Unique macrophage phenotypes activated by BMP signaling in breast cancer bone metastases

New insights into Gremlin-1: A tumour microenvironment landscape re-engineer and potential therapeutic target

Gremlin-1 (GREM1), a well-known bone morphogenetic protein (BMP) antagonist, is highly expressed in various malignant tumours. However, the specific role of GREM1 in tumours remains controversial and may be attributed to the heterogeneity and complexity of the tumour microenvironment (TME). It is currently believed that GREM1 regulates the complex landscape of the TME, primarily by antagonising BMP signalling or BMP-independent pathways. Both GREM1 and BMP play dual roles in tumour progression. Therefore, the mutual crosstalk between tumour cells and tumour-associated fibroblasts and the regulation of various secreted factors in the TME affect the secretion level of GREM1, which in turn regulates the amplitude balance between GREM1 and BMP, affecting tumour progression. The inhibition of GREM1 activity in the TME can disrupt this amplitude balance and prevent the formation of a tumour-supportive microenvironment, demonstrating that GREM1 is a potential therapeutic target. In this study, we reviewed the specific signalling pathways via which GREM1 in the TME regulates epithelial-mesenchymal transition, construction of the tumour immune microenvironment, and maintenance of tumour cell stemness via BMP-dependent and BMP-independent regulation, and also summarised the latest clinical progress of GREM1.

Immune microenvironment of cancer bone metastasis

Bone is a common and frequent site of metastasis in cancer patients, leading to a significant reduction in quality of life and increased mortality. Bone marrow, the primary site of hematopoiesis, also serves as both a primary and secondary lymphoid organ. It harbors and supports a diverse array of immune cells, thereby creating a distinct immune microenvironment. These immune cells engage in a range of activities, including anti-tumor, pro-tumor, or a combination of both, which influence the development and progression of bone metastases. Rapid advances in cancer immunotherapy have underscored its potential to eradicate bone metastases. However, clinical outcomes have not yet met expectations. To improve the efficacy of immunotherapy, it is crucial to gain a comprehensive and in-depth understanding of the immune microenvironment within bone metastases. This review provides an overview of the current understanding of the role of different immune cells, their anti-tumor and pro-tumor activities, and their overall contribution to bone metastasis.

Macrophage diversity in cancer dissemination and metastasis

Invasion and metastasis are hallmarks of cancer. In addition to the well-recognized hematogenous and lymphatic pathways of metastasis, cancer cell dissemination can occur via the transcoelomic and perineural routes, which are typical of ovarian and pancreatic cancer, respectively. Macrophages are a universal major component of the tumor microenvironment and, in established tumors, promote growth and dissemination to secondary sites. Here, we review the role of tumor-associated macrophages (TAMs) in cancer cell dissemination and metastasis, emphasizing the diversity of myeloid cells in different tissue contexts (lungs, liver, brain, bone, peritoneal cavity, nerves). The generally used models of lung metastasis fail to capture the diversity of pathways and tissue microenvironments. A better understanding of TAM diversity in different tissue contexts may pave the way for tailored diagnostic and therapeutic approaches.

Macrophage heterogeneity in bone metastasis

Previous studies illustrated that macrophage, a type of innate immune cell, plays critical roles in tumour progression and metastasis. Bone is the most frequent site of metastasis for several cancer types including breast, prostate, and lung. In bone metastasis, osteoclast, a macrophage subset specialized in bone resorption, was heavily investigated in the past. Recent studies illustrated that other macrophage subsets, e.g. monocyte-derived macrophages, and bone resident macrophages, promoted bone metastasis independent of osteoclast function. These novel mechanisms further improved our understanding of macrophage heterogeneity in the context of bone metastasis and illustrated new opportunities for future studies.