Bone niche and bone metastases

Bone function and bone-residing cells

The skeleton is a structural organ and a major site of calcium and phosphate storage in the body, supporting locomotion, protecting vital organs, and providing the niche for hematopoiesis [ ]. Despite its immobile and inert appearance, the bone actually is highly dynamic and continuously undergoes remodeling. To maintain bone architecture integrity and facilitate its functional roles, the continuous adjustment of the structure and composition is achieved by different types of bone cells [ ]. The bone remodeling process regulates the gain and loss of bone mineral density in the adult skeleton, and requires the proper coupling of bone resorption and bone formation.

Osteoclasts, multinucleated giant cells, which are formed by fusion of mononuclear cells of the hematopoietic lineage are responsible for bone resorption ( Fig. 9.1 ). During the bone resorption process, four membrane domains can be observed in the polarized osteoclast surface: the sealing zone (SZ), ruffled border (RB), basolateral, and functional secretory domain. In the unique confined space on the bone surface formed by the SZ and RB of osteoclasts, protons and proteases are released to demineralize and degrade the bone matrix [ ]. The polarization of osteoclasts is achieved by rearranging their actin cytoskeleton into F-actin rings. The vacuolar-type H ⁺ -ATPase (V-ATPase) in the RB helps to acidify the resorption lacuna and dissolve the hydroxyapatite. The products of this degradation by enzymes, such as tartrate-resistant acid phosphatase (TRAP), cathepsin K (CTSK), and matrix metalloproteinase-9 (MMP-9), are endocytosed across the RB and transcytosed to the functional secretory domain, which is away from bone surface [ ].

The osteoblasts originate from mesenchymal stem cells (MSCs) in the bone marrow stroma and are involved in bone matrix synthesis and subsequent mineralization. Although osteoblasts comprise only about 5% of the total bone cells, they can secrete collagen proteins, mainly type I collagen (COLI), noncollagen proteins (osteocalcin [OCN], osteonectin [ONN], bone sialoprotein [BSP], and osteopontin [OPN]), and proteoglycan including decorin and biglycan. The osteocytes, which comprise 90%–95% of the total bone cells, are embedded in the mineralized bone matrix and show a dendritic morphology. Osteocytes are derived from MSCs lineage through osteoblast differentiation, with downregulated expression of osteoblast markers such as OCN, BSP, and COLI and upregulated osteocyte markers including dentine matrix protein 1 (DMP1) and sclerostin (SOST). Osteocytes are important for maintaining phosphate metabolism and calcium availability, sensing of mechanical loading, and acting as an orchestrator of bone remodeling [ ] ( Fig. 9.1 ).

The bone-residing cells other than osteoclasts and osteoblasts in the bone microenvironment include chondrocytes, adipocytes, fibroblasts, endothelial cells, pericytes, blood cells, immune cells, nerve cells, as well as mesenchymal and hematopoietic stem cells, lineage-committed progenitors and precursors [ ].

Besides bone cells, the extracellular matrix (ECM), which is comprised of inorganic salts and an organic matrix, is a critical component of the bone and provides mechanical support for bone-residing cells. The organic matrix contains collagenous and noncollagenous proteins [ ]. The most abundant form of collagen in the bone matrix is COLI and among the noncollagenous proteins, the top five most characterized proteins are BSP, OPN, fibronectin (Fn), proteoglycans, and matrix metalloproteinases (MMPs) [ ]. The inorganic material consists of phosphate and calcium ions, as well as bicarbonate, sodium, potassium, citrate, and magnesium. The bone matrix also exerts essential roles in maintaining bone homeostasis. Bone matrix protein concentrations may vary with nutrition condition, age, disease, and antiosteoporosis therapy, which could affect bone formation and fracture healing [ ].

Communications between bone cells

The most well-characterized example of communications in normal bone physiology is the osteoclast–osteoblast cross talk, which is crucial for sustaining the balance between bone formation and resorption. Osteoclast differentiation requires macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor κB ligand (RANKL), which are primarily secreted by nearby osteoblasts and osteocytes [ ] ( Fig. 9.1 ). In contrast, osteoprotegerin (OPG), which is produced by cells including osteoblasts and fibroblasts in the bone marrow, can bind to RANKL to inhibit osteoclastogenesis [ , ] ( Fig. 9.1 ). Additional factors can also induce osteoclast differentiation and function directly or through regulating RANKL/OPG ratio, for example, Semaphorin 3A (SEMA3A) [ ], Wnt gene family 5A (WNT5A) [ ], Wnt gene family 16 (WNT16) [ , ], and sclerostin (SOST) [ ] ( Fig. 9.1 ). Osteocyte-secreted SOST also regulates osteoblast differentiation and bone formation in addition to bone resorption [ , ]. Osteoclast-derived factors, including bone morphogenetic protein 6 (BMP6) [ ], collagen triple helix repeat containing 1 (CTHRC1) [ ], Sphingosine 1-phosphate (S1P) [ ], EphrinB2 (EFNB2) [ ], Semaphorin 4D (SEMA4D) [ ], and Cardiotrophin-1 (CT-1) [ ], influence differentiation abilities and functions of osteoblast lineage cells [ ] ( Fig. 9.1 ).

The coupling of angiogenesis and the process of bone formation and resorption are also important for bone remodeling. Osteoblast lineage cells produce vascular endothelial growth factor A (VEGFA), which is a major proangiogenic factor that can promote endothelial cells proliferation, survival, and migration [ , ] ( Fig. 9.1 ). Hypoxia-inducible factor 1α (HIF1α) can promote angiogenesis in bone by inducing VEGFA expression in chondrocytes and preosteoblasts [ ]. MMPs-mediated ECM remodeling affects VEGF release and is also essential for angiogenesis [ , ]. Meanwhile, preosteoclast-secreted platelet-derived growth factor-BB (PDGF-BB) is also an angiogenesis factor which can induce type H vessel formation and thereby activate bone formation [ ] ( Fig. 9.1 ). It is clear that vessel formation is crucial for promoting bone formation; however, angiogenesis also exerts impacts on hematopoietic and mesenchymal stem/progenitor cell and therefore maintains bone homeostasis and tumor growth in bone [ ].

The bone marrow is the main site of hematopoietic cell production for all circulating blood and immune cell lineages in adults. The marrow also contains resident cells that not only participate in maintaining bone homeostasis but also regulate hematopoiesis and immune cell fate [ , ]. Stem niches provide cues to maintain a balance between proliferation and quiescence of stem cells, as well as self-renewal and differentiation ability of adult stem cells [ , ]. For example, mesenchymal progenitors and osteoblasts have been identified in regulating distinct stages of hematopoiesis by releasing C-X-C motif chemokine 12 (CXCL-12), stem cell factor (SCF), and other factors [ ]. The hematopoietic cells such as HSCs, lymphocytes, megakaryocytes, and HSCs-derived osteoclasts in parallel play roles in regulating osteoblast proliferation, differentiation, and function [ ].

Endocrine functions of bone tissue

In addition to its structural role, bone is also considered an endocrine organ that regulates whole body energy homeostasis. Bone-derived factors are involved in the constitution of the endocrine system to ensure homeostatic balance [ ]. Osteoblasts and osteocytes secrete fibroblast growth factor 23 (FGF23) to regulate phosphate metabolism by inhibiting renal phosphate reabsorption and the production of 1,25-dihydroxyvitamin D ₃ [1,25(OH) ₂ D ₃ ] in the kidney, as well as parathyroid hormone (PTH) synthesis in the parathyroid gland [ , ] ( Fig. 9.1 ).

Through a series of studies using mouse genetic models and clinical observations, osteoblast-secreted OCN has been identified as an additional bone-derived endocrine hormone that regulates systemic glucose and energy metabolism, reproduction, and cognition [ ]. It is reported that osteoblasts produce C-carboxylated OCN (GlaOCN) into bone ECM [ ]. GlaOCN is then decarboxylated by the acidic pH environment in the bone resorption lacunae and turns into active undercarboxylated OCN (GluOCN) and enters the circulation as a hormone [ ]. GluOCN enhances glucose uptake [ ], insulin production, and sensitivity [ , ]; increases adiponectin expression [ ]; as well as promotes β-cell proliferation [ ]. Moreover, OCN promotes male fertility by stimulating testosterone synthesis in Leydig cells [ , ] and improves cognitive function through regulating neurotransmitter synthesis and hippocampus development [ ] ( Fig. 9.1 ).

Increased food intake is observed in osteoblast-ablated mice, while additional OCN administration does not further affect appetite, suggesting the possible presence of additional bone-derived hormones to help regulate food intake [ ]. Recent research demonstrates that osteoblast-derived lipocalin-2 (LCN2) inhibits appetite in the hypothalamus and helps maintain glucose homeostasis through regulating glucose tolerance and insulin sensitivity [ ] ( Fig. 9.1 ).

Cancer is a systemic disease. Tumor growth and malignant progression are affected by tumor-induced systemic factors. The concept that bone functions as an endocrine organ suggests bone-derived factors are involved in establishing microenvironments in distant organs, which may contribute to primary tumor growth or metastasis. Notably, Engblom et al. report that bone-resident OCN-expressing (Ocn+) osteoblastic cells can remotely activate neutrophils to promote tumor growth in the lung [ ]. Molecular cross talk between bone and other organs has begun to draw consideration. However, our understanding of bone dynamics in the context of cancer progression at sites distant from the local bone microenvironment remains limited. To address this knowledge gap, additional factors from the bone niches need to be identified to investigate their function through a systemic perspective.