FGF-2– and TGF-β1–Induced Downregulation of Lumican and Keratocan in Activated Corneal Keratocytes by JNK Signaling Pathway

Abstract

Downregulation of lumican and keratocan expression is an undesirable phenotypic change that occurs during corneal wound healing. The present study was intended to determine whether the activation of Jun N-terminal kinase (JNK)-signaling pathway is involved in their downregulation in TGF-β1- and FGF-2-activated keratocytes. Keratocytes, isolated from rabbit corneal stroma, and cultured in a serum-free medium, pretreated or not treated with JNK inhibitor (SP600125), were activated with FGF-2/heparin sulfate (HS) or TGF-β1 in the presence or absence of SP600125. In another set of experiments, keratocytes were transfected with JNK1/2 Dicer-substrate RNA (DsiRNA) and then activated with TGF-β1 or FGF-2/HS. Specific phenotypic changes were analyzed immunocytochemically and correlated with Western blot analyses. The relative levels of specific mRNAs were estimated by quantitative RT-PCR using specific reagents. The FGF-2/HS- or TGF-β-induced activation of corneal stromal keratocytes to fibroblast- or myofibroblast-phenotype, respectively, resulted in marked decreases in cell surface-associated and secreted keratan sulfate proteoglycans (KSPGs). Both keratocan and lumican proteins and their mRNAs were downregulated in the activated keratocytes. However, JNK inhibition during the activation of keratocytes, pretreated with the JNK inhibitor, suppressed the reduction in the cell-surface associated and secreted KSPGs (lumican and keratocan), and their mRNA transcripts. Downregulation of total KSPGs and their mRNAs was also inhibited by decreasing JNK1 and JNK2 levels via JNK1/2 DsiRNA transfection of keratocytes before their activation. Extrapolating from the present study, FGF-2- and TGF-β1-activation of JNK signaling pathway may be partly responsible for the downregulation of keratocan and lumican expression in activated corneal keratocytes observed during corneal stromal wound healing.