CD40 ligand gene therapy directly induces apoptosis in carcinomas

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Background: CD40 is a member of the TNFReceptor superfamily expressed on a variety of immune cells. Interaction of CD40 with its ligand (CD40L) plays a key role in orchestrating immune responses. We have shown that CD40 is also expressed on a variety of carcinomas. CD40 ligation in this context can induce cancer cell death. Thus CD40 represents a rational target with direct cytotoxicity and induction of anti-tumour immunity. Methods: We generated an adenovirus encoding CD40L (AdCD40L) and investigated its effect in CD40-expressing carcinoma cells (bladder, ovary, hepatocellular) as follows:(i)comparison of Ad-delivered CD40L with recombinant soluble ligand (rsCD40L);(ii)inhibition of cleavage of CD40L from the cell membrane using metalloproteinase inhibitors (MMPI);(iii)generation of a mutant CD40L resistant to MMP cleavage. Results: (i)CD40 ligation in carcinomas stimulates survival and apoptotic signaling. rsCD40L is only cytotoxic in the presence of protein synthesis inhibition, which blocks survival pathways and allows apoptosis. In contrast, AdCD40L is cytotoxic even in the absence of protein synthesis inhibitor;(ii)AdCD40L results in expression at the cell membrane but this is cleaved from the membrane and detected as soluble ligand in supernatant of infected cells. Cleavage is inhibited by MMPI and this increases apoptosis, suggesting that membrane-bound CD40L is a more potent apoptotic stimulus than soluble ligand;(iii)we identified the cleavage site of CD40L and generated a mutant by deletion of this region and cloned this mCD40L into an adenovirus. AdmCD40L results in membrane expression of CD40L, with no detectable soluble CD40L, confirming its resistance to cleavage. AdmCD40L is significantly more cytotoxic than wild-type AdCD40L. Conclusions: The effect of CD40 ligation depends on the cellular context and the method of CD40L delivery: adenovirus delivery results in membrane-bound expression and this provides a stronger apoptotic stimulus than rsCD40L. Apoptosis is further enhanced by a mutant CD40L that is resistant to cleavage from the cell membrane. Thus AdmCD40L is a novel therapeutic that is a potent inducer of apoptosis. Studies are underway to assess these effects in vivo and to elucidate the contribution of immunostimulatory effects in syngeneic models.

No significant financial relationships to disclose.

Expression of the cellular FLICE-inhibitory protein (c-FLIP) protects Hodgkin's lymphoma cells from autonomous Fas-mediated death

Hodgkin's lymphoma (HL) is characterized by the presence of malignant so-called Hodgkin's/Reed-Sternberg (HRS) cells, which display resistance to certain apoptotic stimuli, including a lack of sensitivity to Fas-mediated cell death. However, the mechanisms responsible for their resistance to apoptosis inducers have not been elucidated. Here we confirm that both HL-derived cell lines and the HRS cells of primary HL tissues express Fas ligand (FasL) along with the inhibitory c-FLIP protein. Down-regulation of cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) through the use of specific small inhibitory RNAs (siRNAs) leads to reduced viability of the L428 and L591 HL-derived cell lines. To determine whether endogenous FasL was responsible for the reduction in cell viability observed after down-regulation of c-FLIP, L428 and L591 cells were treated with c-FLIP-specific siRNAs with and without siRNAs directed to FasL. Treatment of these cells with both c-FLIP- and FasL-specific siRNAs in combination restored cell viability to near control levels. Our results provide a mechanism whereby HRS cells are protected from autonomous FasL-mediated cell death while preserving their ability to evade immunosurveillance. Targeting c-FLIP could provide a novel approach to the treatment of HL.

Mechanism of cell death induced by the novel enzyme-prodrug combination, nitroreductase/CB1954, and identification of synergism with 5-fluorouracil

Virus-directed enzyme prodrug therapy (VDEPT) utilising the bacterial enzyme nitroreductase delivered by a replication-defective adenovirus vector to activate the prodrug CB1954 is a promising strategy currently undergoing clinical trials in patients with a range of cancers. An understanding of the mechanism of tumour cell death induced by activated CB1954 will facilitate this clinical development. Here, we report that activated CB1954 kills cells predominantly by caspase-dependent apoptosis. This may have important implications for the generation of immune-mediated bystander effects. Further, the use of a replication-defective adenovirus vector to deliver nitroreductase may negatively affect cellular apoptotic pathways stimulated by activated CB1954. Finally, examination of nitroreductase/CB1954 in combination with conventional chemotherapy reveals a synergistic interaction with 5-fluorouracil. These data will facilitate the further development and future clinical trial design of this novel therapy.

Induction of apoptosis by chemotherapeutic drugs: the role of FADD in activation of caspase-8 and synergy with death receptor ligands in ovarian carcinoma cells

Although ovarian tumours initially respond to chemotherapy, they gradually acquire drug resistance. The aims of this study were to identify how chemotherapeutic drugs with diverse cellular targets activate apoptotic pathways and to investigate the mechanism by which exposure to a combination of drugs plus death receptor ligands can increase tumour cell kill. The results show that drugs with distinct cellular targets differentially up-regulate TRAIL and TNF as well CD95L, but do not require interaction of these ligands with their receptor partners to induce cell death. Factors that were critical in drug-induced apoptosis were activation of caspases, with caspase-8 being activated by diverse drugs in a FADD-independent manner. Certain drugs also demonstrated some dependence on FADD in the induction of cell death. Caspase-9 was activated more selectively by chemotherapeutic agents. Combining ligation of death receptors with exposure to drugs increased tumour cell kill in both drug resistant cell lines and primary ovarian carcinoma cells, even though these cells were not sensitive to death receptor ligation alone. CD95L was more consistent at combining with drugs than TRAIL or TNF. Investigation of the mechanism by which a combination of drugs plus CD95 ligation can increase cell death showed that caspase-8 was activated in cells exposed to a combination of cisplatin and anti-CD95, but not in cells exposed to either agent alone.

Isolation of methicillin-resistant Staphylococcus aureus from non-sterile sites: evaluation of a new selective medium

Methicillin-resistant Staphylococcus aureus (MRSA) are relatively common nosocomial isolates, causing problems for health-care professionals worldwide. Therefore, early detection of the organisms by the laboratory is essential. A new selective medium for MRSA is described, comprising a DNA-containing base and a combination of stains that permit direct visualisation of DNase activity around colonies. The medium is made partially selective by adding a number of antibiotics (aztreonam, polymyxin B, nystatin and oxacillin). When compared to other media used for the isolation of MRSA, it was found that the new medium allowed earlier detection of colonies and provided a good direct method of identification, reducing the need for time-consuming replating of colonies, and, therefore, the turnaround time for specimens entering the laboratory for MRSA screening.

The structure of the site on adenovirus early region 1A responsible for binding to TATA-binding protein determined by NMR spectroscopy

Previous detailed mutational analysis has shown that the binding site on adenovirus (Ad) early region 1A (E1A) for TATA-binding protein (TBP) is located toward the N terminus of conserved region 3 (CR3). Here we demonstrate that synthetic peptides of between 15 and 22 amino acids, identical to amino acid sequences of CR3 present in the larger Ad5 E1A (13 S product) and in both the Ad12 E1A (13 and 12 S products) proteins that lie N-terminal to the zinc finger motif, can disrupt binding of E1A to TBP. These findings suggest that the peptides are biologically active in terms of interacting with TBP and must therefore comprise some, if not all, of the TBP binding site on E1A. The interaction between Ad12 E1A and TBP was confirmed by direct co-precipitation experiments. In 1H NMR studies of CR3 peptides, regular patterns of NOEs were observed from which their conformational preferences in aqueous solution were determined. Both Ad5 and Ad12 peptides were shown to contain regions of helical backbone structure in 50% trifluoroethanol. In each case, the type and intensities of NOE cross-peaks observed correlated best to alpha-helical turns. These helices are more extensive in larger peptides and extend from Glu141 to Val147 and from Arg144 to Pro152 in the full-length Ad5 and Ad12 13S E1A proteins, respectively. The structure of a 19-residue Ad5 CR3 peptide carrying the V147L mutation in the full-length protein that abolishes TBP binding was examined. No significant differences between the substituted and wild type peptides were observed, suggesting that this substitution in the intact protein may cause disruption of global rather than local structures.

Adenovirus early region 1A protein binds to mammalian SUG1-a regulatory component of the proteasome

Adenovirus early region 1A (Ad E1A) is a multifunctional protein which is essential for adenovirus-mediated transformation and oncogenesis. Whilst E1A is generally considered to exert its influence on recipient cells through regulation of transcription it also increases the level of cellular p53 by increasing the protein half-life. With this in view, we have investigated the relationship of Ad E1A to the proteasome, which is normally responsible for degradation of p53. Here we have shown that both Ad5 and Ad12 E1A 12S and 13S proteins can be co-immunoprecipitated with proteasomes and that the larger Ad12 E1A protein binds strongly to at least three components of the 26S but not 20S proteasome. One of these interacting species has been identified as mammalian SUGI, a proteasome regulatory component which also plays a role in the cell as a mediator of transcription. In vitro assays have demonstrated a direct interaction between Ad12 E1A 13S protein and mouse SUGI. Following infection of human cells with Ad5 wt and Ad5 mutants with lesions in the E1A gene it has been shown that human SUG1 can be co-immunoprecipitated with full-length E1A and with E1A carrying a deletion in conserved region 1 which is the region considered to be responsible for increased expression of p53. We have concluded therefore that Ad EIA binds strongly to SUGI but that this interaction is not responsible for inhibition of proteasome activity. This is consistent with the observation that purified Ad12 E1A inhibits the activity of the purified 20S but not 26S proteasomes. We have also demonstrated that SUGI can be co-immunoprecipitated with SV40 T and therefore we suggest that this may represent a common interaction of transforming proteins of DNA tumour viruses.

Structural determinants present in the C-terminal binding protein binding site of adenovirus early region 1A proteins

The C-terminal binding protein (CtBP) has previously been shown to bind to a highly conserved six-amino acid motif very close to the C terminus of adenovirus early region 1A (Ad E1A) proteins. We have developed an enzyme-linked immunosorbent assay that has facilitated the screening of synthetic peptides identical or similar to the binding site on Ad E1A for their ability to bind CtBP and thus inhibit its interaction with Ad12 E1A. It has been shown that amino acids both C-terminal and N-terminal to the original proposed binding site contribute to the interaction of peptides with CtBP. Single amino acid substitutions across the binding site appreciably alter the Kd of the peptide for CtBP, indicative of a marked reduction in the affinity of the peptide for CtBP. The solution structures of synthetic peptides equivalent to the C termini of both Ad5 and Ad12 E1A and two substituted forms of these have been determined by proton NMR spectroscopy. Both the Ad12 and Ad5 peptides dissolved in trifluoroethanol/water mixtures were found to adopt regular secondary structural conformations seen as a series of beta-turns. An Ad12 peptide bearing a substitution that resulted in only very weak binding to CtBP (Ad12 L258G) was found to be random coil in solution. However, a second mutant (Ad12 V256K), which bound to CtBP rather more strongly (although not as well as the wild type), adopted a conformation similar to that of the wild type. We conclude that secondary structure (beta-turns) and an appropriate series of amino acid side chains are necessary for recognition by CtBP.

Human cells arrest in S phase in response to adenovirus 12 E1A

It has previously been shown that following viral infection, Ad5 E1A induces cell cycle progression of quiescent rodent cells, leading to DNA synthesis and mitosis. Here we have examined the effect of Ad12 E1A on the cell cycle characteristics of human cells. Human tumor (A549, KB, and HeLa) cells were infected with Ad12 d/620, a mutant virus which has a lesion in the E1B gene and essentially expresses only E1A. These infected cells progressed from being largely in G1 into S phase, where they arrested. Even up to 96 h postinfection (p.i.) the cells remained blocked in S phase. DNA synthesis did, however, proceed in Ad12 d/620-infected cells, giving rise to multiple copies of cellular DNA. Similar results were obtained when primary human skin fibroblasts were infected, although the polyploidy was less marked. The expression of cyclins A, B1, and E in the tumor cells increased appreciably in response to E1A. In contrast, there was a dramatic reduction in the levels of cyclin D1 and D3. Increases in cyclin D1 expression could be detected at very late times p.i. In those cell lines expressing low levels of cdc2 and cdk2 an appreciable increase in expression was seen soon after Ad12 E1A could be detected. The elevated levels of cyclins A, B1, and E were associated with increased protein kinase activity directed against histone H1. An increase in cyclin D1-associated kinase activity against Rb1 was also observed at late times. This deregulation of the cell cycle was not solely dependent on E1A inactivation of Rb, since similar effects were seen in Ad12 d/620-infected retinoblastoma (Y-79) cells, implicating p107 and p130 in E1A-mediated changes in cell cycle progression. We propose that the E1A-induced levels of cyclins A, B1, and E by Ad12 E1A in human cells may lead to an uncoupling of S phase from cell cycle progression.

Regeneration of the binding properties of adenovirus 12 early region 1A proteins after preparation under denaturing conditions

Adenovirus 12 early region 1A (Ad12 E1A) was expressed in Escherichia coli. Protein was purified in good yield in the presence of 8 M urea and then renatured by dialysis against dilute NH4HCO3 buffer. The affinity of this protein for pRb, C-terminal binding protein (CtBP), TATA binding protein (TBP), and SUG1 was similar to, or greater than, that of Ad12 E1A prepared by immunoaffinity chromatography under nondenaturing conditions. While the binding of the 266- and 235-amino-acid (aa) E1A components to TBP showed similar characteristics the larger E1A protein had a higher affinity for CtBP, pRb, and SUG1. Using nuclear magnetic resonance (NMR) spectroscopy it was shown that structural perturbations occurred in the 266-aa protein in the presence of Zn2+ consistent with binding--no such changes were seen for the 235-aa protein. Limited proteolysis of the 266- and 235-aa E1A proteins gave rise to comparable polypeptide products, suggesting overall similarities in structure. However, the different affinities of the 266- and 235-aa proteins for the partner proteins and the differences seen in the NMR spectra from the two proteins suggested structural differences.

Homology between a human apoptosis specific protein and the product of APG5, a gene involved in autophagy in yeast

Apoptosis specific proteins (ASP) are expressed in the cytoplasm of cultured mammalian cells of various lineages following induction of apoptosis. The cDNA encoding ASP has been cloned from a human expression library and has significant homology to the Saccharomyces cerevisiae APG5 gene which is essential for yeast autophagy. The ASP gene, known as hAPG5, can be transcribed to give mRNAs of 3.3 kbp, 2.5 kbp and 1.8 kbp which are present at comparable levels in viable and apoptotic cells, demonstrating that protein expression must be regulated at the translational level. These data indicate a possible relationship between apoptosis and autophagy and suggest evolutionary conservation in mammalian apoptosis of a degradative process present in yeast.

Differential effects of BCL-2 on survival and proliferation of human B-lymphoma cells following gamma-irradiation

Bcl-2 can inhibit apoptosis induced by a variety of stimuli, including radiation and its presence in tumour cells would be expected to indicate poor prognosis. Bcl-2-expressing tumours, however, are often low-grade and highly responsive to therapy. To investigate this apparent paradox, we analysed in vitro the responses of Burkitt lymphoma (BL) cells to gamma-irradiation in the presence and absence of Bcl-2. High-level expression of Bcl-2 was shown to promote BL cell survival following irradiation. However, a significant proportion of Bcl-2-rescued cells subsequently underwent apoptosis after an extended period in culture. In addition, in different BL lines, Bcl-2 was found either to promote or to inhibit long-term proliferative activity following gamma-irradiation. This differential regulation of proliferation correlated both with differential effects of Bcl-2 on the cell cycle and with differences in p53 status. Thus, by one week after irradiation, BL cells expressing only wild-type p53 (wt/wt) had arrested in G1, whereas those with a mutant allele (wt/mu) were arrested in all phases of the cell cycle. The proportion of Bcl-2-rescued cells that subsequently underwent apoptosis was reduced by ligation of CD40 at the time of irradiation in wt/wt BL cells, but not in wt/mu cells. CD40-ligation reduced both G1-arrest and apoptosis in parallel. These results indicate that, whilst Bcl-2 can delay apoptosis in BL cells following gamma-irradiation, the protein can also cause growth-arrest and thereby promote apoptosis. Long-term survival following Bcl-2-mediated rescue of gamma-irradiated cells may depend on p53 status and require additional death-repressing or growth-promoting signals.

Bcl-2 delays macrophage engulfment of human B cells induced to undergo apoptosis

The capacity to be recognized and engulfed by phagocytes is an important characteristic of cells dying by apoptosis. Phagocytosis of apoptotic cells occurs rapidly in vivo, probably prior to plasma membrane breakdown. While the molecular mechanisms mediating phagocytosis of apoptotic cells are beginning to be defined, little is yet known of the relationship between the cell-death program itself and the surface changes on the dying cells that signal for engulfment. Here, we investigate to what extent the apoptosis repressor Bcl-2 can modulate the recognition and phagocytosis of human B cells exposed to triggers of apoptosis. Burkitt lymphoma (BL)-derived, Bcl-2- B cells were induced into apoptosis either by the Ca(2+)-ionophore ionomycin or by the inhibitor of protein synthesis cycloheximide. Apoptotic BL cells, but not viable BL cells, were recognized and phagocytosed by monocyte-derived macrophages. bcl-2-transfected BL populations showed a reduced capacity both to undergo apoptosis in response to these inducing agents and to interact with macrophages. Like their Bcl-2- counterparts, Bcl-2+ BL cells interacted with macrophages only after activation of their apoptotic program as assessed by changes in nuclear morphology. These results demonstrate not only that continued protein synthesis in B cells undergoing apoptosis is not essential for their recognition by macrophages, but also that macrophage recognition of apoptotic B cells cannot be uncoupled from the cell-death program that is controlled by Bcl-2. In this respect, the behavior of B cells contrasts markedly with that of neutrophils in which Bcl-2 has been reported to inhibit apoptosis without affecting phagocytic clearance.

Repression of apoptosis in human B-lymphoma cells by CD40-ligand and Bcl-2: relationship to the cell-cycle and role of the retinoblastoma protein

Using a Burkitt lymphoma cell line to model human B-cell apoptosis in vitro, we observed that crosslinking, by antibody, of cell surface immunoglobulin induced G1 growth-arrest followed by apoptosis. By contrast, cells treated with the Ca(2+)-ionophore, ionomycin, generated apoptotic signals in G2/M as well as in G1. Both ionomycin and anti-immunoglobulin treatment induced rapid dephosphorylation of Rb prior to apoptosis. Apoptosis was repressed following exposure to CD40-ligand and was accompanied by hyperphosphorylation of Rb and cell-cycle progression but not Bcl-2 expression. Expression of Bcl-2 protein in stable bcl-2-transfectants, also resulted in repression of apoptosis and anti-immunoglobulin-treated cells no longer underwent growth-arrest. In Bcl-2-expressing cells in which apoptosis was repressed, Rb remained hyperphosphorylated, even during G1-arrest induced by ionomycin. TGF beta treatment of Bcl-2-expressing cells induced G1-arrest, de-phosphorylation of Rb and apoptosis. These results suggest that the functional activity of Bcl-2 in B-lymphoma cells is dependent upon, or leads to, sustained hyperphosphorylation of Rb and that Rb hyperphosphorylation can be uncoupled from cell-cycle progression.

A novel protein expressed in mammalian cells undergoing apoptosis

Human and rodent cells undergoing apoptosis were observed to express high levels of a novel 45,000 M(r) protein. The protein, which we have termed apoptosis specific protein (ASP), was found in Burkitt lymphoma (BL) cells and in adenovirus-transformed human and rat embryo cells induced into apoptosis by a variety of stimuli, including serum deprivation, exposure to the Ca2+ ionophore, ionomycin, treatment with inhibitors of macromolecular synthesis (cycloheximide and actinomycin D), and cold shock. In BL cells treated with apoptotic stimuli, expression of the oncoprotein Bcl-2 was found to both protect from apoptosis and prevent expression of ASP. ASP was not detected either in viable cells or in cells dying passively by necrosis. Laser scanning confocal microscopy showed high levels of ASP in the cytoplasm of cells displaying the chromatin condensation and fragmentation patterns typical of apoptosis. Retention of ASP was observed even when DNA was no longer detectable, and two-color immunofluorescence staining indicated that the protein primarily colocalized with, but was clearly distinct from, non-muscle actin. These findings, together with the observation that biochemical extraction of ASP was only possible under conditions which caused solubilization of the cytoskeleton, leads us to conclude that ASP forms part of, or at least strongly associates with, a modified cytoskeleton unique to cells undergoing apoptosis. While elucidation of its function will require further work, ASP constitutes a powerful marker for the diagnosis and quantitation of apoptosis in vivo and in vitro.

Effects of interferon-alpha on human B cells: repression of apoptosis and prevention of cell growth are independent responses of Burkitt lymphoma lines

We have previously shown that interferon-alpha (IFN-alpha) can repress apoptosis in Burkitt lymphoma (BL) cells. In this study, we have compared this protective response with a further, well-established effect of IFN-alpha on BL cells, that of growth arrest. Of a panel of BL lines comprising (i) EBV-positive and -negative lines that retain the phenotype of the parental tumour cells and (ii) the prototype IFN-alpha-growth-inhibited line, Daudi, only Daudi cells were found to undergo substantial growth inhibition in response to the cytokine. By contrast, all lines, with the notable exception of Daudi, were protected by IFN-alpha from high-rate apoptosis initiated by the Ca2+ ionophore ionomycin. Ionomycin failed to elicit an IFN-alpha-repressible apoptotic response in either wild-type Daudi cells or IFN-resistant sublines that were refractory to the growth-arresting effects of the cytokine. Analysis of c-myc protein levels confirmed previous observations that repression of apoptosis in IFN-alpha-rescuable BL cells was associated with an early inhibition of myc that was followed by a return to high-level expression. Significantly, ionomycin alone induced a comparable transient inhibition of myc protein in Daudi cells. In Daudi cells, but not in IFN-alpha-rescuable BL cells, renewed expression of myc observed after the early, transient down-regulation was followed by sustained down-regulation of the protein, which paralleled growth arrest. Our results indicate that long-term growth arrest and repression of apoptosis in BL are distinct cellular responses to IFN-alpha.

Regulation of cell survival in Burkitt lymphoma: implications from studies of apoptosis following cold-shock treatment

Burkitt lymphoma (BL) tumour-cell populations are known to display high rates both of proliferation and of apoptosis in vivo, but the mechanisms which determine whether a BL cell continues to cycle or engages its cell-death programme are not understood. Group-I BL-derived cell lines, which retain in vitro the proliferative and apoptotic capacities of the parental cells, selectively entered apoptosis when returned to 37 degrees C after a brief period at low temperature (1 degree C). The induction of apoptosis by cold treatment, as determined by morphological characteristics and DNA fragmentation, was readily detectable within the first 1 to 2 hr of re-incubation at 37 degrees C, reaching a maximum at 4 to 6 hours. Commitment to enter apoptosis occurred after as little as 20 to 30 min at 1 degree C. Significant cell death at 1 degree C occurred only during prolonged incubation in the cold and displayed the characteristics of necrosis. Both bcl-2-dependent and -independent survival pathways were found to provide protection from cold-induced apoptosis, but only if engaged before cold-shock treatment. These results indicate that continued cycling of group-I BL cells is dependent upon their capacity to inhibit or circumvent their normally constitutively active apoptotic programme, and are consistent with the notion that the synthesis of one or more critical "survival" proteins of short half-life is necessary to guarantee successful passage through the cell cycle. Notably, high levels of apoptosis were also inducible in group-I BL cells by inhibitors of RNA and protein synthesis.

Apoptosis in Burkitt lymphoma cells is driven by c-myc

Chromosomal translocation and subsequent de-regulation of the c-myc proto-oncogene are considered to be critical events in the multi-stage evolution of Burkitt lymphoma (BL). It is widely accepted that Myc protein functions as a competence factor for proliferation. However, recent studies indicate that it can also act in some cell types as a regulator of apoptosis. BL cell populations display a high frequency of apoptosis in vivo, a property which is also readily demonstrable in vitro in group I BL cell lines. Such lines are known to retain the cell surface marker characteristics of the parental tumour cells and, in the case of Epstein-Barr virus-positive tumours, their restricted viral protein expression. We have shown previously that apoptosis in a group I BL cell line is inhibited by interferon (IFN)-alpha. Here we show that IFN-alpha-mediated suppression of apoptosis in group I BL cells corresponds temporally with inhibition of Myc protein levels. Furthermore, inhibition of Myc expression following treatment with c-myc anti-sense oligonucleotides markedly enhanced survival of group I BL cells. These results indicate that, whilst c-myc may facilitate cycling of tumour cells in which it is de-regulated, it also stimulates their apoptosis.

Suppression of apoptosis in normal and neoplastic human B lymphocytes by CD40 ligand is independent of Bc1-2 induction

The tendency of isolated germinal center (GC) B cells to undergo apoptosis was suppressed by recombinant cell-bound CD40 ligand (CD40L): after 2 days at 37 degrees C, > 80% of cells remained viable in the presence of CD40L as compared to < 1% in control cultures. CD40L sustained a high rate of DNA synthesis in GC cells and was more effective than monoclonal antibody to CD40 in this regard. Group I Burkitt lymphoma (BL) cell lines induced to undergo apoptosis with anti-immunoglobulin or calcium ionophore were also protected by CD40L. In BL cells, this route of rescue was not accompanied by induction of Bc1-2 protein, the expression of which has been linked to hemopoietic cell survival. Bc1-2 was induced in GC cells responding to CD40L, but its appearance was a relatively late event not reaching significant levels over controls until day 2 of culture. Thus induction of Bc1-2 appears to be secondary to the survival signal imparted by CD40L. These findings are discussed in relation to a potential role for CD40L in supporting B cell tumors in vivo and the discovery that the molecular defect in the X-linked Hyper-IgM syndrome is targeted to the CD40L gene.

IL-2 expands and maintains IgM plasmablasts from a CD5+ subset contained within the germinal centre cell-enriched (surface IgD-/CD39- buoyant) fraction of human tonsil

IL-2 was found to promote the rapid growth of a minority population contained within the germinal centre (GC) cell-enriched (CD39- and/or IgD- buoyant) fraction of human tonsillar B lymphocytes. The cells emerging in response to IL-2 had a high mitotic index and morphologically resembled plasmablasts. Cultures could be maintained in the absence of feeder cells for up to 3 weeks in IL-2 and were characterized by large amounts of IgM in their supernatants: approximately 40% of the cells contained readily detectable cytoplasmic IgM by day 10 of culture. Negligible quantities of IgG and IgA were found. The target population for IL-2-driven expansion and IgM secretion was smIg+/CD38+ and was subject to suppression by anti-IgM antibody. While only 8% of cells within the GC cell-enriched fraction were CD5+ (compared with 15% of high density resting B cells), their removal led to an 83% reduction in the amount of IgM produced in response to IL-2, IL2 selectively expanded this minor CD5+ subset such that by day 6 of culture they comprised 57% of all viable cells. Cultures established with IL-2 showed increasing expression of cytoplasmic Bcl-2 and withdrawal of growth factor resulted in cell death via apoptosis. We discuss these results in relation to CD5+ B cells and their potential role in antibody responses to TD antigens.

bcl-2 modulation of apoptosis induced by anticancer drugs: resistance to thymidylate stress is independent of classical resistance pathways

The hypothesis was tested that expression of bcl-2 could provide protection against apoptosis induced by cytotoxic drugs via a mechanism which was different from the classical determinants of drug resistance. Sensitivity and resistance to inhibitors of thymidylate synthase (EC 2.1.1. 45) were chosen for study since these drugs have a well-defined and quantifiable locus of action with similarly well defined biochemical sequelae resulting from enzyme inhibition. Human lymphoma cells transfected with the vector alone readily underwent apoptosis after a 36-h exposure to various drugs. For example, 5-fluorodeoxyuridine (0.1 microM) induced 67% apoptosis in vector control cells 24 h after removal of the drug. In contrast, cells treated under identical conditions, but which expressed the bcl-2 protein, showed only basal levels of apoptosis (8%), with no significant fall in viability. Similar results were obtained using two quinazoline-based inhibitors of thymidylate synthase, N10-propargyl-5,8-dideazafolic acid (CB3717) and ICI M247496. Determinants of resistance to these three drugs were investigated. Analysis of the cell cycle, thymidylate synthase levels, and activity showed these to be unchanged by expression of bcl-2. Addition of the drugs brought about equivalent inhibition of proliferation in the presence or absence of bcl-2 expression. 5-Fluorodeoxyuridine treatment reduced TTP synthesis, induced strand breaks in nascent DNA, measured by alkaline elution, and increased the synthesis of thymidylate synthase; these changes preceded the onset of apoptosis and were identical in the vector controls and bcl-2 transfectants. Resistance to thymidylate stress in bcl-2-expressing cells therefore occurred by a mechanism different from those which classically define resistance to this type of cytotoxic drug.

A correlation between DNA-nuclear matrix binding and relative radiosensitivity in two human squamous cell carcinoma cell lines

Three aspects of DNA topology were examined in two human squamous cell carcinoma lines of differing radiosensitivity (SQ-9G, D0 = 1.46 Gy; and SQ-20B, D0 = 2.36 Gy). High-salt-extracted nuclei (nucleoids) were taken from gamma-irradiated cells, stained with ethidium bromide and examined by flow cytometry. After 5 Gy, nucleoids from SQ-9G cells became 30% less efficient at adopting positive DNA supercoils than were unirradiated controls. In contrast, only a 4% difference was found with the radioresistant SQ-20B line. Both lines produced positive supercoils more efficiently after irradiation if first exposed to the topoisomerase II inhibitor VP16. Ethidium bromide titration of nucleoids was consistent with each containing similar numbers and sizes of DNA loops. In each line approximately 30-35% of DNA was accessible to trioxsalen, as shown by inter-strand crosslinking after UV photo-activation. Exhaustive digestion of nuclear DNA by DNase I removed more DNA from the radiosensitive than from the radioresistant cell line (12% vs 28% remaining). This difference was thought to be due to the increased accessibility of SQ-9G DNA in vitro. We suggest that a looser association of SQ-9G DNA with the nuclear matrix both promotes DNase I digestion and affects the ability of SQ-9G nucleoids to maintain positive DNA supercoils after irradiation. These data implicate the DNA matrix attachment region in the expression of radiation sensitivity in the cell lines studied.

Second-messenger pathways involved in the regulation of survival in germinal-centre B cells and in Burkitt lymphoma lines

Spontaneous apoptosis in germinal-centre (GC) B cells can be prevented by treatment with anti-immunoglobulin (Ig). By contrast, susceptible group-I Burkitt lymphoma (BL) cells can be driven to apoptosis by anti-Ig. The second-messenger pathways involved in the regulation of apoptosis in GC B lymphocytes and in BL cell lines were studied using pharmacological agonists or inhibitors of intracellular calcium ([Ca2+]i) and protein kinase C (PKC). Anti-Ig was found to mobilize Ca2+ in group-I cells. Pre-incubation with the Ca2+ chelator EGTA partially reduced apoptosis induced by anti-Ig or by Ca2+ ionophore in group-I BL cells. Activation of PKC with phorbol ester reduced such Ca(2+)-driven programmed cell death (PCD) to control levels of apoptosis. Apoptosis in group-I BL cell lines could also be triggered by the kinase inhibitors staurosporine and Ro-31-8220 at concentrations selective for PKC activity. Expression of the bcl-2 protein in BL group-I cells following gene transfer affords protection from apoptosis induced by ionomycin or anti-Ig. In the present study, bcl-2 was additionally found to protect from apoptosis driven by staurosporine. The high levels of spontaneous apoptosis exhibited by normal GC B cells were reduced, but not abrogated, by co-culture with phorbol ester. These results indicate that, in group-I BL cells, imbalance in the phosphoinositide pathway of signalling, in favour of [Ca2+]i and away from PKC, results in apoptosis: constitutive phosphorylation of key proteins by PKC may therefore suppress apoptosis in BL as well as in GC B cells.

Prevention of programmed cell death in Burkitt lymphoma cell lines by bcl-2-dependent and -independent mechanisms

Burkitt lymphoma (BL) cell lines which retain the phenotypic characteristics of the freshly-isolated tumour cells (group I cells) readily enter programmed cell death (apoptosis) in response to a variety of triggers. By contrast, isogenic BL cells which are phenotypically altered as a result of activation of their resident EBV genome (group-III cells) are highly protected from apoptosis. Phenotypic changes in group-III cells include the up-regulation of the oncogene, bcl-2. Expression of the 26-kDa bcl-2 protein in group-I BL cells following gene transfer was found to afford protection from apoptosis: the degree of protection was proportional to the amount of bcl-2 protein expressed. When group-I bcl-2 transfectants were compared with their group-III counterparts it was found that, whilst bcl-2 made a significant contribution in protecting from entry into apoptosis, hyper-expression of bcl-2 protein in group-I cells (well beyond that of group-III cells) was necessary to attain the high levels of protection observed in group-III cells. These results suggested that additional, bcl-2-independent, survival mechanisms could operate in BL cells. In support of this notion it was also found that: (1) prolonged culture of group-I lines in vitro resulted in enhanced survival in the absence of bcl-2 up-regulation, and (2) exposure of group-I cells to interferon-alpha triggered a bcl-2-independent protective response. The molecular mechanisms of both the bcl-2-dependent and -independent survival pathways remain to be determined.

Analysis and discrimination of necrosis and apoptosis (programmed cell death) by multiparameter flow cytometry

Necrosis and apoptosis are two distinct modes of cell death which differ in morphology, mechanism and incidence. Membrane disruptants, respiratory poisons and hypoxia cause ATP depletion, metabolic collapse, cell swelling and rupture leading to inflammation. These are typical features of necrosis. Apoptosis plays a crucial role in embryogenesis and development and is also prevalent in tumours. It is characterised by cell shrinkage, chromatin condensation and systematic DNA cleavage. Apoptotic cells are rapidly engulfed by phagocytes, thus preventing inflammatory reaction to degradative cell contents. In vivo, apoptosis is almost impossible to quantify due to problems of heterogeneity and the short half-life of an apoptotic cell. In vitro, mechanistic studies are further complicated by a late phase of apoptosis where the cell membrane becomes permeable to vital dyes and which occurs in the absence of phagocytes. Here we describe a novel and rapid multiparameter flow cytometric assay which discriminates and quantifies viable, apoptotic and necrotic cells via measurement of forward and side light scatter (proportional to cell diameter and internal granularity, respectively) and the DNA-binding fluorophores Hoechst 33342 and propidium. It is anticipated that mechanistic studies of apoptosis in a variety of cell types will greatly benefit from this mode of analysis.

Cellular radiosensitivity in V79 cells is linked to alterations in chromatin structure

V79 cells grown as spheroids are more radioresistant than those grown as monolayers. Viable cells from spheroid culture contain restraints to ethidium bromide driven rewinding of DNA supercoils that are absent in monolayer cells. Spheroid cells also contain a DNA-protein matrix that is more resistant to detergent-induced degradation. The increase in structural integrity may be related to a 55-60 kD protein in the nucleoids of spheroid, but not monolayer cells. Spheroid cell radioresistance may therefore be related to a more stable chromatin platform for high fidelity repair of DNA damage.

The increase in radioresistance of Chinese hamster cells cultured as spheroids is correlated to changes in nuclear morphology

Chinese hamster V79 cells grown as spheroids in roller culture are more radioresistant than those grown as monolayers. The supercoiled structure of chromatin, as salt-extracted nucleoids, has been examined using flow cytometry. Irradiated viable cells from spheroid culture contain restraints to supercoil relaxation that are absent in monolayer cells. Further analysis of the chromatin organization from each growth form shows that the radioresistant spheroid cells contain a DNA-protein matrix that is more resistant to detergent-induced degradation. The increase in structural integrity may be due to the retention of a 55-60 kDa protein that is apparent in the nucleoids of spheroid, but not monolayer cells. The increase in structural integrity of the spheroid cells may explain their greater radioresistance by providing a more stable platform for high-fidelity DNA damage repair.

Conformational changes in chromatin structure induced by the radioprotective aminothiol, WR 1065

WR 1065, 2-[(minopropyl) amino] ethanethiol is an effective scavenger of free radicals. When present during irradiation it reduces cellular DNA damage as analysed by alkaline elution from filters. The same technique indicates that without irradiation, WR 1065 has no effect of DNA integrity. Using nucleoid analysis, where DNA damage is detected at the level of replicon clusters, WR 1065 distorts replicon supercoiling without breaking the DNA molecule. This confirmational change in nucleoid structure occurs with no detectable change in nucleoid protein content. It is proposed that perturbation of replicon supercoiling affects the process of normal DNA synthesis and strand break rejoining, allowing a longer time for the accurate repair of DNA damage.

The use of flow cytometry in the detection of radiation- and chemically induced nuclear damage in human mononuclear cells

Viable human mononuclear cells were treated with three known genotoxic agents, gamma radiation, bleomycin sulphate (BLM) and acetylaminofluorene (AAF). Following treatment, a high salt concentration buffer was used to disrupt the cells, releasing supercoiled DNA structures known as nucleoids. The nucleoids were treated with the fluorescent dye ethidium bromide, and exposed to a beam of laser light within a fluorescence activated cell sorter (FACS). Treatment of human mononuclear cells with the genotoxic agent BLM or with radiation prior to cell disruption and staining produced a dose-dependent increase in light scatter. However, treatment of human mononuclear cells with AAF, which requires metabolic activation, produced a dose response in light scatter that proved reversible and was not dependent on the formation of reactive metabolites. In the case of BLM and radiation, these changes in light scatter are interpreted as structural alterations in the nucleus partially induced by free radical attack, and thus provide an indication of the DNA damaging ability of the agent used. In contrast the reversible changes in light scatter seen with AAF may only indicate the ability of this agent to penetrate the genome. The significance of this reversible interaction with respect to genotoxicity is not known.

Structural damage to lymphocyte nuclei by H2O2 or gamma irradiation is dependent on the mechanism of OH. radical production

Normal human lymphocytes were exposed to OH. radicals produced indirectly by exposure to H2O2 or directly by gamma irradiation. Using a flow cytometry technique to measure changes in nucleoid size, it was found that generation of OH. in each system produced a characteristic relaxation in nuclear supercoiling. Exposure of cells to H2O2 produced a metal-dependent step-wise relaxation in extracted nucleoids, while gamma irradiation induced a gradual dose-dependent increase in nucleoid size. The site-specific metal-dependent changes produced in lymphocytes incubated in H2O2 should also occur in gamma irradiated cells, but the characteristic effects on nuclear supercoiling would not be detected within the background of random DNA damage. The importance of metals in maintaining the supercoiled loop configuration of DNA within the protein matrix suggests that free radical damage at metal locations may be particularly toxic for the cell.

Measurement of DNA damage in mammalian cells using flow cytometry

A technique for the detection of DNA damage induced by radiation insult has been developed. Cells were lysed with a buffer containing 2 M sodium chloride to release the DNA in a supercoiled form, the nucleoid. These were stained with the DNA intercalating dye, ethidium bromide, and exposed to laser light within a flow cytometer. Scattered and fluorescent light was analyzed from the laser/nucleoid interaction following irradiation of viable cells with gamma rays. The addition of ethidium bromide to prepared nucleoids caused a reduction in scattered light due to condensation of the nucleoid. Irradiation of cells prior to nucleoid production and ethidium bromide treatment restricted this condensation and produced a dose-dependent increase in laser scatter. Nucleoids derived from human lymphocytes showed enhanced light scatter from 5 Gy, compared to Chinese hamster ovary (CHO) fibroblasts where doses above 10 Gy were required. Up to 30 Gy CHO nucleoids showed a dose-dependent reduction in the ethidium bromide fluorescence. This technique allows detection of altered light scattering and fluorescent behavior of nucleoids after cellular irradiation; these may be related to structural changes within the nucleus induced by the radiation. The use of flow cytometry compared to other methods allows a rapid analysis of nuclear damage within individual cells.

Control of human B-lymphocyte replication. I. Characterization of novel activation states that precede the entry of G0 B cells into cycle

Tonsillar B lymphocytes of a particularly high buoyant density were prepared essentially free of contaminating monocytes and T cells. When exposed to anti-immunoglobulin, such cells initiated the hydrolysis of inositol phospholipids. This provides a postulated 'dual signal' for growth through the liberation of intracellular calcium stores and the activation of protein kinase C. Nevertheless, neither anti-immunoglobulin nor direct agonists of this bifurcating pathway (respectively, calcium ionophore and the phorbol ester TPA) were capable, when used alone, of driving cells out of G0 and into RNA synthesis. All three agents did, however, induce two activation antigens at the surface of G0 B cells, which included CD23, p45 and a lineage-unrestricted antigen identified by the monoclonal antibody BK.19.9. Cells that had been exposed to calcium ionophore, but not those activated with either TPA or anti-immunoglobulin, revealed further change indicated by an increased accessibility of their native DNA for the intercalating dye acridine orange. Cells receiving full mitogenic signals in the form of Staphylococcus aureus Cowan Strain I (SAC) or a combination of TPA and calcium ionophore showed the same initial sequelae but continued to enter the cell cycle and progress through to DNA synthesis. The observations identify two phases in the early activation of human B cells, both in terms of various temporal events, and the signals required to promote each activation state. before entering the proliferative cycle. Thus, the exit of human B cells from G0 appears subject to multiple controls that precede those associated with G1 and later phases of the cell cycle.

Detection of antithrombin III microheterogeneity

Antithrombin III microheterogeneity was investigated by isoelectric focusing and immunofixation in healthy individuals and in patients with clinical conditions in which antithrombin III is known to vary (liver disease, nephrotic syndrome, after surgery and anticoagulant therapy). In normal plasma microheterogeneity was present with ten bands of varying intensity being visible in a pI range from 5.0-5.7. One variant was observed which was not associated with a clinical disorder. Low concentrations of antithrombin III were detected in some patients with liver disease, nephrotic syndrome and those on anticoagulant therapy and these demonstrated a decrease of intensity in all bands. Alterations in microheterogeneity were seen in patients tested after surgery and those with nephrotic syndrome. This indicates that changes in the subpopulations of antithrombin III can occur and may be relevant to clinical abnormalities.