Non-enzymatic triggering of the ceramide signalling cascade by solar UVA radiation

Ceramide is a key component of intracellular stress responses. Evidence is provided for a novel mechanism of ceramide formation that mediates solar ul traviolet (UV) A radiation-induced expression of the intercellular adhesion molecule (ICAM)-1, Similarly to UVA radiation, ceramide stimulation of hum an keratinocytes induced ICAM-1 mRNA expression and activated the ICAM-1 pr omoter through transcription factor AP-2, Ceramide-activated AP-2 and ceram ide-induced ICAM-1 reporter gene activation were abrogated through deletion of the AP-2 binding site. UVA radiation increased the level of ceramide in keratinocytes and inhibition of sphingomyelin synthesis prevented UVA radi ation-induced ICAM-1 expression. Hitherto, two pathways have been identifie d for ceramide accumulation: hydrolysis from sphingomyelin through neutral and acid sphingomyelinases, and de novo synthesis by ceramide synthase, WA radiation did not activate any of these enzymes. Ceramide generation in WA- irradiated cells, however, was inhibited by singlet oxygen quenchers and mi micked in unirradiated cells by a singlet oxygen-generating System, In addi tion, UVA radiation and singlet oxygen both generated ceramide in protein-f ree, sphingomyelin-containing liposomes, This study indicates that singlet oxygen triggers a third, non-enzymatic mechanism of ceramide formation.