Rhododendrin inhibits toll-like receptor-7-mediated psoriasis-like skin inflammation in mice.

Many active compounds present in Rhododendron brachycarpum have been used in traditional Oriental medicine for the treatment of various skin diseases. However, the precise mechanism of action of the compounds isolated from R. brachycarpum and their relevance as therapeutics for the treatment of psoriasis remain elusive. In this study, we report that rhododendrin isolated from R. brachycarpum strongly inhibits imiquimod (IMQ)-induced psoriasis-like skin inflammation in mice. We showed that topical treatment with rhododendrin reduces IMQ-induced skin hyperplasia, inflammatory mononuclear cell infiltration and the expression of pro-inflammatory mediators in mouse skin. In addition, we found that rhododendrin inhibits the activation of the TLR-7/NF-κB and mitogen-activated protein kinase pathways in both IMQ-induced psoriasis-like skin inflammation in mice and in normal human epidermal keratinocytes treated with IMQ. These results suggest that rhododendrin has an anti-inflammatory effect and can be used as a therapeutic to fight against psoriasis and other inflammatory skin diseases.

Rhododendrin ameliorates skin inflammation through inhibition of NF-κB, MAPK, and PI3K/Akt signaling.

A wide range of active compounds isolated from nature is used in clinical applications and as a source of lead compounds for drug development. Rhododendron brachycarpum has been used as an oriental herbal medicine for skin inflammatory diseases. In this study, we isolated rhododendrin from Rhododendron brachycarpum leaves and investigated its molecular mechanisms for anti-inflammatory effect. Rhododendrin showed intracellular reactive oxygen species scavenging activity and suppressed nuclear translocation of nuclear factor-κB (NF-κB) by inhibiting phosphorylation of NF-κB, inhibitor of NF-κB(IκBα), and IκBα kinase(IKKα/ß). Furthermore, rhododendrin inhibited mitogen-activated protein kinases (MAPKs), including ERK1/2, p38, and decreased c-Jun N-terminal kinase (JNK) and phosphoinositide 3-kinase (PI3K)/Akt signaling. As a result, rhododendrin reduced expression of pro-inflammatory mediators, such as cyclooxygenase-2 (COX-2), intracellular adhesion molecule-1 (ICAM-1), interleukin-1α (IL-1α), IL-1ß, IL-6, IL-8, tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), chemokine (C-X-C) motif ligand 1 (CXCL1), and chemokine (C-C motif) ligand 17 (CCL17) in TNF-α/IFN-γ-stimulated keratinocytes. Notably, we demonstrated that topically applied rhododendrin alleviated skin inflammation in trinitrochlorobenzene (TNCB)-treated mouse ear skins. Collectively, these results indicate that rhododendrin is a biologically active compound that exhibits anti-inflammatory activity and is a promising candidate molecule to treat inflammatory skin diseases, such as psoriasis.