Role of Upregulated Mucin 21 in Detached Lung Cancer Cells

The molecular mechanisms underlying metastasis still remain unclear. We previously established a suspension culture using low-attachment culture dishes and demonstrated that cell lines adapted through 2 months suspension culture (termed FL sublines) exhibited higher metastatic potential than their parental counterparts. In this study, we identified the molecules involved in acquiring these phenotypes under low-adhesion conditions. We showed that detached tumor cells in suspension culture formed spheroids that recapitulated tumor cells in the spread-through-air space (STAS), and demonstrated that the anti-adhesion molecule mucin 21 was upregulated in detached lung cancer cells independent of driver mutations. Analyses of both cell lines and primary tumors revealed that mucin 21 is overexpressed in terminal respiratory unit-type lung adenocarcinomas. Mucin 21-knockout cells showed reduced viability and proliferation under adherent and low-attachment conditions, accompanied by enhanced anoikis. Transmission electron microscopy revealed that the intercellular spaces observed in FL sublines during suspension culture were reduced in mucin 21-knockout cells, suggesting impaired acquisition of low adhesive properties. Thus, mucin 21 appears crucial for the survival of terminal respiratory unit-type lung adenocarcinoma cells under both adherent and low-adhesion conditions.