Interplay between apolipoprotein E and scavenger receptor class B type I controls coronary atherosclerosis and lifespan in the mouse

Macrophages in Atherosclerosis, First or Second Row Players?

Macrophages represent a cell type that has been widely described in the context of atherosclerosis since the earliest studies in the 17th century. Their role has long been considered to be preponderant in the onset and aggravation of atherosclerosis, in particular by participating in the establishment of a chronic inflammatory state by the release of pro-inflammatory cytokines and by uncontrolled engorgement of lipids resulting in the formation of foam cells and later of the necrotic core. However, recent evidence from mouse models using an elegant technique of tracing vascular smooth muscle cells (VSMCs) during plaque development revealed that resident VSMCs display impressive plastic properties in response to an arterial injury, allowing them to switch into different cell types within the plaque, including mesenchymal-like cells, macrophage-like cells and osteochondrogenic-like cells. In this review, we oppose the arguments in favor or against the influence of macrophages versus VSMCs in all stages of atherosclerosis including pre-atherosclerosis, formation of lipid-rich foam cells, development of the necrotic core and the fibrous cap as well as calcification and rupture of the plaque. We also analyze the relevance of animal models for the investigation of the pathophysiological mechanisms of atherosclerosis in humans, and discuss potential therapeutic strategies targeting either VSMCs or macrophage to prevent the development of cardiovascular events. Overall, although major findings have been made from animal models, efforts are still needed to better understand and therefore prevent the development of atherosclerotic plaques in humans.

PMP22 is critical for actin-mediated cellular functions and for establishing lipid rafts

Haploinsufficiency of peripheral myelin protein 22 (PMP22) causes hereditary neuropathy with liability to pressure palsies, a peripheral nerve lesion induced by minimal trauma or compression. As PMP22 is localized to cholesterol-enriched membrane domains that are closely linked with the underlying actin network, we asked whether the myelin instability associated with PMP22 deficiency could be mediated by involvement of the protein in actin-dependent cellular functions and/or lipid raft integrity. In peripheral nerves and cells from mice with PMP22 deletion, we assessed the organization of filamentous actin (F-actin), and actin-dependent cellular functions. Using in vitro models, we discovered that, in the absence of PMP22, the migration and adhesion capacity of Schwann cells and fibroblasts are similarly impaired. Furthermore, PMP22-deficient Schwann cells produce shortened myelin internodes, and display compressed axial cell length and collapsed lamellipodia. During early postnatal development, F-actin-enriched Schmidt-Lanterman incisures do not form properly in nerves from PMP22(-/-) mice, and the expression and localization of molecules associated with uncompacted myelin domains and lipid rafts, including flotillin-1, cholesterol, and GM1 ganglioside, are altered. In addition, we identified changes in the levels and distribution of cholesterol and ApoE when PMP22 is absent. Significantly, cholesterol supplementation of the culture medium corrects the elongation and migration deficits of PMP22(-/-) Schwann cells, suggesting that the observed functional impairments are directly linked with cholesterol deficiency of the plasma membrane. Our findings support a novel role for PMP22 in the linkage of the actin cytoskeleton with the plasma membrane, likely through regulating the cholesterol content of lipid rafts.

rs5888 variant of SCARB1 gene is a possible susceptibility factor for age-related macular degeneration

Major genetic factors for age-related macular degeneration (AMD) have recently been identified as susceptibility risk factors, including variants in the CFH gene and the ARMS2 LOC387715/HTRA1locus. Our purpose was to perform a case-control study in two populations among individuals who did not carry risk variants for CFHY402H and LOC387715 A69S (ARMS2), called “study” individuals, in order to identify new genetic risk factors. Based on a candidate gene approach, we analyzed SNP rs5888 of the SCARB1 gene, coding for SRBI, which is involved in the lipid and lutein pathways. This study was conducted in a French series of 1241 AMD patients and 297 controls, and in a North American series of 1257 patients with advanced AMD and 1732 controls. Among these individuals, we identified 61 French patients, 77 French controls, 85 North American patients and 338 North American controls who did not carry the CFH nor ARMS2 polymorphisms. An association between AMD and the SCARB1 gene was seen among the study subjects. The genotypic distribution of the rs5888 polymorphism was significantly different between cases and controls in the French population (p<0.006). Heterozygosity at the rs5888 SNP increased risk of AMD compared to the CC genotypes in the French study population (odds ratio (OR) = 3.5, CI95%: 1.4–8.9, p<0.01) and after pooling the 2 populations (OR = 2.9, 95% CI: 1.6–5.3, p<0.002). Subgroup analysis in exudative forms of AMD revealed a pooled OR of 3.6 for individuals heterozygous for rs5888 (95% CI: 1.7–7.6, p<0.0015). These results suggest the possible contribution of SCARB1, a new genetic factor in AMD, and implicate a role for cholesterol and antioxidant micronutrient (lutein and vitamin E) metabolism in AMD.

Hepatic uptake and metabolism of phosphatidylcholine associated with high density lipoproteins

BACKGROUND

Phosphatidylcholine (PC) is the predominant phospholipid associated with high density lipoproteins (HDL). Although the hepatic uptake of cholesteryl esters from HDL is well characterized, much less is known about the fate of PC associated with HDL. Thus, we investigated the uptake and subsequent metabolism of HDL-PC in primary mouse hepatocytes.

METHODS AND RESULTS

The absence of scavenger receptor-BI resulted in a 30% decrease in cellular incorporation of [(3)H]PC whereas [(3)H]cholesteryl ether uptake was almost completely abolished. Although endocytosis is not involved in the uptake of cholesteryl esters from HDL, we demonstrate that HDL internalization accounts for 40% of HDL-PC uptake. Extracellular remodeling of HDL by secretory phospholipase A(2) significantly enhances HDL lipid uptake. HDL-PC taken up by hepatocytes is partially converted to triacylglycerols via PC-phospholipase C-mediated hydrolysis of PC and incorporation of diacylglycerol into triacylglycerol. The formation of triacylglycerol is independent of scavenger receptor-BI and occurs in extralysosomal compartments.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These findings indicate that HDL-associated PC is incorporated into primary hepatocytes via a pathway that differs significantly from that of HDL-cholesteryl ester, and shows that HDL-PC is more than a framework molecule, as evidenced by its partial conversion to hepatic triacylglycerol.

Apolipoprotein E-/- mice have delayed skeletal muscle healing after hind limb ischemia-reperfusion

INTRODUCTION

Classic studies of limb ischemia-reperfusion injury have been performed using young healthy mice. However, patients with peripheral vascular disease are older and often exhibit metabolic derangements that may delay healing after revascularization. Mice with genetic deletion of apolipoprotein E (ApoE(-/-)) have been used as a model in various experimental scenarios of hypercholesterolemia. These experiments evaluated the inflammatory response and changes in skeletal muscle morphology during the acute and chronic phases of limb ischemia-reperfusion injury in aged ApoE(-/-) mice.

METHODS

Age-matched ApoE(-/-) and wild-type (Wt) mice underwent 1.5 hours of unilateral hind limb ischemia, followed by 1, 7, or 14 days of reperfusion (DR). Histologic analysis of skeletal muscle fiber injury was assessed at 1DR. Morphologic evidence of muscular fiber maturation was assessed at 14DR. Levels of MyoD and myogenin, markers of skeletal muscle differentiation, were assessed at 7 and 14DR using Western blots. Markers of inflammation, including myeloperoxidase, macrophage inflammatory protein-2 (MIP-2), monocyte chemotactic protein-1 (MCP-1), and osteopontin, were assayed using enzyme-linked immunosorbent assay and chemokine (C-C motif) receptor 2 (CCR2) using Western blots at 1, 7, and 14DR. After 1DR, tissue adenosine 5'-triphosphate (ATP) levels were measured to assess metabolic activity. Unpaired t test and Mann-Whitney test were used for comparisons.

RESULTS

Histologic evaluation of skeletal muscle after 1DR showed no difference in the degree of injury between Wt and ApoE(-/-) mice. However, at 14DR, ApoE(-/-) mice had higher percentage of immature muscle fibers than Wt mice. Myogenin level was lower in the ApoE(-/-) mice at 7DR. Injured skeletal muscle of ApoE(-/-) mice had lower levels of myeloperoxidase than Wt mice at 7 DR and higher levels of MCP-1 at 14DR. There was no difference in the levels of tissue ATP, MIP-2, osteopontin, or CCR2 at all experimental intervals.

CONCLUSION

Although there was no difference between the injured muscle of Wt and ApoE(-/-) mice during the acute phase of reperfusion, ApoE(-/-) mice showed delay in skeletal muscle healing during the chronic phase of reperfusion. This lag in muscle regeneration was associated with lower levels of myogenin at 7DR and an increased level of MCP-1 at 14DR in the ApoE(-/-) mice. The delay in skeletal muscle healing in the ApoE(-/-) mice may have broader implications for poor tissue healing and functional recovery in elderly patients who have vascular risk factors such as hypercholesterolemia.

Reduced oxidant stress and extended lifespan in mice exposed to a low glycotoxin diet: association with increased AGER1 expression

Aging is accompanied by increased oxidative stress (OS) and accumulation of advanced glycation end products (AGEs). AGE formation in food is temperature-regulated, and ingestion of nutrients prepared with excess heat promotes AGE formation, OS, and cardiovascular disease in mice. We hypothesized that sustained exposure to the high levels of pro-oxidant AGEs in normal diets (Reg(AGE)) contributes to aging via an increased AGE load, which causes AGER1 dysregulation and depletion of anti-oxidant capacity, and that an isocaloric, but AGE-restricted (by 50%) diet (Low(AGE)), would decrease these abnormalities. C57BL6 male mice with a life-long exposure to a Low(AGE) diet had higher than baseline levels of tissue AGER1 and glutathione/oxidized glutathione and reduced plasma 8-isoprostanes and tissue RAGE and p66(shc) levels compared with mice pair-fed the regular (Reg(AGE)) diet. This was associated with a reduction in systemic AGE accumulation and amelioration of insulin resistance, albuminuria, and glomerulosclerosis. Moreover, lifespan was extended in Low(AGE) mice, compared with Reg(AGE) mice. Thus, OS-dependent metabolic and end organ dysfunction of aging may result from life-long exposure to high levels of glycoxidants that exceed AGER1 and anti-oxidant reserve capacity. A reduced AGE diet preserved these innate defenses, resulting in decreased tissue damage and a longer lifespan in mice.

Macrophage apolipoprotein E reduces atherosclerosis and prevents premature death in apolipoprotein E and scavenger receptor-class BI double-knockout mice

OBJECTIVE

Mice null for both apolipoprotein (apo)E and scavenger receptor (SR)-BI (DKO) develop severe hypercholesterolemia, occlusive coronary atherosclerosis, myocardial infarction, and premature death. The current study examines the ability of macrophage apoE to improve the dyslipidemia, reduce atherosclerosis, and rescue the lethal phenotype of DKO mice.

METHODS AND RESULTS

Initially, bone marrow transplantation (BMT) was unsuccessful, because the DKO mice died from a rapidly fatal anemia 3 to 5 days after lethal irradiation. Therefore, probucol was used to rescue the DKO mice during BMT and was discontinued 2-weeks after BMT, allowing successful reconstitution with donor marrow. Twelve male apoE(-/-)SR-BI(-/-) mice fed 0.5% probucol in a chow diet were lethally irradiated and transplanted with either wild-type (WT) or DKO bone marrow. Two-weeks after BMT, apoE was detected in serum in WT-->DKO mice, and mean serum cholesterol levels were reduced by 70% versus DKO-->DKO mice. Lipoprotein profiles and HDL subpopulations in WT-->DKO mice were similar to apoE(+/+)SR-BI(-/-)-->DKO mice and resembled those of SR-BI(-/-) mice. In WT-->DKO mice, aortic atherosclerosis was reduced by 88% to 90% versus DKO-->DKO mice. Furthermore, the DKO-->DKO mice died &8 weeks after BMT, whereas WT-->DKO mice exhibited a life span >40 weeks after BMT.

CONCLUSIONS

Macrophage apoE is able to rescue the lethal phenotype of apoE(-/-)SR-BI(-/-) mice by improving the dyslipidemia and dramatically reducing atherosclerotic lesion development.