Thymocyte apoptosis

MODULATION OF DEXAMETHASONE-INDUCED THYMOCYTE APOPTOSIS BY HEAT-SHOCK PROTEIN 90-BINDING AGENTS

Heat-shock protein 90 (HSP90) is known to affect a variety of cellular activities. The present study showed that the HSP90-binding agents, geldanamycin, herbimycin A and radicicol, inhibited the murine thymocyte apoptosis induced by dexamethasone and was accompanied by the inhibition of the reduction of the mitochondrial transmembrane potential (delta psi m). HSP90-binding agents did not inhibit etoposide-induced apoptosis. The inhibition of dexamethasone-induced apoptosis was in part due to the interference of HSP90 with the glucocorticoid receptor, resulting in the inhibition of nuclear translocation of the receptor. The expression of inositol 1,4,5-triphosphate receptors, which were shown to be involved in dexamethasone-induced apoptosis, did not participate in the inhibition of apoptosis.

S-Nitrosoglutathione-induced mouse thymocyte apoptosis studied by fluorescence near-field scanning optical microscopy

This study is an attempt to deeply understand the mechanisms ensuring self-tolerance of T cells via clonal deletion of thymocytes and exploring T lymophocyte homeostasis by observing the apoptosis of single mouse thymocyte induced by S-nitrosoglutathione (GSNO, a nitric oxide donor) using fluorescence near-field scanning optical microscopy (NSOM) in illumination mode. The GSNO-induced thymocytes were stained with propidium iodide containing 0.01% Triton X-100 and excited with light of 488 nm and the emitting fluorescence at 525 nm. According to the NSOM fluorescence image and the simultaneously obtained topography image, the feature of mouse thymocyte apoptosis was characterized by scattering pattern of the fluorescence spots with the size 0.2-2.1 micro m at the full width at half-maximum of fluorescence intensity 78-80 kHz in the GSNO-treated thymocyte nucleus. Whereas there is no fluorescence from the untreated thymocyte. The intensity of the fluorescence from the dexamethasone-treated thymocyte was much stronger than that from GSNO-induced thymocytes. Furthermore, the fluorescence distribution in the latter were concentrated in the nucleus. Those results also demonstrate the advantages of NSOM such as high spatial resolution and the topography of biology samples.

Protein kinase activity is central to rat germ cell apoptosis induced by methoxyacetic acid

Methoxyacetic acid (MAA) is a major metabolite of ethylene glycol monomethyl ether (EGME). Previous investigations of the testicular lesion induced by EGME have found that dividing meiotic cells are the most sensitive, although several stages of spermatocytes are also vulnerable. Preliminary data from this lab suggested the involvement of protein kinase activity in the development of this lesion, a hypothesis explored in the present studies. We used cultured seminiferous tubules (STs) from juvenile rats (25-day-old), exposed in vitro to MAA and several inhibitors of protein kinases. Nineteen h following a 5-h exposure to 5 mM MAA (the plasma level in vivo after a toxic dose of EGME), apoptotic spermatocytes were seen in early- and late-stage STs. Cell death was prevented by cotreatment with broad-spectrum inhibitors of protein kinases such as H-7, H-8, K-252a, W-7, and genistein. In corroboration, immunocytochemistry with antibodies to various kinases (PKCmu, zeta, and gamma, AKAP220, CaMKII, MLCK, and Src) showed increased staining around dying spermatocytes following EGME treatment in vivo. 2D-PAGE, autoradiography, and nanospray mass spectrometry was used to separate and identify proteins whose phosphorylation status was most greatly changed following exposure to MAA. One protein was identified by sequence analysis as being glucose-regulated protein 94 (grp94). Westem blotting and immunocytochemistry confirmed this finding. The data we present implicate kinase activities in the pathogenesis of this lesion and suggest the involvement of Sertoli cells.

Novel protein kinase C delta isoform insensitive to caspase-3

Protein kinase C delta (PKC delta) plays a key regulatory role in a variety of cellular functions, including apoptosis, as well as cell growth and differentiation. We previously reported that apoptosis was induced by pretreatment with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), an inhibitor of PKC, in mouse thymocytes. In the present study, we showed that a novel PKC delta isoform (PKC deltaII) was transiently expressed when thymocytes were pretreated with H-7. The analysis of the cDNA encoding PKC deltaII indicated that a 78 bp fragment was inserted into the caspase-3 sensitive site of the original PKC delta (PKC deltaI), presumably by alternative splicing. The PKC deltaII expressed in COS-1 cells was one product with a molecular mass of 81 kDa and with kinase activity similar to that of PKC deltaI. The expressed PKC deltaI protein (78 kDa) was in part cleaved into a 38 kDa fragment in vivo and in vitro, but the PKC deltaII protein was not. Cleavage of the PKC deltaI protein was inhibited by a specific inhibitor of caspase-3, indicating that PKC deltaII is insensitive to caspase-3. The PKC deltaII was highly expressed in the testis and ovary, and at a lower level in the thymocytes, brain and kidney, whereas PKC deltaI was detected in most tissues, suggesting that the function of PKC deltaII is different from that of PKC deltaI.

Topoisomerase II inhibitor-induced apoptosis in thymocytes and lymphoma cells

DNA topoisomerase II is a nuclear enzyme that modulates DNA topology during several metabolic processes and is the target of several antitumor drugs. The primary effect of anticancer agents is to induce apoptosis. The present study showed that etoposide, a topoisomerase II inhibitor which forms cleavable complexes, induced apoptosis in nonproliferative thymocytes and proliferative RVC cells, whereas ICRF-154, a bis(2,6-dioxopiperazine) derivative which does not form a cleavable complex, induced apoptosis only in thymocytes. Both etoposide and ICRF-154 inhibited topoisomerase II activity in thymocytes and RVC cells to a similar extent. Etoposide had no effect on the cell cycle of RVC cells, but ICRF-154 induced cell cycle arrest at the G2/M stage followed by cell death without forming a DNA ladder on an agarose gel. Incubation with ICRF-154 reduced the expression of topoisomerase IIa in thymocytes and IIb in RVC cells. These findings suggest that the catalytic inhibitor, ICRF-154, has a mechanism of cytotoxicity which differs from that of etoposide. In RVC cells exposed to etoposide, we identified two clones that were suppressed early in the incubation. One was highly homologous to hnRNP A1 which modulates splicing of selected transcripts or stabilizes mRNAs. The other was a novel gene of which the function remains unknown. These genes were also altered in RVC cells exposed to camptothecin, which underwent apoptosis, but not in those incubated with ICRF-154, indicating that the suppression of these genes is related to inhibitor-induced DNA breaks resulting in apoptosis. In thymocytes, however, a cleavable complex by topoisomerase II inhibitors is not essential for the induction of apoptosis, since it was induced by ICRF-154. This suggests that tissue-specific nuclear matrix proteins other than topoisomerase II, including SATP-1 in the thymus, should also be considered. The present findings also suggest that bis(2,6-dioxopiperazine) derivatives are useful agents with which to study the role of topoisomerase II in the regulation of gene expression as well as the role of the nuclear matrix.

Fas antigen/APO-1 (CD95) expression on myeloma cells

Many factors involved in the proliferation of myelomas have been reported, and the relationship between these factors and the pathogenesis of multiple myeloma has been discussed. We found that most myeloma cells express Fas antigen/APO-1 (CD95), a cell surface antigen that mediates apoptosis. However only some cells are sensitive to anti-Fas antibody and undergo apoptosis. These data indicate that some multiple myelomas are generated not only by cell proliferation but also by cell immortalization. The mechanism by which myelomas are immortalized is still unclear, but Bcl-2, Bcl-xL, adult T cell leukemia derived factor (ADF), soluble Fas are all candidate factors for this mechanism. The possibility also exists that inducers of apoptosis, e.g. tumor necrosis factor(TNF), interleukin-1 beta-converting enzyme(ICE), Bcl-xS, or Bax, do not have a lethal effect. In this review, we focus on the system that immortalizes myeloma cells, and suggest the possibility that multiple myeloma constitutes one group of cells which cannot undergo apoptosis in the bone marrow.

Contrasting effects of protein synthesis inhibition and of cyclic AMP on apoptosis in the developing retina

The role of protein synthesis in apoptosis was investigated in the retina of developing rats. In the neonatal retina, a ganglion cell layer, containing neurons with long, centrally projecting axons, is separated from an immature neuroblastic layer by a plexiform layer. This trilaminar pattern subsequently evolves to five alternating cell and plexiform layers that constitute the mature retina and a wave of programmed neuron death sweeps through the layers. Apoptosis due to axon damage was found in ganglion cells of retinal explants within 2 days in vitro and was prevented by inhibition of protein synthesis. Simultaneously, protein synthesis blockade induced apoptosis among the undamaged cells of the neuroblastic layer, which could be selectively prevented by an increase in intracellular cyclic AMP. Both the prevention and the induction of apoptosis among ganglion cells or neuroblastic cells, respectively, occurred after inhibition of protein synthesis in vivo. The results show the coexistence of two mechanisms of apoptosis within the organized retinal tissue. One mechanism is triggered in ganglion cells by direct damage and depends on the synthesis of proteins acting as positive modulators of apoptosis. A distinct, latent mechanism is found among immature neuroblasts and may be repressed by continuously synthesized negative modulators, or by an increase in intracellular cyclic AMP.

The dual role of S-nitrosoglutathione (GSNO) during thymocyte apoptosis

Nitric oxide (NO) is known to regulate redox-sensitive signalling pathways in physiology and pathophysiology. Depending on its concentration, the NO-releasing compound S-nitrosoglutathione (GSNO) causes negative and positive regulation of thymocyte apoptosis. At levels below 0.6 mM, GSNO produces deoxyribonucleic acid (DNA) laddering, which is inhibited by activation of protein kinase C (PKC), cycloheximide treatment, and calcium chelation. Higher concentrations of the NO donor (1-2 mM) suppress thymocyte apoptosis initiated by the classical agonist dexamethasone. Inhibition of apoptosis by NO is analogous to the action of the thiol-blocking compound N-ethylmaleimide (NEM) and the glutathione-S-transferase substrate 1-chloro-2,4-dinitrobenzene (CDNB). Inhibition of apoptosis results from thiol modification of critical proteins in response to NO treatment. Depending on the concentration, GSNO can be involved either in toxic or in protective signalling in thymocyte biology.

Dopamine-induced programmed cell death in mouse thymocytes

Exposure of mouse thymocytes to dopamine caused apoptosis (programmed cell death). This was manifested by cellular condensation and membrane damage shown by flow cytometry measurements and scanning electron microscopic study. Dopamine also affected thymocytic nuclei and their genomic DNA integrity. Most of the DNA molecules accumulated in a subdiploid peak in flow cytometry analysis, indicating DNA fragmentation to small particles. DNA analysis showed the typical pattern of 'DNA ladder' caused by internucleosomal DNA cleavage. X-ray microanalysis of the cellular elements of dopamine-treated cells showed elevation of sodium (Na), chloride (Cl) and calcium (Ca) peaks, accompanied by reduction in phosphate (P) concentrations. Comparison of the potassium (K) and P concentrations showed significant differences between the two major death processes: necrosis (induced by exposure to sodium azide (NaN3)) and apoptosis (induced by dopamine). High concentrations of K indicated cell viability while reductions in P and elevations in Ca levels were found to be typical of apoptotic cell death. The antioxidant dithiothreitol (DTT) suppressed dopamine-induced apoptosis in thymocytes, suggesting that its toxicity may be mediated via generation of reactive oxygen radicals. Our study suggests that under certain circumstances, dopamine and/or its metabolites, may induce a process of apoptotic cell death of the dopamine-producing cells in the substantia nigra. Increased accessibility of dopamine to the nigral cell nucleus or inability to scavenge excess free radicals generated from dopamine oxidation triggering programmed cell death, may cause the progressive nigral degeneration in Parkinson's disease.

Lack of protective effect of R(-)-deprenyl on programmed cell death of mouse thymocytes induced by dexamethasone

R(-)-Deprenyl, an archetypical MAO-B inhibitor, has been shown to delay the onset of the disabling syndrome of Parkinson's disease and to be useful in the treatment of Alzheimer's disease. Recently, R(-)-deprenyl has been claimed to be capable of preventing apoptosis of PC12 cells, which had been primed with nerve growth factor (NGF) and followed by withdrawal of serum. We investigated the effect of R(-)-deprenyl in a non-neuronal cell model, namely, apoptosis of mouse thymocytes induced by dexamethasone. Trypan blue exclusion and lactate dehydrogenase activity were applied to assess the cell survival. R(-)-Deprenyl did not exhibit any detectable protective effect to the thymocytes from apoptosis. The result is further confirmed by examining the apoptotic DNA fragmentation using gel electrophoresis and assessing the soluble DNA released by a spectrophotometric method.

A role for metals and free radicals in the induction of apoptosis in thymocytes

Recent reports have implicated a possible but undefined role for reactive oxygen species in the induction and mediation of apoptosis. In the present study, the role of free radicals and metal ions in apoptosis induced in rat thymocytes by dexamethasone and etoposide was examined. Copper chelators, but not iron specific chelators, inhibited apoptosis induced by both these stimuli. Several antioxidants also possessed potent inhibitory effects. We therefore propose that diverse agents may induce apoptosis in thymocytes by a common mechanism involving a copper mediated Fenton reaction, generating site specific hydroxyl radicals, possibly as a result of activation of the redox sensitive transcription factor NF-kappa B.