Involvement of caspase activation and mitochondrial stress in trichostatin A-induced apoptosis of Burkitt's lymphoma cell line, Akata

Effect of trichostatin A on Burkitt's lymphoma cells: Inhibition of EPS8 activity through Phospho-Erk1/2 pathway

Histone deacetylase inhibitors (HDACi) manifest great potential for treatment of Burkitt's lymphoma (BL), an aggressive B-cell lymphoma. Epidermal growth factor receptor pathway substrate 8 (EPS8) is confirmed overexpressed and associated with poor prognosis in solid tumors and leukemia. However, EPS8 expression and the relationship between EPS8 and HDACi on BL remains obscure. Here, we hypothesized that trichostatin A (TSA), a pan-HDACi, could inhibit BL cells by downregulating EPS8. We demonstrated that TSA reduced cell viability, induced apoptosis and cell arrest at G0/G1. Mechanismly, TSA attenuated EPS8 and downstream Phospho-Erk1/2 pathway. Knockdown of EPS8 resulted in a significant reduction in cellular proliferation and suppressed Phospho-Erk1/2 pathway activity, particularly when combined with TSA. Conversely, overexpression of EPS8 rescued this phenomenon. Then we showed that the combination of TSA and Epirubicin had a more significant effect when compared with TSA or Epirubicin alone. Finally, knockdown of EPS8 and TSA had a synergistic suppression effect on BALB/c nude mice. In conclusion, this study reveals that TSA affects BL cells by suppressing Phospho-Erk1/2 pathway through downregulating EPS8.

Histone acetyltransferases and histone deacetylases in B- and T-cell development, physiology and malignancy

The development of B and T cells from hematopoietic precursors and the regulation of the functions of these immune cells are complex processes that involve highly regulated signaling pathways and transcriptional control. The signaling pathways and gene expression patterns that give rise to these developmental processes are coordinated, in part, by two opposing classes of broad-based enzymatic regulators: histone acetyltransferases (HATs) and histone deacetylases (HDACs). HATs and HDACs can modulate gene transcription by altering histone acetylation to modify chromatin structure, and by regulating the activity of non-histone substrates, including an array of immune-cell transcription factors. In addition to their role in normal B and T cells, dysregulation of HAT and HDAC activity is associated with a variety of B- and T-cell malignancies. In this review, we describe the roles of HATs and HDACs in normal B- and T-cell physiology, describe mutations and dysregulation of HATs and HDACs that are implicated lymphoma and leukemia, and discuss HAT and HDAC inhibitors that have been explored as treatment options for leukemias and lymphomas.

EBV interferes with the sensitivity of Burkitt lymphoma Akata cells to etoposide

Burkitt lymphoma (BL) commonly exhibits Epstein-Barr virus (EBV) positivity associated with latent chronic infection. Models of acute EBV infection have been associated with cellular resistance to apoptosis. However, the effect of latent long-term EBV infection on apoptosis induced by drugs is not well defined. To determine this, we have studied the response of the Akata EBV+ cell line (type I latency) to etoposide, before and after downregulating EBV gene expression. We observed that downregulating EBV nuclear antigen-1 (EBNA-1) expression with siRNAs reverted cellular sensitivity to etoposide. In accordance with this finding, Akata EBV+ cells showed increased sensitivity to etoposide, when compared to the Akata EBV- cells. We also observed that Akata EBV+ cells presented increased apoptosis levels and decreased Bcl-xL mRNA and protein levels, when compared to the Akata EBV- cells. In addition, Akata EBV+ cells contained less endoplasmic reticulum (ER) than EBV- cells. Finally, downregulation of EBV with EBNA-1 siRNAs caused an increase in the expression of Bcl-xL indicating that EBV is responsible for the differences found between the Akata EBV+ and EBV- cell lines.

Trichostatin A, a histone deacetylase inhibitor, suppresses JAK2/STAT3 signaling via inducing the promoter-associated histone acetylation of SOCS1 and SOCS3 in human colorectal cancer cells

Aberrant janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling is involved in the oncogenesis of several cancers. Suppressors of cytokine signaling (SOCS) genes and SH2-containing protein tyrosine phosphatase 1 (SHP1) proteins, which are negative regulators of JAK/STAT signaling, have been reported to have tumor suppressor functions. However, in colorectal cancer (CRC) cells, the mechanisms that regulate SOCS and SHP1 genes, and the cause of abnormalities in the JAK/STAT signaling pathway, remain largely unknown. The present study shows that trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, leads to the hyperacetylation of histones associated with the SOCS1 and SOCS3 promoters, but not the SHP1 promoter in CRC cells. This indicates that histone modifications are involved in the regulation of SOCS1 and SOCS3. Moreover, upregulation of SOCS1 and SOCS3 expression was achieved using TSA, which also significantly downregulated JAK2/STAT3 signaling in CRC cells. We also demonstrate that TSA suppresses the growth of CRC cells, and induces G1 cell cycle arrest and apoptosis through the regulation of downstream targets of JAK2/STAT3 signaling, including Bcl-2, survivin and p16(ink4a) . Therefore, our data demonstrate that TSA may induce SOCS1 and SOCS3 expression by inducing histone modifications and consequently inhibits JAK2/STAT3 signaling in CRC cells. These results also establish a mechanistic link between the inhibition of JAK2/STAT3 signaling and the anticancer action of TSA in CRC cells.

Cr(VI) induces mitochondrial-mediated and caspase-dependent apoptosis through reactive oxygen species-mediated p53 activation in JB6 Cl41 cells

Cr(VI) compounds are known to cause serious toxic and carcinogenic effects. Cr(VI) exposure can lead to a severe damage to the skin, but the mechanisms involved in the Cr(VI)-mediated toxicity in the skin are unclear. The present study examined whether Cr(VI) induces cell death by apoptosis or necrosis using mouse skin epidermal cell line, JB6 Cl41 cells. We also investigated the cellular mechanisms of Cr(VI)-induced cell death. This study showed that Cr(VI) induced apoptotic cell death in a dose-dependent manner, as demonstrated by the appearance of cell shrinkage, the migration of cells into the sub-G1 phase, the increase of Annexin V positively stained cells, and the formation of nuclear DNA ladders. Cr(VI) treatment resulted in the increases of mitochondrial membrane depolarization and caspases activation. Electron spin resonance (ESR) and fluorescence analysis revealed that Cr(VI) increased intracellular levels of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion radical in dose-dependent manner. Blockage of p53 by si-RNA transfection suppressed mitochondrial changes of Bcl-2 family composition, mitochondrial membrane depolarization, caspase activation and PARP cleavage, leading to the inhibition of Cr(VI)-induced apoptosis. Further, catalase treatment prevented p53 phosphorylation stimulated by Cr(VI) with the concomitant inhibition of caspase activation. These results suggest that Cr(VI) induced a mitochondrial-mediated and caspase-dependent apoptosis in skin epidermal cells through activation of p53, which are mainly mediated by reactive oxidants generated by the chemical.

Tubacin kills Epstein-Barr virus (EBV)-Burkitt lymphoma cells by inducing reactive oxygen species and EBV lymphoblastoid cells by inducing apoptosis

Tubacin is a small molecule inhibitor of histone deacetylase 6 and blocks aggresome activity. We found that Epstein-Barr virus (EBV)-positive Burkitt lymphoma (BL) cells were generally killed by lower doses of tubacin than EBV-transformed lymphoblastoid cells (LCLs) or EBV-negative BL cells. Tubacin induced apoptosis of LCLs, which was inhibited by pretreatment with a pancaspase inhibitor but not by butylated hydroxyanisole, which inhibits reactive oxygen species. In contrast, tubacin killed EBV-positive BL cells in a caspase-3-independent pathway that involved reactive oxygen species and was blocked by butylated hydroxyanisole. Previously, we showed that bortezomib, a proteasome inhibitor, induces apoptosis of EBV LCLs and that LCLs are killed by lower doses of bortezomib than EBV-positive BL cells. Here we found that the combination of bortezomib and tubacin acted in synergy to kill EBV-positive BL cells and LCLs. Tubacin or the combination of bortezomib and tubacin did not induce EBV lytic replication. These findings suggest that the combination of a proteasome inhibitor and an HDAC6 inhibitor may represent a useful strategy for the treatment of certain EBV-associated B cell lymphomas.

Therapeutic potential of AIF-mediated caspase-independent programmed cell death

Resistance to anticancer drugs is often related to deficient cell death execution pathways in cancer cells. Apoptosis, which denotes a form of cell death executed by caspases, was traditionally considered as the only physiological and programmed form of cell death. However, recent evidence indicates that programmed cell death (PCD) can occur in complete absence of caspase activation. Indeed, a large number of caspase-independent models are now defined and a key protein implicated in this type of PCD, apoptosis-inducing factor (AIF), has been identified. AIF is a mitochondrial protein with two faces looking in opposite life/death directions. Recently, the identification of five different isoforms allowed a better characterization of AIFs life/mitochondrial versus death/nuclear functions, as well as definition of its pro-apoptotic region and some of its nuclear partners. Importantly, much work on caspase-independent PCD has revealed that AIF participates in more PCD systems than initially thought. A wider molecular knowledge of AIF, and of the caspase-independent PCDs in which it is involved, are key to provide new insights into the role of PCD. There is no doubt that these insights will lead to the development of more selective and efficient drugs against cancer, degenerative diseases, and other pathological disorders implicating AIF.

Lysosomal cysteine cathepsins: signaling pathways in apoptosis

Apoptosis is the major mechanism by which eukaryotic organisms eliminate potentially dangerous, superfluous and damaged cells. Initially, nuclei and mitochondria were found to be the key organelles involved in the process. However, recent data suggest that lysosomes and the endoplasmic reticulum also play important roles in the process. A number of different stimuli were found to directly or indirectly target the lysosomal membrane, thereby inducing lysosomal permeabilization and the release of cysteine cathepsins and the aspartic protease cathepsin D into the cytosol. Once in the cytosol, cathepsins can trigger cell death by different mechanisms. Here we discuss the different signaling pathways used by lysosomal proteases to trigger apoptosis and their potential role in physiological processes.