Mouse macrophage receptor for acetylated low density lipoprotein: demonstration of a fully functional subunit in the membrane and with purified receptor

Class A scavenger receptors, macrophages, and atherosclerosis

The scope of this review is to discuss the new advances in our understanding of the role of scavenger receptor class A in the initiation and modulation of the atherosclerotic process. Through the approaches of gene manipulation in the mouse model, a substantial body of literature has accumulated that depicts scavenger receptor class A as a central player in atherogenesis. In studies of scavenger receptor class A overexpression in macrophages through bone marrow transplantation using transgenic donor material, recipient mice with hyperlipidemia caused either by apolipoprotein E or LDL receptor deficiency did not show convincing changes in the degree of atherosclerosis development compared with controls. Conversely, the deletion of the scavenger receptor class A gene in the mouse has shown, in a consistent and significant fashion, that this receptor serves a pro-atherogenic function under hyperlipidemic conditions, as both apolipoprotein E and LDL receptor-deficient mice had reduced atherosclerosis in the absence of scavenger receptor class A. In addition, we have recently shown that C57BL/6 mice are protected from diet-induced atherosclerosis when they lack scavenger receptor class A, and that the macrophage is the cell type responsible for the effect of scavenger receptor class A deficiency in reducing lesion formation in C57BL/6 and LDL receptor null mice. Together, these results demonstrate that macrophage scavenger receptor class A contributes significantly to atherosclerotic lesion formation, and suggest that the uptake of oxidized or modified lipoproteins by vessel wall macrophages is a central process in atherogenesis.

The binding in vitro of modified LDL to the intermediate filament protein vimentin

Membrane-associated proteins with specific binding properties to modified LDL were investigated in J774 macrophages and Mono Mac 6 sr cells. Ligand blotting of membrane proteins revealed a 54-kDa protein which bound oxidized and acetylated but not native LDL. The 54-kDa protein, isolated by 2D-PAGE, was identified as vimentin. (125)I-AcLDL bound to purified vimentin and desmin in a saturable manner, with an approximate K(d) of 1.7 x 10(-7) M (89 microgram/ml) and 8.0 x 10(-8) M (41 microgram/ml), respectively. Blots of vimentin mutant proteins with deletions in the positively charged N-terminal head domain showed that amino acids 26-39 are essential for the binding of AcLDL by vimentin. Taken together, our data indicate that vimentin binds modified LDL, but not native LDL, in a specific and saturable manner. Vimentin filaments extend throughout the cytoplasm as far as the inner surfaces of plasma and vesicular membranes. Vimentin may thus play a role in membrane-associated steps involved in the intracellular processing of oxidized LDL, contributing to its unregulated uptake and intracellular retention by cells of the atherogenic plaque.

The active streptococcal hyaluronan synthases (HASs) contain a single HAS monomer and multiple cardiolipin molecules

The functional sizes of the two streptococcal hyaluronan synthases (HASs) were determined by radiation inactivation analysis of isolated membranes. The native enzymes in membranes from Group A Streptococcus pyogenes HAS and Group C Streptococcus equisimilis HAS were compared with the recombinant proteins expressed in Escherichia coli membranes. Based on their amino acid sequences, the masses of these four proteins as monomers are approximately 48 kDa. In all cases, loss of enzyme activity was a simple single exponential function with increasing radiation dose. The functional sizes calculated from these data were identical for the four HASs at approximately 64 kDa. In contrast, the sizes of the proteins estimated by the loss of antibody reactivity on Western blots were essentially identical at 41 kDa for the four HAS species, consistently lower than the functional size by approximately 23 kDa. Matrix-assisted laser desorption time of flight mass spectrometry analysis of purified S. pyogenes HAS-H6 and S. equisimilis HAS-H6 gave masses that differed by <0.07% from the predicted monomer sizes, which confirms that neither protein is posttranslationally modified or covalently attached to another protein. Ongoing studies indicate that the purified HAS enzymes require cardiolipin (CL) for maximal activity and stability. When irradiated membranes were detergent solubilized and the extracts were incubated with exogenous CL, the residual level of HAS activity increased. Consequently, the calculated functional size decreased by approximately 23 kDa to the expected size of the HAS monomer. The approximately 23-kDa larger size of the functional HAS enzyme, compared with the HAS monomer, is due, therefore, to CL molecules. We propose that the active streptococcal HA synthases are monomers in complex with approximately 16 CL molecules.

Human beta-migrating very low density lipoprotein induces foam cell formation in human mesangial cells

To elucidate the mechanism of foam cell formation in the mesangial region of a kidney observed in a familial type III hyperlipoproteinemic patient presenting with diabetes mellitus and nephrotic syndrome, we have examined, in the present study, the effect of human beta-VLDL (apo E2/E2) on foam cell formation in human mesangial cells, since an increase in beta-VLDL is a characteristic feature of this patient. Human beta-VLDL (apo E2/E2) induced foam cell formation in human mesangial cells. The binding of [125I]LDL to human mesangial cells was inhibited completely by both LDL and beta-VLDL. On the other hand, the binding of [125I]beta-VLDL was completely inhibited by beta-VLDL, but partially by LDL. The LDL receptor, but not the VLDL receptor was down-regulated by accumulation of cholesteryl esters. These results suggest that human beta-VLDL (apo E2/E2)-induced foam cell formation in mesangial cells is mediated through both the LDL receptor pathway and the beta-VLDL specific pathway, in which the VLDL receptor is one of the candidates.

Hepatic overexpression of bovine scavenger receptor type I in transgenic mice prevents diet-induced hyperbetalipoproteinemia

Hepatic scavenger receptors (SR) may play a protective role by clearing modified lipoproteins before they target the artery wall. To gain insight into this hypothesized function, transgenic mice expressing hepatic bovine SR (TgSR) were created and studied when fed chow, and during diet-induced hyperlipidemia. SR overexpression resulted in extensive hepatic parenchymal cell uptake of fluorescently labeled acetylated human low density lipoprotein (DiI ac-hLDL) and a twofold increase in 125I-acetylated-LDL clearance. Food intake and cholesterol absorption was indistinguishable between control and TgSR mice. In chow-fed mice, lipoprotein cholesterol was similar in control and TgSR mice. However, on a 3-wk high fat/cholesterol (HFHC) diet, the rise in apoB containing lipoproteins was suppressed in TgSR+/- and TgSR+/+ mice. The rise in HDL was similar in control and TgSR+/- mice, but significantly elevated in the TgSR+/+ mice. Overall, on chow, the ratio of apo-B containing lipoprotein cholesterol to HDL cholesterol was similar for all groups (control = 0.33; TgSR+/- = 0.32; TgSR+/+ = 0.38). However, after 3 wk on the HFHC diet, this ratio was markedly higher in control (2.34 +/- 0.21) than in either TgSR+/- (1.00 +/- 0.24) or TgSR+/+ (1.00 +/- 0.19) mice. In TgSR+/- mice, hepatic cholesteryl esters were reduced by 59%, 7 alpha-hydroxylase mRNA levels were elevated twofold, and a significant increase in fecal bile acid flux was observed after the 3-wk HFHC diet. These results suggest SR may play a protective role in liver by preventing diet-induced increases in apoB containing lipoproteins.

Evidence for low-density lipoprotein receptor-mediated uptake of benzoporphyrin derivative

Plasma lipoproteins, such as low-density lipoprotein (LDL), have been proposed to enhance the delivery of hydrophobic photosensitisers to malignant tissue since tumour cells have been shown to have increased numbers of LDL receptors. We have investigated the role of this receptor in the cellular accumulation of the photosensitiser benzoporphyrin derivative (BPD). We observed that: (1) [14C]BPD-LDL accumulation by LDL receptor-negative fibroblast cell lines was insignificant compared with normal cell lines; (2) there was no evidence that BPD dissociated from LDL during incubation with the cells; and (3) chemical acetylation of LDL markedly decreased the uptake of [14C]BPD-LDL. We conclude, therefore, that virtually all of the photosensitiser accumulated by the cells was due to specific binding and internalisation via the LDL receptor. Subsequent in vivo studies in M-1 (methylcholanthrene-induced rhabdomyosarcoma) tumour-bearing DBA/2J mice showed that tumour accumulation of BPD associated with native LDL was significantly (P < 0.01) enhanced over that of acetyl-LDL-associated BPD. These results indicate that the LDL receptor is responsible for the accumulation of LDL-associated BPD both in vitro and in vivo. Thus, utilisation of this delivery system may provide for improvements in photodynamic therapy in clinical practice.

Dominant negative mutations of the scavenger receptor. Native receptor inactivation by expression of truncated variants

The bovine scavenger receptor was truncated at amino acid 266 or 310 to delete either all or part, respectively, of the collagen-like domain. The truncated receptors were inactive in the binding and internalization of acetyl (Ac) low density lipoprotein (LDL). Coexpression of truncated receptor with the native receptor dramatically reduced the percentage of cells internalizing fluorescently labeled Ac LDL, compared with cells expressing the native receptor alone. The mutant truncated at amino acid 266 was most effective in receptor inactivation, resulting in a 42% or 80% decrease in the percentage of cells expressing active receptor when transfected in a 1:1 or 1:2 molar ratio (native:mutant), respectively, with native receptor. Degradation of 125I-Ac LDL was reduced up to 90% when the native and truncated mutant receptors were coexpressed. Scavenger receptor inhibition was specific because the activity of the LDL receptor was not altered. Transient transfection of the mouse macrophage cell line P388D1 with truncated scavenger receptor resulted in a 65% decrease in the uptake and degradation of Ac LDL but did not decrease the degradation of beta-migrating very low density lipoprotein, which is LDL receptor-mediated. These results demonstrate that expression of truncated bovine scavenger receptor inactivates both the native bovine and murine scavenger receptors, producing a dominant negative phenotype in vitro.