Previous studies identified that exosomes could promote the proliferation and migration of gastric cancer cells via the phosphatidylinositol-3 kinase/Akt, nuclear factor-κB and Hedgehog signaling pathways (11–13). Through this exosomal regulation, gastric cancer cells may become more prone to invade the serosa and exfoliate into the cavity with increased proliferative and migratory abilities. Epithelial cells express high levels of epithelial (E)-cadherin, while mesenchymal cells express neural (N)-cadherin, fibronectin and vimentin (14). Dysfunction of E-cadherin has been implicated in gastric cancer progression and may predominantly contribute to invasion of the gastric wall and migration of cancer cells into the free abdominal space (3). Exosomes may induce epithelial-to-mesenchymal transition (EMT), triggering the loss of cell-cell adhesion to facilitate tumor cell invasion and remodeling of the extracellular matrix. In particular, exosomes derived from gastric cancer have been reported to promote EMT by mediating the transfer of long noncoding RNA ZFAS1 (15). Furthermore, matrix metalloproteinases (MMPs), as the effectors of EMT, have also been identified in exosomes (14). MMP-7 expression has been significantly associated with serosal infiltration and TNM stage in gastric cancer (16). Additionally, Tanaka et al (17) identified that serosal peritoneal mesothelial cells covering the stomach surface could infiltrate the gastric wall to create a novel niche via exosomes containing Wnt family member 3A, as well as guide gastric cancer invasion to the stomach subserosa and facilitate peritoneal metastasis.

Cell-cell interaction is important for cancer cell survival in arduous environments. Protocadherin β-9 may promote the formation of spheroid colonies of gastric cancer cells, and thus aid the survival of cancer cells in the ascites environment and prevent anoikis and apoptosis induced by chemotherapeutic agents (18). Additionally, intraperitoneal mesenchymal cells may support long-distance migration of disseminated tumor cells by maintaining direct contract, as observed with MKN45 gastric cancer cells (19).

Exosomes exhibit another mechanism of promoting tumor survival through hypoxia inducible factor-1α (HIF-1α), and hypoxia can in turn promote exosome release to increase tumor aggressiveness (20). HIF-1α itself participates in exosome-mediated pre-metastatic effects in recipient cells. For instance, incubation of naïve breast cancer cells with exosomes shed by hypoxic breast cancer cells promoted focal adhesion, invasion and metastasis (21). HIF-1α promotes EMT through upregulation of EMT-associated transcription factors, and exosome-mediated delivery of HIF-1α has been reported to induce changes in the expression of E- and N-cadherin associated with EMT in nasopharyngeal carcinoma (22). Notably, HIF-1α expression has been positively correlated with gastric cancer peritoneal dissemination, poor overall and disease-free survival rates, and may be key in the adaptation of cancer cells to the hypoxic microenvironment in the abdominal cavity (23). Gastric cancer stem cells enter and are maintained in the milky spots via the expression and function of HIF-1α (23). HIF-1α may increase cancer cell adhesion via integrin β1 (ITGβ1). Conversely, dextran sulfate may inhibit peritoneal dissemination of gastric cancer cells through a reduction in HIF-1α and ITGβ1 expression (24). At present, it remains to be elucidated whether HIF-1α is associated with exosomes in peritoneal dissemination.

The peritoneum is composed of a single layer of flat mesothelial cells and a thin sub-mesothelial connective tissue (25). The mesothelial cells serve as the first barrier against tumor adhesion and invasion (26). If integrity of the barrier is damaged, tumor cells can invade more efficiently into the sub-mesothelial layer (26).

Arita et al (27) cultured mesothelial cells (MeT-5A) and the gastric cancer cell line MKN45 with exosomes derived from MKN45 or MKN74 cells. They observed that TEX were internalized in both mesothelial and gastric cancer cells, which elevated cancer cell migratory ability; furthermore, the expression of the adhesion molecules fibronectin 1 and laminin γ1 was upregulated in the Met-5A cells. It was concluded that exosomes may assist gastric cancer cells in adhering to the peritoneum (27).

Cluster of differentiation (CD)44 is the receptor of hyaluronan, expressed on both mesothelial cells and cancer cells. As an adhesive molecule, it is important for cancer cell attachment to the peritoneum (28). The expression of CD44 was identified to be significantly increased in individuals whose gastric lesions progressed along the gastric precancerous cascade infected with Helicobacter pylori (29). Nakashio et al (30) observed that CD44H (standard isoform) and integrins mediate the initial attachment of gastric cancer cells to mesothelial cells, and that increased expression of CD44H promotes early development of peritoneal dissemination. The expression level of CD44 mRNA has also been correlated with peritoneal dissemination (31). CD44 expression is upregulated under the postoperative inflammatory response, which involves elevated levels of reactive oxygen species, transforming growth factor-β1 (TGF-β1), interleukin-1β and tumor necrosis factor-α, leading to enhancement of cancer cell adhesion (32). Recently, Nakamura et al (33) co-cultured human peritoneal mesothelial cells (HPMCs) with exosomes derived from ovarian cancer cells and observed that HPMCs, which internalized TEX, exhibited higher level of CD44 expression and underwent mesothelial-to-mesenchymal transition (MMT). Additionally, CD44 overexpression in HPMCs promoted cancer invasion by inducing the HPMCs to secrete MMP9, in order to cross the mesothelial barrier. By contrast, by knockdown of CD44 expression, TEX exerted fewer effects on HPMCs (33). Similarly, isolated exosomes from human gastric cancer cells have demonstrated expression of CD44H and CD44 variant exon 6 isoforms, which may serve a role in the attachment of TEX to cancer cells (34).

Pathological fluid accumulation, or malignant ascites, in the abdominal cavity, is a result of an imbalance between increased capillary fluid filtration and decreased fluid clearance due to lymphatic obstruction (35). Wei et al (36) investigated the effects of malignant ascite-derived exosomes on the HPMC line HMrSV5. They observed that those exosomes could increase the proliferation and migration of the HPMCs, and induce carcinoma-associated fibroblast transition with overexpression of fibroblast activation protein, α-smooth muscle actin and fibronectin. TGF-β1 was also identified to be overexpressed in malignant ascites-derived exosomes, which was associated with peritoneal fibrosis. Furthermore, malignant ascite-derived exosomes could regulate xenograft tumor growth in vivo (36). Lv et al (37) confirmed that the expression of TGF-β1 was markedly increased during the interaction between gastric cancer cells and peritoneal mesothelial cells, and that inhibition of TGF-β1 by P17 peptides could attenuate peritoneal fibrosis and prevent peritoneal dissemination.

Deng et al (38) pretreated nude mice with MGC803 cell-derived exosomes and injected MGC803 cells into the abdominal cavity, and observed that the total weight of the tumor nodules was significantly increased compared with in PBS-treated mice in vivo. Mesothelial cells (HMrSV5) were also cultured with exosomes, which revealed that exosomes induced injury of peritoneal mesothelial cells by promoting apoptosis and MMT, via upregulation of the extracellular signal-regulated kinase pathway (38).

Collectively these previous studies indicate that exosomes may not only facilitate adhesion between gastric cancer cells and mesothelial cells, but also diminish the peritoneal barrier by inducing peritoneal fibrosis, apoptosis and MMT.

Metastatic cancer cells require high levels of nutrition and oxygen to be supplied through angiogenesis. TEX have been confirmed to be crucial in the angiogenesis of cancers (39–41). TEX induce angiogenesis via transportation of paracrine signaling factors and pro-angiogenic molecules including epidermal growth factor receptor (EGFR) mRNA (42). Zhang et al (43) recently verified that gastric cancer-derived exosomes served an important role in liver metastasis; EGFR levels were enriched in exosomes isolated from the serum of gastric cancer patients but not in healthy subject serum exosomes, and the number of serum exosomes increased with the staging of gastric cancer. In a mouse experiment, it was identified that exosomes of the SGC7901 gastric carcinoma cell line could transport EGFR into the liver and ultimately to the membrane of stromal liver cells. As a consequence, SGC-exosome-derived EGFR activated liver hepatocyte growth factor (HGF) by suppressing expression of the microRNA (miRNA/miR) miR-26a/b, establishing the liver as an ideal site for tumor cell metastasis (43). The EGFR positivity rate of metastatic tumors in patients with peritoneal metastasis is reportedly significantly higher (70.1%) than that of metastatic tumors in patients with liver metastasis (37.5%) (44), which indicates that EGFR expression is associated with peritoneal metastasis. Additionally, a study revealed that angiogenesis was significantly increased when GC1415 cells were co-cultured with TEX for 24 h, as demonstrated by measurement of hemoglobin content (34). However, the phenomenon of angiogenesis induced by TEX was not well explained and requires further investigation.