The ubiquitin-proteasome pathway plays a major role in regulating proteins involved in cell cycle regulation and apoptosis. Three different types of enzymatic activity have been reported for proteasomes: chymotrypsin-like (Ch-L), trypsin-like (Tr-L), and caspase-like (Cas-L) (postglutamyl peptide hydrolytic-like). Various proteasome inhibitors affect each of the three activities differently and at different concentrations. For example, NPI-0052 inhibits Ch-L and Tr-L activities at lower concentrations than does bortezomib, while bortezomib inhibits Cas-L at lower concentrations than does NPI-0052. These enzymatic activities are usually measured in normal or tumor cells to monitor therapy with proteasome inhibitors. We developed fluorogenic kinetic assays using peptide-AMC (7-amino 4-methylcoumoran) substrates to measure Ch-L, Tr-L, and Cas-L activities in cell-free plasma. We evaluated plasma activities in 57 patients with acute lymphoblastic leukemia (ALL) and assessed their correlations with clinical behavior. Significantly (P < 0.001) higher Ch-L, Tr-L, and Cas-L activities were detected in patients with ALL (medians: 1.40, 2.06, and 2.04 pmol AMC/sec/mL, respectively) than in healthy volunteers (n = 42) (medians: 0.80, 0.74, and 0.81 pmol AMC/sec/mL, respectively). While there was no significant difference between Ch-L and Cas-L activities in healthy controls, there was a significant difference between the 2 activities in patients with ALL. Cas-L, Tr-L, and Ch-L all correlated positively with lactic dehydrogenase (P <0.01). However, ALL patients with Cas-L activity ≥2.7 pmol AMC/sec/mL had significantly longer survival (P = 0.01) than did patients with lower activity. The increased cell-free circulating proteasome activities most likely reflect the leukemic cells and may represent a marker not only for the disease, but also for monitoring therapy. These data also suggest that patients with ALL may benefit differentially from proteasome inhibitors depending on the specific therapeutic properties of the inhibitor.
Disclosure: No relevant conflicts of interest to declare.