Abstract. Background Trastuzumab, also known as Herceptin, is a monoclonal antibody that interrupts HER2-mediated downstream signaling by various mechanisms (1-3). In combination with chemotherapy, trastuzumab has been shown to increase overall survival in HER2 + breast cancer patients (4). However, only 30% of all HER2 + breast cancer patients respond to trastuzumab, and often duration of response to trastuzumab lasts only 5 to 9 months, indicating that both primary and acquired resistance to trastuzumab is common. Combination therapies with lapatinib and gefitinib have also been fraught with resistance arising from compensatory signaling molecules or pathways (1, 5). There is, therefore, a need for studying other effectors or modulators that influence the durability of response and/or by-pass resistance to HER2-targeted therapies.Several ETS transcription factors, such as ETS-1, ESE-1/Elf-3, ESE-2/Elf5 and PEA-3, appear to be important in human breast cancer. ESE-1 is particularly relevant in HER2 + breast cancer, because ESE-1 regulates the HER2 + promoter activity and protein levels (6). In parental BT474 and SKBR3 cells, ESE-1 controls HER2 dependent signaling and tumorigenesis (7). Moreover, neuregulin and other growth factor ligands induce ESE-1 expression (8), revealing an additional feedforward level of control of ESE-1 and downstream effectors targeted by trastuzumab.In this paper, we investigated the role of ESE-1 in controlling transformation in trastuzumab-resistant HER2 -positive cell lines derived from parental HER2 + , ER + BT474 and HER2 + , ER -SKBR3 breast cancer cell lines. These trastuzumab-resistant cell lines develop an interaction between HER2, HER3/ErbB3 and IGF-1R to form a hetero-trimeric receptor signaling complex as the mechanism mediating trastuzumab resistance (9). Investigating ESE-1's role in the context of trastuzumab-resistance showed that knockdown of ESE-1 inhibits proliferation, clonogenicity and anchorageindependent growth in both resistant cell lines specifically by modulating several signaling molecules. Furthermore, lack of any synergistic inhibitory response of trastuzumab plus ESE-1 knockdown in the parental lines revealed that HER2 and ESE-1 function in the same pathway. Taken together, these studies highlight ESE-1 as a by-pass effector in HER2 resistance and establish the utility of pursuing ESE-1 as a future therapeutic target to inhibit the counter-regulatory responses to trastuzumab-mediated tumor inhibition.
Materials and MethodsCell lines and cell culture. Cell lines BT474 and SKBR3 were purchased from the tissue culture core at University of Colorado Anschutz Medical Campus and the trastuzumab resistant cell lines HR20 and Pool2 were provided by Dr. Bolin Liu. All cell lines were 6583 This Īrticle is freely accessible online.