Molecular signatures of triple-negative breast cancer cells acquiring palbociclib resistance via continuous exposure.

Palbociclib, a cyclin-dependent kinase (CDK)-4/6 inhibitor, exhibits therapeutic potential for triple-negative breast cancer (TNBC), for which effective treatments remain limited. However, understanding the mechanisms underlying drug resistance is essential for its clinical application. Although the mechanisms underlying resistance to CDK4/6 inhibitors in hormone receptor-positive breast cancer have been investigated, the mechanisms by which continuous drug exposure induces resistance in TNBC cells remain unclear. Therefore, the present study aimed to establish palbociclib-resistant TNBC cells using a continuous drug exposure model and to elucidate their characteristics. MDA-MB-231 cells, a human TNBC cell type with wild-type retinoblastoma (Rb) protein, were continuously exposed to palbociclib at gradually increasing concentrations (0.01-1 µM) to establish resistant cells, which were named MB231/PalR cells. Drug sensitivity was subsequently evaluated using cell viability assays; gene and protein expression levels were analyzed by quantitative polymerase chain reaction and western blotting, respectively; and P-glycoprotein-mediated efflux capacity was evaluated using rhodamine 123 staining with a flow cytometer. Compared with parental cells, MB231/PalR cells exhibited reduced sensitivity to palbociclib and abemaciclib. Rb protein expression levels were decreased, without affecting RB1 mRNA expression, in MB231/PalR cells. The expression levels of CCNE1, CDK6, ATP-binding cassette (ABC)B1 mRNA and cyclin E1 protein were increased, whereas those of ABCG2 and CD274 mRNA were decreased in resistant cells. Furthermore, MB231/PalR cells exhibited reduced sensitivity to the CDK2 inhibitor CVT-313, and co-treatment with CVT-313 failed to restore palbociclib sensitivity. Although P-glycoprotein efflux capacity and ABCB1 mRNA expression were increased in resistant cells, verapamil treatment did not affect palbociclib sensitivity. In conclusion, continuous exposure of MDA-MB-231 cells to palbociclib induced alterations associated with CDK4/6 inhibitor action, resulting in drug resistance. These findings provide mechanistic insights into CDK4/6 inhibitor resistance in TNBC and may support the development of biomarkers and therapeutic strategies to overcome or prevent resistance.