Identification of the regulatory mechanism for conferring metastasis of CD26-expressing colorectal cancer stem cells

Abstract

Cancer stem cells are a subpopulation of cells needed for cancer initiation and progression. Previous works have revealed CD26-expressing colorectal cancer (CRC) stem cells are not only endowed with tumor-initiating properties, but also capable of conferring metastasis. However, whether the CD26 molecule plays role in metastasis and the underlying mechanism by which CD26 may mediate metastasis remain unclear. This study aims to reveal the biology and the molecular characteristics of the CD26-expressing CRC stem cells.

Here, by the gene manipulation experiment, we showed that CD26 molecule is a functional marker that confers metastasis as transient and stable knock-down of the CD26 molecule in the CRC stem cells resulted in reduced wound healing, migration and invasion abilities in vitro and the capability to generate metastatic liver nodules in vivo, respectively. With the use of genome-wide expression array and immuno-blotting analysis, Smad-dependent TGF-β signaling, orchestrated by the SMAD2, SMAD3 and SMAD4 molecules, was up-regulated and activated in the CD26 expressing colorectal CSCs. In addition, expressions of the SMAD2 and SMAD3 molecules were found to be positively correlated with the CD26 molecule in clinical samples by qPCR and immunohistochemistry studies. Furthermore, no metastasis through EMT could be achieved once the Smad-dependent TGF-β signaling was down-regulated in the CD26 expressing CRC stem cells, which suggested that Smad-dependent TGF-β signaling was necessary for CD26-expressing CRC stem cells to induce metastasis. Finally, our result showed that the Smad-dependent TGF-β signaling was regulated by the CD26 molecule possibly through the down-regulation of CAV1 protein.

To conclude, our findings have not only revealed the functional role of CD26 molecule, but have also unveiled a linkage between the CD26 molecule and Smad-dependent TGF-β signaling. Further study of this connection may introduce a novel mechanism, through which CRC metastasis can be induced by this functional CD26 marker of CRC stem cells.