Osteoblasts, the cells that build up our skeleton, are remarkably versatile and important cells that need tight regulation in all the phases of their differentiation to guarantee proper skeletal development and homeostasis. Although we know many of the key pathways involved in osteoblast differentiation and signaling, it is becoming clearer and clearer that this is just the tip of the iceberg, and we are constantly discovering novel concepts in osteoblast physiology. In this review, we discuss well-established pathways of osteoblastic differentiation, i.e., the classical ones committing mesenchymal stromal cells to osteoblast, and then osteocytes as well as recently emerged players. In particular, we discuss micro (mi)RNAs, long non-coding (lnc)RNAs, circular (circ)RNAs, and extracellular vesicles, focusing on the mechanisms through which osteoblasts are regulated by these factors, and conversely, how they use extracellular vesicles to communicate with the surrounding microenvironment.

The term osteoimmunology was coined many years ago to describe the research field that deals with the cross-regulation between bone cells and the immune system. As a matter of fact, many factors that are classically considered immune-related, such as InterLeukins (i.e., IL-6, -11, -17, and -23), Tumor Necrosis Factor (TNF)-α, Receptor-Activator of Nuclear factor Kappa B (RANK), and its Ligand (RANKL), Nuclear Factor of Activated T-cell, cytoplasmatic-1 (NFATc1), and others have all been found to be crucial in osteoclast and osteoblast biology. Conversely, bone cells, which we used to think would only regulate each other and take care of remodeling bone, actually regulate immune cells, by creating the so-called “endosteal niche.” Both osteoblasts and osteoclasts participate to this niche, either by favoring engraftment, or mobilization of Hematopoietic Stem Cells (HSCs). In this review, we will describe the main milestones at the base of the osteoimmunology and present the key cellular players of the bone-immune system cross-talk, including HSCs, osteoblasts, osteoclasts, bone marrow macrophages, osteomacs, T- and B-lymphocytes, dendritic cells, and neutrophils. We will also briefly describe some pathological conditions in which the bone-immune system cross-talk plays a crucial role, with the final aim to portray the state of the art in the mechanisms regulating the bone-immune system interplay, and some of the latest molecular players in the field. This is important to encourage investigation in this field, to identify new targets in the treatment of bone and immune diseases.

Bone is the most frequent site of metastasis of the most common cancers in men and women. Bone metastasis incidence has been steadily increasing over the years, mainly because of higher life expectancy in oncologic patients. Although bone metastases are sometimes asymptomatic, their consequences are most often devastating, impairing both life quality and expectancy, due to the occurrence of the skeletal-related events, including bone fractures, hypercalcemia and spinal cord compression. Up to 75% of patients endure crippling cancer-induced bone pain (CIBP), against which we have very few weapons. This review’s purpose is to discuss the molecular and cellular mechanisms that lead to CIBP, including how cancer cells convert the bone “virtuous cycle” into a cancer-fuelling “vicious cycle”, and how this leads to the release of molecular mediators of pain, including protons, neurotrophins, interleukins, chemokines and ATP. Preclinical tests and assays to evaluate CIBP, including the incapacitance tester (in vivo), and neuron/glial activation in the dorsal root ganglia/spinal cord (ex vivo) will also be presented. Furthermore, current therapeutic options for CIBP are quite limited and nonspecific and they will also be discussed, along with up-and-coming options that may render CIBP easier to treat and let patients forget they are patients.

Besides its role as oxygen transporter, recent findings suggest that haemoglobin beta (HBB) may have roles in other contexts. We evaluated the impact of HBB expression in primary human breast cancers, and in breast cancer cell lines overexpressing HBB by in vitro and in vivo studies. Publicly available microarray databases were used to perform multivariate survival analyses. A significantly higher expression of HBB was observed in invasive carcinoma histotypes vs in situ counterparts, along with a positive correlation between HBB and the Ki67 proliferation marker. HBB-overexpressing breast cancer cells migrate and invade more, show HIF-1α upregulation and their conditioned media enhances angiogenesis. Blocking the oxygen-binding site of HBB reverts the increase of migration and HIF-1α upregulation observed in HBB-overexpressing breast cancer cells. Orthotopically implanted MDA-MB-231 overexpressing HBB (MDA-HBB) generated tumours with faster growth rate and increased neoangiogenesis. Moreover, local recurrence and visceral metastases were observed only in MDA-HBB-implanted mice. Similar results were observed with 4T1 mouse breast cancer cells. Finally, bioinformatics analyses of public data sets correlated high HBB expression with lower overall survival. HBB expression increases breast cancer cells aggressiveness and associates with poor prognosis, pointing to HBB as a novel biomarker for breast cancer progression.

Extracellular vesicles (EVs) are emerging as mediators of a range of pathological processes, including cancer. However, their role in bone metastases has been poorly explored. We investigated EV‐mediated effects of osteotropic breast cancer cells (MDA‐MB‐231) on bone resident cells and endothelial cells. Pretreatment of osteoblasts with conditioned medium (CM) of MDA‐MB‐231 (MDA) cells promoted pro‐osteoclastogenic and ‐angiogenic effects by osteoblast EVs (OB‐EVs), as well as an increase of RANKL‐positive OB‐EVs. Moreover, when treating osteoblasts with MDA‐EVs, we observed a reduction of their number, metabolic activity and Alp activity. MDA‐EVs also reduced transcription of Cyclin D1 and of the osteoblast‐differentiating genes, while enhancing the expression of the pro‐osteoclastogenic factors Rankl, Lcn2, Il1b and Il6. Interestingly, a cytokine array on CM from osteoblasts treated with MDA‐EVs showed an increase of the cytokines CCL3, CXCL2, Reg3G and VEGF, while OPG and WISP1 were downregulated. MDA‐EVs contained mRNAs of genes involved in bone metabolism, as well as cytokines, including PDGF‐BB, CCL3, CCL27, VEGF and Angiopoietin 2. In line with this profile, MDA‐EVs increased osteoclastogenesis and in vivo angiogenesis. Finally, intraperitoneal injection of MDA‐EVs in mice revealed their ability to reach the bone microenvironment and be integrated by osteoblasts and osteoclasts. In conclusion, we demonstrated a role for osteoblast‐ and tumor cell‐derived‐EVs in the deregulation of bone and endothelial cell physiology, thus fueling the vicious cycle induced by bone tumors via EVs.