Recent studies showed that arsenic trioxide (As 2 O 3 ) could induce apoptosis and partial differentiation of leukemic promyelocytes. Here, we addressed the possible mechanisms underlying these two different effects. 1.0 M As 2 O 3 -induced apoptosis was associated with condensation of the mitochondrial matrix, disruption of mitochondrial transmembrane potentials (⌬⌿m) and activation of caspase-3 in acute promyelocytic leukemia (APL) cells regardless of their sensitivity to all-trans retinoic acid (ATRA). All these effects were inhibited by dithiothreitol (DTT) and enhanced by buthionine sulfoximine (BSO). Furthermore, BSO could also render HL60 and U937 cells, which had the higher cellular catalase activity, sensitive to As 2 O 3 -induced apoptosis. Surprisingly, 1.0 M As 2 O 3 did not induce the ⌬⌿m collapse and apoptosis, while 0.1 M As 2 O 3 induced partial differentiation of fresh BM cells from a de novo APL patient. In this study, we also showed that 0.2 mM DTT did not block low-dose As 2 O 3 -induced NB4 cell differentiation, and 0.1ෂ0.5 M As 2 O 3 did not induce differentiation of ATRA-resistant NB4-derived sublines, which were confirmed by cytomorphology, expression of CD11b, CD33 and CD14 as well as NBT reduction. Another interesting finding was that 0.1ෂ0.5 M As 2 O 3 could also induce differentiation-related changes in ATRA-sensitive HL60 cells. However, the differentiationinducing effect could not be seen in ATRA-resistant HL60 sublines with RAR␣ mutation. Moreover, low-dose As 2 O 3 and ATRA yielded similar gene expression profiles in APL cells. These results encouraged us to hypothesize that As 2 O 3 induces APL cell differentiation through direct or indirect activation of retinoic acid receptor-related signaling pathway(s), while ⌬⌿m collapse is the common mechanism of As 2 O 3 -induced apoptosis. Leukemia (2000) 14, 262-270.

The RIG-G gene, originally isolated from an acute promyelocytic leukemia cell line NB4, codes for a 60-kDa cytoplasmic protein that is induced by all-trans retinoic acid (ATRA) treatment along with the induction of morphological differentiation of NB4 cells. Here, we provide evidence that ectopic expression of Rig-G in U937 cells can lead to a significant accumulation of cells at G 1͞S transition. Growth arrest seems to occur by modulating several major cell cycle regulatory players. Interestingly, Rig-G alters JAB1 cellular distribution through interacting with this protein and increases the intracellular level of p27 by preventing it from the JAB-1-dependent and ubiquitin͞proteasome-mediated degradation. Furthermore, we demonstrate a role of Rig-G for c-myc down-regulation that results in an up-regulation of p21, tightly associated with cell cycle arrest. In addition, our studies reveal that Rig-G is a direct target of STAT1, a key transcription factor in regulating IFN responses, and may be one of the first experimentally proven molecular mediators for the antiproliferative effect of IFN-␣. Considering that IFN-␣ and ATRA synergistically inhibit growth along the intracellular pathways triggered by the two compounds in many cell types, we suggest that Rig-G may also represent one of the key molecular nodes of signaling cross-talk between ATRA and IFN-␣.cell growth inhibition ͉ retinoic acid ͉ Rig-G ͉ STAT1

Retinoic acid-induced gene G (RIG-G), a gene originally identified in all-trans retinoic acid-treated NB4 acute promyelocytic leukemia cells, is also induced by IFNA in various hematopoietic and solid tumor cells. Our previous work showed that RIG-G possessed a potent antiproliferative activity. However, the mechanism for the transcriptional regulation of RIG-G gene remains unknown. Here, we report that signal transducer and activator of transcription (STAT) 2 together with IFN regulatory factor (IRF)-9 can effectively drive the transcription of RIG-G gene by their functional interaction through a STAT1-independent manner, even without the tyrosine phosphorylation of STAT2. The complex IRF-9/STAT2 is both necessary and sufficient for RIG-G gene expression. In addition, IRF-1 is also able to induce RIG-G gene expression through an IRF-9/STAT2-dependent or IRF-9/ STAT2-independent mechanism. Moreover, the induction of RIG-G by retinoic acid in NB4 cells resulted, to some extent, from an IFNA autocrine pathway, a finding that suggests a novel mechanism for the signal cross-talk between IFNA and retinoic acid. Taken together, our results provide for the first time the evidence of the biological significance of IRF-9/STAT2 complex, and furnish an alternative pathway modulating the expression of IFN-stimulated genes, contributing to the diversity of IFN signaling to mediate their multiple biological properties in normal and tumor cells. [Cancer Res 2009;69(8):3673-80]