No abstract

Background: Ras signaling is known to be critical for tumor progression.Results: Ras-PI3K regulates H3K56 acetylation (H3K56ac) via the MDM2-dependent degradation of CBP/p300. H3K56ac is revealed to be associated with the transcription, proliferation, and migration of tumor cells.Conclusion: H3K56 acetylation is a critical component of the oncogenic Ras-PI3K pathway.Significance: The Ras-PI3K-AKT-H3K56ac pathway is a potential target for cancer therapy.

No abstract

dEfficient DNA double-strand break (DSB) repair is critical for the maintenance of genome stability. Unrepaired or misrepaired DSBs cause chromosomal rearrangements that can result in severe consequences, such as tumorigenesis. RAD6 is an E2 ubiquitin-conjugating enzyme that plays a pivotal role in repairing UV-induced DNA damage. Here, we present evidence that RAD6 is also required for DNA DSB repair via homologous recombination (HR) by specifically regulating the degradation of heterochromatin protein 1␣ (HP1␣). Our study indicates that RAD6 physically interacts with HP1␣ and ubiquitinates HP1␣ at residue K154, thereby promoting HP1␣ degradation through the autophagy pathway and eventually leading to an open chromatin structure that facilitates efficient HR DSB repair. Furthermore, bioinformatics studies have indicated that the expression of RAD6 and HP1␣ exhibits an inverse relationship and correlates with the survival rate of patients. Double-strand breaks (DSBs) in DNA are considered the most deleterious types of DNA damage and pose a great threat to the integrity of the genome. Two pathways, homologous recombination (HR) and nonhomologous end joining (NHEJ), have evolved in mammals to repair the broken ends that characterize DSBs (1). The HR pathway is a precise repair pathway, wherein missing and damaged sequence information is copied from sister chromatids to catalyze the repair (2, 3). In contrast, the repair of DNA DSBs by NHEJ is more error prone and often leads to insertions, deletions, or other types of chromosomal rearrangements. The accumulation of DNA mutations, due to either unrepaired broken ends or improper repair, is thought to increase the incidence rate of cancer and other types of diseases (4, 5).Mounting evidence indicates that the ubiquitination of DSB repair proteins plays an important role in regulating DSB repair in mammals (6-8). Ubiquitination is classified into two types, monoubiquitination and polyubiquitination, depending on the number of ubiquitin molecules that become posttranslationally attached to target proteins. Monoubiquitinated proteins have been shown to participate in nonproteolytic pathways such as receptor trafficking, signal transduction, gene transcription, and DNA repair, while the polyubiquitination of substrates often leads to protein degradation either through the 26S proteasome pathway or through the autophagy pathway (9-12).Ubiquitination is catalyzed by a series of enzymes that includes the ubiquitin activation enzyme (E1), the ubiquitin-conjugating enzyme (E2), and the ubiquitin ligase (E3) (13). RAD6 is an E2 ubiquitin-conjugating enzyme with a well-described role in stimulating the repair of UV-induced DNA damage (7,14). In budding yeast, RAD6 interacts with RAD18 to catalyze the monoubiquitination of proliferating cell nuclear antigen (PCNA) on lysine 164, thereby promoting the error-prone DNA damage repair pathway by recruiting low-fidelity polymerases. Interestingly, the interaction between the RAD6-RAD18 complex and the Ubc13-MMS2-Rad5 complex facilita...

Maintaining an appropriate cellular concentration of p53 is critical for cell survival and normal development in various organisms. In this study, we provide evidence that the human E2 ubiquitin-conjugating enzyme RAD6 plays a critical role in regulating p53 protein levels under both normal and stress conditions. Knockdown and overexpression of RAD6 affected p53 turnover and transcription. We showed that RAD6 can form a ternary complex with MDM2 and p53 that contributes to the degradation of p53. Chromatin immunoprecipitation (ChIP) analysis showed that RAD6 also binds to the promoter and coding regions of the p53 gene and modulates the levels of H3K4 and K79 methylation on local chromatin. When the cells were exposed to stress stimuli, the RAD6-MDM2-p53 ternary complex was disrupted; RAD6 was then recruited to the chromatin of the p53 gene, resulting in an increase in histone methylation and p53 transcription. Further studies showed that stress-induced p53 transcriptional activation, cell apoptosis, and disrupted cell cycle progression are all RAD6 dependent. Overall, this work demonstrates that RAD6 regulates p53 levels in a "yin-yang" manner through a combination of two distinct mechanisms in mammalian cells. The ubiquitin system plays a critical role in numerous cellular events, such as cell cycle regulation, DNA repair, stress responses, metabolic homeostasis, organelle biosynthesis, apoptosis, and gene expression (12,17). The protein ubiquitin pathway involves a multistep ubiquitin thioester cascade, which requires the ubiquitin-activating enzyme (E1), ubiquitinconjugating enzymes (UBC or E2), and the assistance of a ubiquitin-protein ligase (E3). Polyubiquitination is thought to mark proteins for degradation, whereas monoubiquitination may have other functions (10).Rad6 belongs to a group of E2 enzymes (20) that are involved in DNA damage repair by catalyzing the ubiquitination of different target proteins (18,23,27,28,34,35,48). It has been shown that Rad6 interacts with Rad18 to catalyze the monoubiquitination of PCNA (proliferating cell nuclear antigen) on lysine 164 (K164), thereby promoting the error-prone DNA damage repair pathway (4, 5, 6). However, another mechanism has been shown to respond to DNA damage; through this mechanism, a complex containing Ubc13-MMS2-Rad5/Rad18-Rad6 promotes the polyubiquitination of PCNA and activates the error-free repair pathway (18,48). Mutations in the catalytic site of Rad6 have been shown to confer hypersensitivity to a variety of DNA damage agents (40, 57). The Rad6 mutant has been shown to cause slow growth, severe defects in induced mutagenesis, and hypersensitivity to UV, X-ray, and chemical mutagens (33,58).The human homologs of yeast Rad6, HHR6A/RAD6A and HHR6B/RAD6B (human homologs of Rad6), have nearly 70% sequence identity with yeast Rad6, and more than 90% sequence identity is shared between these two human homologs (27, 28). The products of both genes are able to complement the DNA repair and UV mutagenesis defects of the Saccharomyces cerevisiae Rad6 ...