The establishment of a tumor-induced microenvironment in distant organs is needed for the colonization, adaptation and survival of metastatic cells. This metastatic niche, located in distant organs, is termed the PMN. The initial event in HCC metastasis is the formation of the PMN, which may even occur during the early stages of HCC development (25). EVs, secreted by various cell types, serve as intercellular communication carriers; the unique receptors and specific intercellular adhesion molecules on their surfaces enable targeted recognition of recipient cells and facilitate signal transduction. Subpopulations of EVs derived from different cellular sources play distinct roles and collaborate in this process. Specifically, integrins on the surface of tumor-derived EVs are essential for their interactions with distant organs during PMN formation (26); therefore, EVs act as key mediators in the formation of PMNs.

A hallmark of aggressive malignancies is excessive angiogenesis. Neovascularization is essential for providing oxygen, nutrients and energy, all of which support cancer cell growth, metastasis and the development of multidrug resistance (27). Remodeling of the microvasculature is a prerequisite for PMN formation, including multi-step collaborative processes such as increased vascular permeability, neovascularization and functional reprogramming of endothelial cells (28,29). The heterogeneity of EVs is reflected in their precision in regulating endothelial cells. Small EV subpopulations derived from HCC, rich in miRNA cargo, play a central role in regulating endothelial cell function; for instance, an EV subpopulation enriched with miR-183-5p activates the PI3K/AKT pathway by downregulating SIK1, thereby enhancing endothelial cell proliferation, migration, angiogenesis and vascular permeability, ultimately promoting metastasis in vivo (30). By contrast, large EVs or apoptotic body subpopulations may be more inclined to carry and deliver intact protein growth factors, such as vascular endothelial growth factor (VEGF) and fibroblast growth factor, directly providing strong pro-proliferative signals to endothelial cells. For instance, Golgi protein 73 secreted by EVs derived from HCC can enhance VEGF production in HCC cells and bolster mitogenic signaling in vascular endothelial cells, thereby promoting angiogenesis in the TME (31). Studies by Xu et al (15) and Yokota et al (16) established the key paradigm of HCC-derived EVs disrupting vascular barriers, remodeling angiogenesis and initiating PMN by delivering effector molecules such as basigin and miR-638, while downregulating critical vascular endothelial junction proteins (such as VE-cadherin, ZO-1). These findings are notable but focusing solely on these functions risks oversimplifying a dynamic and heterogeneous biological process. At the mechanistic level, while individual effector molecules have been identified, little is known about their upstream regulation; which tumor cell subpopulations selectively load these cargos and under what conditions? In addition, increased vascular permeability is only the initial step in PMN formation. Furthermore, could the damaged vascular endothelium release specific EVs or factors that feedback and enhance the invasiveness of the primary tumor? In the context of hepatocirrhosis, where HCC is already associated with inflammation and vascular abnormalities, do tumor-derived EVs exacerbate this ‘fragile foundation’ or initiate a novel destructive program? Differentiating between these two scenarios is essential for understanding the specificity of HCC metastasis.

In the metastatic niche, cancer-associated fibroblasts (CAFs) play an active role in tumor metastasis progression (32). Fang et al (17) showed that high-metastatic HCC cells have a greater ability to convert normal fibroblasts into CAFs compared with low-metastatic HCC cells during PMN formation. Fibroblasts induced by EVs from high-metastatic HCC cells exhibit elevated expression of pro-inflammatory genes such as interleukin-1β, transforming growth factor β (TGF-β) and collagen types I, III and IV, which are critical for modulating the TME and promoting carcinogenesis. Exosome microarray detection confirmed the notable role of HCC-derived EVs in converting fibroblasts to CAFs by activating the β1-integrin-NF-κB signaling within the PMN. This research revealed that specific subpopulations of EVs released by highly metastatic HCC cells express unique integrin combinations on their surface, which dictate their targeting to specific organs such as the liver and lungs. Upon uptake by resident cells in these target organs, EVs release their cargo, such as miR-638, which activates relevant signaling pathways, promoting angiogenesis, inflammatory factor release and initiating PMN formation.

The TIME encompasses various immune cells, extracellular immune factors, endothelial cells and CAFs, all contributing to tumor survival (33). The recurrence and metastasis of HCC is a multi-step process in which tumor cells not only evade apoptosis and immune responses but also rely on immune cell communication within the TIME (34). Immune cells, including tumor-associated macrophages (TAMs) and regulatory T cells (Tregs), accumulate within the TIME and facilitate the establishment of an immunosuppressive environment. Conversely, immune cells such as natural killer (NK) cells, CD8⁺ T cells, and CD4⁺ T cells work together to counteract tumor-promoting effects (35). EVs are abundant in the immune microenvironment of HCC, acting as mediators of intercellular communication. The transfer of RNA or proteins via EVs between cells can influence the composition and homeostasis of the TIME, contributing to immune escape and promoting HCC metastasis and recurrence (36,37).

Numerous studies have confirmed that EV-mediated crosstalk regulates immune cell activity within the HCC TIME. The functional impairment of NK cells is recognized as a key mechanism for tumor cells to evade immune surveillance. Liu et al (38) demonstrated in an HCC mouse model that EVs secreted by hepatocytes lacking fructose-1,6-bisphosphatase 1 specifically target and infiltrate NK cells, suppressing NK-mediated tumor surveillance and promoting immune remodeling within the HCC TME. Exosomes secreted by HCC cells, which carry major histocompatibility complex class I-related chain A, may inhibit NK cell function by competitively suppressing agonistic NKG2D receptor signaling, thereby reducing NK cell cytotoxicity against HCC in vivo (39,40). Additionally, microRNA carried by EVs derived from HCC can further contribute to NK cell dysfunction and immune evasion. For instance, miR-92b, mediated by HCC-EVs, downregulates the activation marker CD69 on NK cells, impairing their activity (18).

Macrophages, notable players in the innate immune response of the liver, are also involved in HCC proliferation, metastasis, invasion and angiogenesis (41). TAMs are increasingly recognized as essential components of the HCC TIME (42,43). Tumor-derived factors can polarize macrophages into either the M1 phenotype, which exhibits anti-tumor activity, or the M2 phenotype, which promotes tumor growth (44). EVs derived from HCC cells promote macrophage activation and M2 polarization through various mechanisms, enabling tumors to evade immune surveillance (45,46). For example, EVs secreted by HCC deliver long non-coding (lnc)RNA PART1 to TAMs, upregulating TLR4 expression and thus inducing M2 polarization. Polarized M2 macrophages in turn promote HCC cell proliferation, metastasis and tumor growth in vivo (19). In another study, M2 polarization induced by HCC-EVs released exosomes that transferred miR-15b to HCC cells, inhibiting the Hippo pathway and promoting HCC proliferation, invasion and metastasis by targeting large tumor suppressor kinase 1 (20). The T cell-mediated immune response is central to cancer immunity, critical for immune surveillance and tumor cell clearance (47). In the TIME, T cell regeneration often leads to the deterioration of T cell responses and HCC progression. EVs may modulate the function and activity of CD4⁺ T cells, CD8⁺ T cells and Tregs, influencing the homeostasis of TIME and contributing to immune suppression, HCC progression and metastasis (19,48). Gong et al (49) demonstrated that norcholic acid increases programmed death ligand 1 (PD-L1) levels on the surface of HCC and its exosomes, notably inhibiting CD4⁺ T cell function and inducing an immunosuppressive microenvironment that promotes HCC proliferation, invasion and metastasis. Furthermore, EVs from other immune cells in HCC contribute to the depletion of CD8⁺ T cells, another mechanism of immune evasion. For example, HCC-EVs carrying miR-146a-5p induce macrophage polarization towards the CD206⁺PD-L1⁺CD80⁺ M2 phenotype, and these EVs-induced macrophages impair CD3⁺ T cell function by upregulating expression of inhibitory receptors (such as programmed cell death-1 and cytotoxic T-lymphocyte associated protein 4) on co-cultured T cells (50). Another mechanism for creating an immunosuppressive microenvironment in HCC is the recruitment of Tregs to TIME. The acidic microenvironment in HCC promotes the upregulation of miR-21 expression in EVs; miR-21 enhances Treg proliferation and recruitment to TIME while reducing the proportion of Th17 cells. The resulting imbalance between Th17 and Treg cells leads to immune suppression, fostering HCC invasion and metastasis (51,52).

CAFs are key components of the TIME, with increasing evidence supporting their role in promoting HCC recurrence and metastasis through immune suppression (53,54). EVs carrying miR-20a-5p, released by CAFs, activate the Wnt/β-catenin signaling pathway by downregulating LIM domain and actin binding 1, thereby facilitating immune evasion and enhancing HCC metastasis and invasion (55,56). Under hypoxic conditions, CAFs inhibit the activity and cytotoxicity of CD8⁺ T cells via an EV-dependent mechanism, secreting EVs rich in circHIF1A and binding to HuR, upregulating PD-L1 expression, which in turn promotes HCC cell proliferation, migration and invasion. These findings suggest that the interaction between CAFs and the TIME, mediated by EVs, is a notable factor driving HCC progression (57). Further investigation into the activation of CAFs by HCC-derived EVs may offer new insights into the mechanisms underlying recurrence and metastasis, as well as potential therapeutic strategies.

In conclusion, EVs do not promote HCC metastasis in a general or indiscriminate manner. Their high heterogeneity, reflected in the diversity of cellular origins, sizes and molecular cargos, results in functionally specialized subpopulations. These subpopulations collaborate with high precision in both space and time; some prepare the environment by directing remote organs to form the PMN, others remodel microvessels to facilitate transport and others destroy the immune surveillance system along the way, enabling immune evasion. Consequently, future research into liver cancer metastasis and the development of EV-based liquid biopsy and treatments must focus on identifying, isolating and analyzing key pathogenic EV subpopulations, rather than considering EVs as a homogenous entity.

EMT is a process in which epithelial cells lose their adhesive properties and acquire mesenchymal traits, including enhanced migration and invasion capabilities (58). This mechanism plays a marked role in the metastasis of advanced HCC. EMT influences tumor cell proliferation and metastasis through various mechanisms, including the active release of CTCs from primary tumors, facilitating distant metastasis and promoting HCC recurrence (59,60). The activation of EMT in CTCs primarily occurs within the bloodstream, involving dynamic processes related to immune evasion and autophagy (61,62).

During EMT-mediated metastasis in HCC, CTCs contribute to the degradation of the basement membrane and extracellular matrix (ECM). Under hypoxic conditions and through paracrine signaling, CTCs activate transcription factors, miRNAs and other regulatory elements, leading to the disruption of tight and adhesive cell connections and reorganization of the cytoskeleton. This upregulates mesenchymal markers and downregulates epithelial markers, enabling tumor cells to migrate through the matrix and enter the bloodstream (63,64). CTCs can be classified into three phenotypes based on EMT markers: Epithelial, mesenchymal and mixed CTCs (65). A study of 33 patients with HCC found that the number of epithelial CTCs was positively correlated with tumor size, the number of mixed CTCs was positively correlated with tumor number and the number of mesenchymal CTCs (M-CTCs) was positively correlated with metastasis. Mixed CTCs are key players in the EMT transition of HCC and contribute notably to intrahepatic metastasis, whereas M-CTCs may serve as predictors of extrahepatic metastasis (66). Both experimental and clinical studies highlight the marked role of EMT-associated CTCs in HCC metastasis and recurrence (67,68). A study of 112 patients with HCC showed that elevated CTC counts and a higher percentage of M-CTCs preceded the clinical detection of HCC recurrence. Notably, BCAT1, a gene markedly upregulated in these patients, may trigger the EMT process (69). Therefore, classifying CTCs based on EMT characteristics could serve as an early predictor of HCC recurrence, metastasis and survival (70). Furthermore, Lu et al (71) demonstrated that platelets, by binding to CTCs, induce autophagy via the AMPK/mTOR signaling pathway, which promotes EMT and enhances the motility of HCC cells.

The activation of EMT in CTCs may also be linked to immune responses. Huang et al (72) confirmed through in vitro experiments that there is a significant positive correlation between the number of Treg cells, CTC count and vascular infiltration, suggesting that Treg cells may serve as a key regulator of CTC release. Treg cells can enhance the invasive potential of HCC cells by secreting TGF-β1, thus triggering EMT. Therefore, targeting Treg cells within the TME, for example, through the depletion of Tregs via CCR8 targeting (73), or by inhibiting Treg-derived TGF-β1 signaling to suppress EMT (72), may represent a promising therapeutic strategy to prevent CTC release and, consequently, HCC metastasis and recurrence. Another study found that lncRNA FOXD1-AS1 promotes HCC immune evasion via PD-L1 and affects CTC production and metastasis in HCC mouse models by upregulating the PI3K/AKT signaling pathway, thereby regulating EMT and ultimately driving HCC proliferation, invasion, and metastasis (74).

MVI, also known as microvascular tumor thrombus, is characterized by the infiltration of malignant cells into the microvessels surrounding the liver tumor. It is one of the notable pathological features in HCC, strongly associated with poor prognosis following surgical resection and transplantation, early recurrence and intrahepatic metastasis (75). Studies suggest that CTCs released by HCC can form vascular thrombi from individual cells, serving as key components that further promote HCC recurrence and metastasis. Preoperative CTC counting may serve as a predictive tool for MVI and provide dynamic monitoring of HCC prognosis (9). Several mechanisms are thought to be involved in this process.

Firstly, CTCs derived from HCC trigger a coagulation response in peripheral blood, forming circulating tumor microemboli (CTM), which facilitates CTC migration across the endothelial barrier and metastasis both within and outside the liver. Studies have shown that tissue factor is overexpressed in the peripheral blood of patients with HCC and is closely associated with extrahepatic metastasis, lymphatic spread and portal vein thrombosis (76). Tissue factor, notably expressed in cancer cells and CTCs, promotes the binding of CTCs with coagulation factors, leading to the formation of clumps composed of both tumor and non-tumor cell components, such as platelets and fibrin. This coagulation response ultimately facilitates the aggregation and adhesion of CTCs. When tissue factor enters the bloodstream, it activates coagulation factor VII and initiates the extrinsic coagulation pathway, leading to the formation of CTMs in circulation (77). Moreover, once CTMs are formed, tissue factor can regulate the proliferation and growth of HCC cells by activating protease-activated receptors, making it a key factor in reshaping the TME and promoting the formation and progression of MVI.

Secondly, CTCs facilitate the dissemination and colonization of HCC tumor cells by activating platelets, thereby creating a favorable environment for tumor growth. Growing evidence suggests that platelets play a pivotal role in promoting the invasion and metastasis of HCC cells through multiple mechanisms. The ability of platelets to protect tumor cells in circulation is key to their support of hematogenous dissemination (78). CTCs induce platelet activation and aggregation, with activated platelets binding to CTCs, shielding them from the shear forces exerted by blood flow and vessel walls. This protection is essential for the survival of CTCs and their subsequent adhesion to the vascular endothelium at metastatic sites (79). Moreover, CTC-induced platelet activation also aids HCC cells in evading immune surveillance. Sun et al (80) demonstrated that CTC-platelet adhesion helps cancer cells escape NK cell-mediated innate immune responses by upregulating the immune checkpoint CD155-TIGIT, thereby promoting distant metastasis. Activated platelets also secrete numerous growth factors, including TGF-β, VEGF and platelet-derived growth factor. VEGF, secreted by CTCs, enhances microvascular permeability by enlarging the spaces between adjacent endothelial cells. The interaction between CTCs and platelets markedly increases the likelihood of MVI (81). In summary, platelets activated by CTCs not only protect the CTCs but also contribute to the formation of new blood vessels in HCC by transporting cytokines that provide nutrients essential for tumor cell growth (82,83). Additionally, platelets regulate the integrity of tumor vasculature, promoting CTC adhesion and transendothelial migration, which leads to HCC metastasis and colonization. While the role of platelets in HCC metastasis is well-documented, further research is needed to elucidate the specific mechanisms underlying the activation and maintenance of the CTC-platelet interaction.

Tumor cells consisting of two or more CTCs are referred to as CTC clusters or CTMs. CTC clusters have been proposed as indicators of tumor metastasis. Although the number of CTC clusters in circulation is smaller than that of single CTCs, their metastatic potential is notably higher, increasing by 23–50 times (84). Understanding the formation mechanisms of CTC clusters is therefore critical for elucidating the metastasis of HCC. Currently, two widely accepted mechanisms explain the formation of CTC clusters; the first involves tumor cells directly forming cluster-like structures that invade microvessels after detaching from the primary tumor (85). Aceto et al (86) were the first to demonstrate that CTC clusters originate from oligoclonal tumor cell populations rather than from cell aggregation within blood vessels. The authors also identified plakoglobin-dependent intercellular adhesion as critical for CTC cluster formation. The second mechanism suggests that tumor cells detach from the primary tumor due to shear forces from blood flow, diffuse into blood vessels and aggregate into clusters (87). Sun et al (62) prospectively measured CTCs in five key vascular sites in patients with HCC and detected CTCs in tumor-feeding vessels in approximately half of the patients. Notably, CTCs were found in the hepatic veins of 15 patients but not in the surrounding arteries, and CTC recurrence was observed in peripheral veins in five of these patients. This suggests that individual CTCs may aggregate again in the bloodstream. While this contradicts conclusions made by Aceto et al (86), it warrants further investigation. The formation of CTC clusters is a multistage and dynamic process, with mechanisms exhibiting spatiotemporal heterogeneity. The two main pathways currently recognized are collective shedding from the primary tumor and single-cell re-aggregation in circulation, which likely to coexist and complement each other. CTC clusters undergo a complex life cycle, beginning with clonal collective shedding from the primary tumor, followed by dynamic structural evolution in the challenging environment of circulation, partially disintegrating and partially reassembling. Ultimately, the metastasis efficiency may depend on the ability of certain cell populations to maintain an aggregated state or effectively reorganize within distant blood vessels. The unique hemodynamics of the portal venous system in HCC, the abundance of immune cells in the liver and the altered vascular structure in cirrhosis may make the dynamic evolution of CTC clusters particularly active in the intrahepatic circulation. This may play a key role in the intrahepatic dissemination of HCC.

At the onset of metastasis, EVs enhance CTC shedding by promoting EMT and ECM remodeling, stimulating angiogenesis, increasing vascular permeability and strengthening the adhesion ability of CTCs. In advanced HCC, target organ metastasis is common, with EMT activation playing a marked role in initiating metastasis (88). Studies have demonstrated that EVs are pivotal in the metastatic process of HCC, inducing EMT activation and weakening the adhesion of tumor cells, thus facilitating the shedding of tumor cells to form CTCs (89). EVs regulate key genes (for example, PRDM16 and LHX6) in the EMT pathway, activating the Wnt/β-catenin, Notch and MAPK/ERK signaling pathways, among others, to promote the detachment of CTCs from the primary tumor site and enhance their shedding. Chen et al (90) observed that HCC cells treated with EVs showed elevated expression of α-SMA and vimentin, along with reduced expression of E-cadherin, promoting mesenchymal marker expression. Western blot analysis confirmed that these processes were mediated through the MAPK/ERK pathway, facilitating tumor cell migration, chemotaxis and invasion. Further research has revealed that EVs derived from hepatic stellate cells (HSCs) activate the Notch signaling pathway in HCC cells via PRDM16, leading to increased proliferation, migration, invasion and EMT progression (91). Hypoxia-induced EVs in HCC activate EMT through the Wnt/β-catenin signaling pathway, promoting HCC cell proliferation, migration and invasion (92). Moreover, tumor-derived EVs can directly or indirectly promote angiogenesis and increase vascular permeability by releasing pro-angiogenic factors (for example, VEGF and IL-8), which play a pivotal role in the formation of shed CTCs at the primary tumor site (93). Hypoxic HCC cell-derived EVs co-cultured with M2 macrophages secrete higher levels of VEGF, granulocyte colony-stimulating factor and IL-4, which further stimulate angiogenesis and increase vascular permeability, enabling tumor cells to shed into circulation as CTCs, thereby enhancing metastasis (94). Disruption of vascular barriers is a pivotal step in CTC-mediated HCC metastasis, requiring the breakdown of tight junctions and the integrity of vascular endothelial cells (95). Tumor-derived EVs enhance vascular permeability and promote CTC production and shedding by transferring their contents to endothelial cells. The protein nidogen 1, secreted by HCC-derived EVs, increases the permeability of lung endothelial cells by disrupting the integrity of the endothelial barrier and promoting angiogenesis, raises tumor necrosis factor receptor 1 expression and facilitates the formation of the PMN in the lungs (96).

In summary, EVs are key initiators driving HCC metastasis, initiating EMT to internally transform tumor cells and endow them with migratory potential. Simultaneously, EVs directly target and downregulate key junction proteins (such as VE-cadherin and p120-catenin) in endothelial cells by delivering specific cargoes, such as miR-103. This disruption of tight junctions compromises the vascular endothelial barrier, thereby reducing the physical obstacles to CTC intravasation into the circulatory system. This dual approach facilitates CTC generation and shedding, driving HCC metastasis.

CTCs are notable drivers of distant metastasis in HCC. The survival of CTCs in circulation faces notable challenges, including the need to withstand intravascular shear stress and evade immune surveillance. EVs can interact with various hematopoietic and immune cells. For example, EVs induce platelet activation by delivering CD97. Activated platelets can protect CTCs from hemodynamic shear forces, thereby enhancing CTC survival in the bloodstream. Additionally, EVs can induce neutrophils to form neutrophil extracellular traps, which bind to CTCs and promote their adhesion to blood vessels, helping CTCs survive and initiate metastasis (97,98). EVs carry molecules that interact with circulating immune cells, inducing immunosuppression and further protecting CTCs from immune system attacks. Research has demonstrated that PD-L1 is markedly expressed on CTCs, and EVs promote immune escape by upregulating PD-L1 expression on CTCs, thereby hindering CD8⁺ T cell-mediated immune responses in HCC (99–101). The delivery of immune checkpoint molecules such as PD-L1 by EVs is a recognized immune evasion mechanism in various solid tumors. In HCC, however, this process occurs within a unique liver immune microenvironment. In the specific context of chronic inflammation caused by HBV/HCV infection or non-alcoholic fatty liver disease and alcoholic liver disease, Kupffer cells undergo marked functional remodeling and spatial repositioning, while being in a ‘pre-activated’ immune regulatory state, characterized by upregulation of immune checkpoint molecules such as PD-L1 (102,103). This pre-existing immune tolerant microenvironment in the liver, shaped by chronic inflammation, makes Kupffer cells more sensitive to EVs from tumor sources. Therefore, the EVs in HCC not only directly inhibit immune effector cells, but also may synergistically amplify the immunosuppressive effect by acting on these already sensitized Kupffer cells, thereby achieving dual protection against CTCs; however, to the best of our knowledge, there is currently no direct evidence that reveals whether the specific mechanism of EV-mediated CTC survival specifically occurs in HCC with different etiological backgrounds such as HBV, HCV, non-alcoholic fatty liver disease or alcoholic liver disease. Therefore, this hypothesis still requires targeted verification through the design of rigorous experiments. EVs can enhance CTC adhesion, facilitating their colonization in specific organ microenvironments, highlighting the importance of the PMN.

The TME of HCC is dynamic and complex, comprising cancer cells, stromal cells, blood vessels, nerve fibers and related decellularized components (104). EVs have emerged as key mediators of intercellular communication within the TME, driving tumor progression (105). Accumulating evidence suggests that EVs transfer functional biological factors to recipient cells, reshaping the TME by reprogramming both cancer cell and stromal cell metabolism (106). Fang et al (17) demonstrated that EVs released by metastatic HCC cells carry miR-1247-3p, which directly targets B4GALT3 in fibroblasts. This activates the β1-integrin-NF-κB signaling pathway, inducing the transformation of fibroblasts into CAFs, and promotes lung metastasis in patients with HCC.

Furthermore, EVs contribute to the formation of a PMN for CTCs, creating a favorable foundation for their colonization. Tumor-derived EVs serve as key intercellular messengers that recruit inhibitory immune cells (such as TAMs and Tregs) to distant organs by carrying cytokines (such as TGF-β1 and IL-10). These recruited immune cells suppress antitumor immunity through functional reprogramming, including M2 polarization of macrophages and enhancement of Treg-mediated suppression. Concurrently, EVs promote the establishment of a PMN in distant organs by increasing vascular permeability, activating fibroblasts and remodeling the extracellular matrix. This coordinated remodeling of the immunosuppressive and stromal microenvironment facilitates the colonization and outgrowth of CTCs at distant sites (107). EVs released by HCC cells form a liver PMN rich in fibrotic stroma and immunosuppressive cells by activating HSCs and recruiting myeloid-derived suppressor cells; this creates an environment that promotes CTC colonization. For example, TGF-β1-enriched EVs induce HSCs to secrete fibronectin, thereby promoting CTC adhesion and invasion (108). Conversely, EVs released by CTCs can further recruit immunosuppressive cells (such as Tregs) and remodel the liver TME, forming a positive feedback loop. For instance, CCL5 secreted by CTCs is transmitted to Kupffer cells through EVs, stimulating the release of tumor necrosis factor-α, exacerbating liver inflammation and fibrosis and accelerating HCC metastasis (61).

In summary, EVs support CTCs by constructing a dynamic, multi-layered protective network. In circulation, platelet-derived EVs encapsulate CTCs, forming clusters that shield them from shear stress, aiding their survival in the bloodstream. Before colonization, EVs act as messengers, modifying the target organ microenvironment and creating niches for CTCs. However, most current research is based on in vitro models, and whether the complex in vivo microenvironment fully aligns with this protective model remains uncertain. Future research should focus on longitudinal clinical cohort studies to dynamically monitor changes in EV cargo and CTC characteristics in patients with HCC at various stages, before and after treatment, to establish causal relationships. Additionally, novel liquid biopsy strategies targeting specific molecular markers of ‘CTC-protective EVs’ should be developed to enable early and functional prediction of metastatic risk.