HIPK2 controls cytokinesis and prevents tetraploidization by phosphorylating histone H2B at the midbody.

Failure in cytokinesis, the final step in cell division, by generating tetra- and polyploidization promotes chromosomal instability, a hallmark of cancer. Here we show that HIPK2, a kinase involved in cell fate decisions in development and response to stress, controls cytokinesis and prevents tetraploidization through its effects on histone H2B. HIPK2 binds and phosphorylates histone H2B at S14 (H2B-S14(P)), and the two proteins colocalize at the midbody. HIPK2 depletion by targeted gene disruption or RNA interference results in loss of H2B-S14(P) at the midbody, prevention of cell cleavage, and tetra- and polyploidization. In HIPK2 null cells, restoration of wild-type HIPK2 activity or expression of a phosphomimetic H2B-S14D derivative abolishes cytokinesis defects and rescues cell proliferation, showing that H2B-S14(P) is required for a faithful cytokinesis. Overall, our data uncover mechanisms of a critical HIPK2 function in cytokinesis and in the prevention of tetraploidization.

Small Molecule Inhibitors of KDM5 Histone Demethylases Increase the Radiosensitivity of Breast Cancer Cells Overexpressing JARID1B.

Background: KDM5 enzymes are H3K4 specific histone demethylases involved in transcriptional regulation and DNA repair. These proteins are overexpressed in different kinds of cancer, including breast, prostate and bladder carcinomas, with positive effects on cancer proliferation and chemoresistance. For these reasons, these enzymes are potential therapeutic targets. Methods: In the present study, we analyzed the effects of three different inhibitors of KDM5 enzymes in MCF-7 breast cancer cells over-expressing one of them, namely KDM5B/JARID1B. In particular we tested H3K4 demethylation (western blot); radio-sensitivity (cytoxicity and clonogenic assays) and damage accumulation (COMET assay and kinetics of H2AX phosphorylation). Results: we show that all three compounds with completely different chemical structures can selectively inhibit KDM5 enzymes and are capable of increasing sensitivity of breast cancer cells to ionizing radiation and radiation-induced damage. Conclusions: These findings confirm the involvement of H3K4 specific demethylases in the response to DNA damage, show a requirement of the catalytic function and suggest new strategies for the therapeutic use of their inhibitors.

AKTIP/Ft1, a New Shelterin-Interacting Factor Required for Telomere Maintenance.

Telomeres are nucleoprotein complexes that protect the ends of linear chromosomes from incomplete replication, degradation and detection as DNA breaks. Mammalian telomeres are protected by shelterin, a multiprotein complex that binds the TTAGGG telomeric repeats and recruits a series of additional factors that are essential for telomere function. Although many shelterin-associated proteins have been so far identified, the inventory of shelterin-interacting factors required for telomere maintenance is still largely incomplete. Here, we characterize AKTIP/Ft1 (human AKTIP and mouse Ft1 are orthologous), a novel mammalian shelterin-bound factor identified on the basis of its homology with the Drosophila telomere protein Pendolino. AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes and has been previously implicated in the control of apoptosis and in vesicle trafficking. RNAi-mediated depletion of AKTIP results in formation of telomere dysfunction foci (TIFs). Consistent with these results, AKTIP interacts with telomeric DNA and binds the shelterin components TRF1 and TRF2 both in vivo and in vitro. Analysis of AKTIP- depleted human primary fibroblasts showed that they are defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint. Accordingly, AKTIP physically interacts with PCNA and the RPA70 DNA replication factor. Ft1-depleted p53-/- MEFs did not arrest in the S phase but displayed significant increases in multiple telomeric signals (MTS) and sister telomere associations (STAs), two hallmarks of defective telomere replication. In addition, we found an epistatic relation for MST formation between Ft1 and TRF1, which has been previously shown to be required for replication fork progression through telomeric DNA. Ch-IP experiments further suggested that in AKTIP-depleted cells undergoing the S phase, TRF1 is less tightly bound to telomeric DNA than in controls. Thus, our results collectively suggest that AKTIP/Ft1 works in concert with TRF1 to facilitate telomeric DNA replication.

Centrosomal Che-1 protein is involved in the regulation of mitosis and DNA damage response by mediating pericentrin (PCNT)-dependent Chk1 protein localization.

To combat threats posed by DNA damage, cells have evolved mechanisms, collectively termed DNA damage response (DDR). These mechanisms detect DNA lesions, signal their presence, and promote their repair. Centrosomes integrate G2/M checkpoint control and repair signals in response to genotoxic stress, acting as an efficient control mechanism when G2/M checkpoint function fails and mitosis begins in the presence of damaged DNA. Che-1 is an RNA polymerase II-binding protein involved in the regulation of gene transcription, induction of cell proliferation, and DDR. Here we provide evidence that in addition to its nuclear localization, Che-1 localizes at interphase centrosomes, where it accumulates following DNA damage or spindle poisons. We show that Che-1 depletion generates supernumerary centrosomes, multinucleated cells, and multipolar spindle formation. Notably, Che-1 depletion abolishes the ability of Chk1 to bind pericentrin and to localize at centrosomes, which, in its turn, deregulates the activation of centrosomal cyclin B-Cdk1 and advances entry into mitosis. Our results reinforce the notion that Che-1 plays an important role in DDR and that its contribution seems to be relevant for the spindle assembly checkpoint.

Hard-metal (WC-Co) particles trigger a signaling cascade involving p38 MAPK, HIF-1α, HMOX1, and p53 activation in human PBMC.

Hard-metals are made of tungsten carbide (WC) and metallic cobalt (Co) particles and are important industrial materials produced for their extreme hardness and high wear resistance properties. While occupational exposure to metallic Co alone is apparently not associated with an increased risk of cancer, the WC-Co particle mixture was shown to increase the risk of lung cancer in exposed workers. We have previously shown that WC-Co specifically induces a burst of reactive oxygen species (ROS) and in vitro mutagenic/apoptogenic effects in human peripheral blood mononucleated cells (PBMC) used as a validated experimental model. In the present study, PBMCs were treated during a short period (15 min) to focus on the very rapid ROS burst induced by WC-Co. We investigated by microarray the response to WC-Co versus Co(2+) ions (CoCl(2)) after 15 min exposure and found that the oxidative stress response HMOX1 gene was highly expressed in WC-Co-treated samples. This result was confirmed by qRT-PCR, and western blotting was carried out to analyze translational and post-translational regulation of genes belonging to the HMOX1 pathway. We show here that WC-Co, and metallic Co particles although with slower kinetics, but not CoCl(2) or WC alone, induced a temporally ordered cascade of events. This cascade implies p38/MAP kinase activation, HIF-1α stabilization, HMOX1 transcriptional activation, and ATM-independent p53 stabilization. These events, and in particular HIF-1α stabilization, could contribute to the carcinogenic activity of WC-Co dusts.

The FANC pathway is activated by adenovirus infection and promotes viral replication-dependent recombination.

Deciphering the crosstalk between a host cell and a virus during infection is important not only to better define viral biology but also to improve our understanding of cellular processes. We identified the FANC pathway as a helper of viral replication and recombination by searching for cellular targets that are modified by adenovirus (Ad) infection and are involved in its outcome. This pathway, which is involved in the DNA damage response and checkpoint control, is altered in Fanconi anaemia, a rare cancer predisposition syndrome. We show here that Ad5 infection activates the FANC pathway independent of the classical DNA damage response. Infection with a non-replicating Ad shows that the presence of viral DNA is not sufficient to induce the monoubiquitination of FANCD2 but still activates the DNA damage response coordinated by phospho-NBS1 and phospho-CHK1. E1A expression alone fails to induce FANCD2 monoubiquitination, indicating that a productive viral infection and/or replication is required for FANC pathway activation. Our data indicate that Ad5 infection induces FANCD2 activation to promote its own replication. Specifically, we show that FANCD2 is involved in the recombination process that accompanies viral DNA replication. This study provides evidence of a DNA damage-independent function of the FANC pathway and identifies a cellular system involved in Ad5 recombination.

Synchronous protein cycling in batch cultures of the yeast Saccharomyces cerevisiae at log growth phase.

The assumption that cells are temporally organized systems, i.e. showing relevant dynamics of their state variables such as gene expression or protein and metabolite concentration, while tacitly given for granted at the molecular level, is not explicitly taken into account when interpreting biological experimental data. This conundrum stems from the (undemonstrated) assumption that a cell culture, the actual object of biological experimentation, is a population of billions of independent oscillators (cells) randomly experiencing different phases of their cycles and thus not producing relevant coordinated dynamics at the population level. Moreover the fact of considering reproductive cycle as by far the most important cyclic process in a cell resulted in lower attention given to other rhythmic processes. Here we demonstrate that growing yeast cells show a very repeatable and robust cyclic variation of the concentration of proteins with different cellular functions. We also report experimental evidence that the mechanism governing this basic oscillator and the cellular entrainment is resistant to external chemical constraints. Finally, cell growth is accompanied by cyclic dynamics of medium pH. These cycles are observed in batch cultures, different from the usual continuous cultures in which yeast metabolic cycles are known to occur, and suggest the existence of basic, spontaneous, collective and synchronous behaviors of the cell population as a whole.

Excess TPX2 Interferes with Microtubule Disassembly and Nuclei Reformation at Mitotic Exit.

The microtubule-associated protein TPX2 is a key mitotic regulator that contributes through distinct pathways to spindle assembly. A well-characterised function of TPX2 is the activation, stabilisation and spindle localisation of the Aurora-A kinase. High levels of TPX2 are reported in tumours and the effects of its overexpression have been investigated in cancer cell lines, while little is known in non-transformed cells. Here we studied TPX2 overexpression in hTERT RPE-1 cells, using either the full length TPX2 or a truncated form unable to bind Aurora-A, to identify effects that are dependent-or independent-on its interaction with the kinase. We observe significant defects in mitotic spindle assembly and progression through mitosis that are more severe when overexpressed TPX2 is able to interact with Aurora-A. Furthermore, we describe a peculiar, and Aurora-A-interaction-independent, phenotype in telophase cells, with aberrantly stable microtubules interfering with nuclear reconstitution and the assembly of a continuous lamin B1 network, resulting in daughter cells displaying doughnut-shaped nuclei. Our results using non-transformed cells thus reveal a previously uncharacterised consequence of abnormally high TPX2 levels on the correct microtubule cytoskeleton remodelling and G1 nuclei reformation, at the mitosis-to-interphase transition.

X-ray irradiated cultures of mouse cortical neural stem/progenitor cells recover cell viability and proliferation with dose-dependent kinetics.

Exposure of the developing or adult brain to ionizing radiation (IR) can cause cognitive impairment and/or brain cancer, by targeting neural stem/progenitor cells (NSPCs). IR effects on NSPCs include transient cell cycle arrest, permanent cell cycle exit/differentiation, or cell death, depending on the experimental conditions. In vivo studies suggest that brain age influences NSPC response to IR, but whether this is due to intrinsic NSPC changes or to niche environment modifications remains unclear. Here, we describe the dose-dependent, time-dependent effects of X-ray IR in NSPC cultures derived from the mouse foetal cerebral cortex. We show that, although cortical NSPCs are resistant to low/moderate IR doses, high level IR exposure causes cell death, accumulation of DNA double-strand breaks, activation of p53-related molecular pathways and cell cycle alterations. Irradiated NSPC cultures transiently upregulate differentiation markers, but recover control levels of proliferation, viability and gene expression in the second week post-irradiation. These results are consistent with previously described in vivo effects of IR in the developing mouse cortex, and distinct from those observed in adult NSPC niches or in vitro adult NSPC cultures, suggesting that intrinsic differences in NSPCs of different origins might determine, at least in part, their response to IR.

Survival of aneuploid, micronucleated and/or polyploid cells: crosstalk between ploidy control and apoptosis.

Microtubule inhibitors are known to block the cell cycle at M-phase, by damaging the mitotic spindle. However, under certain circumstances, cells can escape these effects and become aneuploid, polyploid and/or micronucleated. It is well known that aneuploidy can have adverse effects on human health such as pregnancy wastage, birth defects and the development of human tumours. The present paper aims at reviewing the data our laboratory has accumulated during the last years about the relation between aneuploidy/polyploidy/presence of micronuclei and the induction of apoptosis in human cells after in vitro exposure to the microtubule inhibitor nocodazole. Exposure to high doses of nocodazole results in polyploidy due to mitotic slippage in the absence of a functional spindle. Depending on their p53-status polyploid cells may eventually arrest, die or continue cycling. In these experimental conditions, our data showed that polyploidy does not constitute a strong apoptotic signal. In case of exposure to low concentrations of nocodazole, microtubule depolymerization is disturbed resulting in a spindle with damaged microtubules. This can give rise to chromosome loss and non-disjunction. Our data showed that in particular micronucleated cells, originating from chromosome loss can be eliminated by apoptosis. In addition, nocodazole-induced apoptosis involves the apical caspase-8 and -9 and the effector caspase-3. We show evidence that caspase-3, in addition to its function in apoptosis, plays a role in the formation of micronuclei.

Mitotic checkpoints and the maintenance of the chromosome karyotype.

The goal of the mitotic cell division is the faithful transmission of chromosomes to the daughter cells. To fulfil a correct separation of sister chromatids, kinetochores of all chromosomes should be correctly attached to spindle microtubules of opposite poles and should all be under tension. These events are monitored by the spindle checkpoint, which delays mitotic progression allowing time for corrections when errors occur in the dynamic interactions between chromosomes and microtubules. The G(1) post-mitotic checkpoint constitutes an additional checkpoint preventing further proliferation of cells that have undergone massive spindle damage. This review concentrates on the key structural and protein components which are pivotal for an accurate segregation of chromosomes during anaphase: the chromosome scaffold, sister chromatid cohesion and segregation and the kinetochores in higher eukaryotes. Furthermore, recent advances in understanding spindle and G(1) post-mitotic checkpoint and how they prevent aneuploidization and polyploidization are presented. In a last part the impact of aneuploidy and polyploidy on human health and in particular on cancer development is highlighted.

Assessment of the genotoxic risk of Punica granatum L. (Punicaceae) whole fruit extracts.

Punica granatum L. (Punicaceae) whole fruit extracts, have been used in Cuban traditional medicine as an effective drug for the treatment of respiratory diseases. This species showed interesting anti-viral activity, e.g. aqueous or hydroalcoholic extracts of whole fruits have proved highly active against the influenza virus. However, some toxic properties of this extract have also been reported and, to date, very little is known about its genotoxic properties. In the present study, the genotoxicity of a Punica granatum (pomegranate) whole fruit extract was assessed using different in vitro and in vivo assays that detect DNA damage at different expression levels. Results from reversion and gene-conversion test in microorganisms, sister chromatid exchanges, micronuclei and sperm-shape abnormality assays in mice, clearly showed that the hydroalcoholic extract of P. granatum whole fruits is genotoxic when tested both in vitro and in vivo.

Molecular dissection of cytokinesis by RNA interference in Drosophila cultured cells.

We have used double-stranded RNA-mediated interference (RNAi) to study Drosophila cytokinesis. We show that double-stranded RNAs for anillin, acGAP, pavarotti, rho1, pebble, spaghetti squash, syntaxin1A, and twinstar all disrupt cytokinesis in S2 tissue culture cells, causing gene-specific phenotypes. Our phenotypic analyses identify genes required for different aspects of cytokinesis, such as central spindle formation, actin accumulation at the cell equator, contractile ring assembly or disassembly, and membrane behavior. Moreover, the cytological phenotypes elicited by RNAi reveal simultaneous disruption of multiple aspects of cytokinesis. These phenotypes suggest interactions between central spindle microtubules, the actin-based contractile ring, and the plasma membrane, and lead us to propose that the central spindle and the contractile ring are interdependent structures. Finally, our results indicate that RNAi in S2 cells is a highly efficient method to detect cytokinetic genes, and predict that genome-wide studies using this method will permit identification of the majority of genes involved in Drosophila mitotic cytokinesis.

Towards a New Paradigm in Nano-Genotoxicology: Facing Complexity of Nanomaterials' Cellular Interactions and Effects.

Changes in paradigm contribute to advances in research. The current paradigms for the evaluation of toxicity of chemicals refer to linear or curvilinear dose-response curves with or without threshold and to surface-dependent induction of oxidative damage for particles. The unique physicochemical properties and biological/genotoxic activity of engineered nanomaterials (NMs) require the development of a new paradigm. Because of their unusual dosimetry and their multiple interactions at NM level (agglomeration/aggregation) and at different cellular and extracellular levels, NMs are likely to have complex modes of action (multiple hits at multiple targets) leading to complex thresholded-non-thresholded dose-response curves. Understanding their cellular targets and their modes of action will contribute to the production of safe-by-design NMs. An integrative, cell-by-cell approach for genotoxic effects should be applied to tackle this emerging paradigm in nano-genotoxicology.

Mitotic cell death induction by targeting the mitotic spindle with tubulin-inhibitory indole derivative molecules.

Tubulin-targeting molecules are widely used cancer therapeutic agents. They inhibit microtubule-based structures, including the mitotic spindle, ultimately preventing cell division. The final fates of microtubule-inhibited cells are however often heterogeneous and difficult to predict. While recent work has provided insight into the cell response to inhibitors of microtubule dynamics (taxanes), the cell response to tubulin polymerization inhibitors remains less well characterized. Arylthioindoles (ATIs) are recently developed tubulin inhibitors. We previously identified ATI members that effectively inhibit tubulin polymerization in vitro and cancer cell growth in bulk cell viability assays. Here we characterise in depth the response of cancer cell lines to five selected ATIs. We find that all ATIs arrest mitotic progression, yet subsequently yield distinct cell fate profiles in time-lapse recording assays, indicating that molecules endowed with similar tubulin polymerization inhibitory activity in vitro can in fact display differential efficacy in living cells. Individual ATIs induce cytological phenotypes of increasing severity in terms of damage to the mitotic apparatus. That differentially triggers MCL-1 down-regulation and caspase-3 activation, and underlies the terminal fate of treated cells. Collectively, these results contribute to define the cell response to tubulin inhibitors and pinpoint potentially valuable molecules that can increase the molecular diversity of tubulin-targeting agents.

Tetraploid cells produced by absence of substrate adhesion during cytokinesis are limited in their proliferation and enter senescence after DNA replication.

Tetraploidy has been proposed as an intermediate state in neoplastic transformation due to the intrinsic chromosome instability of tetraploid cells. Despite the identification of p53 as a major factor in growth arrest of tetraploid cells, it is still unclear whether the p53-dependent mechanism for proliferation restriction is intrinsic to the tetraploid status or dependent on the origin of tetraploidy. Substrate adherence is fundamental for cytokinesis completion in adherent untransformed cells. Here we show that untransformed fibroblast cells undergoing mitosis in suspension produce binucleated tetraploid cells due to defective cleavage furrow constriction that leads to incomplete cell abscission. Binucleated cells obtained after loss of substrate adhesion maintain an inactive p53 status and are able to progress into G1 and S phase. However, binucleated cells arrest in G2, accumulate p53 and are not able to enter mitosis as no tetraploid metaphases were recorded after one cell cycle time. In contrast, tetraploid metaphases were found following pharmacological inhibition of Chk1 kinase, suggesting the involvement of the ATR/Chk1 pathway in the G2 arrest of binucleated cells. Interestingly, after persistence in the G2 phase of the cell cycle, a large fraction of binucleated cells become senescent. These findings identify a new pathway of proliferation restriction for tetraploid untransformed cells that seems to be specific for loss of adhesion-dependent cytokinesis failure. This involves Chk1 and p53 activation during G2. Inhibition of growth and entrance into senescence after cytokinesis in suspension may represent an important mechanism to control tumor growth. In fact, anchorage independent growth is a hallmark of cancer and it has been demonstrated that binucleated transformed cells can enter a cycle of anchorage independent growth.

Xanthium strumarium extract inhibits mammalian cell proliferation through mitotic spindle disruption mediated by xanthatin.

ETHNOPHARMACOLOGICAL RELEVANCE: Xanthium strumarium L. is a member of the Asteraceae family popularly used with multiple therapeutic purposes. Whole extracts of this plant have shown anti-mitotic activity in vitro suggesting that some components could induce mitotic arrest in proliferating cells. AIM OF THE SUDY: Aim of the present work was to characterize the anti-mitotic properties of the X. strumarium whole extract and to isolate and purify active molecule(s). MATERIALS AND METHODS: The capacity of the whole extract to inhibit mitotic progression in mammalian cultured cells was investigated to identify its anti-mitotic activity. Isolation of active component(s) was performed using a bioassay-guided multistep separation procedure in which whole extract was submitted to a progressive process of fractionation and fractions were challenged for their anti-mitotic activity. RESULTS: Our results show for the first time that X. strumarium whole extract inhibits assembly of the mitotic spindle and spindle-pole separation, thereby heavily affecting mitosis, impairing the metaphase to anaphase transition and inducing apoptosis. The purification procedure led to a fraction with an anti-mitotic activity comparable to that of the whole extract. Chemical analysis of this fraction showed that its major component was xanthatin. CONCLUSIONS: The present work shows a new activity of X. strumarium extract, i.e. the alteration of the mitotic apparatus in cultured cells that may be responsible for the anti-proliferative activity of the extract. Anti-mitotic activity is shown to be mainly exerted by xanthatin.

The telomeric protein AKTIP interacts with A- and B-type lamins and is involved in regulation of cellular senescence.

AKTIP is a shelterin-interacting protein required for replication of telomeric DNA. Here, we show that AKTIP biochemically interacts with A- and B-type lamins and affects lamin A, but not lamin C or B, expression. In interphase cells, AKTIP localizes at the nuclear rim and in discrete regions of the nucleoplasm just like lamins. Double immunostaining revealed that AKTIP partially co-localizes with lamin B1 and lamin A/C in interphase cells, and that proper AKTIP localization requires functional lamin A. In mitotic cells, AKTIP is enriched at the spindle poles and at the midbody of late telophase cells similar to lamin B1. AKTIP-depleted cells show senescence-associated markers and recapitulate several aspects of the progeroid phenotype. Collectively, our results indicate that AKTIP is a new player in lamin-related processes, including those that govern nuclear architecture, telomere homeostasis and cellular senescence.

Mammalian cell transduction and internalization properties of lambda phages displaying the full-length adenoviral penton base or its central domain.

In recent years a strong effort has been devoted to the search for new, safe and efficient gene therapy vectors. Phage lambda is a promising backbone for the development of new vectors: its genome can host large inserts, DNA is protected from degradation by the capsid and the ligand-exposed D and V proteins can be extensively modified. Current phage-based vectors are inefficient and/or receptor-independent transducers. To produce new, receptor-selective and transduction-efficient vectors for mammalian cells we engineered lambda by inserting into its genome a GFP expression cassette, and by displaying the penton base (Pb) of adenovirus or its central region (amino acids 286-393). The Pb mediates attachment, entry and endosomal escape of adenovirus in mammalian cells, and its central region (amino acids 286-393) includes the principal receptor-binding motif ((340)RGD(342)). Both the phage chimerae lambda Pb and lambda Pb (286-393) were able to transduce cell lines and primary cultures of human fibroblasts. Competition experiments showed that the transduction pathway was receptor-dependent. We also describe the different trafficking properties of lambda Pb and lambda Pb (286-393). Bafilomycin, which blocks endosome maturation, influenced the intracellular distribution of lambda Pb (286-393), but not that of lambda Pb. The proteasome inhibitor MG-132 improved the efficiency of lambda Pb (286-393)-mediated transduction, but not that of lambda Pb. In summary, this work shows the feasibility of using lambda phage as an efficient vector for gene transfer into mammalian cells. We show that lambda Pb and lambda Pb (286-393) can both mediate receptor-dependent transduction; while only lambda Pb is able to promote endosomal escape and proteasome resistance of phage particles.

Amorphous silica nanoparticles alter microtubule dynamics and cell migration.

Amorphous silica nanoparticles (SiO2-NPs) have been studied for their toxic and genotoxic potential. Although contradictory data have been reported and the possible modes of action are not fully elucidated, aneugenic events have been reported, indicating the microtubule (MT) network as a potential target. To investigate this, we examined the effects of 59 nm (10 µg/ml) and 174 nm (7.5 µg/ml) SiO2-NPs on MTs in mitotic and interphase A549 human lung carcinoma cells. No gross morphological changes of the mitotic spindle or induction of multipolar spindles were observed upon SiO2-NPs treatment. The influence of SiO2-NPs on the interphase MTs network dynamics was investigated by in situ depolymerisation/repolymerisation experiments. Results showed a clear increase in MT dynamics after SiO2-NP treatment. Consistent with this, reduced levels of MT acetylation were observed. In addition, live cell microscopy demonstrated that SiO2-NP treatment reduced A549 cell motility. The SiO2-NP doses and conditions (serum-free) used in this study did not induce significant cell toxicity or MN frequencies. Therefore, the effects on MT dynamics, MT acetylation and migration observed, are direct effects of the SiO2-NPs and not a consequence of NP overload or toxic or genotoxic effects.