Regulation of RNA polymerase III transcription during hypertrophic growth

Antitumor mechanisms and future clinical applications of the natural product triptolide

Triptolide (TPL) is a compound sourced from Tripterygium wilfordii Hook. F., a traditional Chinese medicinal herb recognized for its impressive anti-inflammatory, anti-angiogenic, immunosuppressive, and antitumor qualities. Notwithstanding its favorable attributes, the precise mechanism through which TPL influences tumor cells remains enigmatic. Its toxicity and limited water solubility significantly impede the clinical application of TPL. We offer a comprehensive overview of recent research endeavors aimed at unraveling the antitumor mechanism of TPL in this review. Additionally, we briefly discuss current strategies to effectively manage the challenges associated with TPL in future clinical applications. By compiling this information, we aim to enhance the understanding of the underlying mechanisms involved in TPL and identify potential avenues for further advancement in antitumor therapy.

Murine Gammaherpesvirus 68 ORF45 Stimulates B2 Retrotransposon and Pre-tRNA Activation in a Manner Dependent on Mitogen-Activated Protein Kinase (MAPK) Signaling

RNA polymerase III (RNAPIII) transcribes a variety of noncoding RNAs, including tRNA (tRNA) and the B2 family of short interspersed nuclear elements (SINEs). B2 SINEs are noncoding retrotransposons that possess tRNA-like promoters and are normally silenced in healthy somatic tissue. Infection with the murine gammaherpesvirus MHV68 induces transcription of both SINEs and tRNAs, in part through the activity of the viral protein kinase ORF36. Here, we identify the conserved MHV68 tegument protein ORF45 as an additional activator of these RNAPIII loci. MHV68 ORF45 and ORF36 form a complex, resulting in an additive induction RNAPIII and increased ORF45 expression. ORF45-induced RNAPIII transcription is dependent on its activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) signaling pathway, which in turn increases the abundance of the RNAPIII transcription factor Brf1. Other viral and nonviral activators of MAPK/ERK signaling also increase the levels of Brf1 protein, B2 SINE RNA, and tRNA, suggesting that this is a common strategy to increase RNAPIII activity. IMPORTANCE Gammaherpesviral infection alters the gene expression landscape of a host cell, including through the induction of noncoding RNAs transcribed by RNA polymerase III (RNAPIII). Among these are a class of repetitive genes known as retrotransposons, which are normally silenced elements and can copy and spread throughout the genome, and transfer RNAs (tRNAs), which are fundamental components of protein translation machinery. How these loci are activated during infection is not well understood. Here, we identify ORF45 from the model murine gammaherpesvirus MHV68 as a novel activator of RNAPIII transcription. To do so, it engages the MAPK/ERK signaling pathway, which is a central regulator of cellular response to environmental stimuli. Activation of this pathway leads to the upregulation of a key factor required for RNAPIII activity, Brf1. These findings expand our understanding of the regulation and dysregulation of RNAPIII transcription and highlight how viral cooption of key signaling pathways can impact host gene expression.

RNA polymerase III transcription and cancer: A tale of two RPC7 subunits

RNA polymerase III composition is shaped by the mutually exclusive incorporation of two paralogous subunits, RPC7α and RPC7β, encoded by genes POLR3G and POLR3GL in vertebrates. The expression of POLR3G and POLR3GL is spatiotemporally regulated during development, and multiple reports point to RPC7α-enhanced Pol III activity patterns, indicating that Pol III identity may underly dynamic Pol III transcription patterns observed in higher eukaryotes. In cancer, upregulation of POLR3G, but not POLR3GL, is associated with poor survival outcomes among patients, suggesting differences between RPC7α and RPC7β further influence disease progression and may translate into future biomarkers and therapeutic strategies. Here, we outline our current understanding of Pol III identity and transcription and reexamine the distinct protein characteristics of Pol III subunits RPC7α and RPC7β. Drawing on both structural and genomic studies, we discuss differences between RPC7α and RPC7β and the potential mechanisms by which Pol III identity may establish differential activities during development and disease.

Cross-regulome profiling of RNA polymerases highlights the regulatory role of polymerase III on mRNA transcription by maintaining local chromatin architecture

BACKGROUND

Mammalian cells have three types of RNA polymerases (Pols), Pol I, II, and III. However, the extent to which these polymerases are cross-regulated and the underlying mechanisms remain unclear.

RESULTS

We employ genome-wide profiling after acute depletion of Pol I, Pol II, or Pol III to assess cross-regulatory effects between these Pols. We find that these enzymes mainly affect the transcription of their own target genes, while certain genes are transcribed by the other polymerases. Importantly, the most active type of crosstalk is exemplified by the fact that Pol III depletion affects Pol II transcription. Pol II genes with transcription changes upon Pol III depletion are enriched in diverse cellular functions, and Pol III binding sites are found near their promoters. However, these Pol III binding sites do not correspond to transfer RNAs. Moreover, we demonstrate that Pol III regulates Pol II transcription and chromatin binding of the facilitates chromatin transcription (FACT) complex to alter local chromatin structures, which in turn affects the Pol II transcription rate.

CONCLUSIONS

Our results support a model suggesting that RNA polymerases show cross-regulatory effects: Pol III affects local chromatin structures and the FACT-Pol II axis to regulate the Pol II transcription rate at certain gene loci. This study provides a new perspective for understanding the dysregulation of Pol III in various tissues affected by developmental diseases.

tRNA dysregulation and disease

tRNAs are key adaptor molecules that decipher the genetic code during translation of mRNAs in protein synthesis. In contrast to the traditional view of tRNAs as ubiquitously expressed housekeeping molecules, awareness is now growing that tRNA-encoding genes display tissue-specific and cell type-specific patterns of expression, and that tRNA gene expression and function are both dynamically regulated by post-transcriptional RNA modifications. Moreover, dysregulation of tRNAs, mediated by alterations in either their abundance or function, can have deleterious consequences that contribute to several distinct human diseases, including neurological disorders and cancer. Accumulating evidence shows that reprogramming of mRNA translation through altered tRNA activity can drive pathological processes in a codon-dependent manner. This Review considers the emerging evidence in support of the precise control of functional tRNA levels as an important regulatory mechanism that coordinates mRNA translation and protein expression in physiological cell homeostasis, and highlights key examples of human diseases that are linked directly to tRNA dysregulation.

MAP kinases are involved in RNA polymerase III regulation upon LPS treatment in macrophages

Macrophages are transcriptionally highly dynamic cell type, rapidly adapting to a changing environment to execute innate immune functions. Activation of macrophages with lipopolysaccharides (LPS), a major component of the outer membrane of most Gram-negative bacteria, induces rapid transcriptional changes and within a few hours transcription of several hundred genes is altered. Within these genes are tRNAs, which are synthesised by RNA Polymerase (Pol) III, and whose expression is rapidly upregulated in response to LPS. However, the mechanisms that govern Pol III activation are not fully elucidated. LPS engage the Toll-like receptor (TLR) 4 and induce various signalling pathways, including mitogen-activated protein kinases (MAPK). MAPKs are serine/threonine kinases that catalyse the phosphorylation of transcription factors, protein kinases, and many other substrates including functional proteins, play a central role in mediating cellular responses to extracellular signals, including inflammatory cues. Here we show that ERK and p38 MAP kinases contribute to the activation of Pol III in macrophages stimulated with LPS. We also demonstrate that MAP kinases effector MSK1/2 kinases are involved in tRNA upregulation. Our data show that ERK, p38, and MSK kinases are required for upregulation of Pol III activity in macrophages stimulated by LPS. The possible modes of their action are discussed.

Role of betaine in inhibiting the induction of RNA Pol III gene transcription and cell growth caused by alcohol

Alcohol has been classified as carcinogenic to humans by the International Agency for Research on Cancer (IARC). Studies have demonstrated that alcohol intake increases the risk of breast cancer, and alcohol also stimulates breast cancer cell growth. Deregulation of Pol III genes is tightly associated with tumour development. Transcription factor II-B (TFIIB)-related factor 1 (Brf1) is a transcription factor that specifically regulates Pol III gene transcription. Our in vivo and in vitro studies have indicated that alcohol enhances the transcription of Pol III genes to cause an alteration of cellular phenotypes, which is closely related with human breast cancer. Betaine is a vegetable alkaloid and has antitumor functions. Most reports about betaine show that the consumption level of betaine is inversely associated with a risk of breast cancer. Although different mechanisms of betaine against tumour have been investigated, nothing has been reported on the effect of betaine on the deregulation of Brf1 and Pol III genes. In this study, we determine the role of betaine in breast cancer cell growth and colony formation and explore its mechanism. Our results indicate that alcohol increases the rates of growth and colony formation of breast cancer cells, whereas betaine is able to significantly inhibit the effects of alcohol on these cell phenotypes. Betaine decreases the induction of Brf1 expression and Pol III gene transcription caused by ethanol to reduce the rates of cell growth and colony formation. Together, these studies provide novel insights into the role of betaine in alcohol-caused breast cancer cell growth and deregulation of Brf1 and Pol III genes. These results suggest that betaine consumption is able to prevent alcohol-associated human cancer development.

The significance of Runx2 mediating alcohol-induced Brf1 expression and RNA Pol III gene transcription

Runx2 (Runt-related transcription factor 2) is a key transcription factor which is associated with osteoblast differentiation and expressed in ER+ (estrogen receptor positive) human breast cancer cell lines. Runx2 also participates in mammary gland development. Deregulation of RNA Pol III genes (polymerase III-dependent genes) is tightly linked to tumor development, while Brf1 (TFIIB-related factor 1) specifically regulates these gene transcription. However, nothing is known about the effect of Runx2 on Brf1 expression and Pol III gene transcription. Expression of Runx2, Brf1 and Pol III genes from the samples of human breast cancer and cell culture model were determined by the assays of RT-qPCR, immunoblot, luciferase reporter activity, immunohistochemistry, chromatin immunoprecipitation and Immunofluorescence. High expression of Runx2 is observed in the cases of breast cancer. The patients of high Runx2 expression at early stages display longer survival period, whereas the cases of high Runx2 at advanced stages reveal faster recurrence. The identification of signaling pathway indicates that JNK1 and c-Jun mediate Runx2 transcription. Repression of Runx2 reduces Brf1 expression and Pol III gene transcription. Further analysis indicates that Runx2 is colocalized with Brf1 in nucleus of breast cancer tissue. Both Runx2 and Brf1 synergistically modulate Pol III gene transcription. These studies indicate that Brf1 overexpression is able to be used as an early diagnosis biomarker of breast cancer, while high Runx2 expression indicates long survival period and faster recurrence. Runx2 mediates the deregulation of Brf1 and Pol III genes and its abnormal expression predicts the worse prognosis of breast cancer.

BRF1 accelerates prostate tumourigenesis and perturbs immune infiltration

BRF1 is a rate-limiting factor for RNA Polymerase III-mediated transcription and is elevated in numerous cancers. Here, we report that elevated levels of BRF1 associate with poor prognosis in human prostate cancer. In vitro studies in human prostate cancer cell lines demonstrated that transient overexpression of BRF1 increased cell proliferation whereas the transient downregulation of BRF1 reduced proliferation and mediated cell cycle arrest. Consistent with our clinical observations, BRF1 overexpression in a Pten-deficient mouse (PtenΔ/Δ BRF1Tg) prostate cancer model accelerated prostate carcinogenesis and shortened survival. In PtenΔ/Δ BRF1Tg tumours, immune and inflammatory processes were altered, with reduced tumoral infiltration of neutrophils and CD4 positive T cells, which can be explained by decreased levels of complement factor D (CFD) and C7 components of the complement cascade, an innate immune pathway that influences the adaptive immune response. We tested if the secretome was involved in BRF1-driven tumorigenesis. Unbiased proteomic analysis on BRF1-overexpresing PC3 cells confirmed reduced levels of CFD in the secretome, implicating the complement system in prostate carcinogenesis. We further identify that expression of C7 significantly correlates with expression of CD4 and has the potential to alter clinical outcome in human prostate cancer, where low levels of C7 associate with poorer prognosis.

Inhibition of tRNA Gene Transcription by the Immunosuppressant Mycophenolic Acid

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil, a drug that is widely used for immunosuppression in organ transplantation and autoimmune diseases, as well as anticancer chemotherapy. It inhibits IMP dehydrogenase, a rate-limiting enzyme in de novo synthesis of guanidine nucleotides.

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil, a drug that is widely used for immunosuppression in organ transplantation and autoimmune diseases, as well as anticancer chemotherapy. It inhibits IMP dehydrogenase, a rate-limiting enzyme in de novo synthesis of guanidine nucleotides. MPA treatment interferes with transcription elongation, resulting in a drastic reduction of pre-rRNA and pre-tRNA synthesis, the disruption of the nucleolus, and consequently cell cycle arrest. Here, we investigated the mechanism whereby MPA inhibits RNA polymerase III (Pol III) activity, in both yeast and mammalian cells. We show that MPA rapidly inhibits Pol III by depleting GTP. Although MPA treatment can activate p53, this is not required for Pol III transcriptional inhibition. The Pol III repressor MAF1 is also not responsible for inhibiting Pol III in response to MPA treatment. We show that upon MPA treatment, the levels of selected Pol III subunits decrease, but this is secondary to transcriptional inhibition. Chromatin immunoprecipitation (ChIP) experiments show that Pol III does not fully dissociate from tRNA genes in yeast treated with MPA, even though there is a sharp decrease in the levels of newly transcribed tRNAs. We propose that in yeast, GTP depletion may lead to Pol III stalling.

Brf1 loss and not overexpression disrupts tissues homeostasis in the intestine, liver and pancreas

RNA polymerase III (Pol-III) transcribes tRNAs and other small RNAs essential for protein synthesis and cell growth. Pol-III is deregulated during carcinogenesis; however, its role in vivo has not been studied. To address this issue, we manipulated levels of Brf1, a Pol-III transcription factor that is essential for recruitment of Pol-III holoenzyme at tRNA genes in vivo. Knockout of Brf1 led to embryonic lethality at blastocyst stage. In contrast, heterozygous Brf1 mice were viable, fertile and of a normal size. Conditional deletion of Brf1 in gastrointestinal epithelial tissues, intestine, liver and pancreas, was incompatible with organ homeostasis. Deletion of Brf1 in adult intestine and liver induced apoptosis. However, Brf1 heterozygosity neither had gross effects in these epithelia nor did it modify tumorigenesis in the intestine or pancreas. Overexpression of BRF1 rescued the phenotypes of Brf1 deletion in intestine and liver but was unable to initiate tumorigenesis. Thus, Brf1 and Pol-III activity are absolutely essential for normal homeostasis during development and in adult epithelia. However, Brf1 overexpression or heterozygosity are unable to modify tumorigenesis, suggesting a permissive, but not driving role for Brf1 in the development of epithelial cancers of the pancreas and gut.

Maf1 ameliorates cardiac hypertrophy by inhibiting RNA polymerase III through ERK1/2

Rationale: An imbalance between protein synthesis and degradation is one of the mechanisms of cardiac hypertrophy. Increased transcription in cardiomyocytes can lead to excessive protein synthesis and cardiac hypertrophy. Maf1 is an RNA polymerase III (RNA pol III) inhibitor that plays a pivotal role in regulating transcription. However, whether Maf1 regulates of cardiac hypertrophy remains unclear.

Methods: Cardiac hypertrophy was induced in vivo by thoracic aortic banding (AB) surgery. Both the in vivo and in vitro gain- and loss-of-function experiments by Maf1 knockout (KO) mice and adenoviral transfection were used to verify the role of Maf1 in cardiac hypertrophy. RNA pol III and ERK1/2 inhibitor were utilized to identify the effects of RNA pol III and ERK1/2. The possible interaction between Maf1 and ERK1/2 was clarified by immunoprecipitation (IP) analysis.

Results: Four weeks after surgery, Maf1 KO mice exhibited significantly exacerbated AB-induced cardiac hypertrophy characterized by increased heart size, cardiomyocyte surface area, and atrial natriuretic peptide (ANP) expression and by exacerbated pulmonary edema. Also, the deficiency of Maf1 causes more severe cardiac dilation and dysfunction than wild type (WT) mice after pressure overload. In contrast, compared with adenoviral-GFP injected mice, mice injected with adenoviral-Maf1 showed significantly ameliorated AB-induced cardiac hypertrophy. In vitro study has demonstrated that Maf1 could significantly block phenylephrine (PE)-induced cardiomyocyte hypertrophy by inhibiting RNA pol III transcription. However, application of an RNA pol III inhibitor markedly improved Maf1 knockdown-promoted cardiac hypertrophy. Moreover, ERK1/2 was identified as a regulator of RNA pol III, and ERK1/2 inhibition by U0126 significantly repressed Maf1 knockdown-promoted cardiac hypertrophy accompanied by suppressed RNA pol III transcription. Additionally, IP analysis demonstrated that Maf1 could directly bind ERK1/2, suggesting Maf1 could interact with ERK1/2 and then inhibit RNA pol III transcription so as to attenuate the development of cardiac hypertrophy.

Conclusions: Maf1 ameliorates PE- and AB-induced cardiac hypertrophy by inhibiting RNA pol III transcription via ERK1/2 signaling suppression.

Inhibition of RNA polymerase III transcription by Triptolide attenuates colorectal tumorigenesis

Background

Upregulation of RNA polymerase (Pol) III products, including tRNAs and 5S rRNA, in tumor cells leads to enhanced protein synthesis and tumor formation, making it a potential target for cancer treatment. In this study, we evaluated the inhibition of Pol III transcription by triptolide and the anti-cancer effect of this drug in colorectal tumorigenesis.

Methods

The effect of triptolide on colorectal cancer development was assessed in colorectal cancer mouse models, 3D organoids, and cultured cells. Colorectal cancer cells were treated with triptolide. Pol III transcription was measured by real-time quantitative polymerase chain reaction (PCR). The formation of TFIIIB, a multi-subunit transcription factor for Pol III, was determined by chromatin immunoprecipitation (ChIP), co-immunoprecipitation (Co-IP), and fluorescence resonance energy transfer (FRET).

Results

Triptolide reduced both tumor number and tumor size in adenomatous polyposis coli (Apc) mutated (Apc^Min/+) mice as well as AOM/DSS-induced mice. Moreover, triptolide effectively inhibited colorectal cancer cell proliferation, colony formation, and organoid growth in vitro, which was associated with decreased Pol III target genes. Mechanistically, triptolide treatment blocked TBP/Brf1interaction, leading to the reduced formation of TFIIIB at the promoters of tRNAs and 5S rRNA.

Conclusions

Together, our data suggest that inhibition of Pol III transcription with existing drugs such as triptolide provides a new avenue for developing novel therapies for colorectal cancer.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1232-x) contains supplementary material, which is available to authorized users.

Alcohol Intake and Abnormal Expression of Brf1 in Breast Cancer

Breast cancer is the most common malignant disease of females. Overall, one woman in every nine will get breast cancer at some time in her life. Epidemiological studies have indicated that alcohol consumption has most consistently been associated with breast cancer risk. However, the mechanism of alcohol-associated breast cancer remains to be addressed. Little is known about the effects of alcohol consumption on Brf1 (TFIIIB-related factor 1) expression and RNA Pol III gene (RNA polymerase III-dependent gene) transcription, which are responsible for protein synthesis and tightly linked to cell proliferation, cell transformation, and tumor development. Emerging evidences have indicated that alcohol induces deregulation of Brf1 and Pol III genes to cause the alterations of cell phenotypes and tumor formation. In this paper, we summarize the progresses regarding alcohol-caused increase in the expression of Brf1 and Pol III genes and analysis of its molecular mechanism of breast cancer. As the earlier and accurate diagnosis approach of breast cancer is not available yet, exploring the molecular mechanism and identifying the biomarker of alcohol-associated breast cancer are especially important. Recent studies have demonstrated that Brf1 is overexpressed in most ER+ (estrogen receptor positive) cases of breast cancer and the change in cellular levels of Brf1 reflects the therapeutic efficacy and prognosis of this disease. It suggests that Brf1 may be a potential diagnosis biomarker and a therapeutic target of alcohol-associated breast cancer.

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

TFIIB-related factor 1 (Brf1) modulates the transcription of RNA Pol III genes (polymerase-dependent genes). Upregulation of Pol III genes enhances tRNA and 5S RNA production and increases the translational capacity of cells to promote cell transformation and tumor development. However, the significance of Brf1 overexpression in human breast cancer (HBC) remains to be investigated. Here, we investigate whether Brf1 expression is increased in the samples of HBC, and we explore its molecular mechanism and the significance of Brf1 expression in HBC. Two hundred and eighteen samples of HBC were collected to determine Brf1 expression by cytological and molecular biological approaches. We utilized colocalization, coimmunoprecipitation, and chromatin immunoprecipitation methods to explore the interaction of Brf1 with estrogen receptor alpha (ERα). We determined how Brf1 and ERα modulate Pol III genes. The results indicated that Brf1 is overexpressed in most cases of HBC, which is associated with an ER-positive status. The survival period of the cases with high Brf1 expression is significantly longer than those with low levels of Brf1 after hormone treatment. ERα mediates Brf1 expression. Brf1 and ERα are colocalized in the nucleus. These results indicate an interaction between Brf1 and ERα, which synergistically regulates the transcription of Pol III genes. Inhibition of ERα by its siRNA or tamoxifen reduces cellular levels of Brf1 and Pol III gene expression and decreases the rate of colony formation of breast cancer cells. Together, these studies demonstrate that Brf1 is a good biomarker for the diagnosis and prognosis of HBC. This interaction of Brf1 with ERα and Brf1 itself are potential therapeutic targets for this disease.

Abnormal expression of TFIIIB subunits and RNA Pol III genes is associated with hepatocellular carcinoma

The levels of the products of RNA polymerase III-dependent genes (Pol III genes), including tRNAs and 5S rRNA, are elevated in transformed and tumor cells, which potentiate tumorigenesis. TFIIB-related factor 1 (Brf1) is a key transcription factor and specifically regulates the transcription of Pol III genes. In vivo and in vitro studies have demonstrated that a decrease in Brf1 reduces Pol III gene transcription and is sufficient for inhibiting cell transformation and tumor formation. Emerging evidence indicates that dysregulation of Brf1 and Pol III genes is linked to the development of hepatocellular carcinoma (HCC) in humans and animals. We have reported that Brf1 is overexpressed in human liver cancer patients and that those with high Brf1 levels have shorter survivals. This review summarizes the effects of dysregulation of these genes on HCC and their regulation by signaling pathways and epigenetics. These novel data should help us determine the molecular mechanisms of HCC from a different perspective and guide the development of therapeutic approaches for HCC patients.

Exploring a common mechanism of alcohol-induced deregulation of RNA Pol III genes in liver and breast cells

Alcohol intake is associated with numbers of different human cancers, such as hepatocellular carcinoma (HCC) and breast cancer. However, the molecular mechanism remains to be elucidated. RNA polymerase III-dependent genes (Pol III genes) deregulation elevates cellular production of tRNAs and 5S rRNA, resulting in an increase in translational capacity, which promote cell transformation and tumor formation. To explore a common mechanism of alcohol-associated human cancers, we have comparably analyzed that alcohol causes deregulation of Pol III genes in liver and breast cells. Our results reveal that alcohol enhances RNA Pol III gene transcription in both liver and breast cells. The induction of Pol III genes caused by alcohol in ER+ breast cancer lines or liver tumor lines are significantly higher than in their non-tumor cell lines. Alcohol increases cellular levels of Brf1 mRNA and protein, (which depeted) Brf1 is a key transcription factor and specifically regulate Pol III gene activity. Alcohol activates JNK1 to upregulate transcription of Brf1 and Pol III genes, whereas inhibition of JNK1 by SP600125 or its siRNA significantly decreases the induction of these genes. Furthermore, alcohol increases the rates of transformation of liver and breast cells, repressed JNK1 and Brf1 expression decrease transcription of Pol III genes and reduce the rates of colony formation of AML-12 and MCF-10 cells. Together, these studies support the idea that alcohol induces deregulation of Brf1 and RNA Pol III genes in liver and breast cells, which share a common signaling pathway to promote cell transformation. Through the common mechanism, alcohol-induced deregulation of RNA Pol III genes brings about greater phenotypic changes.

Correlation between Ribosome Biogenesis and the Magnitude of Hypertrophy in Overloaded Skeletal Muscle

External loads applied to skeletal muscle cause increases in the protein translation rate, which leads to muscle hypertrophy. Although some studies have demonstrated that increases in the capacity and efficiency of translation are involved in this process, it remains unclear how these two factors are related to the magnitude of muscle hypertrophy. The present study aimed to clarify the roles played by the capacity and efficiency of translation in muscle hypertrophy. We used an improved synergist ablation in which the magnitude of compensatory hypertrophy could be controlled by partial removal of synergist muscles. Male rats were assigned to four groups in which the plantaris muscle was unilaterally subjected to weak (WK), moderate (MO), middle (MI), and strong (ST) overloading by four types of synergist ablation. Fourteen days after surgery, the weight of the plantaris muscle per body weight increased by 8%, 22%, 32% and 45%, in the WK, MO, MI and ST groups, respectively. Five days after surgery, 18+28S rRNA content (an indicator of translational capacity) increased with increasing overload, with increases of 1.8-fold (MO), 2.2-fold (MI), and 2.5-fold (ST), respectively, relative to non-overloaded muscle (NL) in the WK group. rRNA content showed a strong correlation with relative muscle weight measured 14 days after surgery (r = 0.98). The phosphorylated form of p70S6K (a positive regulator of translational efficiency) showed a marked increase in the MO group, but no further increase was observed with further increase in overload (increases of 22.6-fold (MO), 17.4-fold (MI), and 18.2-fold (ST), respectively, relative to NL in the WK group). These results indicate that increases in ribosome biogenesis at the early phase of overloading are strongly dependent on the amount of overloading, and may play an important role in increasing the translational capacity for further gain of muscular size.

Alteration of BRCA1 expression affects alcohol-induced transcription of RNA Pol III-dependent genes

Emerging evidence has indicated that alcohol consumption is an established risk factor for breast cancer. Deregulation of RNA polymerase III (Pol III) transcription enhances cellular Pol III gene production, leading to an increase in translational capacity to promote cell transformation and tumor formation. We have reported that alcohol intake increases Pol III gene transcription to promote cell transformation and tumor formation in vitro and in vivo. Studies revealed that tumor suppressors, pRb, p53, PTEN and Maf1 repress the transcription of Pol III genes. BRCA1 is a tumor suppressor and its mutation is tightly related to breast cancer development. However, it is not clear whether BRCA1 expression affects alcohol-induced transcription of Pol III genes. At the present studies, we report that restoring BRCA1 in HCC 1937 cells, which is a BRCA1 deficient cell line, represses Pol III gene transcription. Expressing mutant or truncated BRCA1 in these cells does not affect the ability of repression on Pol III genes. Our analysis has demonstrated that alcohol induces Pol III gene transcription. More importantly, overexpression of BRCA1 in estrogen receptor positive (ER+) breast cancer cells (MCF-7) decreases the induction of tRNA(Leu) and 5S rRNA genes by alcohol, whereas reduction of BRCA1 by its siRNA slightly increases the transcription of the class of genes. This suggests that BRCA1 is associated with alcohol-induced deregulation of Pol III genes. These studies for the first time demonstrate the role of BRCA1 in induction of Pol III genes by alcohol and uncover a novel mechanism of alcohol-associated breast cancer.

Tamoxifen represses alcohol-induced transcription of RNA polymerase III-dependent genes in breast cancer cells

Alcohol consumption in women has been associated with an increased risk of breast cancer, particular in estrogen receptor positive (ER+) cases. Deregulation of RNA polymerase III-dependent (Pol III) transcription enhances cellular tRNAs and 5S rRNA production, leading to an increase in translational capacity to promote cell transformation and tumor formation. Our recent studies demonstrated that alcohol induces Brf1 expression and Pol III gene transcription via ER. Here, we report that Tamoxifen (Tam) inhibits the induction of Brf1 and Pol III genes in ER+ breast cancer cells. Further analysis indicates that alcohol increases c-Jun expression to upregulate the transcription of Brf1 and Pol III genes, whereas Tam reduces c-Jun expression to repress the transcription of Brf1. Repression of cJun decreases cellular levels of ERα and Brf1. Alcohol-dependent increased occupancy of Brf1 in Pol III gene promoters is reduced by Tam. The repression of Brf1 and Pol III genes by Tam reduces alcohol-induced cell proliferation and colony formation. Together, these results indicate that Tam inhibits alcohol-induced Brf1 expression through c-Jun and ERα to downregulate Pol III gene transcription. Our studies uncover a new mechanism of Tam-treated ER+ breast cancer, by which Tam inhibits tumor growth through repressing Pol III gene transcription.

Why should cancer biologists care about tRNAs? tRNA synthesis, mRNA translation and the control of growth

Transfer RNAs (tRNAs) are essential for mRNA translation. They are transcribed in the nucleus by RNA polymerase III and undergo many modifications before contributing to cytoplasmic protein synthesis. In this review I highlight our understanding of how tRNA biology may be linked to the regulation of mRNA translation, growth and tumorigenesis. First, I review how oncogenes and tumour suppressor signalling pathways, such as the PI3 kinase/TORC1, Ras/ERK, Myc, p53 and Rb pathways, regulate Pol III and tRNA synthesis. In several cases, this regulation contributes to cell, tissue and body growth, and has implications for our understanding of tumorigenesis. Second, I highlight some recent work, particularly in model organisms such as yeast and Drosophila, that shows how alterations in tRNA synthesis may be not only necessary, but also sufficient to drive changes in mRNA translation and growth. These effects may arise due to both absolute increases in total tRNA levels, but also changes in the relative levels of tRNAs in the overall pool. Finally, I review some recent studies that have revealed how tRNA modifications (amino acid acylation, base modifications, subcellular shuttling, and cleavage) can be regulated by growth and stress cues to selectively influence mRNA translation. Together these studies emphasize the importance of the regulation of tRNA synthesis and modification as critical control points in protein synthesis and growth. This article is part of a Special Issue entitled: Translation and Cancer.

Ribosome biogenesis: emerging evidence for a central role in the regulation of skeletal muscle mass

The ribosome is a supramolecular ribonucleoprotein complex that functions at the heart of the translation machinery to convert mRNA into protein. Ribosome biogenesis is the primary determinant of translational capacity of the cell and accordingly has an essential role in the control of cell growth in eukaryotes. Cumulative evidence supports the hypothesis that ribosome biogenesis has an important role in the regulation of skeletal muscle mass. The purpose of this review is to, first, summarize the main mechanisms known to regulate ribosome biogenesis and, second, put forth the hypothesis that ribosome biogenesis is a central mechanism used by skeletal muscle to regulate protein synthesis and control skeletal muscle mass in response to anabolic and catabolic stimuli. The mTORC1 and Wnt/β-catenin/c-myc signaling pathways are discussed as the major pathways that work in concert with each of the three RNA polymerases (RNA Pol I, II, and III) in regulating ribosome biogenesis. Consistent with our hypothesis, activation of these two pathways has been shown to be associated with ribosome biogenesis during skeletal muscle hypertrophy. Although further study is required, the finding that ribosome biogenesis is altered under catabolic states, in particular during disuse atrophy, suggests that its activation represents a novel therapeutic target to reduce or prevent muscle atrophy. Lastly, the emerging field of ribosome specialization is discussed and its potential role in the regulation of gene expression during periods of skeletal muscle plasticity.

MYC regulation of cell growth through control of transcription by RNA polymerases I and III

MYC's tumorigenic potential involves increased ribosome biogenesis and translational capacity, which supply the cell with protein required for enhanced cell growth and subsequent cell division. In addition to activation of protein-encoding genes transcribed by RNA polymerase II, MYC must stimulate transcription by RNA polymerase I and RNA polymerase III to meet this synthetic demand. In the past decade our knowledge of the mechanisms and importance of MYC regulation of RNA polymerases I and III has flourished. Here we discuss MYC's influence on transcription by these "odd" RNA polymerases and the physiological impact of this regulation is evaluated with relevance to cancer development and treatment.

Role of phosphorylated histone H3 serine 10 in DEN-induced deregulation of Pol III genes and cell proliferation and transformation

The products of Pol III genes (RNA polymerase III-dependent genes), such as tRNAs and 5S rRNA, are elevated in both transformed and tumor cells suggesting that they play a crucial role in tumorigenesis. An increase in Brf1 (TFIIIB-related factor 1), a subunit of TFIIIB, augments Pol III gene transcription and is sufficient for cell transformation and tumor formation. We have demonstrated that enhancement of Brf1 and Pol III gene expression is associated with the occurrences of hepatocellular carcinoma (HCC) in mice. This suggests that Brf1 may be a key molecule during HCC development. Diethylnitrosamine (DEN), a chemical carcinogen, has been used to induce HCC in rodents. To determine the role of Brf1 and the epigenetic-regulating events in cell proliferation and transformation, hepatocytes were treated with DEN. The results indicate that DEN increases proliferation and transformation of AML-12 cells. DEN enhanced Brf1 expression and tRNA(Leu) and 5S rRNA transcription, as well as H3S10ph (phosphorylation of histone H3 serine 10). Interestingly, DEN-induced Pol III gene transcription and H3S10ph in tumor cells of liver are significantly higher than in non-tumor cells. Inhibition of H3S10ph by H3S10A attenuates the induction of Brf1 and Pol III genes. Further analysis indicates that H3S10ph occupies the promoters of Brf1 and Pol III genes to modulate their expression. Blocking H3S10ph represses cell proliferation and transformation. These results demonstrate that DEN induces H3S10ph, which mediate Brf1 expression, including but not limited Brf1-dependent genes, to upregulate Pol III gene transcription, resulting in an increase in cell proliferation and transformation.

Elk1 and AP-1 sites in the TBP promoter mediate alcohol-induced deregulation of Pol III-dependent genes

The major risk factors for hepatocellular carcinoma (HCC) are chronic liver diseases that include hepatitis B, hepatitis C, alcoholic liver disease and non-alcoholic steatohepatitis. However, the mechanisms of alcohol-associated HCC remain to be elucidated. The products of RNA Pol III (RNA polymerase III) dependent genes are elevated in both transformation cells and tumor cells. TBP (TATA-box binding protein) is a central transcription factor, which regulates Pol I, Pol II and Pol III gene activity. Our studies have demonstrated that alcohol increases TBP expression and Pol III gene transcription to promote liver tumor formation. We continue to investigate how ethanol mediates TBP expression. Here, we report that ethanol induces TBP promoter activity and the induction is ethanol dose dependent. Blocking the JNK1 pathway by a chemical inhibitor and siRNA reduces this ethanol-induced activity. Furthermore, mutating G>A at a -46 bp Elk1 binding site of the TBP promoter or mutating AP-1 binding site at -37 bp (A>G) and -38 bp (C>T) reduces the TBP promoter activity. Mutation of both Elk1 and AP-1 binding sites dramatically represses this induction. Together, these studies demonstrate that, for the first time, alcohol increases Pol III gene transcription through a response element, which is composed of the overlapping Elk1 and AP-1 binding sites of the TBP promoter and affected by alcohol. It suggests that these binding sites may play a critical role in alcohol-induced deregulation of Pol III genes in liver tumor development.

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.

ERα mediates alcohol-induced deregulation of Pol III genes in breast cancer cells

The association of alcohol consumption and breast cancer is more pronounced in cases that are positive for estrogen receptor (ER+) than in cases that are negative (ER-). Its mechanism remains to be determined. Deregulation of RNA polymerase III (Pol III) transcription enhances cellular tRNAs and 5S rRNA production, increasing translational capacity to promote cell transformation and tumor formation. Here, we report that alcohol increases Pol III gene transcription in both normal and cancer breast cell lines. The induction in ER+ breast cancer cells (MCF-7) is significantly higher than in ER- normal breast cells (MCF-10A, MCF-10F and MCF-12A) and is correlated with ER expression. E2 causes <2-fold increase in Pol III gene transcription. The addition of ethanol to this system now produces a 10-15-fold increase. Ethanol increases ERα expression, resulting in an increase in Brf1 protein and mRNA levels. In addition, ethanol markedly stimulates phosphorylation of JNK1, but not JNK2. Inhibition of JNK1 decreases ERE-Luc reporter activity and represses expression of ERα, Brf1 and Pol III genes. Reduction of ERα by its small interfering RNA represses Brf1 and Pol III gene transcription. Ethanol with E2 produces larger and more numerous colonies. Repression of ERα or Brf1 inhibits alcohol-induced cell transformation. Together, these results support the idea that alcohol increases ERα expression through JNK1 to elevate Brf1 expression and Pol III gene transcription to bring about greater phenotypic changes. These studies demonstrate that ERα mediates Pol III gene transcription through Brf1, suggesting that ERα may play a critical role in alcohol-induced deregulation of Pol III genes in ER+ breast cancer development.

Hepatitis B virus X protein and c-Myc cooperate in the upregulation of ribosome biogenesis and in cellular transformation

Viral and cellular oncogenes are well known to enhance rRNA synthesis, leading to increased ribosome biogenesis and cell proliferation. Our study on the molecular underpinnings of the interaction between viral HBx and c-Myc, which is implicated in the development of hepatocellular carcinoma, showed a marked increase in the biosynthesis of rRNA, ribosomes and protein in hepatoma cells. A profound alteration in the nucleolar morphology and biochemical content of these cells was also observed. Increased biosynthetic activity was associated with increased cell proliferation and transformation of immortalized human hepatocytes. Furthermore, inhibition of RNA polymerase III activity impaired the proliferative advantage of hepatoma cells and transformation of immortalized hepatocytes as effectively as cisplatin treatment. These findings were corroborated in a transgenic HBx-myc microenvironment, in which an elevated hepatic level of rRNA was associated with conspicuous morphological and biochemical changes in the hepatocytic nucleoli. Thus, HBx and c-Myc seem to work cooperatively to support ribosome biogenesis and cellular transformation.

Role of D-type cyclins in heart development and disease

A defining feature of embryonic cardiomyocytes is their relatively high rates of proliferation. A gradual reduction in proliferative capacity throughout development culminates in permanent cell cycle exit by the vast majority of cardiomyocytes around the perinatal period. Accordingly, the adult heart has severely limited capacity for regeneration in response to injury or disease. The D-type cyclins (cyclin D1, D2, and D3) along with their catalytically active partners, the cyclin dependent kinases, are positive cell cycle regulators that play important roles in regulating proliferation of cardiomyocytes during normal heart development. While expression of D-type cyclins is generally low in the adult heart, expression levels are augmented in association with cardiac hypertrophy, but are uncoupled from myocyte cell division. Accordingly, re-activation of D-type cyclin expression in the adult heart has been implicated in pathophysiological processes via mechanisms distinct from those that drive proliferation during cardiac development. Growth factors and other exogenous agents regulate D-type cyclin production and activity in embryonic and adult cardiomyocytes. Understanding differences in the precise intracellular mediators downstream from these signalling molecules in embryonic versus adult cardiomyocytes could prove valuable for designing strategies to reactivate the cell cycle in cardiomyocytes in the setting of cardiovascular disease in the adult heart.

Therapeutic potential of c-Myc inhibition in the treatment of hypertrophic cardiomyopathy

Investigating the pathophysiological importance of the molecular and mechanical development of cardiomyopathy is critical to find new and broader means of protection against this disease that is increasing in prevalence and impact. The current available treatment options for cardiomyopathy mainly focus on treating symptoms and strive to make the patient more comfortable while preventing progression of disease and sudden death. The proto-oncogene c-Myc (Myc) has been shown to be increased in many different types of heart disease, including hypertrophic cardiomyopathy, before any signs of the disease are present. As the mechanisms of action and multiple pathways of dependent actions of Myc are being dissected by many research groups, inhibition of Myc is becoming an attractive paradigm for prevention and treatment of cardiomyopathy and heart failure. Elucidating the role Myc plays in the development, propagation and perpetuation of cardiomyopathy and heart failure will one day translate into potential therapeutics for cardiomyopathy.

Molecular mechanisms of cardiomyocyte aging

Western societies are rapidly aging, and cardiovascular diseases are the leading cause of death. In fact, age and cardiovascular diseases are positively correlated, and disease syndromes affecting the heart reach epidemic proportions in the very old. Genetic variations and molecular adaptations are the primary contributors to the onset of cardiovascular disease; however, molecular links between age and heart syndromes are complex and involve much more than the passage of time. Changes in CM (cardiomyocyte) structure and function occur with age and precede anatomical and functional changes in the heart. Concomitant with or preceding some of these cellular changes are alterations in gene expression often linked to signalling cascades that may lead to a loss of CMs or reduced function. An understanding of the intrinsic molecular mechanisms underlying these cascading events has been instrumental in forming our current understanding of how CMs adapt with age. In the present review, we describe the molecular mechanisms underlying CM aging and how these changes may contribute to the development of cardiovascular diseases.

Phosphorylation of histone H3 serine 28 modulates RNA polymerase III-dependent transcription

Deregulation of RNA polymerase III (Pol III) transcription enhances cellular tRNAs and 5S rRNA production, leading to an increase in translational capacity to promote cell proliferation, transformation and tumor formation. Phosphorylation of histone H3 (H3ph) is induced by tumor promoters (EGF, UV and TPA) and immediate early genes, such as c-myc, c-jun and c-fos. However, it remains to be determined whether H3ph is involved in RNA Pol III transcription. Here, we report that EGF strongly induced H3ph at serine 28 (H3S28ph). EGF significantly increased transcription of RNA Pol III-dependent genes (Pol III genes), tRNA^Leu, tRNA^Tyr, 5S rRNA, and 7SL RNA. Inhibition of EGFR, but not PI3K, reduced both H3S28ph and tRNA^Leu and 5S rRNA transcription. EGF enhanced occupancy of H3S28ph in the promoters of tRNA^Leu and 5S rRNA. Further analysis indicates that EGF augmented cellular levels of protein and mRNA of TFIIIB subunits, Brf1 and TBP. Brf1 is a specific transcription factor for RNA Pol III genes. EGF enhanced occupancy of H3S28ph in the Brf1 and TBP promoters. Inhibition of H3S28ph by mutant H3S28A repressed Brf1, TBP and tRNA^Leu and 5S rRNA expression and decreased occupancy of H3S28ph in their promoters. Reduction of Brf1 significantly decreased tRNA^Leu and 5S rRNA transcription and repressed EGF-induced anchorage-independent growth. Blocking H3S28ph signaling by using mutant H3S28A reduced EGF-induced cell transformation. Together, these results indicate that EGF activates EGFR signaling to induce H3S28ph, which, in turn, upregulates tRNA^Leu and 5S rRNA transcription through Brf1 and TBP and promotes cell transformation. The studies demonstrate that epigenetic modification of H3S28ph plays a critical role in the activity of Pol III genes.

Recent insights into regulation of transcription by RNA polymerase III and the cellular functions of its transcripts

The products of transcription by the multisubunit enzyme RNA polymerase III (Pol III), such as 5S rRNA, tRNAs, U6 snRNA, are important for cell growth, proliferation and differentiation. The known range of the Pol III transcriptome has expanded over recent years, and novel functions of the newly discovered and already well known transcripts have been identified, including regulation of stress responses and apoptosis. Furthermore, transcription by Pol III has turned out to be strongly regulated, differing between diverse class III genes, among cell types and under stress conditions. The mechanisms involved in regulation of Pol III transcription are being elucidated and disturbances in that regulation have been implicated in various diseases, including cancer. This review summarizes the novel data on the regulation of RNA polymerase III transcription, including epigenetic and gene specific mechanisms and outlines recent insights into the cellular functions of the Pol III transcriptome, in particular of SINE RNAs.

Cell growth- and differentiation-dependent regulation of RNA polymerase III transcription

RNA polymerase III transcribes small untranslated RNAs that fulfill essential cellular functions in regulating transcription, RNA processing, translation and protein translocation. RNA polymerase III transcription activity is tightly regulated during the cell cycle and coupled to growth control mechanisms. Furthermore, there are reports of changes in RNA polymerase III transcription activity during cellular differentiation, including the discovery of a novel isoform of human RNA polymerase III that has been shown to be specifically expressed in undifferentiated human H1 embryonic stem cells. Here, we review major regulatory mechanisms of RNA polymerase III transcription during the cell cycle, cell growth and cell differentiation.

Regulation of TFIIIB during F9 cell differentiation

BACKGROUND

Differentiation of F9 embryonal carcinoma (EC) cells into parietal endoderm (PE) provides a tractable model system for studying molecular events during early and inaccessible stages of murine development. PE formation is accompanied by extensive changes in gene expression both in vivo and in culture. One of the most dramatic is the ~10-fold decrease in transcriptional output by RNA polymerase (pol) III. This has been attributed to changes in activity of TFIIIB, a factor that is necessary and sufficient to recruit pol III to promoters. The goal of this study was to identify molecular changes that can account for the low activity of TFIIIB following F9 cell differentiation.

RESULTS

Three essential subunits of TFIIIB decrease in abundance as F9 cells differentiate; these are Brf1 and Bdp1, which are pol III-specific, and TBP, which is also used by pols I and II. The decreased levels of Brf1 and Bdp1 proteins can be explained by reduced expression of the corresponding mRNAs. However, this is not the case for TBP, which is regulated post-transcriptionally. In proliferating cells, pol III transcription is stimulated by the proto-oncogene product c-Myc and the mitogen-activated protein kinase Erk, both of which bind to TFIIIB. However, c-Myc levels fall during differentiation and Erk becomes inactive through dephosphorylation. The diminished abundance of TFIIIB is therefore likely to be compounded by changes to these positive regulators that are required for its full activity. In addition, PE cells have elevated levels of the retinoblastoma protein RB, which is known to bind and repress TFIIIB.

CONCLUSION

The low activity of TFIIIB in PE can be attributed to a combination of changes, any one of which could be sufficient to inhibit pol III transcription. Declining levels of essential TFIIIB subunits and of activators that are required for maximal TFIIIB activity are accompanied by an increase in a potent repressor of TFIIIB. These events provide fail-safe guarantees to ensure that pol III output is appropriate to the diminished metabolic requirements of terminally differentiated cells.

The JNKs differentially regulate RNA polymerase III transcription by coordinately modulating the expression of all TFIIIB subunits

RNA polymerase (pol) III-dependent transcription is subject to stringent regulation by tumor suppressors and oncogenic proteins and enhanced RNA pol III transcription is essential for cellular transformation and tumorigenesis. Since the c-Jun N-terminal kinases (JNKs) display both oncogenic and tumor suppressor properties, the roles of these proteins in regulating RNA pol III transcription were examined. In both mouse and human cells, loss or reduction in JNK1 expression represses RNA pol III transcription. In contrast, loss or reduction in JNK2 expression induces transcription. The JNKs coordinately regulate expression of all 3 TFIIIB subunits. While JNK1 positively regulates TBP expression, the RNA pol III-specific factors, Brf1 and Bdp1, JNK2 negatively regulates their expression. Brf1 is coregulated with TBP through the JNK target, Elk-1. Reducing Elk-1 expression decreases Brf1 expression. Decreasing JNK1 expression reduces Elk-1 occupancy at the Brf1 promoter, while decreasing JNK2 expression enhances recruitment of Elk-1 to the Brf1 promoter. In contrast, regulation of Bdp1 occurs through JNK-mediated alterations in TBP expression. Altered TBP expression mimics the effect of reduced JNK1 or JNK2 levels on Bdp1 expression. Decreasing JNK1 expression reduces the occupancy of TBP at the Bdp1 promoter, while decreasing JNK2 expression enhances recruitment of TBP to the Bdp1 promoter. Together, these results provide a molecular mechanism for regulating RNA pol III transcription through the coordinate control of TFIIIB subunit expression and elucidate opposing functions for the JNKs in regulating a large class of genes that dictate the biosynthetic capacity of cells.

Regulation by c-Myc of ncRNA expression

Deregulated activity of the proto-oncogene product c-Myc is instrumental in promoting many human cancers. As it is a transcription factor, priority has been given to identifying the genes that it regulates. Until recently, all the attention was focused on protein-encoding genes. It is now clear, however, that c-Myc also controls the production of many non-coding (nc) RNAs, including tRNA, rRNA and miRNAs. This involves it regulating the transcriptional activity of three different RNA polymerases. These ncRNAs are likely to contribute substantially to the complex biology and pathology that is associated with c-Myc.

RNA polymerases I and III, non-coding RNAs and cancer

Oncogenically transformed cells overexpress the non-coding RNAs, such as pre-ribosomal RNA (rRNA) and transfer RNA (tRNA), which are produced by RNA polymerases (Pols) I and III. Recent results indicate that levels of pre-rRNA have prognostic value and that a tRNA has oncogenic potential. Transcription by Pols I and III is restrained in healthy cells by the tumour suppressors RB, p53, ARF and PTEN. Such restraints are compromised during cell transformation and the problem is accentuated by oncogene products, such as c-Myc, that stimulate the output of Pol I and Pol III. The resultant increases in rRNA and tRNA expression might promote the generation of cancers.

Differential expression of the TFIIIB subunits Brf1 and Brf2 in cancer cells

Background

RNA polymerase (pol) III transcription is specifically elevated in a variety of cancers and is a target of regulation by a variety of tumor suppressors and oncogenes. Accurate initiation by RNA pol III is dependent on TFIIIB. In higher eukaryotes, two forms of TFIIIB have been characterized. TFIIIB required for proper initiation from gene internal RNA pol III promoters is comprised of TBP, Bdp1, and Brf1. Proper initiation from gene external RNA pol III promoters requires TBP, Bdp1, and Brf2. We hypothesized that deregulation of RNA polymerase III transcription in cancer may be a consequence of altered TFIIIB expression

Results

Here, we report: (1) the TFIIIB subunits Brf1 and Brf2 are differentially expressed in a variety of cancer cell lines: (2) the Brf1 and Brf2 promoters differ in activity in cancer cell lines, and (3) VAI transcription is universally elevated, as compared to U6, in breast, prostate and cervical cancer cells.

Conclusion

Deregulation of TFIIIB-mediated transcription may be an important step in tumor development. We demonstrate that Brf1 and Brf2 mRNA are differentially expressed in a variety of cancer cells and that the Brf2 promoter is more active than the Brf1 promoter in all cell lines tested. We also demonstrate, that Brf1-dependent VAI transcription was significantly higher than the Brf2-dependent U6 snRNA transcription in all cancer cell lines tested. The data presented suggest that Brf2 protein expression levels correlate with U6 promoter activity in the breast, cervical and prostate cell lines tested. Interestingly, the Brf1 protein levels did not vary considerably in HeLa, MCF-7 and DU-145 cells, yet Brf1 mRNA expression varied considerably in breast, prostate and cervical cancer cell lines tested. Thus, Brf1 promoter activity and Brf1 protein expression levels did not correlate well with Brf1-dependent transcription levels. Taken together, we reason that deregulation of Brf1 and Brf2 expression could be a key mechanism responsible for the observed deregulation of RNA pol III transcription in cancer cells.

Enhanced RNA polymerase III-dependent transcription is required for oncogenic transformation

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 TATA-binding 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.

Elevated tRNA(iMet) synthesis can drive cell proliferation and oncogenic transformation

Characteristics of transformed and tumor cells include increased levels of protein synthesis and elevated expression of RNA polymerase (pol) III products, such as tRNAs and 5S rRNA. However, whether deregulated pol III transcription contributes to transformation has been unclear. Generating cell lines expressing an inducible pol III-specific transcription factor, Brf1, allowed us to raise tRNA and 5S rRNA levels specifically. Brf1 induction caused an increase in cell proliferation and oncogenic transformation, whereas depletion of Brf1 impeded transformation. Among the gene products induced by Brf1 is the tRNA(iMet) that initiates polypeptide synthesis. Overexpression of tRNA(iMet) is sufficient to stimulate cell proliferation and allow immortalized fibroblasts to form foci in culture and tumors in mice. The data indicate that elevated tRNA synthesis can promote cellular transformation.

A cyclin D2-Rb pathway regulates cardiac myocyte size and RNA polymerase III after biomechanical stress in adult myocardium

Normally, cell cycle progression is tightly coupled to the accumulation of cell mass; however, the mechanisms whereby proliferation and cell growth are linked are poorly understood. We have identified cyclin (Cyc)D2, a G(1) cyclin implicated in mediating S phase entry, as a potential regulator of hypertrophic growth in adult post mitotic myocardium. To examine the role of CycD2 and its downstream targets, we subjected CycD2-null mice to mechanical stress. Hypertrophic growth in response to transverse aortic constriction was attenuated in CycD2-null compared with wild-type mice. Blocking the increase in CycD2 in response to hypertrophic agonists prevented phosphorylation of CycD2-target Rb (retinoblastoma gene product) in vitro, and mice deficient for Rb had potentiated hypertrophic growth. Hypertrophic growth requires new protein synthesis and transcription of tRNA genes by RNA polymerase (pol) III, which increases with hypertrophic signals. This load-induced increase in RNA pol III activity is augmented in Rb-deficient hearts. Rb binds and represses Brf-1 and TATA box binding protein (TBP), subunits of RNA pol III-specific transcription factor B, in adult myocardium under basal conditions. However, this association is disrupted in response to transverse aortic constriction. RNA pol III activity is unchanged in CycD2(-/-) myocardium after transverse aortic constriction, and there is no dissociation of TBP from Rb. These investigations identify an essential role for the CycD2-Rb pathway as a governor of cardiac myocyte enlargement in response to biomechanical stress and, more fundamentally, as a regulator of the load-induced activation of RNA pol III.

Regulation of RNA polymerase III transcription by Maf1 in mammalian cells

RNA polymerase (pol) III produces essential components of the biosynthetic machinery; therefore, its output is tightly coupled with the rate of cell growth and proliferation. In Saccharomyces cerevisiae, Maf1 is an essential mediator of pol III repression in response to starvation. We demonstrate that a Maf1 ortholog is also used to restrain pol III activity in mouse and human cells. Mammalian Maf1 represses pol III transcription in vitro and in transfected fibroblasts. Furthermore, genetic deletion of Maf1 elevates pol III transcript expression, thus confirming the role of endogenous Maf1 as an inhibitor of mammalian pol III output. Maf1 is detected at chromosomal pol III templates in rodent and human cells. It interacts with pol III as well as its associated initiation factor TFIIIB and is phosphorylated in a serum-sensitive manner in vivo. These aspects of Maf1 function have been conserved between yeast and mammals and are therefore likely to be of fundamental importance in controlling pol III transcriptional activity.

Mutation of RNA Pol III subunit rpc2/polr3b Leads to Deficiency of Subunit Rpc11 and disrupts zebrafish digestive development

The role of RNA polymerase III (Pol III) in developing vertebrates has not been examined. Here, we identify a causative mutation of the second largest Pol III subunit, polr3b, that disrupts digestive organ development in zebrafish slim jim (slj) mutants. The slj mutation is a splice-site substitution that causes deletion of a conserved tract of 41 amino acids in the Polr3b protein. Structural considerations predict that the slj Pol3rb deletion might impair its interaction with Polr3k, the ortholog of an essential yeast Pol III subunit, Rpc11, which promotes RNA cleavage and Pol III recycling. We engineered Schizosaccharomyces pombe to carry an Rpc2 deletion comparable to the slj mutation and found that the Pol III recovered from this rpc2-delta yeast had markedly reduced levels of Rpc11p. Remarkably, overexpression of cDNA encoding the zebrafish rpc11 ortholog, polr3k, rescued the exocrine defects in slj mutants, indicating that the slj phenotype is due to deficiency of Rpc11. These data show that functional interactions between Pol III subunits have been conserved during eukaryotic evolution and support the utility of zebrafish as a model vertebrate for analysis of Pol III function.

Direct detection of RNA transcription by FRET imaging using fluorescent protein probe

We have constructed a reporter system for intracellular direct detection of RNA transcription that consists of two biomolecular components. The first part is a GFP-based recombinant protein probe (YRG0C-11ad) containing the RNA-binding Rev-peptide between ECFP and EYFP. The second component is RRE-RNA, which specifically binds to the Rev-peptide. Cells stably expressing YRG0C-11ad were identified by an increased FRET signal after direct transfection or intracellular transcription of RRE-RNA. In addition, the signal increase is more noticeable if tandemly repeated RRE-RNA is used as the reporter. Untranslatable non-coding RNAs are regarded as regulators of cellular gene expression, but they are difficult to study using indirect reporter systems that are dependent on translational products. Direct detection of reporter RNA would be a useful method for the detection of intracellular promoter activity during transcription of untranslatable RNAs.