Stromal cells in breast cancer microenvironment: molecular mechanisms involved in tumor progression and potential therapeutic targets

Abstract

Stromal cells in the tumor microenvironment (TME) play an important role in breast cancer progression, metastasis and therapeutic outcome. Among stromal cells, Tumor-Associated Macrophages (TAMs) and Mesenchymal Stem Cells (MSCs) have been shown to sustain breast tumor progression and worsen breast cancer prognosis. Elucidating the molecular mechanisms of epithelial/stromal cell interactions and discovering new therapeutic targets within the breast TME represent the main challenge of current research to increase the chances of successful treatment of breast cancer patients. Here, we firstly investigated the role of ligand-activated Peroxisome Proliferator Activator Receptor γ (PPARγ), a well-known tumor suppressor gene, to modulate breast TAM functional phenotype. We found that the treatment with natural and synthetic PPARγ ligands reduced the cytokine secretion by TAMs generated by exposure of conditioned media (CM) from breast cancer cells (BCCs). Interestingly, this effect was reversed by the PPARγ antagonist GW9662, suggesting the potential involvement of PPARγ in the attenuation of TAM polarization. Next, since it has been reported that soluble factors released in the TME mediate the tumor/stroma interactions, we mainly focused on the role of leptin which has been reported to sustain macrophage recruitment. Thus, we explored the impact of the leptin receptor knockdown (ObR sh) on BCCs in mediating the interaction between tumor cells and macrophages. In co-culture experiments between monocytes and BCCs, the absence of ObR reduced the recruitment of macrophages and affected their cytokine mRNA expression profile toward a less aggressive phenotype. We confirmed a decreased macrophage infiltration and reduced breast cancer growth in xenograft tumors of mice injected with ObR sh BCC. Furthermore, we explored the interaction between BCCs and MSCs within the breast TME. To this aim, we generated BCCs engulfing MSCs which result in hybrid cancer cells characterized by a multinucleated phenotype with increased dormancy and chemoresistance. In mouse models of breast cancer metastasis, hybrid cells had a reduced ability to form metastasis, but upon doxorubicin treatment they acquired resistance, inducing the metastatic spread of breast cancer. Collectively, our findings provide novel insights into the role of PPARγ and leptin signaling in modulating TAM polarization, opening new avenues for therapeutic intervention in breast cancer. Moreover, we identified and characterized a hybrid cell population, generated through. MSC engulfment by BBCs, with phenotypic features of malignancy, highlighting the potential of targeting stromal cells, to overcome drug resistance and metastasis in breast cancer.