Abstract P4-02-20: Plasticity of the ER and HER2 Signaling Networks Results in Compensatory Escape Pathways Contributing to Treatment Resistance
Authors: Fu X , Wang Y-C , Huang C , Ward RM , Botero MF , Shea M , Hilsenbeck SG , Osborne CK , Schiff R. 
Published: April 2011

Abstract
Bidirectional crosstalk between the estrogen receptor (ER) and HER2 has been demonstrated in preclinical model systems. We have previously shown in HER2+ breast cancer cells with low ER that potent HER2- targeted therapy results in ER reactivation that functions as an escape pathway to evade therapeutic inhibition. Here we investigated the role of HER pathway components in resistance to endocrine therapies using UACC812 cells, which are endogenously amplified for HER2. Endocrine resistant (R) derivatives of the UACC812 line were developed through long-term (&gt;6 months) culturing with estrogen deprivation (ED) alone or with 4-OH-tamoxifen (Tam, 10-7M). Endocrine treatment [continued ED, estrogen (E2, 10-9M), Tam (10-7M), or fulvestrant (Ful, 10-7M)] of parental cells slowed cell growth compared to estrogen treatment (P&lt;0.01). EDR and TamR derivatives treated with any of the endocrine therapies, however, exhibited the same growth rates as with estrogen, validating the completely resistant phenotype of our clones. By immunoblot analysis, the resistant derivatives, TamR and EDR, showed a striking and complete loss of ER expression and its downstream gene products including PR, IGF-1R, and IRS-1. In contrast, both resistant lines exhibited significant upregulation of HER pathway components including marked increases in total and phosphorylated (p) EGFR, further upregulation of pHER2, and marked increases in the downstream signaling moieties pMAPK, pAkt, pFAK, pSrc, and pFOXO3a. In contrast, levels of pHER3 were markedly reduced at resistance. Since our in vitro data suggested that ER and HER2 compensate for inhibition of each other's pathways, we next examined whether simultaneous inhibition of both pathways can circumvent the development of resistance in an in vivo xenograft model. Athymic nude mice bearing UACC812 xenografts, established in the presence of E2 supplementation, were randomized to continued E2, endocrine therapies alone (ED, Tam, or Ful), anti-HER2 alone [lapatinib (L), trastuzumab (T), or L+T], or the combination of endocrine and anti-HER2 therapies (ED with L, T, or L+T). We found that tumors were stimulated by E2, as well as by Tam (as a mechanism of de novo resistance), while still being inhibited, though only temporarily (2-3 months), by ED and Ful. Anti-HER2 therapy alone only slowed down (L or T) or stabilized (L+T) tumor growth. In contrast, in the presence of both ED and anti-HER2 therapies, most tumors completely regressed. However, only the potent L+T plus ED regimen achieved continuous long term (&gt;200 days) complete regression in all mice. Our results further illustrate the plasticity of the ER and HER2 pathways and provide additional support for the role of the HER network, especially EGFR, in resistance to endocrine therapy in ER+/HER2+ breast cancer. The data emphasize the need for complete HER blockade for optimal therapy.
Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-02-20.