Differential regulation of osteopontin expression in the clipped and nonclipped kidney of two-kidney, one-clip hypertensive rats.

BACKGROUND Osteopontin (Opn) is highly upregulated in many different animal models of renal disease, where it is suspected to participate in progression of the disease. In some models, angiotensin II (Ang II) seems to induce the elevated Opn production. Therefore, we examined the regulation of Opn in two-kidney, one-clip (2K1C) hypertensive rats, in which Ang II mediates the elevated blood pressure. METHODS At days 7, 14, and 28, the clipped and nonclipped kidneys of hypertensive or sham-operated rats were analyzed for osteopontin protein, mRNA expression and mononuclear cell infiltration by imumunohistochemistry, in situ hybridization, and Northern blot analysis. Rats were treated with the Ang II type 1 receptor antagonist Valsartan starting 14 days after clipping. RESULTS In sham-operated rats, Opn was mainly localized to cells of the thin ascending limbs of the outer medulla. No significant Opn staining was observed in cortical tubules. Focally defined tubular cortical Opn staining was observed in clipped and contralateral kidneys of hypertensive animals at days 14 and 28. Osteopontin protein expression correlated with the mRNA expression detected by in situ hybridization and Northern blot. Treatment with Valsartan reduced osteopontin staining by 51%, mRNA by 47%, and mononuclear cell number by 97% in nonclipped kidneys compared to untreated two-kidney, one-clip animals. In clipped kidneys, however, Opn protein and mRNA expression was not reduced, but a 240% increase in interstitial mononuclear cell number was observed. CONCLUSIONS Osteopontin is involved in the induction of nephrosclerosis in renovascular hypertension, probably by a mechanism augmenting monocyte infiltration. Angiotensin II appears to be an important inducer of Opn in the nonclipped kidney. Ischemic conditions may regulate Opn expression in the clipped kidney.