Delineating the nexus between gut-intratumoral microbiome and osteo-immune system in bone metastases

Abstract

Emerging insights in osteoimmunology have enabled researchers to explore in depth the role of immune modulation in regulating bone health. Bone is one of the common sites of metastasis notably in case of breast cancer, prostate cancer and several other cancer types. High calcium ion concentration and presence of several factors within the mineralized bone matrix including TGF-β, BMP etc., aid in tumor growth and proliferation. Accumulating evidence has substantiated the role of the gut-microbiota (GM) in tumorigenesis, further providing a strong impetus for the growing "immune-cancer-gut microbiota" relationship. Recent advancements in research further highlight the importance of the intra-tumor microbiota in conjunction with GM in cancer metastasis. Intratumoral microbiota owing to their ability to cause genetic instability, mutations, and epigenetic modifications within the tumor microenvironment, has been recognized to affect cancer cell physiology. The host microbiota and immune system crosstalk shapes the innate and adaptive arms of the immune system, which is the key player in cancer progression. In this review, we aim to decipher the role of microorganisms mediating bone metastasis by shedding light on the immuno-onco-microbiome (IOM) axis. We discussed the feasible cancer therapeutic interventions based on the modulation of the microbiome-immune cell axis which includes prebiotics, probiotics, and postbiotics. Here, we leverage the conceptual framework based on the published articles on microbiota-based therapies to target bone metastases. Understanding this complicated nexus will provide insights into fundamental factors governing bone metastases which will subsequently help in managing this malignancy with better efficacy.

Keywords: Biotics; Bone cancer; Bone metastases; Gut microbiota; Immune system; Intratumoral microbiota; Osteo-immuno-oncology.

Fig. 1

The osteo-immuno-oncology triad. Development of immune cells from hematopoietic stem cells (HSCs) in bone marrow indicates that bone and immune system are in constant interaction (Ponzetti and Rucci, 2019). Interaction of bone resident cells (osteoclasts, osteoblasts, osteocytes) with the immune cells is crucial to maintain bone homeostasis. Further, establishment of an immunosuppressive microenvironment and failed anti-tumor immune responses promote bone colonization of tumor cells from advanced cancers of breast, prostate, and lung.

Fig. 2

The interaction between immune cells and tumor cells during bone metastasis. Under the innate arm of the immune system, macrophages, dendritic cells (DCs), neutrophils, and myeloid-derived suppressor cells (MDSCs) release pro-tumorigenic cytokines that aid in the successful establishment of disseminated tumor cells (DTCs) in bone. Natural killer (NK) cells, however, deploy anti-tumorigenic cytokines that promote tumor regression. On the other hand, CD4 + T cells and Bregs release certain cytokines that stimulate cancer cells to metastasize to bone. Differentiation of CD8 + T cells into cytotoxic T lymphocytes (CTLs) exert anti-tumoral attack by secretion of proinflammatory cytokines- TNF-α and IFN-γ, consequently an anti-metastatic effect. As shown in the figure, the osteolytic tumor cells majorly release pro-osteoclastogenic cytokines such as receptor activator for nuclear factor kappa-B ligand (RANKL), parathyroid hormone-related protein (PTHrP), and C-X-C Chemokine Receptor 4 (CXCR4) which activate osteoclastogenesis and drive formation of osteolytic lesion.

Fig. 3

The nexus between gut microbiota (GM) and bone. The GM maintains bone homeostasis and under conditions of dysbiosis, results in abnormal bone remodeling contributing towards osteo-pathologies. A leaky gut allows the translocation of GM species from the intestinal lumen into the lamina propria which hosts a multitude of immune cells. GM species and derived short chain fatty acids (SCFAs) induce the dendritic cells (DCs) to stimulate Th17 and Th1 cells which further participate in inhibiting osteoclastogenesis via the production of cytokines such as interferon (IFN)-γ, interleukin (IL)-10, IL-4, and transforming growth factor (TGF)-β. Secretion of a variety of interleukins drive activation of Th17 cells which promote osteoclastogenesis via tumor necrosis factor (TNF)-α, IL-17, and receptor activator for nuclear factor kappa-B (RANK)- receptor activator for nuclear factor kappa-B ligand (RANKL) signaling. Also, production of IL-22 by Th17 and innate lymphoid cells (ILCs) maintains gut membrane integrity. Bregs are activated directly by the bacteria-derived lipopolysaccharide (LPS) via toll like receptor-4 (TLR4) signaling which inhibits maturation of osteoclast precursors directly and indirectly by inducing Tregs to suppress bone resorption. Secretion of TGF-β by Tregs function to convert pre-osteoblasts into osteoblasts, thus inducing bone formation.

Fig. 4

Microbial inducers of carcinogenesis. Genotoxin-producing bacteria E. coli and B. fragilis drive carcinogenesis by inducing direct DNA damage and cyclooxygenase (COX)-2 mediated signaling, respectively. H. pylori produces cytotoxin-associated gene A (CagA) bacterial protein which bind to the phosphorylated Src homology 2 (SH2) domain leading to an increase in ERK levels driving oncogenesis. The virulence factors produced by F. nucleatum and S.enterica induce activation of β-catenin which upon translocation to the nucleus results in overexpression of a variety of genes involved in cell proliferation, such as Myc, CycinD1, Wnt etc.

Fig. 5

Intratumoral microbiota governing cancer progression: Microbiota residing within the tumor tissues affect the cancer progression by several mechanisms A. Epigenetic modifications involving histone acetylation; B. Genetic modification; C. Immunomodulation; D. Epithelial to Mesenchymal transition; E. Increased cancer cell proliferation; F. Increased Angiogenesis. Either of these mechanisms individually or collectively, allows the tumor cells to metastasize which is usually to bone in case of Breast, Prostate, Lung and Renal cancer. Within the bone, microbiome along with the tumor cells travel to the site of secondary tumor and create microenvironment conducive for metastasis. The translocated microbiota in conjunction with the resident microbiota of that tissue decide the survival of the patient and their response to immune checkpoint inhibitors (ICI) therapy.

Fig. 6

Modulation of gut microbiota (GM) for therapeutic purposes. Prebiotics and probiotics, including the gut-associated short-chain fatty acids (SCFAs) exert anti-cancer effects by preventing the epithelial breach by pathogenic bacteria. This is mediated by activation of the immune machinery to enhance the integrity of gut barrier function by release of interleukin (IL)-22 from innate lymphoid cell (ILC)3s. Activation of dendritic cells (DC)s stimulate Th17 cells to produce IL-17A which has an anti-tumor effect. SCFAs also play a crucial role in maintaining luminal pH and mucin secretions, keeping up with the epithelial integrity. Synbiotics and postbiotics, which include bacterial cell lysates, enzymes, vitamins, SCFAs, and cell wall fragments, also have anti-cancer effects resulting in tumor regression.