Fisetin activates Hippo pathway and JNK/ERK/AP‐1 signaling to inhibit proliferation and induce apoptosis of human osteosarcoma cells via ZAK overexpression

Osteosarcoma (OS) is a tumor entity that can cause a large number of cancer‐related deaths. Although chemotherapy can decrease proliferation and increase apoptosis of human OS cells, the clinical prognosis remains poor. Fisetin is a flavonol found in fruits and vegetables and is reported to inhibit cell growth in numerous cancers. But the molecular mechanism underlying fisetin in human OS cells is not clear. It is known that sterile‐alpha motif and leucine zipper containing kinase (ZAK), a kinase in the MAP3K family, is involved in various cell processes, including proliferation and apoptosis. In our lab, we have demonstrated that overexpression of ZAK can induce apoptosis in human OS cells. In the previous studies, MAP4K, the upstream of MAP3K, can act in parallel to MST1/2 to activate LATS1/2 in the Hippo pathway. Turning on the Hippo pathway can decrease proliferation and otherwise cause cell apoptosis in cancer cells. In this study, we found that fisetin can upregulate ZAK expression to induce the Hippo pathway and mediate the activation of JNK/ERK, the downstream of ZAK, to trigger cell apoptosis via AP‐1 dependent manner in human OS cells. These findings reveal a novel molecular mechanism underlying fisetin effect on human OS cells.