Breast cancer is highly curable when it is localized to the breast and regional lymph nodes; however, residual and/or micrometastatic disease are often present and can progress to gross metastasis that is largely incurable. Radiation and chemotherapy are standard treatments for residual and/or metastatic disease; however, approximately 70% of breast cancer patients with microscopic metastasis and nearly all with gross metastasis have disease that is resistant to treatment. Because of this, understanding mechanisms that result in chemotherapy and radiation resistance is crucial to improving breast cancer survival.

The tumor microenvironment plays an important role in breast cancer treatment resistance. Understanding the resistance pathways that are engaged between breast cancer and stromal fibroblasts, the major cell type in the tumor microenvironment, is needed to make this knowledge actionable and to decrease mortality associated with metastatic disease. We previously demonstrated that a gene signature comprised of interferon-stimulated genes (ISGs), which are normally induced by anti-viral responses, that identifies breast cancer patients who are sensitive or resistant to chemotherapy and radiation. These ISGs are regulated by stromal fibroblasts, suggesting that ISGs might contribute to stroma-mediated resistance. In fact, patients with ISG(-) tumors have a high likelihood of cure by chemotherapy. However, for patients with ISG(+) tumors, lack of mechanistic insight has hindered strategies to reverse resistance.

Now, we have discovered that regulation of breast cancer ISG expression is controlled by stroma-derived exosomes, which are small membrane vesicles similar in size to viruses and thought to play a role in cell-to-cell communication. Like viruses, these exosomes activate antiviral signaling pathways. Unexpectedly, this antiviral signaling facilitates chemotherapy and radiation resistance by enhancing NOTCH3 signaling, a pathway previously implicated in cancer treatment resistance and the regulation of resistance cancer stem-like cells. Drugs that block NOTCH signaling have already been developed. One such drug is a gamma secretase inhibitor (GSI). We have discovered that use of a GSI reverses stroma-mediated chemotherapy and radiation resistance in experimental systems. In this setting, the effectiveness of GSI treatment in reversing treatment resistance is specific for breast cancers that are ISG(+) as a result of coercing stroma to produce exosomes. These results suggest that exosome analysis can identify ISG(+) patients that benefit from GSI treatment or other methods to block NOTCH3, including less toxic NOTCH3-specific antibodies.

Based on our results, the purpose of our proposal is to study (1) how stroma uses antiviral pathways to regulate NOTCH3, (2) evaluate NOTCH3 blockade as a means to reverse stroma-mediated resistance, and (3) examine whether exosome analysis can be used to identify patients with ISG(+) tumors that can be effectively treated with NOTCH3 blockade to reverse stroma-mediated chemotherapy/radiation resistance. Our overarching goal is to decrease mortality of metastatic breast cancer by identifying resistant tumors and providing an effective and less toxic strategy to reverse resistance. Since methods to examine exosomes and drugs that block NOTCH3 signaling already exists, we envision that the results of our study can catalyze initial clinical trials shortly after the 3-year duration of this proposal.