Abstract
Many severely hypoxic cells fail to initiate DNA replication, but the mechanism underlying this observation is unknown. Specifically, although the ataxia-telangiectasia-rad3 related (ATR) kinase has been shown to be activated in hypoxic cells, several studies have not been able to document down-stream consequences of ATR activation in these cells. By clearly defining the DNA replication initiation checkpoint in hypoxic cells, we now demonstrate that ATR is responsible for activating this checkpoint. We show that the hypoxic activation of ATR leads to the phosphorylation-dependent degradation of the cdc25a phosphatase. Downregulation of cdc25a protein by ATR in hypoxic cells decreases CDK2 phosphorylation and activity, which results in the degradation of cdc6 by APC/C(Cdh1). These events do not occur in hypoxic cells when ATR is depleted, and the initiation of DNA replication is maintained. We therefore present a novel mechanism of cdc6 regulation in which ATR can have a central role in inhibiting the initiation of DNA replication by the regulation of cdc6 by APC/C(Cdh1). This model provides insight into the biology and therapy of hypoxic tumors.