Hypoxia-Inducible Factor 2 Alpha Is Essential for Hepatic Outgrowth and Functions via the Regulation of leg1 Transcription in the Zebrafish Embryo

Abstract

The liver plays a vital role in metabolism, detoxification, digestion, and the maintenance of homeostasis. During development, the vertebrate embryonic liver undergoes a series of morphogenic processes known as hepatogenesis. Hepatogenesis can be separated into three interrelated processes: endoderm specification, hepatoblast differentiation, and hepatic outgrowth. Throughout this process, signaling molecules and transcription factors initiate and regulate the coordination of cell proliferation, apoptosis, differentiation, intercellular adhesion, and cell migration. Hifs are already recognized to be essential in embryonic development, but their role in hepatogenesis remains unknown. Using the zebrafish embryo as a model organism, we report that the lack of Hif2-alpha but not Hif1-alpha blocks hepatic outgrowth. While Hif2-alpha is not involved in hepatoblast specification, this transcription factor regulates hepatocyte cell proliferation during hepatic outgrowth. Furthermore, we demonstrated that the lack of Hif2-alpha can reduce the expression of liver-enriched gene 1 (leg1), which encodes a secretory protein essential for hepatic outgrowth. Additionally, exogenous mRNA expression of leg1 can rescue the small liver phenotype of hif2-alpha morphants. We also showed that Hif2-alpha directly binds to the promoter region of leg1 to control leg1 expression. Interestingly, we discovered overrepresented, high-density Hif-binding sites in the potential upstream regulatory sequences of leg1 in teleosts but not in terrestrial mammals. We concluded that hif2-alpha is a key factor required for hepatic outgrowth and regulates leg1 expression in zebrafish embryos. We also proposed that the hif2-alpha-leg1 axis in liver development may have resulted from the adaptation of teleosts to their environment.

The zebrafish miR‐462/miR‐731 cluster is induced under hypoxic stress via hypoxia‐inducible factor 1α and functions in cellular adaptations

“…Under hypoxic conditions, HIF‐1 mediates a metabolism shift from mitochondrial oxidative phosphorylation to aerobic glycolysis by activating downstream metabolism factors, including pyruvate dehydrogenase kinase 1, lactate dehydrogenase, and pyruvate kinase M2 (21, 22). In addition, mammalian embryonic development occurs in a low‐oxygen environment (23), and HIFs are known to function in tissue formation and many developmental systems, such as angiogenesis and osteogenesis (24, 25), hematopoiesis (26), and hepatogenesis (27). Moreover, hypoxia response and HIF signaling may control innate and adaptive immunity by acting on immune cells in pathologic states (28).…”

Targeting hypoxia-inducible factor-2α enhances sorafenib antitumor activity via β-catenin/C-Myc-dependent pathways in hepatocellular carcinoma

“…HIF-1 was reported to mediate the expression of hundreds of genes, including those for vascular endothelial growth factors, glycolytic enzymes and glucose transporters (14,15). In addition, HIF-2 was identified to be involved in implementing the hypoxic (14,15).…”

“…HIF-1 was reported to mediate the expression of hundreds of genes, including those for vascular endothelial growth factors, glycolytic enzymes and glucose transporters (14,15). In addition, HIF-2 was identified to be involved in implementing the hypoxic (14,15). HIF-1β was reported to be able to form a heterodimer complex wither either HIF-1α or HIF-2α, both of which undergo oxygen-dependent degradation via the von Hippel-Lindau protein pathway (14,15).…”

“…In addition, HIF-2 was identified to be involved in implementing the hypoxic (14,15). HIF-1β was reported to be able to form a heterodimer complex wither either HIF-1α or HIF-2α, both of which undergo oxygen-dependent degradation via the von Hippel-Lindau protein pathway (14,15). In addition, it was reported that targeting HIF-1α may significantly enhance sorafenib antitumor sensitivity under hypoxic conditions (10); however, the association between HIF-2α and sorafenib sensitivity remains to be elucidated.…”

Id2a is required for hepatic outgrowth during liver development in zebrafish

“…Moreover, bHLH-PAS (Per-ARNT-Sim) factors, such as the hypoxia inducible factors (HIFs), participate in hepatic disease, regeneration, fibrosis, and hepatocellular carcinoma (Nath and Szabo, 2012). Hif2α (renamed as Epas1b) binds hypoxia response elements (HREs) and regulates hepatic outgrowth in zebrafish (Lin et al, 2014). The activity of bHLH factors can be regulated by the inhibitor of DNA binding (ID) family of proteins.…”