Macrophage renewal modes affect acquired resistance to gefitinib in EGFR-mutant lung cancer PC-9 cells

EGFR tyrosine kinase inhibitors (TKIs) have been successfully used in lung cancer treatment. Nevertheless, tumor cells may develop resistant phenotypes in the tumor microenvironment after a period of treatment with any generation of EGFR TKIs. Macrophages play a pivotal role in carcinogenesis and development. At the same time, macrophages renew continuously, and their polarized phenotype changes dynamically. Although macrophage polarization is generally considered to be involved in secondary resistance, it would be time consuming if renewed macrophages would only protect tumor cells from drug attacks after polarization. It is unclear how dynamic renewal of macrophage contributes to acquired resistance. By using two co-culture approaches, the present study modeled two types of renewal of macrophages: Migrated macrophages (Mm) and locally self-renewal resident macrophages (Mr). The EGFR-mutant lung cancer cell line PC-9 was induced to develop resistance to gefitinib in these co-culture approaches and without macrophages co-culture. PC-9 cells induced in Mm co-culture recovered fast from gefitinib attack, and formed the gefitinib resistant (GR) subline PC-9/Mm/GR with the strongest resistant ability. PC-9/Mm/GR cells exhibited epithelial characteristics, including stable expression of EGFR, phosphorylated EGFR, insulin-like growth factor 1 receptor, TGF-beta II receptor and E-cadherin. PC-9 cells induced in Mr co-culture and those without co-culture developed similar weak resistance to gefitinib. TGF-beta secretion was inhibited when PC-9 cells were in Mr co-culture, but not in Mm co-culture. Macrophages were not polarized in either co-culture type. The present findings suggested that macrophages may affect resistance acquirement by different renewal modes. Mm could promote resistance acquirement by stabilizing tumor cell phenotypes, which would happen before polarization. It would be helpful to monitor the response to EGFR-TKIs and design novel treatment strategies if macrophages from two renewal modes were distinguished effectively.