Specific growth suppression of human cancer cells by targeted delivery of Dictyostelium mitochondrial ribosomal protein S4

BACKGROUND

In general, growth and differentiation are mutually exclusive but are cooperatively regulated throughout development. Thus, the process of a cell's switching from growth to differentiation is of great importance not only for the development of organisms but also for malignant transformation, in which this process is reversed. We have previously demonstrated using a Dictyostelium model system that the Dictyostelium mitochondrial ribosomal protein S4 (Dd-mrp4) gene expression is essential for the initiation of cell differentiation: Dd-mrp4-null cells fail to initiate differentiation, while the initial step of cell differentiation and the subsequent morphogenesis are markedly enhanced in mrp4 (OE) cells overexpressing the Dd-mrp4 in the extramitochondrial cytoplasm. This raised a possibility that the ectopically enforced expression of the Dd-mrp4 in human cells might inhibit their growth, particularly of malignant tumor cells, by inducing cell differentiation.

METHODS

FOUR KINDS OF HUMAN TUMOR CELL LINES WERE TRANSFECTED BY THREE KIND OF VECTOR CONSTRUCTS (THE EMPTY VECTOR: pcDNA3.1 (Mock); pcDNA3.1-rps4 bearing Dictyostelium cytoplasmic ribosomal protein S4; pcDNA3.1-mrp4 bearing Dictyostelium mitochondrial ribosomal protein S4). As controls, four kinds of human primary cultured cells were similarly transfected by the above vector constructs. After transfection, growth kinetics of cells was analyzed using cell viability assay, and also the TUNEL method was used for evaluation of apoptotic cells.

RESULTS

Ectopically expressed Dd-mrp4 suppressed cell proliferation through inducing apoptotic cell death specifically in the human lung adenocarcinoma (A549), epithelial cervical cancer (HeLa), hepatocellular carcinoma (HepG2) and colonic carcinoma (Caco-2), but not in primary cultured normal cells, such as human brain microvascular endothelial cells (HBMECs); human umbilical vein endothelial cells (HUVECs) and human normal hepatocytes (hHeps™), with one exception (human cardiac fibloblasts (HCF)).

CONCLUSION

The present finding that the ectopically enforced expression of Dd-mrp4 in human several tumor cell lines specifically suppresses their proliferation suggests strongly that the Dd-mrp4 gene derived from Dictyostelium mitochondria may provide a new promising therapeutic strategy for disrupting cell viability pathways in human cancers.

Properties of a non-bioactive fluorescent derivative of differentiation-inducing factor-3, an anti-tumor agent found in Dictyostelium discoideum

Differentiation-inducing factor-3 (DIF-3), found in the cellular slime mold Dictyostelium discoideum, and its derivatives, such as butoxy-DIF-3 (Bu-DIF-3), are potent anti-tumor agents. To investigate the activity of DIF-like molecules in tumor cells, we recently synthesized a green fluorescent DIF-3 derivative, BODIPY-DIF-3G, and analyzed its bioactivity and cellular localization. In this study, we synthesized a red (orange) fluorescent DIF-3 derivative, BODIPY-DIF-3R, and compared the cellular localization and bioactivities of the two BODIPY-DIF-3s in HeLa human cervical cancer cells. Both fluorescent compounds penetrated the extracellular membrane within 0.5 h and localized mainly to the mitochondria. In formalin-fixed cells, the two BODIPY-DIF-3s also localized to the mitochondria, indicating that the BODIPY-DIF-3s were incorporated into mitochondria independently of the mitochondrial membrane potential. After treatment for 3 days, BODIPY-DIF-3G, but not BODIPY-DIF-3R, induced mitochondrial swelling and suppressed cell proliferation. Interestingly, the swollen mitochondria were stainable with BODIPY-DIF-3G but not with BODIPY-DIF-3R. When added to isolated mitochondria in vitro, BODIPY-DIF-3G increased dose-dependently the rate of O2 consumption, but BODIPY-DIF-3R did not. These results suggest that the bioactive BODIPY-DIF-3G suppresses cell proliferation, at least in part, by altering mitochondrial activity, whereas the non-bioactive BODIPY-DIF-3R localizes to the mitochondria but does not affect mitochondrial activity or cell proliferation.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

G17-modified hammerhead ribozymes are active in vitro and in vivo

Natural hammerhead ribozymes (HHRz) feature tertiary interactions between hairpin loops or bulges in two of three helices that surround the catalytic core of conserved nucleotides. Their conservation was originally established on minimal versions lacking the tertiary interactions. While those sequence requirements in general also apply to natural versions, we show here differences for the HHRz cleavage site N17. A guanosine at this position strongly impairs cleavage activity in minimal versions, whereas we observe for the G17 variants of four tertiary stabilized HHRz significant cleavage and ligation activity in vitro. Kinetic analyses of these variants revealed a reduced rate and extent of cleavage, compared with wild-type sequences, while variants with distorted tertiary interactions cleaved at a reduced rate, but to the same extent. Contrary to this, G17 variants exhibit similar in vitro ligation activity as compared with the respective wild-type motif. To also address the catalytic performance of these motifs in vivo, we have inserted HHRz cassettes in the lacZ gene and tested this β-galactosidase reporter in Dictyostelium discoideum. In colorimetric assays, we observe differences in the enzymatic activity of β-galactosidase, which correlate well with the activity of the different HHRz variants in vitro and which can be unambiguously attributed to ribozyme cleavage by primer extension analysis.

The cyclic AMP phosphodiesterase RegA critically regulates encystation in social and pathogenic amoebas

Amoebas survive environmental stress by differentiating into encapsulated cysts. As cysts, pathogenic amoebas resist antibiotics, which particularly counteracts treatment of vision-destroying Acanthamoeba keratitis. Limited genetic tractability of amoeba pathogens has left their encystation mechanisms unexplored. The social amoeba Dictyostelium discoideum forms spores in multicellular fruiting bodies to survive starvation, while other dictyostelids, such as Polysphondylium pallidum can additionally encyst as single cells. Sporulation is induced by cAMP acting on PKA, with the cAMP phosphodiesterase RegA critically regulating cAMP levels. We show here that RegA is deeply conserved in social and pathogenic amoebas and that deletion of the RegA gene in P. pallidum causes precocious encystation and prevents cyst germination. We heterologously expressed and characterized Acanthamoeba RegA and performed a compound screen to identify RegA inhibitors. Two effective inhibitors increased cAMP levels and triggered Acanthamoeba encystation. Our results show that RegA critically regulates Amoebozoan encystation and that components of the cAMP signalling pathway could be effective targets for therapeutic intervention with encystation.

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Amoebas differentiate into dormant encapsulated cysts when exposed to environmental stress

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Encystation renders pathogenic amoebas resistant to antibiotics and biocides

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The social amoeba Polysphondylium pallidum is amenable to genetic approaches to resolve encystation mechanisms

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The cAMP phosphodiesterase RegA and the sensor histidine kinases that regulate RegA activity are deeply conserved

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RegA regulates encystation in P.pallidum and in the pathogen Acanthamoeba castellani

AmpA protein functions by different mechanisms to influence early cell type specification and to modulate cell adhesion and actin polymerization in Dictyostelium discoideum

The Dictyostelium discoideum ampA gene encodes a multifunctional regulator protein that modulates cell-cell and cell-substrate adhesions and actin polymerization during growth and is necessary for correct cell type specification and patterning during development. Insertional inactivation of the ampA gene results in defects that define two distinct roles for the ampA gene during development. AmpA is necessary in a non-cell autonomous manner to prevent premature expression of a prespore gene marker. It is also necessary in a cell autonomous manner for the anterior like cells, which express the ampA gene, to migrate to the upper cup during culmination. It is also necessary to prevent excessive cell-cell agglutination when cells are developed in a submerged suspension culture. Here, we demonstrate that a supernatant source of AmpA protein, added extracellularly, can prevent the premature mis-expression of the prespore marker. Synthetic oligopeptides are used to identify the domain of the AmpA protein that is important for preventing cells from mis-expressing the prespore gene. We further demonstrate that a factor capable of inducing additional cells to express the prespore gene marker accumulates extracellularly in the absence of AmpA protein. While the secreted AmpA acts extracellularly to suppress prespore gene expression, the effects of AmpA on cell agglutination and on actin polymerization in growing cells are not due to an extracellular role of secreted AmpA protein. Rather, these effects appear to reflect a distinct cell autonomous role of the ampA gene. Finally, we show that secretion of AmpA protein is brought about by elevating the levels of expression of ampA so that the protein accumulates to an excessive level.

Hydrolysis of aromatic β-glucosides by non-pathogenic bacteria confers a chemical weapon against predators

Bacteria present in natural environments such as soil have evolved multiple strategies to escape predation. We report that natural isolates of Enterobacteriaceae that actively hydrolyze plant-derived aromatic β-glucosides such as salicin, arbutin and esculin, are able to avoid predation by the bacteriovorous amoeba Dictyostelium discoideum and nematodes of multiple genera belonging to the family Rhabditidae. This advantage can be observed under laboratory culture conditions as well as in the soil environment. The aglycone moiety released by the hydrolysis of β-glucosides is toxic to predators and acts via the dopaminergic receptor Dop-1 in the case of Caenorhabditis elegans. While soil isolates of nematodes belonging to the family Rhabditidae are repelled by the aglycone, laboratory strains and natural isolates of Caenorhabditis sp. are attracted to the compound, mediated by receptors that are independent of Dop-1, leading to their death. The β-glucosides-positive (Bgl(+)) bacteria that are otherwise non-pathogenic can obtain additional nutrients from the dead predators, thereby switching their role from prey to predator. This study also offers an evolutionary explanation for the retention by bacteria of 'cryptic' or 'silent' genetic systems such as the bgl operon.

Collective behavior of Dictyostelium discoideum monitored by impedance analysis

Dictyostelium discoideum cells respond to periodic signals of extracellular cAMP by collective changes of cell-cell and cell-substrate contacts. This was confirmed by dielectric analysis employing electric cell-substrate impedance sensing (ECIS) and impedance measurements involving cell-filled micro channels in conjunction with optical microscopy providing a comprehensive picture of chemotaxis under conditions of starvation.

A SAP domain-containing protein shuttles between the nucleus and cell membranes and plays a role in adhesion and migration in D. discoideum

The AmpA protein reduces cell adhesion, thereby influencing cell migration in Dictyostelium. To understand how ampA influences cell migration, second site suppressors of an AmpA overexpressing cell line were created by REMI mutagenesis. Mutant candidates were identified by their ability to suppress the large plaques that the AmpA overexpressing cells form on bacterial lawns as a result of their increased rate of migration. One suppressor gene, sma, encodes an uncharacterized protein, which contains a SAP DNA-binding domain and a PTEN-like domain. Using sma gene knockouts and Sma-mRFP expressing cell lines, a role for sma in influencing cell migration was uncovered. Knockouts of the sma gene in a wild-type background enhanced chemotaxis. An additional role for Sma in influencing cell–cell adhesion was also demonstrated. Sma protein transitions between cytosolic and nuclear localizations as a function of cell density. In growing cells migrating to folic acid it is localized to regions of actin polymerization and absent from the nucleus. A role for Sma in influencing ampA mRNA levels is also demonstrated. Sma additionally appears to be involved in ampA pathways regulating cell size, actin polymerization, and cell substrate adhesion. We present insights to the SAP domain-containing group of proteins in Dictyostelium and provide evidence of a role for a SAP domain-containing protein shuttling from the nucleus to sites of actin polymerization during chemotaxis to folic acid and influencing the efficiency of migration.

A cell number counting factor alters cell metabolism

It is still not clear how organisms regulate the size of appendages or organs during development. During development, Dictyostelium discoideum cells form groups of approximately 2 x 10(4) cells. The cells secrete a protein complex called counting factor (CF) that allows them to sense the local cell density. If there are too many cells in a group, as indicated by high extracellular concentrations of CF, the cells break up the group by decreasing cell-cell adhesion and increasing random cell motility. As a part of the signal transduction pathway, CF decreases the activity of glucose-6-phosphatase to decrease internal glucose levels. CF also decreases the levels of fructose-1,6-bisphosphate and increases the levels of glucose-6-phosphate and fructose-6-phosphate. In this report, we focus on how a secreted signal used to regulate the size of a group of cells regulates many basic aspects of cell metabolism, including the levels of pyruvate, lactate, and ATP, and oxygen consumption.

Formation and Stimuli-Directed Migration of D. discoideum Slugs in Microchips

This paper presents a microfluidic device that geometrically constrains the development of individual Dictyostelium discoideum cells into multicellular organisms (slugs). A microchip for the stimuli-directed migration of slugs is also presented. To demonstrate the formation of slugs in a predetermined shape, a microchip is designed to confine the slugs in the vertical direction. In the microchip, sufficient oxygen is supplied to the cells via a membrane, allowing the formation and adaptation of slugs to the shape of the channel. In addition, the manipulation of slug migration direction in a microchip via external stimuli, such as light and temperature gradients, that induce phototaxis and thermotaxis of slugs, respectively, is demonstrated. To direct slug migration with external stimuli, an optical fiber is used for phototaxis and an electrical wire heater is used for thermotaxis. Experimental results show slug formation in a predefined geometry in the microchip, suggesting that this chip is potentially useful for understanding the relationship between the shape and function of cells or tissue. The controlled migration of slugs demonstrated in the microchips can potentially be employed in biologically based microactuators or microrobots.

Light-Directed Migration of D. discoideum Slugs in Microfabricated Confinements

This paper investigates the light-driven migration of the multi-cellular microorganism Dictyostelium discoideum as a potential bio-actuation mechanism in microsystems. As a platform for slug migration we use microscale confinements, which consist of intersecting microchannels fabricated from solidified agar-water solution. The agar surface provides necessary moisture to the slugs during the experiment while remaining sufficiently stiff to allow effective slug migration. The movements of the slugs in the microchannels are driven and guided by phototaxis via controlling light transmitted through optical fibers. The microchannels impose geometrical confinements on the migrating slugs, improving the spatial precision of the migration. We demonstrate that slugs that form in a microchamber can be driven to migrate through the microchannels, as well as steered to a particular direction at microchannel intersections. Our experimental results indicate that slug movements can be more effectively controlled in microchannels, and potentially useful for bio-actuation applications.

Fast characterisation of cell-derived extracellular vesicles by nanoparticles tracking analysis, cryo-electron microscopy, and Raman tweezers microspectroscopy

The joint use of 3 complementary techniques, namely, nanoparticle tracking analysis (NTA), cryo-electron microscopy (Cryo-EM) and Raman tweezers microspectroscopy (RTM), is proposed for a rapid characterisation of extracellular vesicles (EVs) of various origins. NTA is valuable for studying the size distribution and concentration, Cryo-EM is outstanding for the morphological characterisation, including observation of vesicle heterogeneity, while RTM provides the global chemical composition without using any exogenous label. The capabilities of this approach are evaluated on the example of cell-derived vesicles of Dictyostelium discoideum, a convenient general model for eukaryotic EVs. At least 2 separate species differing in chemical composition (relative amounts of DNA, lipids and proteins, presence of carotenoids) were found for each of the 2 physiological states of this non-pathogenic microorganism, that is, cell growth and starvation-induced aggregation. These findings demonstrate the specific potency of RTM. In addition, the first Raman spectra of human urinary exosomes are reported, presumably constituting the primary step towards Raman characterisation of EVs for the purpose of human diseases diagnoses.

Expression, identification and purification of Dictyostelium acetoacetyl-coa thiolase expressed in Escherichia coli

Acetoacetyl-CoA thiolase (AT) is an enzyme that catalyses the CoA-dependent thiolytic cleavage of acetoacetyl-CoA to yield 2 molecules of acetyl-CoA, or the reverse condensation reaction. A full-length cDNA clone pBSGT-3, which has homology to known thiolases, was isolated from Dictyostelium cDNA library. Expression of the protein encoded in pBSGT-3 in Escherichia coli, its thiolase enzyme activity, and the amino acid sequence homology search revealed that pBSGT-3 encodes an AT. The recombinant AT (r-thiolase) was expressed in an active form in an E. coli expression system, and purified to homogeneity by selective ammonium sulfate fractionation and two steps of column chromatography. The purified enzyme exhibited a specific activity of 4.70 mU/mg protein. Its N-terminal sequence was (NH₂)-Arg-Met-Tyr-Thr-Thr-Ala-Lys-Asn-Leu-Glu-, which corresponds to the sequence from positions 15 to 24 of the amino acid sequence deduced from pBSGT-3 clone. The r-thiolase in the inclusion body expressed highly in E. coli was the precursor form, which is slightly larger than the purified r-thiolase. When incubated with the cell-free extract of Dictyostelium cells, the precursor was converted to the same size to the purified r-thiolase, suggesting that the presequence at the N-terminus is removed by a Dictyostelium processing peptidase.

Localization of a DNA topoisomerase II to mitochondria inDictyostelium discoideum: Deletion mutant analysis and mitochondrial targeting signal presequence

DNA topoisomerase II ofDictyostelium discoideum (TopA), the gene (topA) encoding which we cloned, was shown to have an additional N-terminal region which contains a putative mitochondrial targeting signal presequence. We constructed overexpression mutants which expressed the wild-type or the N-terminally deleted enzyme, and examined its localization by immunofluorescence microscopy and proteinase K digestion experiment. These experiments revealed that the enzyme is located in the mitochondria by virtue of the additional N-terminal region. Furthermore, in the cell extract depleted the enzyme by immunoprecipitation, nuclear DNA topoisomerase II activity was not decreased. These results confirmed that TopA is located in the mitochondria, even through its amino acid sequence is highly similar to those of nuclear type topoisomerase II of other organisms. Thus, this report is the first to establish the location of the mitochondrial targeting signal presequence in DNA topoisomerase II and in proteins ofD. discoideum directly by analyzing deletion mutants.

Relevance of histone H1 kinase activity to the G2/M transition during the cell cycle ofDictyostelium discoideum

The implication of histone H1 kinase activity for the G2/M transition during the cell cycle was investigated usingDictyostelium discoideum Ax-2. Histone H1 kinase with its activity was purified from cell extracts by the use of p13(suc1) affinity gel. In the vegetative cell cycle, the activity of histone H1 kinase including Cdc2 kinase was found using synchronized Ax-2 cells to be highest just before the entry into mitosis. The activity also was markedly enhanced just prior to the M phase from which developing cells (possibly prespore cells) reinitiate their cell cycle at the mound-tipped aggregate stage. These results strongly suggest the importance of Cdc2 kinase activity in the G2 to M phase transition during the cell cycle, as the case for other eukaryotic cells.

A study of PstB cells during Dictyostelium migration and culmination reveals a unidirectional cell type conversion process

The prestalk region of the Dictyostelium slug has recently been shown by Williams and his collaborators to consist of two distinct cell types, pstA and pstB cells. Here the movement of these cells in both the slug and culmination stages has been examined with the use of vital dyes. In the slug some of the pstB cells are continually lost from the prestalk region as small clusters of cells. These cells move through the prespore region and temporarily lie in the rearguard region at the posterior end of the slug. They are finally left in the slug's slime track as single cells or groups of a few cells. When culmination is initiated the pstB cells move as a whole from the prestalk region to the base where they join the rearguard cells to form the basal disc of the fruiting body. Transplantation experiments reveal that the rearguard cells form an outer ring portion of the basal disc and the pstB cells form an inner portion to which the stalk attaches. The continuous loss of one cell type during the slug stage without any change in cell type proportions suggests that cell types are redifferentiating. Grafting and transplantation experiments reveal that there is a unidirectional flow of cells through successive steps of cell type conversion. Prespore cells redifferentiate as anterior-like cells which migrate to the prestalk region and become pstA cells. The pstA cells then replace the pstB cells that are lost from the slug.

Quantitative analysis of the spatial distribution of ultrastructural differentiation markers during development ofDictyostelium discoideum

The appearance and spatial distrubution of ultrastructural markers ofDictyostelium discoideum differentiation were quantitatively analysed. Our results combined with data from the literature on the functions of cells at various stages of development lead to the following conclusions. When food is no longer available all amoebae initially develop an autophagic apparatus in order to sustain metabolism. After slugs have been formed, autophagy is suppressed in the prespore cells. During aggregation a number of cells gradually form prespore characteristics. These cells arise at random but later they become located in the basal part of the tip-forming aggregate. From the early slug stage onwards, cells of the posterior two third region gradually enter into the prespore pathway. During prolonged slug migration the optimal acquirement of prespore characteristics is blocked. Cells of the anterior region show no active differentiation but they maintain the morphology and most of the functions of aggregating cells. At the rear-guard of the slug and later on in the basal region of the maturing fruiting body, a second anteriorlike region appears. Actual stalk cell differentiation takes place only at the apex and at the base of the developing fruiting body.