CD44 and cancer: The findings of several studies showed that activation of β2-AR promotes EMT and endows GC cells with stem cell-like properties through hypoxia inducible factor-1α (HIF-1α)-Snail, ERK, matrix metalloproteinases (MMPs) and STAT3-CD44 pathways, all of which are closely related to the invasion, migration, and dissemination of cancer [59,60,61,62].