Down regulation of Wnt signaling mitigates hypoxia-induced chemoresistance in human osteosarcoma cells.

Osteosarcoma (OS) is the most common type of solid bone cancer and remains the second leading cause of cancer-related death for children and young adults. Hypoxia is an element intrinsic to most solid-tumor microenvironments, including that of OS, and is associated with resistance to therapy, poor survival, and a malignant phenotype. Cells respond to hypoxia through alterations in gene expression, mediated most notably through the hypoxia-inducible factor (HIF) class of transcription factors. Here we investigate hypoxia-induced changes in the Wnt/β-catenin signaling pathway, a key signaling cascade involved in OS pathogenesis. We show that hypoxia results in increased expression and signaling activation of HIF proteins in human osteosarcoma cells. Wnt/β-catenin signaling is down-regulated by hypoxia in human OS cells, as demonstrated by decreased active β-catenin protein levels and axin2 mRNA expression (p<0.05). This down-regulation appears to rely on both HIF-independent and HIF-dependent mechanisms, with HIF-1α standing out as an important regulator. Finally, we show that hypoxia results in resistance of human OS cells to doxorubicin-mediated toxicity (6-13 fold increase, p<0.01). These hypoxic OS cells can be sensitized to doxorubicin treatment by further inhibition of the Wnt/β-catenin signaling pathway (p<0.05). These data support the conclusion that Wnt/β-catenin signaling is down-regulated in human OS cells under hypoxia and that this signaling alteration may represent a viable target to combat chemoresistant OS subpopulations in a hypoxic niche.