Sulfur Dioxide Inhibits TNF-α-induced Photoreceptor Cell Apoptosis Through Sulfenylation of Caspase-3 at Cysteine 163

Photoreceptor cell apoptosis contributes significantly to vision loss in retinal degenerative diseases. Gasotransmitter sulfur dioxide (SO2) has been implicated in the regulation of apoptotic processes. This study investigated changes in the endogenous SO2/aspartate aminotransferase (AAT) system during tumor necrosis factor-α (TNF-α)-induced photoreceptor cell apoptosis.

We examined the effect of endogenous SO2 on the TNF-α-stimulated apoptosis in 661 W photoreceptor cells and investigated the underlying molecular mechanisms. TNF-α-stimulated 661 W photoreceptor cells served as a cellular model to assess apoptosis via a TdT-mediated dUTP nick-end labeling (TUNEL) assay. We evaluated caspase-3 activation using western blotting and quantified its activity using a colorimetric assay. Site-directed mutagenesis and biotin-switch assays were performed to detect the effects of caspase-3 sulfenylation.

During TNF-α-induced apoptosis in photoreceptor cells, the endogenous SO₂ system was markedly upregulated, which correlated with the effective inhibition of apoptosis. SO2 directly suppresses caspase-3 activity in retinal photoreceptor cells and recombinant proteins. Importantly, site-directed mutagenesis revealed that the C163S mutation in caspase-3 abolished SO2-mediated sulfenylation, thereby reducing its anti-apoptotic effects. These findings indicate that endogenous SO2 exerts its protective effects by sulfenylating caspase-3 at cysteine 163.

Our study demonstrates that upregulation of the endogenous SO2 system directly inhibits caspase-3 activation through sulfenylating cysteine 163 in caspase-3, effectively reducing TNF-α-induced apoptosis in 661 W cells. These findings highlight the precise regulatory role of SO2 in apoptosis.

This study provides novel insights into potential therapeutic targets for retinal degenerative diseases.