APOE and glioblastoma: For example, increased arginase-1 (an enzyme that converts arginine to ornithine and urea) in M2 macrophage-derived exosomes is critical for glioblastoma cell proliferation (21), and exosomal CD11b/CD18 and apolipoprotein E (ApoE) separately activate the MMP-9 and PI3K/AKT pathways in recipient tumor cells to promote their migration (20, 22).