(Cancer Sci 2010; 101: 673–678)

Similar to normal tissue stem cells, cancer stem cells (CSCs) are thought to be quiescent or slow‐cycling and, thereby, insensitive to chemo‐ and radiotherapies. CD44, a cell surface component that interacts with the extracellular matrix, has been found to be highly expressed in CSCs of several solid tumors. However, the relevancy between CD44⁺ cells and slow‐cycling cells and the underlying mechanisms for the emergence of CD44⁺ CSCs during tumorigenesis have not been elucidated. Here we show that a gastric gland residing at the squamo‐columnar junction (SCJ) in normal mouse stomach contains CD44⁺ stem cell‐like slow‐cycling cells and that this characteristic CD44⁺ gland was expanded by prostaglandin E2 (PGE₂) and Wnt signaling in K19‐Wnt1/C2mE mouse, a genetic mouse model for gastric tumorigenesis. The analysis of three transgenic mouse lines, K19‐Wnt1, K19‐C2mE and K19‐Wnt1/C2mE, revealed that the expansion of CD44⁺ SCJ cells is triggered by PGE₂‐mediated signaling and is prominently enhanced by the addition of Wnt activation. Furthermore, each expanded CD44⁺ gland in gastric tumor of K19‐Wnt1/C2mE mouse contains a few BrdU label‐retaining quiescent or slow‐cycling cells, suggesting that the CD44⁺ SCJ cells in normal mouse are candidates for the cell‐of‐origin of gastric CSCs. These observations suggest that PGE₂‐mediated inflammatory signaling and Wnt signaling cooperatively trigger the expansion of CD44⁺ slow‐cycling stem‐like cells in SCJ, leading to development of lethal gastric tumors in mice.