ABSTRACT
The activity of the CDK inhibitor p21 is associated with diverse biological activities, including cell proliferation, senescence, and tumorigenesis. However, the mechanisms governing transcription of p21 need to be extensively studied. In this study, we demonstrate that the high-mobility group box-containing protein 1 (HBP1) transcription factor is a novel activator of p21 that works as part of a complex mechanism during senescence and tumorigenesis. We found that HBP1 activates the p21 gene through enhancing p53 stability by inhibiting Mdm2-mediated ubiquitination of p53, a well known positive regulator of p21. HBP1 was also found to enhance p21 transcription by inhibiting Wnt/ß-catenin signaling. We identified histone methyltransferase EZH2, the catalytic subunit of polycomb repressive complex 2, as a target of Wnt/ß-catenin signaling. HBP1-mediated repression of EZH2 through Wnt/ß-catenin signaling decreased the level of trimethylation of histone H3 at lysine 27 of overall and specific histone on the p21 promoter, resulting in p21 transactivation. Although intricate, the reciprocal partnership of HBP1 and p21 has exceptional importance. HBP1-mediated elevation of p21 through the Mdm2/p53 and TCF4/EZH2 pathways contributes to both cellular senescence and tumor inhibition. Together, our results suggest that the HBP1 transcription factor orchestrates a complex regulation of key genes during cellular senescence and tumorigenesis with an impact on protein ubiquitination and overall histone methylation state.
Subject(s)
Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/genetics , Cyclin-Dependent Kinase Inhibitor p21/genetics , Enhancer of Zeste Homolog 2 Protein/genetics , High Mobility Group Proteins/genetics , Proto-Oncogene Proteins c-mdm2/genetics , Repressor Proteins/genetics , Transcription Factors/genetics , Tumor Suppressor Protein p53/genetics , A549 Cells , Animals , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/metabolism , Blotting, Western , Carcinogenesis/genetics , Cell Line , Cell Line, Tumor , Cell Survival/genetics , Cellular Senescence/genetics , Cyclin-Dependent Kinase Inhibitor p21/metabolism , Enhancer of Zeste Homolog 2 Protein/metabolism , Female , Gene Expression Regulation , HEK293 Cells , Hep G2 Cells , High Mobility Group Proteins/metabolism , Humans , Mice, Nude , Protein Binding , Proto-Oncogene Proteins c-mdm2/metabolism , RNA Interference , Repressor Proteins/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction/genetics , Transcription Factor 4 , Transcription Factors/metabolism , Transplantation, Heterologous , Tumor Suppressor Protein p53/metabolismABSTRACT
The tumor suppressor PTEN plays a critical role in the regulation of multiple cellular processes that include survival, cell cycle, proliferation, and apoptosis. PTEN is frequently mutated or deleted in various human cancer cells to promote tumorigenesis. PTEN is regulated by SUMOylation, but the SUMO E3 ligase involved in the SUMOylation of PTEN remains unclear. Here, we demonstrated that PIASxα is a SUMO E3 ligase for PTEN. PIASxα physically interacted with PTEN both in vitro and in vivo. Their interaction depended on the integrity of phosphatase and C2 domains of PTEN and the region of PIASxα comprising residues 134-347. PIASxα enhanced PTEN protein stability by reducing PTEN ubiquitination, whereas the mutation of PTEN SUMO1 conjugation sites neutralized the effect of PIASxα on PTEN protein half-life. Functionally, PIASxα, as a potential tumor suppressor, negatively regulated the PI3K-Akt pathway through stabilizing PTEN protein. Overexpression of PIASxα led to G0/G1 cell cycle arrest, thus triggering cell proliferation inhibition and tumor suppression, whereas PIASxα knockdown or deficiency in catalytic activity abolished the inhibition. Together our studies suggest that PIASxα is a novel SUMO E3 ligase for PTEN, and it positively regulates PTEN protein level in tumor suppression.
