HIF-2α phosphorylation by CK1δ promotes erythropoietin secretion in liver cancer cells under hypoxia

Abstract

Hypoxia Inducible Factor 2 (HIF-2) is a transcriptional activator implicated in the cellular response to hypoxia. Regulation of its inducible subunit, HIF-2α, involves post-translational modifications. Here, we demonstrate that casein kinase 1δ (CK1δ) phosphorylates HIF-2α at Ser383 and Thr528 in vitro. Disruption of these phosphorylation sites and silencing or chemical inhibition of CK1δ reduced the expression of HIF-2 target genes and the secretion of erythropoietin (EPO) in two hepatic cancer cell lines, Huh7 and HepG2, without affecting levels of HIF-2α protein expression. Furthermore, when CK1δ-dependent phosphorylation of HIF-2α was inhibited, we observed significant cytoplasmic mislocalization of HIF-2α, which was reversed upon addition of the nuclear protein export inhibitor Leptomycin B. Taken together these data suggest that CK1δ enhances EPO secretion from liver cancer cells under hypoxia by modifying HIF-2α and promoting its nuclear accumulation. This modification represents a novel mechanism of HIF-2 regulation that may allow differentiation of HIF isoform function.