Doctoral theses
Current Record: 2069 of 2491
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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
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Μόσιαλου, Ιωάννα
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Thesis advisor
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Καρδάσης, Δ.
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Reviewer
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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.
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Language |
Greek |
Subject |
ABCA-1 |
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Biochemistry |
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HOL |
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LET-7 |
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MIR-122 |
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RXR |
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Αθηροσκλήρωση |
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Ανοσοαποτύπωση |
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Απολιποπρωτείνη |
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Μεταγραφική ρύθμιση |
Issue date |
2010 |
Collection
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School/Department--School of Medicine--Department of Medicine--Doctoral theses
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Type of Work--Doctoral theses
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Permanent Link |
https://elocus.lib.uoc.gr//dlib/b/7/c/metadata-dlib-1332913699-496975-23868.tkl
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Views |
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