DDX3, a DEAD Box RNA Helicase with Tumor Growth–Suppressive Property and Transcriptional Regulation Activity of the p21waf1/cip1 Promoter, Is a Candidate Tumor Suppressor

Abstract

DDX3 is a DEAD box RNA helicase with diverse biological functions. Using colony formation assay, our results revealed that DDX3 inhibited the colony formation ability of various tumor cells, and this inhibition might be due to a reduced growth rate caused by DDX3. Additionally, we identified p21 waf1/cip1 , a cyclin-dependent kinase inhibitor, as a target gene of DDX3, and the up-regulation of p21 waf1/cip1 expression accounted for the colony-suppressing activity of DDX3. Moreover, DDX3 exerted its transactivation function on p21 waf1/cip1 promoter through an ATPase-dependent but helicase-independent mechanism, and the four Sp1 sites located within the À123 to À63 region, relative to the transcription start site of p21 waf1/cip1 promoter, were essential for the response to DDX3. Furthermore, DDX3 interacted and cooperated with Sp1 to up-regulate the promoter activity of p21 waf1/cip1 . To determine the relevance of DDX3 in clinical cancers, the expression profile of DDX3 in various tumors was also examined. A declined expression of DDX3 mRNA and protein was found in f58% to 73% of hepatoma specimens, which led to the reduction of p21 waf1/cip1 expression in a manner independent of p53 status. Additionally, an alteration of subcellular localization from nuclei to cytoplasm was also observed in >70% of cutaneous squamous cell carcinoma samples. Because DDX3 exhibits tumor suppressor functions, such as a growth-suppressive property and transcriptional activation of the p21 waf1/cip1 promoter, and is inactivated through down-regulation of gene expression or alteration of subcellular localization in tumor cells, all these features together suggest that DDX3 might be a candidate tumor suppressor. (Cancer Res 2006; 66(13): 6579-88)

Candidate tumor suppressor DDX3 RNA helicase specifically represses cap-dependent translation by acting as an eIF4E inhibitory protein

“…The DDX3-eIF4E interaction is required for the tumor growth suppressive ability of DDX3 Our previous studies have demonstrated that the DDX3-mediated upregulation of p21 waf1/cip1 promoter activity could account for the tumor suppressor function of DDX3 (Chang et al, 2006;Chao et al, 2006). Based on this, we extended our studies to address whether the DDX3-eIF4E interaction is required for the aforementioned property of DDX3.…”

“…More recently, we demonstrated that DDX3 harbors tumor-growth suppressive ability and can modulate the transcriptional activity of the p21 waf1/cip1 promoter through physical interaction with Sp1 (Chao et al, 2006). Furthermore, we have also reported that DDX3 is inactivated through deregulation of expression or nuclear exclusion in tumor cells (Chang et al, 2006;Chao et al, 2006). These findings suggest a regulatory role for DDX3 in cell growth and tumorigenesis.…”

“…Presumably, this DDX3-mediated stimulatory effect on IRES-dependent translation is through sequestration of the available eIF4E, which may augment HCV viral replication or selectively stimulate IRES-directed translation of cellular transcripts. Most importantly, this eIF4E-binding ability or translation regulatory effect of DDX3 is required for the tumor growth suppressor function of DDX3 that has been reported recently (Chang et al, 2006;Chao et al, 2006).…”

“…Given that DDX3 harbors the trans-acting ability (You et al, 1999;Chao et al, 2006), we utilized an RNA reporter transfection assay to further investigate whether DDX3 specifically inhibits cap-dependent translation in vivo. To this end, various amounts of FLAG-DDX3 expression plasmid and a fixed amount of bicistronic reporter transcript were sequentially transfected into 293T cells.…”

“…Most interestingly, DDX3 is not only required for HIV Rev-RRE export (Yedavalli et al, 2004), but also interacts with hepatitis C virus (HCV) core protein (Mamiya and Worman, 1999;Owsianka and Patel, 1999;You et al, 1999), suggesting a role in the life cycle or pathogenesis of certain viruses. More recently, we demonstrated that DDX3 harbors tumor-growth suppressive ability and can modulate the transcriptional activity of the p21 waf1/cip1 promoter through physical interaction with Sp1 (Chao et al, 2006). Furthermore, we have also reported that DDX3 is inactivated through deregulation of expression or nuclear exclusion in tumor cells (Chang et al, 2006;Chao et al, 2006).…”