The emergence of BoM is of significant prognostic importance in BRCA, underscoring the necessity of elucidating the intricate pathogenesis and molecular regulatory networks governing this phenomenon. In this study, we undertook a comprehensive investigation of BRCA, with a particular focus on both LN and BoM. Our meticulous analysis provides valuable insights into the nuanced intricacies of BRCA progression, with special attention given to the metastatic niche of the BoM. Specifically, we observed a distinct increase in CAFs and a decrease in immune cells within the bone metastatic microenvironment. These findings enhance our understanding of the disease and present potential avenues for therapeutic interventions.
Several recent studies have extensively explored the intricate microenvironments within the BoM, shedding light on the niche that supports tumor colonization 25–27. The dynamic interplay of tumor-stromal interactions orchestrates the progression from initial seeding to the development of overt macrometastasis. Consistent with these findings, our investigation of early-stage colonization of BRCA BoM aligns with the observed increase in heterotypic adherens junctions and increase in calcium influx. A pivotal outcome of our research was the identification of a distinct subtype of BRCA BoM cells. This specific subtype demonstrates a close correlation with the occurrence of BRCA BoM and serves as an indicator of an unfavourable prognosis. Through a comparative analysis of cancer hallmarks between BoMs and PTs, as well as between BoMs and PTs, we revealed a predominant upregulation of metabolic and Toll-like receptor signalling pathways in BoMs. This highlights significant molecular distinctions in the metastatic microenvironment.
To gain further insight, we further stratified BoMs into three distinct states using stemness scores. Intriguingly, State 1, characterized by the highest stemness, was found to coexist at both the primary and metastatic sites, acting as the initiating point for BoM. KEGG functional enrichment analysis of State 1 revealed its involvement in critical biological processes, including cell growth, development, proliferation, differentiation, and cell adhesion. By comparing our findings with the conclusions drawn in recent publications 28, we contribute to the ongoing discourse on bone metastatic microenvironments. Our identification of a specific BRCA BoM cell subtype and the delineation of distinct functional pathways provide novel perspectives for understanding and potentially targeting the unique aspects of metastasis within the bone microenvironment.
Ma et al. recently identified a specific subset of protumorigenic macrophages derived from CCL2-recruited inflammatory monocytes that promote BRCA BoM in an IL-4R-dependent manner 29. Our exploration has illuminated the intricate dynamics of communication between BRCA cells and immune cells, providing a nuanced understanding of the immune landscape. Employing distinct biomarkers for immune cell identification, we observed significant upregulation of myeloid cells in BoMs compared with PTs and LNs. Further elucidating the myeloid cell landscape through dimensionality reduction and clustering revealed that specific clusters of TAMs were notably elevated in BoMs. These clusters were found to predominantly engage in processes associated with cell adhesion and the immune response, as substantiated by KEGG functional enrichment analysis.
Remarkably, our examination of cell‒cell interactions revealed a substantial augmentation in interactions specific to the formation of the metastatic niche within the BoM. Notably, major histocompatibility complex class I (MHC-I) was identified as a central mediator facilitating communication between tumor cells and immune cells, as well as orchestrating intercellular interactions among immune cells specifically within the BoM microenvironment, a phenomenon not as prominently observed in the PT or LN. Intriguingly, our focused analysis of interactions involving FN1, SPP1, and MDK with their target genes yielded additional insights. These interactions were found to significantly contribute to an increase in myeloid cells, B cells, naive T cells, and cytotoxic T cells within the dynamic milieu of the BoM microenvironment. This intricately orchestrated interplay emphasizes the influential role of specific signalling pathways in shaping the immune landscape of BRCA BoMs.
Our study provides a nuanced understanding of the roles played by cancer-associated myofibroblasts and inflammatory CAFs within the metastatic niche. Particularly in the BoM cohort, a significant increase in myofibroblasts and FAP− inflammatory CAFs was observed in comparison to those in the PT and LN cohorts, while the number of FAP− inflammatory CAFs was reduced. These identified myofibroblasts and FAP+ inflammatory CAFs emerged as pivotal contributors that are primarily involved in crucial cellular functions such as proliferation, adhesion, and extracellular matrix organization. The intricate interplay orchestrated by CD46, MDK, PTN, and their target genes emerged as a driving force behind the activation and proliferation of myofibroblasts, significantly contributing to tissue remodelling within the BoM. Furthermore, the interactions facilitated by MDK, PTN, FN1, and their respective target genes were found to stimulate the activation and proliferation of FAP+ CAFs, concurrently promoting cell adhesion and migration within the BoM microenvironment. Our in-depth exploration of immune-stromal cell communication revealed critical genes, including PTN, MK, SPP1, and FN1. Through interactions with their target genes, these genes were implicated in fostering the activation and proliferation of myofibroblasts while concurrently playing a pivotal role in orchestrating inflammatory responses within the dynamic context of BoM.
In conclusion, in our study, a comprehensive single-cell map was constructed, providing a detailed portrayal of the metastatic niche throughout the spectrum of BRCA progression, encompassing in situ conditions, LNs, and BoMs. The systematic delineation of the metastatic niche in BoMs has revealed distinctive features, revealing the intricate mechanisms that govern the immunosuppression induced by cancer cells upon metastasizing to bone.