The clinical use of tacrolimus (FK506) is limited by nephrotoxicity. The pathogenesis of fibrosis in chronic FK506 nephrotoxicity remains unknown. Because transforming growth factor (TGF)-β plays a key role in the fibrogenesis of many diseases, including cyclosporine nephrotoxicity we studied a salt-depleted rat model of chronic FK506 nephropathy in which clinically relevant FK506 blood levels are obtained and which shows similarities to the lesions described in patients receiving FK506. Pair-fed rats were treated with either FK506 (1 mg/kg/day s.c.) or an equivalent dose of vehicle and were killed at 7 or 28 days. Characteristic histologic changes of tubular injury, interstitial fibrosis, and arteriolopathy developed in FK506-treated rats at 28 days and were accompanied by worsening kidney function, decreased concentrating ability, and enzymuria. FK506-treated kidneys had a progressive increase in the expression of TGF-β1 and matrix proteins (biglycan, tenascin, fibronectin, and type I collagen). This effect seems to be specific because the expression of type IV collagen, a basement membrane collagen, was not affected. Matrix deposition was present mostly in the tubulointerstitium and vessels in accordance with the FK506 chronic lesion. The expression of plasminogen activator inhibitor-1, a protease inhibitor influenced by TGF-β, followed TGF-β1 and matrix proteins, suggesting that the fibrosis of chronic FK506 nephropathy likely involves the dual action of TGF-β1 on matrix deposition and degradation. Since both peripheral and tissue renin expression were elevated with FK506, the renin- angiotensin system may play a role in the pathogenesis of this condition.
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