Abstract. Mutant KRAS and BRAF are associated with primary EGFR inhibitor resistance in colorectal cancer (CRC). However, other biomarkers that could predict EGFR inhibitor resistance remain elusive. In the present study, immunoblotting and cell proliferation results revealed that yes-associated pro

Mutant KRAS and BRAF are associated with primary EGFR inhibitor resistance in colorectal cancer (CRC). However, other biomarkers that could predict EGFR inhibitor resistance remain elusive. In the present study, immunoblotting and cell proliferation results revealed that yes-associated protein (YAP), a downstream effector of the Hippo pathway, was positively associated with primary cetuximab resistance in CRC cells. YAP knockdown enhanced the cytotoxicity of cetuximab in CRC cells. Simvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor of the mevalonate pathway that inhibits YAP bioactivity through nuclear translocation and total YAP expression, increased the cytotoxicity of EGFR inhibitors (cetuximab and gefitinib) against CRC cells. The combination of simvastatin and EGFR inhibitors inhibited YAP and EGFR signaling more markedly than each agent alone. Adding back geranylgeranyl pyrophosphate (GGPP), a key product of the mevalonate pathway, reversed the YAP bioactivity inhibition induced by simvastatin and the cell proliferation inhibition induced by the combination of simvastatin and EGFR inhibitors. Collectively, these results revealed that YAP may be useful in identifying cetuximab resistance in CRC and indicated that targeting of both YAP and EGFR signals may present a promising therapeutic approach for CRC.