Singlet oxygen-induced attenuation of growth factor signaling: Possible role of ceramides

Singlet oxygen, an electronically excited form of molecular oxygen, is a primary mediator of the activation of stress-activated protein kinases elicited by ultraviolet A (UVA; 320-400 nm). Here, the effects of singlet oxygen (O-1(2) ) on the extracellular signal-regulated kinase (ERK) 1/2 and Akt/protein kinase B pathways were analyzed in human dermal fibroblasts. While basal ERK 1/2 phosphorylation was lowered in cells exposed to either O-1(2) , UVA or photodynamic treatment, Akt was moderately activated by photochemically generated O-1(2) in a phosphoinositide 3-kinase (PI3K)-dependent fashion, resulting in the phosphorylation of glycogen synthase kinase-3 (GSK3). The activation of ERK 1/2 and Akt as induced by stimulation with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) was inhibited by O-1(2) generated intracellularly upon photoexcitation of rose Bengal (RB). Photodynamic therapy (PDT)-induced apoptosis is known to be associated with increased formation of ceramides. Likewise, both O-1(2) and UVA induced ceramide generation in human skin fibroblasts. The attenuation of EGF- and PDGF-induced activation of ERK 1/2 and Akt by O-1(2) was mimicked by stimulation of fibroblasts with the cell-permeable C-2 -ceramide. Interestingly, EGF-induced tyrosine phosphorylation of the EGF receptor was strongly attenuated by O-1(2) but unimpaired by C-2 -ceramide, implying that, although ceramide formation may mediate the above attenuation of ERK and Akt phosphorylation induced by O-1(2) , mechanisms beyond ceramide formation exist that mediate impairment of growth factor signaling by singlet oxygen. In summary, these data point to a novel mechanism of O-1(2) toxicity: the known O-1(2)-induced activation of proapoptotic kinases such as JNK and p38 is paralleled by the prevention of activation of growth factor receptor-dependent signaling and of anti-apoptotic kinases, thus shifting the balance towards apoptosis.