Arsenite-induced apoptosis of human neuroblastoma cells requires p53 but occurs independently of c-Jun.

Arsenite treatment of human SH-SY5Y neuroblastoma cells leads to an upregulation of caspase-3/7 activity and to the fragmentation of chromatin that is accompanied by elevated p53 and c-Jun levels. Expression of a truncated mutant of p53, p53DD, which interfered with the oligomerization of p53, suppressed the arsenite-induced upregulation of caspase-3/7 activity and the fragmentation of chromatin, indicating that p53 is required for arsenite-induced cell death. These data were corroborated by knockdown experiments of p53 following expression of a p53-specific short hairpin RNA. Likewise, expression of either p53DD or knockdown of p53 prevented caspase-3/7 activation and chromatin fragmentation induced by nutlin-3, a compound that prevents the interaction between p53 and the E3 ubiquitin ligase MDM2. Transcriptional upregulation of a chromatin-embedded p53-responsive reporter gene in either arsenite or nutlin-3 stimulated neuroblastoma cells revealed that the transcriptional activity of p53 was increased under these conditions. Expression of a c-Jun-specific short hairpin RNA failed to impair arsenite-induced caspase-3/7 activation and fragmentation of chromatin. Likewise, inhibition of c-Jun target gene expression by expression of a dominant-negative mutant of c-Jun did not interfere with arsenite-induced caspase-3/7 activation and chromatin fragmentation. However, this approach successfully reduced caspase-3/7 activity induced as a result of forced expression of a constitutively active mutant of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase (MEKK)-1. Together, these data show that the upregulation of p53 is causally linked with arsenite-induced cell death in neuroblastoma cells, whereas the upregulation of c-Jun is not part of this apoptotic signaling cascade.

Rearrangement of structured RNA via branch migration structures catalysed by the highly related DEAD-box proteins p68 and p72.

RNA helicases, like their DNA-specific counterparts, can function as processive enzymes, unwinding RNA with a defined step size in a unidirectional fashion. Recombinant nuclear DEAD-box protein p68 and its close relative p72 are reported here to function in a similar fashion, though the processivity of both RNA helicases appears to be limited to only a few consecutive catalytic steps. The two proteins resemble each other also with regard to other biochemical properties. We have found that both proteins exhibit an RNA annealing in addition to their helicase activity. By using both these activities the enzymes are able in vitro to catalyse rearrangements of RNA secondary structures that otherwise are too stable to be resolved by their low processive helicase activities. RNA rearrangement proceeds via protein induced formation and subsequent resolution of RNA branch migration structures, whereby the latter step is dependent on ATP hydrolysis. The analysed DEAD-box proteins are reminiscent of certain DNA helicases, for example those found in bacteriophages T4 and T7, that catalyse homologous DNA strand exchange in cooperation with the annealing activity of specific single strand binding proteins.

Structure and expression of the human p68 RNA helicase gene.

Nuclear DEAD box protein p68 is immunologically related to SV40 large tumour antigen and both proteins possess RNA helicase activity. In this report, we describe the structural organisation of the human p68 gene and aspects of the regulation of its expression. Northern blot and primer extension analyses indicate that, although its level is variable, the p68 RNA helicase appears to be expressed from a single transcription start site in all tissues tested. Sequence analysis revealed that the p68 promoter harbours a 'TATA', a 'CAAT' and an initiator element and contains high affinity binding sites for Sp1, AP-2, CRE and Myc. This and functional promoter analyses in transient expression assays suggest that transcriptional regulation of the p68 gene is complex. Furthermore, there are indications that p68 expression is also regulated post-transcriptionally. Steady-state pools of poly(A)(+)RNA from human cells contain completely spliced p68 mRNA and alternatively spliced forms that contain introns 8-11 or 8-12 (from a total of 12 introns) and are not translated. Analysis of a conditionally p68-overproducing HeLa cell line points to negative autoregulation at the level of splicing, which is confirmed by a recently reported association of p68 with spliceosomes in human cells.