Topical β-hydroxybutyrate suppresses allergic dermatitis via HCAR2-CaMKK-AMPK-mediated barrier protection

Ketone bodies, particularly β-hydroxybutyrate (BHB), have attracted attention for their anti-inflammatory and immunomodulatory properties. We previously demonstrated that nutritional interventions elevating circulating BHB levels suppress immediate-type allergic responses in dinitrofluorobenzene (DNFB)-induced allergic contact dermatitis (ACD). However, whether this effect is mediated by systemic BHB or via direct local activity in the skin remains unclear. Here, we investigated the effect of topical BHB application on mast cell degranulation and skin barrier function during an allergen challenge. Pretreatment with BHB on auricular skin 2 h before DNFB challenge significantly reduced ear swelling and mast cell degranulation. This suppressive effect was abrogated by mepenzolate bromide, a hydroxycarboxylic acid receptor 2 (HCAR2) antagonist, and was also reversed by dorsomorphin, a direct AMPK inhibitor, and by STO-609, a CaMKK inhibitor. Consistently, topical BHB enhanced AMPK phosphorylation in the epidermis, particularly in keratinocyte layers, whereas this effect was attenuated by HCAR2 or AMPK blockade. Moreover, BHB pretreatment preserved the continuous localization of tight junction proteins Zo-1 and occludin, preventing DNFB-induced barrier disruption. These findings demonstrate that topical BHB exerts a local anti-allergic effect through HCAR2-dependent AMPK activation and barrier preservation. Our results highlight topical BHB as a potential non-steroidal strategy for protecting the skin against immediate allergic responses via HCAR2-CaMKK-AMPK-mediated barrier preservation.