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

Susanne Grether-Beck¹, Giuseppina Bonizzi², Heidi Schmitt-Brenden¹, Ingo Felsner¹, Andreas Timmer¹, Helmut Sies^3,4, Judith P. Johnson⁵, Jacques Piette², and Jean Krutmann^1,4,6

¹Clinical and Experimental Photodermatology, Department of Dermatology, Heinrich-Heine-University, Moorenstraße 5, D-40225 Düsseldorf, ³Institute of Physiological Chemistry I, Heinrich-Heine-University, Universitätsstraße 1, D-40225 Düsseldorf, ⁴Biologisch-Medizinisches Forschungszentrum, Heinrich-Heine-University, Universitätsstraße 1, D-40225 Düsseldorf, ⁵Institute of Immunology, University of Munich, Goethestraße 31, D-8000 Munich 2, Germany and ²Laboratory of Virology and Immunology, Institute of Pathology, CHU B35, Sart-Tilman, University of Liege, 4000 Liege, Belgium ⁶Corresponding author e-mail: krutmann{at}rz.uni-duesseldorf.de

Abstract: Ceramide is a key component of intracellular stress responses. Evidence is provided for a novel mechanism of ceramide formation that mediates solar ultraviolet (UV) A radiation-induced expression of the intercellular adhesion molecule (ICAM)-1. Similarly to UVA radiation, ceramide stimulation of human keratinocytes induced ICAM-1 mRNA expression and activated the ICAM-1 promoter through transcription factor AP-2. Ceramide-activated AP-2 and ceramide-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 radiation-induced ICAM-1 expression. Hitherto, two pathways have been identified for ceramide accumulation: hydrolysis from sphingomyelin through neutral and acid sphingomyelinases, and de novo synthesis by ceramide synthase. UVA radiation did not activate any of these enzymes. Ceramide generation in UVA-irradiated cells, however, was inhibited by singlet oxygen quenchers and mimicked in unirradiated cells by a singlet oxygen-generating system. In addition, UVA radiation and singlet oxygen both generated ceramide in protein-free, sphingomyelin-containing liposomes. This study indicates that singlet oxygen triggers a third, non-enzymatic mechanism of ceramide formation.

Key Words: Keywords: ceramide/human keratinocytes/signal transduction/singlet oxygen/ultraviolet radiation