Study question: Can selenium (Se) independent, epididymal-specific glutathione peroxidase 5 (GPX5) protect CHO-K1 cells from oxidative damage and, more specifically, from lipid peroxidation and DNA mutation?
Summary answer: CHO-K1 cells expressing GPX5 have increased resistance to oxidative challenge and, more specifically, decreased levels of lipid peroxidation and decreased levels of the downstream DNA lesion 8-oxo-7,8-dihydroguanine (8-oxodG) compared with control cells.
What is known already: GPX5 associates with sperm during transit of the epididymis, and has been postulated to protect sperm from peroxide-mediated attack. However, its function as an active glutathione peroxidase has been questioned due to substitution of the classical selenocysteine residue at its active site. Indirect evidence for a functional role for GPX5 has been provided by in vivo studies, in particular from the GPX5 knockout mouse whereby offspring sired by GPX5(-/-) males have a higher rate of spontaneous abortion and developmental defects, attributed to increased oxidative injury (8-oxodG) to sperm DNA, but only when the GPX5(-/-) males are over 1 year of age. Interestingly, we have previously shown severely reduced levels of GPX5 in humans.
Study design, size, duration: To look more directly at its role in protection against oxidative damage, we have used an in vitro system, generating a CHO-K1 mammalian cell line expressing recombinant rat GPX5.
Participants/materials, setting, methods: We have used the recombinant CHO-K1 cells to determine whether GPX5 is able to protect these cells from an administered oxidative challenge, using a range of approaches. We compared the viability of GPX5-expressing cells with control cells by both MTT and trypan blue exclusion assays. We next investigated whether GPX5 protects the cells specifically from lipid peroxidation, by using the fluorescent reporter molecule C11-BODIPY(581/591), and thus from downstream DNA mutation, by comparing levels of the DNA lesion 8-oxodG. We also investigated whether GPX5 can be transferred to rat sperm via epididymosomes.
Main results and the role of chance: GPX5-expressing CHO-K1 cells had increased viability compared with control cells following oxidative challenge (P < 0.005). We also found that GPX5-expressing CHO-K1 cells had significantly lower levels of C11-BODIPY(581/591) oxidation, and hence lipid peroxidation, compared with control cells. Levels of 8-oxodG DNA damage were also markedly lower in the nuclei of GPX5-expressing cells than in control cells. Finally, we showed that GPX5 can be transferred to rat sperm via epididymosomes.
Limitations, reasons for caution: GPX5 is not active in glutathione peroxidase assays using H₂O₂ as the substrate. However, the related non-mammalian Se-independent GPXs show preference for electron donors other than glutathione, with a number utilizing thioredoxin as a reducing equivalent. Hence, the in vitro activity of GPX5 needs to be assessed using a range of alternative substrates and electron donors. GPX5 is secreted by the epididymis and associates with the sperm plasma membrane. We showed that this transfer can occur via epididymosomes; however, the mechanism for transfer and the identity of a potential binding partner in the sperm membrane needs to be determined. Finally, our study utilized an in vitro system that needs to be translated to human sperm.
Wider implications of the findings: Our study supports an important role for GPX5 as an antioxidant, possibly acting as a phospholipid hydroperoxidase and participating in the maintenance of cell and DNA integrity.
Keywords: DNA damage; GPX5; antioxidants; lipid peroxidation; sperm.