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Identifier 000373247
Title Μηχανισμοί μεταγραφικής ρύθμισης των γονιδίων των απολιποπρωτεϊνών in vivo
Alternative Title Mechanisms of transcriptional regulation of human apolipoprotein genes in vivo.
Author Μόσιαλου, Ιωάννα
Thesis advisor Καρδάσης, Δ.
Reviewer Τσατσάνης, Χρήστος
Σπιλιανάκη Χαράλαμπο
Μπούμπας, Χαράλαμπος
Στουρνάρας, Χρήστος
Ηλιόπουλος, Αριστείδης
Γραβάνης, Αχιλλέας
Κρασαγάκης, Κωνσταντίνος
Abstract Numerous epidemiological and clinical trials have established that High Density Lipoprotein (HDL) is a strong and independent risk factor for the development of atherosclerosis and Coronary heart disease (CHD) which is the leading cause of morbidity and mortality in Western countries. These studies showed that there is an inverse correlation between plasma HDL cholesterol levels and incidence of acute cardiovascular events. On the other hand, it has been well documented in recent years that the concentration of plasma HDL is not by itself a good biomarker for CHD risk prediction but the functionality of HDL particles is also important. HDL has multiple atheroprotective functions which include the removal of excess cholesterol from peripheral cells and protective functions on the vascular endothelium. These functions are disturbed in subjects with mutations in key genes of the HDL biogenesis/maturation pathway as well as in patients with chronic inflammatory diseases. HDL levels are regulated by genetic and environmental factors as well as by drugs. Existing drugs raise HDL levels to a minor extent suggesting that novel strategies are needed. In the present study, we investigated in detail the molecular mechanisms that control the expression of genes which are involved in lipoprotein metabolism. More specifically: In Parts I and II, we explored the mechanisms that regulate the expression of the gene encoding apolipoprotein M (apoM) in hepatic cells. ApoM is a recently described apolipoprotein that plays an essential role in HDL maturation in plasma. We analyzed the human apoM promoter and we identified a novel Hormone Response Element (HRE) that serves as a binding site for various members of the hormone nuclear receptor superfamily such as HNF-4 and heterodimers of Retinoid X Receptor (RXR) with receptors for oxysterols (LXR), thyroid hormone (TRβ1) and fibrates (PPAR). This element was further characterized by site directed mutagenesis and was found to mediate the response of the apoM promoter to the above agonists. In a region adjacent to this novel HRE, we identified and characterized a dual specificity regulatory element that binds HNF-1 and 3 members of the AP-1 family of transcription factors (c-Jun and JunB). Using in vitro and in vivo assays we established that activation of PKC by phosrbol esters leads to the activation of Jun proteins which bind to the dual specificity response element and displace HNF-1 leading to transcription repression. Based on these findings, we propose a model that could account for the downregulation of apoM gene expression during infection and inflammation. We also found that a similar mechanism could apply to a second HDL-related gene which encodes for apolipoprotein A-II. In Part III, we investigated the mechanism of downregulation of the gene endocing the ATP Binding Cassettee Transporter A1 (ABCA1) by inflammatory factors. Similar to ApoM, we found that the proximal ABCA1 promoter contains an AP-1 element that binds c-Jun. Activation of c-Jun by Protein Kinase C was found to inhibit ABCA1 promoter activity in hepatic cells. In Parts IV and V we focused on the role of miRNAs in the regulation of the expression of genes that are involved in lipoprotein metabolism. The focus was on the role of let-7b in the regulation of apoE gene expression in macrophages (Part IV) and on the role of miR-122 in the expression of several lipoprotein-related genes in hepatic cells (Part V). We found that the apoE gene, which plays several atheroprotective roles including the biogenesis of HDL, is a direct target of let-7b which binds to apoE 3’ UTR and downregulates its expression. The let-7b target site in apoE was verified by in vitro mutagenesis and functional assays. We also found that let-7b plays a role in the inhibition of apoE gene expression by lipopolysaccharide (LPS, model of bacterial infection) in macrophages. Finally, we found that overexpression of miR122 in hepatic cells was associated with a marked increase in the expression of genes that are involved in lipid metabolism including apoB, ABCA1 and Sterol regulatory Element Binding Proteins (SREBPs). Our findings are in line with previous overexpression and knockdown studies in mice which had shown a positive association between miR-122 expression levels and plasma cholesterol and triglyceride levels, hepatic cholesterol levels and lipid accumulation, as well as fatty acid and cholesterol synthesis. 4 In conclusion, understanding the mechanisms of HDL physiology and regulation is very important for developing novel strategies to increase HDL levels in plasma, to correct its functional abnormalities and to effectively treat patients with CHD.
Language Greek
Subject ABCA-1
Biochemistry
HOL
LET-7
MIR-122
RXR
Αθηροσκλήρωση
Ανοσοαποτύπωση
Απολιποπρωτείνη
Μεταγραφική ρύθμιση
Issue date 2010
Collection   School/Department--School of Medicine--Department of Medicine--Doctoral theses
  Type of Work--Doctoral theses
Permanent Link https://elocus.lib.uoc.gr//dlib/b/7/c/metadata-dlib-1332913699-496975-23868.tkl Bookmark and Share
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