Hypoxia-inducible transcription factor 2α promotes steatohepatitis through augmenting lipid accumulation, inflammation, and fibrosis

Abstract

Oxygen dynamics in the liver is a central signaling mediator controlling hepatic homeostasis, and dysregulation of cellular oxygen is associated with liver injury. Moreover, the transcription factor relaying changes in cellular oxygen levels, hypoxia-inducible factor (HIF), is critical in liver metabolism and sustained increase in HIF signaling can lead to spontaneous steatosis, inflammation, and liver tumorigenesis. However, the direct responses and genetic networks regulated by HIFs in the liver are unclear. To help define the HIF signal transduction pathway, an animal model of HIF overexpression was generated and characterized. In this model, overexpression was achieved by Von Hippel-Lindau (Vhl) disruption in a liver-specific temporal fashion. Acute disruption of Vhl induced hepatic lipid accumulation in a HIF-2α-dependent manner. In addition, HIF-2α activation rapidly increased liver inflammation and fibrosis demonstrating that steatosis and inflammation are primary responses of the liver to hypoxia. To identify downstream effectors, a global microarray expression analysis was performed using livers lacking Vhl for 24-hours and 2-weeks, revealing a time-dependent effect of HIF on gene expression. Increase in genes involved in fatty acid synthesis were followed by an increase in fatty acid uptake-associated genes, and an inhibition of fatty acid β-oxidation. A rapid increase in pro-inflammatory cytokines and fibrogenic gene expression was also observed. In vivo chromatin immunoprecipitation assays revealed novel direct targets of HIF signaling that may contribute to hypoxia-mediated steatosis and inflammation. These data suggest that HIF-2α is a critical mediator in progression from clinically manageable steatosis to more severe steatohepatitis and liver cancer, and may be a potential therapeutic target.

Cellular and molecular mechanisms in liver fibrogenesis

“…A very elegant and recent study has provided further evidence for relationships between HIFs and hepatic fibrogenesis by employing hepatocyte-specific von Hippel Lindau protein (VHL) and HIF-1 or HIF-2 mouse mutants [88]. When mice with liver conditional disruption of VHL (and then with an increased expression of both HIF-1 and HIF-2) were treated for 2 weeks with an ethanol-containing diet, they developed increased fibrosis.…”

“…When mice with liver conditional disruption of VHL (and then with an increased expression of both HIF-1 and HIF-2) were treated for 2 weeks with an ethanol-containing diet, they developed increased fibrosis. Interestingly, this increase was prevented when simultaneous deletion of HIF-2 (but not HIF-1), was carried out, suggesting that HIF-2 may prevail in regulating hepatic fibrogenesis [88].…”

Suppression of Hepatocyte Proliferation by Hepatocyte Nuclear Factor 4α in Adult Mice

“…The Vhl F/F;AlbERT2cre mice were described previously and treated with tamoxifen for 5 days and then returned to a chow diet for 11 days prior to euthanasia (22). Hepatic deletion of VHL results in constitutive activation of HIF2␣ in the liver, leading to rapid inflammation, lipid accumulation, and fibrosis (22). For BrdU incorporation experiments, miniosmotic pumps containing sterile BrdU were implanted subcutaneously, and mice were euthanized 6 days later.…”

“…A recent study found elevated BMP7 levels in a hepatitis B mouse model of HCC and further identified elevated BMP7 in human cirrhotic liver and HCC samples (36). To assess whether the induction of Bmp7 in Hnf4a F/F;AlbERT2cre ϩ tamoxifen mice was associated with steatosis and liver damage or cellular proliferation, Bmp7 levels were measured in the Vhl F/F;AlbERT2cre mouse, which displays hepatic steatosis, inflammation, and fibrosis but not cellular proliferation (22). Transglutaminase 2 (Tgm2), a marker of fibrosis, is significantly up-regulated in Vhl F/F;AlbERT2cre ϩ tamoxifen mice, but Bmp7 levels remain nearly undetectable (Fig.…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…HIF-induced transcription promotes angiogenesis, erythropoiesis, metastasis and metabolic reprogramming, such as shifting cell metabolism from oxidative phosphorylation to glycolysis. HIF activation due to hypoxia or loss of VHL function also reprograms lipid metabolism leading to lipid accumulation (Huss et al, 2001;Boström et al, 2006;Rankin et al, 2009;Kucejova et al, 2011;Qu et al, 2011;Walter et al, 2014).…”