EGF receptor in organ development, tissue homeostasis and regeneration

Abstract

The epidermal growth factor receptor (EGFR) is a protein embedded in the outer membrane of epithelial and mesenchymal cells, bone cells, blood and immune cells, heart cells, glia and stem neural cells. It belongs to the ErbB family, which includes three other related proteins: HER2/ErbB2/c-neu, HER3/ErbB3, and HER4/ErbB4. EGFR binds to seven known signaling molecules, including epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-α). This binding triggers the formation of receptor pairs (dimers), self-phosphorylation of EGFR, and the activation of several signaling pathways within the cell. These pathways influence various cellular processes like proliferation, differentiation, migration, and survival. EGFR plays a critical role in both development and tissue homeostasis, including tissue repair and adult organ regeneration. Altered expression of EGFR is linked to disruption of tissue homeostasis and various diseases, among which cancer. This review focuses on how EGFR contributes to the development of different organs like the placenta, gut, liver, bone, skin, brain, T cell regulation, pancreas, kidneys, mammary glands and lungs along with their associated pathologies. The involvement of EGFR in organ-specific branching morphogenesis process is also discussed. The level of EGFR activity and its impact vary across different organs. Factors as the affinity of its ligands, recycling or degradation processes, and transactivation by other proteins or environmental factors (such as heat stress and smoking) play a role in regulating EGFR activity. Understanding EGFR's role and regulatory mechanisms holds promise for developing targeted therapeutic strategies.

Keywords: EGF ligands; EGFR; Mammalian development; Signaling; Tissue homeostasis.

Fig. 1

EGFR structure and activation. The EGFR has an extracellular domain, trans- and juxtamembrane segments, and an intracellular kinase domain. A Crystal structure of the EGF-bound EGFR ectodomain homodimer (PDB: 8HGS) [252]. The EGFR extracellular domain has four flexible domains (I, II, III and IV). In the absence of ligand, the monomeric extracellular domain assumes a tethered conformation that precludes dimerization. Upon EGF binding (between domains I and III), a conformational change occurs in particular at domain IV (green arrows), resulting in exposure of the dimerization arm of domain II (red circle), thereby increasing dimerization. B Crystal structure of the active EGFR kinase domain dimer (PDB: 2GS2) [253]. The domain is intrinsically autoinhibited when monomeric, and is activated by intermolecular interaction, with the formation of an asymmetric dimer. C–E Cartoons describing EGFR activation. C The EGFR monomer is inactive. D An inactive dimer may be formed, according to Arkhipov et al. [254]. E Activation occurs upon EGF binding, that stabilizes the dimeric form, determining dimerization of the juxtamembrane segments, and formation of asymmetric (active) kinase dimers

Fig. 2

EGFR ligands and signaling pathways activation. A EGFR activation: Seven ligands, including EGF, HB-EGF, AREG, TGF-α, epiregulin, EREG, BTC, and EPGN bind EGFR, promoting its dimerization and autophosphorylation which activates downstream signaling pathways, such as ERK MAPK, PI3K-AKT, SRC, PLC-γ1-PKC, JNK and JAK-STAT. These proteins regulate several genes involved in the regulation of different cellular process: cell growth, survival, proliferation, differentiation, angiogenesis and migration. B, C EGFR trans-activation: G protein-coupled receptor (GPCR)-mediated process leads to EGFR signaling activation by two mechanisms: B Activation of metalloprotease group, ADAMs protein that cleave EGFR ligands and release them into the extracellular space. These ligands bind EGFR triggering the signaling cascade activation. C Activation of intracellular SRC protein tyrosine kinases involved in the phosphorylation of EGFR in its cytosolic domain, representing a ligand-independent mechanism. D, E EGFR internalization: Upon EGFR activation, it undergoes to two endocytosis mechanism due to different EGF concentration. Calcium release from ER helps EGFR internalization. D At low EGF level, EGFR is incorporated into clathrin-coated pits and is rapidly internalized to endosomes in a complex process named clathrin-mediated endocytosis (CME). Then, EGFR is recycled back to the plasma membrane, leading to signal propagation. E At high EGF level, EGFR is internalized, ubiquitinated by E3 ligase Cbl and sorted to lysosomes for protein degradation through a process named non-clathrin-mediated endocytosis (NCE)

Fig. 3

The EGFR role in the placenta development. This image shows both EGFR role in the generation of healthy placenta and in the Preeclampsia disease, due to its dysregulation. A, B Healthy placenta: A EGFR leads to a reduction of KISS1, via AKT activation and ID3 downregulation, promoting trophoblast cell invasion, essential to anchor placental villi to the maternal decidua. B Trophoblast cellular invasion and embryo implantation are favored by extracellular matrix (ECM) remodeling due to activation of different metalloproteases expression, such as MMP9 and MMP2, upon EGF/EGFR signaling in trophoblast cells. C, D, Preeclampsia disorder: C The upregulation of EGFR, caused by hypoxic condition, leads to the activation of ERK1/2 and STAT3, and induces the secretion of the antiangiogenic molecule sFlt-1 (soluble FMS-like tyrosine kinase-1) from placenta. sFlt-1 sequesters VEGF and impairs VEGFR activation, affecting angiogenesis process in the placenta. D Bisphenol S (BPS) plays a role as ligand of EGFR, impairing its downstream signaling pathways activation and causing a reduction of proliferation and invasion of extravillous trophoblast cells. These events affect the development of placenta

Fig. 4

The EGFR roles in IECs proliferation and gut regeneration. This image highlights the various ways EGFR contributes to IEC proliferation and gut regeneration. Here's a breakdown of the mechanisms: A EGF-EGFR Binding: High levels of EGFR and its ligand EGF are found in epithelial intestinal cells. EGF binding to EGFR triggers proliferation both directly and indirectly. Directly, EGFR signaling activation induces phosphorylation of the nuclear repressor factor Capicua, promoting its translocation into the cytoplasm, and the consequent transcription of cell cycle regulators, such as Cdc25 and Cyclin E. Indirectly, EGFR leads to the activation of Pnt and ETZ transcription factors involved in the mitochondrial biogenesis pathways. B EGFR Internalization and Activation: Following gut injury (e.g., from radiation), a protein called Cdc42 interacts with clathrin in the endocytosis complex, favoring EGFR internalization and activation of downstream signaling pathways, that ultimately lead to cellular proliferation and tissue regeneration. C YAP and EGFR Signaling: YAP is another interactor of EGFR. YAP induces EGFR signaling by promoting the production of a growth factor called EREG in the surrounding tissue (stroma). EREG can then bind to EGFR, further controlling the balance between proliferating IECs and the differentiation of these cells into mature intestinal epithelial cells. D Heat Stress and EGFR: Heat stress disrupts EGFR signaling pathways, leading to the shedding of cells and ultimately to a breakdown of the intestinal barrier

Fig. 5

The EGFR roles in liver regeneration and fibrosis. This figure illustrates the multifaceted role of the epidermal growth factor receptor (EGFR) in both liver regeneration after injury and the development of liver fibrosis. A, B, C: EGFR Activation Promotes Liver Regeneration. A The ZBTB20 gene promotes the activation of EGFR and AKT signalling pathways, contributing to liver regeneration. B P2Y2R, a G protein-coupled receptor, activates EGFR upon binding to extracellular ATP. This activation increases phosphorylated ERK (p-ERK) levels and promotes cellular proliferation. C During liver regeneration, various ligands such as HB-EGF, TGF-α, AREG, and epiregulin become elevated. These ligands bind to EGFR, leading to the activation of downstream signalling pathways that promote cell proliferation and ultimately liver regeneration. D, E: EGFR and Liver Fibrosis. D Accumulation of bile acids (BAs) triggers EGFR transactivation, which in turn activates the MAPK/ERK and JNK signalling pathways. These pathways are involved in repressing Cyp7a1, a gene that regulates BA synthesis. Additionally, BAs activate the FXR-SOCS3 pathway, leading to STAT3 inhibition. This allows for the transcription of AREG and other genes by nuclear EGFR. E EGFR activation can induce the expression of genes involved in lipogenesis, a process that contributes to non-alcoholic fatty liver disease (NAFLD). NAFLD can progress to non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and even hepatocellular carcinoma

Fig. 6

The EGFR roles in bone development. A Intramembranous ossification: EGFR activation induces ERK phosphorylation, which increases IGFBP-3 expression. IGFBP-3 represses the IGF-1R signalling involved in the regulation of the mTOR pathways, leading to osteoblast proliferation. B Osteoarthritis: EGFR promotes anabolic and catabolic processes. EGFR loss impairs the synthesis of lubricants Prg4 and hyaluronic acid (HA), resulting in a disorganization of collagen fibers. Upon HB-EGF-EGFR binding, ERK phosphorylation and p38 MAP kinases activation leads to phosphorylation of Smad1 and inhibition of OP-1, affecting the synthesis of matrix proteins and favoring osteoarthritis. C Endochondral ossification: A crosstalk between EGFR and BMP signals regulates chondrocyte maturation, promoting a correct long bone growth and bone development

Fig. 7

The EGFR role in skin. EGFR protein is prominently expressed in the lower layers of epidermis, whereas it is downregulated during the cornification process. Altered EGFR expression promotes changes in the expression of keratinocytes markers which participate in the cellular proliferation and differentiation. A, B Upregulation of EGFR: A The increased EGFR level leads to upregulation of DcR3 that induces elevated expression of CE markers, such as involucrin and TGase, and reduced expression of early differentiation markers, including keratin 10 and loricrin. As a result, keratinocytes showed an enhanced proliferation. B A similar effect is also directed by TRPV3, which plays a role in increasing of calcium influx, promoting the CAMKII, involved in the TGFα release and EGFR activation. C, D Downregulation of EGFR and side-effects on cancer patients: C The lack of ADAMs in mice leads to inactivation of EGFR and immature production of stratum corneum, due to an increase of loricrin levels and a decrease of TGM expression. Consequently, an accumulation of inflammatory and T cell population has been reported, affecting the homeostasis of epidermis and skin inflammation development. D Use of EGFRIs in cancer patients leads to side effects such as cutaneous complications. The downregulation of EGFR impairs cellular proliferation in the basal epidermis layers as demonstrated by reduction of Ki67 and leads to pro-differentiation process, upregulating filaggrin, involucrin and Desmoglein-1. As a result, the skin barrier is affected. E EGFR and inflammation process: TGF-α leads to EGFR activation and strengthen TNF-α effect, leading to high GM- CSF expression through regulation of activator protein (AP)−1 and NFkB expression. Moreover, TNF-α promotes the phosphorylation of c-Jun involved in the transactivation of AP-1. These events lead to the increase of cytokines and macrophages that are involved in chronic inflammatory disease progression

Fig. 8

The EGFR role in brain development. This image shows the different roles of EGFR in the brain, including neurogenesis and neurodegenerative diseases and glioblastoma development. A–C EGFR regulation: A The interplay between EGFR and Notch signaling regulates the balance between NSC and NPC. EGFR reprimes Notch through Numb expression, which interacts with E3 ubiquitin ligases degrading Notch and promoting NPC proliferation and expansion. B EGFR regulates the expression of SOX2 that, in turn, binds with EGFR promoter leading to EGFR expression and signaling activation. C SHH promotes transactivation of EGFR influencing NPC pool proliferation. D–F EGFR in brain injuries: D Kainic Acid (KA) treatment and E HB-EGF ligand and zinc induce EGFR expression and MAPK-ERK signaling activation, resulting in the differentiation of NCP into reactive neural stem cells (React-NSCs) that contribute to astrocytes formation, impairing neurogenesis. F On the contrary, EGFR inhibition preserves NSC pool, stimulating the neurogenesis process through the migration of neurons towards injured regions and generation of new neurons. G EGFR role in neurodegenerative diseases: Pharmacological inhibition of EGFR leads to the decrease of oxidative stress and inflammation and to the increase of the autophagy process involved in the reduction of peptides accumulation. H EGFR role in glioblastoma: the inhibition of EGFR affects the downstream signaling pathway activation, decreasing the levels of pro-inflammatory chemokines, such as CCL2 levels, and the infiltration of tumor-associated macrophages involved in the glioblastoma development

Fig. 9

The EGFR role in T cell regulation. A EGFR role in Tregs activity: AREG-EGFR binding leads to activation of MAPK signaling that enhances the suppressive activity of Treg cells. B–D EGFR role in T helper cells activity: B Following gastrointestinal helminth infection, activated EGFR in CD4+ helper T forms a complex with IL-33R, leading to IL-13 secretion and Th2 expression. C A similar pathway promotes the differentiation of Th9 cells. The binding of AREG-EGFR, boosted by IL-33 exposure, promotes the activation of EGFR signaling pathway, including the activation of HIF1α that binds IL9 promoter, resulting in IL-9 production and enhancement of Th9 cell differentiation. D The binding of HB-EGF to EGFR causes STAT5 expression, that leads to IL-2 production involved in the survival of CD4+ cells, at the expense of Th17 cells differentiation. E Engineered CD8 + lymphocytes expressed EGFR in mice bearing B16-OVA (melanoma murine cell lines) and PM299L (hepatocarcinoma cell lines) tumors. Upon EGF ligands, CD8+ T cells respond better to EGFR signaling activation, producing more IFN-γ and leading to a delay in the tumor growth

Fig. 10

The EGFR roles in fetal and postnatal pancreas development. A β-cell commitment and differentiation: Dynamic cell polarity changes are regulated by EGFR, which drives β-cell commitment. It depends on the different EGFR ligands. EGF binds EGFR and activates PI3K and RAC1 proteins, which reduce apical polarity induction in the early pancreatic progenitor epithelium, leading to endocrine commitment towards a β-cell fate (first transition). Upon BTC-EGFR binding, RAC1 inhibits αPKC, which leads to downregulation of Notch signaling and activation of Ngn3, involved in β-cell differentiation (second transition). B β-cell proliferation: Excess of nutrients as glucose induces ChREBP expression, that promotes HB-EGF transcription and SRC kinase. SRC kinase triggers EGFR transactivation via ADAM metalloproteases. As a result, HB-EGF binds EGFR and activates its signalling cascades, including MAPK, PI3K/AKT and mTOR, leading to increase of cellular transcription and β-cell proliferation. These events are associated to diabetes development. C EGFR transactivation via hormones such as GLP1 and Placental lactogen (PL). D miR-124a acts as negative regulator of β-cells proliferation and insulin resistance. EGFR signalling activates, through ERK and AKT downstream signalling pathways, the transcription factor ETS2, which represses miR-124a, leading to expression of genes involved in insulin secretion and control β-cells development

Fig. 11

The EGFR roles in kidney fibrosis. A Kidney injury increases EGFR levels in the epithelial renal cells. EGFR signalling activation induces cellular proliferation and migration of myofibroblasts toward the sites of the injury. B, C EGFR transactivation can be caused by different proteins such as TGF‐β and Angiotensin II. TGF‐β acts through the activation of SRC kinase that phosphorylates EGFR, leading to persistent activation of EGFR; Angiotensin II increases ROS production that mediates SRC kinase activation. EGFR transactivation promotes Smad3 and fibronectin upregulation. This EGFR downstream signalling cascade favours the development of fibrosis disease

Fig. 12

The EGFR roles in mammary gland development. A EGFR role in the switch between the EMT and MET: the balance between EMT and MET represents one of the important processes for mammary gland development, regulated by EGFR signalling. EGF binds EGFR, and promotes p-ERK activation, which increases the expression of mesenchymal marker ZEB1 and decreases the expression of the epithelial marker miR-205, favouring mesenchymal phenotypic changes. Upon activation, EGFR undergoes ubiquitination and degradation, reducing EGFR expression on plasma membrane. On the contrary AREG, binds EGFR and induces a weak activation of p-ERK and increases the expression of miR-205 at the expense of ZEB1, reverting the cells characteristic from mesenchymal to epithelial phenotype. AREG-EGFR binding leads to EGFR recycling back to the plasma membrane and, consequently, prolonged signal activation. B Hormones levels and EGFR activation: hormones stimulation increases the expression of AREG, which binds EGFR. This leads to IL-17B upregulation, which induces NF-κB dependent cytokine expression, responsible for macrophages infiltration in the mammary gland duct. C–E EGFR regulation: C Sprouty and D Rasgrp1 are negative regulators of EGFR. The former inhibits EGFR-MEK-ERK signalling, impairing ECM remodelling, collagen deposition and cellular migration; the latter inhibits EGFR-AKT axis regulation, impairing mTOR signalling and TEBs cellular proliferation during late stage of puberty. E Upon hormones stimulation, AREG positively regulates R-spondin expression, while EGF expression negatively regulates Wnt and R-spondin, that are involved in mammary stem cell (MaSC) self-renewal

Fig. 13

The EGFR roles in lung fibrosis. A–C EGFR role in lung injury: A Lung injury induces an upregulation of EGFR ligands such as EGF and AREG, that promote EGFR activation signalling leading to airway remodelling. Specifically, EGF-EGFR binding increases squamous metaplasia of basal cells and alters barrier integrity, due to suppression of tight junction. Moreover, EGF promotes EGFR degradation. AREG-EGFR binding induces hyperplasia of basal cells and mucous cell hyperplasia and stimulates expression of genes related to mucous differentiation. Furthermore, AREG leads to EGFR prolonged signal pathways due to EGFR recycling process on the plasma membrane. B Oxidative stress activates EGFR-PI3K-AKT signaling pathways, such as FOXO3A, that inhibits NF-κB, leading to the expression of several cytokines that favors lung inflammation. Moreover, AKT activation regulates cell survival. Altogether these events contribute to lung fibrosis development. C Oxidative stress induces the formation of IL-33-RACE-EGFR complex that favors the mucin secretion and globet cell differentiation in lung epithelium. D, E EGFR activation: D EGFR transactivation by TNF-α induces activation of caspase8/3-mediated apoptosis, leading to cell death. However, AREG-EGFR binding triggers EGFR phosphorylation, activating AKT signaling that inhibits apoptosis processes. E TGF-β increases the expression of AREG on lung fibroblasts via ADAM17, which promotes the ligand release. AREG-EGFR binding induces ERK, JNK and AP‐1 expression, promoting the expression of profibrotic and mesenchymal genes, such as α-smooth muscle actin, collagen 1-α1/α2, fibronectin and tenascin, involved in the ECM remodeling and in the EMT process