Connexin hemichannels and pannexin channels in toxicity: Recent advances and mechanistic insights

Connexin hemichannels and pannexin channels are two types of transmembrane channels that allow autocrine/paracrine signalling through the exchange of ions and molecules between the intra- and extracellular compartments. However, owing to the poor selectivity of permeable ions and metabolites, the massive opening of these plasma membrane channels can lead to an excessive influx of toxic substances and an outflux of essential metabolites, such as adenosine triphosphate, glutathione, glutamate and ions, resulting in unbalanced cell homeostasis and impaired cell function. It is becoming increasingly clear that these channels can be activated in response to external stimuli and are involved in toxicity, yet their concrete mechanistic roles in the toxic effects induced by stress and various environmental changes remain poorly defined. This review provides an updated understanding of connexin hemichannels and pannexin channels in response to multiple extrinsic stressors and how these activated channels and their permeable messengers participate in toxicological pathways and processes, including inflammation, oxidative damage, intracellular calcium imbalance, bystander DNA damage and excitotoxicity.

Silver nanoparticles increase connexin43-mediated gap junctional intercellular communication in HaCaT cells through activation of reactive oxygen species and mitogen-activated protein kinase signal pathway

Silver nanoparticles (AgNPs) are widely used in health and consumer products that routinely contact skin. However, the biological effects and possible mechanisms of AgNPs on skin remain unclear. Gap junctional intercellular communication (GJIC) plays a critical role in multicellular organisms to maintain tissue homeostasis. The aim of this study is to examine if non-coated AgNPs affect GJIC in human keratinocytes (HaCaT cells), and to identify the possible molecular mechanisms responsible for the effects. GJIC, connexin (Cx)43 protein and mRNA expression, and the effect of siRNA-mediated knockdown of Cx43 on GJIC were assessed. HaCaT cells exposed to non-coated AgNPs at different doses after a 24 hour exposure. To explore further the underlying mechanism, reactive oxygen species and mitogen-activated protein kinase pathway were evaluated after 2, 6, 12 and 24 hours. Our results revealed that non-coated AgNP exposure at subcytotoxic doses increase GJIC partially via Cx43 upregulation. Reactive oxygen species and extracellular signal-regulated kinase and activation of c-Jun N-terminal kinase were involved in the AgNP-induced upregulation of Cx43. This study provides new insight into the potential mechanism of AgNP biological activity.

Inhibition of gap junction intercellular communication is involved in silica nanoparticles-induced H9c2 cardiomyocytes apoptosis via the mitochondrial pathway

Gap junction intercellular communication (GJIC) between cardiomyocytes is essential for synchronous heart contraction and relies on connexin-containing channels. Connexin 43 (Cx43) is a major component involved in GJIC in heart tissue, and its abnormal expression is closely associated with various cardiac diseases. Silica nanoparticles (SNPs) are known to induce cardiovascular toxicity. However, the mechanisms through which GJIC plays a role in cardiomyocytes apoptosis induced by SNPs remain unknown. The aim of the present study is to determine whether SNPs-decreased GJIC promotes apoptosis in rat cardiomyocytes cell line (H9c2 cells) via the mitochondrial pathway using CCK-8 Kit, scrape-loading dye transfer technique, Annexin V/PI double-staining assays, and Western blot analysis. The results showed that SNPs elicited cytotoxicity in H9c2 cells in a time- and concentration-dependent manner. SNPs also reduced GJIC in H9c2 cells in a concentration-dependent manner through downregulation of Cx43 and upregulation of P-Cx43. Inhibition of gap junctions by gap junction blocker carbenoxolone disodium resulted in decreased survival and increased apoptosis, whereas enhancement of the gap junctions by retinoic acid led to enhanced survival but decreased apoptosis. Furthermore, SNPs-induced apoptosis through the disrupted functional gap junction was correlated with abnormal expressions of the proteins involved in the mitochondrial pathway-related apoptosis such as Bcl-2/Bax, cytochrome C, Caspase-9, and Caspase-3. Taken together, our results provide the first evidence that SNPs-decreased GJIC promotes apoptosis in cardiomyocytes via the mitochondrial pathway. In addition, downregulation of GJIC by SNPs in cardiomyocytes is mediated through downregulation of Cx43 and upregulation of P-Cx43. These results suggest that in rat cardiomyocytes cell line, GJIC plays a protective role in SNPs-induced apoptosis and that GJIC may be one of the targets for SNPs-induced biological effects.

Effects of carbon nanotubes on intercellular communication and involvement of IL-1 genes

An increasing amount of products containing engineered nanoparticles is emerging. Among these particles are carbon nanotubes (CNTs) which are of interest for a wide range of industrial and biomedical applications. There have been raised concerns over the effects of CNTs on human health. Some types of CNTs are classified as group 2B carcinogens by the International Agency for Research on Cancer. CNTs may also induce pulmonary inflammatory and fibrotic effects. By utilizing CNTs of different lengths, we investigated the role of the proinflammatory cytokine, interleukin-1 (IL-1) on gap junctional intercellular communication (GJIC) by using IL-1 wild-type (IL1-WT) and IL-1 knock-out (IL1-KO) cells. GJIC decreased equally in both cell types after CNT exposure. Immunofluorescence staining showed Gja1 and Gjb2 in gap junctions and hemichannels for both cell types. Gjb1 and Gjb2 expression was low in IL1-KO cells, which was confirmed by protein analysis. Gja1 was upregulated with both CNTs, whereas Gjb1 was down-regulated by CNT-2 in IL1-WT cells. Connexin mRNA expression was regulated differently by the CNTs. CNT-1 affected Gja1 and Gjb2, whereas CNT-2 had an effect on Gjb1. CNTs negatively affect GJIC through gap junctions independently of the length of CNT and IL-1 status. Furthermore, connexin gene expression was affected by IL-1 at transcriptional and translational levels. As both CNTs used in this study are cytotoxic to the cells and reduce cell survival, we suggest that CNT-induced reduction in GJIC may be important for inhibiting transfer of cell survival signals between cells.

Genome-wide transcriptional analysis of silica nanoparticle-induced toxicity in zebrafish embryos

Although silica nanoparticles (SiNPs) have a promising application in biomedical fields, there is still a lack of comprehensive understanding of genome-wide transcriptional analysis. This study aims to clarify the toxic effect and molecular mechanisms of SiNPs in zebrafish embryos based on microarray analysis and bioinformatics analysis. Microarray data analysis demonstrated that SiNP-induced toxicity in zebrafish embryos affected expression of 2515 genes, including 1107 genes that were up-regulated and 1408 genes that were down-regulated. These differentially expressed genes were subjected to bioinformatics analysis for exploring the biological processes triggered by SiNPs in zebrafish embryos. Gene ontology analysis showed that SiNPs caused significant changes in gene expression patterns related to many important functions, including response to stimuli, immune response, cellular processes, and embryonic development. In addition, pathway analysis and Signal-net analysis indicated that the gap junction, vascular smooth muscle contraction, and metabolic pathways, apoptosis, the MAPK signaling pathway, the calcium signaling pathway and the JAK-STAT signaling pathway were the most prominent significant pathways in SiNP-induced toxicity in zebrafish embryos. In addition, the results from qRT-PCR and western blot analysis showed that the IL-6 dependent JAK1/STAT3 signaling pathway was activated by SiNPs in zebrafish embryos. In summary, our data will provide compelling clues for further exploration of SiNP-induced toxicity in zebrafish embryos.

Excess titanium dioxide nanoparticles on the cell surface induce cytotoxicity by hindering ion exchange and disrupting exocytosis processes

To date, considerable effort has been devoted to determine the potential toxicity of nanoparticles to cells and organisms. However, determining the mechanism of cytotoxicity induced by different types of nanoparticles remains challenging. Herein, typically low toxicity nanomaterials were used as a model to investigate the mechanism of cytotoxicity induced by low toxicity nanomaterials. We studied the effect of nano-TiO2, nano-Al2O3 and nano-SiO2 deposition films on the ion concentration on a cell-free system simulating the cell membrane. The results showed that the ion concentration of K(+), Ca(2+), Na(+), Mg(2+) and SO4(2-) decreased significantly following filtration of the prepared deposition films. More specifically, at a high nano-TiO2 concentration (200 mg L(-1)) and a long nano-TiO2 deposition time (48 h), the concentration of Na(+) decreased from 2958.01 to 2775.72, 2749.86, 2757.36, and 2719.82 mg L(-1), respectively, for the four types of nano-TiO2 studied. Likewise, the concentration of SO4(2-) decreased from 38.83 to 35.00, 35.80, 35.40, and 35.27 mg L(-1), respectively. The other two kinds of typical low toxicity nanomaterials (nano-Al2O3 and nano-SiO2) have a similar impact on the ion concentration change trend. Adsorption of ions on nanoparticles and the hydrated shell around the ions strongly hindered the ions through the nanoparticle films. The endocytosed nanoparticles could be released from the cells without inducing cytotoxicity. Hindering the ion exchange and disrupting the exocytosis process are the main factors that induce cytotoxicity in the presence of excess nano-TiO2 on the cell surface. The current findings may offer a universal principle for understanding the mechanism of cytotoxicity induced by low toxicity nanomaterials.

Carbon black and titanium dioxide nanoparticles induce distinct molecular mechanisms of toxicity

Increasing evidence link nanomaterials with adverse biological outcomes and due to the variety of applications and potential human exposures to nanoparticles, it is thus important to evaluate their toxicity for the risk assessment of workers and consumers. It is crucial to understand the underlying mechanisms of their toxicity as observation of similar effects after different nanomaterial exposures does not reflect similar intracellular processing and organelle interactions. A thorough understanding of mechanisms is needed not only for accurate prediction of potential toxicological impacts but also for the development of safer nanoapplications by modulating the physicochemical characteristics. Furthermore biomedical applications may also take advantage of an in depth knowledge about the mode of action of nanotoxicity to design new nanoparticle-derived drugs. In the present manuscript we discuss the similarities and differences in molecular pathways of toxicity after carbon black (CB) and titanium dioxide (TiO₂) nanoparticle exposures and identify the main toxicity mechanisms induced by these two nanoparticles which may also be indicative for the mode of action of other insoluble nanomaterials. We address the translocation, cell death induction, genotoxicity, and inflammation induced by TiO₂ and CB nanoparticles which depend on their internalization, reactive oxygen species (ROS) production capacities and/or protein interactions. We summarize their distinct cellular mechanisms of toxicity and the crucial steps which may be targeted to avoid adverse effects or to induce them for nanomedical purposes. Several physicochemical characteristics could influence these general toxicity pathways depicted here and the identification of common toxicity pathways could support the grouping of nanomaterials in terms of toxicity.

1,4-naphthoquinones: from oxidative damage to cellular and inter-cellular signaling

Naphthoquinones may cause oxidative stress in exposed cells and, therefore, affect redox signaling. Here, contributions of redox cycling and alkylating properties of quinones (both natural and synthetic, such as plumbagin, juglone, lawsone, menadione, methoxy-naphthoquinones, and others) to cellular and inter-cellular signaling processes are discussed: (i) naphthoquinone-induced Nrf2-dependent modulation of gene expression and its potentially beneficial outcome; (ii) the modulation of receptor tyrosine kinases, such as the epidermal growth factor receptor by naphthoquinones, resulting in altered gap junctional intercellular communication. Generation of reactive oxygen species and modulation of redox signaling are properties of naphthoquinones that render them interesting leads for the development of novel compounds of potential use in various therapeutic settings.

Biological effects induced by BSA-stabilized silica nanoparticles in mammalian cell lines

Much of the concerns regarding engineered nanoparticle (NP) toxicity are based on knowledge from previous studies on particles in ambient air or occupational situations. E.g., the effects of exposure to silica dust particles have been studied intensely due to the carcinogenicity of crystalline silica. However, the increasing usage of engineered amorphous silica NPs has emphasized the need for further mechanistic insight to predict the consequences of exposure to the amorphous type of silica NPs. The present study focused on the in vitro biological effects following exposure to well-dispersed, BSA-stabilized, amorphous silica NPs whereas unmodified silica NPs where included for reasons of comparison. The cytotoxicity of the silica NPs was investigated in six different cell lines (A549, THP-1, CaCo-2, ASB-XIV, J-774A.1, and Colon-26) selected to explore the significance of organ and species sensitivity in vitro. Viability data demonstrated that macrophages were most sensitive to silica NP and interestingly, murine cell lines were generally found to be more sensitive than comparable human cell lines. Further studies were conducted in the human epithelial lung cell line, A549, to explore the molecular mechanism of silica toxicity. Generation of reactive oxygen species, one of the proposed toxicological mechanisms of NPs, was investigated in A549 cells by the dichlorofluorescin (DCF) assay to be significantly induced at NP concentrations above 113 μg/mL. However, induction of oxidative stress related pathways was not found after silica NP exposure for 24 h in gene array studies conducted in A549 cells at a relatively low NP concentration (EC20). Up-regulated genes (more than 2-fold) were primarily related to lipid metabolism and biosynthesis whereas down-regulated genes included several processes such as transcription, cell junction, extra cellular matrix (ECM)-receptor interaction and others. Thus, gene expression data proposes that several cellular processes other than oxidative stress could be affected by exposure to silica NPs.

Biocompatibility assessment of Si-based nano- and micro-particles

Silicon is one of the most abundant chemical elements found on the Earth. Due to its unique chemical and physical properties, silicon based materials and their oxides (e.g. silica) have been used in several industries such as building and construction, electronics, food industry, consumer products and biomedical engineering/medicine. This review summarizes studies on effects of silicon and silica nano- and micro-particles on cells and organs following four main exposure routes, namely, intravenous, pulmonary, dermal and oral. Further, possible genotoxic effects of silica based nanoparticles are discussed. The review concludes with an outlook on improving and standardizing biocompatibility assessment for nano- and micro-particles.

Force dependent internalization of magnetic nanoparticles results in highly loaded endothelial cells for use as potential therapy delivery vectors

PURPOSE

To investigate the kinetics, mechanism and extent of MNP loading into endothelial cells and the effect of this loading on cell function.

METHODS

MNP uptake was examined under field on/off conditions, utilizing varying magnetite concentration MNPs. MNP-loaded cell viability and functional integrity was assessed using metabolic respiration, cell proliferation and migration assays.

RESULTS

MNP uptake in endothelial cells significantly increased under the influence of a magnetic field versus non-magnetic conditions. Larger magnetite density of the MNPs led to a higher MNP internalization by cells under application of a magnetic field without compromising cellular respiration activity. Two-dimensional migration assays at no field showed that higher magnetite loading resulted in greater cell migration rates. In a three-dimensional migration assay under magnetic field, the migration rate of MNP-loaded cells was more than twice that of unloaded cells and was comparable to migration stimulated by a serum gradient.

CONCLUSIONS

Our results suggest that endothelial cell uptake of MNPs is a force dependent process. The in vitro assays determined that cell health is not adversely affected by high MNP loadings, allowing these highly magnetically responsive cells to be potentially beneficial therapy (gene, drug or cell) delivery systems.

Mitigation of quantum dot cytotoxicity by microencapsulation

When CdSe/ZnS-polyethyleneimine (PEI) quantum dots (QDs) are microencapsulated in polymeric microcapsules, human fibroblasts are protected from acute cytotoxic effects. Differences in cellular morphology, uptake, and viability were assessed after treatment with either microencapsulated or unencapsulated dots. Specifically, QDs contained in microcapsules terminated with polyethylene glycol (PEG) mitigate contact with and uptake by cells, thus providing a tool to retain particle luminescence for applications such as extracellular sensing and imaging. The microcapsule serves as the “first line of defense” for containing the QDs. This enables the individual QD coating to be designed primarily to enhance the function of the biosensor.