Edited by Vladimir Skulachev Keywords:Manganese superoxide dismutase Reactive oxygen species Extracellular regulated kinase Nuclear translocation p53 a b s t r a c t Following our previous finding that sodium selenite induces apoptosis in human leukemia NB4 cells, we now show that the expression of the critical antioxidant enzyme manganese superoxide dismutase (MnSOD) is remarkably elevated during this process. We further reveal that reactive oxygen species (ROS), especially superoxide radicals, play a crucial role in selenite-induced MnSOD upregulation, with extracellular regulated kinase (ERK) and p53 closely implicated. Specifically, ERK2 translocates into the nucleus driven by ROS, where it directly phosphorylates p53, leading to dissociation of p53 from its inhibitory protein mouse double minute 2 (MDM2). Active p53 directly mediates the expression of MnSOD, serving as the link between ERK2 translocation and MnSOD upregulation.

No abstract

Selenium at a low concentration has a chemopreventive role against cancer, while at a high concentration, it exerts a direct antitumor effect. However, the mechanisms remain elusive. In this article, we discovered that Na(2)SeO(3) at 20 micromol/l concentration could significantly inhibit the proliferation of NB4 cells, affect the cell cycle distribution of cell population, and induce cellular changes characteristic of apoptotic cells, while this same compound at 2 micromol/l concentration had no such effects. The mechanisms underlying these overt differences caused by treatment of different concentrations of selenium were further investigated. cDNA microarray analysis showed that after treatment by 20 micromol/l Na(2)SeO(3), 34 genes were changed in expression, while treatment by 2 micromol/l Na(2)SeO(3) resulted in the changes of 29 genes. Nine genes were regulated in both groups, among which three showed opposite changes caused by 2 and 20 micromol/l Na(2)SeO(3). The majority of regulated genes did not coincide between the two experiment groups. In conclusion, 2 and 20 micromol/l Na(2)SeO(3) could have different effects on NB4 cells, and some genes might be involved in the underlying mechanisms. Our findings could provide basis for further uncovering the molecular mechanisms of the chemopreventive and antitumor effects of selenium and, in turn, for probing the rationality of treating leukemia with selenium.

Selenium (Se) is an essential trace element possessing anticarcinogenic properties. Sodium selenite (Na2SeO3) induced apoptosis in human acute promyelocytic leukemia (APL) cell line NB4 with dose and time dependency. In this study, proteomic techniques were used to study the apoptosis of NB4 cells induced by sodium selenite. Twenty-six downregulated and four upregulated proteins were identified, which exhibited a 1.5-fold change or greater. The identified proteins included key regulators of signal transduction such as Rho GDP dissociation inhibitor (Rho GDI) alpha and beta members of the MAPK family, and proteins involved in the regulation of c-fos or c-myc expression. Importantly, the identified proteins, hnRNP D0B and Rho GDI beta, which were related with the regulation of c-myc, c-fos, and c-jun, were determined by reverse transcription-polymerase chain reaction (RT-PCR) to confirm their downregulation in proteomic study. Western blot analysis and RT-PCR were then performed on three associated proteins: c-Myc, c-Fos, and c-Jun, and their expression were observed to be significantly downregulated. Results showed that certain regulation involved in c-myc, c-fos, and c-jun was present in the apoptosis, and the c-Myc dependent-on and Jun N-terminal kinase (JNK) pathway also play roles.

Our previous study has shown that sodium selenite can cause apoptosis in acute promyelocytic leukemia-derived NB4 cells in a caspase-dependent manner involving Deltapsim disruption and cleavage of Bcl-2, but more detailed mechanism(s) remain unclear. Here we showed that mitochondrial apoptosis signaling pathway played a vital role in apoptosis induced by sodium selenite based on the following findings: 1) cytochrome c release, activation of caspase 9, mitochondrial targeting, and oligermerization of Bax; 2) caspase 9, but not caspase 8, inhibitor could attenuate apoptosis; 3) downregulation of Bax and Bad by siRNA could delay sodium selenite-induced apoptosis. Further investigation showed that ROS was an essential inducer of deltapsim disruption and apoptosis by sodium selenite. Our findings here demonstrate that sodium selenite can induce apoptosis in NB4 cells through a mechanism involving ROS, activation of proapoptotic proteins Bad and Bax, Deltapsim disruption, release of cytochrome c, and consequent initiation of caspase cascade.