2. Department of Biochemistry, Cerrahpasa Medical Faculty, Istanbul University, Istanbul, Turkey.
3. Department of Medical Oncology, Vrije Universiteit, Holland.
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 E2 (PGE2) 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 A2 (PLA2); 2) Formation of prostaglandin H2 (PGH2) from AA by the action of cyclooxygenases (COXs); 3) Specific PG synthesis metabolism from PGH2. PG biosynthesis can be regulated via activation or inhibition of these steps. In this study, we examined the effects of PGE2 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 PGH2 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 PGE2 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 PGE2 changes, these granules may be storage units of AA, functional changes in RMICs by PGE2 may influence HA quantity of medullary interstitium and urine volume, and finally PGE2 inhibition may lead to RMIC apoptosis.
Keywords: Kidney, Non steroidal anti-inflammatory drugs, Prostaglandin E2, Renomedullary interstitial cells, Cyclooxygenase-2