AMIGO2 accelerates tumor progression by inducing a cancer stem cell-like phenotype.

Amphoterin-induced gene and open reading frame 2 (AMIGO2) was previously identified as a driver gene for liver metastasis. AMIGO2 has also been found to be a prognostic factor for cancer patients with a low frequency of spontaneous liver metastasis. Here, we investigated whether AMIGO2 expression affects the acquisition of cancer stem cell-like properties in cancer cells by using human gastric (MKN45) and colon (DLD-1) cancer cell lines. Knockdown of AMIGO2 expression in MKN45 and DLD-1 cells via transfection with short hairpin RNA-AMIGO2 suppressed cell proliferation under several starvation conditions in two-dimensional culture. AMIGO2 knockdown cells showed reduced ability to form multicellular spheres in a three-dimensional culture. Half-maximal inhibitory concentration values of anticancer drugs in sphere-forming cells were decreased by AMIGO2 knockdown. Silencing of AMIGO2 suppressed tumor formation and growth of subcutaneously injected tumors in NOD/SCID mice. Expression levels of AMIGO2 and cancer stem cell markers, such as CD44, CD133, and EpCAM, were almost identical in spherically grown cancer cells. AMIGO2 expression was positively correlated with expression of existing cancer stem cell markers in multiple organ cancer cell lines. These results demonstrate that AMIGO2 accelerates the malignant progression of human cancer cells by inducing a cancer stem cell-like phenotype.