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INTRODUCTION AND BACKGROUND
Superparamagnetic iron oxide nanoparticles (SPION) have been widely used in the diagnosis
and treatment for cardiovascular diseases.
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Correspondingly, the myocardial tissue safety of SPION is becoming a bottleneck to
seriously restrict its clinical translation. In recent years, in vitro and in vivo
experiments have confirmed that SPION‐induced oxidative stress of normal myocardium
in mice, leading to myocardial cell injury, apoptosis or necrosis.
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More alarmingly, SPION applied to ischemic myocardium could accumulate in the target
sites for a long time with high concentration,
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thereby probably further aggravating oxidative stress injury and cardiomyocytes death.
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So far, however, the specific molecular mechanism of cardiotoxicity of SPION remains
unclear. Previous studies have reported that SPION‐induced apoptosis of murine macrophage
(J774) cells
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and necrosis of human endothelial cells.
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SPION can selectively induce autophagy‐mediated cell death of human cancer cells (A549).
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After SPION pre‐treatment, H9C2 cardiomyocytes were exposed to acrolein or H2O2, leading
to reactive oxygen species (ROS) dependent cell necrosis.
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Our in vitro experiment showed that SPION significantly increased oxidative stress
damage to overactivate autophagy and endoplasmic reticulum stress, eventually resulting
in cardiomyocyte apoptosis.
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Furthermore, SPION could elicit IL‐1βrelease and pyroptosis in macrophages, especially
with the octapod and plate morphology.
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Notably, it has been recently reported that sorafenib or cisplatin assembled into
nano‐devices containing SPION, which are phagocytized by tumour cells and degraded
into free divalent iron to accelerate Fenton reaction, leading to the lipid peroxidation
burst to promote ferroptosis of tumour cells.
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,
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Taken together, SPION can induce apoptosis, necrosis, autophagy, pyroptosis or ferroptosis
in vitro and in vivo studies. The discrepancy may be attributed to distinct cell types
and experiments design. It has already been well documented that the toxicity of SPION
is mainly due to its degradation and release of free iron to catalyse Fenton reaction,
leading to oxidative stress by a large number of ROS generation.
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Then, what is the downstream molecular mechanism of SPION mediated cardiotoxicity?
Ferroptosis is a novel form of regulated cell death characterized by the iron‐dependent
accumulation of lipid peroxides to lethal levels, which is morphologically, biochemically,
and genetically distinct from apoptosis, necroptosis and autophagy.
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Recent studies found that ferroptosis is not only an important pathological mechanism
in the case of circulating iron overload of hemochromatosis,
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but also a key molecular mechanism of cellular iron overload in doxorubicin (DOX)
induced cardiomyopathy.
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DOX induced mitochondria iron overload by down‐regulating ABCB8,
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a mitochondrial protein that facilitates iron export, to elicit lipid peroxidation
and mitochondria dysfunction, eventually causing cardiomyocytes ferroptosis.
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Mice that were subjected to 30 minutes of myocardial ischemia followed by 24 hours
of reperfusion had significantly higher levels of cardiac non‐heme iron, cardiac ferritin
H, ferritin L and Ptgs2 mRNA. Both ferroptosis inhibitor Ferrostatin‐1 (Fer‐1) and
iron chelator Dexrazoxane (DXZ) pre‐treatment significantly reduced I/R‐induced cardiac
remodelling and fibrosis, indicating that ischemia‐reperfusion could also induce cardiomyocytes
iron overload to cause ferroptosis and subsequent left ventricular remodelling.
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Myocardial haemorrhage is a frequent complication after successful myocardial reperfusion,
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,
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which is associated with residual myocardial iron in post‐myocardial infarction (MI)
patients received reperfusion therapy.
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It is reasonable to infer that this iron accumulation has a potential to generate
excessive ROS and trigger pathological events such as ferroptosis. A previous study
also confirmed that ferroptosis is a significant type of cell death in cardiomyocytes;
moreover, mechanistic target of rapamycin (mTOR) was found to play an important role
in protecting cardiomyocytes against excess iron and ferroptosis by regulating ROS
production.
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In addition, glutathione peroxidase 4 (GPX4), which protects cells from ferroptosis,
was down‐regulated in the early and middle stages of MI mouse model, suggesting that
ferroptosis during MI was in part due to a reduction in GPX4 protein.
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Even though signalling pathways of ferroptosis in cardiovascular diseases is not yet
well characterized, it has been confirmed that ischemia‐reperfusion (I/R) could induce
mitochondrial iron overload in cardiomyocytes rather than the increase of iron content
in cytoplasm.
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In this study, mice treated with 2,2′‐bipyridyl (BPD), which has high membrane permeability
and thus is able to access mitochondria, had demonstrated protective effects on I/R
myocardium, while deferoxamine (DFO) failed to protect mice against I/R damage due
to poor penetrance into mitochondria. Notably, overexpression of ABCB8 in cardiomyocytes
in mice reduces mitochondrial iron and protects against I/R damage,
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suggesting that ABCB8 might play an important role in maintaining iron homeostasis
in myocardial mitochondria and regulating ferroptosis after I/R injury. Thus, it is
not difficult to speculate that SPION could aggravate mitochondrial iron load in I/R
myocardium. SPION applied in ischemic myocardium could be directly degraded by cardiomyocytes,
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leading to severe mitochondrial iron overload.
We detected prominently mitochondrial lipid peroxidation (malondialdehyde, MDA), mitochondrial
membrane potential (MMP) loss and ATP depletion at 24 hours and 4 weeks after SPION
injected into the peri‐infarcted zones of myocardial ischemia‐reperfusion rats compared
with the control group (all P < .01). We found that iron content of mitochondria was
significantly higher than that in the control group (P < .001), and the distorted
mitochondria were observed by transmission electron microscopy in the SPION group,
suggesting that SPION have the potential to destroy mitochondrial structure and function
by inducing mitochondria iron overload (data not published). Mitochondria are the
major site of iron metabolism and ROS production, thereby cardiomyocytes iron accumulation
is especially prone to induce mitochondria iron overload to trigger mitochondrial
oxidative damage. Based on the above results, we speculate that SPION might further
promote ferroptosis to aggravate left ventricular remodelling and cardiac deterioration
by inducing severe mitochondria iron overload to promote lipid peroxidation burst.
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HYPOTHESIS
To summarize, we speculate that SPION applied to ischemic myocardium could exacerbate
cardiomyocytes ferroptosis to worsen left ventricular negative remodelling through
inducing mitochondria iron overload to catalyse sustained Fenton reaction, eliciting
lipid peroxidation burst(as shown in Figure 1). This hypothesis needs to be verified
by animal experiments. Firstly, the mitochondrial iron metabolism, lipid peroxidation,
morphology and function of mitochondria and ferroptosis should be carefully detected
after SPION injected into the peri‐infarcted zones of myocardial ischemia‐reperfusion
rats. Secondly, the rat models of myocardial ischemia‐reperfusion were randomly divided
into different groups to, respectively, treated with apoptosis inhibitor, necrosis
inhibitor, autophagy inhibitor and ferroptosis inhibitor, in order to verify whether
ferroptosis play a pivotal role in cardiomyocytes death induced by SPION. Thirdly,
SPION were injected into the myocardium of I/R mice model, in which Mlkl
−/− or Fadd
−/−
Mlkl
−/− mice were employed to respectively block the pathways of myocardial cell necroptosis
or apoptosis, in order to further illustrate whether ferroptosis is the main pathway
of SPION‐induced cardiomyocytes death. Fourthly, developing SPION modified by mitochondrial
iron chelator or mitochondrial‐targeted antioxidant, the effects of two strategies
to improve the myocardial safety of SPION should be comprehensively investigated in
vitro and in vivo experiments, potentially promoting the clinical transformation of
SPION in cardiovascular field.
FIGURE 1
SPION were internalized into cardiomyocytes and further degraded into free ferrous
iron in lysosomes. The free ferrous iron entered into mitochondria, resulting in lipid
peroxidation of mitochondria to trigger ferroptosis in cardiomyocytes by a large amount
of ROS produced via Fenton reaction
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IMPLICATION
The cardiotoxicity of SPION limits its diagnostic or therapeutic application in the
cardiovascular field. It is helpful to promote the clinical transformation of SPION
in cardiovascular field through rescuing the key target of SPION‐induced cardiomyocyte
ferroptosis to improve the myocardial tissue safety. If our hypothesis is true, given
that SPION mainly induce mitochondria iron overload of ischemic cardiomyocytes to
catalyse lipid peroxidation and exacerbate ferroptosis, and then it is expected to
significantly inhibit ferroptosis induced negative remodelling of ischemic myocardium
by mitochondrial iron chelator or mitochondrial‐targeted antioxidant peptide modifying
SPION to effectively protect mitochondria.
CONFLICT OF INTEREST
The authors confirm that there are no conflicts of interest.