Differences in Proteasome Activity in Plasma of Patients with Hematologic Malignancy.

The proteasome-dependent proteolytic pathway plays a crucial role in pathogenesis of hematologic malignancy, and as such it is a novel target for therapeutic drugs. High proteasome levels measured by conventional technique ELISA have been demonstrated in human malignant cells and in the plasma of patients with some hemopoietic malignancies and solid tumors. The objective of this study was to evaluate the activity of the 20S proteasome in the plasma from patients with hematologic malignancy.

The study included normal subjects (n=15) and untreated patients with acute lymphoblastic leukemia (ALL, n=13), acute myeloblastic leukemia (AML, n=21), multiple myeloma (MM, n= 26) and chronic lymphocytic leukemia (CLL, n=30). The 20S proteasome activity in plasma (50μg of protein) was assayed spectrofluorimetrically using synthetic peptide substrate, Suc-Leu-Leu-Val-Tyr-AMC (100μmol/L) in the presence of 0.03% SDS that selectively activates the chymotrypsin-like activity of 20S proteasome. Enzyme units (U) were expressed as pmol AMC/min.

The average activity of 20S proteasome in plasma from normal subjects was found to be 4.42 ± 0.56U/mg. The highest activity of the 20S proteasome was found in plasma from patients with ALL with a median of 55,5 U/mg (range 10.5–225.6 U/mg, p<0.01). High proteasome activity was also observed in plasma from patients with AML with a median of 28.6 U/mg (13.0–64 U/mg; p<0.01). In patients with MM, the average plasma proteasome activity was 11.1 ± 3.1 U/mg (range 8–18.8 U/mg; p<0.01). In contrast, in patient with CLL plasma proteasome activity was similar to that in normal subjects (4.2± 1.1; p=0.05). There was a significant positive correlation with plasma lactic dehydrogenase activity (LDH) (a global marker of cell lysis), but not with white blood cell (WBC) and blast cell counts in all the examined groups of patients.

The obtained results indicate the presence of high level of 20S proteasome in the plasma of patients with ALL, AML and MM, but not with CLL, which has potential chymotrypsin-like activity.