Time-lapse microscopic observation of non-dividing cells in cultured human osteosarcoma MG-63 cell line.

Cancer stem cells resemble normal tissue-specific stem cells in many aspects, such as self-renewal and plasticity. Like their non-malignant counterparts, cancer stem cells are suggested to exhibit a relative quiescence. The established cancer cell lines reportedly harbor slow-proliferating cells that are positive for some cancer stem cells markers. However, the fate of these cells and their progeny remains unknown. We used time-lapse microscopy and the contrast-based segmentation algorithm to identify and monitor actively dividing and non-dividing cells in human osteosarcoma MG-63 cell line. Within the monitored field of view the non-dividing cells were represented by three cells that never divided, and one cell that attempted to divide, but failed cytokinesis, and later, after significantly prolonged division, produced the progeny with enlarged segmented nuclei, thus pointing to a possible mitotic catastrophe. Together, these cells initially constituted about 6.2% of the total number of seeded cells, yet only 0.02% of all cells at the end of the observation period when cells became confluent. Non-dividing cells were characterized by rounded shape, dark nuclei, random cytoplasmic streaming and subtle oscillatory movement, however, they did not migrate and rarely formed cell-cell contacts as compared to actively dividing cells. Our data indicate that the observed non-dividing MG-63 cells do not have a growth advantage over other cells and, therefore, they do not contribute to the cancer stem cells pool.

microRNA-34a promotes DNA damage and mitotic catastrophe.

Efficient and error-free DNA repair is critical for safeguarding genome integrity, yet it is also linked to radio- and chemoresistance of malignant tumors. miR-34a, a potent tumor suppressor, influences a large set of p53-regulated genes and contributes to p53-mediated apoptosis. However, the effects of miR-34a on the processes of DNA damage and repair are not entirely understood. We explored tet-inducible miR-34a-expressing human p53 wild-type and R273H p53 mutant GBM cell lines, and found that miR-34a influences the broad spectrum of 53BP1-mediated DNA damage response. It escalates both post-irradiation and endogenous DNA damage, abrogates radiation-induced G 2/M arrest and drastically increases the number of irradiated cells undergoing mitotic catastrophe. Furthermore, miR-34a downregulates 53BP1 and inhibits its recruitment to the sites of DNA double-strand breaks. We conclude that whereas miR-34a counteracts DNA repair, it also contributes to the p53-independent elimination of distressed cells, thus preventing the rise of genomic instability in tumor cell populations. These properties of miR-34a can potentially be exploited for DNA damage-effecting therapies of malignancies.

The p53-microRNA-34a axis regulates cellular entry receptors for tumor-associated human herpes viruses.

A growing number of reports indicate the frequent presence of DNA sequences and gene products of human cytomegalovirus in various tumors as compared to adjacent normal tissues, the brain tumors being studied most intensely. The mechanisms underlying the tropism of human cytomegalovirus to the tumor cells or to the cells of tumor origin, as well as the role of the host's genetic background in virus-associated oncogenesis are not well understood. It is also not clear why cytomegalovirus can be detected in many but not in all tumor specimens. Our in silico prediction results indicate that microRNA-34a may be involved in replication of some human DNA viruses by targeting and downregulating the genes encoding a diverse group of proteins, such as platelet-derived growth factor receptor-alpha, complement component receptor 2, herpes simplex virus entry mediators A, B, and C, and CD46. Notably, while their functions vary, these surface molecules have one feature in common: they serve as cellular entry receptors for human DNA viruses (cytomegalovirus, Epstein-Barr virus, human herpes virus 6, herpes simplex viruses 1 and 2, and adenoviruses) that are either proven or suspected to be linked with malignancies. MicroRNA-34a is strictly dependent on its transcriptional activator tumor suppressor protein p53, and both p53 and microRNA-34a are frequently mutated or downregulated in various cancers. We hypothesize that p53-microRNA-34a axis may alter susceptibility of cells to infection with some viruses that are detected in tumors and either proven or suspected to be associated with tumor initiation and progression.

Effects of the ß-agonist, isoprenaline, on the down-regulation, functional responsiveness and trafficking of ß2-adrenergic receptors with N-terminal polymorphisms.

The ß2-AR (ß2-adrenergic receptor) is an important target for respiratory and CVD (cardiovascular disease) medications. Clinical studies suggest that N-terminal polymorphisms of ß2-AR may act as disease modifiers. We hypothesized that polymorphisms at amino acids 16 and 27 result in differential trafficking and down-regulation of ß2-AR variants following ß-agonist exposure. The functional consequences of the four possible combinations of these polymorphisms in the human ß2-AR (designated ß2-AR-RE, ß2-AR-GE, ß2-AR-RQ and ß2-AR-GQ) were studied using site-directed mutagenesis and recombinant expression in HEK-293 cells (human embryonic kidney cells). Ligand-binding assays demonstrated that after 24 h exposure to 1 µM isoprenaline, isoforms with Arg16 (ß2-AR-RE and ß2-AR-RQ) underwent increased down-regulation compared with isoforms with Gly16 (ß2-AR-GE and ß2-AR-GQ). Consistent with these differences in down-regulation between isoforms, prolonged isoprenaline treatment resulted in diminished cAMP response to subsequent isoprenaline challenge in ß2-AR-RE relative to ß2-AR-GE. Confocal microscopy revealed that the receptor isoforms had similar co-localization with the early endosomal marker EEA1 following isoprenaline treatment, suggesting that they had similar patterns of internalization. None of the isoforms exhibited significant co-localization with the recycling endosome marker Rab11 in response to isoprenaline treatment. Furthermore, we found that prolonged isoprenaline treatment led to a higher degree of co-localization of ß2-AR-RE with the lysosomal marker LAMP1 (lysosome-associated membrane protein 1) compared with that of ß2-AR-GE. Taken together, these results indicate that a mechanism responsible for differential responses of these receptor isoforms to the ß-agonist involves differences in the efficiency with which agonist-activated receptors are trafficked to the lysosomes for degradation, or differences in degradation in the lysosomes.

Cooperation between c-Met and focal adhesion kinase family members in medulloblastoma and implications for therapy.

We previously showed the involvement of the tyrosine kinase receptor c-Met in medulloblastoma malignancy. The nonreceptor tyrosine kinases focal adhesion kinase (FAK) and Pyk2 are key players in the progression of different cancers. However, their role in medulloblastoma malignancy is not well understood. In this study, using a protein array approach, we found that c-Met induces FAK and Pyk2 phosphorylation in medulloblastoma cells. We therefore studied the interactions between c-Met and FAK/Pyk2 and their implications for medulloblastoma therapy. We found that c-Met activates FAK and Pyk2 in several medulloblastoma cell lines. We also found that FAK and Pyk2 mediate the malignant effects of c-Met on medulloblastoma cell proliferation, migration, and invasion. On the basis of these findings, we hypothesized that combined c-Met and FAK inhibitions would have additive effects on the inhibition of medulloblastoma malignancy. To test this hypothesis, we assessed the effects on medulloblastoma malignancy parameters of single or combined treatments of medulloblastoma cells with c-Met and FAK small-molecule kinase inhibitors. We found a significant increase in the inhibitory effect of both inhibitors on medulloblastoma cell migration and cell invasion as compared with single inhibitions (P < 0.05). In addition, oral gavage treatment with c-Met inhibitor of mice bearing medulloblastoma xenografts significantly reduced in vivo tumor growth. Therefore, combining c-Met inhibitors with FAK inhibitors constitutes a new potential strategy for medulloblastoma therapy. Altogether, our study describes a role for FAK and Pyk2 in medulloblastoma malignancy, uncovers new interactions between c-Met and FAK/Pyk2, and proposes for the first time combining anti-c-Met and anti-FAK inhibitors as a new strategy for medulloblastoma therapy.

When tumor cells make blood vessels: implications for glioblastoma therapy.

The paper by Soda et al. provides experimental evidence for the plasticity of glioblasoma multiforme (GBM) cells, specifically their ability to form vascular endothelial cells (ECs). The study demonstrates the existence of tumor-derived ECs (TDECs) in GBM blood vessels of transgenic mice and humans. Blood vessels with TDECs were functional and were more frequently found in hypoxic tumor regions. In vitro hypoxic conditions enhanced the transition of tumor-initiating cells to an endothelial-like morphology and the formation of tube-like structures. Contrary to normal ECs, TDECs did not express VEGF receptors, and treatment of experimental GBM tumors with anti-VEGF/VEGF receptor agents led to an increase in the proportion of TDECs relative to normal ECs. These findings identify a new potential mechanism of resistance of GBM tumors to anti-VEGF therapies. Future strategies for GBM therapy will likely require the combined targeting of normal ECs and TDECs, as well as the development of strategies that prevent the conversion of tumor cells into vascular ECs.

The roles of viruses in brain tumor initiation and oncomodulation.

While some avian retroviruses have been shown to induce gliomas in animal models, human herpesviruses, specifically, the most extensively studied cytomegalovirus, and the much less studied roseolovirus HHV-6, and Herpes simplex viruses 1 and 2, currently attract more and more attention as possible contributing or initiating factors in the development of human brain tumors. The aim of this review is to summarize and highlight the most provoking findings indicating a potential causative link between brain tumors, specifically malignant gliomas, and viruses in the context of the concepts of viral oncomodulation and the tumor stem cell origin.

The hepatic stem cell niche: identification by label-retaining cell assay.

UNLABELLED: Label retention assays remain the state-of-the-art approach to identify the location of intraorgan epithelial stem cell niches, in situ and in vivo. They are commonly used in organs with rapid cell turnover but have not been applied to the liver, where cell turnover is very slow. We used a sublethal dose of acetaminophen administered coincident with bromodeoxyuridine to load possible hepatic stem cells in mice with label and then administered a second, sublethal chase of acetaminophen to accomplish "washout" of label from transit amplifying cell populations. CONCLUSION: Four possible hepatic stem cell niches are identified by this approach: the canal of Hering (proximal biliary tree), intralobular bile ducts, periductal "null" mononuclear cells, and peribiliary hepatocytes. These results confirm several different and often contradictory lines of investigation regarding the intrahepatic location of stem/progenitor cells and suggest that the liver has a multi-tiered, flexible system of regeneration rather than a single stem/progenitor cell location.

Hematopoietic mobilization in mice increases the presence of bone marrow-derived hepatocytes via in vivo cell fusion.

The mechanisms for in vivo production of bone marrow-derived hepatocytes (BMDHs) remain largely unclear. We investigated whether granulocyte colony-stimulating factor (G-CSF)-mediated mobilization of hematopoietic cells increases the phenomenon. Recurrent liver injury in mice expressing green fluorescent protein (EGFP) in all hematopoietic-derived cells was produced by 3 months of carbon tetrachloride (CCL4) injections. Histologically, there were necrotic foci with histiocyte-rich infiltrates, but little oval cell proliferation. Subsequently, some animals were mobilized with G-CSF for 1, 2, or 3 weeks. Animals were sacrificed 1 month after growth factor treatment. BMDH percentages were lower than previously reported, though G-CSF mobilization significantly augmented BMDH production in injured livers. BMDHs originating from in vivo fusion were evaluated by transplanting female EGFP+ cells into male mice. Binucleated, EGFP+ hepatocytes with one Y chromosome, indicating fusion, were identified. In conclusion, (1) mobilization of hematopoietic cells increases BMDH production and (2) as with the FAH-null model, the first model demonstrating hematopoietic/hepatocyte fusion, recurring CCl4-induced injury has macrophage-rich infiltrates, a blunted oval cell response, and a predominantly in vivo fusion process for circulating cell engraftment into the liver. These findings open the possibility of using hematopoietic growth factors to treat nonhematopoietic degenerative diseases.

Dose- and time-dependent oval cell reaction in acetaminophen-induced murine liver injury.

We examined the response of murine oval cells, that is, the putative liver progenitor cells, to acetaminophen. Female C57BL/6J mice were injected intraperitoneally with varying doses of N-acetyl-paraaminophen (APAP) (250, 500, 750, and 1,000 mg/kg of weight) and sacrificed at 3, 6, 9, 24, and 48 hours. In preliminary studies, we showed that anticytokeratin antibodies detected A6-positive cells with a sensitivity and specificity of greater than 99%. The oval cell reaction was quantified, on immunostaining for biliary-type cytokeratins, as both number and density of oval cells per portal tract, analyzed by size of portal tract. Acetaminophen injury was followed by periportal oval cell accumulation displaying a moderate degree of morphological homogeneity. Oval cell response was biphasic, not temporally correlating with the single wave of injury seen histologically. Increases in oval cells were largely confined to the smallest portal tracts, in keeping with their primary derivation from the canals of Hering, and increased in a dose-dependent fashion. The timing of the two peaks of the oval cell reaction also changed with increasing dose, the first becoming earlier and the second later. In conclusion, our studies indicate a marked oval cell activation during the height of hepatic injury. Oval cells appear to be resistant to acetaminophen injury. The close fidelity of mechanism and histology of acetaminophen injury between mouse and human livers makes it a useful model for investigating liver regeneration and the participation of stem/progenitor cells in that process.

Co-influence of transgene expression in mammalian cells. Mutual influence of transgenes on their expression in mammalian cells.

It becomes increasingly clear that therapeutic gene delivery should provide not only for the sustained high level of gene expression but also, in most cases, for the regulated expression of transgenes as much as it occurs under natural conditions. Over the past few years a variety of different systems have been developed in order to regulate the amounts of transcribed RNA upon administration of exogenous agents, or in autoregulated manner. While efforts were focused on optimizing gene expression at the transcriptional level, other levels are still overlooked. In the meantime, regulation of gene expression is not restricted to transcription, but is also executed at the post-transcriptional level, i.e. mRNA stability, processing, transport, translation, protein stability, and modification. Codon usage is considered to be one of the critical factors that limit the expression rate of heterologous genes in different organisms at the posttranscriptional level. HIV-1 structural genes gag, pol, and env represent one of the most extensively utilized models for studying codon usage-mediated effects on transgene expression. In the current work we demonstrate that the codon content affects not only CMV-driven HIV-1 gag expression but also the expression of luciferase reporter gene transcribed independently from the SV40 promoter. The expression levels of both transgenes co-transfected into the human H1299 were inversely co-dependent. The observed phenomenon may be described as sequence-independent post-transcriptional gene silencing, which reflects the existing limitation of transgene expression in mammalian cells at the post-transcriptional level. Optimization of the codon usage may provide for the additional level of regulation of transgene expression in gene transfer experiments in order to maintain the concentration of the protein at the therapeutic levels.

HIV-1 gag expression is quantitatively dependent on the ratio of native and optimized codons.

There is a significant variation of codon usage bias among different species and even among genes within the same organisms. Codon optimization, this is, gene redesigning with the use of codons preferred for the specific expression system, results in improved expression of heterologous genes in bacteria, plants, yeast, mammalian cells, and transgenic animals. The mechanisms preventing expression of genes with rare or low-usage codons at adequate levels are not completely elucidated. Human immunodeficiency virus (HIV) represents an interesting model for studying how differences in codon usage affect gene expression in heterologous systems. Construction of synthetic genes with optimized codons demonstrated that the codon-usage effects might be a major impediment to the efficient expression of HIV gag/pol and env gene products in mammalian cells. According to another hypothesis, the poor expression of HIV structural proteins even without HIV context is attributed to the so-called cis-acting inhibitory elements (INS), which are located within the protein-coding region. They consist of AU-rich sequences and may be inactivated through the introduction of multiple mutations over the large regions of gag gene. In our work, we evaluated expression of hybrid HIV-1 gag mRNAs where wild-type (A-rich) gag sequences were combined with artificial sequences. In such "humanized" gag fragments with adapted codon usage, AT-content was significantly reduced in favor of G and C nucleotides without any changes in protein sequence. We show that wild-type gag sequences negatively influence expression of gag-reporter, and the addition of fragments with optimized codons to gag mRNA partially rescues its expression. The results demonstrate that the expression of HIV-1 gag is determined by the ratio of optimized and rare codons within mRNA. Our data also indicates that some wtgag fragments counteract the influence of the other wtgag sequences, which cause the inhibition of gag expression. The presented data do not contradict the concept of INS; yet, it makes the definition of INS more complex. This supports the idea of a broader role of the selected codon usage in influencing the expression of HIV proteins in mammalian cells.

Codon usage-mediated inhibition of HIV-1 gag expression in mammalian cells occurs independently of translation.

Codon usage is considered one of the critical factors that limit the expression rate of heterologous genes. Impaired translation efficiency, specifically insufficient amount of corresponding tRNAs and changed startcodon context, are believed to account for the low translation initiation and elongation rates during the protein biosynthesis in unicellular organisms. Translational efficiency is probably not the primary factor influencing codon usage diversity in mammalian cells. However, the other possible mechanisms preventing expression of genes with low-usage such as mRNA stability, processing and nucleocytoplasmic transport, are not adequately explored. In our work, we addressed the question of whether codon usage differences affect exclusively translational efficiency of mammalian gene products. We demonstrated that the CMV-induced expression of gag-reporter in human H1299 cell line was influenced by the nucleotide composition of the mRNA, and the limitation of gag expression appeared to be inversely related to the level of codon optimization. However, cytoplasmic expression of the gag-reporter driven by vaccinia virus/T7 RNA polymerase hybrid system rescued its expression independently of HIV-1 gag mRNA nucleotide content. We concluded that impaired HIV-1 gag expression may be caused by translation-independent mechanisms, which probably play a major role in codon usage-mediated defects in heterologous gene expression in mammalian cells.