies.

RESULTS

All the enzymes showed similar activity in the cortex and nucleus, regardless of age, but were inactivated to various extents in the older lenses. In the TTase system, both TTase and GR showed activity loss over the five decades, with 70% remaining in the seventh decade, whereas the GSH pool was depleted extensively, with only 35% left in the older lenses. In the TRx system, TRx activity was not affected as much as TR for which only 70% of the activity was found in the seventh decade compared with the second to third decades. Overall, G3PD was more sensitive to age because only 50% activity remained after the sixth decade.

CONCLUSIONS

With increasing age there is a gradual activity loss in both the TTase and the TRx systems and a lowered GSH pool. These alterations, compounded with the age-related loss in G3PD activity, may lead to redox and energy imbalance, likely contributing to a higher risk to cataract formation in the aging population.

Revival of Glutathione Reductase in Human Cataractous and Clear Lens Extracts by Thioredoxin and Thioredoxin Reductase, in Conjunction with α -Crystallin or Thioltransferase

Glutathione reductase (GR) plays a key role in maintaining thiol groups in the lens, and its activity decreases with aging and cataract formation. Mammalian thioredoxin (Trx) and thioredoxin reductase (TrxR), or the Trx/TrxR system, participates in the repair of oxidatively damaged lens proteins and enzymes. Alpha-crystallin, a molecular chaperone, prevents the aggregation of partially denatured proteins under various stress conditions. Thioltransferase (TTase, or glutaredoxin) can maintain the homeostasis of lens protein thiols thus protecting against oxidative stress. We investigated whether the Trx/TrxR system can revive GR activity in both the cortex and nucleus of human cataract and clear aged lenses and whether alpha-crystallin and TTase can help this effect. The GR activity in the cortex and nucleus of the cataractous lenses was significantly lower than that of the aged clear lenses. The highest activity in the cortex was observed in the clear aged lenses. The combination of Trx and TrxR revived the activity of GR from both the cortex and nucleus of aged clear lenses. However, in cataract lenses (grade II and grade IV), there was a statistically significant recovery of GR activity in the cortex, but not in the nucleus. No recovery was observed when Trx or TrxR were used separately. Alpha-crystallin successfully revived GR activity in the cortex of cataract grade II lenses, but not in the nucleus. The combination of alpha-crystallin and Trx/TrxR gave a further increase of activity. TTase alone revived some of the GR activity but together with the Trx/TrxR system gave no statistically significant enhancement of GR activity. These results indicate that both disulfide bond formation and protein unfolding are responsible for GR inactivation.

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

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.

Thioredoxin reductase may be essential for the normal growth of hyperbaric oxygen-treated human lens epithelial cells

We have shown previously with in vivo and in vitro animal models that the lens epithelium, in contrast to the nucleus, is remarkably resistant to hyperoxia. The main purpose of this study was to investigate the mRNA response of cultured human lens epithelial cells (LECs) to challenge by a high level of hyperbaric oxygen. Cells were treated for 3 hr with 50 atm of 99% O2, and then cultured normally for various times up to 11 days. Although the cells appeared normal immediately after the O2-treatment, they failed to grow and suffered 50% cell loss, as well as significant mitochondrial damage, during normal post-culture. Growth of the cells resumed after 3 days and by day 11, the number of O2-treated cells was the same as the controls. Remarkably, the 3 hr O2-treatment produced no immediate effects on either the cellular level of GSH, or on the activities of a number of antioxidant enzymes including glyceraldehyde-3-phosphate dehydrogenase, which is generally regarded as being highly sensitive to oxidation. In contrast, the activity of thioredoxin reductase (TrxR) was severely affected by the O2, decreasing by 51% after the 3 hr exposure. O2-induced death of the cells appeared to be caused by loss of ATP since a 31% decrease in ATP level occurred immediately after the O2-treatment, in spite of a 46% increase in lactate production. Analysis with real-time PCR showed a maximum 3-6-fold increase in mRNA levels 9 hr after the 3 hr O2-exposure for the enzymes heme oxygenase-1 (HO-1), MnSOD and TrxR1 (the cytoplasmic form of TrxR). These results were confirmed with the use of one-step RT-PCR and Northern blotting. Initial upregulation of message for HO-1 occurred a few hours before any upregulation of MnSOD could be detected, suggesting that release of free iron from the degradation of heme by HO-1 may have played a role in the upregulation of the dismutase. No significant changes in mRNA levels were observed for the antioxidant enzymes catalase, CuZnSOD, glutathione reductase and glutathione peroxidase, or for the antioxidant protein thioredoxin. Recovery of TrxR activity over a 4-day period appeared to parallel the return of the cells to a normal rate of growth. The results indicate that damaging effects of hyperoxia on cultured LECs occur primarily in the mitochondria, rather than in the cytoplasm. Cells avoid O2-induced cell death, and return to a normal rate of proliferation by upregulating mRNA levels for HO-1, MnSOD and TrxR1. It appears that full activity of TrxR1, an enzyme required for the production of deoxyribonucletides for DNA synthesis, is essential for the normal growth of O2-challenged LECs.

Absolute gene expression patterns of thioredoxin and glutaredoxin redox systems in mouse

This work provides the first absolute expression patterns of genes coding for all known components of both thioredoxin (Trx) and glutaredoxin (Grx) systems in mouse: Trx1, Trx2, Grx1, Grx2, TrxR1, TrxR2, thioredoxin/glutathione reductase, and glutathione reductase. We devised a novel assay that, combining the advantages of multiplex and real-time PCR, streamlines the quantitation of the actual mRNA copy numbers in whole-animal experiments. Quantitations reported establish differences among adult organs and embryonic stages, compare mRNA decay rates, explore the significance of alternative mRNA isoforms derived from TrxR1 and Grx2 genes, and examine the time-course expression upon superoxide stress promoted by paraquat. Collectively, these quantitations show: i) unique expression profiles for each transcript and mouse organ examined, yet with some general trends like the higher amounts of mRNA species coding for thioredoxins than those coding for the reductases that control their redox states and activities; ii) continuous expression during embryogenesis with outstanding up-regulations of Trx1 and TrxR1 mRNAs in specific temporal sequences; iii) drastic differences in mRNA stability, liver decay rates range from 2.8 h (thioredoxin/glutathione reductase) to >/= 35 h (Trx1 and Trx2), and directly correlate with mRNA steady-state values; iv) testis-specific differences in the amounts (relative to total isoforms) of transcripts yielding the mitochondrial Grx2a and 67-kDa TrxR1 variants; and v) coordinated up-regulation of TrxR1 and glutathione reductase mRNAs in response to superoxide stress in an organ-specific manner. Further insights into in vivo roles of these redox systems should be gained from more focused studies of the mechanisms underlying the vast differences reported here at the transcript level.

Redox regulation in the lens

The high content of glutathione (GSH) in the lens is believed to protect thiols in structural proteins and enzymes for proper biological functions. The lens has both biosynthetic and regenerating systems for GSH to maintain its large pool size. However, ageing lenses or lenses under oxidative stress show an extensively diminished size of GSH pool with some protein thiols being S-thiolated by oxidized non-protein thiols to form protein-thiol mixed disulfides, either as protein-S-S-glutathione (PSSG) or protein-S-S-cysteine (PSSC) or protein-S-S-gamma-glutamylcysteine. It was shown in an H(2)O(2)-induced cataract model that PSSG formation precedes a cascade of events before cataract formation, starting with protein disulfide crosslinks, protein solubility loss and high molecular weight aggregation. Furthermore, this early oxidative damage in protein thiols can be spontaneously reversed in H(2)O(2) pretreated lenses if the oxidant is removed in time. This dethiolation process appears to have mediated through a redox-regulating enzyme, thioltransferase (TTase), which is ubiquitously present in microbial, plant and animal tissues, including the lens. The GSH-dependent, low molecular weight (11.8 kDa) cytosolic enzyme plays an important role in oxidative defense and can modulate key metabolic enzymes in the glycolytic pathway. The enzyme repairs oxidatively damaged proteins/enzymes through its unique catalytic site with a vicinal cysteine moiety, which can specifically dethiolate protein-S-S-glutathione and restore protein free SH groups for proper enzymatic or protein functions. Most importantly, it has been demonstrated that thioltransferase has a remarkable resistance to oxidation (H(2)O(2)) in cultured human and rabbit lens epithelial cells under oxidative stress conditions when other oxidation defense systems of GSH peroxidase and GSH reductase are severely inactivated. A second repair enzyme, thioredoxin (TRx), which is NADPH-dependent, is widely found in many lower and higher life forms of life. It can dethiolate protein disulfides and thus is an extremely important regulator for redox homeostasis in the cells. Thioredoxin has been recently found in the lens and has been shown to participate in the repair process of oxidatively damaged lens proteins/enzymes. These two enzymes may work synergistically to regulate and repair thiols in lens proteins and enzymes, keeping a balanced redox potential to maintain the function of the lens.