An Endoplasmic Reticulum Stress-Initiated Sphingolipid Metabolite, Ceramide-1-Phosphate, Regulates Epithelial Innate Immunity by Stimulating β-Defensin Production

Antimicrobial peptides (AMP) are ubiquitous innate immune elements in epithelial tissues. We recently discovered that a signaling lipid, the ceramide metabolite sphingosine-1-phosphate (S1P), regulates production of a major AMP, cathelicidin antimicrobial peptide (CAMP), in response to a subtoxic level of endoplasmic reticulum (ER) stress that can be induced by external perturbants in keratinocytes. We hypothesized that an ER stress-initiated signal could also regulate production of another major class of AMPs: i.e., the human beta-defensins 2 (hBD2) and 3 (hBD3). Keratinocytes stimulated with a pharmacological ER stressor, thapsigargin (Tg), increased hBD2/hBD3 as well as CAMP mRNA expression. While inhibition of sphingosine-1-phosphate production did not alter hBD expression following ER stress, blockade of ceramide-1-phosphate (C1P) suppressed Tg-induced hBD2/hBD3 but not CAMP expression. Exogenous C1P also increased hBD2/hBD3 production, indicating that C1P stimulates hBD expression. We showed further that C1P-induced hBD2/hBD3 expression is regulated by a novel pathway in which C1P stimulates downstream hBD via a cPLA2a→15d-PGJ2→PPARα/PPARβ/δ→Src kinase→STAT1/STAT3 transcriptional mechanism. Finally, conditioned medium from C1P-stimulated keratinocytes showed antimicrobial activity against Staphylococcus aureus. In summary, our present and recent studies discovered two new regulatory mechanisms of key epidermal AMP, hBD2/hBD3 and CAMP. The C1P and S1P pathways both signal to enhance innate immunity in response to ER stress.