O-GlcNAc transferase acts as a critical nutritional node for the control of liver homeostasis

Background & Aims

O-GlcNAcylation is a reversible post-translational modification controlled by the activity of two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). In the liver, O-GlcNAcylation has emerged as an important regulatory mechanism underlying normal liver physiology and metabolic disease.

Methods

To address whether OGT acts as a critical hepatic nutritional node, mice with a constitutive hepatocyte-specific deletion of OGT (OGTLKO) were generated and challenged with different carbohydrate- and lipid-containing diets.

Results

Analyses of 4-week-old OGTLKO mice revealed significant oxidative and endoplasmic reticulum stress, and DNA damage, together with inflammation and fibrosis, in the liver. Susceptibility to oxidative and endoplasmic reticulum stress-induced apoptosis was also elevated in OGTLKO hepatocytes. Although OGT expression was partially recovered in the liver of 8-week-old OGTLKO mice, hepatic injury and fibrosis were not rescued but rather worsened with time. Interestingly, weaning of OGTLKO mice on a ketogenic diet (low carbohydrate, high fat) fully prevented the hepatic alterations induced by OGT deletion, indicating that reduced carbohydrate intake protects an OGT-deficient liver.

Conclusions

These findings pinpoint OGT as a key mediator of hepatocyte homeostasis and survival upon carbohydrate intake and validate OGTLKO mice as a valuable model for assessing therapeutical approaches of advanced liver fibrosis.

Impact and Implications

Our study shows that hepatocyte-specific deletion of O-GlcNAc transferase (OGT) leads to severe liver injury, reinforcing the importance of O-GlcNAcylation and OGT for hepatocyte homeostasis and survival. Our study also validates the Ogt liver-deficient mouse as a valuable model for the study of advanced liver fibrosis. Importantly, as the severe hepatic fibrosis of Ogt liver-deficient mice could be fully prevented upon feeding on a ketogenic diet (i.e. very-low-carbohydrate, high-fat diet) this work underlines the potential interest of nutritional intervention as antifibrogenic strategies.

A Specific ChREBP and PPARα Cross-Talk Is Required for the Glucose-Mediated FGF21 Response

While the physiological benefits of the fibroblast growth factor 21 (FGF21) hepatokine are documented in response to fasting, little information is available on Fgf21 regulation in a glucose-overload context. We report that peroxisome-proliferator-activated receptor α (PPARα), a nuclear receptor of the fasting response, is required with the carbohydrate-sensitive transcription factor carbohydrate-responsive element-binding protein (ChREBP) to balance FGF21 glucose response. Microarray analysis indicated that only a few hepatic genes respond to fasting and glucose similarly to Fgf21. Glucose-challenged Chrebp^−/− mice exhibit a marked reduction in FGF21 production, a decrease that was rescued by re-expression of an active ChREBP isoform in the liver of Chrebp^−/− mice. Unexpectedly, carbohydrate challenge of hepatic Pparα knockout mice also demonstrated a PPARα-dependent glucose response for Fgf21 that was associated with an increased sucrose preference. This blunted response was due to decreased Fgf21 promoter accessibility and diminished ChREBP binding onto Fgf21 carbohydrate-responsive element (ChoRE) in hepatocytes lacking PPARα. Our study reports that PPARα is required for the ChREBP-induced glucose response of FGF21.

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Fgf21 is a unique hepatic gene inducible by both catabolic and anabolic signals

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The ChREBP-mediated induction of Fgf21 in hepatocytes requires PPARα

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Loss of PPARα impairs Fgf21 promoter accessibility at the ChoRE

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PPARα is required for the control of sucrose preference in vivo

Transcriptional effect of hypoxia on placental leptin

We observed recentlyl that placental leptin is markedly increased in preeclampsia. Since this disorder is associated with vascular disorders, we have tested the hypothesis that hypoxia regulates leptin expression. We show that hypoxia increased leptin mRNA and secretion in trophoblast-derived BeWo cells. This effect was mediated through leptin promoter activation. 5' deletion analysis allowed us to delineate two regions containing 1.87 kb and 1.20 kb of the promoter which conferred respectively high and low responsiveness to hypoxia. These data indicate that leptin is up-regulated by hypoxia through a transcriptional mechanism likely to involve distinct hypoxia-responsive cis-acting sequences on the promoter.

Prenatal leptin production: evidence that fetal adipose tissue produces leptin

In the adult, circulating leptin is highly correlated to adipose tissue mass. Whether such a relationship exists prenatally is unknown, because the actual source of fetal leptin has not been determined. In the present study, we have assessed the placental contribution to fetal and maternal circulating leptin concentrations and determined whether fetal adipose tissue produces leptin. The rate of leptin production in dually perfused human placenta was 0.036 ng/min.g. Ninety-five percent of the leptin released was delivered into the maternal circulation, vs. only 5% on the fetal side. Leptin messenger RNA and protein were detected in adipose tissue biopsies of 20-38 week human fetuses. However, leptin concentration was twice lower in fetal (0.22 +/- 0.11 ng/mg protein, n = 6) than in adult (0.49 +/- 0.12 ng/mg protein, n = 8) adipose tissue. Umbilical leptin levels closely reflected ponderal index at birth over a wide range of birth weights (1.6--4.1 kg). In sharp contrast, maternal and placental leptin concentrations were increased in pregnancies associated with fetal growth retardation. We conclude that umbilical leptin levels are independent of placental leptin production and can be taken as a marker of fat mass in human fetuses. By contrast, placental leptin production makes a substantial contribution to maternal circulating leptin levels during pregnancy.

Overexpression of placental leptin in diabetic pregnancy: a critical role for insulin

Leptin, a small peptide produced by adipocytes, is implicated in an increasing number of endocrine regulations, including adiposity, satiety, puberty, and fertility. Although the factors involved in controlling maternal and fetal weight gain during pregnancy have not been fully elucidated, leptin has recently emerged as such a potential factor. In our study, we report the presence of high amounts of leptin mRNA and immunoreactive protein in the human placenta, establishing the placental synthesis of this hormone. A large (three- to fivefold) augmentation in leptin mRNA and protein was found in placentas from insulin-treated diabetic women. This finding was associated with increased concentrations of leptin and insulin in venous cord blood without modification of maternal circulating leptin levels. These data provide evidence that the placenta is a site for regulated leptin production in utero. Insulin is likely to play a critical role in this regulation, thus emphasizing the importance of placental leptin signaling in diabetic pregnancy.