The emergence of BoM is of significant prognostic importance in BRCA, underscoring the necessity to delve into 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 to the metastatic niche of BoM. Specifically, we observed a distinct elevation in CAFs alongside a reduction 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 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 into the early-stage colonization of breast cancer bone metastasis (BRCA BoM) aligns with the observed overexpression of heterotypic adherens junctions and an up-regulation of calcium influx. A pivotal outcome of our research is 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 unfavorable prognosis. Through a comparative analysis of cancer hallmarks between BoM and PT, as well as LN, we unveiled a predominant upregulation of metabolic and Toll-like receptor signaling pathways in BoM. This highlights significant molecular distinctions in the metastatic microenvironment.
To gain deeper insights, we further stratified BoM into three distinct states using stemness scores. Intriguingly, State 1, characterized by the highest stemness, was found to coexist in both primary and metastatic sites, acting as the initiating point for BoM. KEGG functional enrichment analysis of State 1 underscored 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 which derived from CCL2-recruited inflammatory monocytes, promoting 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 a significant upregulation of myeloid cells in BoM as opposed to PT and LN. Further elucidating the myeloid cell landscape through dimensionality reduction and clustering revealed that specific clusters of TAM were notably elevated in BoM. These clusters were found to predominantly engage in processes associated with cell adhesion and immune response, as substantiated by KEGG functional enrichment analysis.
Remarkably, our examination of cell-cell interactions has revealed a substantial augmentation in interactions specific to the formation of the metastatic niche within BoM. Noteworthy is the identification of Major Histocompatibility Complex class I (MHC-I) as a central mediator in 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 has yielded additional insights. These interactions were found to significantly contribute to an augmentation 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 signaling pathways in shaping the immune landscape of BRCA BoM.
Our study brings forth a nuanced understanding of the roles played by cancer-associated myofibroblasts and inflammatory CAFs within the metastatic niche. Particularly in BoM, a significant augmentation of myofibroblasts and FAP− inflammatory CAFs was observed in comparison to PT and LN, while FAP− inflammatory CAFs displayed a reduction. These identified myofibroblasts and FAP+ inflammatory CAFs emerged as pivotal contributors, 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 remodeling within 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 unveiled 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, our investigation has meticulously constructed a comprehensive single-cell map, providing a detailed portrayal of the metastatic niche throughout the spectrum of BRCA progression, encompassing in situ conditions, LN, and BoM. The systematic delineation of the metastatic niche in BoM has uncovered distinctive features, unraveling the intricate mechanisms that govern the immunosuppression induced by cancer cells upon metastasizing to the bone.