Hepatic insulin signalling is dispensable for suppression of glucose output by insulin in vivo

Camel milk endogenous peptides ameliorated hyperglycemia in high-fat diet-fed C57BL/6 J mice in association with modulation of gut microbiota and the IRS/Akt and JNK/p38 pathways

The hypoglycemic activity of camel milk are garnering significant and growing attention. In this study, the effect of camel milk endogenous peptides (CMEP) on the hyperglycemia in high-fat diet fed C57BL/6 J mice was studied. Our results exhibited that CMEP ameliorated body weight gain, hyperglycemia and hyperinsulinemia, mitigated dyslipidemia and systemic inflammation, increased the short-chain fatty acids level, repaired the gut microbiota disorders, and decreased the intestinal permeability by boosting the zonula occluden 1 (ZO-1), claudin and occludin protein expressions in high-fat diet-fed mice. Furthermore, CMEP resulted in a reduction of insulin receptor substrate-1 (IRS-1) Ser307, c-Jun NH2-terminal kinase (JNK), and p38 phosphorylation levels, and simultaneously enhancing protein kinase B (Akt) phosphorylation level in the liver tissue. These results provided a theoretical basis for the development of active ingredients in camel milk as auxiliary hypoglycemic products.

Tangeretin Unravels Metabolic Dysfunction-Associated Fatty Liver Disease in Rats by Enhancing the IRS/Akt Pathway

Excessive high-fat diet (HFD) intake can precipitate metabolic dysfunction-associated fatty liver disease (MAFLD). Tangeretin is a citrus flavonoid possessing many biological properties. We examined the impact of tangeretin on MAFLD and its underlying mechanism. Rats were given HFD plus 15% fructose solution for four months to produce metabolic syndrome. Metabolic syndrome rats were administered 100 mg/kg of metformin or 25 mg/kg of tangeretin for the last four weeks. HFD-induced increased body weight, liver weight, adipose tissue weight, fasting blood glucose, serum insulin, total triglyceride, total cholesterol, and free fatty acids and reduced adiponectin and high-density lipoprotein cholesterol levels in metabolic syndrome, which were alleviated by tangeretin (p < 0.05). Tangeretin stabilized alanine transaminase activity, liver catalase, and inflammatory and oxidative stress markers in HFD rats compared to untreated HFD rats (p < 0.05). Tangeretin reduced hepatic steatosis induced by HFD. Downregulation of hepatic insulin receptor substrate-1 (IRS-1) and protein kinase B (Akt) protein expression in metabolic syndrome rats was recovered by tangeretin (p < 0.05). Metformin, an antihyperglycemic drug, produced comparable effects to tangeretin. In conclusion, tangeretin attenuates metabolic disorders and fatty liver induced by HFD in rats. The underlying mechanisms involve reducing oxidative stress, and inflammation and enhancing insulin sensitivity.

Stromal Cell Derived Factor-1 Promotes Hepatic Insulin Resistance via Inhibiting Hepatocyte Lipophagy

Saturated fatty acid (SFA) accumulation in liver decreases hepatocyte lipophagy, a type of selective autophagy that degrades intracellular lipid droplets, leading to hepatic insulin resistance (IR), which contributes to simultaneous increases in liver glucose production and fat synthesis, resulting in hyperglycemia and dyslipidemia traits of type 2 diabetes mellitus (T2DM). Stromal cell derived factor-1 (SDF-1), a cytokine produced by hepatocytes, inhibits autophagy. In this study, we evaluated the hypothesis that SDF-1 promoted hepatic IR via inhibiting hepatocyte lipophagy during T2DM. Furthermore, we probed the downstream pathway participating in the role of SDF-1. The results showed that the neutralising of SDF-1 improved hepatic IR via promoting hepatocyte lipophagy in a mouse high-fat and high sucrose diet (HFHSD)-induced T2DM model. In vitro, SDF-1 expression and release increased in palmitic acid (PA, a kind of SFA)-treated hepatocytes. Meanwhile, SDF-1 bound to up-regulated C-X-C chemokine receptor type 4 (CXCR4) and C-X-C chemokine receptor type 7 (CXCR7) on PA-treated hepatocytes. Subsequently, SDF-1 inhibited lipophagy in PA-treated hepatocytes via CXCR4, rather than CXCR7. Finally, SDF-1/CXCR4/protein kinase B (AKT)/mechanistic target of rapamycin (mTOR) pathway-inhibited lipophagy promotes PA-induced hepatocyte IR. Collectively, this study discovered that SDF-1 might inhibit lipophagy in SFA-treated hepatocytes to promote hepatic IR via CXCR4/AKT/mTOR pathway.

Insulin resistance and vigorous physical activity mediates the relationship between waist circumference and metabolic syndrome in Korean children

BACKGROUND

Little is known about how physical activity (PA) and insulin resistance (IR) influence the impact of obesity on metabolic syndrome in pediatric populations.

OBJECTIVES

This cross-sectional observational study aimed to investigate the mediating effects of PA and IR on the relationship between waist circumference (WC) and continuous metabolic syndrome (cMetS) risk scores in children.

METHODS

A total of 1008 children aged 7-12 years (535 girls and 473 boys) were recruited from our local elementary schools. PA, body mass index (BMI), WC, and percent body fat were measured. cMetS risk scores were calculated by summing the standardized residuals for mean arterial pressure, triglycerides, fasting blood glucose, and high-density lipoprotein cholesterol.

RESULTS

Mediation analysis showed a significant indirect effect of WC on cMetS scores through insulin (β= 0.270, 95 % confidence interval [CI = 0.234-0.306) and vigorous PA (β= 0.007, 95 % CI = 0.001-0.015). The direct effect of WC on cMet scores in the presence of the two mediators was significant (β= 0.193, 95 % CI = 0.040-0.066).

CONCLUSION

Our study findings suggest that while WC is the primary determinant of cMetS scores, fasting insulin and vigorous PA partially mediate the association between the two parameters in this study population.

Hepatic WDR23 proteostasis mediates insulin homeostasis by regulating insulin-degrading enzyme capacity

Maintaining insulin homeostasis is critical for cellular and organismal metabolism. In the liver, insulin is degraded by the activity of the insulin-degrading enzyme (IDE). Here, we establish a hepatic regulatory axis for IDE through WDR23-proteostasis. Wdr23KO mice have increased IDE expression, reduced circulating insulin, and defective insulin responses. Genetically engineered human cell models lacking WDR23 also increase IDE expression and display dysregulated phosphorylation of insulin signaling cascade proteins, IRS-1, AKT2, MAPK, FoxO, and mTOR, similar to cells treated with insulin, which can be mitigated by chemical inhibition of IDE. Mechanistically, the cytoprotective transcription factor NRF2, a direct target of WDR23-Cul4 proteostasis, mediates the enhanced transcriptional expression of IDE when WDR23 is ablated. Moreover, an analysis of human genetic variation in WDR23 across a large naturally aging human cohort in the US Health and Retirement Study reveals a significant association of WDR23 with altered hemoglobin A1C (HbA1c) levels in older adults, supporting the use of WDR23 as a new molecular determinant of metabolic health in humans.

The role of adipose tissue dysfunction in hepatic insulin resistance and T2D

The root cause of type 2 diabetes (T2D) is insulin resistance (IR), defined by the failure of cells to respond to circulating insulin to maintain lipid and glucose homeostasis. While the causes of whole-body insulin resistance are multifactorial, a major contributing factor is dysregulation of liver and adipose tissue function. Adipose dysfunction, particularly adipose tissue-IR (adipo-IR), plays a crucial role in the development of hepatic insulin resistance and the progression of metabolic dysfunction-associated steatotic liver disease (MASLD) in the context of T2D. In this review, we will focus on molecular mechanisms of hepatic insulin resistance and its association with adipose tissue function. A deeper understanding of the pathophysiological mechanisms of the transition from a healthy state to insulin resistance, impaired glucose tolerance, and T2D may enable us to prevent and intervene in the progression to T2D.

Pathophysiological Relationship between Type 2 Diabetes Mellitus and Metabolic Dysfunction-Associated Steatotic Liver Disease: Novel Therapeutic Approaches

Type 2 diabetes mellitus (T2DM), often featuring hyperglycemia or insulin resistance, is a global health concern that is increasing in prevalence in the United States and worldwide. A common complication is metabolic dysfunction-associated steatotic liver disease (MASLD), the hepatic manifestation of metabolic syndrome that is also rapidly increasing in prevalence. The majority of patients with T2DM will experience MASLD, and likewise, individuals with MASLD are at an increased risk for developing T2DM. These two disorders may act synergistically, in part due to increased lipotoxicity and inflammation within the liver, among other causes. However, the pathophysiological mechanisms by which this occurs are unclear, as is how the improvement of one disorder can ameliorate the other. This review aims to discuss the pathogenic interactions between T2D and MASLD, and will highlight novel therapeutic targets and ongoing clinical trials for the treatment of these diseases.

Receptor Tyrosine Kinase Signaling Involves Echinococcus-Host Intercommunication: A Potential Therapeutic Target in Hepatic Echinococcosis

Echinococcosis, one of the most serious and life-threatening parasitic forms of zoonosis worldwide, is caused by the larvae of Echinococcus granulosus (E. granulosus) and Echinococcus multilocularis (E. multilocularis). Various drugs are being applied clinically to treat zoonosis; however, their therapeutic efficacy remains a great challenge, especially with albendazole as the preferred drug of choice. Receptor tyrosine kinase (RTK) signaling controls normal cellular proliferation, differentiation, and metabolism in humans and mammals, which are intermediate hosts of E. granulosus and E. multilocularis. Disruption of RTK signaling can cause various forms of carcinogenesis and exacerbate the progression of certain forms of parasitic disease. As a result, a significant number of studies on tyrosine kinase inhibitors (TKIs) have been conducted for the treatment of cancer and parasitic infection, with some TKIs already approved for clinical use for cancer. Notably, RTK signaling has been identified in the parasites E. granulosus and E. multilocularis; however, the mechanisms of RTK signaling response in Echinococcus-host intercommunication are not fully understood. Thus, understanding the RTK signaling response in Echinococcus-host intercommunication and the potential effect of RTK signaling is crucial for identifying new drug targets for echinococcosis. The present review illustrates that RTK signaling in the host is over-activated following infection by E. granulosus or E. multilocularis and can further facilitate the development of metacestodes in vitro. In addition, some TKIs exert strong parasitostatic effects on E. granulosus or E. multilocularis, both in vitro and/or in vivo, through downregulation of RTK signaling molecules. The summarized findings suggest that RTK signaling may be a promising drug target and that TKIs could be potential anti-Echinococcus drugs warranting further research.

The PI3K/Akt signaling axis and type 2 diabetes mellitus (T2DM): From mechanistic insights into possible therapeutic targets

Type 2 diabetes mellitus (T2DM) is an immensely debilitating chronic disease that progressively undermines the well-being of various bodily organs and, indeed, most patients succumb to the disease due to post-T2DM complications. Although there is evidence supporting the activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway by insulin, which is essential in regulating glucose metabolism and insulin resistance, the significance of this pathway in T2DM has only been explored in a few studies. The current review aims to unravel the mechanisms by which different classes of PI3Ks control the metabolism of glucose; and also to discuss the original data obtained from international research laboratories on this topic. We also summarized the role of the PI3K/Akt signaling axis in target tissues spanning from the skeletal muscle to the adipose tissue and liver. Furthermore, inquiries regarding the impact of disrupting this axis on insulin function and the development of insulin resistance have been addressed. We also provide a general overview of the association of impaired PI3K/Akt signaling pathways in the pathogenesis of the most prevalent diabetes-related complications. The last section provides a special focus on the therapeutic potential of this axis by outlining the latest advances in active compounds that alleviate diabetes via modulation of the PI3K/Akt pathway. Finally, we comment on the future research aspects in which the field of T2DM therapies using PI3K modulators might be developed.

Exploring histone deacetylases in type 2 diabetes mellitus: pathophysiological insights and therapeutic avenues

Diabetes mellitus is a chronic disease that impairs metabolism, and its prevalence has reached an epidemic proportion globally. Most people affected are with type 2 diabetes mellitus (T2DM), which is caused by a decline in the numbers or functioning of pancreatic endocrine islet cells, specifically the β-cells that release insulin in sufficient quantity to overcome any insulin resistance of the metabolic tissues. Genetic and epigenetic factors have been implicated as the main contributors to the T2DM. Epigenetic modifiers, histone deacetylases (HDACs), are enzymes that remove acetyl groups from histones and play an important role in a variety of molecular processes, including pancreatic cell destiny, insulin release, insulin production, insulin signalling, and glucose metabolism. HDACs also govern other regulatory processes related to diabetes, such as oxidative stress, inflammation, apoptosis, and fibrosis, revealed by network and functional analysis. This review explains the current understanding of the function of HDACs in diabetic pathophysiology, the inhibitory role of various HDAC inhibitors (HDACi), and their functional importance as biomarkers and possible therapeutic targets for T2DM. While their role in T2DM is still emerging, a better understanding of the role of HDACi may be relevant in improving insulin sensitivity, protecting β-cells and reducing T2DM-associated complications, among others.

Hepatic selective insulin resistance at the intersection of insulin signaling and metabolic dysfunction-associated steatotic liver disease

Insulin resistance (IR) is a major pathogenic factor in the progression of MASLD. In the liver, insulin suppresses gluconeogenesis and enhances de novo lipogenesis (DNL). During IR, there is a defect in insulin-mediated suppression of gluconeogenesis, but an unrestrained increase in hepatic lipogenesis persists. The mechanism of increased hepatic steatosis in IR is unclear and remains controversial. The key discrepancy is whether insulin retains its ability to directly regulate hepatic lipogenesis. Blocking insulin/IRS/AKT signaling reduces liver lipid deposition in IR, suggesting insulin can still regulate lipid metabolism; hepatic glucose metabolism that bypasses insulin's action may contribute to lipogenesis; and due to peripheral IR, other tissues are likely to impact liver lipid deposition. We here review the current understanding of insulin's action in governing different aspects of hepatic lipid metabolism under normal and IR states, with the purpose of highlighting the essential issues that remain unsettled.

Intestinal Trefoil Factor 3: a new biological factor mediating gut-kidney crosstalk in diabetic kidney disease

PURPOSE

To investigate the effect of TFF3 in the pathogenesis of Diabetic Kidney Disease (DKD), and explore the dynamic changes of TFF3 expression pattern in renal injury process.

METHODS

DKD animal model was established by streptozotocin (STZ) (40 mg/kg/d, ip, for 5 days, consecutively) combined with the high fat diet (HFD) for 12 weeks. While animals were sacrificed at different time stages in DKD process (4 weeks, 8 weeks and 12 weeks, respectively).

RESULTS

STZ combined with high-fat diet induced weight gain, increased blood glucose and decreased glucose tolerance in DKD mice. Compared to the control group, the DKD group exhibits extracellular matrix (ECM) accumulation and the renal injury was aggravated in a time-dependent manner. The TFF3 expression level was decreased in kidney, and increased in colon tissue.

CONCLUSION

TFF3 is not only expressed in colon, but also expressed in renal medulla and cortex. TFF3 might be play a pivotal role in renal mucosal repair by gut-kidney crosstalk, and protect renal from high glucose microenvironment damage.

Insulin-Dendrimer Nanocomplex for Multi-Day Glucose-Responsive Therapy in Mice and Swine

The management of diabetes in a manner offering autonomous insulin therapy responsive to glucose-directed need, and moreover with a dosing schedule amenable to facile administration, remains an ongoing goal to improve the standard of care. While basal insulins with reduced dosing frequency, even once-weekly administration, are on the horizon, there is still no approved therapy that offers glucose-responsive insulin function. Herein, a nanoscale complex combining both electrostatic- and dynamic-covalent interactions between a synthetic dendrimer carrier and an insulin analogue modified with a high-affinity glucose-binding motif yields an injectable insulin depot affording both glucose-directed and long-lasting insulin availability. Following a single injection, it is even possible to control blood glucose for at least one week in diabetic swine subjected to daily oral glucose challenges. Measurements of serum insulin concentration in response to challenge show increases in insulin corresponding to elevated blood glucose levels, an uncommon finding even in preclinical work on glucose-responsive insulin. Accordingly, the subcutaneous nanocomplex that results from combining electrostatic- and dynamic-covalent interactions between a modified insulin and a synthetic dendrimer carrier affords a glucose-responsive insulin depot for week-long control following a single routine injection.

Hepatic sialic acid synthesis modulates glucose homeostasis in both liver and skeletal muscle

OBJECTIVE

Sialic acid is a terminal monosaccharide of glycans in glycoproteins and glycolipids, and its derivation from glucose is regulated by the rate-limiting enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). Although the glycans on key endogenous hepatic proteins governing glucose metabolism are sialylated, how sialic acid synthesis and sialylation in the liver influence glucose homeostasis is unknown. Studies were designed to fill this knowledge gap.

METHODS

To decrease the production of sialic acid and sialylation in hepatocytes, a hepatocyte-specific GNE knockdown mouse model was generated, and systemic glucose metabolism, hepatic insulin signaling and glucagon signaling were evaluated in vivo or in primary hepatocytes. Peripheral insulin sensitivity was also assessed. Furthermore, the mechanisms by which sialylation in the liver influences hepatic insulin signaling and glucagon signaling and peripheral insulin sensitivity were identified.

RESULTS

Liver GNE deletion in mice caused an impairment of insulin suppression of hepatic glucose production. This was due to a decrease in the sialylation of hepatic insulin receptors (IR) and a decline in IR abundance due to exaggerated degradation through the Eph receptor B4. Hepatic GNE deficiency also caused a blunting of hepatic glucagon receptor (GCGR) function which was related to a decline in its sialylation and affinity for glucagon. An accompanying upregulation of hepatic FGF21 production caused an enhancement of skeletal muscle glucose disposal that led to an overall increase in glucose tolerance and insulin sensitivity.

CONCLUSION

These collective observations reveal that hepatic sialic acid synthesis and sialylation modulate glucose homeostasis in both the liver and skeletal muscle. By interrogating how hepatic sialic acid synthesis influences glucose control mechanisms in the liver, a new metabolic cycle has been identified in which a key constituent of glycans generated from glucose modulates the systemic control of its precursor.

Normobaric hypoxia shows enhanced FOXO1 signaling in obese mouse gastrocnemius muscle linked to metabolism and muscle structure and neuromuscular innervation

Skeletal muscle relies on mitochondria for sustainable ATP production, which may be impacted by reduced oxygen availability (hypoxia). Compared with long-term hypoxia, the mechanistic in vivo response to acute hypoxia remains elusive. Therefore, we aimed to provide an integrated description of the Musculus gastrocnemius response to acute hypoxia. Fasted male C57BL/6JOlaHsd mice, fed a 40en% fat diet for six weeks, were exposed to 12% O2 normobaric hypoxia or normoxia (20.9% O2) for six hours (n = 12 per group). Whole-body energy metabolism and the transcriptome response of the M. gastrocnemius were analyzed and confirmed by acylcarnitine determination and Q-PCR. At the whole-body level, six hours of hypoxia reduced energy expenditure, increased blood glucose and tended to decreased the respiratory exchange ratio (RER). Whole-genome transcriptome analysis revealed upregulation of forkhead box-O (FOXO) signalling, including an increased expression of tribbles pseudokinase 3 (Trib3). Trib3 positively correlated with blood glucose levels. Upregulated carnitine palmitoyltransferase 1A negatively correlated with the RER, but the significantly increased in tissue C14-1, C16-0 and C18-1 acylcarnitines supported that β-oxidation was not regulated. The hypoxia-induced FOXO activation could also be connected to altered gene expression related to fiber-type switching, extracellular matrix remodeling, muscle differentiation and neuromuscular junction denervation. Our results suggest that a six-hour exposure of obese mice to 12% O2 normobaric hypoxia impacts M. gastrocnemius via FOXO1, initiating alterations that may contribute to muscle remodeling of which denervation is novel and warrants further investigation. The findings support an early role of hypoxia in tissue alterations in hypoxia-associated conditions such as aging and obesity.

Melatonin associated with a high-fat diet during pregnancy and lactation prevents liver changes in the offspring

In the present study, we set out to determine whether melatonin combined with a high-fat diet during pregnancy and lactation can prevent liver disorders in offspring. Forty rats were divided into four groups: DC - pregnant rats submitted to the standard diet; DC + Mel - pregnant rats submitted to the standard diet combined with melatonin; HFD - pregnant rats submitted to a high-fat diet; HFD + Mel - pregnant rats submitted to a high-fat diet combined with melatonin. Morphophysiological and biochemical parameters were analyzed. Melatonin (5 mg/kg) was administered intraperitoneally. The HFD group offspring showed an increase in AST, ALT, alkaline phosphatase, cholesterol, triglycerides, LDL and glucose levels, and a reduction in HDL and lipase levels. In the liver obseved steatosis, hepatocellular ballooning, increased lobular parenchyma and reduced non-lobular parenchyma, beside reduced liver glycogen and fibrosis. These changes were not observed in the HFD + Mel group. In conclusion, melatonin combined with a high-fat diet preserves the liver architecture and function in the offspring.

BRD7 improves glucose homeostasis independent of IRS proteins

Bromodomain-containing protein 7 (BRD7) has emerged as a player in the regulation of glucose homeostasis. Hepatic BRD7 levels are decreased in obese mice, and the reinstatement of hepatic BRD7 in obese mice has been shown to establish euglycemia and improve glucose homeostasis. Of note, the upregulation of hepatic BRD7 levels activates the AKT cascade in response to insulin without enhancing the sensitivity of the insulin receptor (InsR)-insulin receptor substrate (IRS) axis. In this report, we provide evidence for the existence of an alternative insulin signaling pathway that operates independently of IRS proteins and demonstrate the involvement of BRD7 in this pathway. To investigate the involvement of BRD7 as a downstream component of InsR, we utilized liver-specific InsR knockout mice. Additionally, we employed liver-specific IRS1/2 knockout mice to examine the requirement of IRS1/2 for the action of BRD7. Our investigation of glucose metabolism parameters and insulin signaling unveiled the significance of InsR activation in mediating BRD7's effect on glucose homeostasis in the liver. Moreover, we identified an interaction between BRD7 and InsR. Notably, our findings indicate that IRS1/2 is not necessary for BRD7's regulation of glucose metabolism, particularly in the context of obesity. The upregulation of hepatic BRD7 significantly reduces blood glucose levels and restores glucose homeostasis in high-fat diet-challenged liver-specific IRS1/2 knockout mice. These findings highlight the presence of an alternative insulin signaling pathway that operates independently of IRS1/2 and offer novel insights into the mechanisms of a previously unknown insulin signaling in obesity.

Modeling and therapeutic targeting of inflammation-induced hepatic insulin resistance using human iPSC-derived hepatocytes and macrophages

Hepatic insulin resistance is recognized as a driver of type 2 diabetes and fatty liver disease but specific therapies are lacking. Here we explore the potential of human induced pluripotent stem cells (iPSCs) for modeling hepatic insulin resistance in vitro, with a focus on resolving the controversy about the impact of inflammation in the absence of steatosis. For this, we establish the complex insulin signaling cascade and the multiple inter-dependent functions constituting hepatic glucose metabolism in iPSC-derived hepatocytes (iPSC-Heps). Co-culture of these insulin-sensitive iPSC-Heps with isogenic iPSC-derived pro-inflammatory macrophages induces glucose output by preventing insulin from inhibiting gluconeogenesis and glycogenolysis and activating glycolysis. Screening identifies TNFα and IL1β as the mediators of insulin resistance in iPSC-Heps. Neutralizing these cytokines together restores insulin sensitivity in iPSC-Heps more effectively than individual inhibition, reflecting specific effects on insulin signaling and glucose metabolism mediated by NF-κB or JNK. These results show that inflammation is sufficient to induce hepatic insulin resistance and establish a human iPSC-based in vitro model to mechanistically dissect and therapeutically target this metabolic disease driver.

Insulin Regulation of Hepatic Lipid Homeostasis

The incidence of obesity, insulin resistance, and type II diabetes (T2DM) continues to rise worldwide. The liver is a central insulin-responsive metabolic organ that governs whole-body metabolic homeostasis. Therefore, defining the mechanisms underlying insulin action in the liver is essential to our understanding of the pathogenesis of insulin resistance. During periods of fasting, the liver catabolizes fatty acids and stored glycogen to meet the metabolic demands of the body. In postprandial conditions, insulin signals to the liver to store excess nutrients into triglycerides, cholesterol, and glycogen. In insulin-resistant states, such as T2DM, hepatic insulin signaling continues to promote lipid synthesis but fails to suppress glucose production, leading to hypertriglyceridemia and hyperglycemia. Insulin resistance is associated with the development of metabolic disorders such as cardiovascular and kidney disease, atherosclerosis, stroke, and cancer. Of note, nonalcoholic fatty liver disease (NAFLD), a spectrum of diseases encompassing fatty liver, inflammation, fibrosis, and cirrhosis, is linked to abnormalities in insulin-mediated lipid metabolism. Therefore, understanding the role of insulin signaling under normal and pathologic states may provide insights into preventative and therapeutic opportunities for the treatment of metabolic diseases. Here, we provide a review of the field of hepatic insulin signaling and lipid regulation, including providing historical context, detailed molecular mechanisms, and address gaps in our understanding of hepatic lipid regulation and the derangements under insulin-resistant conditions. © 2023 American Physiological Society. Compr Physiol 13:4785-4809, 2023.

The adipocyte supersystem of insulin and cAMP signaling

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Crosstalk between bone and other organs

Bone has long been considered as a silent organ that provides a reservoir of calcium and phosphorus, traditionally. Recently, further study of bone has revealed additional functions as an endocrine organ connecting systemic organs of the whole body. Communication between bone and other organs participates in most physiological and pathological events and is responsible for the maintenance of homeostasis. Here, we present an overview of the crosstalk between bone and other organs. Furthermore, we describe the factors mediating the crosstalk and review the mechanisms in the development of potential associated diseases. These connections shed new light on the pathogenesis of systemic diseases and provide novel potential targets for the treatment of systemic diseases.

Fu Brick Tea Manages HFD/STZ-Induced Type 2 Diabetes by Regulating the Gut Microbiota and Activating the IRS1/PI3K/Akt Signaling Pathway

The antidiabetic effects of Fu brick tea aqueous extract (FTE) and its underlying molecular mechanism in type 2 diabetes mellitus (T2DM) mice were investigated. FTE treatment significantly relieved dyslipidemia, insulin resistance (IR), and hepatic oxidative stress caused by T2DM. FTE also ameliorated the T2DM-induced gut dysbiosis by decreasing the Firmicutes/Bacteroidota (F/B) ratio at the phylum level and promoting the proliferation of Bifidobacterium, Parabacteroides, and Roseburia at the genus level. Besides, FTE significantly improved colonic short-chain fatty acid levels of T2DM mice. Furthermore, the antidiabetic effects of FTE were proved to be mediated by the IRS1/PI3K/Akt and AMPK-mediated gluconeogenesis signaling pathways. Metabolomics analysis illustrated that FTE recovered the levels of 28 metabolites associated with T2DM to the levels of normal mice. Taken together, these findings suggest that FTE can alleviate T2DM by reshaping the gut microbiota, activating the IRS1/PI3K/Akt pathway, and regulating intestinal metabolites.

IPMK modulates hepatic glucose production and insulin signaling

Hepatic glucose production (HGP) is crucial for the maintenance of normal glucose homeostasis. Although hepatic insulin resistance contributes to excessive glucose production, its mechanism is not well understood. Here, we show that inositol polyphosphate multikinase (IPMK), a key enzyme in inositol polyphosphate biosynthesis, plays a role in regulating hepatic insulin signaling and gluconeogenesis both in vitro and in vivo. IPMK-deficient hepatocytes exhibit decreased insulin-induced activation of Akt-FoxO1 signaling. The expression of messenger RNA levels of phosphoenolpyruvate carboxykinase 1 (Pck1) and glucose 6-phosphatase (G6pc), key enzymes mediating gluconeogenesis, are increased in IPMK-deficient hepatocytes compared to wild type hepatocytes. Importantly, re-expressing IPMK restores insulin sensitivity and alleviates glucose production in IPMK-deficient hepatocytes. Moreover, hepatocyte-specific IPMK deletion exacerbates hyperglycemia and insulin sensitivity in mice fed a high-fat diet, accompanied by an increase in HGP during pyruvate tolerance test and reduction in Akt phosphorylation in IPMK deficient liver. Our results demonstrate that IPMK mediates insulin signaling and gluconeogenesis and may be potentially targeted for treatment of diabetes.

Global-run on sequencing identifies Gm11967 as an Akt-dependent long noncoding RNA involved in insulin sensitivity

The insulin responsive Akt and FoxO1 signaling axis is a key regulator of the hepatic transcriptional response to nutrient intake. Here, we used global run-on sequencing (GRO-seq) to measure the nascent transcriptional response to fasting and refeeding as well as define the specific role of hepatic Akt and FoxO1 signaling in mediating this response. We identified 599 feeding-regulated transcripts, as well as over 6,000 eRNAs, and mapped their dependency on Akt and FoxO1 signaling. Further, we identified several feeding-regulated lncRNAs, including the lncRNA Gm11967, whose expression was dependent upon the liver Akt-FoxO1 axis. Restoring Gm11967 expression in mice lacking liver Akt improved insulin sensitivity and induced glucokinase protein expression, indicating that Akt-dependent control of Gm11967 contributes to the translational control of glucokinase. More broadly, we have generated a unique genome-wide dataset that defines the feeding and Akt/FoxO1-dependent transcriptional changes in response to nutrient availability.

Estradiol (E2) Improves Glucose-Stimulated Insulin Secretion and Stabilizes GDM Progression in a Prediabetic Mouse Model

Female New Zealand obese (NZO) mice are an established model of preconceptional (pc.) prediabetes that progresses as gestational diabetes mellitus (GDM) during gestation. It is known that NZO mice show improvement in insulin sensitivity and glucose-stimulated insulin secretion (GSIS) during gestation in vivo. The latter is no longer detectable in ex vivo perifusion experiments in isolated islets of Langerhans, suggesting a modulation by extrapancreatic factors. Here, we demonstrated that plasma 17β-estradiol (E2) levels increased markedly in NZO mice during gestation. The aim of this work was to determine whether these increased E2 levels are responsible for the improvement in metabolism during gestation. To achieve this goal, we examined its effects in isolated islets and primary hepatocytes of both NZO and metabolically healthy NMRI mice. E2 increased GSIS in the islets of both strains significantly. Hepatic glucose production (HGP) failed to be decreased by insulin in NZO hepatocytes but was reduced by E2 in both strains. Hepatocytes of pregnant NZO mice showed significantly lower glucose uptake (HGU) compared with NMRI controls, whereby E2 stimulation diminished this difference. Hepatocytes of pregnant NZO showed reduced glycogen content, increased cyclic adenosine monophosphate (cAMP) levels, and reduced AKT activation. These differences were abolished after E2 stimulation. In conclusion, our data indicate that E2 stabilizes and prevents deterioration of the metabolic state of the prediabetic NZO mice. E2 particularly increases GSIS and improves hepatic glucose utilization to a lower extent.

Role of Serotonin (5-HT) in GDM Prediction Considering Islet and Liver Interplay in Prediabetic Mice during Gestation

Gestational diabetes (GDM) is characterized by a glucose tolerance disorder. This may first appear during pregnancy or pre-exist before conception as a form of prediabetes, but there are few data on the pathogenesis of the latter subtype. Female New Zealand obese (NZO) mice serve as a model for this subpopulation of GDM. It was recently shown that GDM is associated with elevated urinary serotonin (5-hydroxytryptamine, 5-HT) levels, but the role of the biogenic amine in subpopulations with prediabetes remains unclear. 5-HT is synthesized in different tissues, including the islets of Langerhans during pregnancy. Furthermore, 5-HT receptors (HTRs) are expressed in tissues important for the regulation of glucose homeostasis, such as liver and pancreas. Interestingly, NZO mice showed elevated plasma and islet 5-HT concentrations as well as impaired glucose-stimulated 5-HT secretion. Incubation of isolated primary NZO islets with 5-HT revealed an inhibitory effect on insulin and glucagon secretion. In primary NZO hepatocytes, 5-HT aggravated hepatic glucose production (HGP), decreased glucose uptake (HGU), glycogen content, and modulated AKT activation as well as cyclic adenosine monophosphate (cAMP) increase, indicating 5-HT downstream modulation. Treatment with an HTR2B antagonist reduced this 5-HT-mediated deterioration of the metabolic state. With its strong effect on glucose metabolism, these data indicate that 5-HT is already a potential indicator of GDM before conception in mice.

Acute Deletion of the FOXO1-dependent Hepatokine FGF21 Does not Alter Basal Glucose Homeostasis or Lipolysis in Mice

The hepatic transcription factor forkhead box O1 (FOXO1) is a critical regulator of hepatic and systemic insulin sensitivity. Previous work by our group and others demonstrated that genetic inhibition of FOXO1 improves insulin sensitivity both in genetic and dietary mouse models of metabolic disease. Mechanistically, this is due in part to cell nonautonomous control of adipose tissue insulin sensitivity. However, the mechanisms mediating this liver-adipose tissue crosstalk remain ill defined. One candidate hepatokine controlled by hepatic FOXO1 is fibroblast growth factor 21 (FGF21). Preclinical and clinical studies have explored the potential of pharmacological FGF21 as an antiobesity and antidiabetic therapy. In this manuscript, we performed acute loss-of-function experiments to determine the role of hepatocyte-derived FGF21 in glucose homeostasis and insulin tolerance both in control and mice lacking hepatic insulin signaling. Surprisingly, acute deletion of FGF21 did not alter glucose tolerance, insulin tolerance, or adipocyte lipolysis in either liver-specific FGF21KO mice or mice lacking hepatic AKT-FOXO1-FGF21, suggesting a permissive role for endogenous FGF21 in the regulation of systemic glucose homeostasis and insulin tolerance in mice. In addition, these data indicate that liver FOXO1 controls glucose homeostasis independently of liver-derived FGF21.

Bioactive lipids and metabolic syndrome-a symposium report

Recent research has shed light on the cellular and molecular functions of bioactive lipids that go far beyond what was known about their role as dietary lipids. Bioactive lipids regulate inflammation and its resolution as signaling molecules. Genetic studies have identified key factors that can increase the risk of cardiovascular diseases and metabolic syndrome through their effects on lipogenesis. Lipid scientists have explored how these signaling pathways affect lipid metabolism in the liver, adipose tissue, and macrophages by utilizing a variety of techniques in both humans and animal models, including novel lipidomics approaches and molecular dynamics models. Dissecting out these lipid pathways can help identify mechanisms that can be targeted to prevent or treat cardiometabolic conditions. Continued investigation of the multitude of functions mediated by bioactive lipids may reveal additional components of these pathways that can provide a greater understanding of metabolic homeostasis.

TOX4, an insulin receptor-independent regulator of hepatic glucose production, is activated in diabetic liver

Increased hepatic glucose production (HGP) contributes to hyperglycemia in type 2 diabetes. Hormonal regulation of this process is primarily, but not exclusively, mediated by the AKT-FoxO1 pathway. Here, we show that cAMP and dexamethasone regulate the high-mobility group superfamily member TOX4 to mediate HGP, independent of the insulin receptor/FoxO1 pathway. TOX4 inhibition decreases glucose production in primary hepatocytes and liver and increases glucose tolerance. Combined genetic ablation of TOX4 and FoxO1 in liver has additive effects on glucose tolerance and gluconeogenesis. Moreover, TOX4 ablation fails to reverse the metabolic derangement brought by insulin receptor knockout. TOX4 expression is increased in livers of patients with steatosis and diabetes and in diet-induced obese and db/db mice. In the latter two murine models, knockdown Tox4 decreases glycemia and improves glucose tolerance. We conclude that TOX4 is an insulin receptor-independent regulator of HGP and a candidate contributor to the pathophysiology of diabetes.

Analysis of Insulin Resistance in Nonalcoholic Steatohepatitis

Insulin resistance is a major phenotype observed in nonalcoholic steatohepatitis (NASH), the advanced stage of nonalcoholic fatty liver disease (NAFLD). Insulin resistance in NASH is characterized by reductions in whole body, hepatic, and adipose tissue insulin sensitivity. The mechanisms underlying hepatic insulin resistance is primarily associated with hepatic glucose production (HGP) rate. Hepatic insulin resistance can also be a consequence or a driving factor of hepatic lipid accumulation by increasing free fatty acid synthesis, delivery, and catabolism. The common method to assess hepatic insulin resistance is to measure hepatic glucose production (HGP) using isotope tracer distribution technique. However, non-radioactive approaches have been developed to assess hepatic insulin resistance in the context of NASH. In this chapter, we describe the methods to evaluate hepatic insulin resistance in animal models of NASH by examining insulin sensitivity and glucose tolerance as well as the key molecules in hepatic insulin signaling pathways.

Potential role of microRNAs in selective hepatic insulin resistance: From paradox to the paradigm

The paradoxical action of insulin on hepatic glucose metabolism and lipid metabolism in the insulin-resistant state has been of much research interest in recent years. Generally, insulin resistance would promote hepatic gluconeogenesis and demote hepatic de novo lipogenesis. The underlying major drivers of these mechanisms were insulin-dependent, via FOXO-1-mediated gluconeogenesis and SREBP1c-mediated lipogenesis. However, insulin-resistant mouse models have shown high glucose levels as well as excess lipid accumulation. As suggested, the inert insulin resistance causes the activation of the FOXO-1 pathway promoting gluconeogenesis. However, it does not affect the SREBP1c pathway; therefore, cells continue de novo lipogenesis. Many hypotheses were suggested for this paradoxical action occurring in insulin-resistant rodent models. A "downstream branch point" in the insulin-mediated pathway was suggested to act differentially on the FOXO-1 and SREBP1c pathways. MicroRNAs have been widely studied for their action of pathway mediation via suppressing the intermediate protein expressions. Many in vitro studies have postulated the roles of hepato-specific expressions of miRNAs on insulin cascade. Thus, miRNA would play a pivotal role in selective hepatic insulin resistance. As observed, there were confirmations and contradictions between the outcomes of gene knockout studies conducted on selective hepatic insulin resistance and hepato-specific miRNA expression studies. Furthermore, these studies had evaluated only the effect of miRNAs on glucose metabolism and few on hepatic de novo lipogenesis, limiting the ability to conclude their role in selective hepatic insulin resistance. Future studies conducted on the role of miRNAs on selective hepatic insulin resistance warrant the understanding of this paradoxical action of insulin.

Hepatic IGF2/H19 Epigenetic Alteration Induced Glucose Intolerance in Gestational Diabetes Mellitus Offspring via FoxO1 Mediation

OBJECTIVE

The offspring of women with gestational diabetes mellitus (GDM) have a high predisposition to developing type 2 diabetes during childhood and adulthood. The aim of the study was to evaluate how GDM exposure in the second half of pregnancy contributes to hepatic glucose intolerance through a mouse model.

METHODS

By creating a GDM mouse model, we tested glucose and insulin tolerance of offspring by intraperitoneal glucose tolerance test (IPGTT), insulin tolerance test (ITT), and pyruvate tolerance test (PTT). In addition, we checked the expression of genes IGF2/H19, FoxO1, and DNMTs in the mouse liver by RT-qPCR. Pyrosequencing was used to detect the methylation status on IGF2/H19 differentially methylated regions (DMRs). In vitro insulin stimulation experiments were performed to evaluate the effect of different insulin concentrations on HepG2 cells. Moreover, we detect the interaction between FoxO1 and DNMT3A by chromatin immunoprecipitation-quantitative PCR (Chip-qPCR) and knock-down experiments on HepG2 cells.

RESULTS

We found that the first generation of GDM offspring (GDM-F1) exhibited impaired glucose tolerance (IGT) and insulin resistance, with males being disproportionately affected. In addition, the expression of imprinted genes IGF2 and H19 was downregulated in the livers of male mice via hypermethylation of IGF2-DMR0 and IGF2-DMR1. Furthermore, increased expression of transcriptional factor FoxO1 was confirmed to regulate DNMT3A expression, which contributed to abnormal methylation of IGF2/H19 DMRs. Notably, different insulin treatments on HepG2 demonstrated those genetic alterations, suggesting that they might be induced by intrauterine hyperinsulinemia.

CONCLUSION

Our results demonstrated that the intrauterine hyperinsulinemia environment has increased hepatic FoxO1 levels and subsequently increased expression of DNMT3A and epigenetic alterations on IGF2/H19 DMRs. These findings provide potential molecular mechanisms responsible for glucose intolerance and insulin resistance in the first male generation of GDM mice.

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.

LRG1 is an adipokine that mediates obesity-induced hepatosteatosis and insulin resistance

Dysregulation in adipokine biosynthesis and function contributes to obesity-induced metabolic diseases. However, the identities and functions of many of the obesity-induced secretory molecules remain unknown. Here, we report the identification of leucine-rich alpha-2-glycoprotein 1 (LRG1) as an obesity-associated adipokine that exacerbates high fat diet-induced hepatosteatosis and insulin resistance. Serum levels of LRG1 were markedly elevated in obese humans and mice compared to their respective controls. LRG1 deficiency in mice greatly alleviated diet-induced hepatosteatosis, obesity, and insulin resistance. Mechanistically, LRG1 bound with high selectivity to the liver and promoted hepatosteatosis by increasing de novo lipogenesis and suppressing fatty acid β-oxidation. LRG1 also inhibited hepatic insulin signaling by down-regulating insulin receptor substrates 1 and 2. Our study identified LRG1 as a key molecule that mediates the crosstalk between adipocytes and hepatocytes in diet-induced hepatosteatosis and insulin resistance. Suppressing LRG1 expression and function may be a promising strategy for the treatment of obesity-related metabolic diseases.

Chronic AT1 blockade improves hyperglycemia by decreasing adipocyte inflammation and decreasing hepatic PCK1 and G6PC1 expression in obese rats

Inappropriate activation of the renin-angiotensin system decreases glucose uptake in peripheral tissues. Chronic angiotensin receptor type 1 (AT1) blockade (ARB) increases glucose uptake in skeletal muscle and decreases the abundance of large adipocytes and macrophage infiltration in adipose. However, the contributions of each tissue to the improvement in hyperglycemia in response to AT1 blockade are not known. Therefore, we determined the static and dynamic responses of soleus muscle, liver, and adipose to an acute glucose challenge following the chronic blockade of AT1. We measured adipocyte morphology along with TNF-α expression, F4/80- and CD11c-positive cells in adipose and measured insulin receptor (IR) phosphorylation and AKT phosphorylation in soleus muscle, liver, and retroperitoneal fat before (T0), 60 (T60) and 120 (T120) min after an acute glucose challenge in the following groups of male rats: 1) Long-Evans Tokushima Otsuka (LETO; lean control; n = 5/time point), 2) obese Otsuka Long Evans Tokushima Fatty (OLETF; n = 7 or 8/time point), and 3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day; n = 7 or 8/time point). AT1 blockade decreased adipocyte TNF-α expression and F4/80- and CD11c-positive cells. In retroperitoneal fat at T60, IR phosphorylation was 155% greater in ARB than in OLETF. Furthermore, in retroperitoneal fat AT1 blockade increased glucose transporter-4 (GLUT4) protein expression in ARB compared with OLETF. IR phosphorylation and AKT phosphorylation were not altered in the liver of OLETF, but AT1 blockade decreased hepatic Pck1 and G6pc1 mRNA expressions. Collectively, these results suggest that chronic AT1 blockade improves obesity-associated hyperglycemia in OLETF rats by improving adipocyte function and by decreasing hepatic glucose production via gluconeogenesis.NEW & NOTEWORTHY Inappropriate activation of the renin-angiotensin system increases adipocyte inflammation contributing to the impairment in adipocyte function and increases hepatic Pck1 and G6pc1 mRNA expression in response to a glucose challenge. Ultimately, these effects may contribute to the development of glucose intolerance.

New targets for NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a growing cause of chronic liver disease worldwide. It is characterised by steatosis, liver inflammation, hepatocellular injury and progressive fibrosis. Several preclinical models (dietary and genetic animal models) of NAFLD have deepened our understanding of its aetiology and pathophysiology. Despite the progress made, there are currently no effective treatments for NAFLD. In this review, we will provide an update on the known molecular pathways involved in the pathophysiology of NAFLD and on ongoing studies of new therapeutic targets.

Modulation of Glucose Production by Central Insulin Requires IGF-1 Receptors in AgRP Neurons

Similar to insulin, central administration of IGF-1 can suppress hepatic glucose production (HGP), but it is unclear whether this effect is mediated via insulin receptors (InsRs) or IGF-1 receptors (IGF-1Rs) in the brain. To this end, we used pharmacologic and genetic approaches in combination with hyperinsulinemic-euglycemic clamps to decipher the role of these receptors in mediating central effects of IGF-1 and insulin on HGP. In rats, we observed that intracerebroventricular (ICV) administration of IGF-1 or insulin markedly increased the glucose infusion rate (GIR) by >50% and suppressed HGP (P < 0.001). However, these effects were completely prevented by preemptive ICV infusion with an IGF-1R and InsR/IGF-1R hybrid (HybridR) blocking antibody. Likewise, ICV infusion of the InsR antagonist, S961, which also can bind HybridRs, interfered with the ability of central insulin, but not IGF-1, to increase the GIR. Furthermore, hyperinsulinemic clamps in mice lacking IGF-1Rs in AgRP neurons revealed ∼30% reduction in the GIR in knockout animals, which was explained by an impaired ability of peripheral insulin to completely suppress HGP (P < 0.05). Signaling studies further revealed an impaired ability of peripheral insulin to trigger ribosomal S6 phosphorylation or phosphatidylinositol (3,4,5)-trisphosphate production in AgRP neurons lacking IGF-1Rs. In summary, these data suggest that attenuation of IGF-1R signaling in the mediobasal hypothalamus, and specifically in AgRP neurons, can phenocopy impaired regulation of HGP as previously demonstrated in mice lacking InsRs in these cells, suggesting a previously unappreciated role for IGF-1Rs and/or HybridRs in the regulation of central insulin/IGF-1 signaling in glucose metabolism.

Brain insulin signalling in metabolic homeostasis and disease

Insulin signalling in the central nervous system regulates energy homeostasis by controlling metabolism in several organs and by coordinating organ crosstalk. Studies performed in rodents, non-human primates and humans over more than five decades using intracerebroventricular, direct hypothalamic or intranasal application of insulin provide evidence that brain insulin action might reduce food intake and, more importantly, regulates energy homeostasis by orchestrating nutrient partitioning. This Review discusses the metabolic pathways that are under the control of brain insulin action and explains how brain insulin resistance contributes to metabolic disease in obesity, the metabolic syndrome and type 2 diabetes mellitus.

Metabolic drivers of non-alcoholic fatty liver disease

BACKGROUND

The incidence of non-alcoholic fatty liver disease (NAFLD) is rapidly increasing worldwide parallel to the global obesity epidemic. NAFLD encompasses a range of liver pathologies and most often originates from metabolically driven accumulation of fat in the liver, or non-alcoholic fatty liver (NAFL). In a subset of NAFL patients, the disease can progress to non-alcoholic steatohepatitis (NASH), which is a more severe form of liver disease characterized by hepatocyte injury, inflammation, and fibrosis. Significant progress has been made over the past decade in our understanding of NASH pathogenesis, but gaps remain in our mechanistic knowledge of the precise metabolic triggers for disease worsening.

SCOPE OF REVIEW

The transition from NAFL to NASH likely involves a complex constellation of multiple factors intrinsic and extrinsic to the liver. This review focuses on early metabolic events in the establishment of NAFL and initial stages of NASH. We discuss the association of NAFL with obesity as well as the role of adipose tissue in disease progression and highlight early metabolic drivers implicated in the pathological transition from hepatic fat accumulation to steatohepatitis.

MAJOR CONCLUSIONS

The close association of NAFL with features of metabolic syndrome highlight plausible mechanistic roles for adipose tissue health and the release of lipotoxic lipids, hepatic de novo lipogenesis (DNL), and disruption of the intestinal barrier in not only the initial establishment of hepatic steatosis, but also in mediating disease progression. Human genetic variants linked to NASH risk to date are heavily biased toward genes involved in the regulation of lipid metabolism, providing compelling support for the hypothesis that NASH is fundamentally a metabolic disease.

Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis

The reactions of the tricarboxylic acid (TCA) cycle allow the controlled combustion of fat and carbohydrate. In principle, TCA cycle intermediates are regenerated on every turn and can facilitate the oxidation of an infinite number of nutrient molecules. However, TCA cycle intermediates can be lost to cataplerotic pathways that provide precursors for biosynthesis, and they must be replaced by anaplerotic pathways that regenerate these intermediates. Together, anaplerosis and cataplerosis help regulate rates of biosynthesis by dictating precursor supply, and they play underappreciated roles in catabolism and cellular energy status. They facilitate recycling pathways and nitrogen trafficking necessary for catabolism, and they influence redox state and oxidative capacity by altering TCA cycle intermediate concentrations. These functions vary widely by tissue and play emerging roles in disease. This article reviews the roles of anaplerosis and cataplerosis in various tissues and discusses how they alter carbon transitions, and highlights their contribution to mechanisms of disease. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Loss of FOXO transcription factors in the liver mitigates stress-induced hyperglycemia

Objective

Stress-induced hyperglycemia is associated with poor outcomes in nearly all critical illnesses. This acute elevation in glucose after injury or illness is associated with increased morbidity and mortality, including multiple organ failure. Stress-induced hyperglycemia is often attributed to insulin resistance as controlling glucose levels via exogenous insulin improves outcomes, but the mechanisms are unclear. Forkhead box O (FOXO) transcription factors are direct targets of insulin signaling in the liver that regulate glucose homeostasis via direct and indirect pathways. Loss of hepatic FOXO transcription factors reduces hyperglycemia in chronic insulin resistance; however, the role of FOXOs in stress-induced hyperglycemia is unknown.

Methods

We subjected mice lacking FOXO transcription factors in the liver to a model of injury known to cause stress-induced hyperglycemia. Glucose, insulin, glycerol, fatty acids, cytokines, and adipokines were assessed before and after injury. Liver and adipose tissue were analyzed for changes in glycogen, FOXO target gene expression, and insulin signaling.

Results

Stress-induced hyperglycemia was associated with reduced hepatic insulin signaling and increased hepatic FOXO target gene expression while loss of FOXO1, 3, and 4 in the liver attenuated hyperglycemia and prevented hyperinsulinemia. Mechanistically, the loss of FOXO transcription factors mitigated the stress-induced hyperglycemia response by directly altering gene expression and glycogenolysis in the liver and indirectly suppressing lipolysis in adipose tissue. Reductions were associated with decreased IL-6, TNF-α, and follistatin and increased FGF21, suggesting that cytokines and FOXO-regulated hepatokines contribute to the stress-induced hyperglycemia response.

Conclusions

This study implicates FOXO transcription factors as a predominant driver of stress-induced hyperglycemia through means that include cross-talk between the liver and adipose, highlighting a novel mechanism underlying acute hyperglycemia and insulin resistance in stress.

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Liver forkhead box O (FOXO) target gene expression is increased in critical illness.

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Loss of FOXO1, 3, and 4 in the liver mitigates stress-induced hyperglycemia (SIH).

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Hepatic FOXO drives SIH via direct and indirect means in the liver and adipose.

Hepatic AKT orchestrates adipose tissue thermogenesis via FGF21-dependent and -independent mechanisms

Organismal stressors such as cold exposure require a systemic response to maintain body temperature. Brown adipose tissue (BAT) is a key thermogenic tissue in mammals that protects against hypothermia in response to cold exposure. Defining the complex interplay of multiple organ systems in this response is fundamental to our understanding of adipose tissue thermogenesis. In this study, we identify a role for hepatic insulin signaling via AKT in the adaptive response to cold stress and show that liver AKT is an essential cell-nonautonomous regulator of adipocyte lipolysis and BAT function. Mechanistically, inhibition of forkhead box O1 (FOXO1) by AKT controls BAT thermogenesis by enhancing catecholamine-induced lipolysis in the white adipose tissue (WAT) and increasing circulating fibroblast growth factor 21 (FGF21). Our data identify a role for hepatic insulin signaling via the AKT-FOXO1 axis in regulating WAT lipolysis, promoting BAT thermogenic capacity, and ensuring a proper thermogenic response to acute cold exposure.

Role of Insulin Resistance in MAFLD

Many studies have reported that metabolic dysfunction is closely involved in the complex mechanism underlying the development of non-alcoholic fatty liver disease (NAFLD), which has prompted a movement to consider renaming NAFLD as metabolic dysfunction-associated fatty liver disease (MAFLD). Metabolic dysfunction in this context encompasses obesity, type 2 diabetes mellitus, hypertension, dyslipidemia, and metabolic syndrome, with insulin resistance as the common underlying pathophysiology. Imbalance between energy intake and expenditure results in insulin resistance in various tissues and alteration of the gut microbiota, resulting in fat accumulation in the liver. The role of genetics has also been revealed in hepatic fat accumulation and fibrosis. In the process of fat accumulation in the liver, intracellular damage as well as hepatic insulin resistance further potentiates inflammation, fibrosis, and carcinogenesis. Increased lipogenic substrate supply from other tissues, hepatic zonation of Irs1, and other factors, including ER stress, play crucial roles in increased hepatic de novo lipogenesis in MAFLD with hepatic insulin resistance. Herein, we provide an overview of the factors contributing to and the role of systemic and local insulin resistance in the development and progression of MAFLD.

Triglycerides in Nonalcoholic Fatty Liver Disease: Guilty Until Proven Innocent

End-stage liver disease (ESLD) is a rare but often fatal complication of nonalcoholic fatty liver disease (NAFLD). In NAFLD, insulin resistance, which is clinically defined as the impairment of insulin's ability to maintain glucose homeostasis, is associated with perturbations in insulin action that promote triglyceride accumulation, such as increasing de novo lipogenesis. However, the key step in the development of ESLD is not the accumulation of triglycerides, but hepatocyte injury. Whether and how triglycerides promote hepatocyte injury remains unclear. Consequently, it is difficult to predict whether drugs designed to reduce hepatic triglycerides will prevent the most important complications of NAFLD.

A bioinformatics and transcriptomics based investigation reveals an inhibitory role of Huanglian-Renshen-Decoction on hepatic glucose production of T2DM mice via PI3K/Akt/FoxO1 signaling pathway

BACKGROUND

Excessive hepatic glucose production (HGP) largely promotes the development of type 2 diabetes mellitus (T2DM), and the inhibition of HGP significantly ameliorates T2DM. Huanglian-Renshen-Decoction (HRD), a classic traditional Chinese herb medicine, is widely used for the treatment of diabetes in clinic for centuries and proved effective. However, the relevant mechanisms of HRD are not fully understood.

PURPOSE

Based on that, this study was designed to identify the potential effects and underlying mechanisms of HRD on HGP by a comprehensive investigation that integrated in vivo functional experiments, network pharmacology, molecular docking, transcriptomics and molecular biology.

METHODS

After confirming the therapeutic effects of HRD on T2DM mice, the inhibitory role of HRD on HGP was evaluated by pyruvate and glucagon tolerance tests, liver positron emission tomography (PET) imaging and the detection of gluconeogenic key enzymes. Then, network pharmacology and transcriptomics approaches were used to clarify the underlying mechanisms. Molecular biology, computational docking analysis and in vitro experiments were applied for final mechanism verification.

RESULTS

Here, our results showed that HRD can decrease weight gain and blood glucose, increase fasting insulin, glucose clearance and insulin sensitivity in T2DM mice. Dysregulated lipid profile was also corrected by HRD administration. Pyruvate, glucagon tolerance tests and liver PET imaging all indicated that HRD inhibited the abnormal HGP of T2DM, and the expressions of phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase) were significantly suppressed by HRD as expected. Network pharmacology and transcriptomics approaches illustrated that PI3K/Akt/FoxO1 signaling pathway may be responsible for the inhibitory effect of HRD on HGP. Afterward, further western blot and immunoprecipitation found that HRD did activate PI3K/Akt/FoxO1 signaling pathway in T2DM mice, which confirmed previous results. Additionally, the conclusion was further supported by molecular docking and in vitro experiments, in which identified HRD compound, oxyberberine, was proven to exert an obvious effect on Akt.

CONCLUSION

Our data demonstrated that HRD can treat T2DM by inhibiting hepatic glucose production, the underlying mechanisms were associated with the activation of PI3K/Akt/FoxO1 signaling pathway.

Intercellular Transmission of Hepatic ER Stress in Obesity Disrupts Systemic Metabolism

Endoplasmic reticulum stress (ERS) has a pathophysiological role in obesity-associated insulin resistance. Yet, the coordinated tissue response to ERS remains unclear. Increased connexin 43 (Cx43)-mediated intercellular communication has been implicated in tissue-adaptive and -maladaptive response to various chronic stresses. Here, we demonstrate that in hepatocytes, ERS results in increased Cx43 expression and cell-cell coupling. Co-culture of ER-stressed "donor" cells resulted in intercellular transmission of ERS and dysfunction to ERS-naive "recipient" cells ("bystander response"), which could be prevented by genetic or pharmacologic suppression of Cx43. Hepatocytes from obese mice were able to transmit ERS to hepatocytes from lean mice, and mice lacking liver Cx43 were protected from diet-induced ERS, insulin resistance, and hepatosteatosis. Taken together, our results indicate that in obesity, the increased Cx43-mediated cell-cell coupling allows intercellular propagation of ERS. This novel maladaptive response to over-nutrition exacerbates the tissue ERS burden, promoting hepatosteatosis and impairing whole-body glucose metabolism.

Differential Gene Expression and Biological Analyses of Primary Hepatocytes Following D-Chiro-Inositol Supplement

Dietary supplements have improved the prevention of insulin resistance and metabolic diseases, which became a research hotspot in food science and nutrition. Obesity and insulin resistance, caused by a high-fat diet, eventually result in severe metabolic diseases, can be prevented with the dietary supplement D-chiro-inositol (DCI). In this work, we isolated mice primary hepatocytes with palmitic acid stimulation and DCI was applied to compare and contrast its effects of in primary hepatocyte biology. Before and after intervention with DCI, we used RNA-Seq technology to establish a primary hepatocyte transcriptome gene profile. We found that both PA and DCI cause a wide variation in gene expression. Particularly, we found that DCI plays critical role in this model by acting on glycolysis and gluconeogenesis. Overall, we generated extensive transcripts from primary hepatocytes and uncovered new functions and gene targets for DCI.

Four weeks exercise training enhanced the hepatic insulin sensitivity in high fat- and high carbohydrate-diet fed hyperinsulinemic rats

Aim

Hyperinsulinemia is considered the primary defect underlying the development of type 2 diabetes. The liver is essential for the regular glucose homeostasis. In this study, we examined the effect of physical training on the insulin signaling, oxidative stress enzymes and Glucose-6-phosphatase(G6Pase) activity in the liver of Wistar rats.

Methods

Adult male Wistar rats were divided into Control diet group(C), High carbohydrate diet(HCD), High fat diet(HFD), HCD and HFD with training(HCD Ex & HFD Ex). HFD Ex and HCD Ex were trained on a small animal treadmill running at 20 m/min for 30 min, 5 days/wk. The present work investigated the effect of training on hepatic insulin receptor(InsR) signaling events, oxidative stress marker expressions and G6Pase activity in hyperinsulinemic rats.

Results

High carbohydrate and fat feeding led to hyperinsulinemic status with increased hepatic G6Pase activity and impaired phosphorylation of insulin receptor substrate 1(IRS1) and reduced expression of antioxidant enzymes.Training significantly reduced hepatic G6Pase activity, upregulated phosphoinositide 3 kinase(PI3K) docking site phosphorylation and downregulated the negative IRS1 phosphorylations thereby increasing the glucose transporter(GLUT) expressions (aa(P < 0.001) when compared to HFD, b(P < 0.01),bb (P < 0.001 when compared to HCD). Anti oxidant enzymes like CAT, SOD, eNOS expression were increased with reduction in the expression of inflammatory enzymes like TNF-α and COX-2 (*(P < 0.05),**(P < 0.01),***(P < 0.001) when compared to control, †(P < 0.05),††(P < 0.01),†††(P < 0.001) when compared to HFD and HCD).

Conclusion

Thus, our study shows that four weeks training enhanced the hepatic insulin sensitivity in high fat and high carbohydrate-diet fed hyperinsulinemic rats.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-020-00694-y.

Long-term co-administration of sodium nitrite and sodium hydrosulfide inhibits hepatic gluconeogenesis in male type 2 diabetic rats: Role of PI3K-Akt-eNOS pathway

OBJECTIVE

A deficiency in hydrogen sulfide (H₂S) and nitric oxide (NO) contributes to the development of type 2 diabetes (T2D). An inhibitory effect on liver gluconeogenesis has been reported in rats with T2D with co-administration of sodium nitrite and sodium hydrosulfide (NaSH); the underlying mechanisms have however not yet been elucidated. The aim of this study is to determine the long-term effects of co-administering sodium nitrite and NaSH on expression of genes involved in liver gluconeogenesis in rats with T2D.

METHODS

T2D was induced using a high fat diet combined with low-dose of streptozotocin (30 mg/kg). Rats were divided into 5 groups (n = 7/group): Control, T2D, T2D + nitrite, T2D + NaSH, and T2D + nitrite+NaSH. Nitrite (50 mg/L) and NaSH (0.28 mg/kg) were administered for 9 weeks. Intraperitoneal pyruvate tolerance test (PTT) was performed at the end of the ninth week and mRNA expressions of PI3K, Akt, eNOS, PEPCK, G6Pase, and FBPase were measured in the liver.

RESULTS

Co-administration of nitrite and NaSH decreased elevated serum glucose concentrations during PTT. Compared to T2D + nitrite, co-administration of nitrite and NaSH resulted in significant increases in mRNA expression of PI3K, Akt, and eNOS and significant decreases in mRNA expression of G6Pase and FBPase but had no effect on PEPCK expression.

CONCLUSION

Long-term NaSH administration at low-dose, potentiated the inhibitory effects of nitrite on mRNA expression of key liver gluconeogenic enzymes in rats with T2D. This inhibitory effect of nitrite and NaSH co-administration on gluconeogenesis were associated with increased gene expression of PI3K, Akt, and eNOS in the liver.