The role of the HIF1 signaling pathway in tumorigenesis.

摘要

The hypoxia-inducible factor 1 (HIF1) is the key transcription factor involved in the cellular responses to hypoxia and it also plays an essential role in adapting a cell to the microenvironment of a tumor through regulation of genes involved in angiogenesis, glycolysis, and many other processes. HIF1 activity is regulated in an oxygen dependent manner by a family of prolyl hydroxylases (PHDs), whose activity is required for HIF1alpha binding to the von Hippel-Lindau tumor suppressor protein (pVHL). pVHL is a component of an E3 ubiquitin ligase complex responsible for instigating proteosomal degradation of HIF1alpha. This role of regulating HIF1 activity is one reason for pVHL's tumor suppressor activity. Interestingly, this process is completely dependant upon PHD-mediated posttranslational hydroxylation of HIF1alpha and this requirement raises the possibility of the involvement of PHD in tumor development.; In this study, we characterize the relationship between PHD and HIF1 activity and cellular transformation using a lineage of the MSU1 cell strains of varying tumorigenic potential. We have shown that PHD2 represents the primary HIF prolyl hydroxylase in regulating HIF1 signaling 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 their 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 a growth 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.; The hypoxia signaling pathway has many input signals, including low oxygen, reactive oxygen species, TCA metabolites, and various growth factors. It is currently not known how these disparate signals influence HIF1 activity. To address this knowledge gap, we attempted to characterize the PHD2-protein interaction network (PHD2-PIN) using tandem affinity purification and liquid chromatography coupled tandem mass spectrometric analysis (LC-MS/MS). Our result suggested the possibility that PHD2 might have no specific interaction with other proteins under normoxia and that PHD2 might act to coordinate the various input signals without the aid of accessory factors.