Cucurbitacin B Promotes Tumor Necrosis Factor Receptor 1 Ectodomain Shedding by Selectively Activating the Extracellular Signal-Regulated Kinase Signaling Pathway

Cucurbitacin B belongs to a group of tetracyclic triterpenoids and exerts a number of biological effects, including anti-inflammatory and anticancer activities. We previously demonstrated that cucurbitacin B down-regulated tumor necrosis factor (TNF) receptor 1 (TNF-R1) expression and prevented activation of the transcription factor nuclear factor κB in response to a TNF-α stimulation. The present study shows that cucurbitacin B promoted the ectodomain shedding of TNF-R1 by generating a soluble form that accumulated in the culture medium of human lung adenocarcinoma A549 cells. Of the eight tetracyclic and pentacyclic triterpenoids consisting of an α,β-unsaturated carbonyl group that were examined, only cucurbitacin B promoted TNF-R1 ectodomain shedding. Cucurbitacin B-induced TNF-R1 shedding was attenuated by TNF-α protease inhibitor 2 and the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor U0126, but not by the p38 MAPK inhibitor SB203580 or the c-Jun N-terminal kinase (JNK) inhibitor SP600125. Consistent with these results, cucurbitacin B promoted the rapid phosphorylation of rapidly accelerated fibrosarcoma 1 (RAF1) and ERK, but exerted minimal effects on the phosphorylation of p38 MAPK and JNK. Collectively, these results demonstrate that cucurbitacin B selectively activated the RAF1-MEK-ERK pathway, which was essential for TNF-R1 ectodomain shedding.