Diverse roles for protein kinase C delta and protein kinase C epsilon in the generation of high-fat-diet-induced glucose intolerance in mice: regulation of lipogenesis by protein kinase C delta

Cleavage of protein kinase c δ by caspase-3 mediates proinflammatory cytokine-induced apoptosis in pancreatic islets

In type 1 diabetes (T1D), autoreactive immune cells infiltrate the pancreas and secrete proinflammatory cytokines that initiate cell death in insulin producing islet β-cells. Protein kinase C δ (PKCδ) plays a role in mediating cytokine-induced β-cell death; however, the exact mechanisms are not well understood. To address this, we used an inducible β-cell specific PKCδ KO mouse as well as a small peptide inhibitor of PKCδ. We identified a role for PKCδ in mediating cytokine-induced β-cell death and have shown that inhibiting PKCδ protects pancreatic β-cells from cytokine-induced apoptosis in both mouse and human islets. We determined that cytokines induced nuclear translocation and activity of PKCδ and that caspase-3 cleavage of PKCδ may be required for cytokine-mediated islet apoptosis. Further, cytokine activated PKCδ increases activity both of proapoptotic Bax with acute treatment and C-Jun N-terminal kinase with prolonged treatment. Overall, our results suggest that PKCδ mediates cytokine-induced apoptosis via nuclear translocation, cleavage by caspase-3, and upregulation of proapoptotic signaling in pancreatic β-cells. Combined with the protective effects of PKCδ inhibition with δV1-1, the results of this study will aid in the development of novel therapies to prevent or delay β-cell death and preserve β-cell function in T1D.

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

Revealing myopathy spectrum: integrating transcriptional and clinical features of human skeletal muscles with varying health conditions

Myopathy refers to a large group of heterogeneous, rare muscle diseases. Bulk RNA-sequencing has been utilized for the diagnosis and research of these diseases for many years. However, the existing valuable sequencing data often lack integration and clinical interpretation. In this study, we integrated bulk RNA-sequencing data from 1221 human skeletal muscles (292 with myopathies, 929 controls) from both databases and our local samples. By applying a method similar to single-cell analysis, we revealed a general spectrum of muscle diseases, ranging from healthy to mild disease, moderate muscle wasting, and severe muscle disease. This spectrum was further partly validated in three specific myopathies (97 muscles) through clinical features including trinucleotide repeat expansion, magnetic resonance imaging fat fraction, pathology, and clinical severity scores. This spectrum helped us identify 234 genuinely healthy muscles as unprecedented controls, providing a new perspective for deciphering the hallmark genes and pathways among different myopathies. The newly identified featured genes of general myopathy, inclusion body myositis, and titinopathy were highly expressed in our local muscles, as validated by quantitative polymerase chain reaction.

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.

LPIN1 Induces Gefitinib Resistance in EGFR Inhibitor-Resistant Non-Small Cell Lung Cancer Cells

Drug resistance limits the efficacy of targeted therapies, including tyrosine kinase inhibitors (TKIs); however, a substantial portion of the drug resistance mechanisms remains unexplained. In this study, we identified LPIN1 as a key factor that regulates gefitinib resistance in epidermal growth factor receptor (EGFR)-mutant non-small cell lung cancer (NSCLC) cells. Unlike TKI-sensitive HCC827 cells, gefitinib treatment induced LPIN1 expression and increased diacylglycerol concentration in TKI-resistant H1650 cells, followed by the activation of protein kinase C delta and nuclear factor kappa B (NF-κB) in an LPIN1-dependent manner, resulting in cancer cell survival. Additionally, LPIN1 increased the production of lipid droplets, which play an important role in TKI drug resistance. All results were recapitulated in a patient-derived EGFR-mutant NSCLC cell line. In in vivo tumorigenesis assay, we identified that both shRNA-mediated depletion and pharmaceutical inhibition of LPIN1 clearly reduced tumor growth and confirmed that gefitinib treatment induced LPIN1 expression and LPIN1-dependent NF-κB activation (an increase in p-IκBα level) in tumor tissues. These results suggest an effective strategy of co-treating TKIs and LPIN1 inhibitors to prevent TKI resistance in NSCLC patients.

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.

Identification of Key LncRNAs and Pathways in Prediabetes and Type 2 Diabetes Mellitus for Hypertriglyceridemia Patients Based on Weighted Gene Co-Expression Network Analysis

Aims

Prevalence of prediabetes and type 2 diabetes mellitus(T2DM) are increasing worldwide. Key lncRNAs were detected to provide a reference for searching potential biomarkers of prediabetes and T2DM in hypertriglyceridemia patients.

Methods

The study included 18 hypertriglyceridemia patients: 6 newly diagnosed type 2 diabetes patients, 6 samples with prediabetes and 6 samples with normal blood glucose. Weighted gene co-expression network analysis (WGCNA) was conducted to construct co-expression network and obtain modules related to blood glucose, thus detecting key lncRNAs.

Results

The green, yellow and yellow module was significantly related to blood glucose in T2DM versus normal controls, T2DM versus prediabetes, prediabetes versus normal controls, respectively. ENST00000503273, ENST00000462720, ENST00000480633 and ENST00000485392 were detected as key lncRNAs for the above three groups, respectively.

Conclusions

For hypertriglyceridemia patients with different blood glucose levels, ENST00000503273, ENST00000462720 and ENST00000480633 could be potential biomarkers of T2DM.

Comparative analysis of oral and intraperitoneal glucose tolerance tests in mice

Objective

The glucose tolerance test (GTT) is widely used in preclinical research to investigate glucose metabolism, but there is no standardised way to administer glucose. The aim of this study was to directly compare the effect of the route of glucose administration on glucose and insulin kinetics during a GTT in mice.

Methods

A GTT was performed in lean male and female mice and obese male mice and glucose was administered via the oral or intraperitoneal (I.P.) route. Samples were collected frequently during the GTT to provide a full time-course of the insulin and glucose excursions. In another cohort of lean male mice, plasma concentrations of insulin, c-peptide, and incretin hormones were measured at early time points after glucose administration. A stable-isotope labelled GTT (SiGTT) was then performed to delineate the contribution of exogenous and endogenous glucose to glycemia during the GTT, comparing both methods of glucose administration. Finally, we present a method to easily measure insulin from small volumes of blood during a GTT by directly assaying whole-blood insulin using ELISA and show a good concordance between whole-blood and plasma insulin measurements.

Results

We report that I.P. glucose administration results in an elevated blood glucose excursion and a largely absent elevation in blood insulin and plasma incretin hormones when compared to oral administration. Utilising stable-isotope labelled glucose, we demonstrate that the difference in glucose excursion between the two routes of administration is mainly due to the lack of suppression of glucose production in I.P. injected mice. Additionally, rates of exogenous glucose appearance into circulation were different between lean and obese mice after I.P., but not after oral glucose administration.

Conclusion

Reflecting on these data, we suggest that careful consideration be given to the route of glucose administration when planning a GTT procedure in mice and that in most circumstances the oral route of glucose administration should be preferred over the I.P. route to avoid possible artifacts originating from a non-physiological route.

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Intraperitoneal glucose administration does not promote insulin secretion.

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Exogenous glucose appearance is delayed in obese mice after intraperitoneal administration.

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Hepatic glucose production is suppressed after administering oral not intraperitoneal glucose.

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Measuring insulin from whole blood is comparable to that from plasma.

Platelet-Activating Factor Promotes the Development of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a multifaceted clinicopathological syndrome characterised by excessive hepatic lipid accumulation that causes steatosis, excluding alcoholic factors. Platelet-activating factor (PAF), a biologically active lipid transmitter, induces platelet activation upon binding to the PAF receptor. Recent studies have found that PAF is associated with gamma-glutamyl transferase, which is an indicator of liver disease. Moreover, PAF can stimulate hepatic lipid synthesis and cause hypertriglyceridaemia. Furthermore, the knockdown of the PAF receptor gene in the animal models of NAFLD helped reduce the inflammatory response, improve glucose homeostasis and delay the development of NAFLD. These findings suggest that PAF is associated with NAFLD development. According to reports, patients with NAFLD or animal models have marked platelet activation abnormalities, mainly manifested as enhanced platelet adhesion and aggregation and altered blood rheology. Pharmacological interventions were accompanied by remission of abnormal platelet activation and significant improvement in liver function and lipids in the animal model of NAFLD. These confirm that platelet activation may accompany a critical importance in NAFLD development and progression. However, how PAFs are involved in the NAFLD signalling pathway needs further investigation. In this paper, we review the relevant literature in recent years and discuss the role played by PAF in NAFLD development. It is important to elucidate the pathogenesis of NAFLD and to find effective interventions for treatment.

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.

The role of proprotein convertase subtilisin-kexin type 9 (PCSK9) in the vascular aging process - is there a link?

Lately there are many new, promising low-density lipoprotein cholesterol reducing therapies with PCSK9 inhibitors. We performed selected sampling of the publications in PubMed and made a review according to selected keywords. It summarizes the effect of PCSK9 on vascular aging, directly associated with lipid and glucose metabolism, chronic inflammation, atherosclerosis and hypertension. Serum level of PCSK9 is different in patients affected by certain illnesses (whose risk increases with age) than in healthy individuals. The same could be observed in the case of chronic inflammation. In this review we summarize what is known about the role PCSK9 in human metabolism and how this could affect the vascular aging process. Based on the available sources, we prove that PCSK9 is involved in many biochemical pathways associated with vascular aging. In the future, treatments using PCSK9 inhibition may not only reduce the cardiovascular risk but also slow down this process.

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.

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.

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.

Acylated ghrelin induces but deacylated ghrelin prevents hepatic steatosis and insulin resistance in lean rats: Effects on DAG/ PKC/JNK pathway

This study investigated the molecular effects of acylated (AG) and unacylated ghrelin (UAG) or their combination on hepatic lipogenesis pathways and DAG/PKC/JNK signaling in the livers of lean rats fed standard diet. Male rats (n = 10) were classified as control + vehicle (saline, 200 μl), AG, UAG, and AG + UAG-treated groups. All treatments were given at final doses of 200 ng/kg of for 14 days (twice/day, S.C). Administration of AG significantly enhanced circulatory levels of AG and UAG turning the normal ratio of AG/UAG from 1:2.5 to 1:1.2. However, while UAG didn't affect circulatory levels of AG, administration of UAG alone or in combination with AG resulted in AG/UAG ratios of 1:7 and 1:3, respectively. Independent of food intake nor the development of peripheral IR, AG increased hepatic DAG, TGs and CHOL contents and induced hepatic IR. Mechanism of action include 1) upregulation of mRNA and protein levels of DGAT-2 and mtGPAT-1, SREBP-1 and SCD-1, and 2) inhibition of fatty acids (FAs) oxidation mediated by inhibition of AMPK/ PPAR-α/CPT-1 axis. Consequently, AG induced membranous translocation of PKCδ and PKCε leading to activation of JNK and significant inhibition of insulin signaling under basal and insulin stimulation as evident by decreases in the phosphorylation levels of IRS (Tyr⁶¹²) and Akt (Thr³¹⁸) and increased phosphorylation of IRS (Ser³⁰⁷). However, while UAG only activated FAs oxidation in control rats, it reversed all alterations in all measured biochemical endpoints seen in the AG-treated group, when administered in combination with AG, leading to significant decreases in hepatic fat accumulation and prevention of hepatic IR. In conclusion, while exogenous administration of AG is at high risk of developing steatohepatitis and hepatic IR, co-administration of a balanced dose of UAG reduces this risk and inhibits hepatic lipid accumulation and enhance hepatic insulin signaling.

Protein kinase C-delta (PKCδ), a marker of inflammation and tuberculosis disease progression in humans, is important for optimal macrophage killing effector functions and survival in mice

We previously demonstrated that protein kinase C-δ (PKCδ) is critical for immunity against Listeria monocytogenes, Leishmania major, and Candida albicans infection in mice. However, the functional relevance of PKCδ during Mycobacterium tuberculosis (Mtb) infection is unknown. PKCδ was significantly upregulated in whole blood of patients with active tuberculosis (TB) disease. Lung proteomics further revealed that PKCδ was highly abundant in the necrotic and cavitory regions of TB granulomas in multidrug-resistant human participants. In murine Mtb infection studies, PKCδ^-/- mice were highly susceptible to tuberculosis with increased mortality, weight loss, exacerbated lung pathology, uncontrolled proinflammatory cytokine responses, and increased mycobacterial burdens. Moreover, these mice displayed a significant reduction in alveolar macrophages, dendritic cells, and decreased accumulation of lipid bodies (lungs and macrophages) and serum fatty acids. Furthermore, a peptide inhibitor of PKCδ in wild-type mice mirrored lung inflammation identical to infected PKCδ^-/- mice. Mechanistically, increased bacterial growth in macrophages from PKCδ^-/- mice was associated with a decline in killing effector functions independent of phagosome maturation and autophagy. Taken together, these data suggest that PKCδ is a marker of inflammation during active TB disease in humans and required for optimal macrophage killing effector functions and host protection during Mtb infection in mice.

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.

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.

Flavin-Containing Monooxygenase 3 Reduces Endoplasmic Reticulum Stress in Lipid-Treated Hepatocytes

Isoforms of flavin-containing monooxygenase (FMO) are involved in xenobiotic metabolism but have also been implicated in the regulation of glucose and lipid homeostasis and in the development of atherosclerosis. However, we have recently shown that improved insulin action is associated with increased FMO expression in livers of protein kinase C-deficient mice. Here, we investigated whether FMO3 expression affected insulin signaling, glucose metabolism, and endoplasmic reticulum (ER) stress in hepatocytes. HepG2 and IHH hepatocytes were transfected with FMO3 cDNA for overexpression, or small interfering RNA for knockdown. Cells were treated with palmitate to induce insulin resistance and insulin signaling, phosphoenolpyruvate carboxykinase (PEPCK) gene expression and ER stress markers were examined by immunoblotting and RT-PCR. Glycogen synthesis was measured using [(14)C]glucose. Palmitate treatment reduced insulin signaling at the level of Akt phosphorylation and glycogen synthesis, which were little affected by FMO3 overexpression. However, the fatty acid also increased the levels of several ER stress markers and activation of caspase 3, which were counteracted by FMO3 overexpression and exacerbated by FMO3 knockdown. Although FMO3 expression did not reverse lipid effects on protein thiol redox in hepatocytes, it did prevent up-regulation of the gluconeogenic enzyme PEPCK by pharmacological ER stress inducers or by palmitate. ER stress and PEPCK levels were also reduced in livers of fat-fed protein kinase Cδ-deficient mice. Our data indicate that FMO3 can contribute to the regulation of glucose metabolism in the liver by reducing lipid-induced ER stress and the expression of PEPCK, independently of insulin signal transduction.

Expression of Protein Kinase C Isoforms in Pancreatic Islets and Liver of Male Goto-Kakizaki Rats, a Model of Type 2 Diabetes

Protein kinase C (PKC) is a family of protein kinases controlling protein phosphorylation and playing important roles in the regulation of metabolism. We have investigated expression levels of PKC isoforms in pancreatic islets and liver of diabetic Goto-Kakizaki (GK) rats with and without insulin treatment to evaluate their association with glucose homeostasis. mRNA and protein expression levels of PKC isoforms were assessed in pancreatic islets and liver of Wistar rats and GK rats with or without insulin treatment. PKCα and PKCζ mRNA expressions were down-regulated in islets of GK compared with Wistar rats. PKCα and phosphorylated PKCα (p-PKCα) protein expressions were decreased in islets of GK compared with insulin-treated GK and Wistar rats. PKCζ protein expression in islets was reduced in GK and insulin-treated GK compared with Wistar rats, but p-PKCζ was decreased only in GK rats. Islet PKCε mRNA and protein expressions were lower in GK compared with insulin-treated GK and Wistar rats. In liver, PKCδ and PKCζ mRNA expressions were decreased in both GK and insulin-treated GK compared with Wistar rats. Hepatic PKCζ protein expression was diminished in both GK rats with and without insulin treatment compared with Wistar rats. Hepatic PKCε mRNA expression was down-regulated in insulin-treated GK compared with GK and Wistar rats. PKCα, PKCε, and p-PKCζ expressions were secondary to hyperglycaemia in GK rat islets. Hepatic PKCδ and PKCζ mRNA expressions were primarily linked to hyperglycaemia. Additionally, hepatic PKCε mRNA expression could be under control of insulin.

Does Diacylglycerol Accumulation in Fatty Liver Disease Cause Hepatic Insulin Resistance?

Numerous studies conducted on obese humans and various rodent models of obesity have identified a correlation between hepatic lipid content and the development of insulin resistance in liver and other tissues. Despite a large body of the literature on this topic, the cause and effect relationship between hepatic steatosis and insulin resistance remains controversial. If, as many believe, lipid aggregation in liver drives insulin resistance and other metabolic abnormalities, there are significant unanswered questions as to which lipid mediators are causative in this cascade. Several published papers have now correlated levels of diacylglycerol (DAG), the penultimate intermediate in triglyceride synthesis, with development of insulin resistance and have postulated that this occurs via activation of protein kinase C signaling. Although many studies have confirmed this relationship, many others have reported a disconnect between DAG content and insulin resistance. It has been postulated that differences in methods for DAG measurement, DAG compartmentalization within the cell, or fatty acid composition of the DAG may explain these discrepancies. The purpose of this review is to compare and contrast some of the relevant findings in this area and to discuss a number of unanswered questions regarding the relationship between DAG and insulin resistance.

Hepatic Steatosis as a Marker of Metabolic Dysfunction

Nonalcoholic fatty liver disease (NAFLD) is the liver manifestation of the complex metabolic derangements associated with obesity. NAFLD is characterized by excessive deposition of fat in the liver (steatosis) and develops when hepatic fatty acid availability from plasma and de novo synthesis exceeds hepatic fatty acid disposal by oxidation and triglyceride export. Hepatic steatosis is therefore the biochemical result of an imbalance between complex pathways of lipid metabolism, and is associated with an array of adverse changes in glucose, fatty acid, and lipoprotein metabolism across all tissues of the body. Intrahepatic triglyceride (IHTG) content is therefore a very good marker (and in some cases may be the cause) of the presence and the degree of multiple-organ metabolic dysfunction. These metabolic abnormalities are likely responsible for many cardiometabolic risk factors associated with NAFLD, such as insulin resistance, type 2 diabetes mellitus, and dyslipidemia. Understanding the factors involved in the pathogenesis and pathophysiology of NAFLD will lead to a better understanding of the mechanisms responsible for the metabolic complications of obesity, and hopefully to the discovery of novel effective treatments for their reversal.

[Role of metabolic lipases and lipotoxicity in the development of non-alcoholic steatosis and non-alcoholic steatohepatitis]

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disease in developed countries, covering a spectrum of pathological conditions ranging from single steatosis to non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. Its pathogenesis has been often interpreted by the "double-hit" hypothesis, where the lipid accumulation in the liver is followed by proinflammatory mediators inducing inflammation, hepatocellular injury and fibrosis. Nowadays, a more complex model suggests that free fatty acids and their metabolites could be the true lipotoxic agents that contribute to the development of NAFLD and hepatic insulin resistance, suggesting a central role for metabolic lipases in that process.

Proteomic analysis of livers from fat-fed mice deficient in either PKCδ or PKCε identifies Htatip2 as a regulator of lipid metabolism

Insulin resistance contributes to the development of Type 2 diabetes, and is associated with lipid oversupply. Deletion of isoforms of the lipid-activated protein kinase C (PKC) family, PKCδ or PKCε, improves insulin action in fat-fed mice, but differentially affects hepatic lipid metabolism. To investigate the mechanisms involved, we employed an in vivo adaptation of SILAC to examine the effects of a fat diet together with deletion of PKCδ or PKCε on the expression of liver proteins. We identified a total of 3359 and 3488 proteins from the PKCδ and PKCε knockout study groups, respectively, and showed that several enzymes of lipid metabolism were affected by the fat diet. In fat-fed mice, 23 proteins showed changes upon PKCδ deletion while 19 proteins were affected by PKCε deletion. Enzymes of retinol metabolism were affected by the absence of either PKC. Pathway analysis indicated that monosaccharide metabolism was affected only upon PKCδ deletion, while isoprenoid biosynthesis was affected in a PKCε-specific manner. Certain proteins were regulated inversely, including HIV-1 tat interactive protein 2 (Htatip2). Overexpression or knockdown of Htatip2 in hepatocytes affected fatty acid storage and oxidation, consistent with a novel role in mediating the differential effects of PKC isoforms on lipid metabolism. All MS data have been deposited in the ProteomeXchange with identifier PXD000971 (http://proteomecentral.proteomexchange.org/dataset/PXD000971).

PKCδ regulates hepatic triglyceride accumulation and insulin signaling in Lepr(db/db) mice

PKCδ has been linked to key pathophysiological features of non-alcoholic fatty liver disease (NAFLD). Yet, our knowledge of PKCδ's role in NAFLD development and progression in obese models is limited. PKCδ(-/-)/Lepr(db)(/)(db) mice were generated to evaluate key pathophysiological features of NAFLD in mice. Hepatic histology, oxidative stress, apoptosis, gene expression, insulin signaling, and serum parameters were analyzed in Lepr(db)(/)(db) and PKCδ(-/-)/Lepr(db)(/)(db) mice. The absence of PKCδ did not abrogate the development of obesity in Lepr(db)(/)(db) mice. In contrast, serum triglyceride levels and epididymal white adipose tissue weight normalized to body weight were reduced in PKCδ(-/-)/Lepr(db)(/)(db) mice compared Lepr(db)(/)(db) mice. Analysis of insulin signaling in mice revealed that hepatic Akt and GSK3β phosphorylation were strongly stimulated by insulin in PKCδ(-/-)/Lepr(db)(/)(db) compared Lepr(db)(/)(db) mice. PKCδ may be involved in the development of obesity-associated NAFLD by regulating hepatic lipid metabolism and insulin signaling.

Abrogating monoacylglycerol acyltransferase activity in liver improves glucose tolerance and hepatic insulin signaling in obese mice

Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol (DAG), a lipid that has been linked to the development of hepatic insulin resistance through activation of protein kinase C (PKC). The expression of genes that encode MGAT enzymes is induced in the livers of insulin-resistant human subjects with nonalcoholic fatty liver disease, but whether MGAT activation is causal of hepatic steatosis or insulin resistance is unknown. We show that the expression of Mogat1, which encodes MGAT1, and MGAT activity are also increased in diet-induced obese (DIO) and ob/obmice. To probe the metabolic effects of MGAT1 in the livers of obese mice, we administered antisense oligonucleotides (ASOs) against Mogat1 to DIO and ob/ob mice for 3 weeks. Knockdown of Mogat1 in liver, which reduced hepatic MGAT activity, did not affect hepatic triacylglycerol content and unexpectedly increased total DAG content. Mogat1 inhibition also increased both membrane and cytosolic compartment DAG levels. However, Mogat1 ASO treatment significantly improved glucose tolerance and hepatic insulin signaling in obese mice. In summary, inactivation of hepatic MGAT activity, which is markedly increased in obese mice, improved glucose tolerance and hepatic insulin signaling independent of changes in body weight, intrahepatic DAG and TAG content, and PKC signaling.

Molecular Mechanisms of Insulin Resistance Development

Insulin resistance (IR) is a phenomenon associated with an impaired ability of insulin to stimulate glucose uptake by target cells and to reduce the blood glucose level. A response increase in insulin secretion by the pancreas and hyperinsulinemia are compensatory reactions of the body. The development of IR leads to the inability of target cells to respond to insulin that results in developing type 2 diabetes mellitus (T2DM) and metabolic syndrome. For this reason, the metabolic syndrome is defined in practice as a combination of IR with one or more pathologies such as T2DM, arterial hypertension, dyslipidemia, abdominal obesity, non-alcoholic fatty liver disease, and some others. However, a combination of high blood glucose and insulin levels always serves as its physiological criterion. IR should be considered as a systemic failure of the endocrine regulation in the body. Physiological causes of IR are diverse. The main ones are nutritional overload and accumulation of certain lipids and their metabolites in cells, low physical activity, chronic inflammation and stress of various nature, including oxidative and endoplasmic reticulum stress (impairment of damaged protein degradation in the cell). Recent studies have demonstrated that these physiological mechanisms likely act through a single intracellular scenario. This is the impairment of signal transduction from the insulin receptor to its targets via the negative feedback mechanism in intracellular insulin-dependent signaling cascades. This review describes the physiological and intracellular mechanisms of insulin action and focuses on their abnormalities upon IR development. Finally, feasible trends in early molecular diagnosis and therapy of IR are discussed.

Nonalcoholic fatty liver disease, hepatic insulin resistance, and type 2 diabetes

Nonalcoholic fatty liver disease (NAFLD), hepatic insulin resistance, and type 2 diabetes are all strongly associated and are all reaching epidemic proportions. Whether there is a causal link between NAFLD and hepatic insulin resistance is controversial. This review will discuss recent studies in both humans and animal models of NAFLD that have implicated increases in hepatic diacylglycerol (DAG) content leading to activation of novel protein kinase Cϵ (PKCϵ) resulting in decreased insulin signaling in the pathogenesis of NAFLD-associated hepatic insulin resistance and type 2 diabetes. The DAG-PKCϵ hypothesis can explain the occurrence of hepatic insulin resistance observed in most cases of NAFLD associated with obesity, lipodystrophy, and type 2 diabetes.

Lipid metabolism, oxidative stress and cell death are regulated by PKC delta in a dietary model of nonalcoholic steatohepatitis

Steatosis, oxidative stress, and apoptosis underlie the development of nonalcoholic steatohepatitis (NASH). Protein kinase C delta (PKCδ) has been implicated in fatty liver disease and is activated in the methionine and choline-deficient (MCD) diet model of NASH, yet its pathophysiological importance towards steatohepatitis progression is uncertain. We therefore addressed the role of PKCδ in the development of steatosis, inflammation, oxidative stress, apoptosis, and fibrosis in an animal model of NASH. We fed PKCδ^−/− mice and wildtype littermates a control or MCD diet. PKCδ^−/− primary hepatocytes were used to evaluate the direct effects of fatty acids on hepatocyte lipid metabolism gene expression. A reduction in hepatic steatosis and triglyceride levels were observed between wildtype and PKCδ^−/− mice fed the MCD diet. The hepatic expression of key regulators of β-oxidation and plasma triglyceride metabolism was significantly reduced in PKCδ^−/− mice and changes in serum triglyceride were blocked in PKCδ^−/− mice. MCD diet-induced hepatic oxidative stress and hepatocyte apoptosis were reduced in PKCδ^−/− mice. MCD diet-induced NADPH oxidase activity and p47^phox membrane translocation were blunted and blocked, respectively, in PKCδ^−/− mice. Expression of pro-apoptotic genes and caspase 3 and 9 cleavage in the liver of MCD diet fed PKCδ^−/− mice were blunted and blocked, respectively. Surprisingly, no differences in MCD diet-induced fibrosis or pro-fibrotic gene expression were observed in 8 week MCD diet fed PKCδ^−/− mice. Our results suggest that PKCδ plays a role in key pathological features of fatty liver disease but not ultimately in fibrosis in the MCD diet model of NASH.

PKCδ is activated in the liver of obese Zucker rats and mediates diet-induced whole body insulin resistance and hepatocyte cellular insulin resistance

Insulin resistance can arise when pathological levels of free fatty acids (FFAs) and proinflammatory cytokines disrupt insulin signaling. Protein kinase C delta (PKCδ) is a FFA- and a proinflammatory cytokine-regulated protein kinase that is associated with inhibition of insulin signaling and action. To gain insight into the role of PKCδ in insulin resistance, PKCδ activation was studied in a genetic model of obesity-linked insulin resistance. PKCδ was found to be activated in the liver of obese insulin-resistant Zucker rats and in isolated cultured hepatocytes. PKCδ was further studied in PKCδ-null mice and their wild-type littermates fed a high-fat or control diet for 10 weeks. PKCδ-null mice on a high-fat diet had improved insulin sensitivity and hepatic insulin signaling compared to wild-type littermates. Additionally, the deleterious effect of a high-fat diet on glucose tolerance in wild-type mice was completely blocked in PKCδ-null mice. To directly test the role of PKCδ in cellular insulin resistance, primary hepatocytes from the high-fat diet mice were isolated and stimulated with insulin. Primary hepatocytes from PKCδ-null mice had improved insulin-stimulated Akt and FOXO phosphorylation compared to hepatocytes from wild-type littermates. Consistent with this result, tumor necrosis factor alpha-mediated inhibition of insulin signaling was blocked in PKCδ knockdown primary hepatocytes. These results indicate that PKCδ plays a role in insulin resistance and is consistent with the hypothesis that PKCδ is a negative regulator of insulin signaling and thus may be a therapeutic target for the treatment of type 2 diabetes.

The tail wagging the dog--regulation of lipid metabolism by protein kinase C

Upon their discovery almost 40 years ago, isoforms of the lipid-activated protein kinase C (PKC) family were initially regarded only as downstream effectors of the second messengers calcium and diacylglycerol, undergoing activation upon phospholipid hydrolysis in response to acute stimuli. Subsequently, several isoforms were found to be associated with the inhibitory effects of lipid over-supply on glucose homeostasis, especially the negative cross-talk with insulin signal transduction, observed upon accumulation of diacylglycerol in insulin target tissues. The PKC family has therefore attracted much attention in diabetes and obesity research, because intracellular lipid accumulation is strongly correlated with defective insulin action and the development of type 2 diabetes. Causal roles for various isoforms in the generation of insulin resistance have more recently been confirmed using PKC-deficient mice. However, during characterization of these animals, it became increasingly evident that the enzymes play key roles in the modulation of lipid metabolism itself, and may control the supply of lipids between tissues such as adipose and liver. Molecular studies have also demonstrated roles for PKC isoforms in several aspects of lipid metabolism, such as adipocyte differentiation and hepatic lipogenesis. While the precise mechanisms involved, especially the identities of protein substrates, are still unclear, the emerging picture suggests that the currently held view of the contribution of PKC isoforms to metabolism is an over-simplification. Although PKCs may inhibit insulin signal transduction, these enzymes are not merely downstream effectors of lipid accumulation, but in fact control the fate of fatty acids, thus the tail wags the dog.

Two protein kinase C isoforms, δ and ε, regulate energy homeostasis in mitochondria by transmitting opposing signals to the pyruvate dehydrogenase complex

Energy production in mitochondria is a multistep process that requires coordination of several subsystems. While reversible phosphorylation is emerging as the principal tool, it is still unclear how this signal network senses the workloads of processes as different as fuel procurement, catabolism in the Krebs cycle, and stepwise oxidation of reducing equivalents in the electron transfer chain. We previously proposed that mitochondria use oxidized cytochrome c in concert with retinol to activate protein kinase Cδ, thereby linking a prominent kinase network to the redox balance of the ETC. Here, we show that activation of PKCε in mitochondria also requires retinol as a cofactor, implying a redox-mechanism. Whereas activated PKCδ transmits a stimulatory signal to the pyruvate dehdyrogenase complex (PDHC), PKCε opposes this signal and inhibits the PDHC. Our results suggest that the balance between PKCδ and ε is of paramount importance not only for flux of fuel entering the Krebs cycle but for overall energy homeostasis. We observed that the synthetic retinoid fenretinide substituted for the retinol cofactor function but, on chronic use, distorted this signal balance, leading to predominance of PKCε over PKCδ. The suppression of the PDHC might explain the proapoptotic effect of fenretinide on tumor cells, as well as the diminished adiposity observed in experimental animals and humans. Furthermore, a disturbed balance between PKCδ and PKCε might underlie the injury inflicted on the ischemic myocardium during reperfusion. dehydrogenase complex.

Diacylglycerol activation of protein kinase Cε and hepatic insulin resistance

Nonalcoholic fatty liver disease (NAFLD) is now the most frequent chronic liver disease in Western societies, affecting one in four adults in the USA, and is strongly associated with hepatic insulin resistance, a major risk factor in the pathogenesis of type 2 diabetes. Although the cellular mechanisms underlying this relationship are unknown, hepatic accumulation of diacylglycerol (DAG) in both animals and humans has been linked to hepatic insulin resistance. In this Perspective, we discuss the role of DAG activation of protein kinase Cε as the mechanism responsible for NAFLD-associated hepatic insulin resistance seen in obesity, type 2 diabetes, and lipodystrophy.

Glycerolipid/free fatty acid cycle and islet β-cell function in health, obesity and diabetes

Pancreatic β-cells secrete insulin in response to fluctuations in blood fuel concentrations, in particular glucose and fatty acids. However, chronic fuel surfeit can overwhelm the metabolic, signaling and secretory capacity of the β-cell leading to its dysfunction and death - often referred to as glucolipotoxicity. In β-cells and many other cells, glucose and lipid metabolic pathways converge into a glycerolipid/free fatty acid (GL/FFA) cycle, which is driven by the substrates, glycerol-3-phosphate and fatty acyl-CoA, derived from glucose and fatty acids, respectively. Although the overall operation of GL/FFA cycle, consisting of lipolysis and lipogenesis, is "futile" in terms of energy expenditure, this metabolic cycle likely plays an indispensable role for various β-cell functions, in particular insulin secretion and excess fuel detoxification. In this review, we discuss the significance of GL/FFA cycle in the β-cell, its regulation and role in generating essential metabolic signals that participate in the lipid amplification arm of glucose stimulated insulin secretion and in β-cell growth. We propose the novel concept that the lipolytic segment of GL/FFA cycle is instrumental in producing signals for insulin secretion, whereas, the lipogenic segment generates signals relevant for β-cell survival/death and growth/proliferation.

Mechanisms for insulin resistance: common threads and missing links

Insulin resistance is a complex metabolic disorder that defies explanation by a single etiological pathway. Accumulation of ectopic lipid metabolites, activation of the unfolded protein response (UPR) pathway, and innate immune pathways have all been implicated in the pathogenesis of insulin resistance. However, these pathways are also closely linked to changes in fatty acid uptake, lipogenesis, and energy expenditure that can impact ectopic lipid deposition. Ultimately, these cellular changes may converge to promote the accumulation of specific lipid metabolites (diacylglycerols and/or ceramides) in liver and skeletal muscle, a common final pathway leading to impaired insulin signaling and insulin resistance.

Defining protein kinase/phosphatase isoenzymic regulation of mGlu₅ receptor-stimulated phospholipase C and Ca²⁺ responses in astrocytes

BACKGROUND AND PURPOSE

Cyclical phosphorylation and dephosphorylation of a key residue within the C-terminal domain of the activated type 5 metabotropic glutamate (mGlu₅) receptor is believed to cause the synchronous, oscillatory changes in inositol 1,4,5-trisphosphate and Ca²⁺ levels observed in a variety of cell types. Here, we have attempted to better define the kinase and phosphatase enzymes involved in this modulation.

EXPERIMENTAL APPROACH

Ca²⁺ and [³H]inositol phosphate ([³H]IP(x) ) measurements in astrocyte preparations have been used to evaluate the effects of pharmacological inhibition of protein kinase C (PKC) and protein phosphatase activities and small interfering RNA-mediated specific PKC isoenzymic knock-down on mGlu₅ receptor signalling.

KEY RESULTS

Ca²⁺ oscillation frequency or [³H]IP(x) accumulation in astrocytes stimulated by mGlu₅ receptors, was concentration-dependently decreased by protein phosphatase-1/2A inhibition or by PKC activation. PKC inhibition also increased [³H]IP(x) accumulation two- to threefold and changed the Ca²⁺ response into a peak-plateau response. However, selective inhibition of conventional PKC isoenzymes or preventing changes in [Ca²⁺](i) concentration by BAPTA-AM loading was without effect on mGlu₅ receptor-stimulated [³H]IP(x) accumulation. Selective knock-down of PKCδ was without effect on glutamate-stimulated Ca²⁺ responses; however, selective PKCε knock-down in astrocytes changed Ca²⁺ responses from oscillatory into peak-plateau type.

CONCLUSION AND IMPLICATIONS

These data confirm the acute regulation of mGlu₅ receptor signalling by protein kinases and protein phosphatases and provide novel data pinpointing the isoenzymic dependence of this regulation in the native mGlu₅ receptor-expressing rat cortical astrocyte. These data also highlight a potential alternative mechanism by which mGlu₅ receptor signalling might be therapeutically manipulated.

High-fat diet activates pro-inflammatory response in the prostate through association of Stat-3 and NF-κB

BACKGROUND

Signal transducer and activator of transcription (Stat)-3 and nuclear factor-kappa B (NF-κB) are important signaling pathways constitutively activated during inflammation. We previously reported that high-fat diet (HFD) intake induces oxidative stress in the prostate through elevated expression of NADPH oxidase subunits causing NF-κB activation. We sought to determine whether Stat-3 is involved in the activation of NF-κB in the prostate as a result of HFD feeding, leading to inflammation.

METHODS

C57BL/6 mice were either fed with regular diet (RD) or HFD for 4 and 8 weeks. Plasma cytokine levels were determined by multiplex analysis. Western blotting was performed to determine the expression of NF-κB, Stat-3, Akt, PDK1, PKCε, and their phosphorylated forms along with pathologic evaluation of the prostate. Immunoprecipitation and electrophoretic mobility shift assay (EMSA) were conducted to study the association between Stat-3 and NF-κB.

RESULTS

C57BL/6 mice fed with HFD showed a significant increase in the plasma levels of IL-1ß, IL-6, IL-17, and TNFα after 4 and 8 weeks of feeding, compared with RD controls. HFD feeding elevated the intraprostatic expression of IL-6 and caused activation of PKCε and Akt, the upstream kinase regulating Stat-3 and NF-κB. Nuclear extracts from the prostates of mice fed with HFD exhibited constitutively activated levels of Stat-3 and NF-κB/p65. Increased association between the activated forms of Stat-3 and NF-κB/p65 was observed in the nucleus as a result of HFD feeding, a finding that was accompanied by morphologic evidence of increased intraprostatic inflammation.

CONCLUSIONS

Our findings suggest that HFD activates Stat-3 and NF-κB/p65 in the prostate, and their interaction is associated with increased inflammation in the prostate.

Palmitate-activated macrophages confer insulin resistance to muscle cells by a mechanism involving protein kinase C θ and ε

Background

Macrophage-derived factors contribute to whole-body insulin resistance, partly by impinging on metabolically active tissues. As proof of principle for this interaction, conditioned medium from macrophages treated with palmitate (CM-PA) reduces insulin action and glucose uptake in muscle cells. However, the mechanism whereby CM-PA confers this negative response onto muscle cells remains unknown.

Methodology/Principal Findings

L6-GLUT4myc myoblasts were exposed for 24 h to palmitate-free conditioned medium from RAW 264.7 macrophages pre-treated with 0.5 mM palmitate for 6 h. This palmitate-free CM-PA, containing selective cytokines and chemokines, inhibited myoblast insulin-stimulated insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, AS160 phosphorylation, GLUT4 translocation and glucose uptake. These effects were accompanied by a rise in c-Jun N-terminal kinase (JNK) activation, degradation of Inhibitor of κBα (IκBα), and elevated expression of proinflammatory cytokines in myoblasts. Notably, CM-PA caused IRS1 phosphorylation on Ser1101, and phosphorylation of novel PKCθ and ε. Co-incubation of myoblasts with CM-PA and the novel and conventional PKC inhibitor Gö6983 (but not with the conventional PKC inhibitor Gö6976) prevented PKCθ and ε activation, JNK phosphorylation, restored IκBα mass and reduced proinflammatory cytokine production. Gö6983 also restored insulin signalling and glucose uptake in myoblasts. Moreover, co-silencing both novel PKC θ and ε isoforms in myoblasts by RNA interference, but not their individual silencing, prevented the inflammatory response and restored insulin sensitivity to CM-PA-treated myoblasts.

Conclusions/Clinical Significance

The results suggest that the block in muscle insulin action caused by CM-PA is mediated by novel PKCθ and PKCε. This study re-establishes the participation of macrophages as a relay in the action of fatty acids on muscle cells, and further identifies PKCθ and PKCε as key elements in the inflammatory and insulin resistance responses of muscle cells to macrophage products. Furthermore, it portrays these PKC isoforms as potential targets for the treatment of fatty acid-induced, inflammation-linked insulin resistance.

Time-dependent effects of Prkce deletion on glucose homeostasis and hepatic lipid metabolism on dietary lipid oversupply in mice

AIMS/HYPOTHESIS

We examined the time-dependent effects of deletion of the gene encoding protein kinase C epsilon (Prkce) on glucose homeostasis, insulin secretion and hepatic lipid metabolism in fat-fed mice.

METHODS

Prkce(-/-) and wild-type (WT) mice were fed a high-fat diet for 1 to 16 weeks and subjected to i.p. glucose tolerance tests (ipGTT) and indirect calorimetry. We also investigated gene expression and protein levels by RT-PCR, quantitative protein profiling (isobaric tag for relative and absolute quantification; iTRAQ) and immunoblotting. Lipid levels, mitochondrial oxidative capacity and lipid metabolism were assessed in liver and primary hepatocytes.

RESULTS

While fat-fed WT mice became glucose intolerant after 1 week, Prkce(-/-) mice exhibited normal glucose and insulin levels. iTRAQ suggested differences in lipid metabolism and oxidative phosphorylation between fat-fed WT and Prkce(-/-) animals. Liver triacylglycerols were increased in fat-fed Prkce(-/-) mice, resulting from altered lipid partitioning which promoted esterification of fatty acids in hepatocytes. In WT mice, fat feeding elevated oxygen consumption in vivo and in isolated liver mitochondria, but these increases were not seen in Prkce(-/-) mice. Prkce(-/-) hepatocytes also exhibited reduced production of reactive oxygen species (ROS) in the presence of palmitate. After 16 weeks of fat feeding, however, the improved glucose tolerance in fat-fed Prkce(-/-) mice was instead associated with increased insulin secretion during ipGTT, as we have previously reported.

CONCLUSIONS/INTERPRETATION

Prkce deletion ameliorates diet-induced glucose intolerance via two temporally distinct phenotypes. Protection against insulin resistance is associated with changes in hepatic lipid partitioning, which may reduce the acute inhibitory effects of fatty acid catabolism, such as ROS generation. In the longer term, enhancement of glucose-stimulated insulin secretion prevails.

PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

C57BL/6J and 129S6/Sv (B6 and 129) mice differ dramatically in their susceptibility to developing diabetes in response to diet- or genetically induced insulin resistance. A major locus contributing to this difference has been mapped to a region on mouse chromosome 14 that contains the gene encoding PKCδ. Here, we found that PKCδ expression in liver was 2-fold higher in B6 versus 129 mice from birth and was further increased in B6 but not 129 mice in response to a high-fat diet. PRKCD gene expression was also elevated in obese humans and was positively correlated with fasting glucose and circulating triglycerides. Mice with global or liver-specific inactivation of the Prkcd gene displayed increased hepatic insulin signaling and reduced expression of gluconeogenic and lipogenic enzymes. This resulted in increased insulin-induced suppression of hepatic gluconeogenesis, improved glucose tolerance, and reduced hepatosteatosis with aging. Conversely, mice with liver-specific overexpression of PKCδ developed hepatic insulin resistance characterized by decreased insulin signaling, enhanced lipogenic gene expression, and hepatosteatosis. Therefore, changes in the expression and regulation of PKCδ between strains of mice and in obese humans play an important role in the genetic risk of hepatic insulin resistance, glucose intolerance, and hepatosteatosis; and thus PKCδ may be a potential target in the treatment of metabolic syndrome.

Deletion of protein kinase Cδ in mice modulates stability of inflammatory genes and protects against cytokine-stimulated beta cell death in vitro and in vivo

AIMS/HYPOTHESIS

Proinflammatory cytokines contribute to beta cell destruction in type 1 diabetes, but the mechanisms are incompletely understood. The aim of the current study was to address the role of the protein kinase C (PKC) isoform PKCδ, a diverse regulator of cell death, in cytokine-stimulated apoptosis in primary beta cells.

METHODS

Islets isolated from wild-type or Prkcd(-/-) mice were treated with IL-1β, TNF-α and IFNγ and assayed for apoptosis, nitric oxide (NO) generation and insulin secretion. Activation of signalling pathways, apoptosis and endoplasmic reticulum (ER) stress were determined by immunoblotting. Stabilisation of mRNA transcripts was measured by RT-PCR following transcriptional arrest. Mice were injected with multiple low doses of streptozotocin (MLD-STZ) and fasting blood glucose monitored.

RESULTS

Deletion of Prkcd inhibited apoptosis and NO generation in islets stimulated ex vivo with cytokines. It also delayed the onset of hyperglycaemia in MLD-STZ-treated mice. Activation of ERK, p38, JNK, AKT1, the ER stress markers DDIT3 and phospho-EIF2α and the intrinsic apoptotic markers BCL2 and MCL1 was not different between genotypes. However, deletion of Prkcd destabilised mRNA transcripts for Nos2, and for multiple components of the toll-like receptor 2 (TLR2) signalling complex, which resulted in disrupted TLR2 signalling.

CONCLUSIONS/INTERPRETATION

Loss of PKCδ partially protects against hyperglycaemia in the MLD-STZ model in vivo, and against cytokine-mediated apoptosis in vitro. This is accompanied by reduced NO generation and destabilisation of Nos2 and components of the TLR2 signalling pathway. The results highlight a mechanism for regulating proinflammatory gene expression in beta cells independently of transcription.

PKC{delta} is activated in a dietary model of steatohepatitis and regulates endoplasmic reticulum stress and cell death

Hepatic steatosis can progress to the clinical condition of non-alcoholic steatohepatitis (NASH), which is a precursor of more serious liver diseases. The novel PKC isoforms δ and ε are activated by lipid metabolites and have been implicated in lipid-induced hepatic disease. Using a methionine- and choline-deficient (MCD) dietary model of NASH, we addressed the question of whether hepatic PKCδ and PKCε are activated. With progression from steatosis to steatohepatitis, there was activation and increased PKCδ protein content coincident with hepatic endoplasmic reticulum (ER) stress parameters. To examine whether similar changes could be induced in vitro, McA-RH 7777 (McA) hepatoma cells were used. We observed that McA cells stored triglyceride and released alanine aminotransferase (ALT) when treated with MCD medium in the presence of fatty acids. Further, MCD medium with palmitic acid, but not oleic or linoleic acids, maximally activated PKCδ and stimulated ER stress. In PKCδ knockdown McA cells, MCD/fatty acid medium-induced ALT release and ER stress induction were completely blocked, but triglyceride storage was not. In addition, a reduction in the uptake of propidium iodide and the number of apoptotic nuclei and a significant increase in cell viability and DNA content were observed in PKCδ knockdown McA cells incubated in MCD medium with palmitic acid. Our studies show that PKCδ activation and protein levels are elevated in an animal model of steatohepatitis, which was recapitulated in a cell model, supporting the conclusion that PKCδ plays a role in ALT release, the ER stress signal, and cell death.

Protein kinase C-delta is involved in the inflammatory effect of IL-6 in mouse adipose cells

AIMS/HYPOTHESIS

The aim of the study was to address the role of protein kinase C-delta (PKCdelta) on phosphorylation of signal transducer and activator of transcription 3 (STAT3) and activation of inflammatory genes in response to IL-6 in adipose cells.

METHODS

Differentiated mouse 3T3-L1 adipocytes preincubated with the PKCdelta inhibitor rottlerin and mouse embryonic fibroblasts (MEFs) lacking PKCdelta were incubated with IL-6 and/or insulin. RNA was extracted and the gene expression was analysed by real-time PCR, while the proteins from total, nuclear and cytoplasmic lysates were analysed by immunoblotting.

RESULTS

Inhibition of PKCdelta by rottlerin significantly reduced both Ser-727 and Tyr-705 phosphorylation of STAT3. Consequently, nuclear translocation of STAT3 and the IL-6-induced gene transcription and protein release of the inflammatory molecule serum amyloid A 3 (SAA3) were reduced. Similarly, the IL-6-regulated gene transcription of Il-6 (also known as Il6) to Hp and the feedback inhibitor of IL-6, Socs3, were also attenuated by rottlerin. Furthermore, PKCdelta was found to translocate to the nucleus following IL-6 treatment and this was also reduced by rottlerin. In agreement with the effect of rottlerin, Pkcdelta (also known as Prkcd) ( -/- ) MEFs also displayed a markedly reduced ability of IL-6 to activate the transcription of Saa3, Hp, Socs3 and Il6 genes compared with wild-type MEFs. These results correlated with a reduced nuclear translocation and phosphorylation of STAT3.

CONCLUSIONS/INTERPRETATION

These results show that PKCdelta plays a key role in the inflammatory effect of IL-6 in adipose cells and may be a suitable target for novel anti-inflammatory agents.

Metformin Improves Insulin Signaling in Obese Rats via Reduced IKKbeta Action in a Fiber-Type Specific Manner

Metformin is a widely used insulin-sensitizing drug, though its mechanisms are not fully understood. Metformin has been shown to activate AMPK in skeletal muscle; however, its effects on the inhibitor of kappaB kinasebeta (IKKbeta) in this same tissue are unknown. The aim of this study was to (1) determine the ability of metformin to attenuate IKKbeta action, (2) determine whether changes in AMPK activity are associated with changes in IKKbeta action in skeletal muscle, and (3) examine whether changes in AMPK and IKKbeta function are consistent with improved insulin signaling. Lean and obese male Zuckers received either vehicle or metformin by oral gavage daily for four weeks (four groups of eight). Proteins were measured in white gastrocnemius (WG), red gastrocnemius (RG), and soleus. AMPK phosphorylation increased (P < .05) in WG in both lean (57%) and obese (106%), and this was supported by an increase in phospho-ACC in WG. Further, metformin increased IkappaBalpha levels in both WG (150%) and RG (67%) of obese rats, indicative of reduced IKKbeta activity (P < .05), and was associated with reduced IRS1-pSer(307) (30%) in the WG of obese rats (P < .02). From these data we conclude that metformin treatment appears to exert an inhibitory influence on skeletal muscle IKKbeta activity, as evidenced by elevated IkappaBalpha levels and reduced IRS1-Ser(307) phosphorylation in a fiber-type specific manner.