The primary route of human exposure to MNPLs occurs through the gastrointestinal system, via the ingestion of contaminated food and beverages. Upon consumption, MNPLs can penetrate the intestinal epithelium and enter the circulatory system, allowing them to distribute to various tissues and organs (4,33-35). These MNPLs have been identified in a plethora of everyday consumables, including beer, honey, sugar, salt, seafood, rice, fruits and vegetables, beverages, as well as both tap and bottled water (4,21,33).

Seafood has been recognized as a main source of human exposure to MNPL particles (20,33,36). Therefore, in countries where seafood consumption is high due to dietary customs and cultural practices, such as China and Japan, exposure to MPLs tends to be greater (37). It is noteworthy that limited information exists regarding the ingestion of MNPLs through alternative protein sources (e.g., poultry or beef), which may alter the understanding of overall human exposure (37). Of particular concern is the detection of MPLs in various fruits and vegetables (7,33,36,38), with apples identified as among the most heavily contaminated (7,38).

The consumption of bottled water markedly elevates exposure to MPLs, with individuals who exclusively consume bottled water ingesting up to 90,000 particles annually, compared to ~4,000 particles for those who consume tap water (37). Furthermore, research conducted by two separate groups revealed MPL leakage from silicone-rubber infant teats caused by elevated temperatures during sterilization (39,40), leading to ~660,000 MPL particles entering the body of an infant within 1 year of regular bottle feeding (16). Moreover, food packaging represents another source of human exposure to MPLs, as plastics are extensively used, and their degradation can lead to food contamination (16,36,41). For instance, steeping a plastic teabag at 95°C releases ~11.6 billion MPLs and 3.1 billion NPLs into a single cup (42).

Human stool studies have consistently demonstrated the presence of MNPLs, providing direct evidence of their ingestion and subsequent gastrointestinal excretion (4,9,35,43). Significant discrepancies in documented concentrations arise from variations in analytical techniques used (4). Notably, polypropylene is frequently identified as the predominant polymer (44-46), while higher PET concentrations have been observed in infants compared to adults (47). Additionally, MPLs have been detected in human urine samples (43), indicating another pathway of excretion.

Numerous studies have indicated that inhalation constitutes an additional route through which humans are exposed to MNPLs. Major sources include synthetic fabrics, tire wear, household dust and industrial emissions (16,18,33,36,41). The study conducted by Geng et al (48) revealed that among 30 individuals from Shanghai, China, the majority of MPLs inhaled indoors (mainly small fragments and fibers) were retained within the lungs and airways, with only a minor proportion being exhaled (48). Moreover, Vianello et al (49) demonstrated that an average male would inhale up to 272 MPLs daily during light physical activity. A study involving surgical patients revealed that 87% of lung tissue samples contained plastic or cellulose fibers, demonstrating that these microfibers are capable of penetrating lung tissue (50). When considering both ingestion and inhalation pathways, overall human exposure is significantly increased (37). Notably, workers in plastic manufacturing, recycling facilities, or MPL-related environments are at a higher risk of experiencing respiratory irritation (7).

While the primary routes of human exposure to MPLs are ingestion and inhalation, dermal absorption also constitutes a significant pathway (16,21,34). MPLs are present in various products, including cosmetics and personalized items. Although a number of countries have recently banned the use of microbeads in the aforementioned products (51,52), these small, sphere-shaped plastics are still found in scrubs, facial cleansers, shampoos, and toothpastes (36,41). Studies have demonstrated that MPLs can cross the skin barrier, primarily penetrating via hair follicles and glands (21,34,36). Another noteworthy example of skin contact with plastics was demonstrated in the study by Li et al (53). They demonstrated that 3 months of using plastic mobile phone cases resulted in the release of MPLs, with a large amount of MPLs detected on both the phone case and the user's palms (53). Furthermore, textiles are another route of human exposure to MPLs. Fabrics composed of fibers such as polyester, nylon, and acrylic release MPLs during wear and laundering, which can directly contact the skin and become airborne, thereby increasing exposure risks (36). Additionally, workers in the plastic and textile sectors may experience higher exposure levels through contact with materials and dust (34,36).

Although dermal contact with MNPLs may occur through cosmetics, personal care products, textiles and other plastic-containing materials, current evidence indicates that the contribution of this route to overall human exposure is relatively limited. Specifically, experimental studies have demonstrated that the penetration of plastic particles through intact skin is minimal, with the majority of particles remaining confined to the outer layers of the stratum corneum; deeper penetration appears to depend on factors, such as particle size and skin integrity. Nevertheless, data regarding the dermal absorption of MNPLs in human skin remains scarce and warrants further investigation (4).

Characterizing external exposure through these three routes is essential, yet insufficient for a comprehensive risk assessment. Translating ingested, inhaled, or dermally absorbed particle burdens into doses for target tissues requires physiologically based kinetic/pharmacokinetic modeling adapted to particulates, combined with the quantitative in vitro to in vivo extrapolation (QIVIVE) of mechanistic endpoints. Such biokinetic-mechanistic coupling provides the quantitative link between the exposome and the key events discussed below. Moreover, this approach is increasingly feasible for MNPLs as parameters concerning size, shape and surface charge-resolved uptake become available (54). Framing MNPLs as a discrete class of environmental contaminants, however, underestimates their biological complexity. Within the life-course exposome paradigm, MNPLs should be regarded as multicomponent stressors that jointly deliver the polymer matrix, residual monomers, processing additives and a sorbed mixture of co-contaminants acquired during environmental aging. Their inclusion in human biomonitoring and mixtures-risk infrastructures, most notably the human biomonitoring for Europe (HBM4EU) and its successor, the European partnership for the assessment of risks from chemicals (PARC), represents not merely an analytical expansion, but also a conceptual reframing: MNPLs function concurrently as carriers, sources, and exposure matrices (55-57).

Among the proposed mechanisms, oxidative stress has emerged as one of the most extensively studied and consistently reported effects of MNPL exposure. While low levels of ROS are vital for normal cellular function, elevated ROS levels are detrimental, leading to DNA and RNA damage, lipid peroxidation and protein oxidation. The disruption of the balance between oxidant production and the antioxidant defense system within cells leads to oxidative stress (80), a condition widely recognized as a critical molecular mechanism in carcinogenesis (81). Specifically, oxidative stress can induce mutations in essential genes, compromise genome integrity, and modulate signaling pathways crucial for cell survival, proliferation and apoptosis, such as the mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), and Kelch-like ECH-associated protein 1 (Keap-1)/nuclear factor erythroid 2-related factor 2 (Nrf2) pathways (81).

Oxidative stress is recognized as a pivotal mechanism underlying the toxicity of MNPLs (63), and its magnitude appears to depend on particle size, concentration and duration of exposure (63,82-84). Notably, smaller plastic particles tend to induce greater oxidative modifications in cells, likely due to enhanced cellular uptake. Additionally, ROS generation generally escalates with increased concentrations and prolonged exposure periods. These effects have been evidenced in various in vitro models, including HCT-116 colorectal cancer cells (85), the HepG2 hepatocellular carcinoma cells (86), CT26-WT colon carcinoma cells (87) and A549 lung adenocarcinoma cells (88), among others (89).

MNPLs can affect and neutralize both enzymatic and non-enzymatic cellular antioxidants. Enzymes, such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidases (GPxs), as well as non-enzymatic antioxidants, such as L-γ-glutamyl-L-cysteinyl glycine [glutathione (GSH)], are frequently disrupted following exposure to MNPLs. For example, in Carassius auratus, exposure to 50-500 μm polystyrene MPLs was shown to result in a concentration-dependent decrease in the activities of CAT, glutathione S-transferase (GST), and SOD enzymes (90), indicating that the antioxidant defense system was unable to counteract excessive ROS production. By contrast, shorter exposures in zebrafish (Danio rerio) have been shown to be associated with increased SOD and CAT activities (82,91), likely reflecting an early compensatory response to oxidative stress. Similarly, a 14-day exposure to 50 nm polystyrene NPLs increased ROS levels and elevated SOD, CAT and GST activities in Oryzias melastigma, with more pronounced effects observed in the liver compared to the gut of the animals (92). Conversely, 45 μm MPLs administered over the same duration induced a gut dysbiosis-related response in the same model (92), pinpointing the distinct effects induced by MPLs and NPLs in vivo. The exposure of BV2 microglial cells and Aurelia coerulea polyps to 50 nm polystyrene NPLs for 12 and 48 h, respectively, led to reduced CAT levels and increased malondialdehyde (MDA) levels, an indicator of lipid peroxidation, suggesting heightened oxidative stress, while brain tissue and serum samples from ICR mice also confirmed these results (93). Importantly, transcriptomic analyses of BV2 cells and Aurelia coerulea revealed that polystyrene NPLs triggered activation of the MAPK signaling pathways in both models; however, distinct genes were expressed in each case (93).

The primary MAPK sub-families include the extracellular regulated kinases (ERKs), the c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. These kinases play a crucial role in regulating transcription factors involved in cellular processes such as growth, survival, and apoptosis (94,95). The persistent activation of MAPK signaling pathways has been shown to be associated with uncontrolled cellular proliferation, the evasion of apoptosis and metastasis. Furthermore, the dysregulation of these signaling cascades has been reported in a plethora of cancers (96). The detrimental effects associated with MNPLs have been linked to the activation of the MAPK signaling cascade, a process that can be triggered or facilitated by ROS (63,81). Supporting this, the ingestion of 0.05 μm polystyrene microbeads by Paracyclopina nana was shown to result in oxidative stress, which subsequently activated phospho-ERK, phospho-p38 and Nrf2, thereby affecting the growth rate and fecundity of the animals. Of note, the co-administration of a ROS scavenger decreased ROS production and reduced phosphorylation levels of ERK and p38 (97). Similar findings were reported in Brachionus koreanus, where elevated ROS levels were associated with the increased phosphorylation of JNK and p38, along with the reduced growth rate, fecundity and lifespan of the animals (98). Furthermore, Amado et al (99) reported that polystyrene NPLs increased ROS production in Daphnia pulex, resulting in a concentration-dependent increase in ERK gene and protein expression levels. They also demonstrated that the expression of JNK, p38 and VEGFA was significantly upregulated, whereas higher concentrations caused a decline. At the protein level, the expression of JNK, AKT, hypoxia inducible factor 1 subunit α (HIF1α), and nuclear factor κ-light-chain-enhancer of activated B-cells (NF-κB) p65 increased in a concentration-dependent manner, while the activities of CAT and SOD enzymes decreased (99). In another study, healthy Balb/c mice subjected to micro-polystyrene exposure for 6 weeks exhibited increased levels of oxidative stress, marked by elevated ROS and MDA levels. The exposure also triggered the phosphorylation of p38 and JNK in a concentration-dependent manner (100). Furthermore, as demonstrated in another study, in C57BL/6J mice, inhalation exposure to micro- (0.25 μm) and nano- (20 nm) polystyrene particles for 6 and 12 weeks elicited a sustained activation of the MAPK signaling cascade in the lungs, accompanied by an enrichment of pathways associated with carcinogenesis and an increased expression of genes involved in the epidermal growth factor receptor (EGFR)/MAPK signaling pathway. Notably, the upregulation of amphiregulin, an EGFR ligand, and mitogen-activated protein kinase kinase kinase 13, a kinase involved in the activation of the MAPK pathway was observed. Additionally, elevated phosphorylation levels of EGFR and JNK further substantiated the activation of the EGFR/MAPK signaling axis (101).

In addition to the MAPK signaling pathways, MNPL-induced oxidative stress has been suggested to contribute to the activation of the Wnt/β-catenin pathway. The Wnt/β-catenin pathway is a critical pathway implicated in development, tissue homeostasis, proliferation, apoptosis and migration, and its aberrant activation has been associated with gastrointestinal cancers, leukemia, melanoma and breast cancer, among others (102,103). In a recent study, young primary cells (dermal fibroblasts and epidermal keratinocytes) isolated from healthy SKH1-hr hairless immunocompetent mice were exposed to polystyrene microspheres of various sizes, either once or repeatedly (104). During acute exposure, MNPLs stimulated ROS production, which led to Nrf2 activation and elevated mRNA levels of oxidative stress-related genes, such as heme oxygenase-1, NAD(P)H:quinone oxidoreductase 1 (NQO1), SOD1, CAT, GPX2, GSR and GSTA1, reflecting the effort of the cell to manage oxidative stress (Fig. 2). Moreover, the expression of p53, BAX and p21 was increased as a response to DNA damage, as evidenced by accumulated phosphorylated histone H2AX (γ-H2AX). Concurrently, the expression of CTNNB1 and CDH1 was augmented, accompanied by higher levels of lymphoid enhancer-binding factor 1 (LEF1) mRNA, indicating the activation of the Wnt/β-catenin pathway (104). Under chronic exposure, while ROS generation persisted, Nrf2 remained activated, accompanied by the downregulation of NQO1, CAT, GPX2 and GSR, indicating the cell's inability to neutralize ROS and counteract MNPL-induced oxidative stress. In addition, the decrease in B-cell lymphoma 2 (Bcl-2) expression suggested increased resistance to apoptosis induced by DNA damage. The activation of the Wnt/β-catenin signaling pathway was further evidenced by β-catenin translocating to the nucleus and an increase in the mRNA levels of Wnt1, Wnt7a, LEF1, c-JUN and matrix metalloproteinase 7. Simultaneously, cytoskeletal changes occurred, including disruption of actin filaments, modifications in keratin expression (the upregulation of keratin 1 and the downregulation of keratin 14), alterations in proteins of adhesive integrin complexes (the increased levels of focal adhesion kinase and vinculin, and the upregulation of integrin ITGB1), and markers indicative of fibroblast-to-myofibroblast transition (the upregulation of α smooth muscle actin and collagen COL1A) (104).

In study using healthy rats, exposure to polystyrene MPLs for 90 days led to their accumulation in all tissues except the heart, with the highest concentrations detected in gastric tissues and the spleen (105). Notably, there was damage to cellular membranes, abnormal nuclear shapes, mitochondrial impairment, elevated MDA levels, decreased levels of CAT, SOD and GSH-Px, and increased levels of DNA damage markers [8-hydroxy-2'-deoxyguanosine (8-oxodG) and γ-H2AX] in gastric epithelial cells of rats. These findings were also observed in human gastric epithelial GES-1 cells (105), where polystyrene MPLs caused excessive ROS production, leading to the dissociation of the nucleoredoxin/dishevelled 1 complex, a redox-sensitive inhibitor of the Wnt/β-catenin signaling pathway. Such a dissociation resulted in the increased expression of β-catenin, indicating the activation of the Wnt/β-catenin signaling pathway. At the same time, the increased expression of yes-associated protein (YAP) was observed (Fig. 2) (105). YAP protein is a transcriptional activator in the Hippo pathway, playing a significant role in cellular proliferation, survival, and differentiation, and has been linked to cancer (106,107). In addition, crosstalk between the Wnt/β-catenin and Hippo pathways has been described in cancer development (102,108).

Finally, oxidative stress induced by MNPLs is closely associated with mitochondrial dysfunction, as illustrated in Fig. 2. Mitochondria are considered significant targets of MNPL toxicity, since exposure has been shown to diminish mitochondrial membrane potential and impair ATP production in vitro. Such modifications may activate the intrinsic apoptotic pathway, ultimately leading to apoptosis and increasing MNPL-induced cytotoxicity (19). In zebrafish larvae, exposure to aged polystyrene MPLs induced oxidative stress and mitochondrial impairment, thereby leading to developmental toxicity (109). These MPLs increased ROS, lipid peroxidation and DNA damage, thereby disrupting mitochondrial function through the reduction of ATP synthesis, compromising mitochondrial membrane potential and promoting the release of cytochrome c. Consequently, the activities of caspase-3 and caspase-9 increased, accompanied by upregulation of apoptosis-related genes, such as bax, caspase-3, caspase-9 and apoptotic protease activating factor 1 (apaf-1), while bcl-2 was downregulated. These findings collectively indicate the activation of a mitochondrial-mediated apoptotic pathway (109).

Overall, oxidative stress, driven by excessive ROS production, represents a key mechanism underlying MNPL toxicity and has been consistently documented in both in vitro and in vivo experimental models. Nonetheless, its potential contribution to human carcinogenesis remains to be fully elucidated. Key indicators, including Nrf2 and MAPK activation, DNA and mitochondrial damage and the depletion of antioxidants, may serve as quantitative KE descriptors within regulatory-relevant AOP frameworks. The harmonization of these biomarkers across various experimental models could enhance cross-study integration and facilitate the derivation of point of departure values for MNPL mixtures (32,78). An overview of the principal pathways involved in MNPL-induced oxidative stress and the associated downstream signaling pathways is summarized in Fig. 2.

Maintaining genome integrity is fundamental for cell survival; however, DNA damage and genomic instability can facilitate carcinogenesis (110). Research indicates that exposure to MNPLs is associated with DNA damage and genotoxic effects (8,19,36,59,75,111), with the extent of impact seemingly influenced by particle size, concentration, and cell type. Exposure to MNPL has been shown to be associated with numerous DNA alterations, including single-strand breaks (SSBs), double-strand breaks (DSBs) and oxidative modifications of purines and pyrimidines (64,89). Malinowska et al (112) demonstrated that polystyrene NPLs of varying sizes induced genotoxic effects in human peripheral blood mononuclear cells (PBMCs), as evidenced by the formation of SSBs and DSBs, increased levels of 8-oxodG, and oxidation of purines and pyrimidines. Notably, although all tested NPLs caused DNA damage, the smallest particles (29 nm) exhibited the most significant effect, underscoring a size-dependent phenomenon. Furthermore, in the same study, while DNA damage induced by NPLs of 44 and 72 nm in size was repaired within 2 h, that caused by NPLs of 29 nm in size remained incomplete (112), indicating the detrimental impact of smaller particles. Similarly, in another study, the exposure of human intestinal organoids to 1 μm and 100 nm polystyrene particles resulted in significant γ-H2AX accumulation, accompanied by mitochondrial dysfunction and ROS generation, with NPLs eliciting a stronger genotoxic response than MPLs (113). Additionally, the induction of micronuclei and nuclear buds was observed in human foreskin fibroblasts (Hs27) following treatment with polystyrene NPLs of 100 nm in size, with more pronounced effects at the concentration of 75 μg/ml, where frequencies of micronuclei and nuclear buds reached 52 and 50%, respectively. Of note, although NPLs also increased ROS production, co-treatment with the antioxidant extract of Crocus sativus L. reduced ROS generation (114). However, the researchers did not evaluate DNA damage markers after administering the extract, thus limiting mechanistic insights. Furthermore, in A549 human cells exposed to plastic particles from a subway station or a nearby heavily trafficked urban street, concentration-dependent DNA damage was observed, as evidenced by comet assay results and elevated levels of 8-oxodG. The most concerning finding was that the most significant genotoxic effects were associated with subway particles (115). Additional evidence from in vitro studies further supports the notion that MNPLs induce genomic instability. Cytotoxicity and genotoxicity have been documented in human lymphocytes exposed to polystyrene NPLs of 50 nm in size (116) and polyethylene MPLs of 10-45 μm in size, which resulted in increased frequencies of micronuclei formation and nuclear abnormalities, such as nucleoplasm bridges and nuclear buds (117). Cellular responses appeared highly cell-type-specific. The study by Rubio et al (111) demonstrated differential responses among three cell lines exposed to spherical polystyrene NPLs of 50 nm in size. Specifically, THP-1 monocytes did not exhibit genotoxic lesions despite the highest particle internalization among the studied cell lines. TK6 lymphoblasts exhibited increased ROS production leading to oxidative DNA damage, while DNA breaks were observed in Raji-B-cells (B-lymphocytes), indicating distinct mechanisms of action (111).

In vivo models provide additional, albeit heterogeneous, evidence. The exposure of Scrobicularia plana to polystyrene MPLs of 20 μm in size resulted in their accumulation within the gills and digestive glands, subsequently inducing oxidative stress. The MPLs elevated levels of antioxidant enzymes such as CAT, SOD, GPx, and GST, and also induced genotoxicity and neurotoxicity, with the severity of these effects dependent on exposure duration (118). A meta-analysis revealed that aquatic organisms exposed to MNPLs exhibited genotoxic damage compared to control groups, as evidenced by multiple genotoxic indicators (119). The significance of these effects was linked to the size of the plastics (smaller particles causing greater effects), the environmental conditions in which the organisms live and the species tested, with freshwater organisms demonstrating greater susceptibility than marine species (119). In another study, exposure to MPLs for 15 days also increased the activities of SOD, CAT, total peroxides and MDA in Oreochromis niloticus, accompanied by lipid peroxidation and DNA fragmentation. Following a 15-day recovery period, all the previously mentioned indicators returned to normal levels (120). In another study, the administration of virgin and aged (post-120 days of ultraviolet exposure) low-density polyethylene MPLs (average size, 500 μm) reduced the proportion of normal cells in Perca fluviatilis by increasing the number of binucleated cells, cells containing a micronucleus and cells with a micronucleus connected to the main nucleus (121). Notably, virgin polyethylene MPLs had more harmful effects compared to the aged ones. DNA damage was also observed in the liver and muscle tissues of Perca fluviatilis in both MPL groups, with the liver showing the most significant damage. Although the link between ROS generation and DNA damage was not experimentally tested, the authors of that study attributed the DNA damage observed to excessive ROS production induced by MPL treatment (121). A separate study highlighted the differences between acute and chronic exposure to spherical and fibrous MPLs in Mytilus galloprovincialis (122). Both particle types caused morphological disorders in the gills and digestive glands, with effects varying according to exposure duration and particle dose; fibrous particles were more toxic. These effects were accompanied by increased DNA fragmentation and apoptosis, as evidenced by the upregulation of caspase 3/7. In fact, MPL exposure activated both extrinsic (upregulation of Fas-associated death domain) and intrinsic (downregulation of Bcl-1 and Bcl-2, and upregulation of Bax) apoptotic pathways. Additionally, transcript levels of Gadd45α, Gadd45γ, and p53, genes involved in growth arrest and DNA damage response, were elevated, while the PI3K/AKT and ERK pathways, essential for cell survival, were inhibited (122), indicative of genotoxic stress.

Despite the above, it is important to note that not all studies report genotoxic effects. For instance, intestinal and placental co-cultures exposed to various concentrations of polystyrene MNPLs did not exhibit genotoxicity, as assessed by p53 expression levels and micronuclei formation (123). In line with these findings, in a previous study, negatively charged carboxylated polystyrene (COOH-PS) and positively charged amino-modified (NH2) polystyrene NPLs were administered to cancerous HeLa cells and normal NIH 3T3 fibroblasts. Both cell lines internalized NPLs, which remained in the cytoplasm. It is noteworthy that these particles were distributed across all phases of mitosis without causing chromosomal abnormalities or disruption of mitotic spindle formation, although G0/G1 cell cycle arrest was observed, particularly in response to NH2 polystyrene NPLs (124). Additionally, NPLs are divided equally between daughter cells (124), which aligns with the study conducted by Brynzak-Schreiber et al (62). Of note, both types of NPLs were more toxic to normal cells than to cancerous cells (124). These discrepancies likely reflect differences in particle properties, exposure conditions and model systems, highlighting the necessity for standardized experimental approaches.

These findings collectively suggest that DNA damage and genotoxicity may serve as crucial molecular mechanisms linking MNPL exposure to carcinogenesis. Although the exact mechanisms of DNA damage caused by MNPLs are not yet fully understood, an overview of the current understanding of the genotoxic mechanisms induced by MNPLs and the corresponding cellular responses is presented in Fig. 3.

MNPLs are recognized by the immune system as foreign entities, thereby activating both innate and adaptive immune responses (65,125). Within the innate immune system, MNPLs stimulate neutrophils, macrophages and dendritic cells, leading to the increased production of pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, tumor necrosis factor α (TNF-α) and interferon γ (IFN-γ). Concurrently, MNPLs can modulate adaptive immunity by affecting T-cell differentiation and facilitating immune evasion through anti-inflammatory cytokines, such as IL-10 and transforming growth factor-β (TGF-β). As a result, their presence triggers an immune response, increasing the levels of neutrophils, macrophages (M1 and M2), T-cells [T helper 2 (Th2), Th17, regulatory T-cells (Tregs)], B lymphocytes and phagocytes at the 'injury site' (65,125). In the event that MNPLs are not combated by the immune system, this persistent immune activation may lead to chronic inflammation (65), which is known to promote DNA damage, cytokine and chemokine release, and the activation of signaling pathways (e.g., NF-κB) that may contribute to tumor growth, metastasis and progression (19,126-129).

Experimental studies support the pro-inflammatory potential of MNPLs, although the responses appear to be dependent on factors, such as particle size, duration of exposure and the specific model system used. For instance, in the study by Hwang et al (130), smaller propylene MPLs (20 μm in size) elicited more potent inflammatory responses compared with larger particles (25-200 μm), as evidenced by the increased secretion of IL-6 and TNF-α in PBMCs and RAW 264.7 murine macrophages, along with elevated histamine release from the human mast cell line 1 (HMC-1). Notably, the anti-inflammatory cytokines, IL-2 and IL-10, exhibited no significant alterations (130), suggesting that the cell-mediated immune response was not activated. Similarly, the exposure of primary dermal fibroblasts and epidermal keratinocytes, isolated from SKH1-hr mice, to polystyrene microspheres increased the mRNA expression of TNF-α, IL-1β and IL-6. When comparing acute and chronic exposure, the latter was found to be more detrimental (104). In another study, polystyrene NPLs of 50 nm in size also induced an inflammatory response in BV2 microglial cells by elevating ROS levels, upregulating the CD32 and CD11b genes, both associated with promoting an M1 polarization shift, and increasing mRNA levels of IL-1β and TNF-α. Following 12 h of exposure, the apoptotic rate was higher compared to the control-treated cells (93). Furthermore, the type of polymer appears to influence immune responses. Han et al (131) demonstrated that treating PBMCs with 25-75 μm acrylonitrile butadiene styrene (ABS), a thermoplastic polymer, increased IL-6 production, whereas treatment with similarly sized PVC increased TNF-α secretion, while both ABS and PVC inhibited histamine release (131). However, they did not reveal the exact mechanism of action.

Similar findings have been reported in animal models. In zebrafish, polystyrene MPLs (0.5 μm in size) significantly increased the mRNA and protein levels of IL-1α, IL-1β and IFN following 14 days of exposure (132), while in Poecilia reticulata, larger particles (32-40 μm in size) elevated TNF-α, IFN-γ, Toll-like receptor 4 (TLR4) and IL-6 levels in a dose-dependent manner (133), implicating TLR4 signaling in MNPL-induced inflammation. TLR4 is a receptor for pathogens that triggers downstream signaling pathways associated with the inflammatory response (126,129). In C57BL/6J mice, a 3-week exposure to 0.1 and 2 μm polystyrene MPLs also increased the mRNA expression of TNF-α and calgranulins S100a8 and S100a9, which are involved in mediating inflammatory responses (134), further supporting activation of inflammatory pathways.

However, not all studies report consistent immune activation. For example, THP-1 monocytes internalized polystyrene particles of various sizes (1, 4 and 10 μm), without significant changes in M1/M2 polarization markers [signal transducer and activator of transcription (STAT)-1 and STAT-6], chemokine expression (CXCL10, CCL22), or surface receptors (CD209, CD206)], indicating that particle uptake alone may not be sufficient to trigger inflammatory responses under these certain experimental conditions (135).

MNPLs may also interfere with adaptive immune function. Li et al (136) demonstrated that polystyrene NPLs suppressed T-cell activation and differentiation in murine splenic lymphocytes by diminishing the expression of CD3⁺CD69⁺ and CD3⁺CD25⁺, as well as impairing CD8⁺ T-cell differentiation. This was accompanied by decreased IL-2 and INF-γ production (136). At the molecular level, the authors of that study found that polystyrene NPLs significantly reduced the expression of phospho-protein kinase Cθ (PKCθ), p65, phospho-inhibitor of κBα, phospho-inhibitory kappa B kinase β and phospho-STAT-5 in T-cells; these essential proteins are involved in the PKCθ/NF-κB and IL-2 receptor/STAT-5 pathways that regulate T-cell activation and function (136) (Fig. 4), indicating a potential immunosuppressive effect that may compromise anti-tumor immunity.

Mechanistically, MNPL-induced inflammation has been closely linked to oxidative stress, DNA damage and innate immune pathways (8,36,65,125). In both HL-7702 human liver cells and C57L/5 mice, polystyrene MPLs (0.1 and 1 μm in size) caused DNA damage in the nucleus and mitochondria, leading to the presence of DNA fragments in the cell cytoplasm. This leakage triggered the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) signaling pathway by stimulating cGAS, a double-strand DNA sensor, which in turn activated STING (Fig. 4). As a result, NF-κB translocated to the nucleus, where phospho-NF-κB induced the mRNA expression of IL-1β, IL-6 and TNF-α, ultimately contributing to liver fibrosis in mice (137). Consistent with the involvement of NF-κB signaling in MNPL-induced toxicity, a recent study conducted by Ye et al (138) reported that in clear cell renal cell carcinoma cells, exposure to polystyrene MPLs promoted cellular proliferation, migration, and invasion, while concurrently inhibiting apoptosis through activation of the NF-κB and TGF-β signaling pathways. Notably, the pharmacological inhibition of these pathways mitigated the pro-tumorigenic effects in both cellular and preclinical models, thereby supporting their potential involvement in cancer progression (138). In addition, the activation of the NOD-like receptor protein 3 (NLRP3) inflammasome has emerged as a key pathway involved in MNPLs-induced inflammation (129). In normal human epithelial cells (HaCat), 1 μm polyethylene MPLs inhibited proliferation in a concentration- and time-dependent manner by impairing cell cycle progression (139). This effect was caused by an inflammatory response (elevated levels of TNF-α, IL-6 and IL-1β), which activated the NLRP3 inflammasome, ultimately leading to cell pyroptosis, a form of programmed cell necrosis (139). Conversely, in cancer cells (SCL-1 and A431), polyethylene MPLs induced ROS accumulation, mitochondrial membrane potential changes and mitochondrial DNA damage. The damaged mitochondrial DNA entered the cytoplasm through the mitochondrial permeability transition pore, which triggered the NLRP3 inflammasome, resulting in increased TNF-α, IL-6 and IL-1β levels, as well as enhanced cell proliferation (139). Consistently, the activation of the NLRP3/caspase-1 signaling pathway and the induction of pyroptosis following exposure to polystyrene MPLs have also been observed in in vivo models (140-142).

Taken together, these findings highlight the complex role of MNPL-induced inflammation, which varies according to particle characteristics and exposure conditions by modulating inflammatory responses and exerting both pro-tumorigenic and immunomodulatory effects. As inflammation predisposes individuals to cancer development and facilitates all stages of tumorigenesis (128), the precise mechanisms through which MNPLs could initiate tumor proliferation remain limited. An overview of the primary pathways involved in MNPL-induced inflammation and signaling dysregulation is summarized in Fig. 4.

Recent evidence highlights the crucial role of the microbiome in the initiation and progression of cancer (143-145). The gut microbiota, a complex and dynamic consortium of microorganisms including bacteria, viruses, fungi and archaea, plays a vital role in maintaining host homeostasis through the regulation of immune responses and support of metabolic equilibrium. The disruption of this bidirectional communication between the microbiome and the host results in dysbiosis. Such dysbiosis has been associated with carcinogenesis through multiple mechanisms, including the activation of oncogenic signaling pathways (e.g., Wnt/β-catenin), the induction of DNA damage and genomic instability, the modulation of immune signaling pathways (e.g., NF-κB, JAK/STAT and the inflammasome), epigenetic modifications and the induction of aberrant metabolic processes. Additionally, specific microbes and their metabolites may exert protective and anti-tumor effects. Beyond tumor initiation, the microbiome is increasingly recognized as a fundamental regulator of the tumor microenvironment, influencing cancer progression, metastasis, and response to therapies, such as chemotherapy, radiotherapy and immunotherapy (143-145). Notably, environmental factors, including exposure to MPLs, have also been shown to disrupt microbial composition, potentially contributing to inflammation, metabolic imbalance, and impaired intestinal immunity (65).

Experimental evidence indicates that alterations in the microbiome induced by MNPLs are affected by particle type, size, and the model system used. Lei et al (146) demonstrated that various types of MPLs (polyamides, polyethylene, polypropylene, polystyrene and PVC) caused intestinal damage in zebrafish, characterized by villi cracking and enterocyte splitting, whereas in Caenorhabditis elegans, they induced lethality and developmental toxicity. Notably, the authors of that study established that the observed toxicity is dependent on particle size rather than chemical composition (146).

Consistent with these observations, several studies using aquatic models have reported significant shifts in gut microbiota composition following exposure to MNPLs. In zebrafish, exposure to 0.5 and 50 μm polystyrene MPLs for 14 days resulted in decreased relative abundance of Proteobacteria and Bacteroidetes, and increased levels of Fusobacteria and Firmicutes, along with changes in 29 bacterial genera. These microbiome modifications were accompanied by elevated mRNA and protein levels of IL-1α, IL-1β and IFN (132), indicative of an inflammatory response. Comparable outcomes were observed following exposure to 5 μm polystyrene MPLs in the same model for 21 days, which also altered the intestinal metabolomic profile and induced oxidative stress (82). In crustaceans, such as Eriocheir sinensis, exposure to 5 μm polystyrene MPLs decreased the relative abundance of Firmicutes and Bacteroidetes, whereas Fusobacteria and Proteobacteria increased. Additionally, MPL exposure altered the levels of enzymes that play key roles in crustaceans' immune response (147). It is noteworthy that in a recent study, the Firmicutes/Bacteroidetes ratio was found to be significantly decreased in patients with breast cancer compared to healthy individuals, while the risk of developing breast cancer was associated with this ratio (148). However, such associations require further evaluation.

Evidence from mammalian models further supports the link between MNPL exposure and microbiome dysbiosis. In ICR mice, exposure to 0.5 and 50 μm polystyrene MPLs reduced the relative abundance of Firmicutes and α-Proteobacteria in both cecal contents and feces; a reduction in Actinobacteria was observed only in cecal contents exposed to 50 μm MPLs (149). Additionally, there were changes in the number of gut microbes in the cecal content and colon at the genus level, while genes related to glucose and lipid metabolism were also affected (149). Similarly, exposure to 5 μm polystyrene MPLs caused modifications in fifteen bacterial species within the same animal model, accompanied by metabolic disorders (150). Moreover, in C57BL/6 mice, polyethylene particles increased bacterial abundance and enhanced the diversity of the intestinal microbiota, as evidenced by increases in the Staphylococcus genus (Firmicutes) and decreases in the Parabacteroides genus, among others (151). These microbiota modifications were associated with modulation of immune responses, including elevated levels of IL-1α and IL-9 and altered T-cell populations characterized by reduced proportions of Th17 and Treg cells. Furthermore, inflammation was observed in the colon and duodenum, concomitant with activation of the TLR4 signaling pathway, as demonstrated by elevated expression levels of TLR4, AP-1 transcription factor subunit and interferon regulatory factor 5 in the intestine (151).

Recent metagenomic analyses in patients with colorectal cancer further indicate that MPL exposure may be associated not only with alterations in gut microbial composition, but also with modifications in the functional profiles of the tumor-associated microbiome. Specifically, exposure to polystyrene was found to be associated with pathways involved in inflammation, immune regulation and cancer-related signaling, including HIF-1, Wnt, JAK/STAT, IL-17 and NOD-like receptor signaling pathways. This underscores a potential role of MNPL-associated dysbiosis in tumor progression (152).

Collectively, existing evidence indicates that MNPL-induced dysbiosis may indirectly contribute to carcinogenesis through its influence on inflammation, immune modulation and metabolic disruption. Nevertheless, the majority of these findings are derived from experimental models, and definitive causal associations in humans have yet to be established.

MNPLs, together with the chemical additives incorporated to enhance their properties, such as BPA and phthalates, may influence hormonal signaling pathways, potentially leading to endocrine disruption. These compounds, known as endocrine-disrupting chemicals (EDCs), can mimic or inhibit hormones and affect hormonally sensitive organs, including the breast, ovaries, prostate, and thyroid, thereby potentially increasing the risk of carcinogenesis (8,36,75,153).

Among these, BPA is one of the most extensively studied EDCs. It functions as an endocrine disruptor and has been linked to carcinogenesis through its involvement in cancer cell survival, proliferation, migration, and invasion, among other processes (28,154). Mechanistically, BPA can modulate cancer-related cellular signaling pathways by binding to or activating various receptors, including estrogen receptor α/β, androgen receptor, G protein-coupled estrogen receptor, insulin-like growth factor-1 receptor, and estrogen receptor γ (28,154). Current evidence suggests that BPA affects mammary gland development and alters gene methylation patterns, thereby elevating the risk of developing cancer following early-life exposure even at low doses in rodents (28). Although BPA is considered a potential risk factor for breast cancer development (28), its involvement in other malignancies remains less clearly defined and warrants further research. This topic has been extensively reviewed elsewhere (28,154).

Moreover, a significant yet insufficiently discussed aspect is the landscape of substitution. The progressive regulatory restrictions on BPA have resulted in its replacement by structurally related bisphenols, such as bisphenol S, bisphenol F and bisphenol AF, several of which exhibit comparable or enhanced estrogenic, androgenic and metabolic-disrupting activities. In matrices abundant in MNPLs, where polymer aging continuously releases residual additives, this substitution pattern may therefore sustain rather than mitigate endocrine-disrupting exposures. Integrating mechanistic data from NAMs into SSbD substitution decisions is imperative to disrupt this cycle (29-31).

Another well-characterized EDC is DEHP, which is recognized for its potential role in both cancer initiation and progression. DEHP exerts its effects by interacting with estrogen, progesterone, and androgen receptors, among others and functions as an endocrine disruptor by activating pathways associated with cancer development, including PI3K/AKT, TGF-β, MAPK and Wnt/β-catenin (27). The involvement of DEHP in carcinogenesis has been thoroughly examined in the review by Zhou et al (27).

MNPLs have a wide range of mechanisms capable of potentially contributing to carcinogenesis, as previously described. Nonetheless, owing to their intrinsic properties, they may absorb or even facilitate the transport of other compounds, including established carcinogens, toxic chemicals (e.g., heavy metals), and pathogenic microorganisms (155). Lee et al (156) demonstrated that polystyrene, polyethylene and polypropylene exhibit high sorption capacities for hydrophobic organic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), hexachlorocyclohexanes and chlorinated benzenes that may be present in aquatic environments. As a result, MPLs may serve as vectors or 'sponges' for toxic compounds within the marine ecosystem, thereby influencing their bioavailability (156). Notably, under realistic conditions where contaminants exist as mixtures, sorption behavior may vary. Bakir et al (157) observed that in mixed-contaminated environments, compounds such as dichlorodiphenyltrichloroethane can competitively inhibit the sorption of other pollutants such as phenanthrene onto polyethylene and PVC, indicating that interactions among contaminants can modify their affinity for plastics. These findings suggest that MNPLs may function as 'Trojan horses' and may serve as dynamic carriers, with their toxicological impacts depending on the chemical context in which they are present. Accordingly, the toxicological consequences of MNPLs may be substantially modified by co-exposure to sorbed contaminants, with outcomes varying from changes in bioavailability to additive or synergistic toxicity, depending on the characteristics of both the particles and the associated contaminants (155-157). Various studies support the notion that MNPL-associated contaminants can enhance toxicological outcomes. Sharma et al (5) demonstrated that smaller MPLs (<0.1 μm) can absorb and concentrate PAHs at levels significantly higher than those found in water, thereby potentially increasing the estimated lifetime cancer risk associated with ingestion through drinking water, salt and seafood. In another study, in vitro experiments further revealed that polyethylene and polystyrene MPLs can absorb pyrene, a PAH, in a dose- and time-dependent manner (158). When pyrene-treated polyethylene and polystyrene MPLs were administered to Mytilus galloprovincialis, pyrene-contaminated MPLs were detected and bioaccumulated in the hemolymph, gills, and digestive glands at higher levels compared to virgin MPLs. Although nuclear abnormalities were observed in all treatments, pyrene-contaminated MPLs induced greater frequencies of micronuclei formation, along with alterations in processes related to DNA repair, antioxidative functions, lysosomal metabolism, immune responses and apoptosis (158). Conversely, when a combination of polyethylene microspheres (1-5 μm) and pyrene was administered to Pomatoschistus microps, pyrene-induced death was delayed (159), indicating that sorption to MPLs may, in certain cases, temporarily reduce bioavailability and alter the timing of toxic effects.

Similarly, co-exposure to MNPLs and mixtures of environmental contaminants has been shown to be associated with enhanced toxicity. In zebrafish, polyethylene MPLs (125-250 μm) combined with a mixture of chemical pollutants present in the aquatic environments, including polychlorinated biphenyls, BFRs, perfluorinated chemicals and methylmercury, resulted in increased liver damage and overexpression of genes related to oxidative stress and detoxification processes, compared to exposure to MPLs or chemicals alone (160). In Tegillarca granosa, polystyrene MPLs combined with hydrophobic persistent organic pollutants (POPs), particularly 17β-estradiol (E2) and benzo[a]pyrene (B[a]P), induced immunotoxicity in a dose- and size-dependent manner; smaller particles (500 nm) amplified POP toxicity, likely by enhancing bioavailability and facilitating internalization, whereas larger particles (30 μm) mitigated toxicity, possibly due to limited internalization and more rapid elimination from the organism. Both pollutants, when combined with MPLs, produced effects proportional to the increases in B[a]P or E2 concentrations (161). Supporting these findings, in C. elegans, co-exposure to B[a]P and 80 nm polystyrene NPLs accelerated aging and shortened nematodes' lifespan by elevating ROS levels. This process compromised mitochondrial integrity and function, triggered the mitochondrial unfolded protein response, and induced ferroptosis (162). In a recent study, researchers examined the long-term effects of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and PET MPLs on Daphnia magna, assessing both individual and combined exposures. All chemicals tested adversely affected development, reproduction, growth, and sexual maturation, with PFOS showing higher toxicity than PFOA. The combined chemical effects were predominantly additive (59%), with a significant proportion (41%) exhibiting synergistic interactions (163). Of particular concern is that responses to these chemicals varied according to genotype; Daphnia magna populations previously exposed to chemical stressors exhibited diminished tolerance compared to populations with no history of chemical exposure, underscoring the importance of genetic background in modulating sensitivity to chemical stressors (163).

Evidence from human cellular models further supports synergistic effects. Exposing human lymphocytes to 1 μm polystyrene MPLs and BPA increased the frequency of micronuclei and nuclear bud formation compared to treatment with each compound independently (164), thereby highlighting a synergistic genotoxic effect. Moreover, the combination of 70 nm polystyrene NPLs or 1 μm polystyrene MPLs with triphenyl phosphate (TPHP) increased intracellular ROS levels, impaired mitochondrial function, and ultimately induced apoptosis in HepG2 cells more effectively than TPHP alone. Significantly, the effects observed with TPHP in conjunction with NPLs were more pronounced than with MPLs, potentially due to the enhanced absorption and better cell membrane penetration capabilities of NPLs (165), further strengthening the notion that smaller particles induce more significant biological effects.

The synergistic effects of plastics were also assessed in conjunction with metals. In zebrafish, combined exposure to polypropylene or PVC and cadmium induced inflammation (elevated IL-1β) and oxidative stress (increased SOD and MDA), along with alterations in microbiota abundance by increasing Proteobacteria and Vibrio and decreasing Gemmobacter; the combination of polypropylene and cadmium resulted in more pronounced effects than PVC (166). These results were also replicated in mice. The administration of polystyrene MPLs and cadmium affected the livers of the animals and activated the Keap-1/Nrf2 antioxidant signaling pathway, as well as the endogenous apoptosis signaling pathway p53/Bcl-2/Bax (167). The study by Feng et al (168) is particularly noteworthy as it provides a mechanistic explanation of the mechanisms through which co-exposure to polystyrene MPLs and lead exacerbates ovarian toxicity in C57BL/6 mice. This combination affected ovarian tissue by disrupting sex hormone levels and inducing oxidative damage through excessive ROS production. Moreover, this co-exposure stimulated endoplasmic reticulum (ER) stress and activated the protein kinase RNA-like endoplasmic reticulum kinase/eukaryotic initiation factor 2α/C/EBP homologous protein signaling pathway, ultimately leading to apoptosis through the downregulation of Bcl-2 and the upregulation of caspase-3 and caspase-12. The administration of salubrinal, an ER-stress inhibitor, and N-acetylcysteine, an antioxidant, mitigated the observed toxicity (168), further confirming the initial findings.

Finally, MNPLs may also interact with biological agents. In a previous study, co-exposure to polyethylene MPLs and Salmonella enterica serovar Typhimurium in chicken ceca resulted in more pronounced microbiome disruption compared to either stressor alone (169). More specifically, while treatment with polyethylene fibers exerted a greater impact on the cecal microbiome, resulting in a lower mean relative abundance of Firmicutes and a higher abundance of Bacteroidetes, the addition of Salmonella induced an even more significant disruption. This included a reduction in the Firmicutes/Bacteroidetes ratio, an increase in Lactobacillus, and alterations in biotin- and arginine-related metabolites (169).

Overall, current evidence indicates that MNPLs can influence the toxicity of co-existing environmental contaminants. However, the magnitude of these effects is highly dependent on experimental conditions, and while several studies suggest increased toxicity (158,160,163-166,169), others demonstrate decreased bioavailability (159,161). Therefore, although MNPLs may indirectly contribute to carcinogenesis by acting as carriers of hazardous substances, further research is necessary to elucidate their impact under realistic exposure conditions.

From an exposome perspective, this 'Trojan' phenomenon exemplifies a quintessential mixture problem: The particulate functions as both a vector and a concentrator, thus rendering internal dose prediction from the free-phase concentration of co-contaminants. The implications are particularly critical during vulnerable life-course windows (prenatal development, infancy, and puberty), when biotransformation capacity is limited and developmental plasticity is at its peak. During these stages, even minor additive or synergistic shifts in internal dose can lead to long-lasting phenotypic effects. Quantitative mixture frameworks established through HBM4EU and PARC, along with PBPK-coupled AOPs, provide the methodological basis for translating findings from in vitro and animal studies into human risk estimates (54,55,57).

The primary mechanisms by which MNPL exposure may influence carcinogenic processes are summarized in Table II. The table integrates the main experimental models, molecular and cellular events, representative mechanistic findings, and their possible relevance to carcinogenesis, providing an overview of the interconnected biological responses discussed throughout the present review.