In vitro activity: Celastrol at 5 μM inhibits the chymotrypsin-like, PGPH-like, and trypsin-like activities of the purified 20S proteasome by 80%, 5%, and<1%, respectively, whereas at 10 μM, it inhibits these three proteasomal activities by ~90%, 15%, and <1%, respectively. Celastrol significantly inhibits the proteasomal chymotrypsin activity in PC-3 cells in a concentration-dependent manner. Celastrol at 2.5 μM to 5 μM induces caspase-3 activity by 4.7-fold to 5.5-fold in PC-3 cells. Celastrol (5 μM) treated cells, the levels of the proteasome target proteins, IκB-α and Bax, are increased after 1 hour and further increased to its peak for 4 hours to 12 hours. Celastrol (2.5 μM) treatment induces proteasome inhibition by 40%, as shown by the decreased levels of chymotrypsin-like activity and increased accumulation of ubiquitinated proteins in LNCaP cells. Celastrol (2.5 μM) induces apoptosis in the Celastrol-treated LNCaP cells, as shown by increased levels of caspase-3 activity (up to 3.5-fold), PARP cleavage, and apoptotic morphology. Celastrol (300 nM) is found to suppress LPS-induced production of TNF-alpha and IL-1beta by human monocytes and macrophages. Celastrol (100 nM) also decreases LPS-induced expression of class II MHC molecules by microglia. Celastrol strongly inhibits LPS and IFN-y-induced NO production with IC50 of 200 nM in macrophage lineage cells. Celastrol strongly inhibits TNF-α and IFN-γ-induced NO production with IC50 of 200 nM in endothelial cells.

Kinase Assay: A purified rabbit 20S proteasome (0.1 μg) is incubated with 40 μM of various fluorogenic peptide substrates in 100 μL assay buffer (20 mM Tris-HCl (pH 7.5)), in the presence of Celastrol at different concentrations or in the solvent DMSO for 2 hours at 37 ℃, followed by measurement of inhibition of each proteasomal activity.

Cell Assay: The anti-proliferative effect of celastrol on various human tumor cell lines is determined by the MTT uptake method. Briefly, 5×103 cells are incubated with Celastrol in triplicate in a 96-well plate at 37 ℃. MTT solution is then added to each well. After a 2 hours incubation at 37 ℃, extraction buffer (20% SDS, 50% dimethylformamide) is added, cells are incubated overnight at 37 ℃, and the optical density is then measured at 570 nm using a Tecan plate reader.

Cancer Res. 2006 May 1;66(9):4758-65; Prog Neuropsychopharmacol Biol Psychiatry. 2001 Oct;25(7):1341-57.