P5-06-01: Gene Expression Analysis of Resistance to Bevacizumab in a VEGF-Reinforced Xenograft Model of ER-Positive Breast Cancer

Background: Bevacizumab, a monoclonal antibody targeting vascular endothelial growth factor (VEGF), had promising therapeutic efficacy in breast cancer. However, intrinsic or acquired resistance is common in the clinic. To improve our understanding of the underlying mechanisms of resistance to bevacizumab (BEV), we report the gene expression analysis of resistance to bevacizumab in a VEGF-overexpressing xenograft model of ER-positive breast cancer.

Methods: We developed a nude mouse xenograft model of resistance to anti-VEGF therapy with BEV in which MCF-7 control (ML20) or MCF-7 VEGF (MV165) transfectants were implanted in mammary fat pads, allowed to grow, then treated with BEV, with collection of tumor at early or late time points (while responding (R) to or progressing (NR) on anti-VEGF therapy). To elucidate differentially expressed gene profiling associated with tumor resistance to BEV, we performed whole-genome gene expression analysis (Human WG-6v2 Expression Beadchips, Illumina) and miRNA profiling (TaqMan ***ArrayHuman MicroRNAA+B Cards Set v3.0, Applied Biosystems). Validation of the chosen genes was performed using quantitative real-time RT-PCR (qRT-PCR).

Results: Gene expression analysis revealed differentially regulated genes in the MV165-NR group compared with the MV165-R group. Among the significant genes, Follistatin (FST) and HEY2 were the top genes upregulated in NR compared to R by ANOVA. Expression of HEY2 is induced by the Notch signaling pathway. Using qRT-PCR, we validated the expression of FST and Notch in our system. FST was significantly decreased (Fold change= −3.2; P=0.03) in the R group compared with vehicle in MV165 xenografts. In contrast to R group, FST was upregulated significantly (Fold change= 9.3; P=0.05) in the NR group. Notch4 displayed increased levels of expression in NR group, but it did not reach significance (P=0.23). In addition, correlation of mRNA and miRNA profiles showed that miRNAs targeting FST and Notch4 were differentially regulated in NR group compared to R group in MV165 xenograft tumors. Among the miRNAs, TGF-β-induced oncomiR miR-181a is up-regulated in NR and targets both FST and Notch4. Other miRNAs that target both Notch4 and FST include miR-1, miR-133a, miR-133b, and mir-449b. Conclusion: Our data serve as a potential mechanistic explanation for acquired resistance to bevacizumab. These data may shed light on the transitory effect of BEV observed in the E2100 firstline metastatic breast cancer trial, where VEGF-targeted therapy prolongs progression-free survival in metastatic breast cancer without improving overall survival.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P5-06-01.

P3-04-02: Bevacizumab Treatment Alters Intrinsic Subtypes in a VEGF-Reinforced Xenograft Model of ER-Positive Breast Cancer

Background: Anti-vascular endothelial growth factor (anti-VEGF) therapy improves disease-free but not overall survival in metastatic breast cancer. To seek further insight on resistance to anti-VEGF antibody bevacizumab (BEV) at the molecular level, we developed breast cancer xenograft models allowing comparison of tumor response at different time-points. Here we report the gene expression and miRNA analyses of response and non-response to BEV in these models.

Methods: MCF-7 cells transfected with control vector (ML20) or VEGF (MV165) were implanted into the mammary fat pads of athymic mice. Tumors from short-term treatment with BEV (3 weeks; Responders to BEV, R) or long-term treatment (8 weeks; Non-Responders, NR) or with vehicle control group (V) were subjected to whole-genome gene expression analysis (Human WG-6v2 Expression Beadchips, Illumina) and miRNA profiling (TaqMan ArrayHuman MicroRNA A+B Cards Set v3.0, Applied Biosystems).Validation of the chosen genes was performed using quantitative real-time RT-PCR (qRT-PCR) and Immunohistochemistry (IHC). Results: Short-term treatment to BEV (3 weeks; 5 mg/kg, i.p./twice weekly) inhibited primary tumor growth significantly in MV165 xenografts compared with vehicle control, whereas BEV treatment did not affect the tumor growth in the ML20 model. MV165 xenografts progressed after 8 weeks of BEV treatment. Gene set enrichment analysis (GSEA) revealed that luminal A-related gene sets were enriched in MV165-R compared to MV165-NR group including DESMEDT (ESR1), SMID_Breast_Cancer_Luminal_A_up, and MASSARWEH_ Tamoxifen_Resistance_ Down. Myoepithelial-specific gene sets were upregulated in both the R and NR groups compared with the vehicle group. qRT-PCR analysis showed that estrogen receptor alpha (ESR1) representative for luminal A decreased significantly in the MV-165-NR group (P=0.001) compared to vehicle. In contrast, Cytokeratin 5 (KRT5) levels increased significantly in both R (P=0.02) and NR (P=0.03) groups. In addition, KRT14 was upregulated in R (P= 0.04) and in NR (P=0.14) group in comparison with the vehicle group, suggesting the upregulation of myoepithelial phenotype specific to BEV treated MV165 model, but not ML20 model. Similar results were obtained by IHC. Consistent with mRNA changes, ESR1 regulated miRNA such as miR-107 (P=0.007) and miRNA important in tamoxifen resistance such as mir-451 (P= 0.0003) were also altered in MV165-NR group compared to vehicle. Conclusion: These results suggest that treatment with BEV may alter the intrinsic subtypes in the presence of VEGF expression. These data may help to explain the variable results to anti-VEGF therapy based on the duration of BEV treatment.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-04-02.

Dual targeting of EphA2 and ER restores tamoxifen sensitivity in ER/EphA2-positive breast cancer

Abstract #3020

Background: Overexpression and altered function of EphA2 receptor tyrosine kinase are critical in the progression of breast cancer and provide a novel target for breast cancer therapy. We have previously demonstrated that EphA2 overexpression decreases estrogen dependence and tamoxifen sensitivity both in vitro and in vivo. EA5, a novel monoclonal antibody that mimicks the binding of ephrin A to EphA2, reverses the effect of EphA2 overexpression and restores tamoxifen sensitivity in EphA2-transfected MCF-7 cells (MCF-7EphA2) in vitro. Here we report the impact of EA5 on in vivo tumor growth and its ability to overcome in vivo tamoxifen resistance in MCF-7EphA2 xenografts. Furthermore, we investigated the mechanisms by which EphA2 overexpression decreases the estrogen dependence and contributes to tamoxifen resistance in ER+ breast cancer models.
 Material and Methods: MCF-7 cells transfected with vector (MCF-7neo) or EphA2 (MCF-7EphA2) were implanted in the right and left mammary fat pads of athymic mice. Treatment with EA5 (5 mg/kg/i.p./5 days/week), vehicle, tamoxifen (1 mg/oral gavage/5 days/week), or EA5 and tamoxifen in combination was initiated once tumors were established. To explore the role of EphA2 overexpression on ER-dependent mechanisms, we used two different ER+/EphA2-transfected cell line models (MCF-7neo/ MCF-7EphA2 and T47Dneo/ T47DEphA2). We measured ER activity and expression of ER-dependent proteins in response to 17β-estradiol (E2;10-10 M), 4-hydroxy-tamoxifen (10-6 M, and EA5 antibody (3μg/mL). Interaction of ER and EphA2 was investigated using GST-pulldown and co-immunoprecipitation approaches. Gene array studies suggested that EphA2 may impact ER via noncanonical pathways, leading to an investigation of focal adhesion kinase (FAK) signaling.
 Results: EA5 inhibits primary tumor growth and restores tamoxifen sensitivity in the MCF-7EphA2 in vivo model;EA5 had no impact on in vivo tumor growth in MCF-7neo xenografts. Using T47DEphA2 in vitro model, we verified that EphA2 decreases ER activation in response to E2 stimulation consistent with our earlier results in MCF-7EphA2 model. We found no direct interaction between ER and EphA2 and no difference in expression of canonical ER-dependent proteins, ER coactivators or corepressors. However, E2 stimulation phosphorylates FAKTyr925 in ER+/EphA2+ cell lines but not in ER+/EphA2- cell lines. Treatment of T47DEphA2 cells with EA5 and tamoxifen leads to dephosphorylation of FAKTyr925 in the presence of E2 stimulation.
 Conclusion: Our data demonstrate that dual targeting of EphA2 and ER is a promising approach for delaying resistance to tamoxifen. The data support our hypothesis that EphA2 impacts ER function via a FAK dependent pathway.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3020.