Background: Nonsteroidal anti-inflammatory drugs (NSAIDs) induce an inhibition of cyclooxygenase (COX), an enzyme that makes prostaglandins. Two isoforms of COX exist: COX-1 represents the constitutively expressed enzyme, whereas COX-2 is the inducible isoform. This study investigated the role of COX-2 in the inflammatory processes of the kidneys of rats with passive Heymann nephritis (PHN), and focused of the effect of a selective COX-2 inhibitor, flosulide. COX-2-selective inhibitors are thought to represent potent anti-inflammatory agents without major renal side effects.
Methods: PHN was induced by injecting heterologous Fx1A antiserum into female Wistar rats. Two treatment groups, each consisting of 12 rats with PHN, received either 3 or 9 mg of flosulide/kg body wt/day and were compared with untreated controls. After four weeks, the generation of thromboxane B2 (TxB2) and 6-keto-PGF1alpha were determined in renal tissue and in urine. COX-2 protein expression was investigated by Western blotting using a selective antibody.
Results: Rats with PHN exhibited a marked proteinuria of 71 +/- 8 mg/24 hr as compared with 2.0 +/- 0.3 mg/24 hr in healthy controls (P < 0.01). Treatment with flosulide reduced the proteinuria to 26.1 +/- 9 mg/24 hr at 3 mg flosulide/kg body wt/day and 35.5 +/- 6 mg/24 hr at 9 mg/kg body wt/day, which was equivalent to a reduction of proteinuria by a maximum of 65% (P < 0.05). This was accompanied by an increase in glomerular TxB2 from 3073 +/- 355 to 5255 +/- 1041 pg/mg glomerular protein and 6-keto-PGF1alpha from 1702 +/- 161 to 2724 +/- 770 pg/mg glomerular protein in rats with PHN. COX-2 protein expression was also highly elevated in comparison to healthy controls. Low-dose flosulide treatment had no effect on COX protein expression and renal prostaglandin formation. High-dose flosulide treatment reduced renal prostaglandin production and caused a marked decline in COX-1 and COX-2 protein expression. Urine prostanoid excretion remained unchanged in all therapeutic groups. There was a small though significant reduction in renal creatinine clearance from 0.86 ml +/- 0.2/min in untreated controls to 0.6 ml +/- 0.1/min in flosulide-treated rats with PHN (P < 0.01) after four weeks.
Conclusions: Under the influence of flosulide, a highly COX-2-selective inhibitor, we observed an antiproteinuric drug effect. The inflammation in PHN induced COX-2 protein expression that was not affected by low-dose flosulide. COX-1 and COX-2 protein expression was affected by high-dose flosulide, which therefore might lose its selectivity. High-dose flosulide induced a decrease in glomerular prostanoid production possibly because of COX-1 inhibition. Our results suggest that the therapeutic use of flosulide in proteinuria seems advantageous and deserves further studies because the basal prostaglandin levels remain unchanged in the low-dose-treated group, indicating that the compensatory capacity of prostaglandin production, which is essential for the regulation of renal hemodynamics, is maintained.