Methionine-sulfoxide reductases are unique, in that their ability to repair oxidized proteins and MsrA, which reduces S-methionine sulfoxide, can protect lens cells against oxidative stress damage. To date, the roles of MsrB1, -B2 and -B3 which reduce R-methionine sulfoxide have not been established for any mammalian system. The present study was undertaken to identify those MsrBs expressed by the lens and to evaluate the enzyme activities, expression patterns, and abilities of the identified genes to defend lens cells against oxidative stress damage.
Enzyme activities were determined with bovine lens extracts. The identities and spatial expression patterns of MsrB1, -B2, and -B3 transcripts were examined by RT-PCR in human lens and 21 other tissues. Oxidative stress resistance was measured using short interfering (si)RNA–mediated gene-silencing in conjunction with exposure to tert-butyl hydroperoxide (tBHP) and MTS viability measurements in SRA04/01 human lens epithelial cells.
Forty percent of the Msr enzyme activity present in the lens was MsrB, whereas the remaining enzyme activity was MsrA. MsrB1 (selenoprotein R, localized in the cytosol and nucleus), MsrB2 (CBS-1, localized in the mitochondria), and MsrB3 (localized in the endoplasmic reticulum and mitochondria) were all expressed by the lens. These genes exhibit asymmetric expression patterns between different human tissues and different lens sublocations, including lens fibers. All three genes are required for lens cell viability, and their silencing in lens cells results in increased oxidative-stress–induced cell death.
The present data suggest important roles for both MsrA and -Bs in lens cell viability and oxidative stress protection. The differential tissue distribution and lens expression patterns of these genes, coupled with increased oxidative-stress–induced cell death on their deletion provides evidence that they are important for lens cell function, resistance to oxidative stress, and, potentially, cataractogenesis.