Tenascin C interacts with integrin receptors to promote breast cancer metastasis to the lungs

To date, despite significant advances in early detection and treatment of breast cancer, metastatic progression remains a major cause of morbidity and mortality in breast cancer patients. Importantly, patient stratification according to biomarker expression has led to continuous improvement of therapeutic strategies and better understanding of the diseases. However, the aggressive triple-negative subtype of breast cancer is still lacking effective therapeutic options. The important role of the tumor microenvironment in the formation of metastasis is well recognized. The extracellular matrix glycoprotein tenascin-C (TNC) has been previously described as a functional player of the metastatic niche, supporting the growth of breast cancer cells at the distant site. In this study, we investigated the clinical prognostic value of TNC in breast cancer patient cohorts. We found that TNC predicts poor clinical outcome only in the triple-negative subtype and that these tumors are enriched for TNC expression. Interestingly, while other subtypes rely on the stromal compartment as a source of TNC, triple-negative tumors express TNC in an autocrine manner. In addition, we confirmed that TNC promotes the growth of triple-negative breast cancer cells in the lungs in vivo. Therefore, we suggest that triple-negative breast tumors benefit from high, autocrine TNC expression to promote metastasis. Several cell surface receptors have been suggested to interact with TNC. However, the receptor(s) mediating the pro-metastatic signaling downstream of TNC remained unclear. We identified two integrin receptor subunits, integrin beta 1 (ITGB1) and integrin beta 3 (ITGB3), as TNC receptors. We demonstrated the binding of these molecules to TNC in an endogenous setting and showed that ITGB1 and ITGB3 support the growth of breast cancer cells in the lungs in vivo. Furthermore, we observed that the expression of these receptors, similar to TNC, is enriched in the triple-negative subtype. Using a large patient cohort, we showed that the prognostic value of TNC depends on the expression of the identified receptors, underscoring the clinical relevance of our findings. Importantly, ITGB1 and ITGB3 are supporting stem cell properties in triple-negative breast cancer cells. We found that the TNC knockout phenotype was associated with a decrease in stem cell properties of the normal mammary gland epithelium. In addition, we showed that TNC signaling members are upregulated during stages of the mammary gland development and maturation associated with expansion of the stem cell compartment. More importantly, TNC knockout mice showed an impairment in the formation of alveolar structures during pregnancy. All in all, our data strongly suggest a functional role of the TNC signaling in mammary stem cell biology. In this study, we identified two integrin receptors of the mammary gland (ITGB1 and ITGB3) as the receptors mediating the TNC pro-metastatic signaling in triple-negative breast cancer. Furthermore, we showed that TNC supports stem cell properties in the mammary gland. Therefore, we propose that the TNC signaling might play an important role in the mammary stem cell to support its activity and that triple-negative breast cancer cells benefit from high expression of TNC and its receptors to promote metastatic growth in this subtype. Deeper understanding of the mammary stem cell biology might support the development of targeted therapy for triple-negative breast cancer patients.