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

Abstract

DDX3 is a human RNA helicase with plethoric functions. Our previous studies have indicated that DDX3 is a transcriptional regulator and functions as a tumor suppressor. In this study, we use a bicistronic reporter to demonstrate that DDX3 specifically represses cap-dependent translation but enhances hepatitis C virus internal ribosome entry site-mediated translation in vivo in a helicase activity-independent manner. To elucidate how DDX3 modulates translation, we identified translation initiation factor eukaryotic initiation factor 4E (eIF4E) as a DDX3-binding partner. Interestingly, DDX3 utilizes a consensus eIF4E-binding sequence YIPPHLR to interact with the functionally important dorsal surface of eIF4E in a similar manner to other eIF4E-binding proteins. Furthermore, cap affinity chromatography analysis suggests that DDX3 traps eIF4E in a translationally inactive complex by blocking interaction with eIF4G. Point mutations within the consensus eIF4E-binding motif in DDX3 impair its ability to bind eIF4E and result in a loss of DDX3's regulatory effects on translation. All these features together indicate that DDX3 is a new member of the eIF4E inhibitory proteins involved in translation initiation regulation. Most importantly, this DDX3-mediated translation regulation also confers the tumor suppressor function on DDX3. Altogether, this study demonstrates regulatory roles and action mechanisms for DDX3 in translation, cell growth and likely viral replication.

The current understanding of Ded1p/DDX3 homologs from yeast to human

“…While DDX3's role in global translation remains debatable, its overexpression was reported to inhibit translation in vivo. 30 The same study also showed that such inhibition may be mediated by DDX3's direct interaction with eIF4E, which presumably disrupts the eIF4E-eIF4G interaction 30 (Fig. 1).…”

“…30 This result is somewhat intriguing because neither eIF4E nor eIF4G appears to be essential for the HCV IRESmediated internal initiation. 45 Nevertheless, it would be curious to know whether and how DDX3 acts in concert with the HCV core protein in IRES-mediated initiation and perhaps even in capdependent translation.…”

“…The first report showed that global translation is moderately enhanced by downregulating DDX3 in HeLa cell derivatives, suggesting that DDX3 is instead a general translation suppressor. 30 In contrast, another study showed that depletion of DDX3 significantly reduces the protein production from reporter constructs perhaps via its interaction with the translation initiation factor eIF3, indicating an important role for DDX3 in translation 22 ( Fig. 1).…”

“…Finally, DDX3, like Ded1p, has been implicated in cell growth control, although whether it plays a positive or negative role remains unclear. 22,30,37,46 At least in some cases, increased expression of DDX3 was found to promote cell growth, proliferation and even transformation. 22,30,37,46 Because a number of cellular proteins involved in regulating cell growth and programmed cell death are produced via IRES-mediated translation under stressed conditions, 47 it would be of interest to examine whether and how DDX3 participates in such cellular IRES-mediated translation.…”

Human DEAD-box protein 3 has multiple functions in gene regulation and cell cycle control and is a prime target for viral manipulation

“…It was demonstrated that 10 DDX3 interacts with several translation initiation factors, namely eIF4e [27], eIF4a, eIF2 , PABP [13] and eIF3 [14] (Figure 1, C). However, in one of the initial studies, human DDX3 was described to be a repressor rather than a facilitator of cap-dependent protein translation while it enhanced IRES-dependent viral protein translation: DDX3 was shown to interact with eIF4e and to prevent the recruitment of eIF4G to eIF4e, which is required for the initiation of cap-dependent translation [27]. The authors suggested that DDX3 acts like cellular eIF4e-binding proteins (4eBPs) which also prevent translation initiation [27].…”