Persistent Inflammatory Stimulation Drives the Conversion of MSCs to Inflammatory CAFs That Promote Pro-Metastatic Characteristics in Breast Cancer Cells

SIMPLE SUMMARY: Breast cancer progression is prominently regulated by the persistent presence of inflammatory mediators and by stromal cells that are located in the tumors. Here, we connected these two elements by demonstrating that potent pro-inflammatory cytokines (tumor necrosis factor α and interleukin 1β) lead to the conversion of mesenchymal stem cells (MSCs) to inflammatory cancer-associated fibroblasts (CAFs). These inflammation-driven CAFs secrete metastasis-promoting factors that elevate the dispersion, scattering, and migration of breast cancer cells via activation of tumor cell receptors that signal through Ras proteins and via Gαi proteins; the latter receptors were identified as the chemokine receptors CCR2, CCR5, and CXCR1/2. Together, these findings demonstrate that, in breast tumors, chronic inflammation can induce the deleterious process of MSC-to-CAF conversion and thus sets pro-inflammatory mediators as key targets for inhibition, potentially leading to lower levels of pro-metastatic CAFs in breast tumors. ABSTRACT: The pro-inflammatory cytokines tumor necrosis factor α (TNFα) and interleukin 1β (IL-1β) are expressed simultaneously and have tumor-promoting roles in breast cancer. In parallel, mesenchymal stem cells (MSCs) undergo conversion at the tumor site to cancer-associated fibroblasts (CAFs), which are generally connected to enhanced tumor progression. Here, we determined the impact of consistent inflammatory stimulation on stromal cell plasticity. MSCs that were persistently stimulated by TNFα + IL-1β (generally 14–18 days) gained a CAF-like morphology, accompanied by prominent changes in gene expression, including in stroma/fibroblast-related genes. These CAF-like cells expressed elevated levels of vimentin and fibroblast activation protein (FAP) and demonstrated significantly increased abilities to contract collagen gels. Moreover, they gained the phenotype of inflammatory CAFs, as indicated by the reduced expression of α smooth muscle actin (αSMA), increased proliferation, and elevated expression of inflammatory genes and proteins, primarily inflammatory chemokines. These inflammatory CAFs released factors that enhanced tumor cell dispersion, scattering, and migration; the inflammatory CAF-derived factors elevated cancer cell migration by stimulating the chemokine receptors CCR2, CCR5, and CXCR1/2 and Ras-activating receptors, expressed by the cancer cells. Together, these novel findings demonstrate that chronic inflammation can induce MSC-to-CAF conversion, leading to the generation of tumor-promoting inflammatory CAFs.