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.

The role of metallothionein IIa in defending lens epithelial cells against cadmium and TBHP induced oxidative stress.

PURPOSE: Heavy metals and other forms of oxidative stress have been implicated as key factors in the formation of age-related cataract in humans. Metallothioneins are a group of proteins known to play important roles in defending cells against the cytotoxic effects of heavy metals. However, little is known about their involvement in defending against other forms of oxidative stress. Here, we examined the ability of metallothionein IIa (MTIIa) to protect human lens epithelial cells against cadmium and tertiary butyl hydroperoxide (TBHP)-induced oxidative stress. METHODS: MTIIa over-expressing human lens epithelial cells (SRA01/04) were created by retroviral mediated gene transfer. Normal and MTIIa over-expressing cells were exposed to various concentrations of cadmium and TBHP and subsequently monitored for cell death, changes in cell phenotype and differences in growth rate. In addition, expression levels of three other important antioxidant genes, heme oxygenase-1, thioredoxin reductase-1, and manganese superoxide dismutase were monitored by real-time RT-PCR following exposure to TBHP. RESULTS: Analysis of the over expressing cell lines revealed an approximate 3-4 fold increase in MTIIa expression relative to control cells, resulting in as much as 20% protection against cadmium-induced oxidative stress (p<0.001). The MTIIa over expressing cells were also significantly more resistant to TBHP treatment while control cells exhibited significant shrinking and rounding-up following 3-6 h TBHP treatment, no changes were observed in TBHP-treated over expressing cells. When control cells were treated for 3 h or overnight with TBHP, 40-45% cell death occurred by day three. However, no cell death was observed at this time for the treated MTIIa over-expressing cell line. In addition, TBHP induced the expression of MTIIa, heme oxygenase-1, thioredoxin reductase-1, and MnSOD in both normal and MTIIa over-expressed cell lines. Interestingly the latter three genes were induced at 2-3 fold higher levels in TBHP-treated MTIIa over-expressing cells, compared to treated control cells (p=0.001, p=0.02, and p=0.01, respectively). CONCLUSIONS: These data indicate that over-expression of MTIIa in human lens epithelial cells results in protection against cadmium and TBHP-induced oxidative stress. In addition, the results suggest that MTIIa, and/or its ability to chelate metals, may play a role in regulating expression of other important antioxidant genes in response to oxidative stress.