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Previous studies from our laboratory showed that p21Cip1/WAF1 can be phosphorylated by Pim-1 kinase in vitro, implying that part of the function of Pim-1 might involve influencing the cell cycle. In the present study, site-directed mutagenesis and phosphorylated-specific antibodies were used as tools to identify the sites phosphorylated by Pim-1 and the consequences of this phosphorylation. What , it localizes primarily in the cytoplasm and the effect of phosphorylation on stability is minimal. Cotransfection of wild-type Pim-1 with p21 increases the rate of proliferation compared with cotransfection of p21 with kinase-dead Pim-1. Knocking down Pim-1 expression greatly decreases the rate of proliferation of H1299 cells and their ability to grow in soft agar. These data suggest that Pim-1 overexpression may contribute to tumorigenesis in part by influencing the cellular localization and stability of p21 and by promoting cell proliferation. (Mol Cancer Res 2007;5(9):909 -22)

In this report, we employed a lentiviral RNA interference screen to discover nucleolar DEAD/DEAH-box helicases involved in RNA polymerase I (Pol I)-mediated transcriptional activity. Our screen identified DHX33 as an important modulator of 47S rRNA transcription. We show that DHX33 is a cell cycle-regulated nucleolar protein that associates with ribosomal DNA (rDNA) loci, where it interacts with the RNA Pol I transcription factor upstream binding factor (UBF). DHX33 knockdown decreased the association of Pol I with rDNA and caused a dramatic decrease in levels of rRNA synthesis. Wild-type DHX33 overexpression, but not a DNA binding-defective mutant, enhanced 47S rRNA synthesis by promoting the association of RNA polymerase I with rDNA loci. In addition, an NTPase-defective DHX33 mutant (K94R) acted as a dominant negative mutant, inhibiting endogenous rRNA synthesis. Moreover, DHX33 deficiency in primary human fibroblasts triggered a nucleolar p53 stress response, resulting in an attenuation of proliferation. Thus, we show the mechanistic importance of DHX33 in rRNA transcription and proliferation.RNA is a highly structured macromolecule whose secondary and tertiary conformations facilitate an array of specific interactions with proteins. The DEAD/DEAH-box family of RNA helicases (here referred to as DDX/DHX) (3) is one such classification of RNA binding proteins that are capable of modifying the higher-ordered structures of RNA through the hydrolysis of ATP/nucleoside triphosphate (NTP) (41). DDX/ DHX proteins often form large multiprotein complexes that participate in fundamental biological activities such as RNA transcription, RNA editing, pre-mRNA splicing, ribosome biogenesis, and RNA decay (3).DDX/DHX helicases are named and characterized by the conserved DEAD/DEAH motif common among all family members. Through site-directed mutagenesis analysis, DEAD/ DEAH along with seven conserved peptide motifs have been found to participate in ATP/NTP binding, hydrolysis, and substrate binding (28). Despite the conservation of these peptide motifs, the remaining sequences within each RNA helicase family member vary widely. Specifically, differences exist between the two categories of DDX and DHX proteins. DDX proteins contain a unique Q motif at their N termini that distinguishes them from DHX proteins. It was proposed previously that the Q motif might sense the state of ATP in vivo (40), given that DHX-box proteins are promiscuous in their ability to utilize NTP (16).Ribosome biogenesis is a complex multistep process, the majority of which occurs in the nucleolus of the cell (24, 43). The transcription of ribosomal DNA (rDNA) is the initial and rate-limiting step in ribosome biogenesis, and as such, it is influenced by multiple levels of regulation (25). One of the key regulators of rDNA transcription is the upstream binding factor (UBF), a transcriptional transactivator that binds to the upstream core element of rDNA and subsequently bends rDNA (37). This change in the rDNA structure favors the binding of SL.1 as wel...

IntroductionThe DDX21 RNA helicase has been shown to be a nucleolar and nuclear protein involved in ribosome RNA processing and AP-1 transcription. DDX21 is highly expressed in colon cancer, lymphomas, and some breast cancers, but little is known about how DDX21 might promote tumorigenesis.MethodsImmunohistochemistry was performed on a breast cancer tissue array of 187 patients. In order to study the subcellular localization of DDX21 in both tumor tissue and tumor cell lines, indirect immunofluorescence was applied. The effect of DDX21 knockdown was measured by cellular apoptosis, rRNA processing assays, soft agar growth and mouse xenograft imaging. AP-1 transcriptional activity was analyzed with a luciferase reporter and bioluminescence imaging, as well as qRT-PCR analysis of downstream target, cyclin D1, to determine the mechanism of action for DDX21 in breast tumorigenesis.ResultsHerein, we show that DDX21 is highly expressed in breast cancer tissues and established cell lines. A significant number of mammary tumor tissues and established breast cancer cell lines exhibit nuclear but not nucleolar localization of DDX21. The protein expression level of DDX21 correlates with cell proliferation rate and is markedly induced by EGF signaling. Mechanistically, DDX21 is required for the phosphorylation of c-Jun on Ser73 and DDX21 deficiency markedly reduces the transcriptional activity of AP-1. Additionally, DDX21 promotes rRNA processing in multiple breast cancer cell lines. Tumor cells expressing high levels of endogenous DDX21 undergo apoptosis after acute DDX21 knockdown, resulting in significant reduction of tumorigenicity in vitro and in vivo.ConclusionsOur findings indicate that DDX21 expression in breast cancer cells can promote AP-1 activity and rRNA processing, and thus, promote tumorigenesis by two independent mechanisms. DDX21 could serve as a marker for a subset of breast cancer patients with higher proliferation potential and may be used as a therapeutic target for a subset of breast cancer patients.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-014-0449-z) contains supplementary material, which is available to authorized users.