Mast Cells and Lipid Cross-Talk in Skin Inflammation

Atopic dermatitis (AD) is an inflammatory skin disease whose pathogenic mechanisms remain unclear. Using a validated human AD-like mouse model, we observed that skin remodeling started at a pre-symptomatic stage of AD that included cellular infiltration of the hypodermis, accompanied with activated/degranulated mast cells (MC). Local MC activation was quantified using a novel method of computer-assisted image analysis we developed and reported. Using a variation of this method, we defined morphometric parameters allowing for quantitation rather than scoring of cellular infiltration. Cell recruitment correlated with MC activation, chemokine production and increased levels of sphingolipid sphingosine-1-phosphate (S1P), produced by sphingosine kinase-1 (SphK1). MC or SphK1 deficiency significantly hindered early AD inflammation. Chronic AD features skin barrier dysfunction leading to skin lesions due to decreased lipid ceramide (CER) content. Interestingly, local CER species C16 and C24 were significantly increased in pre-symptomatic AD. Accordingly, skin CER synthase (CerS, CER synthesis) CerS4, 5 and 6 mRNA mRNA levels were statistically augmented. Because of the overall proapoptotic functions of CER, we next measured local cleaved/activated caspase 3 levels, the executioner caspase in apoptosis. Skin cleaved caspase 3/apoptosis was significantly augmented in early AD and correlated with increased endoplasmic reticulum (ER) stress-related molecular players. To substantiate the importance of MC in ER stress-induced apoptosis, CER profiling was similarly conducted in treated skin samples collected from MC-deficient mice. MC deficiency prevented CER increase and local apoptosis that were restored following MC reconstitution. Epigenetic regulation of the molecular pathways that drive AD is unknown. We identified and validated a microRNA triad 34a-485-486 whose downregulation promoted the AD-related pathogenic pathways we have unraveled. We conclude that MC may initiate AD by driving early skin remodeling and cell recruitment through local chemokine and S1P production and CER-elicited apoptosis. Moreover, the down-regulation of a miRNA triad de-represses these key players of AD pathogenesis. Targeting these pre-symptomatic effector mechanisms may offer new prophylactic strategies for AD whose treatment remains a clinical challenge

Sphingosine-1-Phosphate/Sphingosine-1-Phosphate Receptor 2 Axis Can Promote Mouse and Human Primary Mast Cell Angiogenic Potential through Upregulation of Vascular Endothelial Growth Factor-A and Matrix Metalloproteinase-2

Mast cells (MC) are present in most vascularized tissues around the vasculature likely exerting immunomodulatory functions. Endowed with diverse mediators, resident MC represent first-line fine-tuners of local microenvironment. Sphingosine-1-phosphate (S1P) functions as a pluripotent signaling sphingolipid metabolite in health and disease. S1P formation occurs at low levels in resting MC and is upregulated upon activation. Its export can result in type 2 S1P receptor-(S1PR2-) mediated stimulation of MC, further fueling inflammation. However, the role of S1PR2 ligation in proangiogenic vascular endothelial growth factor-(VEGF-) A and matrix metalloproteinase-(MMP-) 2 release from MC is unknown. Using a preclinical MC-dependent model of acute allergic responses and in vitro stimulated primary mouse bone marrow-derived MC (BMMC) or human primary skin MC, we report that S1P signaling resulted in substantial amount of VEGF-A release. Similar experiments using S1pr2-deficient mice or BMMC or selective S1P receptor agonists or antagonists demonstrated that S1P/S1PR2 ligation on MC is important for VEGF-A secretion. Further, we show that S1P stimulation triggered transcriptional upregulation of VEGF-A and MMP-2 mRNA in human but not in mouse MC. S1P exposure also triggered MMP-2 secretion from human MC. These studies identify a novel proangiogenic axis encompassing MC/S1P/S1PR2 likely relevant to inflammation

Sphingosine-1-Phosphate/Sphingosine-1-Phosphate Receptor 2 Axis Can Promote Mouse and Human Primary Mast Cell Angiogenic Potential through Upregulation of Vascular Endothelial Growth Factor-A and Matrix Metalloproteinase-2

Mast cells (MC) are present in most vascularized tissues around the vasculature likely exerting immunomodulatory functions. Endowed with diverse mediators, resident MC represent first-line fine-tuners of local microenvironment. Sphingosine-1-phosphate (S1P) functions as a pluripotent signaling sphingolipid metabolite in health and disease. S1P formation occurs at low levels in resting MC and is upregulated upon activation. Its export can result in type 2 S1P receptor- (S1PR2-) mediated stimulation of MC, further fueling inflammation. However, the role of S1PR2 ligation in proangiogenic vascular endothelial growth factor- (VEGF-) A and matrix metalloproteinase- (MMP-) 2 release from MC is unknown. Using a preclinical MC-dependent model of acute allergic responses and in vitro stimulated primary mouse bone marrow-derived MC (BMMC) or human primary skin MC, we report that S1P signaling resulted in substantial amount of VEGF-A release. Similar experiments using S1pr2-deficient mice or BMMC or selective S1P receptor agonists or antagonists demonstrated that S1P/S1PR2 ligation on MC is important for VEGF-A secretion. Further, we show that S1P stimulation triggered transcriptional upregulation of VEGF-A and MMP-2 mRNA in human but not in mouse MC. S1P exposure also triggered MMP-2 secretion from human MC. These studies identify a novel proangiogenic axis encompassing MC/S1P/S1PR2 likely relevant to inflammation