Protection from age-related increase in lipid biomarkers and inflammation contributes to cardiovascular protection in Gilbert's syndrome

Association between bilirubin and biomarkers of metabolic health and oxidative stress in the MARK-AGE cohort

Recent studies have shown that elevated concentrations of unconjugated bilirubin (UCB) may be a protective host factor against the development of noncommunicable diseases (NCDs), whereas low levels of UCB are associated with the opposite effect. The results of this European study, in which 2,489 samples were tested for their UCB concentration using high-performance liquid chromatography (HPLC) and additional data from the MARK-AGE database were used for analysis, provide further evidence that elevated UCB concentrations are linked to a lower risk of developing NCDs and may act as a predictive marker of biological aging as individuals with elevated UCB concentrations showed favorable outcomes in metabolic health and oxidative-stress-related biomarkers. These findings underline the significance of studying individuals with moderate hyperbilirubinemia and investigate UCB routinely, also in the setting of aging, since this condition affects millions of people worldwide but has been underrepresented in clinical research and practice until now.

Combination of UGT1A1 polymorphism and baseline plasma bilirubin levels in predicting the risk of antipsychotic-induced dyslipidemia in schizophrenia patients

The prolonged usage of atypical antipsychotic drugs (AAPD) among individuals with schizophrenia often leads to metabolic side effects such as dyslipidemia. These effects not only limit one's selection of AAPD but also significantly reduce compliance and quality of life of patients. Recent studies suggest that bilirubin plays a crucial role in maintaining lipid homeostasis and may be a potential pre-treatment biomarker for individuals with dyslipidemia. The present study included 644 schizophrenia patients from two centers. Demographic and clinical characteristics were collected at baseline and 4 weeks after admission to investigate the correlation between metabolites, episodes, usage of AAPDs, and occurrence of dyslipidemia. Besides, we explored the combined predictive value of genotypes and baseline bilirubin for dyslipidemia by employing multiple PCR targeted capture techniques to sequence two pathways: bilirubin metabolism-related genes and lipid metabolism-related genes. Our results indicated that there existed a negative correlation between the changes in bilirubin levels and triglyceride (TG) levels in patients with schizophrenia. Among three types of bilirubin, direct bilirubin in the baseline (DBIL-bl) proved to be the most effective in predicting dyslipidemia in the ROC analysis (AUC = 0.627, p < 0.001). Furthermore, the odds ratio from multinomial logistic regression analysis showed that UGT1A1*6 was a protective factor for dyslipidemia (ß = -12.868, p < 0.001). The combination of baseline DBIL and UGT1A1*6 significantly improved the performance in predicting dyslipidemia (AUC = 0.939, p < 0.001). Schizophrenia patients with UGT1A1*6 mutation and a certain level of baseline bilirubin may be more resistant to dyslipidemia and have more selections for AAPD than other patients.

Mildly elevated serum bilirubin and its correlations with lipid levels among male patients undergoing health checkups

BACKGROUND

Bilirubin's ability to lower lipid levels was confirmed by several studies, but those studies mainly focused on total bilirubin (TBil). The present study aimed to elucidate the correlations of the two subtypes of bilirubin with lipid levels.

METHODS

A total of 1732 male patients undergoing health checkups were categorized into three groups according to the levels of direct bilirubin (DBil) and indirect bilirubin (IBil). The differences in medical characteristics among the three groups were analysed.

RESULTS

Subjects in the elevated DBil group had the lowest serum alanine aminotransferase (ALT), total cholesterol (TC), blood urea nitrogen (BUN), γ-glutamyl transpeptidase (γ-GT), fasting blood glucose (FBG), haemoglobin (HGB), and triglyceride (TG) levels in contrast to the other groups (P < 0.01), while subjects in the elevated IBil group had the highest ALT, γ-GT, BUN, serum creatinine (SCR), HGB, TC, and TG levels among the three groups (P < 0.01). DBil levels exhibited a significant negative correlation with TC (r = -0.777, P < 0.01) and TG (r = -0.397, P < 0.01) levels, while IBil levels exhibited a significant positive correlation with TC (r = 0.790, P < 0.01) and TG (r = 0.302, P < 0.01) levels. The frequencies of abnormal TC, TG, HGB and BUN levels were the lowest in the elevated DBil group, while the levels of these four variables were the highest in the elevated IBil group. Mildly elevated DBil levels were related to lower TG (OR = 0.112, 95% CI = 0.027-0.458) and TC (OR = 0.097, 95% CI = 0.013-0.700), and mildly elevated IBil levels were connected with increased TC (OR = 3.436, 95% CI = 2.398-4.924) and TG (OR = 1.636, 95% CI = 1.163-2.303). DBil was an independent protective factor against increased TC (OR = 0.702, 95% CI = 0.602-0.817, P < 0.01) and TG (OR = 0.632, 95% CI = 0.541-0.739, P < 0.01) levels, and IBil was an independent risk factors for increased TC (OR = 1.251, 95% CI = 1.176-1.331, P < 0.01).

CONCLUSIONS

DBil was an independent protective factor against high TC and TG levels. IBil was an independent risk factors for elevated TC levels. The prognostic value of IBil levels warrants further attention.

UGT1A1 morpholino antisense oligonucleotides produce mild unconjugated hyperbilirubinemia in cyclosporine A-induced cardiovascular disorders in BLC57 mice

This study aimed to investigate the induction of mild unconjugated hyperbilirubinemia in hepatic UGT1A1 inhibition by Morpholinos antisense in CsA-treated BLC57 mice in comparison with the efficacy of chitosan (CH) as an anti-hypolipidemic natural product. Antisense morpholino oligonucleotides were injected intravenously into CsA-treated mice for 14 days thrice a week. Serum biochemical parameters, antioxidant status, and gene expression analysis of eNOS, PPAR-α, NF-kB, cFn, AT1-R, and ETA-R were determined in cardiac tissues with confirmation by histopathology. Inhibition of UGT1A1 significantly elevated serum unconjugated bilirubin within a physiological range. Furthermore, induced mild hyperbilirubinemia reduces hyperlipidemia, improves antioxidant status, and significantly increases the expression of the cardiac PPAR-α gene while decreasing, ETA-R, iNOS, NF-kB, cFn and AT1-R gene expression in CsA-treated mice. Importantly, mild unconjugated hyperbilirubinemia within physiological ranges may be used as a novel therapeutic strategy to lower hyperlipidemia, atherosclerosis, hypertension, and the CVD outcomes in CsA- treated transplant recipients.

Association of Different Total Bilirubin Levels with Prognosis of Peritoneal Dialysis-Associated Peritonitis

Background and Objectives: Peritoneal dialysis-associated peritonitis (PDAP) poses significant challenges in peritoneal dialysis (PD) patient management and outcomes. Total bilirubin has gained attention due to its antioxidant and immunomodulatory properties. However, its relationship with PDAP prognosis remains underexplored. Materials and Methods: We conducted a retrospective single-center study involving 243 PDAP patients stratified into tertile-based groups according to total bilirubin levels. The association between total bilirubin levels and treatment failure risk was investigated through statistical analyses and restricted cubic spline curve analysis. Results: Our analysis revealed a non-linear correlation between total bilirubin levels and PDAP treatment failure risk. At total bilirubin levels below 8.24 µmol/L, a protective effect was observed, while levels exceeding this threshold heightened the risk of treatment failure. Conclusions: This study unveils a dual role of total bilirubin in PDAP prognosis. Below a certain threshold, it confers protection, while higher levels exacerbate the risk of treatment failure. These findings emphasize the need for further investigation in larger, multicenter prospective studies to validate and elucidate the mechanisms behind bilirubin's impact on PDAP, potentially guiding the development of targeted therapeutic strategies.

Development and Validation of Pretreatment Serum Total Bilirubin as a Biomarker to Predict the Clinical Outcomes in Primary Central Nervous System Lymphoma: A Multicenter Cohort Study

Primary central nervous system lymphoma (PCNSL) is a predominantly aggressive neoplasm isolated to the central nervous system or vitreoretinal space. Bilirubin is an important biomarker reflecting hepatic function and oxidative stress status that is associated with the occurrence and development of various tumors. However, its prognostic role in PCNSL has yet to be evaluated. Therefore, we conducted a prospective-retrospective study to analyze the predictive value of serum total bilirubin (STB) in PCNSL patients. The association between the pretreatment STB and clinical outcomes in PCNSL was developed in the discovery cohort (retrospective [n = 44] and prospective [n = 45]) and validated in an independent retrospective cohort (n = 69). A generalized additive model, Kaplan-Meier curve, and Cox analysis were applied. In the discovery cohort, the STB showed a linear relationship with overall survival (OS, p = 0.011) and progression-free survival (PFS, p = 0.0476). The median STB level of 12.0 µmol/L was determined as the cutoff value to predict the clinical outcomes with area under the receiver operating characteristic curve (AUROC) values of 0.9205 and 0.8464 for OS and PFS, respectively. The median STB level resulted in similar accuracy for predicting the clinical outcomes in the validation cohort with AUROC values of 0.8857 and 0.8589 for OS and PFS, respectively. In both the discovery and validation cohorts, the Kaplan-Meier survival curve and Cox regression analysis showed that the upper median STB groups showed significantly worse OS than the lower median STB groups. In conclusion, the pretreatment STB could be considered a novel biomarker to predict the clinical outcomes in patients with PCNSL receiving high-dose methotrexate-based combination immunochemotherapy.

The physiology of bilirubin: health and disease equilibrium

Bilirubin has several physiological functions, both beneficial and harmful. In addition to reactive oxygen species-scavenging activities, bilirubin has potent immunosuppressive effects associated with long-term pathophysiological sequelae. It has been recently recognized as a hormone with endocrine actions and interconnected effects on various cellular signaling pathways. Current studies show that bilirubin also decreases adiposity and prevents metabolic and cardiovascular diseases. All in all, the physiological importance of bilirubin is only now coming to light, and strategies for increasing plasma bilirubin levels to combat chronic diseases are starting to be considered. This review discusses the beneficial effects of increasing plasma bilirubin, incorporates emerging areas of bilirubin biology, and provides key concepts to advance the field.

Suppressing Hepatic UGT1A1 Increases Plasma Bilirubin, Lowers Plasma Urobilin, Reorganizes Kinase Signaling Pathways and Lipid Species and Improves Fatty Liver Disease

Several population studies have observed lower serum bilirubin levels in patients with non-alcoholic fatty liver disease (NAFLD). Yet, treatments to target this metabolic phenotype have not been explored. Therefore, we designed an N-Acetylgalactosamine (GalNAc) labeled RNAi to target the enzyme that clears bilirubin from the blood, the UGT1A1 glucuronyl enzyme (GNUR). In this study, male C57BL/6J mice were fed a high-fat diet (HFD, 60%) for 30 weeks to induce NAFLD and were treated subcutaneously with GNUR or sham (CTRL) once weekly for six weeks while continuing the HFD. The results show that GNUR treatments significantly raised plasma bilirubin levels and reduced plasma levels of the bilirubin catabolized product, urobilin. We show that GNUR decreased liver fat content and ceramide production via lipidomics and lowered fasting blood glucose and insulin levels. We performed extensive kinase activity analyses using our PamGene PamStation kinome technology and found a reorganization of the kinase pathways and a significant decrease in inflammatory mediators with GNUR versus CTRL treatments. These results demonstrate that GNUR increases plasma bilirubin and reduces plasma urobilin, reducing NAFLD and inflammation and improving overall liver health. These data indicate that UGT1A1 antagonism might serve as a treatment for NAFLD and may improve obesity-associated comorbidities.

Development and validation of a cardiovascular diseases risk prediction model for Chinese males (CVDMCM)

Background

Death due to cardiovascular diseases (CVD) increased significantly in China. One possible way to reduce CVD is to identify people at risk and provide targeted intervention. We aim to develop and validate a CVD risk prediction model for Chinese males (CVDMCM) to help clinicians identify those males at risk of CVD and provide targeted intervention.

Methods

We conducted a retrospective cohort study of 2,331 Chinese males without CVD at baseline to develop and internally validate the CVDMCM. These participants had a baseline physical examination record (2008–2016) and at least one revisit record by September 2019. With the full cohort, we conducted three models: A model with Framingham CVD risk model predictors; a model with predictors selected by univariate cox proportional hazard model adjusted for age; and a model with predictors selected by LASSO algorithm. Among them, the optimal model, CVDMCM, was obtained based on the Akaike information criterion, the Brier's score, and Harrell's C statistic. Then, CVDMCM, the Framingham CVD risk model, and the Wu's simplified model were all validated and compared. All the validation was carried out by bootstrap resampling strategy (TRIPOD statement type 1b) with the full cohort with 1,000 repetitions.

Results

CVDMCM's Harrell's C statistic was 0.769 (95% CI: 0.738–0.799), and D statistic was 4.738 (95% CI: 3.270–6.864). The results of Harrell's C statistic, D statistic and calibration plot demonstrated that CVDMCM outperformed the Framingham CVD model and Wu's simplified model for 4-year CVD risk prediction.

Conclusions

We developed and internally validated CVDMCM, which predicted 4-year CVD risk for Chinese males with a better performance than Framingham CVD model and Wu's simplified model. In addition, we developed a web calculator–calCVDrisk for physicians to conveniently generate CVD risk scores and identify those males with a higher risk of CVD.

Role of bilirubin in the prognosis of coronary artery disease and its relationship with cardiovascular risk factors: a meta-analysis

Background

Bilirubin is a heme catabolism product with antioxidant, anti-inflammatory, and anti-apoptotic properties and is implicated in the prognosis of several diseases. This study evaluates the prognostic role of bilirubin in coronary artery disease (CAD) patients.

Methods

After identifying studies from the literature, meta-analyses were performed to achieve a) overall estimates of serum total bilirubin levels in patients with myocardial infarction (MI), non-MI CAD and healthy individuals; b) odds ratios (OR) of adverse outcomes between higher and lower total bilirubin levels; c) standardized mean difference (SMD) in total bilirubin levels in patients with high vs low CAD severity; and d) correlation between disease severity and total bilirubin. Metaregression analyses were performed to examine the relationship between cardiovascular risk factors and increasing quantiles of total bilirubin levels.

Results

Forty-three studies were identified. Pooled serum total bilirubin levels were 0.72 mg/dl [95% confidence interval (CI): 0.60, 0.83] in MI patients; 0.65 mg/dl [95% CI: 0.60, 0.69] in non-MI CAD patients; and 0.66 mg/dl [95% CI: 0.56, 0.75] in healthy individuals. Higher total bilirubin levels were associated with greater odds of adverse outcomes in MI patients (OR: 1.08 [95% CI: 0.99, 1.18]) but lower odds in non-MI CAD patients (OR: 0.80 [95%CI: 0.73, 0.88]). Compared to non-severe cases, total bilirubin levels were higher in patients with severe MI (SMD 0.96 [95% CI: − 0.10, 2.01]; p = 0.074) but were lower in severe non-MI CAD patients (SMD − 0.30 [95%CI: − 0.56, − 0.03]; p = 0.02). Total bilirubin levels correlated positively with MI severity (r = 0.41 [95% CI: 0.24, 0.59]; p < 0.01) but correlated negatively with non-MI CAD severity (r =  − 0.17 [95% CI: − 0.48, 0.14]; p = 0.28). Female sex was inversely associated with increasing quantiles of bilirubin (meta-regression coefficient: − 8.164 [− 14.531, − 1.769]; p = 0.016) in MI patients.

Conclusion

Prognostic role of bilirubin for CAD appears complicated, as different odds are observed for MI and non-MI CAD patients which weakens the case of causal involvement of bilirubin in CAD etiology or prognosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-022-02899-w.

Bilirubin as an indicator of cardiometabolic health: a cross-sectional analysis in the UK Biobank

BACKGROUND

Mildly elevated bilirubin, a by-product of hemoglobin breakdown, might mitigate cardiometabolic risk factors including adiposity, dyslipidemia, and high blood pressure (BP). We investigated the cross-sectional relationship between (total) bilirubin and baseline cardiometabolic risk factors in 467,519 UK Biobank study participants.

METHODS

We used multivariable-adjusted linear regression to estimate associations between bilirubin levels and risk factors of cardiometabolic diseases including body mass index (BMI), waist and hip circumferences (WC, HC), waist-to-hip ratio (WHR), fat mass (FM), and trunk FM, and the blood lipids: apolipoprotein A-I (apoA-I), apolipoprotein B (apoB), apoB/apoA-I, lipoprotein (a), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), LDL/HDL, TC/HDL, triglycerides (TG). Log-transformed bilirubin was modelled with restricted cubic splines and predicted mean values with 99% confidence intervals (CI) for each risk marker were estimated, separately. Second, we applied principal component analysis (PCA) for dimension reduction to in turn six anthropometric traits (height, weight, BMI, WC, HC, and WHR) and all above lipids. Last, we estimated associations (99%CI) between bilirubin and three components of the metabolic syndrome, i.e. WC, TG, and BP using logistic regression.

RESULTS

After multivariable adjustments, higher levels of bilirubin were inversely associated with indicators of general adiposity (BMI and FM) and of body fat distribution (WC, HC, WHR, and trunk FM) in both men and women. For example, women with mildly elevated bilirubin (95th percentile equal to 15.0 µmol/L), compared to women with low bilirubin (5th percentile equal to 4.5 µmol/L), had on average a 2.0 kg/m2 (99% CI 1.9-2.1) lower BMI. Inverse associations were also observed with dyslipidemia among men and women. For example, mildly elevated bilirubin among men (95th percentile equal to 19.4 µmol/L) compared to low levels of bilirubin (5th percentile equal to 5.5 µmol/L) were associated with approx. 0.55 mmol/L (99% CI 0.53-0.56) lower TG levels, with similar inverse associations among women. Multiple-trait analyses using PCA confirmed single-trait analyses. Men and women with mildly elevated bilirubin levels ≥ 17.1 µmol/L, compared to low-normal bilirubin < 10 µmol/L had 13% (99% CI 8%-18%) and 11% (99% CI 4%-17%) lower odds of exceeding systolic BP levels of ≥ 130 mm Hg, respectively.

CONCLUSIONS

Higher levels of bilirubin were inversely associated with cardiometabolic risk factors including adiposity, dyslipidemia, and hypertension.

Translational Approach to the Protective Effect of Bilirubin in Diabetic Kidney Disease

Bilirubin has been regarded as a powerful endogenous antioxidant and anti-inflammatory molecule, able to act on cellular pathways as a hormone. Diabetic kidney disease (DKD) is a common chronic complication of diabetes, and it is the leading cause of end-stage renal disease. Here, we will review the clinical and molecular features of mild hyperbilirubinemia in DKD. The pathogenesis of DKD involves oxidative stress, inflammation, fibrosis, and apoptosis. Serum bilirubin levels are positively correlated with the levels of the antioxidative enzymes as superoxide dismutase, catalase, and glutathione peroxidase, while it is inversely correlated with C-reactive protein, TNF-α, interleukin (IL)-2, IL-6, and IL-10 release in diabetic kidney disease. Bilirubin downregulates NADPH oxidase, reduces the induction of pro-fibrotic factor HIF-1α expression, cleaved caspase-3, and cleaved PARP induction showing lower DNA fragmentation. Recent experimental and clinical studies have demonstrated its effects in the development and progression of renal diseases, pointing out that only very mild elevations of bilirubin concentrations result in real clinical benefits. Future controlled studies are needed to explore the precise role of bilirubin in the pathogenesis of DKD and to understand if the use of serum bilirubin levels as a marker of progression or therapeutic target in DKD is feasible and realistic.

Sexual Dimorphism: increased sterol excretion leads to hypocholesterolaemia in female hyperbilirubinaemic Gunn rats

Abstract

Circulating bilirubin is associated with reduced serum cholesterol concentrations in humans and in hyperbilirubinaemic Gunn rats. However, mechanisms contributing to hypocholesterolaemia remain unknown. Therefore, this study aimed to investigate cholesterol synthesis, transport and excretion in mutant Gunn rats. Adult Gunn and control rats were assessed for daily faecal sterol excretion using metabolic cages, and water was supplemented with [1‐¹³C]‐acetate to determine cholesterol synthesis. Bile was collected to measure biliary lipid secretion. Serum and liver were collected for biochemical analysis and for gene/protein expression using RT‐qPCR and western blot, respectively. Additionally, serum was collected and analysed from juvenile rats. A significant interaction of sex, age and phenotype on circulating lipids was found with adult female Gunn rats reporting significantly lower cholesterol and phospholipids. Female Gunn rats also demonstrated elevated cholesterol synthesis, greater biliary lipid secretion and increased total faecal cholesterol and bile acid excretion. Furthermore, they possessed increased hepatic low‐density lipoprotein (LDL) receptor and SREBP2 expression. In contrast, there were no changes to sterol metabolism in adult male Gunn rats. This is the first study to demonstrate elevated faecal sterol excretion in female hyperbilirubinaemic Gunn rats. Increased sterol excretion creates a negative intestinal sterol balance that is compensated for by increased cholesterol synthesis and LDL receptor expression. Therefore, reduced circulating cholesterol is potentially caused by increased hepatic uptake via the LDL receptor. Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).

Key points

Female adult hyperbilirubinaemic (Gunn) rats demonstrated lower circulating cholesterol, corroborating human studies that report a negative association between bilirubin and cholesterol concentrations.

Furthermore, female Gunn rats had elevated sterol excretion creating a negative intestinal sterol balance that was compensated for by elevated cholesterol synthesis and increased hepatic low‐density lipoprotein (LDL) receptor expression.

Therefore, elevated LDL receptor expression potentially leads to reduced circulating cholesterol levels in female Gunn rats providing an explanation for the hypocholesterolaemia observed in humans with elevated bilirubin levels.

This study also reports a novel interaction of sex with the hyperbilirubinaemic phenotype on sterol metabolism because changes were only reported in females and not in male Gunn rats.

Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).

Paradoxical Association Between Baseline Apolipoprotein B and Prognosis in Coronary Artery Disease: A 36,460 Chinese Cohort Study

Background

Apolipoprotein B (ApoB) and low-density lipoprotein cholesterol (LDL-C) were identified targets for blood lipid management among coronary artery disease (CAD) patients. However, previous studies reported an inverse correlation between baseline LDL-C concentration and clinical outcomes. This study aims to explore the definite association between baseline ApoB and long-term prognosis.

Methods

A total of 36,460 CAD patients admitted to Guangdong Provincial People's Hospital were enrolled and categorized into two groups: high ApoB (≥65 mg/dL) group and low ApoB (<65 mg/dL) group. The association between baseline ApoB and long-term all-cause mortality was evaluated by the Kaplan-Meier method, Cox regression analyses and restricted cubic splines.

Results

The overall mortality was 12.49% (n = 4,554) over a median follow-up period of 5.01 years. Patients with low baseline ApoB levels were paradoxically more likely to get a worse prognosis. There was no obvious difference in risk of long-term all-cause mortality when only adjusted for age, gender, and comorbidity (aHR: 1.07, 95% CI: 0.99–1.16). When CONUT and total bilirubin were adjusted, the risk of long-term all-cause mortality would reduce in the low-ApoB (<65 mg/dL) group (aHR: 0.86, 95% CI: 0.78–0.96). In the fully covariable-adjusted model, patients in the ApoB <65 mg/d group had a 10.00% lower risk of long-term all-cause mortality comparing to patients with ApoB ≥65 mg/dL (aHR: 0.90; 95% CI:0.81–0.99).

Conclusion

This study found a paradoxical association between baseline ApoB and long-term all-cause mortality. Malnutrition and bilirubin mainly mediate the ApoB paradox. Increased ApoB concentration remained linearly associated with an increased risk of long-term all-cause mortality.

Serum metabolomics analysis reveals increased lipid catabolism in mildly hyperbilirubinemic Gilbert's syndrome individuals

BACKGROUND

The protective role of mildly elevated bilirubin against CVD and diabetes mellitus type 2 (DMT2) is associated with a favorable lipid phenotype. As the mechanistic understanding of this protection in humans remains elusive, we aimed to assess the metabolomics profile of mildly hyperbilirubinemic (Gilbert's syndrome; GS) individuals especially targeting lipid catabolism.

METHODS AND RESULTS

Using NMR serum metabolomics of 56 GS individuals and 56 age and gender-matched healthy controls, GS individuals demonstrated significantly greater concentrations of acetylcarnitine (+20%, p < 0.001) and the ketone bodies, 3-hydroxybutyric acid (+132%, p < 0.001), acetoacetic acid (+95%, p < 0.001) and acetone (+46%, p < 0.001). Metabolites associated with an increased mitochondrial lipid metabolism such as citrate (+15%, p < 0.001), anaplerotic amino acid intermediates and creatinine were significantly greater and creatine significantly reduced in GS individuals. Stimulators of lipid catabolism including AMPK (+59%, p < 0.001), pPPARα (+24%, p < 0.001) and T3 (+9%, p = 0.009) supported the metabolomics data while concomitantly blood glucose and insulin (-33%, p = 0.002) levels were significantly reduced. We further showed that the increased lipid catabolism partially mediates the favorable lipid phenotype (lower triglycerides) of GS individuals. Increased trimethylamine (+35%, p < 0.001) indicated changes in trimethylamine metabolism, an emerging predictor of metabolic health.

CONCLUSION

We showed an enhanced lipid catabolism in mildly hyperbilirubinemic individuals, novel evidence as to why these individuals are leaner and protected against chronic metabolic diseases emphasizing bilirubin to be a promising future target in obese and dyslipidemia patients.

Gilbert's Syndrome and the Gut Microbiota - Insights From the Case-Control BILIHEALTH Study

The heme catabolite bilirubin has anti-inflammatory, anti-oxidative and anti-mutagenic effects and its relation to colorectal cancer (CRC) risk is currently under evaluation. Although the main metabolic steps of bilirubin metabolism, including the formation of stercobilin and urobilin, take place in the human gastrointestinal tract, potential interactions with the human gut microbiota are unexplored. This study investigated, whether gut microbiota composition is altered in Gilbert's Syndrome (GS), a mild form of chronically elevated serum unconjugated bilirubin (UCB) compared to matched controls. Potential differences in the incidence of CRC-associated bacterial species in GS were also assessed. To this end, a secondary investigation of the BILIHEALTH study was performed, assessing 45 adults with elevated UCB levels (GS) against 45 age- and sex-matched controls (C). Fecal microbiota analysis was performed using 16S rRNA gene sequencing. No association between mildly increased UCB and the composition of the gut microbiota in this healthy cohort was found. The alpha and beta diversity did not differ between C and GS and both groups showed a typical representation of the known dominant phyla. Furthermore, no difference in abundance of Firmicutes and Proteobacteria, which have been associated with the mucosa of CRC patients were observed between the groups. A sequence related to the Christensenella minuta strain YIT 12065 was identified with a weak association value of 0.521 as an indicator species in the GS group. This strain has been previously associated with a lower body mass index, which is typical for the GS phenotype. Overall, sex was the only driver for an identifiable difference in the study groups, as demonstrated by a greater bacterial diversity in women. After adjusting for confounding factors and multiple testing, we can conclude that the GS phenotype does not affect the composition of the human gut microbiota in this generally healthy study group.

Oxidative Stress and Related Biomarkers in Gilbert's Syndrome: A Secondary Analysis of Two Case-Control Studies

Bilirubin is an important antioxidant and a modulator of biological functions. However, most of the protection against oxidative stress was shown in vitro or ex vivo. The aim of this case-control study was to investigate whether subjects with Gilbert’s syndrome (GS) experience different levels of lipid and protein oxidation (as well as differences in oxidative stress related markers) compared to healthy controls. GS subjects (n = 119) demonstrated higher serum levels of unconjugated bilirubin (p < 0.001), a lower BMI (p < 0.001), 37% higher antioxidant potential assessed as ferric reducing ability potential (p < 0.001), higher advanced oxidation protein products (p < 0.01) andlower apolipoprotein B (p < 0.05), hs-C-reactive protein (p < 0.05), interleukin 6 (p < 0.001) and interleukin 1 beta (p < 0.05) values compared to healthy controls (n = 119). Furthermore, the resting heart rate was significantly lower in the GS group (p < 0.05). Stronger protective effects for GS subjects were demonstrated in the older subgroup (n = 104, average age 50 years) compared to those of the younger group (n = 134, average age 27 years). Although not all markers related to oxidative stress were different between the groups (e.g., malondialdehyde, homocysteine, oxLDL, and myeloperoxidase; p > 0.05), the observed differences contribute to the explanation of why GS serves as an important protector in the pathogenesis of metabolic, oxidative stress related diseases.

Heme-oxygenase and lipid mediators in obesity and associated cardiometabolic diseases: Therapeutic implications

Obesity-mediated metabolic syndrome remains the leading cause of death worldwide. Among many potential targets for pharmacological intervention, a promising strategy involves the heme oxygenase (HO) system, specifically its inducible form, HO-1. This review collects and updates much of the current knowledge relevant to pharmacology and clinical medicine concerning HO-1 in metabolic diseases and its effect on lipid metabolism. HO-1 has pleotropic effects that collectively reduce inflammation, while increasing vasodilation and insulin and leptin sensitivity. Recent reports indicate that HO-1 with its antioxidants via the effect of bilirubin increases formation of biologically active lipid metabolites such as epoxyeicosatrienoic acid (EET), omega-3 and other polyunsaturated fatty acids (PUFAs). Similarly, HO-1and bilirubin are potential therapeutic targets in the treatment of fat-induced liver diseases. HO-1-mediated upregulation of EET is capable not only of reversing endothelial dysfunction and hypertension, but also of reversing cardiac remodeling, a hallmark of the metabolic syndrome. This process involves browning of white fat tissue (i.e. formation of healthy adipocytes) and reduced lipotoxicity, which otherwise will be toxic to the heart. More importantly, this review examines the activity of EET in biological systems and a series of pathways that explain its mechanism of action and discusses how these might be exploited for potential therapeutic use. We also discuss the link between cardiac ectopic fat deposition and cardiac function in humans, which is similar to that described in obese mice and is regulated by HO-1-EET-PGC1α signaling, a potent negative regulator of the inflammatory adipokine NOV.

Association of Serum Bilirubin Level with Metabolic Syndrome and Non-Alcoholic Fatty Liver Disease: A Cross-Sectional Study of 1672 Obese Children

As in adults, obesity also plays a central role in the development of metabolic syndrome (MS) in children. Non-alcoholic fatty liver disease (NAFLD) is considered a manifestation of MS. Not only MS but also NAFLD seem to be inversely associated with serum bilirubin concentrations, an important endogenous tissue protector when only mild elevated. The aim of the study was to evaluate the association between serum bilirubin levels and the prevalence of MS and NAFLD in Italian obese children and adolescents. A retrospective cross-sectional study was performed in 1672 patients aged from 5 to 18 years. Clinical and laboratory parameters were assessed. NAFLD was measured by liver ultrasonography. The study was approved by the Ethical Committee of the Istituto Auxologico Italiano (research project code 1C021_2020, acronym BILOB). MS was present in 24% and fatty liver (FL) in 38% of this population. Bilirubin was not associated with FL and MS as a whole, but it was inversely associated only with selected components of MS, i.e., large WC, high blood pressure and high triglycerides. Our data suggest that bilirubin is not protective against MS and NAFLD in the presence of severe obesity.

Association between serum bilirubin levels and progression of albuminuria in Taiwanese with type 2 diabetes mellitus

BACKGROUND

To investigate the association between serum bilirubin (BIL) levels and the progression of albuminuria in type 2 diabetic Taiwanese.

METHODS

Longitudinal data from January 2001 to June 2015 were retrospectively reviewed from Chang Gung Memorial Hospital in Taiwan. A total of 2877 type 2 diabetic patients with normal total BIL levels were divided into 4 groups according to BIL, with the highest BIL in the fourth group. The urinary albumin/creatinine ratio (UACR) trend and progression, as well as other laboratory measurements, were evaluated among the four groups. The cumulative incidence and Cox proportional hazard model analysis were performed to examine the relationship between BIL and the risk of albuminuria progression (AUPr).

RESULTS

The mean duration of follow-up was 1.5 years (±1.37 years). The mean patient age, glycosylated hemoglobin level, and duration of diabetes were 62.52 years, 7.9%, and 3.94 years, respectively. A significant correlation was observed between BIL and both the UACR at baseline (P < 0.001) and the cumulative incidence of AUPr (log-rank test, P = 0.031). Hazard ratio (HR) analysis revealed that patients in the fourth BIL quartile had the lowest HR risk of AUPr among the four groups (adjusted HR = 0.70; 95% Confidence Interval = 0.56-0.89, P < 0.05).

CONCLUSIONS

Higher serum BIL levels are associated with a lower risk of AUPr in type 2 diabetes patients in Taiwan.

Life-Long Hyperbilirubinemia Exposure and Bilirubin Priming Prevent In Vitro Metabolic Damage

Background: Unconjugated bilirubin (UCB) is more than the final product of heme catabolism. Mildly elevated systemic bilirubin concentrations, such as in Gilbert syndrome (GS), protect against various oxidative stress-mediated and metabolic diseases, including cardiovascular disease, type 2 diabetes mellitus, metabolic syndrome, cancer, and age-related disease. The Gunn rat is an animal model of hereditary hyperbilirubinemia widely used in assessing the effect of high serum bilirubin concentration in various organs. The present work aims to understand if life-long hyperbilirubinemia and bilirubin-priming might contribute to protection against atherosclerosis and diabetic nephropathy (DN) at the cellular level.

Methods: Primary aortic endothelial cells and podocytes obtained from hyperbilirubinemic homozygous jj and normobilirubinemic heterozygous Nj Gunn rats were exposed to Palmitic Acid (PA) and Angiotensin II (Ang II), respectively, and the effects on cell viability and the activation of damage-related metabolic pathways evaluated. Results were validated on immortalized H5V and HK2 cells exposed to damage after UCB pretreatment.

Results: In both primary cell models, cells obtained from jj Gunn rats showed as significantly higher than Nj Gunn rats at any dose of the toxic agent. Reduction in CHOP expression and IL-6 release was observed in jj primary aortic endothelial cells exposed to PA compared to Nj cells. The same occurred on H5V pretreated with Unconjugated bilirubin. Upon Ang II treatment, primary podocytes from jj Gunn rats showed lower DNA fragmentation, cleaved caspase-3, and cleaved PARP induction than primary podocytes from Nj Gunn rats. In HK2 cells, the induction by Ang II of HIF-1α and LOXl2 was significantly reduced by UCB pretreatment.

Conclusion: Our data suggest that in models of atherosclerosis and DN life–long hyperbilirubinemia exposure or bilirubin-priming significantly contribute to decrease the injury by enhancing thecellular defensive response,

Mitochondrial Function, Fatty Acid Metabolism, and Body Composition in the Hyperbilirubinemic Gunn Rat

Background: Circulating bilirubin is associated with reduced adiposity in human and animal studies. A possible explanation is provided by in vitro data that demonstrates that bilirubin inhibits mitochondrial function and decreases efficient energy production. However, it remains unclear whether hyperbilirubinemic animals have similar perturbed mitochondrial function and whether this is important for regulation of energy homeostasis. Aim: To investigate the impact of unconjugated hyperbilirubinemia on body composition, and mitochondrial function in hepatic tissue and skeletal muscle. Materials and Methods: 1) Food intake and bodyweight gain of 14-week old hyperbilirubinemic Gunn (n = 19) and normobilirubinemic littermate (control; n = 19) rats were measured over a 17-day period. 2) Body composition was determined using dual-energy X-ray absorptiometry and by measuring organ and skeletal muscle masses. 3) Mitochondrial function was assessed using high-resolution respirometry of homogenized liver and intact permeabilized extensor digitorum longus and soleus fibers. 4) Liver tissue was flash frozen for later gene (qPCR), protein (Western Blot and citrate synthase activity) and lipid analysis. Results: Female hyperbilirubinemic rats had significantly reduced fat mass (Gunn: 9.94 ± 5.35 vs. Control: 16.6 ± 6.90 g, p < 0.05) and hepatic triglyceride concentration (Gunn: 2.39 ± 0.92 vs. Control: 4.65 ± 1.67 mg g^-1, p < 0.01) compared to normobilirubinemic controls. Furthermore, hyperbilirubinemic rats consumed fewer calories daily (p < 0.01) and were less energetically efficient (Gunn: 8.09 ± 5.75 vs. Control: 14.9 ± 5.10 g bodyweight kcal^-1, p < 0.05). Hepatic mitochondria of hyperbilirubinemic rats demonstrated increased flux control ratio (FCR) via complex I and II (CI+II) (Gunn: 0.78 ± 0.16 vs. Control: 0.62 ± 0.09, p < 0.05). Similarly, exogenous addition of 31.3 or 62.5 μM unconjugated bilirubin to control liver homogenates significantly increased CI+II FCR (p < 0.05). Hepatic PGC-1α gene expression was significantly increased in hyperbilirubinemic females while FGF21 and ACOX1 was significantly greater in male hyperbilirubinemic rats (p < 0.05). Finally, hepatic mitochondrial complex IV subunit 1 protein expression was significantly increased in female hyperbilirubinemic rats (p < 0.01). Conclusions: This is the first study to comprehensively assess body composition, fat metabolism, and mitochondrial function in hyperbilirubinemic rats. Our findings show that hyperbilirubinemia is associated with reduced fat mass, and increased hepatic mitochondrial biogenesis, specifically in female animals, suggesting a dual role of elevated bilirubin and reduced UGT1A1 function on adiposity and body composition.

Bilirubin as a metabolic hormone: the physiological relevance of low levels

Recent research on bilirubin, a historically well-known waste product of heme catabolism, suggests an entirely new function as a metabolic hormone that drives gene transcription by nuclear receptors. Studies are now revealing that low plasma bilirubin levels, defined as "hypobilirubinemia," are a possible new pathology analogous to the other end of the spectrum of extreme hyperbilirubinemia seen in patients with jaundice and liver dysfunction. Hypobilirubinemia is most commonly seen in patients with metabolic dysfunction, which may lead to cardiovascular complications and possibly stroke. We address the clinical significance of low bilirubin levels. A better understanding of bilirubin's hormonal function may explain why hypobilirubinemia might be deleterious. We present mechanisms by which bilirubin may be protective at mildly elevated levels and research directions that could generate treatment possibilities for patients with hypobilirubinemia, such as targeting of pathways that regulate its production or turnover or the newly designed bilirubin nanoparticles. Our review here calls for a shift in the perspective of an old molecule that could benefit millions of patients with hypobilirubinemia.

Inhibition of Lipid Accumulation in Skeletal Muscle and Liver Cells: A Protective Mechanism of Bilirubin Against Diabetes Mellitus Type 2

Ectopic lipid accumulation in skeletal muscle and liver drives the pathogenesis of diabetes mellitus type 2 (DMT2). Mild hyperbilirubinaemia has been repeatedly suggested to play a role in the prevention of DMT2 and is known for its capacity to shape an improved lipid phenotype in humans and in animals. To date, the effect of bilirubin on lipid accumulation in tissues that are prone to ectopic lipid deposition is unclear. Therefore, we analyzed the effect of bilirubin on lipid accumulation in skeletal muscle and liver cell lines. C2C12 skeletal mouse muscle and HepG2 human liver cells were treated with physiological concentrations of free fatty acids (FFA) (0.5 mM and 1 mM) and unconjugated bilirubin (UCB) (17.1 and 55 µM). The intracellular presence of UCB upon exogenous UCB administration was confirmed by HPLC and the lipid accumulation was assessed by using Nile red. Exposure of both cell lines to UCB significantly reduced lipid accumulation by up to 23% (p ≤ 0.001) in HepG2 and by up to 17% (p ≤ 0.01) in C2C12 cells at 0.5 and 5 h under hypoglycaemic conditions. Simultaneously, UCB slightly increased FFA uptake in HepG2 cells after 0.5 and 5 h and in C2C12 cells after 12 h as confirmed by gas chromatographic analyses of the remaining FFA content in the incubation media. The effects of UCB on lipid accumulation and uptake were abolished in the presence of higher glucose concentrations. Monitoring the uptake of a radiolabeled glucose analogue [18F]FDG: (2-deoxy-2-[¹⁸F]fluoro-D-glucose) into both cell types further indicated higher glucose consumption in the presence of UCB. In conclusion, our findings show that UCB considerably decreases lipid accumulation in skeletal muscle and liver cells within a short incubation time of max. 5 h which suggests that mildly elevated bilirubin levels could lower ectopic lipid deposition, a major key element in the pathogenesis of DMT2.

Serum Bilirubin Level Is Increased in Metabolically Healthy Obesity

OBJECTIVES

Bilirubin is a biochemical substance with metabolic benefits. The objective of this research was to elucidate the association between serum bilirubin levels and metabolic alterations in different obesity phenotypes.

METHODS

In total, 1,042 drug-naive participants were included in the study. Of them, 541 were obese patients and 501 were age-matched and sex-matched healthy control subjects. The obese patients were divided into metabolically healthy obesity (MHO) group and metabolically unhealthy obesity (MUHO) group according to the levels of fasting plasma glucose (FBG), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C) and blood pressure (BP). Clinical and biochemical parameters including total bilirubin (TBil), indirect bilirubin (IBil) and direct bilirubin (DBil) were measured. ANOVA or Kruskal-Wallis H test was used to test differences among the three groups. Pearson and Spearman correlations were used to analyze the relationships between two parameters. The relationships between bilirubin and other variables were analyzed using Multivariate regression analysis.

RESULTS

MHO group had favorable blood pressure, glucose and lipids profiles, along with increased TBil and DBil, and decreased high-sensitivity C-reactive protein (hsCRP) and homeostasis model assessment of insulin resistance (HOMA-IR) levels when compared to MUHO group (P < 0.05 for all). TBil and DBil were negatively correlated with total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), fasting insulin (FINS), hsCRP and HOMA-IR, even after adjusted for age, gender and BMI (all P <0.01). Multivariate regression analysis demonstrated that HOMA-IR was independently correlated with TBil and DBIi levels (β = -0.400, P < 0.01).

CONCLUSION

MHO group harbors increased bilirubin level compared with MUHO group. HOMA-IR was independently correlated with TBil and DBIi levels.

Bilirubin Nanoparticles Reduce Diet-Induced Hepatic Steatosis, Improve Fat Utilization, and Increase Plasma β-Hydroxybutyrate

The inverse relationship of plasma bilirubin levels with liver fat accumulation has prompted the possibility of bilirubin as a therapeutic for non-alcoholic fatty liver disease. Here, we used diet-induced obese mice with non-alcoholic fatty liver disease treated with pegylated bilirubin (bilirubin nanoparticles) or vehicle control to determine the impact on hepatic lipid accumulation. The bilirubin nanoparticles significantly reduced hepatic fat, triglyceride accumulation, de novo lipogenesis, and serum levels of liver dysfunction marker aspartate transaminase and ApoB100 containing very-low-density lipoprotein. The bilirubin nanoparticles improved liver function and activated the hepatic β-oxidation pathway by increasing PPARα and acyl-coenzyme A oxidase 1. The bilirubin nanoparticles also significantly elevated plasma levels of the ketone β-hydroxybutyrate and lowered liver fat accumulation. This study demonstrates that bilirubin nanoparticles induce hepatic fat utilization, raise plasma ketones, and reduce hepatic steatosis, opening new therapeutic avenues for NAFLD.

Circulating bilirubin levels and risk of colorectal cancer: serological and Mendelian randomization analyses

BACKGROUND

Bilirubin, a byproduct of hemoglobin breakdown and purported anti-oxidant, is thought to be cancer preventive. We conducted complementary serological and Mendelian randomization (MR) analyses to investigate whether alterations in circulating levels of bilirubin are associated with risk of colorectal cancer (CRC). We decided a priori to perform analyses separately in men and women based on suggestive evidence that associations may differ by sex.

METHODS

In a case-control study nested in the European Prospective Investigation into Cancer and Nutrition (EPIC), pre-diagnostic unconjugated bilirubin (UCB, the main component of total bilirubin) concentrations were measured by high-performance liquid chromatography in plasma samples of 1386 CRC cases and their individually matched controls. Additionally, 115 single-nucleotide polymorphisms (SNPs) robustly associated (P < 5 × 10-8) with circulating total bilirubin were instrumented in a 2-sample MR to test for a potential causal effect of bilirubin on CRC risk in 52,775 CRC cases and 45,940 matched controls in the Genetics and Epidemiology of Colorectal Cancer Consortium (GECCO), the Colon Cancer Family Registry (CCFR), and the Colorectal Transdisciplinary (CORECT) study.

RESULTS

The associations between circulating UCB levels and CRC risk differed by sex (Pheterogeneity = 0.008). Among men, higher levels of UCB were positively associated with CRC risk (odds ratio [OR] = 1.19, 95% confidence interval [CI] = 1.04-1.36; per 1-SD increment of log-UCB). In women, an inverse association was observed (OR = 0.86 (0.76-0.97)). In the MR analysis of the main UGT1A1 SNP (rs6431625), genetically predicted higher levels of total bilirubin were associated with a 7% increase in CRC risk in men (OR = 1.07 (1.02-1.12); P = 0.006; per 1-SD increment of total bilirubin), while there was no association in women (OR = 1.01 (0.96-1.06); P = 0.73). Raised bilirubin levels, predicted by instrumental variables excluding rs6431625, were suggestive of an inverse association with CRC in men, but not in women. These differences by sex did not reach formal statistical significance (Pheterogeneity ≥ 0.2).

CONCLUSIONS

Additional insight into the relationship between circulating bilirubin and CRC is needed in order to conclude on a potential causal role of bilirubin in CRC development.

Lower Serum Indirect Bilirubin Levels are Inversely Related to Carotid Intima-Media Thickness Progression

BACKGROUND

Bilirubin has been recognized as a potential endogenous inhibitor of atherosclerosis, being inversely associated with carotid intima-media thickness (CIMT). However, little information is available concerning the correlation between serum indirect bilirubin (IBIL), especially long-term IBIL level, and early atherosclerosis progression. This study was designed to evaluate the relationship between serum IBIL level and CIMT progression.

METHODS

A total of 2205 participants were enrolled in this Asymptomatic Polyvascular Abnormalities Community study (APAC study). CIMT was measured at baseline and 2-year follow-up. The participants were divided into four groups based on their serum IBIL levels at baseline. Both baseline and average serum IBIL values during the 2-year follow up were used in the analysis. Multivariable logistic regression and linear regression were used to assess the associations between serum IBIL and CIMT progression.

RESULTS

The results showed that 51.93% (1145/2205) of participants were diagnosed with CIMT progression during the 2-year follow-up. Baseline serum IBIL level was significantly associated with the incidence of CIMT progression after adjusting for other potential confounding factors. Compared with the first quartile, adjusted odds ratios (OR) of the second, third, and fourth quartiles of IBIL were 0.70 [95% confidence interval (CI), 0.55-0.90], 0.68 (95% CI, 0.52-0.87), and 0.63 (95% CI, 0.49-0.82) (P = 0.0006), respectively. Serum IBIL level during the follow-up was also associated with CIMT progression in the univariate analysis (P = 0.0022), although no longer significant after adjusting for potential confounders in the multiple linear regression.

CONCLUSION

The study demonstrated the inverse relationship between serum IBIL and CIMT progression. Lower serum IBIL level is an independent predictor of subclinical atherosclerosis.

Bilirubin in metabolic syndrome and associated inflammatory diseases: New perspectives

Diabetes mellitus is one of the major global health issues, which is closely related to metabolic dysfunction and the chronic inflammatory diseases. Multiple studies have demonstrated that serum bilirubin is negatively correlated with metabolic syndrome and associated inflammatory diseases, including atherosclerosis, hypertension, etc. However, the roles of bilirubin in metabolic syndrome and associated inflammatory diseases still remain unclear. Here, we explain the role of bilirubin in metabolic syndrome and chronic inflammatory diseases and its therapeutic potential. Understanding the role of bilirubin activities in diabetes may serve as a therapeutic target for the treatment of chronic inflammatory diseases in diabetic patients.

Association between serum bilirubin and asymptomatic intracranial atherosclerosis: results from a population-based study

INTRODUCTION

The effects of bilirubin on asymptomatic intracranial atherosclerosis (aICAS) remain uncertain.

OBJECTIVES

To investigate the association between bilirubin and aICAS in rural-dwelling Chinese people.

METHODS

This population-based study included 2013 participants from the Kongcun Town Study, which aimed to investigate the prevalence of aICAS in people aged ≥ 40 years who were free of stroke and hepatic and gall disease history. Baseline data were collected via interviews, clinical examinations, and laboratory tests. Total bilirubin (Tbil), direct bilirubin (Dbil), and indirect bilirubin (Ibil) levels were divided into high-concentration group and low-concentration group, respectively. We diagnosed aICAS and moderate-to-severe aICAS (m-saICAS) (≥ 50% stenosis) by integrating transcranial Doppler ultrasound with magnetic resonance angiography. The association between bilirubin and aICAS, as well as m-saICAS, was analyzed using logistic regression.

RESULTS

Of the 2013 participants, those in the high-concentration group of Tbil (odds ratio (OR), 0.50; 95% confidence interval (CI), 0.42-0.87), Dbil (OR 0.60, 95%CI 0.41-0.87), and Ibil (OR 0.67; 95%CI 0.47-0.97) had a lower risk of aICAS than those in the low-concentration group after adjusting all confounders. The high concentrations of Tbil, Dbil, and Ibil were also negatively associated with m-saICAS. After stratification according to age, Tbil, Dbil, and Ibil were significantly negatively associated with aICAS among participants aged ≥ 60 years.

CONCLUSION

Tbil, Dbil, and Ibil might be independent protective factors for aICAS and moderate-to-severe aICAS in rural-dwelling Chinese people, especially among older participants aged ≥ 60 years.

Bilirubin Safeguards Cardiorenal and Metabolic Diseases: a Protective Role in Health

PURPOSE OF REVIEW

To discuss recent advances indicating that bilirubin safeguards against cardiorenal and metabolic diseases.

RECENT FINDINGS

Several investigations from human patient populations and experimental animal models have shown that bilirubin improves cardiorenal and metabolic dysfunction. The latest studies found an entirely new function of bilirubin suggesting that it acts as a hormone signaling molecule capable of activating nuclear receptors for burning fat, which may explain several of its protective actions. This review highlights the current findings (within the last 3 years) regarding cardiorenal and metabolic protective effects of bilirubin and the latest mechanism(s) that may be mediating these effects.

Is serum total bilirubin a predictor of prognosis in arteriosclerotic cardiovascular disease? A meta-analysis

The protective role of serum total bilirubin, a widely recognized antioxidant, has been approved by numerous updating studies. However, regarding the effect of high serum total bilirubin level (STBL) in arteriosclerotic cardiovascular disease (ASCVD) are conflicting in different sources of data. We, therefore, performed this meta-analysis to evaluate the influence of STBL on risk of ASCVD.Four databases were used to identify the literature with a date of search of January, 2019. Finally, a total of 20 studies had been adopted. ASCVD was defined as acute coronary syndrome, stable angina, coronary revascularization, atherosclerotic stroke or transient ischemic attack, and peripheral arterial disease (PAD). All relevant data were collected from studies meeting the inclusion criteria.A total of 20 published studies (323,891 cases) met the inclusion criteria. The meta-analysis revealed that, in studies excluding heterogeneity, STBL was significantly positively related to in-hospital cardiovascular mortality (odds ratio [OR] 2.82, 95% confidence interval [CI] 1.83-4.36, Z = 4.69, P < .001) and major adverse cardiac events (OR 1.88, 95% CI 1.414-2.491, Z = 4.36, P < .001), also negatively associated with prognosis of acute myocardial infarction, pooled hazard ratio (HR) = 0.804 (95% CI 0.700-0.923, Z = 3.08, P = .002). The correlation similarity was also reflected in terms of patients with stroke (HR 0.78, 95% CI 0.70-0.88, Z = 4.24, P = .003). Combined analysis revealed that lower STBL was significantly associated with PAD, pooled OR = 0.91 (95% CI 0.85-0.98, Z = 2.39, P = .017). In general analysis, a conclusion can be drawn, that higher STBL was significantly negative correlated with cardiovascular disease, pooled HR = 0.83 (95% CI 0.73-0.94, Z = 3.02, P = .003).Higher STBL significantly improved the prognosis of ASCVD; furthermore, STBL was an important factor in the long-term prognosis of vascular-related disease prevention and can be used as a predictor in vascular-related disease risk prediction.

RNA sequencing in human HepG2 hepatocytes reveals PPAR-α mediates transcriptome responsiveness of bilirubin

Bilirubin is a potent antioxidant that reduces inflammation and the accumulation of fat. There have been reports of gene responses to bilirubin, which was mostly attributed to its antioxidant function. Using RNA sequencing, we found that biliverdin, which is rapidly reduced to bilirubin, induced transcriptome responses in human HepG2 hepatocytes in a peroxisome proliferator-activated receptor (PPAR)-α-dependent fashion (398 genes with >2-fold change; false discovery rate P < 0.05). For comparison, a much narrower set of genes demonstrated differential expression when PPAR-α was suppressed via lentiviral shRNA knockdown (23 genes). Gene set enrichment analysis revealed the bilirubin-PPAR-α transcriptome mediates pathways for oxidation-reduction processes, mitochondrial function, response to nutrients, fatty acid oxidation, and lipid homeostasis. Together, these findings suggest that transcriptome responses from the generation of bilirubin are mostly PPAR-α dependent, and its antioxidant function regulates a smaller set of genes.

Bilirubin, a new therapeutic for kidney transplant?

In patients with end-stage renal disease, kidney transplantation has been associated with numerous benefits, including increased daily activity, and better survival rates. However, over 20% of kidney transplants result in rejection within five years. Rejection is primarily due to a hypersensitive immune system and ischemia/reperfusion injury. Bilirubin has been shown to be a potent antioxidant that is capable of potentially reversing or preventing damage from reactive oxygen species generated from ischemia and reperfusion. Additionally, bilirubin has several immunomodulatory effects that can dampen the immune system to promote organ acceptance. Increased bilirubin has also been shown to have a positive impact on renal hemodynamics, which is critical post-transplantation. Lastly, bilirubin levels have been correlated with biomarkers of successful transplantation. In this review, we discuss a multitude of potentially beneficial effects that bilirubin has on kidney acceptance of transplantation based on numerous clinical trials and animal models. Exogenous bilirubin delivery or increasing endogenous levels pre- or post-transplantation may have therapeutic benefits.

Biliverdin reductase and bilirubin in hepatic disease

The buildup of fat in the liver (hepatic steatosis) is the first step in a series of incidents that may drive hepatic disease. Obesity is the leading cause of nonalcoholic fatty liver disease (NAFLD), in which hepatic steatosis progresses to liver disease. Chronic alcohol exposure also induces fat accumulation in the liver and shares numerous similarities to obesity-induced NAFLD. Regardless of whether hepatic steatosis is due to obesity or long-term alcohol use, it still may lead to hepatic fibrosis, cirrhosis, or possibly hepatocellular carcinoma. The antioxidant bilirubin and the enzyme that generates it, biliverdin reductase A (BVRA), are components of the heme catabolic pathway that have been shown to reduce hepatic steatosis. This review discusses the roles for bilirubin and BVRA in the prevention of steatosis, their functions in the later stages of liver disease, and their potential therapeutic application.

Uridine 5'-diphospho-glucronosyltrasferase: Its role in pharmacogenomics and human disease

Biotransformation is an enzyme-catalyzed process in which the body converts endogenous compounds, xenobiotics and toxic substances into harmless or easily excreted metabolites. The biotransformation reactions are classified as phase I and II reactions. Uridine 5'-diphospho (UDP)-glucuronosyltransferases (UGTs) are a superfamily of phase II enzymes which have roles in the conjugation of xenobiotics or endogenous compounds, including drugs and bilirubin, with glucuronic acid to make them easier to excrete. The method the human body uses to achieve glucuronidation may be affected by a large interindividual variation due to changes in the sequences of the genes encoding these enzymes. In the last five years, the study of the genetic variants of the UGTs at a molecular level has become important due to its association with several diseases and the ability to predict adverse events due to drug metabolism. In the present review, the structure and the prominent genetic variants of the UGT1A subfamily and their metabolic and clinical implications are described.

Bilirubin acts as a multipotent guardian of cardiovascular integrity: more than just a radical idea

Bilirubin, a potentially toxic catabolite of heme and indicator of hepatobiliary insufficiency, exhibits potent cardiac and vascular protective properties. Individuals with Gilbert's syndrome (GS) may experience hyperbilirubinemia in response to stressors including reduced hepatic bilirubin excretion/increased red blood cell breakdown, with individuals usually informed by their clinician that their condition is of little consequence. However, GS appears to protect from all-cause mortality, with progressively elevated total bilirubin associated with protection from ischemic heart and chronic obstructive pulmonary diseases. Bilirubin may protect against these diseases and associated mortality by reducing circulating cholesterol, oxidative lipid/protein modifications, and blood pressure. In addition, bilirubin inhibits platelet activation and protects the heart from ischemia-reperfusion injury. These effects attenuate multiple stages of the atherosclerotic process in addition to protecting the heart during resultant ischemic stress, likely underpinning the profound reduction in cardiovascular mortality in hyperbilirubinemic GS. This review outlines our current knowledge of and uses for bilirubin in clinical medicine and summarizes recent progress in revealing the physiological importance of this poorly understood molecule. We believe that this review will be of significant interest to clinicians, medical researchers, and individuals who have GS.

Bilirubin in the Liver-Gut Signaling Axis

Bilirubin is a component of the heme catabolic pathway that is essential for liver function and has been shown to reduce hepatic fat accumulation. High plasma bilirubin levels are reflective of liver disease due to an injurious effect on hepatocytes. In healthy liver, bilirubin is conjugated and excreted to the intestine and converted by microbes to urobilinoids, which are reduced to the predominant pigment in feces, stercobilin, or reabsorbed. The function of urobilinoids in the gut or their physiological relevance of reabsorption is not well understood. In this review, we discuss the relationship of hepatic bilirubin signaling to the intestinal microbiota and its regulation of the liver-gut axis, as well as its capacity to mediate these processes.

Diagnostic criteria and contributors to Gilbert's syndrome

Hyperbilirubinemia is a well-known condition in the clinical setting; however, the causes of elevated serum bilirubin are diverse, as are the clinical ramifications of this condition. For example, diagnoses of individuals vary depending on whether they exhibit an unconjugated or conjugated hyperbilirubinemia. Diagnoses can include conditions of disordered bilirubin metabolism (Gilbert's, Crigler-Najjar, Rotor, or Dubin-Johnson syndromes) or an acquired disease, including alcoholic/non-alcoholic fatty liver disease, hepatotropic hepatitis, cirrhosis, or hepato-biliary malignancy. Assessment of bilirubin concentrations is typically conducted as part of routine liver function testing. Mildly elevated total bilirubin with normal serum activities of liver transaminases, biliary damage markers, and red blood cell counts, however, may indicate the presence of Gilbert's syndrome (GS), a benign condition that is present in ∼5-10% of the population. In this case, mildly elevated unconjugated bilirubin in GS is strongly associated with "reduced" prevalence of chronic diseases, particularly cardiovascular diseases (CVD) and type 2 diabetes mellitus (and associated risk factors), as well as CVD-related and all-cause mortality. These reports challenge the dogma that bilirubin is simply a potentially neurotoxic by-product of heme catabolism and emphasize the importance of understanding its potential beneficial physiologic and detrimental pathophysiologic effects, in order to appropriately consider bilirubin test results within the clinical laboratory setting. With this information, we hope to improve the understanding of disorders of bilirubin metabolism, emphasize the diagnostic importance of these conditions, and outline the potential impact GS may have on resistance to disease.

Association of serum bilirubin with longevity: Evidence from a retrospective longitudinal study and cross-sectional data

Bilirubin is a potent antioxidant and an important anti-inflammatory factor. Therefore, there has been an increasing focus on serum bilirubin as a negative risk factor of cardiovascular mortality in men and an indicator of improved survival in both sexes, but the direct mechanisms of these links and the causes of sex differences are not well understood. Moreover, the evidence from longitudinal studies on effects of bilirubin on longevity is limited. In this study, we retrospectively analyzed two groups of older adults to explore age-dependent changes in serum bilirubin levels and their associations with long-term survival in both sexes. Longitudinal data from 142 individuals (68 men and 74 women) aged 45 to 70 years were compared with cross-sectional data from 225 individuals (113 men and 112 women). The latter group was divided into four categories of survival, i.e. 53, 63, 68, and 76+ based on data on lifespan. ANOVA, t-test, and regression analysis were run. The analysis of the longitudinal data showed an increase in serum total bilirubin levels in men (0.3038e0.093x, R2 = 0.667) and women (0.1838e0.0187x, R2 = 0.950), while the analysis of cross-sectional data revealed a U-shaped pattern of age-related changes in men (0.001x2 - 0.1263x + 4.4524, R2 = 0.999) but an inverted U-shaped pattern in women (0.0006x2 + 0.072x - 1.6924, R2 = 0.195). On balance, these results suggest that elevated but normal bilirubin levels might confer a survival advantage in older men but not women. Alternatively, the positive relationship between serum total bilirubin and lifespan was not causal but coincidental. Further studies are needed to elucidate the direct mechanisms of the association between serum bilirubin levels and longevity in elderly people of both sexes.

Bilirubin and atherosclerotic diseases

Bilirubin is the final product of heme catabolism in the systemic circulation. For decades, increased serum/plasma bilirubin levels were considered an ominous sign of an underlying liver disease. However, data from recent years convincingly suggest that mildly elevated bilirubin concentrations are associated with protection against various oxidative stress-mediated diseases, atherosclerotic conditions being the most clinically relevant. Although scarce data on beneficial effects of bilirubin had been published also in the past, it took until 1994 when the first clinical study demonstrated an increased risk of coronary heart disease in subjects with low serum bilirubin levels, and bilirubin was found to be a risk factor for atherosclerotic diseases independent of standard risk factors. Consistent with these results, we proved in our own studies, that subjects with mild elevation of serum levels of unconjugated bilirubin (benign hyperbilirubinemia, Gilbert syndrome) have much lower prevalence/incidence of coronary heart as well as peripheral vascular disease. We have also demonstrated that this association is even more general, with serum bilirubin being a biomarker of numerous other diseases, often associated with increased risk of atherosclerosis. In addition, very recent data have demonstrated biological pathways modulated by bilirubin, which are responsible for observed strong clinical associations.

Bilirubin Decreases Macrophage Cholesterol Efflux and ATP-Binding Cassette Transporter A1 Protein Expression

BACKGROUND

Mild but chronically elevated circulating unconjugated bilirubin is associated with reduced total and low-density lipoprotein cholesterol concentration, which is associated with reduced cardiovascular disease risk. We aimed to investigate whether unconjugated bilirubin influences macrophage cholesterol efflux, as a potential mechanism for the altered circulating lipoprotein concentrations observed in hyperbilirubinemic individuals.

METHODS AND RESULTS

Cholesterol efflux from THP-1 macrophages was assessed using plasma obtained from normo- and hyperbilirubinemic (Gilbert syndrome) humans (n=60 per group) or (heterozygote/homozygote Gunn) rats (n=20 per group) as an acceptor. Hyperbilirubinemic plasma from patients with Gilbert syndrome and Gunn rats induced significantly reduced cholesterol efflux compared with normobilirubinemic plasma. Unconjugated bilirubin (3-17.1 μmol/L) exogenously added to plasma- or apolipoprotein A1-supplemented media also decreased macrophage cholesterol efflux in a concentration- and time-dependent manner. We also showed reduced protein expression of the ATP-binding cassette transporter A1 (ABCA1), a transmembrane cholesterol transporter involved in apolipoprotein A1-mediated cholesterol efflux, in THP-1 macrophages treated with unconjugated bilirubin and in peripheral blood mononuclear cells obtained from hyperbilirubinemic individuals. Furthermore, we demonstrated that bilirubin accelerates the degradation rate of the ABCA1 protein in THP-1 macrophages.

CONCLUSIONS

Cholesterol efflux from THP-1 macrophages is decreased in the presence of plasma obtained from humans and rats with mild hyperbilirubinemia. A direct effect of unconjugated bilirubin on cholesterol efflux was demonstrated and is associated with decreased ABCA1 protein expression. These data improve our knowledge concerning bilirubin's impact on cholesterol transport and represent an important advancement in our understanding of bilirubin's role in cardiovascular disease.

Characteristics of the heme catabolic pathway in mild unconjugated hyperbilirubinemia and their associations with inflammation and disease prevention

Heme catabolism exerts physiological functions that impact health through depressing inflammation. Upon reactive pathway progression, as in Gilbert’s Syndrome (GS; UGT1A1*28 polymorphism), aggravated health effects have been determined. Based on lower inflammation and improved metabolic health reported for GS, inter-group differences in heme catabolism were explored. Therefore, a case-control study including 120 fasted, healthy, age- and gender matched subjects with/without GS, was conducted. Genetic expressions of HMOX-1 and BLVRA were measured. Additionally participants were genotyped for those polymorphisms that are known (UGT1A1*28) or likely (HMOX-1 microsatellites) to impact bilirubinemia. Intracellular interleukins (IL-6, IL-1β, TNFα), circulatory C-reactive protein (CRP), serum amyloid A (SAA) and haptoglobin (Hpt) were analysed as inflammatory markers. To assess intracellular heme oxygenase 1 (HO-1) isolated PBMCs were used. In GS vs. C, inflammation markers were significantly decreased. This was supported by an altered heme catabolism, indirectly reflecting in elevated unconjugated bilirubin (UCB; main phenotypic feature of GS) and iron, decreased hemopexin (Hpx) and Hpt and in up-regulated biliverdin reductase (BLVRA) gene expressions. Moreover, HMOX (GT)_n short alleles were non-significantly more prominent in female GS individuals. Herewith, we propose a concept to elucidate why GS individuals encounter lower inflammation, and are thus less prone to oxidative-stress mediated diseases.

Mice with hyperbilirubinemia due to Gilbert's syndrome polymorphism are resistant to hepatic steatosis by decreased serine 73 phosphorylation of PPARα

Gilbert's syndrome in humans is derived from a polymorphism (TA repeat) in the hepatic UGT1A1 gene that results in decreased conjugation and increased levels of unconjugated bilirubin. Recently, we have shown that bilirubin binds directly to the fat-burning nuclear peroxisome proliferator-activated receptor-α (PPARα). Additionally, we have shown that serine 73 phosphorylation [Ser(P)⁷³] of PPARα decreases activity by reducing its protein levels and transcriptional activity. The aim of this study was to determine whether humanized mice with the Gilbert's polymorphism (HuUGT*28) have increased PPARα activation and reduced hepatic fat accumulation. To determine whether humanized mice with Gilbert's mutation (HuUGT*28) have reduced hepatic lipids, we placed them and C57BL/6J control mice on a high-fat (60%) diet for 36 wk. Body weights, fat and lean mass, and fasting blood glucose and insulin levels were measured every 6 wk throughout the investigation. At the end of the study, hepatic lipid content was measured and PPARα regulated genes as well as immunostaining of Ser(P)⁷³ PPARα from liver sections. The HuUGT*28 mice had increased serum bilirubin, lean body mass, decreased fat mass, and hepatic lipid content as well as lower serum glucose and insulin levels. Also, the HuUGT*28 mice had reduced Ser(P)⁷³ PPARα immunostaining in livers and increased PPARα transcriptional activity compared with controls. A chronic but mild endogenous increase in unconjugated hyperbiliubinemia protects against hepatic steatosis through a reduction in Ser(P)⁷³ PPARα, causing an increase in PPARα transcriptional activity.

Hyperbilirubinemia Protects against Aging-Associated Inflammation and Metabolic Deterioration

Mild constitutive hyperbilirubinemia is associated with a reduced risk of cardiovascular diseases, diabetes, and cancer. Since these pathologies are associated with aging, inflammation, and oxidative stress, we investigated whether hyperbilirubinemia interferes with ROS homeostasis in cell cultures and with inflammation, senescence, and mitochondrial dysfunction in aged rats. Human embryonic kidney cells and rat primary fibroblasts showed a dose-dependent decrease in the ratio of oxidized/reduced glutathione, intracellular H2O2 levels, and mitochondrial ROS production, with increasing bilirubin concentrations in the culture media. Compared to their normobilirubinemic siblings, aged hyperbilirubinemic Gunn rats showed significantly smaller amounts of visceral fat, better glucose tolerance, and decreased serum levels of proinflammatory cytokines TNFα, IL-1β, and IL-18. Simultaneously, livers from Gunn rats showed decreased expression of senescence markers and cell cycle inhibitors p21 and p16. Mitochondria from aged Gunn rats showed higher respiration and lower H2O2 production compared to controls. In conclusion, we demonstrated that mildly elevated serum bilirubin is generally associated with attenuation of oxidative stress and with better anthropometric parameters, decreased inflammatory status, increased glucose tolerance, fewer signs of cellular senescence, and enhanced mitochondrial function in aged rats.

Serum Bilirubin Levels and the Presence and Progression of Abdominal Aortic Aneurysms

The relationship between bilirubin levels and peripheral artery disease has been documented. Our aim was to demonstrate the possible relationship between serum bilirubin levels and abdominal aortic aneurysms (AAAs). The study included 219 patients, 110 had a previous diagnosis of AAA and 109 patients were normal controls. Only patients with AAAs which had a size of 40 to 54 mm were included in the study. Baseline laboratory values and 2 computerized tomographic measurements 12 months apart were recorded. Patients with AAA had significantly higher white blood cell (WBC) counts and neutrophil-lymphocyte ratio (NLR) but lower total and direct bilirubin levels compared with the control patients ( P < .05). Multivariate logistic regression analysis showed that WBC, NLR, and total and direct bilirubin levels were independent predictors of the presence of an AAA ( P = .03, P = .001, P = .001, and P = .001, respectively). White blood cells and total bilirubin level were independent predictors of a rapidly enlarging AAA (>10 mm/y, P = .002 and P < .001, respectively). This study demonstrated that increased WBC and decreased total bilirubin levels were independent predictors of an AAA, especially the subgroup in which the AAA was rapidly expanding.

Features of an altered AMPK metabolic pathway in Gilbert's Syndrome, and its role in metabolic health

Energy metabolism, involving the ATP-dependent AMPK-PgC-Ppar pathway impacts metabolic health immensely, in that its impairment can lead to obesity, giving rise to disease. Based on observations that individuals with Gilbert's syndrome (GS; UGT1A1(*)28 promoter mutation) are generally lighter, leaner and healthier than controls, specific inter-group differences in the AMPK pathway regulation were explored. Therefore, a case-control study involving 120 fasted, healthy, age- and gender matched subjects with/without GS, was conducted. By utilising intra-cellular flow cytometry (next to assessing AMPKα1 gene expression), levels of functioning proteins (phospho-AMPK α1/α2, PgC 1 α, Ppar α and γ) were measured in PBMCs (peripheral blood mononucleated cells). In GS individuals, rates of phospho-AMPK α1/α2, -Ppar α/γ and of PgC 1α were significantly higher, attesting to a boosted fasting response in this condition. In line with this finding, AMPKα1 gene expression was equal between the groups, possibly stressing the post-translational importance of boosted fasting effects in GS. In reflection of an apparently improved health status, GS individuals had significantly lower BMI, glucose, insulin, C-peptide and triglyceride levels. Herewith, we propose a new theory to explain why individuals having GS are leaner and healthier, and are therefore less likely to contract metabolic diseases or die prematurely thereof.