Reduced Proteasomal Activity in Marrow Cells From Myelodysplastic Syndrome (MDS) Patients.

The proteasome is a multicatalytic complex that is responsible for the degradation of many intracellular proteins in the cytoplasm and nucleus of all eukaryotic cells. The chymotryptic-like (ChT-L) activity is one of the three proteasomal activities and is rate-limiting in the degradation of intracellular proteins. Such proteins include natural unfolded proteins, oxidized, mutated or damaged proteins and many short–lived proteins such as cell cycle proteins. The correlation between MDS and the presence of increased oxidative stress as well as oxidized proteins within the cell has already been reported. Insufficient proteasomal activity causes the accumulation of oxidatively modified proteins something that may lead to the misregulation of cellular homeostasis. In addition, its role to the pathogenesis of many diseases has already been documented and many studies over the past years strongly indicate the implication of proteasome in many hematological malignancies. Patients and methods: In this study we examined the proteasomal activity of mononuclear cells and immunomagnetically isolated CD34⁺ marrow cells from 24 marrow and 9 peripheral blood samples from MDS patients (10 RA, 2 RARS, 5 RCMD, 10 RAEB I, 2 RAEB II, 4 CMML) compared to the cells from 6 marrow samples of age matched healthy individuals, 8 mobilized peripheral blood samples (mPB) and 14 blood samples from healthy donors. For each sample, cells were lysed with Dithiothreitol (DTT) followed by incubation with the fluorescent conjugated peptide Suc-LLVY-AMC. The fluorescence intensity was directly dependent to ChT-L activity within the cells. The proteasome inhibitor MG-132 was used as negative control. We also examined the amount of the β1 subunit of the proteasome in marrow mononuclear cells from MDS patients, by flow cytometry.

Proteasome activity was significantly reduced in MDS marrow mononuclear cells compared with the cells from healthy individuals. The reduction was 55% and 75% compared with mononuclear cells from marrow samples of healthy donors and mPB samples, respectively. The activity was similarly reduced in both CD34⁺ and CD34^- marrow cells from MDS patients compared to the cells from healthy individuals (70% and 60% reduction respectively). Further analysis showed high variation of the amount of the β1 proteasomal subunit in MDS patients contrary to healthy individuals. Additionally, cells from the mPB samples possessed higher levels of proteasomal activity and β1 subunit compared to the cells from healthy donors. Proteasome activity of blood mononuclear cells from MDS patients was similar to the cells from the blood samples of healthy individuals although the variance was high in both study groups.

Both mononuclear and progenitor CD34⁺ marrow cells from MDS patients have reduced proteasomal activity compared to the same cell populations from healthy individuals. However, no difference in activity was detected between MDS and healthy individuals blood mononuclear cells. The quantitative assessment of the proteasome shows high variation within the MDS group and further investigation is required.

No relevant conflicts of interest to declare.