| Abstract: | Increased intramedullary apoptotic death of hematopoietic cells is thought to contribute to the ineffective hematopoiesis in myelodysplastic syndromes (MDS). Furthermore, high amounts of tumor necrosis factor α (TNFα) have previously been correlated with apoptosis in MDS marrows. The present studies were undertaken to examine the status of two key downstream effectors of TNFα signaling, i.e. Caspase 1 and Caspase 3 enzymes, using a fluorometric assay in the bone marrow aspirate mononuclear cells (BMMNC) in relation to apoptotic DNA fragmentation detected by in situ end-labeling (ISEL) of DNA and with localization of TNFα in the corresponding biopsies from 14 MDS patients. Both Caspase 1 and 3 were detectable in freshly harvested BMMNC, albeit median Caspase 3 levels (47.5 units/mg protein) being almost 10 times higher than Caspase 1 (4.0 units/mg protein). Upon short-term culture for 4 h in a serum-supplemented medium in vitro a significant increase was seen in Caspase 3 activity (58.8±13.9 at 0 h vs. 177.8±55.2 units/mg protein at 4 h, n=14, P=0.017) and in percent cells labeled by ISEL (apoptotic index or AI%: 0.76%±0.25% vs. 3.99%±1.1%, n=14, P=0.004, respectively). Caspase 1 activity increased after 15 min in culture. Interestingly, TNFα levels measured by immunohistochemistry correlated with the net increase in Caspase 3 activity after 4 h (ρ=0.517, n=13, P=0.07) and the starting levels of Caspase 1 at 0 h correlated with the Caspase 3 levels attained at 4 h (ρ=0.593, n=13, P=0.033). Additionally when TNFα-positive bone marrows (8/14) were compared with the negative marrows (6/14) the Caspase 3 levels were significantly higher in the TNFα-positive marrows (189.6±66.2 vs. 25.0±14.6 units/mg protein, respectively, P=0.043). The increase in AI%, though not statistically significant, was also higher in the TNFα-positive marrows. Finally in HL60 cells the effects of different Caspase inhibitors and pentoxifylline (PTX) (interferes with lipid signaling of cytokines) on TNFα-induced apoptosis were evaluated. TNFα treatment significantly increased AI% (P<0.003) as compared to the untreated controls. A co-treatment with three Caspase inhibitors, zVAD.FMK (inhibitor of Caspases 1 and 3, 10 μM/l), Ac.YVAD.FMK (Caspase 1 inhibitor, 1 μM/l), Ac.DEVD.FMK (Caspase 3 inhibitor, 10 μM/l) as well as PTX (250 μM/l) significantly curtailed the AI% induced by TNFα The present studies thus identify the downstream effectors of TNFα-inducible apoptosis in MDS and so also the suppressors of TNFα apoptotic signaling. These results may have significant clinical implications in the therapy of MDS in the future. |
