Pioneering clinical studies in de novo refractoriness to the anti-HER2 monoclonal antibody trastuzumab have suggested that HER2 gene-amplification can take place also in a basal-like molecular background to generate basal/HER2+ tumors intrinsically resistant to trastuzumab. Here, we first investigated the unique histogenesis of the basal/HER2+ phenotype in breast carcinomas. The presence of basal CK5/CK6 cytokeratin expression in HER2+ tumors revealed a significant overlap in the histological features of HER2+/CK5/6+ and basal-like breast carcinomas. Basal/HER2+ tumors were typically poorly differentiated, high-grade invasive ductal carcinomas with large geographic necrosis, pushing margins of invasion, syncytial arrangement of tumor cells, ribbon- or festoon-like architecture, squamous metaplasia, stromal lymphocytic infiltrates, high mitotic index and strong p53 positivity. Secondly, we performed low-scale proteomic approaches in JIMT-1 cells, a unique model of HER2-gene amplified trastuzumab-resistant breast carcinoma with a basal-like phenotype, to develop biomarker signatures that may differentiate trastuzumab-responsive from non-responsive tumors. When applying antibody-based array technology to the extracellular milieu of trastuzumab-refractory JIMT-1 and trastuzumab-sensitive SKBR3 cell cultures, JIMT-1 cells were found to secrete higher amounts of several growth factors including amphiregulin, EGF, IGFBP-6, PDGF-AA, neurotrophins, TGFβ and VEGF. Semi-quantitative signaling node multi-target sandwich ELISAs revealed that JIMT-1 cells drastically overactivate RelA, the prosurvival subunit of NF-κB as compared to trastuzumab-sensitive luminal/HER2+ SKBR3 cells. When simultaneously assessing the activation status of 42 receptor tyrosine kinases (RTK) using a human phospho-RTK array, JIMT-1 cells were found to constitutively display hyperactivation of the insulin-like growth factor-I receptor (IGF-1R). High-content immunofluorescence imaging revealed that activated IGF-1R mainly localized at focal adhesion-like structures in JIMT-1 cells. In vitro wound healing assays suggested that this functional reorganization of the JIMT-1 cytoskeletal reorganization may account for an exacerbated trastuzumab-refractory ‘migratogenic’ phenotype. Forthcoming studies should validate the notion that identification of basal-like immunophenotypes and/or basal-like molecular signatures within HER2+ breast carcinomas may provide rapid means to define subgroups of breast cancer patients likely to display resistance to trastuzumab ab initio.

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