Cellular lipid peroxidation end-products induce apoptosis in human lens epithelial cells

Hydrogen peroxide (H₂O₂), an oxidant present in high concentrations in the aqueous humor of the elderly eyes, is known to impart toxicity to the lens - apoptosis being one of the toxic events. Since H₂O₂ causes lipid peroxidation leading to the formation of reactive end-products, it is important to investigate whether the end-products of lipid peroxidation are involved in the oxidation-induced apoptosis in the lens. 4-Hydroxynonenal (HNE), a major cytotoxic end product of lipid peroxidation, has been shown to mediate oxidative stress-induced cell death in many cell types. It has been shown that HNE is cataractogenic in micromolar concentrations in vitro, however, the underlying mechanism is not yet clearly understood. In the present study we have demonstrated that H₂O₂ and the lipid derived aldehydes, HNE and 4-hydroxyhexenal (HHE), can induce dose- and time-dependent loss of cell viability and a simultaneous increase in apoptosis involving activation of caspases such as caspase-1, -2, -3, and -8 in the cultured human lens epithelial cells. Interestingly, we observed that Z-VAD, a broad range inhibitor of caspases, conferred protection against H₂O₂- and HNE-induced apoptosis, suggesting the involvement of caspases in this apoptotic system. Using the cationic dye JC-1, early apoptotic changes were assessed following 5 h of HNE and H₂O₂ insult. Though HNE exposure resulted in ∼ 50% cells to undergo early apoptotic changes, no such changes were observed in H₂O₂ treated cells during this period. Furthermore, apoptosis, as determined by quantifying the DNA fragmentation, was apparent at a much earlier time period by HNE as opposed to H₂O₂. Taken together, the results demonstrate the apoptotic potential of the lipid peroxidation end-products and suggest that H₂O₂-induced apoptosis may be mediated by these end-products in the lens epithelium.