Epithelial-to-endothelial transition and cancer stem cells: two cornerstones of vasculogenic mimicry in malignant tumors

Abstract

Vasculogenic mimicry (VM) is a functional microcirculation pattern in malignant tumors accompanied by endothelium-dependent vessels and mosaic vessels. VM has been identified in more than 15 solid tumor types and is associated with poor differentiation, late clinical stage and poor prognosis. Classic anti-angiogenic agents do not target endothelium-dependent vessels and are not efficacious against tumors exhibiting VM. Further insight into the molecular signaling that triggers and promotes VM formation could improve anti-angiogenic therapeutics. Recent studies have shown that cancer stem cells (CSCs) and epithelium-to-endothelium transition (EET), a subtype of epithelial-to-mesenchymal transition (EMT), accelerate VM formation by stimulating tumor cell plasticity, remodeling the extracellular matrix (ECM) and connecting VM channels with host blood vessels. VM channel-lining cells originate from CSCs due to expression of EMT inducers such as Twist1, which promote EET and ECM remodeling. Hypoxia and high interstitial fluid pressure in the tumor microenvironment induce a specific type of cell death, linearly patterned programmed cell necrosis (LPPCN), which spatially guides VM and endothelium-dependent vessel networks. This review focuses on the roles of CSCs and EET in VM, and on possible novel anti-angiogenic strategies against alternative tumor vascularization.

Keywords: LPPCN; cancer stem cells; epithelial-to-endothelial transition; hypoxia; vasculogenic mimicry.

Figure 1. Hypoxia and IFP induce VM formation

When endothelium-dependent vessels do not grow into the tumor, the resulting hypoxia promotes VM formation to provide selective perfusion, leading to tumor aggression and metastasis. CSCs and EET accelerateVM formation by inducing tumor cell plasticity, remodeling ECM, and connecting VM channels and host blood vessels. Hypoxia and high IFP in the tumor microenvironment induce LPPCN in some tumor cells, to provide a spatial guide for VM and endothelium-dependent networks. EMT-inducing transcription factors may induce CSCs to differentiate into endothelium-like VE-cadherin+ cells in order to form VM channels with ECM remodeling. These channels then connect to endothelium-dependent vessels and areperfused withblood.