Therapeutic concentrations of cyclosporine A, but not FK506, increase P-glycoprotein expression in endothelial and renal tubule cells

Background. The immunosuppressive drugs cyclosporine A (CsA) and tacrolimus (FK506) are extruded from cells by the multidrug resistance P-glycoprotein (P-gp), an efflux pump for drugs and xenobiotics, which may limit their therapeutic effectiveness and/or incidence of toxic side effects. In the present study, we investigated the effect of therapeutic concentrations of CsA and FK506 on the expression of P-gp in cultured endothelial and proximal tubule cells. Methods. P-gp expression in human arterial endothelial (HAEC) and rat proximal tubule cells (RPTC) was determined by immunoblotting and immunocytochemistry, and correlated with P-gp-mediated transport by measuring the intracellular accumu lation of the fluorescent probe calcein. Results. Following incubation of HAEC with therapeutic concentrations of 0.1 to 1.6 mu M CSA UP to seven days, P-gp expression increased in a time- and concentration-dependent manner, max imally to 291 +/- 42% of controls with 0.8 mu M CsA for seven days. Similar effects of CsA were observed in RPTC. In contrast, therapeutic concentrations of FK506 (0.01 to 0.2 mu M up to 7 days) did not change P-gp expression in either cell type, though at higher, supratherapeutic concentrations of FK506 (0.5 to 1.2 mu M) P-gp expression was also increased. Immunocytochemistry revealed increased P-gp expression in the plasma membrane of HAEC and RPTC treated with 0.8 mu M CsA, which was reflected by a decrease of P-gp-mediated accumulation of calcein in both cell types. Conclusions. The data suggest that the induction of P-gp expression in HAEC and RPTC at concentrations of CsA or FK506 above 0.5 mu M is part of the protective answer of cells to toxic concentrations of the drugs and could therefore interfere with the therapeutic effectiveness of CsA in vivo.