Epstein-Barr virus-mediated protection against etoposide-induced apoptosis in BJA-B B cell lymphoma cells: role of Bcl-2 and caspase proteins

The dynamic character of the BCL2 promoter i-motif provides a mechanism for modulation of gene expression by compounds that bind selectively to the alternative DNA hairpin structure

It is generally accepted that DNA predominantly exists in duplex form in cells. However, under torsional stress imposed by active transcription, DNA can assume nonduplex structures. The BCL2 promoter region forms two different secondary DNA structures on opposite strands called the G-quadruplex and the i-motif. The i-motif is a highly dynamic structure that exists in equilibrium with a flexible hairpin species. Here we identify a pregnanol derivative and a class of piperidine derivatives that differentially modulate gene expression by stabilizing either the i-motif or the flexible hairpin species. Stabilization of the i-motif structure results in significant upregulation of the BCL2 gene and associated protein expression; in contrast, stabilization of the flexible hairpin species lowers BCL2 levels. The BCL2 levels reduced by the hairpin-binding compound led to chemosensitization to etoposide in both in vitro and in vivo models. Furthermore, we show antagonism between the two classes of compounds in solution and in cells. For the first time, our results demonstrate the principle of small molecule targeting of i-motif structures in vitro and in vivo to modulate gene expression.

DNA viruses in human cancer: An integrated overview on fundamental mechanisms of viral carcinogenesis

The first experimental data suggesting that neoplasm development in animals might be influenced by infectious agents were published in the early 1900s. However, conclusive evidence that DNA viruses play a role in the pathogenesis of some human cancers only emerged in the 1950s, when Epstein-Barr virus (EBV) was discovered within Burkitt lymphoma cells. Besides EBV, other DNA viruses consistently associated with human cancers are the hepatitis B virus (HBV), human papillomavirus (HPV), and Kaposi sarcoma herpesvirus (KSHV). Although each virus has unique features, it is becoming clearer that all these oncogenic agents target multiple cellular pathways to support malignant transformation and tumor development.

MLL rearrangements are induced by low doses of etoposide in human fetal hematopoietic stem cells

During fetal development, the liver serves as the primary hematopoietic organ in which hematopoietic stem cells (HSC) comprise a large proportion of hepatic cell populations. Because HSC are capable of initiating long-term hematopoiesis, injury to these cells during pregnancy may play a role in the development of hematopoietic disorders manifested after birth. Of interest is the role of genetic injury to fetal HSC in the etiology of the infant acute leukemias, which are characterized by chromosomal rearrangements in the 11q23 region involving the mixed lineage leukemia (MLL) gene. These gene fusions also occur in leukemias in adults following chemotherapy with etoposide and other inhibitors of DNA topoisomerase II. We used etoposide as a model compound to determine the sensitivity of human fetal HSC to DNA damage and to determine whether we could induce MLL rearrangements in cultured human fetal HSC. Exposure of HSC to etoposide resulted in a dose-dependent loss of viability, with effects observed at low nanomolar concentrations. DNA strand breaks were observed on exposure to 140 nM etoposide, and higher etoposide concentrations stimulated an increase in early lymphoid populations and elicited G2/M cell cycle arrest. Immunophenotyping of MLL translocations revealed a significant increase in positive flow cytometry events at low etoposide concentrations and were consistent with MLL recombination. MLL translocations were confirmed using fluorescent in situ hybridization. In vitro inhibition of DNA topoisomerase II was observed at >or=25 microM etoposide, but was not evident at lower etoposide concentrations associated with DNA damage. Our data indicate that low acute doses of etoposide can cause DNA strand breaks and chromosomal rearrangements involving MLL in human fetal HSC. Ultimately, such injury may have ramifications with regards to transplacental exposures to environmental chemicals linked to the etiology of infant acute leukemias.

Caspase-mediated degradation of human 5-lipoxygenase in B lymphocytic cells

5-Lipoxygenase (5-LO) is a tightly regulated enzyme in the synthesis of bioactive lipids from arachidonic acid. Here, we demonstrate that 5-LO is regulated by caspases, which are signaling molecules that control critical biological processes by means of specific limited proteolysis. Cell splitting of the Epstein-Barr virus-transformed B lymphocytic cell line BL41-E95-A caused a pronounced, but transient, reduction of functional 5-LO protein, accompanied by the appearance of a 62-kDa 5-LO cleavage product. In parallel, splitting of BL41-E95-A cells induced activation of caspase-6 (casp-6) and casp-8. Caspase activation and 5-LO degradation were blocked by the protein-synthesis inhibitor cycloheximide, and cell-permeable peptide inhibitors of casp-6 and casp-8 prevented 5-LO cleavage. Activation of casp-6 and casp-8 was connected to subsequent enhancement of cell proliferation, whereas selective caspase inhibition blocked cell growth. Last, isolated human 5-LO was cleaved by recombinant casp-6 in vitro to a 58-kDa fragment. Based on site-directed mutagenesis studies, 5-LO is cleaved by casp-6 after Asp-170, which in a homology-based 3D model of 5-LO is located on the enzyme periphery. We suggest that splitting of BL41-E95-A cells induces de novo synthesis of a protein involved in the activation of casp-6, which cleaves 5-LO.