Interaction between NO and COX pathways in retinal cells exposed to elevated glucose and retina of diabetic rats

A nonselective inhibitor of cyclooxygenase (COX; high-dose aspirin) and a relatively selective inhibitor of inducible nitric oxide synthase (iNOS; aminoguanidine) have been found to inhibit development of diabetic retinopathy in animals, raising a possibility that NOS and COX play important roles in the development of retinopathy. In this study, the effects of hyperglycemia on retinal nitric oxide (NO) production and the COX-2 pathway, and the interrelationship of the NOS and COX-2 pathways in retina and retinal cells, were investigated using a general inhibitor of NOS [N^G-nitro-L- arginine methyl ester (L-NAME)], specific inhibitors of iNOS [L-N ⁶-(1-iminoethyl)lysine (L-NIL)] and COX-2 (NS-398), and aspirin and aminoguanidine. In vitro studies used a transformed retinal Müller (glial) cell line (rMC-1) and primary bovine retinal endothelial cells (BREC) incubated in 5 and 25 mM glucose with and without these inhibitors, and in vivo studies utilized retinas from experimentally diabetic rats (2 mo) treated or without aminoguanidine or aspirin. Retinal rMC-1 cells cultured in high glucose increased production of NO and prostaglandin E₂ (PGE₂) and expression of iNOS and COX-2. Inhibition of NO production with L-NAME or L-NIL inhibited all of these abnormalities, as did aminoguanidine and aspirin. In contrast, inhibition of COX-2 with NS-398 blocked PGE₂ production but had no effect on NO or iNOS. In BREC, elevated glucose increased NO and PGE₂ significantly, whereas expression of iNOS and COX-2 was unchanged. Viability of rMC-1 cells or BREC in 25 mM glucose was significantly less than at 5 mM glucose, and this cell death was inhibited by L-NAME or NS-398 in both cell types and also by L-NIL in rMC-1 cells. Retinal homogenates from diabetic animals produced significantly greater than normal amounts of NO and PGE₂ and of iNOS and COX-2. Oral aminoguanidine and aspirin significantly inhibited all of these increases. The in vitro results suggest that the hyperglycemia-induced increase in NO in retinal Müller cells and endothelial cells increases production of cytotoxic prostaglandins via COX-2. iNOS seems to account for the increased production of NO in Müller cells but not in endothelial cells. We postulate that NOS and COX-2 act together to contribute to retinal cell death in diabetes and to the development of diabetic retinopathy and that inhibition of retinopathy by aminoguanidine or aspirin is due at least in part to inhibition of this NO/COX-2 axis.