Journal of the American College of Cardiology
Ó 2014 by the American College of Cardiology Foundation
Published by Elsevier Inc.
EDITORIAL COMMENT
MicroRNAs and
Endothelial Function
Many Challenges and
Early Hopes for Clinical Applications*
Nikolaos G. Frangogiannis, MD
Bronx, New York
The human genome contains more than 20,000 proteinencoding 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).
See page 1685
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)b signaling. In vitro, Let-7g exerted anti-inflammatory actions
on endothelial cells, suppressed expression of plasminogen
activator inhibitor (PAI)-1 through downmo-dulation of
*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.
From the Wilf Family Cardiovascular Research Institute, Department of Medicine
(Cardiology), Albert Einstein College of Medicine, Bronx, New York. Dr. Frangogiannis has reported that he has no relationships relevant to the contents of this paper
to disclose.
Vol. 63, No. 16, 2014
ISSN 0735-1097/$36.00
http://dx.doi.org/10.1016/j.jacc.2013.10.056
TGF-b 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–b-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-b/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 oxidativelymodified 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
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JACC Vol. 63, No. 16, 2014
April 29, 2014:1695–6
Frangogiannis
MicroRNAs and Endothelial Function
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-b 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-b 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-b 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-b
signaling. Moreover, interpretation of these in vitro observations is complicated by the notoriously pleiotropic and
context-dependent actions of TGF-b. Even the most robust
and highly-reproducible in vitro actions of TGF-b are often
of limited relevance to the understanding of pathophysiologic
responses, because the in vivo context involves multiple cell
types targeted by TGF-b and implicates many other mediators that may modulate its effects. Unfortunately, direct
in vivo evidence demonstrating a critical role for TGF-b
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 miRNA181b 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.
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. E-mail: nikolaos.
[email protected].
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Key Words: endothelial function
TGF-b pathway.
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Let-7g
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PAI-1
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SIRT-1
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