Mechanisms underlying the inhibitory actions of the pentacyclic triterpene α-amyrin in the mouse skin inflammation induced by phorbol ester 12-O-tetradecanoylphorbol-13-acetate

The present study evaluated some of the mechanisms through which α-amyrin, a pentacyclic triterpene isolated from Protium Kleinii and other plants, exerts its effects against 12-O-tetradecanoylphorbol-acetate (TPA)-induced skin inflammation in mice. Topical application of α-amyrin (0.1–1 mg/ear) dose-dependently inhibited TPA-induced increase of prostaglandin E2 (PGE2) levels. In contrast with the selective cyclooxygenase (COX)-1 SC560 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole] or COX-2 rofecoxib inhibitors, α-amyrin failed to alter either COX-1 or COX-2 activities in vitro. Western blot analysis revealed that α-amyrin dose-dependently inhibited TPA-induced COX-2 expression in the mouse skin. The evaluation of nuclear factor-κB (NF-κB) pathway revealed that topical treatment with α-amyrin is able to prevent IκBα degradation, p65/RelA phosphorylation and NF-κB activation. Moreover, α-amyrin given topically dose-dependently inhibited the activation of upstream protein kinases, namely extracellular signal-regulated protein kinase (ERK), p38 mitogen-activated protein kinase (MAPK) and protein kinase C (PKC)α, following topical TPA treatment. Collectively, present results suggest that topical skin application of α-amyrin exerts a strong and rapid onset inhibition of TPA-induced inflammation. These effects seem to be associated with the suppression of skin PGE2 levels by mechanisms involving the suppression of COX-2 expression, via inhibition of upstream protein kinases – namely ERK, p38 MAPK and PKCα – and blocking of NF-κB activation. These results indicate that α-amyrin-derivative could be potentially relevant for the development of a topical agent for the management of inflammatory diseases.