Protein phosphatase 4 (PP4) functions as a critical regulator in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance

SMEK1 ablation promotes glucose uptake and improves obesity-related metabolic dysfunction via AMPK signaling pathway

Obesity has become a major risk of global public health. SMEK1 is also known as a regulatory subunit of protein phosphatase 4 (PP4). Both PP4 and SMEK1 have been clarified in many metabolic functions, including the regulation of hepatic gluconeogenesis and glucose transporter gene expression in yeast. Whether SMEK1 participates in obesity and the broader metabolic role in mammals is unknown. Thus, we investigated the function of SMEK1 in white adipose tissue and glucose uptake. GWAS/GEPIA/GEO database was used to analyze the correlation between SMEK1 and metabolic phenotypes/lipid metabolism-related genes/obesity. Smek1 KO mice were generated to identify the role of SMEK1 in obesity and glucose homeostasis. Cell culture and differentiation of stromal-vascular fractions (SVFs) and 3T3-L1 were used to determine the mechanism. 2-NBDG was used to measure the glucose uptake. Compound C was used to confirm the role of AMPK. We elucidated that SMEK1 was correlated with obesity and adipogenesis. Smek1 deletion enhanced adipogenesis in both SVFs and 3T3-L1. Smek1 KO protected mice from obesity and had protective effects on metabolic disorders, including insulin resistance and inflammation. Smek1 KO mice had lower levels of fasting serum glucose. We found that SMEK1 ablation promoted glucose uptake by increasing p-AMPKα(T172) and the transcription of Glut4 when the effect on AMPK-regulated glucose uptake was due to the PP4 catalytic subunits (PPP4C). Our findings reveal a novel role of SMEK1 in obesity and glucose homeostasis, providing a potential new therapeutic target for obesity and metabolic dysfunction.NEW & NOTEWORTHY Our study clarified the relationship between SMEK1 and obesity for the first time and validated the conclusion in multiple ways by combining available data from public databases, human samples, and animal models. In addition, we clarified the role of SMEK1 in glucose uptake, providing an in-depth interpretation for the study of its function in glucose metabolism.

LncRNA MEG3 Restrains Hepatic Lipogenesis via the FOXO1 Signaling Pathway in HepG2 Cells

Nonalcoholic fatty liver disease (NAFLD) become a main public health concern, and is characterized by lipid accumulation in the hepatocytes. We found that overexpression of lncRNA MEG3 significantly reduced the expression of FOXO1, ACC1, and FAS, and subsequently decreased the lipid accumulation in HepG2 cells. Moreover, inhibition of lncRNA MEG3 could increase the lipid accumulation and the mRNA and protein levels of FOXO1, ACC1, and FAS. Further study showed that lncRNA MEG3 regulates the lipogenesis process by inhibiting the entry of FOXO1 into the nucleus translocation. Our study demonstrated that lncRNA MEG3 regulates de novo lipogenesis by decreasing the expression and nucleus translocation of FOXO1 in HepG2 cells, suggesting that lncRNA MEG3 could be a promising therapeutic target in lipid metabolic disorders.

Protective effect of manganese treatment on insulin resistance in HepG2 hepatocytes

Introduction

Objectives: manganese (Mn) is closely related to type 2 diabetes mellitus and insulin resistance (IR), but the exact mechanism is unclear. This study aimed to explore the regulatory effects and mechanism of Mn on IR using hepatocyte IR model induced by high palmitate (PA), high glucose (HG) or insulin. Methods: HepG2 cells were exposed to PA (200 μM), HG (25 mM) or insulin (100 nM) respectively, alone or with 5 μM Mn for 24 hours. The expression of key proteins in insulin signaling pathway, intracellular glycogen content and glucose accumulation, reactive oxygen species (ROS) level and Mn superoxide dismutase (MnSOD) activity were detected. Results: compared with control group, the expression of phosphorylated protein kinase B (Akt), glycogen synthase kinase-3β (GSK-3β) and forkhead box O1 (FOXO1) in the three IR groups was declined, and this decrease was reversed by Mn. The reduction of intracellular glycogen content and increase in glucose accumulation in IR groups were also inhibited by Mn. Additionally, the production of ROS was increased in IR models, compared with normal control group, while Mn reduced the excessive production of ROS induced by PA, HG or insulin. However, Mn did not alter the activity of MnSOD in the three IR models. Conclusion: this study demonstrated that Mn treatment can improve IR in hepatocytes. The mechanism is probably by reducing the level of intracellular oxidative stress, enhancing the activity of Akt/GSK-3β/FOXO1 signal pathway, promoting glycogen synthesis, and inhibiting gluconeogenesis.

Protein phosphatases and their targets: Comprehending the interactions in plant signaling pathways

Protein phosphorylation is a vital reversible post-translational modification. This process is established by two classes of enzymes: protein kinases and protein phosphatases. Protein kinases phosphorylate proteins while protein phosphatases dephosphorylate phosphorylated proteins, thus, functioning as 'critical regulators' in signaling pathways. The eukaryotic protein phosphatases are classified as phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine (Ser)/threonine (Thr) specific phosphatases (STPs) that dephosphorylate Ser and Thr residues. The PTP family dephosphorylates Tyr residues while dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. The composition of these enzymes as well as their substrate specificity are important determinants of their functional significance in a number of cellular processes and stress responses. Their role in animal systems is well-understood and characterized. The functional characterization of protein phosphatases has been extensively covered in plants, although the comprehension of their mechanistic basis is an ongoing pursuit. The nature of their interactions with other key players in the signaling process is vital to our understanding. The substrates or targets determine their potential as well as magnitude of the impact they have on signaling pathways. In this article, we exclusively overview the various substrates of protein phosphatases in plant signaling pathways, which are a critical determinant of the outcome of various developmental and stress stimuli.

Action mechanism of hypoglycemic principle 9-(R)-HODE isolated from cortex lycii based on a metabolomics approach

The 9-(R)-HODE is an active compound isolated from cortex lycii that showed significant hypoglycemic effects in our previous in vitro study. In this study, 9-(R)-HODE’s in vivo hypoglycemic activity and effect on alleviating diabetic complications, together with its molecular mechanism, was investigated using a metabolomics approach. The monitored regulation on dynamic fasting blood glucose, postprandial glucose, body weight, biochemical parameters and histopathological analysis confirmed the hypoglycemic activity and attenuation effect, i.e., renal lesions, of 9-(R)-HODE. Subsequent metabolomic studies indicated that 9-(R)-HODE induced metabolomic alterations primarily by affecting the levels of amino acids, organic acids, alcohols and amines related to amino acid metabolism, glucose metabolism and energy metabolism. By mediating the related metabolism or single molecules related to insulin resistance, e.g., kynurenine, myo-inositol and the branched chain amino acids leucine, isoleucine and valine, 9-(R)-HODE achieved its therapeutic effect. Moreover, the mediation of kynurenine displayed a systematic effect on the liver, kidney, muscle, plasma and faeces. Lipidomic studies revealed that 9-(R)-HODE could reverse the lipid metabolism disorder in diabetic mice mainly by regulating phosphatidylinositols, lysophosphatidylcholines, lysophosphatidylcholines, phosphatidylserine, phosphatidylglycerols, lysophosphatidylglycerols and triglycerides in both tissues and plasma. Treatment with 9-(R)-HODE significantly modified the structure and composition of the gut microbiota. The SCFA-producing bacteria, including Rikenellaceae and Lactobacillaceae at the family level and Ruminiclostridium 6, Ruminococcaceae UCG 014, Mucispirillum, Lactobacillus, Alistipes and Roseburia at the genus level, were increased by 9-(R)-HODE treatment. These results were consistent with the increased SCFA levels in both the colon content and plasma of diabetic mice treated with 9-(R)-HODE. The tissue DESI‒MSI analysis strongly confirmed the validity of the metabolomics approach in illustrating the hypoglycemic and diabetic complications-alleviation effect of 9-(R)-HODE. The significant upregulation of liver glycogen in diabetic mice by 9-(R)-HODE treatment validated the interpretation of the metabolic pathways related to glycogen synthesis in the integrated pathway network. Altogether, 9-(R)-HODE has the potential to be further developed as a promising candidate for the treatment of diabetes.

Flavin-Containing Monooxygenase 3 (FMO3) Is Critical for Dioxin-Induced Reorganization of the Gut Microbiome and Host Insulin Sensitivity

Exposure to some environmental pollutants can have potent endocrine-disrupting effects, thereby promoting hormone imbalance and cardiometabolic diseases such as non-alcoholic fatty liver disease (NAFLD), diabetes, and cardiorenal diseases. Recent evidence also suggests that many environmental pollutants can reorganize the gut microbiome to potentially impact these diverse human diseases. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is among the most potent endocrine-disrupting dioxin pollutants, yet our understanding of how TCDD impacts the gut microbiome and systemic metabolism is incompletely understood. Here, we show that TCDD exposure in mice profoundly stimulates the hepatic expression of flavin-containing monooxygenase 3 (Fmo3), which is a hepatic xenobiotic metabolizing enzyme that is also responsible for the production of the gut microbiome-associated metabolite trimethylamine N-oxide (TMAO). Interestingly, an enzymatic product of FMO3 (TMAO) has been associated with the same cardiometabolic diseases that these environmental pollutants promote. Therefore, here, we examined TCDD-induced alterations in the gut microbiome, host liver transcriptome, and glucose tolerance in Fmo3^+/+ and Fmo3^-/- mice. Our results show that Fmo3 is a critical component of the transcriptional response to TCDD, impacting the gut microbiome, host liver transcriptome, and systemic glucose tolerance. Collectively, this work uncovers a previously underappreciated role for Fmo3 in integrating diet-pollutant-microbe-host interactions.

Association between new markers of cardiovascular risk and hepatic insulin resistance in those at high risk of developing type 2 diabetes

AIM/HYPOTHESIS

Hepatic insulin resistance (HIR) is considered to be an independent predictor of metabolic disorders and plays an important role in systemic inflammation, which contributes to abnormalities in cardiovascular disease (CVD) risk factors. The aim of this study was to investigate the relationship between HIR and new markers of cardiovascular risks, including leptin/adiponectin ratio (L/A), lipoprotein(a) [Lp(a)], and tumor necrosis factor alpha (TNF-α), at comparable whole body insulin sensitivity in non-diabetic individuals with or without CVD and at high risk of developing type 2 diabetes.

METHODS

The HIR index, L/A, Lp(a), and TNF-α were measured in 50 participants with CVD and in 200 without CVD (1:4 ratio). These were also matched for the homeostatic model assessment for insulin resistance (HOMA-IR) and Matsuda-insulin sensitivity index (ISI) in an observational study design.

RESULTS

The HIR index (1.52 ± 0.14 vs. 1.45 ± 0.17, p < 0.02), L/A (3.22 ± 3.10 vs. 2.09 ± 2.27, p < 0.004), and levels of Lp(a) (66.6 ± 49.5 vs. 37.9 ± 3 6.8 mg/dL, p < 0.0001) and TNF-α (18.9 ± 21.8 vs. 5.4 ± 7.1 pg/mL, p < 0.0001) were higher in those with CVD than those without CVD. HOMA-IR and ISI were not significantly different (p = 0.88 and p = 0.35, respectively). The HIR index was directly correlated with L/A (r = 0.41, p < 0.0001), Lp(a) (r = 0.20, p < 0.002), TNF- α (r = 0.14, p < 0.03), and diastolic blood pressure (DBP) (r = 0.13, p < 0.03). The stepwise model analysis showed that L/A, Lp(a), and TNF-α explained about 20% of the variation in the HIR indices of all the participants (p < 0.02).

CONCLUSIONS/INTERPRETATIONS

Our results suggest a positive association between HIR and new markers of cardiovascular risk [L/A, Lp(a), and TNF- α] at comparable whole body insulin sensitivity in those with or without CVD and at high risk of developing type 2 diabetes.

Effects of Resistant Starch Interventions on Metabolic Biomarkers in Pre-Diabetes and Diabetes Adults

Simple lifestyle changes can prevent or delay the onset of type 2 diabetes mellitus (T2DM). In addition to maintaining a physically active way of life, the diet has become one of the bases in managing TD2M. Due to many studies linking the ability of resistant starch (RS) to a substantial role in enhancing the nutritional quality of food and disease prevention, the challenge of incorporating RS into the diet and increasing its intake remains. Therefore, we conducted this review to assess the potential benefits of RS on metabolic biomarkers in pre-diabetes and diabetes adults based on available intervention studies over the last decade. Based on the conducted review, we observed that RS intake correlates directly to minimize possible effects through different mechanisms for better control of pre-diabetic and diabetic conditions. In most studies, significant changes were evident in the postprandial glucose and insulin incremental area under the curve (iAUC). Comparative evaluation of RS consumption and control groups also showed differences with inflammatory markers such as TNF-α, IL-1β, MCP-1, and E-selectin. Only RS2 and RS3 were extensively investigated and widely reported among the five reported RS types. However, a proper comparison and conclusion are deemed inappropriate considering the variations observed with the study duration, sample size, subjects and their metabolic conditions, intervention doses, and the intervention base products. In conclusion, this result provides interesting insights into the potential use of RS as part of a sustainable diet in diabetes management and should be further explored in terms of the mechanism involved.

Investigation of insulin resistance through a multiorgan microfluidic organ-on-chip

The development of hepatic insulin resistance (IR) is a critical factor in developing type 2 diabetes (T2D), where insulin fails to inhibit hepatic glucose production but retains its capacity to promote hepatic lipogenesis. Improving insulin sensitivity can be effective in preventing and treating T2D. However, selective control of glucose and lipid synthesis has been difficult. It is known that excess white adipose tissue is detrimental to insulin sensitivity, whereas brown adipose tissue transplantation can restore it in diabetic mice. However, challenges remain in our understanding of liver-adipose communication because the confounding effects of hypothalamic regulation of metabolic function cannot be ruled out in previous studies. There is a lack of in vitro models that use primary cells to study cellular-crosstalk under insulin resistant conditions. Building upon our previous work on the microfluidic primary liver and adipose organ-on-chips, we report for the first time the development of integrated insulin resistant liver-adipose (white and brown) organ-on-chip. The design of the microfluidic device was carried out using computational fluid dynamics; the experimental studies were conducted by carrying out detailed biochemical analysis RNA-seq analysis on both cell types. Further, we tested the hypothesis that brown adipocytes regulated both hepatic insulin sensitivity and lipogenesis. Our results show effective co-modulation of hepatic glucose and lipid synthesis through a platform for identifying potential therapeutics for IR and diabetes.

Protein phosphatase 4 controls circadian clock dynamics by modulating CLOCK/BMAL1 activity

In all organisms with circadian clocks, post-translational modifications of clock proteins control the dynamics of circadian rhythms, with phosphorylation playing a dominant role. All major clock proteins are highly phosphorylated, and many kinases have been described to be responsible. In contrast, it is largely unclear whether and to what extent their counterparts, the phosphatases, play an equally crucial role. To investigate this, we performed a systematic RNAi screen in human cells and identified protein phosphatase 4 (PPP4) with its regulatory subunit PPP4R2 as critical components of the circadian system in both mammals and Drosophila Genetic depletion of PPP4 shortens the circadian period, whereas overexpression lengthens it. PPP4 inhibits CLOCK/BMAL1 transactivation activity by binding to BMAL1 and counteracting its phosphorylation. This leads to increased CLOCK/BMAL1 DNA occupancy and decreased transcriptional activity, which counteracts the "kamikaze" properties of CLOCK/BMAL1. Through this mechanism, PPP4 contributes to the critical delay of negative feedback by retarding PER/CRY/CK1δ-mediated inhibition of CLOCK/BMAL1.

Protein Phosphatase 4 Promotes Hepatocyte Lipoapoptosis by Regulating RAC1/MLK3/JNK Pathway

Lipotoxicity-induced apoptosis, also referred to as lipoapoptosis, is one of the important initial factors promoting the progression from hepatosteatosis to nonalcoholic steatohepatitis (NASH). Saturated free fatty acids (SFAs), which are increased significantly in NASH, are directly hepatotoxic which induce hepatocyte lipoapoptosis. Previously, we reported that protein phosphatase 4 (PP4) was a novel regulator of hepatic insulin resistance and lipid metabolism, but its role in hepatic lipoapoptosis remains unexplored. In this study, we found out that PP4 was upregulated in the livers of western diet-fed-induced NASH mice and SFA-treated murine primary hepatocytes and HepG2 cells. In addition, we found for the first time that suppression of PP4 decreased SFA-induced JNK activation and expression of key modulators of hepatocyte lipoapoptosis including p53-upregulated modulator of apoptosis (PUMA) and Bcl-2-interacting mediator (Bim) and reduced hepatocyte lipoapoptosis level as well both in vitro and in vivo. Further study revealed that PP4 induced JNK activation and lipoapoptosis-related protein expression by regulating the RAC1/MLK3 pathway instead of the PERK/CHOP pathway. The effects of palmitate-treated and PP4-induced lipoapoptosis pathway activation were largely abolished by RAC1 inhibition. Moreover, we identified that PP4 interacted with RAC1 and regulated GTPase activity of RAC1. In conclusion, these results demonstrated that PP4 was a novel regulator of hepatocyte lipoapoptosis and mediated hepatocyte lipoapoptosis by regulating the RAC1/MLK3/JNK signaling pathway. Our finding provided new insights into the mechanisms of this process.

MicroRNA-191 blocking the translocation of GLUT4 is involved in arsenite-induced hepatic insulin resistance through inhibiting the IRS1/AKT pathway

Environmental exposure to arsenic can cause a variety of health problems. Epidemiological and experimental studies have established a diabetogenic role for arsenic, but the mechanisms responsible for arsenic-induced impairment of insulin action are unclear. MicroRNAs (miRNAs) are involved in various metabolic disorders, particularly in the development of insulin resistance. The present study investigated whether arsenite, an active form of arsenic, induces hepatic insulin resistance and the mechanisms underlying it. After male C57BL/6J mice were exposed to arsenite (0 or 20 ppm) in drinking water for 12 months, intraperitoneal glucose tolerance tests (IPGTTs) and insulin tolerance tests (ITTs) revealed an arsenite-induced glucose metabolism disorder. Hepatic glycogen levels were lower in arsenite-exposed mice. Further, for livers of mice exposed to arsenite, miR-191 levels were higher, and protein levels of insulin receptor substrate 1 (IRS1), p-IRS1, and phospho-protein kinase B (p-AKT) were lower. Further, glucose transporter 4 (GLUT4) had lower levels on the plasma membrane. For insulin-treated L-02 cells, arsenite decreased glucose consumption and glycogen levels, increased miR-191 levels, and inhibited the IRS1/AKT pathway and the translocation of GLUT4 from the cytoplasm to the plasma membrane. For insulin-treated L-02 cells, the decreases of glucose consumption, glycogen levels, GLUT4 on the plasma membrane, and p-AKT levels induced by arsenite were reversed by SC79 (agonist of AKT) and an miR-191 inhibitor; these effects caused by miR-191 inhibitor were restored by IRS1 siRNA. In insulin-treated L-02 cells, miR-191, via IRS1, was involved in the arsenite-induced decreases of glucose consumption and glycogen levels and in inhibition of the translocation of GLUT4. Thus, miR-191 blocking the translocation of GLUT4 was involved in arsenite-induced hepatic insulin resistance through inhibiting the IRS1/AKT pathway. Our study reveals a mechanism for arsenite-induced hepatic insulin resistance, which provides clues for discovering biomarkers for the development of type 2 diabetes and for prevention and treatment of arsenic poisoning.

Diagnostic significance of serum PP4R1 and its predictive value for the development of chronic complications in patients with type 2 diabetes mellitus

BACKGROUND

Protein phosphatase 4 regulatory subunit 1 (PP4R1) is one of the regulatory subunits of PP4. It has been determined to be involved in the regulation of TNF-α-induced hepatic insulin resistance and gluconeogenesis. Considering the important role of PP4R1 in hepatic insulin resistance, the current study explored the expression and diagnostic value of PP4R1 in type 2 diabetes mellitus (T2DM) patients and further investigated its predictive value for the development of chronic complications.

METHOD

Hundred and five patients with T2DM and 97 healthy controls were collected. qRT-PCR was used for the measurement of serum PP4R1 mRNA level in both T2DM and control groups. The diagnostic value of PP4R1 in T2DM patients was evaluated using receiver operating characteristic (ROC) curve. Kaplan-Meier methods and Cox regression analysis were used to evaluate the predictive value of PP4R1 for the development of chronic complications in T2DM patients.

RESULTS

PP4R1 was determined to be elevated in the serum of T2DM patients compared with healthy controls. Serum PP4R1 had the potential to distinguish T2DM patients from healthy controls with a sensitivity of 81.9% and specificity of 82.5%. Patients with high PP4R1 expression experienced more chronic complications events. The multivariate Cox analysis results suggested that serum PP4R1 expression was an independent factor for the occurrence of chronic complications for T2DM patients.  CONCLUSION: PP4R1 is elevated in the serum of T2DM patients, had the potential to distinguish T2DM patients from healthy controls. PP4R1 serves as a promising biomarker for predicting the risk of future chronic complications in T2DM patients.

Phosphatases in toll-like receptors signaling: the unfairly-forgotten

Over the past 2 decades, pattern recognition receptors (PRRs) have been shown to be on the front line of many illnesses such as autoimmune, inflammatory, and neurodegenerative diseases as well as allergies and cancer. Among PRRs, toll-like receptors (TLRs) are the most studied family. Dissecting TLRs signaling turned out to be advantageous to elaborate efficient treatments to cure autoimmune and chronic inflammatory disorders. However, a broad understanding of TLR effectors is required to propose a better range of cures. In addition to kinases and E3 ubiquitin ligases, phosphatases emerge as important regulators of TLRs signaling mediated by NF-κB, type I interferons (IFN I) and Mitogen-Activated Protein Kinases signaling pathways. Here, we review recent knowledge on TLRs signaling modulation by different classes and subclasses of phosphatases. Thus, it becomes more and more evident that phosphatases could represent novel therapeutic targets to control pathogenic TLRs signaling. Video Abstract.

Functional roles of protein phosphatase 4 in multiple aspects of cellular physiology: a friend and a foe

Protein phosphatase 4 (PP4), one of serine/threonine phosphatases, is involved in many critical cellular pathways, including DNA damage response (DNA repair, cell cycle regulation, and apoptosis), tumorigenesis, cell migration, immune response, stem cell development, glucose metabolism, and diabetes. PP4 has been steadily studied over the past decade about wide spectrum of physiological activities in cells. Given the many vital functions in cells, PP4 has great potential to develop into the finding of key working mechanisms and effective treatments for related diseases such as cancer and diabetes. In this review, we provide an overview of the cellular and molecular mechanisms by which PP4 impacts and also discuss the functional significance of it in cell health.

Low utilization of glucose in the liver causes diet-induced hypercholesterolemia in exogenously hypercholesterolemic rats

Exogenously hypercholesterolemic (ExHC) rats develop diet-induced hypercholesterolemia (DIHC) when fed with dietary cholesterol. Previously, we reported that, under the high-sucrose-diet-feeding condition, a loss-of-function mutation in Smek2 results in low activity of fatty acid synthase (FAS) followed by the shortage of hepatic triacylglycerol content in ExHC rats and the onset of DIHC. However, the relationship between the Smek2 mutation and FAS dysfunction is still unclear. Here, we focused on carbohydrate metabolism, which provides substrates for FAS, and analyzed carbohydrate and lipid metabolisms in ExHC rats to clarify how the deficit of Smek2 causes DIHC. Male ExHC and SD rats were fed high-sucrose or high-starch diets containing 1% cholesterol for 2 weeks. Serum cholesterol levels of the ExHC rats were higher, regardless of the dietary carbohydrate. Hepatic triacylglycerol levels were higher in only the SD rats fed the high-sucrose diet. Moreover, the ExHC rats exhibited a diabetes-like status and accumulation of hepatic glycogen and low hepatic mRNA levels of liver-type phosphofructokinase (Pfkl), which encodes a rate-limiting enzyme for glycolysis. These results suggest that the glucose utilization, particularly glycolysis, is impaired in the liver of ExHC rats. To evaluate how the diet with extremely low glucose affect to DIHC, ExHC.BN-Dihc2^BN, a congenic strain that does not develop DIHC, and ExHC rats were fed a high-fructose diet containing 1% cholesterol for 2 weeks. The serum cholesterol and hepatic triacylglycerol levels were similar in the strains. Results of water-soluble metabolite analysis with primary hepatocytes, an increase in fructose-6-phosphate and decreases in succinate, malate and aspartate in ExHC rats, support impaired glycolysis in the ExHC rats. Thus, the Smek2 mutation causes abnormal hepatic glucose utilization via downregulation of Pfkl expression. This abnormal glucose metabolism disrupts hepatic fatty acid synthesis and causes DIHC in the ExHC rats.

Identification of Protein Phosphatase 4 Inhibitory Protein That Plays an Indispensable Role in DNA Damage Response

Protein phosphatase 4 (PP4) is a crucial protein complex that plays an important role in DNA damage response (DDR), including DNA repair, cell cycle arrest and apoptosis. Despite the significance of PP4, the mechanism by which PP4 is regulated remains to be elucidated. Here, we identified a novel PP4 inhibitor, protein phosphatase 4 inhibitory protein (PP4IP) and elucidated its cellular functions. PP4IP-knockout cells were generated using the CRISPR/Cas9 system, and the phosphorylation status of PP4 substrates (H2AX, KAP1, and RPA2) was analyzed. Then we investigated that how PP4IP affects the cellular functions of PP4 by immunoprecipitation, immunofluorescence, and DNA double-strand break (DSB) repair assays. PP4IP interacts with PP4 complex, which is affected by DNA damage and cell cycle progression and decreases the dephosphorylational activity of PP4. Both overexpression and depletion of PP4IP impairs DSB repairs and sensitizes cells to genotoxic stress, suggesting timely inhibition of PP4 to be indispensable for cells in responding to DNA damage. Our results identify a novel inhibitor of PP4 that inhibits PP4-mediated cellular functions and establish the physiological importance of this regulation. In addition, PP4IP might be developed as potential therapeutic reagents for targeting tumors particularly with high level of PP4C expression.

Therapeutic Mechanisms of Herbal Medicines Against Insulin Resistance: A Review

Insulin resistance is a condition in which insulin sensitivity is reduced and the insulin signaling pathway is impaired. Although often expressed as an increase in insulin concentration, the disease is characterized by a decrease in insulin action. This increased workload of the pancreas and the consequent decompensation are not only the main mechanisms for the development of type 2 diabetes (T2D), but also exacerbate the damage of metabolic diseases, including obesity, nonalcoholic fatty liver disease, polycystic ovary syndrome, metabolic syndrome, and others. Many clinical trials have suggested the potential role of herbs in the treatment of insulin resistance, although most of the clinical trials included in this review have certain flaws and bias risks in their methodological design, including the generation of randomization, the concealment of allocation, blinding, and inadequate reporting of sample size estimates. These studies involve not only the single-flavored herbs, but also herbal formulas, extracts, and active ingredients. Numerous of in vitro and in vivo studies have pointed out that the role of herbal medicine in improving insulin resistance is related to interventions in various aspects of the insulin signaling pathway. The targets involved in these studies include insulin receptor substrate, phosphatidylinositol 3-kinase, glucose transporter, AMP-activated protein kinase, glycogen synthase kinase 3, mitogen-activated protein kinases, c-Jun-N-terminal kinase, nuclear factor-kappaB, protein tyrosine phosphatase 1B, nuclear factor-E2-related factor 2, and peroxisome proliferator-activated receptors. Improved insulin sensitivity upon treatment with herbal medicine provides considerable prospects for treating insulin resistance. This article reviews studies of the target mechanisms of herbal treatments for insulin resistance.

miR‑338‑3p mediates gluconeogenesis via targeting of PP4R1 in hepatocytes

Hyperglycaemia is a characteristic of type 2 diabetes. In hepatocytes, impaired insulin sensitivity leads to increased gluconeogenesis and decreased glycogenesis. MicroRNA (miR)‑338‑3p is associated with tumour necrosis factor (TNF)‑α‑induced suppression of hepatic glycogenesis via regulation of protein phosphatase 4 regulatory subunit 1 (PP4R1). However, the effect of miR‑338‑3p on gluconeogenesis in hepatocytes remains unknown. In a previous study, it was demonstrated that miR‑338‑3p is downregulated in the livers of mice and in mouse HEPA1‑6 hepatocytes following treatment with TNF‑α. In the present study, the effect of miR‑338‑3p on TNF‑α‑induced gluconeogenesis in hepatocytes was investigated. The levels of phosphorylated‑FOXO1/FOXO1, phosphoenolpyruvate carboxykinase (PEPCK), peroxisome proliferator‑activated receptor γ coactivator (PGC‑1α) and glucose‑6‑phosphatase (G6Pase) were measured by western blotting. The mRNA levels of PEPCK, PGC‑1α and G6Pase were determined by quantitative polymerase chain reaction. Pyruvate tolerance testing was used to determine the gluconeogenesis of mouse livers. The results demonstrated that treatment with TNF‑α resulted in increased levels of gluconeogenesis in the livers of mice and decreased miR‑338‑3p expression levels in HEPA1‑6 cells. Overexpression of miR‑338‑3p reversed TNF‑α‑induced glucose production via enhancement of phosphorylated forkhead box O1 levels and downregulation of the expression levels of genes associated with gluconeogenesis, including peroxisome proliferator‑activated receptor γ coactivator‑1α, phosphoenolpyruvate carboxykinase and glucose‑6‑phosphatase. However, inhibition of miR‑338‑3p expression was revealed to enhance gluconeogenesis in the livers of mice and in HEPA1‑6 cells. Furthermore, downregulation of PP4R1 was revealed to attenuate the effect on glucose production following treatment with miR‑338‑3p inhibitors. In conclusion, the results of the present study revealed that miR‑338‑3p may be involved in TNF‑α‑mediated gluconeogenesis via targeting of PP4R1 in hepatocytes.

MicroRNA-191, acting via the IRS-1/Akt signaling pathway, is involved in the hepatic insulin resistance induced by cigarette smoke extract

Cigarette smoke causes insulin resistance, which is associated with type 2 diabetes mellitus (T2DM). However, the mechanism by which this occurs remains poorly understood. Because the involvement of microRNAs (miRNAs) in the development of insulin resistance is largely unknown, we investigated, in hepatocytes, the roles of miR-191 in cigarette smoke extract (CSE)-induced insulin resistance. In L-02 cells, CSE not only decreased glucose uptake and glycogen levels but also reduced levels of insulin receptor substrate-1 (IRS-1) and Akt activation, effects that were blocked by SC79, an activator of Akt. CSE also increased miR-191 levels in L-02 cells. Furthermore, the inhibition of miR-191 blocked the decreases of IRS-1 and p-Akt levels, which antagonized the decreases of glucose uptake and glycogen levels in L-02 cells induced by CSE. These results reveal a mechanism by which miR-191 is involved in CSE-induced hepatic insulin resistance via the IRS-1/Akt signaling pathway, which helps to elucidate the mechanism for cigarette smoke-induced T2DM.

Vitamin C deficiency aggravates tumor necrosis factor α-induced insulin resistance

Chronic low-grade inflammation plays a major role in the development of insulin resistance. The potential role and underlying mechanism of vitamin C, an antioxidant and anti-inflammatory agent, was investigated in tumor necrosis factor-α (TNF-α)-induced insulin resistance. Gulonolactone oxidase knockout (Gulo^-/-) mice genetically unable to synthesize vitamin C were used to induce insulin resistance by continuously pumping small doses of TNF-α for seven days, and human liver hepatocellular carcinoma cells (HepG2 cells) were used to induce insulin resistance by treatment with TNF-α. Vitamin C deficiency aggravated TNF-α-induced insulin resistance in Gulo^-/- mice, resulting in worse glucose tolerance test (GTT) results, higher fasting plasma insulin level, and the inactivation of the protein kinase B (AKT)/glycogen synthase kinase-3β (GSK3β) pathway in the liver. Vitamin C deficiency also worsened liver lipid accumulation and inflammation in TNF-α-treated Gulo^-/- mice. In HepG2 cells, vitamin C reversed the TNF-α-induced reduction of glucose uptake and glycogen synthesis, which were mediated by increasing GLUT2 levels and the activation of the insulin receptor substrate (IRS-1)/AKT/GSK3β pathway. Furthermore, vitamin C inhibited the TNF-α-induced activation of not only the mitogen-activated protein kinase (MAPKs), but also nuclear factor-kappa B (NF-κB) signaling. Taken together, vitamin C is essential for preventing and improving insulin resistance, and the supplementing with vitamin C may be an effective therapeutic intervention for metabolic disorders.