Peptidyl-prolyl cis-trans isomerase is the cyclosporin A-binding protein cyclophilin

Leukocyte chemotactic activity of FKBP and inhibition by FK506

Cyclophilin, the cyclosporin A binding protein and member of the immunophilin family of proteins, demonstrates leukocyte chemotactic activity. In this study we demonstrate that FKBP, the FK506 and rapamycin binding protein, also displays leukocyte chemotactic activity. The chemotactic activity of FKBP is inhibited by FK506, however, FK506 was unable to inhibit cyclophilin-stimulated chemotactic activity. Rapamycin was unable to prevent the chemotactic activity of FKBP, similarly, the CsA analogue Me6Ala-CsA while displaying cyclophilin binding was unable to block cyclophilin-stimulated chemotactic activity. These results suggest that in addition to their intracellular role the immunophilins may also function as chemotactic agents, furthermore this activity is modulated by immunosuppressants.

Intramolecular catalysis of a proline isomerization reaction in the folding of dihydrofolate reductase

The cis/trans isomerization of the peptide bond preceding proline residues in proteins can limit the rate at which a protein folds to its native conformation. Mutagenic analyses of dihydrofolate reductase (DHFR) from Escherichia coli show that this isomerization reaction can be intramolecularly catalyzed by a side chain from an amino acid which is distant in sequence but adjacent in the native conformation. The guanidinium NH2 nitrogen of Arg 44 forms one hydrogen bond to the imide nitrogen and a second to the carbonyl oxygen of Pro 66 in wild-type DHFR. Replacement of Arg 44 with Leu results in a change of the nature of the two slow steps in refolding from being limited by the acquisition of secondary and/or tertiary structure to being limited by isomerization. The simultaneous replacement of Pro 66 with Ala (i.e., the Leu 44/Ala 66 double mutant) eliminates this isomerization reaction and once again makes protein folding the limiting process. Apparently, one or both of the hydrogen bonds between Arg 44 and Pro 66 accelerate the isomerization of the Gln 65-Pro 66 peptide bond. The replacement of Arg 44 with Leu affects the kinetics of the slow folding reactions in a fashion which indicates that the crucial hydrogen bonds form in the transition states for the rate-limiting steps in folding.

Structural and functional characterization of Escherichia coli peptidyl-prolyl cis-trans isomerases

Peptidyl-prolyl cis-trans isomerases (PPIases), enzymes that catalyze the cis-trans isomerization of peptide bonds to which proline contributes the nitrogen, were purified from Escherichia coli. In this organism, at least two PPIases are present. Both the cationic (periplasmic) and anionic (cytoplasmic) PPIases are inhibited by cyclosporin A with a Ki of 25-50 microM, a concentration 1000-fold higher than that required for eukaryotic PPIases. Although isoelectric focusing indicates that the two enzymes differ in isoelectric point by at least 4.0 pH units, the specific activities of the enzymes toward the tetrapeptide substrate succinyl-Ala-Ala-Pro-Phe-methyl-coumarylamide are equivalent. The activity of both enzymes for a series of substituted succinyl-Ala-Xaa-Pro-Phe-para-nitroanilide tetrapeptides suggests that the structure and function of the active site of the prokaryotic proteins is similar to that of eukaryotic cyclophilins. Both enzymes are capable of catalyzing the refolding of thermally denatured type III collagen. Antibodies against the periplasmic PPIase do not recognize the cytoplasmic enzyme, indicating significant differences in epitopes between the two forms. Circular dichroism spectroscopy indicates that the secondary structure of the cationic protein consists of 17% alpha-helix, 34% beta-sheet, 17% turns, 33% random coil and is very similar to human cytosolic PPIase.

Mechanisms of action of cyclosporine and effects on connective tissues

Cyclosporine is a potent immunomodulatory agent with an increasing number of clinical applications. Its major mode of action is inhibition of the production of cytokines involved in the regulation of T-cell activation. In particular, cyclosporine inhibits the transcription of interleukin 2. Although cyclosporine's major actions are on T cells, there is some evidence that it produces direct effects on other cell types. Its immunosuppressive action is closely linked to its binding of cyclophilin, a member of a family of high-affinity cyclosporine-binding proteins widely distributed in different cell types and in different species. The cyclophilins have been shown to have peptidyl-prolyl cis-trans isomerase enzyme activity that is blocked by cyclosporine. Although this may be a factor in cyclosporine's selective inhibition of cytokine gene transcription, it is still unclear whether inhibition of this activity is the mechanism through which cyclosporine exerts its effects on target cells. The ubiquitous presence of cyclophilins raises the question of why cyclosporine has major effects on T cells. Perhaps the critical proteins affected are transcriptional regulators restricted in their tissue distribution. The effects of cyclosporine on T cells and, directly or indirectly, on connective tissue cells, all of which can produce a range of cytokines, are of interest in relation to the tissue changes that occur in such inflammatory conditions as rheumatoid arthritis.

Limbic seizures increase cyclophilin mRNA levels in rat hippocampus

Limbic seizures lead to dramatic and specific modulation of mRNA levels for many genes in the hippocampus including immediate early, growth factor and neuropeptide genes. In the present study, the influence of hilus lesion (HL)-induced seizures on the abundance of mRNA coding for cyclophilin, a peptide prolyl isomerase, in rat hippocampus was analyzed. By nuclease protection analysis a significant increase in cyclophilin mRNA levels was observed in the hippocampal dentate gyrus/CA1 subfield following HL-induced seizures. The increase began 6 h post-HL, reached a maximum (2.5-fold) at 12 h post-HL and returned to control values by 48 h post-HL. Cyclophilin mRNA levels remained stable in the cerebral cortex throughout the same seizure and post-seizure activity time span.

Purification and N-terminal sequencing of peptidyl-prolyl cis-trans-isomerase from rat liver mitochondrial matrix reveals the existence of a distinct mitochondrial cyclophilin

1. Rat liver mitochondrial matrix peptidyl-prolyl cis-trans-isomerase (PPIase) has been purified. The major form of the enzyme has a molecular mass of 18.6 kDa, with a minor active component of 17.6 kDa. 2. The second-order rate constant for cyclosporin A binding to the enzyme was determined from the time-dependence of the inhibition of PPIase by low concentrations of cyclosporin A and found to be 0.9 microM-1.s-1 at 10 degrees C. 3. The Ki for cyclosporin A inhibition of the enzyme was 3.6 nM, and the half-life for dissociation of the enzyme-inhibitor complex was 3.6 min. 4. From the specific activity of the pure enzyme it can be calculated that isolated liver mitochondria contain approx. 45 pmol of enzyme per mg of total mitochondrial protein. Higher values estimated previously [Halestrap & Davidson (1990) Biochem. J. 268, 153-160] are explained by the use of a short (30 s) preincubation period of the enzyme with cyclosporin, which is insufficient to allow full equilibration of the binding of the inhibitor to the PPIase. 5. N-Terminal sequencing of the 18.6 and 17.5 kDa forms of PPIase show the presence of mitochondrial presequences of 13 and three amino acids respectively, with the remaining sequence having a strong sequence similarity to other cyclophilins. 6. Parallel purification and N-terminal sequencing of rat cytosolic PPIase showed the two proteins to have significant differences, implying that they are probably products of separate genes.

Peptidyl-prolyl cis-trans isomerase improves the efficiency of protein disulfide isomerase as a catalyst of protein folding

The cis-trans isomerization of prolyl peptide bonds and the formation of disulfide bonds are both slow steps in protein folding. By using ribonuclease T1 as a model system, we show that these two processes can become linked in the oxidative folding of reduced proteins and that the formation of the correct disulfide bonds is facilitated in the presence of peptidyl-prolyl cis-trans isomerase. In particular, the efficiency of protein disulfide isomerase (EC 5.3.4.1) as a catalyst of disulfide bond formation in the course of oxidative folding is markedly improved when peptidyl-prolyl cis-trans isomerase is present simultaneously. Possibly, unfolded or partially folded protein chains with correct prolyl isomers are better substrates for catalysis by protein disulfide isomerase. The interdependence of the two enzymatic activities detected during in vitro folding experiments could be of importance for the de novo folding and disulfide bond formation of nascent proteins in the endoplasmic reticulum.

Isolation and partial characterization of membrane-associated cyclophilin and a related 22-kDa glycoprotein

The presence of membrane-associated proteins which stereospecifically bind cyclosporin A and react with anti-cyclophilin antibodies has been documented in rat tissues. Extraction of membranes with 6 M urea or 0.5% Chaps releases cyclosporin-binding activity that is 5-12% of that found in cytosol. Cyclosporin-A-binding proteins are present in most subcellular organelles of liver, but microsomes contain the greatest activity. These proteins can be purified by adsorption onto a cyclosporin-A affinity column and elution with cyclosporin A. Two major fractions are resolved on SDS/PAGE: an 18-kDa fraction is comprised of two isoforms that are similar if not identical to the two major cytosolic isoforms of cyclophilin. In addition, in microsomes an approximately equal quantity of a 22-kDa glycoprotein was detected. Based on partial sequencing (five peptides, 89 amino acids) this protein is similar but not identical to human cyclophilin B. This 22-kDa isoform is poorly recognized by affinity-purified anti-cyclophilin antibodies and comprises several predominant isoforms (pI approximately 9.3-9.6). Selective binding of membrane 22-kDa cyclophilin to peanut lectin suggests the oligosaccharides contain a terminal galactosyl-N-galactosamine residue.

Evidence for a functional receptor for cyclosporin A on the surface of lymphocytes

Cyclosporin A (CsA) is an immunosuppressive agent that inhibits the synthesis of lymphokines by T lymphocytes at the level of transcription. A cytoplasmic protein, cyclophilin, is the most thoroughly studied CsA-binding protein, but its ubiquitous presence in cells of all types raises questions about its role in immunosuppression. In an attempt to ascertain the presence of a cell surface receptor, we synthesized two polyvalent macromolecular CsA derivatives, CsA-BBa-ovalbumin and CsA-BBa-aminodextran (CBD), from the product of the photochemical reaction of CsA and 4-benzoylbenzoic acid (CsA-BBa). (i) They inhibited the peptidylprolyl cis-trans isomerase activity of cyclophilin and the synthesis of interleukin 2 by phorbol ester-activated EL-4 cells. (ii) CBD also inhibited interleukin 2 secretion by Con A-activated T-cell-enriched mouse splenocytes. 4-Benzoylbenzoic acid (BBa)-aminodextran and aminodextran were inactive. (iii) Direct binding and competition studies with [3H]CsA indicated that CBD does not enter EL-4 cells (i.e., it acted at the surface). (iv) CBD caused agglutination of EL-4 cells, murine B and T lymphocytes, human thymocytes, and two T-cell hybridomas. Agglutination was inhibited by a monoclonal antibody to CsA and by CsA and CsA-BBa, but not by BBa. No agglutination was seen with BBa-aminodextran or aminodextran. HeLa cells, Vero (monkey kidney) cells, a mouse plasmacytoma, COS cells, and a poorly differentiated B-cell lymphoma were not agglutinated. (v) EL-4 cells failed to be agglutinated after treatment with trypsin or chymotrypsin. Specific agglutination was again possible after incubation for 5 h at 37 degrees C in the absence of enzyme. (vi) CBD covalently linked to crosslinked agarose beads inhibited interleukin 2 production by phorbol ester-stimulated EL-4 cells. No activity was seen if cell-to-bead contact was prevented by a 0.02-microns microporous filter that did not interfere with the passage of CBD. Our findings support the presence of a functional receptor on the surface of selected cells of the immune system.

Cyclosporin-mediated inhibition of bovine calcineurin by cyclophilins A and B

The Ca(2+)- and calmodulin-dependent protein phosphatase calcineurin is inhibited by the immunosuppressant drug cyclosporin A in the presence of cyclophilin A or B. Of the two isoforms, cyclophilin B is more potent by a factor of 2-5 when either the phosphoprotein [32P]casein or the [32P]phosphoserine [Ser(32P)] form of the 19-residue bovine cardiac cAMP-dependent protein kinase regulatory subunit peptide RII, [Ser(32P)15]RII, is used as substrate. With [Ser(32P15]RII as substrate, the concentrations of the cyclosporin A.cyclophilin A and cyclosporin A.cyclophilin B complexes, which cause 50% inhibition of calcineurin activity, are 120 and 50 nM, respectively. Lowering the concentration of calcineurin 80% with [32P]casein as substrate lowered the apparent inhibition constant for each complex even further; 50% inhibition of calcineurin was observed at 40 nM for cyclosporin A.cyclophilin A, whereas it was less than 10 nM for cyclosporin A.cyclophilin B. In all inhibition assays with [32P]casein or [Ser(32P)15]RII, the concentration of calcineurin required for measurable phosphatase activity is such that these complexes behave as tight-binding inhibitors of calcineurin, and steady-state kinetics cannot be used to assess inhibition patterns or Ki values. Limited trypsinization of calcineurin produces a fragment that is still inhibited, indicating that the interaction of cyclosporin.cyclophilin with calcineurin does not require either calmodulin or Ca2+.

Anti-inflammatory effect of cyclosporin A on human skin mast cells

We have examined the effects of cyclosporin A (CsA) and cyclosporin H (CsH), which bind with different affinity to cyclophilin, to evaluate the role of this protein in the release of preformed (histamine) and de novo synthesized (prostaglandin D2[PGD2]) mediators of inflammatory reactions from human skin mast cells (HSMC). CsA (2.4-800 nM)-inhibited (5-60%) histamine release from HSMC challenged with anti-IgE. CsA exerted little, if any, inhibitory effect on histamine release from HSMC challenged with compound A23187 and substance P, whereas it completely suppressed A23187-induced histamine release from human basophils. Inhibition of histamine release from HSMC challenged with anti-IgE was extremely rapid and was not abolished by washing (three times) the cells before anti-IgE challenge. CsA (2.4-800 nM) markedly inhibited (25-70%) the de novo synthesis of PGD2 from HSMC challenged with anti-IgE. CsH, which has an extremely low affinity for cyclophilin, had no effect on skin mast-cell mediator release. These data suggest that CsA is a potent anti-inflammatory agent acting on HSMC, presumably by interacting with cyclophilin.

Enzymatic biosynthesis of cyclosporin A and analogues

The final assembly of the undecapeptide chain of cyclosporin A and its cyclization is accomplished in Beauveria nivea by cyclosporin synthetase. This multienzyme is the largest integrated enzyme structure so far reported. Its size has been estimated at approximately 1,400 kDa by two different methods: 1), by 3% SDS-PAGE using the related multienzymes ACV synthetase and gramicidin S synthetase 2 as references (420 and 556 kDa, respectively); and 2), by CsCl density gradient centrifugation experiments using fluorescence-labeled cyclosporin synthetase. Besides cyclosporin A and a number of cyclosporins known from fermentation studies cyclosporin synthetase is capable of synthesizing some new cyclosporins which are so far unobtainable by fermentation. So, for example the synthesis of [N-methyl-(+)-2-amino-3-hydroxy-4,4-dimethyloctanoic acid1]CyA, dihydro-CyA, [L-norvaline2,5, N-methyl-L-norvaline11]CyA, [L-allo-isoleucine5, N-methyl-L-allo-isoleucine11]CyA, [D-2-aminobutyric acid8]CyA, [beta-chloro-D-alanine8]CyA and some related compounds could be established. By using a related but different enzyme from Cylindrotrichum Bonorden, the peptolide [L-threonine2, L-leucine5,10, D-2-hydroxyisovaleric acid8]CyA could be synthesized in vitro. We were able to synthesize these cyclosporins in sufficient quantities to examine their structure by FAB mass spectroscopy and explore their immunosuppressivity. It was found that all new cyclosporins so far synthesized in the in vitro system are immunosuppressive.

Inhibition of T cell signaling by immunophilin-ligand complexes correlates with loss of calcineurin phosphatase activity

Calcineurin, a Ca2+, calmodulin-dependent protein phosphatase, was recently found to bind with high affinity to two different immunosuppressant binding proteins (immunophilins) with absolute dependence on the presence of the immunosuppressants FK506 or cyclosporin A (CsA) [Liu et al. (1991) Cell 66, 807-815]. The binding affinities of the immunophilin-drug complexes toward calcineurin and the stoichiometry of the resultant multimeric complexes have now been determined, and structural elements of FK506, CsA, and calcineurin that are critical for mediating their interactions have been identified. Analogues of FK506 (FK520, FK523, 15-O-demethyl-FK520) and CsA (MeBm2t1-CsA and MeAla6-CsA) whose affinities for their cognate immunophilins do not correlate with their immunosuppressive activities have been prepared and evaluated in biochemical and cellular assays. We demonstrate a strong correlation between the ability of these analogues, when bound to their immunophilins, to inhibit the phosphatase activity of calcineurin and their ability to inhibit transcriptional activation by NF-AT, a T cell specific transcription factor that regulates IL-2 gene synthesis in human T cells. In addition, FKBP-FK506 and CyP-CsA do not inhibit members of the PP1, PP2A, and PP2C classes of serine/threonine phosphatases. These data suggest that calcineurin is the relevant cellular target of these immunosuppressive agents and is involved in Ca(2+)-dependent signal transduction pathways in, among others, T cells and mast cells.

Identification of cyclophilin as a proinflammatory secretory product of lipopolysaccharide-activated macrophages

We have isolated an 18-kDa peptide (designated sp18, for 18-kDa secreted protein) from the conditioned medium of lipopolysaccharide-stimulated RAW 264.7 murine macrophages. Purified sp18 had in vivo inflammatory activity and in vitro chemotactic activity for human peripheral blood polymorphonuclear leukocytes and monocytes. Surprisingly, N-terminal sequencing and tryptic mapping studies revealed that sp18 and cyclophilin, an intracellular protein that binds the immunosuppressive drug cyclosporin A, are highly homologous. The in vitro chemotactic activity of sp18 on monocytes was blocked by cyclosporin A but not by cyclosporin H, a structural analog of cyclosporin A that does not bind cyclophilin. Like purified porcine cyclophilin, mouse sp18 exhibited peptidyl-prolyl cis-trans isomerase activity. Medium conditioned by lipopolysaccharide-stimulated resident peritoneal exudate macrophages isolated from C57BL/6 mice contained substantially higher levels of sp18/cyclophilin than medium conditioned by nonstimulated macrophages. The observation that sp18/cyclophilin exhibits proinflammatory activity and is secreted by macrophages in response to endotoxin suggests that this protein may function as a cytokine, and invites the hypothesis that the immunosuppressive action of cyclosporin A results in part from interaction with an extracellular form of cyclophilin released as a mediator of immune and inflammatory functions.

A conformation of cyclosporin A in aqueous environment revealed by the X-ray structure of a cyclosporin-Fab complex

The conformation of the immunosuppressive drug cyclosporin A (CsA) in a complex with a Fab molecule has been established by crystallographic analysis to 2.65 angstrom resolution. This conformation of CsA is similar to that recently observed in the complex with the rotamase cyclophilin, its binding protein in vivo, and totally different from its conformation in an isolated form as determined from x-ray and nuclear magnetic resonance analysis. Because the surfaces of CsA interacting with cyclophilin or with the Fab are not identical, these results suggest that the conformation of CsA observed in the bound form preexists in aqueous solution and is not produced by interaction with the proteins.

Molecular mode of action of cyclosporin and FK506 in human thymocytes

The molecular mode of action of cyclosporin, three of its non-immunosuppressive analogs, N-methyl-l-alanyl cyclosporin, acetyl cyclosporin and cyclosporin S3, and of FK506 was studied in primary cultures of human thymocytes. Nuclear factors derived from thymocytes activated with phorbol myristate acetate and concanavalin A were tested for their ability to bind to a synthetic radiolabelled probe corresponding to the NF-AT region (-285 to -255) of the IL-2 gene. Binding was observed, and it was inhibited by CsA (100 ng/ml), while the analogs at ten-fold higher concentrations (1000 ng/ml) were only partially inhibitory. CsA in combination with FK506 inhibited binding of nuclear factors at the NF-AT site, and acted in concert.

The mechanism of action of cyclosporin A and FK506

CsA and FK506 are powerful suppressors of the immune system, most notably of T cells. They act at a point in activation that lies between receptor ligation and the transcription of early genes. Here, Stuart Schreiber and Gerald Crabtree review recent findings that indicate CsA and FK506 operate as prodrugs: they bind endogenous intracellular receptors, the immunophilins, and the resulting complex targets the protein phosphatase, calcineurin, to exert the immunosuppressive effect.

Minimum energy conformations of proline-containing helices

Proline occurs frequently in transmembrane alpha-helices of transport and receptor proteins even though statistical surveys demonstrate the overwhelming preference of this residue for a non-alpha-helical, hydrophilic environment. As a result, membrane-buried proline has been proposed to be functionally important, with function arising from structural discontinuity or destabilization of the helix. Destabilization may occur by Pro-mediated conformational transitions between discrete states, and may be manifested in membrane protein systems through reversible processes such as channel opening and closing or signal transduction. In this study, computer modeling of a model transmembrane alpha-helix, (Ala)8-Leu-Pro-Phe-(Ala)8, in a medium of low polarity (dielectric = 2), is used to examine the occurrence and energetic accessibility of Pro-mediated conformational interconversions. Leu psi and chi 1, Pro psi, and Phe phi and chi 1 torsion angles were assigned random values so that a data base of 200 conformations for each of the cis and trans states was generated. The conformations were minimized and low-energy structures organized into families. This analysis demonstrated that the most populated lowest energy family is the Trans-I conformation, corresponding to proline in a kinked alpha-helix. Two additional trans structures, Trans-II and Trans-III, as well as a cis conformation, Cis-I, are also energetically competitive. Interconversions between the trans states could thus be mediated by changes at a single torsion angle, accompanied by minor local hydrogen-bonding rearrangements. This work substantiates that membrane-buried proline can provide the basis for conformational transitions between discrete alpha-helix-based structures in a nonpolar environment.

Presence of autoantibodies to peptidyl-prolyl cis-trans isomerase (cyclosporin A-binding protein) in systemic lupus erythematosus

Several autoantibodies against cytoplasmic or nuclear components of cells have been reported in autoimmune diseases. We report here a previously unrecognized autoantibody to peptidyl-prolyl cis-trans isomerase (PPIase) in patients with systemic lupus erythematosus (SLE). PPIase, which catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides, has recently been found to be identical to cyclophilin, a specific binding protein of a potent immunosuppressant, cyclosporin A. IgG and IgM anti-PPIase antibodies were detected in 40 and 20% of unselected patients with SLE, respectively, by ELISA. The reactivity of these sera was confirmed by immunoblotting experiments. Sera from rheumatoid arthritis patients showed no reactivity and 1 of 8 sera from systemic sclerosis patients and 1 of 25 sera from normal controls showed only weak reactivity. Unexpectedly, the anti-PPIase antibody was unable to inhibit PPIase activity, indicating that the autoantibody recognizes an epitope of PPIase which is different from the active site of PPIase. The levels of the anti-PPIase antibody in SLE patients correlated with remissions and flares of the disease. The anti-PPIase antibody was higher in patients with active SLE than those with inactive disease. The prevalence of the active stage of the disease was significantly higher in IgG anti-PPIase antibody-positive SLE patients as compared to antibody-negative SLE patients. These data define the presence of a new autoantibody against PPIase and its association with the activity and certain clinical manifestations in SLE.

Neisseria meningitidis encodes an FK506-inhibitable rotamase

Eukaryotic peptidyl-prolyl cis-trans isomerases (rotamases) fall into two classes, the cyclophilins inhibited by cyclosporin A and the FK506-binding proteins inhibited by the macrolide antibiotic FK506. In prokaryotes homologs of cyclophilins have been identified and found to have rotamase activity. Sequence similarities have been noted between FK506-binding proteins and gene products in a number of bacterial species, but whether these bacterial proteins have rotamase activity is not known. Using the polymerase chain reaction, we have cloned and sequenced a homolog of an FK506-binding protein from Neisseria meningitidis and expressed the gene product as a fusion protein with maltose-binding protein. The fusion protein was purified by affinity chromatography. By measuring the rate of chymotrypsin cleavage of the substrate succinyl-Ala-Ala-Pro-Phe p-nitroanilide, we found that the fusion protein had rotamase activity comparable to that of human FK506-binding protein. This rotamase activity was inhibited by FK506.

Cyclosporine A inhibits ATP net uptake of rat kidney mitochondria

The adenine nucleotide content of mitochondria varies at several physiological and pathological situations. Both net transport and intramitochondrial catabolism of adenine nucleotides has been suggested to be responsible for these changes. Here, the influence of cyclosporine A on the ATP net uptake of isolated rat kidney mitochondria was examined. The ATP net uptake of mitochondria depleted of matrix adenine nucleotides by pyrophosphate treatment was inhibited by cyclosporine A showing a I50 value of about 4 nmol/mg mitochondrial protein. Because intramitochondrial adenine nucleotide content is important for several mitochondrial functions such as oxidative phosphorylation, Ca2+ homoeostasis and mitochondrial biogenesis, it is concluded that the inhibition of adenine nucleotide net transport and a decrease of adenine nucleotide content may be involved in the immunosuppressive and nephrotoxic effects of cyclosporine A.

Covalent binding of cyclosporine inhibits irreversibly T-lymphocyte activation

A diazirine derivative of cyclosporine (PL-CS) was used to photolabel recombinant human cyclophilin (rhCyp), the cytosolic receptor for the immunosuppressant cyclosporine. The affinity of PL-CS for rhCyp and the immunosuppressive activity were 10-fold reduced as compared to cyclosporine A. Whereas cyclosporine immunosuppression was fully reversible, UV cross-linking of PL-CS resulted in permanent inhibition of lymphocyte activation as shown by proliferation of anti-CD3 stimulated human peripheral lymphocyte, interleukin (IL)-2 gene transcription and IL-2 synthesis in the human T-leukemia cell line Jurkat. In vivo photolabeling of viable Jurkat cells revealed that a 21-kDa complex was the major radiolabeled product which was identified as a cyclophilin-cyclosporine complex. In addition, cyclophilin B (25 kDa) and proteins of an unidentified nature at 40, 46 and 60 kDa were observed in Jurkat cells. The cyclosporine-resistant human fibroblast cell line MRC5 displayed a different labeling pattern: cyclophilin B (25 kDa) and a 65-kDa protein were the major labeled products, while the 46- and 60-kDa components were not detectable and cyclophilin was only faintly labeled. In summary, covalent cyclosporine binding caused irreversible lymphocyte inactivation and revealed in addition to cyclophilin other specifically labeled proteins in lymphoid cells. The role and identity of these proteins is presently unknown.

SDZ 280-446, a novel semi-synthetic cyclopeptolide: in vitro and in vivo circumvention of the P-glycoprotein-mediated tumour cell multidrug resistance

SDZ 280-446 is a semi-synthetic derivative of a natural cyclic peptolide. Its ability to sensitise in vitro tumour cells whose resistance is due to P-glycoprotein-mediated anticancer-drug efflux was shown using four different pairs of parental drug-sensitive (Par-) and multidrug-resistant (MDR-) cell lines, from three different species (mouse, human, Chinese hamster) representing four different cell lineages (monocytic leukaemia, nasopharyngeal epithelial carcinoma, colon epithelial carcinoma, ovary fibroblastoid carcinoma), and using four different drug classes (colchicine, vincristine, daunomycin/doxorubicin and etoposide). By measuring its capacity to restore normal drug sensitivity of MDR-cells in culture in vitro, it appeared that SDZ 280-446 belongs to the same class of very potent chemosensitisers as the cyclosporin derivative SDZ PSC 833: both are about one order of magnitude more active than cyclosporin A (CsA), which is itself about one order of magnitude more active than other known chemosensitisers (including verapamil, quinidine and amiodarone which have already entered clinical trials in MDR reversal). Low concentrations of SDZ 280-446 could also restore cellular daunomycin retention in MDR-P388 cells to the levels found in the Par-P388 cells. SDZ 280-446 was also effective as a chemosensitiser when given orally in vivo. In a syngeneic mouse model, combined therapy with vinca alkaloids given i.p. and SDZ 280-446 given per os for 5 consecutive days significantly prolonged the survival of MDR-P388 tumour-bearing mice, when compared with mice receiving vinca alkaloids alone. Another protocol, using three cycles of i.p. doxorubicin at 4 day intervals, could also not increase MDR-P388 tumour-bearing mouse survival unless the mice received SDZ 280-446 orally 4 h before each doxorubicin injection. Though only very few combined therapy treatment protocols have been tested so far, clear increases in survival time of MDR-tumour-bearing mice were regularly obtained, leaving hope for major improvement of the therapy using other dosing schedules.

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.

The yeast cyclophilin multigene family: purification, cloning and characterization of a new isoform

Cyclophilins (Cyps) constitute a highly conserved family of proteins present in a wide variety of organisms. Historically, Cyps were first identified by their ability to bind the immunosuppressive agent cyclosporin A (CsA) with high affinity; they later were found to have peptidyl-prolyl cis-trans isomerase (PPIase) activity, which catalyzes the folding of oligopeptides at proline-peptide bonds in vitro and may be important for protein folding in vivo. Cells of Saccharomyces cerevisiae contain at least two distinct Cyp-related PPIases encoded by the genes CYP1 and CYP2. A yeast strain (GL81) containing genomic disruptions of three known yeast PPIase-encoding genes [CYP1, CYP2 and RBP1 (for rapamycin-binding protein); Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] was previously constructed and found to be viable. Soluble fractions of these cells possess residual CsA-sensitive PPIase activity (2-5% of that present in wild-type cells as assayed in vitro). We have purified an approx. 18-kDa protein exhibiting PPIase activity from a soluble fraction of GL81 cells and determined that its N-terminal amino acid (aa) sequence exhibits significant homology (but nonidentity) to the Cyp1 and Cyp2 proteins. We designate the gene for this new protein, CYP3. Using a degenerate oligodeoxyribonucleotide (oligo) based on the N-terminal aa sequence, plus an internal oligo homologous to a conserved region within the portion of CYP1 and CYP2 that had been deleted in the genome, a CYP3-specific DNA fragment was generated by the polymerase chain reaction (PCR) using GL81 genomic DNA as a substrate. This PCR fragment was used as a probe to isolate CYP3 genomic and cDNA clones.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiplex solution hybridization-ribonuclease protection assay for quantitation of different ribonucleic Acid transcripts from snap-frozen neuroendocrine tissues of individual animals

We have compiled a protocol for simultaneous quantitation of a variety of gene transcripts in multiple individual brain and pituitary gland dissections for studying pretranslational regulation of neuroendocrine systems in vivo, using experimental designs compatible with meaningful statistical power. To facilitate collection of many samples at a time, the tissue was snap-frozen in chilled liquid Freon and stored at -80 °C until further processing. In this way, as many as five different brain and pituitary gland dissections per rat could be collected from eight rats in about an hour. The snap-frozen tissue was suitable for isolation of separate cytoplasmic and nuclear RNA fractions using homogenization in the presence of detergents. To facilitate homogenization of many samples at a time, we devised a method in which the tissue was repeatedly expelled through a 22 gauge hypodermic needle attached to a 1-ml plastic syringe used as a disposable, ready-to-use homogenizer. In order to promote dissolution of lipid membrane structures which are prevalent in the brain, the lysis buffer has been optimized to include the detergent sodium deoxycholate in addition to Nonidet P-40. Specific RNA transcripts were analyzed using a quantitative solution hybridization-ribonuclease protection assay coupled with polyacrylamide gel electrophoresis. The value of this highly sensitive assay has been expanded by including several molecular probes against a variety of neuroendocrine mRNA sequences simultaneously (e.g. progonadotropin-releasing hormone, proopiomelanocortin, tyrosine hydroxylase, dopamine D2 receptor, prolactin), thus increasing the amount of information obtained from each sample in one assay. Furthermore, each sample was routinely co-assayed for cyclophilin mRNA, an abundant, generally non-regulated mRNA whose levels reflected the individual variability in sample processing, thus serving as an internal reference. Once stored in hybridization solution, as many as 100 samples could be analyzed simultaneously for several different RNA transcripts in one assay. This protocol provides a powerful tool for studying regulation of neuroendocrine systems at the molecular level in vivo, using sample sizes suitable for applying statistical analysis of meaningful statistical power.

FK-506 binding protein from tolypocladium inflatum: Resistance of FKBPFK-506 complex against proteolysis

A 12-kDa peptidyl-prolyl-cis/trans-isomerase was purified 225-fold to homogeneity from the cyclosporin producing fungus Tolypocladium inflatum. The enzyme is highly sensitive to the immunosuppressant FK-506 but not to cyclosporin and thus belongs to the class of FK-506 binding proteins (FKBP). Interestingly the FKBP/FK-506 complex is resistant against proteolytic digestion by the endoproteases GluC and LysC, in contrast to the free FKBP, which is readily cleaved by these proteases. This protection may play a role in the effects of FK-506 in the living cell.

Cis-trans isomerization is rate-determining in the reactivation of denatured human carbonic anhydrase II as evidenced by proline isomerase

The refolding of human carbonic anhydrase II is a sequential process. The slowest step involved is the recovery of enzymic activity (t1/2 = 9 min). Kinetic data from 'double-jump' measurements indicate that proline isomerization might be rate determining in the reactivation of the denatured enzyme. Proof of this is provided by the effect of proline isomerase on the reactivation kinetics: the presence of isomerase during reactivation lowers the half-time of the reaction to 4 min, and inhibition of proline isomerase completely abolishes this kinetic effect. A similar acceleration of the refolding process by proline isomerase is also observed for bovine carbonic anhydrase II, in contrast to what has previously been reported. In human carbonic anhydrase II there are two cis-peptidyl-Pro bonds at Pro30 and Pro202. Two asparagine single mutants (P30N and P202N) and a glycine double mutant (P30G/P202G) were constructed to investigate the role of these prolines in the rate limitation of the reactivation process. Both in the presence and absence of PPIase the P202N mutant behaved exactly like the unmutated enzyme. Thus, cis-trans isomerization of the Pro202 cis-peptidyl bond is not rate determining in the reactivation process. The mutations at position 30 led to such extensive destabilization of the protein that the refolding reaction could not be studied.

s-cyclophilin is retained intracellularly via a unique COOH-terminal sequence and colocalizes with the calcium storage protein calreticulin

Cyclophilins (cyclosporin A-binding proteins) are conserved, ubiquitous, and abundant proteins that accelerate the isomerization of XaaPro peptide bonds and the refolding of proteins in vitro. s-Cyclophilin is a member of the cyclophilin family with unique NH2- and COOH-terminal extensions, and with a signal sequence. We now report that s-cyclophilin is retained in the cell, and that the conserved s-cyclophilin-specific COOH-terminal extension VEKPFAIAKE is sufficient to direct a secretory protein to s-cyclophilin containing structures. Antibodies to s-cyclophilin-specific peptides were produced and the location of the protein was determined by an immunocytochemical study at the light microscopic level. s-Cyclophilin colocalized with the Ca(2+)-binding protein calreticulin and, to a lesser extent, with the microsomal Ca(2+)-ATPase in the myogenic cell line L6, and with the Ca(2+)-binding protein calsequestrin in skeletal muscle. In activated platelets, s-cyclophilin immunoreactivity was detected in a ring-like structure that might correspond to the Ca(2+)-storing and -releasing dense tubular network. In spreading cells, s-cyclophilin containing vesicular structures accumulated at actin-rich protrusion sites. While s-cyclophilin consistently codistributed with Ca2+ storage site markers, the distribution of s-cyclophilin immunoreactivity was not identical to that of ER markers. To determine whether the COOH-terminal extension of s-cyclophilin was involved in its intracellular transport we added this sequence to the COOH-terminus of the secretory protein glia-derived nexin. Appropriate constructs were expressed transiently in cultured cells and proteins were detected with specific antibodies. We found that glia-derived nexin with the COOH-terminal sequence VEKPFAIAKE (but not with the control sequence GLVVMNIT) colocalized with endogenous s-cyclophilin, indicating that the sequence contained retention information. These results indicate that s-cyclophilin is a retained component of an intracellular organelle and that it may accumulate in specialized portions of the ER, and possibly in calciosomes. Because of its conserved structure, widespread distribution, and abundance s-cyclophilin may be a useful marker to study the biogenesis and distribution of ER subcompartments.

Novel monoclonal antibodies to cyclosporine A: Characterization and epitope mapping with cyclosporine analogs and cyclophilin

Monoclonal antibodies to cyclosporine A (Cs), a potent immunosuppressant, were generated in BALB/c mice using a novel antigen prepared by linking Cs to a protein carrier via a photoactive cross-linking reagent, 4-benzoylbenzoic acid (BBa). Twenty-two monoclonal anti-Cs antibodies were generated, using Cs-BBa-bovine serum albumin (Cs-BBa-BSA) as the immunogen. They were characterized with respect to affinity by Scatchard analysis of a radioimmunoassay (RIA), and with respect to specificity by an ELISA in which a series of singly substituted Cs derivatives were examined as inhibitors. McAb affinities ranged from 5 x 10(-8) M to 2 x 10(-10) M. Based on ELISA inhibition data with Cs analogs, and on the binding to two Cs-BSA conjugates in which opposite sides of the Cs molecule are exposed, the antibodies fell into five epitope recognition groups. Binding to Cs was also studied by ELISA in competition with cyclophilin (CyP), a Cs-binding protein whose epitope specificity has been well characterized. Competition by CyP was found to correlate with antibody specificity, not with affinity, i.e. CyP competed best with antibodies having specificities most similar to that of CyP. Epitope mapping can, therefore, be accomplished in a system in which two different species of binding proteins compete for the same antigen. This type of characterization may be useful in identifying antibodies whose combining sites mimic those of a receptor.

A specific binding site in Nb2 node lymphoma cells mediates the effects of didemnin B, an immunosuppressive cyclic peptide

Didemnin B (DB) is a cyclic depsipeptide with a variety of biologic effects, including potent antiviral, antitumor, and immunosuppressive activities. Although its mechanism of action has been attributed to inhibition of DNA and protein synthesis, the exact cellular site of interaction has not been previously defined. Since DB is strongly antiproliferative in Nb2 node lymphoma cells, we investigated potential DB binding sites in these cells, using [3H]-DB (2.7 mCi/mg) as the radiolabeled ligand. Time course studies with Nb2 cells showed that steady state [3H]-DB binding was attained after 4 h. Scatchard analysis with resting cells yielded a Kd of 180 nM (200 ng/ml), and 7 x 10(6) binding sites/cell. The IC50 of DB inhibition of ongoing protein and DNA synthesis in Nb2 cells, measured 24 h after prolactin (PRL) stimulation, was also in the range of 100 ng/ml. Didemnin analogs, with alterations at critical amino acid residues, inhibited the synthesis of DNA and protein and competed with [3H]-DB binding with the same rank order of potency. This implies that this binding site may mediate the inhibition of macromolecule synthesis. Subcellular fractionation of [3H]-DB labeled Nb2 cells revealed that specific binding occurred predominantly in the 100,000 g cytosolic fraction. Comparison with cyclophilin and the FK506 binding protein, both cytosolic receptors, suggests that the DB binding site may also belong to the family of immunophilins.

Cyclosporin A. Mode of action and effects on bone and joint tissues

Cyclosporin A is an established immunomodulatory agent with an increasing number of clinical applications. Although its precise mechanisms of action remain elusive, one of the most important known properties of CyA is its ability to inhibit the production of cytokines involved in the regulation of T-cell activation. In particular, CyA inhibits de novo synthesis of interleukin 2(IL-2), the major cytokine involved in T-cell proliferation, as well as other cytokines, probably at the level of gene transcription, as shown by the suppression of mRNA levels in activated T-cells. Although the major actions of CyA are on T-cells, there is some evidence for possible direct effects on other cell types e.g. B-cells, macrophages and, from our own work, on bone and cartilage cells. Cyclosporin A is thought to enter cells and to bind to cyclophilins, which are members of a family of high-affinity cyclosporin A-binding proteins, now known as immunophilins. The binding of cyclosporins to such proteins appears to be closely linked to the immunosuppressive action of cyclosporins. The immunophilins possess enzyme activity, ie. peptidyl-prolyl cis-trans isomerase, also known as rotamase, which can regulate protein folding, and may therefore alter the functional state of many cell proteins. Cyclosporin A blocks peptidyl-prolyl cis-trans isomerase activity but it is not clear whether this plays a part in its selective inhibition of cytokine-gene transcription. Moreover, the ubiquitous presence of cyclophilins and immunophilins raises the question of why cyclosporin A has its apparent major effects only on T-cells. Recent proposals regarding the intracellular mode of action of CyA suggest that it interacts with cyclophilin and other regulatory proteins including calmodulin and calcineurin, which is a serine/threonine phosphatase, and thereby affects the functional state of key regulators of gene transcription in its target cells. The effects of CyA on T-cells and directly or indirectly on connective tissue cells, including bone, cartilage and synovial cells, which all can produce a range of cytokines, are of interest in relation to the tissue changes that occur in inflammatory diseases, such as rheumatoid arthritis. Thus, for example, cyclosporin A inhibits in vitro the bone resorbing activity of interleukin 1, 1,25-dihydroxy-vitamin D3, parathyroid hormone and prostaglandin E2 by apparently non-T-cell effects, while in vivo protects against bone and cartilage loss in adjuvant arthritis. More needs to be known about the direct and indirect modulation of cytokine production by cyclosporin A in connective tissues, in order to understand its potential value in clinical disorders.

Murine cyclophilin-S1: a variant peptidyl-prolyl isomerase with a putative signal sequence expressed in differentiating F9 cells

Fractionation of differentiating murine teratocarcinoma F9 cells and extraction of the nuclear/microsomal pellets with ethidium bromide led to the purification and microsequencing of the protein mCyP-S1, a novel cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase (PPIase). mCyP-S1 is a new member of the cyclophilin class of proteins. Cloning and sequencing of the mCyP-S1 cDNA revealed extended coding capacity for a putative N-terminal signal sequence, suggesting processing of mCyP-S1 during intracellular translocation across the membrane of the endoplasmic reticulum. mCyP-S1 is abundantly expressed in a variety of mouse organ tissues and its mRNA levels increase during F9 cell differentiation. Specific subcellular localization of PPIases is postulated to contribute to functional specificities of this class of enzymes.

The nucleotide sequence of a third cyclophilin-homologous gene from Saccharomyces cerevisiae

The nucleotide sequence of a 1558 bp DNA fragment from the right arm of chromosome III of Saccharomyces cerevisiae contains an open reading frame of 954 nucleotides with coding potential for a protein with high similarity to the ubiquitous cyclophilins which are both peptidyl-prolyl cis-trans isomerases and cyclosporin A-binding proteins. It should, therefore, represent the third gene (SCC3) of this kind from S. cerevisiae. SCC3 is present in a single copy in the genome of S. cerevisiae and results in a constitutively expressed 1.2 kb transcript during cell growth. Its putative protein product (Scc3) contains two hydrophobic cores, one at the amino terminal, 20 amino acids long, which could serve as a signal peptide, and the other one at the carboxyl end with a structure similar to a transmembrane helix. These findings suggest that Scc3 could be a secretory or, more likely, a transmembrane protein. The only cyclophilin with similar structure to that of Scc3 is ninaA from Drosophila melanogaster, a transmembrane protein which seems to be implicated in the correct folding and/or intercalation of rhodopsin in the endoplasmic reticulum of the fly photoreceptors (Stamnes, M.A. et al., Cell 65, 219-227, 1991). In addition, the amino and the carboxy regions of Scc3 and ninaA share a significant level of homology, which suggests that they have a similar function, albeit for different target proteins.

The CYP2 gene of Saccharomyces cerevisiae encodes a cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase with an N-terminal signal sequence

Cells of Saccharomyces cerevisiae contain a major cytosolic cyclophilin (Cyp)-related peptidyl-prolyl cis-trans isomerase (PPIase) which is the target for cyclosporin A (CsA) cytotoxicity and which is encoded by the CYP1 gene [Haendler et al., Gene 83 (1989) 39-46]. We recently identified a second Cyp-related gene in yeast, CYP2 [Koser et al., Nucleic Acids Res. 18 (1990) 1643] which predicts a protein with a hydrophobic leader sequence. A sequence lacking 33 codons from the 5'-end of the CYP2 open reading frame was generated by the polymerase chain reaction and engineered for expression in Escherichia coli. The corresponding recombinant truncated protein was purified and found to exhibit PPIase activity which was inhibited by CsA. The CYP2 gene is genetically unlinked to CYP1. As with CYP1, genomic disruption of CYP2 had no effect on haploid cell viability. Disruption of all three of the known yeast PPIase-encoding genes [CYP1, CYP2, and RBP1 for rapamycin-binding protein; Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] in the same haploid cell also resulted in no apparent cellular phenotype, suggesting either that none of these enzymes have an essential function or that additional PPIases can compensate for their specific absence. Whereas cells containing a genomic disruption of CYP1 exhibited a CsA-resistant phenotype, genomic disruption of CYP2 had no effect on CsA sensitivity. This suggests that the CYP1 gene product is the primary cellular target for CsA toxicity in yeast. Since both purified Cyps display CsA sensitivity in vitro, our data suggest that Cyp1 and Cyp2 differ in terms of their cellular function and/or localization.

Differential patterns of regulated gene expression in the adult rat epididymis

Specialization among the principal epithelial cells of the epididymal tubule is documented following the analysis of transcriptional activity of four distinct species of mRNA. In situ histochemical analysis revealed a unique pattern of expression for each transcript. This observation supports the concept that region-specific patterns of transcriptional expression along the epididymal tubule serve as the major molecular basis underlying region-specific patterns of luminal proteins within the tubule. Additionally, multiple testicular factors appear to regulate expression of these mRNAs. The transcript encoding peptidyl-prolyl cis-trans isomerase is constitutively expressed. Those encoding the major secretory proteins, protein B/C and protein D/E, are directly regulated by testicular androgen. That encoding the opioid peptide precursor, proenkephalin, is regulated by a non-androgen testicular factor(s), specifically, spermatozoa or a spermatozoa-related factor. Thus, a complex array of nuclear events and signals received by the principal cells serve to determine the transcriptional status of genes expressed within this epididymal cell type.