Physiological concentrations of unconjugated bilirubin prevent oxidative stress-induced hepatocanalicular dysfunction and cholestasis

High-normal unconjugated bilirubin is associated with decreased risk of chronic kidney disease in type 2 diabetes: A real-world study

OBJECTIVE

To investigate the association between serum unconjugated bilirubin (UCB) within normal limits and chronic kidney disease (CKD) in T2DM patients.

METHOD

This cross-sectional, real-world study was performed in 8661 hospitalised T2DM patients. The subjects were stratified into quintiles based on serum UCB levels. The clinical characteristics and CKD prevalence were compared among the UCB quantile groups. The associations of serum UCB levels and quintiles with CKD were also analysed by binary logistic regression.

RESULTS

After controlling for age, sex, and diabetes duration (DD), the CKD prevalence (20.4%, 12.2%, 10.6%, 8.3%, and 6.4% for the first, second, third, fourth, and fifth quintiles, respectively, p < 0.001 for trend) was significantly decreased across the serum UCB quintiles. The fully adjusted regression model showed negative associations of serum UCB levels (OR: 0.660, 95% CI: 0.585-0.744; p < 0.001 for trend) and quintiles (p < 0.001) with the presence of CKD. Compared with the subjects in the lowest UCB quintile, the risk of CKD decreased by 36.2%, 54.3%, 53.8%, and 62.1%, respectively, in those from the second to the highest UCB quintile. Additionally, C-reactive protein (CRP) levels were significantly higher in the subjects with CKD than in those without CKD (p < 0.001), and significantly decreased across the UCB quintiles (p < 0.001 for trend).

CONCLUSIONS

Serum UCB levels within the normal range were significantly and negatively linked to CKD in T2DM patients. High-normal UCB may be an independent protective factor for CKD by its antioxidant and the following anti-inflammatory activities through its signalling activity, which was indicated by clearly decreased CRP levels across the UCB quintiles.

Toward nanotechnology-enabled application of bilirubin in the treatment and diagnosis of various civilization diseases

Bilirubin, an open chain tetrapyrrole, has powerful antioxidant, anti-inflammatory, immuno-suppressive, metabolic-modulating and anti-proliferative activities. Bilirubin is a natural molecule that is produced and metabolized within the human body, making it highly biocompatible and well suited for clinical use. However, the use of bilirubin has been hampered by its poor water solubility and instability. With advanced construction strategies, bilirubin-derived nanoparticles (BRNPs) have not only overcome the disadvantages of bilirubin but also enhanced its therapeutic effects by targeting damaged tissues, passing through physiological barriers, and ensuring controlled sustained release. We review the mechanisms underlying the biological activities of bilirubin, BRNP preparation strategies and BRNP applications in various disease models. Based on their superior performance, BRNPs require further exploration of their efficacy, biodistribution and long-term biosafety in nonhuman primate models that recapitulate human disease to promote their clinical translation.

4-Phenylbutyrate protects against rifampin-induced liver injury via regulating MRP2 ubiquitination through inhibiting endoplasmic reticulum stress

Rifampin (RFP), a first-line anti-tuberculosis drug, often induces cholestatic liver injury and hyperbilirubinemia which limits its clinical use. Multidrug resistance-associated protein 2 (MRP2) localizes to the hepatocyte apical membrane and plays a pivotal role in the biliary excretion of bilirubin glucuronides. RFP is discovered to reduce MRP2 expression in liver cells. 4-Phenylbutyrate (4-PBA), a drug used to treat ornithine transcarbamylase deficiency (DILI), is reported to alleviate RFP-induced liver cell injury. However, the underlying mechanism still remains unclear. In the current study, we discovered that RFP induced HepG2 cell viability reduction, apoptosis and MRP2 ubiquitination degradation. Administration of 4-PBA alleviated the effect of RFP on HepG2 cell viability reduction, apoptosis and MRP2 ubiquitination degradation. In mechanism, 4-PBA suppressed RPF-caused intracellular Ca²⁺ disorder and endoplasmic reticulum (ER) stress, as well as the increases of Clathrin and adapter protein 2 (AP2). ER stress marker protein C/EBP homologous protein took part in the modulation of AP2 and clathrin. Besides, 4-PBA reduced the serum bilirubin level in RFP-induced cholestasis mouse model, along with raised the MRP2 expression in liver tissues. These findings indicated that 4-PBA could alleviate RFP-induced cholestatic liver injury and thereby decreased serum total bilirubin concentration via inhibiting ER stress and ubiquitination degradation of MRP2, which provides new insights into the mechanism of 4-PBA in the treatment of RFP-induced cholestasis and liver damage.

Oxidative Stress and Localization Status of Hepatocellular Transporters: Impact on Bile Secretion and Role of Signaling Pathways

Significance: Most hepatopathies are primarily or secondarily cholestatic in nature. Oxidative stress (OS) is a frequent trait among them, and impairs the machinery to generate bile by triggering endocytic internalization of hepatocellular transporters, thus causing cholestasis. This is critical, since it leads to accelerated transporter degradation, which could explain the common post-transcriptional downregulation of transporter expression in human cholestatic diseases. Recent Advances: The mechanisms involved in OS-induced hepatocellular transporter internalization are being revealed. Filamentous actin (F-actin) cytoskeleton disorganization and/or detachment of crosslinking actin proteins that afford transporter stability have been characterized as causal factors. Activation of redox-sensitive signaling pathways leading to changes in phosphorylation status of these structures is involved, including Ca²⁺-mediated activation of "classical" and "novel" protein kinase C (PKC) isoforms or redox-signaling cascades downstream of NADPH oxidase. Critical Issues: Despite the well-known occurrence of hepatocellular transporter internalization in human hepatopathies, the cholestatic implications of this phenomenon have been overlooked. Accordingly, no specific treatment has been established in the clinical practice for its prevention/reversion. Future Directions: We need to improve our knowledge on the pro-oxidant triggering factors and the multiple signaling pathways that mediate this oxidative injury in each cholestatic hepatopathy, so as to envisage tailor-made therapeutic strategies for each case. Meanwhile, administration of antioxidants or heme oxygenase-1 induction to elevate the hepatocellular levels of the endogenous scavenger bilirubin are promising alternatives that need to be re-evaluated and implemented. They may complement current treatments in cholestasis aimed to enhance transcriptional carrier expression, by providing membrane stability to the newly synthesized carriers. Antioxid. Redox Signal. 35, 808-831.

Decreased Bilirubin is Associated With Disease Activity of Primary Sjögren's Syndrome

Objectives

This study aims to determine the serum bilirubin levels in primary Sjögren's syndrome (pSS) patients and to explore clinical significance of bilirubin in pSS.

Patients and methods

Retrospective analysis of electronic medical records was performed in 97 pSS patients (12 males, 85 females; mean age 54±15 years; range, 15 to 91 years) and 100 healthy controls (17 males, 83 females; mean age 51±14 years; range, 25 to 75 years). Serum bilirubin and other variables were compared between pSS patients and healthy controls. The European League Against Rheumatism Sjögren's syndrome disease activity index (ESSDAI) was used to assess the disease activity of pSS, and ESSDAI ≥5 was defined as moderate to high activity. The relationship between bilirubin and ESSDAI was analyzed by Spearman's correlation analysis and multivariable logistic regression.

Results

The median level of serum bilirubin was 9 μmol/L (interquartile range (IQR), 7-13 μmol/L) in pSS patients, much lower than healthy controls (median, IQR, 13, 10-18 μmol/L) (p<0.001). It was positively correlated with age (r=0.255, p=0.012), but negatively with immunoglobulin (Ig) A (r=-0.314, p=0.003), IgG (r=-0.265, p=0.015), erythrocyte sedimentation rate (r=-0.309, p=0.002) and ESSDAI (r=-0.342, p=0.001). Multivariate analysis revealed that increased bilirubin was independently associated with decreased risk of moderate to high disease activity (odds ratio, 95% confidence interval: 0.852, 0.730-0.955).

Conclusion

Serum bilirubin is decreased in pSS patients and may be a useful biomarker for reflecting pSS disease activity.

Endocytosis in the adaptation to cellular stress

Cellular life is challenged by a multitude of stress conditions, triggered for example by alterations in osmolarity, oxygen or nutrient supply. Hence, cells have developed sophisticated stress responses to cope with these challenges. Some of these stress programs such as the heat shock response are understood in great detail, while other aspects remain largely elusive including potential stress-dependent adaptations of the plasma membrane proteome. The plasma membrane is not only the first point of encounter for many types of environmental stress, but given the diversity of receptor proteins and their associated molecules also represents the site at which many cellular signal cascades originate. Since these signaling pathways affect virtually all aspects of cellular life, changes in the plasma membrane proteome appear ideally suited to contribute to the cellular adaptation to stress. The most rapid means to alter the cell surface proteome in response to stress is by alterations in endocytosis. Changes in the overall endocytic flux or in the endocytic regulation of select proteins conceivably can help to counteract adverse environmental conditions. In this review we summarize recent data regarding stress-induced changes in endocytosis and discuss how these changes might contribute to the cellular adaptation to stress in different systems. Future studies will be needed to uncover the underlying mechanisms in detail and to arrive at a coherent picture.

HGF induces protective effects in α-naphthylisothiocyanate-induced intrahepatic cholestasis by counteracting oxidative stress

Cholestasis is a clinical syndrome common to a large number of hepatopathies, in which either bile production or its transit through the biliary tract is impaired due to functional or obstructive causes; the consequent intracellular retention of toxic biliary constituents generates parenchyma damage, largely via oxidative stress-mediated mechanisms. Hepatocyte growth factor (HGF) and its receptor c-Met represent one of the main systems for liver repair damage and defense against hepatotoxic factors, leading to an antioxidant and repair response. In this study, we evaluated the capability of HGF to counteract the damage caused by the model cholestatic agent, α-naphthyl isothiocyanate (ANIT). HGF had clear anti-cholestatic effects, as apparent from the improvement in both bile flow and liver function test. Histology examination revealed a significant reduction of injured areas. HGF also preserved the tight-junctional structure. These anticholestatic effects were associated with the induction of basolateral efflux ABC transporters, which facilitates extrusion of toxic biliary compounds and its further alternative depuration via urine. The biliary epithelium seems to have been also preserved, as suggested by normalization in serum GGT levels, CFTR expression and cholangyocyte primary cilium structure our results clearly show for the first time that HGF protects the liver from a cholestatic injury.

Rifampicin induces clathrin-dependent endocytosis and ubiquitin-proteasome degradation of MRP2 via oxidative stress-activated PKC-ERK/JNK/p38 and PI3K signaling pathways in HepG2 cells

It was reported that antituberculosis medicines could induce liver damage via oxidative stress. In this study, we investigated the effects of rifampicin (RFP) on the membrane expression of multidrug resistance-associated protein 2 (MRP2) and the relationship between oxidative stress and RFP-induced endocytosis of MRP2 in HepG2 cells. We found that RFP (12.5–50 μM) dose-dependently decreased the expression and membrane localization of MRP2 in HepG2 cells without changing the messenger RNA level. RFP (50 μM) induced oxidative stress responses that further activated the PKC-ERK/JNK/p38 (protein kinase C-extracellular signal-regulated kinase/c-JUN N-terminal kinase/p38) and PI3K (phosphoinositide 3-kinase) signaling pathways in HepG2 cells. Pretreatment with glutathione reduced ethyl ester (2 mM) not only reversed the changes in oxidative stress indicators and signaling molecules but also diminished RFP-induced reduction in green fluorescence intensity of MRP2. We conducted co-immunoprecipitation assays and revealed that a direct interaction existed among MRP2, clathrin, and adaptor protein 2 (AP2) in HepG2 cells, and their expression was clearly affected by the changes in intracellular redox levels. Knockdown of clathrin or AP2 with small interfering RNA attenuated RFP-induced decreases of membrane and total MRP2. We further demonstrated that RFP markedly increased the ubiquitin–proteasome degradation of MRP2 in HepG2 cells, which was mediated by the E3 ubiquitin ligase GP78, but not HRD1 or TEB4. In conclusion, this study demonstrates that RFP-induced oxidative stress activates the PKC-ERK/JNK/p38 and PI3K signaling pathways that leads to clathrin-dependent endocytosis and ubiquitination of MRP2 in HepG2 cells, which provides new insight into the mechanism of RFP-induced cholestasis.

Heme oxygenase-1 induction by hemin prevents oxidative stress-induced acute cholestasis in the rat

We previously demonstrated in in vitro and ex vivo models that physiological concentrations of unconjugated bilirubin (BR) prevent oxidative stress (OS)-induced hepatocanalicular dysfunction and cholestasis. Here, we aimed to ascertain, in the whole rat, whether a similar cholestatic OS injury can be counteracted by heme oxygenase-1 (HO-1) induction that consequently elevates endogenous BR levels. This was achieved through the administration of hemin, an inducer of HO-1, the rate-limiting step in BR generation. We found that BR peaked between 6 and 8 h after hemin administration. During this time period, HO-1 induction fully prevented the pro-oxidant tert-butylhydroperoxide (tBuOOH)-induced drop in bile flow, and in the biliary excretion of bile salts and glutathione, the two main driving forces of bile flow; this was associated with preservation of the membrane localization of their respective canalicular transporters, bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2), which are otherwise endocytosed by OS. HO-1 induction counteracted the oxidation of intracellular proteins and membrane lipids induced by tBuOOH, and fully prevented the increase in the oxidized-to-total glutathione (GSHt) ratio, a sensitive parameter of hepatocellular OS. Compensatory elevations of the activity of the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were also prevented. We conclude that in vivo HO-1 induction protects the liver from acute oxidative injury, thus preventing consequent cholestasis. This reveals an important role for the induction of HO-1 and the consequently elevated levels of BR in preserving biliary secretory function under OS conditions, thus representing a novel therapeutic tool to limit the cholestatic injury that bears an oxidative background.

The Relationship between Serum Bilirubin and Elevated Fibrotic Indices among HBV Carriers: A Cross-Sectional Study of a Chinese Population

The study probed the association between bilirubin and hepatitis B virus (HBV) infection and progression. A cross-sectional analysis of 28,500 middle aged and elderly Chinese participants was performed to analyze the differences of bilirubin in terms of hepatitis B surface antigen (HBsAg) positive or negative and the correlation between bilirubin and severity of hepatic fibrosis estimated by non-invasive indices. Bilirubin was significantly higher in the HBsAg (+) group than the HBsAg (−) group. Higher bilirubin levels were consistently associated with elevated liver fibrosis indices among HBsAg carriers. Compared with quartile 1 of total bilirubin (TBil), the multivariable-adjusted ORs (95% CIs) for elevated fibrosis indices of quartile 4 were 2.24 (95% CIs, 1.57–3.21) estimated by fibrosis 4 score (FIB-4) and 2.22 (95% CIs, 1.60–3.08) estimated by aspartate transaminase to platelet ratio index (APRI). In addition, direct bilirubin (DBil) had a stronger association with elevated liver fibrosis indices than did indirect bilirubin (IBil). Furthermore, the relationship between DBil and elevated fibrosis indices was more robust among participants who were female, overweight or had central fat distribution. These findings suggested that bilirubin levels, especially DBil, were independently associated with an increased risk of increased fibrosis indices.

Mitogen-activated protein kinases are involved in hepatocanalicular dysfunction and cholestasis induced by oxidative stress

In previous studies, we showed that the pro-oxidant model agent tert-butyl hydroperoxide (tBuOOH) induces alterations in hepatocanalicular secretory function by activating Ca²⁺-dependent protein kinase C isoforms (cPKC), via F-actin disorganization followed by endocytic internalization of canalicular transporters relevant to bile formation (Mrp2, Bsep). Since mitogen-activated protein kinases (MAPKs) may be downstream effectors of cPKC, we investigated here the involvement of the MAPKs of the ERK1/2, JNK1/2, and p38^MAPK types in these deleterious effects. tBuOOH (100 µM, 15 min) increased the proportion of the active, phosphorylated forms of ERK1/2, JNK1/2, and p38^MAPK, and panspecific PKC inhibition with bisindolylmaleimide-1 (100 nM) or selective cPKC inhibition with Gö6976 (1 μM) prevented the latter two events. In isolated rat hepatocyte couplets, tBuOOH (100 µM, 15 min) decreased the canalicular vacuolar accumulation of the fluorescent Bsep and Mrp2 substrates, cholylglycylamido fluorescein, and glutathione-methylfluorescein, respectively, and selective inhibitors of ERK1/2 (PD098059), JNK1/2 (SP600125), and p38^MAPK (SB203580) partially prevented these alterations. In in situ perfused rat livers, these three MAPK inhibitors prevented tBuOOH (75 µM)-induced impairment of bile flow and the decrease in the biliary output of the Bsep and Mrp2 substrates, taurocholate, and dinitrophenyl-S-glutathione, respectively. The changes in Bsep/Mrp2 and F-actin localization induced by tBuOOH, as assessed by (immuno)fluorescence staining followed by analysis of confocal images, were prevented total or partially by the MAPK inhibitors. We concluded that MAPKs of the ERK1/2, JNK1/2, and p38^MAPK types are all involved in cholestasis induced by oxidative stress, by promoting F-actin rearrangement and further endocytic internalization of canalicular transporters critical for bile formation.