Maternal obesity and fetal metabolic programming: a fertile epigenetic soil

Abstract

The incidence of obesity and overweight has reached epidemic levels in the United States and developed countries worldwide. Even more alarming is the increasing prevalence of metabolic diseases in younger children and adolescents. Infants born to obese, overweight, and diabetic mothers (even when normal weight) have increased adiposity and are at increased risk of later metabolic disease. In addition to maternal glucose, hyperlipidemia and inflammation may contribute to the childhood obesity epidemic through fetal metabolic programming, the mechanisms of which are not well understood. Pregravid obesity, when combined with normal changes in maternal metabolism, may magnify increases in inflammation and blood lipids, which can have profound effects on the developing embryo and the fetus in utero. Fetal exposure to excess blood lipids, particularly saturated fatty acids, can activate proinflammatory pathways, which could impact substrate metabolism and mitochondrial function, as well as stem cell fate, all of which affect organ development and the response to the postnatal environment. Fetal and neonatal life are characterized by tremendous plasticity and the ability to respond to environmental factors (nutrients, oxygen, hormones) by altering gene expression levels via epigenetic modifications. Given that lipids act as both transcriptional activators and signaling molecules, excess fetal lipid exposure may regulate genes involved in lipid sensing and metabolism through epigenetic mechanisms. Epigenetic regulation of gene expression is characterized by covalent modifications to DNA and chromatin that alter gene expression independent of gene sequence. Epigenetic modifications can be maintained through positive and negative feedback loops, thereby creating stable changes in the expression of metabolic genes and their main transcriptional regulators. The purpose of this article is to review current literature on maternal-fetal lipid metabolism and maternal obesity outcomes and to suggest some potential mechanisms for fetal metabolic programming in key organ systems that regulate postnatal energy balance, with an emphasis on epigenetics and the intrauterine environment.

Figures

Fig. 1.

Obesity and pregnancy are associated with insulin resistance and inflammatory changes that exacerbate in combination, increasing lipid transfer earlier in gestation. Obesity is associated with adipose tissue inflammation and systemic insulin resistance, resulting in increased adipose tissue lipolysis and hepatic very-low-density lipoprotein (VLDL) secretion. When combined with pregnancy, this leads to an increase in maternal circulating lipids with advancing gestation. Subsequent hydrolysis of maternal triglycerides (TGs) by placental lipoprotein lipase (LPL) and increased free fatty acid (FFA) uptake and transport by the placenta results in excess lipid transfer to the developing fetus. This increase in fetal lipid exposure may impact the liver, skeletal muscle, adipose tissue, brain, and pancreas to increase the risk for metabolic disease in childhood. MCP-1, monocyte chemotractant protein-1; CM, chylomicron; NAFLD, nonalcoholic liver disease.

Fig. 2.

General example of epigenetic regulation of gene transcription. Epigenetic regulation of gene expression is characterized by stable changes to DNA and chromatin structure that alter gene expression independent of gene sequence. The primary forms of epigenetic control involve DNA methylation by DNA methyl-transferase (DNMTs), and histone tail modifications, such as acetylation/deacetylation, by histone acetyl-transferase (HAT) and histone deacetylase (HDAC) activities, respectively. Additionally, microRNAs have recently been shown to regulate DNA methylation as well. Histone tail acetylation promotes an open-chromatin conformation, and is associated with regions of active gene expression, while histone tail deacetylation promotes a closed-chromatin conformation and is associated with gene silencing. DNA methylation of cytosine guanine (CpG) dinucleotides in the 5′ promoter region of genes generally induces transcriptional silencing, both by blocking transcription factor binding and by promoting the recruitment of transcriptional corepressors or histone-modifying complexes. MeBP, methyl-CpG binding protein; TF, transcription factor; Pol II, DNA polymerase II.