Selective activation of the prostaglandin E2 circuit in chronic injury-induced pathologic angiogenesis.

The successful execution of an acute inflammatory response is nowhere as critical and evolved as in the delicate visual axis because uncontrolled ocular inflammation impairs vision and leads to blindness. In particular, the transparent, avascular, and immune-privileged cornea has a tightly regulated and remarkable inflammatory/reparative response that is highly dependent on the generation and release of specific and temporally defined arrays of mediators.1–4 In this regard, lipid autacoids such as arachidonic acid (AA)-derived cytochrome P450 metabolites, lipoxins, and hydroxyeicosatetraenoic acids (HETEs) have emerged as key mediators of corneal inflammation and pathologic angiogenesis and the resolution of these responses.3,5–7 Lipid autacoids are formed in most tissues and are essential regulators of inflammatory and immune responses.1,8–11 Recent findings have uncovered a critical role for resident protective lipid circuits, namely 12/15-lipoxygenase (12/15-LOX; Alox15), in the cornea. 12/15 LOX is highly expressed in mouse and human corneas12,13 and generates AA- and ω-3 polyunsaturated fatty acid-derived mediators (i.e., lipoxins, resolvins, and protectins) that inhibit inflammation and pathologic angiogenesis and promote wound healing.7,13–15 This emerging field underscores the importance of resident endogenous lipid autacoid circuits in the eye to maintain and execute essential acute inflammatory responses. Cyclooxygenase (COX)-derived prostaglandins (PG) are of primary interest because they are critical and early response mediators that initiate or amplify inflammation. In particular, PGE2 has traditionally been identified as a prevalent inflammatory mediator in many tissues and inflammatory diseases.10,16 More recently, it has been implicated in tumor-driven angiogenesis.17 Not surprisingly, it is a primary target of several important classes of clinically used drugs (i.e., nonsteroidal anti-inflammatory drugs [NSAIDs] and corticosteroids) for inflammation, pain, and colon cancer. PGE2 is produced by many cells of the body, including fibroblasts, macrophages/monocytes, dendritic cells, and some types of malignant cells, and it exerts its effects through binding to G-protein–coupled receptors (EP). Four EP receptor subtypes have been identified—EP-1, EP-2, EP-3, and EP-4—which are coupled to different intracellular signal transduction pathways.18,19 EP-2 and EP-4 receptors are coupled to Gs and mediate increases in cyclic adenosine monophosphate (cAMP) concentrations, whereas activation of EP-3 inhibits adenylate cyclase through Gi; EP-1 stimulates intracellular Ca2+ mobilization through Gq. Hence, the distribution and relative expression of these four receptor subtypes provide an elegant system that can account for the ability of PGE2 to evoke pleiotropic and sometimes opposing bioactions that are tissue- and cell type–specific.18–20 Although PGE2 induces reactions that give rise to the initial cardinal signs of inflammation (i.e., edema),21 it can also inhibit inflammation by downregulating host responses. PGE2 is a negative regulator of neutrophil, monocyte, and lymphocyte function.10,11,18 Specifically, PGE2 has been shown to negatively regulate allergic reactions, including contact hypersensitivity, conjunctivitis, and asthma, through EP-3 in several murine experimental models, and, through EP-4, to maintain intestinal homeostasis by downregulation of the immune response.22–25 Moreover, PGE2 decreases macrophage production of proinflammatory cytokines such as TNF-α26 and stimulates anti-inflammatory cytokine production,27 mechanisms that both contribute to the resolution of inflammation. PGE2 can also promote the resolution of inflammation through the induction of key enzymes, namely 12/15-LOX, in polymorphonuclear leukocytes (PMNs), and the formation of the anti-inflammatory and pro-resolving lipoxin A4.28 Hence, the role of PGE2 in inflammatory and immune responses is complex; it is temporally defined and largely dependent on EP receptor expression. The capacity of ocular tissues to synthesize PGE2 was initially determined on the basis of 14C-AA conversion in vitro and in vivo.29–31 Subsequently, studies that used NSAIDs, PGE2, and PGE2 analogs, as well as EP receptor knockout mice, have demonstrated that in the eye, this prostanoid causes miosis, vasodilation, disrupts the integrity of the blood-aqueous barrier, and, through regulating the dynamics of aqueous humor flow, can increase or reduce intraocular pressure.32–35 Importantly, NSAIDs are a primary treatment option for ocular indications including the inhibition of intraoperative miosis and the management of postoperative inflammation and pain. However, in contrast to other tissues and despite reports of COX-2 upregulation and PG formation in models of ocular inflammation,3 the endogenous role of this prominent prostanoid and primary therapeutic target in the corneal inflammatory/reparative response, surprisingly, remains to be fully defined. To this end, we assessed the role of the PGE2 circuit in mediating inflammatory responses in the cornea. Here, we report that endogenous PGE2 in the cornea is not a general inflammatory mediator. Increased PGE2 formation and upregulation of EP-2 and EP-4 receptor expression were key features of a severe and chronic injury that was associated with inflammatory neovascularization but not a feature of acute self-resolving inflammation. Moreover, topical PGE2 treatment selectively amplified leukocyte recruitment and pathologic angiogenesis in response to chronic inflammation but did not affect wound healing or leukocyte recruitment during acute epithelial injury, providing evidence for a selective role of the PGE2 circuit in mediating corneal inflammatory responses.