Systemic bile acids induce insulin resistance in a TGR5-independent manner

The molecular insights of bile acid homeostasis in host diseases

Bile acids (BAs) function as detergents promoting nutrient absorption and as hormones regulating nutrient metabolism. Most BAs are key regulatory factors of physiological activities, which are involved in the regulation of glucose, lipid, and drug metabolisms. Hepatic and intestinal diseases have close connections with the systemic cycling disorders of BAs. The abnormal in BA absorption came up with overmuch BAs could be involved in the pathophysiology of liver and bowel and metabolic disorders such as fatty liver diseases and inflammatory bowel diseases. The primary BAs (PBAs), which are synthesized in the liver, can be transformed into the secondary BAs (SBAs) by gut microbiota. The transformation processes are tightly associated with the gut microbiome and the host endogenous metabolism. The BA biosynthesis gene cluster bile-acid-inducible operon is essential for modulating BA pool, gut microbiome composition, and the onset of intestinal inflammation. This forms a bidirectional interaction between the host and its gut symbiotic ecosystem. The subtle changes in the composition and abundance of BAs perturb the host physiological and metabolic activity. Therefore, maintaining the homeostasis of BAs pool contributes to the balance of the body's physiological and metabolic system. Our review aims to dissect the molecular mechanisms underlying the BAs homeostasis, assess the key factors sustaining the homeostasis and the role of BA acting on host diseases. By linking the BAs metabolic disorders and their associated diseases, we illustrate the effects of BAs homeostasis on health and potential clinical interventions can be taken under the latest research findings.

Circulating Bile Acids as Biomarkers for Disease Diagnosis and Prevention

CONTEXT

Bile acids (BAs) are pivotal signaling molecules that regulate energy metabolism and inflammation. Recent epidemiological studies have reported specific alterations in circulating BA profiles in certain disease states, including obesity, type 2 diabetes mellitus (T2DM), nonalcoholic fatty liver disease (NAFLD), and Alzheimer disease (AD). In the past decade, breakthroughs have been made regarding the translation of BA profiling into clinical use for disease prediction. In this review, we summarize and synthesize recent data on variation in circulating BA profiles in patients with various diseases to evaluate the value of these biomarkers in human plasma for early diagnosis.

EVIDENCE ACQUISITION

This review is based on a collection of primary and review literature gathered from a PubMed search for BAs, obesity, T2DM, insulin resistance (IR), NAFLD, hepatocellular carcinoma (HCC), cholangiocarcinoma (CCA), colon cancer, and AD, among other keywords.

EVIDENCE SYNTHESIS

Individuals with obesity, T2DM, HCC, CCA, or AD showed specific alterations in circulating BA profiles. These alterations may have existed long before the initial diagnosis of these diseases. The intricate relationship between obesity, IR, and NAFLD complicates the establishment of clear and independent associations between BA profiles and nonalcoholic steatohepatitis. Alterations in the levels of total BAs and several BA species were seen across the entire spectrum of NAFLD, demonstrating significant increases with the worsening of histological features.

CONCLUSIONS

Aberrant circulating BA profiles are an early event in the onset and progression of obesity, T2DM, HCC, and AD. The pleiotropic effects of BAs explain these broad connections. Circulating BA profiles could provide a basis for the development of biomarkers for the diagnosis and prevention of a wide range of diseases.

The acute effect of thiamine on serum insulin levels and some biochemical parameters in excessive alcohol-consuming rats

There are studies that present metabolic disorders in alcohol drinkers are associated with thiamine deficiency. Therefore, in the present study, it was aimed to investigate the effectiveness of thiamine in rats with binge drinking model. For this purpose, total 21 spraque dawley rats were divided into three equal groups as control, alcohol and thiamine+alcohol groups. The thiamine+ethanol group was given thiamine at a daily dose of 100 mg/kg by oral gavage, starting 2 days before the ethanol administration. Alcohol and thiamine+alcohol groups were given 3.45g/kg/day ethanol as 20%. At the end of the study, while serum total bile acid, total bilirubin and insulin levels increased in rats in the alcohol group compared to the rats in the control group; total protein and albumin levels decreased (P<0.05). In the thiamine + alcohol group, LDL-cholesterol, total cholesterol, bile acid levels and AST enzyme activity increased, while ALT enzyme activity and total protein levels decreased compared to the control group (P<0.05). There was no statistically significant result in the values in the thiamine+alcohol group compared to the alcohol group. It has been concluded that acutely administered thiamine supplementation had no effect on alcohol-induced biochemical parameter changes in binge-drinking animals. In this sense, studies with longer-term thiamine use are needed.

FXR in the dorsal vagal complex is sufficient and necessary for upper small intestinal microbiome-mediated changes of TCDCA to alter insulin action in rats

OBJECTIVE

Conjugated bile acids are metabolised by upper small intestinal microbiota, and serum levels of taurine-conjugated bile acids are elevated and correlated with insulin resistance in people with type 2 diabetes. However, whether changes in taurine-conjugated bile acids are necessary for small intestinal microbiome to alter insulin action remain unknown.

DESIGN

We evaluated circulating and specifically brain insulin action using the pancreatic-euglycaemic clamps in high-fat (HF) versus chow fed rats with or without upper small intestinal healthy microbiome transplant. Chemical and molecular gain/loss-of-function experiments targeting specific taurine-conjugated bile acid-induced changes of farnesoid X receptor (FXR) in the brain were performed in parallel.

RESULTS

We found that short-term HF feeding increased the levels of taurochenodeoxycholic acid (TCDCA, an FXR ligand) in the upper small intestine, ileum, plasma and dorsal vagal complex (DVC) of the brain. Transplantation of upper small intestinal healthy microbiome into the upper small intestine of HF rats not only reversed the rise of TCDCA in all reported tissues but also enhanced the ability of either circulating hyperinsulinaemia or DVC insulin action to lower glucose production. Further, DVC infusion of TCDCA or FXR agonist negated the enhancement of insulin action, while genetic knockdown or chemical inhibition of FXR in the DVC of HF rats reversed insulin resistance.

CONCLUSION

Our findings indicate that FXR in the DVC is sufficient and necessary for upper small intestinal microbiome-mediated changes of TCDCA to alter insulin action in rats, and highlight a previously unappreciated TCDCA-FXR axis linking gut microbiome and host insulin action.

Enhanced Bilosomal Properties Resulted in Optimum Pharmacological Effects by Increased Acidification Pathways

INTRODUCTION

Recent studies in our laboratory have shown that some bile acids, such as chenodeoxycholic acid (CDCA), can exert cellular protective effects when encapsulated with viable β-cells via anti-inflammatory and anti-oxidative stress mechanisms. However, to explore their full potential, formulating such bile acids (that are intrinsically lipophilic) can be challenging, particularly if larger doses are required for optimal pharmacological effects. One promising approach is the development of nano gels. Accordingly, this study aimed to examine biological effects of various concentrations of CDCA using various solubilising nano gel systems on encapsulated β-cells.

METHODS

Using our established cellular encapsulation system, the Ionic Gelation Vibrational Jet Flow technology, a wide range of CDCA β-cell capsules were produced and examined for morphological, biological, and inflammatory profiles.

RESULTS AND CONCLUSION

Capsules' morphology and topographic characteristics remained similar, regardless of CDCA or nano gel concentrations. The best pharmacological, anti-inflammatory, and cellular respiration, metabolism, and energy production effects were observed at high CDCA and nano gel concentrations, suggesting dose-dependent cellular protective and positive effects of CDCA when incorporated with high loading nano gel.

Fasting serum total bile acid levels are associated with insulin sensitivity, islet β-cell function and glucagon levels in response to glucose challenge in patients with type 2 diabetes

Type 2 diabetes (T2D) is characterized by islet β-cell dysfunction and impaired suppression of glucagon secretion of α-cells in response to oral hyperglycaemia. Bile acid (BA) metabolism plays a dominant role in maintaining glucose homeostasis. So we evaluated the association of fasting serum total bile acids (S-TBAs) with insulin sensitivity, islet β-cell function and glucagon levels in T2D. Total 2,952 T2D patients with fasting S-TBAs in the normal range were recruited and received oral glucose tolerance tests for determination of fasting and postchallenge glucose, C-peptide and glucagon. Fasting and systemic insulin sensitivity were assessed by homeostasis model assessment (HOMA) and Matsuda index using C-peptide, i.e., IS_HOMA-cp and ISI_M-cp, respectively. Islet β-cell function was assessed by the insulin-secretion-sensitivity-index-2 using C-peptide (ISSI2_cp). The area under the glucagon curve (AUC_gla) was used to assess postchallenge glucagon. The results showed IS_HOMA-cp, ISI_M-cp and ISSI2_cp decreased, while AUC_gla notably increased, across ascending quartiles of S-TBAs but not fasting glucagon. Moreover, S-TBAs were inversely correlated with IS_HOMA-cp, ISI_M-cp and ISSI2_cp (r = -0.21, -0.15 and -0.25, respectively, p < 0.001) and positively correlated with AUC_gla (r = 0.32, p < 0.001) but not with fasting glucagon (r = 0.033, p = 0.070). Furthermore, after adjusting for other clinical covariates by multiple linear regression analyses, the S-TBAs were independently associated with IS_HOMA-cp (β = -0.04, t = -2.82, p = 0.005), ISI_M-cp (β = -0.11, t = -7.05, p < 0.001), ISSI2_cp (β = -0.15, t = -10.26, p < 0.001) and AUC_gla (β = 0.29, t = 19.08, p < 0.001). Increased fasting S-TBAs are associated with blunted fasting and systemic insulin sensitivity, impaired islet β-cell function and increased glucagon levels in response to glucose challenge in T2D.

Maternal glucose homeostasis is impaired in mouse models of gestational cholestasis

Women with intrahepatic cholestasis of pregnancy (ICP), a disorder characterised by raised serum bile acids, are at increased risk of developing gestational diabetes mellitus and have impaired glucose tolerance whilst cholestatic. FXR and TGR5 are modulators of glucose metabolism, and FXR activity is reduced in normal pregnancy, and further in ICP. We aimed to investigate the role of raised serum bile acids, FXR and TGR5 in gestational glucose metabolism using mouse models. Cholic acid feeding resulted in reduced pancreatic β-cell proliferation and increased apoptosis in pregnancy, without altering insulin sensitivity, suggesting that raised bile acids affect β-cell mass but are insufficient to impair glucose tolerance. Conversely, pregnant Fxr-/- and Tgr5-/- mice are glucose intolerant and have reduced insulin secretion in response to glucose challenge, and Fxr-/- mice are also insulin resistant. Furthermore, fecal bile acids are reduced in pregnant Fxr-/- mice. Lithocholic acid and deoxycholic acid, the principal ligands for TGR5, are decreased in particular. Therefore, we propose that raised serum bile acids and reduced FXR and TGR5 activity contribute to the altered glucose metabolism observed in ICP.