BCL6 overexpression prevents increase in reactive oxygen species and inhibits apoptosis induced by chemotherapeutic reagents in B-cell lymphoma cells

Abstract

Chromosomal translocations and somatic mutations occurring in the 5 0 noncoding region of the BCL6 gene, encoding a transcriptional repressor, are most frequent genetic abnormalities associated with non-Hodgkin B-cell lymphoma and result in deregulated expression of BCL6. However, the significance of deregulated expression of BCL6 in lymphomagenesis and its effect on clinical outcomes of lymphoma patients have remained elusive. In the present study, we established Daudi and Raji B-cell lymphoma cell lines that overexpress BCL6 or its mutant, BCL6-Ala333/343, in which serine residues required for degradation through the proteasome pathway in B-cell receptor-stimulated cells are mutated. BCL6 overexpression did not have any significant effect on cell proliferation, but significantly inhibited apoptosis caused by etoposide, which induced a proteasome-dependent degradation of BCL6. BCL6-Ala333/343 was not degraded after etoposide treatment and strongly inhibited apoptosis. In these lymphoma cell lines, etoposide increased the generation of reactive oxygen species (ROS) and reduced mitochondria membrane potential, both of which were inhibited by the antioxidant N-acetyl-l-cysteine (NAC). NAC also inhibited apoptosis. Furthermore, BCL6 overexpression was found to inhibit the increase in ROS levels and apoptosis in response to etoposide and other chemotherapeutic reagents. These results raise the possibility that deregulated expression of BCL6 may endow lymphoma cells with resistance to chemotherapeutic reagents, most likely by enhancing the antioxidant defense systems.

Reactive oxygen species and aldehyde dehydrogenase activity in Hodgkin lymphoma cells

“…30 To examine whether this is the case in HL, Bcl-6 expression levels were examined in ROS-high and -low L1236 and L428 cells. No significant difference in Bcl-6 expression was detected between these two populations, indicating that Bcl-6 was not involved in the regulation of ROS production, at least in L1236 and L428 cells (Figure 4e).…”

Rottlerin synergistically enhances imatinib-induced apoptosis of BCR/ABL-expressing cells through its mitochondrial uncoupling effect independent of protein kinase C-δ

“…Apoptosis and Dc m were analysed by flow cytometry as described previously (Kurosu et al, 2003). In brief, cells were stained with Annexin V-FITC and propidium iodide (PI) using the TACS Annexin V Kit (Trevigen, Gaithersburg, MD, USA) and analysed using a Becton Dickinson FACSscan flow cytometer (Mountain View, CA, USA) for apoptosis.…”

“…Analyses of ROS and MMP Intracellular ROS levels were measured fluorometrically using a fluorescent probe, HE, which is oxidized to the fluorescent intercalator ethidium by cellular oxidants, particularly superoxide radicals (Rothe and Valet, 1990), as described previously (Kurosu et al, 2003). In brief, 1 Â 10 6 cells were incubated with 2 mM HE for 30 min at 371C, resuspended in phosphatebuffered saline (PBS), and analysed by flow cytometry.…”

ERK and p38 upregulation versus Bcl-6 downregulation in rat kidney epithelial cells exposed to prolonged hypoxia

“…60 An increase in its expression can inhibit the apoptosis of lymphoma cells treated with chemotherapeutic reagents, most likely through enhancement of the antioxidant defense systems. 55 It is also indicated that inhibition of Bcl-6 function by a dominantnegative form of Bcl-6 can inhibit the cell cycle progression and induce apoptosis in Raji cells. 61 Previous studies showed that the activation of peroxisome proliferator-activated receptor (PPAR)-d increased both total and free Bcl-6 levels and inhibited the activation of p38 and ERK1/2, 62 and the activated form of p38 was detected in spermatocytes of adult Bcl-6-deficient mice.…”