Although CDH17 belongs to the cadherin superfamily accounting for the intercellular conjunction, unlike the classical cadherins, CDH17 is seemingly distinct that it can still retain its adhesive function without interacting with other cytoplasmic components.25
It has recently become more evident that such adhesion molecule plays an important role during the process of invasion or metastasis in colorectal, liver and gastric cancers,13,26-28
not only considered as a static intercellular glue. During the last couple of years, accumulating evidence suggested that high expression of CDH17 is associated with high metastatic potential, positive lymph node metastasis and short overall survival in gastric cancer patients.14,15
However, several researchers hold the opposite opinion that CDH17 predicts a better prognosis in GC with N0 status.18
In the context of controversial role of CDH17 in LNM of GC, we explored the relationship between CDH17 and LNMM to figure out whether it participated in the initiating process of LNM. As a result, we found that CDH17 was the independent risk factor of LNMM, predicting a poor prognosis for GC.19
Accordingly, our present study demonstrates the knockdown of CDH17 in a poorly differentiated GC cell line (MKN-45) by using RNAi, rendered the cancer cells less proliferative and invasive both in vitro and in vivo, which implies that CDH17 could be an attractive therapeutic target for GC.
Up to now, the Met and Wnt signaling pathways have been reported to interact with CDH17 in hepatic cell carcinoma.29,30
Despite that, the mechanism of CDH17 activation in GC has not been well elucidated yet.31,32
NFκB signaling pathway, as an eminent key factor regulating inflammation-induced carcinogenesis and lymphangiogenesis, is consecutively activated in GC.20,22,33
The primary NFκB family consists of RelA (p65), NF-κB1 (p50), RelB, c-Rel and NFκB2 (p52), among which the p50/p65 heterodimers are the main complexes regulating the transcription of responsive genes.34
Comparison with adjacent normal epithelial cells, increased nuclear translocation of p65 in GC is correlated with the depth of invasion, tumor size and metastases.35
Especially in recent study, Flister et al. suggests that the induction of p50/p65 enhances the responsiveness of preexisting lymphatic endothelium to VEGF-C and VEGF-D, ultimately resulting in robust lymphangiogenesis.23
Taken together, we presumed the implicit relationship between CDH17 and the NFκB signaling pathway.
Based on the alterations of proteins in vitro and in vivo, we inference that the NFκB signaling pathway might be the key mediator in the process of CDH17 affecting gastric cancer, on the other hand, knocking down CDH17 expression by miRNA is responsible for the retention of NFκB and a concomitant reduction in downstream proteins (e.g., VEGF-C), as presented in . It is of note that VEGF-C has been identified as major regulator of the development of lymphatic vessels (lymphangiogenesis),36
expressing positively in LNM of human cancers including thyroid, prostate, gastric, colorectal and lung cancers.37
Although there has been considerable effort to decipher the specific activation mechanisms and contribution of NFκB to oncogenesis and metastasis, the crosstalk between NFκB and other important oncogenic signaling pathways pathways including Ras, p53, Notch,38
and LTβR signalings,40
remains poorly defined. Additionally, there is also considerable conservation of signaling intermediates upstream of the IKK complex, such as the receptor-interacting proteins (RIPs) and TNFR-associated factors (TRAFs) which are critical to IKK activation,38
hence it is still difficult to assert that CDH17 regulates NFκB directly.
Figure 5. Schematic exhibition of the proposed signaling pathway modulated by CDH17 in gastric cancer cells. Coupling CDH17 activates the IKK complex, which subsequently phosphorylates the IκB-α. Then the phosphorylated IκB-α (more ...)
The dorsal subcutaneous xenografted tumor model was used to evaluate the effect of CDH17 on proliferative potency of gastric cancer. On the other hand, the claw pad lymph node metastasis model was selected specifically to probe the association between CDH17 and LNM, since we identified that CDH17 was the independent risk factor of LNMM previously.19
That is why we employed two different animal models to investigate the impact of CDH17 on tumorigenesis and metastasis respectively. Although orthotopic model is established to simulate metastasis more distinctively, the heterotopic subcutaneous lymphatic metastasis model including implantation in back, claw pad, tail, penis or femoribus internus, remains less invasive and more convenient to be monitored. Among them, subcutaneous implantation in claw pad is a mouse model with moderate frequency of lymph node metastasis, of which the tumor cells are supposed to shift from the lymphoreticular tissue in claw to the popliteal LN, inguinal LN, axillary LN and lung tissue.41,42
With abundant lymphatic capillary networks and explicit drainage, the claw pad lymphatic metastasis model is superior to the other models in construction and inspection. From our analysis, node-positive mice in control groups accounted for 33%. By contrast, there was no metastasis in miR-CDH17 group. Owing to the relatively small sample capacity, statistical difference was not found between the control groups and the knockdown group (p = 0.276). Nevertheless, the trend that antagonizing CDH17 inhibits LNM could still be observed, which shed light on the further investigations.