Hepatocyte HIF-1 and Intermittent Hypoxia Independently Impact Liver Fibrosis in Murine Nonalcoholic Fatty Liver Disease

Abstract

Obstructive sleep apnea is associated with insulin resistance, lipid dysregulation, and hepatic steatosis and fibrosis in nonalcoholic fatty liver disease (NAFLD). We have previously shown that hepatocyte HIF-1 (hypoxia-inducible factor-1) mediates the development of liver fibrosis in a mouse model of NAFLD. We hypothesized that intermittent hypoxia (IH) modeling obstructive sleep apnea would worsen hepatic steatosis and fibrosis in murine NAFLD, via HIF-1. Mice with hepatocyte-specific deletion of Hif1a (Hif1a 2/2 hep) and wild-type (Hif1a F/F ) controls were fed a high trans-fat diet to induce NAFLD with steatohepatitis. Half from each group were exposed to IH, and the other half were exposed to intermittent air. A glucose tolerance test was performed just prior to the end of the experiment. Mitochondrial efficiency was assessed in fresh liver tissue at the time of death. The hepatic malondialdehyde concentration and proinflammatory cytokine levels were assessed, and genes of collagen and fatty acid metabolism were examined. Hif1a 2/2 hep mice gained less weight than wild-type Hif1a mice (22.3 g, P = 0.029). There was also a genotype-independent effect of IH on body weight, with less weight gain in mice exposed to IH (P = 0.003). Fasting glucose, homeostatic model assessment for insulin resistance, and glucose tolerance test results were all improved in Hif1a 2/2 hep mice. Liver collagen was increased in mice exposed to IH (P = 0.033) and was reduced in Hif1a 2/2 hep mice (P , 0.001), without any significant exposure/genotype interaction being demonstrated. Liver TNF-a and IL-1b were significantly increased in mice exposed to IH and were decreased in Hif1a 2/2 hep mice. We conclude that HIF-1 signaling worsens the metabolic profile and hastens NAFLD progression and that IH may worsen liver fibrosis. These effects are plausibly mediated by hepatic inflammatory stress.

Sleep Apnea, Hypoxia Inducible Factor, and Fatty Liver: More Questions Than Answers?

“…This hypothesis was tested by Mesarwi and colleagues (pp. 390–402 ) and is reported in this issue of the Journal ( 15 ). The investigators examined the role of HIF-1α in IH-induced progression of NAFLD using two protocols: 1 ) mice with hepatocyte-specific deletion of Hif1-α ( Hif1-α −/− hep ) and wild type ( Hif1-α F/ F ) were fed a trans-fat diet for 26 weeks and exposed to IH or intermittent air in the last 6 weeks, and 2 ) C57BL/6J mice were fed with the same diet for 26 weeks and treated with HIF-1α antisense oligonucleotides (ASO), scrambled ASO, or saline while exposed to IH.…”

“… Summary of the main findings of Mesarwi and colleagues ( 15 ). Independent effects of intermittent hypoxia and HIF-1α knockout were observed on a model of nonalcoholic fatty liver disease, but there was no significant interaction.…”

Hypoxia-Inducible Factors as Key Players in the Pathogenesis of Non-alcoholic Fatty Liver Disease and Non-alcoholic Steatohepatitis

“…In patients with OSA and obesity, more severe OSA was associated with worsened fibrosis (29), and circulating levels of LOX (which is regulated by HIFs) are higher in patients with obesity and more severe OSA (64). In mice fed a high trans-fat diet and exposed to CIH (97), and in rats fed a HFD to induce NASH and injected with sodium nitrite to mimic CIH (70) fibrosis worsened. It is not clear what mechanisms contributed to this.…”

“…VEGF receptor neutralising antibodies attenuated the development of fibrosis in CCl 4 induced fibrosis, and VEGF stimulated HSC proliferation in vitro (98), further supporting a role for pathological angiogenesis. However, the link between CIH and fibrosis may not always be HIF1α mediated, as, while HIF1α deletion improved liver fibrosis and inflammation in trans-fat diet fed mice with or without CIH, it did so without significant interaction with CIH effects (97). Further research is needed to understand the mechanisms involved in the link between CIH/OSA and liver fibrosis.…”

“…This pathological phenotype was averted by concomitant deletion of Hif2a, but not Hif1a, suggesting a greater importance for HIF2α in driving steatohepatitis in hepatocytes. Similarly, in patients with NAFLD, hepatic levels of HIF2α and HIF1α are increased in early, noninflamed stages of NAFLD, but only HIF2α levels are further increased in the livers of patients with NASH vs. patients with non-inflamed NAFLD (40), though studies in animal models do suggest a possible role for HIF1α as well (97).…”