Activated estrogen receptor-mitogen-activated protein kinases cross talk confer acquired resistance to lapatinib.

Background: The efficacy of lapatinib is limited by the development of acquired resistance. The aim of this study was to investigate the role of estrogen receptor (ER) signaling compensatory activation in acquired resistance to lapatinib in breast cancer cells BT474 and the related mechanism.

Methods: Acquired resistant cell model resistant (r)BT474 was generated with an increasing concentration of lapatinib. Real-time polymerase chain reaction and Western blotting were used to determine the changes of human epidermal growth factor receptor (HER)2 and ER pathways in breast cancer cell BT474 after treatment with lapatinib and the distinction between BT474 and rBT474. Methyl thiazolyl tetrazolium and colony formation assays were employed to detect the proliferation of rBT474 and BT474 cells treated with lapatinib and/or an ER inhibitor, fulvestrant, respectively.

Results: Lapatinib could inhibit phosphorylation of HER2 and induce expression of forkhead-box protein O3a and progesterone receptor. Acquired resistant cell model rBT474 could grow in the presence of 5 μM lapatinib, with an apoptosis rate of only 5%. Significant inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT) pathway and the activation of the mitogen-activated protein kinases (MAPK) and ER pathways were detected in rBT474, compared with BT474. Furthermore, the expressions of Src phosphorylation and caveolin-1 were also upregulated. The viability of rBT474 was markedly suppressed by the lapatinib/fulvestrant combination in vitro, confirmed by the BT474 xenograft model.

Conclusion: ER signaling compensatory activation may partly contribute to lapatinib acquired resistance in HER2-overexpressing/ERα-positive breast cancer cells, which might be related to PI3K/AKT inhibition and MAPK pathway activation.