2026 年 49 巻 1 号 p. 24-29
Neuregulin 1 (NRG1), a member of the epidermal growth factor (EGF) family, regulates the development, differentiation, proliferation, and plasticity in multiple tissues through its binding to ErbB3 and ErbB4 receptors. In the cardiovascular system, NRG1 plays a crucial role in cardiac development, physiological function, and cell survival. Since NRG1 exerts cardioprotective effects through its interaction with ErbB2/ErbB4 and ErbB4 homodimers on cardiomyocytes, the administration of recombinant human NRG1 has the potential for the treatment of heart failure. ErbB2 is known as human epidermal growth factor receptor 2 (HER2), which is overexpressed in approximately 20% of breast cancers. Trastuzumab (TRZ), a humanized monoclonal antibody targeting ErbB2/HER2, is used for the therapy of HER2-positive breast cancer. However, cardiotoxicity has been observed in approximately 5–10% of patients treated with TRZ. Risk factors for the onset of cardiotoxicity include the use of anthracyclines, hypertension, and diabetes. However, the mechanism linking diabetes and TRZ-induced cardiotoxicity remains unclear. Recently, we reported that the serum levels of NRG1 were elevated in the mouse model of diabetic cardiomyopathy. We found that the up-regulated NRG1 compensates for insulin deficiency to maintain systolic function in the early stage of diabetic cardiomyopathy. This review aims to discuss the physiological roles of NRG1–ErbB2 signaling in the cardiovascular system, the cardioprotective effects of NRG1 and its clinical applications, and the molecular mechanisms of TRZ-induced cardiotoxicity through the blockade of the NRG1–ErbB2 signaling pathway.
Neuregulin1 (NRG1) is a member of the epidermal growth factor (EGF) family. It undergoes processing by metalloproteinase and then binds to receptor tyrosine kinase erythroblastic leukemia viral oncogene homolog (ErbB). The binding of NRG1 to ErbB induces homodimerization (ErbB4/4) or heterodimerization (ErbB2/3 or ErbB2/4), promoting the proliferation, differentiation, and protection of cells in multiple tissues, including the cardiovascular system.1–3)
Human epidermal growth factor receptor 2 (HER2), also known as ErbB2 and Neu, is up-regulated in approximately 20% of breast cancers. HER2 overexpression is associated with aggressive tumor growth and poor prognosis.4–6) Trastuzumab (TRZ), a humanized monoclonal antibody targeting HER2, is the standard treatment for HER2-positive breast cancer and gastric cancer.7,8) TRZ may cause cardiotoxicity as a side effect. Epidemiological studies have reported various risk factors, including the use of anthracycline-based drugs.9–12) The block of the cardioprotective NRG1–ErbB2/HER2 signaling pathway by TRZ is considered to manifest the anthracycline-induced cardiac dysfunction.
In this review, we will provide a brief description of NRG1–ErbB2 signaling, followed by an overview of the cardioprotective effects of NRG1 and its clinical applications. Furthermore, we will discuss the molecular mechanism of TRZ-induced cardiotoxicity (TIC) and its relationship with diabetes as a risk factor.
NRG1 consists of an N-terminal sequence, an EGF-like domain, and a transmembrane domain. Multiple isoforms of NRG1 are produced through alternative splicing.13–15) NRG1 is synthesized as a single transmembrane protein, translocated to the cell membrane, and activated by the protease ADAM metallopeptidase domain 17 (ADAM17).2,16) The EGF-like domain sequence of NRG1 binds specifically to its receptors ErbB3 and ErbB4.17) NRG1 plays a crucial role primarily in the nervous system, heart, and breasts. There is also evidence that the NRG1/ErbB signaling pathway is involved in the development and function of multiple organ systems, as well as human diseases.13) In the heart, NRG-1 is expressed exclusively in the endocardium, vascular endothelium, and epicardium.18,19)
ErbB receptors include four isoforms: ErbB1, ErbB2, ErbB3, and ErbB4, among which NRG-1 binds to ErbB3 and ErbB4.15) ErbB2 cannot interact directly with NRG-1, but it is an important receptor involved in NRG1 signaling by forming heterodimers with ErbB3 or ErbB4. In recent years, three-dimensional crystal structural analysis has revealed the binding conformation and activation mechanism of NRG1 and ErbB receptors.20,21) Downstream signals are reported to activate various pathways, including the Ras/Raf/MEK/extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT), Src/FAK, and Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) signaling pathways2,3,22,23) (Fig. 1).
NRG1 interacts with ErbB4 and induces the formation of a heterodimer with ErbB2 and/or a homodimer with ErbB4. Activated ErbB receptors trigger intracellular signaling pathways, including the PI3K/Akt, ERK/MAPK, and JAK/STAT3 pathways. NRG1 also activates the ErbB2-dependent FAK signaling pathway. The NRG1/ErbB signaling pathway mediates downstream alterations in cardiac function.
The NRG1–ErbB signaling pathway plays a crucial role in the development, survival, and homeostasis of cardiomyocytes. In the embryonic stage, NRG1 is expressed in the endocardial endothelial cells, whereas ErbB2 and ErbB4 are expressed in the cardiomyocytes.18,24) Knockout mice lacking ErbB2 or ErbB4 are lethal at embryonic day 10.5 due to defective ventricular septum formation. ErbB2-deficient embryos die before embryonic day 11 due to a lack of cardiac trabeculae.25) Mice lacking ErbB4 are lethal during the mid-embryogenesis stage due to the abnormal development of myocardial trabeculae in the ventricle.26) NRG1 knockout mice are also lethal at embryonic day 10.5 due to impaired cardiac formation.1) Nakaoka et al. reported that there is bidirectional crosstalk between vascular endothelial cells and cardiomyocytes mediated by NRG1 and Angiopoietin-1 (Ang1).27) NRG1 derived from vascular endothelial cells binds to the ErbB2/ErbB4 receptors on cardiomyocytes, resulting in the activation of downstream ERK1/2 and AKT signaling. In contrast, Ang1 is secreted from cardiomyocytes and interacts with the Tie2 receptor, a receptor-type tyrosine kinase localized on vascular endothelial cells or endocardial endothelial cells, contributing to vascular formation and maintenance.27) The NRG1/ErbB signaling pathway plays a critical role during cardiac development.
In the adult heart, NRG1 is expressed in the endocardium and vascular endothelial cells, while ErbB2 is expressed in cardiomyocytes, suggesting cell-to-cell communication. The ventricle-specific ErbB2 conditional knockout mice are viable at birth but develop dilated cardiomyopathy (DCM), characterized by chamber dilation, wall thinning, and systolic dysfunction.28) Endothelium-selective deletion of NRG1 in the mice leads to increased ischemic injury.29) After doxorubicin treatment, the heterozygous knockout mice of NRG1 show worsened survival and left ventricular function.30) Recent studies have reported that NRG1 exerts a cardioprotective effect and improves diastolic dysfunction.19,31–33) Since NRG1–ErbB signaling plays a crucial role in maintaining and protecting cardiac function in the adult heart, it has attracted attention as a therapeutic target for heart failure.
Since NRG1/ErbB signaling has been reported to exert cardioprotective effects, the potential of administering recombinant NRG1 for treating heart failure has been investigated. Both pre-clinical studies using animal models and clinical trials in patients with heart failure are currently underway. Liu et al. demonstrated that the intravenous administration of recombinant human NRG-1 (rhNRG1β; a 61-amino-acid peptide containing an EGF-like domain) improved systolic function and survival of animal models of ischemic, dilated, and viral cardiomyopathy.34) Gu et al. also indicated that the intravenous infusion of rhNRG1 to rats with myocardial infarction (MI) significantly improved the fractional shortening through up-regulating cardiac myosin light chain kinase.35) In patients with heart failure with reduced ejection fraction (HFrEF), Phase 2 clinical trials have been completed, confirming a sustained improvement in left ventricular ejection fraction (LVEF).36,37) A Phase 3 trial is currently in progress.
In the cardiomyocyte, both ErbB2 and ErbB4, but not ErbB3, are co-expressed, and the ErbB2/ErbB4 receptor heterodimer exerts potent cardioprotection.3) Conversely, NRG1 exerts oncogenic activity in cancer cells through the ErbB2/ErbB3 heterodimer.38,39) In order to achieve cardioprotective effects via ErbB2/ErbB4 without promoting cancer cell proliferation via ErbB2/ErbB3, engineered bivalent NRG1 (NN) was designed.40) NN was produced by linking two identical ErbB receptor binding domains of NRG1 via a hydrophilic, protease-resistant spacer.40) NN successfully inhibited the formation of ErbB2/ErbB3 heterodimer by promoting dimerization (homodimerization and heterodimerization) of ErbB3 and ErbB4 receptors. Both NN and monomeric NRG1 EGF-like domain exhibited cardioprotective effects in doxorubicin-exposed cardiomyocytes, but only NN prevented the proliferation of tumor cell lines expressing ErbB2 and ErbB3 receptors.41)
Recently, Cools et al. identified a small molecule (EF-1) that induces homodimerization of ErbB4.42) EF-1 suppressed cardiac cell death and hypertrophy through an ERBB4-dependent mechanism in vitro.42) In mice, EF-1 attenuated angiotensin II-induced fibrosis and reduced myocardial damage caused by doxorubicin and myocardial infarction.42)
The engineered NRG1 ligands and the inducer of ErbB4 dimerization are expected to promote cardioprotective effects without stimulating the proliferation of cancer cells. Further advancements are anticipated.
As mentioned above, the NRG1–ErbB signaling pathway is a promising therapeutic target for treating heart failure. On the other hand, the pathophysiological role of NRG1 in the heart had not been fully elucidated.
TRZ demonstrates significant efficacy in treating HER2-positive cancers. Cancer therapeutics-related cardiac dysfunction (CTRCD) is classified into Type 1, caused by anthracycline-based anti-cancer agents, and Type 2, caused by TRZ.43,44) Anthracycline-induced cardiomyopathy is irreversible, dose-dependent, and cumulative. By contrast, TIC is generally reversible and does not exhibit dose dependency. However, the reversibility of cardiomyopathy is controversial, and there have been reports of irreversible cases even with TIC.45,46) In addition to these two types, there have been reports of heart failure caused by various other anti-cancer therapies.11)
The presence of various risk factors exacerbates the onset of TIC.47) According to meta-analysis reports, the use of anthracycline, hypertension, and diabetes mellitus (DM) were highlighted as risk factors of TIC.48–50) There have been reports that elderly patients with TIC tend to present with risk factors such as a history of cardiovascular disease and DM.51) DM is reportedly a significant risk factor for the development of heart failure and cardiomyopathy after TRZ administration in breast cancer patients.47) We recently elucidated one of the molecular mechanisms underlying TIC in patients with a background of DM.19) In the streptozotocin (STZ)-induced type 1 DM model mice, at the early stage of diabetic cardiomyopathy, diastolic function was impaired as has been previously reported.52) On the other hand, systolic function was preserved. In these mice, the serum levels of NRG1 were significantly up-regulated compared to control mice. We observed the upregulation of NRG1 expression in the liver and kidney, suggesting that circulating NRG1 acts on cardiomyocytes in the left ventricle (LV). The physiological role of NRG1–ErbB2 signaling was elucidated by administering TRZ to mice. TRZ did not cause significant changes in either diastolic or systolic function in the control group. In sharp contrast, in the TRZ-treated STZ (STZ-TRZ) group, LV systolic function was significantly reduced without change in diastolic function. The phosphorylation levels of Akt in the STZ group were significantly lower than in the control group, and it was even lower in the STZ-TRZ group. The up-regulated NRG1–ErbB2–Akt signaling pathway appears to compensate for insulin deficiency to maintain systolic function in the early stages of diabetic cardiomyopathy (Fig. 2). Based on our results, the higher TIC risk of DM may be explained as that the TRZ treatment deprives the compensation by NRG1–ErbB2–Akt signaling pathway and then promotes the development of diabetic cardiomyopathy in patients with DM.
In a non-diabetic heart (left), insulin binds to the insulin receptor, and maintains the phosphorylation of Akt. Both LV diastolic and systolic functions are normal. In the cardiomyocyte at the early stage of diabetic cardiomyopathy (middle), impaired insulin secretion and insulin resistance reduce the phosphorylation levels of Akt. However, a compensatory increase in NRG1 prevents the progression to systolic dysfunction. TRZ administration further reduces Akt phosphorylation levels in STZ-4W (right). Blockade of the NRG1–ErbB2/4–Akt axis accelerates the progression to systolic dysfunction.
Our previous study suggests that NRG1 is up-regulated to compensate for insulin deficiency and mediates cardioprotective effects against systolic dysfunction in the early stage of diabetic cardiomyopathy.19) Interestingly, the levels of NRG1 have also been reported in different types of cardiomyopathies and heart failure. We re-analyzed a dataset of LV from T2DM patients (transcriptome analysis of DM and non-DM patients affected by post-ischemic heart failure),19,53) which revealed that the NRG1–ErbB2–Akt axis was significantly up-regulated in a subgroup with DM. In the dog model of late pacing-induced cardiomyopathy, the ventricular levels of NRG1 activated ErbB2 and ErbB4, and the levels of phosphorylated Akt increased progressively.54) Ky et al. quantified serum NRG-1β in patients in the Penn Heart Failure Study, a referral cohort representing a broad spectrum of systolic heart failure.55) They found that the circulating NRG-1β is independently associated with heart failure severity and risk of death or cardiac transplantation. Rohrbach et al. reported that the expression levels of NRG1 mRNA and protein are increased in the human left ventricular tissue of failing hearts.56) In patients undergoing mechanical unloading with a ventricular assist device (VAD), total NRG1 mRNA levels were significantly reduced compared to the very high levels prior to VAD use.56) The effect of endogenous serum NRG1β concentration in patients with systolic heart failure was examined, and a positive correlation was observed between LVEF values and the baseline levels of serum NRG1.57) Recent single-cell and single-nucleus RNA sequencing studies have shown that NRG1 expression is enhanced in the ventricle of DCM patients.58,59) In nonischemic DCM, heart rate (HR) reduction induced by ivabradine promotes LV reverse remodeling. In the endomyocardial biopsy samples collected from patients with nonischemic DCM treated with ivabradine, the gene expression levels of NRG1 correlated with LV reverse remodeling by HR reduction.60) These results suggest that the upregulation of NRG1 in the endothelial cells of DCM may promote reverse remodeling and improve systolic function. Although further evidence is required, it is possible that the compensatory role of NRG1 is a common mechanism in various types of cardiomyopathies. Furthermore, since NRG1 is also considered to be a mediator of intrinsic reverse remodeling,61) circulating NRG1 may be utilized as a diagnostic marker in cardiomyopathy.
This review has primarily discussed the crucial roles of the NRG1–ErbB signaling pathway in maintaining cardiovascular function. Our study and the clinical data suggest that NRG1 is up-regulated in the LV of the hearts with cardiomyopathy and plays a compensatory cardioprotective role. Since NRG1 is expressed in various cell types, it may exert its effects on the heart through intercellular and interorgan interactions (Fig. 3). Clinical trials of rhNRG1 in the cardiovascular system are ongoing, offering hope for the development of safer and more effective therapeutic drugs. Understanding the role of NRG1 through intercellular and interorgan communication will provide new insights into pathophysiological mechanisms and therapeutic targets for cardiovascular diseases and the cardiotoxicity of anti-cancer drugs.
The NRG1/ErbB signaling pathway promotes neurogenesis and synapse formation in the developing brain. Furthermore, NRG1 produced by axons plays a role in the control of myelination. In the adult brain, NRG1 and ErbB4 are expressed in multiple regions and regulate synaptic plasticity. Single-nucleotide polymorphisms (SNPs) in NRG1 are associated with schizophrenia. In the cardiovascular system, NRG1 plays a crucial role in maintaining systolic function, survival of cardiomyocytes, and homeostatic metabolism. The blockade of the NRG1–ErbB2 signaling pathway by TRZ increases the risk of cardiotoxicity. Compensatory upregulation of NRG1 has been suggested to occur in the hearts of patients with various cardiomyopathies, including diabetic cardiomyopathy.
This work was supported by JSPS KAKENHI [Grant numbers: JP21K06810 and JP24K10043 to Y.M.]
The authors declare no conflict of interest.
