Enhanced RNA Polymerase III-dependent Transcription Is Required for Oncogenic Transformation*

Abstract

RNA polymerase (pol) III transcription, responsible for the synthesis of various stable RNAs, including 5 S rRNAs and tRNAs, is regulated by oncogenic proteins and tumor suppressors. Although it is well established that RNA pol III-dependent transcription is deregulated in transformed cells and malignant tumors, it has not been determined whether this represents a cause or consequence of these processes. We show that Rat1a fibroblasts undergoing oncogenic transformation by the TATAbinding protein or c-Myc display enhanced RNA pol III transcription. Decreased expression of the RNA pol III-specific transcription factor Brf1 prevented this increase in RNA pol III transcription. Although the overall proliferation rates of these cells remained unchanged, the ability of cells to grow in an anchorage-independent manner and form tumors in mice was markedly reduced. Although overexpression of Brf1 modestly stimulated RNA pol III transcription, expression of a phosphomimic, Brf1-T145D, more significantly induced transcription. However, these increases in transcription were not sufficient to promote cellular transformation. Together, these results demonstrate that enhanced RNA pol III transcription is essential for anchorage-independent growth and tumorigenesis and that these events can be uncoupled from effects on anchorage-dependent proliferation. RNA polymerase (pol)2 III synthesizes a variety of small untranslated RNAs, including tRNA, 5 S rRNA, U6 RNA, and 7SL RNA. RNA pol III products play essential roles in protein synthesis, RNA processing, and protein trafficking. rRNA synthesis by RNA pol I and pol III is a limiting step in ribosome accumulation (1), and the overall rate of translation is determined by the number of ribosomes (2). Thus, the rate of RNA pol I and pol III transcription dictates the biosynthetic capacity of cells and may be an important determinant of a cell's capacity to grow. Consistent with this idea, decreasing RNA pol I transcription has been shown to result in reduced cellular growth rates (3). Recent results showed that decreased RNA pol III transcription by the bacterial compound Tagetin prevented hypertropic enlargement of cardiomyocytes (4). Activation of RNA pol III-dependent transcription may thus be needed for increased cell mass.RNA pol III transcription is regulated by a variety of tumor suppressors and oncogenes. Oncogenic Ras (5), c-Myc (6), and activated phosphatidylinositol 3-kinase (7) all serve to induce transcription, whereas the tumor suppressors Rb (8), p53 (9), and PTEN (7) repress RNA pol III transcription. The RNA pol III-specific transcription factor (TF) IIIB complex, composed of TATA-binding protein (TBP), Brf1, and Bdp1, is the target of these regulatory proteins. Both Rb and p53 interact directly with components of the TFIIIB complex to inhibit its function, whereas c-Myc stimulates RNA pol III transcription by directly associating with the TFIIIB complex and enhancing its recruitment to promoters. In contrast, through its ability to regulate phosphatidylinositol 3-ki...

Role of Brf1 interaction with ERα, and significance of its overexpression, in human breast cancer

“…Repression of ERα decreases ethanol‐caused induction of Brf1 and Pol III genes (Figs 4 and 6), whereas human sample studies have shown that Brf1 overexpression is significantly linked to ER+ and PR+ statuses of HBC cases (Figs 1 and 4A, Table 1). Increase in transcription of Brf1 and Pol III genes promotes tumor formation (Johnson et al ., 2008; Zhong et al ., 2011), while high Brf1 expression results in shorter survival period for the patients of hepatocellular carcinoma (Zhong et al ., 2016). However, the HBC cases with low Brf1 show worse prognosis, compared to those with high Brf1 expression (Fig.…”

“…tRNA and 5S rRNA control the translational and growth capacity of cells (Goodfellow et al ., 2006; White, 2004). Studies have indicated that oncogenic proteins, such as c‐Myc, c‐Jun, c‐Fos, and Ras, increase Pol III gene transcription (Goodfellow et al ., 2006; Johnson et al ., 2008; Zhang et al ., 2011, 2013; Zhong and Johnson, 2007; Zhong and Johnson, 2009; Zhong et al ., 2011). In contrast, tumor suppressors, such as BRCA1, p53, PTEN, and pRB, decrease transcription of these genes (Johnson et al ., 2008; White, 2004; Woiwode et al ., 2008; Zhong et al ., 2015).…”

“…The products of the tRNA and 5S rRNA genes are elevated in both transformed cells and tumor cells and biopsies of human cancer, suggesting that they play a crucial role in tumorigenesis (Johnson et al ., 2008; Woiwode et al ., 2008; Zhang et al ., 2011, 2013; Zhong and Johnson, 2007, 2009; Zhong et al ., 2011). A decrease in Brf1 expression reduces Pol III gene transcription and is sufficient for repressing cell transformation and xenograft tumor formation (Johnson et al ., 2008; Woiwode et al ., 2008; Zhong et al ., 2013a, 2015, 2016). Alcohol increases Brf1 expression to upregulate Pol III gene transcription in vivo and in vitro (Zhang et al ., 2013; Zhong et al ., 2011).…”

Covalent Small Ubiquitin-like Modifier (SUMO) Modification of Maf1 Protein Controls RNA Polymerase III-dependent Transcription Repression

“…RNA pol III products have important functions in protein synthesis, protein trafficking, and RNA processing. Enhanced expression of these genes is a hallmark of human cancers, and this has been shown to be required for cellular transformation and tumorigenesis (1,2). Maf1 is a transcription repressor that was first characterized in Saccharomyces cerevisiae as a central node for repression of RNA pol III-dependent gene expression (3,4).…”

“…RNA polymerase (pol) 2 III transcribes a variety of small untranslated RNAs, including tRNAs, 5 S rRNAs, U6 RNA, and 7SL RNA. RNA pol III products have important functions in protein synthesis, protein trafficking, and RNA processing.…”