Immunolocalization of high-density lipoproteins in arterial walls of rats

Visualization of the uptake of high-density lipoprotein by rat aortic endothelial cells and smooth muscle cells in vitro

High-density lipoproteins (HDL) were conjugated to Fluorescein 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) or colloidal gold for the investigation of ultrastructural aspects of binding and uptake of HDL by cholesterol-loaded cultured endothelial and smooth muscle cells from rat aorta. When cells were incubated for 2 h at 4 degrees C, HDL-DiI and HDL-gold conjugates were seen only on the cell surface. When cells were returned to incubation at 37 degrees C for 5 min, HDL-DiI appeared in the cytoplasm and colocalized with the fluorescent cholesteryl ester tag BODIPY-FL-C12. HDL-gold conjugates appeared in the plasmalemmal invaginations and plasmalemmal vesicles. After incubation for 15 min, most of the HDL-gold conjugates reappeared on the cell surface. After incubation for 30 min, only a few conjugates were observed and they localized in lysosomal-like bodies. Quantitative data indicated that when the cholesterol-loaded cells were incubated at 4 degrees C for 2 h, the numbers of HDL-gold associated in clusters on the endothelial cell surface was 1.18 clusters/microm. When cells were returned to incubation at 37 degrees C for 5 min, this value decreased to 0.7, increased again to 1.13 at 15 min, and decreased to 0.29 at 30 min. The numbers of clusters in the plasmalemmal invaginations were 0.06 clusters/microm at 4 degrees C for 2 h, increased to 0.34 at 37 degrees C for 5 min and decreased gradually to 0.19 and 0.04 at 15 and 30 min, respectively. The incidence of clusters in the plasmalemmal vesicles per non-nuclear cytoplasm was 0.01 clusters/microm2 at 4 degrees C for 2 h, increased significantly to 1.08 at 37 degrees C for 5 min, and decreased to 0.43 and 0.14 at 15 and 30 min, respectively. This work supports that the plasmalemmal invaginations and plasmalemmal vesicles are linked to the HDL uptake in cholesterol-loaded aortic endothelial cells and smooth muscle cells.

Identification and expression of scavenger receptor SR-BI in endothelial cells and smooth muscle cells of rat aorta in vitro and in vivo

In this study, we used immunoelectron microscopy to investigate the subcellular localization of scavenger receptor class B type I (SR-BI) in the arterial walls of rats. The expression of SR-BI in cultured endothelial and smooth muscle cells of rat aorta after exposure to high-density lipoprotein (HDL) was also investigated by immunofluorescence microscopy and immunoblotting analysis. A peptide containing residues 495-509 from mouse SR-BI (mSR-BI) plus an NH2-terminal cysteine was coupled to hemocyanin to generate mSR-BI antiserum in rabbits. Reactivity of antiserum against the synthetic peptides was confirmed with an enzyme-linked immunosorbent assay (ELISA). The results showed that SR-BI was specifically localized on the surface of the endothelial cells and smooth muscle cells. SR-BI was also observed in the cytoplasm of smooth muscle cells. Immunoblotting analysis indicated that SR-BI was expressed in the cell membrane. The levels of SR-BI increased gradually from 1 to 3 h and decreased at 24 and 48 h after cholesterol-loaded cells were incubated in the culture medium containing HDL. We conclude that SR-BI, a functional receptor for HDL, is expressed in the aortic endothelial cells as well as in smooth muscle cells. This receptor also responds to the presence of HDL in the culture medium.

Effects of high-cholesterol diet on the interendothelial clefts and the associated junctional complexes in rat aorta

The arterial endothelial intercellular cleft (AEC) and its associated junctional complex (JC) are the determinants of permeability to macromolecules. This study analyzed frequencies of AEC and JC profile types in the rat thoracic aorta at 1 and 12 months after feeding the animals with a normal or a high-cholesterol diet. Rats on either a normal diet or high-cholesterol diet for 12 months showed more of the simple 'end to end' or 'overlap' types (P < 0.01) but fewer complex 'interdigitating' type (P < 0.01) of AEC compared to the 1 month group. With regard to JC, the frequencies of gap junctions were decreased (P < 0.01) while the tight junctions and the normal junctionless complex were increased (P < 0.01) after 12 months of normal diet as compared with 1 month on the normal diet. These changes in frequencies for gap junction and tight junction were even greater for the high-cholesterol diet than for the normal diet treatment. Moreover, the incidence of open junctions was also noticeably increased after 12 months of high-cholesterol diet. These findings suggest that the proportions of the AEC and JC were highly responsive to aging whereas those of JC were more susceptible to the high-cholesterol diet treatment.

Biglycan, a vascular proteoglycan, binds differently to HDL2 and HDL3: role of apoE

Lipoprotein retention by vascular extracellular matrix proteoglycans is important in atherogenesis. Proteoglycans bind apolipoprotein (apo)B- and apoE-containing lipoproteins. However, the colocalization of apoA-I and apoE with biglycan in atherosclerotic lesions suggests that vascular proteoglycans also may trap high density lipoproteins (HDLs). Because the major HDL subclasses may be atheroprotective to different degrees, we investigated the role of apoE in mediating HDL(2) and HDL(3) binding to the extracellular vascular proteoglycan, biglycan. ApoE-free HDL(2) and HDL(3) did not bind to purified [(35)S]SO(4)-biglycan, whereas apoE-containing HDL(2) and HDL(3) (HDL+E) did. The extent of binding correlated positively with the apoE content for both HDL(2) and HDL(3), although HDL(2)+E had a 3.5-fold higher affinity than did HDL(3)+E. ApoE on HDL(3) was cleaved into 22- and 12-kDa fragments, whereas apoE on HDL(2) remained intact. These results suggest that the cleaved apoE on HDL(3) results in diminished biglycan binding of HDL(3)+E relative to HDL(2)+E. Reducing positive charges on lysine and arginine residues on HDL+E eliminated biglycan binding, suggesting an ionic interaction. Thus, apoE is an important determinant of HDL binding to extracellular vascular proteoglycans and may play a role in HDL retention in the artery wall.