Alantolactone Activates the Extracellular Signal-Regulated Kinase Signaling Pathway to Promote Tumor Necrosis Factor Receptor 1 Ectodomain Shedding

Alantolactone is a sesquiterpene lactone that possesses anticancer and anti-inflammatory properties. We previously demonstrated that several sesquiterpenes, including alantolactone, induced the ectodomain shedding of tumor necrosis factor receptor 1 (TNF-R1). In the present study, we investigated the upstream signaling pathway underlying alantolactone-induced TNF-R1 ectodomain shedding. Alantolactone down-regulated the expression of full-length TNF-R1 on the cell surface of human lung adenocarcinoma A549 cells, and this was accompanied by an increase in soluble TNF-R1 in the culture medium. TNF-R1 ectodomain shedding was also detected in human embryonic kidney 293T cells and human fibrosarcoma HT-1080 cells, indicating the conservation of this effect in multiple cell lines. The metalloproteinase inhibitor GM6001 markedly suppressed alantolactone-induced soluble TNF-R1 release and restored cell-surface TNF-R1 expression. Among specific inhibitors targeting mitogen-activated protein kinase (MAPK) signaling pathways, TNF-R1 ectodomain shedding was markedly suppressed by the MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor U0126, whereas the suppressive effects of the c-Jun N-terminal kinase (JNK) inhibitor SP600125 or the p38 MAPK inhibitor SB203580 were negligible. Consistent with these results, alantolactone increased phospho-ERK and phospho-RAF1 levels within 60–120 min, while p38 MAPK and JNK were minimally phosphorylated during the 120-min incubation. Collectively, these results indicate that alantolactone-induced TNF-R1 ectodomain shedding is mediated by the activation of the RAF1–ERK signaling pathway.