Palmitic acid aggravates atopic dermatitis by regulating SGK1/NEDD4L-involved cutaneous neuroimmune inflammation through driving TRPV1 and MRGPRB2 S-palmitoylation

OBJECTIVE: To determine how cutaneous palmitic acid (PA) modulates transient receptor potential vanilloid-1(TRPV1) in nociceptor and dorsal-root-ganglions (DRGs), and Mas-related G protein-coupled receptor B2 (MRGPRB2) in mast cells (MCs), and to investigate their associations with serum- and glucocorticoid-regulated kinase-1 (SGK1)/neural precursor cell expressed developmentally down regulated 4-like (NEDD4L) in atopic dermatitis (AD). METHODS: AD was induced in mice with nedd4l or sgk1 conditional knock-out(cKO) in nociceptor, mrgprb2, nedd4l, or sgk1 cKO in MCs. Intradermal PA, substance P(SP), or pan-palmitoylation inhibitor 2BP was administered. Isolated DRGs and mouse bone-marrow-derived-MCs (mBMMCs) were used. RESULTS: nociceptor-SP-MCs MRGPRB2-tryptase-AD axis. nedd4l cKO in nociceptor up-regulated cutaneous SP expression, which was further enhanced by PA. sgk1 cKO in nociceptor slightly reduced SP levels, which were further decreased by PA or 2BP. SP levels in mice with nedd4l or sgk1 cKO in MCs were increased by PA. In DRGs, supernatants from MC903-treated keratinocytes induced SGK1 and NEDD4L phosphorylation, TRPV1 S-palmitoylation, and SP production, all of which were up-regulated by PA; total and S-palmitoylated TRPV1 levels and SP production were increased following nedd4l knockdown, whereas they were slightly reduced following sgk1 knockdown and further decreased by PA. SP induced weak phosphorylation of SGK1 and NEDD4L in MCs. SP induced MRGPRB2 S-palmitoylation and tryptase release in wild-type, nedd4l or sgk1 knock-out MCs, and these effects were enhanced by PA; 2BP caused MRGPRB2 reduction in wild-type and sgk1 knock-out MCs. CONCLUSIONS: The increased cutaneous PA exacerbates AD by promoting TRPV1 S-palmitoylation and SP production in nociceptor, followed by MRGPRB2 S-palmitoylation and tryptase release in MCs. S-palmitoylation promotes TRPV1 whereas inhibits MRGPRB2 reduction via lysosome when NEDD4L and its upstream SGK1 are not phosphorylated.