PSMB2 and central nervous system cancer: Shikonin treatment significantly increased the protein levels of constitutive subunits (PSMB1, PSMB2, and PSMB5) but reduced those of the immunosubunits subunit (i.e., PSMB8, PSMB9, PSMB10, PSME1, PSME2, and PSME3; Figures 9 and 10), indicating that the Shikonin-induced decrease in proteasome activity in glioma is mainly mediated by decreased expression of the immunoproteasome.