Dual Targeting of EGFR with PLK1 Exerts Therapeutic Synergism in Taxane-Resistant Lung Adenocarcinoma by Suppressing ABC Transporters

SIMPLE SUMMARY: Our previous studies led us to hypothesize that downregulation of PLK1 expression or its activity can overcome the hurdles of taxane resistance by downregulating ABC transporters. Targeting PLK1 with shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in paclitaxel-resistant lung adenocarcinoma (LUAD(TXR)), similar to the downregulation effects from treatment with PLK1 inhibitors. Since EGFR is highly expressed in LUAD(TXR) cells, gefitinib was combined with PLK1 inhibitors. Under these conditions, LUAD(TXR) cells tend to undergo apoptosis more effectively than parental cells, showing a synergistic effect on downregulation of ABC transporters through c-Myc or AP-1. Clinical data provide evidence for the relationship between survival rates and expressions of PLK1 and EGFR in LUAD patients. Taken together, our data suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD(TXR), providing a benefit to overcome the limitations associated with taxanes. ABSTRACT: To overcome the limitations of chemoresistance, combination therapies using druggable targets have been investigated. Our previous studies led us to hypothesize that the downregulation of PLK1 expression or activity can be one strategy to overcome the hurdles of taxane resistance by the downregulation of ABC transporters. To explore this, various versions of PLK1 including a constitutively active version, kinase-dead form, and polo-box domain mutant were expressed in paclitaxel-resistant lung adenocarcinoma (LUAD(TXR)). Targeting PLK1 using shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in LUAD(TXR) cells, which was similar to the downregulation effects from treatment with PLK1 inhibitors. The high expression of EGFR in LUAD led us to administer gefitinib, showing a markedly reduced EGFR level in LUAD(TXR) cells. When gefitinib and PLK1 inhibitors were combined, LUAD(TXR) cells tended to undergo apoptosis more effectively than parental cells, showing a synergistic effect on the downregulation of ABC transporters through c-Myc and AP-1. Clinical data provide evidence for the relevance between survival rates and expressions of PLK1 and EGFR in LUAD patients. Based on these results, we suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD(TXR), which helps to overcome the limitations associated with taxanes.