5-Azacytidine engages an IRE1α-EGFR-ERK1/2 signaling pathway that stabilizes the LDL receptor mRNA

Hepatic low-density lipoprotein receptor (LDLR) is the primary conduit for the clearance of plasma LDL-cholesterol and increasing its expression represents a central goal for treating cardiovascular disease. However, LDLR mRNA is unstable and undergoes rapid turnover mainly due to the three AU-rich elements (ARE) in its proximal 3'-untranslated region (3'-UTR). Herein, our data revealed that 5-azacytidine (5-AzaC), an antimetabolite used in the treatment of myelodysplastic syndrome, stabilizes the LDLR mRNA through a previously unrecognized signaling pathway resulting in a strong increase of its protein level in human hepatocytes in culture. 5-AzaC caused a sustained activation of the inositol-requiring enzyme 1α (IRE1α) kinase domain and c-Jun N-terminal kinase (JNK) independently of endoplasmic reticulum stress. This resulted in activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase1/2 (ERK1/2) that, in turn, stabilized LDLR mRNA. Systematic mutation of the AREs (ARE1-3) in the LDLR 3'UTR and expression of each mutant coupled to a luciferase reporter in Huh7 cells demonstrated that ARE1 is required for rapid LDLR mRNA decay and 5-AzaC-induced mRNA stabilization via the IRE1α-EGFR-ERK1/2 signaling cascade. The characterization of this pathway will help to reveal potential targets to enhance plasma LDL clearance and novel cholesterol-lowering therapeutic strategies.

Nestin expression is dynamically regulated in cardiomyocytes during embryogenesis

The transcriptional factors implicated in the expression of the intermediate filament protein nestin in cardiomyocytes during embryogenesis remain undefined. In the heart of 9,5-10,5 day embryonic mice, nestin staining was detected in atrial and ventricular cardiomyocytes and a subpopulation co-expressed Tbx5. At later stages of development, nestin immunoreactivity in cardiomyocytes gradually diminished and was absent in the heart of 17,5 day embryonic mice. In the heart of wild type 11,5 day embryonic mice, 54 ± 7% of the trabeculae expressed nestin and the percentage was significantly increased in the hearts of Tbx5^+/- and Gata4^+/- embryos. The cell cycle protein Ki67 and transcriptional coactivator Yap-1 were still prevalent in the nucleus of nestin⁽⁺⁾ -cardiomyocytes identified in the heart of Tbx5^+/- and Gata4^+/- embryonic mice. Phorbol 12,13-dibutyrate treatment of neonatal rat ventricular cardiomyocytes increased Yap-1 phosphorylation and co-administration of the p38 MAPK inhibitor SB203580 led to significant dephosphorylation. Antagonism of dephosphorylated Yap-1 signalling with verteporfin inhibited phorbol 12,13-dibutyrate/SB203580-mediated nestin expression and BrdU incorporation of neonatal cardiomyocytes. Nestin depletion with an AAV9 containing a shRNA directed against the intermediate filament protein significantly reduced the number of neonatal cardiomyocytes that re-entered the cell cycle. These findings demonstrate that Tbx5- and Gata4-dependent events negatively regulate nestin expression in cardiomyocytes during embryogenesis. By contrast, dephosphorylated Yap-1 acting via upregulation of the intermediate filament protein nestin plays a seminal role in the cell cycle re-entry of cardiomyocytes. Based on these data, an analogous role of Yap-1 may be prevalent in the heart of Tbx5^+/- and Gata4^+/- mice.

Lower Methylation of the ANGPTL2 Gene in Leukocytes from Post-Acute Coronary Syndrome Patients

DNA methylation is believed to regulate gene expression during adulthood in response to the constant changes in environment. The methylome is therefore proposed to be a biomarker of health through age. ANGPTL2 is a circulating pro-inflammatory protein that increases with age and prematurely in patients with coronary artery diseases; integrating the methylation pattern of the promoter may help differentiate age- vs. disease-related change in its expression. We believe that in a pro-inflammatory environment, ANGPTL2 is differentially methylated, regulating ANGPTL2 expression. To test this hypothesis we investigated the changes in promoter methylation of ANGPTL2 gene in leukocytes from patients suffering from post-acute coronary syndrome (ACS). DNA was extracted from circulating leukocytes of post-ACS patients with cardiovascular risk factors and from healthy young and age-matched controls. Methylation sites (CpGs) found in the ANGPTL2 gene were targeted for specific DNA methylation quantification. The functionality of ANGPTL2 methylation was assessed by an in vitro luciferase assay. In post-ACS patients, C-reactive protein and ANGPTL2 circulating levels increased significantly when compared to healthy controls. Decreased methylation of specific CpGs were found in the promoter of ANGPTL2 and allowed to discriminate age vs. disease associated methylation. In vitro DNA methylation of specific CpG lead to inhibition of ANGPTL2 promoter activity. Reduced leukocyte DNA methylation in the promoter region of ANGPTL2 is associated with the pro-inflammatory environment that characterizes patients with post-ACS differently from age-matched healthy controls. Methylation of different CpGs in ANGPTL2 gene may prove to be a reliable biomarker of coronary disease.

Epigenetic Regulatory Effect of Exercise on Glutathione Peroxidase 1 Expression in the Skeletal Muscle of Severely Dyslipidemic Mice

Exercise is an effective approach for primary and secondary prevention of cardiovascular diseases (CVD) and loss of muscular mass and function. Its benefits are widely documented but incompletely characterized. It has been reported that exercise can induce changes in the expression of antioxidant enzymes including Sod2, Trx1, Prdx3 and Gpx1 and limits the rise in oxidative stress commonly associated with CVD. These enzymes can be subjected to epigenetic regulation, such as DNA methylation, in response to environmental cues. The aim of our study was to determine whether in the early stages of atherogenesis, in young severely dyslipidemic mice lacking LDL receptors and overexpressing human ApoB100 (LDLR-/-; hApoB+/+), exercise regulates differentially the expression of antioxidant enzymes by DNA methylation in the skeletal muscles that consume high levels of oxygen and thus generate high levels of reactive oxygen species. Expression of Sod2, Txr1, Prdx3 and Gpx1 was altered by 3 months of exercise and/or severe dyslipidemia in 6-mo dyslipidemic mice. Of these genes, only Gpx1 exhibited changes in DNA methylation associated with dyslipidemia and exercise: we observed both increased DNA methylation with dyslipidemia and a transient decrease in DNA methylation with exercise. These epigenetic alterations are found in the second exon of the Gpx1 gene and occur alongside with inverse changes in mRNA expression. Inhibition of expression by methylation of this specific locus was confirmed in vitro. In conclusion, Gpx1 expression in the mouse skeletal muscle can be altered by both exercise and dyslipidemia through changes in DNA methylation, leading to a fine regulation of free radical metabolism.

GRP94 Regulates Circulating Cholesterol Levels through Blockade of PCSK9-Induced LDLR Degradation

Clearance of circulating low-density lipoprotein cholesterol (LDLc) by hepatic LDL receptors (LDLR) is central for vascular health. Secreted by hepatocytes, PCSK9 induces the degradation of LDLR, resulting in higher plasma LDLc levels. Still, it remains unknown why LDLR and PCSK9 co-exist within the secretory pathway of hepatocytes without leading to complete degradation of LDLR. Herein, we identified the ER-resident GRP94, and more precisely its client-binding C-terminal domain, as a PCSK9-LDLR inhibitory binding protein. Depletion of GRP94 did not affect calcium homeostasis, induce ER stress, nor did it alter PCSK9 processing or its secretion but greatly increased its capacity to induce LDLR degradation. Accordingly, we found that hepatocyte-specific Grp94-deficient mice have higher plasma LDLc levels correlated with ∼ 80% reduction in hepatic LDLR protein levels. Thus, we provide evidence that, in physiological conditions, binding of PCSK9 to GRP94 protects LDLR from degradation likely by preventing early binding of PCSK9 to LDLR within the ER.

The epigenetic drug 5-azacytidine interferes with cholesterol and lipid metabolism

DNA methylation and histone acetylation inhibitors are widely used to study the role of epigenetic marks in the regulation of gene expression. In addition, several of these molecules are being tested in clinical trials or already in use in the clinic. Antimetabolites, such as the DNA-hypomethylating agent 5-azacytidine (5-AzaC), have been shown to lower malignant progression to acute myeloid leukemia and to prolong survival in patients with myelodysplastic syndromes. Here we examined the effects of DNA methylation inhibitors on the expression of lipid biosynthetic and uptake genes. Our data demonstrate that, independently of DNA methylation, 5-AzaC selectively and very potently reduces expression of key genes involved in cholesterol and lipid metabolism (e.g. PCSK9, HMGCR, and FASN) in all tested cell lines and in vivo in mouse liver. Treatment with 5-AzaC disturbed subcellular cholesterol homeostasis, thereby impeding activation of sterol regulatory element-binding proteins (key regulators of lipid metabolism). Through inhibition of UMP synthase, 5-AzaC also strongly induced expression of 1-acylglycerol-3-phosphate O-acyltransferase 9 (AGPAT9) and promoted triacylglycerol synthesis and cytosolic lipid droplet formation. Remarkably, complete reversal was obtained by the co-addition of either UMP or cytidine. Therefore, this study provides the first evidence that inhibition of the de novo pyrimidine synthesis by 5-AzaC disturbs cholesterol and lipid homeostasis, probably through the glycerolipid biosynthesis pathway, which may contribute mechanistically to its beneficial cytostatic properties.

Angiopoietin-like 2 promotes atherogenesis in mice

BACKGROUND

Angiopoietin like-2 (angptl2), a proinflammatory protein, is overexpressed in endothelial cells (ECs) from patients with coronary artery disease (CAD). Whether angptl2 contributes to atherogenesis is unknown. We tested the hypothesis that angptl2 promotes inflammation and leukocyte adhesion onto ECs, thereby accelerating atherogenesis in preatherosclerotic dyslipidemic mice.

METHODS AND RESULTS

In ECs freshly isolated from the aorta, basal expression of TNF-α and IL-6 mRNA was higher in 3-month-old severely dyslipidemic mice (LDLr(-/-); hApoB100(+/+) [ATX]) than in control healthy wild-type (WT) mice (P<0.05) and was increased in both groups by exogenous angptl2 (100 nmol/L). Angptl2 stimulated the adhesion of leukocytes ex vivo on the native aortic endothelium of ATX, but not WT mice, in association with higher expression of ICAM-1 and P-selectin in ECs (P<0.05). Antibodies against these endothelial adhesion molecules prevented leukocyte adhesion. Intravenous administration of angptl2 for 1 month in preatherosclerotic 3-month-old ATX mice increased (P<0.05) total cholesterol and LDL-cholesterol levels, strongly induced (P<0.05) the expression of endothelial proinflammatory cytokines and adhesion molecules while accelerating atherosclerotic lesion formation by 10-fold (P<0.05). Plasma and aortic tissue levels of angptl2 increased (P<0.05) with age and were higher in 6- and 12-month-old ATX mice than in age-matched WT mice. Angptl2 accumulated to high levels in the atherosclerotic lesions (P<0.05). Finally, angptl2 was greatly expressed (P<0.05) in ECs cultured from CAD patients, and circulating angptl2 levels were 6-fold higher in CAD patients compared with age-matched healthy volunteers.

CONCLUSIONS

Angptl2 contributes to the pathogenesis of atherosclerosis.

Nestin expression is lost in ventricular fibroblasts during postnatal development of the rat heart and re-expressed in scar myofibroblasts

Studies have reported that the intermediate filament protein nestin was expressed in various non-stem/progenitor cells during development, downregulated during postnatal growth and re-expressed following injury. The present study tested the hypothesis that an analogous paradigm was prevalent for ventricular fibroblasts. In the neonatal rat heart, nestin protein levels were significantly higher than the adult heart and the isolation of cardiac cells revealed a selective expression in ventricular fibroblasts. In adult ventricular fibroblasts, nestin protein expression was markedly lower compared to neonatal ventricular fibroblasts. Following ischemic damage to the rat heart, nestin staining was detected in a subpopulation of scar myofibroblasts (37%) and the percentage of immunoreactive cells was greater than adult ventricular fibroblasts (7%) but significantly lower than neonatal ventricular fibroblasts (86%). Moreover, dissimilar rates of (3)H-thymidine uptake were observed among the fibroblast populations and may be related in part to the disparate percentage of nestin(+) cells. To assess the role of nestin in DNA synthesis, neonatal ventricular fibroblasts were infected with a lentivirus containing a shRNAmir directed against the intermediate filament protein. The partial depletion of nestin expression in neonatal ventricular fibroblasts significantly reduced basal DNA synthesis, in the absence of an apoptotic response. Thus, postnatal development of the rat heart was associated with a selective loss of nestin expression in ventricular fibroblasts and subsequent induction in a subpopulation of myofibroblasts following ischemic injury. The re-expression of nestin in scar myofibroblasts may represent an adaptive response to enhance their proliferative rate and accelerate the healing process.

Stress-induced senescence predominates in endothelial cells isolated from atherosclerotic chronic smokers

Age-associated telomere shortening leads to replicative senescence of human endothelial cells (EC). Risk factors for cardiovascular disease (CVD) accelerate ageing, while there is a concomitant rise in oxidative stress known to promote stress-induced senescence (SIS) in vitro. Of all risk factors for CVD, smoking is most associated with the development of inflammation and accelerated atherosclerosis due to a prooxidant-antioxidant imbalance. We tested the hypothesis that SIS predominates in EC isolated from chronic smokers with premature atherosclerosis undergoing coronary artery bypass graft surgery (CABG). We isolated and cultured EC from segments of internal mammary arteries from smoker, former smoker, and nonsmoker coronary patients. Senescence of EC was induced by serial passage and quantified by the measurement of telomere length and senescence-associated beta-galactosidase activity. Compared with nonsmokers, smoker patients were 10 years younger at the time of CABG, evidence of premature atherosclerosis. Cellular senescence was independent of telomere length and directly related to oxidative damage. EC exhibited higher expression levels of markers of oxidative stress (lipid peroxydation level and caveolin-1 mRNA), inflammation (angiopoietin-like 2 mRNA), hypoxia (vascular endothelial growth factor (VEGF)-A mRNA), and cell damage (p53 mRNA). In conclusion, a high oxidative stress environment in EC isolated from atherosclerotic chronic smokers predisposes to SIS rather than replicative senescence.

Post-translational processing of beta-secretase (beta-amyloid-converting enzyme) and its ectodomain shedding. The pro- and transmembrane/cytosolic domains affect its cellular activity and amyloid-beta production

Processing of the beta-amyloid precursor protein (betaAPP) by beta- and gamma-secretases generates the amyloidogenic peptide Abeta, a major factor in the etiology of Alzheimer's disease. Following the recent identification of the beta-secretase beta-amyloid-converting enzyme (BACE), we herein investigate its zymogen processing, molecular properties, and cellular trafficking. Our data show that among the proprotein convertase family members, furin is the major converting enzyme of pro-BACE into BACE within the trans-Golgi network of HK293 cells. While we demonstrate that the 24-amino acid prosegment is required for the efficient exit of pro-BACE from the endoplasmic reticulum, it may not play a strong inhibitory role since we observe that pro-BACE can produce significant quantities of the Swedish mutant betaAPP(sw) beta-secretase product C99. BACE is palmitoylated at three Cys residues within its transmembrane/cytosolic tail and is sulfated at mature N-glycosylated moieties. Data with three different antibodies show that a small fraction of membrane-bound BACE is shed into the medium and that the extent of ectodomain shedding is palmitoylation-dependent. Overexpression of full-length BACE causes a significant increase in the production of C99 and a decrease in the alpha-secretase product APPsalpha. Although there is little increase in the generation of Abeta by full-length BACE, overexpression of either a soluble form of BACE (equivalent to the shed form) or one lacking the prosegment leads to enhanced Abeta levels. These findings suggest that the shedding of BACE may play a role in the amyloidogenic processing of betaAPP.

cDNA structure, tissue distribution, and chromosomal localization of rat PC7, a novel mammalian proprotein convertase closest to yeast kexin-like proteinases

By using reverse transcription-coupled PCR on rat anterior pituitary RNA, we isolated a 285-bp cDNA coding for a novel subtilisin/kexin-like protein convertase (PC), called rat (r) PC7. By screening rat spleen and PC12 cell lambda gt11 cDNA libraries, we obtained a composite 3.5-kb full-length cDNA sequence of rPC7. The open reading frame codes for a prepro-PC with a 36-amino acid signal peptide, a 104-amino acid prosegment ending with a cleavable RAKR sequence, and a 747-amino acid type I membrane-bound glycoprotein, representing the mature form of this serine proteinase. Phylogenetic analysis suggests that PC7 represents the most divergent enzyme of the mammalian convertase family and that it is the closest member to the yeast convertases krp and kexin. Northern blot analyses demonstrated a widespread expression with the richest source of rPC7 mRNA being the colon and lymphoid-associated tissues. In situ hybridization revealed a distinctive tissue distribution that sometimes overlaps with that of furin, suggesting that PC7 has widespread proteolytic functions. The gene for PC7 (Pcsk7) was mapped to mouse chromosome 9 by linkage analysis of an interspecific backcross DNA panel.