Recent advances in the knowledge of the EGFR pathway have revealed its contribution to distinct immune/inflammatory functions of the epidermis. The purpose of our study was to evaluate the role of EGFR in the regulation of keratinocyte GM-CSF expression. In cultured human keratinocytes, proinflammatory cytokines synergized with TGF-alpha to induce GM-CSF expression. Accordingly, high epidermal levels of EGFR activation are associated with enhanced expression of GM-CSF in lesional skin of patients with psoriasis or allergic contact dermatitis. In cultured keratinocytes, pharmacological inhibition of EGFR activity reduced GM-CSF promoter transactivation, whereas genetic inhibition of AP-1 reduced expression of GM-CSF. Furthermore, EGFR activation enhanced TNF-alpha-induced c-Jun phosphorylation and DNA binding, whereas c-Jun silencing reduced GM-CSF expression. Using two different mouse models, we showed that the lack of a functional EGFR pathway was associated with reduced cytokine-induced phosphorylation of ERK1/2, JNK1/2, c-Jun and reduced keratinocyte-derived GM-CSF expression both in vitro and in vivo. Finally, the analysis of GM-CSF expression in the skin of cancer patients treated with anti EGFR drugs showed an association between ERK activity, c-Jun phosphorylation, and epidermal GM-CSF expression. These data demonstrate that the EGFR pathway is critical for the upregulation of keratinocyte GM-CSF expression under conditions of cytokine stimulation.
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