Improved antitumor therapy by dual targeting of estrogen and growth factor receptor signaling in human breast cancer cells

Abstract

637 Background: Breast cancer growth is tightly regulated by interactions between growth factor and estrogen receptors. Dimerization is crucial for activation of HER receptors (EGFR, HER2, HER3, HER4) that contribute to modulation of tumor progression and the failure of antiestrogen therapy. HER2 is the preferred dimerization partner in a process stimulated by the several HER receptor ligands. HER2 antibody (trastuzumab) shows antitumor efficacy in HER2-overexpressing cancers. New antibodies that disrupt HER2 dimerization (pertuzumab) may be useful in treating cancers with normal HER2 levels. Methods: Estrogen receptor (ER)-positive tumor cells with normal HER levels (MCF-7, ZR75), as well as cells engineered for HER2-overexpression (MCF-7/HER2, ZR75/HER2) or tamoxifen resistance (MCF-7/TAM), were grown in vitro and as xenografts in nude mice and treated with tamoxifen, fulvestrant (Faslodex), trastuzumab or pertuzumab, alone or combined. Effects of agents on phosphorylation of ER and coactivator AIB1, ER-dependent transcripts, p27 and VEGF secretion were assessed. Results: Growth of ER-positive tumors with low HER is reduced by tamoxifen and fulvestrant. In contrast, HER2-overexpressing tumors are tamoxifen-resistant but retain sensitivity to fulvestrant and show greater, synergistic antitumor effects with trastuzumab-fulvestrant combined. Similarly, non-HER2-overexpressing tumors with or without tamoxifen resistance are sensitive to fulvestrant, but fulvestrant-pertuzumab elicits more profound antitumor effects (P<0.001). Enhanced inhibitory action of fulvestrant-pertuzumab is related, in part, to changes in phosphorylation of ER and AIB1, leading to blockade of ER-induced transcripts, as well as nuclear localization of p27, a CDK-inhibitor that downregulates cell proliferation. Further, enhanced tumor secretion of VEGF after estrogen and HER signaling is markedly reduced by fulvestrant-pertuzumab as compared with either agent alone (P<0.001). Conclusions: Pertuzumab promotes antitumor efficacy of the pure antiestrogen, fulvestrant, and dual disruption of ER and HER signaling may be a new way to optimize hormonal therapy and overcome antiestrogen resistance. Supported by US Army BCRP and Stiles Program. [Table: see text]