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Hypoxia-inducible factor 1 (HIF-1), a transcriptional activator that mediates cellular response to hypoxia and a promising target of anticancer therapy, is essential for adaptation to low oxygen conditions, embryogenesis and tumor progression. HIF-1 is a heterodimer of HIF-1α, expression of which is controlled by oxygen levels as well as by various oxygen-independent mechanisms, and HIF-1β (or ARNT), which is constitutively expressed. In this work, we investigate the phosphorylation of the N-terminal heterodimerization (PAS) domain of HIF-1α and identify Ser247 as a major site of in vitro modification by casein kinase 1δ (CK1δ). Mutation of this site to alanine, surprisingly, enhanced the transcriptional activity of HIF-1α, a result phenocopied by inhibition or small interfering RNA (siRNA)-mediated silencing of CK1δ under hypoxic conditions. Conversely, overexpression of CK1δ or phosphomimetic mutation of Ser247 to aspartate inhibited HIF-1α activity without affecting its stability or nuclear accumulation. Immunoprecipitation and in vitro binding experiments suggest that CK1-dependent phosphorylation of HIF-1α at Ser247 impairs its association with ARNT, a notion also supported by modeling the structure of the complex between HIF-1α and ARNT PAS-B domains. We suggest that modification of HIF-1α by CK1 represents a novel mechanism that controls the activity of HIF-1 during hypoxia by regulating the interaction between its two subunits.

  • Accepted May 21, 2010.
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