Variations in Proteasome Enzymatic Activities in Plasma of Patients with Acute Myeloid Leukemia and Myelodysplastic Syndrome and Their Value in Predicting Clinical Behavior.

The ubiquitin-proteasome pathway is responsible for multiple pathways in cancer cells; proteasome inhibition causes rapid apoptosis of tumor cells. Three different types of peptidase activities 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 3 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. Because rapidly proliferating leukemic cells pour their proteins, DNA, and RNA into the circulation, we developed fluorogenic kinetic assays using peripheral blood plasma. The assays used peptide-AMC (7-amino 4-methylcoumoran) substrates to measure Ch-L, Tr-L, and Cas-L activities. We measured proteasome activities in plasma from 188 patients with acute myeloid leukemia (AML) and 58 patients with myelodysplastic syndrome (MDS) and assessed their correlations with clinical behavior. Significantly (P < 0.001) higher Ch-L, Tr-L, and Cas-L activities were seen in AML patients (medians: 1.39, 1.51, and 2.40 pmol AMC/sec/mL, respectively) and MDS patients (medians: 1.16, 1.40, and 1.67 pmol AMC/sec/mL, respectively) than in healthy volunteers (n=42) (medians: 0.80, 0.74, and 0.81 pmol AMC/sec/mL, respectively). The difference in Cas-L activity between AML and MDS was significant (P <0.001). 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 both AML and MDS. Cas-L and Ch-L, but not Tr-L, correlated with WBC count and lactic dehydrogenase in AML and MDS patients. In AML patients, higher levels of Ch-L and Cas-L were associated with poor response to a variety of therapies (P = 0.004 and P = 0.001, respectively). Cas-L correlated strongly with survival in AML patients when used as an activity-dependent variable (P <0.001) or when the median was used as a cut-off (P = 0.004). This was independent of cytogenetic abnormalities, age, and performance status. Patients with intermediate-risk cytogenetic abnormalities and Cas-L activity >3 pmol AMC/sec/mL had significantly shorter survival (P = 0.04). Ch-L activity was also predictive of survival in AML independent of age and cytogenetic and performance status, but not independent of Cas-L. In MDS, higher levels of Cas-L, but not Ch-L, correlated with shorter survival and this was independent of cytogenetic abnormalities. The increased cell-free circulating proteasome activities most likely reflect the leukemic cells and may be a marker not only for disease, but also potentially for monitoring therapy. These data also suggest that patients with AML may benefit differentially from proteasome inhibitors depending on the specific therapeutic effect of the inhibitor.