Biliverdin Reductase A Attenuates Hepatic Steatosis by Inhibition of Glycogen Synthase Kinase (GSK) 3β Phosphorylation of Serine 73 of Peroxisome Proliferator-activated Receptor (PPAR) α

Dual phosphorylation of glycogen synthase kinase 3β differentially integrates metabolic programs to determine T cell immunity across vertebrates

The integration of metabolic programs with T cell signaling establishes a molecular foundation for immune metabolism. As a key metabolic regulator, GSK3β's activity is dynamically modulated by phosphorylation at Ser9 and Tyr216. However, the contribution of these phosphorylation sites on metabolism-driven T cell response remains unclear. Using tilapia and mouse models, we investigated the regulation of GSK3β on T cell metabolism and its evolutionary variation. In tilapia, T cell activation induces GSK3β signaling, linking to both glycolysis and oxidative phosphorylation (OXPHOS). Tyr216 phosphorylation preferentially promotes glycolysis, facilitating T cell activation, proliferation, and antibacterial immunity; while inhibition of Ser9 phosphorylation specifically enhances OXPHOS to sustain T cell responses. Differently, Tyr216 phosphorylation supports both glycolysis and OXPHOS in mouse, ensuring CD4+ T and CD8+ T cell activation, proliferation, and cytokine production. Although Ser9 phosphorylation controls OXPHOS, its inhibition impairs rather than enhances OXPHOS and CD4+ T cell responses in mouse. We thus revealed a previously unknown mechanism underlying T cell metabolism and proposed that, through evolution, GSK3β has restructured the regulatory strategy, enabling bidirectional control of T cell metabolism and immunity in mammals and enhancing the flexibility of the adaptive immune system.

Alcohol-Associated Hepatocarcinogenesis: Wnt/β-Catenin in Action

Long-term alcohol consumption is a leading global health concern, primarily due to its deleterious effects on liver function and its well-established association with hepatocellular carcinoma. Alcohol-related liver disease (ALD) encompasses a continuum-from reversible hepatic steatosis and steatohepatitis through progressive fibrosis and cirrhosis to overt hepatocellular carcinoma. Accumulating studies have revealed that the Wnt/β-catenin signaling pathway is an essential regulator in ALD pathogenesis, orchestrating diverse molecular, immunologic, and epigenetic processes. Aberrant β-catenin activity disrupts redox homeostasis, promotes chronic inflammation, drives extracellular matrix remodeling, and alters hepatocyte cell fate, thereby creating a microenvironment that is highly conducive to carcinogenesis. This article provides a systematic review of the significant function of Wnt/β-catenin signaling in ALD, emphasizing its regulatory impact on liver fat accumulation, its inflammatory role in steatohepatitis, its involvement in fibrogenesis, and its tumor-promoting effects in alcohol-related hepatocellular carcinoma. In addition, emerging therapeutic strategies that offer potential for early identification and tailored therapy of ALD are explored-including direct Wnt modulators, combinatory therapeutics, and precision medicine approaches.

Bilirubin metabolism in relation to cancer

Bilirubin, a metabolite of hemoglobin, was long thought to be a harmful waste product, but recent studies have found it to have antioxidant and anti-tumor effects. With the extensive research on the mechanism of malignant tumor development, the antioxidant effect of bilirubin is increasingly becoming a hotspot in anti-cancer research. At present, there are two main views on the relationship between bilirubin and cancer, namely, its pro-cancer and anti-cancer effects, and in recent years, studies on the relationship between bilirubin and cancer have not been systematically summarized, which is not conducive to the further investigation of the role of bilirubin on cancer. To understand the multifaceted role of bilirubin in tumorigenesis as well as to develop more effective and affordable antitumor therapies, this review provides an overview of the effects of bilirubin on tumors in terms of oxidative, inflammatory, and cellular signaling pathways, as well as the resulting therapeutic ideas and approaches.

Nrf2/Bach1 signaling axis: A promising multifaceted therapeutic strategy for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia, which continues to elude effective treatment despite decades of research and numerous clinical trials. While existing therapeutic strategies have primarily targeted neuropathological hallmarks such as amyloid plaques and tau tangles, they have failed to halt disease progression, leaving patients with limited options. This persistent failure reveals a critical gap in our understanding of AD and calls for a fresh perspective - one that goes beyond the traditional targets and dives deeper into the fundamental cellular processes that drive neurodegeneration. Recent advances in molecular biology underscore the significance of nuclear factor E2-related factor 2 (Nrf2), often termed the "guardian of redox homeostasis," in the pathophysiology of AD. Nrf2 orchestrates cellular responses to oxidative stress and neuroinflammation - two interlinked pathological features of AD. In the brains of AD patients, Nrf2 activity is diminished, weakening the brain's ability to counteract oxidative damage. Additionally, the BTB and CNC homology 1 (Bach1) protein, a transcriptional repressor of Nrf2, has emerged as a potential therapeutic target. Here, we review the current landscape of clinical trials in AD and identify the limitations of the conventional approaches. We then explore the prospects of a novel approach that combines Nrf2 activation with Bach1 inhibition to achieve a multipronged defense against oxidative stress, neuroinflammation, and other molecular culprits driving AD. This innovative strategy holds promise for synergistically modulating multiple neuroprotective pathways to advance AD treatment.

The Role of Renal Medullary Bilirubin and Biliverdin Reductase in Angiotensin II-Dependent Hypertension

BACKGROUND

Increased circulating bilirubin attenuates angiotensin (Ang) II-induced hypertension and improves renal hemodynamics. However, the intrarenal mechanisms that mediate these effects are not known. The goal of the present study was to test the hypothesis that bilirubin generation in the renal medulla plays a protective role against Ang II-induced hypertension.

METHODS

Twenty-week-old male C57Bl/6J mice were implanted with intrarenal medullary interstitial (IRMI) catheters following unilateral nephrectomy. After this time, biliverdin IXα was specifically infused into the kidney (3.6 mg/day) for 3 days before implantation with an osmotic minipump delivering Ang II (1,000 ng/kg/min). BP was recorded for 3 days, 1 week after minipump infusion, in conscious mice. To further explore the antihypertensive role of renal medullary bilirubin generation, mice with specific deletion of biliverdin reductase-A (Blvra) in the thick ascending loop of Henle were generated. At 20 weeks, BlvraTALHKO and control mice (Blvrafl/fl) were infused with Ang II for 2 weeks.

RESULTS

IRMI infusion of biliverdin significantly decreased blood pressure compared with mice infused with vehicle (118 ± 4 vs. 158 ± 2 mmHg, p < 0.05). Angiotensin-II infusion resulted in significantly higher blood pressure measured in conscious mice 7 days after implantation in BlvraTALHKO as compared to Blvrafl/fl mice (152 ± 2 vs. 140 ± 3 mmHg, P < 0.05).

CONCLUSIONS

Altogether, these findings show that medullary bilirubin and biliverdin reductase can improve hypertension and that mechanisms that increase bilirubin and biliverdin reductase in the renal medulla could be an effective approach to treat hypertension.

Pharmacological treatment for metabolic dysfunction-associated steatotic liver disease and related disorders: Current and emerging therapeutic options

Metabolic dysfunction-associated steatotic liver disease (MASLD; formerly known as nonalcoholic fatty liver disease) is a chronic liver disease affecting over a billion individuals worldwide. MASLD can gradually develop into more severe liver pathologies, including metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis, and liver malignancy. Notably, although being a global health problem, there are very limited therapeutic options against MASLD and its related diseases. While a thyroid hormone receptor agonist (resmetirom) is recently approved for MASH treatment, other efforts to control these diseases remain unsatisfactory. Given the projected rise in MASLD and MASH incidence, it is urgent to develop novel and effective therapeutic strategies against these prevalent liver diseases. In this article, the pathogenic mechanisms of MASLD and MASH, including insulin resistance, dysregulated nuclear receptor signaling, and genetic risk factors (eg, patatin-like phospholipase domain-containing 3 and hydroxysteroid 17-β dehydrogenase-13), are introduced. Various therapeutic interventions against MASH are then explored, including approved medication (resmetirom), drugs that are currently in clinical trials (eg, glucagon-like peptide 1 receptor agonist, fibroblast growth factor 21 analog, and PPAR agonist), and those failed in previous trials (eg, obeticholic acid and stearoyl-CoA desaturase 1 antagonist). Moreover, given that the role of gut microbes in MASLD is increasingly acknowledged, alterations in the gut microbiota and microbial mechanisms in MASLD development are elucidated. Therapeutic approaches that target the gut microbiota (eg, dietary intervention and probiotics) against MASLD and related diseases are further explored. With better understanding of the multifaceted pathogenic mechanisms, the development of innovative therapeutics that target the root causes of MASLD and MASH is greatly facilitated. The possibility of alleviating MASH and achieving better patient outcomes is within reach. SIGNIFICANCE STATEMENT: Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease worldwide, and it can progress to more severe pathologies, including steatohepatitis, cirrhosis, and liver cancer. Better understanding of the pathogenic mechanisms of these diseases has facilitated the development of innovative therapeutic strategies. Moreover, increasing evidence has illustrated the crucial role of gut microbiota in the pathogenesis of MASLD and related diseases. It may be clinically feasible to target gut microbes to alleviate MASLD in the future.

Bilirubin bioconversion to urobilin in the gut-liver-kidney axis: A biomarker for insulin resistance in the Cardiovascular-Kidney-Metabolic (CKM) Syndrome

The rising rates of obesity worldwide have increased the incidence of cardiovascular disease (CVD), making it the number one cause of death. Higher plasma bilirubin levels have been shown to prevent metabolic dysfunction and CVD. However, reducing levels leads to deleterious outcomes, possibly due to reduced bilirubin half-life that escalates the production of its catabolized product, urobilinogen, produced by gut bacteria and naturally oxidized to urobilin. Recent findings suggest that the involvement of the microbiome catabolism of bilirubin to urobilin and its absorption via the hepatic portal vein contributes to CVD, suggesting a liver-gut axis involvement. We discuss the studies that demonstrate that urobilin is frequently raised in the urine of persons with CVD and its probable role in acquiring the disease. Urobilin is excreted from the kidneys into the urine and may serve as a biomarker for Cardiovascular-Kidney-Metabolic (CKM) Syndrome. We deliberate on the newly discovered bilirubin reductase (BilR) bacterial enzyme that produces urobilin. We discuss the bacterial species expressing BilR, how they impact CVD, and whether suppressing urobilin production and increasing bilirubin may provide new therapeutic strategies for CKM. Possible therapeutic mechanisms for achieving this goal are discussed.

Heme oxygenase, biliverdin reductase, and bilirubin pathways regulate oxidative stress and insulin resistance: a focus on diabetes and therapeutics

Metabolic and insulin-resistant diseases, such as type 2 diabetes mellitus (T2DM), have become major health issues worldwide. The prevalence of insulin resistance in the general population ranges from 15.5% to 44.6%. Shockingly, the global T2DM population is anticipated to double by 2050 compared with 2021. Prior studies indicate that oxidative stress and inflammation are instrumental in causing insulin resistance and instigating metabolic diseases. Numerous methods and drugs have been designed to combat insulin resistance, including metformin, thiazolidinediones (TZDs), sodium-glucose cotransporter 2 inhibitors (SGLT2i), glucagon-like peptide 1 receptor agonists (GLP1RA), and dipeptidyl peptidase 4 inhibitors (DPP4i). Bilirubin is an antioxidant with fat-burning actions by binding to the PPARα nuclear receptor transcription factor, improving insulin sensitivity, reducing inflammation, and reversing metabolic dysfunction. Potential treatment with antioxidants like bilirubin and increasing the enzyme that produces it, heme oxygenase (HMOX), has also gained attention. This review discusses the relationships between bilirubin, HMOX, and insulin sensitivity, how T2DM medications affect HMOX levels and activity, and potentially using bilirubin nanoparticles to treat insulin resistance. We explore the sex differences between these treatments in the HMOX system and how bilirubin levels are affected. We discuss the emerging concept that bilirubin bioconversion to urobilin may have a role in metabolic diseases. This comprehensive review summarizes our understanding of bilirubin functioning as a hormone, discusses the HMOX isoforms and their beneficial mechanisms, analyzes the sex differences that might cause a dichotomy in responses, and examines the potential use of HMOX and bilirubin nanoparticle therapies in treating metabolic diseases.

Urobilin Derived from Bilirubin Bioconversion Binds Albumin and May Interfere with Bilirubin Interacting with Albumin: Implications for Disease Pathology

Background/Objectives: Bilirubin is a hydrophobic molecule that binds the carrier protein albumin for transport through systemic circulation. Bilirubin is cleared from the body through the liver and excreted into the intestines, where the microbiota modifies the chemical structure, forming urobilin, which can be reabsorbed into circulation by the hepatic portal vein. Urobilin has no known function. It is also unknown whether urobilin binds albumin for transport in circulation. We hypothesized that because of the likeness of their chemical structures, urobilin would also bind albumin like bilirubin does. Methods: First, we used in silico docking to predict if urobilin would bind to albumin and compared it to the bilirubin binding sites. To test this binding in vitro, we applied bilirubin's fluorescent property, which occurs when it is bound to a protein, including albumin, and exposed to light. We also used this method to determine if urobilin could exhibit autofluorescence when protein bound. Results: We found that bilirubin was predicted to bind albumin at amino acids E208, K212, D237, and K240 through hydrogen bonds. However, urobilin was predicted to bind albumin using different hydrogen bonds at amino acids H67, K240, and E252. We found that urobilin has a fluorescent property that can be quantified when bound to albumin. We performed a concentration response for urobilin-albumin fluorescent binding and observed a direct relationship between the urobilin level and the fluorescence intensity. Conclusions: The in silico docking analysis and autofluorescence results demonstrate that urobilin binds to albumin and might compete with bilirubin. This is the first study to identify a urobilin-binding protein and the important aspects of its physiological function and transport in circulation.

Liver function and Alzheimer's brain pathologies: A longitudinal study: Liver and Alzheimer's pathologies

IMPORTANCE

The neuropathological links underlying the association between changes in liver function and AD have not yet been clearly elucidated.

OBJECTIVE

We aimed to examine the relationship between liver function markers and longitudinal changes in Alzheimer's disease (AD) core pathologies.

DESIGN

Data from the Korean Brain Aging Study for the Early Diagnosis and Prediction of Alzheimer's Disease, a longitudinal cohort study initiated in 2014, were utilized.

SETTING

Community and memory clinic setting.

PARTICIPANTS

Three hundred forty-seven older adults.

MAIN OUTCOME AND MEASURES

Participants underwent baseline and 2-year follow-up evaluations, including liver function assessments and various brain imaging techniques, such as amyloid and tau PET, FDG-PET, and MRI). Liver function indicators [alanine aminotransferase (ALT), aspartate aminotransferase (AST), and total bilirubin] were examined as exposure variables.

RESULTS

Higher baseline ALT levels were associated with a greater increase in beta-amyloid deposition over 2 years [β = 0.166, Bonferroni-corrected P (PB) = 0.012], while lower total bilirubin levels were associated with a greater increase in tau deposition over the same period (β = -0.570, PB < 0.001). In contrast, AST alone showed no significant association with changes of AD pathologies.

CONCLUSIONS AND RELEVANCE

The findings suggest a possible link between lower liver function and the accumulation of core AD pathologies in the brain. These results also support the possibility that the liver-brain axis could be a potential target for therapeutic or preventive strategies against AD.

Knockdown of ketohexokinase versus inhibition of its kinase activity exert divergent effects on fructose metabolism

Excessive fructose intake is a risk factor for the development of obesity and its complications. Targeting ketohexokinase (KHK), the first enzyme of fructose metabolism, has been investigated for the management of metabolic dysfunction-associated steatotic liver disease (MASLD). We compared the effects of systemic, small molecule inhibitor of KHK enzymatic activity with hepatocyte-specific, N-acetylgalactosamine siRNA-mediated knockdown of KHK in mice on an HFD. We measured KHK enzymatic activity, extensively quantified glycogen accumulation, performed RNA-Seq analysis, and enumerated hepatic metabolites using mass spectrometry. Both KHK siRNA and KHK inhibitor led to an improvement in liver steatosis; however, via substantially different mechanisms, KHK knockdown decreased the de novo lipogenesis pathway, whereas the inhibitor increased the fatty acid oxidation pathway. Moreover, KHK knockdown completely prevented hepatic fructolysis and improved glucose tolerance. Conversely, the KHK inhibitor only partially reduced fructolysis, but it also targeted triokinase, mediating the third step of fructolysis. This led to the accumulation of fructose-1 phosphate, resulting in glycogen accumulation, hepatomegaly, and impaired glucose tolerance. Overexpression of wild-type, but not kinase-dead, KHK in cultured hepatocytes increased hepatocyte injury and glycogen accumulation after treatment with fructose. The differences between KHK inhibition and knockdown are, in part, explained by the kinase-dependent and -independent effects of KHK on hepatic metabolism.

Advanced deep-tissue imaging and manipulation enabled by biliverdin reductase knockout

We developed near-infrared (NIR) photoacoustic and fluorescence probes, as well as optogenetic tools from bacteriophytochromes, and enhanced their performance using biliverdin reductase-A knock-out model (Blvra-/-). Blvra-/- elevates endogenous heme-derived biliverdin chromophore for bacteriophytochrome-derived NIR constructs. Consequently, light-controlled transcription with IsPadC-based optogenetic tool improved up to 25-fold compared to wild-type cells, with 100-fold activation in Blvra-/- neurons. In vivo , light-induced insulin production in Blvra-/- reduced blood glucose in diabetes by ∼60%, indicating high potential for optogenetic therapy. Using 3D photoacoustic, ultrasound, and two-photon fluorescence imaging, we overcame depth limitations of recording NIR probes. We achieved simultaneous photoacoustic imaging of DrBphP in neurons and super-resolution ultrasound localization microscopy of blood vessels ∼7 mm deep in the brain, with intact scalp and skull. Two-photon microscopy provided cell-level resolution of miRFP720-expressing neurons ∼2.2 mm deep. Blvra-/- significantly enhances efficacy of biliverdin-dependent NIR systems, making it promising platform for interrogation and manipulation of biological processes.

On the Therapeutic Potential of Heme Oxygenase-1 and Its Metabolites

Over the past 55 years, the heme oxygenase (HO) system has emerged as a pivotal player in a myriad of cellular, tissue, and integrative physiological processes [...].

PPARα suppresses low-intensity-noise-induced body weight gain in mice: the activated HPA axis plays an critical role

BACKGROUND

As the second most risky environmental pollution, noise imposes threats to human health. Exposure to high-intensity noise causes hearing impairment, psychotic disorders, endocrine modifications. The relationship among low-intensity noise, obesity and lipid-regulating nuclear factor PPARα is not yet clear.

METHODS

In this study, male wild-type (WT) and Pparα-null (KO) mice on a high-fat diet (HFD) were exposed to 75 dB noise for 12 weeks to explore the effect of low-intensity noise on obesity development and the role of PPARα. 3T3-L1 cells were treated with dexamethasone (DEX) and sodium oleate (OA) to verify the down-stream effect of hypothalamic-pituitary-adrenal (HPA) axis activation on the adipose tissues.

RESULTS

The average body weight gain (BWG) of WT mice on HFD exposed to noise was inhibited, which was not observed in KO mice. The mass and adipocyte size of adipose tissues accounted for the above difference of BWG tendency. In WT mice on HFD, the adrenocorticotropic hormone level was increased by the noise challenge. The aggravation of fatty liver by noise exposure occurred in both mouse lines, and the transport of hepatic redundant lipid to adipose tissues were similar. The lipid metabolism in adipose tissue driven by HPA axis accorded with the BWG inhibition in vivo, validated in 3T3-L1 adipogenic stem cells.

CONCLUSION

Chronic exposure to low-intensity noise aggravated fatty liver in both WT and KO mice. BWG inhibition was observed only in WT mice, which covered up the aggravation of fatty liver by noise exposure. PPARα mediates the activation of HPA axis by noise exposure in mice on HFD. Elevated adrenocorticotropic hormone (ACTH) promoted lipid metabolism in adipocytes, which contributed to the disassociation of BWG and fatty liver development in male WT mice. Summary of PPARα suppresses noise-induced body weight gain in mice on high-fat-diet. Chronic exposure to low-intensity noise exposure inhibited BWG by PPARα-dependent activation of the HPA axis.

Glucocorticoid resistance remodels liver lipids and prompts lipogenesis, eicosanoid, and inflammatory pathways

We have previously demonstrated that the glucocorticoid receptor β (GRβ) isoform induces hepatic steatosis in mice fed a normal chow diet. The GRβ isoform inhibits the glucocorticoid-binding isoform GRα, reducing responsiveness and inducing glucocorticoid resistance. We hypothesized that GRβ regulates lipids that cause metabolic dysfunction. To determine the effect of GRβ on hepatic lipid classes and molecular species, we overexpressed GRβ (GRβ-Ad) and vector (Vec-Ad) using adenovirus delivery, as we previously described. We fed the mice a normal chow diet for 5 days and harvested the livers. We utilized liquid chromatography-mass spectrometry (LC-MS) analyses of the livers to determine the lipid species driven by GRβ. The most significant changes in the lipidome were monoacylglycerides and cholesterol esters. There was also increased gene expression in the GRβ-Ad mice for lipogenesis, eicosanoid synthesis, and inflammatory pathways. These indicate that GRβ-induced glucocorticoid resistance may drive hepatic fat accumulation, providing new therapeutic advantages.

Heme (dys)homeostasis and liver disease

Heme is essential for a variety of proteins involved in vital physiological functions in the body, such as oxygen transport, drug metabolism, biosynthesis of steroids, signal transduction, antioxidant defense and mitochondrial respiration. However, free heme is potentially cytotoxic due to the capacity of heme iron to promote the oxidation of cellular molecules. The liver plays a central role in heme metabolism by significantly contributing to heme synthesis, heme detoxification, and recycling of heme iron. Conversely, enzymatic defects in the heme biosynthetic pathway originate multisystemic diseases (porphyrias) that are highly associated with liver damage. In addition, there is growing evidence that heme contributes to the outcomes of inflammatory, metabolic and malignant liver diseases. In this review, we summarize the contribution of the liver to heme metabolism and the association of heme dyshomeostasis with liver disease.

Biliverdin Reductase-A integrates insulin signaling with mitochondrial metabolism through phosphorylation of GSK3β

Brain insulin resistance links the failure of energy metabolism with cognitive decline in both type 2 Diabetes Mellitus (T2D) and Alzheimer's disease (AD), although the molecular changes preceding overt brain insulin resistance remain unexplored. Abnormal biliverdin reductase-A (BVR-A) levels were observed in both T2D and AD and were associated with insulin resistance. Here, we demonstrate that reduced BVR-A levels alter insulin signaling and mitochondrial bioenergetics in the brain. Loss of BVR-A leads to IRS1 hyper-activation but dysregulates Akt-GSK3β complex in response to insulin, hindering the accumulation of pGSK3βS9 into the mitochondria. This event impairs oxidative phosphorylation and fosters the activation of the mitochondrial Unfolded Protein Response (UPRmt). Remarkably, we unveil that BVR-A is required to shuttle pGSK3βS9 into the mitochondria. Our data sheds light on the intricate interplay between insulin signaling and mitochondrial metabolism in the brain unraveling potential targets for mitigating the development of brain insulin resistance and neurodegeneration.

iLight2: A near-infrared optogenetic tool for gene transcription with low background activation

Optogenetic tools (OTs) operating in the far-red and near-infrared (NIR) region offer advantages for light-controlling biological processes in deep tissues and spectral multiplexing with fluorescent probes and OTs acting in the visible range. However, many NIR OTs suffer from background activation in darkness. Through shortening linkers, we engineered a novel NIR OT, iLight2, which exhibits a significantly reduced background activity in darkness, thereby increasing the light-to-dark activation contrast. The resultant optimal configuration of iLight2 components suggests a molecular mechanism of iLight2 action. Using a biliverdin reductase knock-out mouse model, we show that iLight2 exhibits advanced performance in mouse primary cells and deep tissues in vivo. Efficient light-controlled cell migration in wound healing cellular model demonstrates the possibility of using iLight2 in therapy and, overall, positions it as a valuable addition to the NIR OT toolkit for gene transcription applications.

Cardiac lipotoxicity and fibrosis underlie impaired contractility in a mouse model of metabolic dysfunction-associated steatotic liver disease

The leading cause of death among patients with metabolic dysfunction-associated steatotic liver disease (MASLD) is cardiovascular disease. A significant percentage of MASLD patients develop heart failure driven by functional and structural alterations in the heart. Previously, we observed cardiac dysfunction in hepatocyte-specific peroxisome proliferator-activated receptor alpha knockout (Ppara HepKO), a mouse model that exhibits hepatic steatosis independent of obesity and insulin resistance. The goal of the present study was to determine mechanisms that underlie hepatic steatosis-induced cardiac dysfunction in Ppara HepKO mice. Experiments were performed in 30-week-old Ppara HepKO and littermate control mice fed regular chow. We observed decreased cardiomyocyte contractility (0.17 ± 0.02 vs. 0.24 ± 0.02 μm, p < 0.05), increased cardiac triglyceride content (0.96 ± 0.13 vs. 0.68 ± 0.06 mM, p < 0.05), collagen type 1 (4.65 ± 0.25 vs. 0.31 ± 0.01 AU, p < 0.001), and collagen type 3 deposition (1.32 ± 0.46 vs. 0.05 ± 0.03 AU, p < 0.05). These changes were associated with increased apoptosis as indicated by terminal deoxynucleotidyl transferase dUTP nick end labeling staining (30.9 ± 4.7 vs. 13.1 ± 0.8%, p < 0.006) and western blots showing increased cleaved caspase-3 (0.27 ± 0.006 vs. 0.08 ± 0.01 AU, p < 0.003) and pro-caspase-3 (5.4 ± 1.5 vs. 0.5 ± 0.3 AU, p < 0.02), B-cell lymphoma protein 2-associated X (0.68 ± 0.07 vs. 0.04 ± 0.04 AU, p < 0.001), and reduced B-cell lymphoma protein 2 (0.29 ± 0.01 vs. 1.47 ± 0.54 AU, p < 0.05). We further observed elevated circulating natriuretic peptides and exercise intolerance in Ppara HepKO mice when compared to controls. Our data demonstrated that lipotoxicity, and fibrosis underlie cardiac dysfunction in MASLD.

A Possible Therapeutic Application of the Selective Inhibitor of Urate Transporter 1, Dotinurad, for Metabolic Syndrome, Chronic Kidney Disease, and Cardiovascular Disease

The reabsorption of uric acid (UA) is mainly mediated by urate transporter 1 (URAT1) and glucose transporter 9 (GLUT9) in the kidneys. Dotinurad inhibits URAT1 but does not inhibit other UA transporters, such as GLUT9, ATP-binding cassette transporter G2 (ABCG2), and organic anion transporter 1/3 (OAT1/3). We found that dotinurad ameliorated the metabolic parameters and renal function in hyperuricemic patients. We consider the significance of the highly selective inhibition of URAT1 by dotinurad for metabolic syndrome, chronic kidney disease (CKD), and cardiovascular disease (CVD). The selective inhibition of URAT1 by dotinurad increases urinary UA in the proximal tubules, and this un-reabsorbed UA may compete with urinary glucose for GLUT9, reducing glucose reabsorption. The inhibition by dotinurad of UA entry via URAT1 into the liver and adipose tissues increased energy expenditure and decreased lipid synthesis and inflammation in rats. Such effects may improve metabolic parameters. CKD patients accumulate uremic toxins, including indoxyl sulfate (IS), in the body. ABCG2 regulates the renal and intestinal excretion of IS, which strongly affects CKD. OAT1/3 inhibitors suppress IS uptake into the kidneys, thereby increasing plasma IS, which produces oxidative stress and induces vascular endothelial dysfunction in CKD patients. The highly selective inhibition of URAT1 by dotinurad may be beneficial for metabolic syndrome, CKD, and CVD.

Neuroprotective Roles of the Biliverdin Reductase-A/Bilirubin Axis in the Brain

Biliverdin reductase-A (BVRA) is a multi-functional enzyme with a multitude of important roles in physiologic redox homeostasis. Classically, BVRA is well known for converting the heme metabolite biliverdin to bilirubin, which is a potent antioxidant in both the periphery and the brain. However, BVRA additionally participates in many neuroprotective signaling cascades in the brain that preserve cognition. Here, we review the neuroprotective roles of BVRA and bilirubin in the brain, which together constitute a BVRA/bilirubin axis that influences healthy aging and cognitive function.

Role of Natural Compounds Modulating Heme Catabolic Pathway in Gut, Liver, Cardiovascular, and Brain Diseases

The crucial physiological process of heme breakdown yields biliverdin (BV) and bilirubin (BR) as byproducts. BV, BR, and the enzymes involved in their production (the "yellow players-YP") are increasingly documented as endogenous modulators of human health. Mildly elevated serum bilirubin concentration has been correlated with a reduced risk of multiple chronic pro-oxidant and pro-inflammatory diseases, especially in the elderly. BR and BV per se have been demonstrated to protect against neurodegenerative diseases, in which heme oxygenase (HMOX), the main enzyme in the production of pigments, is almost always altered. HMOX upregulation has been interpreted as a tentative defense against the ongoing pathologic mechanisms. With the demonstration that multiple cells possess YP, their propensity to be modulated, and their broad spectrum of activity on multiple signaling pathways, the YP have assumed the role of an adjustable system that can promote health in adults. Based on that, there is an ongoing effort to induce their activity as a therapeutic option, and natural compounds are an attractive alternative to the goal, possibly requiring only minimal changes in the life style. We review the most recent evidence of the potential of natural compounds in targeting the YP in the context of the most common pathologic condition of adult and elderly life.

Ubiquitination and Metabolic Disease

The increasing incidence of metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic fatty liver disease (NAFLD), in the past decade is a serious concern worldwide. Disruption of cellular protein homeostasis has been considered as a crucial contributor to the pathogenesis of metabolic diseases. To maintain protein homeostasis, cells have evolved multiple dynamic and self-regulating quality control processes to adapt new environmental conditions and prevent prolonged damage. Among them, the ubiquitin proteasome system (UPS), the primary proteolytic pathway for degradation of aberrant proteins via ubiquitination, has an essential role in maintaining cellular homeostasis in response to intracellular stress. Correspondingly, accumulating evidences have shown that dysregulation of ubiquitination can aggravate various metabolic derangements in many tissues, including the liver, skeletal muscle, pancreas, and adipose tissue, and is involved in the initiation and progression of diverse metabolic diseases. In this part, we will summarize the role of ubiquitination in the pathogenesis of metabolic diseases, including obesity, T2DM and NAFLD, and discuss its potential as a therapeutic target.

An enzyme that selectively S-nitrosylates proteins to regulate insulin signaling

Acyl-coenzyme A (acyl-CoA) species are cofactors for numerous enzymes that acylate thousands of proteins. Here, we describe an enzyme that uses S-nitroso-CoA (SNO-CoA) as its cofactor to S-nitrosylate multiple proteins (SNO-CoA-assisted nitrosylase, SCAN). Separate domains in SCAN mediate SNO-CoA and substrate binding, allowing SCAN to selectively catalyze SNO transfer from SNO-CoA to SCAN to multiple protein targets, including the insulin receptor (INSR) and insulin receptor substrate 1 (IRS1). Insulin-stimulated S-nitrosylation of INSR/IRS1 by SCAN reduces insulin signaling physiologically, whereas increased SCAN activity in obesity causes INSR/IRS1 hypernitrosylation and insulin resistance. SCAN-deficient mice are thus protected from diabetes. In human skeletal muscle and adipose tissue, SCAN expression increases with body mass index and correlates with INSR S-nitrosylation. S-nitrosylation by SCAN/SNO-CoA thus defines a new enzyme class, a unique mode of receptor tyrosine kinase regulation, and a revised paradigm for NO function in physiology and disease.

Metabolic-Dysfunction-Associated Steatotic Liver Disease-Its Pathophysiology, Association with Atherosclerosis and Cardiovascular Disease, and Treatments

Metabolic-dysfunction-associated steatotic liver disease (MASLD) is a chronic liver disease that affects more than a quarter of the global population and whose prevalence is increasing worldwide due to the pandemic of obesity. Obesity, impaired glucose metabolism, high blood pressure and atherogenic dyslipidemia are risk factors for MASLD. Therefore, insulin resistance may be closely associated with the development and progression of MASLD. Hepatic entry of increased fatty acids released from adipose tissue, increase in fatty acid synthesis and reduced fatty acid oxidation in the liver and hepatic overproduction of triglyceride-rich lipoproteins may induce the development of MASLD. Since insulin resistance also induces atherosclerosis, the leading cause for death in MASLD patients is cardiovascular disease. Considering that the development of cardiovascular diseases determines the prognosis of MASLD patients, the therapeutic interventions for MASLD should reduce body weight and improve coronary risk factors, in addition to an improving in liver function. Lifestyle modifications, such as improved diet and increased exercise, and surgical interventions, such as bariatric surgery and intragastric balloons, have shown to improve MASLD by reducing body weight. Sodium glucose cotransporter 2 inhibitors (SGLT2i) and glucagon-like peptide-1 receptor agonists (GLP-1RAs) have been shown to improve coronary risk factors and to suppress the occurrence of cardiovascular diseases. Both SGLT2i and GLP-1 have been reported to improve liver enzymes, hepatic steatosis and fibrosis. We recently reported that the selective peroxisome proliferator-activated receptor-alpha (PPARα) modulator pemafibrate improved liver function. PPARα agonists have multiple anti-atherogenic properties. Here, we consider the pathophysiology of MASLD and the mechanisms of action of such drugs and whether such drugs and the combination therapy of such drugs could be the treatments for MASLD.

Hepatic insulin receptor: new views on the mechanisms of liver disease

Over 65 % of people with obesity display the metabolic-associated fatty liver disease (MAFLD), which can manifest as steatohepatitis, fibrosis, cirrhosis, or liver cancer. The development and progression of MAFLD involve hepatic insulin resistance and reduced insulin clearance. This review discusses the relationships between altered insulin signaling, hepatic insulin resistance, and reduced insulin clearance in the development of MAFLD and how this provides the impetus for exploring the use of insulin sensitizers to curb this disease. The review also explores the role of the insulin receptor in hepatocytes and hepatic stellate cells and how it signals in metabolic and end-stage liver diseases. Finally, we discuss new research findings that indicate that advanced hepatic diseases may be an insulin-sensitive state in the liver and deliberate whether insulin sensitizers should be used to manage late-stage liver diseases.

Loss of hepatic PPARα in mice causes hypertension and cardiovascular disease

The leading cause of death in patients with nonalcoholic fatty liver disease (NAFLD) is cardiovascular disease (CVD). However, the mechanisms are unknown. Mice deficient in hepatocyte proliferator-activated receptor-α (PPARα) (PparaHepKO) exhibit hepatic steatosis on a regular chow diet, making them prone to manifesting NAFLD. We hypothesized that the PparaHepKO mice might be predisposed to poorer cardiovascular phenotypes due to increased liver fat content. Therefore, we used PparaHepKO and littermate control mice fed a regular chow diet to avoid complications with a high-fat diet, such as insulin resistance and increased adiposity. After 30 wk on a standard diet, male PparaHepKO mice exhibited elevated hepatic fat content compared with littermates as measured by Echo MRI (11.95 ± 1.4 vs. 3.74 ± 1.4%, P < 0.05), hepatic triglycerides (1.4 ± 0.10 vs. 0.3 ± 0.01 mM, P < 0.05), and Oil Red O staining, despite body weight, fasting blood glucose, and insulin levels being the same as controls. The PparaHepKO mice also displayed elevated mean arterial blood pressure (121 ± 4 vs. 108 ± 2 mmHg, P < 0.05), impaired diastolic function, cardiac remodeling, and enhanced vascular stiffness. To determine mechanisms controlling the increase in stiffness in the aorta, we used state-of-the-art PamGene technology to measure kinase activity in this tissue. Our data suggest that the loss of hepatic PPARα induces alterations in the aortas that reduce the kinase activity of tropomyosin receptor kinases and p70S6K kinase, which might contribute to the pathogenesis of NAFLD-induced CVD. These data indicate that hepatic PPARα protects the cardiovascular system through some as-of-yet undefined mechanism.

Dynamic Changes of BVRA Protein Levels Occur in Response to Insulin: A Pilot Study in Humans

Biliverdin reductase-A (BVRA) is involved in the regulation of insulin signaling and the maintenance of glucose homeostasis. Previous research showed that BVRA alterations are associated with the aberrant activation of insulin signaling in dysmetabolic conditions. However, whether BVRA protein levels change dynamically within the cells in response to insulin and/or glucose remains an open question. To this aim, we evaluated changes of intracellular BVRA levels in peripheral blood mononuclear cells (PBMC) collected during the oral glucose tolerance test (OGTT) in a group of subjects with different levels of insulin sensitivity. Furthermore, we looked for significant correlations with clinical measures. Our data show that BVRA levels change dynamically during the OGTT in response to insulin, and greater BVRA variations occur in those subjects with lower insulin sensitivity. Changes of BVRA significantly correlate with indexes of increased insulin resistance and insulin secretion (HOMA-IR, HOMA-β, and insulinogenic index). At the multivariate regression analysis, the insulinogenic index independently predicted increased BVRA area under curve (AUC) during the OGTT. This pilot study showed, for the first time, that intracellular BVRA protein levels change in response to insulin during OGTT and are greater in subjects with lower insulin sensitivity, supporting the role of BVR-A in the dynamic regulation of the insulin signaling pathway.

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.

Assessing the causal relationships between human blood metabolites and the risk of NAFLD: A comprehensive mendelian randomization study

Background: Non-alcoholic fatty liver disease (NAFLD) is a liver disease associated with obesity, insulin resistance, type 2 diabetes mellitus (T2DM), and metabolic syndrome. The risk factors for NAFLD have not been identified. Metabolic dysfunction has been found to be an important factor in the pathogenesis and progression of NAFLD. However, the causal impact of blood metabolites on NAFLD is unclear.

Methods: We performed a two-sample Mendelian randomization (MR) study. A genome-wide association study (GWAS) with 7824 participants provided data on 486 human blood metabolites. Outcome information was obtained from a large-scale GWAS meta-analysis of NAFLD, which contained 8,434 cases and 770,180 controls of Europeans. The inverse variance weighted (IVW) model was chosen as the primary two-sample MR analysis approach, followed by sensitivity analyses such as the heterogeneity test, horizontal pleiotropy test, and leave-one-out analysis. In addition, we performed replication, meta-analysis, and metabolic pathway analysis. We further conducted colocalization analysis to deeply reflect the causality.

Results: After rigorous genetic variant selection, IVW, sensitivity analysis, replication, and meta-analysis, two known metabolites were identified as being associated with the development of NAFLD [biliverdin: OR = 1.45; 95% CI 1.20-1.75; p = 0.0001; myristoleate: OR = 0.57; 95% CI 0.39-0.83; p = 0.0030].

Conclusion: By combining genomics with metabolomics, our findings provide a new perspective on the underlying mechanisms of NAFLD and have important implications for the screening and prevention of NAFLD.

Heme oxygenase-1 prevents non-alcoholic steatohepatitis through modulating mitochondrial quality control

AIM

Nonalcoholic steatohepatitis (NASH) is a severe form of nonalcoholic fatty liver disease (NAFLD) and lacks effective treatment options. Heme oxygenase-1 (HO-1) is a critical defense against oxidative stress and inflammation in the liver injury. This study aims to investigate the protective role and underlying mechanisms of HO-1 in NASH pathogenesis.

METHODS

The hepatocyte-specific HO-1 knockout (HO-1^HEPKO ) mice on a C57BL/6J background (HO-1^fl/fl /Alb-Cre) were generated and fed a high-fat/western-style diet (HFD) or methionine-choline-deficient diet (MCD). Changes in mitochondrial ultrastructure were observed by transmission electron microscopy and confocal microscopy. A mitochondrial PCR array was used to identify the crucial genes associated with mitochondrial dysfunction.

RESULTS

Hepatocyte-specific HO-1^HEPKO mice developed steatohepatitis with severe steatosis, ballooning, and necroinflammation. Dysregulated hepatic expression of mitochondria-related proteins, including DRP1, Tomm20, MFN1 and MFN2 were detected in NASH animals. Ultrastructural mitochondrial damage was observed in HO-1^HEPKO mice. Mitochondrial dysfunction was recapitulated in HO-1-knockdown cells in vitro, as evidenced by decreased membrane potential, reduced ATP content, and mtDNA damage. Conversely, HO-1 overexpression restored these changes in vitro. Mechanistically, HO-1 deficiency reduced the inhibitory effect on Tomm20, leading to mitochondrial dysfunction, and thereby causing steatohepatitis.

CONCLUSIONS

HO-1 attenuates diet-induced steatohepatitis by preventing mitochondrial dysfunction, indicating that HO-1 may constitute a potential therapeutic target for NASH.

Phosphorus Overload Promotes Hepatic Lipolysis by Suppressing GSK3β-Dependent Phosphorylation of PPARα at Ser84 and Thr265 in a Freshwater Teleost

Excessive phosphorus (Pi) contributes to eutrophication in an aquatic environment, which threatens human and fish health. However, the mechanisms by which Pi overload influences aquatic animals remain largely unexplored. In the present study, Pi supplementation increased the Pi content, inhibited lipid accumulation and lipogenesis, and stimulated lipolysis in the liver. Pi supplementation increased the phosphorylation of glycogen synthase kinase-3 β (GSK3β) at serine 9 (S9) but inhibited the phosphorylation of GSK3α at tyrosine 279 (Y279), GSK3β at tyrosine 216 (Y216), and peroxisome proliferator-activated receptor α (PPARα) at serine 84 (S84) and threonine 265 (T265). Pi supplementation also upregulated PPARα protein expression and stimulated its transcriptional activity, thereby inducing lipolysis. Pi suppressed GSK3β activity and prevented GSK3β, but not GSK3α, from interacting with PPARα, which in turn alleviated PPARα phosphorylation. GSK3β-induced phosphorylation of PPARα was dependent on GSK3β S9 dephosphorylation rather than Y216 phosphorylation. Mechanistically, underphosphorylation of PPARα mediated Pi-induced lipid degradation through transcriptionally activating adipose triglyceride lipase (atgl) and very long-chain-specific acyl-CoA dehydrogenase (acadvl). Collectively, our findings uncovered a new mechanism by which Pi facilitates lipolysis via the GSK3β-PPARα pathway and highlighted the importance of S84 and T265 phosphorylation in PPARα action.

Bilirubin Nanoparticle Treatment in Obese Mice Inhibits Hepatic Ceramide Production and Remodels Liver Fat Content

Studies have indicated that increasing plasma bilirubin levels might be useful for preventing and treating hepatic lipid accumulation that occurs with metabolic diseases such as obesity and diabetes. We have previously demonstrated that mice with hyperbilirubinemia had significantly less lipid accumulation in a diet-induced non-alcoholic fatty liver disease (NAFLD) model. However, bilirubin's effects on individual lipid species are currently unknown. Therefore, we used liquid chromatography-mass spectroscopy (LC-MS) to determine the hepatic lipid composition of obese mice with NAFLD treated with bilirubin nanoparticles or vehicle control. We placed the mice on a high-fat diet (HFD) for 24 weeks and then treated them with bilirubin nanoparticles or vehicle control for 4 weeks while maintaining the HFD. Bilirubin nanoparticles suppressed hepatic fat content overall. After analyzing the lipidomics data, we determined that bilirubin inhibited the accumulation of ceramides in the liver. The bilirubin nanoparticles significantly lowered the hepatic expression of two essential enzymes that regulate ceramide production, Sgpl1 and Degs1. Our results demonstrate that the bilirubin nanoparticles improve hepatic fat content by reducing ceramide production, remodeling the liver fat content, and improving overall metabolic health.

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.

Bilirubin Levels Are Negatively Correlated with Adiposity in Obese Men and Women, and Its Catabolized Product, Urobilin, Is Positively Associated with Insulin Resistance

Bilirubin levels in obese humans and rodents have been shown to be lower than in their lean counterparts. Some studies have proposed that the glucuronyl UGT1A1 enzyme that clears bilirubin from the blood increases in the liver with obesity. UGT1A1 clearance of bilirubin allows more conjugated bilirubin to enter the intestine, where it is catabolized into urobilin, which can be then absorbed via the hepatic portal vein. We hypothesized that when bilirubin levels are decreased, the urobilin increases in the plasma of obese humans, as compared to lean humans. To test this, we measured plasma levels of bilirubin and urobilin, body mass index (BMI), adiposity, blood glucose and insulin, and HOMA IR in a small cohort of obese and lean men and women. We found that bilirubin levels negatively correlated with BMI and adiposity in obese men and women, as compared to their lean counterparts. Contrarily, urobilin levels were positively associated with adiposity and BMI. Only obese women were found to be insulin resistant based on significantly higher HOMA IR, as compared to lean women. The urobilin levels were positively associated with HOMA IR in both groups, but women had a stronger linear correlation. These studies indicate that plasma urobilin levels are associated with obesity and its comorbidities, such as insulin resistance.

Cutting edge concepts: Does bilirubin enhance exercise performance?

Exercise performance is dependent on many factors, such as muscular strength and endurance, cardiovascular capacity, liver health, and metabolic flexibility. Recent studies show that plasma levels of bilirubin, which has classically been viewed as a liver dysfunction biomarker, are elevated by exercise training and that elite athletes may have significantly higher levels. Other studies have shown higher plasma bilirubin levels in athletes and active individuals compared to general, sedentary populations. The reason for these adaptions is unclear, but it could be related to bilirubin's antioxidant properties in response to a large number of reactive oxygen species (ROS) that originates from mitochondria during exercise. However, the mechanisms of these are unknown. Current research has re-defined bilirubin as a metabolic hormone that interacts with nuclear receptors to drive gene transcription, which reduces body weight. Bilirubin has been shown to reduce adiposity and improve the cardiovascular system, which might be related to the adaption of bilirubin increasing during exercise. No studies have directly tested if elevating bilirubin levels can influence athletic performance. However, based on the mechanisms proposed in the present review, this seems plausible and an area to consider for future studies. Here, we discuss the importance of bilirubin and exercise and how the combination might improve metabolic health outcomes and possibly athletic performance.

Hepatic kinome atlas: An in-depth identification of kinase pathways in liver fibrosis of humans and rodents

BACKGROUND AND AIMS

Resolution of pathways that converge to induce deleterious effects in hepatic diseases, such as in the later stages, have potential antifibrotic effects that may improve outcomes. We aimed to explore whether humans and rodents display similar fibrotic signaling networks.

APPROACH AND RESULTS

We assiduously mapped kinase pathways using 340 substrate targets, upstream bioinformatic analysis of kinase pathways, and over 2000 random sampling iterations using the PamGene PamStation kinome microarray chip technology. Using this technology, we characterized a large number of kinases with altered activity in liver fibrosis of both species. Gene expression and immunostaining analyses validated many of these kinases as bona fide signaling events. Surprisingly, the insulin receptor emerged as a considerable protein tyrosine kinase that is hyperactive in fibrotic liver disease in humans and rodents. Discoidin domain receptor tyrosine kinase, activated by collagen that increases during fibrosis, was another hyperactive protein tyrosine kinase in humans and rodents with fibrosis. The serine/threonine kinases found to be the most active in fibrosis were dystrophy type 1 protein kinase and members of the protein kinase family of kinases. We compared the fibrotic events over four models: humans with cirrhosis and three murine models with differing levels of fibrosis, including two models of fatty liver disease with emerging fibrosis. The data demonstrate a high concordance between human and rodent hepatic kinome signaling that focalizes, as shown by our network analysis of detrimental pathways.

CONCLUSIONS

Our findings establish a comprehensive kinase atlas for liver fibrosis, which identifies analogous signaling events conserved among humans and rodents.

Glycogen Synthase Kinase 3β (GSK3β) Regulates Myogenic Differentiation in Skeletal Muscle Satellite Cells of Sheep

Glycogen synthase kinase 3β (GSK3β) has a vital role in the regulation of many cellular processes. However, the role of GSK3β in muscle cell differentiation in sheep remains unknown. In this study, we investigated the function of GSK3β in skeletal muscle satellite cells (SMSCs) of sheep. An overexpression of GSK3β significantly inhibited myotube formation as well as the mRNA levels of myogenic genes (MyoD, MyoG, MyHC1, and MyHC2a) in sheep SMSCs. SB216763 treatment had a time-course effect on the phosphorylation levels of sheep GSK3β. In addition, reducing the activity of GSK3β lead to the promotion of sheep SMSCs differentiation as well as the mRNA levels of myogenic genes (MyoD, MyoG, MyHC1, and MyHC2a). This study illustrated the function of GSK3β to inhibit myogenesis in sheep SMSCs, which provided evidence for studying the mechanisms involved in the regulation of sheep SMSCs differentiation by GSK3β.

Molecular mechanisms of metabolic associated fatty liver disease (MAFLD): functional analysis of lipid metabolism pathways

The metabolic-associated fatty liver disease (MAFLD) is a condition of fat accumulation in the liver in combination with metabolic dysfunction in the form of overweight or obesity and insulin resistance. It is also associated with an increased cardiovascular disease risk, including hypertension and atherosclerosis. Hepatic lipid metabolism is regulated by a combination of the uptake and export of fatty acids, de novo lipogenesis, and fat utilization by β-oxidation. When the balance between these pathways is altered, hepatic lipid accumulation commences, and long-term activation of inflammatory and fibrotic pathways can progress to worsen the liver disease. This review discusses the details of the molecular mechanisms regulating hepatic lipids and the emerging therapies targeting these pathways as potential future treatments for MAFLD.

Heme oxygenase-1 mitigates liver injury and fibrosis via modulation of LNX1/Notch1 pathway in myeloid cells

Activation of resident macrophages (Mϕ) and hepatic stellate cells is a key event in chronic liver injury. Mice with heme oxygenase-1 (HO-1; Hmox1)-deficient Mϕ (LysM-Cre:Hmox1 ^flfl ) exhibit increased inflammation, periportal ductular reaction, and liver fibrosis following bile duct ligation (BDL)-induced liver injury and increased pericellular fibrosis in NASH model. RiboTag-based RNA-sequencing profiling of hepatic HO-1-deficient Mϕ revealed dysregulation of multiple genes involved in lipid and amino acid metabolism, regulation of oxidative stress, and extracellular matrix turnover. Among these genes, ligand of numb-protein X1 (LNX1) expression is strongly suppressed in HO-1-deficient Mϕ. Importantly, HO-1 and LNX1 were expressed by hepatic Mϕ in human biliary and nonbiliary end-stage cirrhosis. We found that Notch1 expression, a downstream target of LNX1, was increased in LysM-Cre:Hmox1 ^flfl mice. In HO-1-deficient Mϕ treated with heme, transient overexpression of LNX1 drives M2-like Mϕ polarization. In summary, we identified LNX1/Notch1 pathway as a downstream target of HO-1 in liver fibrosis.

Role of Biliverdin Reductase A in the Regulation of Insulin Signaling in Metabolic and Neurodegenerative Diseases: An Update

Insulin signaling is a conserved pathway that orchestrates glucose and lipid metabolism, energy balance, and inflammation, and its dysregulation compromises the homeostasis of multiple systems. Insulin resistance is a shared hallmark of several metabolic diseases, including obesity, metabolic syndrome, and type 2 diabetes, and has been associated with cognitive decline during aging and dementia. Numerous mechanisms promoting the development of peripheral and central insulin resistance have been described, although most of them were not completely clarified. In the last decades, several studies have highlighted that biliverdin reductase-A (BVR-A), over its canonical role in the degradation of heme, acts as a regulator of insulin signaling. Evidence from human and animal studies show that BVR-A alterations are associated with the aberrant activation of insulin signaling, metabolic syndrome, liver steatosis, and visceral adipose tissue inflammation in obese and diabetic individuals. In addition, recent findings demonstrated that reduced BVR-A levels or impaired BVR-A activation contribute to the development of brain insulin resistance and metabolic alterations in Alzheimer's disease. In this narrative review, we will provide an overview on the literature by focusing on the role of BVR-A in the regulation of insulin signaling and how BVR-A alterations impact on cell dysfunctions in both metabolic and neurodegenerative disorders.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

Fibroblast growth factor 21: A "rheostat" for metabolic regulation?

Fibroblast growth factor 21 is an evolutionarily conserved factor that plays multiple important roles in metabolic homeostasis. During the past two decades, extensive investigations have improved our understanding of its delicate metabolic roles and identified its pharmacological potential to mitigate metabolic disorders. However, most clinical trials have failed to obtain the desired results, which raises issues regarding its clinical value. Fibroblast growth factor 21 is dynamically regulated by nutrients derived from food intake and hepatic/adipose release, which in turn act on the central nervous system, liver, and adipose tissues to influence food preference, hepatic glucose, and adipose fatty acid output. Based on this information, we propose that fibroblast growth factor 21 should not be considered merely an anti-hyperglycemia or anti-obesity factor, but rather a means of balancing of nutrient fluctuations to maintain an appropriate energy supply. Hence, the specific functions of fibroblast growth factor 21 in glycometabolism and lipometabolism depend on specific metabolic states, indicating that its pharmacological effects require further consideration.

FGF9 Alleviates the Fatty Liver Phenotype by Regulating Hepatic Lipid Metabolism

Although the fatty liver has been linked to numerous impairments of energy homeostasis, the molecular mechanism responsible for fatty liver development remains largely unknown. In the present study, we show that fibroblast growth factors 9 (FGF9) expression is increased in the liver of diet-induced obese (DIO), db/db, and ob/ob mice relative to their respective controls. The long-term knockdown of hepatic FGF9 expression mediated by adeno-associated virus expressing FGF9-specific short hairpin RNA (AAV-shFGF9) aggravated the fatty liver phenotype of DIO mice. Consistently, downregulation of FGF9 expression mediated by adenovirus expressing FGF9-specific shRNA (Ad-shFGF9) in the primary hepatocyte promoted the cellular lipid accumulation, suggesting that FGF9 exerts its effects in an autocrine manner. In contrast, adenoviruses expressing FGF9 (Ad-FGF9) mediated FGF9 overexpression in the liver of DIO mice alleviated hepatic steatosis and improved the insulin sensitivity and glucose intolerance. Moreover, the liver-specific FGF9 transgenic mice phenocopied the Ad-FGF9-infected mice. Mechanistically, FGF9 inhibited the expression of genes involved in lipogenesis and increased the expression of genes involved in fatty acid oxidation, thereby reducing cellular lipid accumulation. Thus, targeting FGF9 might be exploited to treat nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome.

Biliverdin reductase B impairs cholangiocarcinoma cell motility by inhibiting the Notch/Snail signaling pathway

Cholangiocarcinoma (CCA) is one of the most lethal types of solid tumors worldwide. Lymph node metastasis is common in the early stage, which is associated with recurrence and reduced survival time after CCA resection. The molecular pathogenesis of CCA is complex and requires extensive investigation. It involves multiple genomic alterations and the dysregulation of signaling pathways. Biliverdin reductase B (BLVRB) is a non-redundant NAD(P)H-dependent biliverdin reductase that regulates cellular redox status by reducing biliverdin to bilirubin. This study aimed at describing the biological functions and molecular mechanisms of BLVRB in human CCA. Prognostic clinical data showed that low expression BLVRB was associated with poor prognosis and lymph node metastasis. BLVRB depletion accelerated epithelial-mesenchymal transition (EMT), cell migration and invasion. In contrast, BLVRB overexpression was associated with reduced EMT and cell migration and invasion in CCA. BLVRB suppression activated Notch signaling, and activated c-Notch enhanced EMT by upregulating Snail expression levels, thereby increasing cell migration and invasion in CCA. Our results identified an unexpected function of BLVRB in CCA migration and invasion through the regulation of Notch/Snail signaling.

Novel Function for Bilirubin as a Metabolic Signaling Molecule: Implications for Kidney Diseases

Bilirubin is the end product of the catabolism of heme via the heme oxygenase pathway. Heme oxygenase generates carbon monoxide (CO) and biliverdin from the breakdown of heme, and biliverdin is rapidly reduced to bilirubin by the enzyme biliverdin reductase (BVR). Bilirubin has long been thought of as a toxic product that is only relevant to health when blood levels are severely elevated, such as in clinical jaundice. The physiologic functions of bilirubin correlate with the growing body of evidence demonstrating the protective effects of serum bilirubin against cardiovascular and metabolic diseases. Although the correlative evidence suggests a protective effect of serum bilirubin against many diseases, the mechanism by which bilirubin offers protection against cardiovascular and metabolic diseases remains unanswered. We recently discovered a novel function for bilirubin as a signaling molecule capable of activating the peroxisome proliferator-activated receptor α (PPARα) transcription factor. This review summarizes the new finding of bilirubin as a signaling molecule and proposes several mechanisms by which this novel action of bilirubin may protect against cardiovascular and kidney diseases.

Heme Oxygenase-1: An Anti-Inflammatory Effector in Cardiovascular, Lung, and Related Metabolic Disorders

The heme oxygenase (HO) enzyme system catabolizes heme to carbon monoxide (CO), ferrous iron, and biliverdin-IXα (BV), which is reduced to bilirubin-IXα (BR) by biliverdin reductase (BVR). HO activity is represented by two distinct isozymes, the inducible form, HO-1, and a constitutive form, HO-2, encoded by distinct genes (HMOX1, HMOX2, respectively). HO-1 responds to transcriptional activation in response to a wide variety of chemical and physical stimuli, including its natural substrate heme, oxidants, and phytochemical antioxidants. The expression of HO-1 is regulated by NF-E2-related factor-2 and counter-regulated by Bach-1, in a heme-sensitive manner. Additionally, HMOX1 promoter polymorphisms have been associated with human disease. The induction of HO-1 can confer protection in inflammatory conditions through removal of heme, a pro-oxidant and potential catalyst of lipid peroxidation, whereas iron released from HO activity may trigger ferritin synthesis or ferroptosis. The production of heme-derived reaction products (i.e., BV, BR) may contribute to HO-dependent cytoprotection via antioxidant and immunomodulatory effects. Additionally, BVR and BR have newly recognized roles in lipid regulation. CO may alter mitochondrial function leading to modulation of downstream signaling pathways that culminate in anti-apoptotic, anti-inflammatory, anti-proliferative and immunomodulatory effects. This review will present evidence for beneficial effects of HO-1 and its reaction products in human diseases, including cardiovascular disease (CVD), metabolic conditions, including diabetes and obesity, as well as acute and chronic diseases of the liver, kidney, or lung. Strategies targeting the HO-1 pathway, including genetic or chemical modulation of HO-1 expression, or application of BR, CO gas, or CO donor compounds show therapeutic potential in inflammatory conditions, including organ ischemia/reperfusion injury. Evidence from human studies indicate that HO-1 expression may represent a biomarker of oxidative stress in various clinical conditions, while increases in serum BR levels have been correlated inversely to risk of CVD and metabolic disease. Ongoing human clinical trials investigate the potential of CO as a therapeutic in human disease.

Acute exercise rapidly activates hepatic mitophagic flux

Exercise is critical for improving metabolic health and putatively maintains or enhances mitochondrial quality control in metabolic tissues. Although previous work has shown that exercise elicits hepatic mitochondrial biogenesis, it is unknown if acute exercise activates hepatic mitophagy, the selective degradation of damaged or low-functioning mitochondria. We tested if an acute bout of treadmill running increased hepatic mitophagic flux both right after and 2-h postexercise in 15- to 24-wk-old C57BL/6J female mice. Acute exercise did not significantly increase markers of autophagic flux, however, mitophagic flux was activated 2-h post-treadmill running as measured by accumulation of both LC3-II and p62 in isolated mitochondria in the presence of leupeptin, an inhibitor of autophagosome degradation. Furthermore, mitochondrial-associated ubiquitin, which recruits the autophagy receptor protein p62, was also significantly increased at 2 h. Further examination via Western blot and proteomics analysis revealed that acute exercise elicits a time-dependent, dynamic activation of mitophagy pathways. Moreover, the results suggest that exercise-induced hepatic mitophagy is likely mediated by both polyubiquitination and receptor-mediated signaling pathways. Overall, we provide evidence that acute exercise activates hepatic mitophagic flux while also revealing specific receptor-mediated proteins by which exercise maintains mitochondrial quality control in the liver.NEW & NOTEWORTHY This study provides evidence that acute exercise activates hepatic mitophagic flux and mitochondrial polyubiquitination while additionally revealing specific receptor-mediated proteins by which exercise maintains mitochondrial quality control in the liver.

Transforming Growth Factor-Beta (TGF-β) Signaling in Cancer-A Betrayal Within

A ubiquitously expressed cytokine, transforming growth factor-beta (TGF-β) plays a significant role in various ongoing cellular mechanisms. The gain or loss-of-function of TGF-β and its downstream mediators could lead to a plethora of diseases includes tumorigenesis. Specifically, at the early onset of malignancy TGF-β act as tumour suppressor and plays a key role in clearing malignant cells by reducing the cellular proliferation and differentiation thus triggers the process of apoptosis. Subsequently, TGF-β at an advanced stage of malignancy promotes tumorigenesis by augmenting cellular transformation, epithelial-mesenchymal-transition invasion, and metastasis. Besides playing the dual roles, depending upon the stage of malignancy, TGF-β also regulates cell fate through immune and stroma components. This oscillatory role of TGF-β to fight against cancer or act as a traitor to collaborate and crosstalk with other tumorigenic signaling pathways and its betrayal within the cell depends upon the cellular context. Therefore, the current review highlights and understands the dual role of TGF-β under different cellular conditions and its crosstalk with other signaling pathways in modulating cell fate.