Mesenchymal stem/stromal cells (MSCs) are multipotent cells involved in numerous physiological events, including organogenesis, the maintenance of tissue homeostasis, regeneration, or tissue repair. MSCs are increasingly recognized as playing a major, dual, and complex role in cancer pathophysiology through their ability to limit or promote tumor progression. Indeed, these cells are known to interact with the tumor microenvironment, modulate the behavior of tumor cells, influence their functions, and promote distant metastasis formation through the secretion of mediators, the regulation of cell–cell interactions, and the modulation of the immune response. This dynamic network can lead to the establishment of immunoprivileged tissue niches or the formation of new tumors through the proliferation/differentiation of MSCs into cancer-associated fibroblasts as well as cancer stem cells. However, MSCs exhibit also therapeutic effects including anti-tumor, anti-proliferative, anti-inflammatory, or anti-oxidative effects. The therapeutic interest in MSCs is currently growing, mainly due to their ability to selectively migrate and penetrate tumor sites, which would make them relevant as vectors for advanced therapies. Therefore, this review aims to provide an overview of the double-edged sword implications of MSCs in tumor processes. The therapeutic potential of MSCs will be reviewed in melanoma and lung cancers.

Background/Objectives: Metallic nanoparticles (NPs) exhibit interesting radiosensitizing effects, and finding a way to accurately deliver them appears to be crucial. Due to their tumor tropism, mesenchymal stem cells (MSCs) represent a strategic approach. Therefore, we aimed to evaluate the impact of core–shell Fe₃O₄@Au NPs on the functionality of human pulmonary MSCs (HPMSCs). Methods/Results: The results showed that 100 µg/mL Fe₃O₄@Au NPs, accumulated in HPMSCs (revealed by Prussian blue staining), did not alter cell viability as assessed by cell counting, MTT, and LDH assays. However, caspase 9 and Bcl2 gene expression, evaluated by RT-qPCR, was regulated 72 h after exposure to the NPs. Moreover, the NPs also decreased proinflammatory cytokine/chemokine secretions, except for CXCL8 (ELISA). These modulations were associated with the downregulation of AMPK gene expression at 24 h. In contrast, the NPs did not modulate VEGF, PI3K, or PDGF gene expression. Nevertheless, a decrease in VEGF secretion was observed after 24 h of exposure to the NPs. Interestingly, the Fe₃O₄@Au NPs did not modulate Nrf2 gene expression, but they did regulate the expression of the genes encoding Nox4 and HMOX-1. Additionally, the NPs increased ROS production, suggesting a redox imbalance. Conclusions: Finally, the Fe₃O₄@Au NPs did not affect the HPMSCs’ viability or proangiogenic/tumorigenic markers. These findings are encouraging for investigating the effects of Fe₃O₄@Au NPs delivered by HPMSCs to tumor sites in combination with radiation.