Fluorescence characterization of structural transitions at the strong actin binding motif in skeletal myosin affinity labeled at cysteine 540 with novel spectroscopic cysteaminyl mixed disulfides

We have synthesized the luminescent and fluorescent lanthanide chelate S-(2-nitro-5-thiobenzoic acid)cysteaminyldiethylenetriaminepentaacetate-5-[(2-aminoethyl)am ino ]naphthalene-1-sulfonic acid as well as the fluorescent analogue S-(2-nitro-5-thiobenzoic acid)cysteaminyl-5-carboxyfluorescein using the procedure we recently described [Bertrand, R., Capony, J.-P., Derancourt, J., and Kassab, R. (1999) Biochemistry 38, 11914-11925]. Both mixed disulfides react with the skeletal myosin motor domain (S-1) as actin site-directed agents and label exclusively and stoichiometrically Cys 540 in the hydrophobic strong actin binding helix-loop-helix motif, causing only a 1.9-2.4-fold decrease in the V(max) for acto-S-1 ATPase. The covalently attached cysteaminyl probe side chain spans maximally 17 and 8 A, respectively, and the fluorophores have different polarity, volume, and flexibility. Thus, they may provide complementary spectroscopic information on the environmental properties of this critical actin binding region. Here, we have analyzed by extrinsic fluorescence spectroscopy S-1 derivatized with the fluorescein label or with the Tb(3+) or Eu(3+) chelate of the other label to assess the conformational transitions precisely occurring at this site upon interaction with F-actin, nucleotides, or phosphate analogues. For either label, specific spectral changes of significant amplitude were obtained, identifying at least two major structural states. One was mediated by rigor binding of F-actin in the absence or presence of MgADP. It was abolished by MgATP, and it was not produced by the binding of nonpolymerizable G-actin. A modeling of the corresponding changes in the intensity and lambda(max) of the fluorescence emission spectra, achieved using the fluorescent adducts of 2-mercaptoethanol in varying concentrations of dimethylformamide, illustrates the predicted apolar nature of the strong acto-S-1 interface. A second state was promoted by the binding of ATP, AMP-PNP, ADP.AlF4, ADP. BeFx, or PP(i). It should be prevalent in the weak acto-S-1 binding complexes. The accompanying fluorescence intensity reduction, observed with each label, in both the absence and presence of F-actin, would result from a specific modification by these ligands of the probe orientation and/or solvent accessibility as suggested by acrylamide quenching experiments. It could represent the spectral manifestation of the predicted allosteric linkage from the ATPase site to the strong actin binding site of S-1 that modulates the acto-S-1 affinity. Our study offers the basis necessary for further detailed spectroscopic investigations on the conformational dynamics in solution of the stereospecific and hydrophobic actin binding motif during the skeletal cross-bridge cycle.

Characterization of Bax-sigma, a cell death-inducing isoform of Bax

The Ced-9/Bcl-like family of genes codes for proteins that have antiapoptotic and proapoptotic activity. Several Bax isoproteins have been detected by 2-D gel electrophoresis, and a novel human member, designated as Bax-sigma, has been identified and cloned from human cancer promyelocytic cells. Bax-sigma contains BH-3, BH-1, and BH-2 domains, putative alpha-5 and alpha-6 helices, and the carboxy-terminal hydrophobic transmembrane domain but lacks amino acids 159 to 171 compared to Bax-alpha. mRNA expression analysis by reverse transcription-polymerase chain reaction and RNase protection assays have revealed that Bax-sigma is expressed in a variety of human cancer cell lines and normal tissues. To investigate the potential role of Bax-sigma in apoptosis, first its effects were compared to those of Bax-alpha by transient expression in human B lymphoma Namalwa cells. Both Bax-sigma and Bax-alpha promoted apoptosis, as detected by DNA fragmentation and morphological analysis by electron microscopy. The apoptosis induced by Bax-sigma and Bax-alpha was correlated with their expression, cytochrome c release, and caspase activation. In a yeast two-hybrid system, Bax-sigma interacted with several Ced-9/Bcl family members but had no affinity for the human Egl-1 homologs Bik and Bad and the Ced-4 homolog Apaf-1. In human cells, Bax-sigma function was counteracted by Bcl-xL overexpression, and co-immunoprecipitation experiments indicated that Bax-sigma was associated with Bcl-xL. Furthermore, Bax-sigma overexpression increased cell death induced by various concentrations of genotoxic agents with the most pronounced effect occurring at low camptothecin and vinblastine dose levels. Our results suggest that Bax-sigma, a novel variant of Bax, encodes a protein with a proapoptotic effect and mode of action similar to those of Bax-alpha.

Detection of nucleotide- and F-actin-induced movements in the switch II helix of the skeletal myosin using its differential oxidative cleavage mediated by an iron-EDTA complex disulfide-linked to the strong actin binding site

We have synthesized the mixed disulfide, S-(2-nitro-5-thiobenzoic acid) cysteaminyl-EDTA, using a rapid procedure and water-soluble chemistry. Its disulfide-thiol exchange reaction with rabbit myosin subfragment-1 (S-1), analyzed by spectrophotometry, ATPase assays, and peptide mapping, led to the incorporation of the cysteaminyl-EDTA group into only Cys 540 on the heavy chain and into the unique cysteine on the alkali light chains. The former thiol, residing in the strong actin binding site, reacted at a much faster rate with a concomitant 3-fold decrease in the V(max) for acto-S-1 ATPase but without change in the essential enzymatic functions of S-1. Upon chelation of Fe(3+) ions to the Cys 540-bound EDTA and incubation of the S-1 derivative-Fe complex with ascorbic acid at pH 7.5, the 95 kDa heavy chain underwent a conformation-dependent, single-cut oxidative fragmentation within 5-15 A of Cys 540. Three pairs of fragments were formed which, after specific fluorescent labeling and SDS-PAGE, could be positioned along the heavy chain sequence as 68 kDa-26 kDa, 62 kDa-32 kDa, and 54 kDa-40 kDa. Densitometric measurements revealed that the yield of the 54 kDa-40 kDa pair of bands, but not that for the two other pairs, was very sensitive to S-1 binding to nucleotides or phosphate analogues as well as to F-actin. In binary complexes, all the former ligands specifically lowered the yield to 40% of S-1 alone, roughly in the following order: ADP = AMP-PNP > ATP = ADP.AlF(4) > ADP.BeF(x)() > PP(i). By contrast, rigor binding to F-actin increased the yield to 130%. In the ternary acto-S-1-ADP complex, the yield was again reduced to 80%, and it fell to 25% in acto-S-1-ADP.AlF(4), the putative transition state analogue complex of the acto-S-1 ATPase. These different quantitative changes reflect distinct ligand-induced conformations of the secondary structure element whose scission generates the 54 kDa-40 kDa species. According to the S-1 crystal structure, this element could be unambiguously assigned to the switch II helix (residues 475-507) whose N-terminus lies 14.2 A from Cys 540 and would include the ligand-responsive cleavage site. This motif is thought to be crucial for the transmission of sub-nanometer structural changes at the ATPase site to both the actin site and the lever arm domain during energy transduction. Our study illustrates this novel, actin site-specific chemical proteolysis of S-1 as a direct probe of the switch II helix conformational transitions in solution most likely associated with the skeletal cross-bridge cycle.

Caspase inhibition in camptothecin-treated U-937 cells is coupled with a shift from apoptosis to transient G1 arrest followed by necrotic cell death

Leukemia U-937 cells rapidly undergo characteristic morphological changes, caspase activation, and DNA fragmentation typical of apoptosis on treatment with the DNA topoisomerase I inhibitor camptothecin (CPT). In a previous report (Sané, A. T., and Bertrand, R., Cancer Res., 58: 3066-3072, 1998), we showed that, after CPT treatment, caspase inhibition by the tripeptide derivative benzyloxycarbonyl-Val-Ala-Asp(Ome)-fluoromethyl ketone (zVAD-fmk) blocked apoptosis and slowed passage of the cells through S-G2 and caused a transient accumulation of these cells at the G1 phase of the cell cycle. Accumulation of these cells at G1 is not associated with major changes in expression level of cyclin-dependent kinase (cdk)2, cdk4, and cdk6; cyclin D1 and cyclin E; or p16, p21, p27, and p57 after CPT treatment. Furthermore, cdk2, cdk4, and cdk6 kinase activities remain unaffected after CPT treatment. These results indicate that the G1 arrest of these cells does not correlate with a classical driven cell cycle checkpoint but with the known effect of CPT in mediating inhibition of DNA replication and RNA transcription after stabilization of topoisomerase I-linked DNA strand breaks. However, persistent caspase inhibition after CPT treatment also results in cells falling into necrosis after the transient G1 arrest. These results indicate that the enforced inhibition of caspase activities does not confer a survival advantage upon CPT-treated cells but is coupled with a shift from apoptosis to transient G1 arrest followed by massive necrosis.

Distinct steps in DNA fragmentation pathway during camptothecin-induced apoptosis involved caspase-, benzyloxycarbonyl- and N-tosyl-L-phenylalanylchloromethyl ketone-sensitive activities

Monocytic-like leukemia U-937 cells rapidly undergo morphological changes and DNA fragmentation that is typical of apoptosis following treatment with DNA topoisomerase I inhibitor [20-S-camptothecin lactone (CPT)]. The tripeptide derivative benzyloxycarbonyl-Val-Ala-Asp(OMe)fluoromethyl ketone blocks Asp-Glu-Val-Asp-ase (DEVDase) activity and prevents the occurrence of high molecular weight and oligonucleosome-sized DNA fragments associated with apoptosis in CPT-treated cells. In contrast, N-tosyl-L-phenylalanylchloromethyl ketone (TPCK) does not prevent DEVDase activity and high molecular weight DNA fragmentation but completely abrogates the appearance of oligonucleosome-sized DNA fragmentation. These results suggest that caspase 3-like activities are involved with high molecular weight DNA fragmentation pathway, whereas TPCK-sensitive activities are involved in oligonucleosome-sized DNA fragmentation pathway in CPT-treated cells. Electron micrographs reveal that caspase inhibition by benzyloxycarbonyl-Val-Ala-Asp(OMe)fluoromethyl ketone also abrogates the typical morphological changes associated with apoptosis, whereas TPCK does not delay these morphological changes that are typical of apoptosis. Caspase inhibition slows passage of the cells through G2 and causes a transient accumulation of these cells at the G0/G1 phase of the cell cycle following CPT treatment. In a cell-free system, when purified nuclei are incubated with apoptotic cytosolic extracts obtained from CPT-treated U-937 cells, TPCK causes a similar effect in abrogating the oligonucleosome-sized DNA fragmentation but does not affect DEVDase activity. Addition of either benzyloxycarbonyl-Val-Ala-Asp-free carboxyl group or acetyl-Asp-Glu-Val-Asp-aldehyde completely inhibits DEVDase activity in these extracts. However, acetyl-Asp-Glu-Val-Asp-aldehyde does not affect the occurrence of oligonucleosome-sized DNA fragmentation in the cell-free system, whereas the benzyloxycarbonyl derivatives benzyloxycarbonyl-Val-Ala-Asp-free carboxyl group, benzyloxycarbonyl-Val-Ala-free hydroxyl group, benzyloxycarbonyl-Val-free hydroxyl group, and benzyloxycarbonyl hydrazide abolish it markedly. Taken together, these observations show the pivotal role of DEVDase activity in triggering the apoptotic process and high molecular weight DNA fragmentation, whereas TPCK- and benzyloxycarbonyl-sensitive activities are involved in the oligonucleosome-sized DNA fragmentation pathway induced by CPT.

Interaction of caldesmon with actin subdomain-2

The polymerization-resistant maleimidobenzoyl-G-actin (MBS-G-actin), which behaves as a functional analogue of native G-actin [Bettache, N., Bertrand, R. & Kassab, R. (1989) Proc. Natl Acad. Sci. USA 86, 6028-6032; Bettache, N., Bertrand, R. & Kassab, R. (1990) Biochemistry 29, 9085-9091) has been employed to probe the solution interaction between monomeric actin and smooth muscle caldesmon, using fluorescence measurements, limited proteolysis and covalent cross-linking reactions. MBS-G-actin associates, without polymerization, to turkey gizzard caldesmon, at about 50 mM ionic strength and 25 degrees C, with a high affinity (Kd approximately 0.04 microM) and with a 1:1 stoichiometry. However, the binding strength of the complex including caldesmon and MBS-G-actin cleaved at the subdomain-2 loop with subtilisin decreased fivefold (Kd approximately 0.20 microM). Conversely, caldesmon strongly protected subdomain-2 of MBS-G-actin from tryptic digestion at the susceptible peptide bond at positions 68-69. Furthermore, caldesmon induced the dissociation of native G-actin from its complex with DNase I, as assessed by cosedimentation assays, and increasing concentrations of the latter protein inhibited the MBS-G-actin-caldesmon interaction, suggesting mutual exclusion binding of caldesmon and DNase I to monomeric actin. MBS-G-actin was specifically coupled, via a maleimidobenzoyl group incorporated into its subdomain-2, to caldesmon, producing in high yield a 205-kDa covalent complex consisting of one actin monomer joined to Cys 580 of caldesmon. A similar conjugation process was observed with the complex of caldesmon and polymerized MBS-F-actin. MBS-G-actin could be also cross-linked to caldesmon by 1-ethyl-3[3-(dimethylamino)propyl]carbodiimide, producing a three-band pattern identical to that of F-actin and caldesmon and previously shown to reflect the covalent union between the NH2-terminal segment of actin and the COOH-terminal actin-binding domain of caldesmon. The overall data point to a direct interaction of the latter region with actin subdomain-2 and suggest that during its binding to monomeric or filamentous actin, the caldesmon functional domain spans the entire length of a single actin and closely contacts the bottom of its subdomain-1 as well as the top portion of its subdomain-2.

Bax-alpha promotes apoptosis induced by cancer chemotherapy and accelerates the activation of caspase 3-like cysteine proteases in p53 double mutant B lymphoma Namalwa cells

Stable transfected human p53 (mt/mt) B lymphoma Namalwa variant lines showing differential expression of the Bax-alpha protein were derived under hygromycin selection. Overexpression of Bax-alpha in these variant cells accelerates cell death induced by short or continuous treatments with various concentrations of camptothecin, etoposide, vinblastine and shows no accelerating cell death activity in cis-platinum and paclitaxel-treated cells. Activation of apoptosis and oligonucleosome-sized DNA fragmentation was observed in the variant lines with more pronounced effect in cells containing high level of Bax-alpha protein. These results suggest that increased cell death mediated by anticancer drugs correlates with Bax-alpha level of expression and that Bax-alpha sensitizes Namalwa cells treated at low drug concentrations. The extent of DNA synthesis inhibition following DNA topoisomerase inhibitor treatments was similar in control and all transfected Namalwa cells suggesting that Bax-alpha acts downstream of DNA topoisomerase-mediated DNA strand breaks. To define further the relation between Bax-alpha expression and apoptosis activation, kinetics of caspase activation was measured in drug-treated cells. Caspase activities were measured using specific fluorogenic peptide derivatives DABCYL-YVADAPV-EDANS and Ac-DEVD-AMC, substrates of the caspase 1-like and caspase 3-like families, respectively. In control and Bax-alpha transfected Namalwa cells no increase in caspase 1-like activity was detected following camptothecin and etoposide treatments. In contrast, a significant difference in Ac-DEVD-AMC hydrolysis activity was observed in Bax-alpha transfected Namalwa cells compared to that of control Namalwa cells after camptothecin and etoposide treatment. Increased caspase 3-like activity correlated also with poly(ADPribosyl) polymerase cleavage. Taken together, these results suggest that Bax-alpha sensitize B lymphoma cells to series of anticancer drugs and accelerates the activation of apoptotic protease cascade.

Bcl-xL modulates apoptosis induced by anticancer drugs and delays DEVDase and DNA fragmentation-promoting activities

Using an episomal eucaryotic expression vector, we derived three stable transfected human leukemic U-937 variant lines showing differential expression of the Bcl-xL protein. Preventive effect of Bcl-xL on cell death induced by various concentrations of camptothecin (DNA topoisomerase I inhibitor; CPT) was observed in the three lines with most pronounced effect in cells containing the highest level of Bcl-xL expression. These results show that increased cell death protection by Bcl-xL is correlated with its level of expression. The extent of DNA strand break formation and DNA synthesis inhibition following CPT treatments was similar in control and transfected U-937 cells, suggesting that Bcl-xL acts downstream of CPT-DNA topoisomerase I-mediated DNA strand breaks. Modulation of cell death by Bcl-xL was also observed in cells treated with etoposide, vinblastine, paclitaxel, and cisplatinum (II) diammine dichloride. To define whether Bcl-xL functions downstream or upstream of apoptogenic proteolytic cascade activation, we compared kinetics of DNA fragmentation in treated cells with kinetics of caspase 1-like, caspase 3-like, and N-tosyl-L-phenylalanylchloromethyl ketone (TPCK)-sensitive activities. In CPT-treated U-937 cells, caspase 3-like and TPCK-sensitive activities promoting DNA fragmentation in a cell-free system were detected much more rapidly in extracts obtained from CPT-treated U-937 cells compared to those obtained from CPT-treated U-937-Bcl-xL variant cells. These results suggest that Bcl-xL delays their activation that correlates with the occurrence of DNA fragmentation. Addition of recombinant Bcl-xL in extracts containing DEVDase and TPCK-sensitive activities did not inhibit these activities, suggesting that Bcl-xL acts primarily upstream of their activation in the apoptotic process. Taken together, these results suggest that Bcl-xL is a primary checkpoint that can block or delay transmission of cell death signals emerging from DNA damage and prevents activation of an apoptogenic proteolytic cascade.

The Bcl-xL and Bax-alpha control points: modulation of apoptosis induced by cancer chemotherapy and relation to TPCK-sensitive protease and caspase activation

Defective control of apoptosis appears to play a central role in the pathogenesis of human diseases including neoplasic, autoimmune, and neurodegenerative diseases. Conversely, cancer chemotherapy and ionizing radiation can induce cancer cell death by apoptosis, and deregulated apoptosis following cancer chemotherapy could define a new category of drug resistance mechanism. By understanding the role that some major regulators of apoptosis play either at the commitment or execution phases of cell death in a given tissue and pathology, we will be in a better position to design and explore new therapeutic modalities. The Ced-9 - Bcl-like and Ced-3 - Ice-like gene family products are intrinsic proteins regulating the decision of a cell to survive or die and executing part of the cell death process itself, respectively. Among the various Bcl-like proteins, the effects and functions of the Bcl-x and Bax proteins in controlling apoptosis induced by cancer chemotherapy have been studied recently. In human cancer variant cell lines showing differential expression of the Bcl-xL protein, a preventive effect of Bcl-xL on cell death induced by various cytotoxic drugs is observed, with greater effects in cells containing the highest level of Bcl-xL expression. Similarly, overexpression of Bax-alpha in cancer cell lines sensitizes these cells to some cancer chemotherapy compounds. Modulation of apoptosis either negatively by Bcl-xL or positively by Bax-alpha resides downstream of the primary mechanism of action of anticancer drugs, suggesting that they act primarily as intrinsic control points following cytotoxic drug injuries. An emerging family of Ced-3 - Ice like cysteine proteases (caspases) has been also identified and several studies have revealed their importance in executing the process of cell death. More recently, activation of a N-tosyl-L-phenylalanylchloromethyl ketone (TPCK)-sensitive pathway was also suggested to play an important role in apoptosis induction following cancer chemotherapy. Evidence obtained using a combination of assays including cell-free systems and enzyme activity assays now suggests that Bcl-xL and Bax-alpha control points function upstream of TPCK-sensitive protease and caspase activation. Bcl-xL delays and prevents activation of apoptotic protease cascades whereas Bax-alpha shows the opposite effect, accelerating their activation.

The CrmA- and TPCK-sensitive pathways that trigger oligonucleosome-sized DNA fragmentation in camptothecin-induced apoptosis: relation to caspase activation and high molecular weight DNA fragmentation

Abstract: In human B lymphoma Namalwa variant cells expressing the serpin-like CrmA protein, the kinetics of oligonucleosome-sized DNA fragmentation was retarded compared with that of control Namalwa cells following camptothecin treatment. However, no difference in the kinetics of high molecular weight DNA fragmentation was observed between the two lines after camptothecin treatment. Similar delay and inhibition of the oligonucleosome-sized DNA fragmentation was observed in human B lymphoma Namalwa and monocytic-like leukemia U-937 cells coincubated in the presence of various concentrations of N-tosyl-L-phenylalanyl chloromethylketone and camptothecin. The effect of N-tosyl-L-phenylalanyl chloromethylketone was similar to that of CrmA and did not prevent the appearance of high molecular weight DNA fragments. Similar suppression of camptothecin-induced internucleosomal DNA fragmentation was also observed in a cell-free system when cytosolic extracts obtained from camptothecin-treated Namalwa and U-937 cells were coincubated with untreated nuclei in the presence of N-tosyl-L-phenylalanyl chloromethylketone. Furthermore, N-tosyl-L-phenylalanyl chloromethylketone had no significant effects on caspase-3-like activities in camptothecin-treated Namalwa and U-937 cells. Hydrolysis of Ac-Asp-Glu-Val-Asp-amino-4-methylcoumarin, a fluorogenic substrate with caspase-3-like activities, was detected in extracts prepared from camptothecin-treated Namalwa and U-937 cells with no apparent difference in the time courses of caspase-3-like activation in the absence or presence of N-tosyl-L-phenylalanyl chloromethylketone. Similarly, N-tosyl-L-phenylalanyl chloromethylketone was a weak inhibitor of caspase-3-like activities in vitro. Taken together, these observations suggest that the pathway sensitive to N-tosyl-L-phenylalanyl chloromethylketone is involved in camptothecin-induced oligonucleosome-sized DNA fragmentation. Furthermore, inhibition of this pathway had no effect on caspase-3-like activation and on the occurrence of high molecular weight DNA fragmentation.

Probing the hydrophobic interactions in the skeletal actomyosin subfragment 1 and its nucleotide complexes by zero-length cross-linking with a nickel-peptide chelate

The complex of Ni(II) and the tripeptide Gly-Gly-His catalyzes, in the presence of monoperoxyphthalic acid, a zero-length protein-protein cross-linking via an oxidative radical pathway involving mainly aromatic amino acids and not at all nucleophilic residues [Brown, K. C., Yang, S.-H., and Kodadek, T. (1995) Biochemistry 34, 4733-4739]. We have taken advantage of this unprecedented cross-linking system to directly and selectively probe the solution structure and functioning of the hydrophobic interface between F-actin and skeletal myosin subfragment 1 (S-1) at the level of its aromatic components, in the absence and in the presence of nucleotides (ATP and ADP) or nucleotide analogs (AMPPNP, PPi, and ADP. AlF4). Following verification of the structure of the Ni(II)-peptide chelate and of its oxidized active form by electrospray mass spectrometry, complexes of F-actin and S-1 or proteolytic S-1 derivatives and complexes of S-1 and proteolytic F-actin derivatives were readily cross-linked under various controlled conditions without apparent alteration of the acto-S-1 recognition. The covalent adducts were identified on electrophoretic gels using specific protein labeling with the oxidation-resistant fluorophor, monobromobimane, combined with immunochemical staining. Two types of actin-heavy chain conjugates were produced. One, with a mass of 180 kDa, was formed in the rigor state or with ADP bound; the other one, with a mass of 200 kDa, was generated from the ternary complexes comprising a gamma-P-containing ligand. They were accumulated with an efficiency of 8 and 6%, respectively. For each reversible complex, the 180 kDa:200 kDa band ratio was essentially as predicted from the nucleotide-dependent A to R equilibrium mechanism of the acto-S-1 interaction in solution [Geeves, A. M., and Conibear, P. B. (1995) Biosphys. J. 68, 194s-201s]. Both covalent species resulted from the cross-linking of an actin monomer to the central 50 kDa segment, and their distinct mobilities reflect gamma-P-mediated structural changes at or near the actin-50 kDa fragment interface. Peptide mapping showed the cross-linking to take place between the 506-561 S-1 segment and the 48-113 actin stretch. The localization of these regions in the atomic F-actin-S-1 model implies that nucleotide-modulated close contacts, involving aromatic residues, are operating between the C-terminal helix of the hydrophobic strong actin-binding motif of S-1 bound to the primary actin monomer and the top portion of the adjacent lower actin subunit. The specificity of the nickel-peptide cross-linking, as assessed with the acto-S-1 complex, makes it a candidate for potential general use in investigations of the hydrophobic interactions within other protein motor-based assemblies.

Cholera toxin triggers apoptosis in human lung cancer cell lines

Cholera toxin (ChT) inhibits signals generated by multiple growth factors in human lung cancer cells, resulting in cell growth inhibition. We now report that ChT triggers apoptosis as shown by DNA fragmentation and activation of caspases cleaving poly(ADP-ribose) polymerase and lamin B. Apoptosis induced by ChT in a small cell lung cancer cell line is not affected by manipulations of intracellular cAMP through preincubation with isobutylmethylxanthine but can be modestly increased through inhibition of protein kinase C with chelerythrine. Thus, apoptosis is actively suppressed in lung cancer cells by a ChT-sensitive-growth regulatory pathway, and these observations may have significant implications in the development of novel strategies for lung cancer treatment.

Decreased Fas antigen receptor expression in testicular tumor cell lines derived from polyomavirus large T-antigen transgenic mice

MT-PVLT-10 transgenic mice express large T-antigen of polyomavirus under the control of the mouse metallothionein-1 promoter and mates of this transgenic line develop testicular tumors at advanced ages. The differential display technique was employed to compare mRNA expression from immortalized cell lines derived from normal or adenomatous testis from MT-PVLT-10 transgenic males. Using this technique, a complementary DNA fragment corresponding to the mouse Fas antigen receptor was recovered from normal testicular cells but not from tumor cells. RNAse protection assays with the Fas antigen specific fragment confirmed its differential expression. Normal testicular cells from the transgenic animals responded to treatment of interferon-gamma by increasing the expression of Fas antigen specific mRNA and were sensitive to the proliferative inhibitory effect of anti-Fas antibody in vitro. This proliferative inhibition was characterized by an accumulation of cells in S phase of the cell cycle. In contrast, the testicular tumor cells did not respond to either interferon-gamma or to anti-Fas antibody in vitro. These results suggest that the toss of proliferative inhibitory effect mediated by the Fas antigen pathway in tumor cells may be an important step in testicular tumor progression in the MT-PVLT-10 transgenic mice.

[Apoptosis of human leukemic cells induced by topoisomerase I and II inhibitors]

Comparison between five human leukemic lines (BV173, HL60, U937, K562, KCL22) suggest that the main determinant of their sensitivity to topoisomerase I (camptothecin) and II (VP-16) inhibitors is their ability to regulate cell cycle progression in response to specific DNA damage, then to die through apoptosis: the more the cells inhibit cell cycle progression, the less sensitive they are. The final pathway of apoptosis induction involves a cytoplasmic signal, active at neutral pH, needing magnesium, sensitive to various protease inhibitors and activated directly by staurosporine. Modulators of intracellular signaling (calcium chelators, calmodulin inhibitors, PKC modulators, kinase and phosphatase inhibitors) have no significant influence upon apoptosis induction. Conversely, apoptosis induction pathway is modified during monocytic differentiation of HL60 cells induced by phorbol esters. Lastly, poly(ADP-ribosyl)ation and chromatine structure should regulate apoptotic DNA fragmentation that is prevented by 3-aminobenzamide and spermine, respectively.

Biochemical and molecular studies of human methenyltetrahydrofolate synthetase

5-FormylH4folate is administered clinically under the name Leucovorin in association with the antineoplastic agent 5-fluorouracil (5-FU) to enhance the cytotoxic effects of 5-FU. The combination has been shown to be superior to 5-FU alone in the treatment of patients with metastatic colorectal carcinoma. Methenyltetrahydrofolate synthetase (MTHFS) catalyzes the transformation of 5-formyl-tetrahydrofolate to methenylH4folate, which is the obligatory initial metabolic step prior to the intracellular conversion of 5-formylH4folate to other reduced folates and the increase in intracellular folate pools required for 5-FU potentiation. In the following paper, we will summarize results of biochemical and molecular studies of human MTHFS.

Cloning and characterization of the human 5,10-methenyltetrahydrofolate synthetase-encoding cDNA

Methenyltetrahydrofolate synthetase (MTHFS) catalyses the obligatory initial metabolic step in the intracellular conversion of 5-formyltetrahydrofolate to other reduced folates. We have isolated and sequenced a human MTHFS cDNA which is 872-bp long and codes for a 203-amino-acid protein of 23,229 Da. Escherichia coli BL21(DE3), transfected with pET11c plasmids containing an open reading frame encoding MTHFS, showed a 100-fold increase in MTHFS activity in bacterial extracts after IPTG induction. Northern blot studies of human tissues determined that the MTHFS mRNA was expressed preferentially in the liver and Southern blot analysis of human genomic DNA suggested the presence of a single-copy gene.

Production and properties of skeletal myosin subfragment 1 selectively labeled with fluorescein at lysine-553 proximal to the strong actin-binding site

We describe, for the first time, the reaction of skeletal myosin subfragment 1 (S-1) with the succinimido ester of 6-[fluorescein-5(and 6)-carboxamido]hexanoic acid (FHS), which takes place at pH 7.0, 20 degrees C, within a 15 min period, in the presence of 1.5-1.8-fold molar excess of reagent over protein. As a result, 0.9-1.0 mol of fluorescyl group/mol of S-1 was covalently incorporated exclusively into the 95 kDa heavy chain as monitored by spectroscopic measurements. The central 50 kDa segment included the main site of fluorescence attachment as assessed by gel electrophoresis. The extent of S-1--FHS conjugation is strongly sensitive to F-actin binding but not to the interaction of nucleotides. The formation of the rigor F-actin--S-1 complex decreased the level of S-1 labeling to 20% without any competition between actin and S-1 for FHS binding. The derivatization of S-1 did not alter the K(+)-ATPase activity, but it enhanced the Ca(2+)-ATPase and Mg(2+)-ATPase to 150% and 225%, respectively, whereas it lowered the actin-activated ATPase to only 75% of the original activity. A double-reciprocal plot of the ATPase rate against actin concentration indicated a 2-fold decrease of the Vmax value for modified S-1, while the Km for actin was unchanged. Cosedimentation experiments did not reveal disruption of the rigor acto-S-1 interaction by the bound fluorophore. The labeled S-1 heavy chain was isolated, and its total tryptic digest was fractionated by reverse-phase HPLC.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of human mitochondrial methenyltetrahydrofolate synthetase activity

We present evidence for the presence of the folate metabolism enzyme methenyltetrahydrofolate synthetase (MTHFS) in mitochondria. MTHFS activity was identified in the matrix of mitochondria purified from human liver biopsies. Mitochondrial and cytoplasmic MTHFS specific activities are similar, 85% of the total cellular MTHFS activity is in the cytoplasm and both native enzymes have similar molecular weights (approximately 25 kDa). Studies using purified mitochondrial MTHFS from CA46 human Burkitt lymphoma cells reveal that mitochondrial MTHFS behaves kinetically like the cytoplasmic enzyme with Km values of 4.7, 0.8 and 22 microM respectively for (6R,S)-5-formyltetrahydrofolate monoglutamate, (6S)-5-formyltetrahydrofolate pentaglutamate and ATP. This finding adds to previous observations that various folate-dependent enzymes reside in the mitochondria of eucaryotic cells. Intracellular tetrahydrofolate metabolism is highly compartmentalized and mitochondrial MTHFS activity is necessary for the entry of mitochondrial 5-formyltetrahydrofolate into the mitochondrial folate pool.