The helicase protein DHX29 promotes translation initiation, cell proliferation, and tumorigenesis

Abstract

Translational control plays an important role in cell growth and tumorigenesis. Cap-dependent translation initiation of mammalian mRNAs with structured 5 UTRs requires the DExH-box protein, DHX29, in vitro. Here we show that DHX29 is important for translation in vivo. Down-regulation of DHX29 leads to impaired translation, resulting in disassembly of polysomes and accumulation of mRNA-free 80S monomers. DHX29 depletion also impedes cancer cell growth in culture and in xenografts. Thus, DHX29 is a bona fide translation initiation factor that potentially can be exploited as a target to inhibit cancer cell growth.I nitiation is a tightly regulated rate-limiting step in the translation of eukaryotic mRNAs. Ribosome recruitment to the mRNA commences with binding of translation initiation factor 4F (eIF4F) to the 7-methyl guanosine cap structure, which is present at the 5Ј end of all nuclear-encoded eukaryotic mRNAs (1). eIF4F (comprising the cap-binding protein eIF4E, the DEAD-box RNA helicase eIF4A and eIF4G, a scaffold for binding eIF4E and eIF4A) binds to the cap, unwinds (with the aid of eIF4A) the cap-proximal region of the mRNA, and, through interaction with the ribosome-bound eIF3, recruits the 40S ribosomal subunit to the mRNA (2-4). The 40S subunit then scans the 5Ј UTR in a 5Ј to 3Ј direction until it encounters an initiation codon. A subsequent joining of the 60S ribosomal subunit and release of eIFs result in formation of an elongationcompetent 80S ribosome.Secondary structures in 5ЈUTRs of mRNAs are thought to become unwound to allow ribosomal complexes to move along the mRNA in search of the initiation codon. Thus, in addition to its role in the initial attachment of ribosomal complexes to mRNA, eIF4A is believed to assist ribosomal complexes during scanning (5). Recent observations suggest that the process of eukaryotic initiation requires additional members of the DEAD/DExH-box protein family; for instance, a DEAD-box protein, yeast Ded1, and its mammalian homologue, DDX3, are biochemically and genetically implicated in translation initiation on long structured 5ЈUTRs (6), and another DExH-box protein, DHX29, strongly stimulates cap-dependent initiation on mRNAs with structured 5ЈUTRs in vitro (7). Here we studied the importance of DHX29 for translation in vivo and characterized it as a novel factor required for cell proliferation. Results DHX29 Is a Ubiquitously Expressed Cytoplasmic Protein That Associ-ates with the 40S Ribosomal Subunit. DHX29 has been found to interact with the 40S ribosomal subunit in vitro and to associate with 40S ribosomal complexes in a rabbit reticulocyte lysate (7). To determine how general these findings are, we examined the distribution of DHX29 in polysome preparations from HeLa cells using 2 commercial DHX29 antibodies (Fig. 1A). The specificity of these antibodies to human DHX29 was confirmed by immunoblotting against the purified native protein [supporting information (SI) Fig. S1] and recombinant DHX29, as well as by the loss of immunoreactivity following DHX29 RNAi ...

Regulation of global and specific mRNA translation by the mTOR signaling pathway

“…72 These RNA helicases may assist eIF4A during the initiation step to enhance the process of ribosomal scanning on structured mRNAs, 97 as was recently described for the DEAH (Asp-Glu-Ala-His)-box protein DHX29. 98 The DEAD-box helicase DDX3 is not essential for global translation, but is required for the translation of specific transcripts (e.g., Cyclin E1) that contain secondary structures within their 5' UTRs. 99,100 We found that mTOR promotes the recruitment of DDX3 to the 5' mRNA cap in a 4E-BP1/2-dependent manner, 72 suggesting a mechanism whereby mTOR activity is required for the translation of DDX3-bound transcripts.…”

DHX29 and eIF3 cooperate in ribosomal scanning on structured mRNAs during translation initiation

“…Efficient translation initiation on mRNAs with structured 5 ′ -UTRs, which are more prevalent in higher eukaryotes, requires the specific DExH-box helicase DHX29 in vitro and in vivo (Pisareva et al 2008;Parsyan et al 2009). DHX29 depletion is also shown to impede cancer cell growth in culture and in xenografts, which suggests a role for this protein in tumorigenesis (Parsyan et al 2009).…”

“…DHX29 depletion is also shown to impede cancer cell growth in culture and in xenografts, which suggests a role for this protein in tumorigenesis (Parsyan et al 2009). Until the recent discovery of mammalian-specific DHX29, eIF4A was the only reported helicase with a confirmed role in canonical eukaryotic translation initiation.…”

DHX33 Transcriptionally Controls Genes Involved in the Cell Cycle

“…Many characterized RNA helicases have been found to be fundamental in essential cellular processes (1,26). In recent years, a few RNA helicases have been found to participate in cancer development even though their detailed molecular mechanisms remain largely undiscovered (27)(28)(29)(30)(31)(32)(33). Furthermore, despite the characterization of several DEAD/DEAH proteins, few specific inhibitors targeting these ATP-dependent helicases have been developed.…”