COX-2 lack of function in hypoxia-induced ocular surface inflammation

Injury to the ocular surface increases corneal epithelial production of cyclooxygenase (COX)-derived eicosanoids but this increase correlates poorly to the inflammatory sequelae. Moreover, corticosteroids are effective in treatment of this inflammation but NSAIDs are not. The discovery of COX-2 that is differentially affected by common NSAIDs reopened the question of the role of prostaglandins in ocular surface inflammation. We examined the presence and inducibility of COX-2 in the corneal epithelium following hypoxia-induced injury in vivo and in vitro. COX-2, but not COX-1, protein levels markedly increased following hypoxia. Use of the selective COX-2 inhibitor, NS-398, indicated that COX activity in hypoxic corneas or cells is essentially that of COX-2; in control cells, both COX-1 and COX-2 contributed equally to the production of PGE2. COX-2 protein overexpression induced by hypoxia was not associated with a parallel increase in PGE2 accumulation but the enzyme regained full catalytic activity when cells were re-exposed to normoxia in the presence of heme and arachidonic acid. Decreases in the levels of oxygen and heme, essential substrates/cofactors for COX catalytic activity, contributed to a diminished prostanoid production during hypoxia. These results suggest that in hypoxic injury, molecules other than COX-derived prostanoids play a major pro-inflammatory role. Furthermore, this study provides an explanation for the ineffectiveness of classical NSAIDs in the treatment of hypoxia-related ocular surface inflammation.

Hypoxia-induced production of 12-hydroxyeicosanoids in the corneal epithelium: involvement of a cytochrome P-4504B1 isoform

The corneal epithelium metabolizes arachidonic acid by a cytochrome P-450 (CYP)-mediated activity to 12-hydroxy-5,8,11, 14-eicosatetraenoic acid (12(R)-HETE) and 12-hydroxy-5,8, 14-eicosatrienoic acid (12(R)-HETrE ). Both metabolites possess potent inflammatory properties, with 12(R)-HETrE being a powerful angiogenic factor, and they assume the role of inflammatory mediators in hypoxia- and chemical-induced injury in the cornea in vivo and in vitro. We used a model of corneal organ culture that exhibits hypoxia-induced epithelial CYP-dependent 12(R)-HETE and 12(R)-HETrE synthesis for isolating, identifying, and characterizing the CYP protein responsible for these eicosanoid syntheses. Northern analysis revealed the presence of a CYP4A-hybridizable mRNA, the levels of which were increased after hypoxia. Reverse transcription-polymerase chain reaction analysis with primers specific for the CYP4A family led to the isolation of a 671-base pair fragment with a 98.8% sequence homology to the rabbit lung CYP4B1 isoform, of which the levels in the corneal epithelium were greatly increased under hypoxic conditions. Moreover, phenobarbital, an inducer of hepatic CYP4B1 in the rabbit, also induced 12-HETE and 12-HETrE synthesis. Antibodies against CYP4B1, but not against CYP4A1, inhibited hypoxia-, clofibrate-, and phenobarbital-induced 12-HETE and 12-HETrE synthesis. These results suggest the involvement of a CYP4B1 isoform in the corneal epithelial synthesis of these eicosanoids in response to hypoxia.

NO-independent vasodilation to acetylcholine in the rat isolated kidney utilizes a charybdotoxin-sensitive, intermediate-conductance Ca(++)-activated K+ channel

The role of K+ channels in the nitric oxide-independent renal vasodilator effect of acetylcholine (Ach) was examined to address the hypothesis that the mechanism underlying this response was different from that of bradykinin, because an earlier study indicated the possibility of different mediators. We used the rat isolated, perfused kidney that was constricted with phenylephrine and treated with nitroarginine and indomethacin to inhibit nitric oxide synthase and cyclooxygenase, respectively. The nonspecific K+ channel inhibitors, procaine and tetraethylammonium (TEA), reduced vasodilator responses to Ach and cromakalim, but not those to nitroprusside. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, reduced vasodilator responses to cromakalim but did not affect those to Ach or nitroprusside. Charybdotoxin, an inhibitor of Ca(++)-activated K+ channels, reduced vasodilator responses to Ach without affecting those to cromakalim or nitroprusside. Iberiotoxin and apamin, inhibitors of large- and small-conductance Ca(++)-activated K+ channels, respectively, did not reduce vasodilation induced by Ach, cromakalim or nitroprusside. The inhibitor of cytochrome P450, clotrimazole, reduced the renal vasodilator effects of Ach and bradykinin but not those of nitroprusside or SCA 40, an agonist for Ca(++)-activated K+ channels. These results suggest that in the rat kidney, Ach, like bradykinin, utilizes a charybdotoxin-sensitive Ca(++)-activated K+ channel of intermediate conductance to elicit vasodilation and that this effect may be dependent on cytochrome P450 activity.

Contribution of calcium-activated potassium channels to the vasodilator effect of bradykinin in the isolated, perfused kidney of the rat

1. NO- and prostaglandin-independent, endothelium-dependent vasodilator responses to bradykinin are attributed to release of a hyperpolarizing factor. Therefore, the contribution of K+ channels to the renal vasodilator effect of bradykinin was examined in rat perfused kidneys that were preconstricted with phenylephrine and treated with NG-nitro-L-arginine (L-NOARG) and indomethacin to inhibit NO and prostaglandin synthesis. 2. The non-specific K+ channel inhibitors, TEA and TBA reduced vasodilator responses to bradykinin and cromakalim but not those to nitroprusside. 3. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, blocked the vasodilator response to cromakalim without affecting responses to bradykinin. 4. Charybdotoxin, a selective inhibitor of Ca(2+)-activated K+ channels, greatly attenuated vasodilator responses to bradykinin without affecting those to cromakalim or nitroprusside. 5. Iberiotoxin and leiurotoxin, inhibitors of large and small conductance Ca(2+)-activated K+ channels, respectively, were without effect on vasodilator responses to bradykinin, cromakalim or nitroprusside. 6. These results implicate K+ channels, specifically Ca(2+)-activated K+ channels of intermediate conductance, in the renal vasodilator effect of bradykinin and, thereby, support a role for a hyperpolarizing factor.