The association between bilirubin levels, and the incidence of metabolic syndrome and diabetes mellitus: a systematic review and meta-analysis of cohort studies

OBJECTIVES

The objective of this meta-analysis was to investigate the association between plasma bilirubin levels and the incidence of metabolic syndrome and diabetes mellitus across all populations.

METHODS

Several databases were searched, including PubMed (Medline), Scopus, Web of Science, and Embase (Elsevier), to identify relevant cohort studies. All cohort studies that reported the risk ratio along with a 95% confidence interval were included. The association between bilirubin levels and metabolic syndrome or diabetes was reported as a pooled RR with a 95% CI in the forest plot. All analyses were conducted using STATA version 17, with a significance level of 0.05.

RESULTS

Out of the 10 studies included in the analysis, four investigated the effect of hyperbilirubinemia on the incidence of type 2 diabetes. When these four studies were combined, the pooled RR was 0.78 (95% CI: 0.73, 0.83; I2: 88.61%; P heterogeneity <  0.001), indicating a significant association between hyperbilirubinemia and decreased risk of type 2 diabetes. Five of the 10 studies evaluated the effect of hyperbilirubinemia on the incidence of metabolic syndrome, and the pooled RR was 0.70 (95% CI: 0.67, 0.73; I2: 78.13%; P heterogeneity <  0.001), indicating a significant association between hyperbilirubinemia and decreased risk of metabolic syndrome.

CONCLUSION

The findings suggest that elevated levels of bilirubin may have a significant protective effect against the development of diabetes mellitus and metabolic syndrome.

Association of Low Serum Bilirubin Concentrations and Promoter Variations in the UGT1A1 and HMOX1 Genes with Type 2 Diabetes Mellitus in the Czech Population

Bilirubin has potent biological beneficial effects, protecting against atherosclerosis, obesity, and metabolic syndrome. The aim of this study was to assess serum bilirubin concentrations and (TA)n and (GT)n microsatellite variations in the promoter regions of the UGT1A1 and HMOX1 genes, respectively, in patients with type 2 diabetes mellitus (T2DM). The study was carried out in 220 patients with T2DM and 231 healthy control subjects, in whom standard biochemical tests were performed. The (TA)n and (GT)n dinucleotide variations were determined by means of fragment (size-based) analysis using an automated capillary DNA sequencer. Compared to controls, both male and female patients with T2DM had lower serum bilirubin concentrations (9.9 vs. 12.9 μmol/L, and 9.0 vs. 10.6 μmol/L, in men and women, respectively, p < 0.001). Phenotypic Gilbert syndrome was much less prevalent in T2DM patients, as was the frequency of the (TA)7/7UGT1A1 genotype in male T2DM patients. (GT)nHMOX1 genetic variations did not differ between diabetic patients and controls. Our results demonstrate that the manifestation of T2DM is associated with lower serum bilirubin concentrations. Consumption of bilirubin due to increased oxidative stress associated with T2DM seems to be the main explanation, although (TA)n repeat variations in UGT1A1 partially contribute to this phenomenon.

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.

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.

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.

Association between serum total bilirubin levels and the risk of type 2 diabetes mellitus

AIM

To confirm whether serum bilirubin is an independent risk factor of type 2 diabetes mellitus (T2DM) onset in patients with impaired fasting glycemia (IFG) and impaired glucose tolerance (IGT).

METHODS

This was a prospective cohort study carried out at the Diabetic Identification Center of Tianjin Metabolic Diseases Hospital. Serum total bilirubin (TBIL) was measured at baseline and the patients were grouped according to baseline bilirubin quartiles. The outcome was the confirmation of T2DM by oral glucose tolerance test (OGTT) during the 3-year follow-up. Logistic regression was used to determine the risk factors for T2DM development and whether bilirubin levels are independently associated with T2DM development.

RESULTS

Finally, 523 patients were analyzed. After 3 years, 310 participants were diagnosed with diabetes based on OGTT. Baseline quartiles of total bilirubin were inversely associated with diabetes risk, even after multivariable adjustment. The adjusted ORs for diabetes were 1.0 (reference), 0.83 (95% CI 0.74-0.96), 0.78 (95% CI 0.68-0.90), 0.74 (95% CI 0.64-0.87) for the 1st, 2nd, 3rd, and 4th quartiles of baseline serum total bilirubin, respectively (P < 0.001).

CONCLUSION

In patients with IFG or IGT, low levels of serum total bilirubin were associated with a significantly increased risk of T2DM.

Neuroprotective Effect of ω-3 Polyunsaturated Fatty Acids on Bilirubin Encephalopathy In Vitro and In Vivo

Background

Bilirubin encephalopathy is a serious complication in neonatal jaundice and is associated with high mortality and disability in newborns. The present study aimed to investigate the neuroprotective effects of omega-3 polyunsaturated fatty acids (ω-3 PUFA) on bilirubin encephalopathy in vitro and in vivo.

Material/Methods

The cytotoxicity of unconjugated bilirubin (UCB) to neurons and neuroprotection of ω-3 PUFA were investigated using MTT assays and apoptosis evaluations. Superoxide dismutase (SOD) and catalase (CAT) enzyme activity were measured to investigate the anti-oxidative effect of ω-3 PUFA. The differences between eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were also compared. The in vivo neuroprotective effect of DHA was demonstrated in neonatal rats with bilirubin encephalopathy by bilirubin monitoring, neuron-specific enolase (NSE) monitoring, H&E staining of brain tissue, and apoptosis rate evaluations.

Results

Omega-3 PUFA reduced the rate of apoptosis induced by UCB and increased SOD and CAT enzyme activity for anti-oxidation. DHA did not reduce the bilirubin in the serum of neonatal rats with bilirubin encephalopathy, but did reduce the damage caused by bilirubin with decreased NSE and apoptosis rate as well as improved neuron morphology.

Conclusions

Omega-3 PUFA, particularly DHA, can reduce neurological damage in neonatal rats with bilirubin encephalopathy by increasing anti-apoptosis and anti-oxidation effects against UCB, providing a theoretical basis for the clinical treatment of bilirubin encephalopathy in newborns.

Elevated Serum Total Bilirubin Concentrations Are Negatively Associated with Diabetic Retinopathy among the Chinese Northeastern Population

OBJECTIVE

To evaluate the association between serum total bilirubin concentration (STBC) and diabetic retinopathy (DR) among the Chinese northeastern population.

METHODS

A cross-sectional study was conducted in Liaoning between January 2015 and May 2017.

RESULTS

A total of 742 subjects (419 men and 323 women) with type 2 diabetes mellitus (DM) who visited an ophthalmic clinic were included in this study. The mean age of the subjects was 59.55 ± 10.63 years, and 43.5% of the subjects were women. The mean of DM duration was 11.01 ± 7.35 years. STBC were negatively correlated with DM duration, urea nitrogen, serum creatinine, uric acid, and urine microalbumin. After adjusting for confounding factors, as a continuous variable, STBC was inversely associated with the risk of DR in total subjects (OR: 0.95, 95% CI: 0.93-0.99). When STBC was used as a tertiary variable, compared with the first tertile, the OR in the third tertile was 0.37 (95% CI: 0.22-0.64) in total subjects.

CONCLUSION

Our results demonstrate that a significant negative association was found between STBC and DR. STBC might be an early clinical marker for predicting the occurrence of DR.

Gilbert syndrome in patients with type 1 diabetes-Prevalence, glycemic control, and microalbuminuria

BACKGROUND

Gilbert syndrome (GS) is a common hereditary condition, characterized by intermittent unconjugated hyperbilirubinemia. In adults with type 2 diabetes and GS, a markedly lower prevalence of nephropathy was documented, suggesting a beneficial effect of hyperbilirubinemia. We investigated the prevalence of GS among individuals with type 1 diabetes mellitus (T1DM), and the prevalence of microalbuminuria.

METHODS

The prevalence of GS was assessed in 401 (204 female) patients with T1DM, median age 21.0 years, (interquartile range [IQR], 15.7-27.9), median disease duration 10.8 years (IQR, 5.7-15.8); and was compared with GS prevalence in 181 children (control). The prevalence of microalbuminuria was assessed in patients with T1DM and GS (group I) and compared with that of patients with T1DM alone (group II), in a ratio of 1:2 matched by gender, age, and duration of diabetes.

RESULTS

The prevalence of GS in TIDM patients was significantly higher than in the control group (10.7% vs 3.3% respectively, p = .004), with no gender difference. Patients with T1DM and GS had significantly lower HbA1c levels than did those with T1DM alone 7.3 ± 1.2 vs 7.9 ± 1.3% respectively (56 ± 13 vs 63 ± 14 mmol/mol), p = .02. The rate of microalbuminuria was 14.0% vs 11.0% for patients with T1DM and GS, compared with those with T1DM alone (p = .6).

CONCLUSIONS

The occurrence of GS was 3-fold higher among individuals with T1DM than in a healthy control group. Despite better glycemic control, the rate of microalbuminuria was similar among young individuals with T1DM and GS, and those with T1DM alone, suggesting no protective value to elevated bilirubin.

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.

Serum bilirubin levels and risk of type 2 diabetes: results from two independent cohorts in middle-aged and elderly Chinese

Serum bilirubin is a potent endogenous antioxidant and has been identified as cardiovascular risk in cohort studies, while the relation to type 2 diabetes (T2D) in the elderly remains unclear. We investigated both cross-sectional and prospective associations between serum bilirubin levels and T2D risk in the Dongfeng-Tongji (DFTJ) cohort, and replicated the prospective findings in a nested case-control study (509 cases and 509 controls) within the Singapore Chinese Health Study (SCHS). In the cross-sectional analysis of DFTJ cohort (15,575 participants with 2,532 diabetes cases), serum bilirubin levels (total, direct and indirect) increased in new on-set diabetes and decreased with the diabetic duration. In the longitudinal analysis of DFTJ cohort (772 incident diabetes cases during 4.5 years of follow-up among 12,530 diabetes-free participants at baseline), positive association was found between direct bilirubin and T2D risk comparing extreme quartiles, similar results were observed in the nested case-control study within SCHS. Total and indirect bilirubin levels were not significantly associated with T2D in either cohort. In conclusion, our findings do not support the protective association between serum bilirubin levels and incident T2D in the middle-aged and elderly adults; instead, direct bilirubin levels were associated with increased risk of T2D.

Elevated serum total bilirubin levels are negatively associated with major diabetic complications among Chinese senile diabetic patients

AIMS

This study aimed to evaluate the association between total bilirubin (TBiL) and major diabetic complications among Chinese senile diabetic patients.

METHODS

A cross-sectional study was conducted in all the cadre sanitariums in Beijing, between May 2012 and December 2014. All the diagnoses of diabetic complications were extracted from the medical records including chronic kidney disease, retinopathy, peripheral vascular disease, coronary heart disease, and ischemic stroke.

RESULTS

The mean age of the 1839 senile diabetic patients were 87.4±4.0 (80-102years) and mean TBiL level was 13.2±6.0μmol/L. The prevalence of chronic kidney disease, retinopathy, peripheral vascular disease, coronary heart disease and ischemic stroke was 13.8%, 17.8%, 5.8%, 62.5%, and 44.5%, respectively. TBiL level was negatively correlated with age, weight, SBP, TC, FBG, 2hPG, diabetic duration and positively correlated with HDL-C. Prevalence of all five diseases gradually decreased with the increase of TBiL level (p<0.05). Also, number of diabetic complications showed a decreasing trend along with TBiL tertiaries. After adjusted age, education, marital status, current smoking, current drinking, physical activity ≥0.5h/day, BMI, hypertension, dyslipidemia, treatment and control status of diabetes, patients with higher TBiL level were at significantly decreased ORs for prevalence of major diabetic complications. The ORs were 0.97 (95%CI: 0.96-0.99), 0.90 (95%CI: 0.87-0.93), 0.98 (95%CI: 0.97-0.99), 0.97 (95%CI: 0.95-.99) and 0.98 (95%CI: 0.97-0.99) for chronic kidney disease, retinopathy, peripheral vascular disease, coronary heart disease and ischemic stroke (p<0.005). Similar results were obtained when TBiL was used as tertiary variable.

CONCLUSION

Higher TBiL was significantly associated with lower prevalence of major diabetic complication among senile diabetic patients, and this association was graded with TBiL level and independent of age and control status of diabetes.

Metabolomics Study of Roux-en-Y Gastric Bypass Surgery (RYGB) to Treat Type 2 Diabetes Patients Based on Ultraperformance Liquid Chromatography-Mass Spectrometry

Roux-en-Y gastric bypass (RYGB) is one of the most effective treatments for long-term weight loss and diabetes remission; however, the mechanisms underlying these changes are not clearly understood. In this study, the serum metabolic profiles of 23 remission and 12 nonremission patients with type 2 diabetes mellitus (T2DM) were measured at baseline, 6- and 12-months after RYGB. A metabolomics analysis was performed based on ultra-performance liquid chromatography-mass spectrometry. Clinical improvements in insulin sensitivity, energy metabolism, and inflammation were related to metabolic alterations of free fatty acids (FFAs), acylcarnitines, amino acids, bile acids, and lipids species. Differential metabolic profiles were observed between the two T2DM subgroups, and patients with severity fat accumulation and oxidation stress may be more suitable for RYGB. Baseline levels of tryptophan, bilirubin, and indoxyl sulfate measured prior to surgery as well as levels of FFA 16:0, FFA 18:3, FFA 17:2, and hippuric acid measured at 6 months after surgery best predicted the suitability and efficacy of RYGB for patients with T2DM. These metabolites represent potential biomarkers that may be clinically helpful in individualized treatment for T2DM patients by RYGB.

Innovative biomarkers for predicting type 2 diabetes mellitus: relevance to dietary management of frailty in older adults

Type 2 diabetes mellitus (T2DM) increases in prevalence in the elderly. There is evidence for significant muscle loss and accelerated cognitive impairment in older adults with T2DM; these comorbidities are critical features of frailty. In the early stages of T2DM, insulin sensitivity can be improved by a "healthy" diet. Management of insulin resistance by diet in people over 65 years of age should be carefully re-evaluated because of the risk for falling due to hypoglycaemia. To date, an optimal dietary programme for older adults with insulin resistance and T2DM has not been described. The use of biomarkers to identify those at risk for T2DM will enable clinicians to offer early dietary advice that will delay onset of disease and of frailty. Here we have used an in silico literature search for putative novel biomarkers of T2DM risk and frailty. We suggest that plasma bilirubin, plasma, urinary DPP4-positive microparticles and plasma pigment epithelium-derived factor merit further investigation as predictive biomarkers for T2DM and frailty risk in older adults. Bilirubin is screened routinely in clinical practice. Measurement of specific microparticle frequency in urine is less invasive than a blood sample so is a good choice for biomonitoring. Future studies should investigate whether early dietary changes, such as increased intake of whey protein and micronutrients that improve muscle function and insulin sensitivity, affect biomarkers and can reduce the longer term complication of frailty in people at risk for T2DM.

Haeme oxygenase signalling pathway: implications for cardiovascular disease

Evidence now points to the haeme oxygenase (HO) pathway as a possible actor in modulating risk for cardiovascular disease (CVD). In particular, the HO pathway may represent a key endogenous modulator of oxidative, inflammatory, and cytotoxic stress while also exhibiting vasoregulatory properties. In this review, we summarize the accumulating experimental and emerging clinical data indicating how activity of the HO pathway and its products may play a role in mechanisms underlying the development of CVD. We also identify gaps in the literature to date and suggest future directions for investigation. Because HO pathway activity can be influenced not only by genetic traits and environmental stimuli but also by a variety of existing pharmacologic interventions, the pathway could serve as a prime target for reducing the overall burden of CVD. Further work is needed to determine the role of HO pathway products as possible prognostic markers of risk for clinical CVD events and the extent to which therapeutic augmentation or inhibition of HO pathway activity could serve to modify CVD risk.

Bilirubin as a potential causal factor in type 2 diabetes risk: a Mendelian randomization study

Circulating bilirubin, a natural antioxidant, is associated with decreased risk of type 2 diabetes (T2D), but the nature of the relationship remains unknown. We performed Mendelian randomization in a prospective cohort of 3,381 participants free of diabetes at baseline (age 28-75 years; women 52.6%). We used rs6742078 located in the uridine diphosphate-glucuronosyltransferase locus as an instrumental variable (IV) to study a potential causal effect of serum total bilirubin level on T2D risk. T2D developed in a total of 210 participants (6.2%) during a median follow-up period of 7.8 years. In adjusted analyses, rs6742078, which explained 19.5% of bilirubin variation, was strongly associated with total bilirubin (a 0.68-SD increase in bilirubin levels per T allele; P < 1 × 10(-122)) and was also associated with T2D risk (odds ratio [OR] 0.69 [95% CI 0.54-0.90]; P = 0.006). Per 1-SD increase in log-transformed bilirubin levels, we observed a 25% (OR 0.75 [95% CI 0.62-0.92]; P = 0.004) lower risk of T2D. In Mendelian randomization analysis, the causal risk reduction for T2D was estimated to be 42% (causal OR for IV estimation per 1-SD increase in log-transformed bilirubin 0.58 [95% CI 0.39-0.84]; P = 0.005), which was comparable to the observational estimate (Durbin-Wu-Hausman χ(2) test, P for difference = 0.19). These novel results provide evidence that an elevated bilirubin level is causally associated with the risk of T2D and support its role as a protective determinant.

Metabolomic characteristics of arsenic-associated diabetes in a prospective cohort in Chihuahua, Mexico

Chronic exposure to inorganic arsenic (iAs) has been linked to an increased risk of diabetes, yet the specific disease phenotype and underlying mechanisms are poorly understood. In the present study we set out to identify iAs exposure-associated metabolites with altered abundance in nondiabetic and diabetic individuals in an effort to understand the relationship between exposure, metabolomic response, and disease status. A nested study design was used to profile metabolomic shifts in urine and plasma collected from 90 diabetic and 86 nondiabetic individuals matched for varying iAs concentrations in drinking water, body mass index, age, and sex. Diabetes diagnosis was based on measures of fasting plasma glucose and 2-h blood glucose. Multivariable models were used to identify metabolites with altered abundance associated with iAs exposure among diabetic and nondiabetic individuals. A total of 132 metabolites were identified to shift in urine or plasma in response to iAs exposure characterized by the sum of iAs metabolites in urine (U-tAs). Although many metabolites were altered in both diabetic and nondiabetic 35 subjects, diabetic individuals displayed a unique response to iAs exposure with 59 altered metabolites including those that play a role in tricarboxylic acid cycle and amino acid metabolism. Taken together, these data highlight the broad impact of iAs exposure on the human metabolome, and demonstrate some specificity of the metabolomic response between diabetic and nondiabetic individuals. These data may provide novel insights into the mechanisms and phenotype of diabetes associated with iAs exposure.

Serum bilirubin concentrations are positively associated with serum C-peptide levels in patients with Type 2 diabetes

AIMS

To investigate the relationship between physiological serum total bilirubin concentrations and serum C-peptide levels in Korean patients with Type 2 diabetes.

METHODS

A total of 588 patients with Type 2 diabetes were investigated in this cross-sectional study. Fasting C-peptide level, 2-h postprandial C-peptide level and ΔC-peptide (postprandial C-peptide minus fasting C-peptide) level were measured in all patients.

RESULTS

Fasting C-peptide level, postprandial C-peptide level and ΔC-peptide level tended to be higher in patients with higher bilirubin concentrations. Partial correlation analysis showed that serum bilirubin levels were significantly correlated with fasting C-peptide level (r = 0.159, P < 0.001), postprandial C-peptide level (r = 0.209, P < 0.001) and ΔC-peptide level (r = 0.186, P < 0.001) after adjustment for other covariates. In the multivariate model, the association between serum bilirubin concentrations and serum C-peptide levels remained significant after adjustment for confounding factors including age, gender, familial diabetes, hypertension, hyperlipidaemia, BMI, HbA1c , duration of diabetes and associated liver function tests (fasting C-peptide level: β = 0.083, P = 0.041; postprandial C-peptide level: β = 0.106, P = 0.005; ΔC-peptide level: β = 0.096, P = 0.015, respectively).

CONCLUSIONS

Serum bilirubin concentrations within the physiological range were positively associated with serum C-peptide levels in patients with Type 2 diabetes.

Bilirubin activates transcription of HIF-1α in human proximal tubular cells cultured in the physiologic oxygen content

The expression of hypoxia-inducible factor (HIF) is influenced by reactive oxygen species (ROS). Effect of bilirubin on HIF-1 expression in proximal tubular cells was investigated under physiological oxygen concentration, which is relative hypoxic condition mimicking oxygen content in the medulla of renal tissue. The human kidney (HK2) cells were cultured in 5% oxygen with or without bilirubin. HIF-1α protein expression was increased by bilirubin treatment at 0.01-0.2 mg/dL concentration. The messenger RNA expression of HIF-1α was increased by 1.69±0.05 folds in the cells cultured with 0.1 mg/dL bilirubin, compared to the control cells. The inhibitors of PI3K/mTOR, PI3K/AKT, and ERK 1/2 pathways did not attenuate increased HIF-1α expression by bilirubin. HIF-1α expression decreased by 10 µM exogenous hydrogen peroxide (H2O2); scavenger of ROS with or without bilirubin in the HK2 cells increased HIF-1α concentration more than that in the cells without bilirubin. Exogenous H2O2 decreased the phosphorylation of P70S6 kinase, which was completely reversed by bilirubin treatment. Knockdown of NOX4 gene by small interfering RNA (siRNA) increased HIF-1α mRNA expression. In coonclusion, bilirubin enhances HIF-1α transcription as well as the up-regulation of HIF-1α protein translation through the attenuation of ROS and subunits of NADPH oxidase.

Effect of bilirubin on triglyceride synthesis in streptozotocin-induced diabetic nephropathy

We aimed to elucidate the effect of bilirubin on dyslipidemia and nephropathy in a diabetes mellitus (DM) type I animal model. Sprague-Dawley rats were separated into control, DM, and bilirubin-treated DM (Bil) groups. The Bil group was injected intraperitoneally with 60 mg/kg bilirubin 3 times per week and hepatoma cells were cultured with bilirubin at a concentration of 0.3 mg/dL. The Bil group showed lower serum creatinine levels 5 weeks after diabetes onset. Bilirubin treatment also decreased the amount of mesangial matrix, lowered the expression of renal collagen IV and transforming growth factor (TGF)-β1, and reduced the level of apoptosis in the kidney, compared to the DM group. These changes were accompanied by decreased tissue levels of hydrogen superoxide and NADPH oxidase subunit proteins. Bilirubin decreased serum total cholesterol, high-density lipoprotein cholesterol (HDL-C), free fatty acids, and triglycerides (TGs), as well as the TG content in the liver tissues. Bilirubin suppressed protein expression of LXRα, SREBP-1, SCD-1, and FAS, factors involved in TG synthesis that were elevated in the livers of DM rats and hepatoma cells under high-glucose conditions. In conclusion, bilirubin attenuates renal dysfunction and dyslipidemia in diabetes by suppressing LXRα and SREBP-1 expression and oxidative stress.

Mid to distal small bowel resection with the preservation of the terminal ileum improves glucose homeostasis in diabetic rats by activating the hindgut-dependent mechanism

BACKGROUND

The aim of this study was to develop a novel surgical model to test the "hindgut hypothesis" and thereby study the role of the gut in glucose homeostasis and the mechanism of action of bariatric surgery.

METHOD

Sprague-Dawley rats were given a high-fat and high-sugar diet and treated with 25 mg/kg streptozotocin (STZ). The fat-sugar-fed/STZ-treated rats were randomized into mid to distal small bowel resection with the preservation of the terminal ileum (DBRPI) and sham operation (which had a formal celiotomy with bowel manipulation only) groups. Rats were observed for 12 weeks after the operation. The main outcome measures were weight, food intake, non-fasting glucose, an oral glucose tolerance test (OGTT), an insulin tolerance test (ITT), the levels of fasting and glucose-induced insulin, glucagon-like peptide-1 (GLP-1), peptide YY (PYY), serum bile acids, and lipid profile.

RESULT

The DBRPI and sham groups exhibited no difference in weight and food intake after surgery. When compared to the sham controls, the DBRPI group displayed an improvement in non-fasting glucose, oral glucose tolerance, and insulin tolerance at 4 and 12 weeks postresection. DBRPI elicited an increased serum insulin, PYY and GLP-1 levels at 12 weeks postoperation; furthermore, DBRPI resulted in higher serum levels of triglyceride, total bile acids, total bilirubin, and direct bilirubin levels and lower free fatty acid level at 12 weeks.

CONCLUSIONS

This study provides strong evidences for the key role of hindgut in the amelioration of diabetes after bariatric surgery. Moreover, these findings confirm that DBRPI is a simple and effective surgical model for testing the "hindgut hypothesis" and focused study of biliary enterohepatic recycling in the context of bariatric operations.

Bilirubin and beyond: a review of lipid status in Gilbert's syndrome and its relevance to cardiovascular disease protection

Gilbert's syndrome (GS) is characterized by a benign, mildly elevated bilirubin concentration in the blood. Recent reports show clear protection from cardiovascular disease in this population. Protection of lipids, proteins and other macromolecules from oxidation by bilirubin represents the most commonly accepted mechanism contributing to protection in this group. However, a recent meta-analysis estimated that bilirubin only accounts for ~34% of the cardioprotective effects within analysed studies. To reveal the additional contributing variables we have explored circulating cholesterol and triacylglycerol concentrations, which appear to be decreased in hyperbilirubinemic individuals/animals, and are accompanied by lower body mass index in highly powered studies. These results suggest that bilirubin could be responsible for the development of a lean and hypolipidemic state in GS. Here we also discuss the possible contributing mechanisms that might reduce circulating cholesterol and triacylglycerol concentrations in individuals with syndromes affecting bilirubin metabolism/excretion, which we hope will stimulate future research in the area. In summary, this article is the first review of lipid status in animal and human studies of hyperbilirubinemia and explores possible mechanisms that could contribute to lowering circulating lipid parameters and further explain cardiovascular protection in Gilbert's syndrome.

Endothelial Cells Derived from the Blood-Brain Barrier and Islets of Langerhans Differ in their Response to the Effects of Bilirubin on Oxidative Stress Under Hyperglycemic Conditions

Unconjugated bilirubin (UCB) is a neurotoxic degradation product of heme. Its toxic effects include induction of apoptosis, and ultimately neuronal cell death. However, at low concentrations, UCB is a potent antioxidant that may protect cells and tissues against oxidative stress by neutralizing toxic metabolites such as reactive oxygen species (ROS). High glucose levels (hyperglycemia) generate reactive metabolites. Endothelial cell dysfunction, an early vascular complication in diabetes, has been associated with hyperglycemia-induced oxidative stress. Both glucose and UCB are substrates for transport proteins in microvascular endothelial cells of the blood-brain barrier (BBB). In the current study we show that UCB (1–40 μM) induces apoptosis and reduces survival of bEnd3 cells, a mouse brain endothelial cell line which serves as an in vitro model of the BBB. These deleterious effects of UCB were enhanced in the presence of high glucose (25 mM) levels. Interestingly, the bEnd3 cells exhibited an increased sensitivity to the apoptotic effects of UCB when compared to the MS1 microcapillary endothelial cell line. MS1 cells originate from murine pancreatic islets of Langerhans, and are devoid of the barrier characteristics of BBB-derived endothelial cells. ROS production was increased in both bEnd3 and MS1 cells exposed to high glucose, as compared with cells exposed to normal (5.5 mM) glucose levels. While UCB (0.1–40 μM) did not alter ROS production in cells exposed to normal glucose, relatively low (“physiological”) UCB concentrations (0.1–5 μM) attenuated ROS generation in both cell lines exposed to high glucose levels. Most strikingly, higher UCB concentrations (20–40 μM) increased ROS generation in bEnd3 cells exposed to high glucose, but not in similarly treated MS1 cells. These results may be of critical importance for understanding the vulnerability of the BBB endothelium upon exposure to increasing UCB levels under hyperglycemic conditions.

The role of bilirubin in diabetes, metabolic syndrome, and cardiovascular diseases

Bilirubin belongs to a phylogenetically old superfamily of tetrapyrrolic compounds, which have multiple biological functions. Although for decades bilirubin was believed to be only a waste product of the heme catabolic pathway at best, and a potentially toxic compound at worst; recent data has convincingly demonstrated that mildly elevated serum bilirubin levels are strongly associated with a lower prevalence of oxidative stress-mediated diseases. Indeed, serum bilirubin has been consistently shown to be negatively correlated to cardiovascular diseases (CVD), as well as to CVD-related diseases and risk factors such as arterial hypertension, diabetes mellitus, metabolic syndrome, and obesity. In addition, the clinical data are strongly supported by evidence arising from both in vitro and in vivo experimental studies. This data not only shows the protective effects of bilirubin per se; but additionally, of other products of the heme catabolic pathway such as biliverdin and carbon monoxide, as well as its key enzymes (heme oxygenase and biliverdin reductase); thus, further underlining the biological impacts of this pathway. In this review, detailed information on the experimental and clinical evidence between the heme catabolic pathway and CVD, and those related diseases such as diabetes, metabolic syndrome, and obesity is provided. All of these pathological conditions represent an important threat to human civilization, being the major killers in developed countries, with a steadily increasing prevalence. Thus, it is extremely important to search for novel markers of these diseases, as well as for novel therapeutic modalities to reverse this unfavorable situation. The heme catabolic pathway seems to fulfill the criteria for both diagnostic purposes as well as for potential therapeutical interventions.

Induction of protective genes leads to islet survival and function

Islet transplantation is the most valid approach to the treatment of type 1 diabetes. However, the function of transplanted islets is often compromised since a large number of β cells undergo apoptosis induced by stress and the immune rejection response elicited by the recipient after transplantation. Conventional treatment for islet transplantation is to administer immunosuppressive drugs to the recipient to suppress the immune rejection response mounted against transplanted islets. Induction of protective genes in the recipient (e.g., heme oxygenase-1 (HO-1), A20/tumor necrosis factor alpha inducible protein3 (tnfaip3), biliverdin reductase (BVR), Bcl2, and others) or administration of one or more of the products of HO-1 to the donor, the islets themselves, and/or the recipient offers an alternative or synergistic approach to improve islet graft survival and function. In this perspective, we summarize studies describing the protective effects of these genes on islet survival and function in rodent allogeneic and xenogeneic transplantation models and the prevention of onset of diabetes, with emphasis on HO-1, A20, and BVR. Such approaches are also appealing to islet autotransplantation in patients with chronic pancreatitis after total pancreatectomy, a procedure that currently only leads to 1/3 of transplanted patients being diabetes-free.