Oxidative stress-induced temporal activation of ERK1/2 phosphorylates coreceptor of Wnt/β-catenin for myofibroblast formation in human lens epithelial cells.

Abstract

Purpose: Posterior capsular opacification (PCO) is the most common complication postcataract surgery, and its underlying mechanisms involve epithelial-mesenchymal transition (EMT) of remnant lens epithelial cells (LECs) in response to drastic changes in stimuli in the intraocular environment, such as oxidative stress and growth factors. Wnt/β-catenin signaling is a major pathway mediating oxidative stress-induced EMT in LECs, but its interplay with other transduction pathways remains little known in the development of PCO. ERK1/2 signaling is the downstream component of a phosphorelay pathway in response to extracellular stimuli (e.g., reactive oxygen species), and its activation regulates multiple cellular processes, including proliferation and EMT. Thus, this study aimed to investigate how ERK1/2 signaling and Wnt/β-catenin pathway crosstalk in oxidative stress-induced EMT in LECs.

Methods: Hydrogen peroxide (H₂O₂) at 50 μM treatment for 48 h was used to establish a moderate oxidative stress-induced EMT model in LECs. ERK1/2 signaling was inhibited using MEK1/2 inhibitor U0126 at 20 μM. Western blotting was used to quantify protein expression of various biomarkers of EMT and phosphorylated components in ERK1/2 and Wnt/β-catenin signaling. LEC proliferation was determined using an EdU staining assay and expression of proliferating cellular nuclear antigen (PCNA). Subcellular localization of biomarker proteins was visualized with immunofluorescent staining.

Results: Under the moderate level of H₂O₂-induced EMT in LECs, ERK1/2 signaling was activated, as evidenced by a marked increase in the ratio of phosphorylated ERK1/2 to total ERK1/2 at early (i.e., 5-15 min) and late time points (i.e., 12 h); the canonical Wnt/β-catenin pathway was activated by H₂O₂ at 48 h. LECs exposed to H₂O₂ exhibited hyperproliferation and EMT; however, these were restored by inhibition of ERK1/2 signaling demonstrated by reduced DNA synthesis and PCNA expression for cellular proliferation and altered expression of various EMT protein markers, including E-cadherin, α-SMA, and vimentin. More importantly, inhibition of ERK1/2 signaling reduced β-catenin accumulation in the activated Wnt/β-catenin signaling cascade. Specifically, there was significant downregulation in the phosphorylation level of LRP6 at Ser 1490 and GSK-3β at Ser 9, the key coreceptor of Wnt and regulator of β-catenin, respectively.

Conclusions: ERK1/2 signaling plays a crucial role in the moderate level of oxidative stress-induced EMT in LECs. Pharmacologically blocking ERK1/2 signaling significantly inhibited LEC proliferation and EMT. Mechanistically, ERK1/2 signaling regulated Wnt/β-catenin cascade by phosphorylating Wnt coreceptor LRP6 at Ser 1490 in the plasma membrane. These results shed light on a potential molecular switch of ERK1/2 and Wnt/β-catenin crosstalk underlying the development of PCO.