High glucose induced differential expression of lysyl oxidase and its isoform in ARPE-19 cells

Lysyl oxidase (LOX) stabilizes the extracellular matrix (ECM) by cross-linking collagen and elastin molecules. In proliferative diabetic retinopathy (PDR), there is ECM remodeling with neovascularization and basement membrane changes. While protease activities are well reported, the role of LOX in the pathogenesis of diabetic retinopathy is less studied. This study was done to see the effect of high glucose on the activity and expression of LOX and its isoforms in ARPE-19 cells.

ARPE-19 cells were exposed to high glucose up to 48 h, and LOX activity was determined by N-acetyl-3,7-dihydroxyphenoxazine assay. The mRNA expression of LOX and its isoforms was done by real-time PCR and the protein expression by ELISA. Immunohistochemistry for LOX was done in epiretinal membrane from PDR.

With an increase in glucose concentration LOX activity and protein was reduced significantly at 30 mM glucose at 48 h. mRNA expression of LOX, LOXL1, and LOXL2 varied with time and concentration of glucose. Vascular endothelial growth factor (VEGF) increased the LOX activity as well as the mRNA expression. Pigment epithelium-derived factor (PEDF) downregulated the mRNA expression of LOX, LOXL1, and LOXL2. The matrix metalloprotease (MMP) activity increased significantly with the increase in glucose concentration. The diabetic neovascular membrane showed increased immunostaining of LOX.

This study suggests that although the LOX activity, which is composite of all the isoforms, was reduced under high glucose conditions, there was a differential mRNA expression with increased LOX and LOXL1 and decreased LOXL2 expression.