A Nontranscriptional Role for HIF-1α as a Direct Inhibitor of DNA Replication

In response to hypoxia, HIF-1α induces cell cycle arrest by inhibiting a helicase complex that unwinds DNA in preparation for replication. Staying Wound Up During Hypoxia Decreased oxygen availability (a condition called hypoxia) triggers various cellular responses, such as increased activity of hypoxia-inducible factor 1 (HIF-1), a transcription factor that activates genes involved in helping cells to survive oxygen deprivation. Hubbi et al. found that the HIF-1α subunit also had a nontranscriptional role in cellular responses to hypoxia. Hypoxic cells undergo cell cycle arrest, and HIF-1α altered interactions between the minichromosome maintenance (MCM) helicase, which unwinds DNA in preparation for replication, and various binding partners, leading to inhibition of MCM helicase activation and decreased DNA replication. HIF-1α mutants lacking transcriptional activity retained the ability to decrease DNA replication and to induce cell cycle arrest. HIF-1α can therefore mediate adaptive cellular responses to hypoxia through both transcriptional and nontranscriptional mechanisms. Cell cycle arrest in response to hypoxia is a fundamental physiological mechanism to maintain a balance between O2 supply and demand. Many of the cellular responses to reduced O2 availability are mediated through the transcriptional activity of hypoxia-inducible factor 1 (HIF-1). We report a role for the isolated HIF-1α subunit as an inhibitor of DNA replication, and this role was independent of HIF-1β and transcriptional regulation. In response to hypoxia, HIF-1α bound to Cdc6, a protein that is essential for loading of the minichromosome maintenance (MCM) complex (which has DNA helicase activity) onto DNA, and promoted the interaction between Cdc6 and the MCM complex. Although the interaction between Cdc6 and the MCM complex increased the association of the MCM proteins with chromatin, the binding of HIF-1α to the complex decreased phosphorylation and activation of the MCM complex by the kinase Cdc7. As a result, HIF-1α inhibited firing of replication origins, decreased DNA replication, and induced cell cycle arrest in various cell types. These findings establish a transcription-independent mechanism by which the stabilization of HIF-1α leads to cell cycle arrest in response to hypoxia.

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