A novel vascular pattern promotes metastasis of hepatocellular carcinoma in an epithelial-mesenchymal transition-independent manner

Early metastasis is responsible for frequent relapse and high mortality of hepatocellular carcinoma (HCC), but its underlying mechanisms remain unclear. Epithelial-mesenchymal transition (EMT) has been considered a key event in metastasis. Based on histological examination of serial HCC sections and three-dimensional reconstruction, we found a novel and prevalent vascular pattern, vessels that encapsulated tumor clusters (VETC) and formed cobweb-like networks. The presence of VETC (VETC+) predicted higher metastasis and recurrence rates of HCC. Using clinical samples and mouse xenograft models, we further showed that VETC was composed of functional vessels with blood perfusion and induced by tumor cells at the early stage of HCC. Subsequent investigations revealed that HCC cell-derived angiopoietin-2 was a prerequisite for VETC formation and that the VETC pattern was a critical factor promoting HCC metastasis as knockdown of angiopoietin-2 abolished this vascular pattern and consequently attenuated in vivo tumor metastasis. Interestingly, abrogation of EMT by knockdown of Snail or Slug significantly diminished in vivo metastasis of VETC- xenografts but did not affect that of VETC+ ones, although silencing of Snail or Slug substantially reduced the in vitro migration of both VETC+ and VETC- HCC cells. In contrast to human VETC- cases, EMT signatures were rarely observed in VETC+ cases with metastatic potential. Further analysis revealed that VETC provided an efficient metastasis mode by facilitating the release of whole tumor clusters into the bloodstream. Conclusion: Our findings identify a novel metastasis mechanism that relies on vascular pattern but is independent of EMT, which may provide new targets for antimetastasis therapy and offer a basis for selecting patients who may benefit from certain molecularly targeted drugs. (Hepatology 2015;62:452-465