Biomedical subjects

Stephen T Thuahnai

Publications and source records attributed to Stephen T Thuahnai.

4 recordsLinked to original sources

PubMed2005

Class B scavenger receptor-mediated intestinal absorption of dietary beta-carotene and cholesterol.

There is now a general consensus that the intestinal absorption of water-insoluble, dietary lipids is protein-mediated, but the assignment of protein(s) to this function is still a matter of debate. To address this issue, we measured beta-carotene and cholesterol absorption in wild-type and SR-BI knockout mice and the uptake of these lipids in vitro using brush border membrane (BBM) vesicles. From the comparison of the in vivo and in vitro results we conclude that both BBM-resident class B scavenger receptors, SR-BI and CD36, can facilitate the absorption of beta-carotene and cholesterol. SR-BI is essential for beta-carotene absorption, at least in mice on a high fat diet. This is due to the fact that the absorption of beta-carotene is restricted to the duodenum and SR-BI is the predominant receptor in the mouse duodenum. In contrast, SR-BI may be involved but is not essential for cholesterol absorption in the small intestine. The question of whether SR-BI contributes to cholesterol absorption in vivo is still unresolved. Transfection of COS-7 cells with SR-BI or CD36 confers on these cells lipid uptake properties closely resembling those of enterocytes and BBM vesicles. Both scavenger receptors facilitate the uptake of dietary lipids such as beta-carotene, free and esterified cholesterol, phospholipids, and fatty acids into COS-7 cells. This lipid uptake is effected from three different lipid donor particles: mixed bile salt micelles, phospholipid small unilamellar vesicles, and trioleoylglycerol emulsions which are all likely to be present in the small intestine. Ezetimibe, a representative of a new class of drugs that inhibit intestinal cholesterol absorption, blocks SR-BI- and CD36-facilitated uptake of cholesterol into COS-7 cells.

Animal Feed↗

PubMed2004

Scavenger receptor class B type I-mediated cholesteryl ester-selective uptake and efflux of unesterified cholesterol. Influence of high density lipoprotein size and structure.

Scavenger receptor (SR)-BI catalyzes the selective uptake of cholesteryl ester (CE) from high density lipoprotein (HDL) by a two-step process that involves the following: 1) binding of HDL to the receptor and 2) diffusion of the CE molecules into the cell plasma membrane. We examined the effects of the size of discoidal HDL particles containing wild-type (WT) apoA-I on selective uptake of CE and efflux of cellular free (unesterified) cholesterol (FC) from COS-7 cells expressing SR-BI to determine the following: 1) the influence of apoA-I conformation on the lipid transfer process, and 2) the contribution of receptor binding-dependent processes to the overall efflux of cellular FC. Large (10 nm diameter) reconstituted HDL bound to SR-BI better (B(max) approximately 420 versus 220 ng of apoA-I/mg cell protein), delivered more CE, and promoted more FC efflux than small ( approximately 8 nm) particles. When normalized to the number of reconstituted HDL particles bound to the receptor, the efficiencies of either CE uptake or FC efflux with these particles were the same indicating that altering the conformation of WT apoA-I modulates binding to the receptor (step 1) but does not change the efficiency of the subsequent lipid transfer (step 2); this implies that binding induces an optimal alignment of the WT apoA-I.SR-BI complex so that the efficiency of lipid transfer is always the same. FC efflux to HDL is affected both by binding of HDL to SR-BI and by the ability of the receptor to perturb the packing of FC molecules in the cell plasma membrane.

Animals↗

PubMed2003

High density lipoprotein structure.

HDL particles possess important antiatherogenic functionalities and understanding of the molecular mechanisms underlying these effects requires detailed knowledge of HDL structure. This review summarizes current understanding of HDL structure. The various HDL subclasses are compared in terms of their lipid and protein compositions. The lipid-binding properties of the principal HDL apolipoprotein, apo A-I, permit plasticity in HDL structure. The amphipathic alpha-helical domains that are the major element of secondary structure mediate the interaction of apo A-I with phospholipid. Low resolution models of the structures of both discoidal and spherical HDL particles are evaluated. HDL particles are dynamic in that they are being remodeled constantly in vivo by interaction with lipases, lipid transfer proteins, and cell-surface HDL receptors. Current knowledge of the ways in which HDL particle structure is modulated by interactions with proteins such as LCAT, CETP, SR-BI and ABCA1 is reviewed.

Humans↗

PubMed2003

A quantitative analysis of apolipoprotein binding to SR-BI: multiple binding sites for lipid-free and lipid-associated apolipoproteins.

Competitive binding experiments were performed using Y1-BS1 adrenal cells to provide information about the interaction of HDL apolipoproteins with scavenger receptor class B, type I (SR-BI). Exchangeable apolipoproteins apolipoprotein A-I (apoA-I), apoA-II, apoE-2, apoE-3, and apoE-4 as phospholipid complexes bind like HDL3 to SR-BI via their multiple amphipathic alpha-helices; the concentrations required to reduce the binding of HDL3 to SR-BI by 50% (IC50) were similar and in the range of 35-50 microgram protein/ml. In the case of apoA-I, peptides corresponding to segments 1-85, 44-65, 44-87, 149-243, and 209-241 all had the same IC50 as each other (P = 0.86), showing that a specific amino acid sequence in apoA-I is not responsible for the interaction with SR-BI. The distribution of charged residues in the amphipathic alpha-helix affects the interaction, with class A and Y helices binding better than class G* helices. Synthetic alpha-helical peptides composed of either l or d amino acids can bind equally to the receptor. Association with phospholipid increases the amount of apolipoprotein binding to SR-BI without altering the affinity of binding. Lipid-free apolipoproteins compete only partially with the binding of HDL to SR-BI, whereas lipidated apolipoproteins compete fully. These results are consistent with the existence of more than one type of apolipoprotein binding site on SR-BI.

Animals↗