Regulation of Cyclooxygenase-2 by Hypoxia and Peroxisome Proliferators in the Corneal Epithelium

Albino Bonazzi,Vladimir Mastyugin,Paul A Mieyal,Michael W Dunn,Michal Laniado-Schwartzman

doi:10.1074/jbc.275.4.2837

Abstract

Hypoxic injury provokes inflammation of many tissues including the ocular surface. In rabbit corneal epithelial cells, both peroxisome proliferator-activated receptor (PPAR)-inducible cytochrome P450 4B1 and cyclooxygenase-2 (COX-2) mRNAs were increased by hypoxia. PPAR alpha and beta but not gamma mRNAs were detected in these cells. The PPAR activator, WY-14,643 increased COX-2 expression. Similarly, non-steroidal anti-inflammatory drugs with the ability to activate PPARs induced COX-2 independently of prostaglandin synthesis inhibition. COX-2 protein overexpression by hypoxia and PPAR activation was not associated with a parallel increase in prostaglandin E(2) accumulation. However, the enzyme regained full catalytic activity when: 1) hypoxic cells were re-exposed to normoxic conditions in the presence of heme and arachidonic acid, and 2) WY-14,643-treated cells were depleted of intracellular GSH. Consistent with previous observations showing that the corneal production of cytochrome P450-derived inflammatory eicosanoids is elevated by hypoxia and inflammation, the current data suggest that hypoxic injury is a model of inflammation in which molecules other than COX-derived arachidonic acid metabolites play a major proinflammatory role. This study also suggests that increased cellular GSH may be the mechanism responsible for the characteristic dissociation of PPAR-induced COX-2 expression and activity. Moreover, we provide new insights into the commonly observed lack of efficacy of classical non-steroidal anti-inflammatory drugs in the treatment of hypoxia-related ocular surface inflammation.

Highlights

Eye closure during sleep as well as contact lens wear creates an environment that has been described as a state of subclinical inflammation characterized by corneal swelling, conjunctival vasodilation, and significant levels of polymorphonuclear leukocytes in the tear film
CYP4B1, a cytochrome P450 protein whose expression has been correlated with the synthesis of the inflammatory eicosanoids 12(R)HETE and 12(R)-HETrE by the corneal epithelium [10], showed a similar time course of expression in response to hypoxia
Studies in our laboratory identified the formation of the CYP-derived arachidonic acid metabolites 12(R)-HETE, a Na,K-ATPase inhibitor with chemotactic activity, and 12(R)-HETrE, a powerful proinflammatory mediator that has been shown to stimulate vasodilation, increase anterior chamber proteins, chemoattract polymorphonuclear leukocytes, and elicit angiogenesis

Summary

Introduction

Eye closure during sleep as well as contact lens wear creates an environment that has been described as a state of subclinical inflammation characterized by corneal swelling, conjunctival vasodilation, and significant levels of polymorphonuclear leukocytes in the tear film. Similar to WY-14,643, addition of these NSAIDs to hypoxia-treated cells did not cause a further increase in COX-2 protein levels (data not shown).

Results

Conclusion