Cancer-associated fibroblast-derived protein S100-A11 influences the response to anti-HER2 therapies in HER2-positive breast cancer

Breast cancer is the most commonly diagnosed cancer worldwide and includes the HER2-positive (HER2+) subtype, characterised by HER2 overexpression. HER2+ breast cancer is treated with neoadjuvant anti-HER2 therapy combined with taxane-based chemotherapy, yet a substantial proportion of patients fail to achieve pathological complete response. Increasing evidence indicates that the tumour microenvironment (TME), particularly cancer-associated fibroblasts (CAFs), contributes to therapy resistance through paracrine signalling. In this study, we investigated the role of stromal/CAF-derived S100-A11 in the response to trastuzumab, pertuzumab, and docetaxel (TPD) therapy. Proteomic analysis and ELISA confirmed increased S100-A11 secretion by TPD-treated CAF-200 fibroblasts. Recombinant S100-A11 reduced sensitivity to TPD in multiple HER2+ breast cancer cell lines, whereas S100A11 silencing in CAF-200 attenuated the resistance-promoting effect of CAF-conditioned medium in BT-474 and EFM-192A cells. Mechanistically, S100-A11 exposure was associated with increased STAT3 phosphorylation, and pharmacological inhibition of STAT3 or RAGE partially reversed the S100-A11-associated resistance phenotype. In a reductionist xenograft model, RAGE inhibition with azeliragon attenuated the effect of exogenous S100-A11 on tumour response to TPD. In a retrospective cohort of early-stage HER2+ breast cancer, high stromal S100-A11 expression was associated with residual disease after neoadjuvant therapy and with increased tumour p-STAT3 levels. Dual staining for S100-A11 and α-SMA further supported the presence of S100-A11-expressing CAFs in patient tumours. Together, these findings support a role for stromal S100-A11 in modulating response to anti-HER2 therapy and suggest that the S100-A11/RAGE/STAT3 axis may represent a therapeutically relevant stromal signalling pathway. Further validation in independent clinical cohorts and more physiologically representative models is required. Resistance to anti-HER2 therapies remains a major clinical challenge in the treatment of HER2+ breast cancer. This study identifies the stromal protein S100-A11, secreted by CAFs, as a candidate mediator associated with reduced sensitivity to TPD therapy. We show that extracellular S100-A11 promotes tumour cell proliferation and is associated with activation of the RAGE/STAT3 signalling axis. Pharmacological inhibition of this pathway using azeliragon (a RAGE antagonist) or stattic (a STAT3 inhibitor) partially reverses the S100-A11-associated resistance phenotype in vitro and in a reductionist xenograft model. In parallel, high stromal S100-A11 expression is associated with poorer pathological response in a retrospective cohort of early-stage HER2+ breast cancer patients. While these findings are exploratory, they support the potential relevance of stromal S100-A11 as a component of the TME linked to therapy response. Overall, this work provides a rationale for further investigation of the S100-A11/RAGE/STAT3 axis as a potential target in stromal-mediated resistance to anti-HER2 therapy. This study provides further insight into the mechanisms underlying resistance to anti-HER2 therapy by identifying a therapy-associated, stroma-related adaptive response that may contribute to reduced treatment sensitivity beyond tumour-intrinsic alterations. While resistance in HER2+ breast cancer has traditionally been attributed to oncogenic signalling rewiring within tumour cells, our findings suggest that anti-HER2 therapy may also influence CAFs, promoting the secretion of factors that support tumour cell survival under therapeutic pressure. In this context, we describe a paracrine signalling interaction in which CAF-associated S100-A11 is linked to activation of the RAGE/STAT3 axis in tumour cells, accompanied by increased proliferation and reduced sensitivity to treatment. These observations support a model in which the TME acts as an active contributor to adaptive resistance, rather than a passive component. Moreover, the partial reversibility of this phenotype through pharmacological inhibition of RAGE or STAT3 suggests that stromal signalling pathways may represent potential targets for therapeutic intervention. Overall, these findings expand current perspectives on resistance by incorporating the contribution of therapy-associated stromal responses and support further investigation into combinatorial strategies targeting both tumour cells and their TME.