Synthesis of sphingosine is essential for oxidative stress-induced apoptosis of photoreceptors

ABRAHAN C.E.; MIRANDA G.E.; AGNOLAZZA CL; POLITI L.E.; ROTSTEIN N.P.

INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE

ASSOC RESEARCH VISION OPHTHALMOLOGY INC

Año: 2010 vol. 51 p. 1171 - 1180

0146-0404

PURPOSE. Oxidative stress is involved in inducing apoptosis of photoreceptors in many retinal neurodegenerative diseases. We have previously shown that oxidative stress increases in photoreceptors the synthesis of ceramide, a sphingolipid precursor that then activates apoptosis. In several cell types ceramide is converted by ceramidases to sphingosine (Sph), another apoptosis mediator; hence, we investigated whether Sph participates in triggering photoreceptor apoptosis. METHODS. Rat retina neurons were incubated with [3H]palmitic acid and treated with the oxidant paraquat (PQ) to evaluate Sph synthesis. Sph was added to cultures with or without docosahexaenoic acid (DHA), the major retina polyunsaturated fatty acid and a photoreceptor survival factor, to evaluate apoptosis. Synthesis of Sph and sphingosine-1-phosphate (S1P), a prosurvival signal, were inhibited with alkaline ceramidase or sphingosine kinase inhibitors, respectively, before adding PQ, C2-ceramide or Sph. Apoptosis, mitochondrial membrane polarization, cytochrome c localization and reactive oxygen species (ROS) production were determined. RESULTS. PQ increased [3H]Sph synthesis in photoreceptors and blocking this synthesis by inhibiting alkaline ceramidase decreased PQ-induced apoptosis. Sph addition induced photoreceptor apoptosis, increased ROS production and promoted cytochrome C release from mitochondria. While DHA prevented this apoptosis, inhibiting Sph conversion to S1P blocked DHA protection. CONCLUSIONS. These results suggest that oxidative stress enhances formation of ceramide and its subsequent breakdown to Sph; ceramide and/or Sph would then trigger photoreceptor apoptosis. Preventing Sph synthesis or promoting its phosphorylation to S1P rescued photoreceptors, suggesting that Sph is a mediator of their apoptosis and modulation of Sph metabolism might be crucial for promoting photoreceptor survival.