Activation of ribosomal RNA transcription by hepatitis C virus involves upstream binding factor phosphorylation via induction of cyclin D1.
Hepatitis C virus (HCV) causes chronic infection in humans leading to liver cirrhosis and hepatocellular carcinoma. rRNA transcription, catalyzed by RNA polymerase I (Pol I), plays a critical role in ribosome biogenesis, and changes in Pol I transcription rate are associated with profound alterations in the growth rate of the cell. Because rRNA synthesis is intimately linked to cell growth and frequently up-regulated in many cancers, we hypothesized that HCV might have the ability to activate rRNA synthesis in infected cells. We show here that rRNA promoter-mediated transcription is significantly (10- to 12-fold) activated in human liver-derived cells following infection with type 2 JFH-1 HCV or transfection with the subgenomic type 1 HCV replicon. Further analysis revealed that HCV nonstructural protein 5A (NS5A) was responsible for activation of rRNA transcription. Both the NH(2)-terminal amphipathic helix and the polyproline motifs of NS5A seem to be essential for rRNA transcription activation. The NS5A-dependent activation of rRNA transcription seems to be due to hyperphosphorylation and consequent activation of upstream binding factor (UBF), a Pol I DNA binding transcription factor. We further show that hyperphosphorylation of UBF occurs as a result of up-regulation of both cyclin D1 and cyclin-dependent kinase 4 by the HCV NS5A polypeptide. These results suggest that the endoplasmic reticulum-associated NS5A is able to transduce signals into the nucleoplasm via UBF hyperphosphorylation leading to rRNA transcription activation. These results could, at least in part, explain a mechanism by which HCV contributes to transformation of liver cells.
Raychaudhuri
Santanu
S
Department of Microbiology, Immunology, and Molecular Genetics, David Geffen school of Medicine at University of California at Los Angeles, 250C Biomedical Sciences Research Building, 615 Charles E. Young Drive South, Los Angeles, CA 90095, USA.
Fontanes
Vanessa
V
Barat
Bhaswati
B
Dasgupta
Asim
A
D002478:Cells, Cultured; D019938:Cyclin D1 / Q000032:analysis / Q000502:physiology*; D051358:Cyclin-Dependent Kinase 4 / Q000032:analysis; D004256:DNA Polymerase I / Q000235:genetics; D016174:Hepacivirus / Q000502:physiology*; D006801:Humans; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins / Q000378:metabolism*; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D014158:Transcription, Genetic; D017361:Viral Nonstructural Proteins / Q000737:chemistry / Q000502:physiology; D012324:RNA-Dependent RNA Polymerase
D019938:Cyclin D1; D051358:Cyclin-Dependent Kinase 4; D004256:DNA Polymerase I; D035463:Pol1 Transcription Initiation Complex Proteins; D012335:RNA, Ribosomal; D017361:Viral Nonstructural Proteins; C530104:CCND1 protein, human; C063844:transcription factor UBF; C495900:CDK4 protein, human; C087036:NS-5 protein, hepatitis C virus; D012324:RNA-Dependent RNA Polymerase