Vitamin D Receptor And Its Link To Reduced Endogenous Crosslinking In Keratoconus

Keratoconus(KC)is a corneal ectatic condition ,characterized by global ultrastructural & molecular changes,resulting in focal macrostructural change as steepening.These changes are mainly in epithelium,epithelial basement membrane,Bowman layer,stroma.KC is associated with defective collagen crosslinking(CXL)driven by inflammation associated with oxidative stress response in cornea.Vitamin D,a natural hormone is an endogenous antiinflammatory defense mechanism which works by binding to vitamin D receptor(VDR),which is shown to be reduced in affected tissues ,confirming that it is critical in mediating cellular homeostasis & dampening inflammation.Our study aims to determine relevance of VDR on endogenous & therapeutic CXL in KC.

The cross-sectional study was approved by institutional ethics committee & informed consent was obtained from the study subjects.Corneal epithelium from 30 KC patients (Amsler Krumeich grades 1-3)undergoing T-PRK & collagen crosslinking & 10 controls undergoing PRK were obtained. Corneal epithelium from KC patients were demarcated as those from the ectatic & non-ectatic zone and the expression of VDR,lysyl oxidase (LOX),MMP9 & collagens in corneal epithelium of study subjects were determined using western blotting and/or quantitative PCR. Further, the effect of oxidative stress on VDR, LOX, MMP9 & collagens and the beneficial role of vitamin D on the expression of these factors in human corneal epithelial cells (HCE) were studied, in vitro.

The expression of VDR, LOX & collagen in KC epithelium was significantly (P<0.05) lower & MMP9 was significantly higher compared to epithelium from non-KC control subjects. Further, expression of VDR & LOX in the ectatic epithelium of KC was markedly lower compared to matched epithelium from the non-ectatic zone. As the severity/clinical grades of KC increase, VDR expression was observed to be down regulated. We also observed a significant reduction in expression of VDR & LOX, with elevated levels of MMP9 when HCE cells were exposed to oxidative stress, in vitro. Vitamin D treatment to HCE cells, in vitro, improved expression of VDR & LOX, & reduced levels of MMP9

The findings indicate defect in VDR status, the tissues endogenous anti-inflammatory mechanism in KC epithelium. This emphasizes the critical role of vitamin D – VDR axis in regulating endogenous collagen cross-linking components (LOX) and extracellular matrix remodeling factors such as collagen and MMP9. Thus, suggesting VDR activation through vitamin D supplementation could be a novel strategy in the management of KC.