Pathological Roles of Advanced Glycation End Product Receptors SR-A and CD36

The pathological significance of advanced glycation end product (AGE)-modified proteins deposited in several lesions is generally accounted for by their cellular interaction via the AGE receptors and subsequent acceleration of the inflammatory process. In this study, we focused on two AGE receptors-specifically, the role of SR-A in pathogenesis of diabetic nephropathy and the role of CD36 in AGE-induced downregulation of leptin by adipocytes. In terms of SR-A, diabetic wild-type mice exhibited increased urinary albumin excretion, glomerular hypertrophy, and mesangial matrix expansion, whereas SR-A-knockout mice showed reduced glomerular size and mesangial matrix area. In these diabetic SR-A-knockout mice, the number of macrophages that infiltrated into glomeruli was remarkably reduced (P < 0.05), suggesting that SR-A-dependent glomerular migration of macrophages plays an important role in the pathogenesis of diabetic nephropathy. In terms of CD36, incubation of glycolaldehyde-modified bovine serum albumin (GA-BSA) with 3T3-L1 adipocytes reduced leptin secretion by these cells. The binding of GA-BSA to these cells and subsequent endocytic degradation were effectively inhibited by a neutralizing anti-CD36 antibody. AGE-induced downregulation of leptin was protected by N-acetyl-cysteine, an antioxidant. These results indicate that the interaction of AGE ligands with 3T3-L1 adipocytes via CD36 induces oxidative stress and leads to inhibition of leptin expression by these cells, suggesting a potential link of this phenomenon to exacerbation of the insulin sensitivity in metabolic syndrome.

Glycolaldehyde-modified bovine serum albumin downregulates leptin expression in mouse adipocytes via a CD36-mediated pathway

Previous observations by us have clarified that proteins modified by advanced glycation end products (AGEs) are recognized as effective ligands by CD36-overexpressed CHO cells and undergo receptor-mediated endocytosis. CD36, a member of the class B scavenger receptor family, also acts as a fatty acid transporter in adipocytes. Oxidized low-density lipoprotein (Ox-LDL), a ligand for CD36, is known to upregulate CD36 by activating peroxisome proliferator-activated receptor gamma (PPAR-gamma) in macrophages, whereas PPAR-gamma ligands such as troglitazone and 15-deoxy-delta12,14-prostaglandin J2 decrease leptin secretion from adipocytes. The purpose of this study was to examine effects of AGE ligands on leptin expression in adipocytes. Glycolaldehyde-modified bovine serum albumin (GA-BSA) decreased leptin expression at both the protein and mRNA levels in 3T3-L1 adipocytes and mouse epididymal adipocytes. The binding to and subsequent endocytic degradation of GA-BSA by 3T3-L1 adipocytes were effectively inhibited by a neutralizing anti-CD36 antibody. These results indicate that the ligand interaction of GA-BSA with CD36 leads to downregulation of leptin expression in 3T3-L1 adipocytes, suggesting that AGE-induced leptin downregulation is linked to reduction of the insulin sensitivity in metabolic syndrome.

Human urotensin II accelerates foam cell formation in human monocyte-derived macrophages

Human urotensin II (U-II), the most potent vasoconstrictor peptide identified to date, and its receptor (UT) are involved in hypertension and atherosclerosis. Acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) converts intracellular free cholesterol into cholesterol ester (CE) for storage in lipid droplets and plays an important role in the formation of macrophage-derived foam cells in atherosclerotic lesions. We examined the effects of U-II on ACAT-1 expression and CE accumulation in human monocyte-derived macrophages. U-II increased ACAT activity in a concentration-dependent manner after 7 days in monocyte primary culture. Immunoblotting analysis showed that U-II at 25 nmol/L increased ACAT-1 protein expression level by 2.5-fold, which was completely abolished by anti-U-II antibody, selective UT receptor antagonists (urantide and 4-aminoquinoline), a G-protein inactivator (GDP-beta-S), a c-Src protein tyrosine kinase inhibitor (PP2), a protein kinase C (PKC) inhibitor (rottlerin), a mitogen-activated protein kinase kinase (MEK) inhibitor (PD98059), or a Rho kinase (ROCK) inhibitor (Y27632). Northern blotting analysis indicated that among the 4 ACAT-1 mRNA transcripts (2.8-, 3.6-, 4.3-, and 7.0-kb), the 2.8- and 3.6-kb transcript levels were selectively upregulated by approximately 1.7-fold by U-II (25 nmol/L). Further, U-II (25 nmol/L) significantly increased acetylated LDL (acetyl-LDL)-induced CE accumulation in monocyte-derived macrophages but not scavenger receptor class A (SR-A) function as assessed by endocytic uptake of [(125)I]acetyl-LDL. Our results suggest that U-II may play a novel role in the formation of macrophage-derived foam cells by upregulating ACAT-1 expression via the UT receptor/G-protein/c-Src/PKC/MEK and ROCK pathways but not by SR-A, thus contributing to the relatively rapid development of atherosclerosis in hypertension.

Advanced glycation end products-modified proteins and oxidized LDL mediate down-regulation of leptin in mouse adipocytes via CD36

Advanced glycation end products (AGE)-modified proteins as well as oxidized-LDL (Ox-LDL) undergo receptor-mediated endocytosis by CHO cells overexpressing CD36, a member of class B scavenger receptor family. The purpose of the present study was to examine the effects of glycolaldehyde-modified BSA (GA-BSA) as an AGE-ligand and Ox-LDL on leptin expression in adipocytes. GA-BSA decreased leptin expression at both protein and mRNA levels in 3T3-L1 adipocytes and mouse epididymal adipocytes. Ox-LDL showed a similar inhibitory effect on leptin expression in 3T3-L1 adipocytes, which effect was protected by N-acetylcysteine, a reactive oxygen species (ROS) inhibitor. Binding of (125)I-GA-BSA or (125)I-Ox-LDL to 3T3-L1 adipocytes and subsequent endocytic degradation were inhibited by a neutralizing anti-CD36 antibody. Furthermore, this antibody also suppressed Ox-LDL-induced leptin down-regulation. These results clarify that the interaction of GA-BSA and Ox-LDL with CD36 leads to down-regulation of leptin expression via ROS system(s) in 3T3-L1 adipocytes, suggesting that a potential link of AGE- and/or Ox-LDL-induced leptin down-regulation might be linked to insulin-sensitivity in metabolic syndrome.

Cytotoxic effect of oxidized low density lipoprotein on macrophages

Macrophage or macrophage-derived foam cell death is one of the characteristic events in the development of cell-poor lipid-rich cores of the advanced atherosclerotic plaques. Although the in vivo mechanism for the death of macrophages is unclear, one possible candidate for the agent which induces macrophage cell death is oxidized low density lipoprotein (Ox-LDL). To investigate the mechanism of Ox-LDL-induced macrophage cell death, we have recently employed macrophage cell genetics and isolated mutant cells resistant to the cytotoxic effect of Ox-LDL from mutagenized populations of murine macrophage-derived J774 cells (Hakamata, H., Miyazaki, A., Sakai, M., Matsuda, H., Suzuki, H., Kodama, T., and Horiuchi, S. (1998) J. Lipid Res. 39, 482-494). The results obtained showed that one mutant form, JO21b cells, was characterized by reduced expression of type I and type II class A macrophage scavenger receptors (MSR-AI/AII) with a concomitant decrease in the uptake of Ox-LDL. Moreover, peritoneal macrophages obtained from MSR-AI/AII-knockout mice showed a higher resistance to the cytotoxic effect of Ox-LDL compared to those of their wild-type littermates. From these results, we have concluded that Ox-LDL cytotoxicity to macrophages is enhanced by effective endocytic uptake of Ox-LDL through MSR-AI/AII. These findings imply a possibility that formation of the cell-poor lipid-rich core is also enhanced by MSR-AI/AII-mediated uptake of Ox-LDL and subsequent macrophage cell death in atherosclerotic lesions.

Reconstituted high density lipoprotein reduces the capacity of oxidatively modified low density lipoprotein to accumulate cholesteryl esters in mouse peritoneal macrophages

Oxidized low density lipoprotein (ox-LDL) was incubated with discoidal complexes of apolipoprotein A-I (apo A-I) and dimyristoylphosphatidylcholine (DMPC) (DMPC/apo A-I) in a cell-free system and re-isolated on Sephacryl S-400 gel filtration chromatography. Analyses of re-isolated ox-LDL showed that apo A-I was transferred from DMPC/apo A-I to ox-LDL, which accounted for 10% of the total protein of ox-LDL. Re-isolated ox-LDL also showed a 2.2-fold increase in phospholipid and a 14% decrease in cholesterol content on an apo B basis. The electrophoretic mobility of re-isolated ox-LDL was markedly reduced almost to that of native LDL. Moreover, the amounts of re-isolated ox-LDL to be degraded by mouse peritoneal macrophages as well as the capacity of re-isolated ox-LDL to accumulate cholesteryl esters (CE) in these cells were markedly reduced (60% and 80% reduction, respectively), suggesting that the ligand activity of ox-LDL for the scavenger receptor was significantly reduced upon treatment with DMPC/apo A-I. Parallel incubation of ox-LDL with free apo A-I led to a similar incorporation of apo A-I into ox-LDL. However, it had no effects on the ligand activity of ox-LDL. Thus, it is likely that the reduction in the ligand activity of ox-LDL by DMPC/apo A-I is explained by the change in the lipid moiety (mainly phospholipid) of ox-LDL. Since discoidal high density lipoprotein (HDL) is known to occur in vivo, this phenomenon might explain one of the anti-atherogenic functions of HDL.

Differential effects of an acyl-coenzyme A:cholesterol acyltransferase inhibitor on HDL-induced cholesterol efflux from rat macrophage foam cells

When rat macrophages were converted to foam cells with acetylated low density lipoprotein (acetyl-LDL) and then reacted with high density lipoprotein (HDL) and an inhibitor of acyl-coenzyme A:cholesterol acyltransferase (58-035) (sequential incubation system), 58-035 did not enhance HDL-induced cholesterol efflux. In contrast, when macrophages were exposed to acetyl-LDL in the presence of both HDL and 58-035 (simultaneous incubation system), HDL-induced cholesterol efflux was enhanced 1.6-fold by 58-035. Cholesterol efflux with HDL alone was 2-fold greater in simultaneous incubation than in sequential incubation. These results suggest the presence of an efficient cholesterol efflux pathway in simultaneous incubation which is not available in sequential incubation. This pathway, which we refer to as the neutral cholesterol ester hydrolase-independent pathway, is characterized by the efflux of lysosome-derived cholesterol without re-esterification.