1. Introduction
Since the first discovery of the epidermal growth factor receptor (EGFR) in the mid-1970s, over 40,000 papers have described roles in nearly every known cellular process and in numerous disease states. In this review, we focus on the reported activity of EGFR and its ligands in cardiovascular disease.
EGFR is the inaugural member of the ErbB family of receptor tyrosine kinases and consists of an extracellular ligand binding domain, a single α-helical trans-membrane domain, an intracellular tyrosine kinase domain, and a carboxy (
C)-terminal region that contains autophosphorylation sites. Upon ligand binding, EGFR undergoes a transition from an inactive monomer to an active homodimer or heterodimer with other ErbB family members (ErbB2/HER2/neu, ErbB3, and ErbB4). EGFR dimerization stimulates its intracellular protein tyrosine kinase activity, leading to autophosphorylation of several tyrosine residues in the
C-terminal domain of the EGFR; consequently, it elicits downstream activation and signaling by several other proteins, including mitogen-activated protein kinases (MAPK), phosphoinositide-3 kinase (PI3K), phospholipase C-γ (PLC-γ), and c-Src [
1]. As such, EGFR serves as signaling hub, engaging in cross-talk with multiple pathways (
Figure 1). The downstream signaling proteins initiate several signal transduction cascades, principally the extracellular signal-regulated kinase (ERK)/MAPK, PI3K/Akt and jun kinase (JNK) pathways, thereby regulating cell proliferation, survival, differentiation, migration, inflammation, and matrix homeostasis [
1]. In addition to the classical pathway of activation, EGFR can be activated through G protein-coupled receptors (GPCR) without direct interaction with GPCR agonists, an event referred to as transactivation [
2,
3]. EGFR transactivation mediates several downstream signaling cascades such as ERK activation [
4,
5]. These events are mediated by pathways involving classic second messengers (Ca
2+, diacylglycerol (DAG)) as well as protein kinases, non-receptors tyrosine kinases, matrix metalloproteinases (MMP), and reactive oxygen species (ROS) [
4,
6].
In addition to canonical cell-surface EGFR activity, EGFR also localizes to the nucleus via retrograde translocation, where it regulates cell growth via transcriptional activation of cyclin D1 [
7,
8]. More recently, it has been shown that EGFR is also proteolytically processed, and the intracellular domain fragment is present in both the cytosol and the nucleus [
9,
10]. Thus, multiple mechanisms exist whereby activated EGFR controls cellular processes.
Ligands of EGFR, which include epidermal growth factor (EGF), transforming growth factor-α (TGF-α), heparin-binding EGF (HB-EGF), amphiregulin (AREG), betacellulin (BTC), and epiregulin (EREG), are synthesized as transmembrane precursors that must be cleaved by MMPs, including the ADAM (a disintegrin and metalloproteinase) family of proteases, to release mature ligands. EGFR ligands are derived from glycoprotein precursors consisting of an extracellular region, a transmembrane domain and a cytoplasmic domain; therefore, proteolytic cleavage can result in release of soluble growth factors [
11]. Cleavage occurs between the first and second motifs of the immature ligand precursor, leading to release of the subunit adjacent to the plasma membrane as a mature ligand [
12].
While in general the mechanism of cleavage is consistent among the EGFR ligands, some differences have been described. For example, ADAM-mediated liberation of EGF occurs via binding of the ADAM disintegrin domain to EGF and cleavage by its metalloproteinase domain, leading to shedding of mature active EGF [
13]. TGF-α is cleaved at two sites within its extracellular domain. The well characterized ADAM17 (also known as tumor necrosis factor-α converting enzyme/TACE) and ADAM10 are capable of cleaving TGF-α at its
N-terminal site, but the identity of the
C-terminal protease remains unclear [
14]. HB-EGF is primarily cleaved by MMP-3 and MMP-7 [
15,
16]. Thus, activation of EGFR is complex and multifactorial.
2. EGFR in Cardiac Development
EGFR is implicated in proliferation and development of epithelial cells in multiple organs based on evidence that EGFR-deficient mice suffer from abnormalities in the skin, kidney, brain and gastrointestinal tract [
17–
21]. In these surviving mice and in the
wa-2 mice (mice with a global reduced EGFR kinase activity) the major cardiac defect is valvular, specifically related to the aortic valve. For example, many
wa-2 mice die before weaning. Those surviving to three months develop significantly thicker aortic cusps, culminating in severe aortic stenosis, left ventricular hypertrophy and heart failure [
22,
23]. Histological examination of hearts from
wa-2 mice revealed extensive calcification and inflammatory changes mimicking those of human aortic stenosis [
23]. The cell types mostly affected are epithelial and glial. Roles for the other ErbB family members in cardiac development have also been provided by studies of mice deficient in ErbB2 and ErbB4 [
24,
25]. Furthermore, a single nucleotide polymorphism of ErbB4 is associated with congenital cardiovascular abnormalities in humans, specifically left ventricular outflow tract defects [
26]. In addition, mice deficient in EGF-like ligands, such as HB-EGF, have decreased survival and abnormalities in aortic valve and ventricular development similar to those in
wa-2 mice and ErbB2-deficient mice [
27]. Finally, deficiency of ADAM10 yields similar impairment of heart development, presumably through alterations in EGF-like ligand bioavailability [
28]. Taken together, these studies demonstrate essential roles for EGFR family members in normal cardiac development.
5. Therapeutic Implications
As we have reviewed herein, there is growing evidence for causal roles for EGFR and its ligands in the development and progression of many cardiovascular abnormalities. Mutations in EGFR have been documented in many cancers and impact response to therapy. However, it is important to note that the majority of these mutations are somatic rather than germline mutations and, to date, no mutations in EGFR have been reported that alter its activity in the cardiovascular system.
Based on the role of EGFR and its ligands in cardiovascular disease, EGFR inhibitors should be explored as a therapeutic strategy. Numerous inhibitors of EGFR family members have been developed for clinical applications in solid tumors. To date, three small molecule tyrosine kinase inhibitors (TKIs, EGFR–gefitinib and erlotinib; EGFR/HER2–lapatinib) and four monoclonal antibodies (mAbs, EGFR–cetuximab and panitumumab; HER2–trastuzumab and pertuxumab) have been FDA-approved for use in breast, colorectal, head and neck, non-small cell lung, and pancreatic cancers. The TKIs are ATP analogs that bind in the ATP binding pocket of the tyrosine kinase domain with high affinity, thereby preventing receptor activation. Due to the fairly distinct structure of the tyrosine kinase domain in EGFR, HER2, and HER4, the TKIs targeting these receptors have little effect on catalytic activity of other tyrosine kinase receptors. All FDA-approved TKIs are reversible inhibitors, though irreversible inhibitors are being tested in clinical trials. The mAbs bind to the extracellular domain to antagonize ligand binding and induce receptor internalization without activating the receptor (
i.e., cetuximab) or prevent dimerization (
i.e., pertuzumab). In addition, some studies of mAbs (
i.e., trastuzumab) have demonstrated additional therapeutic benefit due to antibody-dependent cell-mediated cytotoxicity, a process in which binding of the antibody to target cancer cells elicits an anti-tumor immune response [
98].
Highlighting the pivotal role for EGFR family members in cardiovascular homeostasis, the EGFR family targeted therapies are associated with adverse cardiac effects, the most notable of which is cardiotoxicity associated with the HER2 mAb trastuzumab [
99]. Toxicity ranges from subclinical abnormalities like asymptomatic decline in ejection fraction to more severe events such as congestive heart failure and acute coronary syndrome [
100]. While inhibition of EGFR may produce adverse cardiovascular effects, they are generally manageable [
99] and do not necessarily preclude consideration of EGFR inhibitors for the treatment of cardiovascular disorders.