Understanding the Biology of Bone Sarcoma from Early Initiating Events through Late Events in Metastasis and Disease Progression

Abstract

The two most common primary bone malignancies, osteosarcoma (OS), and Ewing sarcoma (ES), are both aggressive, highly metastatic cancers that most often strike teens, though both can be found in younger children and adults. Despite distinct origins and pathogenesis, both diseases share several mechanisms of progression and metastasis, including neovascularization, invasion, anoikis resistance, chemoresistance, and evasion of the immune response. Some of these processes are well-studies in more common carcinoma models, and the observation from adult diseases may be readily applied to pediatric bone sarcomas. Neovascularization, which includes angiogenesis and vasculogenesis, is a clear example of a process that is likely to be similar between carcinomas and sarcomas, since the responding cells are the same in each case. Chemoresistance mechanisms also may be similar between other cancers and the bone sarcomas. Since OS and ES are mesenchymal in origin, the process of epithelial-to-mesenchymal transition is largely absent in bone sarcomas, necessitating different approaches to study progression and metastasis in these diseases. One process that is less well-studied in bone sarcomas is dormancy, which allows micrometastatic disease to remain viable but not growing in distant sites - typically the lungs - for months or years before renewing growth to become overt metastatic disease. By understanding the basic biology of these processes, novel therapeutic strategies may be developed that could improve survival in children with OS or ES.

Keywords: Ewing sarcoma; anoikis resistance; cancer signaling; intravasation; metastasis; neovascularization; osteosarcoma; tumor dormancy.

Figure 1

Hypoxia and angiogenesis. Inset, upper right: HIF1a has been stabilized in the hypoxic center of a tumor, allowing it to bind to the VEGF promotor. Additionally, EGFR signaling, acting through SRC and the MAPK cascade, induce STAT3, which helps promote VEGF transcription. Larger picture: the hypoxic tumor, in purple, releases VEGF that establishes a gradient from the tumor to the nearby blood vessel. VEGF stimulates release of MMPs and plasmin, which help mediate digestion of extracellular matrix proteins, facilitating migration of both endothelial cells and tumor cells. In response to the VEGF gradient, some endothelial cells acquire a “tip cell” phenotype, pushing long extensions toward the tumor. These tip cells will eventually form the new blood vessels that provide a blood supply to the growing tumor.

Figure 2

Invasion and intravasation. (A) The growing tumor advances to the basement membrane of a nearby blood vessel, with the proteolytic functions of MMPs and plasmins clearing a pathway and allowing the leading cells to approach (bottom) and eventually push between (middle and top) the endothelial cells. With mass migration, other tumor cells may follow behind the leading cells. These trailing cells do not necessarily have as much invasive capacity as the lead cell, but may have their invasion and intravasation facilitated by the more invasive lead cells. (B) Migrating tumor cells effectively form a small “beachhead” in the spaces where lead cells have pushed between and through gaps in the endothelial cells. (C) With further mass migration, tumor cells begin obstructing the flow of blood through the vessel, and encounter red cells, platelets, and other blood components. A tumor thrombus may occlude blood flow and, is the vessel is large enough, cause symptoms of deep venous thrombosis. Clumps of tumor and individual tumor cells detach from the main mass of tumors and are carried away in the blood stream. Individual cells would be subject to anoikis when loose in the blood stream. Some cells in tumor clumps may be protected from anoikis by their attachment to other cells in the clump.

Figure 3

Signaling in anoikis resistance. The α4 and β4 integrins, paired either with each other or with other integrins, initiate signals vital to anoikis resistance, through interaction with Ezrin in the case of β4, and Src for α4. Src-mediated activation of PI3K may be independent of its well-described role in activating FAK. Her-4, induced by E-cadherin, signals strongly through AKT and mediates anoikis resistance, certainly for Ewing sarcoma and probably in osteosarcoma as well. While many receptor tyrosine kinases may mediate Ras activation and Ras-dependent and – independent activation of the MAP kinase cascade, IGF1R is an especially important source of these signals, especially for anoikis resistance in Ewing sarcoma.

Figure 4

Evasion of immune response. To survive, tumor cells must evade several components of cellular immunity. Cytotoxic T cells, through their T cell receptors (TCR), recognize foreign antigens, and kill cells bearing these antigens presented by class I HLA. Tumor-specific antigens can potentially be treated as foreign by the immune system. Tumor cells, such as the one in the lower right, typically downregulate HLA expression as a means of evading T cell response. Natural killer (NK) cells (left side of figure) recognize specific surface molecules expressed on “stressed” cells, especially tumor cells. One requirement for full NK response is the absence of self-HLA, so the down-regulation of HLA that allows tumors to evade cytotoxic T cells may facilitate their recognition by NK cells. Macrophages may ingest and destroy tumor cells in an antigen non-specific manner, though this process may require macrophage activation such as typically occurs in the sites of infection. The macrophage-activating drug L-MTP-PE has been shown to induce macrophage activation (top of figure) and improves survival in high-risk osteosarcoma patients when given in first complete remission or at a time of minimal residual disease, presumably by increasing phagocytosis on tumor cells.