Peter Hiestand

Identification of cyclophilin as the erythrocyte ciclosporin-binding protein

Previous studies on the distribution of circulating ciclosporin have shown that the majority of the drug is associated with erythrocytes. In order to investigate the nature of ciclosporin-erythrocyte binding, binding studies were performed on isolated erythrocytes. At therapeutic concentrations (approx. 0.5 microgram/ml in whole blood) greater than 90% of the erythrocyte associated ciclosporin was found in the cytosol. The cytosolic binding capacity was approximately (2-2.5).10(5) molecules of ciclosporin per cell. A lower affinity binding of the drug to the plasma membrane occurred only at higher ciclosporin concentrations. The ciclosporin-binding species was purified from erythrocyte cytosol using ciclosporin-Affigel affinity chromatography. This revealed a 16 kDa protein, similar in size to the ciclosporin-binding protein, cyclophilin, previously identified in lymphocyte cytosol. Immunochemical analysis using rabbit anti-bovine spleen cyclophilin antisera revealed that the erythrocyte ciclosporin-binding protein was either cyclophilin or a closely related protein. It is concluded that intracellular ciclosporin-binding within erythrocytes is mostly attributable to the presence of a single protein or protein family represented by cyclophilin. The presence of (2-2.5).10(5) copies of this binding protein within each erythrocyte is responsible for the ciclosporin found associated with erythrocytes.

Identification of several cyclosporine binding proteins in lymphoid and non-lymphoid cells in vivo

The immunosuppressant cyclosporine A (CSA) has been shown to bind to the ubiquitous cellular protein, cyclophilin, and to inhibit its rotamase activity. In the present study, 3H-cyclosporine diazirine analogue was used to photolabel viable human cells of lymphoid and fibroblast origin in order to identify the intracellular targets for the drug. While cyclophilin was strongly labeled in situ, additional minor cyclosporine-protein complexes of 25, 40, 46 and 60 kDa were identified in the T cell leukemia cell line Jurkat. These proteins bound specifically, since only active CSA but not inactive CSH or FK506 competed for binding. Photolabeling of MRC5 cells, a CSA resistant human fibroblast cell line, revealed a 25 kDa complex as the major product, while the 46 and 60 kDa bands were not detectable and cyclophilin labeling was only faint, even though both MRC5 and Jurkat cells contain similar cyclophilin concentrations. Thus, our data suggest that the intracellular targets of CSA and/or the accessibility to cyclophilin varies considerably in drug sensitive and resistant cell types, which may contribute to explaining the lymphocyte selectivity of the drug.

A comparison of cyclosporine binding by cyclophilin and calmodulin and the identification of a novel 45 KD cyclosporine-binding phosphoprotein in Jurkat cells.

Cyclosporine mediates its immunosuppressive effect by preventing the synthesis of lymphokine mRNA during the process of T lymphocyte activation. Although the detailed molecular mechanism by which CsA achieves this effect is unknown, two proteins have been identified as putative intracellular CsA‐receptor proteins. One of these, calmodulin, is an important Ca++‐binding protein and enzyme cofactor and the other, cyclophilin, is a novel protein that is reported to have protein kinase activity. In this study the CsA‐binding capacity of both these proteins has been assessed using CsA‐coated ELISA plates and CsA‐affinity gel matrices. CsA binding was shown by cyclophilin whereas no CsA‐calmodulin binding could be detected under identical conditions. However, it was not possible to demonstrate any cyclophilin‐associated protein kinase activity. Jurkat cells were probed for the presence of CsA‐binding proteins using the CsA‐affinity gel matrix; a 17 KD protein, most probably cyclophilin, was identified as the major CsA‐binding protein. In addition, a previously unidentified CsA‐binding 45 KD phosphoprotein was precipitated from 32P‐labeled Jurkat cells. These results would support cyclophilin as the major, if not only, intracellular receptor protein for CsA. However, the relationship between binding of CsA to cyclophilin and/or the 45 KD phosphoprotein and the immunosuppressive effects of CsA is still unknown.

Evidence that rapamycin has differential effects on IL-4 function : multiple IL-4 signaling pathways and implications for in vivo use

The immunosuppressive drug rapamycin, which inhibits the response of T cells to growth-promoting lymphokines, has been considered to act as a general inhibitor of cytokine action. Our investigations into the effect of rapamycin on human IL-4, a cytokine controlling B and T cell function, show this not to be the case. Unexpectedly, rapamycin actually synergized with IL-4 in both the upregulation of CD23 expression and the down-regulation of the type II (p75) TNF receptor, while in the same B cell line, rapamycin simultaneously inhibited the IL-4-dependent production of TNFα and β. These results raise the possibility that multiple IL-4 signaling pathways may be responsible for the pleiotropic effects of IL-4, and have important implications for both the experimental and possible clinical in vivo use of rapamycin as a selective immunosuppressant.