The biphasic role of prolyl hydroxylase domain protein 2 (PHD2/EGLN1) in modulating tumor-forming potential

LB-202

Hypoxia is a common feature of solid tumors and the ability of cancer cells to adapt to hypoxia is important for tumor progression. The cellular response to hypoxic stress is controlled by a family of prolyl hydroxylases (PHDs) and the transcription factor hypoxia-inducible factor 1 (HIF1). To investigate the relationship between PHD, HIF1 activity, and cellular transformation we characterized the expression levels of PHD isoforms across a lineage of human fibroblast cell strains. These cells, derived one from the other, each one having acquired a characteristic related to malignant transformation, until the last change gave rise to a cell strain that by acquiring one more change, e.g., an RAS oncogene, becomes capable of forming malignant tumors in athymic mice with a short latency. Using these cells we have shown that PHD2 represents the primary HIF prolyl hydroxylase within this lineage of cells and that PHD2 levels decrease as the cell exhibits more transformed characteristics. When PHD2 levels were altered with RNAi in non-tumorigenic fibroblasts we found that moderate decreases in PHD2 activity can lead to malignant transformation, whereas cells with a more severe loss of PHD2 were unable to form tumors. Consistent with these results, direct chemical inhibition of PHD2 activity in transformed cells reverses a cell’s transformed characteristics. Moreover, we found that overexpression of PHD2 in malignant fibroblasts leads to loss of the tumor-forming ability. These changes correlated with HIF1-activated glycolytic rates, vascularization, and the ability to grow under hypoxic stress. These findings suggest a biphasic model for the relationship between PHD2 activity and malignant transformation: With a slight decrease in PHD2 activity the cells gain an advantage such as enhanced glycolysis and angiogenesis and become malignantly transformed. As PHD2 activity is further decreased, the pro-death response might become the dominant signal and the increased adaptation would be overwhelmed. The dual nature of this response is presumably due to PHD2’s ability to alter the cellular balance between hypoxic-induced adaptation and pro-death responses.