Protective effect of mycophenolate mofetil against nephrotoxicity and hepatotoxicity induced by tacrolimus in Wistar rats

Tacrolimus (TAC), a calcineurin inhibitor (CNI), is clinically used as an immunosuppressive agent in the transplant recipient; however, the use of TAC is greatly limited by its nephrotoxicity and hepatotoxicity. Mycophenolate mofetil (MMF), an inhibitor of the purine synthesis, has been used in combination with many immunosuppressive drugs such as TAC. The association TAC/MMF was used in organ transplantation to increase the efficiency and reduce acute rejection rates, but the effects of MMF on TAC-induced kidney and liver injuries are still not well investigated. The aims of this study are to explore whether MMF co-administration with TAC has a renoprotective and hepatoprotective effect against TAC-induced renal and hepatic injuries and to check the implication of oxidative stress in the MMF's possible protective effect. Our results showed that MMF (at 50 mg kg(-1) body weight (b.w.)) restored creatinine, in addition to increased AST and ALT levels by TAC (at 60 mg kg(-1) b.w.). Furthermore, MMF decreased DNA damage induced by TAC in the kidney and liver of rats as assessed by comet assay. This renoprotective and hepatoprotective effect of MMF was associated with an antioxidant effect. In fact, MMF co-treatment with TAC decreased oxidative damage induced by TAC. It reduced malondialdehyde (MDA) and protein carbonyl (PC) levels as well as catalase and superoxide dismutase (SOD) activities. We conclude that the co-administration MMF with TAC protect liver and kidney against TAC toxicity via an antioxidant process.

Possible therapeutic effect of lipid supplementation on neurological complications in liver transplant recipients

Neurological complications (NCs) represent a serious problem following liver transplantation and may develop either because of various peri-operative factors or the toxicity of immunosuppression. Although the causality assessment of NCs can be particularly difficult in the setting of organ transplantation, calcineurin inhibitors (CNIs) might influence NCs to a certain extent, regardless of the etiology. Therefore, minimizing the influence of CNIs could be a reasonable strategy for alleviating NCs. Based on our hypothesis that lipid supplementation prevents lipophilic CNIs from crossing the blood-brain barrier, soybean oil was administered to five liver transplant patients with NCs. In all of these patients, the neurological symptoms improved without discontinuing or reducing the dose of CNIs. Thus, lipid supplementation might be able to reduce the adverse neurological effects of CNIs.

Raloxifene does not affect the changes in bone histomorphometric parameters induced by low dose tacrolimus in male rats

Raloxifene is a selective estrogen receptor modulator. The drug reduces bone loss and prevents fractures in postmenopausal women. Tacrolimus, an immunosuppressant, is used to prevent organ transplant rejection. The effect of raloxifene and tacrolimus on the osseous bone in men has not been exhaustively determined. To study the effects of raloxifene, tacrolimus as well as concurrent administration of raloxifene and tacrolimus on the osseous tissue in male rats, a preliminary assessment of the drug action on histomorphometric parameters of rat bones was made. The experiments were carried out on mature male Wistar rats. The animals were divided into six groups, 7 animals each: I--control rats; II--rats which were administered raloxifene (5 mg/kg po daily); III-- rats which were administered tacrolimus (0.3 mk/kg po daily); IV - rats which were administered tacrolimus (0.6 mg/kg po daily; V-- rats which were administered raloxifene (5 mg/kg po daily) and tacrolimus (0.3 mg/kg po daily); VI - rats which were administered raloxifene (5 mg/kg po daily) and tacrolimus (0.6 mg/kg po daily). The drugs were administered for 4 weeks. Body mass, macrometric parameters of the tibia, femur and L-4 vertebra, histomorphometric parameters of tibia (transverse growth, width of osteoid, area of the transverse cross section of bone marrow cavity and cortical bone), and the femur (width of trabeculae, width of epiphyseal cartilage) were examined. The action of raloxifene in male rats was demonstrated through increased width of osteoid. An increased traverse growth of bone and osteoid width as well as transverse cross section of the cortical bone and the marrow cavity and increased thickness of trabeculae were observed in male rats receiving tacrolimus at 0.3 mg/kg. The administration of tacrolimus at 0.6 mg/kg resulted in increased traverse growth of bone and increased thickness of osteoid, whereas the thickness of trabeculae remained unaffected. The results obtained in the rats administered concurrently raloxifene and tacrolimus (at 0.3 mg/kg or at 0.6 mg/kg) were similar to those obtained in the group of rats receiving tacrolimus at 0.3 mg/kg. It seems that the most valuable in entire experimental system of the study are the results obtained in the group receiving tacrolimus at 0.3 mg/kg po, which are indicative of intensified bone remodeling processes with dominant the bone formation process.

Protective effect of calcineurin inhibitors on acoustic injury of the cochlea

This study examined the effect of immunosuppressants, cyclosporin A, FK506 and rapamycin on functional recovery of the cochlea after acoustic overexposure, in guinea pigs and mice. Thirty guinea pigs were exposed to a 2 kHz pure tone at 120 dB SPL for 10 min. The compound action potential threshold shift induced by acoustic overexposure was examined. Twenty-five mice were exposed to a 4 kHz pure tone at 128 dB SPL for 4h. Auditory brainstem response was used to examine the hearing threshold shift. In both the guinea pig and mouse experiments, cyclosporin A and FK506, intraperitonally given just before acoustic overexposure, significantly decreased the hearing threshold shift one or two weeks after acoustic overexposure. However, neither rapamycin nor the FK506 and rapamycin combined treatment groups showed improvement of the threshold shift. The present findings suggest that these two calcineurin inhibitors have a protective effect against acoustic injury of the cochlea, whereas the non-calcineurin inhibitor, rapamycin, not only has no effect against acoustic injury, but rather blocked the effect of FK506. This indicated a possible role of calcineurin against acoustic injury.

Tacrolimus and Posttransplant Diabetes Mellitus in Renal Transplantation

The most prominent side effect of tacrolimus is the induction of posttransplant diabetes mellitus (PTDM). In this review, the authors discuss the incidence, mechanism, prevention, and treatment of tacrolimus-induced PTDM in renal patients.

Prograf produces a molecular environment favoring antifibrosis, an effect reversed by the addition of rapamune

Rapamune, an inhibitor of the mammalian target of rapamycin, exhibits antiproliferative actions and is increasingly used as adjuvant therapy with calcineurin inhibitors. This study investigated the effect of Rapamune on functional and molecular markers in a rat model of calcineurin inhibitor-induced graft dysfunction. Prograf (6 mg), with or without addition of Rapamune (1 mg), was administered to salt-depleted male rats (n = 6/group). Urinary protein excretion and serum creatinine were measured. Rats were culled at 28 days, and messenger RNA expression of TGF-beta, MMP-2, MMP-9, TIMP-1, and collagen III was evaluated with reverse transcriptase polymerase chain reaction. Serum creatinine increased with Prograf (P = .01), but not Rapamune (P = .69) treatment, compared to controls at 28 days. The combination of Rapamune and Prograf produced a rise in serum creatinine at 7 (P = .007) and 14 (P = .01) days, but this was not observed at later time points. Urinary protein excretion was unaltered by any drug or combination. While confirming a synergistic effect of Rapamune and calcineurin inhibitors on renal function, these results suggest that sole therapy with Prograf produces inhibition of fibrotic gene expression. Rapamune alone has no deleterious effect on gene expression but addition of Rapamune cancels out the beneficial effects of Prograf.

INHIBITORY EFFECTS OF POMELO ON THE METABOLISM OF TACROLIMUS AND THE ACTIVITIES OF CYP3A4 AND P-GLYCOPROTEIN

We recently reported a case of increase in the blood level of tacrolimus following intake of pomelo in a renal transplant recipient. To clarify the mechanism of this increase in the blood level of tacrolimus, we investigated the effect of pomelo juice extract on the activities of CYP3A4 and P-glycoprotein, in comparison with that of extract of grapefruit juice (GFJ). The 10% ethyl acetate extracts of the juice of three pomelos of different origins (Banpeiyu, pomelo I; Hirado Buntan, pomelo II; and Tosa Buntan, pomelo III) and GFJ significantly inhibited 6beta-hydroxylation of testosterone in human liver microsomes by 76.4, 67.2, 37.5, and 83.9%, respectively. The extract of pomelo I was as potent as that of GFJ. The metabolism of tacrolimus itself was also inhibited by the extract of pomelo I, as well as that of GFJ. Furthermore, the inhibition of both 6beta-hydroxylation of testosterone and metabolism of tacrolimus by pomelo I and GFJ was preincubation time-dependent. On the other hand, the extract of pomelo I had little effect on the transcellular transport of tacrolimus or [(3)H]digoxin across a monolayer of LLC-GA5-COL150 cells (a porcine kidney epithelial cell line, LLC-PK1, transfected with human MDR1 cDNA and overexpressing human P-glycoprotein). In conclusion, pomelo constituents inhibit the activity of CYP3A4 and may thereby produce an increase in the blood level of tacrolimus.

Identification of Ald6p as the target of a class of small-molecule suppressors of FK506 and their use in network dissection

FK506 inhibits the Ca2+/calmodulin-dependent protein phosphatase calcineurin, which plays a critical role in yeast subjected to salt stress. A chemical genetic screen for small molecules that suppress growth inhibition by high NaCl plus FK506 identified a structurally related class of suppressors of FK506 (SFKs) named SFKs 2-4. To identify possible protein targets for these small molecules, a genome-wide screen of approximately 4,700 haploid yeast deletion strains was undertaken for strains showing resistance to high NaCl plus FK506. This screen yielded a number of genes not previously implicated in salt stress, including ALD6, which encodes an NADP(+)-dependent aldehyde dehydrogenase, and UTR1, which encodes an NAD+ kinase. Transcriptional profiling of yeast treated with SFK2 indicated that the SFKs target the Ald6p pathway. In addition, screening of the deletion strains for hypersensitivity to SFK2 yielded ZWF1, encoding glucose-6-phosphate dehydrogenase, which has been shown to play an overlapping role with Ald6p in NADPH production. Furthermore, the SFKs inhibited the activity of Ald6p in vitro. Having established that the SFKs target Ald6p, they were used as tools to implicate systematically other gene products in the Ald6p pathway, including Utr1p, which may function by supplying Ald6p with its NADP+ cofactor. Furthermore, growth improvement by the SFKs on high NaCl plus FK506 was shown to require GPD1, which encodes an NADH-dependent glycerol-3-phosphate dehydrogenase that is important for the production of glycerol in response to osmotic stress.

Calcineurin inhibitor-associated early renal insufficiency in cardiac transplant recipients: risk factors and strategies for prevention and treatment

Cardiac transplantation is the definitive treatment for eligible patients with end-stage cardiac failure. Techniques have evolved to reduce surgical mortality to under 5%. Immediate and subsequent long-term survival is more dependent on acute and chronic rejection and the complications of immunosuppressive therapy. Ten-year survival is greater than 50%.The success of transplantation over the last 20 years has been largely due to the advances in immunosuppression. The most notable and dramatic milestone was the introduction of cyclosporine in the early 1980s, which resulted in a significant improvement in allograft and patient survival. Cyclosporine is a peptide that inhibits the immune system by suppressing T-helper cell activation via inhibition of calcineurin, a critical intracellular enzyme. Tacrolimus has a similar (but not identical) mechanism of action, and was introduced in the 1990s. Drugs such as cyclosporine and tacrolimus, generically referred to as calcineurin inhibitors, have become the cornerstones of immunosuppressive protocols. As a group, calcineurin inhibitors have adverse effects, including neurotoxicity, hypertension, and nephrotoxicity, which complicate their use. Early renal insufficiency manifests as postoperative oliguria (<50 mL/h urine output) or rising serum creatinine levels. There are a variety of postulated causes for calcineurin inhibitor-associated early renal insufficiency including direct calcineurin inhibitor-mediated renal arteriolar vasoconstriction, increased levels of endothelin-1 (a potent vasoconstrictor), as well as decreased nitric oxide production and alterations in the kidney's ability to adjust to changes in serum tonicity. Once early renal insufficiency occurs, no single treatment has been shown to be effective. Approaches discussed in this paper include reduction in calcineurin inhibitor dosages, as well as various drugs to promote increased renal perfusion such as misoprostol and dopamine. In addition, the paper emphasizes the importance of ruling out other causes of renal insufficiency in the early postoperative period, including volume depletion, depressed cardiac output, and mechanical obstruction to urine flow. Given that there is no highly efficacious treatment for this syndrome, ways to avoid its occurrence are desirable. One paper is referenced that suggests that avoidance of rapid changes in tacrolimus level during the first three days of therapy is associated with a low occurrence of early renal insufficiency.

Rapamycin antagonizes cyclosporin A- and tacrolimus (FK506)-mediated augmentation of linker for activation of T cell expression in T cells

The discovery of new immunosuppressive drugs such as rapamycin, cyclosporin A (CsA) and tacrolimus (FK506) has been very useful for preventing graft rejection and autoimmune disease. However, these drugs are not specific, and are associated with side-effects and toxicities. Therefore, understanding the molecular mechanisms of these drugs is important for designing specific immunosuppressants. Here, we show that in contrast to CsA and FK506, rapamycin blocks activation-induced expression of the linker for activation of T cells (LAT), a signaling molecule critical for initiating TCR signaling. Thus, whereas CsA and FK506 strongly enhanced TCR- and phorbol myristate acetate-induced LAT expression in T cells, rapamycin effectively inhibited activation-induced LAT expression. Importantly, these opposite effects were mutually antagonistic, as rapamycin acted as a potent antagonist for both CsA and FK506. Because CsA, unlike FK506 and rapamycin, does not bind to the intracellular immunophilin FK-binding protein (FKBP), the antagonism between these drugs is not simply due to competition for intracellular FKBP. Accordingly, RNA and protein stability analyses suggest inhibition by rapamycin at the translational level. Given the important role of LAT in initiating T cell activation, our data suggests that the effects of rapamycin, CsA and FK506 on T cell activation involve regulating early T cell signaling. These findings refine our understanding of the manifold effects of these immunosuppressants, thus providing insight into the drastic physiological contrasts observed between these drugs.

Reversal of acute tacrolimus-induced renal vasoconstriction by theophylline in rats

OBJECTIVE

To determine whether theophylline, a nonselective adenosine receptor antagonist and phosphodiesterase inhibitor, reverses the acute declines in renal blood flow and glomerular filtration rate induced by high-dose tacrolimus in rats.

DESIGN

Prospective, randomized, placebo-controlled experimental study.

SETTING

University-based basic science research laboratory.

SUBJECTS

Adult male Sprague-Dawley rats.

INTERVENTIONS

After mechanical ventilation and instrumentation under isoflurane and nitrous oxide anesthesia, animals received either tacrolimus 0.5 mg/kg intravenously or vehicle and 1 hr later either theophylline 4 mg/kg intravenously or vehicle.

MEASUREMENTS AND MAIN RESULTS

By using radiolabeled microspheres, renal blood flow was measured in three groups: control (n = 5), tacrolimus plus vehicle (n = 6), and tacrolimus plus theophylline (n = 6) at four time points-baseline and 60, 75, and 90 mins after tacrolimus or vehicle (the latter two time points being 15 and 30 mins after theophylline or vehicle, respectively). Whole blood tacrolimus and serum theophylline concentrations were measured. In a separate group of animals, by using (51)Cr-EDTA, glomerular filtration rate was measured in two groups: tacrolimus plus vehicle (n = 5) and tacrolimus plus theophylline (n = 5) at baseline and over two consecutive 20-min time periods beginning 61 mins posttacrolimus. Urine flow rate also was measured. Following tacrolimus, both renal blood flow and glomerular filtration rate declined in parallel by approximately 33% and 50% from baseline after 75 and 90 mins, respectively (p <.05 by two-way repeated-measures analysis of variance). Theophylline completely reversed these tacrolimus-induced decreases in renal blood flow and glomerular filtration rate. Urine flow rate also increased in response to theophylline.

CONCLUSIONS

Low-dose theophylline reverses tacrolimus-induced declines in renal blood flow and glomerular filtration rate observed in an acute model of tacrolimus toxicity. Theophylline's effect in chronic toxicity remains to be determined.

A small molecule suppressor of FK506 that targets the mitochondria and modulates ionic balance in Saccharomyces cerevisiae

FK506 inhibits the evolutionarily conserved, Ca(2+)-dependent phosphatase calcineurin, which in yeast is essential for growth during sodium stress. We undertook a chemical genetic modifier screen to identify small molecules that suppress the ability of FK506 to inhibit yeast growth in high NaCl. One of these small molecule suppressors, SFK1 (suppressor of FK506 1), causes a mitochondrially induced death in low salt, concomitant with the release of reactive oxygen species. Biochemically, SFK1 interacts with Por1p, a channel protein in the outer mitochondrial membrane, suggesting that SFK1 interacts with the mitochondria directly. A genome-wide screen of yeast deletion strains for hypersensitivity to SFK1 yielded several strains with impaired mitochondrial function, as well as several with reduced sodium tolerance. Our data link ionic balance to mitochondrial function and suggest a role for calcineurin in mediating this signaling network.

Protective effects of FK506 against glutamate-induced neurotoxicity in retinal cell culture

PURPOSE

To examine the effects of FK506 on glutamate neurotoxicity in cultured retinal neurons.

METHODS

Experiments were performed with primary retinal cultures obtained from 17- to 19-day-old rat fetuses. To assess the effects of FK506 and other drugs on glutamate neurotoxicity, cultures were treated with a drug beginning 10 minutes before application of glutamate and continuing during the subsequent 10 minutes of glutamate exposure. The treated cells were then incubated for 1 hour in a drug-free and glutamate-free medium. After a 1-hour incubation, cell viability was quantitatively measured by the trypan blue exclusion method.

RESULTS

Brief exposure to glutamate markedly decreased cell viability. FK506 protected against glutamate neurotoxicity in a dose-dependent manner. Rapamycin is a competitive inhibitor of FK506 that binds FK506 binding protein. Simultaneous application of rapamycin and FK506 negated the protective effects of FK506. Cyclosporin A, which binds and inhibits calcineurin, mimicked the protective effects of FK506. Treatment with FK506 did not affect the intracellular maximum Ca2+ concentration induced by glutamate application. Although FK506 exhibited protective action against Ca2+ ionophore-induced neurotoxicity, it had no effect on nitric oxide-induced neurotoxicity. Treatment with FK506 reduced the activity of nitric oxide synthase (NOS).

CONCLUSION

FK506 protected against glutamate neurotoxicity by inhibiting NOS activity in cultured retinal neurons.

Inhibition of cell cycle progression by a synthetic peptide corresponding to residues 65-79 of an HLA class II sequence: functional similarities but mechanistic differences with the immunosuppressive drug rapamycin

A synthetic peptide corresponding to a region of the alpha1 alpha-helix of DQA03011 (DQ 65-79) inhibits the proliferation of human PBL and T cells in an allele-nonspecific manner. It blocks proliferation stimulated by anti-CD3 mAb, PHA-P, and alloantigen, but not by PMA and ionomycin. Substitution of each amino acid with serine shows that residues 66, 68, 69, 71-73, and 75-79 are critical for function. Inhibition of proliferation is long lasting and is not reversible with exogenous IL-2. The peptide can be added 24 to 48 h after stimulation and still block proliferation. The DQ 65-79 peptide does not affect expression of IL-2 or IL-2R; however, IL-2-stimulated proliferation is inhibited. Cell cycle progression is blocked at the G1/S transition, and the activity of cdk2 (cyclin-dependent kinase 2) kinase is impaired by the continued presence of p27. Although these results suggest a mechanism similar to that of rapamycin, the peptide inhibition is not reversed with FK-506, which indicates a distinct mechanism.

[Selective immunosuppression with monoclonal antibodies against ICAM-1 and LFA-1 with FK 506 after experimental small intestine transplantation in the rat]

Prevention of allograft rejection remains a major problem after small bowel transplantation (SBT) and requires potent immunosuppressive regimens. In a variety of transplant models, e.g. liver or heart transplantation, the supplementary application of monoclonal antibodies (mabs) against adhesion molecules is effective in both prolonging graft survival and inducing long-term graft acceptance or tolerance in some cases. In this study anti ICAM-1 (1A29) and anti LFA-1 (WT1) mabs were used in combination with a subtherapeutic dose of FK 506 for 13 days after allogeneic SBT in the rat. The results of this study indicate that in contrast to the induction of allospecific tolerance after liver transplantation, the addition of anti ICAM-1 and/or of anti LFA-1 mabs after SBT reverses the FK 506 effect and results in early graft rejection. It has been shown that higher dosages of these mabs, given alone or in combination with CsA or FK 506, result in long-term survival of cardiac allografts in both, rats and mice [4-8]. Graft survival after pancreas transplantation is prolonged [6]. However, these negative therapeutic effects of the mabs 1A29 and WT1 after SBT have to be critically reevaluated and further analysed.

Antascomicins A, B, C, D and E. Novel FKBP12 binding compounds from a Micromonospora strain

5 novel ascomycin-like compounds, antascomicins A, B, C, D and E were isolated from a strain of Micromonospora. The antascomicins bind strongly to the FK506-binding protein FKBP12 and antagonize the immunosuppressive activity of FK506 and rapamycin. The strain description, fermentation, structure elucidation and biological activity of these compounds are described.

Induction of CD5 on B and T cells is suppressed by cyclosporin A, FK-520 and rapamycin

The expression of CD5 can be induced on murine B-2 cells by anti-IgM, a recognized analog of thymus-independent 2 type (TI-2) antigen. Given that cyclosporin A (CsA) sensitivity is a distinguishing feature of TI-2 type B cell activation, we asked whether the in vitro induction of CD5 on B cells by anti-mu is CsA sensitive. We report that anti-mu induced CD5 expression on B-2 cells was inhibited by CsA as well as FK-520 and rapamycin. When L-685,818, a FK-520 and rapamycin antagonist, was added to anti-mu stimulated B cell cultures containing FK-520 or rapamycin, but not CsA, suppression was abrogated and complete induction of CD5 was seen. When we used either CD4+CD8+ thymocytes or peripheral T cells activated by phorbol ester and ionomycin, the cell surface induction of CD5 was also partially blocked by CsA, FK-520 and rapamycin. Moreover, in both B and T cells, the same immunosuppressive drugs did not affect constitutive CD5 expression but only blocked de novo induction. To determine the level of CD5 regulation, we activated T cells using phorbol myristate acetate (PMA)/ionomycin and report that CD5 induction was sensitive to actinomycin D (AcD). Similarly, the induction of CD5 on anti-mu activated B cells was blocked by AcD. In addition, T cells that were activated by PMA/ionomycin expressed more abundant CD5 mRNA than CsA or FK-520 treated cells. Based on the CsA-sensitive regulation of CD5 we thought that the CsA-sensitive nuclear factor of activated T cells (NFAT) might be involved in CD5 regulation. We report evidence by Western blot analysis that NFATp is expressed by both resting and TI activated B cells but apparently not CD4+CD8+CD5+ thymocytes. We conclude that in both B and T cells the induction of CD5 requires transcriptional regulation, and that the inhibition of CD5 expression by the immunosuppressive drugs CsA, FK-520 and rapamycin requires drug-immunophilin complex formation.

Calcineurin-mediated protein dephosphorylation in brain nerve terminals regulates the release of glutamate

In response to Ca2+ entry, several prominent brain nerve terminal phosphoproteins undergo dephosphorylation, but the relation between dephosphorylation and neurotransmitter release is unknown. Using the immunosuppressants cyclosporin A (CsA) and L-683,590 (FK-520) to inhibit specifically the Ca2+/calmodulin-dependent protein phosphatase calcineurin, we demonstrate here that Ca(2+)-dependent dephosphorylation in isolated rat brain nerve terminals (synaptosomes) is mediated by calcineurin. Pretreatment with micromolar CsA resulted in a 76-95% inhibition of stimulation-induced decreases in 32P-labeled dynamin (previously referred to as dephosphin), a phosphoprotein of M(r) = 145,000 (145-kDa protein), and a phosphoprotein of M(r) = 170,000 (170-kDa protein). Pretreatment with FK-520 also inhibited Ca(2+)-dependent dephosphorylation. Using hypotonic lysates of 32P-labeled synaptosomes, the addition of Ca2+ plus calmodulin, but not either agent alone, induced dynamin dephosphorylation. CsA and FK-520 had little to no effect on the release of glutamate induced by either K(+)-depolarization or the Ca2+ ionophore ionomycin. In contrast, calcineurin inhibition led to a substantial enhancement of glutamate release evoked by the K(+)-channel blocker 4-aminopyridine, an agent whose action most closely mimics physiological stimulation. Calcineurin inhibition had no effect on stimulation-induced changes in synaptosomal Ca2+ levels. Based on our findings, we hypothesize that Ca(2+)-dependent protein dephosphorylation resulting from calcineurin activation during physiological stimulation limits neurotransmitter release from brain nerve terminals, perhaps being dependent upon cyclic repolarization of the membrane during stimulation.

Quantitative and temporal analysis of the cellular interaction of FK-506 and rapamycin in T-lymphocytes

The structurally related immunosuppressive macrolides FK-506 and rapamycin (RAP) exert distinct biological effects: inhibition of interleukin-2 production and inhibition of interleukin-2-induced proliferation, respectively, through binding to intracellular receptors, termed FKBPs. Although the interaction of these drugs with purified FKBPs in vitro has been well characterized, little is known about their interaction with FKBPs in living cells. Here, we used [3H]-dihydro-FK-506 as a probe to examine the binding of these macrolides in both normal mouse splenic T-cells and the human Jurkat T-cell lymphoma. These cells were found to accumulate the radioligand, predominantly in the cytosol, to a saturable level corresponding to an estimated concentration of 6 to 7 microM. Half-maximal suppression of T-cell activation was shown to require radioligand occupancy of only 3 to 5% of the pool of available intracellular binding sites (FKBPs). Moreover, the binding and immunosuppressive effect of the radioligand could not be removed by extensive washing and remained stable for at least 6 hr upon incubation of the cells at 37 degrees C. However, a molar excess of either FK-506 or RAP was found to rapidly displace [3H]-dihydro-FK-506 from its cellular binding sites. Consistently, FK-506 and RAP were able to antagonize mutually their immunosuppressive activities even when added several hr after each other to T-cell cultures. We took advantage of the reciprocal antagonism of FK-506 and RAP to define their apparent affinities for the functionally relevant cellular receptors by Schild analysis. This indicated that the drugs compete for a single cellular receptor with similar KdS and, therefore, may mediate their immunosuppressive action upon interaction with similar or highly related FKBPs.

Inhibition of cAMP-responsive element-mediated gene transcription by cyclosporin A and FK506 after membrane depolarization

The cAMP-responsive element (CRE) and its cognate transcription factor CREB can mediate induction of gene transcription in response to membrane depolarization and calcium influx. In this study, the effect of cyclosporin A (CsA) and FK506 on depolarization-induced glucagon gene transcription was investigated in a pancreatic islet cell line by transfection of reporter fusion genes. CsA and FK506 inhibited depolarization-induced glucagon gene transcription, FK506 being more potent than CsA. CsA/FK506 responsiveness was mediated by the glucagon CRE and also by well characterized CREs of the choriogonadotropin and somatostatin genes. Rapamycin antagonized the inhibitory effect of FK506 but not CsA, suggesting that FK506 and CsA may act through complex formation with distinct intracellular immunophilins. Overexpression of calcineurin, which is known to be inhibited by drug-immunophilin complexes, rendered pancreatic islet cells more resistant to the inhibitory effects of CsA and FK506. These results demonstrate an inhibition by CsA and FK506 of CRE-mediated, calcium-induced transcription and suggest that membrane depolarization relies on calcineurin phosphatase activity for activation of CREB/CRE-mediated gene transcription. The interference with CRE-mediated gene transcription represents a novel mechanism of CsA/FK506 action, which may underlie pharmacological effects and toxic manifestations of these potent immunosuppressive drugs.

FK-506-binding protein: three-dimensional structure of the complex with the antagonist L-685,818

L-685,818 differs only slightly in structure from the immunosuppressive drug FK-506, and both compounds bind with comparable affinity to the 12-kDa FK-506-binding protein (FKBP12), the major intracellular receptor for the drug. Despite these similarities, L-685,818 is a potent antagonist of both the immunosuppressive and toxic effects of the drug. Here, we present a structural analysis of this problem. Although FK-506 and L-685,818 differ greatly in pharmacology, we have found that the three-dimensional structures of their complexes with FKBP12 are essentially identical. Approximately half of each ligand is in contact with the receptor protein, and half is exposed to solvent; the exposed region includes the two sites where the compounds differ. These results indicate that the profound differences in the pharmacology of these two compounds are not caused by any difference in their interaction with FKBP12. Rather, these effects arise because relatively minor changes in the exposed part of a bound ligand have a strong effect on how FKBP12-ligand complexes interact with calcineurin, their putative intracellular target. In addition, FK-506 complexes with FKBP12 proteins from several species all inhibit mammalian calcineurin. Analysis of the three-dimensional structure of the complex with respect to residues conserved among these proteins suggests a small number of surface residues near the bound ligands that may play a critical role in interactions between the protein-drug complex and calcineurin.

A role for calcineurin in degranulation of murine cytotoxic T lymphocytes

The immunosuppressive drugs cyclosporin A (CsA) and FK506 bind to distinct families of intracellular proteins, cyclophilins, and FK506 binding proteins (FKBP) respectively, termed immunophilins. Immuno-suppressant-immunophilin complexes bind to and inhibit the activity of calcineurin, a calcium-dependent serine/threonine phosphatase. CsA is known to inhibit degranulation in CTL as assessed by N benzyloxylcarbonyl-L-lysine thiobenzyl ester-esterase release assays. We have investigated whether calcineurin phosphatase activity is involved in this degranulation. Both CsA and FK506 are shown to inhibit N benzyloxylcarbonyl-L-lysine thiobenzyl esteresterase release in murine CTL clones induced either by cognate target or by PMA and the calcium ionophore A23187. Inhibition is concentration dependent and is observed at drug concentrations that specifically inhibit cellular calcineurin. The FK506-binding immunophilin FKBP12, as well as calcineurin, are shown to be present in these cells by immunoblotting analysis. Rapamycin, a macrolide antibiotic thought to compete with FK506 for binding to common FKBP receptor sites, antagonizes the effects of FK506 on both degranulation and calcineurin activity. Neither the degranulation nor the effect of the immunosuppressants is affected by the protein synthesis inhibitor cycloheximide. These observations suggest a role for calcineurin in CTL degranulation. Thus, in addition to its previously described role in lymphokine gene activation, calcineurin also appears to be involved in T cell activation processes which do not require protein synthesis.

The immunosuppressive and toxic effects of FK-506 are mechanistically related: pharmacology of a novel antagonist of FK-506 and rapamycin

FK-506 inhibits Ca(2+)-dependent transcription of lymphokine genes in T cells, and thereby acts as a powerful immunosuppressant. However, its potential therapeutic applications may be seriously limited by several side effects, including nephrotoxicity and neurotoxicity. At present, it is unclear whether these immunosuppressive and toxic effects result from interference with related biochemical processes. FK-506 is known to interact with FK-binding protein-12 (FKBP-12), an abundant cytosolic protein with cis-trans peptidyl-prolyl isomerase activity (PPIase) activity. Because rapamycin (RAP) similarly binds to FKBP-12, although it acts in a manner different from FK-506, by inhibiting T cell responses to lymphokines, such an interaction with FKBP-12 is not sufficient to mediate immunosuppression. Recently, it was found that the complex of FKBP-12 with FK-506, but not with RAP, inhibits the phosphatase activity of calcineurin. Here, we used L-685,818, the C18-hydroxy, C21-ethyl derivative of FK-506, to explore further the role of FKBP-12 in the immunosuppressive and toxic actions of FK-506. Although L-685,818 bound with high affinity to FKBP-12 and inhibited its PPIase activity, it did not suppress T cell activation, and, when complexed with FKBP-12, did not affect calcineurin phosphatase activity. However, L-685,818 was a potent antagonist of the immunosuppressive activity of both FK-506 and RAP. Moreover, L-685,818 did not induce any toxicity in dogs and rats or in a mouse model of acute FK-506 nephrotoxicity, but it blocked the effect of FK-506 in this model. Therefore, FK-506 toxicity involves the disruption of biochemical mechanisms related to those implicated in T cell activation. Like immunosuppression, this toxicity is not due to the inhibition of the PPIase activity of FKBP-12, but may be linked to the inhibition of the phosphatase activity of calcineurin by the drug FKBP-12 complex.

Differential effects of the immunosuppressive macrolides FK-506 and rapamycin on activation-induced T-cell apoptosis

Activation of certain T-cell lines induces, besides lymphokine production, a suicide process (apoptosis) mediated by fragmentation of the cell's genome. This also occurs intrathymically during negative selection of the T-cell receptor (TcR) repertoire. Cyclosporin A (CsA) has been shown to block activation-driven T-cell apoptosis, an effect which may account for the perturbations of TcR repertoire selection caused by this agent in vivo. Recently, the macrolide FK-506 was demonstrated to suppress T-cell activation by inhibiting lymphokine production in a manner apparently similar to CsA. Thus, it seemed important to determine whether FK-506 would also prevent T-cell apoptosis. For the purpose of comparison, we also investigated rapamycin (RAP), a macrolide structurally related to FK-506, but that does not block lymphokine production and antagonizes the immunosuppressive action of FK-506. The DO-11.10 T-cell hybridoma stimulated with ionomycin plus PMA was used as a model system. FK-506 (1.2 nM) totally prevented DNA fragmentation detectable by agarose gel electrophoresis at 16 h of culture. FK-506 still inhibited this phenomenon when added 2 h after the initiation of the cultures but not later. In contrast, concentrations of RAP as high as 1 microM failed to block apoptosis. However, RAP (110 nM) reversed the apoptosis-inhibitory effect of FK-506, even if added 1-2 h after the latter to the cultures. Consistent with this antagonism, RAP also reversed the binding of a radiolabeled derivative of FK-506 in DO-11.10 cells. Therefore, FK-506 interferes with an early event of T-cell activation that leads to apoptosis whereas RAP does not.(ABSTRACT TRUNCATED AT 250 WORDS)