Atherosclerosis Journal Highlights July 2016
27 July 2016
Posted by: Carmel Hayes
Volume 250 Issue July 2016
By Simona Negrini and Arnold von Eckardstein (Editor–in-Chief)
Low ankle-brachial index (ABI) is associated with increased mortality and increased incidence of cardiovascular events. Tanaka and collaborators investigated the value of borderline ABI in predicting clinical outcomes.
Genome-wide association studies (GWAS) for plasma lipid levels have mapped numerous genomic loci, with each region often containing many protein-coding genes. Targeted re-sequencing of exons is a strategy to pinpoint causal variants and genes.
Data were derived from the Shinken Database 2004–2012, from a single hospital-based cohort study. ABI was measured in 5205 subjects; 4756 subjects, whose ABI was 0.91–1.39, and having no history of peripheral artery disease, were enrolled.
Subjects were classified into two groups: borderline ABI (0.91–1.00; n = 324) and normal ABI (1.01–1.39; n = 4432). Subjects in the borderline ABI group had more comorbidities, including diabetes mellitus, aortic disease, and stroke. Moreover, the borderline ABI group had higher levels of hemoglobin A1c and brain natriuretic peptide, larger diameters of left atrium and left ventricle, and lower levels of estimated glomerular filtration rate and left ventricular ejection fraction. All-cause death and cardiovascular death occurred in 9.3% and 4.6% of the subjects in the borderline ABI group, and in 2.0% and 0.8% of the subjects in the normal ABI group, respectively. An adjusted Cox regression model showed that borderline ABI was associated with a higher incidence of all-cause death and cardiovascular death.
In conclusion, the authors suggested that a borderline ABI was independently associated with worse clinical outcomes in a relatively high-risk population. However, data should be confirmed in larger populations, including those with low-risk profiles.
Strengths and limitations of the approach used and conclusions drawn are further discussed in the commentary by Trovato and Tamura.
Patel and colleagues performed solution-based hybrid selection of 9008 exons at 939 genes within 95 GWAS loci, for plasma lipid levels. Using next-generation sequencing technology, samples from individuals with extremely high and low-to-normal levels of low-density lipoprotein cholesterol, triglycerides, and high-density lipoprotein cholesterol were sequenced.
15,002 missense, nonsense, or splice site variants were identified with a frequency <5%.
Association of coding sequence variants, individually or aggregated within a gene, with plasma lipid levels was assessed, and sequencing analysis was then repeated in 6424 independent participants to reproduce the results obtained.
Across discovery and replication sequencing, the authors identified 6 variants with significant associations with plasma lipids levels. Among them, a novel association was found: p.Ser147Asn variant in APOA4 with triglyceride levels. Moreover, in gene-level association analyses, where rare variants within each gene are collapsed, APOC3 and LDLR were associated with plasma lipid levels.
In summary, after sequencing genes from 95 GWAS loci, in participants with extremely high plasma lipid levels, one new coding variant associated with triglycerides levels was identified.
The authors suggested that these results provide further insights concerning the design of similar sequencing studies with respect to sample size, follow-up, and analysis methodology.
Coronary artery disease (CAD) risk is associated with non-coding genetic variants at the phosphatase and actin regulating protein 1(PHACTR1) gene locus. The PHACTR1 gene encodes an actin-binding protein with phosphatase regulating activity. The mechanism whereby PHACTR1 influences CAD risk is unknown.
Reschen and colleagues hypothesized that PHACTR1 could be expressed in human cell types relevant to CAD and regulated by atherogenic or genetic factors.
The authors showed that PHACTR1 protein was strongly expressed in human atherosclerotic plaque macrophages, lipid-laden foam cells, adventitial lymphocytes and endothelial cells. Moreover, PHACTR1 was expressed as multiple previously uncharacterized transcripts in macrophages, foam cells, lymphocytes and endothelial cells. Immunoblotting confirmed a total absence of PHACTR1 in vascular smooth muscle cells. Real-time quantitative PCR showed that PHACTR1 expression is regulated by atherogenic and inflammatory stimuli.
In aortic endothelial cells, oxLDL and TNF-alpha upregulated the expression of an intermediate-length transcript. A short transcript, expressed only in immune cells, was upregulated in macrophages by oxidized low-density lipoprotein, and oxidized phospholipids, but suppressed by lipopolysaccharide or TNF-alpha. In primary human macrophages, a novel expression quantitative trait locus (eQTL), specific for this short transcript, was identified, whereby the risk allele at CAD risk SNP rs9349379 was associated with reduced PHACTR1 expression, similar to the effect of an inflammatory stimulus.
In conclusion, the authors demonstrated that PHACTR1 is a key atherosclerosis candidate gene since it is regulated by atherogenic stimuli in macrophages and endothelial cells; in addition, they showed an effect of the genetic risk variant on PHACTR1 expression in macrophages similar to that of an inflammatory stimulus.
Macrophage differentiation is associated with development of atherosclerosis and plaque vulnerability and is regulated by the transcription factor MafB.
Hasegawa et al. previously reported that MafB attenuates macrophage apoptosis, which is associated with atherosclerotic plaque instability. With this study, they aimed to further elucidate the role of MafB in the progression of the atherosclerotic plaque.
Macrophage-specific dominant-negative MafB transgenic (DN-MafB) mice were generated and intercrossed with apolipoprotein E knockout (ApoE KO) mice.
Nine weeks after high-cholesterol diet, there was no significant difference in the advanced atherosclerotic lesion area between DN-MafB/ApoE KO mice and littermate control ApoE KO mice. However, DN-MafB/ApoE KO mice showed significantly larger necrotic cores and lower collagen content in atherosclerotic plaques than ApoE KO mice.
Although there was no difference in intraplaque macrophage infiltration and efferocytosis, DN-MafB/ApoE KO mice showed significantly more apoptotic macrophages at the plaque edges than ApoE KO mice. Moreover, after lipopolysaccharide stimulation, peritoneal macrophages of DN-MafB/ApoE KO mice showed a greater increase in matrix metalloproteinase-9 and inflammatory/M1 macrophage markers (tissue necrosis factor-α, interleukin-6, CD11c, and p47phox) than ApoE KO mice.
In conclusion, the authors suggested that macrophage-specific inhibition of MafB may destabilize atherosclerotic plaques in advanced lesions.
Atherosclerosis is both a chronic inflammatory disease and a lipid metabolism disorder. C/EBPβ is well documented for its role in the development of hematopoietic cells and integration of lipid metabolism. However, C/EBPβ role in atherosclerotic progression has not been examined so far.
Rahman et al. assessed the impact of hematopoietic CEBPβ deletion on hyperlipidemia, inflammatory responses, and lesion formation in the aorta, in ApoE−/− mice.
ApoE−/− mice were reconstituted with bone marrow cells derived from either wild type or C/EBPβ−/− mice and placed on low-fat or high-fat/high-cholesterol diet for 11 weeks.
Hematopoietic C/EBPβ deletion in ApoE−/− mice reduced blood and hepatic lipids levels and gene expression of hepatic stearoyl CoA desaturase 1 and fatty acid synthase, while expression of ATP binding cassette transporter G1, cholesterol 7-alpha-hydroxylase, and liver X receptor alpha genes was significantly increased.
Aortic sinuses of ApoE−/− mice reconstituted with C/EBPβ−/− bone marrow cells showed significantly reduced blood cytokine levels and lesion area compared with ApoE−/− mice reconstituted with wild type bone marrow cells.
Silencing of C/EBPβ in RAW264.7 macrophage cells, in conditioned medium, prevented oxLDL-mediated foam cell formation and inflammatory cytokine secretion.
The authors concluded that C/EBPβ in hematopoietic cells is crucial to regulate diet-induced inflammation, hyperlipidemia and atherosclerosis development.
In his commentary, S. Thomas further highlights how this study identifies C/EBPβ transcription factor as a potential therapeutic target relevant for lowering hepatic and circulating lipid levels, as well as attenuating macrophage foam cell formation and inflammatory responses in the arterial wall. Given the emerging role of C/EBPβ in foam cells and the plasticity of macrophages, targeting this transcription factor may represent a viable option for reducing the inflammatory phenotype, while promoting the viability and migratory capacity of lesion macrophages, thereby favouring lesion regression.