The effects of different inhibitory pathways of prostaglandin E2 biosynthesis on renomedullary interstitial cells in rats: a multidisciplinary study

Abstract

Renomedullary interstitial cells (RMICs) are the most dominant cell type in inner renal medulla. Their most distinct characteristic is the presence of multiple lipid droplets in their cytoplasm. These lipid droplets are believed to be the storage units for precursors of prostaglandins (PGs), prostacyclin and medullipin. Especially prostaglandin E 2 (PGE 2 ) is synthesized by RMICs in kidney. PGs are produced by three key steps: 1) Arachidonic acid (AA) release from membrane phospholipids by the action of phospholipase A 2 (PLA 2 ); 2) Formation of prostaglandin H 2 (PGH 2 ) from AA by the action of cyclooxygenases (COXs); 3) Specific PG synthesis metabolism from PGH 2 . PG biosynthesis can be regulated via activation or inhibition of these steps. In this study, we examined the effects of PGE 2 inhibition in different steps on RMIC function, the number of lipid droplets, medullary hyaluronan (HA) content and cell viability. We formed four groups (n=8): First group was control and treated with intraperitoneal (ip) 0.9% saline. Second group in which we inhibited AA release from membrane phospholipids was injected with ip dexamethasone (DEX) (2 mg/kg, 10 days); third group was treated with ip indomethasine (IND) (1 mg/kg, 10 days) to inhibit non-specific COX at the stage of PGH 2 formation from AA; and the fourth group was injected with ip celecoxib (CXB) (1 mg/ kg, 10 days) to examine selective cyclooxygenase-2 (COX-2) inhibition. We dissected renal medulla of the sacrificed animals after 10 days to analyze with light and electron microscopy. We counted the lipid droplets in 50 random RIMCs for each animal (x6.000 magnification) in electron microscopy. Our morphometric analysis showed that the number of lipid droplets was significantly decreased in DEX group and was significantly increased in IND and CXB groups when compared to control. In addition, medullary HA content and CD44 immunoreactivity were significantly increased in all groups when compared to control. When we analyzed cell viability, we found that RMIC apoptosis was significantly higher in PGE 2 inhibited groups when compared to control. Besides this, 24-hour urine values collected on the 10th day were significantly increased in dexamethasone and indomethacin groups; but in celecoxib group the values were similar to control. These results indicate that lipid granules may be numerical and functionally influenced from PGE 2 changes, these granules may be storage units of AA, functional changes in RMICs by PGE 2 may influence HA quantity of medullary interstitium and urine volume, and finally PGE 2 inhibition may lead to RMIC apoptosis.