Regulation of phosphatidylserine exposure at the cell surface by the serine--base exchange enzyme system during CD95-induced apoptosis

Understanding the role of OXPHOS dysfunction in the pathogenesis of ECHS1 deficiency

Mitochondria provide the main source of energy for eukaryotic cells, oxidizing fatty acids and sugars to generate ATP. Mitochondrial fatty acid β-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are two key pathways involved in this process. Disruption of FAO can cause human disease, with patients commonly presenting with liver failure, hypoketotic glycaemia and rhabdomyolysis. However, patients with deficiencies in the FAO enzyme short-chain enoyl-CoA hydratase 1 (ECHS1) are typically diagnosed with Leigh syndrome, a lethal form of subacute necrotizing encephalomyelopathy that is normally associated with OXPHOS dysfunction. Furthermore, some ECHS1-deficient patients also exhibit secondary OXPHOS defects. This sequela of FAO disorders has long been thought to be caused by the accumulation of inhibitory fatty acid intermediates. However, new evidence suggests that the mechanisms involved are more complex, and that disruption of OXPHOS protein complex biogenesis and/or stability is also involved. In this review, we examine the clinical, biochemical and genetic features of all ECHS1-deficient patients described to date. In particular, we consider the secondary OXPHOS defects associated with ECHS1 deficiency and discuss their possible contribution to disease pathogenesis.

Combinatory Evaluation of Transcriptome and Metabolome Profiles of Low Temperature-induced Resistant Ascites Syndrome in Broiler Chickens

To select metabolic biomarkers and differentially expressed genes (DEGs) associated with resistant-ascites syndrome (resistant-AS), we used innovative techniques such as metabolomics and transcriptomics to comparatively examine resistant-AS chickens and AS controls. Metabolomic evaluation of chicken serum using ultra-performance liquid chromatography-quadruple time-of-flight high-sensitivity mass spectrometry (UPLC-QTOF/HSMS) showed significantly altered lysoPC(18:1), PE(18:3/16:0), PC(20:1/18:3), DG(24:1/22:6/0:0), PS(18:2/18:0), PI(16:0/16:0), PS(18:0/18:1), PS(14:1/14:0), dihydroxyacetone, ursodeoxycholic acid, tryptophan, L-valine, cycloserine, hypoxanthine, and 4-O-Methylmelleolide concentrations on day 21 and LysoPC(18:0), LysoPE(20:1/0:0), LysoPC(16:0), LysoPE(16:0/0:0), hypoxanthine, dihydroxyacetone, 4-O-Methylmelleolide, LysoPC(18:2), and PC(14:1/22:1) concentrations on day 35, between the susceptible and resistant groups. Compared to the susceptible group, transcriptomic analysis of liver samples using RNA-seq revealed 413 DEGs on day 21 and 214 DEGs on day 35 in the resistant group. Additional evaluations using gene ontology (GO) indicate that significant enrichment occurred in the oxygen transportation, defensive reactions, and protein modifications of the decreased DEGs as well as in the cell morphological formation, neural development, and transforming growth factor (TGF)-beta signalling of the increased DEGs on day 21. Oxygen transportation was also significantly enriched for downregulated DEGs on day 35. The combinatory evaluation of the metabolome and the transcriptome suggests the possible involvement of glycerophospholipid metabolism in the development of resistant-AS in broilers.

The cytotoxic effects of Scilla nervosa (Burch.) Jessop (Hyacinthaceae) aqueous extract on cultured HepG2 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Bulbs of Scilla nervosa, a medicinal plant indigenous to Southern Africa, are traditionally used in aqueous decoctions to treat a diverse range of illnesses. The bulbs contain homoisoflavanones and stilbenoids. Little information is known about the plant's toxicity on the liver, a major detoxifying organ. This study investigated the effects of an aqueous extract of the bulbs in cultured HepG2 liver cells, a model system for investigating the toxicity of xenobiotics.

MATERIALS AND METHODS

The concentration that reduced cell viability to 50% (IC(50)) after 24h treatment was derived. Potential mechanisms of toxicity using the IC(50) were investigated as changes in metabolic activity, apoptosis, oxidative damage and DNA fragmentation. In addition, cytochrome P450 3A4 (CYP3A4) activity, which is implicated in drug metabolism and interactions, was also assayed.

RESULTS

Cell viability decreased in a concentration-dependent manner and the IC(50) was determined as 0.03 mg/mL. Treating the cells at the IC(50) for 24h resulted in increased intracellular ATP levels, no significant change in phosphatidylserine externalisation, increased caspase-8 activity, decreased caspase-9 activity, no significant change in mitochondrial membrane potential, increased lipid peroxidation, evidence for genotoxicity as demonstrated by DNA fragmentation, and slightly induced CYP3A4 activity.

CONCLUSION

Results suggest that liver cells are sensitive to an aqueous extract of the bulbs and there is an increased potential to induce apoptosis, oxidative stress and genotoxicity in vitro.

The ins and outs of phospholipid asymmetry in the plasma membrane: roles in health and disease

A common feature of all eukaryotic membranes is the non-random distribution of different lipid species in the lipid bilayer (lipid asymmetry). Lipid asymmetry provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions. Alteration of lipid asymmetry plays a prominent role during cell fusion, activation of the coagulation cascade, and recognition and removal of apoptotic cell corpses by macrophages (programmed cell clearance). Here we discuss the origin and maintenance of phospholipid asymmetry, based on recent studies in mammalian systems as well as in Caenhorhabditis elegans and other model organisms, along with emerging evidence for a conserved role of mitochondria in the loss of lipid asymmetry during apoptosis. The functional significance of lipid asymmetry and its disruption during health and disease is also discussed.

Monitoring of chemotherapy-induced cell death in melanoma tumors by N,N'-Didansyl-L-cystine

Early assessment of the efficacy of anticancer agents is a highly desirable and an unmet need in clinical oncology. Clinical imaging of cell-death may be useful in addressing this need, as induction of tumor cell-death is the primary mechanism of action of most anticancer drugs. In this study, we examined the performance of N,N'-Didansyl-L-cystine (DDC), a member of the ApoSense family of novel small molecule detectors of cell-death, as a potential tool for monitoring cell-death in cancer models. Detection of cell-death by DDC was examined in fluorescent studies on B16 melanoma cells both in vitro and ex vivo following its in vivo administration. In vitro, DDC manifested selective uptake and accumulation within apoptotic cells that was highly correlated with Annexin-V binding, changes in mitochondrial membrane potential, and caspase activation. Uptake was not ATP-dependent, and was inducible by calcium mobilization. In vivo, DDC selectively targeted cells undergoing cell-death in melanoma tumors, while not binding to viable tumor cells. Chemotherapy caused marked tumor cell-death, evidenced by increased DDC uptake, which occurred before a detectable change in tumor size and was associated with increased animal survival. These data confirm the usefulness of imaging of cell-death by DDC as a tool for early monitoring of tumor response to anti-cancer therapy.

Fluorescent detection of apoptotic cells by using zinc coordination complexes with a selective affinity for membrane surfaces enriched with phosphatidylserine

The appearance of phosphatidylserine on the membrane surface of apoptotic cells (Jurkat, CHO, HeLa) is monitored by using a family of bis(Zn2+-2,2'-dipicolylamine) coordination compounds with appended fluorescein or biotin groups as reporter elements. The phosphatidylserine affinity group is also conjugated directly to a CdSe/CdS quantum dot to produce a probe suitable for prolonged observation without photobleaching. Apoptosis can be detected under a wide variety of conditions, including variations in temperature, incubation time, and binding media. Binding of each probe appears to be restricted to the cell membrane exterior, because no staining of organelles or internal membranes is observed.

Resistance to UV-induced apoptosis in Chinese-hamster ovary cells overexpressing phosphatidylserine synthases

Externalization of PtdSer (phosphatidylserine) is an important event in signalling removal of apoptotic cells. In contrast with previous work [Yu, Byers, Ridgway, McMaster and Cook (2000) Biochim. Biophys. Acta 1487, 296-308] with U937 cells showing that specific stimulation of PtdSer biosynthesis during apoptosis was caspase dependent, PtdSer biosynthesis in CHO (Chinese-hamster ovary)-K1 increased 2.5-fold during UV-induced apoptosis but was not reversed by a caspase inhibitor, Z-VAD-FMK (benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone). Also, in CHO-K1 cells, stimulation of synthesis was less specific for PtdSer as similar levels of stimulation were observed for sphingomyelin biosynthesis. Involvement of PtdSer synthase isoforms was tested in CHO-K1 cells overexpressing PSS I (PtdSer synthase I) and PSS II. Both types of transformed cells showed resistance to UV-induced apoptosis based on the decreased levels of caspase 3 activation and morphology changes; externalization of PtdSer was reduced with UV treatment even though expression of endogenous scramblase increased slightly. Serine-labelling experiments showed that PSS I- or PSS II-expressing cells had higher basal levels of PtdSer biosynthesis compared with vector control cells. When cells were exposed to UV light to induce apoptosis, PtdSer biosynthesis was further stimulated 1.5- and 2-fold in PSS I- and PSS II-expressing cells respectively compared with UV-treated vector cells. Caspase activation was not required, as Z-VAD-FMK did not change PtdSer synthesis. Although enhanced PtdSer synthesis was supposed to facilitate apoptosis, cells overexpressing PSS I and II were actually resistant to UV-induced apoptosis. Whereas enhanced PtdSer synthesis was associated with apoptosis, potential anti-apoptotic effects were observed when excess activity of these synthetic enzymes was present. This suggests a tightly regulated role for PtdSer synthesis and/or an important dependence on compartmentation of PSS enzymes in association with scramblase facilitated enrichment of this phospholipid at the cell surface.

Plant-derived abrin-a induces apoptosis in cultured leukemic cell lines by different mechanisms

Abrin-a consists of A-chain with N-glycosidase activity, which inhibits protein synthesis, and lectin-like B-chain responsible for binding with cell-surface receptors and penetrating of abrin-a molecule into the cells. As a lectin component, the B-chain can also participate in cell signal transduction. It has been reported that abrin induces apoptosis, but the molecular mechanism(s) of this induction have been obscure and several alternative variants have been discussed. The present study demonstrates that abrin-a induces apoptosis in human cultured cell lines, derived from acute lymphoblastic leukemia (ALL) (Jurkat, CCRF-CEM, MOLT-4, HPB-ALL). The apoptosis was estimated by: phosphatidylserine (PSer) exposure at the cell surface, activation of caspase cascade, and DNA fragmentation. The penetrating of abrin-a into the cells was detected by fluorescent confocal microscopy, using fluorescein isothiocyanate (FITC) as a fluorescent marker. It was established that the effect of abrin-a on the apoptosis induction in leukemic cells was dose- and time-dependent. The process was initiated 1 h after abrin-a application (before its penetrating into the cells) and was characterized with PSer translocation from the inner to the outer monolayer of plasma membrane, caspase activation on the first to second hour after beginning of treatment, with maximum on the third to fourth hour, and DNA fragmentation on the fourth to sixth hour, depending of the cell line. The exposure of PSer on the cell surface was detected in Jurkat, CCRF-CEM, and MOLT-4 cells. In HPB-ALL, no significant changes in PSer exposure on the cell surface was observed. Activation of caspase-3, -8, and -9 was detected in Jurkat, MOLT-4, and HPB-ALL. Surprisingly, the activity of caspase-3 increased on the first hour after beginning of treatment, while the activity of caspase-8 and -9 began to increase on the second hour. In CCRF-CEM, activation of caspases was not measured, but the apoptosis progressed to DNA fragmentation in a dose- and time-dependent manner. DNA fragmentation was also detected in Jurkat, but not in MOLT-4 and HPB-ALL cells. It seems that the mechanisms of abrin-a-induced apoptosis are different and the progress of apoptosis depends of the cell line. There was a very good positive correlation between the agglutinating activity of abrin-a and development of apoptosis to DNA fragmentation. The time-dependent effects of abrin-a on apoptosis as well as its time-dependent penetration into the cells suggest that the B-chain probably triggers the apoptosis, while the A-chain and breakage of the disulfide bond are responsible for its progress.

Initial exposed phosphatidylserine levels correlate with cellular response to cytotoxic drugs

Phosphatidylserine's (PS) membranal distribution is associated with an expanding variety of biological processes. We studied the relevance of preliminarily exposed membranal PS levels to cellular effects of cytotoxic agents. PBL of normal controls (n = 18) and patients with doxorubicin-treated breast carcinoma (n = 27) or 5'-fluorouracil-treated colorectal cancer (n = 32) were assayed before and after drug infusion. Membranal expression levels of PS, adhesion molecules (CD18, CD11a-c, CD63) and Fas-R of leukocyte subtypes were assessed by flow cytometer. Statistical analysis was implemented. Our results demonstrate external expression of PS on all leukocyte subpopulations despite non-apoptotic light scatter characteristics. Several distinct features were observed of which the more prominent were: leukocyte subtypes each display characteristic PS levels; cancer patients' PBL display higher preliminary PS levels than normal controls in all cell groups; and existence of negative correlations between initial membranal PS levels and drug-induced changes in its expression. Our findings underscore the complex involvement of PS in PBL apoptosis and possibly drug resistance.

Metabolism and functions of phosphatidylserine in mammalian brain

Phosphatidylserine (PtdSer) is involved in cell signaling and apoptosis. The mechanisms regulating its synthesis and degradation are still not defined. Thus, its role in these processes cannot be clearly established at molecular level. In higher eukaryotes, PtdSer is synthesized from phosphatidylethanolamine or phosphatidylcholine through the exchange of the nitrogen base with free serine. PtdSer concentration in the nervous tissue membranes varies with age, brain areas, cells, and subcellular components. At least two serine base exchange enzymes isoforms are present in brain, and their biochemical properties and regulation are still largely unknown because their activities vary with cell type and/or subcellular fraction, developmental stage, and differentiation. These peculiarities may explain the apparent contrasting reports. PtdSer cellular levels also depend on its decarboxylation to phosphatidylethanolamine and conversion to lysoPtdSer by phospholipases. Several aspects of brain PtdSer metabolism and functions seem related to the high polyunsaturated fatty acids content, particularly docosahexaenoic acid (DHA).

Group B streptococcus (GBS) modifies macrophage phosphatidylserine metabolism during induction of apoptosis

Group B streptococcus (GBS) induced macrophage apoptosis by which it could avoid host defence mechanisms. Macrophages, which constitutively express phosphatidylserine (PtdSer) on the outer leaflet of plasma membrane, increased PtdSer exposure during GBS-induced apoptosis. Induction of apoptosis decreased PtdSer radioactivity of macrophages incubated with [(3)H]serine. The effect appeared not due to increasing conversion of PtdSer to phosphatidylethanolamine or phosphatidylcholine nor to the release of radioactive membrane vesicles. The radioactivity in lysoPtdSer was also reduced. These results confirm that induction of apoptosis involves a modification of PtdSer metabolism and point out the typical features of the GBS-induced apoptosis with respect to other models of apoptosis.

Activated T lymphocytes from patients with high risk of type I diabetes mellitus have different ability to produce interferon-gamma, interleukin-6 and interleukin-10 and undergo anti-CD95 induced apoptosis after insulin stimulation

Type I Diabetes mellitus (DM1) is the effect of T cell dependent autoimmune destruction of insulin producing beta cells in the pancreas islet. T cells are activated in response to islet dominant autoantigens, the result being the development of DM1. Insulin is one of the islet autoantigens responsible for activation of T lymphocyte functions, inflammatory cytokine production and development of DM1. The experiments reported in this study have shown the spontaneous increase of CD95 molecule expression on lymphocytes of the first-degree relatives of DM1 patients. The autoantigen insulin is responsible for stimulation in vitro of potentially hazardous 'memory' lymphocytes to produce interleukin-6 (IL-6) and interleukin-10 (IL-10) interleukins. Insulin induced stimulation of lymphocytes in vitro was observed in patients at high risk of developing diabetes mellitus (prediabetics). Phytohaemagglutinin (PHA) stimulates lymphocytes of all groups in the same way. Stimulated lymphocytes in second cultures undergo apoptosis induced with anti-Fas specific antibodies. The deletion in vitro of resting peripheral lymphocytes is nonfunctional. Insulin activated T lymphocytes, which undergo apoptosis were not observed in peripheral blood of healthy people and in patients with DM1. This observation suggests that insulin is involved as autoantigen in DM1 progression in patients with high risk of diabetes type I. The autoreactive T lymphocytes may persist in peripheral blood of patients with high risk DM1. Defective elimination of autoreactive T cells may result in autodestructive damage of islets beta cells in the prediabetic stage and disease progression to DM1.

Modulation of apoptosis by procaspase-2 short isoform: selective inhibition of chromatin condensation, apoptotic body formation and phosphatidylserine externalization

Procaspase-2 is one of the cysteine aspartate proteases involved in apoptotic cell death. Alternative splicing of CASP-2 messenger RNA generates a long isoform, procaspase-2L, whose overexpression induces cell death and a truncated isoform, procaspase-2S, whose function remains poorly defined. The present study explored the consequences of procaspase-2S overexpression in U937 human leukemic cells exposed to the topoisomerase II inhibitor etoposide as an apoptotic stimulus. Overexpression of procaspase-2S in U937 cells partially prevented nuclear changes associated with etoposide-induced cell death, as determined by Hoechst 33342 staining of nuclear chromatin and electron microscopy studies. Procaspase-2S also prevented the maturation of apoptotic bodies, delayed phosphatidylserine externalization on the plasma membrane and prevented the cleavage and activation of procaspase-2L. These effects were not observed when the cysteine 289 in the consensus QACRG motif was mutated into a serine. Wild-type procaspase-2S overexpression did not influence the cleavage of procaspase-3, procaspase-7 and poly(ADP-ribose)polymerase nor the fragmentation of nuclear DNA into nucleosome-sized fragments. Altogether, these results indicate that the short isoform of procaspase-2 negatively interferes with selective features of apoptosis, an activity that is suppressed by mutation of the cysteine 289.