A wrinkle in the unfolding of hypoxic response: HIF and ATF4

Köditz et al describe the interaction of activating transcription factor-4 (ATF4) with the HIF hydroxylase PHD3, suggesting that oxygen-dependent regulatory pathways mediated by PHDs might not be confined to the HIF system.

Hypoxia is a dynamic feature of ischemic disease and the tumor microenvironment. Survival pathways help cells adapt and overcome the hypoxic stress. The hypoxia-inducible factor (HIF) is the master regulator of the cellular adaptation to hypoxia.¹  In hypoxia, the α/β heterodimeric HIF complex induces transcription of target genes such as EPO, Glut-1, VEGF, and iNOS. Under normoxic conditions the von Hippel-Lindau (VHL) tumor suppressor protein binds the HIF-α subunits, which leads to rapid proteasomal degradation. Essential for binding of VHL to the HIF-α subunit is an oxygen-dependent posttranslational hydroxylation. This reaction is catalyzed by 3 oxygenases termed PHD1, PHD2, and PHD3 (PHD: prolyl hydroxylase domain enzymes), which are dioxygenases belonging to the 2-oxoglutarate (2OG) and Fe(II)-dependent oxygenase superfamily.²  Prolyl hydroxylation through PHDs, and also asparagyl hydroxylation by the factor inhibiting HIF (FIH), has been shown to link cellular oxygen availabilty to the transcriptional response of the HIF system.

Apart from HIF-regulated adaptation processes, several HIF-independent pathways exist to sustain oxygen deprivation. Recent evidence suggests that hypoxia activates components of the unfolded protein response (UPR). The stress-responsive activating transcription factor-4 (ATF4) is a downstream UPR gene that is up-regulated by a variety of stimuli such as hypoxia/anoxia, ER stress, and metabolic and oxidative stress. ATF4 can function as a transcriptional activator of UPR genes but has also been implicated in cellular defense processes. Studies in transgenic animals indicate that ATF4 is required for skeletal and eye development, cellular proliferation, and hematopoiesis.³  Moreover, ATF4 has been observed in greater levels in tumors than in normal tissue, suggesting that ATF4 signaling and the UPR as a whole are active in hypoxic areas of tumors and might regulate processes relevant to cancer progression.⁴ 

As Köditz and coworkers now demonstrate, the HIF prolyl hydroxylase PHD3 interacts with ATF4, thus establishing a link between the 2 adaptive systems. They convincingly show that ATF4 binds to PHD3 and that ATF4 protein stability is increased by PHD inhibition or hypoxia. Further characterization of the interaction identified a proline-rich domain in ATF4, which is important for oxygen-dependent degradation. This process does not involve VHL. But the molecular mechanism of the posttranslational modification of ATF4 by PHD3 is not fully understood yet, and it remains elusive whether hydroxylation of the prolines or the binding of other factors influence the stability of ATF4.

The functional significance of the findings of Köditz and colleagues lies in the demonstration of a molecular link between 2 central systems of cellular adaptive processes to stress. It raises the possibility that hydroxylation, and ultimately function, of HIF-α or HIF target genes might be influenced by this novel role for PHD3, and might as well have impact on the general understanding of cell-fate decisions in hypoxia.