The combination of time and order-dependent chemotherapeutic strategies has demonstrated enhanced efficacy in killing cancer cells while minimizing adverse effects. However, the precise mechanism remains elusive. Our results showed that pre-treatment of MCF-7 and MDA-MB-468 cells with epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor erlotinib or lapatinib significantly enhanced the cytotoxic effects of DNA-damaging agents compared to coadministration of the EGFR inhibitor and DNA-damaging agent. Sequential application of erlotinib and doxorubicin increased activated caspase-8 by promoting pro-caspase-8 homodimerization and autocatalytical cleavage, whereas coadministration did not. We found that EGFR inhibitors promoted pro-caspase-8 homodimerization by inhibiting ERK pathway signaling, while doxorubicin promoted it. Our data highlight that ERK has the potential to inhibit the formation of pro-caspase-8 homodimers by phosphorylating pro-caspase-8 at S387. In conclusion, the pretreatment of EGFR tyrosine kinase inhibitors promote pro-caspase-8 dimerization that sensitizes cancer cells to DNA-damaging agents. Our findings provide rationale for novel strategies for the implementation of combined targeted and cytotoxic chemotherapy within a new framework of time and order-dependent therapy.
Keywords: EGFR inhibitors; breast cancer; caspase-8; doxorubicin; sequential application.