A Novel Role of a Lipid Species, Sphingosine-1-Phosphate, in Epithelial Innate Immunity

Abstract

A variety of external perturbations can induce endoplasmic reticulum (ER) stress, followed by stimulation of epithelial cells to produce an innate immune element, the cathelicidin antimicrobial peptide (CAMP). ER stress also increases production of the proapoptotic lipid ceramide and its antiapoptotic metabolite, sphingosine-1-phosphate (S1P). We demonstrate here that S1P mediates ER stress-induced CAMP generation. Cellular ceramide and S1P levels rose in parallel with CAMP levels following addition of either exogenous cell-permeating ceramide (C2Cer), which increases S1P production, or thapsigargin (an ER stressor), applied to cultured human skin keratinocytes or topically to mouse skin. Knockdown of S1P lyase, which catabolizes S1P, enhanced ER stress-induced CAMP production in cultured cells and mouse skin. These and additional inhibitor studies show that S1P is responsible for ER stress-induced upregulation of CAMP expression. Increased CAMP expression is likely mediated via S1P-dependent NF-B-C/EBP␣ activation. Finally, lysates of both ER-stressed and S1P-stimulated cells blocked growth of virulent Staphylococcus aureus in vitro, and topical C2Cer and LL-37 inhibited invasion of Staphylococcus aureus into murine skin. These studies suggest that S1P generation resulting in increased CAMP production comprises a novel regulatory mechanism of epithelial innate immune responses to external perturbations, pointing to a new therapeutic approach to enhance antimicrobial defense.

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

“…The latter downstream metabolite then stimulates production of a major AMP (i.e., CAMP [7]). While VDR-dependent stimulation of CAMP production has been well characterized (46), we found that VDR transactivation instead is suppressed in cells under similar ERstressed conditions, suggesting that VDR-and S1P-dependent pathways maintain CAMP production under basal versus stressed conditions, respectively (8). We now show here that another ER stress-induced Cer metabolite, C1P, upregulates two other key AMP (i.e., hBD2 and hBD3) in epithelial cells exposed to similar conditions.…”

“…When investigating how these perturbants stimulate AMP production, we recently found that these various unrelated forms of external perturbations induce ER stress, which in turn increases production of Cer and S1P (8). The latter downstream metabolite then stimulates production of a major AMP (i.e., CAMP [7]).…”

“…Instead, VDR-induced transactivity is suppressed under the same ER-stressed conditions (7). We then identified the ER stress-induced signal: i.e., increased production of ceramide (Cer) and its metabolite, sphingosine-1-phosphate (S1P), which serves as an upstream activator of NF-B in epithelial cells (8). Thus, VDRand S1P-dependent mechanisms likely regulate CAMP production under basal and stressed conditions, respectively.…”

“…Normal human keratinocytes (KC), isolated from neonatal foreskins, were grown in serum-free KC growth medium containing 0.07 mM calcium chloride and growth supplements (Invitrogen, Carlsbad, CA), as described previously (7). Culture media were switched to a medium containing 0.07 mM calcium chloride, without growth supplements, prior to treatment with Tg, C2Cer, or C16C1P, as described previously (8).…”

Antimicrobial lipids: Emerging effector molecules of innate host defense

“…These inflammation resolving lipids are derivatives of the essential omega-6 and omega-3 PUFAs and include resolvins (coined after their inflammation resolving function), lipoxins, protectins and maresins [16][17][18] . Moreover, there is new evidence that lipids may also trigger increased antimicrobial peptide production as shown for the sphingolipid S1P which increased CAMP production [19] , or for sebum FFA which induced beta-defensin production [18] . However, lipids can also exert direct antimicrobial activity, which is not only supported by in vitro testing but also by the association of some infectious diseases with defects in lipid metabolism.…”