Author + information
- Nikolaos G. Frangogiannis, MD∗ ()
- Wilf Family Cardiovascular Research Institute, Department of Medicine (Cardiology), Albert Einstein College of Medicine, Bronx, New York
- ↵∗Reprint requests and correspondence:
Dr. Nikolaos G. Frangogiannis, The Wilf Family Cardiovascular Research Institute, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Forchheimer G46B, Bronx, New York 10461.
The human genome contains more than 20,000 protein-encoding genes and at least as many noncoding RNA (ncRNA) genes, which are not translated into protein but may contribute to regulation of gene expression by modulating transcriptional or post-transcriptional processes (1). Although they represent only a small fraction of the noncodingRNAs, the members of the evolutionarily ancient family of microRNAs (miRNAs) appear to play a uniquely important role in the pathogenesis of disease (2). Over the last 5 years, several members of the family have been identified as key modulators in a wide range of cardiovascular pathophysiologic conditions ranging from vascular disease (3) to heart failure and cardiac fibrosis (4).
One of the best-studied and highly-conserved miRNAs, the Let-7 group, consists of 12 members in mammals and corresponds to a Caenorhabditis elegans counterpart that is an essential regulator of cell differentiation (5). Although a mounting body of evidence implicates Let-7 transcripts in the pathobiology of cancer as tumor suppressors, their potential role in cardiovascular disease remains unknown. In this issue of the Journal, Liao et al. (6) explore the role of Let-7g, one of the best-characterized members of the Let-7 family, in endothelial function. Using in vitro experiments, animal investigations, and samples from human patients, the authors found a protective effect of Let-7g on the endothelium, mediated through transforming growth factor (TGF)-β signaling. In vitro, Let-7g exerted anti-inflammatory actions on endothelial cells, suppressed expression of plasminogen activator inhibitor (PAI)-1 through downmo-dulation of TGF-β signaling, and reduced endothelial cell senescence by increasing sirtuin 1 expression. In vivo, injection of a Let-7g inhibitor in hyperlipidemic apolipoprotein E knockout mice increased vascular expression of TGF–β-inducible genes (including PAI-1), accentuated macrophage infiltration in the carotid artery, and caused overgrowth of the intima media. In patients with lacunar stroke, low serum Let-7g levels were associated with higher plasma PAI-1 levels. Taken together, the findings suggest that Let-7g preserves endothelial function and suppresses inflammation induced by metabolic dysregulation through effects dependent, at least in part, on downmodulation of the TGF-β/PAI-1 axis. Considering the recent explosion in study results implicating various miRNAs in phenotypic modulation of vascular cells, how does the current study impact our understanding of the pathophysiology of vascular disease?
Let-7g as an endogenous inhibitor of endothelial inflammatory activation
Vascular homeostasis is dependent on preserved structure and optimal function of the endothelial layer (7). Normal, healthy endothelial cells prevent leukocyte adhesion, inhibit thrombogenic processes, and serve as an effective barrier between the blood and the vascular wall. In the presence of dyslipidemia, proinflammatory activation of the endothelium by oxidatively-modified lipoproteins plays an important role in the pathogenesis of atherosclerosis (8). In the presence of metabolic dysregulation or increased shear stress, a healthy endothelial layer may serve as a source of inhibitory mediators that prevent or restrain the inflammatory reaction. The current study suggests that Let-7g may be a key endogenous suppressor of endothelial inflammatory activation; in its absence, endothelial cells acquire a proinflammatory phenotype that triggers vascular injury and may also exert prothrombotic actions. Associative evidence in both mice and human patients links hyperlipidemia with lower Let-7g levels and suggests that down-regulation of Let-7g is associated with increased circulating levels of PAI-1, a marker of endothelial dysfunction and thrombogenic activation. Although the study reveals a novel and potentially important role for Let-7g in the pathogenesis of vascular disease, the findings also raise several important questions.
Is reduced Let-7g expression causally involved in the pathogenesis of atherosclerotic disease?
Although hyperlipidemia is associated with reduction in Let-7g levels (9), it is unclear whether down-modulation of Let-7g is a primary event that triggers inflammatory activation in subjects with metabolic dysregulation or simply represents an epiphenomenon. The mechanisms of Let-7g regulation in endothelial cells remain unknown. Growth factors and mitogen-activated protein kinase signaling have been demonstrated to reduce Let-7g expression in carcinoma cells (10) and may have similar effects on the endothelium. A growing body of evidence suggests that inflammatory activation is associated with suppression of Let-7g expression. In human volunteers, endotoxin infusion reduced leukocyte Let-7g levels (11); however, whether innate immune signals are implicated in the regulation of Let-7g remains unknown. In tumor cells, loss of Let-7g increases invasiveness by accentuating matrix metalloproteinase activity (10). The involvement of matrix metalloproteinases in plaque rupture raises the intriguing possibility that Let-7g downregulation in susceptible individuals may promote formation of unstable lesions.
Which molecular signals mediate Let-7g-induced preservation of endothelial function?
The current investigation suggests the involvement of TGF-β down-modulation in mediating the effects of Let-7g on the vasculature. This novel mechanistic finding is an important contribution of the study and, considering the broad involvement of the TGF-β system in tissue injury, neoplasia, and angiogenesis, may have implications in understanding the actions of Let-7g in a wide range of pathophysiologic conditions. However, regulation of TGF-β signaling is unlikely to be the only (or even the most important) action of Let-7g. Most miRNAs target multiple messenger RNAs; as a result, their actions are rarely explained through modulation of a single mediator. For example, the current study also demonstrates that Let-7g down-regulates expression of thrombospondin (TSP)-1, a matricellular protein with potent direct angiostatic properties (12,13). Thus, the angiogenic effects of Let-7g may be explained by the reduction of endothelial TSP-1 levels without the involvement of TGF-β signaling. Moreover, interpretation of these in vitro observations is complicated by the notoriously pleiotropic and context-dependent actions of TGF-β. Even the most robust and highly-reproducible in vitro actions of TGF-β are often of limited relevance to the understanding of pathophysiologic responses, because the in vivo context involves multiple cell types targeted by TGF-β and implicates many other mediators that may modulate its effects. Unfortunately, direct in vivo evidence demonstrating a critical role for TGF-β modulation in mediating effects of Let-7g is lacking.
Are miRNAs therapeutic targets in vascular disease?
If Let-7g is required for suppression of endothelial inflammation under conditions of metabolic dysregulation, mimicking Let-7g actions may represent a promising therapy for atherosclerotic disease. Modulation of other miRNAs may also have therapeutic potential: systemic delivery of miRNA-181b inhibits vascular inflammation and attenuates atherosclerosis in apolipoprotein E knockout mice (14), whereas miRNA-146 has been shown to inhibit endothelial proinflammatory activation (15). Despite these promising early observations, implementation of miRNA targeting strategies in vascular disease remains a daunting task. In addition to the technical challenges in the development of reproducible approaches for inhibition or repletion of selected miRNAs, concerns regarding the lack of specificity of miRNAs, their complex actions, and the apparent redundancy of the miRNA system pose major challenges to therapeutic applications. Because most miRNAs target multiple pleiotropic mediators, the consequences of their modulation in human patients could be unpredictable. Moreover, because several distinct miRNAs can target the same gene, redundancy in their functions may complicate attempts at therapeutic modulation. These pitfalls are reflected in the conflicting findings often reported in the literature with the use of various approaches targeting specific miRNAs. In this rapidly evolving field, more robust in vivo experimentation and careful dissection of the pathophysiologic pathways are needed to support the potential clinical usefulness of miRNA-targeting strategies.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Frangogiannis has reported that he has no relationships relevant to the contents of this paper to disclose.
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- Let-7g as an endogenous inhibitor of endothelial inflammatory activation
- Is reduced Let-7g expression causally involved in the pathogenesis of atherosclerotic disease?
- Which molecular signals mediate Let-7g-induced preservation of endothelial function?
- Are miRNAs therapeutic targets in vascular disease?