Hypoxic Regulation of Cell Cycle Checkpoints: Most normal cells undergo a cell cycle arrest when rendered hypoxic. Many neoplastic cells, when hypoxic, can proliferate. We have delineated two cell cycle checkpoints that are activated in normal hypoxic cells. Under moderate hypoxic conditions, CDK2 activity is diminished, leading to Rb hypo-phosphorylation and a G1 cell cycle arrest. Cells deficient in Rb, or p27, can proliferate when moderately hypoxic. Anoxic cells undergo an S phase arrest. We have determined that under these conditions the elongation component of DNA replication is unaffected, but the initiation of DNA replication is suppressed. We are currently evaluating the role of ATR activation in this anoxia-induced S phase checkpoint. We have also determined that expression of the E1a oncoprotein permits immortalized cells to proliferate even when anoxic, thus bypassing these checkpoints. We are interested in understanding how E1a bypasses these checkpoints, and the significance of hypoxic proliferation in genomic instability and tumorigenesis. |
Hypoxic Regulation of Id-1: Id-1 acts as a dominant negative against several helix-loop-helix transcription factors and has been reported to play an important role in differentiation, neoplastic proliferation and neovascularization. We have determined that Id-1 is down-regulated in most hypoxic cells. This down-regulation is dependent on the hypoxic-activation of the Unfolded Protein Response, and the induction of the transcriptional repressor ATF-3. The Unfolded Protein Response is not activated in hypoxic neuroblastoma cells, and instead of down-regulation of Id-1 it is up-regulated by the hypoxia-induced transcription factor HIF-1. We are interested in the mechanism for the aberrant UPR in neuroblastoma, and will use animal models to determine the in vivo significance of hypoxic regulation of Id-1 in tumorigenesis. We are also interested in the role Id-1 regulation plays in the epithelial to mesenchymal transformation (EMT) which we have observed in hypoxic epithelial cells. |
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