SPP1 and glioblastoma: CD44/OPN have a perivascular expression pattern and their interaction induces the activation of the γ-secretase-regulated intracellular domain of CD44, which promotes migration, invasion, and stem cell-like phenotypes of GBM cells via CBP (CREB binding protein)/p300-dependent enhancement of HIF (hypoxia-inducible factor)-2α activity [82].