Pathogenesis of selective insulin resistance in isolated hepatocytes

Organoid-guided precision hepatology for metabolic liver disease

Metabolic dysfunction-associated steatotic liver disease affects millions of people worldwide. Progress towards a definitive cure has been incremental and treatment is currently limited to lifestyle modification. Hepatocyte-specific lipid accumulation is the main trigger of lipotoxic events, driving inflammation and fibrosis. The underlying pathology is extraordinarily heterogenous, and the manifestations of steatohepatitis are markedly influenced by metabolic communications across non-hepatic organs. Synthetic human tissue models have emerged as powerful platforms to better capture the mechanistic diversity in disease progression, while preserving person-specific genetic traits. In this review, we will outline current research efforts focused on integrating multiple synthetic tissue models of key metabolic organs, with an emphasis on organoid-based systems. By combining functional genomics and population-scale en masse profiling methodologies, human tissues derived from patients can provide insights into personalised genetic, transcriptional, biochemical, and metabolic states. These collective efforts will advance our understanding of steatohepatitis and guide the development of rational solutions for mechanism-directed diagnostic and therapeutic investigation.

Exploring the underlying mechanisms of fisetin in the treatment of hepatic insulin resistance via network pharmacology and in vitro validation

OBJECTIVE

To characterize potential mechanisms of fisetin on hepatic insulin resistance (IR) using network pharmacology and in vitro validation.

METHODS

Putative targets of fisetin were retrieved from the Traditional Chinese Medicine Systems Pharmacology database, whereas the potential genes of hepatic IR were obtained from GeneCards database. A protein-protein interaction (PPI) network was constructed according to the intersection targets of fisetin and hepatic IR using the Venn diagram. The biological functions and potential pathways related to genes were determined using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. Cell experiments were also conducted to further verify the mechanism of fisetin on hepatic IR.

RESULTS

A total of 118 potential targets from fisetin were associated with hepatic IR. The areas of nodes and corresponding degree values of TP53, AKT1, TNF, IL6, CASP3, CTNNB1, JUN, SRC, epidermal growth factor receptor (EGFR), and HSP90AA1 were larger and could be easily found in the PPI network. Furthermore, GO analysis revealed that these key targets were significantly involved in multiple biological processes that participated in oxidative stress and serine/threonine kinase activity. KEGG enrichment analysis showed that the PI3K/AKT signaling pathway was a significant pathway involved in hepatic IR. Our in vitro results demonstrated that fisetin treatment increased the expressions of EGFR and IRS in HepG2 and L02 cells under normal or IR conditions. Western blot results revealed that p-AKT/AKT levels were significantly up-regulated, suggesting that fisetin was involved in the PI3K/AKT signaling pathway to regulate insulin signaling.

CONCLUSION

We explored the pharmacological actions and the potential molecular mechanism of fisetin in treating hepatic IR from a holistic perspective. Our study lays a theoretical foundation for the development of fisetin for type 2 diabetes.

Liver insulinization as a driver of triglyceride dysmetabolism

Metabolic dysfunction-associated fatty liver disease (MAFLD) is an increasingly prevalent fellow traveller with the insulin resistance that underlies type 2 diabetes mellitus. However, the mechanistic connection between MAFLD and impaired insulin action remains unclear. In this Perspective, we review data from humans to elucidate insulin's aetiological role in MAFLD. We focus particularly on the relative preservation of insulin's stimulation of triglyceride (TG) biosynthesis despite its waning ability to curb hepatic glucose production (HGP). To explain this apparent 'selective insulin resistance', we propose that hepatocellular processes that lead to TG accumulation require less insulin signal transduction, or 'insulinization,' than do those that regulate HGP. As such, mounting hyperinsulinaemia that barely compensates for aberrant HGP in insulin-resistant states more than suffices to maintain hepatic TG biosynthesis. Thus, even modestly elevated or context-inappropriate insulin levels, when sustained day and night within a heavily pro-lipogenic metabolic milieu, may translate into substantial cumulative TG biosynthesis in the insulin-resistant state.

Dysregulation of Lipid and Glucose Metabolism in Nonalcoholic Fatty Liver Disease

Non-Alcoholic Fatty Liver Disease (NAFLD) is a highly prevalent condition affecting approximately a quarter of the global population. It is associated with increased morbidity, mortality, economic burden, and healthcare costs. The disease is characterized by the accumulation of lipids in the liver, known as steatosis, which can progress to more severe stages such as steatohepatitis, fibrosis, cirrhosis, and even hepatocellular carcinoma (HCC). This review focuses on the mechanisms that contribute to the development of diet-induced steatosis in an insulin-resistant liver. Specifically, it discusses the existing literature on carbon flux through glycolysis, ketogenesis, TCA (Tricarboxylic Acid Cycle), and fatty acid synthesis pathways in NAFLD, as well as the altered canonical insulin signaling and genetic predispositions that lead to the accumulation of diet-induced hepatic fat. Finally, the review discusses the current therapeutic efforts that aim to ameliorate various pathologies associated with NAFLD.

ETNPPL modulates hyperinsulinemia-induced insulin resistance through the SIK1/ROS-mediated inactivation of the PI3K/AKT signaling pathway in hepatocytes

Hyperinsulinemia is a critical risk factor for the pathogenesis of insulin resistance (IR) in metabolic tissues, including the liver. Ethanolamine phosphate phospholyase (ETNPPL), a newly discovered metabolic enzyme that converts phosphoethanolamine (PEA) to ammonia, inorganic phosphate, and acetaldehyde, is abundantly expressed in liver tissue. Whether it plays a role in the regulation of hyperinsulinemia-induced IR in hepatocytes remains elusive. Here, we established an in vitro hyperinsulinemia-induced IR model in the HepG2 human liver cancer cell line and primary mouse hepatocyte via a high dose of insulin treatment. Next, we overexpressed ETNPPL by using lentivirus-mediated ectopic to investigate the effects of ETNPPL per se on IR without insulin stimulation. To explore the underlying mechanism of ETNPPL mediating hyperinsulinemia-induced IR in HepG2, we performed genome-wide transcriptional analysis using RNA sequencing (RNA-seq) to identify the downstream target gene of ETNPPL. The results showed that ETNPPL expression levels in both mRNA and protein were significantly upregulated in hyperinsulinemia-induced IR in HepG2 and primary mouse hepatocytes. Upon silencing ETNPPL, hyperinsulinemia-induced IR was ameliorated. Under normal conditions without IR in hepatocytes, overexpressing ETNPPL promotes IR, reactive oxygen species (ROS) generation, and AKT inactivation. Transcriptome analysis revealed that salt-inducible kinase 1 (SIK1) is markedly downregulated in the ETNPPL knockdown HepG2 cells. Moreover, disrupting SIK1 prevents ETNPPL-induced ROS accumulation, damage to the PI3K/AKT pathway and IR. Our study reveals that ETNPPL mediates hyperinsulinemia-induced IR through the SIK1/ROS-mediated inactivation of the PI3K/AKT signaling pathway in hepatocyte cells. Targeting ETNPPL may present a potential strategy for hyperinsulinemia-associated metabolic disorders such as type 2 diabetes.

Excessive gluconeogenesis causes the hepatic insulin resistance paradox and its sequelae

Background

Hepatic insulin signaling suppresses gluconeogenesis but promotes de novo lipid synthesis. Paradoxically, hepatic insulin resistance (HIR) enhances both gluconeogenesis and de novo lipid synthesis. Elucidation of the etiology of this paradox, which participates in the pathogenesis of non-alcoholic fatty liver disease (NAFLD), cardiovascular disease, the metabolic syndrome and hepatocellular carcinoma, has not been fully achieved.

Scope of review

This article briefly outlines the previously proposed hypotheses on the etiology of the HIR paradox. It then discusses literature consistent with an alternative hypothesis that excessive gluconeogenesis, the direct effect of HIR, is responsible for the aberrant lipogenesis. The mechanisms involved therein are explained, involving de novo synthesis of fructose and uric acid, promotion of glutamine anaplerosis, and induction of glucagon resistance. Thus, gluconeogenesis via lipogenesis promotes hepatic steatosis, a component of NAFLD, and dyslipidemia. Gluconeogenesis-centred mechanisms for the progression of NAFLD from simple steatosis to non-alcoholic steatohepatitis (NASH) and fibrosis are suggested. That NAFLD often precedes and predicts type 2 diabetes is explained by the ability of lipogenesis to cushion against blood glucose dysregulation in the earlier stages of NAFLD.

Major conclusions

HIR-induced excessive gluconeogenesis is a major cause of the HIR paradox and its sequelae. Such involvement of gluconeogenesis in lipid synthesis rationalizes the fact that several types of antidiabetic drugs ameliorate NAFLD. Thus, dietary, lifestyle and pharmacological targeting of HIR and hepatic gluconeogenesis may be a most viable approach for the prevention and management of the HIR-associated network of diseases.

Bile acids-gut microbiota crosstalk contributes to the improvement of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) occurs that cannot effectively use the insulin. Insulin Resistance (IR) is a significant characteristic of T2DM which is also an essential treatment target in blood glucose regulation to prevent T2DM and its complications. Bile acids (BAs) are one group of bioactive metabolites synthesized from cholesterol in liver. BAs play an important role in mutualistic symbiosis between host and gut microbiota. It is shown that T2DM is associated with altered bile acid metabolism which can be regulated by gut microbiota. Simultaneously, BAs also reshape gut microbiota and improve IR and T2DM in the bidirectional communications of the gut-liver axis. This article reviewed the findings on the interaction between BAs and gut microbiota in improving T2DM, which focused on gut microbiota and its debinding function and BAs regulated gut microbiota through FXR/TGR5. Meanwhile, BAs and their derivatives that are effective for improving T2DM and other treatments based on bile acid metabolism were also summarized. This review highlighted that BAs play a critical role in the glucose metabolism and may serve as therapeutic targets in T2DM, providing a reference for discovering and screening novel therapeutic drugs.

From Diabetes Care to Heart Failure Management: A Potential Therapeutic Approach Combining SGLT2 Inhibitors and Plant Extracts

Diabetes is a complex chronic disease, and among the affected patients, cardiovascular disease (CVD)is the most common cause of death. Consequently, the evidence for the cardiovascular benefit of glycaemic control may reduce long-term CVD rates. Over the years, multiple pharmacological approaches aimed at controlling blood glucose levels were unable to significantly reduce diabetes-related cardiovascular events. In this view, a therapeutic strategy combining SGLT2 inhibitors and plant extracts might represent a promising solution. Indeed, countering the main cardiometabolic risk factor using plant extracts could potentiate the cardioprotective action of SGLT2 inhibitors. This review highlights the main molecular mechanisms underlying these beneficial effects that could contribute to the better management of diabetic patients.

Chronic hyperinsulinemia promotes human hepatocyte senescence

OBJECTIVE

Cellular senescence, an irreversible proliferative cell arrest, is caused by excessive intracellular or extracellular stress/damage. Increased senescent cells have been identified in multiple tissues in different metabolic and other aging-related diseases. Recently, several human and mouse studies emphasized the involvement of senescence in development and progression of NAFLD. Hyperinsulinemia, seen in obesity, metabolic syndrome, and other conditions of insulin resistance, has been linked to senescence in adipocytes and neurons. Here, we investigate the possible direct role of chronic hyperinsulinemia in the development of senescence in human hepatocytes.

METHODS

Using fluorescence microscopy, immunoblotting, and gene expression, we tested senescence markers in human hepatocytes subjected to chronic hyperinsulinemia in vitro and validated the data in vivo by using liver-specific insulin receptor knockout (LIRKO) mice. The consequences of hyperinsulinemia were also studied in senescent hepatocytes following doxorubicin as a model of stress-induced senescence. Furthermore, the effects of senolytic agents in insulin- and doxorubicin-treated cells were analyzed.

RESULTS

Results showed that exposing the hepatocytes to prolonged hyperinsulinemia promotes the onset of senescence by increasing the expression of p53 and p21. It also further enhanced the senescent phenotype in already senescent hepatocytes. Addition of insulin signaling pathway inhibitors prevented the increase in cell senescence, supporting the direct contribution of insulin. Furthermore, LIRKO mice, in which insulin signaling in the liver is abolished due to deletion of the insulin receptor gene, showed no differences in senescence compared to their wild-type counterparts despite having marked hyperinsulinemia indicating these are receptor-mediated effects. In contrast, the persistent hyperinsulinemia in LIRKO mice enhanced senescence in white adipose tissue. In vitro, senolytic agents dasatinib and quercetin reduced the prosenescent effects of hyperinsulinemia in hepatocytes.

CONCLUSION

Our findings demonstrate a direct link between chronic hyperinsulinemia and hepatocyte senescence. This effect can be blocked by reducing the levels of insulin receptors or administration of senolytic drugs, such as dasatinib and quercetin.

Association of cord blood asprosin concentration with atherogenic lipid profile and anthropometric indices

BACKGROUND

Elevated lipids in umbilical cord blood affect fetal programming, leading to a higher risk of developing cardiovascular disease in later life. However, the causes of changes in the lipid profile of umbilical cord blood are not clear yet. This study aimed for the first time to determine the association of asprosin concentration with TAG, TC, HDL-C, LDL-C concentrations and TAG/HDL-C, TC/HDL-C, LDL-C/HDL-C and non-HDL-C/HDL-C ratio in umbilical cord blood as well as newborn anthropometric indices. This cross-sectional study was based on 450 mother- newborn pairs of a birth cohort study in Sabzevar, Iran. Multiple linear regression was used to estimate the association of lipid concentration and lipid ratios as well as birth weight (BW), birth length (BL), head circumference (HC) and chest circumference (CC) with asprosin in cord blood samples controlled for the relevant covariates.

RESULT

In fully adjusted models, each 1 ng/mL increase in asprosin was associated with 0.19 (95% CI 0.06, 0.31, P < 0.01), 0.19 (95% CI 0.10, 0.29, P < 0.01), 0.17 (95% CI 0.09, 0.25, P < 0.01), 0.17 (95% CI 0.09, 0.25, P < 0.01), 0.01 (95% CI 0.00, 0.013, P < 0.01), 0.01 (95% CI 0.01, 0.01, P < 0.01), 0.01 (95% CI 0.01, 0.01, P < 0.01) and 0.01 (95% CI 0.01, 0.01, P < 0.01) increase in TAG, TC, LDL-C, TAG/HDL-C, TC/HDL-C, LDL-C/HDL-C and non-HDL-C/HDL-C ratio respectively. Moreover, higher asprosin levels was positively associated with newborn BW, BL, HC and CC; however, these associations were not statistically significant.

CONCLUSION

Overall, our findings support the positive association between cord asprosin concentration and the development of atherogenic lipid profile in newborns. Further studies are needed to confirm the findings of this study in other populations.

The Dose-Response Effects of Consuming High Fructose Corn Syrup-Sweetened Beverages on Hepatic Lipid Content and Insulin Sensitivity in Young Adults

Increased hepatic lipid content and decreased insulin sensitivity have critical roles in the development of cardiometabolic diseases. Therefore, our objective was to investigate the dose-response effects of consuming high fructose corn syrup (HFCS)-sweetened beverages for two weeks on hepatic lipid content and insulin sensitivity in young (18-40 years) adults (BMI 18-35 kg/m2). In a parallel, double-blinded study, participants consumed three beverages/day providing 0% (aspartame: n = 23), 10% (n = 18), 17.5% (n = 16), or 25% (n = 28) daily energy requirements from HFCS. Magnetic resonance imaging for hepatic lipid content and oral glucose tolerance tests (OGTT) were conducted during 3.5-day inpatient visits at baseline and again at the end of a 15-day intervention. During the 12 intervening outpatient days participants consumed their usual diets with their assigned beverages. Significant linear dose-response effects were observed for increases of hepatic lipid content (p = 0.015) and glucose and insulin AUCs during OGTT (both p = 0.0004), and for decreases in the Matsuda (p = 0.0087) and Predicted M (p = 0.0027) indices of insulin sensitivity. These dose-response effects strengthen the mechanistic evidence implicating consumption of HFCS-sweetened beverages as a contributor to the metabolic dysregulation that increases risk for nonalcoholic fatty liver disease and type 2 diabetes.

Porphyromonas gingivalis Induces Increases in Branched-Chain Amino Acid Levels and Exacerbates Liver Injury Through livh/livk

Porphyromonas gingivalis, a keystone periodontal pathogen, has emerged as a risk factor for systemic chronic diseases, including non-alcoholic fatty liver disease (NAFLD). To clarify the mechanism by which this pathogen induces such diseases, we simultaneously analyzed the transcriptome of intracellular P. gingivalis and infected host cells via dual RNA sequencing. Pathway analysis was also performed to determine the differentially expressed genes in the infected cells. Further, the infection-induced notable expression of P. gingivalis livk and livh genes, which participate in branched-chain amino acid (BCAA) transfer, was also analyzed. Furthermore, given that the results of recent studies have associated NAFLD progression with elevated serum BCAA levels, which reportedly, are upregulated by P. gingivalis, we hypothesized that this pathogen may induce increases in serum BCAA levels and exacerbate liver injury via livh/livk. To verify this hypothesis, we constructed P. gingivalis livh/livk-deficient strains (Δlivk, Δlivh) and established a high-fat diet (HFD)-fed murine model infected with P. gingivalis. Thereafter, the kinetic growth and exopolysaccharide (EPS) production rates as well as the invasion efficiency and in vivo colonization of the mutant strains were compared with those of the parental strain. The serum BCAA and fasting glucose levels of the mice infected with either the wild-type or mutant strains, as well as their liver function were also further investigated. It was observed that P. gingivalis infection enhanced serum BCAA levels and aggravated liver injury in the HFD-fed mice. Additionally, livh deletion had no effect on bacterial growth, EPS production, invasion efficiency, and in vivo colonization, whereas the Δlivk strain showed a slight decrease in invasion efficiency and in vivo colonization. More importantly, however, both the Δlivk and Δlivh strains showed impaired ability to upregulate serum BCAA levels or exacerbate liver injury in HFD-fed mice. Overall, these results suggested that P. gingivalis possibly aggravates NAFLD progression in HFD-fed mice by increasing serum BCAA levels, and this effect showed dependency on the bacterial BCAA transport system.

Dysregulation of hepatic metabolism with obesity: factors influencing glucose and lipid metabolism

The liver is a key metabolic organ that undertakes a multitude of physiological processes over the course of a day, including intrahepatic lipid and glucose metabolism which plays a key role in the regulation of systemic lipid and glucose concentrations. It serves as an intermediary organ between exogenous (dietary) and endogenous energy supply to extrahepatic organs. Thus, perturbations in hepatic metabolism can impact widely on metabolic disease risk. For example, the accumulation of intra-hepatocellular TAG (IHTG), for which adiposity is almost invariably a causative factor may result in dysregulation of metabolic pathways. Accumulation of IHTG is likely due to an imbalance between fatty acid delivery, synthesis and removal (via oxidation or export as TAG) from the liver; insulin plays a key role in all of these processes.

An integrative transcriptional logic model of hepatic insulin resistance

Abnormalities of lipid/lipoprotein and glucose metabolism are hallmarks of hepatic insulin resistance in type 2 diabetes. The former antedate the latter, but the latter become progressively refractory to treatment and contribute to therapeutic failures. It's unclear whether the two processes share a common pathogenesis and what underlies their progressive nature. In this study, we investigated the hypothesis that genes in the lipid/lipoprotein pathway and those in the glucose metabolic pathway are governed by different transcriptional regulatory logics that affect their response to physiologic (fasting/refeeding) as well as pathophysiologic cues (insulin resistance and hyperglycemia). To this end, we obtained genomic and transcriptomic maps of the key insulin-regulated transcription factor, FoxO1, and integrated them with those of CREB, PPAR-α, and glucocorticoid receptor. We found that glucose metabolic genes are primarily regulated by promoter and intergenic enhancers in a fasting-dependent manner, while lipid genes are regulated through fasting-dependent intron enhancers and fasting-independent enhancerless introns. Glucose genes also showed a remarkable transcriptional resiliency (i.e., the ability to compensate following constitutive FoxO1 ablation through an enrichment of active marks at shared PPAR-α/FoxO1 regulatory elements). Unexpectedly, insulin resistance and hyperglycemia were associated with a "spreading" of FoxO1 binding to enhancers and the emergence of unique target sites. We surmise that this unusual pattern correlates with the progressively intractable nature of hepatic insulin resistance. This transcriptional logic provides an integrated model to interpret the combined lipid and glucose abnormalities of type 2 diabetes.

Impaired glucocorticoid receptor expression in liver disrupts feeding-induced gene expression, glucose uptake, and glycogen storage

The transition from a fasted to a fed state is associated with extensive transcriptional remodeling in hepatocytes facilitated by hormonal- and nutritional-regulated transcription factors. Here, we use a liver-specific glucocorticoid receptor (GR) knockout (L-GRKO) model to investigate the temporal hepatic expression of GR target genes in response to feeding. Interestingly, in addition to the well-described fasting-regulated genes, we identify a subset of hepatic feeding-induced genes that requires GR for full expression. This includes Gck, which is important for hepatic glucose uptake, utilization, and storage. We show that insulin and glucocorticoids cooperatively regulate hepatic Gck expression in a direct GR-dependent manner by a 4.6 kb upstream GR binding site operating as a Gck enhancer. L-GRKO blunts preprandial and early postprandial Gck expression, which ultimately affects early postprandial hepatic glucose uptake, phosphorylation, and glycogen storage. Thus, GR is positively involved in feeding-induced gene expression and important for postprandial glucose metabolism in the liver.

Insulin at 100 years - is rebalancing its action key to fighting obesity-related disease?

One hundred years ago, insulin was purified and administered to people with diabetes to lower blood glucose, suppress ketogenesis and save lives. A century later, insulin resistance (IR) lies at the heart of the obesity-related disease pandemic. Multiple observations attest that IR syndrome is an amalgamation of gain and loss of insulin action, suggesting that IR is a misnomer. This misapprehension is reinforced by shortcomings in common model systems and is particularly pronounced for the tissue growth disorders associated with IR. It is necessary to move away from conceptualisation of IR as a pure state of impaired insulin action and to appreciate that, in the long term, insulin can harm as well as cure. The mixed state of gain and loss of insulin action, and its relationship to perturbed insulin-like growth factor (IGF) action, should be interrogated more fully in models recapitulating human disease. Only then may the potential of rebalancing insulin action, rather than simply increasing global insulin signalling, finally be appreciated.

Insulin resistance in cardiovascular disease, uremia, and peritoneal dialysis

Diabetic nephropathy is highly correlated with the occurrence of other complications of type 1 diabetes (T1D) and type 2 diabetes (T2D) mellitus; for example, hypertension with cardiovascular disease (CVD) being the most frequent cause of death in patients with end-stage renal disease and undergoing renal dialysis. Hyperglycemia and insulin resistance (IR) are responsible for the micro- and macrovascular complications of diabetes through different mechanisms. In particular, IR plays a key role in the etiology of atherosclerosis in both diabetic and non-diabetic patients. IR - exacerbated by organ-level selectivity - is more important than glycemic control per se in determining cardiovascular outcomes. This may be exacerbated by the fact that IR is organ and pathway specific due to the only selective loss of sensitivity to insulin action of specific pathways/processes. Therefore, it is counterintuitive that the use of peritoneal dialysis (PD) in (frequently) diabetic renal disease patients should involve their exposure to high daily doses of glucose peritoneally. In view of the controversy about the causal association between glucose load and CVD in PD patients, we discuss the role that selective IR may play in the progression of CVD in diabetic renal end-stage patients. In discussing these associations, we propose that reducing glucose exposure in PD solutions may be beneficial especially if coupled with strategies that address IR directly, and the avoidance of excessive use of insulin treatment in T2D.

Role of 2‑series prostaglandins in the pathogenesis of type 2 diabetes mellitus and non‑alcoholic fatty liver disease (Review)

Nowadays, metabolic syndromes are emerging as global epidemics, whose incidence are increasing annually. However, the efficacy of therapy does not increase proportionately with the increased morbidity. Type 2 diabetes mellitus (T2DM) and non-alcoholic fatty liver disease (NAFLD) are two common metabolic syndromes that are closely associated. The pathogenic mechanisms of T2DM and NAFLD have been studied, and it was revealed that insulin resistance, hyperglycemia, hepatic lipid accumulation and inflammation markedly contribute to the development of these two diseases. The 2-series prostaglandins (PGs), a subgroup of eicosanoids, including PGD₂, PGE₂, PGF_2α and PGI₂, are converted from arachidonic acid catalyzed by the rate-limiting enzymes cyclooxygenases (COXs). Considering their wide distribution in almost every tissue, 2-series PG pathways exert complex and interlinked effects in mediating pancreatic β-cell function and proliferation, insulin sensitivity, fat accumulation and lipolysis, as well as inflammatory processes. Previous studies have revealed that metabolic disturbances, such as hyperglycemia and hyperlipidemia, can be improved by treatment with COX inhibitors. At present, an accumulating number of studies have focused on the roles of 2-series PGs and their metabolites in the pathogenesis of metabolic syndromes, particularly T2DM and NAFLD. In the present review, the role of 2-series PGs in the highly intertwined pathogenic mechanisms of T2DM and NAFLD was discussed, and important therapeutic strategies based on targeting 2-series PG pathways in T2DM and NAFLD treatment were provided.

Physiologically inspired culture medium prolongs the lifetime and insulin sensitivity of human hepatocytes in micropatterned co-cultures

Given significant species-specific differences in liver functions, cultures of primary human hepatocytes (PHHs) are useful for assessing drug metabolism and to mitigate the risk of drug-induced hepatotoxicity in humans. While significant advances have been made to keep PHHs highly functional for 2-4 weeks in vitro, especially upon co-culture with both liver- and non-liver-derived non-parenchymal cells (NPCs), the functional lifespan of PHHs is 200-400 days in vivo. Therefore, it is desirable to determine culture conditions that can further prolong PHHs functions in vitro for modeling chronic drug exposure, disease pathogenesis, and to provide flexibility to the end-user for staggering drug incubations across multiple culture batches. Most PHH culture platforms utilize supraphysiologic levels of glucose and insulin and bovine-derived serum when including NPCs, which can alter PHH functions. Therefore, here we developed a culture medium containing physiologic levels of glucose (5 mM), insulin (500 pM), and human serum (10 % v/v) and tested its effects on micropatterned co-cultures (MPCCs) in which PHHs are organized onto collagen domains of empirically optimized dimensions and surrounded by 3T3-J2 murine fibroblasts that express liver-like molecules and induce higher PHH functions than liver-derived NPCs. Our physiologically-inspired culture medium allowed better retention of PHH morphology, polarity, and functions (albumin and urea, cytochrome-P450 activities, and sensitivity to insulin-mediated inhibition of gluconeogenesis) for up to 10 weeks relative to the traditional medium. Finally, PHHs in the physiologic medium displayed clinically-relevant responses to prototypical drugs for hepatoxicity and cytochrome-P450 induction. Ultimately, our physiologic culture medium could find broader utility for the continued development of PHH-NPC co-cultures for drug development, investigating the effects of patient-derived sera on PHH functions and disease phenotypes, and for use in cell-based therapies.

Type 2 diabetes - unmet need, unresolved pathogenesis, mTORC1-centric paradigm

The current paradigm of type 2 diabetes (T2D) is gluco-centric, being exclusively categorized by glycemic characteristics. The gluco-centric paradigm views hyperglycemia as the primary target, being driven by resistance to insulin combined with progressive beta cells failure, and considers glycemic control its ultimate treatment goal. Most importantly, the gluco-centric paradigm considers the non-glycemic diseases associated with T2D, e.g., obesity, dyslipidemia, hypertension, macrovascular disease, microvascular disease and fatty liver as 'risk factors' and/or 'outcomes' and/or 'comorbidities', rather than primary inherent disease aspects of T2D. That is in spite of their high prevalence (60-90%) and major role in profiling T2D morbidity and mortality. Moreover, the gluco-centric paradigm fails to realize that the non-glycemic diseases of T2D are driven by insulin and, except for glycemic control, response to insulin in T2D is essentially the rule rather than the exception. Failure of the gluco-centric paradigm to offer an exhaustive unifying view of the glycemic and non-glycemic diseases of T2D may have contributed to T2D being still an unmet need. An mTORC1-centric paradigm maintains that hyperactive mTORC1 drives the glycemic and non-glycemic disease aspects of T2D. Hyperactive mTORC1 is proposed to act as double-edged agent, namely, to interfere with glycemic control by disrupting the insulin receptor-Akt transduction pathway, while concomitantly driving the non-glycemic diseases of T2D. The mTORC1-centric paradigm may offer a novel perspective for T2D in terms of pathogenesis, clinical focus and treatment strategy.

Regulation of basal expression of hepatic PEPCK and G6Pase by AKT2

Hepatic glucose metabolism signaling downstream of insulin can diverge to multiple pathways including AKT. Genetic studies suggest that AKT is necessary for insulin to suppress gluconeogenesis. To specifically address the role of AKT2, the dominant liver isoform of AKT in the regulation of gluconeogenesis genes, we generated hepatocytes lacking AKT2 (Akt2-/-). We found that, in the absence of insulin signal, AKT2 is required for maintaining the basal level expression of phosphoenolpyruvate carboxyl kinase (PEPCK) and to a lesser extent G6Pase, two key rate-limiting enzymes for gluconeogenesis that support glucose excursion due to pyruvate loading. We further showed that this function of AKT2 is mediated by the phosphorylation of cyclic AMP response element binding (CREB). Phosphorylation of CREB by AKT2 is needed for CREB to induce the expression of PEPCK and likely represents a priming event for unstimulated cells to poise to receive glucagon and other signals. The inhibition of gluconeogenesis by insulin is also dependent on the reduced FOXO1 transcriptional activity at the promoter of PEPCK. When insulin signal is absent, this activity appears to be inhibited by AKT2 in manner that is independent of its phosphorylation by AKT. Together, this action of AKT2 on FOXO1 and CREB to maintain basal gluconeogenesis activity may provide fine-tuning for insulin and glucocorticoid/glucagon to regulate gluconeogenesis in a timely manner to meet metabolic needs.

Loss of Hepatic Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 Links Nonalcoholic Steatohepatitis to Atherosclerosis

Patients with nonalcoholic fatty liver disease/steatohepatitis (NAFLD/NASH) commonly develop atherosclerosis through a mechanism that is not well delineated. These diseases are associated with steatosis, inflammation, oxidative stress, and fibrosis. The role of insulin resistance in their pathogenesis remains controversial. Albumin (Alb)Cre⁺Cc1^flox(fl)/fl mice with the liver‐specific null deletion of the carcinoembryonic antigen‐related cell adhesion molecule 1 (Ceacam1; alias Cc1) gene display hyperinsulinemia resulting from impaired insulin clearance followed by hepatic insulin resistance, elevated de novo lipogenesis, and ultimately visceral obesity and systemic insulin resistance. We therefore tested whether this mutation causes NAFLD/NASH and atherosclerosis. To this end, mice were propagated on a low‐density lipoprotein receptor (Ldlr)^−/− background and at 4 months of age were fed a high‐cholesterol diet for 2 months. We then assessed the biochemical and histopathologic changes in liver and aortae. Ldlr^−/−AlbCre⁺Cc1^fl/fl mice developed chronic hyperinsulinemia with proatherogenic hypercholesterolemia, a robust proinflammatory state associated with visceral obesity, elevated oxidative stress (reduced NO production), and an increase in plasma and tissue endothelin‐1 levels. In parallel, they developed NASH (steatohepatitis, apoptosis, and fibrosis) and atherosclerotic plaque lesions. Mechanistically, hyperinsulinemia caused down‐regulation of the insulin receptor followed by inactivation of the insulin receptor substrate 1–protein kinase B–endothelial NO synthase pathway in aortae, lowering the NO level. This also limited CEACAM1 phosphorylation and its sequestration of Shc‐transforming protein (Shc), activating the Shc–mitogen‐activated protein kinase–nuclear factor kappa B pathway and stimulating endothelin‐1 production. Thus, in the presence of proatherogenic dyslipidemia, hyperinsulinemia and hepatic insulin resistance driven by liver‐specific deletion of Ceacam1 caused metabolic and vascular alterations reminiscent of NASH and atherosclerosis. Conclusion: Altered CEACAM1‐dependent hepatic insulin clearance pathways constitute a molecular link between NASH and atherosclerosis.

Diabetic Cardiomyopathy Modelling Using Induced Pluripotent Stem Cell Derived Cardiomyocytes: Recent Advances and Emerging Models

The global burden of diabetes has drastically increased over the past decades and in 2017 approximately 4 million deaths were caused by diabetes and cardiovascular complications. Diabetic cardiomyopathy is a common complication of diabetes with early manifestations of diastolic dysfunction and left ventricular hypertrophy with subsequent progression to systolic dysfunction and ultimately heart failure. An in vitro model accurately recapitulating key processes of diabetic cardiomyopathy would provide a useful tool for investigations of underlying disease mechanisms to further our understanding of the disease and thereby potentially advance treatment strategies for patients. With their proliferative capacity and differentiation potential, human induced pluripotent stem cells (iPSCs) represent an appealing cell source for such a model system and cardiomyocytes derived from induced pluripotent stem cells have been used to establish other cardiovascular related disease models. Here we review recently made advances and discuss challenges still to be overcome with regard to diabetic cardiomyopathy models, with a special focus on iPSC-based systems. Recent publications as well as preliminary data presented here demonstrate the feasibility of generating cardiomyocytes with a diabetic phenotype, displaying insulin resistance, impaired calcium handling and hypertrophy. However, capturing the full metabolic- and functional phenotype of the diabetic cardiomyocyte remains to be accomplished.

Carob fruit extract-enriched meat improves pancreatic beta-cell dysfunction, hepatic insulin signaling and lipogenesis in late-stage type 2 diabetes mellitus model

The inclusion of functional bioactive compounds of dietary fiber in meat products has been demonstrated to exert a significant impact on human health. Carob fruit extract (CFE) is a dietary fiber rich in proanthocyanidins with known antioxidant, hypolipidemic and hypoglycemic effects. Consumption of CFE-enriched meat (CFE-RM) may provide interesting benefits in late-stage type 2 diabetes mellitus (T2DM). To explore the antidiabetic mechanisms of CFE-RM, we used a model of late-stage T2DM in Wistar rats fed a high-saturated-fat/high-cholesterol diet (Chol-diet) and injected streptozotocin plus nicotinamide (D group). The effects of CFE-RM were tested by incorporating it into the diet as preventive strategy (ED group) or curative treatment (DE group). CFE-RM had a positive effect on glycemia, enhancing hepatic insulin sensitivity and improving pancreatic β-cell regeneration in both ED and DE groups. Western blotting and immunohistochemistry suggested that CFE-RM increased levels of insulin receptor β and phosphatidylinositol-3-kinase, as well as the downstream target phospho-Akt (at Ser⁴⁷³). CFE-RM also up-regulated glucose transporter 2, which improves the insulin-mediated glucose uptake by the liver, and promoted phosphorylation of glycogen synthesis kinase-3βprotein (at ser⁹), consequently increasing the hepatic glycogen content. In addition, CFE-RM decreased fatty liver by suppressing de novo lipogenesis activation due to down-regulation of liver X receptor-α/β, sterol regulatory element binding protein-1c and carbohydrate-response element-binding protein transcription factors. Our findings suggest that the consumption of CFE-RM included in the diet as a functional food should be considered as a suitable nutritional strategy to prevent or manage late-stage T2DM.

Branched-Chain Amino Acids Exacerbate Obesity-Related Hepatic Glucose and Lipid Metabolic Disorders via Attenuating Akt2 Signaling

Branched chain amino acids (BCAAs) are associated with the progression of obesity-related metabolic disorders, including type 2 diabetes and nonalcoholic fatty liver disease. However, whether BCAAs disrupt the homeostasis of hepatic glucose and lipid metabolism remains unknown. In this study, we observed that BCAAs supplementation significantly reduced high-fat (HF) diet-induced hepatic lipid accumulation while increasing the plasma lipid levels and promoting muscular and renal lipid accumulation. Further studies demonstrated that BCAAs supplementation significantly increased hepatic gluconeogenesis and suppressed hepatic lipogenesis in HF diet-induced obese (DIO) mice. These phenotypes resulted from severe attenuation of Akt2 signaling via mTORC1- and mTORC2-dependent pathways. BCAAs/branched-chain α-keto acids (BCKAs) chronically suppressed Akt2 activation through mTORC1 and mTORC2 signaling and promoted Akt2 ubiquitin-proteasome-dependent degradation through the mTORC2 pathway. Moreover, the E3 ligase Mul1 played an essential role in BCAAs/BCKAs-mTORC2-induced Akt2 ubiquitin-dependent degradation. We also demonstrated that BCAAs inhibited hepatic lipogenesis by blocking Akt2/SREBP1/INSIG2a signaling and increased hepatic glycogenesis by regulating Akt2/Foxo1 signaling. Collectively, these data demonstrate that in DIO mice, BCAAs supplementation resulted in serious hepatic metabolic disorder and severe liver insulin resistance: insulin failed to not only suppress gluconeogenesis but also activate lipogenesis. Intervening BCAA metabolism is a potential therapeutic target for severe insulin-resistant disease.

Amelioration of High-Insulin-Induced Skeletal Muscle Cell Insulin Resistance by Resveratrol Is Linked to Activation of AMPK and Restoration of GLUT4 Translocation

Insulin resistance, the hallmark of type 2 diabetes mellitus (T2DM), is linked to hyperinsulinemia, which develops to counterbalance initial peripheral hormone resistance. Studies indicate that chronically elevated levels of insulin lead to skeletal muscle insulin resistance by deregulating steps within the insulin signaling cascade. The polyphenol resveratrol (RSV) has been shown to have antidiabetic properties in vitro and in vivo. In the present study, we examined the effect of RSV on high insulin (HI)-induced insulin resistance in skeletal muscle cells in vitro and investigated the mechanisms involved. Parental and GLUT4myc-overexpressing L6 rat skeletal muscle cells were used. [3H]2-deoxyglucose (2DG) uptake was measured, and total and phosphorylated levels of specific proteins were examined by immunoblotting. Exposure of L6 cells to HI levels (100 nM) for 24 h decreased the acute-insulin-stimulated 2DG uptake, indicating insulin resistance. HI increased ser307 and ser636/639 phosphorylation of IRS-1 (to 184% ± 12% and 225% ± 28.9% of control, with p < 0.001 and p < 0.01, respectively) and increased the phosphorylation levels of mTOR (174% ± 6.7% of control, p < 0.01) and p70 S6K (228% ± 33.5% of control, p < 0.01). Treatment with RSV abolished these HI-induced responses. Furthermore, RSV increased the activation of AMPK and restored the insulin-mediated increase in plasma membrane GLUT4 glucose transporter levels. These data suggest that RSV has a potential to counteract the HI-induced muscle insulin resistance.

Metabolic Health, Insulin, and Breast Cancer: Why Oncologists Should Care About Insulin

Studies investigating the potential link between adult pre-menopausal obesity [as measured by body mass index (BMI)] and triple-negative breast cancer have been inconsistent. Recent studies show that BMI is not an exact measure of metabolic health; individuals can be obese (BMI > 30 kg/m²) and metabolically healthy or lean (BMI < 25 kg/m²) and metabolically unhealthy. Consequently, there is a need to better understand the molecular signaling pathways that might be activated in individuals that are metabolically unhealthy and how these signaling pathways may drive biologically aggressive breast cancer. One key driver of both type-2 diabetes and cancer is insulin. Insulin is a potent hormone that activates many pathways that drive aggressive breast cancer biology. Here, we review (1) the controversial relationship between obesity and breast cancer, (2) the impact of insulin on organs, subcellular components, and cancer processes, (3) the potential link between insulin-signaling and cancer, and (4) consider time points during breast cancer prevention and treatment where insulin-signaling could be better controlled, with the ultimate goal of improving overall health, optimizing breast cancer prevention, and improving breast cancer survival.

mir-101-3p Downregulation Promotes Fibrogenesis by Facilitating Hepatic Stellate Cell Transdifferentiation During Insulin Resistance

Insulin resistance (IR) and microRNAs (miRNAs), which regulate cell-to-cell communication between hepatocytes and hepatic stellate cells (HSCs), may intertwine in nonalcoholic fatty liver disease (NAFLD) pathogenesis. The aim of this study was to evaluate whether epigenetics and environmental factors interact to promote progressive NAFLD during IR. We examined the miRNA signature in insulin receptor haploinsufficient (InsR+/−) and wild-type (wt) HSCs by RNAseq (n = 4 per group). Then, we evaluated their impact in an IR-NASH (nonalcoholic steatohepatitis) model (InsR+/− mice fed standard or methionine choline deficient (MCD) diet, n = 10 per group) and in vitro. InsR+/− HSCs displayed 36 differentially expressed miRNAs (p < 0.05 vs. wt), whose expression was then analyzed in the liver of InsR+/− mice fed an MCD diet. We found that miR-101-3p negatively associated with both InsR+/− genotype and MCD (p < 0.05) and the histological spectrum of liver damage (p < 0.01). miR-101-3p was reduced in InsR+/− hepatocytes and HSCs and even more in InsR+/− cells exposed to insulin (0.33 µM) and fatty acids (0.25 mM), resembling the IR-NASH model. Conversely, insulin induced miR-101-3p expression in wt cells but not in InsR+/− ones (p < 0.05). In conclusion, IR combined with diet-induced liver injury favors miR-101-3p downregulation, which may promote progressive NAFLD through HSC and hepatocyte transdifferentiation and proliferation.

From "two hit theory" to "multiple hit theory": Implications of evolution of pathogenesis concepts for treatment of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is becoming a burgeoning and burdensome public health problem worldwide, along with diabetes and metabolic syndrome. In the NAFLD spectrum, non-alcoholic steatohepatitis can progress to hepatic fibrosis, especially progressive fibrosis, which can lead to cirrhosis or even hepatocellular carcinoma. However, the pathogenesis of NAFLD is extremely complex and has not yet been fully elucidated, thus there is a lack of effective treatment. In recent years, the classic "two-hit" hypothesis has been gradually surpassed and supplemented by a great deal of findings, and the "multiple hit" hypothesis has been proposed and is being accepted. The study on the interaction among cellular and molecular mechanisms, environmental and genetic factors has revealed a number of critical targets in the pathogenesis of NAFLD, providing broad directions for the development of diagnostic markers and targeted therapeutic drugs. Here we elaborate the latest advances in understanding the pathogenesis of NAFLD from multiple perspectives, in order to analyze and evaluate the prospect of developing diagnostic biomarkers and therapeutic targets based on those pathogeneses.

Hepatic Insulin Clearance: Mechanism and Physiology

Upon its secretion from pancreatic β-cells, insulin reaches the liver through the portal circulation to exert its action and eventually undergo clearance in the hepatocytes. In addition to insulin secretion, hepatic insulin clearance regulates the homeostatic level of insulin that is required to reach peripheral insulin target tissues to elicit proper insulin action. Receptor-mediated insulin uptake followed by its degradation constitutes the basic mechanism of insulin clearance. Upon its phosphorylation by the insulin receptor tyrosine kinase, carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) takes part in the insulin-insulin receptor complex to increase the rate of its endocytosis and targeting to the degradation pathways. This review summarizes how this process is regulated and how it is associated with insulin-degrading enzyme in the liver. It also discusses the physiological implications of impaired hepatic insulin clearance: Whereas reduced insulin clearance cooperates with increased insulin secretion to compensate for insulin resistance, it can also cause hepatic insulin resistance. Because chronic hyperinsulinemia stimulates hepatic de novo lipogenesis, impaired insulin clearance also causes hepatic steatosis. Thus impaired insulin clearance can underlie the link between hepatic insulin resistance and hepatic steatosis. Delineating these regulatory pathways should lead to building more effective therapeutic strategies against metabolic syndrome.

Honey extracts inhibit PTP1B, upregulate insulin receptor expression, and enhance glucose uptake in human HepG2 cells

Honey is a food known for its medical properties. In this work, we have studied the impact of different types of honey on insulin signalling pathway. We found that honey extracts inhibit the enzyme PTP1B, one of the main negative regulators of insulin receptor signalling. HPLC-MS analysis allowed us to confirm the presence of several natural PTP1B inhibitors in the honey extracts analysed. Statistical analysis methods show a correlation between specific ¹H-NMR resonance frequencies/HPLC peaks and the inhibitory power of the samples. This finding will allow the prediction of the biological properties of honey samples applying relative simple analytical methods. Finally, we demonstrated that the treatment of HepG2 cells with honey extracts enhances the expression of insulin receptor, and stimulates glucose uptake. For the first time, our results demonstrate that bioactive components of honey could improve glycaemic control by both inhibiting PTP1B and stimulating the expression of insulin receptor in liver cells.

Liver-specific insulin receptor isoform A expression enhances hepatic glucose uptake and ameliorates liver steatosis in a mouse model of diet-induced obesity

Among the main complications associated with obesity are insulin resistance and altered glucose and lipid metabolism within the liver. It has previously been described that insulin receptor isoform A (IRA) favors glucose uptake and glycogen storage in hepatocytes compared with isoform B (IRB), improving glucose homeostasis in mice lacking liver insulin receptor. Thus, we hypothesized that IRA could also improve glucose and lipid metabolism in a mouse model of high-fat-diet-induced obesity. We addressed the role of insulin receptor isoforms in glucose and lipid metabolism in vivo. We expressed IRA or IRB specifically in the liver by using adeno-associated viruses (AAVs) in a mouse model of diet-induced insulin resistance and obesity. IRA, but not IRB, expression induced increased glucose uptake in the liver and muscle, improving insulin tolerance. Regarding lipid metabolism, we found that AAV-mediated IRA expression also ameliorated hepatic steatosis by decreasing the expression of Fasn, Pgc1a, Acaca and Dgat2 and increasing Scd-1 expression. Taken together, our results further unravel the role of insulin receptor isoforms in hepatic glucose and lipid metabolism in an insulin-resistant scenario. Our data strongly suggest that IRA is more efficient than IRB at favoring hepatic glucose uptake, improving insulin tolerance and ameliorating hepatic steatosis. Therefore, we conclude that a gene therapy approach for hepatic IRA expression could be a safe and promising tool for the regulation of hepatic glucose consumption and lipid metabolism, two key processes in the development of non-alcoholic fatty liver disease associated with obesity.

This article has an associated First Person interview with the first author of the paper.

Summary: Adeno-associated-virus-mediated gene therapy for insulin receptor isoform A expression in the liver improves glucose disposal and alleviates lipid accumulation in wild-type mice under a high-fat diet.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

microRNA-205-5p is a modulator of insulin sensitivity that inhibits FOXO function

Objectives

Hepatic insulin resistance is a hallmark of type 2 diabetes and obesity. Insulin receptor signaling through AKT and FOXO has important metabolic effects that have traditionally been ascribed to regulation of gene expression. However, whether all the metabolic effects of FOXO arise from its regulation of protein-encoding mRNAs is unknown.

Methods

To address this question, we obtained expression profiles of FOXO-regulated murine hepatic microRNAs (miRNAs) during fasting and refeeding using mice lacking Foxo1, 3a, and 4 in liver (L-Foxo1,3a, 4).

Results

Out of 439 miRNA analyzed, 175 were differentially expressed in Foxo knockouts. Their functions were associated with insulin, Wnt, Mapk signaling, and aging. Among them, we report a striking increase of miR-205-5p expression in L-Foxo1,3a,4 knockouts, as well as in obese mice. We show that miR-205-5p gain-of-function increases AKT phosphorylation and decreases SHIP2 in primary hepatocytes, resulting in FOXO inhibition. This results in decreased hepatocyte glucose production. Consistent with these observations, miR-205-5p gain-of-function in mice lowered glucose levels and improved pyruvate tolerance.

Conclusions

These findings reveal a homeostatic miRNA loop regulating insulin signaling, with potential implications for in vivo glucose metabolism.

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A comprehensive analysis of Foxo-dependent miRNA.

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miRNAs recapitulate the transcriptional effects of Foxo on insulin signaling.

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Foxo regulates miRNA transcription during the fasting/refeeding transition.

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miR205 regulates insulin sensitivity through a homeostatic loop with Foxo.

Increased intracellular iron in mouse primary hepatocytes in vitro causes activation of the Akt pathway but decreases its response to insulin

BACKGROUND

An iron-overloaded state has been reported to be associated with insulin resistance. On the other hand, conditions such as classical hemochromatosis (where iron overload occurs primarily in the liver) have been reported to be associated with increased insulin sensitivity. The reasons for these contradictory findings are unclear. In this context, the effects of increased intracellular iron per se on insulin signaling in hepatocytes are not known.

METHODS

Mouse primary hepatocytes were loaded with iron in vitro by incubation with ferric ammonium citrate (FAC). Intracellular events related to insulin signaling, as well as changes in gene expression and hepatocyte glucose production (HGP), were studied in the presence and absence of insulin and/or forskolin (a glucagon mimetic).

RESULTS

In vitro iron-loading of hepatocytes resulted in phosphorylation-mediated activation of Akt and AMP-activated protein kinase. This was associated with decreased basal and forskolin-stimulated HGP. Iron attenuated forskolin-mediated induction of the key gluconeogenic enzyme, glucose-6-phosphatase. It also attenuated activation of the Akt pathway in response to insulin, which was associated with decreased protein levels of insulin receptor substrates 1 and 2, constituting insulin resistance.

CONCLUSIONS

Increased intracellular iron has dual effects on insulin sensitivity in hepatocytes. It increased basal activation of the Akt pathway, but decreased activation of this pathway in response to insulin.

GENERAL SIGNIFICANCE

These findings may help explain why both insulin resistance and increased sensitivity have been observed in iron-overloaded states. They are of relevance to a variety of disease conditions characterized by hepatic iron overload and increased risk of diabetes.

Liver-specific rescuing of CEACAM1 reverses endothelial and cardiovascular abnormalities in male mice with null deletion of Ceacam1 gene

Objective

Mice with global null mutation of Ceacam1 (Cc1^−/−), display impairment of insulin clearance that causes hyperinsulinemia followed by insulin resistance, elevated hepatic de novo lipogenesis, and visceral obesity. In addition, they manifest abnormal vascular permeability and elevated blood pressure. Liver-specific rescuing of Ceacam1 reversed all of the metabolic abnormalities in Cc1^{−/−liver+} mice. The current study examined whether Cc1^−/− male mice develop endothelial and cardiac dysfunction and whether this relates to the metabolic abnormalities caused by defective insulin extraction.

Methods and results

Myography studies showed reduction of agonist-stimulated nitric oxide production in resistance arterioles in Cc1^−/−, but not Cc1^{−/−liver+} mice. Liver-based rescuing of CEACAM1 also attenuated the abnormal endothelial adhesiveness to circulating leukocytes in parallel to reducing plasma endothelin-1 and recovering plasma nitric oxide levels. Echocardiography studies revealed increased septal wall thickness, cardiac hypertrophy and reduced cardiac performance in Cc1^−/−, but not Cc1^{−/−xliver+} mice. Insulin signaling experiments indicated compromised IRS1/Akt/eNOS pathway leading to lower nitric oxide level, and activated Shc/MAPK pathway leading to more endothelin-1 production in the aortae and hearts of Cc1^−/−, but not Cc1^{−/−xliver+} mice. The increase in the ratio of endothelin-1 receptor A/B indicated an imbalance in the vasomotor activity of Cc1^−/− mice, which was normalized in Cc1^{−/−xliver+} mice.

Conclusions

The data underscore a critical role for impaired CEACAM1-dependent hepatic insulin clearance pathways and resulting hyperinsulinemia and lipid accumulation in aortae and heart in regulating the cardiovascular function.

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Mice with global deletion of Ceacam1 gene (Cc1^−/−) manifest endothelial dysfunction which is reversed by liver-specific rescuing of CEACAM1.

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Restoring CEACAM1 expression in the liver reversed cardiac hypertrophy and rescued cardiac performance.

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Hyperinsulinemia emerging from impaired insulin clearance regulates endothelial and cardiovascular functions.

Specific plasma amino acid disturbances associated with metabolic syndrome

PURPOSE

The primary objective of the present study was to examine the association between branched chain and aromatic amino acid profiles (BCAA and AAA respectively) and the metabolic syndrome (MS), and to evaluate the clinical utility of these associations in the diagnostic process.

METHODS

Two hundred and sixty three healthy men with MS [MS(+): n = 165] and without MS [MS(-): n = 98] were enrolled in the observational study. Anthropometrical, biochemical, and amino acid measurements were performed. The ability of the BCAA and AAA to discriminate subjects with MS and insulin resistance was tested. Based on logistic discrimination, a multivariate early MS diagnostic model was built, and its discrimination properties were evaluated.

RESULTS

Two functionally independent amino acid clusters were identified. BCAA and phenylalanine differed significantly between MS(+) and MS(-) participants (P = 0.003). These factors were also found to be indicators of MS(+) individuals (AUC: 0.66; 95% CI: 0.5757-0.7469), and correlated with cardiometabolic factors. No statistically significant differences in amino acid concentrations between those with and without insulin resistance were noted, and none of the amino groups were indicators of insulin resistance. The proposed MS multivariate diagnostic model consisted of phenylalanine, insulin, leptin, and adiponectin, and had good discrimination properties [AUC 0.79; 95% CI: 0.7239-0.8646].

CONCLUSIONS

MS is associated with selective BCAA and AAA profile disturbances, which could be part of cardiometabolic disease pathogenesis and derive neither directly from insulin sensitivity impairment, nor obesity or muscle mass. The MS diagnostic model developed and described herein should be validated in future studies.

Liver-specific reconstitution of CEACAM1 reverses the metabolic abnormalities caused by its global deletion in male mice

AIMS/HYPOTHESIS

The carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) promotes insulin clearance. Mice with global null mutation (Cc1 -/-) or with liver-specific inactivation (L-SACC1) of Cc1 (also known as Ceacam1) gene display hyperinsulinaemia resulting from impaired insulin clearance, insulin resistance, steatohepatitis and obesity. Because increased lipolysis contributes to the metabolic phenotype caused by transgenic inactivation of CEACAM1 in the liver, we aimed to further investigate the primary role of hepatic CEACAM1-dependent insulin clearance in insulin and lipid homeostasis. To this end, we examined whether transgenic reconstitution of CEACAM1 in the liver of global Cc1 -/- mutant mice reverses their abnormal metabolic phenotype.

METHODS

Insulin response was assessed by hyperinsulinaemic-euglycaemic clamp analysis and energy balance was analysed by indirect calorimetry. Mice were overnight-fasted and refed for 7 h to assess fatty acid synthase activity in the liver and the hypothalamus in response to insulin release during refeeding.

RESULTS

Liver-based rescuing of CEACAM1 restored insulin clearance, plasma insulin level, insulin sensitivity and steatohepatitis caused by global deletion of Cc1. It also reversed the gain in body weight and total fat mass observed with Cc1 deletion, in parallel to normalising energy balance. Mechanistically, reversal of hyperphagia appeared to result from reducing fatty acid synthase activity and restoring insulin signalling in the hypothalamus.

CONCLUSIONS/INTERPRETATION

Despite the potential confounding effects of deleting Cc1 from extrahepatic tissues, liver-based rescuing of CEACAM1 resulted in full normalisation of the metabolic phenotype, underscoring the key role that CEACAM1-dependent hepatic insulin clearance pathways play in regulating systemic insulin sensitivity, lipid homeostasis and energy balance.

Selective Inhibition of FOXO1 Activator/Repressor Balance Modulates Hepatic Glucose Handling

Insulin resistance is a hallmark of diabetes and an unmet clinical need. Insulin inhibits hepatic glucose production and promotes lipogenesis by suppressing FOXO1-dependent activation of G6pase and inhibition of glucokinase, respectively. The tight coupling of these events poses a dual conundrum: mechanistically, as the FOXO1 corepressor of glucokinase is unknown, and clinically, as inhibition of glucose production is predicted to increase lipogenesis. Here, we report that SIN3A is the insulin-sensitive FOXO1 corepressor of glucokinase. Genetic ablation of SIN3A abolishes nutrient regulation of glucokinase without affecting other FOXO1 target genes and lowers glycemia without concurrent steatosis. To extend this work, we executed a small-molecule screen and discovered selective inhibitors of FOXO-dependent glucose production devoid of lipogenic activity in hepatocytes. In addition to identifying a novel mode of insulin action, these data raise the possibility of developing selective modulators of unliganded transcription factors to dial out adverse effects of insulin sensitizers.

Indirect Regulation of Endogenous Glucose Production by Insulin: The Single Gateway Hypothesis Revisited

On the basis of studies that investigated the intraportal versus systemic insulin infusion and transendothelial transport of insulin, we proposed the "single gateway hypothesis," which supposes an indirect regulation of hepatic glucose production by insulin; the rate-limiting transport of insulin across the adipose tissue capillaries is responsible for the slow suppression of free fatty acids (FFAs), which in turn is responsible for delayed suppression of hepatic endogenous glucose production (EGP) during insulin infusion. Preventing the fall in plasma FFAs during insulin infusion either by administering intralipids or by inhibiting adipose tissue lipolysis led to failure in EGP suppression, thus supporting our hypothesis. More recently, mice lacking hepatic Foxo1 in addition to Akt1 and Akt2 (L-AktFoxo1TKO), all required for insulin signaling, surprisingly showed normal glycemia. Inhibiting the fall of plasma FFAs in these mice prevented the suppression of EGP during a clamp, reaffirming that the site of insulin action to control EGP is extrahepatic. Measuring whole-body turnover rates of glucose and FFAs in L-AktFoxo1TKO mice also confirmed that hepatic EGP was regulated by insulin-mediated control of FFAs. The knockout mouse model in combination with sophisticated molecular techniques confirmed our physiological findings and the single gateway hypothesis.

New insight into inter-organ crosstalk contributing to the pathogenesis of non-alcoholic fatty liver disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver dysfunction and a significant global health problem with substantial rise in prevalence over the last decades. It is becoming increasingly clear that NALFD is not only predominantly a hepatic manifestation of metabolic syndrome, but also involves extra-hepatic organs and regulatory pathways. Therapeutic options are limited for the treatment of NAFLD. Accordingly, a better understanding of the pathogenesis of NAFLD is critical for gaining new insight into the regulatory network of NAFLD and for identifying new targets for the prevention and treatment of NAFLD. In this review, we emphasize on the current understanding of the inter-organ crosstalk between the liver and peripheral organs that contributing to the pathogenesis of NAFLD.

Insulin resistance promotes Lysyl Oxidase Like 2 induction and fibrosis accumulation in non-alcoholic fatty liver disease

In patients with non-alcoholic fatty liver disease (NAFLD), insulin resistance (IR) associates with fibrosis progression independently of the hepatic inflammation, but the mechanisms are still unclear. We modeled the independent contribution of inflammation (non-alcoholic steatohepatitis: NASH) by exploiting the methionine-choline deficient (MCD) diet, and that of IR by insulin receptor (InsR) haploinsufficiency (InsR+/-) in the pathogenesis of liver fibrosis in C57BL/6 mice. We confirmed the study findings in 96 patients with NAFLD. InsR+/- enhanced hepatic fat content and impaired hepatic insulin signaling leading to Forkhead box protein O1 (FoxO1) accumulation in MCD-fed mice. Remarkably, despite reduced inflammation and hampered transdifferentiation of hepatic stellate cells (HSCs), InsR+/- promoted hepatic fibrosis accumulation, which correlated with the induction of the Lysyl Oxidase Like 2 (Loxl2), involved in matrix stabilization. Loxl2 up-regulation was not a cell autonomous property of insulin resistant HSCs, but was dependent on microparticles (MPs) released specifically by insulin resistant hepatocytes (HEPs) exposed to fatty acids. The mechanism entailed FoxO1 up-regulation, as FoxO1 silencing normalized Loxl2 expression reversing fibrosis in InsR+/- MCD-fed mice. Loxl2 up-regulation was similarly detected during IR induced by obesity, but not by lipogenic stimuli (fructose feeding). Most importantly, LOXL2 up-regulation was observed in NAFLD patients with type 2 diabetes (T2D) and LOXL2 hepatic and circulating levels correlated with histological fibrosis progression. IR favors fibrosis deposition independently of the classic 'inflammation - HSC transdifferentiation' pathway. The mechanism entails a cross-talk between enhanced lipotoxicity in insulin resistant HEPs and Loxl2 production by HSCs, which was confirmed in patients with diabetes, thereby facilitating extracellular matrix (ECM) stabilization.

Lack of liver glycogen causes hepatic insulin resistance and steatosis in mice

Disruption of the Gys2 gene encoding the liver isoform of glycogen synthase generates a mouse strain (LGSKO) that almost completely lacks hepatic glycogen, has impaired glucose disposal, and is pre-disposed to entering the fasted state. This study investigated how the lack of liver glycogen increases fat accumulation and the development of liver insulin resistance. Insulin signaling in LGSKO mice was reduced in liver, but not muscle, suggesting an organ-specific defect. Phosphorylation of components of the hepatic insulin-signaling pathway, namely IRS1, Akt, and GSK3, was decreased in LGSKO mice. Moreover, insulin stimulation of their phosphorylation was significantly suppressed, both temporally and in an insulin dose response. Phosphorylation of the insulin-regulated transcription factor FoxO1 was somewhat reduced and insulin treatment did not elicit normal translocation of FoxO1 out of the nucleus. Fat overaccumulated in LGSKO livers, showing an aberrant distribution in the acinus, an increase not explained by a reduction in hepatic triglyceride export. Rather, when administered orally to fasted mice, glucose was directed toward hepatic lipogenesis as judged by the activity, protein levels, and expression of several fatty acid synthesis genes, namely, acetyl-CoA carboxylase, fatty acid synthase, SREBP1c, chREBP, glucokinase, and pyruvate kinase. Furthermore, using cultured primary hepatocytes, we found that lipogenesis was increased by 40% in LGSKO cells compared with controls. Of note, the hepatic insulin resistance was not associated with increased levels of pro-inflammatory markers. Our results suggest that loss of liver glycogen synthesis diverts glucose toward fat synthesis, correlating with impaired hepatic insulin signaling and glucose disposal.

Glucagon and Insulin Cooperatively Stimulate Fibroblast Growth Factor 21 Gene Transcription by Increasing the Expression of Activating Transcription Factor 4

Previous studies have shown that glucagon cooperatively interacts with insulin to stimulate hepatic FGF21 gene expression. Here we investigated the mechanism by which glucagon and insulin increased FGF21 gene transcription in primary hepatocyte cultures. Transfection analyses demonstrated that glucagon plus insulin induction of FGF21 transcription was conferred by two activating transcription factor 4 (ATF4) binding sites in the FGF21 gene. Glucagon plus insulin stimulated a 5-fold increase in ATF4 protein abundance, and knockdown of ATF4 expression suppressed the ability of glucagon plus insulin to increase FGF21 expression. In hepatocytes incubated in the presence of insulin, treatment with a PKA-selective agonist mimicked the ability of glucagon to stimulate ATF4 and FGF21 expression. Inhibition of PKA, PI3K, Akt, and mammalian target of rapamycin complex 1 (mTORC1) suppressed the ability of glucagon plus insulin to stimulate ATF4 and FGF21 expression. Additional analyses demonstrated that chenodeoxycholic acid (CDCA) induced a 6-fold increase in ATF4 expression and that knockdown of ATF4 expression suppressed the ability of CDCA to increase FGF21 gene expression. CDCA increased the phosphorylation of eIF2α, and inhibition of eIF2α signaling activity suppressed CDCA regulation of ATF4 and FGF21 expression. These results demonstrate that glucagon plus insulin increases FGF21 transcription by stimulating ATF4 expression and that activation of cAMP/PKA and PI3K/Akt/mTORC1 mediates the effect of glucagon plus insulin on ATF4 expression. These results also demonstrate that CDCA regulation of FGF21 transcription is mediated at least partially by an eIF2α-dependent increase in ATF4 expression.

Reduced Hepatic Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 Level in Obesity

Impairment of insulin clearance is being increasingly recognized as a critical step in the development of insulin resistance and metabolic disease. The carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) promotes insulin clearance. Null deletion or liver-specific inactivation of Ceacam1 in mice causes a defect in insulin clearance, insulin resistance, steatohepatitis, and visceral obesity. Immunohistological analysis revealed reduction of hepatic CEACAM1 in obese subjects with fatty liver disease. Thus, we aimed to determine whether this occurs at the hepatocyte level in response to systemic extrahepatic factors and whether this holds across species. Northern and Western blot analyses demonstrate that CEACAM1 mRNA and protein levels are reduced in liver tissues of obese individuals compared to their lean age-matched counterparts. Furthermore, Western analysis reveals a comparable reduction of CEACAM1 protein in primary hepatocytes derived from the same obese subjects. Similar to humans, Ceacam1 mRNA level, assessed by quantitative RT-PCR analysis, is significantly reduced in the livers of obese Zucker (fa/fa, ZDF) and Koletsky (f/f) rats relative to their age-matched lean counterparts. These studies demonstrate that the reduction of hepatic CEACAM1 in obesity occurs at the level of hepatocytes and identify the reduction of hepatic CEACAM1 as a common denominator of obesity across multiple species.

Loss of Hepatic CEACAM1: A Unifying Mechanism Linking Insulin Resistance to Obesity and Non-Alcoholic Fatty Liver Disease

The pathogenesis of human non-alcoholic fatty liver disease (NAFLD) remains unclear, in particular in the context of its relationship to insulin resistance and visceral obesity. Work on the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in mice has resolved some of the related questions. CEACAM1 promotes insulin clearance by enhancing the rate of uptake of the insulin-receptor complex. It also mediates a negative acute effect of insulin on fatty acid synthase activity. This positions CEACAM1 to coordinate the regulation of insulin and lipid metabolism. Fed a regular chow diet, global null mutation of Ceacam1 manifest hyperinsulinemia, insulin resistance, obesity, and steatohepatitis. They also develop spontaneous chicken-wire fibrosis, characteristic of non-alcoholic steatohepatitis. Reduction of hepatic CEACAM1 expression plays a significant role in the pathogenesis of diet-induced metabolic abnormalities, as bolstered by the protective effect of hepatic CEACAM1 gain-of-function against the metabolic response to dietary fat. Together, this emphasizes that loss of hepatic CEACAM1 links NAFLD to insulin resistance and obesity.

Aniline Induces Oxidative Stress and Apoptosis of Primary Cultured Hepatocytes

The toxicity and carcinogenicity of aniline in humans and animals have been well documented. However, the molecular mechanism involved in aniline-induced liver toxicity and carcinogenesis remains unclear. In our research, primary cultured hepatocytes were exposed to aniline (0, 1.25, 2.50, 5.0 and 10.0 μg/mL) for 24 h in the presence or absence of N-acetyl-l-cysteine (NAC). Levels of reactive oxygen species (ROS), malondialdehyde (MDA), and glutathione (GSH), activities of superoxide dismutase (SOD) and catalase (CAT), mitochondrial membrane potential, DNA damage, cell viability, and apoptosis were detected. Levels of ROS and MDA were significantly increased and levels of GSH and CAT, activity of SOD, and mitochondrial membrane potential in hepatocytes were significantly decreased by aniline compared with the negative control group. The tail moment and DNA content of the tail in exposed groups were significantly higher than those in the negative control group. Cell viability was reduced and apoptotic death was induced by aniline in a concentration-dependent manner. The phenomena of ROS generation, oxidative damage, loss of mitochondrial membrane potential, DNA damage and apoptosis could be prevented if ROS inhibitor NAC was added. ROS generation is involved in the loss of mitochondrial membrane potential and DNA injury, which may play a role in aniline-induced apoptosis in hepatocytes. Our study provides insight into the mechanism of aniline-induced toxicity and apoptosis of hepatocytes.