Yael Zilberman

Role of mitochondrial glucocorticoid receptor in glucocorticoid-induced apoptosis.

The mechanisms by which glucocorticoid receptor (GR) mediates glucocorticoid (GC)-induced apoptosis are unknown. We studied the role of mitochondrial GR in this process. Dexamethasone induces GR translocation to the mitochondria in GC-sensitive, but not in GC-resistant, T cell lines. In contrast, nuclear GR translocation occurs in all cell types. Thymic epithelial cells, which cause apoptosis of the PD1.6 T cell line in a GR-dependent manner, induce GR translocation to the mitochondria, but not to the nucleus, suggesting a role for mitochondrial GR in eliciting apoptosis. This hypothesis is corroborated by the finding that a GR variant exclusively expressed in the mitochondria elicits apoptosis of several cancer cell lines. A putative mitochondrial localization signal was defined to amino acids 558–580 of human GR, which lies within the NH2-terminal part of the ligand-binding domain. Altogether, our data show that mitochondrial and nuclear translocations of GR are differentially regulated, and that mitochondrial GR translocation correlates with susceptibility to GC-induced apoptosis.

Glucocorticoid-induced apoptosis revisited: a novel role for glucocorticoid receptor translocation to the mitochondria.

Recent data cast new light on the mechanisms by which glucocorticoids (GCs)elicit apoptosis of thymocytes and leukemia cells. Here we attempt to integrate recentstudies by others and us, which provide a novel insight to this apoptotic process. In thelast few years it was made clear that there is a tight cooperation between genomic andnon-genomic effects exerted by GC receptors (GRs). GC invokes major alterations in thegene expression profile through GR-mediated transactivation and transrepression, whichultimately tip the balance between pro-survival and pro-apoptotic proteins. Althoughessential in shaping the cell’s proteome, these genomic effects are insufficient to elicitapoptotic death and additional signals are required for activating the pro-apoptoticproteins. Several non-genomic effects have been described that occur immediatelyfollowing exposure to GC, which are imperative for the induction of apoptosis. We haverecently observed that GC induces instant GR translocation to the mitochondria in GCsensitive,but not in GC-resistant, T lymphoid cells. This response contrasts the nucleartranslocation of GR occurring in both cell types. We propose that the sustained elevationof GR in the mitochondria following GC exposure is crucial for triggering apoptosis.

Eosinophils Maintain Their Capacity to Signal and Release Eosinophil Cationic Protein Upon Repetitive Stimulation with the Same Agonist

Eosinophils contain in their granules eosinophil cationic protein (ECP) and other basic proteins that have been implicated in immunity to parasites and pathophysiology of chronic allergic responses. In a model of eosinophil degranulation, we show that eosinophils release ECP upon short-term GM-CSF priming and stimulation with either platelet-activating factor (PAF) or the anaphylatoxin C5a, but not eotaxin. Restimulation with the same agonist (PAF or C5a) was unsuccessful as assessed by monitoring intracellular calcium concentration and ECP release. In contrast, upon an intermediate washing step, eosinophils rapidly transduced PAF and C5a signals followed by significant ECP releases. Ligand-binding studies demonstrated that only a proportion of PAF receptors is internalized upon cell stimulation and that washing of the cells removes the agonist from the cell surface. Upon repetitive stimulation, eosinophils with less than 50% of the original ECP content were obtained. Such eosinophils did not increase cellular ECP levels even in the presence of the eosinophil survival factor GM-CSF in overnight cultures. In vivo studies revealed that eosinophils always express detectable amounts of ECP under chronic inflammatory conditions. In conclusion, we have shown that eosinophils maintain their capacity to degranulate upon repetitive stimulation with the same agonist as long as the receptor is not occupied from a previous stimulation. The cellular content of ECP appears to be a no limiting factor in the case of repetitive stimulation, implying that mature eosinophils may not require a significant ECP resynthesis.

Staurosporine Sensitizes T Lymphoma Cells to Glucocorticoid-Induced Apoptosis: Role of Nur77 and Bcl-2

Glucocorticoids (GCs) are used for treatment of various hematopoietic malignancies owing to their ability to induce apoptosis. A major obstacle in leukemia therapy is the emergence of GC-resistant cells. Hence, combinatory treatment protocols should be developed that convert GC-resistant leukemia cells into sensitive ones. Here we demonstrate that the broad-acting kinase inhibitor staurosporine (STS) confers GC-sensitivity on GC-resistant T lymphoma cells expressing elevated levels of either Bcl-2 or Bcl-XL, but not on GC-resistant myelogenic leukemia cells expressing Mcl-1 in addition to Bcl-2 and/or Bcl-XL. In T lymphoma cells, STS induces the expression of the pro-apoptotic orphan receptor Nur77 that overcomes the anti-apoptotic effect of Bcl-2, thus enabling GC-induced apoptosis. However, in the myelogenic leukemia cells, STS does not up-regulate Nur77. In these cells, the glucocorticoid receptor (GR) is rapidly downregulated by GC and the anti-apoptotic Mcl-1 protein is upregulated by STS, thereby leading to an even more resistant phenotype. Altogether, our data provide a molecular basis for the differential apoptotic response of T lymphoma versus myelogenic leukemia cells to STS and GC. The former being sensitized to GC-induced apoptosis by STS, whereas in the latter, STS intensifies GC resistance. The cell type specific responses should be taken into consideration when combinatory therapy is used for treating hematopoietic malignancies.

CD44 co-stimulates apoptosis in thymic lymphomas and T cell hybridomas

Thymic lymphomas and hybridomas vary in their sensitivity to dexamethasone (DEX). Identical variance has been demonstrated in our laboratory for apoptosis of such cells by primary thymic epithelial cells or a cell line (TEC). We have also shown that apoptosis induced by TEC was partially mediated by TEC-derived glucocorticoids (GC). We studied the responses of various thymic lymphomas and hybridomas to TEC and DEX. Of these cells, PD1.6 and 2B4 were sensitive whereas B10 were relatively resistant to either inducer. In the present study we found that TEC and DEX synergize in inducing B10 cell apoptosis. B10 cells could also undergo apoptosis by TEC, conditional upon the presence of a TEC-sensitive cell (PD1.6 or 2B4). Contact between TEC and B10 was essential for apoptosis to occur. Thus, TEC may provide two signals, one mediated by GC and the other requiring cell to cell contact. We then analyzed the involvement of co-stimulatory or adhesion molecules in the TEC-induced apoptosis of thymic lymphoma cells. Soluble anti-CD44 antibodies but not anti-CD18, CD2 or CD28, inhibited TEC-induced apoptosis of PD1.6. Dimerization of CD44 by immobilized antibodies augmented DEX-induced apoptosis of all the lymphomas tested. CD44 cross-linkage up-regulated expression of the pro-apoptotic protein Bax, and down-regulated the anti-apoptotic protein, Bclx(L), in the presence of DEX. Taken together, the data suggest that CD44 enhances the apoptotic response of T lymphoma cells to DEX, and that CD44 modulates TEC-induced apoptosis of thymic lymphomas.

The effect of verapamil, lanthanum and local anesthetics on serotonin release from rabbit platelets

The effect of calcium blockers (verapamil, local anesthetics and lanthanum chloride) on serotonin release from rabbit platelets was studied. The following results were obtained: (1) Verapamil and tetracaine (but not lanthanum) caused a time- and dose-dependent release of serotonin. The curves describing the time-course and those describing the concentration dependence of the release were sigmoid, suggesting cooperativity. (2) Thrombin-induced release from the platelets was dependent upon extracellular sodium ions, while no dependence was observed for the drug-induced release. (3) The release by verapamil was partially inhibited by prostaglandin E1 and theophylline which are known to raise intracellular cAMP levels, but was unaffected by the prostaglandin-synthesis inhibitor, indomethacin. (4) Verapamil, tetracaine and lanthanum inhibited thrombin-induced release of serotonin. The curve of dose dependence of the inhibition by verapamil and tetracaine was not sigmoid. The inhibition by verapamil and tetracaine was reversed by extracellular calcium ions, but no effect of this ion on the drug-induced release reaction was observed. It is concluded that the serotonin release induced by some calcium blockers and the inhibition of the thrombin-induced release by the same drugs are two separate phenomena. It is suggested that verapamil and tetracaine-induced release are mediated by exocytotic processes brought about by the interference of the drugs with calcium distribution between the cytosol and storage compartments within the platelet.

Surface curvature and mobility in phospholipid bilayers. NMR studies of lecithin-deoxycholate mixed micelles

Abstract In mixed micelles of phospholipids and bile salts, the phospholipids are arranged as a flat bilayer with bile salts distributed along the hydrophobic edges. The linewidths of signals in the proton magnetic resonance spectra of such mixed micelles depend strongly on the size of the micelles, which is a function of the molar ratio of the components. The signals in the spectra of such mixed micelles are much broader than the corresponding signals in the much larger, curved bilayers of lecithin vesicles obtained by ultrasonic irradiation. This result strongly supports the idea that curvature is a prime factor in determining proton NMR linewidths, probably through a disruptive effect of curvature on the molecular packing in phospholipid bilayers.

Multiple effects of staurosporine, a kinase inhibitor, on thymocyte functions: Comparison with the effect of tyrosine kinase inhibitors

The effects of staurosporine, a protein kinase inhibitor, on the signal transduction and proliferation of thymocytes were studied. Signal transduction in response to Concanavalin A (Con A) as well as Concanavalin A (Con A)-induced augmentation of [3H]inositol incorporation into phospholipids were inhibited by staurosporine (> or = 10(-8) M). Staurosporine inhibited thymocyte proliferation in response to Con A in the presence or absence of the phorbol ester, phorbol myristate acetate (TPA) (10 nM). This inhibition was observed regardless of whether staurosporine was added together with Con A or 3 hr later. High concentrations of staurosporine (> 10(-8) M) inhibited thymocyte proliferation induced by the calcium ionophore A23187 and the phorbol ester TPA, whereas lower concentrations of the inhibitor (< or = 10(-8) M) enhanced thymidine incorporation in response to these activators. This dual effect of staurosporine was also observed in the presence of the staurosporine-related kinase inhibitor, K252a. In contrast, the tyrosine kinase inhibitor, tyrphostin AG490, inhibited the response to A23187 and TPA at all concentrations of the inhibitor and no augmentation was seen. Interleukin 2 (IL-2)-driven mitogenesis in IL-2-dependent cells was also inhibited by staurosporine. We suggest that the inhibition of thymocyte proliferation by staurosporine results from inhibition of both protein kinase C and tyrosine kinase: the augmentation of the response to A23187 and TPA results from inhibition of protein kinase C. Inhibition of signal transduction as well as inhibition of IL-2-driven mitogenesis result from inhibition of tyrosine kinase.

Attenuation of the Lithium-Induced Diabetes-Insipidus-Like Syndrome by Amiloride in Rats

The effect of amiloride on lithium-induced polydipsia and polyuria and on the lithium concentration in the plasma, brain, kidney, thyroid and red blood cells was investigated in rats, chronically treated with LiCl. Amiloride reduced the drinking and urine volume of rats in an acute (6 or 12 h) and a subacute (3 days) experiment. 6 h after the administration of amiloride, a reduction was observed in the lithium content of the renal medulla but not in the other organs studied. At 12 h, all the tissues showed a slight increase in lithium levels. After 3 days of combined treatment, a marked elevation in plasma and tissue lithium levels accompanied a reduction in water intake. In all the experiments, the attenuation of the lithium-induced diabetes-insipidus-like syndrome by amiloride was accompanied by a reduction of the ratio between the lithium concentration in the renal medulla and its levels in the blood and an elevation in the plasma potassium level. It is concluded that acute amiloride administration to lithium-treated patients suffering from polydipsia and polyuria might relieve these patients but prolonged amiloride supplementation would result in elevated lithium levels and might be hazardous.

Effects of prolonged lithium treatment on the water consumption and lithium content of rats.

Abstract Injection of LiC1 to rats (3.0 mmole/kg/day) caused a marked increase in water intake. The polydipsic effect was maximal after 8 – 10 days of treatment and then gradually declined. This reduction in polydipsia was accompanied by a decrease in Li + content of plasma, red blood cells, liver, spleen, kidney and brain. Furthermore, the ratio of tissue to plasma lithium concentration (lithium index) declined in all tissues examined. The decrease in polydipsia was abolished by increasing the dose of LiC1 by 50%. These results suggest that tolerance to the polydipsic effect of lithium develops in the rat during chronic treatment with LiC1 as a result of a reduction in the cells capacity to accumulate lithium.