Adiponectin Stimulates Apolipoprotein A-1 Gene Expression in HepG2 Cells via AMPK, PPARα, and LXRs Signaling Mechanisms.

Adiponectin is an adipose tissue hormone, participating in energy metabolism and involved in atherogenesis. Previously, it was found that adiponectin increases expression of the APOA1 (apolipoprotein A-1) gene in hepatocytes, but the mechanisms of this effect remained unexplored. Our aim was to investigate the role of adiponectin receptors AdipoR1/R2, AMP-activated protein kinase (AMPK), nuclear peroxisome proliferator-activated receptor alpha (PPARα) and liver X receptors (LXRs) in mediating the action of adiponectin on hepatic APOA1 expression in human hepatoma HepG2 cells. The level of APOA1 expression was determined by RT-qPCR and ELISA. We showed that the siRNA-mediated knockdown of genes coding for AdipoR1, AdipoR2, AMPK, PPARα, and LXRα and ß prevented adiponectin-induced APOA1 expression in HepG2 cells and demonstrated that interaction of PPARα and LXRs with the APOA1 gene hepatic enhancer is important for the adiponectin-dependent APOA1 transcription. The results of this study point out to the involvement of both types of adiponectin receptors, AMPK, PPARα, and LXRs in the adiponectin-dependent upregulation of the APOA1 expression.

Immunohistochemical analysis of adiponectin in atherosclerotic lesions of human aorta.

BACKGROUND: Metabolic syndrome, a cluster of interrelated disorders including abdominal obesity, insulin resistance, dyslipidemia, and hypertension (HTN) plays an important role in development of atherosclerotic lesions in arterial wall. Dysregulation of adipose tissue hormones (adipokines) production is a possible link between abdominal obesity and other manifestations of metabolic syndrome. Adiponectin is a well-known adipokine which affects metabolism and inflammatory response. However, data on its role in atherogenesis are still controversial. The aim of this study is to investigate whether adiponectin is present in atherosclerotic lesions of human aorta. METHODS: Thirty-five autopsy segments from abdominal, thoracic aortas, and aortic arch of four men (mean age: 57 years) were fixed and stained for lipids [Oil Red O (ORO)], cells [hematoxylin-eosin (H&E)], and adiponectin [indirect immunoperoxidase assay (IPA) method]. Samples of both stable and unstable plaques were selected for analysis. Human adipose tissue, THP-1 monocytes/macrophages, and human endothelial hybrid cell line (EA.hy926) were chosen for detection of adiponectin messenger ribonucleic acid (mRNA) using reverse transcription polymerase chain reaction (RT-PCR). RESULTS: Adiponectin accumulations were found inside endothelial cells covering both stable and unstable atherosclerotic plaques. Focal depositions of adiponectin were also found in fibrous caps of stable lesions and atheromatous core of both stable and unstable plaques and also in adventitia. RT-PCR revealed mRNA expression of adiponectin gene in adipose tissue, but not in mononuclears and endothelial cells. CONCLUSION: Adiponectin is present in aortic plaques of humans, but is not synthesized in endothelial cells and mononuclears, at least in culture conditions. Detection of adiponectin in atherosclerotic lesions can serve as indirect evidence of possible participation of this adipokine in atherogenesis.

Insulin-Mediated Downregulation of Apolipoprotein A-I Gene in Human Hepatoma Cell Line HepG2: The Role of Interaction Between FOXO1 and LXRß Transcription Factors.

Apolipoprotein A-I (ApoA-I) is a key component of high density lipoproteins which possess anti-atherosclerotic and anti-inflammatory properties. Insulin is a crucial mediator of the glucose and lipid metabolism that has been implicated in atherosclerotic and inflammatory processes. Important mediators of insulin signaling such as Liver X Receptors (LXRs) and Forkhead Box A2 (FOXA2) are known to regulate apoA-I expression in liver. Forkhead Box O1 (FOXO1) is a well-known target of insulin signaling and a key mediator of oxidative stress response. Low doses of insulin were shown to activate apoA-I expression in human hepatoma HepG2 cells. However, the detailed mechanisms for these processes are still unknown. We studied the possible involvement of FOXO1, FOXA2, LXRα, and LXRß transcription factors in the insulin-mediated regulation of apoA-I expression. Treatment of HepG2 cells with high doses of insulin (48 h, 100 nM) suppresses apoA-I gene expression. siRNAs against FOXO1, FOXA2, LXRß, or LXRα abrogated this effect. FOXO1 forms a complex with LXRß and insulin treatment impairs FOXO1/LXRß complex binding to hepatic enhancer and triggers its nuclear export. Insulin as well as LXR ligand TO901317 enhance the interaction between FOXA2, LXRα, and hepatic enhancer. These data suggest that high doses of insulin downregulate apoA-I gene expression in HepG2 cells through redistribution of FOXO1/LXRß complex, FOXA2, and LXRα on hepatic enhancer of apoA-I gene. J. Cell. Biochem. 118: 382-396, 2017. © 2016 Wiley Periodicals, Inc.

FOXO1 and LXRα downregulate the apolipoprotein A-I gene expression during hydrogen peroxide-induced oxidative stress in HepG2 cells.

Reactive oxygen species damage various cell components including DNA, proteins, and lipids, and these impairments could be a reason for severe human diseases including atherosclerosis. Forkhead box O1 (FOXO1), an important metabolic transcription factor, upregulates antioxidant and proapoptotic genes during oxidative stress. Apolipoprotein A-I (ApoA-I) forms high density lipoprotein (HDL) particles that are responsible for cholesterol transfer from peripheral tissues to liver for removal in bile in vertebrates. The main sources for plasma ApoA-I in mammals are liver and jejunum. Hepatic apoA-I transcription depends on a multitude of metabolic transcription factors. We demonstrate that ApoA-I synthesis and secretion are decreased during H2O2-induced oxidative stress in human hepatoma cell line HepG2. Here, we first show that FOXO1 binds to site B of apoA-I hepatic enhancer and downregulates apoA-I gene activity in HepG2 cells. Moreover, FOXO1 and LXRα transcription factors participate in H2O2-triggered downregulation of apoA-I gene together with Src, JNK, p38, and AMPK kinase cascades. Mutations of sites B or C as well as the administration of siRNAs against FOXO1 or LXRα to HepG2 cells abolished the hydrogen peroxide-mediated suppression of apoA-I gene.

Association of adipokines with metabolic disorders in patients with schizophrenia: Results of comparative study with mental healthy cohort.

AIM: The role of adipose tissue hormones, adipokines, in formation of metabolic disorders in schizophrenia is not fully understood. The aim was to investigate the association of leptin and adiponectin plasma levels with metabolic parameters in antipsychotic treated patients with schizophrenia and in the group of age, gender and body mass index matched mental healthy persons. METHODS: One hundred patients with diagnosis of schizophrenia, who took antipsychotic medication, and equal number of control subjects, were enrolled for cross-sectional evaluation. Fasting blood plasma levels of glucose, lipids, insulin, adiponectin, leptin concentrations and insulin resistance HOMA index were determined. RESULTS: In both groups plasma leptin concentration positively correlated with body mass index, insulin plasma level and HOMA index, while adiponectin level had negative correlations with adiposity measures and positive associations with high density lipoprotein cholesterol content. At the same time, in schizophrenia group, but not in control subjects, leptin level positively associated with cholesterol and triglycerides concentrations and adiponectin negatively correlated with plasma insulin content, HOMA index and triglycerides levels. After controlling for confounders significant correlations remained for leptin concentration with HOMA index and plasma triglycerides level in schizophrenic patients and for adiponectin concentration with plasma high density lipoprotein cholesterol concentrations in both studied groups. CONCLUSIONS: Both adipokines associate with metabolic parameters in antipsychotic treated patients with schizophrenia. Leptin can play more specific role in pathogenesis of metabolic syndrome in schizophrenic persons than in mental healthy subjects.

Endogenous apolipoprotein A-I stabilizes ATP-binding cassette transporter A1 and modulates Toll-like receptor 4 signaling in human macrophages.

Apolipoprotein A-I (ApoA-I) is the main functional protein component of human high-density lipoproteins. ApoA-I shows various anti-inflammatory and atheroprotective properties toward macrophages; however, endogenous apoA-I expression has not been investigated in macrophages. We have shown that endogenous apoA-I gene is expressed in human macrophages at both mRNA and protein levels. Endogenous ApoA-I is localized in intracellular vesicles and at the external side of the plasma membrane in association with ATP-binding cassette transporter A1 (ABCA1) and lipid rafts in macrophages. We have shown that endogenous ApoA-I stabilizes ABCA1, moreover, down-regulation of ApoA-I by siRNA results in an increase of Toll-like receptor 4 (TLR4) mRNA and membrane surface protein expression, as well as an enhancement of bacterial lipopolysaccharide (LPS)-induced expression of tumor necrosis factor-α (TNF-α), interleukin 1ß (IL-1ß), and inducible nitric oxide synthase (NOS2) genes in human macrophages. TNF-α stimulates ApoA-I expression and secretion (1.2±0.2 vs. 4.3±0.9 ng/mg total protein) in macrophages. Obtained results suggest that endogenous ApoA-I has anti-inflammatory properties, presumably due to ABCA1 stabilization in macrophages; these results elucidate the cell type-specific mechanism of the TNF-α-mediated regulation of apoA-I gene expression in monocytes and macrophages.