Free radicals act as effectors in the growth inhibition and apoptosis of iron-treated Burkitt's lymphoma cells.

The addition of ferric citrate to Burkitt's lymphoma (BL) cell lines inhibits growth, leads to the accumulation of cells in the phase G(2)/M of the cell cycle and to the modulation of translocated c-myc expression. The increase in the labile iron pool (LIP) of iron-treated BL cells leads to cytotoxicity. Indeed, intracellular free iron catalyzes the formation of highly reactive compounds such as hydroxyl radicals and nitric oxide (NO) that damages macromolecular components of cells, eventually resulting in apoptosis. In this report, we have investigated the possible involvement of free radicals in the response of Ramos cells to iron. When added to Ramos cells, iron increased the intracellular levels of peroxide/peroxynitrite and NO. Moreover, the addition of free radicals scavengers (TROLOX and Carboxy-PTIO) neutralized the effects of iron on Ramos cells while addition of an NO donor or hydrogen peroxide (H2O2) to cells generated effects which partially mimicked those induced by iron addition. Collectively, our results suggest the involvement of free radicals as effectors in the iron specific growth inhibition of BL cells observed in vitro.

c-Myc over-expression in Ramos Burkitt's lymphoma cell line predisposes to iron homeostasis disruption in vitro.

Burkitt's lymphoma is an aggressive B-cell neoplasm resulting from deregulated c-myc expression. We have previously shown that proliferation of Burkitt's lymphoma cell lines such as Ramos is markedly reduced by iron treatment. It has been shown that iron induces expression of c-myc which, owing to its transcriptional regulatory functions, regulates genes involved in iron metabolism. Transient enhancement of c-myc expression by iron could increase the expression of genes involved in iron incorporation, which could lead to an accumulation of intracellular free iron. Here, we have investigated whether cells with a high basal level of c-Myc were more likely to accumulate free iron. Our results suggest that the basal level of c-Myc in Ramos cells is twofold higher than what is seen in HL-60 cells. Moreover, in Ramos cells, where c-Myc is expressed at a high level, H-ferritin expression is down-regulated, transferrin receptor (CD71) expression is increased, and ferritin translation is inhibited. These modifications in iron metabolism, resulting from the strong basal expression of c-Myc, and amplified by iron addition, could lead to a disruption in homeostasis and consequently to growth arrest.

Iron specific growth inhibition of Burkitt's lymphoma cells in vitro, associated with a decrease in translocated c-myc expression.

The cellular proto-oncogene c-myc is an important transcription factor that plays a role in several cellular activities such as proliferation, differentiation, and apoptosis. It follows that regulation of c-myc expression is crucial for maintaining cell integrity. Amplification or translocation can convert this proto-oncogene into an activated oncogene, thereby deregulating c-myc expression. Changes in the expression of c-myc leading to malignant cell growth and tumor progression can also be triggered by extrinsic factors. It has been reported that iron can increase cell proliferation, mainly by stimulating DNA synthesis as well as by enhancing c-myc expression. Here, we studied the effect of iron on cells in which c-myc expression is deregulated by either chromosomal translocation or gene amplification. When added to Burkitt's lymphoma cell lines, iron markedly inhibits cell proliferation. This effect is mediated by a cell cycle arrest in G2/M, followed by an important decrease in c-myc expression, whereas no effect could be observed in a cell line harboring amplified c-myc. Moreover, the down-regulation of c-myc expression, which is independent from cell cycle blockade, leads to cell death by apoptosis. Collectively, our results suggest the existence of a novel iron-dependent cell cycle regulatory mechanism involving modulation of translocated c-myc gene expression.

Maintenance of Epstein-Barr virus-derived episomal vectors in the murine Sp2/0 myeloma cell line is dependent upon exogenous expression of human EBP2.

Vectors carrying the origin of replication (oriP) and driving expression of the EBNA-1 protein from Epstein-Barr virus (EBV) replicate as extrachromosomal episomes in human cells. Whether these vectors can be maintained as episomes in murine cells is still controversial. Here we demonstrate that EBNA-1 expression alone was unable to maintain episomal expression of an EBV-based vector in the murine Sp2/0 cell line. However, we were able to obtain long-term episome maintenance in Sp2/0 cells after exogenously expressing human EBP2 by genetic engineering. Our results provide further evidence for the fundamental role of human EBP2 in episomal maintenance of EBV-based vectors. Moreover, we demonstrate that EBV-based vectors can be successfully used in cells presumably incompetent for episomal maintenance.