AASLD Roundtable Discussion: Revisiting the “Gut-Liver-Brain Axis” through the Lens of Bile Acids – Key Advances and Prospects in Basic and Clinical Research
Editor’s Note:
The recently concluded AASLD 2023, a pivotal annual event in the field of hepatology, hosted an engaging forum titled “Bile Acids at The Cross-Road of The Gut-Liver and Brain-Liver Axes”. This session focused on the latest developments in the role of microbiomes and bile acid signaling under physiological and pathological conditions in the “Gut-Liver-Brain Axis”. The discussion offered significant insights for both basic researchers seeking deeper investigation and clinicians aiming to discover new therapeutic targets.
To augment this, the “Hepatology Digest” reporting team organized a trilateral roundtable at the conference. Special invitees included Professor Huiping Zhou from the Virginia Commonwealth University School of Medicine, Professor Grace L. Guo from Rutgers University School of Pharmacy in New Jersey, and Professor Jian-Gao Fan from Shanghai Jiao Tong University School of Medicine’s affiliated Xinhua Hospital in China. They shared their expert views on the basics, translational aspects, and clinical disease prevention and treatment related to “Bile Acids and the Gut-Liver-Brain Axis”.
<Hepatology Digest>: The “Gut-Liver-Brain Axis” has been a hot topic in recent years. What kind of interactions and impacts exist between the intestines, liver, and central nervous system under normal physiological conditions?
Professor Huiping Zhou explained that bile acids and the “Gut-Liver-Brain Axis” are indeed hot topics recently. In normal physiology, this axis is closely related not only to bile acid metabolism but also to various other metabolic processes. Traditionally, bile acids were thought to be mainly associated with the liver, aiding in the digestion and absorption of lipid-soluble nutrients and involved in drug metabolism. However, over the past two decades, especially since the discovery of bile acid receptors, our understanding of their role has expanded. These receptors are not just in the liver but also in the intestines and other organs. They help regulate metabolism and are involved in various other regulatory processes, including recent findings on their role in brain signal regulation.
Bile acids have many functions beyond the usual carbohydrate and lipid metabolism. They are complex compounds, not just simple molecules. Each metabolite of bile acids has its specific mechanism of action, and many aspects are still not fully understood.
The gut microbiota is also crucial under physiological conditions. It is now considered an important organ and tissue of the human body, including not only a significant proportion of bacteria but also fungi and viruses. It’s like an ecosystem within the human body. In other words, a balanced macro environment is essential for maintaining our normal physiological functions. Any issue at any step or stage can potentially lead to various pathological states.
<Hepatology Digest>: about the latest research advancements and the connection between the “Gut-Liver-Brain Axis” and different etiologies of liver diseases.
Professor Grace L. Guo responded that traditionally, liver diseases were thought to be caused by various factors damaging the liver. However, from the perspective of bile acids, the intestine’s circulation, production, and regulation of bile acids significantly influence liver diseases. For instance, a factor in the intestines, Fibroblast Growth Factor 15 (in mice) or 19 (in humans) (FGF15/19), is crucial and can inhibit the liver’s bile acid production. Moreover, this factor not only affects bile acids but also impacts liver lipid metabolism, liver regeneration, and possibly the development of liver cancer. The liver, being the body’s largest biochemical factory, produces thousands of factors that affect the entire body in various ways.
Recent studies have found that bile acids significantly impact many central nervous system diseases, such as Parkinson’s disease and Alzheimer’s disease. Research indicates that, compared to a normal control group, patients exhibit significant differences in the composition and levels of bile acids. However, the specific mechanisms remain unclear. These differences might relate to gut microbiota or bile acids themselves. In conditions like liver diseases, aging, and other systemic diseases or inflammations, the composition of bile acids undergoes significant changes. The gut microbiota can produce secondary bile acids, which are particularly influenced by the gut flora. These secondary bile acids strongly activate several bile acid membrane receptors, such as S1PR2 and TGR5, and also have potent pro-inflammatory effects.
Thus, basic research is ongoing into the interactions and dialogues between the gut microbiota, central nervous system diseases, and some age-related diseases with bile acids. The aim is to identify correlations or causal links, or at least to use bile acids as markers for disease detection and prognosis prediction, providing references for everyone.
<Hepatology Digest>: How do bile acids regulate the “Gut-Liver-Brain Axis” under normal physiological conditions? And in pathological conditions, how do they affect the development and progression of liver diseases by acting on the “Gut-Liver-Brain Axis”?
Professor Huiping Zhou responded: Hepatocytes synthesize bile acids. Typically, after synthesis, bile acids bind with amino acids and are secreted into the bile duct, then stored in the gallbladder. When we eat, the gallbladder contracts, releasing bile acids into the intestine to assist in the digestion and absorption of lipid-soluble substances, forming micelles. After completing their digestive and absorptive tasks, 95% of bile acids are absorbed from the ileum mucosa, known as the enterohepatic circulation.
For instance, if we eat 3-4 meals a day, there are 3-4 enterohepatic circulations, and only 5% of bile acids are metabolized by the gut flora and excreted in feces. Generally, the liver re-synthesizes to compensate for this 5% loss. Hence, the total amount of bile acids is relatively balanced. When needed by the body, bile acids are released into the intestine to aid in digestion and absorption.
The regulation of bile acid circulation is significantly influenced by peptide hormones released by the brain. Why do we sometimes feel hunger or fullness? It’s due to the regulation by these peptide hormones, which also regulate the enterohepatic circulation of bile acids, thus maintaining the normal bile acid pool.
Many diseases, particularly metabolic disorders, arise when this regulation is disrupted. The movement of bile acids from the liver to the intestine and back to the liver involves many transporters. These transporters regulate the entry and exit of bile acids from cells. Any disruption in this process can lead to transport disorders, potentially blocking the enterohepatic circulation and leading to various pathological states. Many diseases, especially digestive and metabolic disorders, are clinically found to be related to these transport disorders, possibly due to genetic mutations causing transporter deficiencies or low expression, thus affecting normal bile acid circulation.
Another important function of bile acids is their action on the intestine. Although bile acids are metabolized by the gut flora, converting primary bile acids into secondary ones, the composition of bile acids in the intestine plays a crucial role in regulating the balance of gut flora. Typically, if certain bile acids are deficient, harmful bacteria overgrow while beneficial bacteria decrease, potentially causing inflammation affecting various body parts, not just the intestine but also the liver and even other organs like the brain. Therefore, the regulation of bile acids is vital.
Additionally, the farnesoid X receptor (FXR) is crucial in regulating bile acid synthesis. If the body has sufficient bile acids, they signal to FXR to reduce bile acid synthesis through negative feedback. Intestinal release of FGF15/19 also activates FXR, signaling the liver that enough bile acids are present, reducing the need for further synthesis. If any step in this process is disrupted, it can lead to various pathological states.
Apart from FXR, other nuclear receptors regulated by bile acids are being intensively studied, including TGR5 from the G-protein-coupled receptor family. TGR5 is involved in regulating glucose and energy metabolism in the liver and intestines. Although hepatocytes lack TGR5, its high expression in immune cells is crucial for regulating the liver’s metabolic functions.
In addition to these two main receptors, our laboratory discovered for the first time that the sphingosine-1-phosphate receptor can also be activated by bile acids. Why are there so many bile acid receptors? It’s because different bile acids activate them, each with its specificity and affinity. This complex physiological system requires various signals to maintain the body’s normal physiological state. Any receptor deficiency, increased expression, or other anomalies can lead to many pathological disorders. Clinical data analysis from patients also shows a close relationship between the expression or activity of these receptors and many diseases, including metabolic-related fatty liver disease, alcoholic fatty liver disease, and cholestatic liver diseases.
<Hepatology Digest>: Regarding bile acid research, establishing experimental models is a crucial prerequisite. Could you share some technical details and practical experiences in this area?
Professor Grace L. Guo shared: In bile acid research, animal models are particularly important. Purely in vitro models, such as cell or tissue cultures, are quite limited for studying bile acids. However, animal models also have their limitations.
Primates, like monkeys, are very similar to humans but have limited application due to the difficulty of obtaining sufficient specimens. Furthermore, human experiments are not feasible, not just due to ethical concerns, but also because of the significant individual differences among people. Therefore, our laboratory frequently uses mice, as they are relatively convenient for experiments.
The primary challenge with using mice in experiments is that the production of bile acids in mice significantly differs from humans. Humans synthesize bile acids through two pathways – neutral and acidic. The primary bile acids produced are potent FXR agonists. However, in mice, bile acids produced via the neutral pathway act as FXR agonists, while those from the acidic pathway act as antagonists. Therefore, half of the bile acids in mice are entirely different from those in humans, although the receptors are the same. When using mice for experiments, it is crucial to be mindful of these differences and interpret the results carefully, considering their relevance to humans. Despite these limitations, mice are still necessary for experiments, especially for disease models, due to the limitations of cell experiments.
Researchers are now utilizing humanized mouse models, but these too have limitations. Although the liver cells in these mice are humanized, the immune cells are different. Moreover, the mouse intestine produces FGF15, which doesn’t recognize human FGF19 receptors, unless FGF15 is converted to FGF19 to mimic human cells. Some laboratories use genetically modified mice that can synthesize human bile acids, but these models also have problems. Mice have never encountered such lipid-soluble bile acids in their evolutionary history, so these mice are born with cholestasis.
Hence, using mice for human experiments presents significant species differences. Currently, there is no ideal model; each model reveals some truth. Researchers hope to make progress and develop mouse models more closely resembling humans for studying human bile acids.
<Hepatology Digest>: Looking at the current progress in basic research on bile acids and the “Gut-Liver-Brain Axis,” does it offer promising directions for clinical treatment of liver diseases?
Professor Huiping Zhou noted that research on bile acids is very promising. Recent preclinical studies in various mouse models, including those for fatty liver disease, diabetes, and fibrosis, have shown effective treatment or preventive effects using FXR and TGR5 agonists. There are also several Phase I or II clinical trials mainly observing the therapeutic effects of these agonists on fatty liver.
The main issue so far is that while all these FXR agonists have some therapeutic effect, they also have side effects. Researchers hope that technological advancements, especially in artificial intelligence (AI), will help identify more precise targets in preclinical mouse models.
Furthermore, significant progress has been made with dual agonists for TGR5 and FXR, showing promising therapeutic effects in clinical studies. The application of technologies like single-cell sequencing and tissue localization techniques will provide more precise scientific methods. These will aid in designing the next generation of TGR5 and FXR agonists or antagonists.
Additionally, research on sphingosine-1-phosphate receptors is ongoing. Currently, there are no effective antagonists, as they have low bioavailability, low specificity, and some toxicity. Preclinical experiments, including cell studies, have found absorption issues. However, experiments using genetically modified mice suggest a definite effect, warranting further exploration.
Moreover, drugs are being developed to regulate gut microbiota and enzymes related to bile acid metabolism. In normal conditions, the body maintains balance because beneficial bacteria produce antibiotics to suppress harmful bacteria. When this balance is disrupted in pathological conditions, harmful bacteria overgrow. Laboratory researchers have isolated such bacteria and identified several types that produce antibiotics, potentially useful for treating metabolic-related diseases and even inflammatory bowel disease. Besides bile acid receptors, many clinical studies have found that inhibitors of bile acid transport, such as intestinal bile acid transport inhibitors, can regulate the entire bile acid circulation process.
<Hepatology Digest>: What treatment or management strategies can be adopted clinically for liver disease patients experiencing abnormalities in the “Gut-Liver-Brain Axis” and bile acid metabolism?
Professor Jian-Gao Fan, from a clinical perspective, acknowledged that the “Gut-Liver-Brain Axis” is involved in more than just end-stage liver diseases or hepatic encephalopathy. Many disorders, including degenerative changes in the brain and conditions like overeating and emotional disturbances, might be related to the gut, and the role of the liver in this context warrants further study.
Some research has already linked these conditions to the liver, focusing recently on abnormal bile acid metabolism. For instance, in alcohol-related diseases, including gastrointestinal, liver, and brain disorders, metabolic liver diseases, and cholestatic liver diseases, especially those progressing to cirrhosis, the “Gut-Liver-Brain Axis” and bile acid metabolism abnormalities may play a more significant role. Clinically focusing on the gut, strategies like healthy eating and increased exercise are important to restore the gut microbiota’s homeostasis and improve bile acid metabolism, potentially benefiting liver diseases and overall health.
Currently, prebiotics, probiotics, synbiotics, and even fecal microbiota transplantation (FMT) are being explored for treating various diseases, including autism and hepatic encephalopathy. The role of bile acids in these treatments, such as why FMT can treat autism or hepatic encephalopathy, is an area of interest. Changes in gut microbiota, ammonia, short-chain fatty acids, and aromatic amino acids, along with bile acid metabolism, play a crucial role. Drugs targeting bile acids, such as obeticholic acid and ursodeoxycholic acid, are used not only for chronic liver diseases like cholestasis but also show benefits outside the liver. Additionally, pig bile acid could be a potential treatment for metabolic dysfunction and related fatty liver disease.
Increasing research is focusing on treatments like FXR agonists obeticholic acid and FGF19 and 21 analogs, which can improve metabolism and have weight loss effects. Thus, focusing on the “Gut-Liver-Brain Axis” and bile acid metabolism could be crucial in preventing and treating many diseases. Recent studies by Professor Jian Wu’s team at Fudan University in Shanghai suggest that ursodeoxycholic acid could reduce the risk of liver cancer. The metabolism of bile acids is linked to hepatocellular carcinoma and cholangiocarcinoma. Their research indicates that ursodeoxycholic acid might not only improve bile acids and alleviate skin itching but also have anti-cancer effects.
In summary, significant progress in basic and clinical research on bile acid metabolism and the “Gut-Liver-Brain Axis” offers more opportunities for breakthroughs in diagnosing, treating, and developing new drugs for related diseases.
Experts introductions :
Huiping Zhou
– Currently a tenured professor and Ph.D. supervisor in the Department of Microbiology and Immunology at Virginia Commonwealth University School of Medicine, USA.
Grace L. Guo, MBBS, PhD, FAASLD
– Tenured professor in the Department of Pharmacology and Toxicology at Rutgers University School of Pharmacy, New Jersey, USA.
Jian-Gao Fan
– Chief physician, second-level professor, Ph.D. supervisor, and head of the Department of Gastroenterology at Xinhua Hospital affiliated with Shanghai Jiao Tong University School of Medicine
