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

Hypoxia inducible factor 2 (HIF-2) is a transcriptional activator implicated in the cellular response to hypoxia. Regulation of its inducible subunit, HIF-2 alpha (also known as EPAS1), involves post-translational modifications. Here, we demonstrate that casein kinase 1d (CK1d; also known as CSNK1D) phosphorylates HIF-2 alpha at Ser383 and Thr528 in vitro. We found that disruption of these phosphorylation sites, and silencing or chemical inhibition of CK1 delta, 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 the levels of HIF-2 alpha protein expression. Furthermore, when CK1 delta-dependent phosphorylation of HIF-2 alpha was inhibited, we observed substantial cytoplasmic mislocalization of HIF-2 alpha, which was reversed upon the addition of the nuclear protein export inhibitor leptomycin B. Taken together, these data suggest that CK1 delta enhances EPO secretion from liver cancer cells under hypoxia by modifying HIF-2 alpha and promoting its nuclear accumulation. This modification represents a new mechanism of HIF-2 regulation that might allow HIF isoforms to undertake differing functions.