Targeting focal adhesion kinase in ER+/HER2+ breast cancer improves trastuzumab response.

The HER2 transmembrane receptor is a well-characterised predictive marker for trastuzumab benefit and may be associated with decreased benefit from endocrine therapy use. Despite the clinical effectiveness of anti-HER2 agents in such cases, resistance represents a significant limiting factor. Focal adhesion kinase (FAK) plays an important role in HER2 signalling, mediating downstream Akt activation in addition to HER2 cross talk with other growth factor receptors. In this study, we investigated the therapeutic potential of FAK in oestrogen receptor-positive (ER+)/HER2+ breast cancer using the novel FAK-specific inhibitor PF4554878 ('PF878'). The activation of the FAK/HER2 signalling pathway was assessed in ER+/HER2- (MCF7 and T47D) and ER+/HER2+ (BT-474 and MDAMB361) breast cancer cells in the presence or absence of PF878 and PF878±trastuzumab. The effects of PF878 on cell growth as a monotherapy and in combination with trastuzumab were assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and Coulter counting with isobologram analysis to determine synergy/additive effects. FAK activation (at Y861 but not at Y397) was highest in ER+/HER2+ cells, which also demonstrated the greatest sensitivity to PF878. As a monotherapy, PF878 prevented heregulin-induced MDA361 cell migration, but had no significant effect on cell growth. The treatment of ER+/HER2+ cells with PF878 and trastuzumab in combination resulted in the synergistic inhibition of cell proliferation. Underlying this was an abrogation of Akt activity and increased poly(ADP-ribose) polymerase cleavage, effects that were greatest in trastuzumab-refractory MDA361 cells. Collectively, these data support a role for FAK in ER+/HER2+ breast cancer, where its targeting has the potential to improve trastuzumab response. This is particularly important in the context of ER+/HER2+, trastuzumab-refractory disease, where FAK inhibition may present an important strategy to restore trastuzumab sensitivity.

Overexpression of CD44 accompanies acquired tamoxifen resistance in MCF7 cells and augments their sensitivity to the stromal factors, heregulin and hyaluronan.

BACKGROUND: Acquired resistance to endocrine therapy in breast cancer is a significant problem with relapse being associated with local and/or regional recurrence and frequent distant metastases. Breast cancer cell models reveal that endocrine resistance is accompanied by a gain in aggressive behaviour driven in part through altered growth factor receptor signalling, particularly involving erbB family receptors. Recently we identified that CD44, a transmembrane cell adhesion receptor known to interact with growth factor receptors, is upregulated in tamoxifen-resistant (TamR) MCF7 breast cancer cells. The purpose of this study was to explore the consequences of CD44 upregulation in an MCF7 cell model of acquired tamoxifen resistance, specifically with respect to the hypothesis that CD44 may influence erbB activity to promote an adverse phenotype. METHODS: CD44 expression in MCF7 and TamR cells was assessed by RT-PCR, Western blotting and immunocytochemistry. Immunofluorescence and immunoprecipitation studies revealed CD44-erbB associations. TamR cells (± siRNA-mediated CD44 suppression) or MCF7 cells (± transfection with the CD44 gene) were treated with the CD44 ligand, hyaluronon (HA), or heregulin and their in vitro growth (MTT), migration (Boyden chamber and wound healing) and invasion (Matrigel transwell migration) determined. erbB signalling was assessed using Western blotting. The effect of HA on erbB family dimerisation in TamR cells was determined by immunoprecipitation in the presence or absence of CD44 siRNA. RESULTS: TamR cells overexpressed CD44 where it was seen to associate with erbB2 at the cell surface. siRNA-mediated suppression of CD44 in TamR cells significantly attenuated their response to heregulin, inhibiting heregulin-induced cell migration and invasion. Furthermore, TamR cells exhibited enhanced sensitivity to HA, with HA treatment resulting in modulation of erbB dimerisation, ligand-independent activation of erbB2 and EGFR and induction of cell migration. Overexpression of CD44 in MCF7 cells, which lack endogenous CD44, generated an HA-sensitive phenotype, with HA-stimulation promoting erbB/EGFR activation and migration. CONCLUSIONS: These data suggest an important role for CD44 in the context of tamoxifen-resistance where it may augment cellular response to erbB ligands and HA, factors that are reported to be present within the tumour microenvironment in vivo. Thus CD44 may present an important determinant of breast cancer progression in the setting of endocrine resistance.

Fulvestrant-induced expression of ErbB3 and ErbB4 receptors sensitizes oestrogen receptor-positive breast cancer cells to heregulin ß1.

INTRODUCTION: We have previously reported that induction of epidermal growth factor receptor and ErbB2 in response to antihormonal agents may provide an early mechanism to allow breast cancer cells to evade the growth-inhibitory action of such therapies and ultimately drive resistant cell growth. More recently, the other two members of the ErbB receptor family, ErbB3 and ErbB4, have been implicated in antihormone resistance in breast cancer. In the present study, we have investigated whether induction of ErbB3 and/or ErbB4 may provide an alternative resistance mechanism to antihormonal action in a panel of four oestrogen receptor (ER)-positive breast cancer cell lines. METHODS: MCF-7, T47D, BT474 and MDAMB361 cell lines were exposed to fulvestrant (100 nM) for seven days, and effects on ErbB3/4 expression and signalling, as well as on cell growth, were assessed. Effects of heregulin ß1 (HRGß1) were also examined in the absence and presence of fulvestrant to determine the impact of ER blockade on the capacity of this ErbB3/4 ligand to promote signalling and cell proliferation. RESULTS: Fulvestrant potently reduced ER expression and transcriptional activity and significantly inhibited growth in MCF-7, T47D, BT474 and MDAMB361 cells. However, alongside this inhibitory activity, fulvestrant also consistently induced protein expression and activity of ErbB3 in MCF-7 and T47D cells and ErbB4 in BT474 and MDAMB361 cell lines. Consequently, fulvestrant treatment sensitised all cell lines to the actions of the ErbB3/4 ligand HRGß1 with enhanced ErbB3/4-driven signalling activity, reexpression of cyclin D1 and significant increases in cell proliferation being observed when compared to untreated cells. Indeed, in T47D and MDAMB361 HRGß1 was converted from a ligand having negligible or suppressive growth activity into one that potently promoted cell proliferation. Consequently, fulvestrant-mediated growth inhibition was completely overridden by HRGß1 in all four cell lines. CONCLUSIONS: These findings suggest that although antihormones such as fulvestrant may have potent acute growth-inhibitory activity in ER-positive breast cancer cells, their ability to induce and sensitise cells to growth factors may serve to reduce and ultimately limit their inhibitory activity.

Antihormone induced compensatory signalling in breast cancer: an adverse event in the development of endocrine resistance.

Using MCF7 breast cancer cells, it has been shown that antihormones promote expression/activity of oestrogen-repressed tyrosine kinases, notably EGFR, HER2 and Src. These inductive events confer responsiveness to targeted inhibitors (e.g., gefitinib, trastuzumab, saracatinib). We observed that these antihormone-induced phenomena are common to ER+HER2- and ER+HER2+ breast cancer models in vitro, where targeting of EGFR, HER2 or Src alongside antihormone improves antitumour response and delays/prevents endocrine resistance. Such targeted inhibitors also subvert acquired endocrine resistant cells which retain increased EGFR, HER2 and Src (e.g., TAMR and FASR models derived after 6-12 months of tamoxifen or Faslodex treatment). Thus, antihormone-induced tyrosine kinases comprise "compensatory signalling" crucial in limiting maximal initial antihormone response and subsequently driving acquired resistance in vitro. However, despite such convincing preclinical findings from our group and others, clinical trials examining equivalent antigrowth factor strategies have proved relatively disappointing. Our new studies deciphering underlying causes reveal that further antihormone-promoted events could be pivotal in vivo. Firstly, Faslodex induces HER3 and HER4 which sensitise ER+ cells to heregulin, a paracrine growth factor that overcomes endocrine response and diminishes antitumour effect of agents targeting EGFR, HER2 or Src alongside antihormone. Secondly, extended antihormone exposure (experienced by ER+ cells prior to adjuvant clinical relapse) can "reprogramme" the compensatory kinase profile in vitro, hindering candidate antigrowth factor targeting of endocrine resistance. Faslodex resistant cells maintained with this antihormone for 3 years in vitro lose EGFR/HER2 dependency, gaining alternative mitogenic/invasion kinases. Deciphering these previously unrecognised antihormone-induced events could provide superior treatments to control endocrine relapse in the clinic.

Characterization of a transplantable hormone-responsive human prostatic cancer xenograft TEN12 and its androgen-resistant sublines.

BACKGROUND: Models for human prostate cancer can facilitate the study of resistance to endocrine therapy, aid drug discovery, and pre-clinical assessment. METHODS: Characteristics thought relevant to the growth in athymic nude mice of TEN12, an androgen-dependent transplantable prostatic cell line derived from a primary prostate carcinoma, and its two androgen-independent sublines, TEN12F and TEN12C, have been assessed immunocytochemically. RESULTS: The xenografts of the parental TEN12 line are moderately differentiated with both papillary and glandular regions, pleomorphic nuclei and abundant mitotic figures and are extremely vascular. The cells express androgen receptor (AR), PSA, VEGF, EGFR, c-erbB2, and TGFalpha. Both TEN12F and TEN12C xenografts possessed a more anaplastic morphology and displayed significantly lower growth rates, reduced blood vessel density (BVD), decreased MIB-1 antigen and E-cadherin expression and increased cytoplasmic AR and HSP90 staining. Elevated EGFR (membrane) but not c-erbB2 expression was demonstrated in the TEN12F line only. Castration of mice bearing TEN12 xenografts rapidly induced the appearance of cytoplasmic AR in the cells, PSA levels decreased initially but recovered to below pre-castration levels whilst reduced TGFalpha and loss of VEGF expression was seen in the long-term castrates. CONCLUSIONS: TEN12 and its sublines offer additional in vivo models to study the factors involved in the progression of prostatic cancer to androgen-independence.

Multiple responses to EGF receptor activation and their abrogation by a specific EGF receptor tyrosine kinase inhibitor.

BACKGROUND: Epidermal growth factor receptor (EGF-R) autophosphorylation is essential for its intracellular mitogenic signaling via the MAPK pathway and for interaction in other cellular processes. Inhibition of this activity in tumor cells that predominantly utilise EGF-R therefore offers an alternative approach to therapy. METHODS: The ability of a specific inhibitor of EGF-R tyrosine kinase, ZM 252868, (TKI) to alter various parameters related to growth in DU145 and PC3 cell lines was investigated, by immunocytochemistry, Northern blotting, Western blotting and invasion assays. RESULTS: In DU145 cultures, the total cell population and number of cells in cell cycle decreased in the presence of TKI whilst the apoptotic rate was significantly increased. Reduction in autophosphorylation of the EGF-R, membrane expression of EGF-R, activation of the MAPK, p38, and JNK enzymes and the invasive capacity of DU145 cells was observed in the TKI treated cells. Under the same conditions, PC3 cell growth and EGF-R expression and MAPK activation were not affected. The use of inhibitors of intracellular signaling indicated that the DU145 cells, in contrast to PC3 cells, predominantly utilize EGF-R activation of the MAPK signaling pathway for growth. CONCLUSIONS: In prostatic cancer patients, in whom androgen resistance has developed and whose tumors have upregulated EGF-R for growth, specific TKI's may offer an important therapy option.