PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

Epoxygenase Cyp2c44 Regulates Hepatic Lipid Metabolism and Insulin Signaling by Controlling FATP2 Localization and Activation of the DAG/PKCδ Axis

Cytochrome P450 epoxygenase Cyp2c44, a murine epoxyeicosatrienoic acid (EET)-producing enzyme, promotes insulin sensitivity, and Cyp2c44-/- mice show hepatic insulin resistance. Because insulin resistance leads to hepatic lipid accumulation and hyperlipidemia, we hypothesized that Cyp2c44 regulates hepatic lipid metabolism. Standard chow diet (SCD)-fed male Cyp2c44-/- mice had significantly decreased EET levels and increased hepatic and plasma lipid levels compared with wild-type mice. We showed increased hepatic plasma membrane localization of the FA transporter 2 (FATP2) and total unsaturated fatty acids and diacylglycerol (DAG) levels. Cyp2c44-/- mice had impaired glucose tolerance and increased hepatic plasma membrane-associated PKCδ and phosphorylated IRS-1, two negative regulators of insulin signaling. Surprisingly, SCD and high-fat diet (HFD)-fed Cyp2c44-/- mice had similar glucose tolerance and hepatic plasma membrane PKCδ levels, suggesting that SCD-fed Cyp2c44-/- mice have reached their maximal glucose intolerance. Inhibition of PKCδ resulted in decreased IRS-1 serine phosphorylation and improved insulin-mediated signaling in Cyp2c44-/- hepatocytes. Finally, Cyp2c44-/- HFD-fed mice treated with the analog EET-A showed decreased hepatic plasma membrane FATP2 and PCKδ levels with improved glucose tolerance and insulin signaling. In conclusion, loss of Cyp2c44 with concomitant decreased EET levels leads to increased hepatic FATP2 plasma membrane localization, DAG accumulation, and PKCδ-mediated attenuation of insulin signaling. Thus, Cyp2c44 acts as a regulator of lipid metabolism by linking it to insulin signaling.

ARTICLE HIGHLIGHTS

Nanoformulations for the diagnosis and treatment of metabolic dysfunction-associated steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) is a progressive phase of metabolic dysfunction-associated steatotic liver disease (MASLD) that develops into irreversible liver cirrhosis and hepatocellular carcinoma, ultimately necessitating liver transplantation as the sole life-saving option. However, given the drawbacks of liver transplantation, including invasiveness, chronic immunosuppression, and a lack of donor livers, prompt diagnosis and effective treatment are indispensable. Due to the limitations of liver biopsy and conventional imaging modalities in diagnosing MASH, as well as the potential hazards associated with liver-protecting medicines, numerous nanoformulations have been created for MASH theranostics. Particularly, there has been significant study interest in artificial nanoparticles, natural biomaterials, and bionic nanoparticles that exhibit exceptional biocompatibility and bioavailability. In this review, we summarized extracellular vesicles (EVs)-based omics analysis and Fe3O4-based functional magnetic nanoparticles as magnetic resonance imaging (MRI) contrast agents for MASH diagnosis. Additionally, artificial nanoparticles such as organic and inorganic nanoparticles, as well as natural biomaterials such as cells and cell-derived EVs and bionic nanoparticles including cell membrane-coated nanoparticles, have also been reported for MASH treatment owing to their specific targeting and superior therapeutic effect. This review has the potential to stimulate advancements in nanoformulation fabrication techniques. By exploring their compatibility with cell biology, it could lead to the creation of innovative material systems for efficient theragnostic uses for MASH. STATEMENT OF SIGNIFICANCE: People with metabolic dysfunction-associated steatohepatitis (MASH) will progress to fibrosis, cirrhosis, or even liver cancer. It is imperative to establish effective theragnostic techniques to stop MASH from progressing into a lethal condition. In our review, we summarize the advancement of artificial, natural, and bionic nanoparticles applied in MASH theragnosis. Furthermore, the issues that need to be resolved for these cutting-edge techniques are summarized to realize a more significant clinical impact. We forecast the key fields that will advance further as nanotechnology and MASH research progress. Generally, our discovery has significant implications for the advancement of nanoformulation fabrication techniques, and their potential to be compatible with cell biology could lead to the creation of innovative materials systems for effective MASH theragnostic.

PKCδ regulates the vascular biology in diabetic atherosclerosis

Diabetes mellitus, known for its complications, especially vascular complications, is becoming a globally serious social problem. Atherosclerosis has been recognized as a common vascular complication mechanism in diabetes. The diacylglycerol (DAG)-protein kinase C (PKC) pathway plays an important role in atherosclerosis. PKCs can be divided into three subgroups: conventional PKCs (cPKCs), novel PKCs (nPKCs), and atypical PKCs (aPKCs). The aim of this review is to provide a comprehensive overview of the role of the PKCδ pathway, an isoform of nPKC, in regulating the function of endothelial cells, vascular smooth muscle cells, and macrophages in diabetic atherosclerosis. In addition, potential therapeutic targets regarding the PKCδ pathway are summarized. Video Abstract.

Rottlerin suppresses lipid accumulation by inhibiting de novo lipogenesis and adipogenesis via LRP6/mTOR/SREBP1C in 3T3-L1 adipocytes

Rottlerin is isolated from Mallotus japonicus, a plant rich in polyphenols. Rottlerin is a selective PKCδ-inhibitor and is also known as an uncoupler of oxidative phosphorylation and anti-neoplastic agent. However, its anti-obesity effect is yet to be established. Therefore, this study tested whether rottlerin inhibits adipogenesis and de novo lipogenesis via the LRP6/mTOR/SREBP1C pathway in 3T3-L1 adipocytes. Rottlerin dramatically decreased lipid accumulation assessed by Oil Red O as evidence to support the cellular phenotype (p < 0.001). Pivotal messenger RNA and protein expressions associated with de novo lipogenesis (SREBP1C, ACC1, FAS, and SCD1) and adipogenesis (PPARγ and C/EBPα) were subsequentially verified by rottlerin in a dose-dependent manner (p < 0.05). Further investigation revealed that rottlerin reduced the AKT/mTOR pathway via diminished total protein of LRP6 (p < 0.05). Collectively, these findings establish a causal link between rottlerin, LRP6, and the altered nutrient-sensing mTOR pathway, in which rottlerin regulates de novo lipogenesis and adipogenesis in white adipocytes.

Hepatic protein kinase Cbeta deficiency mitigates late-onset obesity

Although aging is associated with progressive adiposity and a decline in liver function, the underlying molecular mechanisms and metabolic interplay are incompletely understood. Here, we demonstrate that aging induces hepatic protein kinase Cbeta (PKCβ) expression, while hepatocyte PKCβ deficiency (PKCβHep-/-) in mice significantly attenuates obesity in aged mice fed a high-fat diet. Compared with control PKCβfl/fl mice, PKCβHep-/- mice showed elevated energy expenditure with augmentation of oxygen consumption and carbon dioxide production which was dependent on β3-adrenergic receptor signaling, thereby favoring negative energy balance. This effect was accompanied by induction of thermogenic genes in brown adipose tissue (BAT) and increased BAT respiratory capacity, as well as a shift to oxidative muscle fiber type with an improved mitochondrial function, thereby enhancing oxidative capacity of thermogenic tissues. Furthermore, in PKCβHep-/- mice, we determined that PKCβ overexpression in the liver mitigated elevated expression of thermogenic genes in BAT. In conclusion, our study thus establishes hepatocyte PKCβ induction as a critical component of pathophysiological energy metabolism by promoting progressive hepatic and extrahepatic metabolic derangements in energy homeostasis, contributing to late-onset obesity. These findings have potential implications for augmenting thermogenesis as a means of combating aging-induced obesity.

CD38/ADP-ribose/TRPM2-mediated nuclear Ca2+ signaling is essential for hepatic gluconeogenesis in fasting and diabetes

Hepatic glucose production by glucagon is crucial for glucose homeostasis during fasting, yet the underlying mechanisms remain incompletely delineated. Although CD38 has been detected in the nucleus, its function in this compartment is unknown. Here, we demonstrate that nuclear CD38 (nCD38) controls glucagon-induced gluconeogenesis in primary hepatocytes and liver in a manner distinct from CD38 occurring in the cytoplasm and lysosomal compartments. We found that the localization of CD38 in the nucleus is required for glucose production by glucagon and that nCD38 activation requires NAD⁺ supplied by PKCδ-phosphorylated connexin 43. In fasting and diabetes, nCD38 promotes sustained Ca²⁺ signals via transient receptor potential melastatin 2 (TRPM2) activation by ADP-ribose, which enhances the transcription of glucose-6 phosphatase and phosphoenolpyruvate carboxykinase 1. These findings shed light on the role of nCD38 in glucagon-induced gluconeogenesis and provide insight into nuclear Ca²⁺ signals that mediate the transcription of key genes in gluconeogenesis under physiological conditions.

Uncoupling hepatic insulin resistance - hepatic inflammation to improve insulin sensitivity and to prevent impaired metabolism-associated fatty liver disease in type 2 diabetes

Diabetes mellitus is a metabolic disease clinically-characterized as acute and chronic hyperglycemia. It is emerging as one of the common conditions associated with incident liver disease in the US. The mechanism by which diabetes drives liver disease has become an intense topic of discussion and a highly sought-after therapeutic target. Insulin resistance (IR) appears early in the progression of type 2 diabetes (T2D), particularly in obese individuals. One of the co-morbid conditions of obesity-associated diabetes that is on the rise globally is referred to as non-alcoholic fatty liver disease (NAFLD). IR is one of a number of known and suspected mechanism that underlie the progression of NAFLD which concurrently exhibits hepatic inflammation, particularly enriched in cells of the innate arm of the immune system. In this review we focus on the known mechanisms that are suspected to play a role in the cause-effect relationship between hepatic IR and hepatic inflammation and its role in the progression of T2D-associated NAFLD. Uncoupling hepatic IR/hepatic inflammation may break an intra-hepatic vicious cycle, facilitating the attenuation or prevention of NAFLD with a concurrent restoration of physiologic glycemic control. As part of this review, we therefore also assess the potential of a number of existing and emerging therapeutic interventions that can target both conditions simultaneously as treatment options to break this cycle.

TRIM21 ameliorates hepatic glucose and lipid metabolic disorders in type 2 diabetes mellitus by ubiquitination of PEPCK1 and FASN

Hepatic glucose and lipid metabolism disorders promote the development and progression of type 2 diabetes mellitus (T2DM), yet the underlying mechanisms are not fully understood. Here, we identify tripartite motif-containing protein 21 (TRIM21), a class IV TRIM family member, as a pivotal regulator of hepatic metabolism in T2DM for the first time. Bioinformatic analysis suggests that TRIM21 expression is significantly reduced in T2DM patients. Intriguingly, in a mouse model of obese diabetes, TRIM21 expression is predominantly reduced in the liver rather than in other metabolic organs. It is further demonstrated that hepatic overexpression of TRIM21 significantly ameliorates glucose intolerance, insulin resistance, hepatic steatosis, and dyslipidemia in obese diabetic mice. In contrast, the knockdown of TRIM21 promotes glucose intolerance, insulin resistance, and triglyceride accumulation. Mechanistically, both phosphoenolpyruvate carboxykinase 1 (PEPCK1) and fatty acid synthase (FASN) are the hepatic targets of TRIM21. We revealed that TRIM21 promotes the degradation of PEPCK1 and FASN through a direct protein-protein interaction mediated K48-linked ubiquitination. Notably, overexpression of PEPCK1 and FASN essentially abolished the beneficial effects achieved by TRIM21 overexpression in obese diabetic mice. Overall, our data demonstrate that TRIM21 is a novel regulator of hepatic metabolic disorder, and suggest TRIM21 as a promising therapeutic target for T2DM.

Quantum chemistry rules retinoid biology

This Perspective discusses how retinol catalyzes resonance energy transfer (RET) reactions pivotally important for mitochondrial energy homeostasis by protein kinase C δ (PKCδ). PKCδ signals to the pyruvate dehydrogenase complex, controlling oxidative phosphorylation. The PKCδ-retinol complex reversibly responds to the redox potential of cytochrome c, that changes with the electron transfer chain workload. In contrast, the natural retinoid anhydroretinol irreversibly activates PKCδ. Its elongated conjugated-double-bond system limits the energy quantum absorbed by RET. Consequently, while capable of triggering the exergonic activating pathway, anhydroretinol fails to activate the endergonic silencing path, trapping PKCδ in the ON position and causing harmful levels of reactive oxygen species. However, physiological retinol levels displace anhydroretinol, buffer cyotoxicity and potentially render anhydroretinol useful for rapid energy generation. Intriguingly, apocarotenoids, the primary products of the mitochondrial β-carotene,9'-10'-oxygenase, have all the anhydroretinol-like features, including modulation of energy homeostasis. We predict significant conceptual advances to stem from further understanding of the retinoid-catalyzed RET.

Once a week consumption of Western diet over twelve weeks promotes sustained insulin resistance and non-alcoholic fat liver disease in C57BL/6 J mice

The Western diet (high in fat and sucrose) consumption is a highly prevalent feature in the whole world, mainly due to the increasing consumption of ultra-processed foods (UPF), which are cheaper and easier-to-eat, as compared to fresh and highly nutritive meals. Epidemiological studies have associated UPF consumption with development of obesity, non-alcoholic fat liver disease (NAFLD) and insulin resistance. For molecular studies, mice fed with Western diets have been used to characterize signaling pathways involved in these diet-induced pathologies. However, these studies fed mice continuously with the diets, which is not compatible with what occurs in real life, when consumption is occasional. Here, we fed mice once-a-week with a high fat, high sucrose (HFHS) diet and compared these animals with those fed continuously with HFHS diet or with a standard diet. Our results show that after a single day of consuming HFHS, animals presented impaired oral glucose tolerance test (oGTT) as compared to control group. Although this impairment was reversed after 24 h consuming regular diet, repetition of HFHS consumption once-a-week aggravated the picture such as after 12-weeks, oGTT impairment was not reversed after 6 days under control diet. Liver steatosis, inflammation, impaired insulin signaling pathway and endoplasmic reticulum stress are similar comparing animals that consumed HFHS once-a-week with those that continuously consumed HFHS, though weekly-fed animals did not gain as much weight. Therefore, we conclude that regimen of one day HFHS plus 6 days normal diet over 12 weeks is sufficient to induce insulin resistance and NAFLD in mice.

Integrative multi-omics networks identify PKCδ and DNA-PK as master kinases of glioblastoma subtypes and guide targeted cancer therapy

Despite producing a panoply of potential cancer-specific targets, the proteogenomic characterization of human tumors has yet to demonstrate value for precision cancer medicine. Integrative multi-omics using a machine-learning network identified master kinases responsible for effecting phenotypic hallmarks of functional glioblastoma subtypes. In subtype-matched patient-derived models, we validated PKCδ and DNA-PK as master kinases of glycolytic/plurimetabolic and proliferative/progenitor subtypes, respectively, and qualified the kinases as potent and actionable glioblastoma subtype-specific therapeutic targets. Glioblastoma subtypes were associated with clinical and radiomics features, orthogonally validated by proteomics, phospho-proteomics, metabolomics, lipidomics and acetylomics analyses, and recapitulated in pediatric glioma, breast and lung squamous cell carcinoma, including subtype specificity of PKCδ and DNA-PK activity. We developed a probabilistic classification tool that performs optimally with RNA from frozen and paraffin-embedded tissues, which can be used to evaluate the association of therapeutic response with glioblastoma subtypes and to inform patient selection in prospective clinical trials.

Human genetics uncovers MAP3K15 as an obesity-independent therapeutic target for diabetes

We performed collapsing analyses on 454,796 UK Biobank (UKB) exomes to detect gene-level associations with diabetes. Recessive carriers of nonsynonymous variants in MAP3K15 were 30% less likely to develop diabetes (P = 5.7 × 10-10) and had lower glycosylated hemoglobin (β = -0.14 SD units, P = 1.1 × 10-24). These associations were independent of body mass index, suggesting protection against insulin resistance even in the setting of obesity. We replicated these findings in 96,811 Admixed Americans in the Mexico City Prospective Study (P < 0.05)Moreover, the protective effect of MAP3K15 variants was stronger in individuals who did not carry the Latino-enriched SLC16A11 risk haplotype (P = 6.0 × 10-4). Separately, we identified a Finnish-enriched MAP3K15 protein-truncating variant associated with decreased odds of both type 1 and type 2 diabetes (P < 0.05) in FinnGen. No adverse phenotypes were associated with protein-truncating MAP3K15 variants in the UKB, supporting this gene as a therapeutic target for diabetes.

p66Shc in Cardiovascular Pathology

p66Shc is a widely expressed protein that governs a variety of cardiovascular pathologies by generating, and exacerbating, pro-apoptotic ROS signals. Here, we review p66Shc’s connections to reactive oxygen species, expression, localization, and discuss p66Shc signaling and mitochondrial functions. Emphasis is placed on recent p66Shc mitochondrial function discoveries including structure/function relationships, ROS identity and regulation, mechanistic insights, and how p66Shc-cyt c interactions can influence p66Shc mitochondrial function. Based on recent findings, a new p66Shc mitochondrial function model is also put forth wherein p66Shc acts as a rheostat that can promote or antagonize apoptosis. A discussion of how the revised p66Shc model fits previous findings in p66Shc-mediated cardiovascular pathology follows.

Proteomics and Phosphoproteomics of Circulating Extracellular Vesicles Provide New Insights into Diabetes Pathobiology

The purpose of this study was to define the proteomic and phosphoproteomic landscape of circulating extracellular vesicles (EVs) in people with normal glucose tolerance (NGT), prediabetes (PDM), and diabetes (T2DM). Archived serum samples from 30 human subjects (n = 10 per group, ORIGINS study, NCT02226640) were used. EVs were isolated using EVtrap®. Mass spectrometry-based methods were used to detect the global EV proteome and phosphoproteome. Differentially expressed features, correlation, enriched pathways, and enriched tissue-specific protein sets were identified using custom R scripts. Phosphosite-centric analyses were conducted using directPA and PhosR software packages. A total of 2372 unique EV proteins and 716 unique EV phosphoproteins were identified among all samples. Unsupervised clustering of the differentially expressed (fold change ≥ 2, p < 0.05, FDR < 0.05) proteins and, particularly, phosphoproteins showed excellent discrimination among the three groups. CDK1 and PKCδ appear to drive key upstream phosphorylation events that define the phosphoproteomic signatures of PDM and T2DM. Circulating EVs from people with diabetes carry increased levels of specific phosphorylated kinases (i.e., AKT1, GSK3B, LYN, MAP2K2, MYLK, and PRKCD) and could potentially distribute activated kinases systemically. Among characteristic changes in the PDM and T2DM EVs, “integrin switching” appeared to be a central feature. Proteins involved in oxidative phosphorylation (OXPHOS), known to be reduced in various tissues in diabetes, were significantly increased in EVs from PDM and T2DM, which suggests that an abnormally elevated EV-mediated secretion of OXPHOS components may underlie the development of diabetes. A highly enriched signature of liver-specific markers among the downregulated EV proteins and phosphoproteins in both PDM and T2DM groups was also detected. This suggests that an alteration in liver EV composition and/or secretion may occur early in prediabetes. This study identified EV proteomic and phosphoproteomic signatures in people with prediabetes and T2DM and provides novel insight into the pathobiology of diabetes.

Recombinant humanized IgG1 maintain liver triglyceride homeostasis through Arylacetamide deacetylase in ApoE-/- mice

BACKGROUND & AIMS

Hyperlipidemia is a lipid metabolism disorder associated with elevated serum triglyceride (TG) and/or cholesterol. Over the years, studies have shown that hyperlipidemia is associated with combordities, incluing diabetes and obesity, gradually becoming a public health concern. Current treatment approaches remain limited due to the lack of effective drugs. Here we investigated the function of recombinant humanized IgG1 in maintaining liver TG homeostasis and the underlying mechanisms.

METHODS

ApoE^-/- mice were fed a high-fat diet (HFD) for 20 weeks to induce hyperlipidemia. RNA sequencing (RNA-Seq) was performed to identify differences in gene expression in different groups of ApoE^-/- mice liver. In vitro lipid accumulation in primary mouse hepatocytes was induced using a free fatty acid (FFA) mixture. Gene and protein expression were assessed in primary mouse hepatocytes by qPCR and Western blot. Gene reporter assays and ChIP-PCR were used to determine arylacetamide deacetylase (Aadac) promoter activity.

RESULTS

Recombinant humanized IgG1 could significantly decrease the serum level of TG and low-density lipoproteins (LDL-C). Moreover, hepatic TG and lipid droplets were also reduced compared to the HFD group. Mouse liver RNA-Seq revealed that administration of recombinant humanized IgG1 significantly elevated the expression of Aadac. In vitro, knock-down of Aadac could nullify the effect of recombinant humanized IgG1 on decreasing the lipid droplets induced by FFA in primary mouse hepatocytes. Gene Reporter assays and ChIP-PCR demonstrated that the foxa1 response element in the Aadac promoter played a key role in Aadac expression induced by recombinant humanized IgG1. Moreover, recombinant humanized IgG1 repressed phosphorylation of PKCδ and resulted in foxa1 elevation. Finally, neonatal Fc receptor (FcRn) knock-down reversed the effect of recombinant humanized IgG1 on the expression of PKCδ phosphorylation, foxa1 and Aadac.

CONCLUSIONS

Our findings suggest that recombinant humanized IgG1 plays an important role in maintaining liver TG homeostasis via the FcRn/PKCδ/foxa1/Aadac pathway.

Investigation of the Effect of Curcumin on Protein Targets in NAFLD Using Bioinformatic Analysis

BACKGROUND: Non-alcoholic fatty liver disease (NAFLD) is a prevalent metabolic disorder. Defects in function/expression of genes/proteins are critical in initiation/progression of NAFLD. Natural products may modulate these genes/proteins. Curcumin improves steatosis, inflammation, and fibrosis progression. Here, bioinformatic tools, gene−drug and gene-disease databases were utilized to explore targets, interactions, and pathways through which curcumin could impact NAFLD. METHODS: Significant curcumin−protein interaction was identified (high-confidence:0.7) in the STITCH database. Identified proteins were investigated to determine association with NAFLD. gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were analyzed for significantly involved targets (p < 0.01). Specificity of obtained targets with NAFLD was estimated and investigated in Tissue/Cells−gene associations (PanglaoDB Augmented 2021, Mouse Gene Atlas) and Disease−gene association-based EnrichR algorithms (Jensen DISEASES, DisGeNET). RESULTS: Two collections were constructed: 227 protein−curcumin interactions and 95 NAFLD-associated genes. By Venn diagram, 14 significant targets were identified, and their biological pathways evaluated. Based on gene ontology, most targets involved stress and lipid metabolism. KEGG revealed chemical carcinogenesis, the AGE-RAGE signaling pathway in diabetic complications and NAFLD as the most common significant pathways. Specificity to diseases database (EnrichR algorithm) revealed specificity for steatosis/steatohepatitis. CONCLUSION: Curcumin may improve, or inhibit, progression of NAFLD through activation/inhibition of NAFLD-related genes.

Adipogenesis of ear mesenchymal stem cells (EMSCs): adipose biomarker-based assessment of genetic variation, adipocyte function, and brown/brite differentiation

Ear mesenchymal stem cells (EMSCs) have been investigated to differentiate into adipocytes, chondrocytes, and muscle cells in vitro. However, the factors controlling adipogenesis of this stem cell population in vitro, function, and type of adipocytes raised from them are still unclear. Here we found that genetics have a modest effect on adipogenic capacity of EMSCs. Adipocytes differentiated from EMSCs have a potential function in lipid metabolism as indicated by expression of lipogenic genes and this function of EMSC adipocytes is regulated by genetics. EMSCs failed to be differentiated into brite/brown adipocytes due to their lack of a thermogenic program, but adipocytes raised from EMSCs showed a fate of white adipocytes. Overall, our data suggest that EMSCs differentiate into functional white adipocytes in vitro and this is genetic-dependent.

Insulin resistance and insulin sensitizing agents

Insulin resistance is a common feature of obesity and type 2 diabetes, but novel approaches of diabetes subtyping (clustering) revealed variable degrees of insulin resistance in people with diabetes. Specifically, the severe insulin resistant diabetes (SIRD) subtype not only exhibits metabolic abnormalities, but also bears a higher risk for cardiovascular, renal and hepatic comorbidities. In humans, insulin resistance comprises dysfunctional adipose tissue, lipotoxic insulin signaling followed by glucotoxicity, oxidative stress and low-grade inflammation. Recent studies show that aside from metabolites (free fatty acids, amino acids) and signaling proteins (myokines, adipokines, hepatokines) also exosomes with their cargo (proteins, mRNA and microRNA) contribute to altered crosstalk between skeletal muscle, liver and adipose tissue during the development of insulin resistance. Reduction of fat mass mainly, but not exclusively, explains the success of lifestyle modification and bariatric surgery to improve insulin sensitivity. Moreover, some older antihyperglycemic drugs (metformin, thiazolidinediones), but also novel therapeutic concepts (new peroxisome proliferator-activated receptor agonists, incretin mimetics, sodium glucose cotransporter inhibitors, modulators of energy metabolism) can directly or indirectly reduce insulin resistance. This review summarizes molecular mechanisms underlying insulin resistance including the roles of exosomes and microRNAs, as well as strategies for the management of insulin resistance in humans.

Role of hepatic PKCβ in nutritional regulation of hepatic glycogen synthesis

The signaling mechanisms by which dietary fat and cholesterol signals regulate central pathways of glucose homeostasis are not completely understood. By using a hepatocyte-specific PKCβ-deficient (PKCβHep-/-) mouse model, we demonstrated the role of hepatic PKCβ in slowing disposal of glucose overload by suppressing glycogenesis and increasing hepatic glucose output. PKCβHep-/- mice exhibited lower plasma glucose under the fed condition, modestly improved systemic glucose tolerance and mildly suppressed gluconeogenesis, increased hepatic glycogen accumulation and synthesis due to elevated glucokinase expression and activated glycogen synthase (GS), and suppressed glucose-6-phosphatase expression compared with controls. These events were independent of hepatic AKT/GSK-3α/β signaling and were accompanied by increased HNF-4α transactivation, reduced FoxO1 protein abundance, and elevated expression of GS targeting protein phosphatase 1 regulatory subunit 3C in the PKCβHep-/- liver compared with controls. The above data strongly imply that hepatic PKCβ deficiency causes hypoglycemia postprandially by promoting glucose phosphorylation via upregulating glucokinase and subsequently redirecting more glucose-6-phosphate to glycogen via activating GS. In summary, hepatic PKCβ has a unique and essential ability to induce a coordinated response that negatively affects glycogenesis at multiple levels under physiological postprandial conditions, thereby integrating nutritional fat intake with dysregulation of glucose homeostasis.

Intestine-liver crosstalk in Type 2 Diabetes and non-alcoholic fatty liver disease

Type 2 diabetes (T2D) and Non-Alcoholic Fatty Liver Disease (NAFLD) are pathologies whose prevalence continues to increase worldwide. Both diseases are precipitated by an excessive caloric intake, which promotes insulin resistance and fatty liver. The role of the intestine and its crosstalk with the liver in the development of these metabolic diseases is receiving increasing attention. Alterations in diet-intestinal microbiota interactions lead to the dysregulation of intestinal functions, resulting in altered metabolite and energy substrate production and increased intestinal permeability. Connected through the portal circulation, these changes in intestinal functions impact the liver and other metabolic organs, such as visceral adipose tissue, hence participating in the development of insulin resistance, and worsening T2D and NAFLD. Thus, targeting the intestine may be an efficient therapeutic approach to cure T2D and NAFLD. In this review, we will first introduce the signaling pathways linking T2D and NAFLD. Next, we will address the role of the gut-liver crosstalk in the development of T2D and NAFLD, with a particular focus on the gut microbiota and the molecular pathways behind the increased intestinal permeability and inflammation. Finally, we will summarize the therapeutic strategies which target the gut and its functions and are currently used or under development to treat T2D and NAFLD.

Potential Role of Protein Kinase C in the Pathophysiology of Diabetes-Associated Atherosclerosis

Diabetes mellitus is a metabolic syndrome that affects millions of people worldwide. Recent studies have demonstrated that protein kinase C (PKC) activation plays an important role in hyperglycemia-induced atherosclerosis. PKC activation is involved in several cellular responses such as the expression of various growth factors, activation of signaling pathways, and enhancement of oxidative stress in hyperglycemia. However, the role of PKC activation in pro-atherogenic and anti-atherogenic mechanisms remains controversial, especially under hyperglycemic condition. In this review, we discuss the role of different PKC isoforms in lipid regulation, oxidative stress, inflammatory response, and apoptosis. These intracellular events are linked to the pathogenesis of atherosclerosis in diabetes. PKC deletion or treatment with PKC inhibitors has been studied in the regulation of atherosclerotic plaque formation and evolution. Furthermore, some preclinical and clinical studies have indicated that PKCβ and PKCδ are potential targets for the treatment of diabetic vascular complications. The current review summarizes these multiple signaling pathways and cellular responses regulated by PKC activation and the potential therapeutic targets of PKC in diabetic complications.

Hepatic Knockdown of Endothelin Type A Receptor (ETAR) Ameliorates Hepatic Insulin Resistance and Hyperglycemia Through Suppressing p66Shc-Mediated Mitochondrial Fragmentation in High-Fat Diet-Fed Mice

BACKGROUND

Emerging evidence from animal studies and clinical trials indicates that systemic inhibition of endothelin1 (ET1) signaling by endothelin receptor antagonists improves pathological features of diabetes and its complications. It is indicated that endothelin type A receptor (ETAR) plays a major role in ET1-mediated pathophysiological actions including diabetic pathology. However, the effects as well as the mechanistic targets of hepatic ET1/ETAR signaling inhibition on the pathology of metabolic diseases remain unclear. This study aimed to investigate the beneficial effects as well as the underlying mechanisms of hepatic ETAR knockdown on metabolism abnormalities in high-fat diet (HFD)-fed mice.

METHODS

Mice were fed a HFD to induce insulin resistance and metabolism abnormalities. L02 cells were treated with ET1 to assess the action of ET1/ETAR signaling in vitro. Liver-selective knockdown of ETAR was achieved by tail vein injection of adeno-associated virus 8 (AAV8). Systemic and peripheral metabolism abnormalities were determined in vivo and in vitro. Mitochondrial fragmentation was observed by transmission electron microscope (TEM) and mitoTracker red staining.

RESULTS

Here we provided in vivo and in vitro evidence to demonstrate that liver-selective knockdown of ETAR effectively ameliorated hepatic insulin resistance and hyperglycemia in HFD-fed mice. Mechanistically, hepatic ETAR knockdown alleviated mitochondrial fragmentation and dysfunction via inactivating 66-kDa Src homology 2 domain-containing protein (p66Shc) to recover mitochondrial dynamics, which was mediated by inhibiting protein kinase Cδ (PKCδ), in the livers of HFD-fed mice. Ultimately, hepatic ETAR knockdown attenuated mitochondria-derived oxidative stress and related liver injuries in HFD-fed mice. These ETAR knockdown-mediated actions were confirmed in ET1-treated L02 cells.

CONCLUSION

This study defined an ameliorative role of hepatic ETAR knockdown in HFD-induced metabolism abnormalities by alleviating p66Shc-mediated mitochondrial fragmentation and consequent oxidative stress-related disorders and indicated that hepatic ETAR knockdown may be a promising therapeutic strategy for metabolic diseases.

Hugan Qingzhi medication ameliorates free fatty acid-induced L02 hepatocyte endoplasmic reticulum stress by regulating the activation of PKC-δ

Background

Previous studies have found that Hugan Qingzhi tablet (HQT) has significant lipid-lowering and antioxidant effects on non-alcoholic fatty liver disease (NAFLD). Moreover, the results of proteomic analysis confirmed that various proteins in endoplasmic reticulum stress (ERS) pathway were activated and recovered by HQT. However, its mechanism remains confused. The purpose of this study was to explore the effects of HQT-medicated serum on hepatic ERS and its relevant mechanisms.

Methods

L02 cells were induced by Free Fatty Acid (FFA) for 24 h to establish a model of hepatic ERS and pretreated with the drug-medicated rat serum for 24 h. Accumulation of intracellular lipid was evaluated using Oil Red O staining and Triglyceride detection kit. The morphological changes of ER were observed by TEM. PKC-δ was silenced by specific siRNA. Western blot and RT-qPCR were applied to detect the expression of markers related to ERS, calcium disorder, steatosis and insulin resistance. The fluorescence of Ca²⁺ influx was recorded using fluorescence spectrophotometer.

Results

HQT-medicated serum significantly decreased the intracellular TG content. Furthermore, it caused significant reduction in the expression of ERS markers and an improvement in ER structure of L02 cells. PKC-δ was activated into phosphorylated PKC-δ in FFA-induced L02 hepatocytes while these changes can be reversed by HQT-medicated serum. Silencing PKC-δ in L02 cells can restore the expression and activity of SERCA2 in ER and down-regulate the expression of IP3R protein to maintain intracellular calcium homeostasis, so as to relieve FFA-induced ERS and its lipid accumulation and insulin resistance.

Conclusions

The results concluded that HQT-medicated serum exerts protective effects against hepatic ERS, steatosis and insulin resistance in FFA-induced L02 hepatocyte. And its potential mechanism might be down-regulating the activation of PKC-δ and stabilization of intracellular calcium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-020-03164-3.

A Cell-Autonomous Signature of Dysregulated Protein Phosphorylation Underlies Muscle Insulin Resistance in Type 2 Diabetes

Skeletal muscle insulin resistance is the earliest defect in type 2 diabetes (T2D), preceding and predicting disease development. To what extent this reflects a primary defect or is secondary to tissue cross talk due to changes in hormones or circulating metabolites is unknown. To address this question, we have developed an in vitro disease-in-a-dish model using iPS cells from T2D patients differentiated into myoblasts (iMyos). We find that T2D iMyos in culture exhibit multiple defects mirroring human disease, including an altered insulin signaling, decreased insulin-stimulated glucose uptake, and reduced mitochondrial oxidation. More strikingly, global phosphoproteomic analysis reveals a multidimensional network of signaling defects in T2D iMyos going beyond the canonical insulin-signaling cascade, including proteins involved in regulation of Rho GTPases, mRNA splicing and/or processing, vesicular trafficking, gene transcription, and chromatin remodeling. These cell-autonomous defects and the dysregulated network of protein phosphorylation reveal a new dimension in the cellular mechanisms underlying the fundamental defects in T2D.

Diacylglycerol-evoked activation of PKC and PKD isoforms in regulation of glucose and lipid metabolism: a review

Protein kinase C (PKC) and Protein kinase D (PKD) isoforms can sense diacylglycerol (DAG) generated in the different cellular compartments in various physiological processes. DAG accumulates in multiple organs of the obese subjects, which leads to the disruption of metabolic homeostasis and the development of diabetes as well as associated diseases. Multiple studies proved that aberrant activation of PKCs and PKDs contributes to the development of metabolic diseases. DAG-sensing PKC and PKD isoforms play a crucial role in the regulation of metabolic homeostasis and therefore might serve as targets for the treatment of metabolic disorders such as obesity and diabetes.

Deconstructing the Role of PKC Epsilon in Glucose Homeostasis

The failure of insulin to suppress glucose production by the liver is a key aspect of the insulin resistance seen in type 2 diabetes. Lipid-activated protein kinase C epsilon has long been identified as an important mediator of diet-induced glucose intolerance and hepatic insulin resistance and the current view emphasizes a mechanism involving phosphorylation of the insulin receptor by the kinase to inhibit downstream insulin action. However, the significance of this direct effect in the liver has now been challenged by tissue-specific deletion of PKCε, which demonstrated a more prominent role for the kinase in adipose tissue to promote glucose intolerance. New insights regarding the role of PKCε therefore contribute to the understanding of indirect effects on hepatic glucose metabolism.

Role of c-Jun N-terminal Kinase (JNK) in Obesity and Type 2 Diabetes

Obesity has been described as a global epidemic and is a low-grade chronic inflammatory disease that arises as a consequence of energy imbalance. Obesity increases the risk of type 2 diabetes (T2D), by mechanisms that are not entirely clarified. Elevated circulating pro-inflammatory cytokines and free fatty acids (FFA) during obesity cause insulin resistance and ß-cell dysfunction, the two main features of T2D, which are both aggravated with the progressive development of hyperglycemia. The inflammatory kinase c-jun N-terminal kinase (JNK) responds to various cellular stress signals activated by cytokines, free fatty acids and hyperglycemia, and is a key mediator in the transition between obesity and T2D. Specifically, JNK mediates both insulin resistance and ß-cell dysfunction, and is therefore a potential target for T2D therapy.

Excess Accumulation of Lipid Impairs Insulin Sensitivity in Skeletal Muscle

Both glucose and free fatty acids (FFAs) are used as fuel sources for energy production in a living organism. Compelling evidence supports a role for excess fatty acids synthesized in intramuscular space or dietary intermediates in the regulation of skeletal muscle function. Excess FFA and lipid droplets leads to intramuscular accumulation of lipid intermediates. The resulting downregulation of the insulin signaling cascade prevents the translocation of glucose transporter to the plasma membrane and glucose uptake into skeletal muscle, leading to metabolic disorders such as type 2 diabetes. The mechanisms underlining metabolic dysfunction in skeletal muscle include accumulation of intracellular lipid derivatives from elevated plasma FFAs. This paper provides a review of the molecular mechanisms underlying insulin-related signaling pathways after excess accumulation of lipids.

Novel Paracrine Functions of Smooth Muscle Cells in Supporting Endothelial Regeneration Following Arterial Injury

RATIONALE

Regeneration of denuded or injured endothelium is an important component of vascular injury response. Cell-cell communication between endothelial cells and smooth muscle cells (SMCs) plays a critical role not only in vascular homeostasis but also in disease. We have previously demonstrated that PKCδ (protein kinase C-delta) regulates multiple components of vascular injury response including apoptosis of SMCs and production of chemokines, thus is an attractive candidate for a role in SMC-endothelial cells communication.

OBJECTIVE

To test whether PKCδ-mediated paracrine functions of SMCs influence reendothelialization in rodent models of arterial injury.

METHODS AND RESULTS

Femoral artery wire injury was performed in SMC-conditional Prkcd knockout mice, and carotid angioplasty was conducted in rats receiving transient Prkcd knockdown or overexpression. SMC-specific knockout of Prkcd impaired reendothelialization, reflected by a smaller Evans blue-excluding area in the knockout compared with the wild-type controls. A similar impediment to reendothelialization was observed in rats with SMC-specific knockdown of Prkcd. In contrast, SMC-specific gene transfer of Prkcd accelerated reendothelialization. In vitro, medium conditioned by AdPKCδ-infected SMCs increased endothelial wound closure without affecting their proliferation. A polymerase chain reaction-based array analysis identified Cxcl1 and Cxcl7 among others as PKCδ-mediated chemokines produced by SMCs. Mechanistically, we postulated that PKCδ regulates Cxcl7 expression through STAT3 (signal transducer and activator of transcription 3) as knockdown of STAT3 abolished Cxcl7 expression. The role of CXCL7 in SMC-endothelial cells communication was demonstrated by blocking CXCL7 or its receptor CXCR2, both significantly inhibited endothelial wound closure. Furthermore, insertion of a Cxcl7 cDNA in the lentiviral vector that carries a Prkcd shRNA overcame the adverse effects of Prkcd knockdown on reendothelialization.

CONCLUSIONS

SMCs promote reendothelialization in a PKCδ-dependent paracrine mechanism, likely through CXCL7-mediated recruitment of endothelial cells from uninjured endothelium.

The mitochondrial PKCδ/retinol signal complex exerts real-time control on energy homeostasis

The review focuses on the role of vitamin A (retinol) in the control of energy homeostasis, and on the manner in which certain retinoids subvert this process, leading potentially to disease. In eukaryotic cells, the pyruvate dehydrogenase complex (PDHC) is negatively regulated by four pyruvate dehydrogenase kinases (PDKs) and two antagonistically acting pyruvate dehydrogenase phosphatases (PDPs). The second isoform, PDK2, is regulated by an autonomous mitochondrial signal cascade that is anchored on protein kinase Cδ (PKCδ), where retinoids play an indispensible co-factor role. Along with its companion proteins p66Shc, cytochrome c, and vitamin A, the PKCδ/retinol complex is located in the intermembrane space of mitochondria. At this site, and in contrast to cytosolic locations, PKCδ is activated by the site-specific oxidation of its cysteine-rich activation domain (CRD) that is configured into a complex RING-finger. Oxidation involves the transfer of electrons from cysteine moieties to oxidized cytochrome c, a step catalyzed by vitamin A. The PKCδ/retinol signalosome monitors the internal cytochrome c redox state that reflects the workload of the respiratory chain. Upon sensing demands for energy PKCδ signals the PDHC to increase glucose-derived fuel flux entering the KREBS cycle. Conversely, if excessive fuel flux surpasses the capacity of the respiratory chain, threatening the release of damaging reactive oxygen species (ROS), the polarity of the cytochrome c redox system is reversed, resulting in the chemical reduction of the PKCδ CRD, restoration of the RING-finger, refolding of PKCδ into the inactive, globular form, and curtailment of PDHC output, thereby constraining the respiratory capacity within safe margins. Several retinoids, notably anhydroretinol and fenretinide, capable of displacing retinol from binding sites on PKCδ, can co-activate PKCδ signaling but, owing to their extended system of conjugated double bonds, are unable to silence PKCδ in a timely manner. Left in the ON position, PKCδ causes chronic overload of the respiratory chain leading to mitochondrial dysfunction. This review explores how defects in the PKCδ signal machinery potentially contribute to metabolic and degenerative diseases.

Accuracy of proton magnetic resonance for diagnosing non-alcoholic steatohepatitis: a meta-analysis

Liver biopsy is the reference standard test to differentiate between non-alcoholic steatohepatitis (NASH) and simple steatosis (SS) in non-alcoholic fatty liver disease (NAFLD), but noninvasive diagnostics are warranted. The diagnostic accuracy in NASH using MR imaging modality have not yet been clearly identified. This study was assessed the accuracy of magnetic resonance imaging (MRI) method for diagnosing NASH. Data were extracted from research articles obtained after a literature search from multiple electronic databases. Random-effects meta-analyses were performed to obtain overall effect size of the area under the receiver operating characteristic(ROC) curve, sensitivity, specificity, likelihood ratios(LR), diagnostic odds ratio(DOR) of MRI method in detecting histopathologically-proven SS(or non-NASH) and NASH. Seven studies were analyzed 485 patients, which included 207 SS and 278 NASH. The pooled sensitivity was 87.4% (95% CI, 76.4–95.3) and specificity was 74.3% (95% CI, 62.4–84.6). Pooled positive LR was 2.59 (95% CI, 1.96–3.42) and negative LR was 0.17 (95% CI, 0.07–0.38). DOR was 21.57 (95% CI, 7.27–63.99). The area under the curve of summary ROC was 0.89. Our meta-analysis shows that the MRI-based diagnostic methods are valuable additions in detecting NASH.

A specific small-molecule inhibitor of protein kinase CδI activity improves metabolic dysfunction in human adipocytes from obese individuals

The metabolic consequences and sequelae of obesity promote life-threatening morbidities. PKCδI is an important elicitor of inflammation and apoptosis in adipocytes. Here we report increased PKCδI activation via release of its catalytic domain concurrent with increased expression of proinflammatory cytokines in adipocytes from obese individuals. Using a screening strategy of dual recognition of PKCδI isozymes and a caspase-3 binding site on the PKCδI hinge domain with Schrödinger software and molecular dynamics simulations, we identified NP627, an organic small-molecule inhibitor of PKCδI. Characterization of NP627 by surface plasmon resonance (SPR) revealed that PKCδI and NP627 interact with each other with high affinity and specificity, SPR kinetics revealed that NP627 disrupts caspase-3 binding to PKCδI, and in vitro kinase assays demonstrated that NP627 specifically inhibits PKCδI activity. The SPR results also indicated that NP627 affects macromolecular interactions between protein surfaces. Of note, release of the PKCδI catalytic fragment was sufficient to induce apoptosis and inflammation in adipocytes. NP627 treatment of adipocytes from obese individuals significantly inhibited PKCδI catalytic fragment release, decreased inflammation and apoptosis, and significantly improved mitochondrial metabolism. These results indicate that PKCδI is a robust candidate for targeted interventions to manage obesity-associated chronic inflammatory diseases. We propose that NP627 may also be used in other biological systems to better understand the impact of caspase-3-mediated activation of kinase activity.

Noninvasive Differential Diagnosis of Liver Iron Contents in Nonalcoholic Steatohepatitis and Simple Steatosis Using Multiecho Dixon Magnetic Resonance Imaging

RATIONALE AND OBJECTIVES

The roles of iron stores in nonalcoholic fatty liver disease have not yet been clearly identified, and it is lack of uniform criteria and a standardized study design for assessing the liver iron content (LIC) in nonalcoholic steatohepatitis (NASH). This study was to compare LICs in biopsy-proven simple steatosis (SS) and NASH based on T₂^⁎-relaxometry.

MATERIAL AND METHODS

A total of 32 subjects divided to three groups, consisting of 10 healthy controls, 12 SS and 10 NASH. All MRI examinations were performed on a 3 T MRI with a 32-channel body coil. To measure T₂^⁎-value, we used a gradient echo sequence with six multiechoes within a single breath-hold. Hepatic iron contents among three groups were compared using Kruskal-Wallis H test and Mann-Whitney's posthoc tests. Diagnostic accuracy was determined by calculating the area under the receiver operating characteristics curve. To identify the reliability of iron measurements in the different region of interests, coefficient of variance (CV) was calculated overall CV values for the variability of measurements. Interobserver agreement and reliability were estimated by calculating the intraclass correlation coefficient.

RESULTS

The variations of all LIC measurements are not exceeded 20%, as a mean CV value 18.3%. intraclass correlation coefficients were higher than 0.9. Mean T₂^⁎-values at localized region of interests were healthy controls 45.42 ± 7.19 ms, SS 20.96 ± 4.28 ms, and NASH 15.49 ± 2.87 ms. The mean T₂^⁎-value in NASH is significantly shorter than that in SS (p = 0.008). The area under the receiver operating characteristics curve to distinguish NASH from SS was 0.908 (95% confidence interval 0.775-1.000, p = 0.001) at a cut-off of iron contents greater than 17.95 ms, and its diagnostic accuracy had 0.833 sensitivity and 0.800 specificity.

CONCLUSION

This study demonstrates that the T₂^⁎ calculation can help to differentially diagnose NASH.

NOD1: An Interface Between Innate Immunity and Insulin Resistance

Insulin resistance is driven, in part, by activation of the innate immune system. We have discussed the evidence linking nucleotide-binding oligomerization domain (NOD)1, an intracellular pattern recognition receptor, to the onset and progression of obesity-induced insulin resistance. On a molecular level, crosstalk between downstream NOD1 effectors and the insulin receptor pathway inhibits insulin signaling, potentially through reduced insulin receptor substrate action. In vivo studies have demonstrated that NOD1 activation induces peripheral, hepatic, and whole-body insulin resistance. Also, NOD1-deficient models are protected from high-fat diet (HFD)-induced insulin resistance. Moreover, hematopoietic NOD1 deficiency prevented HFD-induced changes in proinflammatory macrophage polarization status, thus protecting against the development of metabolic inflammation and insulin resistance. Serum from HFD-fed mice activated NOD1 signaling ex vivo; however, the molecular identity of the activating factors remains unclear. Many have proposed that an HFD changes the gut permeability, resulting in increased translocation of bacterial fragments and increased circulating NOD1 ligands. In contrast, others have suggested that NOD1 ligands are endogenous and potentially lipid-derived metabolites produced during states of nutrient overload. Nevertheless, that NOD1 contributes to the development of insulin resistance, and that NOD1-based therapy might provide benefit, is an exciting advancement in metabolic research.

Innate Immune Signaling and Its Role in Metabolic and Cardiovascular Diseases

The innate immune system is an evolutionarily conserved system that senses and defends against infection and irritation. Innate immune signaling is a complex cascade that quickly recognizes infectious threats through multiple germline-encoded cell surface or cytoplasmic receptors and transmits signals for the deployment of proper countermeasures through adaptors, kinases, and transcription factors, resulting in the production of cytokines. As the first response of the innate immune system to pathogenic signals, inflammatory responses must be rapid and specific to establish a physical barrier against the spread of infection and must subsequently be terminated once the pathogens have been cleared. Long-lasting and low-grade chronic inflammation is a distinguishing feature of type 2 diabetes and cardiovascular diseases, which are currently major public health problems. Cardiometabolic stress-induced inflammatory responses activate innate immune signaling, which directly contributes to the development of cardiometabolic diseases. Additionally, although the innate immune elements are highly conserved in higher-order jawed vertebrates, lower-grade jawless vertebrates lack several transcription factors and inflammatory cytokine genes downstream of the Toll-like receptors (TLRs) and retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) pathways, suggesting that innate immune signaling components may additionally function in an immune-independent way. Notably, recent studies from our group and others have revealed that innate immune signaling can function as a vital regulator of cardiometabolic homeostasis independent of its immune function. Therefore, further investigation of innate immune signaling in cardiometabolic systems may facilitate the discovery of new strategies to manage the initiation and progression of cardiometabolic disorders, leading to better treatments for these diseases. In this review, we summarize the current progress in innate immune signaling studies and the regulatory function of innate immunity in cardiometabolic diseases. Notably, we highlight the immune-independent effects of innate immune signaling components on the development of cardiometabolic disorders.

Attenuation of PKCδ enhances metabolic activity and promotes expansion of blood progenitors

A finely tuned balance of self‐renewal, differentiation, proliferation, and survival governs the pool size and regenerative capacity of blood‐forming hematopoietic stem and progenitor cells (HSPCs). Here, we report that protein kinase C delta (PKCδ) is a critical regulator of adult HSPC number and function that couples the proliferative and metabolic activities of HSPCs. PKCδ‐deficient mice showed a pronounced increase in HSPC numbers, increased competence in reconstituting lethally irradiated recipients, enhanced long‐term competitive advantage in serial transplantation studies, and an augmented HSPC recovery during stress. PKCδ‐deficient HSPCs also showed accelerated proliferation and reduced apoptosis, but did not exhaust in serial transplant assays or induce leukemia. Using inducible knockout and transplantation models, we further found that PKCδ acts in a hematopoietic cell‐intrinsic manner to restrict HSPC number and bone marrow regenerative function. Mechanistically, PKCδ regulates HSPC energy metabolism and coordinately governs multiple regulators within signaling pathways implicated in HSPC homeostasis. Together, these data identify PKCδ as a critical regulator of HSPC signaling and metabolism that acts to limit HSPC expansion in response to physiological and regenerative demands.

Is oxidative stress of adipocytes a cause or a consequence of the metabolic syndrome?

Metabolic syndrome is accompanied by oxidative stress in animals and humans. The main source of ROS in experimental metabolic syndrome is NADPH oxidase and possibly adipocyte mitochondria. It is now documented that oxidative stress induces insulin resistance of adipocytes and increases secretion of leptin, MCP-1, IL-6, and TNF-α by adipocytes. It was established that oxidative stress induces a decrease in adiponectin production by adipocytes. It has also been shown that obesity itself can induce oxidative stress. Oxidative stress can cause an alteration of intracellular signaling in adipocytes that apparently leads to the formation of insulin resistance of adipocytes. Chronic stress, glucocorticoids, mineralocorticoids, angiotensin-II, TNF-α also play an important role in the pathogenesis of oxidative stress of adipocytes. Oxidative stress is not only a consequence of metabolic syndrome, but also a reason and a foundational link in the pathogenesis of the metabolic syndrome.

Proteomic Investigation of Murine Neuronal α7-Nicotinic Acetylcholine Receptor Interacting Proteins

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel that is expressed widely in vertebrates and is the principal high-affinity α-bungarotoxin (α-bgtx) binding protein in the mammalian CNS. α7-nAChRs associate with proteins that can modulate its properties. The α7-nAChR interactome is the summation of proteins interacting or associating with α7-nAChRs in a protein complex. To identify an α7-nAChR interactome in neural tissue, we isolated α-bgtx-affinity protein complexes from wild-type and α7-nAChR knockout (α7 KO) mouse whole brain tissue homogenates using α-bgtx-affinity beads. Affinity precipitated proteins were trypsinized and analyzed with an Orbitrap Fusion mass spectrometer. Proteins isolated with the α7-nAChR specific ligand, α-bgtx, were determined to be α7-nAChR associated proteins. The α7-nAChR subunit and 120 additional proteins were identified. Additionally, 369 proteins were identified as binding to α-bgtx in the absence of α7-nAChR expression, thereby identifying nonspecific proteins for α7-nAChR investigations using α-bgtx enrichment. These results expand on our previous investigations of α7-nAChR interacting proteins using α-bgtx-affinity bead isolation by controlling for differences between α7-nAChR and α-bgtx-specific proteins, developing an improved protein isolation methodology, and incorporating the latest technology in mass spectrometry. The α7-nAChR interactome identified in this study includes proteins associated with the expression, localization, function, or modulation of α7-nAChRs, and it provides a foundation for future studies to elucidate how these interactions contribute to human disease.

Mechanisms of Insulin Action and Insulin Resistance

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

Protein kinase C δ-dependent regulation of Ubiquitin-proteasome system function in breast cancer

Besides the crucial role of hyperinsulinemia in the development of breast cancer with Type 2 diabetes mellitus (T2DM), it has been shown that hyperglycemia could contribute to promote cancer progression. A remarkable association within hyperglycemia, PKCδ and Ubiquitin-proteasome system (UPS) has been reported, suggesting that PKCδ may mediate high glucose-induced UPS activation in breast cancer cells. Although the independent effects of PKCδ or UPS on breast cancer and T2DM are increasingly supported by experimental evidence, the complex interactional link between PKCδ and UPS is still unclear. Hence, we focus on the relationship between PKCδ and UPS in breast cancer with T2DM. We hypothesize that PKCδ may have the function to regulate the activity of UPS. Further, we speculate that PKCδ combine with proteasome α2 promoter, that indicate PKCδ regulate the function of UPS by change the composition of proteasome. Therefore, we surmise that PKCδ mediated high glucose-induced UPS activation in breast cancer cells, and specific PKCδ inhibitor rottlerin significantly suppressed elevated glucose induced the activity of UPS. We hope that our paper will stimulate further studies the relationship between PKCδ and UPS, and a new targeted therapy and early medical intervention for PKCδ could be a useful option for breast cancer cases complicated with T2DM or hyperglycemia.

S100A4 protects mice from high-fat diet-induced obesity and inflammation

As a member from S100 calcium-binding protein family, S100A4 is ubiquitous and elevated in tumor progression and metastasis, but its role in regulating obesity has not been well characterized. In this study, we showed that S100A4 was mainly expressed by stromal cells in adipose tissue and the S100A4 level in adipose tissue was decreased after high-fat diet (HFD). S100A4 deficient mice exhibited aggravated symptoms of obesity and suppressed insulin signaling after 12 weeks of HFD. Aggravated obesity in S100A4 deficient mice were found to be positively correlated with higher inflammatory status of the liver. Then, we found that extracellular S100A4 or overexpressed S100A4 inhibited adipogenesis and decreased mRNA levels of inflammation gene in 3T3-L1 adipocytes in vitro; whereas small interfering RNA (siRNA)-mediated suppression of S100A4 displayed the opposite results. Additionally, the protective effect induced by S100A4 during HFD-induced obesity was tightly related with activation of Akt signaling in adipose tissues, as well as livers and muscles. Taken together, we demonstrate that S100A4 is an inhibitory factor for obesity and attenuates the inflammatory reaction, while activating the Akt signaling, which suggest that S100A4 is a potential candidate for the treatment of diet-induced obesity and its complications.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Anarchy in the UPR: A Ca2+-insensitive PKC inhibits SERCA activity to promote ER stress

Nonalcoholic fatty liver disease (NAFLD) is highly prevalent in Western countries, and is linked to the development of liver cancer and Type 2 diabetes (T2D). It is strongly associated with obesity, but the dysregulation of liver lipid storage is not fully understood. Fatty acid oversupply to hepatocytes can establish a vicious cycle involving diminished protein folding, endoplasmic reticulum (ER) stress, insulin resistance and further lipogenesis. This commentary discusses the recent findings of Lai et al. published in Bioscience Reports, that implicate protein kinase C delta (PKCδ) activation by fatty acids in the inhibition of the SERCA Ca2+ pump, resulting in reduced ER Ca2+ loading and protein misfolding. PKCδ therefore represents a target for the treatment of both steatosis and insulin resistance, key to the prevention of NAFLD and T2D.

Angiotensin II type 2 receptor inhibits expression and function of insulin receptor in rat renal proximal tubule cells

Both renin-angiotensin systems and insulin participate in kidney-involved blood pressure regulation. Activation of angiotensin II type 2 receptor (AT₂R) decreases sodium reabsorption in renal proximal tubule (RPT) cells, whereas insulin produces the opposite effect. We presume that AT₂R has an inhibitory effect on insulin receptor expression in RPT cells, which may affect renal sodium transport and therefore be of physiological or pathological significance. Our present study found that activation of AT₂R inhibited insulin receptor expression in a concentration and time-dependent manner in RPT cells from Wistar-Kyoto (WKY) rats. In the presence of a protein kinase C (PKC) inhibitor (PKC inhibitor peptide 19-31, 10^-6 mol/L) or a phosphatidylinositol 3 kinase inhibitor (wortmannin, 10^-6 mol/L), the inhibitory effect of AT₂R on insulin receptor was blocked, indicating that both PKC and phosphatidylinositol 3 kinase were involved in the signaling pathway. There was a linkage between AT₂R and insulin receptor which was determined by both laser confocal microscopy and coimmunoprecipitation. However, the effect of AT₂R activation on insulin receptor expression was different in RPT cells from spontaneously hypertensive rats (SHRs). Being contrary to the effect in WKY RPT cells, AT₂R stimulation increased insulin receptor in SHR RPT cells. Insulin (10^-7 mol/L, 15 minutes) enhanced Na⁺-K⁺-ATPase activity in both WKY and SHR RPT cells. Pretreatment with CGP42112 decreased the stimulatory effect of insulin on Na⁺-K⁺-ATPase activity in WKY RPT cells, whereas pretreatment with CGP42112 increased it in SHR RPT cells. It is suggested that activation of AT₂R inhibits insulin receptor expression and function in RPT cells. The lost inhibitory effect of AT₂R on insulin receptor expression may contribute to the pathophysiology of hypertension.

PKCδ silencing alleviates saturated fatty acid induced ER stress by enhancing SERCA activity

Protein kinase C δ (PKCδ) plays an important role in nonalcoholic fatty liver disease (NAFLD), however, the mechanism remains unknown. The present study explored the role of PKCδ in NAFLD development and investigated the relationships between PKCδ, calcium homeostasis, and endoplasmic reticulum (ER) stress (ERS). Hepatic steatosis cell model was induced by palmitic acid (PA) in L02 cells. Lipid accretion was evaluated using Oil Red O staining and a triglyceride (TG) detection kit. PKCδ was down-regulated by siRNA. RT-PCR and Western blotting were used to detect the expression of ERS markers. The fluorescence of Ca²⁺ influx was recorded using confocal microscopy. Sarco-ER Ca²⁺-ATPase (SERCA) activity was measured by ultramicro-ATP enzyme test kit. PA treatment induced lipid accretion in L02 cells, destroyed the ER structure, and increased PKCδ activation in a time-dependent manner. Further, PA treatment significantly increased the expression of ERS markers, Ig heavy chain binding protein (Bip), and homologous proteins of CCAAT-enhancer binding proteins (CHOP). PKCδ silencing down-regulated Bip and CHOP expression, indicating a successful alleviation of ERS. The increased calcium storage induced by PA stimulation was significantly decreased in L02 cells treated with PKCδ siRNA compared with the negative control. Moreover, diminished SERCA activity caused by PA was recovered in PKCδ siRNA transfected cells. To the best of our knowledge, this is the first report demonstrating that the inhibition of PKCδ alleviates ERS by enhancing SERCA activity and stabilizing calcium homeostasis.

Protein kinase C-δ signaling regulates glucagon secretion from pancreatic islets

Accumulating evidence supports the "glucagonocentric hypothesis", in which antecedent α-cell failure and inhibition of glucagon secretion are responsible for diabetes progression. Protein kinase C (PKC) is involved in glucagon secretion from α-cells, although which PKC isozyme is involved and the mechanism underlying this PKC-regulated glucagon secretion remains unknown. Here, the involvement of PKCδ in the onset and progression of diabetes was elucidated. Immunofluorescence studies revealed that PKCδ was expressed and activated in α-cells of STZ-induced diabetic model mice. Phorbol 12-myristate 13-acetate (PMA) stimulation significantly augmented glucagon secretion from isolated islets. Pre-treatment with quercetin and rottlerin, PKCδ signaling inhibitors, significantly suppressed the PMA-induced elevation of glucagon secretion. While Go6976, a Ca²⁺-dependent PKC selective inhibitor did not suppress glucagon secretion. Quercetin suppressed PMA-induced phosphorylation of Tyr³¹¹ of PKCδ in isolated islets. However, quercetin itself had no effect on either glucagon secretion or glucagon mRNA expression. Our data suggest that PKCδ signaling inhibitors suppressed glucagon secretion. Elucidation of detailed signaling pathways causing PKCδ activation in the onset and progression of diabetes followed by the augmentation of glucagon secretion could lead to the identification of novel therapeutic target molecules and the development of novel therapeutic drugs for diabetes. J. Med. Invest. 64: 122-128, February, 2017.

Regulation of Macrophage Apoptosis and Atherosclerosis by Lipid-Induced PKCδ Isoform Activation

RATIONALE

Activation of monocytes/macrophages by hyperlipidemia associated with diabetes mellitus and obesity contributes to the development of atherosclerosis. PKCδ (protein kinase C δ) expression and activity in monocytes were increased by hyperlipidemia and diabetes mellitus with unknown consequences to atherosclerosis.

OBJECTIVE

To investigate the effect of PKCδ activation in macrophages on the severity of atherosclerosis.

METHODS AND RESULTS

PKCδ expression and activity were increased in Zucker diabetic rats. Mice with selective deletion of PKCδ in macrophages were generated by breeding PKCδ flox/flox mice with LyzM-Cre and ApoE^-/- mice (MPKCδKO/ApoE^-/- mice) and studied in atherogenic (AD) and high-fat diet (HFD). Mice fed AD and HFD exhibited hyperlipidemia, but only HFD-fed mice had insulin resistance and mild diabetes mellitus. Surprisingly, MPKCδKO/ApoE^-/- mice exhibited accelerated aortic atherosclerotic lesions by 2-fold versus ApoE^-/- mice on AD or HFD. Splenomegaly was observed in MPKCδKO/ApoE^-/- mice on AD and HFD but not on regular chow. Both the AD or HFD increased macrophage number in aortic plaques and spleen by 1.7- and 2-fold, respectively, in MPKCδKO/ApoE^-/- versus ApoE^-/- mice because of decreased apoptosis (62%) and increased proliferation (1.9-fold), and not because of uptake, with parallel increased expressions of inflammatory cytokines. Mechanisms for the increased macrophages in MPKCδKO/ApoE^-/- were associated with elevated phosphorylation levels of prosurvival cell-signaling proteins, Akt and FoxO3a, with reduction of proapoptotic protein Bim associated with PKCδ induced inhibition of P85/PI3K.

CONCLUSIONS

Accelerated development of atherosclerosis induced by insulin resistance and hyperlipidemia may be partially limited by PKCδ isoform activation in the monocytes, which decreased its number and inflammatory responses in the arterial wall.