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Doctoral theses

Current Record: 2029 of 2427

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Identifier

000366128

Title

Μοριακοί Μηχανισμοί μεταγραφικής ρύθμισης του γονιδίου του μεμβρανικού μεταφορέα λιπιδίων ΑΒCA1 στο ήπαρ

Alternative Title

Mechanisms of transcriptional regulation of the human ABCA1 gene in the liver.

Author

Θυμιακού, Ευσταθία

Thesis advisor

Καρδάσης, Δημήτρης

Reviewer

Ζαννής, Βασίλειος
Γραβάνης, Αχιλλέας
Στουρνάρα, Χρήστος
Μπούμπας, Δημήτριος
Ηλιόπουλος
Παπακωνσταντή

Abstract

Low levels of plasma HDL cholesterol are an independent risk factor for cardiovascular disease. The membrane ABCA1 transporter plays a key role in the biogenesis of HDL by promoting the efflux of cellular cholesterol and phospholipids to lipid-poor apolipoprotein A-I. Mutations in the ABCA1 gene lead to Tangier disease which is characterized by near absent plasma HDL levels, accumulation of foam cells in various tissues and a moderate increase in the incidence of atherosclerosis. Hepatic ABCA1 protein is the major source of plasma HDL while macrophage ABCA1 protects against foam cell formation and atherosclerosis. Thus, the induction of ABCA1 gene expression has been associated with the increase in functional plasma HDL levels and the protection from atherosclerosis. The best characterized and most significant mechanism of ABCA1 gene regulation is via the activation of the nuclear receptors LXRα/RXRα in response to cholesterol loading. The purpose of the present thesis was the study of the molecular mechanisms that affect the induction of the ABCA1 gene via the above-mentioned pathway as well as the identification of new regulatory elements and factors. Initially, we investigated the role of the transcription factor Sp1 which binds to the proximal ABCA1 gene promoter and is necessary for its basal activity in hepatic cells. We showed that, physical and functional interactions between Sp1 and the nuclear receptors LXRα/RXRα were required for the optimal induction of ABCA1 gene transcription in response to their ligands, oxysterols and retinoids. Next, we identified, for the first time, binding sites for the hepatic transcription factor FOXA2/HNF-3β in the ABCA1 gene promoter. Binding of FOXA2 to these sites repressed the basal ABCA1 gene transcription. Furthermore, we showed that binding of FOXA2 to the TATA-box of the ABCA1 gene promoter inhibited the induction by oxysterols and retinoids possibly via competition with the transcription initiation machinery which is assembled to this site. Finally, we investigated the functional promoter that is present in the first intron of the ABCA1 gene. We showed the transcription factor SREBP-1, which is activated by LXRα/RXRα heterodimers in response to oxysterols and retinoids, binds to multiple sites in the proximal intron 1 promoter region. One of these sites overlaps with an AP1 binding site which binds to the transcription factor JunB during monocyte differentiation to macrophages. The ABCG1 transporter, similarly to ABCA1, plays a key role in cholesterol efflux to mature HDL particles and in the protection against atherosclerosis. We investigated the role of a natural mutation in the ABCG1 gene promoter which was identified in the general population in the City of Copenhagen and is associated with increased incidence of ischemic heart disease, myocardial infarction and dyspnea. We showed that this mutation abolishes a binding site for the transcription factor Sp1 and inhibits the induction of the ABCG1 gene promoter by this factor. Understanding in depth the molecular mechanisms that regulate ABCA1 gene expression in the liver and the role of specific transcription factors in these processes is anticipated to lead to the development of new substances that will increase ABCA1 gene expression and as a result, plasma HDL levels, providing protection against atherosclerosis and cardiovascular disease.

Language

Greek

Subject

Atherosclerosis

Biochemistry

Coronary heart disease

HDL

Hepatic transcription factors

Nuclear receptors

Reverse cholesterol transport

Αθηροσκλήρωση ,

Αντίστροφη μεταφορά χοληστερόλης