Human lens thioredoxin: molecular cloning and functional characterization

PURPOSE: To molecularly clone the human lens thioredoxin (TXN) gene, characterize the recombinant protein (rTrx1) and study the regulation expression of thioredoxin (Trx1) in human lens epithelial cells under oxidative stress. METHODS: The human TXN gene was cloned from a human lens cDNA library. Trx1 activity was measured by insulin reduction assay. For study of the upregulation of Trx1, 1.6 million human lens epithelial cells (HLE B3) were exposed to H(2)O(2) (0.1 mM) for 0, 5, 10, 15, 20, and 30 minutes. The cells were lysed with lysis buffer and used for Trx1 activity assay, Western blot analysis, and real-time PCR. RESULTS: The sequence of the human lens TXN gene was identical with that of other human tissues. Recombinant Trx1 was sensitive to iodoacetic acid but showed strong resistance to oxidation (0.1 mM H(2)O(2)) at its approximate physiological protein level. Western blot analysis and assay of Trx activity revealed that Trx1 was expressed in HLE B3 cells and localized in epithelial, cortical, and nuclear regions of human and porcine lenses. In vivo oxidative stress of HLE B3 cells resulted in a 35% upregulation of the level of Trx1 protein after 10 minutes of H(2)O(2) treatment. Real-time PCR analysis showed an increase of approximately 50% in the level of Trx1 mRNA under the same oxidative stress conditions. CONCLUSIONS: The upregulation of Trx1 in HLE B3 cells under oxidative stress and the presence of Trx1 in the lens suggest that the thioredoxin system may be an effective defense system that protects the lens against oxidative stress.     

硫醇转移酶和硫氧还蛋白系统在白内障中的研究

晶状体透明性的维持需要平衡的氧化还原状态和恰当的巯基／二硫化物比例．其丰富的谷胱甘肽和内在修复酶系统作为重要的防御屏障对于维持晶状体功能非常重要，然而，在老化白内障晶状体中，谷胱甘肽含量显著下降。近年对硫醇转移酶和硫氧还蛋白系统的研究发现：硫醇转移酶特异地使蛋白质与谷胱甘肽形成的二硫化物脱硫醇，恢复蛋白质的自由巯基，维持蛋白质或酶的功能；硫氧还蛋白系统能使蛋白质二硫化物脱硫醇，是细胞内氧化还原稳态的重要调节子。两者协同作用能有效地修复晶状体蛋白质和酶的巯基，维持晶状体的功能.     

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.     

Revival of glutathione reductase in human cataractous and clear lens extracts by thioredoxin and thioredoxin reductase, in conjunction with alpha-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.     

晚期糖基化终产物对人晶状体上皮细胞TTase表达的影响

目的：观察晚期糖基化终产物对人晶状体上皮细胞硫醇转移酶表达及活性的影响。
　　方法：将体外人晶状体上皮细胞用 AGEs-BSA 浓度为1.5mg/mL,胎牛血清体积分数为10%的DMEM培养液培养,同时设定相同浓度的BSA对照组及空白对照组,分别于0,1,2,3,4 d收集细胞,测定晶状体上皮细胞内ROS含量、TTase 活性, qRT-PCR 检测 TTase mRNA 表达情况, Western blot检测TTase蛋白质表达。
　　结果：与对照组相比, AGEs-BSA干预后,细胞内ROS含量呈时间依赖性增高,差异有显著统计学意义(P<0.01), BSA干预后ROS的表达与对照组无显著差异。 AGEs可诱导TTase的 mRNA表达逐渐增高,2 d时达到峰值,约为正常对照组的5.06倍( P<0.01);而BSA处理组和对照组TTase的mRNA表达差异无统计学意义( P>0.05)。与TTase的mRNA表达类似, TTase活性升高,在3 d达到峰值,为正常对照组的2.01倍( P<0.01)。 Western blot检测发现,TTase蛋白质表达逐渐增加,从3d开始TTase表达与对照组相比差异有统计学意义(P<0.05)。
　　结论：AGEs可能是通过诱导人晶状体上皮细胞发生氧化应激,致使TTase表达上调,活性增强。     

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.     

Regulation of thioredoxin by ceramide in retinal pigment epithelial cells

The purpose of this study was to determine the expression, regulation and signaling of a key redoxin family member thioredoxin 1 (Trx1) in normal, oxidant-stimulated and growth factor-pretreated RPE cells. Trx1 is expressed in early passage, human RPE cell cultures. RPE cells exposed to C₂-ceramide for 24 h showed no significant change in expression of Trx1 vs. controls with and without pretreatment for 24 h with hepatocyte growth factor (HGF). Neither hypoxia from 1% O₂ or from CoCl₂ exposure resulted in any alteration in Trx1 expression in RPE cells. C₂-ceramide treatment caused translocation of Trx1 from cytosol to the nucleus, which was abolished by pre-treatment of cells with a p38 MAPK-specific inhibitor. Furthermore, the gene and protein expression of thioredoxin interacting protein (Txnip) increased with ceramide treatment and was significantly (p < 0.001) elevated with HGF preincubation vs. untreated controls. Prominent protection from ceramide-induced RPE cell death by exogenous rTrx1 was demonstrated. Although Trx1 directly interacts with its inhibitor, Txnip, p38 inhibition does not appear to have a role in this interaction. We found no direct interaction between apoptosis signal regulating kinase (ASK-1) and Txnip under the same experimental conditions. In summary, our data demonstrate the expression of Trx1 and Txnip in human RPE cells. Ceramide treatment results in translocation of Trx1 to the nucleus, and upregulation of Txnip expression; exogenous rTrx1 protects from ceramide-induced cell death. These results suggest that Trx1 and Txnip play an important role in the response of RPE to ceramide toxicity.     

Effect of H(2)O(2)on human lens epithelial cells and the possible mechanism for oxidative damage repair by thioltransferase

Human lens epithelial (HLE) B3 cells were used to study the oxidative damage and cellular repair with respect to the redox homeostasis, the oxidative defense enzymes and the glucose metabolic pathway. The effect of oxidative stress on cell growth was initially analyzed by culturing the cells with a bolus amount (0.02--0.1m M) of hydrogen peroxide (H(2)O(2)) in minimal essential medium (MEM) containing 20% fetal bovine serum (FBS) for 1 week. Concentration of H(2)O(2)greater than 0.03m M showed progressive inhibition of cell growth. However, the cells were also shown to tolerate H(2)O(2)concentrations up to 0.5m M by detoxifying the exogenous oxidant within 3hr without any detectable DNA damage. Therefore, this short-term H(2)O(2)exposure model was chosen to study the effect of oxidative stress on the cellular redox homeostasis. HLE B3 cells were first grown to confluence in MEM with 20% FBS. Approximately 1.6 million cells were gradually weaned off serum by subculturing in 2% FBS overnight, followed by serum-free medium for 30 min before subjecting to a bolus of 0.1m M H(2)O(2)for up to 180 min. These cells were used for biochemical analysis, which included H(2)O(2)detoxification (H(2)O(2)in the medium), glutathione (GSH) level and lactate production. Activity measurements were conducted on the oxidation defense enzymes: glutathione-S-transferase (GST), glutathione reductase (GR) and glutathione peroxidase (GPx); the dethiolating enzyme, thioltransferase (TTase); and a key glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (G-3PD). While the B3 cells were shown to tolerate and detoxify 0.1m M H(2)O(2)within 60 min, the GSH pool was transiently depleted in the first 60 min before fully recovered. GPx suffered more than 80% loss in activity and was unable to recover fully. GST showed slight inactivation but neither GR nor TTase was affected. G-3PD was inactivated to < 50% within 15 min of oxidative stress and was reactivated gradually to 80% of normal at the end of 180 min, concurrent with the transient loss of lactate production in the same cells. The reactivation of G-3PD was both temperature- and GSH-dependent, occurring only at physiological temperature and failing to reactivate when the intracellular GSH pool was depleted by BCNU (GR inhibitor) pretreatment. The inactivated cellular G-3PD in the cell extract could be partially reactivated by DTT (6m M) or by recombinant human lens thioltransferase (RHLT) but not by GSH (1m M), GR or GST. HLE cells cultured in the presence of L-(35)S-cystine and cycloheximide displayed an extra radiolabelled protein band on the autoradiograph in the H(2)O(2)treated cells. The labelled band was positively reacted with G-3PD antibody and could be removed by RHLT, indicating that S-thiolation of G-3PD occurred. The H(2)O(2)pre-exposed cells also transiently accumulated proteins modified by thiolation, including protein-S-S-glutathione (PSSG) and protein-S-S-cysteine (PSSC). It can be concluded that HLE could endure up to 0.1m M of H(2)O(2)oxidative stress since the cell could be protected by its effective repair systems, including dethiolating the inactivated key SH-sensitive enzymes. TTase may play a role in this. One of the mechanisms may be through preserving glucose metabolism and supplying ATP needed for maintaining cell viability.     

硫氧还蛋白在糖尿病大鼠视网膜中的动态表达

目的 观察硫氧还蛋白( thioredoxin,Trx)在糖尿病(diabetes mellitus,DM)大鼠视网膜中的表达部位及动态变化规律.方法 成年Sprague Dawley(SD)大鼠随机分为2组,糖尿病组(DM组)一次性腹腔注射链脲佐菌素制作大鼠DM模型;对照组(NC组)注射同等剂量的柠檬酸缓冲液.在注射后4周、8周、12周,采用免疫组织化学法检测Trx在各组大鼠视网膜中的定位表达,采用Real-time PCR测定视网膜Trx mRNA表达.结果 免疫组织化学结果显示,NC组与DM组大鼠视网膜神经纤维层、神经节细胞层、内核层有Trx阳性表达,表达位于细胞浆.Real-time PCR结果显示正常大鼠视网膜有Trx mRNA的表达;与NC组相比,DM组各时间点TrxmRNA表达下降,4周时DM组Trx mRNA相对表达量为0.42±0.03,约为NC组(1.00±0.03)的0.42倍;8周时DM组Trx mRNA相对表达量为0.57±0.04,约为NC组(1.00±0.03)的0.57倍;12周时DM组Trx mRNA相对表达量为0.82±0.08,约为NC组(1.00±0.07)的0.82倍,差异均有统计学意义(均为P＜0.05).DM组视网膜组织Trx mRNA在4～12周表达逐渐升高,至第12周时仍未达到正常水平,第4周与第12周相比差异有统计学意义(P=0.00).结论 在糖尿病视网膜病变时,Trx mRNA表达下降,说明氧化应激反应产生过多的活性氧使得Trx mRNA表达量下降.随着DM病程的延长,视网膜Trx mRNA表达量逐渐升高,这可能是Trx系统功能下降、自由基增多引起的一种代偿性反应.     

表没食子酸酯对高糖诱导的人晶状体上皮细胞氧化应激的影响

目的：：探讨表没食子酸酯( EGCG)对高糖诱导的人晶状体上皮细胞( human lens epithelial cells, HLEC )氧化应激的影响。方法：建立高糖诱导的HLEC氧化损伤模型,用不同浓度的EGCG干预,MTT检测细胞活力,倒置显微镜观察细胞形态,Hoechst-PI染色观察细胞凋亡,分光光度计检测上清液中超氧化物歧化酶( SOD )、谷胱甘肽过氧化物酶( GSH-Px)及丙二醛( MDA)的含量。结果：MTT 结果显示用10μmol/L EGCG 和100μmol/L EGCG处理后,HLEC活性分别提高到50.33%±3.52%和63.33%±4.63%,与氧化损伤组(32.67%±3.10%)比较差异具有统计学意义( P<0.05);EGCG干预的高糖条件下的HLEC较好地保持了细胞的形态,凋亡细胞数量减少,细胞内SOD、GSH-Px水平升高,MDA水平下降。结论：EGCG可能通过提高细胞内SOD、GSH-Px含量,降低MDA含量发挥其较强的抗氧化作用,从而为寻求有效的防治白内障药物提供可靠的实验依据。     

Thiol regulation in the lens.

The high content of glutathione (GSH) in the lens is believed to protect the 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 (4-6 mM). However, we have observed that, in aging lenses or lenses under oxidative stress, the size of GSH pool is diminished; and some protein thiols are being S-thiolated by oxidized nonprotein thiols to form protein-thiol mixed disulfides, either as protein-S-S-glutathione (PSSG) or protein-S-S-cysteine (PSSC). We have shown in an H2O2-induced cataract model that PSSG formation precedes a cascade of events starting with protein disulfide crosslinks, protein solubility loss, and eventual lens opacification. Recently, we discovered that this early oxidative damage in protein thiols could be spontaneously reversed in H2O2 pretreated lenses if the oxidant was removed in time. This dethiolation process is likely mediated through a redox regulating enzyme, thioltransferase (TTase), which has been discovered recently in the lens. To understand if the role of oxidative defense and repair is the physiological function of TTase in the lens, we cloned the TTase gene and purified the recombinant human lens TTase. Although TTase required GSH for its activity, TTase was far more efficient in dethiolating lens proteins than GSH alone. It favored PSSG over PSSC and dethiolated gamma-crystallin-S-S-G better than the alpha-crystallin counterparts. Furthermore, TTase showed a remarkable resistance to oxidation (H2O2) in cultured rabbit lens epithelial cells when GSH peroxidase, GSH reductase, and glyceraldehyde-3-phosphate dehydrogenase were severely inactivated. We further showed that activity loss in those SH sensitive enzymes could be attributed to S-thiolation, but reactivation via dethiolation could be attributed to TTase. We conclude that TTase can regulate and repair the thiols in lens proteins and enzymes through its dethiolase activity, thus contributing to the maintenance of the function of the lens.     

Protection of HLE B-3 cells against hydrogen peroxide- and naphthalene-induced lipid peroxidation and apoptosis by transfection with hGSTA1 and hGSTA2

PURPOSE: To investigate the physiological role of two major alpha-class glutathione S-transferases (GSTs), hGSTA1-1 and hGSTA2-2 in protection against oxidative stress and lipid peroxidation (LPO) in human lens epithelial (HLE B-3) cells. METHODS: Total GSTs were purified from HLE B-3 cells by glutathione (GSH)-affinity chromatography and characterized by Western blot analysis, isoelectric focusing, and kinetic studies. The relative contributions of the alpha-class GSTs and the Se-dependent glutathione peroxidase (GPx)-1 in GSH-dependent reduction of phospholipid hydroperoxide (PL-OOH) were quantitated through immunoprecipitation studies using separately the specific polyclonal antibodies against human alpha-class GSTs and GPx-1. HLE B-3 cell membranes were prepared, peroxidized, and used to examine whether hGSTA1-1 and hGSTA2-2 catalyzes the reduction of membrane PL-OOH in situ using the microiodometric and spectrophotometric assays. The protective effects of the alpha-class GSTs against H2O2- and naphthalene-induced LPO and apoptosis were examined by transfecting HLE B-3 cells with cDNAs of hGSTA1 and hGSTA2. RESULTS. HLE B-3 cells expressed only the alpha and pi class GSTs. The Michaelis-Menten constant (k(m)) and turnover number (k(cat)) of purified total GSTs toward phosphatidylcholine hydroperoxide (PC-OOH) were found to be 30 +/- 4 microM and 1.95 +/- 0.26 seconds, respectively. The alpha-class GSTs accounted for approximately 65% of the total GPx activity of HLE B-3 cells toward PC-OOH. Our results demonstrate for the first time that hGSTA1-1 and hGSTA2-2 effectively catalyzed GSH-dependent reduction of membrane PL-OOH in situ in HLE B-3 cells. Transfection with hGSTA1 or hGSTA2 protected these cells from H2O2- and naphthalene-induced LPO and attenuated H2O2- and naphthalene-induced apoptosis through inhibiting caspase 3 activation. CONCLUSIONS: These results demonstrate that the alpha-class GSTs hGSTA1-1 and hGSTA2-2 play a major role as antioxidant enzymes and are the main determinants of the levels of LPO caused by oxidative stress in human lens epithelial cells.     

Generation of endostatin by matrix metalloproteinase and cathepsin from human limbocorneal epithelial cells cultivated on amniotic membrane

PURPOSE: Cultured human limbocorneal epithelial (HLE) cells secrete endostatin-related molecules that are augmented when the cells are cultivated on denuded amniotic membrane (DAM). This study is to identify mechanisms for enhanced endostatin production by HLE cells cultivated on AM. METHODS: HLE cells were cultured on dish, on intact AM (IAM) or on DAM. Collagen XVIII alpha1 mRNA was analyzed by real-time quantitative PCR. In HLE/DAM cultures, inhibitors of MMPs (GM-6001; 1,10-phenanthroline), cathepsins (E64; cathepsin B inhibitor II), elastase (elastatinal), and serine proteases (AEBSF; aprotinin) were added. Endostatin in the conditioned medium (CM) was detected by Western blot. MMP-7; MMP-9; and cathepsins B, K, L, and V in the CM were quantitated by ELISA. Exogenous cathepsin B or V was added to the concentrated HLE/DAM CM to see the effect on endostatin production. RESULTS: The expression of collagen XVIII alpha1 mRNA in the three groups was similar. Elastatinal, AEBSF, and aprotinin had no effect on endostatin generation. MMP inhibitors inhibited the generation of all the 20- and 28- to 30-kDa endostatin-related fragments, while cathepsin inhibitors inhibited only the 20-kDa endostatin. The level of MMP-7 and cathepsin B but not cathepsin V increased as the culture time increased, and paralleled with endostatin production. However, cathepsins K and L were absent in the CM. Exogenous cathepsins B and V further augmented the generation of endostatin. CONCLUSIONS: MMP-7 and cathepsins B and V are involved in the generation of endostatin by HLE cells. Facilitating endostatin generation may be a novel physiological function of the cornea-specific cathepsin V.     

Bovine eye 1-Cys peroxiredoxin: expression in E. coli and antioxidant properties.

Peroxiredoxins constitute a molecular family of novel antioxidant proteins and are distributed broadly in non-mammalian and mammalian tissues, including the eye. In this study, a recombinant bovine eye 1-Cys peroxiredoxin (BRPrx) was expressed in Escherichia coli (E. coli). The recombinant protein protected glutamine synthetase from oxidative damage caused by a metal ion-catalyzed oxidation system (ascorbate/Fe3+/O2) in the presence of dithiothreitol as an electron donor. The protector activity of BRPrx is attributed to its peroxidase activity exhibited in the presence of dithiothreitol. Both hydrogen peroxide and short chain hydroperoxides are substrates for the protein. Glutathione could not support antioxidant properties of the recombinant protein. The antioxidant activity of BRPrx in the glutamine synthetase protection assay was as high as the activity of catalase and about one order of magnitude lower than that of selenium glutathione peroxidase. These results support the premise that Prx is an important component of the antioxidant defense system in eye tissues.