Roger T. Snowden

Mechanisms of resistance to TRAIL-induced apoptosis in primary B cell chronic lymphocytic leukaemia

Primary B cells from B cell chronic lymphocytic leukaemia (B-CLL) were resistant to the novel selective cytotoxic agent, TNF-related apoptosis-inducing ligand (TRAIL). Low levels of the death-inducing TRAIL receptors, TRAIL-R1 and TRAIL-R2 but not the putative ‘decoy’ receptors, TRAIL-R3 and TRAIL-R4, were expressed on the surface of B-CLL cells. Resistance to TRAIL was upstream of caspase-8 activation, as little or no caspase-8 was processed in TRAIL-treated B-CLL cells. Low levels of a TRAIL death-inducing signalling complex (DISC) were formed in these cells, accompanied by the recruitment of endogenous FADD, caspase-8 and c-FLIPL but not c-FLIPS. Both caspase-8 and c-FLIPL were cleaved to form two stable intermediates of ∼43 kDa, which remained associated with the DISC. Caspase-8 was not further processed to its active heterotetramer. Thus the resistance of B-CLL cells to TRAIL may be due partly to low surface expression of the death receptors resulting in low levels of DISC formation and also to the high ratio of c-FLIPL to caspase-8 within the DISC, which would prevent further activation of caspase-8. Our results highlight the possibility of sensitising B-CLL cells to TRAIL by modulation of c-FLIP levels or by upregulation of surface expression of death receptors.

The comparative toxicology of ethyl- and methylmercury

Neurotoxicity and renotoxicity were compared in rats given by gastric gavage five daily doses of 8.0 mg Hg/kg methyl- or ethylmercuric chloride or 9.6 mg Hg/kg ethylmercuric chloride. Three or 10 days after the last treatment day rats treated with either 8.0 or 9.6 mg Hg/kg ethylmercury had higher total or organic mercury concentrations in blood and lower concentrations in kidneys and brain than methylmercury-treated rats. In each of these tissues the inorganic mercury concentration was higher after ethyl than after methylmercury.Weight loss relative to the expected body weight and renal damage was higher in ethylmercury-treated rats than in rats given equimolar doses of methylmercury. These effects became more severe when the dose of ethylmercury was increased by 20%. Thus in renotoxicity the renal concentration of inorganic mercury seems to be more important than the concentration of organic or total mercury. In methylmercury-treated rats damage and inorganic mercury deposits were restricted to the P2 region of the proximal tubules, while in ethylmercury-treated rats the distribution of mercury and damage was more widespread.There was little difference in the neurotoxicities of methylmercury and ethylmercury when effects on the dorsal root ganglia or coordination disorders were compared. Based on both criteria, an equimolar dose of ethylmercury was less neurotoxic than methylmercury, but a 20% increase in the dose of ethylmercury was enough to raise the sum of coordination disorder scores slightly and ganglion damage significantly above those in methylmercury-treated rats.In spite of the higher inorganic mercury concentration in the brain of ethylmercurythan in the brain of methylmercury-treated rats, the granular layer damage in the cerebellum was widespread only in the methylmercury-treated rats. Thus inorganic mercury or dealkylation cannot be responsible for granular layer damage in alkylmercury intoxication. Moreover, histochemistry demonstrated no inorganic mercury deposits in the granular layer.

A flow-cytometric method for the separation and quantitation of normal and apoptotic thymocytes

Using flow cytometry, we describe a method for separating and quantifying normal and apoptotic thymocytes. Apoptosis was induced in isolated thymocytes from immature rats by treatment with the glucocorticoid dexamethasone or the antitumor agent etoposide. Subsequent incubation with the vital bisbenzimidazole dye Hoechst 33342 and the DNA intercalating agent propidium iodide enabled three distinct populations of cells to be identified and sorted by flow cytometry. Dead cells fluoresced red due to propidium iodide whereas normal and apoptotic cells fluoresced blue due to Hoechst 33342. Apoptotic cells were distinguished from normal thymocytes both by their higher intensity of blue fluorescence and by their smaller size as determined by a reduction in forward light scatter. The larger cells, with low blue fluorescence, showed normal thymocyte morphology by electron microscopy and the absence of any DNA fragmentation as measured by agarose gel electrophoresis. In contrast, the smaller cells showed both the morphological characteristics of apoptosis and extensive internucleosomal fragmentation of DNA to multiples of approximately 180 bp. Using this method, a time-dependent induction of apoptosis by dexamethasone, which was inhibited by cycloheximide, actinomycin D, and aurin tricarboxylate, was observed. The method should facilitate mechanistic studies on the induction of apoptosis in thymocytes.

BCL2/BCL-X(L) inhibition induces apoptosis, disrupts cellular calcium homeostasis, and prevents platelet activation.

Apoptosis in megakaryocytes results in the formation of platelets. The role of apoptotic pathways in platelet turnover and in the apoptotic-like changes seen after platelet activation is poorly understood. ABT-263 (Navitoclax), a specific inhibitor of antiapoptotic BCL2 proteins, which is currently being evaluated in clinical trials for the treatment of leukemia and other malignancies, induces a dose-limiting thrombocytopenia. In this study, the relationship between BCL2/BCL-X(L) inhibition, apoptosis, and platelet activation was investigated. Exposure to ABT-263 induced apoptosis but repressed platelet activation by physiologic agonists. Notably, ABT-263 induced an immediate calcium response in platelets and the depletion of intracellular calcium stores, indicating that on BCL2/BCL-X(L) inhibition platelet activation is abrogated because of a diminished calcium signaling. By comparing the effects of ABT-263 and its analog ABT-737 on platelets and leukemia cells from the same donor, we show, for the first time, that these BCL2/BCL-X(L) inhibitors do not offer any selective toxicity but induce apoptosis at similar concentrations in leukemia cells and platelets. However, reticulated platelets are less sensitive to apoptosis, supporting the hypothesis that treatment with ABT-263 induces a selective loss of older platelets and providing an explanation for the transient thrombocytopenia observed on ABT-263 treatment.

Conformational change and mitochondrial translocation of Bax accompany proteasome inhibitor-induced apoptosis of chronic lymphocytic leukemic cells.

Chemotherapy resistance remains a major clinical problem in patients with B-cell chronic lymphocytic leukemia (B-CLL). Proteasome inhibitors are able to induce apoptosis in chemotherapy-resistant B-CLL cells in vitro. Exposure of B-CLL cells to the proteasome inhibitors, MG132 and lactacystin, resulted in inhibition of proteasomal activity within 30 min of treatment and was accompanied by an increase in the level of ubiquitinated proteins. Proteasome inhibitors did not alter the levels of expression of the proapoptotic Bcl-2 family proteins, Bax and Bid, prior to the onset of apoptosis. Instead, proteasome inhibitors induced a caspase-independent conformational change in Bax (as shown by a conformation-specific Bax antibody) and its translocation to mitochondria, resulting in mitochondrial perturbation, as evidenced by loss of the mitochondrial membrane potential and cytochrome c release. Similar conformational change and subcellular localization of Bax were observed during apoptosis induced with fludarabine, chlorambucil and prednisolone. These data suggest that alteration of Bax conformation and its redistribution to mitochondria are common and early features of B-CLL apoptosis in response to proteasome inhibitors and other chemotherapeutic agents.

Proteasome inhibitor-induced apoptosis of B-chronic lymphocytic leukaemia cells involves cytochrome c release and caspase activation, accompanied by formation of an ∼700 kDa Apaf-1 containing apoptosome complex

Proteasome inhibitors, including lactacystin and MG132 (carbobenzoxyl-leucinyl-leucinyl-leucinal), potently induce apoptosis in leukaemic B cells from patients with B cell chronic lymphocytic leukaemia (B-CLL). This pro-apoptotic effect occurs in cells from patients at all stages of the disease, including those resistant to conventional chemotherapy, suggesting that proteasome inhibitors may be useful for treatment of B-CLL. Following initial inhibition of proteasomal activity, these agents induce mitochondrial cytochrome c release and caspase-dependent apoptosis, involving cleavage/activation of caspases -2, -3, -7, -8 and -9. Pre-treatment with the cell permeable caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp (OMe)fluoromethyl ketone (Z-VAD.fmk), did not prevent the release of cytochrome c or partial processing of caspase-9 but prevented activation of effector caspases and the induction of apoptosis. These results suggest that the release of cytochrome c is caspase independent and that caspase-9 is the initiator caspase in proteasome inhibitor-induced apoptosis of B-CLL cells. Activation of B-CLL lysates with dATP results in the formation of an 700 kDa caspase-activating apoptosome complex containing Apaf-1. We describe for the first time the formation of a similar 700 kDa caspase-activating apoptosome complex in B-CLL cells induced to undergo apoptosis by proteasome inhibitors.

Quantification of Apoptosis and Necrosis by Flow Cytometry

Apoptosis and necrosis are two important mechanisms of cell death. Several methods have recently been described for quantifying apoptotic cells by flow cytometry. We report a novel method for the quantification and separation of viable normal and apoptotic cells. We have applied this method both to immature rat thymocytes treated with a variety of agents and to a murine haemopoetic cell line after withdrawal of a growth factor. The cells were incubated with two dyes which give fluorescent complexes when bound to DNA, the bis-benzimidazole, Hoechst 33342, and propidium iodide. Three populations were identified and characterized. On excitation with UV radiation, dead cells fluoresced red due to the uptake of propidium iodide whereas apoptotic cells fluoresced bright blue; normal cells showed low blue, low red fluorescence. In this paper, we demonstrate how this method may be used to help to distinguish between cell death by apoptosis and necrosis.

CDDO induces apoptosis via the intrinsic pathway in lymphoid cells.

The peroxisome-proliferator-activated receptor (PPAR) γ agonist, CDDO, is under investigation for use in various malignancies. The mechanisms by which CDDO induces apoptosis are controversial. We have therefore sought to determine these mechanisms using primary chronic lymphocyte leukemic (CLL) cells and Jurkat cell lines with defined apoptotic abnormalities. In these cells, CDDO induced-apoptosis involved caspase-independent loss in mitochondrial membrane potential followed by caspase processing. The pattern of CDDO-induced caspase processing, defined by use of a caspase inhibitor, strongly suggested that caspase-9 was the apical caspase. Moreover, CDDO induced apoptosis in caspase-8 and FADD-deficient but not in Bcl-xL overexpressing Jurkat cells. In CLL cells, CDDO induced an early release of mitochondrial cytochrome c and Smac that preceded apoptosis. Thus, in both cell types, CDDO induced apoptosis primarily by the intrinsic pathway with caspase-9 as the apical caspase. This has important implications in the design of novel agents for the treatment of CLL and other malignancies.

Caspases-3 and -7 are activated in goniothalamin-induced apoptosis in human Jurkat T-cells

Goniothalamin, a plant styrylpyrone derivative isolated from Goniothalamus andersonii, induced apoptosis in Jurkat T‐cells as assessed by the externalisation of phosphatidylserine. Immunoblotting showed processing of caspases‐3 and ‐7 with the appearance of their catalytically active large subunits of 17 and 19 kDa, respectively. Activation of these caspases was further evidenced by detection of poly(ADP‐ribose) polymerase cleavage (PARP). Pre‐treatment with the caspase inhibitor benzyloxycarbonyl‐Val‐Ala‐Asp fluoromethyl ketone (Z‐VAD.FMK) blocked apoptosis and the resultant cleavage of these caspases and PARP. Our results demonstrate that activation of at least two effector caspases is a key feature of goniothalamin‐induced apoptosis in Jurkat T‐cells.

Comparative study of the sensitivity of male and female rats to methylmercury.

Male and female rats were dosed daily by gastric gavage four or five times with 8.0 mg/kg Hg as methylmercury. Treatment lowered the body weight in relation to the body weight of untreated rats to the same extent in male and female rats but when body weight was related to the initial body weight, the effect of methylmercury was more pronounced in females than in males. The importance of differences in growth or loss of body weight is that in spite of the similar whole body clearance mercury concentrations were higher in females than in males. After identical doses the brains of females always contained more mercury than those of males and in both sexes the brain concentration of mercury showed a disproportionate elevation when the number of doses was increased from four to five. However, weight change alone does not explain the sex related difference in the brain concentration of mercury as this was evident even 72 h after a single dose. In agreement with the brain concentration of mercury, female rats developed more intensive co-ordination disorders and after five doses they had more extensive damage in the granular layer of the cerebellum than males.