Association of cyclophilin A with renal brush border membranes: Redistribution by cyclosporine A

Abstract

Administration of the immunosuppressive agent cyclosporine A (CsA) is associated with nephrotoxicity. The main target for CsA, cyclophilin A (CypA), was found in high levels in epithelial cells of renal proximal tubules. In the present study, CypA was immunodetected and characterized following CsA treatment in subcellular fractions of renal cortex. The renal content and distribution of CypA was evaluated in untreated rats and in rats treated with a subcutaneous injection of CsA (10 mg. kg-1. day-1) for 10 days. In untreated rats, membrane-bound CypA represents 0.25% of total brush border membrane (BBM) proteins, similar to the proportion found in the soluble fraction. High ionic strength treatment was unable to extract CypA from BBMs, whereas alkaline treatment (Na2CO2, pH 11) and detergent 3 - [(3 - cholamidopropyl) - dimethyl - ammonio] - 1 - propanesulfate (CHAPS) released it from BBMs. These results indicate that CypA is associated with renal BBMs, and that hydrophobic interactions are involved in this association. The CypA distribution was strongly modified in both BBMs and the soluble fraction after CsA treatment, but its affinity for CsA estimated by photoaffinity labeling was unaffected. The CypA expression level decreased by 45% in BBMs, while it increased by 33% in the soluble fraction, compared with control rats. CypA remained associated with the membranes following in vitro incubation of renal BBMs with CsA. However, incubation of CypA with one of its substrates released CypA from renal BBMs. These experiments suggest that renal BBMs contain a significant amount of CypA and chronic exposure to CsA, and acute exposure to one of CypA substrates may modify its subcellular distribution.