Abstract 4611: Growth inhibitory effects of lapatinib on letrozole resistant breast cancer cells via HER2-AIB1-ERα crosstalk
Authors: Kazi Armina A. , Gilani Rabia A. , Sabnis Gauri J. , Brodie Angela H. 
Published: February 2011

Abstract
Aromatase inhibitors (AI), such as letrozole, provide effective endocrine therapy for estrogen receptor positive (ER+) breast cancers, but not all patients respond to AIs and those that do may become resistant. Thus, it is clinically important to determine the factors and signaling mechanisms involved in AI resistance and provide potential therapeutic strategies to overcome and/or prevent it. Previous work from this laboratory comparing long-term letrozole treated MCF-7Ca breast cancer (LTLTCa) cells resistant to growth inhibition by letrozole with their parental letrozole-sensitive MCF-7Ca cells, indicated that AI resistance involves a switch from estrogen (E2)-ERα dependent to HER2/MAPK pathway-dependent growth and that inhibition of HER2/MAPK pathway signaling via trastuzumab can restore ERα levels and letrozole sensitivity. We observed that the upregulation of HER2 is through protein stabilization and not gene amplification. This current study further investigates the factors and mechanism(s) involved in letrozole resistance in LTLTCa cells, specifically steroid receptor coactivator Amplified in Breast Cancer-1 (AIB1/SRC-3). AIB1 is often overexpressed in breast cancer cells, is associated with poor prognosis, and has been linked with both E2-ERα and HER2 signaling. Indeed, western blot analyses showed that LTLTCa cells have 2.8-fold higher AIB1 protein levels than MCF-7Ca cells (p &lt; 0.05, n= 3). In order to determine how AIB1 is involved in the cross talk between HER2 and ERα signaling in LTLTCa cells, the effect of HER1/HER2 kinase inhibitor lapatinib (Lap) on LTLTCa cells. As expected, Lap inhibited cell proliferation of LTLTCa cells in MTT assays (IC50= 1 µM; n= 3). Additional experiments were then conducted comparing vehicle- and 1 µM Lap-treated LTLTCa cells after 24 h (n= 3). Lap had no effect on total HER2 expression, but it did decrease HER2 downstream activation of MAPK and PI3K/Akt pathways, as indicated by decreased protein expression of phosphorylated forms of ERK1/2 by 98.9% (p &lt; 0.001) and of Akt by 55% (p &lt; 0.01), respectively. Concurrently, AIB1 protein levels in LTLTCa cells also decreased by 48.0% (p &lt; 0.05). Lastly, experiments were done to investigate the link between HER2/AIB1/ERα signaling with resistance of LTLTCa cells to letrozole growth inhibition. Compared with vehicle-treated cells, Lap-treated LTLTCa cells had decreased protein expression of cyclin D1 by 65.7% (p &lt; 0.01) and of transporter breast cancer resistance protein (BCRP) by 45.2% (p &lt; 0.05). Cyclin D1 and BCRP are involved in cell proliferation and drug efflux, respectively, and both are known to be regulated by HER2, AIB1, and/or ERα. Overall, these preliminary results suggest that AIB1 plays a role in AI resistance in LTLTCa cells via crosstalk with HER2 and ERα, and that it can be therapeutically targeted by lapatinib.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4611.