Bone microenvironment signals in osteosarcoma development

Abstract

The bone is a complex connective tissue composed of many different cell types such as osteoblasts, osteoclasts, chondrocytes, mesenchymal stem/progenitor cells, hematopoietic cells and endothelial cells, among others. The interaction between them is finely balanced through the processes of bone formation and bone remodeling, which regulates the production and biological activity of many soluble factors and extracellular matrix components needed to maintain the bone homeostasis in terms of cell proliferation, differentiation and apoptosis. Osteosarcoma (OS) emerges in this complex environment as a result of poorly defined oncogenic events arising in osteogenic lineage precursors. Increasing evidence supports that similar to normal development, the bone microenvironment (BME) underlies OS initiation and progression. Here, we recapitulate the physiological processes that regulate bone homeostasis and review the current knowledge about how OS cells and BME communicate and interact, describing how these interactions affect OS cell growth, metastasis, cancer stem cell fate and therapy outcome.

Fig. 1

Endochondral bone formation. a MSC differentiate to chondrocytes through the formation of cell mass condensations (i). This primary ossification centers expand through the proliferation of chondrocytes (ii) and the subsequent hypertrophy of post-proliferative chondrocytes (iii) which direct the mineralization of the surrounding matrix and control the conversion of progenitor perichondral cells to osteoblasts that form the bone collar. After that, hypertrophic chondrocytes undergo apoptosis and the newly formed osteoblasts invade the cartilage mold and form the definitive bone matrix (iv). This mold also creates access to blood vessels within the matrix. b GP are multilayered structures divided into: (1) a reserve layer of resting cells that assure the germinal structure, (2) a proliferative layer of rapidly proliferating chondrocytes, (3) a hypertrophic layer of non-proliferating chondrocytes synthesizing collagen X and proteoglycans, (4) a degenerative layer where chondrocytes undergo apoptosis and (5) an ossification layer (primary spongiosa) where MSC-derived osteoblasts replace the matrix with mineralized bone. The marrow space formed below the GP provides niches for hematopoiesis. Most OS develop during puberty in the GPs of long bones as a result of the oncogenic transformation of MSC or their derived osteogenic progenitors

Fig. 2

In vivo distribution of OS cells within the BME. Histological sections of OS generated orthotopically in immunocompromised NOD/SCID mice, stained with hematoxylin and eosin (HE) or specific antibodies (anti-GFP; anti-human vimentin). a Images of incipient OS originated from GFP-expressing murine P53^−/− RB^−/− MSC, which initiate OS with an incidence of 80 % [11], located in either tibial metaphysis or epiphysis (upper panels) or in tibial diaphysis (lower panels). OS cells (GFP-positive cells) at this stage distribute peripherally to BM cavities, in close relation with host bone, osteoblasts, blood sinusoids and blood vessels. b Images of invasive OS generated from human primary OS cells, taken at the tibial diaphysis level whose integrity is damaged by the invading tumor. Tumor cells inside the mouse setting were identified using an anti-human vimentin antibody, known to react with this particular tumor. This immunohistochemistry shows that tumor cells can be found in close proximity to host-derived tumor stroma and blood vessels. c Formation of metastasis in the lungs of mice intra-bone inoculated with GFP-expressing P53^−/−RB^−/− MSC. The metastatic process includes the extravasation of GFP+ tumoral cells from blood vessels (left panel) and the migration, homing and proliferation of metastatic GFP+ cells (middle panel). Original magnification is shown. B bone, BM bone marrow, BV blood vessels, T tumor mass, M muscle, 1 osteocytes, 2 osteoblasts, 3 blood sinusoids, 4 erythroid precursors, 5 granulocytes, 6 megakaryocytes, 7 tumor stroma

Fig. 3

Interrelation between bone environment cell types and OS. The presence of OS metastatic cells with osteolytic potential in the bone microenvironment initiates a “vicious cycle” in which tumor cells produce factors (PTHrP, TGFβ or IL11) that stimulate the activation of osteoclast through a RANKL–RANK-mediated interaction between osteoblast and osteoclast. This activation results in a dysregulated bone lysis and increased release of growth factors from the bone matrix (BMP, TGFβ, IGF1 or FGF) which, in turn, promotes tumor cell proliferation (blue arrows). In a similar way, primary OS cells might also induce a similar cycle of osteoclastic activity and bone lysis associated with enhanced tumor aggressiveness. OS cells may also induce the production of pro-tumorogenic molecules (lactate, VEGF and IL6) from the bone environment MSC (magenta and orange arrows). In addition, immune-stimulatory (M1) and immune-suppressive (M2) tumor-associated macrophages and other immune cells play a role in OS progression through cytokine- and chemokine-mediated signaling (gray and red arrows). Physical conditions of the tumor microenvironment also play relevant roles in OS development (green arrows). Thus, hypoxia (through HIF1-mediated signaling) and acidic pH favor angiogenesis, stemness and metastatic behavior, stimulate OS cells to produce osteoblast-stimulatory factors (VEGF, PDGF and EDN1) and may also cooperate with TGFβ signaling to potentiate the “vicious cycle” of bone tumors. Finally, extracellular calcium-mediated signaling also contributes to the “vicious cycle” through the induction of PTHrP production or the generation of EMV containing pro-osteoclastic cargo. The final effect of the different bone cells–tumor interactions on OS development is indicated according to the above-indicated color code