In This Issue of Diabetes

A protein called hepatic E4 promoter-binding protein 4, or E4BP4, appears to play a critical role in the promotion of the formation of lipid droplets and liver steatosis, according to Wang et al. (p. 348). Dysregulated lipid droplet formation is central to the liver steatosis seen during nonalcoholic fatty liver disease. Given there is currently no specific medication available for its treatment, identification of potential regulators of hepatic lipid droplet formation could be a first step toward treating the disease. The findings come from a series of experiments with cells and mice that focus on the E4BP4 protein and its role in lipid droplet formation. The studies used a combination of liver-specific E4BP4 knockout mice, high-fat diet feeding, and manipulation with adenovirus to variously overexpress or knock down components in the pathways involved. According to the authors, they found that mice with E4bp4 gene deficiency were protected against liver steatosis following a high-fat diet, and this was independent of obesity or insulin resistance. At the same time, they found that such mice had a marked reduction in the expression of the lipid droplet binding gene termed Fsp27. Using primary mouse hepatocytes, they found that E4BP4 is required to promote FSP27 transcription even though its canonical function is transcriptional repression. This suggested that its actions toward FSP27 were mediated by at least one intermediary transcriptional activator. Further experiments identified CREBH as a likely candidate. They also found that E4BP4 is modified by a process called SUMOylation, that high-fat diet feeding can result in deSUMOylation, and crucially that the process is critical for the downregulation of FSP27 and lipid droplet formation—all potential treatment leads for nonalcoholic fatty liver disease. “For future studies, we will use a novel mouse model that expresses the SUMOylation-resistant E4BP4 mutant specifically in the liver,” said author Xin Tong. “This will allow us to further investigate the significance of its deSUMOylation during the pathogenesis of nonalcoholic fatty liver disease.”

An epigenome-wide association study by Fragoso-Bargas et al. (p. 415) has identified six CpG sites that are inversely associated with insulin resistance across different ancestries. Crucially, the authors were able to replicate the findings for five of them in independent cohorts, significantly strengthening the outcomes. The findings come from the Epigenetics in Pregnancy (EPIPREG) sample, which is part of the STORK Groruddalen cohort. The discovery set consisted of women of European or South Asian origin with DNA collected in gestational week 28. Three independent studies were then used as replication sets. Six CpG sites were inversely associated with insulin resistance, and of those, five could be replicated in at least one of the three replication cohorts. Three sites were in the gene TXNIP, while there was one each in SLC7A11 and ZSCAN26. They were variously associated with several cardiometabolic phenotypes, including inverse associations with C-peptide, insulin, and BMI and at least one other phenotype. Using methylation quantitative trait loci analysis, they then found that the five replicated sites all were related to several genetic variants nominally associated with cardiometabolic and inflammatory traits and diseases in genome-wide association study summary data. Mediation analysis also suggested that the association was mediated by DNA methylation, which they suggest modifies gene expression. The picture that emerges shows that there is still much to learn about the functions of the identified sites. The authors also caution that further Mendelian randomization studies will be needed to explore causality. “The cross-ancestry design in EPIPREG seems to successfully extract DNA methylation marks that may play an important biological role in gestational and type 2 diabetes,” said author Christine Sommer. “Understanding the mechanisms of insulin resistance and other features of gestational and type 2 diabetes is crucial to improve prevention and treatment of diabetes in the longer term. I hope that our study can inspire and stimulate more epigenetic research in general and specifically in diverse populations.”

In terms of postprandial glucose tolerance and β-cell function, the importance of two incretin hormones appears to be altered following two bariatric surgical procedures, according to Hindsø et al. (p. 336). Glucose-dependent insulinotropic polypeptide (GIP) appears to be the most important incretin hormone in unoperated individuals, while both GIP and glucagon-like peptide 1 (GLP-1) appear to be equally important following sleeve gastrectomy. GLP-1 appears to be the most important hormone following Roux-en-Y gastric bypass surgery. Using a crossover design, the authors recruited 12 individuals per group who had received Roux-en-Y gastric bypass surgery or a sleeve gastrectomy or who did not have an operation. All individuals then received four liquid mixed-meal tests while receiving either placebo, a GLP-1 receptor antagonist, a GIP receptor antagonist, or both antagonists. They found that GLP-1 levels were highest in the Roux-en-Y gastric bypass surgery group and elevated in the sleeve gastrectomy group compared with the unoperated control group. Conversely, they found that GIP levels were lowest in the Roux-en-Y gastric bypass surgery group. Using antagonists to blockade receptors, the authors found that the GLP-1 receptor antagonist reduced β-cell glucose sensitivity and generally increased postprandial glucose responses only in the groups with surgery. Conversely, the GIP receptor antagonist had similar effects only in the control and sleeve gastrectomy groups. They also note that combined blockade with both antagonists resulted in decreased β-cell glucose sensitivity in all groups. Commenting further, author Kirstine N. Bojsen-Møller said, “The study demonstrates the tremendous impact of altered GLP-1 and GIP signaling in the regulation of insulin secretion and glucose control. In this study it was mediated by an altered secretion of the hormones after surgical modifications of the gut, but the results are also interesting in light of existing and emerging drugs targeting the same signaling pathways.”

A sedentary lifestyle appears to lower the ability of insulin to inhibit lipolysis in subcutaneous fat cells, according to Andersson et al. (p. 316). The findings suggest that the impaired antilipolytic effects of insulin in sedentary individuals lead to less efficient suppression of circulating fatty acids, with elevated levels contributing to insulin resistance and ultimately type 2 diabetes risk. On that basis, they suggest this is yet another reason to switch from a sedentary lifestyle to a more active one. The findings come from an analysis of subcutaneous fat cells from 204 individuals who self-reported a sedentary lifestyle and 336 individuals who were physically active to some extent. Specifically, the authors examined measures of the inhibition of hydrolysis of triglycerides by insulin (antilipolysis) and stimulation of triglyceride formation (lipogenesis). They found that the responsiveness of insulin for antilipolysis and lipogenesis actions were similar in sedentary and physically active individuals. However, sensitivity for both measures was 10-fold lower in sedentary individuals. Only the association between antilipolysis and physical activity remained after adjusting for a series of factors. They also found that fatty acid levels did not decrease normally following hyperinsulinemia and remained higher overall in sedentary individuals. mRNA levels for the genes for the insulin receptor and its two substrates (IRS1 and IRS2) were also decreased in the sedentary individuals. The authors note that the sedentary individuals did show many differences in measures of cardiometabolic health compared with the physically active individuals, although overall rates of diabetes and hyperlipidemic disease were lower in the sedentary individuals. “This is the first comprehensive study on the relationship between insulin action in adipose tissue and a sedentary lifestyle,” said author Peter Arner. “The novel observation that this tissue is also involved in insulin resistance but differently so than in other target tissues for the hormone has a clear clinical relevance for targeting fat cells in the treatment of impaired insulin action.”