Abstract
Purpose: :
Inflammation and angiogenesis are intimately linked and fundamental responses to injury. In this regard, cyclooxygenase (COX)-derived PGE2 is of primary interest as it can regulate both responses. The four PGE2 receptors, EP1-4, are coupled to different intracellular signal transduction pathways. Hence, the distribution and relative expression of EP1-4 can dictate whether PGE2 exerts an anti/pro-inflammatory and/or pro-angiogenic effect. The physiological role and molecular mechanism of endogenous PGE2 in the cornea, and the regulation of EP receptor expression during a highly dynamic and complex inflammatory/reparative response are unknown, and was the aim of this study.
Methods: :
Acute self-resolving inflammation was induced in mice by epithelial abrasion, while chronic inflammation and neovascularization was induced using the corneal suture model. Re-epithelialization was monitored by fluorescein staining and neovascularization quantified by immunofluorescence using CD31 as an endothelial marker. Myeloperoxidase activity was used as an index for PMN infiltration. PGE2 formation was analyzed by lipidomics. Expression of EPs and inflammatory/angiogenic mediators was assessed by real-time PCR and immunohistochemistry. Mice eyes were treated with PGE2 (100 ng topically, t.i.d.) for up to 7 days.
Results: :
COX-2, EP2 and EP4 expression was upregulated with chronic inflammation which correlated with increased corneal PGE2 formation (6.6-fold increase 4 days post-suture placement) and marked neovascularization. In contrast, corneal levels of PGE2 and EP receptor expression were unaltered following an acute abrasion injury. Moreover, topical PGE2 treatment amplified PMN infiltration and the angiogenic response to chronic inflammation, but did not affect wound healing or PMN infiltration after epithelial injury. Interestingly, exacerbated inflammatory neovascularization seen with PGE2 treatment was independent of VEGF, a traditional and key mediator of corneal neovascularization, but was associated with a significant induction of the eotaxin-CCR3 axis, a novel regulator of ocular angiogenesis.
Conclusions: :
These data provide evidence that the PGE2 circuit is an endogenous lipid autacoid circuit in the cornea. More importantly, inflammatory neovascularization, rather than general inflammation of the cornea, regulates this circuit at the level of both biosynthetic enzyme and receptor expression.
Keywords: inflammation • neovascularization • immunomodulation/immunoregulation