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...

Autophagy is an evolutionarily conserved cellular process that primarily participates in lysosome-mediated protein degradation. Although autophagy is a cytoplasmic event, how epigenetic pathways are involved in the regulation of autophagy remains incompletely understood. Here, we found that H2B monoubiquitination (H2Bub1) is down-regulated in cells under starvation conditions and that the decrease in H2Bub1 results in the activation of autophagy. We also identified that the deubiquitinase USP44 is responsible for the starvation-induced decrease in H2Bub1. Furthermore, the changes in H2Bub1 affect the transcription of genes involved in the regulation of autophagy. Therefore, this study reveals a novel epigenetic pathway for the regulation of autophagy through H2Bub1.

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Background MicroRNAs (miRNAs) are a class of small non-coding single-stranded RNA molecules that inhibit gene expression at post-transcriptional level. Gadd45g (growth arrest and DNA-damage-inducible 45 gamma) is a stress-response protein, which has been implicated in several biological processes, including DNA repair, the cell cycle and cell differentiation. Results In this work, we found that miR-383 is a negative regulator of Gadd45g. Forced expression of miR-383 decreased the expression of Gadd45g through binding to the 3′ untranslated region (3′-UTR), whereas inhibition of miR-383 increased Gadd45g expression. The presence of miR-383 increased the cellular sensitivity to DNA damage in breast cancer cells, which was rescued by ectopic expression of Gadd45g without the 3′-UTR. miR-383 also regulates the expression of Gadd45g in embryonic stem (ES) cells, but not their apoptosis under genotoxic stress. miR-383 was further showed to negatively regulate ES cell differentiation via targeting Gadd45g, which subsequently modulates the pluripotency-associated genes. Taken together, our study demonstrates that miR-383 is a negative regulator of Gadd45g in both tumor cells and ES cells, however, has distinct function in regulating cell apoptosis. miR-383 may be used as antineoplastic agents in cancer chemotherapy. Conclusion We demonstrate for the first time that miR-383 can specifically regulates the expression of Gadd45g by directly targeting to the 3-UTR region of Gadd45g mRNA, a regulatory process conserved in human tumor cells and mouse embryonic stem cells. These two compotents can be potentially used as antineoplastic agents in cancer chemotherapy.

Epigenetics plays critical roles in controlling stem cell self-renewal and differentiation. Histone H1 is one of the most critical chromatin regulators, but its role in adult stem cell regulation remains unclear. Here we report that H1 is intrinsically required in the regulation of germline stem cells (GSCs) in the Drosophila ovary. The loss of H1 from GSCs causes their premature differentiation through activation of the key GSC differentiation factor bam. Interestingly, the acetylated H4 lysine 16 (H4K16ac) is selectively augmented in the H1-depleted GSCs. Furthermore, overexpression of mof reduces H1 association on chromatin. In contrast, the knocking down of mof significantly rescues the GSC loss phenotype. Taken together, these results suggest that H1 functions intrinsically to promote GSC self-renewal by antagonizing MOF function. Since H1 and H4K16 acetylation are highly conserved from fly to human, the findings from this study might be applicable to stem cells in other systems.