UPREGULATION OF PROSTAGLANDIN G/H SYNTHASE (PGHS)-2 TRANSCRIPTION IS THE MAIN SOURCE OF PROSTAGLANDIN (PG) SYNTHESIS IN RETINA EXPOSED TO HYPEROXIA. ▴ 1410

Abstract

Intermittent elevations in retinal O2 concentrations induce an oxidative stress which has been implicated in retinal pathologies including retinopathy of prematurity (ROP) and diabetic retinopathy. We have shown that reactive O2 species increase the activity of PGHS in retina and its products impair retinal vasomotor responses in the newborn (NB) in contrast to the adult (A), resulting in excessive constriction, a significant event which precedes neovascularization. However the source of these PGs has not yet been defined. PGHS-1 is the constitutive PGHS isoform, and PGHS-2 is rapidly induced by various mitogenic and inflammatory stresses. We hypothesized that elevated O2 levels upregulates PGHS-2 synthesis and contributes to the increased synthesis of PG in retina under such oxidative stresses. NB (1-2 days) and A (5-7 months) pig retinas were incubated at varying concentrations of O2 for 0-4 h. Tissues were homogenized immediately after exposure to O2 to measure PG synthesis (30 μM arachidonic acid added) in the presence or absence of PGHS-2 specific inhibitors, NS-398 and DuP697, and to determine tissue concentrations of total glutathione and malondialdehyde (MDA; peroxidation product). PGHS-1 and PGHS-2 mRNA were quantified by RNase protection assay using porcine PGHS-1 and PGHS-2 cRNA probes. PGHS-2 mRNA in NB and A retina increased by 2 h of exposure to 21% O2 (≈3-4 fold higher than normal retinal O2 concentrations), but remained constant at≤5% O2; PGHS-1 mRNA changed minimally. PG synthesis declined by 2 h in retinas exposed to 21% O2. The synthesis of PGI2 recovered in A and partially in NB by 4 h, but not that of PGE2 and PGF2α which are dependent upon glutathione which itself decreased markedly and was associated with an increase in MDA in NB; the renewed PGI2 synthesis was completely inhibited by PGHS-2 inhibitors. Thus, high O2 levels induce PGHS-2 transcription which is responsible for increased PGI2 synthesis in retinas exposed to hyperoxia; but this oxidative stress limits the synthesis of the cytoprotective vasodilator PGI2 in NB, which may ultimately contribute to ROP.