Endoplasmic reticulum stress and lipid dysregulation

An extra honey polyphenols-rich diet ameliorates the high-fat diet induced chronic kidney disease via modulating gut microbiota in C57BL/6 mice

Honey is not equivalent to sugar and possess a worldwide health promoting effects such as antioxidant, antibacterial, anti-inflammatory, and hepatoprotective activities. Nevertheless, the potential impacts of honey on high-fat diet induced chronic kidney disease (CKD) and gut microbiota remain to be explored. Herein a high-fat diet was used to induce a mouse CKD model, and analysis was conducted on liver, kidney, spleen indices, tissue morphology, biochemical parameters, CKD related genes, and gut microbial diversity. The results indicated that significant inhibitory effects on renal damage caused by a high-fat diet in mice and improvement in disease symptoms were observed upon honey treatment. Significant changes were also found in serum TC, TG, UA, and BUN as well as the inflammation-related protein TNF-α and IL-6 levels in renal tissues. Gene expression analysis revealed that honey intake closely relates to gut microbiota diversity, which can regulate the composition of gut microbiota, increase microbial diversity, especially Bifidobacteriales and S24_7 and promote the synthesis of short chain fatty acids (SCFAs). In summary, this study suggests that honey has both preventive and therapeutic effects on CKD, which may be associated with its ability to improve microbial composition, increase microbial diversity, and regulate SCFAs levels.

Hypoxia in the Blue Mussel Mytilus chilensis Induces a Transcriptome Shift Associated with Endoplasmic Reticulum Stress, Metabolism, and Immune Response

The increase in hypoxia events, a result of climate change in coastal and fjord ecosystems, impacts the health and survival of mussels. These organisms deploy physiological and molecular responses as an adaptive mechanism to maintain cellular homeostasis under environmental stress. However, the specific effects of hypoxia on mussels of socioeconomic interest, such as Mytilus chilensis, are unknown. Using RNA-seq, we investigated the transcriptomic profiles of the gills, digestive gland, and adductor muscle of M. chilensis under hypoxia (10 days at 2 mg L-1) and reoxygenation (10 days at 6 mg L-1). There were 15,056 differentially expressed transcripts identified in gills, 11,864 in the digestive gland, and 9862 in the adductor muscle. The response varied among tissues, showing chromosomal changes in Chr1, Chr9, and Chr10 during hypoxia. Hypoxia regulated signaling genes in the Toll-like, mTOR, citrate cycle, and apoptosis pathways in gills, indicating metabolic and immunological alterations. These changes suggest that hypoxia induced a metabolic shift in mussels, reducing reliance on aerobic respiration and increasing reliance on anaerobic metabolism. Furthermore, hypoxia appeared to suppress the immune response, potentially increasing disease susceptibility, with negative implications for the mussel culture industry and natural bed populations. This study provides pivotal insights into metabolic and immunological adaptations to hypoxia in M. chilensis, offering candidate genes for adaptive traits.

Cytotoxic sigma-2 ligands trigger cancer cell death via cholesterol-induced-ER-stress

Sigma-2-ligands (S2L) are characterized by high binding affinities to their cognate sigma-2 receptor, overexpressed in rapidly proliferating tumor cells. As such, S2L were developed as imaging probes (ISO1) or as cancer therapeutics, alone (SV119 [C6], SW43 [C10]) and as delivery vehicles for cytotoxic drug cargoes (C6-Erastin, C10-SMAC). However, the exact mechanism of S2L-induced cytotoxicity remains to be fully elucidated. A series of high-affinity S2L were evaluated regarding their cytotoxicity profiles across cancer cell lines. While C6 and C10 displayed distinct cytotoxicities, C0 and ISO1 were essentially non-toxic. Confocal microscopy and lipidomics analysis in cellular and mouse models revealed that C10 induced increases in intralysosomal free cholesterol and in cholesterol esters, suggestive of unaltered intracellular cholesterol trafficking. Cytotoxicity was caused by cholesterol excess, a phenomenon that contrasts the effects of NPC1 inhibition. RNA-sequencing revealed gene clusters involved in cholesterol homeostasis and ER stress response exclusively by cytotoxic S2L. ER stress markers were confirmed by qPCR and their targeted modulation inhibited or enhanced cytotoxicity of C10 in a predicted manner. Moreover, C10 increased sterol regulatory element-binding protein 2 (SREBP2) and low-density lipoprotein receptor (LDLR), both found to be pro-survival factors activated by ER stress. Furthermore, inhibition of downstream processes of the adaptive response to S2L with simvastatin resulted in synergistic treatment outcomes in combination with C10. Of note, the S2L conjugates retained the ER stress response of the parental ligands, indicative of cholesterol homeostasis being involved in the overall cytotoxicity of the drug conjugates. Based on these findings, we conclude that S2L-mediated cell death is due to free cholesterol accumulation that leads to ER stress. Consequently, the cytotoxic profiles of S2L drug conjugates are proposed to be enhanced via concurrent ER stress inducers or simvastatin, strategies that could be instrumental on the path toward tumor eradication.

Tri-ortho-cresyl phosphate induces hepatic steatosis by mTOR activation and ER stress induction

Tri-ortho-cresyl phosphate (TOCP), an organophosphorus compound (OP), which is widely used as plasticizer, flame retardant and other industrial products, has been reported to cause multiple toxicities including neurotoxicity and reproductive toxicity. However, it remains to be elusive whether TOCP induces hepatotoxicity. The purpose of this study was to investigate the effect of TOCP on hepatocytes and the lipid metabolism in particular. The adult mice were given a single dose of TOCP (800 mg/kg, p.o.) and the histological changes in liver tissue and lipid content in serum were determined. The results showed that more vacuoles and lipid droplets were observed in the liver of the mice exposed to TOCP. And triglyceride concentrations in serum and liver tissue significantly increased. However, the histopathological changes of the liver and the elevated triglyceride levels in the exposed mice can be reversed by endoplasmic reticulum (ER) stress inhibitor 4-phenylbutyric acid and mTOR signal inhibitor rapamycin. It was also found that the changes of expression levels of the biomarkers of ER stress and mTOR signaling pathway, such as GRP78, CHOP, and p-mTOR, in the exposed mice were consistent with those observed in the cultured primary hepatocytes treated with the same chemicals. These results showed that TOCP activated mTOR signal and ER stress to induce de novo lipid synthesis, which led to the hepatic steatosis in mouse.

Endoplasmic reticulum stress: molecular mechanism and therapeutic targets

The endoplasmic reticulum (ER) functions as a quality-control organelle for protein homeostasis, or "proteostasis". The protein quality control systems involve ER-associated degradation, protein chaperons, and autophagy. ER stress is activated when proteostasis is broken with an accumulation of misfolded and unfolded proteins in the ER. ER stress activates an adaptive unfolded protein response to restore proteostasis by initiating protein kinase R-like ER kinase, activating transcription factor 6, and inositol requiring enzyme 1. ER stress is multifaceted, and acts on aspects at the epigenetic level, including transcription and protein processing. Accumulated data indicates its key role in protein homeostasis and other diverse functions involved in various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, achromatopsia, cataracts, ocular tumors, ocular surface diseases, and myopia. This review summarizes the molecular mechanisms underlying the aforementioned ocular diseases from an ER stress perspective. Drugs (chemicals, neurotrophic factors, and nanoparticles), gene therapy, and stem cell therapy are used to treat ocular diseases by alleviating ER stress. We delineate the advancement of therapy targeting ER stress to provide new treatment strategies for ocular diseases.

ORP5 promotes migration and invasion of cervical cancer cells by inhibiting endoplasmic reticulum stress

ORP5 is a transmembrane protein anchored to the endoplasmic reticulum, which mainly functions as a lipid transporter and has reportedly been linked to cancer. However, the specific mechanism of ORP5 action in cervical cancer (CC) is unclear. In this study, we found that ORP5 promotes the migration and invasive ability of CC cells in vitro and in vivo. In addition, ORP5 expression was linked to endoplasmic reticulum stress, and ORP5 encouraged CC metastasis by inhibiting endoplasmic reticulum stress. Mechanistically, ORP5 inhibited endoplasmic reticulum stress in CC cells by stimulating ubiquitination and proteasomal degradation of SREBP1 to reduce its expression. In conclusion, ORP5 promotes the malignant progression of CC by inhibiting endoplasmic reticulum stress, providing a therapeutic target and strategy for CC treatment.

BS148 Reduces the Aggressiveness of Metastatic Melanoma via Sigma-2 Receptor Targeting

The management of advanced-stage melanoma is clinically challenging, mainly because of its resistance to the currently available therapies. Therefore, it is important to develop alternative therapeutic strategies. The sigma-2 receptor (S2R) is overexpressed in proliferating tumor cells and represents a promising vulnerability to target. Indeed, we have recently identified a potent S2R modulator (BS148) that is effective in melanoma. To elucidate its mechanism of action, we designed and synthesized a BS148 fluorescent probe that enters SK-MEL-2 melanoma cells as assessed using confocal microscopy analysis. We show that S2R knockdown significantly reduces the anti-proliferative effect induced by BS148 administration, indicating the engagement of S2R in BS148-mediated cytotoxicity. Interestingly, BS148 treatment showed similar molecular effects to S2R RNA interference-mediated knockdown. We demonstrate that BS148 administration activates the endoplasmic reticulum stress response through the upregulation of protein kinase R-like ER kinase (PERK), activating transcription factor 4 (ATF4) genes, and C/EBP homologous protein (CHOP). Furthermore, we show that BS148 treatment downregulates genes related to the cholesterol pathway and activates the MAPK signaling pathway. Finally, we translate our results into patient-derived xenograft (PDX) cells, proving that BS148 treatment reduces melanoma cell viability and migration. These results demonstrate that BS148 is able to inhibit metastatic melanoma cell proliferation and migration through its interaction with the S2R and confirm its role as a promising target to treat cancer.

Lipoxin and glycation in SREBP signaling: Insight into diabetic cardiomyopathy and associated lipotoxicity

Diabetes and cardiovascular diseases are the leading cause of morbidity and mortality worldwide. Diabetes increases cardiovascular risk through hyperglycemia and atherosclerosis. Chronic hyperglycemia accelerates glycation reaction, which forms advanced glycation end products (AGEs). Additionally, hyperglycemia with enhanced levels of cholesterol, native and oxidized low-density lipoproteins, free fatty acids, and oxidative stress induces lipotoxicity. Accelerated glycation and disturbed lipid metabolism are characteristic features of diabetic heart failure. SREBP signaling plays a significant role in lipid and glucose homeostasis. AGEs increase lipotoxicity in diabetic cardiomyopathy by inhibiting SREBP signaling. While anti-inflammatory lipid mediators, lipoxins resolve inflammation caused by lipotoxicity by upregulating the PPARγ expression and regulating CD36. PPARγ connects the bridge between glycation and lipoxin in SREBP signaling. A summary of treatment modalities against diabetic cardiomyopathy is given in brief. This review indicates the novel therapeutic approach in the crosstalk between glycation and lipoxin in SREBP signaling.

Homocysteine-induced endoplasmic reticulum stress activates FGF21 and is associated with browning and atrophy of white adipose tissue in Bhmt knockout mice

Betaine-homocysteine methyltransferase (BHMT) catalyzes the transfer of methyl groups from betaine to homocysteine (Hcy), producing methionine and dimethylglycine. In this work, we characterize Bhmt wild type (Bhmt-WT) and knockout (Bhmt-KO) mice that were fully backcrossed to a C57Bl6/J background. Consistent with our previous findings, Bhmt-KO mice had decreased body weight, fat mass, and adipose tissue weight compared to WT. Histological analyses and gene expression profiling indicate that adipose browning was activated in KO mice and contributed to the adipose atrophy observed. BHMT is not expressed in adipose tissue but is abundant in liver; thus, a signal must originate from the liver that modulates adipose tissue. We found that, in Bhmt-KO mice, homocysteine-induced endoplasmic reticulum (ER) stress is associated with activation of the hepatic transcription factor cyclic AMP response element binding protein (CREBH), and an increase in hepatic and plasma concentrations of fibroblast growth factor 21 (FGF21), which is known to induce adipose browning. Our data indicate that the deletion of a single gene in one-carbon metabolism modifies adipose biology and energy metabolism. Future studies could focus on identifying if functional polymorphisms in BHMT result in a similar adipose atrophy phenotype.

Current Understanding on the Role of Lipids in Macrophages and Associated Diseases

Lipid metabolism is the major intracellular mechanism driving a variety of cellular functions such as energy storage, hormone regulation and cell division. Lipids, being a primary component of the cell membrane, play a pivotal role in the survival of macrophages. Lipids are crucial for a variety of macrophage functions including phagocytosis, energy balance and ageing. However, functions of lipids in macrophages vary based on the site the macrophages are residing at. Lipid-loaded macrophages have recently been emerging as a hallmark for several diseases. This review discusses the significance of lipids in adipose tissue macrophages, tumor-associated macrophages, microglia and peritoneal macrophages. Accumulation of macrophages with impaired lipid metabolism is often characteristically observed in several metabolic disorders. Stress signals differentially regulate lipid metabolism. While conditions such as hypoxia result in accumulation of lipids in macrophages, stress signals such as nutrient deprivation initiate lipolysis and clearance of lipids. Understanding the biology of lipid accumulation in macrophages requires the development of potentially active modulators of lipid metabolism.

GRP78 Activity Moderation as a Therapeutic Treatment against Obesity

Glucose-regulated protein 78 (GRP78), a molecular chaperone, is overexpressed in patients suffering from obesity, fatty liver, hyperlipidemia and diabetes. GRP78, therefore, can be not only a biomarker to predict the progression and prognosis of obesity and metabolic diseases but also a potential therapeutic target for anti-obesity treatment. In this paper, GRP78 inhibitors targeting its ATPase domain have been reviewed. Small molecules and proteins that directly bind GRP78 have been described. Putative mechanisms of GRP78 in regulating lipid metabolism were also summarized so as to investigate the role of GRP78 in obesity and other related diseases and provide a theoretical basis for the development and design of anti-obesity drugs targeting GRP78.

Dunaliella salina Microalga Restores the Metabolic Equilibrium and Ameliorates the Hepatic Inflammatory Response Induced by Zinc Oxide Nanoparticles (ZnO-NPs) in Male Zebrafish

Microalgae are rich in bioactive compounds including pigments, proteins, lipids, polyunsaturated fatty acids, carbohydrates, and vitamins. Due to their non-toxic and nutritious characteristics, these are suggested as important food for many aquatic animals. Dunaliella salina is a well-known microalga that accumulates valuable amounts of carotenoids. We investigated whether it could restore the metabolic equilibrium and mitigate the hepatic inflammation induced by zinc oxide nanoparticles (ZnO-NPs) using male zebrafish which were exposed to 1/5^th 96 h-LC₅₀ for 4 weeks, followed by dietary supplementation with D. salina at two concentrations (15% and 30%) for 2 weeks. Collectively, ZnO-NPs affected fish appetite, whole body composition, hepatic glycogen and lipid contents, intestinal bacterial and Aeromonas counts, as well as hepatic tumor necrosis factor- α (TNF-α). In addition, the mRNA expression of genes related to gluconeogenesis (pck1, gys2, and g6pc3), lipogenesis (srepf1, acaca, fasn, and cd36), and inflammatory response (tnf-α, tnf-β, nf-kb2) were modulated. D. salina reduced the body burden of zinc residues, restored the fish appetite and normal liver architecture, and mitigated the toxic impacts of ZnO-NPs on whole-body composition, intestinal bacteria, energy metabolism, and hepatic inflammatory markers. Our results revealed that the administration of D. salina might be effective in neutralizing the hepatotoxic effects of ZnO-NPs in the zebrafish model.

Metabolic Injury of Hepatocytes Promotes Progression of NAFLD and AALD

Nonalcoholic liver disease is a component of metabolic syndrome associated with obesity, insulin resistance, and hyperlipidemia. Excessive alcohol consumption may accelerate the progression of steatosis, steatohepatitis, and fibrosis. While simple steatosis is considered a benign condition, nonalcoholic steatohepatitis with inflammation and fibrosis may progress to cirrhosis, liver failure, and hepatocellular cancer. Studies in rodent experimental models and primary cell cultures have demonstrated several common cellular and molecular mechanisms in the pathogenesis and regression of liver fibrosis. Chronic injury and death of hepatocytes cause the recruitment of myeloid cells, secretion of inflammatory and fibrogenic cytokines, and activation of myofibroblasts, resulting in liver fibrosis. In this review, we discuss the role of metabolically injured hepatocytes in the pathogenesis of nonalcoholic steatohepatitis and alcohol-associated liver disease. Specifically, the role of chemokine production and de novo lipogenesis in the development of steatotic hepatocytes and the pathways of steatosis regulation are discussed.

Organelles coordinate milk production and secretion during lactation: Insights into mammary pathologies

The mammary gland undergoes a spectacular series of changes during its development and maintains a remarkable capacity to remodel and regenerate during progression through the lactation cycle. This flexibility of the mammary gland requires coordination of multiple processes including cell proliferation, differentiation, regeneration, stress response, immune activity, and metabolic changes under the control of diverse cellular and hormonal signaling pathways. The lactating mammary epithelium orchestrates synthesis and apical secretion of macromolecules including milk lipids, milk proteins, and lactose as well as other minor nutrients that constitute milk. Knowledge about the subcellular compartmentalization of these metabolic and signaling events, as they relate to milk production and secretion during lactation, is expanding. Here we review how major organelles (endoplasmic reticulum, Golgi apparatus, mitochondrion, lysosome, and exosome) within mammary epithelial cells collaborate to initiate, mediate, and maintain lactation, and how study of these organelles provides insight into options to maintain mammary/breast health.

The Emerging Roles of Intracellular PCSK9 and Their Implications in Endoplasmic Reticulum Stress and Metabolic Diseases

The importance of the proprotein convertase subtilisin/kexin type-9 (PCSK9) gene was quickly recognized by the scientific community as the third locus for familial hypercholesterolemia. By promoting the degradation of the low-density lipoprotein receptor (LDLR), secreted PCSK9 protein plays a vital role in the regulation of circulating cholesterol levels and cardiovascular disease risk. For this reason, the majority of published works have focused on the secreted form of PCSK9 since its initial characterization in 2003. In recent years, however, PCSK9 has been shown to play roles in a variety of cellular pathways and disease contexts in LDLR-dependent and -independent manners. This article examines the current body of literature that uncovers the intracellular and LDLR-independent roles of PCSK9 and also explores the many downstream implications in metabolic diseases.

Selective PPARα Modulator Pemafibrate and Sodium-Glucose Cotransporter 2 Inhibitor Tofogliflozin Combination Treatment Improved Histopathology in Experimental Mice Model of Non-Alcoholic Steatohepatitis

Ballooning degeneration of hepatocytes is a major distinguishing histological feature of non-alcoholic steatosis (NASH) progression that can lead to cirrhosis and hepatocellular carcinoma (HCC). In this study, we evaluated the effect of the selective PPARα modulator (SPPARMα) pemafibrate (Pema) and sodium-glucose cotransporter 2 (SGLT2) inhibitor tofogliflozin (Tofo) combination treatment on pathological progression in the liver of a mouse model of NASH (STAM) at two time points (onset of NASH progression and HCC survival). At both time points, the Pema and Tofo combination treatment significantly alleviated hyperglycemia and hypertriglyceridemia. The combination treatment significantly reduced ballooning degeneration of hepatocytes. RNA-seq analysis suggested that Pema and Tofo combination treatment resulted in an increase in glyceroneogenesis, triglyceride (TG) uptake, lipolysis and liberated fatty acids re-esterification into TG, lipid droplet (LD) formation, and Cidea/Cidec ratio along with an increased number and reduced size and area of LDs. In addition, combination treatment reduced expression levels of endoplasmic reticulum stress-related genes (Ire1a, Grp78, Xbp1, and Phlda3). Pema and Tofo treatment significantly improved survival rates and reduced the number of tumors in the liver compared to the NASH control group. These results suggest that SPPARMα and SGLT2 inhibitor combination therapy has therapeutic potential to prevent NASH-HCC progression.

Induction of Endoplasmic Reticulum Stress-Mediated Apoptosis by Aminosteroid RM-581 Efficiently Blocks the Growth of PC-3 Cancer Cells and Tumors Resistant or Not to Docetaxel

Aminosteroid derivative RM-581 was previously identified as an endoplasmic-reticulum (ER) stress inducer with potent in vitro and in vivo anticancer activities. We report its evaluation in androgen-independent prostate cancer (PC-3) cells. RM-581 efficiently blocks PC-3 cell proliferation with stronger activity than that of a selection of known antineoplastic agents. This later also showed a synergistic effect with docetaxel, able to block the proliferation of docetaxel-resistant PC-3 cells and, contrary to docetaxel, did not induce cell resistance. RM-581 induced an increase in the expression level of ER stress-related markers of apoptosis, potentially triggered by the presence of RM-581 in the ER of PC-3 cells. These in vitro results were then successfully translated in vivo in a PC-3 xenograft tumor model in nude mice, showing superior blockade than that of docetaxel. RM-581 was also able to stop the progression of PC-3 cells when they had become resistant to docetaxel treatment. Concomitantly, we observed a decrease in gene markers of mevalonate and fatty acid pathways, and intratumoral levels of cholesterol by 19% and fatty acids by 22%. Overall, this work demonstrates the potential of an ER stress inducer as an anticancer agent for the treatment of prostate cancers that are refractory to commonly used chemotherapy treatments.

Endoplasmic Reticulum Stress in Diabetic Nephrology: Regulation, Pathological Role, and Therapeutic Potential

Recent progress has been made in understanding the roles and mechanisms of endoplasmic reticulum (ER) stress in the development and pathogenesis of diabetic nephropathy (DN). Hyperglycemia induces ER stress and apoptosis in renal cells. The induction of ER stress can be cytoprotective or cytotoxic. Experimental treatment of animals with ER stress inhibitors alleviated renal damage. Considering these findings, the normalization of ER stress by pharmacological agents is a promising approach to prevent or arrest DN progression. The current article reviews the mechanisms, roles, and therapeutic aspects of these findings.

Evaluation of hepatotoxicity induced by 2-ethylhexyldiphenyl phosphate based on transcriptomics and its potential metabolism pathway in human hepatocytes

Increasing use of organophosphorus flame retardants (OPFRs) has aroused great concern to their uncertain environment risk, especially to human health risk. In our study, hepatotoxicity screening of six aryl-OPFRs, potential hepatotoxicity mechanism of 2-ethylhexyldiphenyl phosphate (EHDPP) using RNA-sequencing and its metabolites were investigated in human hepatocytes (L02). The toxicity results demonstrated that EHDPP should be prioritized for further research with the highest toxicity. Further RNA-seq results through GO and KEGG enrichment analysis indicated that exposure to 10 mg/L of EHDPP significantly affected energy homeostasis, endoplasmic reticulum (ER) stress, apoptosis, cell cycle, and inflammation response in cells. The top 12 hub genes were validated by RT-qPCR and conformed to be mainly related to glycolysis and ER stress, followed by cell cycle and inflammation response. Western blot, apoptosis detection, glycolysis stress test, and cell cycle analysis were further performed to verify the above main pathways. Additionally, it was found in the metabolism experiment that detoxification of EHDPP by phase I and phase II metabolism in cells wasn't significant until 48 h with a metabolic rate of 6.12%. EHDPP was stable and still dominated the induction of toxicity. Overall, this study provided valuable information regarding the toxicity and potential metabolism pathway of EHDPP.

σ2 Receptor and Its Role in Cancer with Focus on a MultiTarget Directed Ligand (MTDL) Approach

Sigma-2 (σ2) is an endoplasmic receptor identified as the Endoplasmic Reticulum (ER) transmembrane protein TMEM97. Despite its controversial identity, which was only recently solved, this protein has gained scientific interest because of its role in the proliferative status of cells; many tumor cells from different organs overexpress the σ2 receptor, and many σ2 ligands display cytotoxic actions in (resistant) cancer cells. These properties have shed light on the σ2 receptor as a potential druggable target to be bound/activated for the diagnosis or therapy of tumors. Additionally, diverse groups have shown how the σ2 receptor can be exploited for the targeted delivery of the anticancer drugs to tumors. As the cancer disease is a multifactorial pathology with multiple cell populations, a polypharmacological approach is very often needed. Instead of the simultaneous administration of different classes of drugs, the use of one molecule that interacts with diverse pharmacological targets, namely MultiTarget Directed Ligand (MTDL), is a promising and currently pursued strategy, that may overcome the pharmacokinetic problems associated with the administration of multiple molecules. This review aims to point out the progress regarding the σ2 ligands in the oncology field, with a focus on MTDLs directed towards σ2 receptors as promising weapons against (resistant) cancer diseases.

Functional Peroxisomes Are Essential for Efficient Cholesterol Sensing and Synthesis

Cholesterol biosynthesis is a multi-step process involving several subcellular compartments, including peroxisomes. Cells adjust their sterol content by both transcriptional and post-transcriptional feedback regulation, for which sterol regulatory element-binding proteins (SREBPs) are essential; such homeostasis is dysregulated in peroxisome-deficient Pex2 knockout mice. Here, we compared the regulation of cholesterol biosynthesis in Chinese hamster ovary (CHO-K1) cells and in three isogenic peroxisome-deficient CHO cell lines harboring Pex2 gene mutations. Peroxisome deficiency activated expression of cholesterogenic genes, however, cholesterol levels were unchanged. 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) protein levels were increased in mutant cells, whereas HMGCR activity was significantly decreased, resulting in reduced cholesterol synthesis. U18666A, an inhibitor of lysosomal cholesterol export, induced cholesterol biosynthetic enzymes; yet, cholesterol synthesis was still reduced. Interestingly, peroxisome deficiency promoted ER-to-Golgi SREBP cleavage-activating protein (SCAP) trafficking even when cells were cholesterol-loaded. Restoration of functional peroxisomes normalized regulation of cholesterol synthesis and SCAP trafficking. These results highlight the importance of functional peroxisomes for maintaining cholesterol homeostasis and efficient cholesterol synthesis.

Cannabinoid Receptor Subtype 2 (CB2R) in a Multitarget Approach: Perspective of an Innovative Strategy in Cancer and Neurodegeneration

The cannabinoid receptor subtype 2 (CB2R) represents an interesting and new therapeutic target for its involvement in the first steps of neurodegeneration as well as in cancer onset and progression. Several studies, focused on different types of tumors, report a promising anticancer activity induced by CB2R agonists due to their ability to reduce inflammation and cell proliferation. Moreover, in neuroinflammation, the stimulation of CB2R, overexpressed in microglial cells, exerts beneficial effects in neurodegenerative disorders. With the aim to overcome current treatment limitations, new drugs can be developed by specifically modulating, together with CB2R, other targets involved in such multifactorial disorders. Building on successful case studies of already developed multitarget strategies involving CB2R, in this Perspective we aim at prompting the scientific community to consider new promising target associations involving HDACs (histone deacetylases) and σ receptors by employing modern approaches based on molecular hybridization, computational polypharmacology, and machine learning algorithms.

Roles of Nrf2 in Liver Diseases: Molecular, Pharmacological, and Epigenetic Aspects

Liver diseases represent a critical health problem with 2 million deaths worldwide per year, mainly due to cirrhosis and its complications. Oxidative stress plays an important role in the development of liver diseases. In order to maintain an adequate homeostasis, there must be a balance between free radicals and antioxidant mediators. Nuclear factor erythroid 2-related factor (Nrf2) and its negative regulator Kelch-like ECH-associated protein 1 (Keap1) comprise a defense mechanism against oxidative stress damage, and growing evidence considers this signaling pathway as a key pharmacological target for the treatment of liver diseases. In this review, we provide detailed and updated evidence regarding Nrf2 and its involvement in the development of the main liver diseases such as alcoholic liver damage, viral hepatitis, steatosis, steatohepatitis, cholestatic damage, and liver cancer. The molecular and cellular mechanisms of Nrf2 cellular signaling are elaborated, along with key and relevant antioxidant drugs, and mechanisms on how Keap1/Nrf2 modulation can positively affect the therapeutic response are described. Finally, exciting recent findings about epigenetic modifications and their link with regulation of Keap1/Nrf2 signaling are outlined.

Mechanistic Study of Triazole Based Aminodiol Derivatives in Leukemic Cells-Crosstalk between Mitochondrial Stress-Involved Apoptosis and Autophagy

Various derivatives that mimic ceramide structures by introducing a triazole to connect the aminodiol moiety and long alkyl chain have been synthesized and screened for their anti-leukemia activity. SPS8 stood out among the derivatives, showing cytotoxic selectivity between leukemic cell lines and human peripheral blood mononuclear cells (about ten times). DAPI nuclear staining and H&E staining revealed DNA fragmentation under the action of SPS8. SPS8 induced an increase in intracellular Ca²⁺ levels and mitochondrial stress in HL-60 cells identified by the loss of mitochondrial membrane potential, transmission electron microscopy (TEM) examination, and altered expressions of Bcl-2 family proteins. SPS8 also induced autophagy through the detection of Atg5, beclin-1, and LC3 II protein expression, as well as TEM examination. Chloroquine, an autophagy inhibitor, promoted SPS8-induced apoptosis, suggesting the cytoprotective role of autophagy in hindering SPS8 from apoptosis. Furthermore, SPS8 was shown to alter the expressions of a variety of genes using a microarray analysis and volcano plot filtering. A further cellular signaling pathways analysis suggested that SPS8 induced several cellular processes in HL-60, including the sterol biosynthesis process and cholesterol biosynthesis process, and inhibited some cellular pathways, in which STAT3 was the most critical nuclear factor. Further identification revealed that SPS8 inhibited the phosphorylation of STAT3, representing the loss of cytoprotective activity. In conclusion, the data suggest that SPS8 induces both apoptosis and autophagy in leukemic cells, in which autophagy plays a cytoprotective role in impeding apoptosis. Moreover, the inhibition of STAT3 phosphorylation may support SPS8-induced anti-leukemic activity.

Recent Insights Into SREBP as a Direct Mediator of Kidney Fibrosis via Lipid-Independent Pathways

Sterol regulatory-element binding proteins (SREBPs) are classical regulators of cellular lipid metabolism in the kidney and other tissues. SREBPs are currently recognized as versatile transcription factors involved in a myriad of cellular processes. Meanwhile, SREBPs have been recognized to mediate lipotoxicity, contributing to the progression of kidney diseases. SREBP1 has been shown to bind to the promoter region of TGFβ, a major pro-fibrotic signaling mechanism in the kidney. Conversely, TGFβ activates SREBP1 transcriptional activity suggesting a positive feedback loop of SREBP1 in TGFβ signaling. Public ChIP-seq data revealed numerous non-lipid transcriptional targets of SREBPs that plausibly play roles in progressive kidney disease and fibrosis. This review provides new insights into SREBP as a mediator of kidney fibrosis via lipid-independent pathways.

Chronic Microcystin-LR Exposure Induces Abnormal Lipid Metabolism via Endoplasmic Reticulum Stress in Male Zebrafish

In order to explore effects of low levels of continuous microcystin-LR (MC-LR) (a cyanotoxin) exposure on hepatic lipid metabolism on the basis of the endoplasmic reticulum stress (ERS) pathway, we exposed adult male zebrafish to MC-LR (0, 1, 5, and 25 μg/L) for 60 days, and hepatic histopathology as well as lipid metabolic parameters were determined with mRNA levels of ERS signal molecules and downstream factors, along with genes associated with lipid metabolism in zebrafish liver. The results revealed that prolonged exposure to MC-LR remarkably altered the levels of hepatic total cholesterol and triglyceride and led to hepatic steatosis, which was also confirmed by hepatic cytoplasmic vacuolization in Hematoxylin/eosin (H&E) stain and lipid droplet accumulation in Oil Red O stain. The severity of hepatic damage and lipidation was increased in a dose-related manner. MC-LR exposure significantly upregulated transcriptional levels of ERS markers including hspa5, mapk8, and chop, indicating the occurrence of ERS in the liver of zebrafish. Concurrently, MC-LR significantly improved mRNA expression of unfolded protein response (UPR) pathway-related genes including atf6, eif2ak3, ern1, and xbp1s, suggesting that all of the three UPR branches were activated by MC-LR. MC-LR also induced significant upregulation of downstream lipid metabolism-related factors and genes including srebf1, srebf2, fatty acid synthase (fasn), acetyl-CoA carboxylase (acaca), stearoyl-CoA desaturase (scd), HMG CoA reductase (hmgcra), and HMG CoA synthase (hmgcs1), and downregulation of genes associated with lipolysis such as triglyceride hydrolase gene (atgl), hormone-sensitive enzyme gene (hsla), and carnitine palmitoyltransferase gene (cpt1aa). Our present results indicated that the cause of hepatic lipid accumulation by MC-LR was mainly by upregulating lipogenic and cholesterol genes but downregulating the expression of lipolytic genes through the induction of srebf1 and srebf2, which were involved in the activation of ERS signal pathways.

Plant Sterol Ester of α-Linolenic Acid Attenuates Nonalcoholic Fatty Liver Disease by Rescuing the Adaption to Endoplasmic Reticulum Stress and Enhancing Mitochondrial Biogenesis

Nonalcoholic fatty liver disease (NAFLD) is becoming more common in the world and is presenting a great challenge concerning prevention and treatment. Plant sterol ester of α-linolenic acid (PS-ALA) has a potential benefit to NAFLD. To examine the effect of PS-ALA on NAFLD, C57BL/6J mice were given a control diet, high fat and high cholesterol diet (HFD), and HFD plus 2% PS, 1.3% ALA, or 3.3% PS-ALA for 16 weeks. Our results showed that PS-ALA treatment suppressed hepatic steatosis, ameliorated lipid disorder, attenuated inflammatory response, and inhibited oxidative stress. In the molecular level, PS-ALA downregulated high transcriptional and translational levels of endoplasmic reticulum (ER) stress markers (Grp78 and Chop) leading to decreased protein expression of transcription factor and key enzymes involved in de novo lipogenesis (Srebp-1c and Fas) and cholesterol synthesis (Srebp-2 and Hmgcr). In parallel, PS-ALA blocked Nlrp3 activation and reduced release of IL-1β and IL-18 via inhibiting ER stress-induced sensitization of unfolded protein response sensors (Ire1α and Xbp1s). Finally, PS-ALA improved HFD-induced mitochondrial damage and fatty acid accumulation as exhibited by higher protein and mRNA expression of key genes administering mitochondrial biogenesis (Pgc-1α, Nrf1, and Tfam) and fatty acid β-oxidation (Pparα and Cpt1a). In conclusion, our study originally demonstrated that PS-ALA rescued ER stress, enhanced mitochondrial biogenesis, and thus ameliorated NAFLD.

Tumor Microenvironment Regulation by the Endoplasmic Reticulum Stress Transmission Mediator Golgi Protein 73 in Mice

The unfolded protein response (UPR) signal in tumor cells activates UPR signaling in neighboring macrophages, which leads to tumor-promoting inflammation by up-regulating UPR target genes and proinflammatory cytokines. However, the molecular basis of this endoplasmic reticulum (ER) stress transmission remains largely unclear. Here, we identified the secreted form of Golgi protein 73 (GP73), a Golgi-associated protein functional critical for hepatocellular carcinoma (HCC) growth and metastasis, is indispensable for ER stress transmission. Notably, ER stressors increased the cellular secretion of GP73. Through GRP78, the secreted GP73 stimulated ER stress activation in neighboring macrophages, which then released cytokines and chemokines involved in the tumor-associated macrophage (TAM) phenotype. Analysis of HCC patients revealed a positive correlation of GP73 with glucose-regulated protein 78 (GRP78) expression and TAM density. High GP73 and CD206 expression was associated with poor prognosis. Blockade of GP73 decreased the density of TAMs, inhibited tumor growth, and prolonged survival in two mouse HCC models. Conclusion: Our findings provide insight into the molecular mechanisms of extracellular GP73 in the amplification and transmission of ER stress signals.

Cellular α-synuclein pathology is associated with bioenergetic dysfunction in Parkinson's iPSC-derived dopamine neurons

Parkinson's disease (PD) is the second most common neurodegenerative disorder and a central role for α-synuclein (αSyn; SNCA) in disease aetiology has been proposed based on genetics and neuropathology. To better understand the pathological mechanisms of αSyn, we generated induced pluripotent stem cells (iPSCs) from healthy individuals and PD patients carrying the A53T SNCA mutation or a triplication of the SNCA locus and differentiated them into dopaminergic neurons (DAns). iPSC-derived DAn from PD patients carrying either mutation showed increased intracellular αSyn accumulation, and DAns from patients carrying the SNCA triplication displayed oligomeric αSyn pathology and elevated αSyn extracellular release. Transcriptomic analysis of purified DAns revealed perturbations in expression of genes linked to mitochondrial function, consistent with observed reduction in mitochondrial respiration, impairment in mitochondrial membrane potential, aberrant mitochondrial morphology and decreased levels of phosphorylated DRP1Ser616. Parkinson's iPSC-derived DAns showed increased endoplasmic reticulum stress and impairments in cholesterol and lipid homeostasis. Together, these data show a correlation between αSyn cellular pathology and deficits in metabolic and cellular bioenergetics in the pathology of PD.

Anti-tumor Efficacy Assessment of the Sigma Receptor Pan Modulator RC-106. A Promising Therapeutic Tool for Pancreatic Cancer

Introduction: Pancreatic cancer (PC) is one of the most lethal tumor worldwide, with no prognosis improvement over the past 20-years. The silent progressive nature of this neoplasia hampers the early diagnosis, and the surgical resection of the tumor, thus chemotherapy remains the only available therapeutic option. Sigma receptors (SRs) are a class of receptors proposed as new cancer therapeutic targets due to their over-expression in tumor cells and their involvement in cancer biology. The main localization of these receptors strongly suggests their potential role in ER unfolded protein response (ER-UPR), a condition frequently occurring in several pathological settings, including cancer. Our group has recently identified RC-106, a novel pan-SR modulator with good in vitro antiproliferative activities toward a panel of different cancer cell lines. In the present study, we investigated the in vitro properties and pharmacological profile of RC-106 in PC cell lines with the aim to identify a potential lead candidate for the treatment of this tumor.

Methods: Pancreatic cancer cell lines Panc-1, Capan-1, and Capan-2 have been used in all experiments. S1R and TMEM97/S2R expression in PC cell lines was quantified by Real-Time qRT-PCR and Western Blot experiments. MTS assay was used to assess the antiproliferative effect of RC-106. The apoptotic properties of RC-106 was evaluated by TUNEL and caspase activation assays. GRP78/BiP, ATF4, and CHOP was quantified to evaluate ER-UPR. Proteasome activity was investigated by a specific fluorescent-based assay. Scratch wound healing assay was used to asses RC-106 effect on cell migration. In addition, we delineated the in vivo pharmacokinetic profile and pancreas distribution of RC-106 in male CD-1 mice.

Results: Panc-1, Capan-1, and Capan-2 express both SRs. RC-106 exerts an antiproliferative and pro-apoptotic effect in all examined cell lines. Cells exposure to RC-106 induces the increase of the expression of ER-UPR related proteins, and the inhibition of proteasome activity. Moreover, RC-106 is able to decrease PC cell lines motility. The in vivo results show that RC-106 is more concentrated in pancreas than plasma.

Conclusion: Overall, our data evidenced that the pan-SR modulator RC-106 is an optimal candidate for in vivo studies in animal models of PC.

Diet-induced hepatic steatosis abrogates cell-surface LDLR by inducing de novo PCSK9 expression in mice

The worldwide prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing rapidly. Although this condition is generally benign, accumulating evidence now suggests that patients with NAFLD are also at increased risk of cardiovascular disease (CVD); the leading cause of death in developed nations. Despite the well-established role of the liver as a central regulator of circulating low-density lipoprotein (LDL) cholesterol levels, a known driver of CVD, the mechanism(s) by which hepatic steatosis contributes to CVD remains elusive. Interestingly, a recent study has shown that circulating proprotein convertase subtilisin/kexin type 9 (PCSK9) levels correlate positively with liver steatosis grade. Given that PCSK9 degrades the LDL receptor (LDLR) and prevents the removal of LDL from the blood into the liver, in the present study we examined the effect of hepatic steatosis on LDLR expression and circulating LDL cholesterol levels. We now report that in a manner consistent with findings in patients, diet-induced steatosis increases circulating PCSK9 levels as a result of de novo expression in mice. We also report the finding that steatosis abrogates hepatic LDLR expression and increases circulating LDL levels in a PCSK9-dependent manner. These findings provide important mechanistic insights as to how hepatic steatosis modulates lipid regulatory genes, including PCSK9 and the LDLR, and also highlights a novel mechanism by which liver disease may contribute to CVD.

Evaluation of MEDAG gene expression in papillary thyroid microcarcinoma: associations with histological features, regional lymph node metastasis and prognosis

Papillary thyroid microcarcinoma accounts for a large proportion of papillary thyroid carcinoma, especially among new cases. Many  PTMC patients have regional lymph node metastasis, with some experiencing recurrence and even death. However, the risk factors and mechanism by which PTMC relates to these factors are unknown. In this study, differentially expressed genes were identified with microarray from The Cancer Genome Atlas, followed by analysis using the Kyoto Encyclopedia of Genes and Genomes. Immunohistochemistry, immunofluorescence, western blot and Oil Red O staining were carried out to evaluate expression levels and functional alterations. Mesenteric Estrogen Dependent Adipogenesis expression was observed in almost all cases of papillary thyroid microcarcinomas, and the location of expression was associated with histological subtype. High expression was correlated with metastasis and poor disease-free survival. Furthermore, the enrichment analysis indicated that Mesenteric Estrogen Dependent Adipogenesis expression may be associated with metabolic reprogramming to influence metastasis and prognosis. These findings contribute to a better understanding of how Mesenteric Estrogen Dependent Adipogenesis affects metastasis and the prognosis of papillary thyroid microcarcinoma patients and suggest that Mesenteric Estrogen Dependent Adipogenesis expression may be a novel prognostic marker in these patients. 

ER Stress Drives Lipogenesis and Steatohepatitis via Caspase-2 Activation of S1P

Nonalcoholic fatty liver disease (NAFLD) progresses to nonalcoholic steatohepatitis (NASH) in response to elevated endoplasmic reticulum (ER) stress. Whereas the onset of simple steatosis requires elevated de novo lipogenesis, progression to NASH is triggered by accumulation of hepatocyte-free cholesterol. We now show that caspase-2, whose expression is ER-stress inducible and elevated in human and mouse NASH, controls the buildup of hepatic-free cholesterol and triglycerides by activating sterol regulatory element-binding proteins (SREBP) in a manner refractory to feedback inhibition. Caspase-2 colocalizes with site 1 protease (S1P) and cleaves it to generate a soluble active fragment that initiates SCAP-independent SREBP1/2 activation in the ER. Caspase-2 ablation or pharmacological inhibition prevents diet-induced steatosis and NASH progression in ER-stress-prone mice. Caspase-2 inhibition offers a specific and effective strategy for preventing or treating stress-driven fatty liver diseases, whereas caspase-2-generated S1P proteolytic fragments, which enter the secretory pathway, are potential NASH biomarkers.

Sigma Receptors as Endoplasmic Reticulum Stress "Gatekeepers" and their Modulators as Emerging New Weapons in the Fight Against Cancer

Despite the interest aroused by sigma receptors (SRs) in the area of oncology, their role in tumor biology remains enigmatic. The predominant subcellular localization and main site of activity of SRs are the endoplasmic reticulum (ER). Current literature data, including recent findings on the sigma 2 receptor subtype (S2R) identity, suggest that SRs may play a role as ER stress gatekeepers. Although SR endogenous ligands are still unknown, a wide series of structurally unrelated compounds able to bind SRs have been identified. Currently, the identification of novel antiproliferative molecules acting via SR interaction is a challenging task for both academia and industry, as shown by the fact that novel anticancer drugs targeting SRs are in the preclinical-stage pipeline of pharmaceutical companies (i.e., Anavex Corp. and Accuronix). So far, no clinically available anticancer drugs targeting SRs are still available. The present review focuses literature advancements and provides a state-of-the-art overview of SRs, with emphasis on their involvement in cancer biology and on the role of SR modulators as anticancer agents. Findings from preclinical studies on novel anticancer drugs targeting SRs are presented in brief.

Translational control of human acetyl-CoA carboxylase 1 mRNA is mediated by an internal ribosome entry site in response to ER stress, serum deprivation or hypoxia mimetic CoCl2

Acetyl-CoA carboxylase 1 (ACC1) is a cytosolic enzyme catalyzing the rate limiting step in de novo fatty acid biosynthesis. There is mounting evidence showing that ACC1 is susceptible to dysregulation and that it is over-expressed in liver diseases associated with lipid accumulation and in several cancers. In the present study, ACC1 regulation at the translational level is reported. Using several experimental approaches, the presence of an internal ribosome entry site (IRES) has been established in the 5' untranslated region (5' UTR) of the ACC1 mRNA. Transfection experiments with the ACC1 5' UTR inserted in a dicistronic reporter vector show a remarkable increase in the downstream cistron translation, through a cap-independent mechanism. The endoplasmic reticulum (ER) stress condition and the related unfolded protein response (UPR), triggered by treatment with thapsigargin and tunicamycin, cause an increase of the cap-independent translation of ACC1 mRNA in HepG2 cells, despite the overall reduction in global protein synthesis. Other stress conditions, such as serum starvation and incubation with hypoxia mimetic agent CoCl₂, up-regulate ACC1 expression in HepG2 cells at the translational level. Overall, these findings indicate that the presence of an IRES in the ACC1 5' UTR allows ACC1 mRNA translation in conditions that are inhibitory to cap-dependent translation. A potential involvement of the cap-independent translation of ACC1 in several pathologies, such as obesity and cancer, has been discussed.

Deciphering the Regulatory Network between the SREBP Pathway and Protein Secretion in Neurospora crassa

Sterol regulatory element binding proteins (SREBPs) are conserved from yeast to mammalian cells and function in the regulation of sterol homeostasis. In fungi, the SREBP pathway has been implicated in the adaptation to hypoxia and in virulence. In Neurospora crassa and Trichoderma reesei, the SREBP pathway also negatively regulates protein secretion under lignocellulolytic conditions. Here we utilized global transcriptional profiling combined with genetic and physiological analyses to address the regulatory link between the SREBP pathway and protein secretion in N. crassa. Our results demonstrated that the function of the SREBP pathway in ergosterol biosynthesis and adaptation to hypoxia was conserved in N. crassa. Under lignocellulolytic conditions, the SREBP pathway was highly activated, resulting in the reduced expression of lytic polysaccharide monooxygenases, which require molecular oxygen for catalytic activity. Additionally, activation of the SREBP pathway under lignocellulolytic conditions repressed a set of genes predicted to be involved in the endoplasmic reticulum stress response. Here we show that the inability of a hac-1 mutant, which bears a deletion of the major regulator of the unfolded protein response (UPR), to efficiently produce cellulases and utilize cellulose was suppressed by mutations in the SREBP pathway. The analyses presented here demonstrated new SREBP pathway functions, including linkages to the UPR, and provide new clues for genetic engineering of filamentous fungi to improve their production of extracellular proteins.

The role of SREBP transcription factors in the regulation of sterol biosynthesis is conserved from humans to yeast. In filamentous fungi, this pathway regulates the secretion of lignocellulolytic enzymes during plant biomass deconstruction. Here we show that the SREBP pathway in Neurospora crassa regulates the production of specific cellulases, lytic polysaccharide monooxygenases that utilize molecular oxygen. Via global transcriptional profile and genetic analyses, a relationship between the SREBP pathway and the unfolded protein response (UPR) pathway was revealed, suggesting a regulatory interplay of these two pathways in the trafficking of plant biomass-degrading enzymes. These findings have implications for our understanding of the cross talk of the SREBP and UPR pathways in other organisms and will guide the rational engineering of fungal strains to improve cellulolytic enzyme production.

Research Progress on Mechanism of Podocyte Depletion in Diabetic Nephropathy

Diabetic nephropathy (DN) together with glomerular hyperfiltration has been implicated in the development of diabetic microangiopathy in the initial stage of diabetic diseases. Increased amounts of urinary protein in DN may be associated with functional and morphological alterations of podocyte, mainly including podocyte hypertrophy, epithelial-mesenchymal transdifferentiation (EMT), podocyte detachment, and podocyte apoptosis. Accumulating studies have revealed that disruption in multiple renal signaling pathways had been critical in the progression of these pathological damages, such as adenosine monophosphate-activated kinase signaling pathways (AMPK), wnt/β-catenin signaling pathways, endoplasmic reticulum stress-related signaling pathways, mammalian target of rapamycin (mTOR)/autophagy pathway, and Rho GTPases. In this review, we highlight new molecular insights underlying podocyte injury in the progression of DN, which offer new therapeutic targets to develop important renoprotective treatments for DN over the next decade.

Macrophage Apoptosis and Necrotic Core Development in Atherosclerosis: A Rapidly Advancing Field with Clinical Relevance to Imaging and Therapy

Cardiovascular diseases represent 1 of the main causes of death worldwide, and atherosclerosis is 1 of the major contributors leading to ischemic heart disease. Macrophages actively participate in all stages of atherosclerosis development, from plaque initiation to the transition to vulnerable plaques. Macrophage apoptosis, in particular, has been recognized as a critical step in the formation of the necrotic core, a key characteristic of unstable lesions. In this review, we discuss the role of macrophage apoptosis and clearance of apoptotic cells by efferocytosis in the development of atherosclerosis, with particular emphasis on their contribution to the development of the necrotic core and the clinical implications of this process for plaque stabilization. We consider the molecular triggers of macrophage apoptosis during atherogenesis, the role of endoplasmic reticulum (ER) stress, the roles of key cellular mediators of apoptosis and efferocytosis, and mechanisms of defective efferocytosis in the progression of atherosclerotic plaques. Finally, we discuss the important clinical implications of rapidly evolving macrophage science, such as novel approaches to imaging vulnerable atherosclerotic plaques with macrophage-sensitive positron emission tomography and magnetic resonance imaging, the role of macrophages in mediating beneficial pleiotropic actions of lipid-lowering therapies, and novel therapeutic modalities targeting ER stress, autophagy, and deficient efferocytosis. Advances in understanding the critical role of macrophages in the progression and destabilization of atherosclerosis have the potential to greatly improve the prevention and management of atherosclerotic diseases over the next decade.

Endoplasmic Reticulum Stress and Ca2+ Depletion Differentially Modulate the Sterol Regulatory Protein PCSK9 to Control Lipid Metabolism

Accumulating evidence implicates endoplasmic reticulum (ER) stress as a mediator of impaired lipid metabolism, thereby contributing to fatty liver disease and atherosclerosis. Previous studies demonstrated that ER stress can activate the sterol regulatory element-binding protein-2 (SREBP2), an ER-localized transcription factor that directly up-regulates sterol regulatory genes, including PCSK9 Given that PCSK9 contributes to atherosclerosis by targeting low density lipoprotein (LDL) receptor (LDLR) degradation, this study investigates a novel mechanism by which ER stress plays a role in lipid metabolism by examining its ability to modulate PCSK9 expression. Herein, we demonstrate the existence of two independent effects of ER stress on PCSK9 expression and secretion. In cultured HuH7 and HepG2 cells, agents or conditions that cause ER Ca²⁺ depletion, including thapsigargin, induced SREBP2-dependent up-regulation of PCSK9 expression. In contrast, a significant reduction in the secreted form of PCSK9 protein was observed in the media from both thapsigargin- and tunicamycin (TM)-treated HuH7 cells, mouse primary hepatocytes, and in the plasma of TM-treated C57BL/6 mice. Furthermore, TM significantly increased hepatic LDLR expression and reduced plasma LDL concentrations in mice. Based on these findings, we propose a model in which ER Ca²⁺ depletion promotes the activation of SREBP2 and subsequent transcription of PCSK9. However, conditions that cause ER stress regardless of their ability to dysregulate ER Ca²⁺ inhibit PCSK9 secretion, thereby reducing PCSK9-mediated LDLR degradation and promoting LDLR-dependent hepatic cholesterol uptake. Taken together, our studies provide evidence that the retention of PCSK9 in the ER may serve as a potential strategy for lowering LDL cholesterol levels.

Lipids and flaviviruses, present and future perspectives for the control of dengue, Zika, and West Nile viruses

Flaviviruses are emerging arthropod-borne pathogens that cause life-threatening diseases such as yellow fever, dengue, West Nile encephalitis, tick-borne encephalitis, Kyasanur Forest disease, tick-borne encephalitis, or Zika disease. This viral genus groups >50 viral species of small enveloped plus strand RNA virus that are phylogenetically closely related to hepatitis C virus. Importantly, the flavivirus life cycle is intimately associated to host cell lipids. Along this line, flaviviruses rearrange intracellular membranes from the endoplasmic-reticulum of the infected cells to develop adequate platforms for viral replication and particle biogenesis. Moreover, flaviviruses dramatically orchestrate a profound reorganization of the host cell lipid metabolism to create a favorable environment for viral multiplication. Consistently, recent work has shown the importance of specific lipid classes in flavivirus infections. For instances, fatty acid synthesis is linked to viral replication, phosphatidylserine and phosphatidylethanolamine are involved on the entry of flaviviruses, sphingolipids (ceramide and sphingomyelin) play a key role on virus assembly and pathogenesis, and cholesterol is essential for innate immunity evasion in flavivirus-infected cells. Here, we revise the current knowledge on the interactions of the flaviviruses with the cellular lipid metabolism to identify potential targets for future antiviral development aimed to combat these relevant health-threatening pathogens.

Update on Mechanisms of Renal Tubule Injury Caused by Advanced Glycation End Products

Diabetic nephropathy (DN) caused by advanced glycation end products (AGEs) may be associated with lipid accumulation in the kidneys. This study was designed to investigate whether Nε-(carboxymethyl) lysine (CML, a member of the AGEs family) increases lipid accumulation in a human renal tubular epithelial cell line (HK-2) via increasing cholesterol synthesis and uptake and reducing cholesterol efflux through endoplasmic reticulum stress (ERS). Our results showed that CML disrupts cholesterol metabolism in HK-2 cells by activating sterol regulatory element-binding protein 2 (SREBP-2) and liver X receptor (LXR), followed by an increase in 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoAR) mediated cholesterol synthesis and low density lipoprotein receptor (LDLr) mediated cholesterol uptake and a reduction in ATP-binding cassette transporter A1 (ABCA1) mediated cholesterol efflux, ultimately causing lipid accumulation in HK-2 cells. All of these responses could be suppressed by an ERS inhibitor, which suggests that CML causes lipid accumulation in renal tubule cells through ERS and that the inhibition of ERS is a potential novel approach to treating CML-induced renal tubular foam cell formation.

Lipid accumulation stimulates the cap-independent translation of SREBP-1a mRNA by promoting hnRNP A1 binding to its 5'-UTR in a cellular model of hepatic steatosis

Non-alcoholic fatty liver disease (NAFLD) is a chronic disease characterized by accumulation of lipid droplets in hepatocytes. Enhanced release of non-esterified fatty acids from adipose tissue accounts for a remarkable fraction of accumulated lipids. However, the de novo lipogenesis (DNL) is also implicated in the etiology of the NAFLD. Sterol Regulatory Element-Binding Protein-1 (SREBP-1) is a transcription factor modulating the expression of several lipogenic enzymes. In the present study, in order to investigate the effect of lipid droplet accumulation on DNL, we used a cellular model of steatosis represented by HepG2 cells cultured in a medium supplemented with free oleic and palmitic fatty acids (FFAs). We report that FFA supplementation induces the expression of genes coding for enzymes involved in the DNL as well as for the transcription factor SREBP-1a. The SREBP-1a mRNA translation, dependent on an internal ribosome entry site (IRES), and the SREBP-1a proteolytic cleavage are activated by FFAs. Furthermore, FFA treatment enhances the expression and the nucleus-cytosolic shuttling of hnRNP A1, a trans-activating factor of SREBP-1a IRES. The binding of hnRNP A1 to the SREBP-1a IRES is also increased upon FFA supplementation. The relocation of hnRNP A1 and the consequent increase of SREBP-1a translation are dependent on the p38 MAPK signal pathway, which is activated by FFAs. By RNA interference approach, we demonstrate that hnRNP A1 is implicated in the FFA-induced expression of SREBP-1a and of its target genes as well as in the lipid accumulation in cells.

The Unfolded Protein Response and Cholesterol Biosynthesis Link Luman/CREB3 to Regenerative Axon Growth in Sensory Neurons

We recently revealed that the axon endoplasmic reticulum resident transcription factor Luman/CREB3 (herein called Luman) serves as a unique retrograde injury signal in regulation of the intrinsic elongating form of sensory axon regeneration. Here, evidence supports that Luman contributes to axonal regeneration through regulation of the unfolded protein response (UPR) and cholesterol biosynthesis in adult rat sensory neurons. One day sciatic nerve crush injury triggered a robust increase in UPR-associated mRNA and protein expression in both neuronal cell bodies and the injured axons. Knockdown of Luman expression in 1 d injury-conditioned neurons by siRNA attenuated axonal outgrowth to 48% of control injured neurons and was concomitant with reduced UPR- and cholesterol biosynthesis-associated gene expression. UPR PCR-array analysis coupled with qRT-PCR identified and confirmed that four transcripts involved in cholesterol regulation were downregulated >2-fold by the Luman siRNA treatment of the injury-conditioned neurons. Further, the Luman siRNA-attenuated outgrowth could be significantly rescued by either cholesterol supplementation or 2 ng/ml of the UPR inducer tunicamycin, an amount determined to elevate the depressed UPR gene expression to a level equivalent of that observed with crush injury. Using these approaches, outgrowth increased significantly to 74% or 69% that of injury-conditioned controls, respectively. The identification of Luman as a regulator of the injury-induced UPR and cholesterol at levels that benefit the intrinsic ability of axotomized adult rat sensory neurons to undergo axonal regeneration reveals new therapeutic targets to bolster nerve repair.

Relationships among injury, fibrosis, and time in human kidney transplants

BACKGROUND

Kidney transplant biopsies offer an opportunity to understand the pathogenesis of organ fibrosis. We studied the relationships between the time of biopsy after transplant (TxBx), histologic fibrosis, diseases, and transcript expression.

METHODS

Expression microarrays from 681 kidney transplant indication biopsies taken either early (n = 282, <1 year) or late (n = 399, >1 year) after transplant were used to analyze the molecular landscape of fibrosis in relationship to histologic fibrosis and diseases.

RESULTS

Fibrosis was absent at transplantation but was present in some early biopsies by 4 months after transplant, apparently as a self-limited response to donation implantation injury not associated with progression to failure. The molecular phenotype of early biopsies represented the time sequence of the response to wounding: immediate expression of acute kidney injury transcripts, followed by fibrillar collagen transcripts after several weeks, then by the appearance of immunoglobulin and mast cell transcripts after several months as fibrosis appeared. Fibrosis in late biopsies correlated with injury, fibrillar collagen, immunoglobulin, and mast cell transcripts, but these were independent of time. Pathway analysis revealed epithelial response-to-wounding pathways such as Wnt/β-catenin.

CONCLUSION

Fibrosis in late biopsies had different associations because many kidneys had potentially progressive diseases and subsequently failed. Molecular correlations with fibrosis in late biopsies were independent of time, probably because ongoing injury obscured the response-to-wounding time sequence. The results indicate that fibrosis in kidney transplants is driven by nephron injury and that progression to failure reflects continuing injury, not autonomous fibrogenesis.

TRIAL REGISTRATION

INTERCOM study (www.clinicalTrials.gov; NCT01299168).

FUNDING

Canada Foundation for Innovation and Genome Canada.

Endoplasmic reticulum stress and Oxidative stress in the pathogenesis of Non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome. The underlying causes of the disease progression in NAFLD are unclear. Recent evidences suggest endoplasmic reticulum stress in the development of lipid droplets (steatosis) and subsequent generation of reactive oxygen species (ROS) in the progression to non-alcoholic steatohepatitis (NASH). The signalling pathway activated by disruption of endoplasmic reticulum (ER) homoeostasis, called as unfolded protein response, is linked with membrane biosynthesis, insulin action, inflammation and apoptosis. ROS are important mediators of inflammation. Protein folding in ER is linked to ROS. Therefore understanding the basic mechanisms that lead to ER stress and ROS in NAFLD have become the topics of immense interest. The present review focuses on the role of ER stress and ROS in the pathogenesis of NAFLD. We also highlight the cross talk between ER stress and oxidative stress which suggest and encourage the development of therapeutics for NAFLD. Further we have reviewed various strategies used for the management of NAFLD/NASH and limitations of such strategies. Our review therefore highlights the need for newer strategies with regards to ER stress and oxidative stress.

Microcystin-LR altered mRNA and protein expression of endoplasmic reticulum stress signaling molecules related to hepatic lipid metabolism abnormalities in mice

To explore the effects of microcystin-LR (MC-LR), a hepatotoxin, on the incidence of liver lipid metabolism abnormality, and the potential molecular mechanisms of action, healthy male Balb/c mice were intraperitoneally injected with MC-LR at doses of 0, 5, 10, and 20 μg/kg/d for 14 days. Hepatic histopathology and serum lipid parameters of mice were determined, and the changes of mRNA and protein expression of endoplasmic reticulum (ER) stress signaling molecules related to the lipid metabolism abnormalities in the livers of mice were investigated by quantitative real-time polymerase chain reaction (qPCR) and Western blotting, respectively. The results indicated that 5-20 μg/kg/d MC-LR altered serum lipid parameters and caused hepatic steatosis. MC-LR treatment at 10 or 20 μg/kg/d changed mRNA and protein expression of ER stress signaling molecules, including upregulation of mRNA and protein expression of activating transcription factor 6 (ATF6), pancreatic ER eukaryotic translation initiation factor 2α (eIF-2α) kinase (PERK), and eIF-2α. MC-LR exposure at 10 or 20 μg/kg/d also altered mRNA and protein expression of downstream factors and genes of ER stress signaling pathways, including the downregulation of sterol regulatory element binding protein 1c (SREBP-1c) and fatty acid synthase (FASn), and upregulation of acetyl-coenzyme A carboxylase α (ACACA) and glycogen synthase kinase 3β (Gsk-3β). Our results reveal that ER stress plays a significant role in hepatic lipid metabolism abnormalities in mice exposed to MC-LR.