Sphingolipids regulate photoreceptor survival during oxidative stress

ROTSTEIN NP; ABRAHAN CE; MIRANDA GE; GERMAN OL

Buenos Aires, Argentina

Congreso; XVII ICER; 2006

ISER

SPHINGOLIPIDS REGULATE PHOTORECEPTOR SURVIVAL DURING OXIDATIVE STRESS Rotstein N., Abrahan C., Miranda G. and German L. Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-CONICET. 8000Bahia Blanca, Buenos Aires, Argentina. inrotste@criba.edu.ar   Apoptosis of photoreceptor leads to blindness in several neurodegenerative diseases of the retina. Improving our understanding of the mechanisms involved in triggering photoreceptor death and the pathways promoting photoreceptor survival is essential for finding new clues for treating retina degeneration. In the last decade increasing evidence shows that different sphingolipids participate in the regulation of apoptosis and cell survival in several biological systems; however, knowledge on their function in the vertebrate retina is scarce. We have investigated whether sphingolipids are involved in the regulation of photoreceptor apoptosis in the rat retina. Several lines of evidence point to a key role of the sphingolipid ceramide in triggering photoreceptor apoptosis. Induction of oxidative stress with the oxidant paraquat (PQ) led to a rapid increase in the endogenous levels of ceramide, which preceded photoreceptor apoptosis, thus suggesting that ceramide might be a key mediator of this death. Moreover, exogenous addition of ceramide triggered photoreceptor apoptosis in culture. Conversely, blocking ceramide de novo synthesis protected photoreceptors from oxidative stress-induced apoptosis. This indicates that an increase in ceramide levels is a key step in the induction of photoreceptor apoptosis after oxidative stress. Ceramide can be metabolized through several pathways: it can be glucosylated to the non apoptotic metabolite glucosylceramide, or metabolized to sphingosine, an established proapototic sphingolipid. In turn, sphingosine can be phosphorylated by sphingosine kinase to form sphingosine-1-phosphate (S1P), shown to have an antiapoptotic function. Addition of sphingosine induced photoreceptor apoptosis, while blocking the endogenous synthesis of this sphingolipid prevented ceramide-induced apoptotic death. This suggests that conversion of ceramide into sphingosine is required for activation of photoreceptor apoptosis. We have previously demonstrated that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, protects photoreceptor from oxidative stress-induced apoptosis. DHA also prevented ceramide-induced apoptosis of photoreceptors; however, pretreatment with an inhibitor of ceramide glucosylation before ceramide or PQ addition blocked DHA protection, suggesting that DHA enhances ceramide glucosylation to decrease ceramide levels and thus avoid induction of apoptosis. Ceramide metabolization to S1P also seems to play a role in DHA protection: preventing S1P synthesis with an inhibitor of sphingosine kinase also abolished DHA protection from ceramide and PQ-induced apoptosis. This suggests that an enhanced synthesis of S1P is instrumental for DHA prevention of apoptosis in photoreceptors. These results suggest that DHA regulation of the balance between the levels of pro, ceramide and sphingosine, and anti, S1P, apoptotic sphingolipids is crucial for determining photoreceptor death or survival. Modulation of sphingolipid metabolism might thus provide a useful therapeutical tool for rescuing photoreceptors in retina degeneration.