REGULATION OF PROXIMAL TUBULAR OSTEOPONTIN IN EXPERIMENTAL HYDRONEPHROSIS IN THE RAT

Background. Osteopontin is a tubular-derived glycoprotein with macroph age chemoattractant properties. Our previous observations demonstrate that osteopontin is involved in the accumulation of macrophages within the renal cortex of rats following unilateral ureteral obstruction (U UO). Methods. The present study performed Northern and Western blot an alyses of isolated proximal tubular cells exposed to exogenous angiote nsin II, and cultured rat proximal tubular cells subjected to one hour of cyclic mechanical stretch, which provided insight into mechanisms involving the proximal tubular renin-angiotensin system in the increas ed expression of cortical osteopontin following hydronephrosis. Result s. In situ hybridization, using a S-35-labeled antisense riboprobe, sh owed osteopontin mRNA transcription localized to the cortical tubules of the obstructed kidney. Freshly isolated proximal tubules incubated with angiotensin II (10(-5) M) for one hour had increased osteopontin mRNA and protein expression by Northern and Western blot analyses, res pectively. Pre-treatment of proximal tubules with losartan (10(-5) M) for one hour prior to the addition of exogenous angiotensin II (10(-5) M) decreased osteopontin mRNA and protein expression. Rat proximal tu bule cells subjected to cyclic mechanical stretch for one hour exhibit ed a 2.1-fold increment in osteopontin mRNA levels, which was normaliz ed following pre-treatment with losartan. Conclusions. This study prov ides evidence that angiotensin II, produced by the proximal tubule in the obstructed kidney as a result of mechanical injury, possibly mecha nical stretch, may stimulate angiotensin II type I receptor activation , leading to up-regulated osteopontin expression and secretion by the proximal tubule, thereby facilitating macrophage recruitment into the renal interstitium.