A Systematic Review of Lipid-Focused Cardiovascular Disease Research: Trends and Opportunities

Lipids are important modifiers of protein function, particularly as parts of lipoproteins, which transport lipophilic substances and mediate cellular uptake of circulating lipids. As such, lipids are of particular interest as blood biological markers for cardiovascular disease (CVD) as well as for conditions linked to CVD such as atherosclerosis, diabetes mellitus, obesity and dietary states. Notably, lipid research is particularly well developed in the context of CVD because of the relevance and multiple causes and risk factors of CVD. The advent of methods for high-throughput screening of biological molecules has recently resulted in the generation of lipidomic profiles that allow monitoring of lipid compositions in biological samples in an untargeted manner. These and other earlier advances in biomedical research have shaped the knowledge we have about lipids in CVD. To evaluate the knowledge acquired on the multiple biological functions of lipids in CVD and the trends in their research, we collected a dataset of references from the PubMed database of biomedical literature focused on plasma lipids and CVD in human and mouse. Using annotations from these records, we were able to categorize significant associations between lipids and particular types of research approaches, distinguish non-biological lipids used as markers, identify differential research between human and mouse models, and detect the increasingly mechanistic nature of the results in this field. Using known associations between lipids and proteins that metabolize or transport them, we constructed a comprehensive lipid-protein network, which we used to highlight proteins strongly connected to lipids found in the CVD-lipid literature. Our approach points to a series of proteins for which lipid-focused research would bring insights into CVD, including Prostaglandin G/H synthase 2 (PTGS2, a.k.a. COX2) and Acylglycerol kinase (AGK). In this review, we summarize our findings, putting them in a historical perspective of the evolution of lipid research in CVD.

Germline loss-of-function PAM variants are enriched in subjects with pituitary hypersecretion

Introduction

Pituitary adenomas (PAs) are common, usually benign tumors of the anterior pituitary gland which, for the most part, have no known genetic cause. PAs are associated with major clinical effects due to hormonal dysregulation and tumoral impingement on vital brain structures. PAM encodes a multifunctional protein responsible for the essential C-terminal amidation of secreted peptides.

Methods

Following the identification of a loss-of-function variant (p.Arg703Gln) in the peptidylglycine a-amidating monooxygenase (PAM) gene in a family with pituitary gigantism, we investigated 299 individuals with sporadic PAs and 17 familial isolated PA kindreds for PAM variants. Genetic screening was performed by germline and tumor sequencing and germline copy number variation (CNV) analysis.

Results

In germline DNA, we detected seven heterozygous, likely pathogenic missense, truncating, and regulatory SNVs. These SNVs were found in sporadic subjects with growth hormone excess (p.Gly552Arg and p.Phe759Ser), pediatric Cushing disease (c.-133T>C and p.His778fs), or different types of PAs (c.-361G>A, p.Ser539Trp, and p.Asp563Gly). The SNVs were functionally tested in vitro for protein expression and trafficking by Western blotting, splicing by minigene assays, and amidation activity in cell lysates and serum samples. These analyses confirmed a deleterious effect on protein expression and/or function. By interrogating 200,000 exomes from the UK Biobank, we confirmed a significant association of the PAM gene and rare PAM SNVs with diagnoses linked to pituitary gland hyperfunction.

Conclusion

The identification of PAM as a candidate gene associated with pituitary hypersecretion opens the possibility of developing novel therapeutics based on altering PAM function.

Type 2 Diabetes risk alleles in Peptidyl-glycine Alpha-amidating Monooxygenase influence GLP-1 levels and response to GLP-1 Receptor Agonists

Patients with type 2 diabetes vary in their response to currently available therapeutic agents (including GLP-1 receptor agonists) leading to suboptimal glycemic control and increased risk of complications. We show that human carriers of hypomorphic T2D-risk alleles in the gene encoding peptidyl-glycine alpha-amidating monooxygenase (PAM), as well as Pam-knockout mice, display increased resistance to GLP-1 in vivo. Pam inactivation in mice leads to reduced gastric GLP-1R expression and faster gastric emptying: this persists during GLP-1R agonist treatment and is rescued when GLP-1R activity is antagonized, indicating resistance to GLP-1's gastric slowing properties. Meta-analysis of human data from studies examining GLP-1R agonist response (including RCTs) reveals a relative loss of 44% and 20% of glucose lowering (measured by glycated hemoglobin) in individuals with hypomorphic PAM alleles p.S539W and p.D536G treated with GLP-1R agonist. Genetic variation in PAM has effects on incretin signaling that alters response to medication used commonly for treatment of T2D.

Germline loss-of-function PAM variants are enriched in subjects with pituitary hypersecretion

Pituitary adenomas (PAs) are common, usually benign tumors of the anterior pituitary gland which, for the most part, have no known genetic cause. PAs are associated with major clinical effects due to hormonal dysregulation and tumoral impingement on vital brain structures. Following the identification of a loss-of-function variant (p.Arg703Gln) in the PAM gene in a family with pituitary gigantism, we investigated 299 individuals with sporadic PAs and 17 familial isolated pituitary adenomas kindreds for PAM variants. PAM encodes a multifunctional protein responsible for the essential C-terminal amidation of secreted peptides. Genetic screening was performed by germline and tumor sequencing and germline copy number variation (CNV) analysis. No germline CNVs or somatic single nucleotide variants (SNVs) were identified. We detected seven likely pathogenic heterozygous missense, truncating, and regulatory SNVs. These SNVs were found in sporadic subjects with GH excess (p.Gly552Arg and p.Phe759Ser), pediatric Cushing disease (c.-133T>C and p.His778fs), or with different types of PAs (c.-361G>A, p.Ser539Trp, and p.Asp563Gly). The SNVs were functionally tested in vitro for protein expression and trafficking by Western blotting, for splicing by minigene assays, and for amidation activity in cell lysates and serum samples. These analyses confirmed a deleterious effect on protein expression and/or function. By interrogating 200,000 exomes from the UK Biobank, we confirmed a significant association of the PAM gene and rare PAM SNVs to diagnoses linked to pituitary gland hyperfunction. Identification of PAM as a candidate gene associated with pituitary hypersecretion opens the possibility of developing novel therapeutics based on altering PAM function.

Lithium treatment extends human lifespan: findings from the UK Biobank

Lithium is a nutritional trace element that is also used pharmacologically for the management of bipolar and related psychiatric disorders. Recent studies have shown that lithium supplementation can extend health and lifespan in different animal models. Moreover, nutritional lithium uptake from drinking water was repeatedly found to be positively correlated with human longevity. By analyzing a large observational aging cohort (UK Biobank, n = 501,461 individuals) along with prescription data derived from the National Health Services (NHS), we here find therapeutic supplementation of lithium linked to decreased mortality (p = 0.0017) of individuals diagnosed with affective disorders. Subsequent multivariate survival analyses reveal lithium to be the strongest factor in regards to increased survival effects (hazard ratio = 0.274 [0.119-0.634 CI 95%, p = 0.0023]), corresponding to 3.641 times lower (95% CI 1.577-8.407) chances of dying at a given age for lithium users compared to users of other anti-psychotic drugs. While these results may further support the use of lithium as a geroprotective supplement, it should be noted that doses applied within the UK Biobank/NHS setting require close supervision by qualified medical professionals.

Targeted Suppression of miRNA-33 Using pHLIP Improves Atherosclerosis Regression

BACKGROUND

miRNA therapeutics have gained attention during the past decade. These oligonucleotide treatments can modulate the expression of miRNAs in vivo and could be used to correct the imbalance of gene expression found in human diseases such as obesity, metabolic syndrome, and atherosclerosis. The in vivo efficacy of current anti-miRNA technologies hindered by physiological and cellular barriers to delivery into targeted cells and the nature of miRNAs that allows one to target an entire pathway that may lead to deleterious off-target effects. For these reasons, novel targeted delivery systems to inhibit miRNAs in specific tissues will be important for developing effective therapeutic strategies for numerous diseases including atherosclerosis.

METHODS

We used pH low-insertion peptide (pHLIP) constructs as vehicles to deliver microRNA-33-5p (miR-33) antisense oligonucleotides to atherosclerotic plaques. Immunohistochemistry and histology analysis was performed to assess the efficacy of miR-33 silencing in atherosclerotic lesions. We also assessed how miR-33 inhibition affects gene expression in monocytes/macrophages by single-cell RNA transcriptomics.

RESULTS

The anti-miR-33 conjugated pHLIP constructs are preferentially delivered to atherosclerotic plaque macrophages. The inhibition of miR-33 using pHLIP-directed macrophage targeting improves atherosclerosis regression by increasing collagen content and decreased lipid accumulation within vascular lesions. Single-cell RNA sequencing analysis revealed higher expression of fibrotic genes (Col2a1, Col3a1, Col1a2, Fn1, etc) and tissue inhibitor of metalloproteinase 3 (Timp3) and downregulation of Mmp12 in macrophages from atherosclerotic lesions targeted by pHLIP-anti-miR-33.

CONCLUSIONS

This study provides proof of principle for the application of pHLIP for treating advanced atherosclerosis via pharmacological inhibition of miR-33 in macrophages that avoid the deleterious effects in other metabolic tissues. This may open new therapeutic opportunities for atherosclerosis-associated cardiovascular diseases via selective delivery of other protective miRNAs.

Grainyhead 1 acts as a drug-inducible conserved transcriptional regulator linked to insulin signaling and lifespan

Aging is impacted by interventions across species, often converging on metabolic pathways. Transcription factors regulate longevity yet approaches for their pharmacological modulation to exert geroprotection remain sparse. We show that increased expression of the transcription factor Grainyhead 1 (GRH-1) promotes lifespan and pathogen resistance in Caenorhabditis elegans. A compound screen identifies FDA-approved drugs able to activate human GRHL1 and promote nematodal GRH-1-dependent longevity. GRHL1 activity is regulated by post-translational lysine methylation and the phosphoinositide (PI) 3-kinase C2A. Consistently, nematodal longevity following impairment of the PI 3-kinase or insulin/IGF-1 receptor requires grh-1. In BXD mice, Grhl1 expression is positively correlated with lifespan and insulin sensitivity. In humans, GRHL1 expression positively correlates with insulin receptor signaling and also with lifespan. Fasting blood glucose levels, including in individuals with type 2 diabetes, are negatively correlated with GRHL1 expression. Thereby, GRH-1/GRHL1 is identified as a pharmacologically malleable transcription factor impacting insulin signaling and lifespan.

Life- and healthspan of organisms can be modulated by dietary, genetic, or pharmacological interventions, which often affect metabolic pathways. Here the authors report that Grainyhead 1 is an evolutionarily conserved, drug-inducible transcription factor that promotes longevity in C. elegans, and thus a potential target for the development of geroprotective drugs.

The Diabetes Gene JAZF1 Is Essential for the Homeostatic Control of Ribosome Biogenesis and Function in Metabolic Stress

The ability of pancreatic β-cells to respond to increased demands for insulin during metabolic stress critically depends on proper ribosome homeostasis and function. Excessive and long-lasting stimulation of insulin secretion can elicit endoplasmic reticulum (ER) stress, unfolded protein response, and β-cell apoptosis. Here we show that the diabetes susceptibility gene JAZF1 is a key transcriptional regulator of ribosome biogenesis, global protein, and insulin translation. JAZF1 is excluded from the nucleus, and its expression levels are reduced upon metabolic stress and in diabetes. Genetic deletion of Jazf1 results in global impairment of protein synthesis that is mediated by defects in ribosomal protein synthesis, ribosomal RNA processing, and aminoacyl-synthetase expression, thereby inducing ER stress and increasing β-cell susceptibility to apoptosis. Importantly, JAZF1 function and its pleiotropic actions are impaired in islets of murine T2D and in human islets exposed to metabolic stress. Our study identifies JAZF1 as a central mediator of metabolic stress in β-cells.

Macrophage deficiency of miR-21 promotes apoptosis, plaque necrosis, and vascular inflammation during atherogenesis

Atherosclerosis, the major cause of cardiovascular disease, is a chronic inflammatory disease characterized by the accumulation of lipids and inflammatory cells in the artery wall. Aberrant expression of microRNAs has been implicated in the pathophysiological processes underlying the progression of atherosclerosis. Here, we define the contribution of miR‐21 in hematopoietic cells during atherogenesis. Interestingly, we found that miR‐21 is the most abundant miRNA in macrophages and its absence results in accelerated atherosclerosis, plaque necrosis, and vascular inflammation. miR‐21 expression influences foam cell formation, sensitivity to ER‐stress‐induced apoptosis, and phagocytic clearance capacity. Mechanistically, we discovered that the absence of miR‐21 in macrophages increases the expression of the miR‐21 target gene, MKK3, promoting the induction of p38‐CHOP and JNK signaling. Both pathways enhance macrophage apoptosis and promote the post‐translational degradation of ABCG1, a transporter that regulates cholesterol efflux in macrophages. Altogether, these findings reveal a major role for hematopoietic miR‐21 in atherogenesis.

Lanosterol Modulates TLR4-Mediated Innate Immune Responses in Macrophages

Macrophages perform critical functions in both innate immunity and cholesterol metabolism. Here, we report that activation of Toll-like receptor 4 (TLR4) in macrophages causes lanosterol, the first sterol intermediate in the cholesterol biosynthetic pathway, to accumulate. This effect is due to type I interferon (IFN)-dependent histone deacetylase 1 (HDAC1) transcriptional repression of lanosterol-14α-demethylase, the gene product of Cyp51A1. Lanosterol accumulation in macrophages, because of either treatment with ketoconazole or induced conditional disruption of Cyp51A1 in mouse macrophages in vitro, decreases IFNβ-mediated signal transducer and activator of transcription (STAT)1-STAT2 activation and IFNβ-stimulated gene expression. These effects translate into increased survival to endotoxemic shock by reducing cytokine secretion. In addition, lanosterol accumulation increases membrane fluidity and ROS production, thus potentiating phagocytosis and the ability to kill bacteria. This improves resistance of mice to Listeria monocytogenes infection by increasing bacterial clearance in the spleen and liver. Overall, our data indicate that lanosterol is an endogenous selective regulator of macrophage immunity.

MicroRNAs as regulators of endothelial cell functions in cardiometabolic diseases

Endothelial cells (ECs) provide nutrients and oxygen essential for tissue homeostasis. Metabolic imbalances and other environmental stimuli, like cytokines or low shear stress, trigger endothelial inflammation, increase permeability, compromise vascular tone, promote cell proliferation, and ultimately cause cell death. These factors contribute to EC dysfunction, which is crucial in the development of cardiometabolic diseases. microRNAs (miRNAs) are small non-coding RNAs that have important functions in the regulation of ECs. In the present review, we discuss the role of miRNAs in various aspects of EC pathology in cardiometabolic diseases like atherosclerosis, type 2 diabetes, obesity, and the metabolic syndrome, and in complication of those pathologies, like ischemia. We also discuss the potential therapeutic applications of miRNAs in promoting angiogenesis and neovascularization in tissues where the endothelium is damaged in different cardiometabolic diseases. This article is part of a Special Issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

miR-27b inhibits LDLR and ABCA1 expression but does not influence plasma and hepatic lipid levels in mice

RATIONALE

Recently, there has been significant interest in the therapeutic administration of miRNA mimics and inhibitors to treat cardiovascular disease. In particular, miR-27b has emerged as a regulatory hub in cholesterol and lipid metabolism and potential therapeutic target for treating atherosclerosis. Despite this, the impact of miR-27b on lipid levels in vivo remains to be determined. As such, here we set out to further characterize the role of miR-27b in regulating cholesterol metabolism in vitro and to determine the effect of miR-27b overexpression and inhibition on circulating and hepatic lipids in mice.

METHODS AND RESULTS

Our results identify miR-27b as an important regulator of LDLR activity in human and mouse hepatic cells through direct targeting of LDLR and LDLRAP1. In addition, we report that modulation of miR-27b expression affects ABCA1 protein levels and cellular cholesterol efflux to ApoA1 in human hepatic Huh7 cells. Overexpression of pre-miR-27b in the livers of wild-type mice using AAV8 vectors increased pre-miR-27b levels 50-fold and reduced hepatic ABCA1 and LDLR expression by 50% and 20%, respectively, without changing circulating and hepatic cholesterol and triglycerides. To determine the effect of endogenous miR-27b on circulating lipids, wild-type mice were fed a Western diet for one month and injected with 5 mg/kg of LNA control or LNA anti-miR-27b oligonucleotides. Following two weeks of treatment, the expression of ABCA1 and LDLR were increased by 10-20% in the liver, demonstrating effective inhibition of miR-27b function. Intriguingly, no differences in circulating and hepatic lipids were observed between treatment groups.

CONCLUSIONS

The results presented here provide evidence that short-term modulation of miR-27b expression in wild-type mice regulates hepatic LDLR and ABCA1 expression but does not influence plasma and hepatic lipid levels.

ANGPTL4 deficiency in haematopoietic cells promotes monocyte expansion and atherosclerosis progression

Lipid accumulation in macrophages has profound effects on macrophage gene expression and contributes to the development of atherosclerosis. Here, we report that angiopoietin-like protein 4 (ANGPTL4) is the most highly upregulated gene in foamy macrophages and it's absence in haematopoietic cells results in larger atherosclerotic plaques, characterized by bigger necrotic core areas and increased macrophage apoptosis. Furthermore, hyperlipidemic mice deficient in haematopoietic ANGPTL4 have higher blood leukocyte counts, which is associated with an increase in the common myeloid progenitor (CMP) population. ANGPTL4-deficient CMPs have higher lipid raft content, are more proliferative and less apoptotic compared with the wild-type (WT) CMPs. Finally, we observe that ANGPTL4 deficiency in macrophages promotes foam cell formation by enhancing CD36 expression and reducing ABCA1 localization in the cell surface. Altogether, these findings demonstrate that haematopoietic ANGPTL4 deficiency increases atherogenesis through regulating myeloid progenitor cell expansion and differentiation, foam cell formation and vascular inflammation.

Angiopoietin-like 4 protein (ANGPTL4) is a regulator of lipoprotein metabolism whose role in atherosclerosis has been controversial. Here the authors show that ANGPTL4 deficiency in haematopoietic cells increases atherogenesis by promoting myeloid progenitor cell expansion and differentiation, foam cell formation and vascular inflammation.

Chronic miR-29 antagonism promotes favorable plaque remodeling in atherosclerotic mice

Abnormal remodeling of atherosclerotic plaques can lead to rupture, acute myocardial infarction, and death. Enhancement of plaque extracellular matrix (ECM) may improve plaque morphology and stabilize lesions. Here, we demonstrate that chronic administration of LNA‐miR‐29 into an atherosclerotic mouse model improves indices of plaque morphology. This occurs due to upregulation of miR‐29 target genes of the ECM (col1A and col3A) resulting in reduced lesion size, enhanced fibrous cap thickness, and reduced necrotic zones. Sustained LNA‐miR‐29 treatment did not affect circulating lipids, blood chemistry, or ECM of solid organs including liver, lung, kidney, spleen, or heart. Collectively, these data support the idea that antagonizing miR‐29 may promote beneficial plaque remodeling as an independent approach to stabilize vulnerable atherosclerotic lesions.

VEGF-Induced Expression of miR-17-92 Cluster in Endothelial Cells Is Mediated by ERK/ELK1 Activation and Regulates Angiogenesis

Supplemental Digital Content is available in the text.

Several lines of evidence indicate that the regulation of microRNA (miRNA) levels by different stimuli may contribute to the modulation of stimulus-induced responses. The miR-17–92 cluster has been linked to tumor development and angiogenesis, but its role in vascular endothelial growth factor–induced endothelial cell (EC) functions is unclear and its regulation is unknown.

MicroRNA-148a regulates LDL receptor and ABCA1 expression to control circulating lipoprotein levels

The hepatic low-density lipoprotein receptor (LDLR) pathway is essential for clearing circulating LDL-cholesterol (LDL-C). While the transcriptional regulation of LDLR is well-characterized, the post-transcriptional mechanisms which govern LDLR expression are just beginning to emerge. Here, we developed a high-throughput genome-wide screening assay to systematically identify microRNAs (miRNAs) that regulate LDLR activity in human hepatic cells. From this screen, we characterize miR-148a as a negative regulator of LDLR expression and activity, and define a novel SREBP1-mediated pathway by which miR-148a regulates LDL-C uptake. Importantly, inhibition of miR-148a increases hepatic LDLR expression and decreases plasma LDL-C in vivo. We also provide evidence that miR-148a regulates hepatic ABCA1 expression and circulating HDL-C levels. Collectively, these studies uncover miR-148a as an important regulator of hepatic LDL-C clearance through direct regulation of LDLR expression, and demonstrate the therapeutic potential of inhibiting miR-148a to ameliorate the elevated LDL-C/HDL-C ratio, a prominent risk factor for cardiovascular disease.

Fibrosis and hypoxia-inducible factor-1α-dependent tumors of the soft tissue on loss of von Hippel-Lindau in mesenchymal progenitors

The hypoxia-inducible factor (Hif)-1α (Hif-1α) and Hif-2α (Epas1) have a critical role in both normal development and cancer. von Hippel Lindau (Vhl) protein, encoded by a tumor suppressor gene, is an E3 ubiquitin ligase that targets Hif-1α and Epas1 to the proteasome for degradation. To better understand the role of Vhl in the biology of mesenchymal cells, we analyzed mutant mice lacking Vhl in mesenchymal progenitors that give rise to the soft tissues that form and surround synovial joints. Loss of Vhl in mesenchymal progenitors of the limb bud caused severe fibrosis of the synovial joints and formation of aggressive masses with histologic features of mesenchymal tumors. Hif-1α and its downstream target connective tissue growth factor were necessary for the development of these tumors, which conversely still developed in the absence of Epas1, but at lower frequency. Human tumors of the soft tissue are a very complex and heterogeneous group of neoplasias. Our novel findings in genetically altered mice suggest that activation of the HIF signaling pathway could be an important pathogenetic event in the development and progression of at least a subset of these tumors.

Therapeutic Potential of Modulating microRNAs in Atherosclerotic Vascular Disease

Atherosclerosis (also known as arteriosclerotic vascular disease) is a chronic inflammatory disease of the arterial wall, characterized by the formation of lipid-laden lesions. The activation of endothelial cells at atherosclerotic lesion-prone sites in the arterial tree results in the up-regulation of cell adhesion molecules and chemokines, which mediate the recruitment of circulating monocytes. Accumulation of monocytes and monocyte-derived phagocytes in the wall of large arteries leads to chronic inflammation and the development and progression of atherosclerosis. The lesion experiences the following steps: foam cell formation, fatty streak accumulation, migration and proliferation of vascular smooth muscle cells, and fibrous cap formation. Finally, the rupture of the unstable fibrous cap causes thrombosis in complications of advanced lesions that leads to unstable coronary syndromes, myocardial infarction and stroke. MicroRNAs have recently emerged as a novel class of gene regulators at the post-transcriptional level. Several functions of vascular cells, such as cell differentiation, contraction, migration, proliferation and inflammation that are involved in angiogenesis, neointimal formation and lipid metabolism underlying various vascular diseases, have been found to be regulated by microRNAs and are described in the present review as well as their potential therapeutic application.

Hematopoietic Akt2 deficiency attenuates the progression of atherosclerosis

Atherosclerosis is the major cause of death and disability in diabetic and obese subjects with insulin resistance. Akt2, a phosphoinositide-dependent serine-threonine protein kinase, is highly express in insulin-responsive tissues; however, its role during the progression of atherosclerosis remains unknown. Thus, we aimed to investigate the contribution of Akt2 during the progression of atherosclerosis. We found that germ-line Akt2-deficient mice develop similar atherosclerotic plaques as wild-type mice despite higher plasma lipids and glucose levels. It is noteworthy that transplantation of bone marrow cells isolated from Akt2(-/-) mice to Ldlr(-/-) mice results in marked reduction of the progression of atherosclerosis compared with Ldlr(-/-) mice transplanted with wild-type bone marrow cells. In vitro studies indicate that Akt2 is required for macrophage migration in response to proatherogenic cytokines (monocyte chemotactic protein-1 and macrophage colony-stimulating factor). Moreover, Akt2(-/-) macrophages accumulate less cholesterol and have an alternative activated or M2-type phenotype when stimulated with proinflammatory cytokines. Together, these results provide evidence that macrophage Akt2 regulates migration, the inflammatory response and cholesterol metabolism and suggest that targeting Akt2 in macrophages might be beneficial for treating atherosclerosis.

Loss of VHL in mesenchymal progenitors of the limb bud alters multiple steps of endochondral bone development

Adaptation to low oxygen tension (hypoxia) is a critical event during development. The transcription factors Hypoxia Inducible Factor-1α (HIF-1α) and HIF-2α are essential mediators of the homeostatic responses that allow hypoxic cells to survive and differentiate. Von Hippel-Lindau protein (VHL) is the E3 ubiquitin ligase that targets HIFs to the proteasome for degradation in normoxia. We have previously demonstrated that the transcription factor HIF-1α is essential for survival and differentiation of growth plate chondrocytes, whereas HIF-2α is not necessary for fetal growth plate development. We have also shown that VHL is important for endochondral bone development, since loss of VHL in chondrocytes causes severe dwarfism. In this study, in order to expand our understanding of the role of VHL in chondrogenesis, we conditionally deleted VHL in mesenchymal progenitors of the limb bud, i.e. in cells not yet committed to the chondrocyte lineage. Deficiency of VHL in limb bud mesenchyme does not alter the timely differentiation of mesenchymal cells into chondrocytes. However, it causes structural collapse of the cartilaginous growth plate as a result of impaired proliferation, delayed terminal differentiation, and ectopic death of chondrocytes. This phenotype is associated to delayed replacement of cartilage by bone. Notably, loss of HIF-2α fully rescues the late formation of the bone marrow cavity in VHL mutant mice, though it does not affect any other detectable abnormality of the VHL mutant growth plates. Our findings demonstrate that VHL regulates bone morphogenesis as its loss considerably alters size, shape and overall development of the skeletal elements.

Improved repair of dermal wounds in mice lacking microRNA-155

Wound healing is a well-regulated but complex process that involves haemostasis, inflammation, proliferation and maturation. Recent reports suggest that microRNAs (miRs) play important roles in dermal wound healing. In fact, miR deregulation has been linked with impaired wound repair. miR-155 has been shown to be induced by inflammatory mediators and plays a central regulatory role in immune responses. We have investigated the potential role of miR-155 in wound healing. By creating punch wounds in the skin of mice, we found an increased expression of miR-155 in wound tissue when compared with healthy skin. Interestingly, analysis of wounds of mice lacking the expression of miR-155 (miR-155(-/-) ) revealed an increased wound closure when compared with wild-type animals. Also, the accelerated wound closing correlated with elevated numbers of macrophages in wounded tissue. Gene expression analysis of wounds tissue and macrophages isolated from miR-155(-/-) mice that were treated with interleukin-4 demonstrated an increased expression of miR-155 targets (BCL6, RhoA and SHIP1) as well as, the finding in inflammatory zone-1 (FIZZ1) gene, when compared with WT mice. Moreover, the up-regulated levels of FIZZ1 in the wound tissue of miR-155(-/-) mice correlated with an increased deposition of type-1 collagens, a phenomenon known to be beneficial in wound closure. Our data indicate that the absence of miR-155 has beneficial effects in the wound healing process.

Autoregulation of glypican-1 by intronic microRNA-149 fine tunes the angiogenic response to FGF2 in human endothelial cells

MicroRNA-149 (miR-149) is located within the first intron of the glypican-1 (GPC1) gene. GPC1 is a low affinity receptor for fibroblast growth factor (FGF2) that enhances FGF2 binding to its receptor (FGFR1), subsequently promoting FGF2-FGFR1 activation and signaling. Using bioinformatic approaches, both GPC1 and FGFR1 were identified and subsequently validated as targets for miR-149 (both the mature strand, miR-149, and the passenger strand, miR-149*) in endothelial cells (ECs). As a consequence of their targeting activity towards GPC1 and FGFR1, both miR-149 and miR-149* regulated FGF2 signaling and FGF2-induced responses in ECs, namely proliferation, migration and cord formation. Moreover, lentiviral overexpression of miR-149 reduced in vivo tumor-induced neovascularization. Importantly, FGF2 transcriptionally stimulated the expression of miR-149 independently of its host gene, therefore assuring the steady state of FGF2-induced responses through the regulation of the GPC1-FGFR1 binary complex in ECs.

MiR-155 has a protective role in the development of non-alcoholic hepatosteatosis in mice

Hepatic steatosis is a global epidemic that is thought to contribute to the pathogenesis of type 2 diabetes. MicroRNAs (miRs) are regulators that can functionally integrate a range of metabolic and inflammatory pathways in liver. We aimed to investigate the functional role of miR-155 in hepatic steatosis. Male C57BL/6 wild-type (WT) and miR-155^−/− mice were fed either normal chow or high fat diet (HFD) for 6 months then lipid levels, metabolic and inflammatory parameters were assessed in livers and serum of the mice. Mice lacking endogenous miR-155 that were fed HFD for 6 months developed increased hepatic steatosis compared to WT controls. This was associated with increased liver weight and serum VLDL/LDL cholesterol and alanine transaminase (ALT) levels, as well as increased hepatic expression of genes involved in glucose regulation (Pck1, Cebpa), fatty acid uptake (Cd36) and lipid metabolism (Fasn, Fabp4, Lpl, Abcd2, Pla2g7). Using miRNA target prediction algorithms and the microarray transcriptomic profile of miR-155^−/− livers, we identified and validated that Nr1h3 (LXRα) as a direct miR-155 target gene that is potentially responsible for the liver phenotype of miR-155^−/− mice. Together these data indicate that miR-155 plays a pivotal role regulating lipid metabolism in liver and that its deregulation may lead to hepatic steatosis in patients with diabetes.

Therapeutic Potential of Modulating microRNAs in Atherosclerotic Vascular Disease

Atherosclerosis (also known as arteriosclerotic vascular disease) is a chronic inflammatory disease of the arterial wall, characterized by the formation of lipid-laden lesions. The activation of endothelial cells at atherosclerotic lesion-prone sites in the arterial tree results in the up-regulation of cell adhesion molecules and chemokines, which mediate the recruitment of circulating monocytes. Accumulation of monocytes and monocyte-derived phagocytes in the wall of large arteries leads to chronic inflammation and the development and progression of atherosclerosis. The lesion experiences the following steps: foam cell formation, fatty streak accumulation, migration and proliferation of vascular smooth muscle cells, and fibrous cap formation. Finally, the rupture of the unstable fibrous cap causes thrombosis in complications of advanced lesions that lead to unstable coronary syndromes, myocardial infarction and stroke. MicroRNAs have recently emerged as a novel class of gene regulators at the post-transcriptional level. Several functions of vascular cells, such as cell differentiation, contraction, migration, proliferation and inflammation that are involved in angiogenesis, neointimal formation and lipid metabolism underlying various vascular diseases, have been found to be regulated by microRNAs and are described in the present review as well as their potential therapeutic application.

Control of cholesterol metabolism and plasma high-density lipoprotein levels by microRNA-144

RATIONALE

Foam cell formation because of excessive accumulation of cholesterol by macrophages is a pathological hallmark of atherosclerosis, the major cause of morbidity and mortality in Western societies. Liver X nuclear receptors (LXRs) regulate the expression of the adenosine triphosphate-binding cassette (ABC) transporters, including adenosine triphosphate-binding cassette transporter A1 (ABCA1) and adenosine triphosphate-binding cassette transporter G1 (ABCG1). ABCA1 and ABCG1 facilitate the efflux of cholesterol from macrophages and regulate high-density lipoprotein (HDL) biogenesis. Increasing evidence supports the role of microRNA (miRNAs) in regulating cholesterol metabolism through ABC transporters.

OBJECTIVE

We aimed to identify novel miRNAs that regulate cholesterol metabolism in macrophages stimulated with LXR agonists.

METHODS AND RESULTS

To map the miRNA expression signature of macrophages stimulated with LXR agonists, we performed an miRNA profiling microarray analysis in primary mouse peritoneal macrophages stimulated with LXR ligands. We report that LXR ligands increase miR-144 expression in macrophages and mouse livers. Overexpression of miR-144 reduces ABCA1 expression and attenuates cholesterol efflux to apolipoproteinA1 in macrophages. Delivery of miR-144 oligonucleotides to mice attenuates ABCA1 expression in the liver, reducing HDL levels. Conversely, silencing of miR-144 in mice increases the expression of ABCA1 and plasma HDL levels. Thus, miR-144 seems to regulate both macrophage cholesterol efflux and HDL biogenesis in the liver.

CONCLUSIONS

miR-144 regulates cholesterol metabolism via suppressing ABCA1 expression and modulation of miRNAs may represent a potential therapeutical intervention for treating dyslipidemia and atherosclerotic vascular disease.

Notch pathway activation targets AML-initiating cell homeostasis and differentiation

Notch behaves as a tumor suppressor in AML, and Notch activation induces cell cycle arrest, differentiation, and apoptosis of AML-initiating cells.

Notch signaling pathway activation is known to contribute to the pathogenesis of a spectrum of human malignancies, including T cell leukemia. However, recent studies have implicated the Notch pathway as a tumor suppressor in myeloproliferative neoplasms and several solid tumors. Here we report a novel tumor suppressor role for Notch signaling in acute myeloid leukemia (AML) and demonstrate that Notch pathway activation could represent a therapeutic strategy in this disease. We show that Notch signaling is silenced in human AML samples, as well as in AML-initiating cells in an animal model of the disease. In vivo activation of Notch signaling using genetic Notch gain of function models or in vitro using synthetic Notch ligand induces rapid cell cycle arrest, differentiation, and apoptosis of AML-initiating cells. Moreover, we demonstrate that Notch inactivation cooperates in vivo with loss of the myeloid tumor suppressor Tet2 to induce AML-like disease. These data demonstrate a novel tumor suppressor role for Notch signaling in AML and elucidate the potential therapeutic use of Notch receptor agonists in the treatment of this devastating leukemia.

International Society for Extracellular Vesicles: first annual meeting, April 17-21, 2012: ISEV-2012

Extracellular micro- and nano-scale membrane vesicles produced by different cells are recognised as an essential entity of physiological fluids in a variety of organisms and function as mediators of intercellular communication employed for the regulation of multiple systemic and local processes. In the last decade, an exponential amount of experimental work was dedicated to exploring the biogenesis and secretion mechanisms, physiological and pathological functions and potential applications of the extracellular vesicles (EVs). Noteworthy is the large heterogeneity of in vitro and in vivo models applied, technical approaches developed in these studies and the diversity of designations assigned to different or similar types of EVs. Hence, there is a clear necessity for a uniform nomenclature and standardisation of methods to isolate and characterise these vesicles. In April 2012, the first meeting of the International Society for Extracellular Vesicles (ISEV) took place bringing together this exponentially grown scientific community. The University of Gothenburg (Krefting Research Centre) together with the Interim Board of the Society created in September 2011 (Jan Lötvall, Clotilde Théry, Xandra Breakefield, Marca Wauben, Yong Song Gho, Lawrence Rajendran, Graça Raposo, Douglas Taylor, Margareta Sjöstrand and Esbjörn Telemo) organised this fantastic event that counted 488 registered and contributing participants. This meeting report provides a retrospective summary of the broad spectrum of ISEV-2012 sessions. Again, we emphasise novel findings, discussions and decisions met by the community during the meeting.

The HIF signaling pathway in osteoblasts directly modulates erythropoiesis through the production of EPO

Osteoblasts are an important component of the hematopoietic microenvironment in bone. However, the mechanisms by which osteoblasts control hematopoiesis remain unknown. We show that augmented HIF signaling in osteoprogenitors results in HSC niche expansion associated with selective expansion of the erythroid lineage. Increased red blood cell production occurred in an EPO-dependent manner with increased EPO expression in bone and suppressed EPO expression in the kidney. In contrast, inactivation of HIF in osteoprogenitors reduced EPO expression in bone. Importantly, augmented HIF activity in osteoprogenitors protected mice from stress-induced anemia. Pharmacologic or genetic inhibition of prolyl hydroxylases1/2/3 in osteoprogenitors elevated EPO expression in bone and increased hematocrit. These data reveal an unexpected role for osteoblasts in the production of EPO and modulation of erythropoiesis. Furthermore, these studies demonstrate a molecular role for osteoblastic PHD/VHL/HIF signaling that can be targeted to elevate both HSCs and erythroid progenitors in the local hematopoietic microenvironment.

VEGF-independent cell-autonomous functions of HIF-1α regulating oxygen consumption in fetal cartilage are critical for chondrocyte survival

Fetal growth plate cartilage is nonvascularized, and chondrocytes largely develop in hypoxic conditions. We previously found that mice lacking the hypoxia-inducible transcription factor HIF-1α in cartilage show massive death of centrally located, hypoxic chondrocytes. A similar phenotype was observed in mice with genetic ablation of either all or specifically the diffusible isoforms of vascular endothelial growth factor (VEGF), a prime angiogenic target of HIF-1α. Here, we assessed whether VEGF is a critical downstream component of the HIF-1α-dependent survival pathway in chondrocytes. We used a genetic approach to conditionally overexpress VEGF164 in chondrocytes lacking HIF-1α, evaluating potential rescuing effects. The effectiveness of the strategy was validated by showing that transgenic expression of VEGF164 in Col2-Cre;VEGF(f/f) mice stimulated angiogenesis in the perichondrium, fully corrected the excessive hypoxia of VEGF-deficient chondrocytes, and completely prevented chondrocyte death. Yet, similarly crossed double-mutant embryos lacking HIF-1α and overexpressing VEGF164 in the growth plate cartilage still displayed a central cell death phenotype, albeit slightly delayed and less severe compared with mice exclusively lacking HIF-1α. Transgenic VEGF164 induced massive angiogenesis in the perichondrium, yet this only partially relieved the aberrant hypoxia present in HIF-1α-deficient cartilage and thereby likely inflicted only a partial rescue effect. In fact, excessive hypoxia and failure to upregulate phosphoglycerate-kinase 1 (PGK1), a key enzyme of anaerobic glycolytic metabolism, were among the earliest manifestations of HIF-1α deficiency in cartilaginous bone templates, and reduced PGK1 expression was irrespective of transgenic VEGF164. These findings suggest that HIF-1α activates VEGF-independent cell-autonomous mechanisms to sustain oxygen levels in the challenged avascular cartilage by reducing oxygen consumption. Hence, regulation of the metabolic pathways by HIF-1α and VEGF-dependent regulation of angiogenesis coordinately act to maintain physiological cartilage oxygenation. We conclude that VEGF and HIF-1α are critical preservers of chondrocyte survival by ensuring an adequate balance between availability and handling of oxygen in developing growth cartilage.

MicroRNA-16 and microRNA-424 regulate cell-autonomous angiogenic functions in endothelial cells via targeting vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1

OBJECTIVE

MicroRNAs play key roles in modulating a variety of cellular processes by posttranscriptional regulation of their target genes. Vascular endothelial growth factor (VEGF), VEGF receptor-2 (VEGFR2), and fibroblast growth factor receptor-1 (FGFR1) were identified by bioinformatic approaches and subsequently validated as targets of microRNA (miR)-16 and miR-424 in endothelial cells (ECs).

METHODS AND RESULTS

Mimetics of these microRNAs reduced VEGF, VEGFR2, and FGFR1 expression, whereas specific antagonists enhanced their expression. Expression of mature miR-16 and miR-424 was upregulated on VEGF or basic fibroblast growth factor (bFGF) treatment. This upregulation was accompanied by a parallel increase in primary transcript (pri-miR)-16-1 and pri-miR-16-2 but not in pri-miR-424 levels, indicating a VEGF/bFGF-dependent transcriptional and posttranscriptional regulation of miR-16 and miR-424, respectively. Reduced expression of VEGFR2 and FGFR1 by miR-16 or miR-424 overexpression regulated VEGF and bFGF signaling through these receptors, thereby affecting the activity of downstream components of the pathways. Functionally, miR-16 or miR-424 overexpression reduced proliferation, migration, and cord formation of ECs in vitro, and lentiviral overexpression of miR-16 reduced the ability of ECs to form blood vessels in vivo.

CONCLUSION

We conclude that these miRNAs fine-tune the expression of selected endothelial angiogenic mediators in response to these growth factors. Altogether, these findings suggest that miR-16 and miR-424 play important roles in regulating cell-intrinsic angiogenic activity of ECs.

A novel tumour-suppressor function for the Notch pathway in myeloid leukaemia

Notch signaling is a central regulator of differentiation in a variety of organisms and tissue types¹. Its activity is controlled by the multi-subunit γ–secretase complex (γSE) complex². Although Notch signaling can play both oncogenic and tumor suppressor roles in solid tumors, in the hematopoietic system, it is exclusively oncogenic, notably in T cell acute lymphoblastic leukemia (T-ALL), a disease characterized by Notch1 activating mutations³. Here we identify novel somatic inactivating Notch pathway mutations in a fraction of chronic myelomonocytic leukemia (CMML) patients. Inactivation of Notch signaling in mouse hematopoietic stem cells (HSC) resulted in an aberrant accumulation of granulocyte/monocyte progenitors (GMP), extramedullary hematopoieisis and the induction of CMML-like disease. Transcriptome analysis revealed that Notch signaling regulates an extensive myelomonocytic-specific gene signature, through the direct suppression of gene transcription by the Notch target Hes1. Our studies identify a novel role for Notch signaling during early hematopoietic stem cell differentiation and suggest that the Notch pathway can play both tumor-promoting and suppressive roles within the same tissue.

Hypoxia, HIFs and bone development

Oxygen is not only an obviously important substrate, but it is also a regulatory signal that controls expression of a specific genetic program. Crucial mediator of the adaptive response of cells to hypoxia is the family of Hypoxia-Inducible Transcription Factors (HIFs).The fetal growth plate, which is an avascular structure of mesenchymal origin, has a unique out-in gradient of oxygenation. HIF-1alpha is necessary for chondrogenesis in vivo by controlling a complex homeostatic response that allows chondrocytes to survive and differentiate in a hypoxic environment. Moreover, HIFs are also essential in osteogenesis and joint development. This brief Perspective summarizes the critical role of HIFs in endochondral bone development.

MicroRNA-140 and the silencing of osteoarthritis

MicroRNAs (miRNAs) have emerged as important modulators in development, tissue homeostasis, and diseases. In this issue of Genes & Development, Miyaki and colleagues (pp. 1173-1185) report that miR-140 is involved in the pathogenesis of osteoarthritis by regulating, at least in part, ADAMTS5. Moreover, mice lacking miR-140 are dwarf as a consequence of impaired chondrocyte proliferation. This study is the first in vivo demonstration that miR-140 has a critical and nonredundant role in cartilage development and homeostasis.