| Clinical reports from Chinese research groups, confirmed by a preliminary study in the United States, indicate that arsenic trioxide (As2O3) is extremely effective for inducing complete remission in patients with acute promyelocytic leukemia (APL) who are resistant to cytotoxic chemotherapy. Recently, there are numerous reports in the literature have shown that this agent exhibits potent growth inhibitory effects on several cell lines of diverse malignant phenotypes in vitro, including malignant lymphoma cells, neuroblastoma, multiple myeloma, prostate cancer cells, colon cancer cells and gastric cancer cells, but As2O3 can induce apoptosis but only at higher concentrations that may be unacceptable in the clinic because of toxicity. Solid tumors are often infiltrated by inflammatory phagocytes which can generate reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide anions and hydroxyl radicals within the tumor tissue. Cancer tissues are reported to have higher levels of generalized oxidative stress. Low doses of ROS, particularly H2O2, are mitogenic and promote cell proliferation via activation of a various of signaling transduction pathway including Phosphoinositide 3-kinase (PI3-K) signaling pathway. Does lowconcentrations of H2O2 interfere with As2O3-induced apoptosis in tumor cells? Is there a crucial signal molecular among H2O2-activated signaling pathway?The human Burkitt's lymphoma cells Raji were studied in vitro models. Cell and molecular biology methods were applied in the investigated of H2O2 effect on As2O3-induced apoptosis in different levels including morphology, DNA and protein.With 2mol/L, 5mol/L and 10mol/L As2O3, the concentration of As2O3 achievable in vivo, treatment for 24 hours, induced human Burkitt's lymphoma cells apoptosis. 200mol/L H2O2 inhibited the occurrence of apoptosis and converted the mode of cell death to pyknosis/necrosis. Membrane-intact apoptotic cells are recognized and phagocytosed by monocyte-derived macrophages, but membrane-intact pyknotic/necrotic cells are not. One potential physiologic ramification of this result is that cells killed in vivo in the presence of H2O2 are not phagocytosed until after they have begun to leak their contents into the extracellular space, thus allowing an inflammatory response to ensue. This may induce a cycle of chronic inflammation and further interference with As2O3 action. Our data show that when cells incubated with the PI3-k inhibitor wortmannin, the protective effects of H2O2 are reversed and PI3-k plays the pivotal role in H2O2 inhibited As2O3-induced apoposis.We further examined the downstream targets of PI3-K signal pathway in H2O2-induced oxidative stress. H2O2 inhibited As2O3-induced activation of caspase-3, down-regulation Bcl-XL protein expression and up-regulation Bax protein expression in a PI3-k-dependent manner and PI3-k is involved in H2O2-induced up-regulation of Bc1-2 protein expression. Meanwhile, PI3-k inhibitor wortmannin reverses H2O2-mediated phosphorylation of Bcl-2 and IKB , and translocation of nuclear factor-kB. Nevertheless, mechanisms of H2O2-induced activation of extracellular regulated kinase (ERK) and inhibition of Mitogen-activated protein kinase (p38MAPK) are PI3-K-independent. These results suggested that up-regulation of Bcl-2 and BC1-XL protein expression, down-regulation of Bax protein expression,inhibition of NF-B translocation and caspase-3 activation, and phosphorylation of Bc1-2 and IKB maybe the downstream of PI3-K signaling pathway. ERK and p38MAPK are not involved in PI3-K survival signaling.Take together, we concluded that the ability of low concentrations H2O2 protects human malignant lymphoma from As2O3-induced toxicity. The protective effect of H2O2 against As2O3-induced apoptosis is dependent on a PI3-kinase signaling pathway, and PI3-kinase activation modulates changes in Bcl-2, Bcl-XL, Bax expression, phosphorylation of Bcl-2 and IKBa, translocation of NF-KB and activation of caspase-3.
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