Full size image. Data gaps and future studies

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Apoptosis

Apoptosis is defined as the self-destruction of a cell regulated by genes through complicated programmes [83, 195]. GO and rGO caused apoptosis and inflammation in mice lungs after inhalation [99], and GFNs also had pro-apoptotic effects in cells [111, 113, 124, 196]. Additionally, graphene and GO physically damaged cell membranes [166], increased the permeabilization of the outer mitochondrial membrane and changed the mitochondrial membrane potential; the increased ROS triggered the MAPK and TGF-β signalling pathways and activated caspase-3 via mitochondrial-dependent apoptotic cascades, prompting the execution of apoptosis [83, 99]. Similarly, rGO caused apoptosis at a low dose and an early time point, triggered by the death-receptor and canonical mitochondrial pathway [110]. Another study showed three different apoptosis pathways by GFNs: GO led to ROS-dependent apoptosis through direct interaction with protein receptors and subsequent activation of the B-cell lymphoma-2 (Bcl-2) pathway; GO-COOH transmitted a passive apoptosis signal to nuclear DNA by binding to protein receptors and activating a ROS-independent pathway; However, GO-PEI severely damaged the membranes of T lymphocytes to trigger apoptosis [105, 197].

Autophagy

Autophagy is the process of self-degradation of cellular components and recently recognized as non-apoptotic cell death [198–200]. Autophagy activation requires autophagosome formation containing Beclin 1, multiple autophagy-related proteins (ATG), microtubule-associated protein light chain 3 (LC3) and p62 [201]. Autophagosome accumulation is associated with exposure to various nanoparticles [202–205], and autophagy can remove extracellular organisms and destruct the organisms in the cytosol [206]. GO and GQDs was shown to induce autophagosome accumulation and the conversion of LC3-I to LC3-II; inhibit the degradation of the autophagic substrate p62 protein [207, 208]. Furthermore, GO can simultaneously trigger TLR4 and TLR9 responses in macrophages [34, 192] and colon cancer cells CT26 [206]. The autophagy pathway is linked to phagocytosis by TLR signalling in macrophages [206, 209].

Necrosis

Necrosis is an alternate form of cell death induced by inflammatory responses or cellular injury. The exposure of cells to pristine graphene causes apoptosis and necrosis at high doses (50 mg/mL) [83]. Reportedly, LDH leakage and the opening of the mitochondrial permeability transition pore, induced by elevated level of cytoplasmic Ca²⁺, lead to apoptosis/necrosis [210]. GO treatment was revealed to induce macrophagic necrosis by activating TLR4 signalling and subsequently partly triggering autocrine TNF-α production [93]. GO combined with CDDP (GO/CDDP) triggered necrosis by decreasing RIP1 and increasing RIP3 proteins, accompanied with the release of high mobility group B1 (HMGB1) into the cytosol from the nucleus and out of CT26 cells [205, 211, 212].

Epigenetic changes

Epigenetics involve DNA methylation, genomic imprinting, maternal effects, gene silencing, and RNA editing [213–215]. DNA methylation, which is one of the best-studied epigenetic modifications, includes phosphorylation, ubiquitination, and ATP-ribosylation and can lead to chromatin remodelling [197, 216, 217]. A recently paper reported that SL-GO/FL-GO exposure resulted in global DNA hypermethylation through upregulating DNMT3B and MBD1 genes; GNP treatment caused hypomethylation by decreasing the expression of DNMT3B and MBD1 genes [216]. GO could activate the miRNA-360 regulation pathway to suppress the DNA damage-apoptosis signalling cascade by affecting the component of CEP-1 [218]. Taken together, these data suggest that GFNs could cause subtle changes in gene expression programming by modulating epigenetic changes. However, studies of GFNs-induced epigenetic changes are few, and the epigenetic mechanism caused by GFNs exposure is not fully understood.

To conclude, many studies have discussed representative mechanisms of GFNs toxicity involving four signalling pathways: TLRs, TGF-β, TNF-α and MAPKs. These four signalling pathways are correlative and cross-modulatory, making the inflammatory response, autophagy, apoptosis and other mechanisms independent and yet connected to each other. Additionally, oxidative stress appears to play the most important role in activating these signalling pathways. It has been reported that there are intersections of apoptosis, autophagy and necrosis in the studies of other nanomaterials toxicity, they inhibit or promote mutually in some conditions. However, the signalling pathways of GFNs toxicity investigated in papers to date are only a small part of an intricate web, and the network of signalling pathways needs to be explored in detail in the future.

Data gaps and future studies

Currently, the literature is insufficient to draw conclusions about the potential hazards of GFNs. Two opposite opinions have begun to emerge: some researchers suggested that graphene materials are biocompatible in a number of studies focused on biomedical applications [119, 154, 162, 219], and other studies reported adverse biological responses and cytotoxicity [32, 118, 135, 138, 192]. These inconsistent results might have been caused by several factors, including the different research groups, various cellular or animal models, and varying physicochemical characterizations of GFNs. When GFNs are explored for in vivo applications in the human body or some other biomedical applications, biocompatibility must be considered, and more detailed and accurate studies of GFNs toxicity are needed.

First, detailed physicochemical characterization is imperative in all future studies of GFNs toxicity. In the experiments, feature descriptions of GFNs should include their size, morphology, surface area, charge, surface modifications, purity, and agglomeration [88, 141, 148, 162]. Because these physicochemical factors largely influence the toxicity and biocompatibility of GFNs, single-factor experimental designs and the exclusion of other interfering factors should be considered. Details of the fabrication process should also be provided because the formed oxidative debris could largely alter the surface structure of graphene and GO during functionalization [151]. Importantly, a single, universal method needs to be established in graphene technology, which will allow for better comparison of data from different studies or different laboratories.

Second, different observational criteria, parameters and selection of experimental methods might induce large inter-laboratory variations [220, 221]. For example, the MTT assay always fails to accurately predict graphene toxicity because the spontaneous reduction results in a false positive signal. Therefore, appropriate alternative assessments should be utilized, such as the water-soluble tetrazolium salt reagent (WST-8), ROS assay, and trypan blue exclusion test [106, 222]. Additionally, the comet assay often shows higher levels of DNA damage than the micronucleus assay because the former measures the repairable injury and the latter measures the gene damage that remains after cell division [159, 223]. Therefore, caution is required in choosing the most appropriate assay to evaluate the toxicity of graphene materials to avoid false-positive results.

Third, the selection of cell lines is of vital importance because cancer cell lines tend to be sensitive or resistant depending upon their genetic background. The same graphene nanoparticles can cause different reactions depending on their various cells origins. Suitable cell lines with good stability must be used to avoid false positive or negative results. Primary cells derived from humans or animals can better simulate the health conditions of humans. A large amount of primary cells have been utilized to test the toxicity of other nanomaterials [224–228], but the culturing of primary cells is extremely rare in the experiments with GFNs to date [210, 229]. Various cell experiments combined with primary cells should be performed to comprehensively evaluate the physicochemical properties and toxicity of GFNs.

Fourth, the administration route of GFNs plays a very important role in toxicity studies, and different delivery methods will result in different toxicological reactions [32, 53]. Thus, the route and period of exposure should be carefully chosen according to the aim of the study. Nasal drug delivery is often used to study the neurotoxicity of nanomaterials [230, 231], but this administration method has rarely been applied in the testing of GFNs toxicity. Toxicological studies of GFNs in the nervous system are rare, and the mechanism is unclear and needs to be studied further in the future. Recent toxicokinetic studies involving the absorption, distribution, metabolism, accumulation, and excretion of GFNs through different exposure routes have yielded some results but are far from sufficient to clarify the internal complex mechanisms. For instance, further studies are needed to understand the specific molecular mechanisms of GFNs passing through the physiological barriers and the amount of accumulation or the excretion period of GFNs in tissues. In addition, given the increased exposure of humans to GFNs, the assessment of systemic toxicity in the human body is indispensable in future studies.

Fifth, another important issue requiring attention is the long-term fate of GFNs after entering the body or being taken up by cells. Most recent studies have consisted of short-term toxicity assessments [89, 232], and long-term toxic injury has not received much attention since the widespread application of GFNs in 2008. Moreover, a functionalized graphene surface can improve its biocompatibility, but the long-term stability of the surface coatings should be considered [233]. If the surface coatings eventually break down, their toxicity may be significantly different from the short-term exposure results. Extended studies are needed to determine if longer treatment times influence the nanotoxic potential of GFNs.

Sixth, more specific signalling pathways in the mechanism of GFNs toxicity need to be discovered and elucidated. Currently, several typical toxicity mechanisms of GFNs have been illustrated and widely accepted, such as oxidative stress, apoptosis, and autophagy. However, these mechanisms have only been described in general terms, and the specific signalling pathways within these mechanisms need to be investigated in detail. The signalling pathways involved in the toxicity of other nanomaterials may also be relevant to the study of GFNs. Therefore, more signalling pathways should be detected in future research. For instance, nano-epigenetics has been considered in numerous studies of nanomaterials, which is also helpful in assessing the limited toxicity and side effects of GFNs. Recent studies have shown that GFNs could cause epigenetic and genomic changes that might stimulate physical toxicity and carcinogenicity [234]. GFNs have high surface areas, smooth continuous surfaces and bio-persistence, similar to the properties of tumorigenic solid-state implants. It is unknown whether GFNs have the potential to induce foreign body sarcomas, and definitive studies of tumour potentialities or risks of graphene should therefore be conducted as soon as possible.