A Novel Transcriptional Inhibitor Induces Apoptosis in Tumor Cells and Exhibits Antiangiogenic Activity

Abstract

Using a high-throughput cell-based assay, we identified a nucleoside analogue 4-amino-6-hydrazino-7-B-D-ribofuranosyl-7H-pyrrolo(2,3-d)-pyrimidine-5-carboxamide (ARC), which has the properties of a general transcriptional inhibitor. Specifically, ARC inhibits the phosphorylation of RNA polymerase II by positive transcription elongation factor-b, leading to a block in transcriptional elongation. ARC was able to potently repress p53 targets p21 and hdm2 (human homologue of mdm2) protein levels, but dramatically increased p53 levels similar to other transcriptional inhibitors, including flavopiridol. This increase in p53 corresponded to the down-regulation of shortlived protein hdm2, which is a well-established negative regulator of p53. Remarkably, ARC induced potent apoptosis in human tumor and transformed, but not in normal cells, and possessed strong antiangiogenic activity in vitro. Although ARC promoted the accumulation of p53, ARC-induced apoptosis in tumor cells was p53-independent, suggesting that it may be useful for the treatment of tumors with functionally inactive p53. Furthermore, cell death induced by ARC had a strong correlation with down-regulation of the antiapoptotic gene survivin, which is often overexpressed in human tumors. Taken together, our data suggests that ARC may be an attractive candidate for anticancer drug development. (Cancer Res 2006; 66(6): 3264-70)

Proapoptotic compound ARC targets Akt and N-myc in neuroblastoma cells

“…Previously, we had characterized ARC on a number of cancer cell lines of epithelial origin (Radhakrishnan and Gartel, 2006b). In this study, we have extended these studies on neuroblastoma cell lines and found that these cells are susceptible to ARC in nanomolar concentrations.…”

“…Furthermore, we observed that ARC was able to induce the cleavage of caspase-3 in NB-1691 and IMR32 cell lines at B10-fold lower concentration than DRB (Figure 1d). The significant difference in apoptosis, despite their similarity in the degree of transcriptional inhibition (Radhakrishnan and Gartel, 2006b) suggests that ARC may employ more than transcriptional repression to induce apoptosis in these neuroblastoma cell lines.…”

“…Nucleoside analog ARC (4-amino-6-hydrazino-7-beta-D-ribofuranosyl-7H-pyrrolo[2,3-d]-pyrimidine-5-carboxamide) is a global transcriptional inhibitor, which we previously identified using a high-throughput cellbased assay (Radhakrishnan and Gartel, 2006b). Treatment of cancer cells with ARC displayed the hallmarks of transcriptional repression such as the induction of p53, and the repression of its target genes mdm2 and p21.…”

“…Previously, we had demonstrated that though ARC and DRB repressed transcription to a similar extent, ARC induced more potent apoptosis in a colon cancer cell line (Radhakrishnan and Gartel, 2006b). To test if the neuroblastoma cell lines display differential sensitivities toward ARC and DRB, we first used NB-1691 cells and treated them with 5 mM of ARC or 50 mM of DRB for 24 h and compared them with dimethyl sulfoxide (DMSO)-treated control.…”

P-TEFb inhibitors interfere with activation of p53 by DNA-damaging agents

“…Recently, we had reported that ARC (4-amino-6-hydrazino-7-b-d-ribofuranosyl-7H-pyrrolo[2,3-d]-pyrimidine-5-carboxamide), a small molecule inhibitor of P-TEFb kinase (Cdk9/CyclinT1 complex) and a transcriptional inhibitor with anticancer properties, increased p53 levels although simultaneously repressing its target genes (Radhakrishnan and Gartel, 2006b) similarly to the well-known anticancer drug flavopiridol (Demidenko and Blagosklonny, 2004). We also found that p53 may be phosphorylated by P-TEFb and ARC inhibits phosphorylation of p53 by P-TEFb kinase in vitro (Radhakrishnan and Gartel, 2006a).…”