Mitochondrial role in the neonatal predisposition to developing nonalcoholic fatty liver disease (original) (raw)

Growing evidence suggests that individuals who develop pediatric NAFLD are born with predisposed risk. Severity of childhood NAFLD correlates with maternal obesity (13), birthweight (14), and shorter duration of breastfeeding (13, 15), even after adjusting for childhood BMI. The fetal programming hypothesis, that environmental conditions during critical periods influence long-term physical development, was first suggested by David Barker regarding maternal undernutrition and low-birthweight infants (16). From the early 1990s to the present, however, human epidemiologic studies combined with functional studies in animal models have demonstrated that increased maternal BMI, insulin resistance, and high-fat diet (HFD) all contribute to increased risk of metabolic disease in offspring later in life. Mothers entering pregnancy with preexisting insulin resistance, such as those with obesity or GDM or consuming a WSD (high in both fat and sugar), display an overabundance of maternal metabolic substrates (17). The fetus might be especially vulnerable to steatosis because immature fetal adipose depots that buffer the excess transplacental lipids or other fuels in maternal obesity are not available until late in gestation. Excess substrates contribute not only to excess fetal growth but also to increased risk for childhood obesity and metabolic disease. Maternal metabolites that indicate altered mitochondrial function and are associated with maternal overnutrition include excess glucose, fatty acids (FAs), triglycerides, branched-chain amino acids (BCAAs), and long-chain acylcarnitine species. These are elevated in maternal plasma during pregnancy (17), are found in cord blood at birth (18), and are intimately associated with maternal BMI, insulin resistance in pregnancy, high maternal dietary fat intake, and high infant birthweight (19).

Limited human data suggest that the maternal environment is important for NAFLD pathogenesis extremely early in life. The most direct evidence derives from a handful of studies in offspring of mothers with diabetes (gestational and pregestational) and/or obesity. In one study, 105 neonates (mean age 11 days) of mothers with a BMI ranging from underweight to obese (16–36 kg/m2) were characterized based on adipose tissue and hepatic lipid content using MRI and NMR spectroscopy (8). Neonatal hepatic fat content, as well as total adiposity, correlated with maternal BMI after correcting for infant sex and gestational age (8). Using a noninvasive MRI method, we described a 68% increase in hepatic lipid content in the 2-week-old offspring of mothers with obesity and GDM versus normal-weight mothers (7). In this study and the former study, follow-up hepatic MRIs were not performed at a later age. Therefore, neither the persistence of increased intrahepatic fat nor the risk of these infants going on to develop NASH was assessed. Finally, in another study, 78% of stillborn offspring of obese mothers with diabetes (n = 33) had evidence of hepatic steatosis versus 17% of stillborn offspring of nondiabetic mothers (n = 48) (20). Diabetes in this study was predominantly gestational (n = 22), but mothers with preexisting diabetes were included as well (20).

No neonatal studies looking at prospective development of NAFLD or testing biomarkers in the setting of increased neonatal hepatic lipid accumulation in humans have been reported. Early detection of NAFLD is challenging because of difficulties of tissue sampling and of performing potentially invasive studies on otherwise asymptomatic children. The Avon Longitudinal Study of Parents and Children (ALSPAC) demonstrated that offspring of insulin-resistant mothers were at higher risk for signs of NAFLD at 17 years of age versus offspring of mothers without insulin resistance (21). Increased maternal BMI was also associated with NAFLD in offspring, largely owing to its correlation with offspring adiposity (21). The Western Australian Pregnancy (Raine) Cohort Study reported similar results: increased maternal pre-pregnancy BMI conferred sex-specific risk for NAFLD in offspring at 17 years of age (22). Offspring of obese or diabetic mothers born with intrauterine growth restriction (IUGR) have a predisposition to develop NAFLD (23). Undernutrition in pregnancy can also result in IUGR in offspring and predispose offspring to develop metabolic syndrome and NAFLD (23).

Noninvasive analyses examining blood from offspring of obese and/or insulin-resistant mothers offer more insight into potential mitochondrial mechanisms of hepatic lipid accumulation in the setting of fetal programming. The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study, one of the largest studies to date, examined metabolites in neonates in relation to maternal obesity (18). Metabolomic analyses on cord blood from 1,600 mother-infant pairs spanning four ethnicities showed that maternal BMI was associated with cord blood BCAAs and their catabolites propionylcarnitine (C3), butyrylcarnitine/isobutyrylcarnitine (C4/Ci4), and isovalerylcarnitine (C5) (18). Maternal glucose was associated with cord blood levels of ketone 3-hydroxybutyrate, its carnitine ester, 3-hydroxy-decanoyl carnitine (C10OH or C8DC), and glycerol (18). These biomarkers are classically linked to hepatic and skeletal muscle mitochondrial dysmetabolism, as well as insulin resistance and risk of type 2 diabetes in adults.

In our unpublished analysis of neonatal plasma at 48 hours of life, we found markers of incomplete mitochondrial lipid oxidation (medium-chain and dicarboxylic acylcarnitines) and higher C5 relative to neonatal adiposity in offspring of overweight/obese, but not normal-weight, mothers (24). C5 is a marker of insulin resistance and related compensatory increased BCAA catabolism (25, 26). Interestingly, our proteomic analysis revealed enrichment of the BCAA catabolism pathway relative to neonatal adiposity in offspring of overweight/obese, but not normal-weight, mothers (24).

Stem cells from offspring of obese mothers might also provide clues regarding in utero programming of altered mitochondrial function. Using umbilical cord–derived mesenchymal stem cells (MSCs), we demonstrated relationships between maternal pre-pregnancy BMI, maternal circulating lipids during pregnancy, neonatal adiposity, and adiposity gain in the first few months of life with markers of altered mitochondrial function (27–30). It should be noted that although the MSCs studied were induced to differentiate toward adipocytes and myocytes, these same precursor cells are known to populate the liver as hepatic stellate cells, which are intricately involved in hepatic fibrosis (31, 32). In myocyte-differentiated MSCs, accumulation of long-chain and dicarboxylic acylcarnitines correlated with neonatal adiposity particularly in offspring of obese mothers (27). In differential gene expression analysis, we observed that pathway enrichment for multiple metabolic processes corresponds to maternal BMI (27). Adipocyte-differentiated MSCs demonstrated broad changes in mitochondrial gene expression related to maternal FA levels in the second trimester. There was upregulation of multiple electron transport chain (ETC) genes coupled with downregulation of genes related to mitochondrial biogenesis, including CREBBP, EP300, and PPARA (27). Also, pathway enrichment was observed for nutrient-sensing pathways, including PI3K/AKT and AMPK, relative to maternal BMI (27).

Rapid gain in adiposity in the first 5 months of life is a known risk factor for childhood obesity and metabolic syndrome and is a risk marker for NAFLD (33, 34). In infants with rapid gains in adiposity, we found markers of incomplete lipid oxidation and upregulation of membrane lipid transport genes in their MSC-derived adipocytes (28). We further found alterations in analytes and genes in the glutathione cycle, including higher cysteine concentrations and upregulation of GCLC, the rate-limiting step in glutathione synthesis and ophthalmate generation (28). Oxidative stress was greater, as suggested by upregulation of SOD2 and HIF1 gene expression in MSCs (28). Taken together, these findings suggest that rapid gain in adiposity in an infant’s first months of life corresponds with potentially lipotoxic alterations in their stem cells. More germane to the discussion of hepatic disease, we also found upregulation of genes in the Kyoto Encyclopedia of Genes and Genomes (KEGG) NAFLD disease pathway as well as upregulation of mitochondrial oxidative phosphorylation genes relative to rapid infant gain in adiposity (28).