Cyclosporin Exerts a Direct Fibrogenic Effect on Human Tubulointerstitial Cells: Roles of Insulin-Like Growth Factor I, Transforming Growth Factor β1, and Platelet-Derived Growth Factor

To assess the direct fibrogenic effects of cyclosporin A (CyA) on the human tubulointerstitium, primary cultures of human renal proximal tubule cells (PTC) and renal cortical fibroblasts (CF) were incubated for 24 h with various concentrations of CyA. Cytotoxicity was confirmed in both cell populations by dose-dependent inhibition of thymidine incorporation, viability, and PTC apical sodium-hydrogen exchange activity (ethylisopropylamiloride-sensitive apical22Na+ uptake). Compared with controls, both 500 and 1000 ng/ml CyA significantly stimulated CF collagen synthesis (proline incorporation 4.6 ± 0.4, 6.5 ± 0.8, and 7.1 ± 1.0%, respectively; p < .05) and inhibited matrix metalloproteinase-2 (100%, 85.7 ± 10.0%, and 38.8 ± 9.2%) and matrix metalloproteinase-9 activity (100%, 110.6 ± 19.0%, and 49.9 ± 12.8%). CyA did not affect CF secretion of transforming growth factor β1, but markedly stimulated insulin-like growth factor-I (IGF-I) secretion and inhibited secretion of both IGF-I binding protein-(IGFBP)-3 and IGFBP-2. CyA-induced CF collagen synthesis was abrogated by 5 μg/ml anti-IGF-I receptor antibody, but not by 5 μg/ml murine nonimmune globulin. Increasing concentrations of CyA progressively augmented PTC secretion of the fibrogenic cytokines transforming growth factor-β1and platelet-derived growth factor. These results indicate that clinically relevant concentrations of CyA are directly toxic to PTC and CF, irrespective of hemodynamic effects, and promote interstitial fibrosis by inhibiting matrix degradation and stimulating cortical fibroblast collagen synthesis via induction of autocrine IGF-I action. The latter effect may be further accentuated by the ability of CyA to augment secretion of transforming growth factor β1 and platelet-derived growth factor by PTCs.