Assessing the effects of silver nanoparticles on monolayers of differentiated Caco-2 cells, as a model of intestinal barrier

Silver nanoparticles induce iron accumulation-associated cognitive impairment via modulating neuronal ferroptosis

Silver nanoparticles (AgNPs) are widely used in daily life and medical fields owing to their unique physicochemical properties. Daily exposure to AgNPs has become a great concern regarding their potential toxicity to human beings, especially to the central nervous system. Ferroptosis, a newly recognized programmed cell death, was recently reported to be associated with the neurodegenerative process. However, whether and how ferroptosis contributes to AgNPs-induced neurotoxicity remain unclear. In this study, we investigated the role of ferroptosis in neurotoxic effects induced by AgNPs using in vitro and in vivo models. Our results showed that AgNPs induced a notable dose-dependent cytotoxic effect on HT-22 cells and cognitive impairment in mice as indicated by a decline in learning and memory and brain tissue injuries. These findings were accompanied by iron overload caused by the disruption of the iron transport system and activation of NCOA4-mediated autophagic degradation of ferritin. The excessive free iron subsequently induced GSH depletion, loss of GPX and SOD activities, differential expression of Nrf2 signaling pathway elements, down-regulation of GPX4 protein and production of lipid peroxides, initiating ferroptosis cascades. The mitigating effects of ferrostatin-1 and deferoxamine on iron overload, redox imbalance, neuronal cell death, impairment of mice learning and memory, Aβ deposition and synaptic plasticity reduction suggested ferroptosis as a potential molecular mechanism in AgNPs-induced neurotoxicity. Taken together, these results demonstrated that AgNPs induced neuronal cell death and cognitive impairment with Aβ deposition and reduction of synaptic plasticity, which were mediated by ferroptosis caused by iron-mediated lipid peroxidation. Our study provides new insights into the underlying mechanisms of AgNPs-induced neurotoxicity and predicts potential preventive strategies.

Investigation of cellular uptake and transport capacity of Cordyceps sinensis exopolysaccharide‑selenium nanoparticles with different particle sizes in Caco-2 cell monolayer

Cordyceps sinensis exopolysaccharide‑selenium nanoparticles (EPS-SeNPs) were successfully constructed, characterized, and its Se release kinetics and mechanism were also evaluated in our previous studies. However, the intestinal cellular uptake and transport capacities of EPS-SeNPs remain unknown. On the basis of our previous researches, this work was designed to evaluate the uptake and transport capacities of EPS-SeNPs (EPS/Se = 20/1, 3/1, 1/1, and 3/4) in intestinal epithelial (Caco-2) cells. Confocal laser scanning microscopy results indicated that the internalization of coumarin-6 labeled EPS-SeNPs was in a time-dependent process and eventually located in the cytoplasm, not in the nucleus. Endocytosis inhibitors were employed to evaluate the cellular uptake pathway of EPS-SeNPs, relevant results revealed that clathrin-, caveolae-, and energy-mediated pathways were participated in the internalization of EPS-SeNPs by Caco-2 cells. In addition, the transportation of EPS-SeNPs across Caco-2 cell monolayers was in a concentration-dependent manner. Different particle sizes of EPS-SeNPs presented different uptake and transport capacities in Caco-2 cells. Noteworthy, EPS/Se = 3/4 with the highest selenium content possessed the most superior cellular uptake and transport abilities in Caco-2 cells. The present work may contribute to illustrate the internalization and transport mechanism of EPS-SeNPs, thus facilitating its application in food and medical industries.

Nanomaterial genotoxicity evaluation using the high-throughput p53-binding protein 1 (53BP1) assay

Toxicity evaluation of engineered nanomaterials is challenging due to the ever increasing number of materials and because nanomaterials (NMs) frequently interfere with commonly used assays. Hence, there is a need for robust, high-throughput assays with which to assess their hazard potential. The present study aimed at evaluating the applicability of a genotoxicity assay based on the immunostaining and foci counting of the DNA repair protein 53BP1 (p53-binding protein 1), in a high-throughput format, for NM genotoxicity assessment. For benchmarking purposes, we first applied the assay to a set of eight known genotoxic agents, as well as X-ray irradiation (1 Gy). Then, a panel of NMs and nanobiomaterials (NBMs) was evaluated with respect to their impact on cell viability and genotoxicity, and to their potential to induce reactive oxygen species (ROS) production. The genotoxicity recorded using the 53BP1 assay was confirmed using the micronucleus assay, also scored via automated (high-throughput) microscopy. The 53BP1 assay successfully identified genotoxic compounds on the HCT116 human intestinal cell line. None of the tested NMs showed any genotoxicity using the 53BP1 assay, except the positive control consisting in (CoO)(NiO) NMs, while only TiO2 NMs showed positive outcome in the micronucleus assay. Only Fe3O4 NMs caused significant elevation of ROS, not correlated to DNA damage. Therefore, owing to its adequate predictivity of the genotoxicity of most of the tested benchmark substance and its ease of implementation in a high throughput format, the 53BP1 assay could be proposed as a complementary high-throughput screening genotoxicity assay, in the context of the development of New Approach Methodologies.

Transformation, Absorption and Toxicological Mechanisms of Silver Nanoparticles in the Gastrointestinal Tract Following Oral Exposure

Oral exposure is known as the primary way for silver nanoparticles (AgNPs), which are commonly used as food additives or antibacterial agents in commercial products, to enter the human body. Although the health risk of AgNPs has been a concern and extensively researched over the past few decades, there are still numerous knowledge gaps that need to be filled to disclose what AgNPs experience in the gastrointestinal tract (GIT) and how they cause oral toxicity. In order to gain more insight into the fate of AgNPs in the GIT, the main gastrointestinal transformation of AgNPs, including aggregation/disaggregation, oxidative dissolution, chlorination, sulfuration, and corona formation, is first described. Second, the intestinal absorption of AgNPs is presented to show how AgNPs interact with epithelial cells and cross the intestinal barrier. Then, more importantly, we make an overview of the mechanisms underlying the oral toxicity of AgNPs in light of recent advances as well as the factors affecting the nano-bio interactions in the GIT, which have rarely been thoroughly elaborated in published literature. At last, we emphatically discuss the issues that need to be addressed in the future to answer the question "How does oral exposure to AgNPs cause detrimental effects on the human body?".

Cytotoxic and genotoxic properties of silver nanoparticles synthesized by ethanolic extract of Salacia chinensis

In this study, in vitro and in vivo methods were used to evaluate the cytotoxicity and genotoxicity properties of silver nanoparticles (Ag-NPs) made from a crude ethanolic extract of Salacia chinensis. The test Ag-NPs had no cytotoxicity on the fibroblast cell line at a concentration of 100 μg/mL, according to the MTT assay results. The Chinese hamster ovary (CHO) cell line treated with varied concentrations of test Ag-NPs, with a maximum concentration of 200 μg/mL, did not exhibit any appreciable genotoxic activity, either by comparing the results with positive controls of genotoxicity caused by Methyl methane sulfonate and Benzo (a) pyrene at the concentration of 20 μg/mL, the lack of genotoxicity was established. An in vivo study in Swiss albino mice using various concentrations (250, 500, and 1000 mg/kg) of test Ag-NPs, which were compared with positive controls, further confirmed this in vitro result pattern. Contrary to the genotoxicity caused by the positive control, mouse bone marrow micronucleus testing findings revealed the absence of genotoxicity. These findings imply that at the measured doses, the Ag-NPs produced from the crude ethanolic extract of Salacia chinensis do not exhibit any cytotoxicity or genotoxicity.

Bedaquiline fumarate microemulsion: formulation optimization, rheological characterization and in vitro studies

Aim: Bedaquiline fumarate (BQF), an antitubercular drug, shows limited bioavailability due to solubility-limited intestinal absorption. In this research, the authors formulated a BQF-loaded microemulsion to improve BQF's oral bioavailability. Methods: Microemulsion was prepared by a spontaneous emulsification method and evaluated for thermodynamic stability, size, dispersibility, transmittance, rheology, microrheology, drug release, cytotoxicity and cellular uptake. Results: Microemulsion showed an average globule size of 26.50 ± 6.29 nm with spherical geometry and revealed gel-sol-gel behavior in microrheological studies. Cytotoxicity and cell uptake studies in Caco-2 cells showed that BQF microemulsion was cytocompatible at the highest concentration of 500 μg/ml with significantly higher cellular uptake than control. Conclusion: The present study indicates that BQF microemulsion could be explored further for effective treatment of multidrug-resistant tuberculosis.

Investigation of eight cellulose nanomaterials' impact on Differentiated Caco-2 monolayer integrity and cytotoxicity

The potential applications of cellulose nanomaterials (CNMs) as food additives or in food packaging, present a possible source of human ingestion. While micron- and macro-scale cellulose products are classified as Generally Regarded As Safe, the safety of ingested nano-scale cellulose is largely unknown. Using fully differentiated Caco-2 cells, the perturbation of intestinal barrier function and cytotoxicity was investigated for four nanocellulose crystals (CNCs) and four nanocellulose fibrils (CNFs) following 24 h of exposure at 50 μg/mL. Scanning electron microscope showed some aggregation of both CNCs and CNFs. X-ray photoelectron spectroscopy analyses showed that carbon and oxygen were the main elements. The zeta-potential for CNMs formulated in cell culture medium showed a negative surface charge. Two CNMs increased cell membrane permeability and three CNMs decreased the cell metabolic activity. While three CNMs lead to cytotoxic responses, no changes in apparent permeability coefficient (P_app) for dextran or tight junction integrity were found. Our results show that three CNMs induce cytotoxicity in differentiated Caco-2 cells, demonstrating the need to understand the role of size and shape. The interaction between CNMs and the intestinal epithelium needs to be evaluated to understand potential intestinal barrier dysfunction and resulting health implications following CNM ingestion.

Effects of soil-extractable metals Cd and Ni from an e-waste dismantling site on human colonic epithelial cells Caco-2: Mechanisms and implications

E-wastes release toxic metals including Cd, Cu, Ni, Pb and Zn into nearby soils during dismantling process. However, their adverse effects and the associated mechanisms on human intestinal epithelium are poorly understood. In this study, their toxic effects on human colonic epithelial cells Caco-2 and the underlying mechanisms were assessed basing on three soils from Wenling e-waste dismantling site. Since soil-extractable metals are more available for gastrointestinal absorption, we used phosphate buffer saline solution to extract metals at solid to liquid ratio of 1:2. Among metals, total Cd and Ni exceeded the risk screening values in three soils, being 3.8-8.8 and 42.4-155 mg/kg. Furthermore, high extractable-metals at 5.9, 1.9, and 0.87 mg/kg Cd (20-67%) and 4.6, 6.4, and 12.4 mg/kg Ni (3.6-29%) were observed for Soil-1, -2 and -3, respectively. All three extracts triggered cytotoxicity, with Soil-2 showing the strongest inhibition of cell viability. Higher production of reactive oxygen species and stronger inhibition of antioxidant enzymes SOD1 and CAT were observed in Soil-2 and -3. Upregulation of proinflammatory mediators (IL-1β, IL-8 and TNF-α) and apoptosis-regulatory genes (GADD45α, Caspase-3, and Caspase-8) were observed. Our data suggest that soil extracts induced cytotoxicity, oxidative damage, inflammatory response, and cell apoptosis in Caco-2 cells, indicating soil ingestion from e-waste dismantling site may adversely impact human health.

Recent Advances and Mechanistic Insights into Antibacterial Activity, Antibiofilm Activity, and Cytotoxicity of Silver Nanoparticles

The substantial increase in multidrug-resistant (MDR) pathogenic bacteria is a major threat to global health. Recently, the Centers for Disease Control and Prevention reported possibilities of greater deaths due to bacterial infections than cancer. Nanomaterials, especially small-sized (size ≤10 nm) silver nanoparticles (AgNPs), can be employed to combat these deadly bacterial diseases. However, high reactivity, instability, susceptibility to fast oxidation, and cytotoxicity remain crucial shortcomings for their uptake and clinical application. In this review, we discuss various AgNPs-based approaches to eradicate bacterial infections and provide comprehensive mechanistic insights and recent advances in antibacterial activity, antibiofilm activity, and cytotoxicity (both in vitro and in vivo) of AgNPs. The mechanistic of antimicrobial activity involves four steps: (i) adhesion of AgNPs to cell wall/membrane and its disruption; (ii) intracellular penetration and damage; (iii) oxidative stress; and (iv) modulation of signal transduction pathways. Numerous factors affecting the bactericidal activity of AgNPs such as shape, size, crystallinity, pH, and surface coating/charge have also been described in detail. The review also sheds light on antimicrobial photodynamic therapy and the role of AgNPs versus Ag⁺ ions release in bactericidal activities. In addition, different methods of synthesis of AgNPs have been discussed in brief.

Nanoparticle Food Applications and Their Toxicity: Current Trends and Needs in Risk Assessment Strategies

ABSTRACT

Nanotechnology has developed into one of the most groundbreaking scientific fields in the last few decades because it exploits the enhanced reactivity of materials at the atomic scale. The current classification of nanoparticles (NPs) used in foods is outlined in relation to the production and physicochemical characteristics. This review aims to concisely present the most popular and widely used inorganic and organic NPs in food industries. Considering that the toxicity of NPs is often associated with chemical reactivity, a series of in vitro toxicity studies are also summarized, integrating information on the type of NP studies and reported specifications, type of cells used, exposure conditions, and assessed end points. The important role of the digestive system in the absorption and distribution of nanoformulated foods within the body and how this affects the resultant cytotoxicity. Examples of how NPs and their accumulation within different organs are presented in relation to the consumption of specific foods. Finally, the role of developing human health risk assessments to characterize both the potential impact of the hazard and the likelihood or level of human exposure is outlined. Uncertainties exist around risk and exposure assessments of NPs due to limited information on several aspects, including toxicity, behavior, and bioaccumulation. Overall, this review presents current trends and needs for future assessments in toxicity evaluation to ensure the safe application of NPs in the food industry.

HIGHLIGHTS

Cellular and Molecular Mechanisms of Toxicity of Ingested Titanium Dioxide Nanomaterials

An exponential increase in products containing titanium dioxide nanomaterials (TiO2), in agriculture, food and feed industry, lead to increased oral exposure to these nanomaterials (NMs). Thus, the gastrointestinal tract (GIT) emerges as a possible route of exposure that may drive systemic exposure, if the intestinal barrier is surpassed. NMs have been suggested to produce adverse outcomes, such as genotoxic effects, that are associated with increased risk of cancer, leading to a concern for public health. However, to date, the differences in the physicochemical characteristics of the NMs studied and other variables in the test systems have generated contradictory results in the literature. Processes like human digestion may change the NMs characteristics, inducing unexpected toxic effects in the intestine. Using TiO2 as case-study, this chapter provides a review of the works addressing the interactions of NMs with biological systems in the context of intestinal tract and digestion processes, at cellular and molecular level. The knowledge gaps identified suggest that the incorporation of a simulated digestion process for in vitro studies has the potential to improve the model for elucidating key events elicited by these NMs, advancing the nanosafety studies towards the development of an adverse outcome pathway for intestinal effects.

A harmonized and standardized in vitro approach produces reliable results on silver nanoparticles toxicity in different cell lines

Despite the widespread use of silver nanoparticles (AgNPs) in different fields and the amount of investigations available, to date, there are many contradictory results on their potential toxicity. In the present study, extensively characterized 20-nm AgNPs were investigated using optimized protocols and standardized methods to test several toxicological endpoints in different cell lines. The agglomeration/aggregation state of AgNPs in culture media was measured by dynamic light scattering (DLS). DNA and chromosomal damage on BEAS-2B and RAW 264.7 cells were evaluated by comet and micronucleus assays, while oxidative DNA damage by modified comet assay and 8-oxodG/8-oxodA detection. We also investigated immunotoxicity and immunomodulation by cytokine release and NO production in RAW 264.7 and MH-S cells, with or without lipopolysaccharide (LPS) stimulus. Transmission electron microscope (TEM) analysis was used to analyze cellular uptake of AgNPs. Our results indicate different values of AgNPs hydrodynamic diameter depending on the medium, some genotoxic effect just on BEAS-2B and no or slight effects on function of RAW 264.7 and MH-S in absence or presence of LPS stimulus. This study highlights the relevance of using optimized protocols and multiple endpoints to analyze the potential toxicity of AgNPs and to obtain reliable and comparable results.

Caco-2 in vitro model of human gastrointestinal tract for studying the absorption of titanium dioxide and silver nanoparticles from seafood

Titanium dioxide nanoparticles (TiO₂ NPs) are widely used in industry as a white pigment (paints, paper industry and toothpastes), photocatalysts (environmental decontamination and photovoltaic cells), inorganic UV filter (sunscreens and personal care products) and as a food additive (E171) and antimicrobial food packaging material. Silver nanoparticles (Ag NPs) are used in photonics, microelectronics, catalysis and medicine due to their catalytic activity, magnetic and optical polarizability, electrical and thermal conductivities and enhanced Raman scattering. They also have antibacterial, antifungal and antiviral activities, as well as anti-inflammatory potential. The huge increase in the use of nano-based products, mainly metallic NPs, implies the presence of nanomaterials in the environment, and hence, the unintentional human ingestion through water or foods (gastrointestinal tract is the main pathway of NPs intake in humans). The presence of TiO₂ NPs and Ag NPs in seafood samples was firstly established using an ultrasound assisted enzymatic hydrolysis procedure and sp-ICP-MS analysis. Several clams, cockles, mussels, razor clams, oysters and variegated scallops, which contain TiO₂ NPs and Ag NPs, were subjected to an in vitro digestion process simulating human gastrointestinal digestion in the stomach and in the small and large intestine to determine the bioaccessibility of these NPs. Caco-2 cells were selected as model of human intestinal epithelium for transport studies because of the development of membrane transporters that are responsible for the uptake of chemicals. Parameters as transepithelial electrical resistance (TEER) and permeability of Lucifer Yellow were studied for establishing cell monolayer integrity. TiO₂ NPs and Ag NPs transport as well as total Ti and Ag concentrations passing through the gastrointestinal epithelial barrier model (0-2 h) were assessed by sp-ICP-MS and ICP-MS in several molluscs.

In vivo assessments for predicting the bioavailability of nanoencapsulated food bioactives and the safety of nanomaterials

Use of nano-sized materials to design novel delivery systems is actually a double-edged sword regarding the enhancement of absorption and bioavailability of encapsulated bioactives as well as the unpredictable phenomena inside the living cells causing health concerns. So, comprehensive investigations on the use of nanomaterials in foods and their biological fate are needed. To reach this goal, both in vitro and in vivo techniques have been extensively applied. Besides the in vitro models such as cell culture and yeast/bacteria, different live animal models like mice, rat, Drosophila melanogaster, Caenorhabditis elegans, Zebrafish and dog can be applied to study bioavailability and safety of nanodelivery systems. However, considering the low correlation between the achieved results of in vitro and in vivo assays, in vivo tests are the first priority due to providing a real physiological condition. On the other hand, uncorrelated results by in vivo assays represent a serious problem to compare them. To defeat the issues in setting an in vivo research for the nanodelivery systems, all restrictions and FDA regulations is likely to be considered to improve the assays authenticity. This review takes a comprehensive look at the different types of in vivo assays and model organisms that has been utilized for the investigation of bioavailability, release profile and possible toxicity of food-based nanomaterials so far.

Ameliorative effect of graphene nanosheets against arsenic-induced toxicity in mice by oral exposure

Arsenic is a highly toxic environmental pollutant that is abundant in waters around the world. Previous studies have indicated that the toxicity of arsenic can be affected by co-exposure to chemicals. As a promising two-dimensional nanomaterial, graphene has received extensive attention in recent years. Owing to the wide application of graphene, its co-exposure with arsenic increases significantly. However, little information is available on the combined toxicity of graphene and arsenic. This study aimed to investigate the effects of graphene on arsenic-induced toxicity by oral exposure in mice. The results showed that the addition of graphene significantly reduced the toxicity of arsenic in the intestine and liver of mice, and the high-concentration graphene exhibited greater toxicity reduction effects. Graphene significantly decreased the bioavailability of arsenic because of its adsorption capability and spatial structure. In addition, graphene adhered to the intestinal tract may also prevent arsenic from passing through the intestinal epithelium freely. Moreover, graphene also affected arsenic toxicity through enhanced metabolism transformation by changing the gut microbiota. This study provides a comprehensive understanding of the combined toxicity of graphene and arsenic.

Model Complexity as Determining Factor for In Vitro Nanosafety Studies: Effects of Silver and Titanium Dioxide Nanomaterials in Intestinal Models

With the rising interest in the effects of orally ingested engineered nanomaterials (ENMs), much effort is undertaken to develop and advance intestinal in vitro models. The cytotoxic, proinflammatory, and DNA damaging properties of polyvinylpyrrolidone-capped silver (Ag-PVP) and titanium dioxide (TiO₂ , P25) ENM in four in vitro models of increasing complexity-from proliferating Caco-2 and HT29-MTX-E12 monocultures to long-term transwell triple cultures including THP-1 macrophages to reproduce the human intestine in healthy versus inflamed-like state-are studied. Results are compared against in vivo effects of the same ENM through intestinal tissue analysis from 28-day oral exposure studies in mice. Adverse responses are only observed in monocultures and suggest toxic potential for both ENM, typically showing stronger effects for Ag-PVP than for TiO₂ . By contrast, no adverse effects are observed in either the transwell cultures or the analyzed murine tissues. The data provide further support that monoculture models represent a cost and time efficient tool for early-phase hazard assessment. However, the observed similarities in morphology and ENM effects in murine intestinal tissue and the in vitro triple culture model suggest that advanced multifacetted research questions concerning oral ENM exposure are more adequately addressed by the more complex and time intensive models.

Repeated exposure of Caco-2 versus Caco-2/HT29-MTX intestinal cell models to (nano)silver in vitro: Comparison of two commercially available colloidal silver products

Colloidal silver products are sold for a wide range of disinfectant and health applications. This has increased the potential for human exposure to silver nanoparticles (AgNPs) and ions (Ag⁺), for which oral ingestion is considered to be a major route of exposure. Our objective was to evaluate and compare the toxicity of two commercially available colloidal silver products on two human intestinal epithelial models under realistic exposure conditions. Mesosilver™ and AgC were characterized and a concentration range between 0.1 and 12 μg/mL chosen. Caco-2 cells vs. co-culture of Caco-2 and mucus-secreting HT29-MTX cells (90/10) were used. Repeated exposure was carried out to determine cell viability over 18 days of cell differentiation in 24-well plates. Selected concentrations (0.1, 1, and 3 μg/mL) were tested on cells cultured in E-plates and Transwells with the same repeated exposure regimen, to determine cell impedance, and cell viability and trans-epithelial electrical resistance (TEER), respectively. Silver uptake, intracellular localisation, and translocation were determined by CytoViva™, HIM-SIMS, and ICP-MS. Genotoxicity was determined on acutely-exposed proliferating Caco-2 cells by γH2AX immunofluorescence staining. Repeated exposure of a given concentration of AgC, which is composed solely of ionic silver, generally exerted more toxic effects on Caco-2 cells than Mesosilver™, which contains a mix of AgNPs and ionic silver. Due to its patchy structure, the presence of mucus in the Caco-2/HT29-MTX co-culture only slightly mitigated the deleterious effects on cell viability. Increased genotoxicity was observed for AgC on proliferating Caco-2 cells. Silver uptake, intracellular localisation, and translocation were similar. In conclusion, Mesosilver™ and AgC colloidal silver products show different levels of gut toxicity due to the forms of distinct silver (AgNPs and/or Ag⁺) contained within. This study highlights the applicability of high-resolution (chemical) imaging to detect and localize silver and provides insights into its uptake mechanisms, intracellular fate and cellular effects.

Differential effect of silver nanoparticles on the microbiome of adult and developing planaria

Silver nanoparticles (AgNPs) are widely incorporated in household, consumer and medical products. Their unintentional release via wastewaters raises concerns on their environmental impact, particularly for aquatic organisms and their associated bacterial communities. It is known that the microbiome plays an important role in its host's health and physiology, e.g. by producing essential nutrients and providing protection against pathogens. A thorough understanding of the effects of AgNPs on bacterial communities and on their interactions with the host is crucial to fully assess AgNP toxicity on aquatic organisms. Our results indicate that the microbiome of the invertebrate Schmidtea mediterranea, a freshwater planarian, is affected by AgNP exposure at the tested 10 μg/ml concentration. Using targeted amplification of the bacterial 16S rRNA gene V3-V4 region, two independent experiments on the microbiomes of adult worms revealed a consistent decrease in Betaproteobacteriales after AgNP exposure, mainly attributed to a decrease in Curvibacter and Undibacterium. Although developing tissues and organisms are known to be more sensitive to toxic compounds, three independent experiments in regenerating worms showed a less pronounced effect of AgNP exposure on the microbiome, possibly because underlying bacterial community changes during development mask the AgNP induced effect. The presence of a polyvinyl-pyrrolidone (PVP) coating did not significantly alter the outcome of the experiments compared to those with uncoated particles. The observed variation between the different experiments underlines the highly variable nature of microbiomes and emphasises the need to repeat microbiome experiments, within and between physiological states of the animal.

Investigating the effects of differently produced synthetic amorphous silica (E 551) on the integrity and functionality of the human intestinal barrier using an advanced in vitro co-culture model

E 551, also known as synthetic amorphous silica (SAS), is the second most produced food additive. However, according to the re-evaluation of E 551 by the European Food Safety Authority (EFSA) in 2018, the amount of available data on the oral toxicity of food grade E 551 is still insufficient for reliable risk assessment. To close this gap, this study aimed to investigate six food-grade SAS with distinct physicochemical properties on their interaction with the intestinal barrier using advanced in vitro intestinal co-cultures and to identify potential structure-activity relationships. A mucus-secreting Caco-2/HT-29/Raji co-culture model was treated with up to 50 µg/ml SAS for 48 h, which represents a dose range relevant to dietary exposure. No effects on cell viability, barrier integrity, microvilli function or the release of inflammatory cytokine were detected after acute exposure. Slight biological responses were observed for few SAS materials on iron uptake and gene expression levels of mucin 1 and G-protein coupled receptor 120 (GPR120). There was no clear correlation between SAS properties (single or combined) and the observed biological responses. Overall, this study provides novel insights into the short-term impact of food-relevant SAS with distinct characteristics on the intestinal epithelium including a range of intestine-specific functional endpoints. In addition, it highlights the importance of using advanced intestinal co-cultures embracing relevant cell types as well as a protective mucus barrier to achieve a comprehensive understanding of the biological response of food additives at the intestinal barrier in vitro.

Implications of the Human Gut-Brain and Gut-Cancer Axes for Future Nanomedicine

Recent clinical and pathological evidence have implicated the gut microbiota as a nexus for modulating the homeostasis of the human body, impacting conditions from cancer and dementia to obesity and social behavior. The connections between microbiota and human diseases offer numerous opportunities in medicine, most of which have limited or no therapeutic solutions available. In light of this paradigm-setting trend in science, this review aims to provide a comprehensive and timely summary of the mechanistic pathways governing the gut microbiota and their implications for nanomedicines targeting cancer and neurodegenerative diseases. Specifically, we discuss in parallel the beneficial and pathogenic relationship of the gut microbiota along the gut-brain and gut-cancer axes, elaborate on the impact of dysbiosis and the gastrointestinal corona on the efficacy of nanomedicines, and highlight a molecular mimicry that manipulates the universal cross-β backbone of bacterial amyloid to accelerate neurological disorders. This review further offers a forward-looking section on the rational design of cancer and dementia nanomedicines exploiting the gut-brain and gut-cancer axes.

Physico-chemical characterisation of the fraction of silver (nano)particles in pristine food additive E174 and in E174-containing confectionery

Silver (E174) is authorised as a food additive in the EU. The unknown particle size distribution of E174 is a specific concern for the E174 risk assessment. This study characterised the fraction of silver (nano)particles in 10 commercially available pristine E174 food additives and 10 E174-containing products by transmission electron microscopy (TEM) and single-particle inductively coupled plasma-mass spectrometry (spICP-MS). TEM analysis showed that all samples contained micrometre-sized flakes and also a fraction of (nano)particles. Energy-dispersive X-ray spectroscopy (EDX) and electron diffraction confirmed that the (nano)particles and micrometre-sized flakes consisted of silver. A higher amount of (nano)particles was observed in the products than in the food additives. In addition, the surface of the micrometre-sized flakes was rougher in products. The median of the minimum external dimension, assessed as minimal Feret diameter, of the fraction of (nano)particles determined by quantitative TEM analysis was 11 ± 4 nm and 18 ± 7 nm (overall mean ± standard deviation), for food additives and products, respectively. Similar size distributions were obtained by spICP-MS and TEM, considering the limit of detection of spICP-MS. The median of the equivalent spherical diameter of the fraction of (nano)particles determined by spICP-MS was 19 ± 4 nm and 21 ± 2 nm (overall mean ± standard deviation), for food additives and products, respectively. In all samples, independent of the choice of technique, the nano-sized particles represented more than 97% (by number) of the silver particles, even though the largest mass of silver was present as flakes.

The impact of synthetic amorphous silica (E 551) on differentiated Caco-2 cells, a model for the human intestinal epithelium

For several decades, food-grade synthetic amorphous silica (SAS) have been used as a technological additive to reduce caking of food powders. Human exposure is thus inevitable and safety concerns are taken seriously. The toxicity of silica in general and SAS in particular has been studied extensively. Overall, there is little evidence that food-grade SAS pose any health risks to humans. However, from the available data it was often not clear which type of silica was used. Accordingly, the latest report of the European food safety authority requested additional toxicity data for well-characterised "real food-grade SAS". To close this gap, we screened a panel of ten well-defined, food-grade SAS for potential adverse effects on differentiated Caco-2 cells. Precipitated and fumed SAS with low, intermediate and high specific surface area were included to determine structure-activity relationships. In a physiological dose-range up to 50 μg/ml and 48 h of incubation, none of the materials induced adverse effects on differentiated Caco-2 cells. This held true for endpoints of acute cytotoxicity as well as epithelial specific measures of barrier integrity. These results showed that despite considerable differences in production routes and material characteristics, food-relevant SAS did not elicit acute toxicity responses in intestinal epithelial cells.

The Nano-Intestine Interaction: Understanding the Location-Oriented Effects of Engineered Nanomaterials in the Intestine

Engineered nanomaterials (ENMs) are used in food additives, food packages, and therapeutic purposes owing to their useful properties, Therefore, human beings are orally exposed to exogenous nanomaterials frequently, which means the intestine is one of the primary targets of nanomaterials. Consequently, it is of great importance to understand the interaction between nanomaterials and the intestine. When nanomaterials enter into gut lumen, they inevitably interact with various components and thereby display different effects on the intestine based on their locations; these are known as location-oriented effects (LOE). The intestinal LOE confer a new biological-effect profile for nanomaterials, which is dependent on the involvement of the following biological processes: nano-mucus interaction, nano-intestinal epithelial cells (IECs) interaction, nano-immune interaction, and nano-microbiota interaction. A deep understanding of NM-induced LOE will facilitate the design of safer NMs and the development of more efficient nanomedicine for intestine-related diseases. Herein, recent progress in this field is reviewed in order to better understand the LOE of nanomaterials. The distant effects of nanomaterials coupling with microbiota are also highlighted. Investigation of the interaction of nanomaterials with the intestine will stimulate other new research areas beyond intestinal nanotoxicity.

Ongoing inflammation enhances the toxicity of engineered nanomaterials: Application of an in vitro co-culture model of the healthy and inflamed intestine

Chronic inflammatory conditions can negatively impact intestinal barrier function and affect the epithelium's interaction with nano-sized materials. We demonstrate the application of a Caco-2/THP-1 co-culture mimicking the intestine in healthy (i.e. stable) or inflamed state in nanotoxicological research. The co-cultures were exposed to non-toxic concentrations of silver nanoparticles (AgNPs) or silver nitrate (AgNO3) for 24 h. The barrier integrity and cytokine release as well as necrotic and apoptotic cell death were investigated. AgNPs and AgNO3 most strongly affected the inflamed co-culture. Higher concentrations of AgNPs induced a significant increase in barrier integrity in the inflamed but not the stable co-culture. Necrotic and apoptotic cell death was detected in both conditions but were significantly more pronounced in the inflamed condition. The exposure to AgNO3 affected barrier integrity in all experimental set-ups, but caused nuclear condensation only in the Caco-2 monoculture and the inflamed co-culture. AgNPs reduced the release of monocyte chemoattractant protein-1 in the stable model. Clear differences were observed in the effects of AgNPs and AgNO3 in relation to the model's health status. The results suggest an increased vulnerability of the inflamed epithelial barrier towards AgNPs underlining the importance to consider the intestinal health status in the safety assessment of nanomaterials.

Transcytosis of Bacillus subtilis extracellular vesicles through an in vitro intestinal epithelial cell model

Bacterial EVs have been related to inter-kingdom communication between probiotic/pathogenic bacteria and their hosts. Our aim was to investigate the transcytosis process of B. subtilis EVs using an in vitro intestinal epithelial cell model. In this study, using Confocal Laser Scanning Microscopy, we report that uptake and internalization of CFSE-labeled B. subtilis EVs (115 nm ± 27 nm) by Caco-2 cells are time-dependent. To study the transcytosis process we used a transwell system and EVs were quantified in the lower chamber by Fluorescence and Nanoparticle Tracking Analysis measurements. Intact EVs are transported across a polarized cell monolayer at 60-120 min and increased after 240 min with an estimated average uptake efficiency of 30% and this process is dose-dependent. EVs movement into intestinal epithelial cells was mainly through Z axis and scarcely on X and Y axis. This work demonstrates that EVs could be transported across the gastrointestinal epithelium. We speculate this mechanism could be the first step allowing EVs to reach the bloodstream for further delivery up to extraintestinal tissues and organs. The expression and further encapsulation of bioactive molecules into natural nanoparticles produced by probiotic bacteria could have practical implications in food, nutraceuticals and clinical therapies.

Interactions of graphene oxide and graphene nanoplatelets with the in vitro Caco-2/HT29 model of intestinal barrier

Carbon-based nanomaterials are being increasingly used, demanding strong information to support their safety in terms of human health. As ingestion is one of the most important exposure routes in humans, we have determined their potential risk by using an in vitro model simulating the human intestinal barrier and evaluated the effects of both graphene oxide (GO) and graphene nanoplatelets (GNPs). A coculture of differentiated Caco-2/HT29 cells presenting inherent intestinal epithelium characteristics (i.e. mucus secretion, brush border, tight junctions, etc.) were treated with GO or GNPs for 24 h. Different endpoints such as viability, membrane integrity, NPs localization, cytokines secretion, and genotoxic damage were evaluated to have a wide view of their potentially harmful effects. No cytotoxic effects were observed in the cells that constitute the barrier model. In the same way, no adverse effects were detected neither in the integrity of the barrier (TEER) nor in its permeability (LY). Nevertheless, a different bio-adhesion and biodistribution behavior was observed for GO and GNPs by confocal microscopy analysis, with a more relevant uptake of GNPs. No oxidative damage induction was detected, either by the DCFH-DA assay or the FPG enzyme in the comet assay. Conversely, both GO and GNPs were able to induce DNA breaks, as observed in the comet assay. Finally, low levels of anti-inflammatory cytokines were detected, suggesting a weak anti-inflammatory response. Our results show the moderate/severe risk posed by GO/GNPs exposures, given the observed genotoxic effects, suggesting that more extensive genotoxic evaluations must be done to properly assess the genotoxic hazard of these nanomaterials.

Genotoxicity of Silver Nanoparticles

Silver nanoparticles (AgNPs) are widely used in diverse sectors such as medicine, food, cosmetics, household items, textiles and electronics. Given the extent of human exposure to AgNPs, information about the toxicological effects of such products is required to ensure their safety. For this reason, we performed a bibliographic review of the genotoxicity studies carried out with AgNPs over the last six years. A total of 43 articles that used well-established standard assays (i.e., in vitro mouse lymphoma assays, in vitro micronucleus tests, in vitro comet assays, in vivo micronucleus tests, in vivo chromosome aberration tests and in vivo comet assays), were selected. The results showed that AgNPs produce genotoxic effects at all DNA damage levels evaluated, in both in vitro and in vivo assays. However, a higher proportion of positive results was obtained in the in vitro studies. Some authors observed that coating and size had an effect on both in vitro and in vivo results. None of the studies included a complete battery of assays, as recommended by ICH and EFSA guidelines, and few of the authors followed OECD guidelines when performing assays. A complete genotoxicological characterization of AgNPs is required for decision-making.

Silver Nanoparticles against Foodborne Bacteria. Effects at Intestinal Level and Health Limitations

Foodborne diseases are one of the factors that endanger the health of consumers, especially in people at risk of exclusion and in developing countries. The continuing search for effective antimicrobials to be used in the food industry has resulted in the emergence of nanotechnology in this area. Silver nanoparticles (Ag-NPs) are the nanomaterial with the best antimicrobial activity and therefore, with great potential of application in food processing and packing. However, possible health effects must be properly addressed to ensure food safety. This review presents a detailed description on the main applications of Ag-NPs as antimicrobial agents for food control, as well as the current legislation concerning these materials. Current knowledge about the impact of the dietary exposure to Ag-NPs in human health with special emphasis on the changes that nanoparticles undergo after passing through the gastrointestinal tract and how they alter the oral and gut microbiota, is also summarized. It is concluded that given their potential and wide properties against foodborne pathogens, research in Ag-NPs is of great interest but is not exempt from difficulties that must be resolved in order to certify the safety of their use.

Potential adverse health effects of ingested micro- and nanoplastics on humans. Lessons learned from in vivo and in vitro mammalian models

In recent years, increasing global attention has focused on "microplastics" (MPs) and "nanoplastics" (NPs) resulting in many studies on the effects of these compounds on ecological and environmental aspects. These tiny particles (<5000 µm), predominantly derived from the degradation of plastics, pollute the marine and terrestrial ecosystems with the ability to enter into the food chain. In this manner, human consumption of food contaminated with MPs or NPs is unavoidable, but the related consequences remain to be determined. The aim of this review is to complement previous reviews on this topic by providing new studies related to exposure, absorption, and toxicity in mammalian in vivo and in vitro systems. With respect to novel information, gaps and limitations hindering attainment of firm conclusions as well as preparation of a reliable risk assessment are identified. Subsequently, recommendations for in vivo and in vitro testing methods are presented in order to perform further relevant and targeted research studies.

Investigation of Twenty Metal, Metal Oxide, and Metal Sulfide Nanoparticles' Impact on Differentiated Caco-2 Monolayer Integrity

The use of engineered nanomaterials (ENMs) in foods and consumer products is rising, increasing the potential for unintentional ingestion. While the cytotoxicity of many ENMs has been investigated, less attention has been given to adverse impact on the intestinal barrier integrity. Chronical disruption of gastrointestinal integrity can have far reaching health implications. Using fully differentiated Caco-2 cells, the perturbation of intestinal barrier function and cytotoxicity were investigated for 20 metal, metal oxide, and metal sulfide ENMs. Caco-2 cells were exposed to 50 μg/mL ENMs for 24 hours. ENM formulations were characterized at 0 and 24 hours, and In Vitro Sedimentation, Diffusion and Dosimetry Modeling was applied to calculate the effective dose of exposure during 24 hours. The apparent permeability coefficient (P_app) was determined for fluorescent labeled dextran (3,000 Da) and tight junction integrity was evaluated by immunofluorescence microscopy. Cytotoxicity was investigated by determining lactate dehydrogenase release (LDH) and cell metabolic activity (tetrazolium based MTS) assays. Four ENMs led to significantly increased P_app, (15.8% w/w% Ag-SiO₂ nanoparticle (NP), 60 nm CdS NP, 100 nm V₂O₅ flakes, and 50 nm ZnO NP), while one ENM (20 nm MgO NP) decreased P_app. With the exception of CdS NP, significantly increased P_app was not connected with cell cytotoxicity. The calculated effective dose concentration was not correlated with increased P_app. Our results illustrate that while many metal, metal oxide, and metal sulfide ENMs do not adversely affect monolayer integrity or induce cytotoxicity in differentiated Caco-2 cells, a subset of ENMs may compromise the intestinal integrity. This study demonstrated the use of differentiated Caco-2 monolayer and P_app as an endpoint to identify and prioritize ENMs that should be investigated further. The interaction between ENMs and the intestinal epithelium needs to be evaluated to understand potential intestinal barrier dysfunction and resulting health implications.

Chang'an II Decoction ( II )-Containing Serum Ameliorates Tumor Necrosis Factor-α-Induced Intestinal Epithelial Barrier Dysfunction via MLCK-MLC Signaling Pathway in Rats

OBJECTIVE

To investigate the effect of Chang'an II Decoction ( II ))-containing serum on intestinal epithelial barrier dysfunction in rats.

METHODS

Tumor necrosis factor (TNF)-α-induced injury of Caco-2 monolayers were established as an inflammatory model of human intestinal epithelium. Caco-2 monolayers were treated with blank serum and Chang'an II Decoction-containing serum that obtained from the rats which were treated with distilled water and Chang'an II Decoction intragastrically at doses of 0.49, 0.98, 1.96 g/(kg·d) for 1 week, respectively. After preparation of containing serum, cells were divided into the normal group, the model group, the Chang'an II-H, M, and L groups (treated with 30 ng/mL TNF-α and medium plus 10% high, middle-, and low-doses Chang'an II serum, respectively). Epithelial barrier function was assessed by transepithelial electrical resistance (TER) and permeability of fluorescein isothiocyanate (FITC)-labeled dextran. Transmission electron microscopy was used to observe the ultrastructure of tight junctions (TJs). Immunofluorescence of zonula occludens-1 (ZO-1), claudin-1 and nuclear transcription factor-kappa p65 (NF-κ Bp65) were measured to determine the protein distribution. The mRNA expression of myosin light chain kinase (MLCK) was measured by real-time polymerase chain reaction. The expression levels of MLCK, myosin light chain (MLC) and p-MLC were determined by Western blot.

RESULTS

Chang'an II Decoction-containing serum significantly attenuated the TER and paracellular permeability induced by TNF-α. It alleviated TNF-α-induced morphological alterations in TJ proteins. The increases in MLCK mRNA and MLCK, MLC and p-MLC protein expressions induced by TNF-α were significantly inhibited in the Chang'an II-H group. Additionally, Chang'an II Decoction significantly attenuated translocation of NF-κ Bp65 into the nucleus.

CONCLUSION

High-dose Chang'an II-containing serum attenuates TNF-α-induced intestinal barrier dysfunction. The underlying mechanism may be involved in inhibiting the MLCK-MLC phosphorylation signaling pathway mediated by NF-κ Bp65.

Micronuclei Detection by Flow Cytometry as a High-Throughput Approach for the Genotoxicity Testing of Nanomaterials

Thousands of nanomaterials (NMs)-containing products are currently under development or incorporated in the consumer market, despite our very limited understanding of their genotoxic potential. Taking into account that the toxicity and genotoxicity of NMs strongly depend on their physicochemical characteristics, many variables must be considered in the safety evaluation of each given NM. In this scenario, the challenge is to establish high-throughput methodologies able to generate rapid and robust genotoxicity data that can be used to critically assess and/or predict the biological effects associated with those NMs being under development or already present in the market. In this study, we have evaluated the advantages of using a flow cytometry-based approach testing micronucleus (MNs) induction (FCMN assay). In the frame of the EU NANoREG project, we have tested six different NMs—namely NM100 and NM101 (TiO₂NPs), NM110 (ZnONPs), NM212 (CeO₂NPs), NM300K (AgNPs) and NM401 (multi-walled carbon nanotubes (MWCNTs)). The obtained results confirm the ability of AgNPs and MWCNTs to induce MN in the human bronchial epithelial BEAS-2B cell line, whereas the other tested NMs retrieved non-significant increases in the MN frequency. Based on the alignment of the results with the data reported in the literature and the performance of the FCMN assay, we strongly recommend this assay as a reference method to systematically evaluate the potential genotoxicity of NMs.

A switch of N-glycosylation of proteome and secretome during differentiation of intestinal epithelial cells

The maintenance of homeostasis of the intestinal epithelium depends on the complex process of epithelial cells differentiation, which repeatedly continues throughout the entire life. Many studies suggest, that cellular differentiation is regulated by glycosylation, or at least that changes of the latter are the hallmark of the process. The detailed description and understanding of this relationship are important in the context of gastrointestinal tract disease, including cancer. Here we employ a broadly used in vitro model of intestinal cell differentiation to track the glycosylation changes in details. We analyzed the glycoproteome- and glycosecretome-derived N-glycomes of undifferentiated Caco-2 adenocarcinoma cells and Caco-2-derived enterocyte-like cells. We used HILIC-HPLC and MALDI-ToF-MS approach together with exoglycosidases digestions to describe qualitative and quantitative N-glycosylation changes upon differentiation. Derived glycan traits analysis revealed, that differentiation results in substantial upregulation of sialylation of glycoproteome and increment of fucosylation within glycosecretome. This was also clearly visible when we analyzed the abundances of individual glycan species. Moreover, we observed the characteristic shift within oligomannose N-glycans, suggesting the augmentation of mannose trimming, resulting in downregulation of H8N2 and upregulation of H5N2 glycan. This was supported by elevated expression of Golgi alpha-mannosidases (especially MAN1C1). We hypothesize, that intensified mannose trimming at the initial steps of N-glycosylation pathway during differentiation, together with the remodeling of the expression of key glycosyltransferases leads to increased diversity of N-glycans and enhanced fucosylation and sialylation of complex structures. Finally, we propose H4N5F1 glycan as a potential biomarker of intestinal epithelial cell differentiation.

Toxic effects and involved molecular pathways of nanoparticles on cells and subcellular organelles

Owing to the increasing application of engineered nanoparticles (NPs), besides the workplace, human beings are also exposed to NPs from nanoproducts through the skin, respiratory tract, digestive tract and vein injection. This review states pathways of cellular uptake, subcellular distribution and excretion of NPs. The uptake pathways commonly include phagocytosis, micropinocytosis, clathrin- and caveolae-mediated endocytosis, scavenger receptor-related pathway, clathrin- or caveolae-independent pathway, and direct penetration or insertion. Then the ability of NPs to decrease cell viability and metabolic activity, change cell morphology, and destroy cell membrane, cytoskeleton and cell function was presented. In addition, the lowest dose decreasing cell metabolic viability compared with the control or IC₅₀ of silver, titanium dioxide, zinc oxide, carbon black, carbon nanotubes, silica, silicon NPs and cadmium telluride quantum dots to some cell lines was gathered. Next, this review attempts to increase our understanding of NP-caused adverse effects on organelles, which have implications in mitochondrial dysfunction, endoplasmic reticulum stress and lysosomal rupture. In particular, the disturbance of mitochondrial biogenesis and mitochondrial dynamic fusion-fission, mitophagy and cytochrome c-dependent apoptosis are involved. In addition, prolonged endoplasmic reticulum stress will result in apoptosis. Rupture of the lysosomal membrane was associated with inflammation, and both induction of autophagy and blockade of autophagic flow can result in cytotoxicity. Finally, the network mechanism of the combined action of multiple organelle dysfunction, apoptosis, autophagy and oxidative stress was discussed.

Nanoparticles and danger signals: Oral delivery vehicles as potential disruptors of intestinal barrier homeostasis

Gut immune system homeostasis involves diverse structural interactions among resident microbiota, the protective mucus layer, and a variety of cells (intestinal epithelial, lymphoid, and myeloid). Due to the substantial surface area in direct contact with an "external" environment and the diversity of xenobiotic, abiotic, and self-interactions coordinating to maintain gut homeostasis, there is enhanced potential for the generation of endogenous danger signals when this balance is lost. Here, we focus on the potential generation and reception of damage in the gut resulting from exposure to nanoparticles (NPs), common food and drug additives. Specifically, we describe recent evidence in the literature showing that certain NPs are potential generators of damage-associated molecular patterns, as well as potential immune-stimulating molecular patterns themselves.

Safety Assessment of Graphene-Based Materials: Focus on Human Health and the Environment

Graphene and its derivatives are heralded as "miracle" materials with manifold applications in different sectors of society from electronics to energy storage to medicine. The increasing exploitation of graphene-based materials (GBMs) necessitates a comprehensive evaluation of the potential impact of these materials on human health and the environment. Here, we discuss synthesis and characterization of GBMs as well as human and environmental hazard assessment of GBMs using in vitro and in vivo model systems with the aim to understand the properties that underlie the biological effects of these materials; not all GBMs are alike, and it is essential that we disentangle the structure-activity relationships for this class of materials.

A Cell Model of Human Small Intestinal Wall Based on Genetically Modified Caco-2 Cells

We propose a cell model of the human small intestinal wall based on genetically modified Caco-2 cells that allows visualization and quantitative assessment of activation of NF-κB factor and related intracellular pathway by using fluorescence microscopy. A dose-dependent increase in fluorescence intensity of the obtained cells in response to TNFα exposure in concentrations of 1-100 ng/ml was demonstrated. It was found that this parameter correlates with a decrease in the transepithelial resistance of the cell monolayer in response to TNFα and can be used to assess the toxic effects of substances on epithelial cells of the human small intestine.

Assessing the relevance of exposure time in differentiated Caco-2/HT29 cocultures. Effects of silver nanoparticles

In vitro models of the intestinal barrier are being increasingly used to evaluate nanoparticles (NPs) exposure risk. Nevertheless, most of these studies have focused on short-term exposures lasting no more than 24 h of duration, which could underestimate the toxic effects of a given compound under a more realistic setting. Since the assessment of longer exposure time-points is crucial to evaluate the risk of cumulative exposure to NPs, we have analyzed the effects of AgNPs at different exposure time-points between 6 h and 4 days on the barrier model system constituted by Caco-2/HT29 cells. Our results indicate that i) the system is stable during this time frame; ii) AgNPs affect the barrier's integrity only at the highest concentration tested (100 μg/mL), and only after 96 h of exposure; iii) cellular uptake of AgNPs showed a time-dependent and concentration-dependent increase; iv) translocation through the barrier was only observed at the highest concentration and only after 96 h of exposure; v) the expression of genes involved in the barrier's structure differs depending on the exposure time analyzed. All these results reinforce our proposal of expanding exposure times beyond 24 h when performing assays for hazard assessment of NPs using in vitro models of the intestinal barrier.