Elastin as a Novel Extracellular Matrix From Aberrant HDAC4 Expression in PDGF-BB-Induced Orbital Fibroblasts From Graves' Ophthalmopathy Patients

The purpose of this study was to investigate fibroblast markers and histone deacetylase (HDAC) 4 in orbital tissues and orbital fibroblasts from patients with Graves' ophthalmopathy (GO) and healthy controls. Hematoxylin and eosin (H&E), Masson's trichrome, and Verhoeff's Van Gieson (VVG) staining were performed on GO and control orbital tissues. Immunohistochemistry on fibroblast markers were investigated. GO orbital fibroblasts (GOFs) and control orbital fibroblasts (COFs) stimulated with platelet-derived growth factor (PDGF)-BB were assessed for collagen type I alpha I (COL1A1), elastin (ELN), and fibrillin-2 (FBN2) mRNA expression. HDAC4 knockdown in GOF and COF were assessed to study its impact on elastin expression. GO orbital tissues and GOF treated with either LMK-235 or tasquinimod were examined. Orbital tissues exhibited accumulation of adipocytes, orbital fibroblasts, collagen, and elastin. After 2 hours, PDGF-BB stimulation induced an 8-fold increase of ELN in GOF and a 5-fold increase of ELN in COF compared to no treatment, whereas COL1A1 and FBN2 mRNA levels were later induced after 24 hours. Elastin protein expression was significantly higher in GOF compared to COF at both the basal level and after PDGF-BB stimulation. HDAC4 knockdown significantly reduced PDGF-BB-induced ELN mRNA expression in GOF but not in COF. LMK-235 inhibited ELN mRNA expression in GOF. However, LMK-235 and tasquinimod did not decrease ELN mRNA level in GO orbital tissues. Orbital tissues exhibit shared and unique characteristics from other tissues. Our in vitro studies showed that elastin mRNA and protein expression increased in response to PDGF-BB stimulation in GOF which might be due to aberrant HDAC4 levels.