Dimitris Kardassis

Transcriptional Regulation of the Human Apolipoprotein Genes

The transcription of eukaryotic genes is controlled by the interaction of regulatory gene sequences (promoter elements) with specific nuclear proteins (transcription factors) (1–3). The interaction of the transcription factors with the promoter elements controls: a) tissue specific gene expression (4–6); b) gene expression during differentiation and development (7,8); and c) gene expression in response to intracellular and extracellular stimuli such as hormones and metabolites (9–12).

Characterization of the promoter elements required for hepatic and intestinal transcription of the human apoB gene: definition of the DNA-binding site of a tissue-specific transcriptional factor.

The promoter elements important for intestinal and hepatic transcription of the human apoB gene have been localized downstream of nucleotide -150. Footprinting analysis using hepatic nuclear extracts identified four protected regions, -124 to -100, -97 to -93, -86 to -33, and +33 to +52. Gel electrophoretic mobility shift assays showed that multiple factors interact with the apoB sequence -86 to -33, while the region -88 to -61 binds a single nuclear factor. Methylation interference analysis and nucleotide substitution mutagenesis identified the binding site of the factor between residues -78 and -68. Binding competition experiments indicate that this factor recognizes the regulatory elements of other liver-specific genes.

Pleiotropic Functions of HDL Lead to Protection from Atherosclerosis and Other Diseases

High density lipoprotein (HDL) is a macromolecular complex of proteins and lipids that is produced primarily by the liver through a complex pathway that requires initially the functions of apolipoprotein A-I (apoA-I), ATP binding cassette transporter A1 (ABCA1) and lecithin:cholesterol acetyl transferase (LCAT) (Zannis et al., 2006b). Following synthesis, HDL affects the functions of the arterial wall cells through signaling mechanisms mediated by scavenger receptor class B type-I (SR-BI) and other cell surface proteins. The impetus for studying HDL has been the inverse correlation that exists between plasma HDL levels and the risk for coronary artery disease (CAD) (Gordon et al., 1989). HDL promotes cholesterol efflux (Gu et al., 2000; Nakamura et al., 2004), prevents oxidation of low density lipoprotein (LDL) (Navab et al., 2000a; Navab et al., 2000b), inhibits expression of proinflammatory cytokines by macrophages (Okura et al., 2010) as well as expression of adhesion molecules by endothelial cells (Cockerill et al., 1995; Nicholls et al,. 2005b). HDL inhibits cell apoptosis (Nofer et al., 2001) and promotes endothelial cell proliferation and migration (Seetharam et al., 2006). HDL stimulates release of nitric oxide (NO) from endothelial cells thus promoting vasodilation (Mineo et al., 2003). HDL also inhibits platelet aggregation and thrombosis (Dole et al., 2008) and has antibacterial, antiparasitic and antiviral activities (Parker et al., 1995; Singh et al., 1999; Vanhollebeke and Pays, 2010). Due to these properties HDL is thought to protect the endothelium and inhibit several steps in the cascade of events that lead to the pathogenesis of atherosclerosis and various other human diseases. This review focuses on two important aspects of contemporary HDL research. The first part considers briefly the structure of apoA-I and HDL and the key proteins that participate in the pathway of the biogenesis of HDL as well as clinical phenotypes associated with HDL abnormalities. The second part considers various physiological functions of HDL and apoA-I and the protective role of HDL against atherosclerosis and other diseases.

Regulation of ApoA-I Gene Expression and Prospects to Increase Plasma ApoA-I and HDL Levels

In humans, the apolipoprotein A-I (apoA-I) gene is expressed abundantly in liver and intestine, and to a lesser extent in other tissues. Following synthesis, apoA-I is secreted in plasma and proceeds to participate in the formation of high density lipoprotein (HDL). In the absence of apoA-I, HDL is not formed.

Molecular Biology of Human Apolipoprotein B and Related Diseases

Apolipoprotein B is the main protein component of LDL and comprises 23.8% of the LDL particle (1,2). ApoB is the ligand for the cellular recognition and catabolism of LDL by the LDL receptor (1,3). The LDL receptor-apoB interaction and subsequent catabolism mediates the clearance of LDL from plasma and regulates cellular cholesterol biosynthesis (1,3). Early studies had shown that the human and rat apoB exist in two primary forms designated apoB-100 and apoB-48 (4–12). Apolipoprotein B seems to be important for lipoprotein assembly and secretion (14,15). Thus, absence of apoR in abetalipoproteinemia (16) or inhibition of its synthesis by cyclohexamide treatment of cells (15) abolishes secretion of chylomicrons and VLDL.