ABSTRACT
The retinoblastoma tumor suppressor protein (Rb) affects gene transcription both negatively and positively and through this regulates distinct cellular responses. Although cell cycle regulation requires gene repression, Rb's ability to promote differentiation and part of its antiproliferative activity appears to rely on the activation of gene transcription. We present evidence here that the RET finger protein (RFP)/tripartite motif protein 27 (TRIM 27) inhibits gene transcription activation by Rb but does not affect gene repression. RFP binds to Rb and prevents the degradation of the EID-1 inhibitor of histone acetylation and differentiation. Furthermore, ablation of RFP in U2OS osteosarcoma cells augments a transcriptional program indicative of lineage-specific differentiation in response to Rb. These findings provide precedent for a regulatory pathway that uncouples different Rb-dependent activities and thus silences specific cellular responses to Rb in a selective way.
Subject(s)
Adenovirus E1A Proteins/metabolism , DNA-Binding Proteins/metabolism , Gene Expression Regulation , Nuclear Proteins/metabolism , Retinoblastoma Protein/metabolism , Transcription, Genetic , Adenovirus E1A Proteins/genetics , Blotting, Western , Cell Cycle Proteins/antagonists & inhibitors , Cell Differentiation , Cell Line, Tumor , Cell Lineage , DNA-Binding Proteins/antagonists & inhibitors , DNA-Binding Proteins/chemistry , DNA-Binding Proteins/genetics , E2F Transcription Factors , Genes, Reporter , Glutathione Transferase/metabolism , Humans , Luciferases/metabolism , Models, Biological , Nuclear Proteins/chemistry , Nuclear Proteins/genetics , Oligonucleotide Array Sequence Analysis , Osteosarcoma , Precipitin Tests , Promoter Regions, Genetic , Protein Structure, Tertiary , RNA, Small Interfering/metabolism , Recombinant Proteins/metabolism , Repressor Proteins , Retinoblastoma Protein/genetics , Structure-Activity Relationship , Transcription Factors/antagonists & inhibitors , Two-Hybrid System TechniquesABSTRACT
In this study we show that E6 of human papillomavirus has the ability to deregulate the cell cycle G1/S transition. In rodent immortalized fibroblasts (NIH3T3) serum deprivation or over-expression of the cyclin-dependent kinase inhibitors, p16(INK4a) or p27(KIP1), leads to G1 cell cycle arrest. HPV16 E6 overcomes the antiproliferative signals, gaining the ability to drive serum-deprived and p16(INK4a) or p27(KIP1) over-expressing cells into S phase. E6 protein from the benign HPV type 1 displays a similar activity to HPV16 E6 to deregulate the G1/S transition. Thus, this activity appears to be conserved between E6 proteins from non-oncogenic and oncogenic HPV types. Furthermore, we show that HPV16 E6 is not able to circumvent a G1 arrest imposed by pRb mutant in which all CDK phosphorylation sites have been mutated. These data indicate that the viral protein acts upstream of pRb and its mechanism in promoting cell cycle progression is dependent on pRb phosphorylation. In summary, this study describes a novel biological function of HPV E6 and shows that the S phase entry, required for viral DNA replication, is not exclusively controlled by E7, but that E6 also is involved in this event.