Magnolol, a compound extracted from Magnolia officinalis, demonstrates potential efficacy in addressing metabolic dysfunction and cardiovascular diseases. Its biological activities encompass anti-inflammatory, antioxidant, anticoagulant, and anti-diabetic effects. Growth/differentiation factor-15 (GDF-15), a member of the transforming growth factor β superfamily, is considered a potential therapeutic target for metabolic disorders. This study investigated the impact of magnolol on GDF-15 production and its underlying mechanism. The research examined the pharmacological effect of magnolol on GDF-15 expression in vitro and in vivo, and determined the involvement of endoplasmic reticulum (ER) stress signaling in this process. Luciferase reporter assays, chromatin immunoprecipitation, and in vitro DNA binding assays were employed to examine the regulation of GDF-15 by activating transcription factor 4 (ATF4), CCAAT enhancer binding protein γ (CEBPG), and CCCTC-binding factor (CTCF). The study also investigated the effect of magnolol and ATF4 on the activity of a putative enhancer located in the intron of the GDF-15 gene, as well as the influence of single nucleotide polymorphisms (SNPs) on magnolol and ATF4-induced transcription activity. Results demonstrated that magnolol triggers GDF-15 production in endothelial cells (ECs), hepatoma cell line G2 (HepG2) and hepatoma cell line 3B (Hep3B) cell lines, and primary mouse hepatocytes. The cooperative binding of ATF4 and CEBPG upstream of the GDF-15 gene or the E1944285 enhancer located in the intron led to full-power transcription of the GDF-15 gene. SNP alleles were found to impact the magnolol and ATF4-induced transcription activity of GDF-15. In high-fat diet ApoE^-/- mice, administration of magnolol induced GDF-15 production and partially suppressed appetite through GDF-15. These findings suggest that magnolol regulates GDF-15 expression through priming of promoter and enhancer activity, indicating its potential as a drug for the treatment of metabolic disorders.
Lignans/pharmacology* ; Growth Differentiation Factor 15/metabolism* ; Animals ; Biphenyl Compounds/pharmacology* ; Endoplasmic Reticulum Stress/drug effects* ; Activating Transcription Factor 4/genetics* ; Mice ; Humans ; Male ; Magnolia/chemistry* ; CCAAT-Enhancer-Binding Proteins/genetics* ; Mice, Inbred C57BL

Triple-negative breast cancer (TNBC) represents an aggressive breast cancer subtype with poor prognosis and limited targeted treatment options. This investigation examined the anti-cancer potential of Caerulomycin A (Cae A), a natural compound derived from marine actinomycetes, against TNBC. Cae A demonstrated selective inhibition of viability and proliferation in TNBC cell lines, including 4T1, MDA-MB-231, and MDA-MB-468, through apoptosis induction. Mechanistic analyses revealed that the compound induced sustained endoplasmic reticulum (ER) stress and subsequent upregulation of C/EBP homologous protein (CHOP) expression, resulting in mitochondrial damage-mediated apoptosis. Inhibition of ER stress or CHOP expression knockdown reversed mitochondrial damage and apoptosis, highlighting the essential role of ER stress and CHOP in Cae A's anti-tumor mechanism. Both oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) decreased in TNBC cells following Cae A treatment, indicating reduced mitochondrial respiratory and glycolytic capacities. This diminished energy metabolism potentially triggers ER stress and subsequent apoptosis. Furthermore, Cae A exhibited significant anti-tumor effects in the 4T1 tumor model in vivo without apparent toxicity. The compound also effectively inhibited human TNBC organoid growth. These results indicate that Cae A may serve as a potential therapeutic agent for TNBC, with its efficacy likely mediated through the disruption of glucose metabolism and the induction of ER stress-associated apoptosis.
Humans ; Endoplasmic Reticulum Stress/drug effects* ; Triple Negative Breast Neoplasms/genetics* ; Apoptosis/drug effects* ; Cell Line, Tumor ; Female ; Animals ; Glucose/metabolism* ; Mice ; Cell Proliferation/drug effects* ; Transcription Factor CHOP/genetics* ; Antineoplastic Agents/pharmacology* ; Mitochondria/metabolism* ; Mice, Inbred BALB C

OBJECTIVE: To explore the clinical phenotype and genetic characteristics of a Chinese Han pedigree with Darier's disease (DD). METHODS: A DD pedigree, who visited Tongji Hospital of Tongji University on October 22, 2023, was selected as the study subject. Clinical data of the pedigree were collected, and whole exome sequencing was performed on the proband. Suspected variant loci were screened, and Sanger sequencing was used to validate the variant in pedigree members. Bioinformatics analysis was performed on the variant loci. This study was approved by the Medical Ethics Committee of Tongji Hospital of Tongji University Ethics No.K-W-2024-004). RESULTS: The proband is a 67-year-old female with clinical features of DD, such as keratotic papules in sebaceous areas. whole exome sequencing revealed a missense variant, c.68G>A (p.Gly23Glu), in the exon 1 of ATP2A2 gene of the proband. Sanger sequencing showed that the proband's eldest daughter also carried this variant. This variant was not detected in other pedigree members, indicating a co-segregation of the variant with the disease phenotype in the pedigree. According to the interpretation principles of gene variants by the American College of Medical Genetics and Genomics (ACMG), this variant was classified as pathogenic (PS1+PM1+PM2_Supporting+PP1+PP3+PP4). CONCLUSION The c.68G>A (p.Gly23Glu) variant in the ATP2A2 gene may be the genetic cause of the disease in this pedigree. This finding further enriches the genetic variant spectrum in DD patients and provides a basis for clinical diagnosis and genetic counseling for patients.
Aged ; Female ; Humans ; Male ; China ; Darier Disease/genetics* ; Exome Sequencing ; Mutation, Missense ; Pedigree ; Phenotype ; East Asian People ; Sarcoplasmic Reticulum Calcium-Transporting ATPases

OBJECTIVES: To investigate the role and mechanism of copper overload-mediated endoplasmic reticulum stress (ERS) in vascular endothelial injury in Kawasaki disease (KD). METHODS: Four-week-old male C57BL/6 mice were randomly divided into four groups: control, KD, KD plus copper chelator tetrathiomolybdate (TTM), and KD plus ERS inhibitor AMG PERK 44 (AMG) (n=20 per group). A KD mouse model was established using Candida albicans extract. Human umbilical vein endothelial cells (HUVECs) were divided into control (intervention with healthy children's serum), KD (intervention with KD patients' serum), and KD+TTM (intervention with KD patients' serum plus 20 µmol/L TTM). Copper deposition in mouse heart tissue was assessed using rubeanic acid staining. Vascular pathological changes were observed using hematoxylin-eosin staining and measurement of abdominal aortic diameter and area. ERS activation was detected by transmission electron microscopy and immunofluorescence. HUVEC viability, apoptosis, and functional changes were evaluated using CCK8, flow cytometry, cell scratch assay, and angiogenesis experiments. ERS marker protein expression levels were measured by Western blot. RESULTS: Compared to the KD group, the KD+TTM and KD+AMG groups showed reduced copper deposition in the vascular wall, decreased swelling of coronary endothelial cells and endoplasmic reticulum, reduced inflammatory cell infiltration, and less abdominal aortic lesion expansion. The abdominal aortic diameter and area, and the fluorescence intensity of ERS marker proteins (GRP78 and CHOP) were significantly lower (P<0.05). Compared to the KD group, the KD+TTM group exhibited increased cell viability, tube number, and scratch healing rate, along with decreased apoptosis rate and expression of ERS marker proteins (GRP78, CHOP, ATF6, and p-PERK) (P<0.05). CONCLUSIONS Copper overload aggravates vascular endothelial injury in KD by activating the ERS pathway. TTM can exert protective effects on the endothelium by regulating copper metabolism and inhibiting the ERS pathway.
Endoplasmic Reticulum Stress ; Copper/toxicity* ; Male ; Mucocutaneous Lymph Node Syndrome/metabolism* ; Animals ; Humans ; Endoplasmic Reticulum Chaperone BiP ; Mice, Inbred C57BL ; Mice ; Human Umbilical Vein Endothelial Cells ; Apoptosis ; Endothelium, Vascular/injuries*

OBJECTIVES: To explore the protective effect of vitexin-4 ″-O-glucoside (VOG) against acetaminophen-induced acute liver injury in mice and its underlying mechanism. METHODS: C57BL/6 mice were randomly divided into 4 groups: normal control group, model control group, low-dose group of VOG (30 mg/kg), and high-dose group of VOG (60 mg/kg). Acute liver injury was induced by intraperitoneal injection of acetaminophen (500 mg/kg). VOG was administrated by gavage 2 h before acetaminophen treatment in VOG groups. The protective effect of VOG against acute liver injury was evaluated by detecting alanine transaminase (ALT), aspartate transaminase (AST) levels and hematoxylin and eosin staining. The malondialdehyde (MDA) content, superoxide dismutase (SOD) and catalase (CAT) activity in liver were detected to evaluate the hepatic oxidative stress. The expression levels of tumor necrosis factor (TNF)-α, Il-1β, and Il-6 in liver were detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR). The expression levels of phosphorylated c-jun N-terminal kinase (JNK)/JNK, phosphorylated p38/p38, inositol-requiring enzyme 1 alpha (IRE-1α), X-box binding protein 1s (XBP1s), and glucose-regulated protein 78 (GRP78) in liver were detected by Western blotting. An endoplasmic reticulum stress model was established in AML-12 cells using tunicamycin. Cell viability was assessed using the CCK-8 assay, and the degree of cell damage was detected by lactate dehydrogenase (LDH) assay. The gene expression levels of Ire-1α, Xbp1s, and Grp78 in the cells were detected using qRT-PCR. RESULTS: In the animal experiments, compared with the model control group, VOG significantly improved plasma ALT and AST levels, liver MDA content, as well as SOD and CAT activities. VOG also reduced the expression levels of Tnf-α, Il-1β, and Il-6 in the liver, and improved protein phosphorylation levels of JNK and p38, as well as the protein expression levels of IRE-1α, XBP1s, and GRP78. In cell experiments, VOG pretreatment enhanced cell viability, reduced LDH release and decreased the mRNA expression of Ire-1α, Xbp1s, and Grp78. CONCLUSIONS VOG can suppress inflammation and oxidative stress, and alleviate acetaminophen-induced acute liver injury in mice by suppressing endoplasmic reticulum stress and modulating the MAPK signaling pathway.
Animals ; Endoplasmic Reticulum Chaperone BiP ; Mice ; Acetaminophen/adverse effects* ; Mice, Inbred C57BL ; Chemical and Drug Induced Liver Injury/prevention & control* ; Glucosides/therapeutic use* ; Oxidative Stress/drug effects* ; Male ; Apigenin/therapeutic use* ; Liver/drug effects* ; Tumor Necrosis Factor-alpha/metabolism* ; Endoplasmic Reticulum Stress/drug effects* ; X-Box Binding Protein 1 ; Endoribonucleases/metabolism* ; Interleukin-1beta/metabolism* ; Interleukin-6/metabolism* ; Protein Serine-Threonine Kinases

BACKGROUND: Hyperthermia (HT), while a cancer treatment approach, isn't always effective alone. Therefore, identifying hyperthermia enhancers is crucial. We demonstrated that Mito-TEMPO ([2-[(1-Hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) amino]-2-oxoethyl]-triphenylphosphanium, MT) acts as a potent thermosensitizer, promoting cell death in human cervical cancer (HeLa) cells. METHODS: Cells were pretreated with 0.4 mM MT for 5 minutes, followed by exposure to hyperthermia (42 °C for 60 minutes). The impacts of MT/HT on cell viability, proliferation, apoptosis, endoplasmic reticulum (ER) stress, apoptosis-related proteins and autophagy, autophagy-related proteins expression were measured. The relationships between autophagy and apoptosis were further investigated using the specific autophagy inhibitor chloroquine (CQ) and the autophagy inducer rapamycin (Rapa). RESULTS: The combined treatment reduced the mitochondrial membrane potential (MMP) and increased ROS production. It also upregulated the pro-apoptotic protein Bax and downregulated anti-apoptotic proteins such as Bcl-2 and MCL-1. As a result, Caspase-3 was activated. Additionally, the combined treatment upregulated the expression of p-PERK/PERK, ATF-4, CHOP proteins. Moreover, the combined treatment also increased the expression of LC3 II and p62, decreased expression of LAMP 1 and Cathepsin D and increased lysosomal pH, indicating coordinated changes in autophagy regulation. Notably, intensification of apoptosis induced by the combined treatment was observed with CQ, whereas attenuation was seen with Rapa. CONCLUSIONS MT effectively enhanced HT-induced apoptosis in HeLa cells. Elevated ER stress and interruption of autophagy flux are the possible underlying molecular mechanisms for this phenomenon. These findings suggested MT can act as a potential thermosensitizer, highlighting its versatility in cancer treatment strategies.
Humans ; Apoptosis/drug effects* ; Autophagy/drug effects* ; HeLa Cells ; Uterine Cervical Neoplasms/therapy* ; Female ; Hyperthermia, Induced ; Spin Labels ; Endoplasmic Reticulum Stress/drug effects* ; Cyclic N-Oxides/pharmacology* ; Cell Survival/drug effects*

OBJECTIVE: To investigate the effects of astragaloside IV (AS-IV) on podocyte injury of diabetic nephropathy (DN) and reveal its potential mechanism. METHODS: In in vitro experiment, podocytes were divided into 4 groups, normal, high glucose (HG), inositol-requiring enzyme 1 (IRE-1) α activator (HG+thapsigargin 1 µmol/L), and IRE-1α inhibitor (HG+STF-083010, 20 µmol/L) groups. Additionally, podocytes were divided into 4 groups, including normal, HG, AS-IV (HG+AS-IV 20 µmol/L), and IRE-1α inhibitor (HG+STF-083010, 20 µmol/L) groups, respectively. After 24 h treatment, the morphology of podocytes and endoplasmic reticulum (ER) was observed by electron microscopy. The expressions of glucose-regulated protein 78 (GRP78) and IRE-1α were detected by cellular immunofluorescence. In in vivo experiment, DN rat model was established via a consecutive 3-day intraperitoneal streptozotocin (STZ) injections. A total of 40 rats were assigned into the normal, DN, AS-IV [AS-IV 40 mg/(kg·d)], and IRE-1α inhibitor [STF-083010, 10 mg/(kg·d)] groups (n=10), respectively. The general condition, 24-h urine volume, random blood glucose, urinary protein excretion rate (UAER), urea nitrogen (BUN), and serum creatinine (SCr) levels of rats were measured after 8 weeks of intervention. Pathological changes in the renal tissue were observed by hematoxylin and eosin (HE) staining. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) and Western blot were used to detect the expressions of GRP78, IRE-1α, nuclear factor kappa Bp65 (NF-κBp65), interleukin (IL)-1β, NLR family pyrin domain containing 3 (NLRP3), caspase-1, gasdermin D-N (GSDMD-N), and nephrin at the mRNA and protein levels in vivo and in vitro, respectively. RESULTS: Cytoplasmic vacuolation and ER swelling were observed in the HG and IRE-1α activator groups. Podocyte morphology and ER expansion were improved in AS-IV and IRE-1α inhibitor groups compared with HG group. Cellular immunofluorescence showed that compared with the normal group, the fluorescence intensity of GRP78 and IRE-1α in the HG and IRE-1α activator groups were significantly increased whereas decreased in AS-IV and IRE-1α inhibitor groups (P<0.05). Compared with the normal group, the mRNA and protein expressions of GRP78, IRE-1α, NF-κ Bp65, IL-1β, NLRP3, caspase-1 and GSDMD-N in the HG group was increased (P<0.05). Compared with HG group, the expression of above indices was decreased in the AS-IV and IRE-1α inhibitor groups, and the expression in the IRE-1α activator group was increased (P<0.05). The expression of nephrin was decreased in the HG group, and increased in AS-IV and IRE-1α inhibitor groups (P<0.05). The in vivo experiment results revealed that compared to the normal group, the levels of blood glucose, triglyceride, total cholesterol, BUN, blood creatinine and urinary protein in the DN group were higher (P<0.05). Compared with DN group, the above indices in AS-IV and IRE-1α inhibitor groups were decreased (P<0.05). HE staining revealed glomerular hypertrophy, mesangial widening and mesangial cell proliferation in the renal tissue of the DN group. Compared with the DN group, the above pathological changes in renal tissue of AS-IV and IRE-1α inhibitor groups were alleviated. Quantitative RT-PCR and Western blot results of GRP78, IRE-1α, NF-κ Bp65, IL-1β, NLRP3, caspase-1 and GSDMD-N were consistent with immunofluorescence analysis. CONCLUSION AS-IV could reduce ERS and inflammation, improve podocyte pyroptosis, thus exerting a podocyte-protective effect in DN, through regulating IRE-1α/NF-κ B/NLRP3 signaling pathway.
Podocytes/metabolism* ; Animals ; Diabetic Nephropathies/metabolism* ; Saponins/therapeutic use* ; Triterpenes/therapeutic use* ; Signal Transduction/drug effects* ; NF-kappa B/metabolism* ; Protein Serine-Threonine Kinases/metabolism* ; Male ; Rats, Sprague-Dawley ; NLR Family, Pyrin Domain-Containing 3 Protein/metabolism* ; Endoribonucleases/metabolism* ; Endoplasmic Reticulum Chaperone BiP ; Rats ; Diabetes Mellitus, Experimental/complications* ; Endoplasmic Reticulum/metabolism* ; Multienzyme Complexes

OBJECTIVE: To explore the mechanism of crocin, a major active component of Crocus sativus (Zanghonghua), in regulating amyloid beta (Aβ) generation, endoplasmic reticulum (ER) stress, and autophagy in neuronal cells, with potential therapeutic applications in Alzheimer's disease (AD). METHODS: Mouse neuroblastoma Neuron2a (N2a) cells stably transfected with the human amyloid precursor protein (APP) Swedish mutant was used as a cellular model for AD (N2a/APP). Control cells were vector transfected (N2a/vector). The effects of 3 different doses of crocin on reactive oxygen species (ROS) generation, cytosolic calcium, and apoptosis were evaluated by flow cytometry. Aβ levels were determined by enzyme-linked immunosorbent assay. APP processing and ER stress proteins expressions were determined by Western blot. Autophagosome formation was evaluated by autophagy detection kit and confocal microscope. RESULTS: Crocin inhibited APP expression in N2a/APP cells and promoted α-cleavage of APP processing, while modestly reduced beta-secretase 1 (BACE1) and presenilin 1 (PS1, P<0.05 or P<0.01). ER stress markers, including the binding immunoglobulin protein/78-kD glucose-regulated protein (Bip/GRP78) and C/EBP homologous protein (CHOP), were elevated in N2a/APP cells compared to N2a/vector cells (P<0.05). Crocin could effectively reduce the levels of ER stress (P<0.05 or P<0.01). In addition, crocin enhanced autophagy by promoting formation of autophagosome (P<0.05 or P<0.01). CONCLUSION Crocin significantly inhibited Aβ generation by promoting α-cleavage of APP processing, inhibiting ER stress-associated unfolded protein response, and regulating autophagy.
Endoplasmic Reticulum Stress/drug effects* ; Autophagy/drug effects* ; Animals ; Endoplasmic Reticulum Chaperone BiP ; Mice ; Amyloid beta-Peptides/metabolism* ; Amyloid beta-Protein Precursor/metabolism* ; Carotenoids/pharmacology* ; Humans ; Cell Line, Tumor ; Reactive Oxygen Species/metabolism* ; Apoptosis/drug effects* ; Calcium/metabolism*

OBJECTIVES: The integrated endoplasmic reticulum stress inhibitor (ISRIB) is a selective inhibitor of the protein kinase R-like endoplasmic reticulum kinase (PERK) signaling pathway within endoplasmic reticulum stress (ERS) and can improve spatial and working memory in aged mice. Although ERS and oxidative stress are tightly interconnected, it remains unclear whether ISRIB alleviates cognitive impairment by restoring the balance between ERS and oxidative stress. This study aims to investigate the effects and mechanisms of ISRIB on lipopolysaccharide (LPS)-induced cognitive impairment in mice. METHODS: Eight-week-old male ICR mice were randomly divided into 3 groups: Normal saline (NS) group, LPS group, and ISRIB+LPS group. NS and LPS groups received daily intraperitoneal injections of normal saline for 7 days; on day 7, LPS group mice received intraperitoneal LPS (0.83 mg/kg) to establish a cognitive impairment model. ISRIB+LPS group received ISRIB (0.25 mg/kg) intraperitoneally for 7 days, with LPS injected 30 minutes after ISRIB on day 7. Cognitive ability was evaluated by the novel place recognition test (NPRT). Real-time fluorogenic quantitative PCR (RT-qPCR) was used to detect changes in nitric oxide synthase (NOS), superoxide dismutase-1 (SOD-1), and catalase (CAT) gene expression in the hippocampus and prefrontal cortex. Oxidative stress markers malondialdehyde (MDA), glutathione (GSH), and oxidized glutathione (GSSG), were measured in hippocampal and prefrontal cortex tissues. RESULTS: Compared with the NS group, mice in LPS group showed a significant reduction in novel place recognition ratio, upregulation of hippocampal NOS-1 and NOS-2 mRNA, downregulation of SOD-1 and CAT mRNA, increased MDA and GSSG, decreased GSH, and reduced GSH/GSSG ratio (all P<0.05). Compared with the LPS group, mice in ISRIB+LPS group exhibited significantly improved novel place recognition, downregulated NOS-1 and NOS-2 mRNA, upregulated SOD-1 and CAT mRNA, decreased MDA and GSSG, increased GSH, and an elevated GSH/GSSG ratio in the hippocampus (all P<0.05). No significant changes were observed in the prefrontal cortex. CONCLUSIONS ISRIB improves LPS-induced cognitive impairment in mice by restoring the oxidative/antioxidant balance in the hippocampus.
Animals ; Lipopolysaccharides ; Male ; Mice, Inbred ICR ; Cognitive Dysfunction/drug therapy* ; Mice ; Oxidative Stress/drug effects* ; Endoplasmic Reticulum Stress/drug effects* ; Hippocampus/drug effects* ; Nitric Oxide Synthase Type II/genetics* ; Guanidines/pharmacology* ; eIF-2 Kinase/antagonists & inhibitors* ; Signal Transduction/drug effects* ; Superoxide Dismutase/metabolism*

OBJECTIVES: To explore the effects of cannabidiol on endoplasmic reticulum stress and neuronal apoptosis in rats with multiple concussions (MCC). METHODS: SD rats were randomized into sham group, MCC group, 1% tween20 (TW) treatment group, and low-dose (10 mg/kg) and high-dose (40 mg/kg) cannabidiol treatment groups. In all but the sham group, MCC models were established using a metal pendulum percussion device, after which the rats received daily intraperitoneal injections of the corresponding agents for 2 weeks. The expressions of PERK, eIF2α, ATF4, CHOP, TRIB3, p-Akt and pro-caspase-3 in the brain tissue of the rats were detected with qRT-PCR, Western blotting and immunofluorescence staining. The core targets of cannabidiol in treatment of traumatic brain injury (TBI) were identified by network pharmacology analysis, and molecular docking was carried out to simulate the interaction of cannabidiol with the factors related to endoplasmic reticulum stress and apoptosis. RESULTS: Compared with the sham-operated rats, the rat models of MCC showed significantly increased mRNA expressions of PERK, eIF2α and CHOP and protein expressions of PERK, eIF2α, ATF4, CHOP, TRIB3, p-AKT and pro-caspase-3 in the cerebral cortex. CBD treatment, especially at the high dose, obviously increased the expression of p-Akt and lowered the expression levels of the other factors tested in the rat models. Network pharmacology analysis indicated interactions of the core targets of CBD with the factors related to endoplasmic reticulum stress and TBI, and molecular docking study showed a high binding energy of CBD with multiple factors pertaining to endoplasmic reticulum stress and apoptosis. CONCLUSIONS MCC induce endoplasmic reticulum stress and apoptosis in rat brain tissues, for which CBD, especially at a high dose, provides neuroprotective effects by inhibiting endoplasmic reticulum stress and cell apoptosis.
Animals ; Endoplasmic Reticulum Stress/drug effects* ; Apoptosis/drug effects* ; Rats, Sprague-Dawley ; Activating Transcription Factor 4/metabolism* ; Transcription Factor CHOP/metabolism* ; Rats ; Eukaryotic Initiation Factor-2/metabolism* ; Signal Transduction/drug effects* ; eIF-2 Kinase/metabolism* ; Cannabidiol/pharmacology* ; Neurons/metabolism* ; Brain Concussion/metabolism* ; Male ; Molecular Docking Simulation