Brain metastasis (BM) is a frequent and detrimental outcome of lung cancer, significantly impairing treatment efficacy and reducing overall survival (48). Notably, SCLC is recognized as the most aggressive histological subtype, with a pronounced tendency for early BM development (43). BM also constitutes a critical cause of mortality in patients with NSCLC (49). In advanced lung adenocarcinoma (LUAD), the incidence of BM has been reported to reach up to 45% (50). Exosomes are deeply implicated in this process through various mechanisms ( Figure 3A ).

The establishment of a brain PMN in lung cancer individuals with BM largely depends on the disruption of the blood-brain barrier (BBB). Astrocytes, as a critical component of the BBB, play a pivotal role in maintaining the homeostasis of the brain microenvironment (51). Exosomes are capable of crossing the BBB, thereby participating in intercellular communication within the central nervous system (CNS), supporting neuronal development, and modulating inflammatory responses (52). For example, exosomes secreted by the lung cancer cell line H1299, containing MAP2K1, TUBA1C, RELA, and CASP6, induce astrocyte apoptosis and trigger the release of pro-inflammatory cytokines, thereby fostering an immune-evading microenvironment (51).

Exosomal miRNAs play diverse and critical roles in the regulation of BM. In particular, serum-derived exosomal miRNAs from lung cancer individuals with BM are involved in several regulatory pathways. miRNA-342-5p, for instance, has been identified as a downstream target of Notch signaling, promoting the differentiation of neural stem cells into neural progenitors and astrocytes. Meanwhile, the overexpression of miRNA-483-5p has been shown to inhibit ALCAM, a key protein in maintaining BBB integrity (53). Furthermore, miR-550a-3-5p, enriched in exosomes isolated from the plasma of lung cancer individuals with BM, inhibits cell viability and migration while inducing apoptosis in human brain microvascular endothelial cells through YAP1 targeting (54). Additional studies on plasma-derived tumor exosomes from patients with cerebellar metastases have identified six upregulated miRNAs, including has-miRNA-331-5p, -542-5p, -26a-2-3p, -99a-3p, -184, and -3065-5p, which may serve as potential biomarkers for BM (48). Interestingly, miR-124-3p targets the 3’UTR of Rab27a, reducing exosome secretion and inhibiting the PI3K/AKT signaling pathway, thereby curtailing NSCLC metastasis (55). Moreover, lysophosphatidylcholine acyltransferase 1 (LPCAT1), an enzyme involved in phospholipid metabolism, has been found to regulate the PI3K/AKT pathway by modulating the MYC transcription factor, thus enhancing cancer cell growth and metastasis (56). Single-cell RNA sequencing of LUAD brain metastases confirmed significantly elevated LPCAT1 expression in lung cancer cells at BM sites compared to primary lung tumor cells (49). Upregulation of LPCAT1 in NSCLC has been associated with an increased incidence of BM, while LPCAT1 inhibition effectively curbs lung cancer cell proliferation and metastasis (49, 56).

LncRNAs and proteins in exosomes are equally pivotal in the development of brain metastasis (BM). For example, exosomes secreted by the NSCLC cell line H1299 that carry LINC01356 play a pivotal role in remodeling the BBB and facilitating BM in lung cancer (57). Additionally, calcium-dependent proteins and S100 proteins are strongly linked to BM formation (58). Notably, exosome-mediated delivery of S100A16 from brain microvascular endothelial cells enhances SCLC cell survival and promotes BM by boosting mitochondrial function (43). Moreover, plasma exosomal MUC5B, a cell surface glycoprotein that stimulates brain metastasis, has shown potential as a biomarker for diagnosing BM in patients with lung cancer (58).

The influence of external environmental factors on lung cancer brain metastasis is also profound. Studies indicate that patients with lung cancer with a history of smoking are more susceptible to BM compared to non-smokers. Chronic nicotine exposure leads to the accumulation of N2-neutrophils around brain metastases, which release exosomes carrying miRNA-4466. These exosomes activate tumor stem cell-associated genes and metabolic regulators, such as SOX2 and CPT1A, thereby promoting brain tumor metastasis and solid tumor growth (59). Similarly, exosomes derived from human bronchial epithelial cells exposed to PM2.5 have been shown to enhance the migration, invasion, and epithelial-mesenchymal transition (EMT) of lung cancer cells, activating the JNK signaling pathway to accelerate lung cancer metastasis in vivo (60).

In summary, the progression of brain metastasis during lung cancer metastasis to the brain, mediated by exosomes, is shaped by both intrinsic and extrinsic environmental factors. The establishment of the PMN within the intrinsic environment further contributes to BM development. However, safeguarding the BBB is paramount in preventing tumor metastasis to the brain. Therefore, future research into strategies aimed at enhancing the protective function of the BBB during tumor cell metastasis holds significant promise.

The skeletal environment plays a pivotal role in supporting lung cancer cell proliferation (61). Bone is one of the most frequent sites for lung cancer metastasis, with an incidence rate of 30-40% among patients with lung cancer (44). The presence of bone metastases drastically reduces the three-year survival rate from 71.6% to 46.8% (62). Bone metastases in lung cancer are classified as either osteoblast-associated or osteoclast-associated. In metastatic NSCLC, osteolytic bone destruction driven by heightened osteoclast activity is the predominant mechanism (63). Exosomes are deeply involved in this pathological process ( Figure 3B ), delivering bioactive molecules—such as proteins, lipids, and non-coding RNAs (ncRNAs)—that play a critical role in bone remodeling by modulating osteoblast function (64).

The interaction between osteoclasts and cancer cells is reciprocal. Exosomes carrying miR-214 and miR-194-5p secreted by lung cancer cells promote osteoclast formation and enhance bone resorption, creating a bone marrow microenvironment conducive to cancer cell survival and metastasis (65, 66). Exosomal miRNAs play a pivotal role in facilitating lung cancer bone metastasis by modulating key signaling pathways. Research suggests that the dysregulation of miRNAs, particularly those involved in the Wnt/β-catenin pathway, significantly drives cancer progression, metastatic spread, and resistance to therapy (67). This pathway is integral to the EMT, a critical process in tumor development and metastasis, and is especially important in CNS maturation (68, 69). Key miRNAs, such as miR-574-5p, miR-328-3p, miR-423-3p, miR-770, miR-433, and miR-1260b, actively participate in the Wnt/β-catenin pathway, influencing EMT. Notably, inhibiting miR-574-5p, miR-328-3p, and miR-423-3p may synergistically disrupt the Wnt/β-catenin signaling pathway, potentially playing a critical role in preventing lung cancer bone metastasis. Further analysis has identified 43 miRNAs, predominantly located on chromosome 14, that are upregulated in bone metastases and linked to EMT and bone metastasis (69). For instance, miR-433 targets the 3′-UTR of TMED5, thereby inhibiting the Wnt/β-catenin pathway (70), while miR-320a from human umbilical cord MSC-derived exosomes modulates this pathway by targeting SOX4 (71). Moreover, circRACGAP1 enhances the stemness and metastatic potential of NSCLC cells by interacting with PTBP1, stabilizing SIRT3, and activating the RIF1-mediated Wnt/β-catenin pathway (72).

The PI3K/AKT signaling pathway also plays a central role in lung cancer bone metastasis. miR-17-5p promotes osteoclast formation by targeting the tumor suppressor PTEN and activating the PI3K/AKT signaling cascade in lung cancer bone metastasis (73). Conversely, miR-124-3p inhibits PI3K/AKT signaling by targeting the 3’UTR of Rab27a, reducing exosome secretion and thereby limiting NSCLC metastasis (55).

Bone marrow mesenchymal stem cells (BMSCs) also contribute to the proliferation and further metastasis of lung cancer cells in the bone marrow microenvironment. Quiescent lung cancer cells release exosomes that are internalized by BMSCs, enhancing the glycolytic capacity of BMSCs via the insulin-like growth factor 1 receptor (IGF-1R) signaling pathway. Blocking IGF-1R signaling or glycolytic pathways effectively slows the proliferation of lung cancer cells within the bone marrow (61).

Both the Wnt/β-catenin and PI3K/AKT signaling pathways play integral roles in lung cancer metastasis to bone via exosomes. By modulating the bone marrow microenvironment, these pathways provide cancer cells with enhanced survival and proliferation capacities, ultimately promoting metastasis. The mutual reinforcement between osteoclasts and cancer cells underscores the importance of osteoclasts in the metastatic process. Future research should focus on understanding osteoclast formation and function, as altering their properties could potentially disrupt the metastatic cycle.

In conclusion, exosomes play a critical role in driving the metastasis of lung cancer to the brain and bone through various molecular mechanisms, making them a promising target for future diagnostic and therapeutic interventions in the management of this lethal disease.

Breast cancer (BC) metastasis accounts for approximately 90% of fatalities associated with the disease, with lung metastasis being a predominant route (45). The 5-year survival rate for patients with metastatic breast cancer is a mere 20% (74), and it is estimated that 60-70% of those with lung metastases eventually succumb to recurrence (75). Exosomes play a pivotal role in facilitating these metastatic processes ( Figure 4A ).

Exosome-mediated PMN formation is essential for the metastasis of breast cancer cells to the lungs. Exosomes derived from metastatic cells are integral in creating a microenvironment conducive to metastasis, particularly through the involvement of specific proteins (76). For instance, exosomes from BC cells containing Cav-1 upregulate inflammatory gene expression in lung epithelial cells, thereby modulating matrix deposition in lung fibroblasts. These exosomes disrupt the PTEN/CCL2/VEGF-A signaling pathway in lung macrophages, promoting M2-type polarization, neovascularization, and PMN formation, all of which facilitate lung metastasis in breast cancer (24). Moreover, exosomes containing Wnt7a protein from low-metastatic cell lines, such as LM.4T1, can enhance the metastatic potential of high-metastatic cells by activating the PI3K/AKT/mTOR signaling pathway. These low-metastatic cells also stimulate angiogenesis within the TME, providing a more favorable setting for highly metastatic cells to evade immune surveillance. Notably, Wnt7a increases the risk of lung metastasis in triple-negative breast cancer, highlighting its potential as a novel therapeutic target (77).

High levels of the lncRNA Lin28B, coupled with low levels of let-7s, are prognostic indicators for poor outcomes and lung metastasis in patients with breast cancer. Lin28B alters the cytokine environment in the lungs before metastasis occurs, facilitating neutrophil recruitment and N2 polarization. Even in the absence of tumor cell migration, Lin28B triggers changes in the PMN, leading to the production of IL-6 and IL-10, which drive the N2-type conversion (22). Comparative analysis of primary tumors, non-tumor tissues, and lung metastases has shown that tumor suppressors miR-200c and let-7a are downregulated in both tumors and metastatic tissues, but their levels in exosomes are elevated, particularly in lung metastases. Exosomal miR-200c may suppress the immune response of F4/80+ macrophages, negating the tumor-suppressive effects on recipient cells. Rab1A facilitates the packaging of miR-200c into exosomes, circumventing its inhibitory effect on tumor proliferation. Thus, targeting anti-Rab1A proteins to enhance miR-200c expression could impede breast cancer metastasis to the lungs (78).

miRNAs within exosomes secreted by breast cancer cells also influence macrophage remodeling and programming. For example, miR-138-5p translocates into macrophages and downregulates KDM6B expression, regulating macrophage polarization (79). M2-type macrophages, in particular, play a tumor-supportive role, aiding in tumor invasion and metastasis (80, 81). Exosomes derived from tumor-associated macrophages (TAMs), containing miR-223-3p, promote lung metastasis by targeting and inhibiting Cbx5, thus facilitating breast cancer cell metastasis to the lungs (82).

In conclusion, breast cancer utilizes exosomes to influence target genes and proteins, modify multiple signaling pathways, and reshape macrophage polarization. These changes ultimately contribute to the formation of the PMN, which drives the initiation of lung metastasis in breast cancer cells.

Osteosarcoma (OS) predominantly arises in regions of rapid bone growth, such as the femur, tibia, and humerus. This malignancy is characterized by cancer cells that produce immature bone or bone-like tissue (83). OS is one of the most common malignant tumors originating in the skeletal system, particularly affecting children and adolescents, and is notorious for its high metastatic potential (84). The 5-year overall survival rate remains relatively low, around 20%, largely due to the presence of lung metastases in 15-30% of patients at the time of diagnosis (85). Exosomes play a critical role in the lung metastasis of bone cancer ( Figure 4B ).

The complex genetic makeup of OS suggests that immune suppression is a key factor in its progression (86). In patients with OS, immunosuppressive cells, supportive cells within the TME, and tumor cells can release programmed death ligand 1 (PD-L1) via exosomes, which collectively contribute to systemic immunosuppression (87). Studies have shown that PD-L1 expression in the TME, both in mouse models and humans, promotes systemic immune suppression, thereby increasing tumor burden and reducing patient survival across various cancers (88). In contrast, lung metastasis was significantly reduced in mice treated with exosomes from PD-L1 knockdown cells, suggesting that exosomal PD-L1 is crucial in promoting lung metastasis in OS (89). Furthermore, tumor-derived exosomes enhance glucose uptake via the TLR2 and NF-κB pathways, leading to increased nitric oxide synthase 2 (NOS2) levels, which inhibit mitochondrial oxidative phosphorylation. This metabolic shift increases PD-L1 expression by converting more pyruvate to lactate, which in turn stimulates NF-κB activity and polarizes macrophages towards an immunosuppressive phenotype (90).

Macrophages, as primary phagocytes involved in the innate immune response, play a critical role in combating OS and other cancers, significantly influencing survival outcomes (91). M1 macrophages exert tumor-suppressive effects, while M2 macrophages promote tumor growth and metastasis. OS cells enhance migration, invasion, EMT, and lung metastasis primarily through exosome-mediated immunomodulatory factors, such as Tim3, which induce M2 macrophage polarization and the secretion of cytokines like IL-4, IL-6, IL-10, TGF-β, and VEGF (81, 91–94). For example, visfatin treatment of chondrosarcoma cells has been shown to increase exosome production, which in turn stimulates M2 macrophage polarization, enhancing chondrosarcoma motility and contributing to lung metastasis (95). However, interactions between tumor cells and macrophages may vary depending on the metastatic potential of the tumor. For instance, EVs from highly metastatic OS interact differently with macrophages compared to those from less metastatic cells. In highly metastatic K7M2 OS cells, EVs decrease the levels of pro-inflammatory cytokines such as INFγ, IL-2, IL-6, and TNFα in macrophages, suggesting that these EVs further promote M2 macrophage differentiation, reinforcing the metastatic process. This represents the first evidence of differential interactions between OS-EVs and macrophages based on their polarization states (96).

Exosomal proteins are pivotal in facilitating the metastasis of OS to the lungs. One key player in this process is the upregulation of the COL6A1 gene in OS tissues, which has been strongly correlated with an increased likelihood of lung metastasis and reduced survival rates in patients with OS. This upregulation occurs through the binding of the transcription factor c-Jun to p300, which enhances the acetylation modification (H3K27ac) of histone H3 lysine 27 in the promoter region of the COL6A1 gene. Moreover, COL6A1 facilitates OS metastasis by downregulating STAT1 expression through its degradation via the ubiquitin-proteasome pathway. OS cells package COL6A1 into exosomes and transfer it to fibroblasts, inducing their activation and stimulating TGF-β secretion, which in turn enhances the invasive and migratory abilities of OS cells (92). Additionally, exosomes derived from OS cells carrying Rab22a-NeoF1 fusion proteins and PYK2 further exacerbate lung metastasis by recruiting bone marrow-derived macrophages to establish a favorable metastatic microenvironment. PYK2 in exosomes activates the RhoA signaling pathway within OS cells and promotes the generation of M2-type macrophages, which are known to support tumor metastasis, thereby accelerating OS metastasis to the lungs (97). Furthermore, Rab27a, a gene critical for exosome-specific protein transport, is essential for OS cell metastasis. Knocking out Rab27a in OS cells reduces exosome secretion, significantly lowering their metastatic potential to the lungs (89).

As discussed, OS cells exploit exosomes to influence several pathways, including macrophage polarization and fibroblast activation, by releasing various cytokines. These exosomal interactions create an immunosuppressive microenvironment that favors metastasis, supporting the migration of OS cells and the development of lung metastasis. Thus, identifying strategies to detect, disrupt, or prevent the formation of this immunosuppressive environment represents a promising avenue for future research. Such approaches may offer new therapeutic insights into mitigating the progression and metastasis of OS to the lungs.

Colorectal cancer (CRC) is the third most common malignancy globally and ranks second in cancer-related mortality (4). Approximately 20-25% of patients with CRC present with synchronous metastatic disease at diagnosis (98), with the liver and lungs being the most frequent sites of metastasis. Approximately 10-25% of patients with CRC eventually develop lung metastases (47). Exosomes play a significant role in mediating CRC lung metastasis through various mechanisms ( Figure 4C ).

The formation of PMN is a critical step in CRC lung metastasis, where exosomal miRNAs facilitate communication between cancer cells and stromal cells (99). Notably, miR-10a expression in CRC-derived exosomes is inversely correlated with the depth of tumor invasion. Exosomal miR-10a from CRC cells inhibits the proliferation and metastatic potential of normal human lung fibroblasts (NHLFs) and reduces the expression of inflammatory cytokines, such as IL-6, IL-8, and IL-1β in NHLFs (100). Additionally, miR-25-3p secreted by CRC cells targets KLF2 and KLF4 in vascular endothelial cells, downregulating the expression of vascular endothelial growth factor receptor 2 (VEGFR2), ZO-1, occludin, and Claudin5, thereby affecting neovascularization and PMN formation, promoting CRC metastasis (36).

CXCL12 is another key component of the PMN that is often upregulated before metastasis occurs. In noncancerous lung tissues of patients with CRC, CXCL12 expression is significantly higher compared to patients with benign lung disease. Immunohistochemistry analysis reveals that CXCR7 and CXCL12, both indicators of colon cancer lung metastasis, are more highly expressed in lung metastatic tissues than in primary tumor sites (101). Moreover, circLONP2 interacts with the DGCR8/Drosha complex in a DDX1-dependent manner, enhancing the maturation of primary microRNA-17. The resulting elevated levels of miR-17-5p can be encapsulated in exosomes and transferred to neighboring cells, thereby promoting migration and spread of CRC cells (102).

Hypoxic conditions in tumors also enhance exosome production in CRC. In a low-oxygen environment, tumor cells alter their metabolic pathways and migrate toward oxygen-rich areas, promoting metastasis (103). For instance, macrophages exposed to exosomes under hypoxic conditions undergo M2-type macrophage differentiation through activation of the AMPK/p38 signaling pathway, which in turn supports lung tumor proliferation and metastasis (104). Hypoxia also elevates miR-4299 levels in CRC cells and hypoxia-derived exosomes in a hypoxia-inducible factor 1 alpha (HIF1α) dependent manner, which directly targets ZBTB4, thereby promoting CRC proliferation and metastasis (103).

Thus, CRC cells utilize mechanisms such as hypoxia and cytokine production to increase exosome release and promote PMN formation, contributing to lung metastasis.

In summary, exosomes play a central role in the metastasis of breast, bone, and colorectal cancers to the lungs by influencing various signaling pathways and facilitating PMN formation. This underscores the importance of exosome-mediated PMNs in metastasis and highlights potential diagnostic and therapeutic targets for mitigating lung metastasis from primary tumors.

Exosomes promote TAMs polarization toward the M2 phenotype by activating the NLRP6/NF-κB signaling pathway (114, 115). Tumor cells also transfer TRIM59 via exosomes to macrophages, where it interacts with ABHD5 to activate the NLRP3 signaling pathway. This promotes the release of pro-inflammatory cytokines such as IL-18 and IL-1β, further contributing to an immunosuppressive environment (114, 115). Additionally, tumor cells undergoing enhanced glycolysis, which supports their proliferation, angiogenesis, and metastasis (116), release exosomes that boost glucose uptake through TLR2 and NF-κB pathways. These exosomes increase NOS2 levels, inhibiting mitochondrial oxidative phosphorylation and shifting energy production towards lactate. This metabolic change amplifies NF-κB activity, which upregulates PD-L1 expression and further drives macrophages towards the M2 phenotype, thereby enhancing lung cancer’s metastatic potential (90).

The STAT3 signaling pathway is another critical factor in macrophage polarization. Activation of this pathway leads to M2 macrophage differentiation. Exosomal MFG-E8, a significant component of small cellular debris exosomes, mitigates inflammatory responses by activating SOCS3; however, this also interferes with the regulation of STAT3 phosphorylation, facilitating M2 polarization (117). PYK2 in tumor exosomes can also trigger STAT3 activation in macrophages by activating the RhoA pathway in OS cells (97). Furthermore, exosomal miR-19b-3p from LUAD cells induces M2 macrophage polarization by targeting PTPRD proteins to activate STAT3 signaling, supporting lung metastasis of tumor cells (118).

Exosomes released under hypoxic conditions further promote M2 macrophage polarization. In NSCLC, circPLEKHM1 in exosomes, regulated by HIF1α, activates the OSMR/JAK/STAT3 signaling pathway, promoting M2 polarization in the TME by enhancing the interaction between PABPC1 and eIF4G (46). Additionally, the AMPK/p38 signaling pathway is upregulated under hypoxia, driving the differentiation of macrophages into the M2 phenotype (104).

Exosomes play a pivotal role in driving macrophage polarization toward the M2 phenotype by transmitting various bioactive molecules. For instance, lung cancer cell-derived exosomes can facilitate this transition by delivering lncRNAs such as PCAT6 and LINC00963, which promote M2 macrophage polarization (119, 120). Additionally, exosomal circ_0001715 internalizes and inhibits miR-205-5p, leading to the upregulation of TREM2, which further stimulates M2 macrophage polarization and creates a feedback loop that accelerates lung cancer growth, metastasis, and EMT (120–122). In contrast, lung cancer cells can also release miR-770-containing exosomes, which suppress M2 macrophage transformation by targeting the MAP3K1 gene, thereby reducing immunosuppression (121). Tumor-derived exosomes additionally enhance I-IFN secretion via the circPIK3R3/miR-872-3p/IRF7 axis, boosting the anti-tumor immune response of CD8+ T cells (123).

Beyond tumor-secreted exosomes, M2-type TAMs contribute to NSCLC metastasis by releasing exosomes carrying miRNAs such as miR-155 and miR-196a-5p, which promote metastasis by directly inhibiting the RASSF4 gene through interaction with its 3’UTR (124). Tumor cells induce M2 macrophage polarization via multiple pathways, including NF-κB, STAT3, and hypoxia-related mechanisms, thereby intensifying the malignant characteristics of the tumor. M2 macrophage polarization not only enhances metastasis of lung cancer to distant sites through exosomes but also facilitates metastasis from other regions, particularly bone cancer, to the lungs. This expands diagnostic options by broadening the range of potential indicators. Moreover, macrophages can interact with other immunosuppressive cells, tumor cells, and various components within the microenvironment, fostering an immunosuppressive setting during M2 polarization. Targeting this process through artificial interventions aimed at engaging macrophages and disrupting the immunosuppressive milieu during the M2 transition could significantly mitigate cancer metastasis.