Macrophage alternative activation confers protection against lipotoxicity-induced cell death

The beneficial impact of curcumin on cardiac lipotoxicity

Lipotoxicity is defined as a prolonged metabolic imbalance of lipids that results in ectopic fat distribution in peripheral organs such as the liver, heart, and kidney. The harmful consequences of excessive lipid accumulation in cardiomyocytes cause cardiac lipotoxicity, which alters the structure and function of the heart. Obesity and diabetes are linked to lipotoxic cardiomyopathy. These anomalies might be caused by a harmful metabolic shift that accumulates toxic lipids and shifts glucose oxidation to less fatty acid oxidation. Research has linked fatty acids, fatty acyl coenzyme A, diacylglycerol, and ceramide to lipotoxic stress in cells. This stress can be brought on by apoptosis, impaired insulin signaling, endoplasmic reticulum stress, protein kinase C activation, p38 Ras-mitogen-activated protein kinase (MAPK) activation, or modification of peroxisome proliferator-activated receptors (PPARs) family members. Curcuma longa is used to extract curcumin, a hydrophobic polyphenol derivative with a variety of pharmacological characteristics. Throughout the years, curcumin has been utilized as an anti-inflammatory, antioxidant, anticancer, hepatoprotective, cardioprotective, anti-diabetic, and anti-obesity drug. Curcumin reduces cardiac lipotoxicity by inhibiting apoptosis and decreasing the expression of apoptosis-related proteins, reducing the expression of inflammatory cytokines, activating the autophagy signaling pathway, and inhibiting the expression of endoplasmic reticulum stress marker proteins.

Crosstalk between P2Y receptors and cyclooxygenase activity in inflammation and tissue repair

The role of extracellular nucleotides as modulators of inflammation and cell stress is well established. One of the main actions of these molecules is mediated by the activation of purinergic receptors (P2) of the plasma membrane. P2 receptors can be classified according to two different structural families: P2X ionotropic ion channel receptors, and P2Y metabotropic G protein-coupled receptors. During inflammation, damaged cells release nucleotides and purinergic signaling occurs along the temporal pattern of the synthesis of pro-inflammatory and pro-resolving mediators by myeloid and lymphoid cells. In macrophages under pro-inflammatory conditions, the expression and activity of cyclooxygenase 2 significantly increases and enhances the circulating levels of prostaglandin E2 (PGE2), which exerts its effects both through specific plasma membrane receptors (EP1-EP4) and by activation of intracellular targets. Here we review the mechanisms involved in the crosstalk between PGE2 and P2Y receptors on macrophages, which is dependent on several isoforms of protein kinase C and protein kinase D1. Due to this crosstalk, a P2Y-dependent increase in calcium is blunted by PGE2 whereas, under these conditions, macrophages exhibit reduced migratory capacity along with enhanced phagocytosis, which contributes to the modulation of the inflammatory response and tissue repair.

Inflammaging and fatty acid oxidation in monocytes and macrophages

Fatty acid oxidation (FAO), primarily known as β-oxidation, plays a crucial role in breaking down fatty acids within mitochondria and peroxisomes to produce cellular energy and preventing metabolic dysfunction. Myeloid cells, including macrophages, microglia, and monocytes, rely on FAO to perform essential cellular functions and uphold tissue homeostasis. As individuals age, these cells show signs of inflammaging, a condition that includes a chronic onset of low-grade inflammation and a decline in metabolic function. These lead to changes in fatty acid metabolism and a decline in FAO pathways. Recent studies have shed light on metabolic shifts occurring in macrophages and monocytes during aging, correlating with an altered tissue environment and the onset of inflammaging. This review aims to provide insights into the connection of inflammatory pathways and altered FAO in macrophages and monocytes from older organisms. We describe a model in which there is an extended activation of receptor for advanced glycation end products, nuclear factor-κB (NF-κB) and the nod-like receptor family pyrin domain containing 3 inflammasome within macrophages and monocytes. This leads to an increased level of glycolysis, and also promotes pro-inflammatory cytokine production and signaling. As a result, FAO-related enzymes such as 5' AMP-activated protein kinase and peroxisome proliferator-activated receptor-α are reduced, adding to the escalation of inflammation, accumulation of lipids, and heightened cellular stress. We examine the existing body of literature focused on changes in FAO signaling within macrophages and monocytes and their contribution to the process of inflammaging.

Simulations of naïve and KLA-activated macrophage plasma membrane models

Macrophages (MAs), which play vital roles in human immune responses and lipid metabolisms, are implicated in the development and progression of atherosclerosis, a major contributor to cardiovascular diseases. Specifically, the abnormal lipid metabolism of oxidized low-density lipids (oxLDLs) in MAs is believed to be a crucial factor. However, the precise mechanism by which the MA membrane contributes to this altered lipid metabolism remains unclear. Lipidomic studies have revealed significant differences in membrane composition between various MA phenotypes. This study serves to provide and characterize complex realistic computational models for naïve (M0) and Kdo2-lipid A-activated (M1) state MA. Analyses of surface area per lipid (SA/lip), area compressibility modulus (KA), carbon‑hydrogen order parameter (SCH), electron density profile (EDP), tilt angles, two-dimension radial distribution functions (2D RDFs), mean squared displacement (MSD), hydrogen bonds (H-bonds), lipid clustering, and lipid wobble were conducted for both models. Results indicate that the M1 state MA membrane is more tightly packed, with increased chain order across lipid species, and forms PSM-DOPG-CHOL and PSM-SLPC-CHOL clusters. Importantly, the bilayer thicknesses reported for the models are in good agreement with experimental data for the thicknesses of transmembrane regions for MA integral proteins. These findings validate the described models as physiologically accurate for future computational studies of MA membranes and their residing proteins.

The Double-Edged Sword of Erythrocytes in Health and Disease via Their Adhesiveness

Their widespread presence throughout the vasculature, coupled with their reactivity, and thereby to their potential to release reactive oxidative species, or to utilize their anti-oxidative capacities, has promoted much discussion of the role(s) of red blood cells (RBCs) in the progression of health or, alternatively, a wide range of disease states. Moreover, these role(s) have been linked to the development of adhesiveness and, in fact, thereby to the essential pathway to their eventual clearance, e.g., by macrophages in the spleen. These disparate roles coupled with the mechanisms involved are reviewed and given. Following an analysis, novel perspectives are provided; these perspectives can lead to novel assays for identifying the potential for RBC adhesiveness as suggested herein. We describe this paradigm, that involves RBC adhesiveness, hemolysis, and ghost formation, with examples including, inter alia, the progression of atherosclerosis and the suppression of tumor growth along with other disease states.

We are what we eat: The role of lipids in metabolic diseases

Lipids play a fundamental role, both structurally and functionally, for the correct functioning of the organism. In the last two decades, they have evolved from molecules involved only in energy storage to compounds that play an important role as components of cell membranes and signaling molecules that regulate cell homeostasis. For this reason, their interest as compounds involved in human health has been gaining weight. Indeed, lipids derived from dietary sources and endogenous biosynthesis are relevant for the pathophysiology of numerous diseases. There exist pathological conditions that are characterized by alterations in lipid metabolism. This is particularly true for metabolic diseases, such as liver steatosis, type 2 diabetes, cancer and cardiovascular diseases. The main issue to be considered is lipid homeostasis. A precise control of fat homeostasis is required for a correct regulation of metabolic pathways and safe and efficient energy storage in adipocytes. When this fails, a deregulation occurs in the maintenance of systemic metabolism. This happens because an increased concentrations of lipids impair cellular homeostasis and disrupt tissue function, giving rise to lipotoxicity. Fat accumulation results in many alterations in the physiology of the affected organs, mainly in metabolic tissues. These alterations include the activation of oxidative and endoplasmic reticulum stress, mitochondrial dysfunction, increased inflammation, accumulation of bioactive molecules and modification of gene expression. In this chapter, we review the main metabolic diseases in which alterations in lipid homeostasis are involved and discuss their pathogenic mechanisms.

Functional Crosstalk between PCSK9 Internalization and Pro-Inflammatory Activation in Human Macrophages: Role of Reactive Oxygen Species Release

Atherosclerosis is a cardiovascular disease caused mainly by dyslipidemia and is characterized by the formation of an atheroma plaque and chronic inflammation. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a protease that induces the degradation of the LDL receptor (LDLR), which contributes to increased levels of LDL cholesterol and the progress of atherosclerosis. Given that macrophages are relevant components of the lipidic and inflammatory environment of atherosclerosis, we studied the effects of PCSK9 treatment on human macrophages. Our data show that human macrophages do not express PCSK9 but rapidly incorporate the circulating protein through the LDLR and also activate the pro-inflammatory TLR4 pathway. Both LDLR and TLR4 are internalized after incubation of macrophages with exogenous PCSK9. PCSK9 uptake increases the production of reactive oxygen species and reduces the expression of genes involved in lipid metabolism and cholesterol efflux, while enhancing the production of pro-inflammatory cytokines through a TLR4-dependent mechanism. Under these conditions, the viability of macrophages is compromised, leading to increased cell death. These results provide novel insights into the role of PCSK9 in the crosstalk of lipids and cholesterol metabolism through the LDLR and on the pro-inflammatory activation of macrophages through TLR4 signaling. These pathways are relevant in the outcome of atherosclerosis and highlight the relevance of PCSK9 as a therapeutic target for the treatment of cardiovascular diseases.

Molecular Mechanism of Lipotoxicity as an Interesting Aspect in the Development of Pathological States-Current View of Knowledge

Free fatty acids (FFAs) play numerous vital roles in the organism, such as contribution to energy generation and reserve, serving as an essential component of the cell membrane, or as ligands for nuclear receptors. However, the disturbance in fatty acid homeostasis, such as inefficient metabolism or intensified release from the site of storage, may result in increased serum FFA levels and eventually result in ectopic fat deposition, which is unfavorable for the organism. The cells are adjusted for the accumulation of FFA to a limited extent and so prolonged exposure to elevated FFA levels results in deleterious effects referred to as lipotoxicity. Lipotoxicity contributes to the development of diseases such as insulin resistance, diabetes, cardiovascular diseases, metabolic syndrome, and inflammation. The nonobvious organs recognized as the main lipotoxic goal of action are the pancreas, liver, skeletal muscles, cardiac muscle, and kidneys. However, lipotoxic effects to a significant extent are not organ-specific but affect fundamental cellular processes occurring in most cells. Therefore, the wider perception of cellular lipotoxic mechanisms and their interrelation may be beneficial for a better understanding of various diseases’ pathogenesis and seeking new pharmacological treatment approaches.

Adipose Tissue Immunometabolism and Apoptotic Cell Clearance

The safe removal of apoptotic debris by macrophages—often referred to as efferocytosis—is crucial for maintaining tissue integrity and preventing self-immunity or tissue damaging inflammation. Macrophages clear tissues of hazardous materials from dying cells and ultimately adopt a pro-resolving activation state. However, adipocyte apoptosis is an inflammation-generating process, and the removal of apoptotic adipocytes by so-called adipose tissue macrophages triggers a sequence of events that lead to meta-inflammation and obesity-associated metabolic diseases. Signals that allow apoptotic cells to control macrophage immune functions are complex and involve metabolites released by the apoptotic cells and also metabolites produced by the macrophages during the digestion of apoptotic cell contents. This review provides a concise summary of the adipocyte-derived metabolites that potentially control adipose tissue macrophage immune functions and, hence, may induce or alleviate adipose tissue inflammation.

Innate Immune System Orchestrates Metabolic Homeostasis and Dysfunction in Visceral Adipose Tissue During Obesity

Arising incidence of metabolic disorders and related diseases caused by obesity is a global health concern. Elucidating the role of the immune system in this process will help to understand the related mechanisms and develop treatment strategies. Here, we have focused on innate immune cells in visceral adipose tissue (VAT) and summarized the roles of these cells in maintaining the homeostasis of VAT. Furthermore, this review reveals the importance of quantitative and functional changes of innate immune cells when the metabolic microenvironment changes due to obesity or excess lipids, and confirms that these changes eventually lead to the occurrence of chronic inflammation and metabolic diseases of VAT. Two perspectives are reviewed, which include sequential changes in various innate immune cells in the steady state of VAT and its imbalance during obesity. Cross-sectional interactions between various innate immune cells at the same time point are also reviewed. Through delineation of a comprehensive perspective of VAT homeostasis in obesity-induced chronic inflammation, and ultimately metabolic dysfunction and disease, we expect to clarify the complex interactive networks among distinct cell populations and propose that these interactions should be taken into account in the development of biotherapeutic strategies.

The Emerging Role of COX-2, 15-LOX and PPARγ in Metabolic Diseases and Cancer: An Introduction to Novel Multi-target Directed Ligands (MTDLs)

Emerging evidence supports an intertwining framework for the involvement of different inflammatory pathways in a common pathological background for a number of disorders. Of importance are pathways involving arachidonic acid metabolism by cyclooxygenase-2 (COX-2) and 15-lipoxygenase (15-LOX). Both enzyme activities and their products are implicated in a range of pathophysiological processes encompassing metabolic impairment leading to adipose inflammation and the subsequent vascular and neurological disorders, in addition to various pro- and antitumorigenic effects. A further layer of complexity is encountered by the disparate, and often reciprocal, modulatory effect COX-2 and 15-LOX activities and metabolites exert on each other or on other cellular targets, the most prominent of which is peroxisome proliferator-activated receptor gamma (PPARγ). Thus, effective therapeutic intervention with such multifaceted disorders requires the simultaneous modulation of more than one target. Here, we describe the role of COX-2, 15-LOX, and PPARγ in cancer and complications of metabolic disorders, highlight the value of designing multi-target directed ligands (MTDLs) modifying their activity, and summarizing the available literature regarding the rationale and feasibility of design and synthesis of these ligands together with their known biological effects. We speculate on the potential impact of MTDLs in these disorders as well as emphasize the need for structured future effort to translate these early results facilitating the adoption of these, and similar, molecules in clinical research.

Nuclear Receptors as Multiple Regulators of NLRP3 Inflammasome Function

Nuclear receptors are important bridges between lipid signaling molecules and transcription responses. Beside their role in several developmental and physiological processes, many of these receptors have been shown to regulate and determine the fate of immune cells, and the outcome of immune responses under physiological and pathological conditions. While NLRP3 inflammasome is assumed as key regulator for innate and adaptive immune responses, and has been associated with various pathological events, the precise impact of the nuclear receptors on the function of inflammasome is hardly investigated. A wide variety of factors and conditions have been identified as modulators of NLRP3 inflammasome activation, and at the same time, many of the nuclear receptors are known to regulate, and interact with these factors, including cellular metabolism and various signaling pathways. Nuclear receptors are in the focus of many researches, as these receptors are easy to manipulate by lipid soluble molecules. Importantly, nuclear receptors mediate regulatory mechanisms at multiple levels: not only at transcription level, but also in the cytosol via non-genomic effects. Their importance is also reflected by the numerous approved drugs that have been developed in the past decade to specifically target nuclear receptors subtypes. Researches aiming to delineate mechanisms that regulate NLRP3 inflammasome activation draw a wide range of attention due to their unquestionable importance in infectious and sterile inflammatory conditions. In this review, we provide an overview of current reports and knowledge about NLRP3 inflammasome regulation from the perspective of nuclear receptors, in order to bring new insight to the potentially therapeutic aspect in targeting NLRP3 inflammasome and NLRP3 inflammasome-associated diseases.

The transcription factor EGR2 is the molecular linchpin connecting STAT6 activation to the late, stable epigenomic program of alternative macrophage polarization

In this study, Daniel et al. examined the mechanisms of macrophage polarization in general and alternative polarization in particular. Using unbiased epigenomic, molecular biology, and genetic GOF and LOF approaches, the authors show that the zinc finger transcription factor EGR2 acts as an essential and evolutionarily conserved broad-acting factor, linking transient polarization signals to stable epigenomic and transcriptional changes in macrophages.

Macrophages polarize into functionally distinct subtypes while responding to microenvironmental cues. The identity of proximal transcription factors (TFs) downstream from the polarization signals are known, but their activity is typically transient, failing to explain the long-term, stable epigenomic programs developed. Here, we mapped the early and late epigenomic changes of interleukin-4 (IL-4)-induced alternative macrophage polarization. We identified the TF, early growth response 2 (EGR2), bridging the early transient and late stable gene expression program of polarization. EGR2 is a direct target of IL-4-activated STAT6, having broad action indispensable for 77% of the induced gene signature of alternative polarization, including its autoregulation and a robust, downstream TF cascade involving PPARG. Mechanistically, EGR2 binding results in chromatin opening and the recruitment of chromatin remodelers and RNA polymerase II. Egr2 induction is evolutionarily conserved during alternative polarization of mouse and human macrophages. In the context of tissue resident macrophages, Egr2 expression is most prominent in the lung of a variety of species. Thus, EGR2 is an example of an essential and evolutionarily conserved broad acting factor, linking transient polarization signals to stable epigenomic and transcriptional changes in macrophages.

Clinical importance of neuregulin-4 and its receptor ErbB4 in periodontal disease pathogenesis

OBJECTIVE

The aim of this clinical trial was to evaluate the levels of Neuregulin-4 (Nrg4), Erb-b2 receptor tyrosine kinase 4 (ErbB4), interleukin (IL)-6, IL-10, nitric oxide synthase (NOS)-2, and arginase (Arg)-1 in periodontal health and disease.

MATERIALS AND METHODS

This study includes systemically healthy 20 periodontally healthy (H), 20 gingivitis (G), 20 stage II periodontitis (P1), and 20 stage III periodontitis (P2) subjects. Periodontal clinical measurements and samples of gingival crevicular fluid (GCF) and serum were obtained at baseline and 4 weeks after non-surgical periodontal treatment (NSPT). Enzyme-linked immunosorbent assay (ELISA) was used to determine ErbB4, Nrg4, IL-6, IL-10, NOS2, and Arg1 levels in all samples.

RESULTS

GCF ErbB4 and Nrg4 total amounts and IL-6/IL-10 ratio were significantly higher in G, P1, and P2 groups than H group. Serum NOS2 levels were significantly lower, whereas serum Arg1 levels were higher in H group than the others. The GCF levels of ErbB4 and Nrg4 were significantly decreased after NSPT in G, P1, and P2 groups. Additionally, the GCF levels of ErbB4 and Nrg4 were positively correlated with all clinical parameters and IL-6/IL-10 ratio.

CONCLUSIONS

Nrg4 and its receptor ErbB4 might have crucial roles in the pathogenesis of periodontal disease. These results should be verified with future prospective studies to further clarify the exact role of those biomarkers.

MDMX phosphorylation-dependent p53 downregulation contributes to an immunosuppressive tumor microenvironment

A role of tumor-suppressive activity of p53 in the tumor microenvironment (TME) has been implicated but remains fairly understudied. To address this knowledge gap, we leveraged our Mdmx^S314A mice as recipients to investigate how implanted tumor cells incapacitate host p53 creating a conducive TME for tumor progression. We found that tumor cell-associated stress induced p53 downregulation in peritumor cells via an MDMX-Ser314 phosphorylation-dependent manner. As a result, an immunosuppressive TME was developed, as reflected by diminished immune cell infiltration into tumors and compromised macrophage M1 polarization. Remarkably, ablation of MDMX-Ser314 phosphorylation attenuated p53 decline in peritumor cells, which was associated with mitigation of immunosuppression and significant tumor growth delay. Our data collectively uncover a novel role of p53 in regulating the tumor immune microenvironment, suggesting that p53 restoration in the TME can be exploited as a potential strategy of anticancer therapy.

Bmal1 integrates mitochondrial metabolism and macrophage activation

Metabolic pathways and inflammatory processes are under circadian regulation. Rhythmic immune cell recruitment is known to impact infection outcomes, but whether the circadian clock modulates immunometabolism remains unclear. We find that the molecular clock Bmal1 is induced by inflammatory stimulants, including Ifn-γ/lipopolysaccharide (M1) and tumor-conditioned medium, to maintain mitochondrial metabolism under metabolically stressed conditions in mouse macrophages. Upon M1 stimulation, myeloid-specific Bmal1 knockout (M-BKO) renders macrophages unable to sustain mitochondrial function, enhancing succinate dehydrogenase (SDH)-mediated mitochondrial production of reactive oxygen species as well as Hif-1α-dependent metabolic reprogramming and inflammatory damage. In tumor-associated macrophages, aberrant Hif-1α activation and metabolic dysregulation by M-BKO contribute to an immunosuppressive tumor microenvironment. Consequently, M-BKO increases melanoma tumor burden, whereas administering the SDH inhibitor dimethyl malonate suppresses tumor growth. Therefore, Bmal1 functions as a metabolic checkpoint that integrates macrophage mitochondrial metabolism, redox homeostasis and effector functions. This Bmal1-Hif-1α regulatory loop may provide therapeutic opportunities for inflammatory diseases and immunotherapy.

Luteolin prevents irinotecan-induced intestinal mucositis in mice through antioxidant and anti-inflammatory properties

BACKGROUND AND PURPOSE

Intestinal mucositis refers to mucosal damage caused by cancer treatment, and irinotecan is one of the agents most associated with this condition. Focusing on the development of alternatives to prevent this important adverse effect, we evaluated the activity of the flavonoid luteolin, which has never been tested for this purpose despite its biological potential.

EXPERIMENTAL APPROACH

The effects of luteolin were examined on irinotecan-induced intestinal mucositis in mice. Clinical signs were evaluated. Moreover, histological, oxidative, and inflammatory parameters were analysed, as well as the possible interference of luteolin in the anti-tumour activity of irinotecan.

KEY RESULTS

Luteolin (30 mg·kg^-1 ; p.o. or i.p.) prevented irinotecan-induced intestinal damage by reducing weight loss and diarrhoea score and attenuating the shortening of the duodenum and colon. Histological analysis confirmed that luteolin (p.o.) prevented villous shortening, vacuolization, and apoptosis of cells and preserved mucin production in the duodenum and colon. Moreover, luteolin treatment mitigated irinotecan-induced oxidative stress, by reducing the levels of ROS and LOOH and augmenting endogenous antioxidants, and inflammation by decreasing MPO enzymic activity, TNF, IL-1β, and IL-6 levels and increasing IL-4 and IL-10. Disruption of the tight junctions ZO-1 and occludin was also prevented by luteolin treatment. Importantly, luteolin did not interfere with the anti-tumour activity of irinotecan.

CONCLUSION AND IMPLICATIONS

Luteolin prevents intestinal mucositis induced by irinotecan and therefore could be a potential adjunct in anti-tumour therapy to control this adverse effect, increasing treatment adherence and consequently the chances of cancer remission.

Cigarette smoking differentially regulates inflammatory responses in a mouse model of nonalcoholic steatohepatitis depending on exposure time point

Cigarette smoke (CS) is a risk factor for the development of nonalcoholic fatty liver disease. However, the role of mainstream CS (MSCS) in the pathogenesis of nonalcoholic steatohepatitis (NASH) remains unclear. During the first (early exposure) or last (late exposure) three weeks of methionine-choline deficient with high fat diet feeding (6 weeks), each diet group was exposed to MSCS (300 or 600 μg/L). Hepatic or serum biochemical analysis showed that MSCS differentially modulated hepatic injury in NASH milieu, depending on exposure time points. Consistently, NASH-related hepatocellular apoptosis and fibrosis were increased in the early exposure group, but decreased in the late exposure group, except for steatosis. Ex vivo experiments showed that CS extract differentially regulated inflammatory responses in co-cultured hepatocytes and macrophages isolated from steatohepatitic livers after 10 days or 3 weeks of diet feeding. Furthermore, CS differentially up- and down-regulated the expression levels of M1/M2 polarization markers and peroxisome proliferator-activated receptor-gamma (PPARγ) in livers (29% and 38%, respectively) or co-cultured macrophages (2 and 2.5 fold, respectively). Collectively, our findings indicate that opposite effects of MSCS on NASH progression are mediated by differential modulation of PPARγ and its-associated M1/M2 polarization in hepatic macrophages, depending on exposure time points.

Self-defense of macrophages against oxidative injury: Fighting for their own survival

Activated macrophages play a central role in both the development and resolution of inflammation. These immune cells need to be functional in harmful conditions with high levels of reactive oxygen and nitrogen species that can damage their basic cell components, which may alter their metabolism. An excessive accumulation of these cell alterations drives macrophages inexorably to cell death, which has been associated to the development of several inflammatory diseases and even with aging in a process termed as "immunosenescence". Macrophages, however, exhibit a prolonged survival in this hostile environment because they equip themselves with a complex network of protective mechanisms. Here we provide an overview of these self-defense mechanisms with special attention being paid to bioactive lipid mediators, NRF2 signaling and metabolic reprogramming.

TRB3 stimulates SIRT1 degradation and induces insulin resistance by lipotoxicity via COP1

Fatty acid-induced lipotoxicity plays an important role in the pathogenesis of diabetes mellitus. Our previous studies have documented that lipotoxicity contributes to the onset and development of diabetes via insulin resistance and/or compromised function of the pancreatic β-cells. However, the underlying molecular mechanisms associating lipotoxicity with insulin resistance remain to be fully elucidated. In this study, we explored the role of TRB3-COP1-SIRT1 in lipotoxicity leading to insulin resistance in hepatocytes. High fat diet (HFD)-fed mice and hepG2 cells stimulated with palmitate were utilized as models of lipid metabolism disorders. We analyzed the interactions of SIRT1 and COP1 with each other and with TRB3 using co-immunoprecipitation, western blotting. SIRT1 ubiquitination was also explored. Animal and cell experiments showed that lipotoxicity induced SIRT1 down-regulation at the protein level without altering the mRNA level, whereas, lipotoxicity led to up-regulation of TRB3 and COP1 at both the gene and protein levels. Mechanistic analysis indicated that COP1 functioned as an E3 Ub-ligase of SIRT1, responsible for its proteasomal degradation under lipotoxic conditions. TRB3 recruited COP1 to SIRT1 to promote its ubiquitination. Our data indicated for the first time that TRB3-COP1-SIRT1 pathway played an important role in lipotoxicity leading to insulin resistance in hepatocytes, and suggested that COP1 could be a potential therapeutic choice for the treatment of diabetes mellitus, with lipotoxicity being the important pathomechanism.

Protein translation associated to PERK arm is a new target for regulation of metainflammation: A connection with hepatocyte cholesterol

Endoplasmic reticulum stress is a cellular phenomenon that has been associated with metabolic disorders, contributing to the development of obesity, fatty liver disease, and dyslipidemias. Under metabolic overload conditions, in cells with a high protein-secretory activity, such as hepatocytes and Langerhans β cells, the unfolded protein response (UPR) is critical in to maintain protein homeostasis (proteostasis). UPR integrated by a tripartite signaling system, through activating transcription factor 6, protein kinase R-like endoplasmic reticulum kinase (PERK), and inositol-requiring enzyme 1, regulates gene transcription and translation to resolve stress and conserve proteostasis. In the current study, we demonstrated in hepatocytes under metabolic overload by saturated palmitic and stearic fatty acids, through activation of PERK signaling and CCAAT-enhancer-binding protein homologous protein (CHOP) transcription factor, an association with the expression of cyclooxygenase 2. More important, isolated exosomes from supernatants of macrophages exposed to lipopolysaccharides can also induce a metainflammation phenomenon, and when treated on hepatocytes, induced a rearrangement in cholesterol metabolism through sterol regulatory element-binding protein 2 (SREBP2), low-density lipoprotein receptor (LDLR), apolipoprotein A-I, and ABCA1. Moreover, we demonstrate the cellular effect of terpene-derived molecules, such as cryptotanshinone, isolated of plant Salvia brandegeei, regulating metainflammatory conditions through PERK pathway in both hepatocytes and β cells. Our data suggest the presence of a modulatory mechanism on specific protein translation process. This effect could be mediated by eukaryotic initiation factor-4A, evaluating salubrinal as a control molecule. Likewise, the protective mechanisms of unsaturated fatty acids, such as oleic and palmitoleic acid were confirmed. Therefore, modulation of metainflammation suggests a new target through PERK signaling in cells with a high secretory activity, and possibly the regulation of cholesterol in hepatocytes is promoted via exosomes.