ABSTRACT
The nucleolus has long been considered to be a pure ribosome factory. However, over the last two decades it became clear that the nucleolus is involved in numerous other functions besides ribosome biogenesis. Our experiments indicate that the activity of RNA polymerase I (Pol I) transcription monitors the integrity of the DNA and influences the response to nucleolar stress as well as the rate of survival. Cells with a repressed ribosomal DNA (rDNA) transcription activity showed an increased and prolonged p53 stabilisation after UVC-irradiation. Furthermore, p53 stabilisation after inhibition and especially after UVC-irradiation might be due to abrogation of the HDM2-p53 degradation pathway by ribosomal proteins (RPs). Apoptosis mediated by highly activated p53 is a typical hallmark of Cockayne syndrome cells and transcriptional abnormalities and the following activation of the RP-HDM2-p53 pathway would be a possible explanation.
Subject(s)
RNA Polymerase I/metabolism , Transcription, Genetic/radiation effects , Ultraviolet Rays , Apoptosis/radiation effects , Cell Line , HCT116 Cells , Humans , Pol1 Transcription Initiation Complex Proteins/antagonists & inhibitors , Pol1 Transcription Initiation Complex Proteins/genetics , Pol1 Transcription Initiation Complex Proteins/metabolism , Protein Stability/radiation effects , Proto-Oncogene Proteins c-mdm2/metabolism , RNA Interference , RNA Polymerase I/genetics , RNA, Ribosomal/metabolism , RNA, Small Interfering/metabolism , Tumor Suppressor Protein p53/metabolismABSTRACT
In addition to performing its canonical function, Telomerase Reverse Transcriptase (TERT) has been shown to participate in cellular processes independent of telomerase activity. Furthermore, although TERT mainly localizes to Cajal bodies, it is also present within the nucleolus. Because the nucleolus is the site of rDNA transcription, we investigated the possible role of telomerase in regulating RNA polymerase I (Pol I). Here we show that TERT binds to rDNA and stimulates transcription by Pol I during liver regeneration and Ras-induced hyperproliferation. Moreover, the inhibition of telomerase activity by TERT- or TERC-specific RNA interference, the overexpression of dominant-negative-TERT, and the application of the telomerase inhibitor imetelstat reduce Pol I transcription and the growth of tumour cells. In vitro, telomerase can stimulate the formation of the transcription initiation complex. Our results demonstrate how non-canonical features of telomerase may direct Pol I transcription in oncogenic and regenerative hyperproliferation.
Subject(s)
Cell Proliferation/physiology , DNA, Ribosomal/genetics , DNA, Ribosomal/physiology , RNA Polymerase I/physiology , Telomerase/physiology , Transcription, Genetic/physiology , Animals , Cell Line , Cell Line, Tumor , Cell Proliferation/genetics , Colorectal Neoplasms/pathology , Humans , Kidney/cytology , Liver/cytology , Liver Regeneration/genetics , Liver Regeneration/physiology , Lung/cytology , Male , Mice , Mice, Inbred C57BL , Myofibroblasts/cytology , Protein Binding/physiology , RNA Polymerase I/genetics , Rabbits , Telomerase/genetics , Transcription, Genetic/geneticsABSTRACT
TFIIH is a multisubunit factor essential for transcription initiation and promoter escape of RNA polymerase II and for the opening of damaged DNA double strands in nucleotide excision repair (NER). In this study, we have analyzed at which step of the transcription cycle TFIIH is essential for transcription by RNA polymerase I. We demonstrate that TFIIH associates with the rDNA promoter and gene-internal sequences and leaves the rDNA promoter in a complex with RNA polymerase I after start of transcription. Moreover, mutations in the TFIIH subunits XPB and XPD found in Cockayne syndrome impair the interaction of TFIIH with the rDNA, but do not influence initiation complex formation or promoter escape of RNA polymerase I, but preclude the productivity of the enzyme by reducing transcription elongation in vivo and in vitro. Our results implicate that reduced RNA polymerase I transcription elongation and ribosomal stress could be one factor contributing to the Cockayne syndrome phenotype.