• History & evolution
• Biosynthesis vs. dietary uptake
• Phosphatidylethanolamines and membranes
• Phosphatidylethanolamines, lipoproteins and liver disease
• Phosphatidylethanolamines, neurology and aging

History and evolution

1884: isolation from brain by Thudichum | | 1952: PE structure by Baer et al. (Vance et al. 2013)

Phosphatidylethanolamine (PE) is a sub-class of phospholipids with a variety of functions in animals, plants and microorganisms. Like other phospholipids, PEs are more than simply the building blocks of membranes: their functions range from regulation of membrane fluidity to modulation of intracellular signaling, protein folding and post-translational modification. Together with phosphatidylserines (PSs), PEs were first isolated from brain tissue in 1884.

They were originally referred to as “cephalin”, deriving their name from the Greek word kephal, meaning head. In the brain, PEs constitute about 45% of all phospholipids. Today, we know that PEs come in many sizes (chain lengths of their fatty acyl groups) and shapes (number of unsaturations) and serve a variety of functions.

Biosynthesis vs. uptake

PEs consist of a glycerol backbone esterified in three positions by two fatty acids and a phosphoric acid. In the head group of this phospholipid, the phosphate group is combined to the small molecule ethanolamine. The latter is sourced from diet and found free in micromolar concentrations in biofluids in mammals (Patel et al. 2017; Vance et al. 2013). PEs are the main phospholipid in egg yolk.

In humans, PEs can be absorbed through the intestine (Cohn et al. 2010; Ikuo et al. 1987). Two main pathways lead to de novo PE synthesis in two sub-cellular compartments (reviewed by Vance et al.2013):

• the Kennedy pathway, with one branch yielding phosphatidylcholines (PCs) and one yielding PEs, which takes place in the endoplasmic reticulum (ER)
• the phosphatidylserine decarboxylation (PSD) pathway, which uses PSs as precursor for PE synthesis and takes place in the mitochondria.

In the ER, PEs are formed in the cytidine diphosphate (CDP)-ethanolamine branch of the Kennedy pathway. A diglyceride (glycerol already bound to two fatty acyl groups) is combined to the ethanolamine group provided by CDP-ethanolamine, producing PE and CDP. The most commonly described rate-limiting step for this pathway takes place in the cytosol; i.e. the conversion of ethanolamine phosphate to CDP-ethanolamine by the enzyme cytidine triphosphate (CTP):phosphoethanolamine cytidylyltransferase.

In the outer aspect of the inner mitochondrial membrane, the PSD pathway converts PSs to PEs in a single step that removes a carbon dioxide group from the head group (Zborowski et al. 1983). Disruption of the PSD pathway is lethal in mice and causes mitochondrial dysfunction (Steenbergen et al. 2005). Here, the rate-limiting step lies in the ATP-dependent translocation of PS from the ER to the site of PSD at the inner mitochondrial membrane.

Interestingly, ER and mitochondria are not as independent as they seem, and these two pathways can take place in relative proximity thanks to mitochondria-associated ER membranes (MAM). It seems that PSs rather than PEs are exchanged between ER and mitochondria, and PE levels in the mitochondria increase via the PSD pathway rather than by direct exchange with ER membranes (Shiao et al. 1995).

To learn more about the contribution of MAMs to phospholipid transport, watch this webinar by Professor Jean Vance, whose research is behind many of the literature cited in this paragraph.

Another noteworthy observation is that in mammals, while PS decarboxylation yields PEs, triple methylation of PEs by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT) yields PCs. This reaction is responsible for about a third of PC synthesis in the liver, providing PCs that are secreted in the bile to help digestion of lipids (Vance et al. 2013). In addition, the two branches of the Kennedy pathway meet in several places to convert intermediaries of PE synthesis to their equivalent in the PC branch (van der Veen et al. 2017).

Phosphatidylethanolamines and membranes

In animals, PEs are enriched in inner membranes. The exact ratio of PEs to total membrane lipids or phospholipids varies depending on species, cell type, and organelle, but PCs and PEs usually top the phospholipid list. PEs constitute roughly 40% of the phospholipids of inner mitochondrial membranes and 15–25% of the phospholipids in other membranes (van der Veen et al. 2017).

As is the case with many lipid classes, “phosphatidylethanolamine” is often used in singular form, as if there were only one chemical structure for PE. However, as for PCs and other lipids that integrate fatty acyl groups in their structure, PEs are a large group of metabolites. Their structures vary according to the length and number of unsaturations of these fatty acyl groups, which confer a range of properties (Hishikawa et al. 2014).

With their typically conical shape, PEs impose a negative curvature to membranes (McMahon et al. 2015). Their detailed structure is also significant, as it influences membrane fluidity (Dawaliby et al. 2016). Typically, saturated fatty acyl groups will reduce membrane fluidity, while unsaturated groups will increase fluidity. Since the inner mitochondrial membrane is rich in PEs and poor in cholesterol (another lipid classically in charge of regulating membrane fluidity), the exact structure of PEs becomes paramount.

PEs also play a role in cytokinesis and the disassembling of the contractile ring needed for parting cells during cell division (Emoto et al. 2000). In the model organism Trypanosoma brucei (a parasitic protozoa), PEs have been studied for their contribution to cell cycle regulation and their involvement in the assembly of glycosylphosphatidylinositol (GPI) anchors of surface proteins (Signorell et al. 2009; Menon et al. 1993).

For more information on the many functions already described for PEs, see the review by Calzada et al. (2016) where the contribution of PEs to these and other mechanisms are detailed.

Phosphatidylethanolamines, lipoproteins and liver disease

Phospholipids are key components of lipoproteins, the vesicles that transport lipids between organs via the circulation. The lipoproteins most loaded in triglycerides (TGs) are also rich in PEs. These are the chylomicrons that form in intestinal cells to support uptake of dietary lipids, and the very-low-density lipoproteins (VLDLs) that form in hepatocytes to distribute lipids (mainly TGs but also cholesteryl esters) to other organs.

Although PCs constitute the bulk of the phospholipids that form the vesicle, PEs are also present, with functions in vesicle assembly and secretion. If PE levels are too low during assembly of VLDL in liver cells, nascent VLDLs are discarded. In addition, PE levels are shown to decrease with VLDL “age”, pointing to a signal for quick uptake of newly formed vesicle based on their high PE content.

Besides absolute phospholipid concentrations, the PC/PE ratio also attracts interest among researchers. A low PC/PE ratio was observed in liver biopsies and in erythrocytes of patients with non-alcoholic fatty liver disease (NAFLD) (Arendt et al. 2013). In knockout mice lacking PEMT activity (the enzyme responsible for the conversion of PEs to PCs in the liver), a low PC/PE ratio is associated with more permeable hepatocytes membranes, leading to hepatic damage (Li et al. 2006). Interestingly, blocking PE synthesis by blocking the CDP-ethanolamine pathway leads to TG accumulation and liver steatosis (Leonardi et al. 2009).

Phosphatidylethanolamines, neurology and aging

PEs also attract attention as metabolites that change with aging (Pradas et al. 2019; Dai et al. 2021). In the human brain, post-mortem analysis has shown that overall PE levels decreased with age in mitochondrial membranes. For example, the levels of the polyunsaturated fatty acid (PUFA)-containing PE 18:0_22:4 decreased by 20% in the mitochondria of the hippocampus in adults aged between 20 and 100 (Hancock et al. 2015).
Naturally, the discovery of links between PEs and aging have triggered research into the use of PE supplementation to increase lifespan. Positive results have been seen in model organisms and human cells via stimulation of autophagy (Rockenfeller et al. 2015).

PEs are also a point of interest for research into neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases, specifically in relation to PEs role in mitochondrial functions, including oxidative phosphorylation. Reducing PE synthesis has been shown to increase mitochondrial membrane potential and reduce activity of the electron transport chain and ATP production (Tasseva et al. 2013).

Synthesis of amyloid beta, a prominent peptide in the pathophysiology of Alzheimer’s disease, is synthesized by enzymes anchored in membranes. In cell models and in Drosophila melanogaster (fruit fly), amyloid beta production was found to increase when membrane PE levels were reduced, and decrease when PE levels were increased. (Nesic et al. 2012).

Learn more about the roles of PEs and other phospholipids in complex chronic diseases such as cancer, Alzheimer’s disease, depression, inflammatory bowel disease, multiple sclerosis and diabetes in our whitepaper “Complex chronic diseases have a common origin”.

References

Calzada E. et al.: Phosphatidylethanolamine Metabolism in Health and Disease. (2016) International review of cell and molecular biology | http://doi.org/10.1016/bs.ircmb.2015.10.001

Cohn J. et al.: Dietary phospholipids and intestinal cholesterol absorption. (2010) Nutrients | http://doi.org/10.3390/nu2020116

Dai Y. et al.: The Crucial Roles of Phospholipids in Aging and Lifespan Regulation. (2021) Frontiers in physiology | http://doi.org/10.3389/fphys.2021.775648

Dawaliby R. et al.: Phosphatidylethanolamine Is a Key Regulator of Membrane Fluidity in Eukaryotic Cells. (2016) The Journal of Biological Chemistry | https://doi.org/10.1074/jbc.M115.706523

Emoto K. et al.: An essential role for a membrane lipid in cytokinesis. Regulation of contractile ring disassembly by redistribution of phosphatidylethanolamine. (2000) The Journal of cell biology | https://doi.org/10.1083/jcb.149.6.1215

Hancock S. et al.: Decreases in Phospholipids Containing Adrenic and Arachidonic Acids Occur in the Human Hippocampus over the Adult Lifespan. (2015) Lipids | https://doi.org/10.1007/s11745-015-4030-z

Hishikawa D. et al.: Diversity and function of membrane glycerophospholipids generated by the remodeling pathway in mammalian cells. (2014) Journal of Lipid Research | https://doi.org/10.1194/jlr.R046094

Ikuo I. et al.: Absorption and transport of base moieties of phosphatidylcholine and phosphatidylethanolamine in rats. (1987) Biochimica et Biophysica Acta (BBA) – Lipids and Lipid Metabolism | https://doi.org/10.1016/0005-2760(87)90024-5

McMahon H. et al.: Membrane curvature at a glance. (2015) Journal of Cell Science | https://doi.org/10.1242/jcs.114454

Menon A. et al.: Phosphatidylethanolamine is the donor of the terminal phosphoethanolamine group in trypanosome glycosylphosphatidylinositols. (1993) The EMBO Journal | https://doi.org/10.1002/j.1460-2075.1993.tb05839.x

Nesic I. et al.: Alterations in phosphatidylethanolamine levels affect the generation of Aβ. (2012) Aging cell | https://doi.org/10.1111/j.1474-9726.2011.00760.x

Patel D. et al.: Ethanolamine and Phosphatidylethanolamine: Partners in Health and Disease. (2017) Oxidative Medicine and Cellular Longevity | https://doi.org/10.1155/2017/4829180

Pradas I. et al.: Exceptional human longevity is associated with a specific plasma phenotype of ether lipids. (2019) Redox Biology | https://doi.org/10.1016/j.redox.2019.101127

Rockenfeller P. et al.: Phosphatidylethanolamine positively regulates autophagy and longevity. (2015) Cell death and differentiation | https://doi.org/10.1038/cdd.2014.219

Shiao Y. et al.: Evidence that phosphatidylserine is imported into mitochondria via a mitochondria-associated membrane and that the majority of mitochondrial phosphatidylethanolamine is derived from decarboxylation of phosphatidylserine. (1995) The Journal of Biological Chemistry | https://doi.org/10.1074/jbc.270.19.11190

Signorell A. et al.: Perturbation of phosphatidylethanolamine synthesis affects mitochondrial morphology and cell-cycle progression in procyclic-form Trypanosoma brucei. (2009) Molecular Microbiology | https://doi.org/10.1111/j.1365-2958.2009.06713.x

Steenbergen R. et al.: Disruption of the phosphatidylserine decarboxylase gene in mice causes embryonic lethality and mitochondrial defects. (2005) The Journal of Biological Chemistry | https://doi.org/10.1074/jbc.M506510200

Tasseva G. et al.: Phosphatidylethanolamine deficiency in Mammalian mitochondria impairs oxidative phosphorylation and alters mitochondrial morphology. (2013) The Journal of Biological Chemistry | https://doi.org/10.1074/jbc.M112.434183

van der Veen J. et al.: The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease. (2017) Biochimica et Biophysica Acta (BBA) – Biomembranes | https://doi.org/10.1016/j.bbamem.2017.04.006

Vance J. et al.: Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells. (2013) Biochimica et biophysica acta | https://doi.org/10.1016/j.bbalip.2012.08.016

Zborowski J. et al.: Phosphatidylserine decarboxylase is located on the external side of the inner mitochondrial membrane. (1983) FEBS Letters | https://doi.org/10.1016/0014-5793(83)81141-7

Patients with Crohn’s Disease (CD) often suffer from liver steatosis, but what connects the two is not completely understood. For treatments that inhibit Tumor-Necrosis-Factor (TNF), which are effective for CD, there is no obvious mode of action for anti-TNF α therapy to improve liver function. Pharmaceuticals can have effects beyond their immediate target, and in anti-TNF α therapy, a potentially positive effect on fatty liver pathology has previously been reported.

Lars Bechmann, Ali Canbay and colleagues from Ruhr-University Bochum investigated the interaction between CD, anti-TNF α therapy and hepatic steatosis. Serum and stool metabolomics and lipidomics were used to analyze changes in lipid and metabolomic profiles and correlated with analysis of gut microbiota composition and clinical parameters.

Interestingly, there were no major differences in fecal calprotectin between CD patients and CD patients under therapy. Selected triglycerides were increased in CD patients compared to controls, but not in patients under therapy. In fecal samples, changes in ceramide levels between patients and controls were less pronounced in those receiving anti-TNF α therapy. Besides metabolic changes, alterations were also found in selected gut bacterial phylae. Lower abundance of Firmicutes and Ruminococcaceae were associated with higher levels of triglycerides.

These findings may be explained by interactions between the gut microbiome, bile acid signaling (such as FGF19 and FXR), and lipid homeostasis, providing a potential mechanistic explanation of how anti-TNF α therapy improves steatosis via the gut-liver axis.

This is not the first study to address the metabolic effects of therapeutics in the context of liver disease. In a recent post, we showed how obeticholic acid changes the metabolome in non-alcoholic steatohepatitis .

Manka P, Sydor S, Wase N, et al.: Anti-TNFα treatment in Crohn’s disease: Impact on hepatic steatosis, gut-derived hormones, and metabolic status (2021) Liver Int. | https://doi.org/10.1111/liv.15003

Metabolomics has one big advantage over other -omics technologies: metabolism is highly conserved throughout evolution, which is hugely helpful for translatability. Although there are a several notable exceptions, most key metabolites found in humans are also found in other higher organisms, including mice, which are the most frequently used animal model for biomedical research. This also facilitates a better understanding of different interventions, so patients can be given the most appropriate, safe and effective treatments.

A new study from researchers in Shanghai and Beijing adds to our knowledge of what happens biochemically in the progression of metabolic liver disease. In a sample of mice with diet-induced obesity (DIO), they used carbon tetrachloride (CCl4) to induce inflammation in the liver (non-alcoholic steatohepatitis, or NASH), resulting in DIO-NASH. The animals were treated with obeticholic acid, a semi-synthetic bile acid and potent FXR receptor agonist, which is being tested as a potential treatment for a variety of diseases in gastroenterology and hepatology.

By profiling serum, liver tissue, and intestinal contents, the study showed progressive metabolic dysregulation with DIO-NASH compared to DIO alone, with the effects most pronounced in the liver. These metabolic profiles correlated with other indicators for liver disease and NAFLD-NASH. The observed changes aligned with findings from clinical and epidemiological studies.

Most altered metabolites in liver were found to be intermediates of lipid metabolism. There were also some changes in amino acids and amino acid metabolites. Tyrosine and tryptophan showed significant increases with increased disease severity. Although not specifically mentioned in the paper, this may result in a reduced ratio of branched-chain amino acids to aromatic amino acids, which would be in line with expectations as reduced ratios have previously been associated with reduced liver function and increased severity of liver disease. Finally, it was shown that the patterns of several metabolites were altered only by DIO or the induction of NASH, suggesting different pathophysiological processes at play.

The effects of obeticholic acid assessed in the liver showed a reversal of many metabolic changes induced by CCl4. Consequently, the liver metabolome of mice treated with obeticholic acid clustered with the metabolic profiles observed in DIO mice. It would be interesting to see the impact of DIO and DIO-NASH, as well as therapy, on the level of endogenous bile acids. Future studies might also investigate the effect of obeticholic acid on the serum metabolome.

In conclusion, this study shows that metabolomics is a valuable tool in increasing our understanding of the biochemical processes involved in the progression of chronic diseases. Metabolomics can also improve the understanding of the molecular action of novel therapeutics beyond the immediate target.

Want to learn more about metabolomics in obesity, cardiometabolic and liver diseases? Check out these articles:
Defining the biochemistry of obesity
Microbiome instability and host metabolic dysfunctions

Zhu N, Huang S, Zhang Q, Zhao Z, Qu H, Ning M, Leng Y, Liu J.: Metabolomic Study of High-Fat Diet-Induced Obese (DIO) and DIO Plus CCl₄-Induced NASH Mice and the Effect of Obeticholic Acid. Metabolites.( 2021) | https://doi.org/10.3390/metabo11060374

Elevated liver fat content is a hallmark of metabolic syndrome. This causes impaired glucagon sensitivity of liver cells and in turn increased plasma levels of amino acids. As consequence, α-cells secrete glucagon release by α-cells is fostered. Hyperglucagonaemia can lead to the development of type 2 diabetes (T2D) due to steady hepatic glucose production. This regulatory circuit is described by the liver-α cell axis. This study examined the association of liver fat content and insulin resistance with the glucagon-alanine index, which is an indicator for the integrity of the liver-α cell axis.

The study, led by Dr Andreas Lechner from Helmholtz Zentrum Munich, is based on data from 79 young women with low levels of liver fat, participating in the Prediction, Prevention and Subclassification of Type 2 Diabetes (PPSDiab) study. Their glucagon-alanine index was calculated and analyzed for an association with liver fat content and inulin resistance.

The results revealed a significant correlation between liver fat and the glucagon-alanine index from a liver fat level of 0.5%, independent of insulin resistance. The correlation between the glucagon-index and insulin resistance also started around 0.5% liver fat content. However, this association was not independent of liver fat content in participants with > 0.5% liver fat content. These results indicate an impairment of the liver-α cell axis already at non-steatotic liver fat levels.

In summary, findings suggest an association of liver fat content and glucagon-alanine index that’s independent of insulin resistance. A disturbed regulatory circuit of liver and α-cells leads to hyperglucagonaemia and might foster the development of T2D. Therefore, the glucagon-alanine index might have potential as predictive biomarker.

If you want to examine the relation between amino acids and regulatory circuits of metabolism, please consider our products.

Gar C, Haschka SJ, Kern-Matschilles S, Rauch B, Sacco V, Prehn C et al.: The liver-alpha cell axis associates with liver fat and insulin resistance: a validation study in women with non-steatotic liver fat levels. (2021) Diabetologia | https://doi.org/s00125-020-05334-x

Prenatal exposure to harmful substances is widely accepted as a health risk later in life. Prospective cohort studies starting during pregnancy are a powerful tool to identify risk factors and to develop prevention strategies for diseases.

In a study under the Human Early Life Exposome (HELIX) project, Dr. Nikos Stratakis and colleagues analyzed the effect of prenatal PFAS exposure (exposure to perfluoralkyl substances) on the prevalence of liver injuries in childhood. PFAS are very stable and ubiquitous chemicals used in the industrial production of a wide variety of consumer goods, which considerably accumulate in the human liver and have been shown to exert hepatotoxic effects in animal models.

The study measured PFAS concentration in the maternal blood during pregnancy as well as liver enzyme levels and serum metabolites in the children during follow-up.

Higher PFAS exposure during pregnancy was associated with higher liver enzyme levels in serum, indicating an increased level of liver injuries in these children. Child serum metabolomics revealed perturbations in the amino acid and glycerophospholipid metabolism. A distinct metabolic profile for children at high risk of liver injury was identified. It was characterized by a high PFAS exposure in utero and included elevated levels of branched-chain amino acids, aromatic amino acids, and glycerophospholipids. Abnormal glycerophospholipid concentrations can induce hepatic lipotoxicity and inflammation. Notably, this is a hallmark of non-alcoholic fatty liver disease (NAFLD), which is increasingly diagnosed in children.

As part of the HELIX project, this study provides profound datasets on mother-child pairs from six countries, observing the effect of exposure to environmental PFAS mixtures during the critical time of fetal development. The increasing prevalence of liver injuries like NAFLD in children highlights the importance of the results for public health and prevention policy.

If you are interested in applying metabolomics in population-based studies, go ahead and watch our free virtual symposium with internationally renowned researchers presenting “The future of population health”.

Stratakis N, V Conti D, Jin R, Margetaki K, Valvi D, Siskos AP et al.: Prenatal Exposure to Perfluoroalkyl Substances Associated With Increased Susceptibility to Liver Injury in Children. (2020) Hepatology | https://doi.org/10.1002/hep.31483

Gut microbiota profiles are as unique to each person as fingerprints. For this reason, when investigating the stability of microbiota composition over time, studies where samples are collected from the same subjects at several time points are ideal. This study focuses on intra-individual comparisons of paired long-term follow-up data collected in the Study of Health in Pomerania (SHIP). Fecal and plasma samples were collected at a 5-year interval to study the links between microbiota composition, plasma metabolome and clinical signs of metabolic dysfunctions.

Using data from over 1200 subjects, the researchers show that at the level of the overall population, the microbiome is quite stable, with a predominance of bacteria of the Bacteroide, Prevotella and Faecalibacterium taxa at both time points. At the individual level however, the picture becomes more complex, and a larger microbial instability appears. Subjects with metabolic conditions such as liver steatosis and diabetes mellitus tend to have higher levels of facultative pathogens such as Enterobacteriaceae, Escherichia or Shigella.

This microbiome instability correlates positively with steatosis and diabetes mellitus, but negatively with species richness (diversity in microbiota species), household net income, being female and proper exocrine pancreatic function.

Plasma metabolomics, coupled with a detailed analysis of the genera linked to steatosis, reveals a pattern of lipids (phosphatidylcholines, lysophosphatidylcholines and sphingomyelins) associated with higher levels of Clostridium XIVa in subjects with steatosis. Metagenomic pathway analysis also highlights a potential impact on short chain fatty acid (SCFA) production pathways consistent with dysbiosis. These results could help in the applications such as human interventional trials aiming to reverse disease processes associated with microbiome instability.

If you are interested in more examples on how metabolic profiling can be applied to microbiome research, please visit our microbiome application page. Metabolomics, as microbiome profiling, can also be performed in feces samples. See our blog on feces metabolomics for more detail.

Frost F, Kacprowski T, Rühlemann M, Pietzner M, Bang C, Franke A, Nauck M, Völker U, Völzke H, Dörr M, Baumbach J, Sendler M, Schulz C, Mayerle J, Weiss FU, Homuth G, Lerch MM. Long-term instability of the intestinal microbiome is associated with metabolic liver disease, low microbiota diversity, diabetes mellitus and impaired exocrine pancreatic function. (2020) Gut | http://dx.doi.org/10.1136/gutjnl-2020-322753

It is well known that microorganisms in the gut are involved in the digestion of our foods. The formation of the gut microbiota and the external factors governing its maturation are two of the hottest topics in microbiome research. In the article, the authors look at the maturation of mice gut microbiota from postnatal to adulthood and the role bile acids play in its development. Bile acids have been shown to have profound influence on gut microbial populations in adult mice and humans but their role in early gut microbiome maturation is not well studied.

A significant observation noticed by the authors is the major metabolic shift that takes place during weaning. Looking at liver gene expression using RT-PCR, the authors observed a major increase towards bile acid production. Such effect was found to be independent of the microbiota signaling to the host as the shift also takes place in germ-free animals.

Due to this shift, bacterial bile salt hydrolase (BSH) gene expression rises steadily in response to liver bile availability. The gut microbiome shifts towards species carrying this gene. While the ratio of primary to secondary bile acid concentrations stabilizes about 21 days after birth (from a high primary/secondary bile acid ratio at birth lowering with time). The extent of the contribution of the single bile acid species on the different bacterial taxa remained elusive. The authors intervened by orally administering, bile acids at a stage (day 7), when the natural shift towards bile acid production has not occurred yet. By externally administrated TCA (taurine-conjugated cholic acid) and βTMCA (taurine-conjugated β-muricholic acid) the global microbiota composition shifted in the intestine towards a more adultlike microbiota composition. This indicates that the onset of bile acid production and its availability does exert an effect on the early forming gut microbiome. In summary, this study shows that bile acids as host factors shape the postnatal intestinal microbiota composition.

Do you want to know more more about metabolomics in the study of microbiome-host interaction? Visit our applications site microbiome.

van Best N, Rolle-Kampczyk U, Schaap FG, Basic M, Olde Damink SWM, Bleich A, Savelkoul PHM, von Bergen M, Penders J, Hornef MW: Bile acids drive the newborn’s gut microbiota maturation (2020) Nat Commun | https://doi.org/10.1038/s41467-020-17183-8

Incidence of non-alcoholic steatohepatitis (NASH), an advanced form of non-alcoholic fatty liver disease (NAFLD), is rapidly increasing globally and may progress to hepatocellular carcinoma (HCC) with or without cirrhosis. The progression cascade from NAFLD via NASH to HCC may be promoted by alterations in the gut microbiome and the bile acid homeostasis, both essential parts of the gut-liver axis.

This is what a recent study led by a German research group from the University Hospital Magdeburg aimed to investigate for NASH-related HCC. Stool and serum samples from 87 subjects divided into five groups (NASH, NASH with cirrhosis, NASH-HCC, NASH-HCC with cirrhosis, and healthy controls) were included in the analysis.

Serum levels of total bile acids and individual conjugated primary bile acids increased with disease severity, which is even more pronounced in patients with cirrhosis. Unlike in NASH, serum levels of the fibroblast growth factor 19 (FGF19), a suppressor of hepatic bile acid synthesis induced by the nuclear receptor farnesoid X receptor (FXR), were elevated in NASH-HCC patients. This indicates that different mechanisms lead to the accumulation of bile acids, independent from cirrhosis. Along with the alteration in bile acid homeostasis, the diversity of certain intestinal bacteria was affected. An increased abundance of Lactobacilli, for instance, is thought to be a consequence of the increased availability of primary conjugated bile acids as a substrate for deconjugating enzymes of these bacteria.

Overall, the researchers propose that an increase in bile acid levels might contribute to fibrogenesis, liver injury, and tumorigenesis in NASH-HCC, while there seem to be two distinct mechanisms in hepatocarcinogenesis, a cirrhosis-dependent and an –independent one, reflected by even higher serum BA levels.

If you are interested in learning more about the gut microbiota and in quantifying bile acids or other microbial-derived metabolites, please visit our products and services webpages, or contact us for support.

Sydor S, Best J, Messerschmidt I, Manka P, Vilchez-Vargas R, Brodesser S, Lucas C, Wegehaupt A, Wenning C, Aßmuth S, Hohenester S, Link A, Faber KN, Moshage H, Cubero FJ, Friedman SL, Gerken G, Trauner M, Canbay A, Bechmann LP: Altered Microbiota Diversity and Bile Acid Signaling in Cirrhotic and Noncirrhotic NASH-HCC (2020) Clin Transl Gastroenterol | https://doi.org/10.14309/ctg.0000000000000131

The liver is a major supplier of fatty acids to the brain for phospholipid and sphingomyelin synthesis. Perturbations of liver metabolism can profoundly affect the brain, highlighting the importance of the liver-brain axis. Disrupted liver methylation by lack of essential methyl-donors of the one-carbon metabolism, such as methionine, choline, and betaine, is known to induce non-alcoholic steatohepatitis (NASH); however, the effects on the brain have not yet been studied. 

The initial hypothesis that methionine-choline-deficient (MCD) diet would induce NASH and simultaneously disrupt brain methylation in a rat model, turned out to be only partially true. Instead, MCD diet induced NASH and altered brain lipid profiles, but did not affect brain methylation. Interestingly, dietary supplementation with betaine prevented NASH by enhanced fatty acid β-oxidation and restored brain lipid profiles, despite elevated homocysteine levels and irrespective of liver methylation activity. The observations could be explained by a catabolic state that provides methionine to the brain for S-adenosylmethionine synthesis via the degradation of peripheral tissue, which is reflected by weight loss in the rats. 

The detected decrease of plasma phospholipids may be an early sign of pre-symptomatic disrupted brain function which could explain why no cognitive decline has yet been observed. Betaine exerts its hepato- and maybe also neuroprotective effect by altering lipid metabolism, thereby preserving the composition of cell membranes.

At first, the results were unexpected, counterintuitive, and contrary to previous findings, but thanks to the comprehensive study design combining mRNA and protein expression data with metabolomics data, the cause-and-effect relationship between lack of methylation-donors and metabolism in liver and brain could be uncovered and conclusively explained.

If you are interested in measuring plasma, liver, or brain metabolites and compare your results to the results gained in this study, please contact us. 

Nur Abu Ahmad, Merav Raizman, Nathalie Weizmann, Brandi Wasek, Erland Arning, Teodoro Bottiglieri, Oren Tirosh, Aron M. Troen, FASEB J. 2019, https://doi.org/10.1096/fj.201802683R