DEAD-box protein family member DDX28 is a negative regulator of HIF-2α and eIF4E2-directed hypoxic translation

ABSTRACT Hypoxia occurs when there is a deficiency in oxygen delivery to tissues and is connected to physiological and pathophysiological processes such as embryonic development, wound healing, heart disease and cancer. The master regulators of oxygen homeostasis in mammalian cells are the heterodimeric hypoxia-inducible transcription factors HIF-1 and HIF-2. The oxygen-labile HIF-2α subunit has not only been implicated in transcription, but also as a regulator of eIF4E2-directed hypoxic translation. Here, we have identified the DEAD-box protein family member DDX28 as a novel interactor and negative regulator of HIF-2α that suppresses its ability to activate eIF4E2-directed translation. We demonstrate that stable silencing of DDX28 via shRNA in hypoxic human U87MG glioblastoma cells caused an increase, relative to control, to: HIF-2α protein levels, the ability of eIF4E2 to bind the m 7 GTP cap structure, and the translation of select eIF4E2 target mRNAs. DDX28 depletion elevated both nuclear and cytoplasmic HIF-2α, but HIF-2α transcriptional activity did not increase possibly due to its already high nuclear abundance in hypoxic control cells. Depletion of DDX28 conferred a proliferative advantage to hypoxic, but not normoxic cells, which is likely a consequence of the translational upregulation of a subset of hypoxia-response mRNAs. DDX28 protein levels are reduced in several cancers, including glioma, relative to normal tissue. Therefore, we uncover a regulatory mechanism for this potential tumor suppressor in the repression of HIF-2α- and eIF4E2-mediated translation activation of oncogenic mRNAs.