Epidermal Growth Factor Receptor Expression in the Corneal Epithelium

Abstract

A properly functioning cornea is critical to clear vision and healthy eyes. As the most anterior portion of the eye, it plays an essential role in refracting light onto the retina and as an anatomical barrier to the environment. Proper vision requires that all layers be properly formed and fully intact. In this article, we discuss the role of the epidermal growth factor receptor (EGFR) in maintaining and restoring the outermost layer of the cornea, the epithelium. It has been known for some time that the addition of epidermal growth factor (EGF) promotes the restoration of the corneal epithelium and patients using EGFR inhibitors as anti-cancer therapies are at increased risk of corneal erosions. However, the use of EGF in the clinic has been limited by downregulation of the receptor. More recent advances in EGFR signaling and trafficking in corneal epithelial cells have provided new insights in how to overcome receptor desensitization. We examine new strategies for overcoming the limitations of high ligand and receptor expression that alter trafficking of the ligand:receptor complex to sustain receptor signaling.

Keywords: cornea; epidermal growth factor receptor (EGFR); epithelium; homeostasis; wound healing.

Figure 1

Histology and schematic of the cornea. (A) Human cornea obtained from cadaver donation. Corneal tissue was placed in 4% paraformaldehyde overnight, washed, and embedded in paraffin for histological sectioning and hematoxylin and eosin staining to visualize the various cell layers and membranes. (B) Schematic of the corneal epithelium that highlights the epithelial layers and innervation; BL = basal epithelial cells, DC = differentiated cells, SL = superficial layer.

Figure 2

Saturation binding in hTCEpi cells. (A) Saturation binding assays were performed using adherent immortalized corneal epithelial (hTCEpi) cells [70] using ¹²⁵I-EGF (Perkin Elmer, Waltham, MA, USA), as previously described [71,72]. Shown are cummulative data from 4 independent experiments (6–7 data points/experiment) that were subjected to Scatchard transformation. The concave-up binding curve (solid line) is consistent with two affinity sites. Saturation binding from one representative experiment is shown in the inset. (B) The apparent Kd and total binding were generated from the summative data of all four experiments. High- and low-affinity binding sites were calculated from each experiment with PRISM utilizing a non-linear, two-site binding analysis. Data are presented as the average ± S.E.M. (n = 4).

Figure 3

Schema summarizing the role of EGFR and ErbB family members in the corneal epithelium. (A) Schematic depicting expression of EGFR (ErbB1), ErbB2 (Her2), and ErbB3, but not ErbB4, in corneal epithelium. (B) Table summarizing the effects of endogenous EGFR ligand on in vitro and in vivo corneal epithelial wound healing. (C) Schematic depicting how EGFR endocytic trafficking affects receptor signaling in corneal epithelial cells. EGF stimulation promotes internalization of the EGF:EGFR complex to the early endosome. During internalization, the receptor is ubiquitylated and targeted for trafficking to lysosome for degradation. EGFR degradation can be diverted from a degradation pathway by promoting receptor recycling. This can be achieved by stimulating with TGFα, a ligand that dissociates in the mildly acidic environment of the early endosome and has reduced receptor ubiquitylation, or inhibiting the ubiquitin ligases of the EGFR in corneal epithelial cells (i.e., c-Cbl and Cbl-b).