Reduction of ryanodine binding and cytosolic Ca2+ levels in liver by the immunosuppressant FK506

Elucidation of the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells: modulation by cyclic ADP-ribose and FK506

The effects of cyclic ADP-ribose (cADPR) and the immunosuppressant drug FK506 on microsomal Ca2+ release through a ryanodine-sensitive mechanism were investigated in rat pancreatic acinar cells. After a steady state of 45Ca2+ uptake into the microsomal vesicles, ryanodine or caffeine was added. Preincubation of the vesicles with cADPR (0.5 microM) shifted the dose-response curve of ryanodine- or caffeine-induced 45Ca2+ release from the vesicles to the left. Preincubation with cADPR shifted the dose-response curve of the FK506-induced 45Ca2+ release upward. Preincubation with FK506 (3 microM) shifted the dose-response curve of the ryanodine- or caffeine-induced 45Ca2+ release to the left by the same extent as that in the case of cADPR. FK506 shifted the dose-response curve of the cADPR-induced 45Ca2+ release upward. The presence of both cADPR and FK506 enhanced the ryanodine (30 microM)- or caffeine (10 mM)-induced 45Ca2+ release by the same extent as that in the case of cADPR alone or FK506 alone. These results indicate that cADPR and FK506 modulate the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells by increasing the ryanodine or caffeine sensitivity to the mechanism. In addition, there is a possibility that the mechanisms of modulation by cADPR and FK506 are the same.

Disruption of filamentous actin diminishes hormonally evoked Ca2+ responses in rat liver

Previous studies have suggested a role for the actin cytoskeleton in hormonally evoked Ca2+ signaling in the liver. Here, we present evidence supporting a connection between filamentous actin (F-actin) organization and the ability of vasopressin and glucagon to increase cytosolic free-Ca2+ ([Ca2+]i) levels. F-actin was disrupted in hepatic cells by perfusion of rat liver with cytochalasin D. Epifluorescence microscopy of subsequently isolated cells showed reduced cortical fluorescent phalloidin staining in cytochalasin D-treated liver cells. Cytochalasin D pretreatment of liver cells reduced the vasopressin-stimulated elevation of [Ca2+]i by 60% and of glucagon by 50%. Experiments performed on cytochalasin D-treated cells using Mn2+ as an indicator of Ca2+ influx quenched fura-2 fluorescence signals following vasopressin administration. This indicates that a structurally intact cortical F-actin web is not a prerequisite for the influx of calcium. Therefore, the attenuation of the increase in cytosolic calcium observed in cytochalasin D-treated liver cells was likely caused either by the depletion of the calcium store by treatment with cytochalasin D or by the need for an intact cytoskeletal structure for its release. Because the resting level of calcium did not change in cells exposed to cytochalasin D, the latter is likely. The reduced [Ca2+]i response may be the mechanism by which cytochalasin D pretreatment inhibits vasopressin-induced metabolic effects. Cytochalasin D pretreatment also decreased the ability of glucagon to stimulate gluconeogenesis and reduced the stimulation of O2 uptake usually observed following glucagon administration. In conclusion, these results suggest that the hormonal elevation of [Ca2+]i and resultant activation of specific metabolic pathways require normal F-actin organization.

Agonist-specific modulation of glucocorticoid receptor-mediated transcription by immunosuppressants

Although the immunosuppressive drugs FK506, rapamycin and cyclosporin A have been reported to potentiate transcriptional activation mediated by a non-saturating concentration of the glucocorticoid receptor agonist dexamethasone, the precise mechanism(s) underlying these responses remains unclear. The murine L-929-derived LMCAT cell line stably transfected with the mouse mammary tumor virus promoter-chloramphenicol acetyl transferase reporter gene construct was utilized in the present study to further investigate the mechanism(s) underlying this dexamethasone potentiation as well as the possible agonist specificity of this potentiation. The present data demonstrate that pretreatment (2 h) of LMCAT cells with 10 microM FK506, rapamycin or cyclosporin A results in the potentiation of reporter gene transcription mediated not only by dexamethasone (approximately 12-fold), but also by hydrocortisone (approximately 6-fold) and triamcinolone acetonide (approximately 2.5-fold). In sharp contrast, the data show for the first time that pretreatment with any one of these immunosuppressive drugs suppresses (approximately 2-8-fold) the transcriptional responses mediated by corticosterone, deoxycorticosterone, and cortexolone. Pretreatment of intact LMCAT cells with FK506 increases the subsequent whole cell specific binding of [3H]dexamethasone, but does not increase specific cytoplasmic binding when the tritiated agonist is added directly to cytosolic extracts prepared from the pretreated cells. These data suggest that the FK506-mediated potentiation of the transcriptional responses induced by some agonists, like dexamethasone, may be related to the ability of this immunosuppressant to inhibit the membrane-associated multidrug resistance (MDR) P-glycoprotein, which actively extrudes some steroids from cells. Identical pretreatment with FK506 has no detectable effect on the subsequent whole cell specific binding of [3H]corticosterone, a steroid which is not effectively extruded by the MDR pump. Two additional MDR pump inhibitors, verapamil and quinidine, potentiate (30-fold) the dexamethasone-mediated transcriptional response as expected, but have no detectable effects on a corticosterone-mediated transcriptional response. Unlike immunosuppressive drugs, these ion channel blockers do not bind to receptor-associated immunophilins (FK506-binding proteins or cyclophilins). Collectively, these results suggest that immunosuppressants potentiate a dexamethasone-mediated transcriptional response in LMCAT cells by inhibiting efflux of this steroid. In contrast, these drugs appear to suppress a corticosterone-mediated transcriptional response by a different mechanism, perhaps one involving their binding to glucocorticoid receptor-associated immunophilins.

Evaluation of the effects of immunosuppressants on neuronal and glial cells in vitro by multinuclear magnetic resonance spectroscopy

The use of the undecapeptide cyclosporine and the macrolide tacrolimus as immunosuppressants in transplantation medicine and for the therapy of immune diseases often provokes side effects, among the most important one is neurotoxicity. Changes in the cellular metabolism of glial cells (C6 rat glioma), neuronal cells (N1E-115 mouse neuroblastoma) and primary glia cells (isolated from rats) after addition of cyclosporine and tacrolimus were investigated using 1H-, 13C- and 31P-NMR spectroscopy in vitro. Cells were exposed to various concentrations of the drugs from 3 h to 42 days. The immunosuppressants (cyclosporine IC50 : 55 mumol/l; tacrolimus IC50 : 47 mumol/l) inhibited cell proliferation in a concentration- and time-dependent fashion. Multinuclear NMR studies of PCA extracts of drug-treated cells showed a significant deterioration in the energy status (a decreasing level of PCr : -46 +/- 11%; an increasing NDP/NTP ratio: +136 +/- 4% and an increasing level of Pi : +248 +/- 15%; mean +/- standard deviation). It also showed decreasing concentrations of major cell metabolites like NAA (-59 +/- 12%) in neuroblastoma cells and myo-inositol (-47 +/- 6%) in glia cells compared with untreated controls. Immunosuppressive treatment caused a large reduction of taurine (-36 +/- 12%) and glutamate (-68 +/- 10%) in all cell cultures, whereas intermediates of phospholipid biosynthesis (PE: +59 +/- 13%; PC: +127 +/- 27%;) and breakdown (GPE: +215 +/- 24%; GPC: +245 +/- 17%) increased. No significant differences were observed between the two immunosuppressants. The toxic effects of immunosuppressants on cell cultures are in line with MRI studies of brain oedema observed in patients under immunosuppressive treatment.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.