E2F1 and p53 transcription factors as accessory factors for nucleotide excision repair

Does the egg capsule protect against chronic UV-B radiation? A study based on encapsulated and decapsulated embryos of cuttlefish Sepia officinalis

Although the egg capsule plays a crucial role in the embryonic development of cephalopods, its ability to protect embryos from Ultraviolet (UV) radiation is unknown. Our study evaluated the photoprotection mechanisms of S. officinalis to UV-B radiation and estimated the ability of the black capsule to act as a physical shield against it. Embryos with and without capsule and juveniles were exposed to four experimental UVB conditions for 55 days. The effects of different UVB doses were evaluated in terms of morphological abnormalities and differences in gene expression between each group. We observed that the development might be severely impaired in embryos exposed to UVB without capsule protection, and these effects were time- and UVB-dose-dependent. In addition, we found variations in gene expression levels (light-sensitive, stress response and DNA repair) in different tissues as a function of UVB doses. We suggest a relationship between morphological abnormalities and the limit of molecular regulation. These results suggest that the quantitative differences in expression are essential for defining the survivability of the embryo face to UVB. Thus, we demonstrated that the egg capsule could ensure successful embryonic development of the cuttlefish S. officinalis even at high doses of UVB.

The E2F1 transcription factor and RB tumor suppressor moonlight as DNA repair factors

The E2F1 transcription factor and RB tumor suppressor are best known for their roles in regulating the expression of genes important for cell cycle progression but, they also have transcription-independent functions that facilitate DNA repair at sites of damage. Depending on the type of DNA damage, E2F1 can recruit either the GCN5 or p300/CBP histone acetyltransferases to deposit different histone acetylation marks in flanking chromatin. At DNA double-strand breaks, E2F1 also recruits RB and the BRG1 ATPase to remodel chromatin and promote loading of the MRE11-RAD50-NBS1 complex. Knock-in mouse models demonstrate important roles for E2F1 post-translational modifications in regulating DNA repair and physiological responses to DNA damage. This review highlights how E2F1 moonlights in DNA repair, thus revealing E2F1 as a versatile protein that recruits many of the same chromatin-modifying enzymes to sites of DNA damage to promote repair that it recruits to gene promoters to regulate transcription.

The Retinoblastoma (RB) Tumor Suppressor: Pushing Back against Genome Instability on Multiple Fronts

The retinoblastoma (RB) tumor suppressor is known as a master regulator of the cell cycle. RB is mutated or functionally inactivated in the majority of human cancers. This transcriptional regulator exerts its function in cell cycle control through its interaction with the E2F family of transcription factors and with chromatin remodelers and modifiers that contribute to the repression of genes important for cell cycle progression. Over the years, studies have shown that RB participates in multiple processes in addition to cell cycle control. Indeed, RB is known to interact with over 200 different proteins and likely exists in multiple complexes. RB, in some cases, acts through its interaction with E2F1, other members of the pocket protein family (p107 and p130), and/or chromatin remodelers and modifiers. RB is a tumor suppressor with important chromatin regulatory functions that affect genomic stability. These functions include the role of RB in DNA repair, telomere maintenance, chromosome condensation and cohesion, and silencing of repetitive regions. In this review we will discuss recent advances in RB biology related to RB, partner proteins, and their non-transcriptional functions fighting back against genomic instability.

RB localizes to DNA double-strand breaks and promotes DNA end resection and homologous recombination through the recruitment of BRG1

The retinoblastoma (RB) tumor suppressor is recognized as a master regulator that controls entry into the S phase of the cell cycle. Its loss leads to uncontrolled cell proliferation and is a hallmark of cancer. RB works by binding to members of the E2F family of transcription factors and recruiting chromatin modifiers to the promoters of E2F target genes. Here we show that RB also localizes to DNA double-strand breaks (DSBs) dependent on E2F1 and ATM kinase activity and promotes DSB repair through homologous recombination (HR), and its loss results in genome instability. RB is necessary for the recruitment of the BRG1 ATPase to DSBs, which stimulates DNA end resection and HR. A knock-in mutation of the ATM phosphorylation site on E2F1 (S29A) prevents the interaction between E2F1 and TopBP1 and recruitment of RB, E2F1, and BRG1 to DSBs. This knock-in mutation also impairs DNA repair, increases genomic instability, and renders mice hypersensitive to IR. Importantly, depletion of RB in osteosarcoma and breast cancer cell lines results in sensitivity to DNA-damaging drugs, which is further exacerbated by poly-ADP ribose polymerase (PARP) inhibitors. We uncovered a novel, nontranscriptional function for RB in HR, which could contribute to genome instability associated with RB loss.

E2F8 is a Potential Therapeutic Target for Hepatocellular Carcinoma

E2F transcriptional factors are widely expressed in a number of tissues and organs, possessing many regulatory functions related to cellular proliferation, differentiation, DNA repair, cell-cycle and cell apoptosis. E2F8 is a recently identified member of the E2F family with a duplicated DNA-binding domain feature discriminated from E2F1-6, controlling gene expression in a dimerization partner-independent manner. It is indispensable for angiogenesis, lymphangiogenesis and embryonic development. Although E2F8 and E2F7 perform complementary and overlapping functions in many cell metabolisms, E2F8, but not E2F7, overexpresses remarkably in hepatocellular carcinoma (HCC) to facilitate the HCC occurrence and development via activating a E2F1/ Cyclin D1 signaling pathway to regulate the G1- to S-phase transition of cell cycle progression or transcriptionally suppressing CDK1 to induce hepatocyte polyploidization. It also involves closely a variety of cellular physiological functions and pathological processes, which may bring a new breakthrough for the treatment of certain diseases, especially the HCC. Here, we summarize the latest progress of E2F8 on its relevant functions and mechanisms as well as potential application.

Long noncoding RNA CRNDE functions as a competing endogenous RNA to promote metastasis and oxaliplatin resistance by sponging miR-136 in colorectal cancer

Colorectal neoplasia differentially expressed (CRNDE) is a novel gene recognized as a long noncoding RNA (lncRNA) that is highly elevated in colorectal cancer and many other solid tumors but its functions on metastasis and oxaliplatin (OXA) resistance are unknown. In our study, we confirmed the upregulation of CRNDE in both primary specimens from colorectal cancer patients and colorectal cancer cell lines. Knockdown of CRNDE expression inhibited the migration and invasion potency of colorectal cancer cells with no effect on cell apoptosis. Overexpression of CRNDE promoted the migration and invasion potency of colorectal cancer cells. Furthermore, we found that CRNDE conferred chemoresistance in colorectal cancer cells. Knockdown of CRNDE with OXA treatment decreased cell viability and promoted DNA damage and cell apoptosis, while the overexpression of CRNDE with OXA treatment reduced DNA damage and cell apoptosis. Further in-depth mechanistic studies revealed that CRNDE functioned as a competing endogenous RNA for miR-136, led to the de-repression of its endogenous target, E2F transcription factor 1 (E2F1). Overall, our findings demonstrate that CRNDE functions as a competing endogenous RNA to promote metastasis and OXA resistance by sponging miR-136 in colorectal cancer.

E2F1 responds to ultraviolet radiation by directly stimulating DNA repair and suppressing carcinogenesis

In response to DNA damage, the E2F1 transcription factor is phosphorylated at serine 31 (serine 29 in mouse) by the ATM or ATR kinases, which promotes E2F1 protein stabilization. Phosphorylation of E2F1 also leads to the recruitment of E2F1 to sites of DNA damage, where it functions to enhance DNA repair. To study the role of this E2F1 phosphorylation event in vivo, a knock-in mouse model was generated, in which serine 29 was mutated to alanine. The S29A mutation impairs E2F1 stabilization in response to ultraviolet (UV) radiation and doxorubicin treatment, but has little effect on the expression of E2F target genes. The apoptotic and proliferative responses to acute UV radiation exposure are also similar between wild-type and E2f1(S29A/) (S29A) mice. As expected, the S29A mutation prevents E2F1 association with damaged DNA and reduces DNA repair efficiency. Moreover, E2f1(S29A/) (S29A) mice display increased sensitivity to UV-induced skin carcinogenesis. This knock-in mouse model thus links the ability of E2F1 to directly promote DNA repair with the suppression of tumor development.

The chemosensitivity of testicular germ cell tumors

Although rare cancers overall, testicular germ cell tumors (TGCTs) are the most common type of cancer in young males below 40 years of age. Both subtypes of TGCTs, i.e., seminomas and non-seminomas, are highly curable and the majority of even metastatic patients may expect to be cured. These high cure rates are not due to the indolent nature of these cancers, but rather to their sensitivity to chemotherapy (and for seminomas to radiotherapy). The delineation of the cause of chemosensitivity at the molecular level is of paramount importance, because it may provide insights into the minority of TGCTs that are chemo-resistant and, thereby, provide opportunities for specific therapeutic interventions aimed at reverting them to chemosensitivity. In addition, delineation of the molecular basis of TGCT chemo-sensitivity may be informative for the cause of chemo-resistance of other more common types of cancer and, thus, may create new therapeutic leads. p53, a frequently mutated tumor suppressor in cancers in general, is not mutated in TGCTs, a fact that has implications for their chemo-sensitivity. Oct4, an embryonic transcription factor, is uniformly expressed in the seminoma and embryonic carcinoma components of non-seminomas, and its interplay with p53 may be important in the chemotherapy response of these tumors. This interplay, together with other features of TGCTs such as the gain of genetic material from the short arm of chromosome 12 and the association with disorders of testicular development, will be discussed in this paper and integrated in a unifying hypothesis that may explain their chemo-sensitivity.

Chk2 and p53 regulate the transmission of healed chromosomes in the Drosophila male germline

When a dicentric chromosome breaks in mitosis, the broken ends cannot be repaired by normal mechanisms that join two broken ends since each end is in a separate daughter cell. However, in the male germline of Drosophila melanogaster, a broken end may be healed by de novo telomere addition. We find that Chk2 (encoded by lok) and P53, major mediators of the DNA damage response, have strong and opposite influences on the transmission of broken-and-healed chromosomes: lok mutants exhibit a large increase in the recovery of healed chromosomes relative to wildtype control males, but p53 mutants show a strong reduction. This contrasts with the soma, where mutations in lok and p53 have the nearly identical effect of allowing survival and proliferation of cells with irreparable DNA damage. Examination of testes revealed a transient depletion of germline cells after dicentric chromosome induction in the wildtype controls, and further showed that P53 is required for the germline to recover. Although lok mutant males transmit healed chromosomes at a high rate, broken chromosome ends can also persist through spermatogonial divisions without healing in lok mutants, giving rise to frequent dicentric bridges in Meiosis II. Cytological and genetic analyses show that spermatid nuclei derived from such meiotic divisions are eliminated during spermiogenesis, resulting in strong meiotic drive. We conclude that the primary responsibility for maintaining genome integrity in the male germline lies with Chk2, and that P53 is required to reconstitute the germline when cells are eliminated owing to unrepaired DNA damage.

A hormone-DNA repair circuit governs the response to genotoxic insult

Alterations in DNA repair promote tumor development, but the impact on tumor progression is poorly understood. Here, discovery of a biochemical circuit linking hormone signaling to DNA repair and therapeutic resistance is reported. Findings show that androgen receptor (AR) activity is induced by DNA damage and promotes expression and activation of a gene expression program governing DNA repair. Subsequent investigation revealed that activated AR promotes resolution of double-strand breaks and resistance to DNA damage both in vitro and in vivo. Mechanistically, DNA-dependent protein kinase catalytic subunit (DNAPKcs) was identified as a key target of AR after damage, controlling AR-mediated DNA repair and cell survival after genotoxic insult. Finally, DNAPKcs was shown to potentiate AR function, consistent with a dual role in both DNA repair and transcriptional regulation. Combined, these studies identify the AR-DNAPKcs circuit as a major effector of DNA repair and therapeutic resistance and establish a new node for therapeutic intervention in advanced disease.

SIGNIFICANCE

The present study identifies for the fi rst time a positive feedback circuit linking hormone action to the DNA damage response and shows the significant impact of this process on tumor progression and therapeutic response. These provocative findings provide the foundation for development of novel nodes of therapeutic intervention for advanced disease.

Cockayne syndrome group B (CSB) protein: at the crossroads of transcriptional networks

Cockayne syndrome (CS) is a rare genetic disorder characterized by a variety of growth and developmental defects, photosensitivity, cachectic dwarfism, hearing loss, skeletal abnormalities, progressive neurological degeneration, and premature aging. CS arises due to mutations in the CSA and CSB genes. Both gene products are required for the transcription-coupled (TC) branch of the nucleotide excision repair (NER) pathway, however, the severe phenotype of CS patients is hard to reconcile with a sole defect in TC-NER. Studies using cells from patients and mouse models have shown that the CSB protein is involved in a variety of cellular pathways and plays a major role in the cellular response to stress. CSB has been shown to regulate processes such as the transcriptional recovery after DNA damage, the p53 transcriptional response, the response to hypoxia, the response to insulin-like growth factor-1 (IGF-1), transactivation of nuclear receptors, transcription of housekeeping genes and the transcription of rDNA. Some of these processes are also affected in combined XP/CS patients. These new advances in the function(s) of CSB shed light onto the etiology of the clinical features observed in CS patients and could potentially open therapeutic avenues for these patients in the future. Moreover, the study of CS could further our knowledge of the aging process.