Abrogating phosphorylation of eIF4B is required for EGFR and mTOR inhibitor synergy in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) patients suffer from a highly malignant and aggressive disease. They have a high rate of relapse and often develop resistance to standard chemotherapy. Many TNBCs have elevated epidermal growth factor receptor (EGFR) but are resistant to EGFR inhibitors as monotherapy. In this study, we sought to find a combination therapy that could sensitize TNBC to EGFR inhibitors. Phospho-mass spectrometry was performed on the TNBC cell line, BT20, treated with 0.5 µM gefitinib. Immunoblotting measured protein levels and phosphorylation. Colony formation and growth assays analyzed the treatment on cell proliferation, while MTT assays determined the synergistic effect of inhibitor combination. A Dual-Luciferase reporter gene plasmid measured translation. All statistical analysis was done on CalucuSyn and GraphPad Prism using ANOVAs. Phospho-proteomics identified the mTOR pathway to be of interest in EGFR inhibitor resistance. In our studies, combining gefitinib and temsirolimus decreased cell growth and survival in a synergistic manner. Our data identified eIF4B, as a potentially key fragile point in EGFR and mTOR inhibitor synergy. Decreased eIF4B phosphorylation correlated with drops in growth, viability, clonogenic survival, and cap-dependent translation. Taken together, these data suggest EGFR and mTOR inhibitors abrogate growth, viability, and survival via disruption of eIF4B phosphorylation leading to decreased translation in TNBC cell lines. Further, including an mTOR inhibitor along with an EGFR inhibitor in TNBC with increased EGFR expression should be further explored. Additionally, translational regulation may play an important role in regulating EGFR and mTOR inhibitor synergy and warrant further investigation.

Fibroblast-secreted hepatocyte growth factor mediates epidermal growth factor receptor tyrosine kinase inhibitor resistance in triple-negative breast cancers through paracrine activation of Met.

INTRODUCTION: Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) have shown clinical efficacy in lung, colon, and pancreatic cancers. In lung cancer, resistance to EGFR TKIs correlates with amplification of the hepatocyte growth factor (HGF) receptor tyrosine kinase Met. Breast cancers do not respond to EGFR TKIs, even though EGFR is overexpressed. This intrinsic resistance to EGFR TKIs in breast cancer does not correlate with Met amplification. In several tissue monoculture models of human breast cancer, Met, although expressed, is not phosphorylated, suggesting a requirement for a paracrine-produced ligand. In fact, HGF, the ligand for Met, is not expressed in epithelial cells but is secreted by fibroblasts in the tumor stroma. We have identified a number of breast cancer cell lines that are sensitive to EGFR TKIs. This sensitivity is in conflict with the observed clinical resistance to EGFR TKIs in breast cancers. Here we demonstrate that fibroblast secretion of HGF activates Met and leads to EGFR/Met crosstalk and resistance to EGFR TKIs in triple-negative breast cancer (TNBC). METHODS: The SUM102 and SUM149 TNBC cell lines were used in this study. Recombinant HGF as well as conditioned media from fibroblasts expressing HGF were used as sources for Met activation. Furthermore, we co-cultured HGF-secreting fibroblasts with Met-expressing cancer cells to mimic the paracrine HGF/Met pathway, which is active in the tumor microenvironment. Cell growth, survival, and transformation were measured by cell counting, clonogenic and MTS assays, and soft agar colony formation, respectively. Student's t test was used for all statistical analysis. RESULTS: Here we demonstrate that treatment of breast cancer cells sensitive to EGFR TKIs with recombinant HGF confers a resistance to EGFR TKIs. Interestingly, knocking down EGFR abrogated HGF-mediated cell survival, suggesting a crosstalk between EGFR and Met. HGF is secreted as a single-chain pro-form, which has to be proteolytically cleaved in order to activate Met. To determine whether the proteases required to activate pro-HGF were present in the breast cancer cells, we utilized a fibroblast cell line expressing pro-HGF (RMF-HGF). Addition of pro-HGF-secreting conditioned fibroblast media to TNBC cells as well as co-culturing of TNBC cells with RMF-HGF fibroblasts resulted in robust phosphorylation of Met and stimulated proliferation in the presence of an EGFR TKI. CONCLUSIONS: Taken together, these data suggest a role for Met in clinical resistance to EGFR TKIs in breast cancer through EGFR/Met crosstalk mediated by tumor-stromal interactions.

Macrophages Enhance Migration in Inflammatory Breast Cancer Cells via RhoC GTPase Signaling.

Inflammatory breast cancer (IBC) is the most lethal form of breast cancer. All IBC patients have lymph node involvement and one-third of patients already have distant metastasis at diagnosis. This propensity for metastasis is a hallmark of IBC distinguishing it from less lethal non-inflammatory breast cancers (nIBC). Genetic profiling studies have been conducted to differentiate IBC from nIBC, but no IBC cancer-cell-specific gene signature has been identified. We hypothesized that a tumor-extrinsic factor, notably tumor-associated macrophages, promotes and contributes to IBC's extreme metastatic phenotype. To this end, we studied the effect of macrophage-conditioned media (MCM) on IBC. We show that two IBC cell lines are hyper-responsive to MCM as compared to normal-like breast and aggressive nIBC cell lines. We further interrogated IBC's hyper-responsiveness to MCM using a microfluidic migration device, which permits individual cell migration path tracing. We found the MCM "primes" the IBC cells' cellular machinery to become extremely migratory in response to a chemoattractant. We determined that interleukins -6, -8, and -10 within the MCM are sufficient to stimulate this enhanced IBC migration effect, and that the known metastatic oncogene, RhoC GTPase, is necessary for the enhanced migration response.

EGFR Tyrosine 845 Phosphorylation-Dependent Proliferation and Transformation of Breast Cancer Cells Require Activation of p38 MAPK.

Phosphorylation of epidermal growth factor receptor (EGFR) on tyrosine 845 by c-Src has been shown to be important for cell proliferation and migration in several model systems. This cross talk between EGFR and Src family kinases (SFKs) is one mechanism for resistance to EGFR inhibitors both in cell models and in the clinic. Here, we show that phosphorylation of tyrosine 845 on EGFR is required for proliferation and transformation using several cell models of breast cancer. Overexpression of EGFR-Y845F or treating cells with the SFK inhibitor dasatinib abrogated tyrosine 845 phosphorylation, yet had little to no effect on other EGFR phosphorylation sites or EGFR kinase activity. Abrogation of Y845 phosphorylation inhibited cell proliferation and transformation, even though extracellular signal-regulated kinase (ERK) and Akt remained active under these conditions. Importantly, cotransfection of mitogen-activated protein kinase (MAPK) kinase 3 and p38 MAPK restored cell proliferation in the absence of EGFR tyrosine 845 phosphorylation. Taken together, these data demonstrate a novel role for p38 MAPK signaling downstream of EGFR tyrosine 845 phosphorylation in the regulation of breast cancer cell proliferation and transformation and implicate SFK inhibitors as a potential therapeutic mechanism for overcoming EGFR tyrosine kinase inhibitor resistance in breast cancer.