Microenvironment of osteosarcoma stem cells
Stem cell niche defines the microenvironment in which stem cells reside. It is comprised of stem cells, neighboring supportive cells, inflammatory cells, microvessels, the extracellular matrix and soluble factors such as chemokines and cytokines. The niche microenvironment facilitates stem cells in entering quiescence and undergo differentiation and to maintain their stemness including self-renewal and to regulate differentiation [72] and [73]. This has been demonstrated in OS99-1 cells: OS99-1 cells with high ALDH activity exhibited CSC-like behavior when the cells were isolated from subcutaneous tumors in vitro and not from adherent cultures [28]. Three types of stem cell microenvironments have been reported in glioblastoma: perivascular niche, hypoxic niche and metastatic niche. Tumor stem cells are thought to induce vascularization and differentiate into endothelial-like cells to mimic microvessels in order to supply oxygen and nutrition to the tumor [74] and [75]. GSCs (Glioblastoma stem cells) have been found adjacent to capillaries and adhered to endothelial cells in brain tumors [76]. This was further demonstrated through increased tumor formation in mice following co-injection of CD133-positive human medulloblastoma cells with endothelial cells [76]. Further investigation revealed that the process was mediated by VEGF/VEGFR and SDF-1/CXCR4 pathways [77] and [78]. Osteosarcoma is a tumor enriched in vasculature, supporting the existence of a perivascular OSC niche, and reports have confirmed that the hypoxic niche is central in osteosarcoma propagation and CSC maintenance [79] and [80]. Hypoxia-inducible factor (HIF) has been reported as highly expressed in CSCs in several tumors; conversely, blocking HIF-1α or HIF-2α activity resulted in dramatic decreases in CSC proliferation and self-renewal [81]. A hypoxic environment in non-stem osteosarcoma cells has been reported to dramatically induce the self-renewal capacity of OSCs [36]. Interestingly, expression of the epigenetic gene TSSC3, which is also known as PHLDA2, reported to repress OSC self-renewal, can be regulated by hypoxia [82]. Both the Notch and Hedgehog pathways have been implicated in the stem cell niche: The Notch pathway was reported to be activated and to promote stem-like characteristics in glioblastoma and colorectal cancer. This was believed to be through Nitric oxide and Notch ligands released from tumor endothelial cells in the stem cell niche [83], [84] and [85]; the Hedgehog pathway was activated in the glioblastoma stem cell (GSC) niche, probably through ligands secreted by endothelial cells, suggesting a paracrined function may support CSCs [6]. Although there is few data for understanding microenvironment of OSCs, it is without doubt that microenvironment could notably influence the biological behaviors of OSCs. Exploration of molecular interactions among OSCs, microvessels and hypoxia in future might develop new therapeutic strategy to target OSCs.
Microenvironment osteosarcoma สเต็มเซลล์Stem cell niche defines the microenvironment in which stem cells reside. It is comprised of stem cells, neighboring supportive cells, inflammatory cells, microvessels, the extracellular matrix and soluble factors such as chemokines and cytokines. The niche microenvironment facilitates stem cells in entering quiescence and undergo differentiation and to maintain their stemness including self-renewal and to regulate differentiation [72] and [73]. This has been demonstrated in OS99-1 cells: OS99-1 cells with high ALDH activity exhibited CSC-like behavior when the cells were isolated from subcutaneous tumors in vitro and not from adherent cultures [28]. Three types of stem cell microenvironments have been reported in glioblastoma: perivascular niche, hypoxic niche and metastatic niche. Tumor stem cells are thought to induce vascularization and differentiate into endothelial-like cells to mimic microvessels in order to supply oxygen and nutrition to the tumor [74] and [75]. GSCs (Glioblastoma stem cells) have been found adjacent to capillaries and adhered to endothelial cells in brain tumors [76]. This was further demonstrated through increased tumor formation in mice following co-injection of CD133-positive human medulloblastoma cells with endothelial cells [76]. Further investigation revealed that the process was mediated by VEGF/VEGFR and SDF-1/CXCR4 pathways [77] and [78]. Osteosarcoma is a tumor enriched in vasculature, supporting the existence of a perivascular OSC niche, and reports have confirmed that the hypoxic niche is central in osteosarcoma propagation and CSC maintenance [79] and [80]. Hypoxia-inducible factor (HIF) has been reported as highly expressed in CSCs in several tumors; conversely, blocking HIF-1α or HIF-2α activity resulted in dramatic decreases in CSC proliferation and self-renewal [81]. A hypoxic environment in non-stem osteosarcoma cells has been reported to dramatically induce the self-renewal capacity of OSCs [36]. Interestingly, expression of the epigenetic gene TSSC3, which is also known as PHLDA2, reported to repress OSC self-renewal, can be regulated by hypoxia [82]. Both the Notch and Hedgehog pathways have been implicated in the stem cell niche: The Notch pathway was reported to be activated and to promote stem-like characteristics in glioblastoma and colorectal cancer. This was believed to be through Nitric oxide and Notch ligands released from tumor endothelial cells in the stem cell niche [83], [84] and [85]; the Hedgehog pathway was activated in the glioblastoma stem cell (GSC) niche, probably through ligands secreted by endothelial cells, suggesting a paracrined function may support CSCs [6]. Although there is few data for understanding microenvironment of OSCs, it is without doubt that microenvironment could notably influence the biological behaviors of OSCs. Exploration of molecular interactions among OSCs, microvessels and hypoxia in future might develop new therapeutic strategy to target OSCs.
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