YC-1 Induces S Cell Cycle Arrest and Apoptosis by Activating Checkpoint Kinases

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Abstract
Hypoxia-inducible factor-1α (HIF-1α) seems central to tumor growth and progression because it up-regulates genes essential for angiogenesis and the hypoxic adaptation of cancer cells, which is why HIF-1α inhibition is viewed as a cancer therapy strategy. Paradoxically, HIF-1α also leads to cell cycle arrest or the apoptosis of cancer cells. Thus, the possibility cannot be ruled out that HIF-1α inhibitors unlock cell cycle arrest under hypoxic conditions and prevent cell death, which would limit the anticancer effect of HIF-1α inhibitors. Previously, we reported on the development of YC-1 as an anticancer agent that inhibits HIF-1α. In the present study, we evaluated the effects of YC-1 on hypoxia-induced cell cycle arrest and cell death. It was found that YC-1 does not reverse the antiproliferative effect of hypoxia, but rather that it induces S-phase arrest and apoptosis at therapeutic concentrations that inhibit HIF-1α and tumor growth; however, YC-1 did not stimulate cyclic guanosine 3′,5′-monophosphate production in this concentration range. It was also found that YC-1 activates the checkpoint kinase–mediated intra-S-phase checkpoint, independently of ataxia-telangiectasia mutated kinase or ataxia-telangiectasia mutated and Rad3-related kinase. These results imply that YC-1 does not promote the regrowth of hypoxic tumors because of its cell cycle arrest effect. Furthermore, YC-1 may induce the combined anticancer effects of HIF-1α inhibition and cell growth inhibition. (Cancer Res 2006; 66(12): 6345-52)