Poly(ADP-ribose) polymerase 1 protein (PARP1) mediates chromatin loosening and activates the transcription of inducible genes, but the mechanism of PARP1 regulation in chromatin is poorly understood. We have found that PARP1 interaction with chromatin is dynamic and that PARP1 is exchanged continuously between chromatin and nucleoplasm, as well as between chromatin domains. Specifically, the PARP1 protein preferentially interacts with nucleosomal particles, and although the nucleosomal linker DNA is not necessary for this interaction, we have shown that the core histones, H3 and H4, are critical for PARP1 binding. We have also demonstrated that the histones H3 and H4 interact preferentially with the C-terminal portion of PARP1 protein and that the N-terminal domain of PARP1 negatively regulates these interactions. Finally, we have found that interaction with the N-terminal tail of the H4 histone triggers PARP1 enzymatic activity. Therefore, our data collectively suggests a model in which both the regulation of PARP1 protein binding to chromatin and the enzymatic activation of PARP1 protein depend on the dynamics of nucleosomal core histone mediation.Eukaryotic chromatin organization involves the fundamental nucleosomal unit, which consists of four core histones plus a linker histone (1). Recently, it has been shown that the activity of transcription complexes at nucleosomes is regulated by the PARP1 3 protein (2, 3). Notwithstanding these findings, major gaps in our present understanding exist that involve the mechanism by which the PARP1 protein binds to specific chromatin domains and the mechanism by which the local PARP1 protein is activated in response to developmental and environmental stimuli.After histones, PARP1 is the most abundant nuclear protein (4). The distribution of PARP1 in chromatin is broad and occurs in regions characterized by distinct cell types (2, 3, 5). Nevertheless, exactly how the PARP1 enzyme interacts with chromatin in vivo has not been thoroughly investigated, and the molecular basis for PARP1 binding to chromatin remains poorly understood. Although zinc fingers within the PARP1 protein contribute to DNA binding in vitro, they specifically recognize damaged DNA (6) and therefore do not contribute to the association of PARP1 with intact chromatin. Moreover, a PARP1 paralog, PARP2, that has no zinc fingers and no direct DNA binding capability, nevertheless exhibits a pattern of chromatin association similar to PARP1 and is able to partially complement PARP1 functions in a PARP1 null mutant (7-9). This suggests that PARP1 and PARP2 both bind chromatin indirectly, through an interaction with one or more DNA-binding proteins.A key aim of this study is to determine the specific mechanisms by which PARP1 protein associates with chromatin in vivo. Considerable evidence now suggests that PARP1 interacts with chromatin by binding to histones (10). For example, histones H1, H2A, and H2B are efficient targets for PARP1 binding in vitro (11) and are enzymatically modified by . This idea is, howev...

According to the histone code hypothesis, histone variants and modified histones provide binding sites for proteins that change the chromatin state to either active or repressed. Here, we identify histone variants that regulate the targeting and enzymatic activity of poly(ADP-ribose) polymerase 1 (PARP1), a chromatin regulator in higher eukaryotes. We demonstrate that PARP1 is targeted to chromatin by association with the histone H2A variant (H2Av)-the Drosophila homolog of the mammalian histone H2A variants H2Az/H2Ax-and that subsequent phosphorylation of H2Av leads to PARP1 activation. This two-step mechanism of PARP1 activation controls transcription at specific loci in a signal-dependent manner. Our study establishes the mechanism through which histone variants and changes in the histone modification code control chromatin-directed PARP1 activity and the transcriptional activation of target genes.poly(ADP-ribosyl)ation | poly(ADP-ribose) glycohydrolase | nucleosome | Hsp70

Animal models of ovarian cancer are crucial for understanding the pathogenesis of the disease and for testing new treatment strategies. A model of ovarian carcinogenesis in the rat was modified and improved to yield ovarian preneoplastic and neoplastic lesions that pathogenetically resemble human ovarian cancer. A significantly lower dose (2 to 5 g per ovary) of 7,12-dimethylbenz(a)anthracene (DMBA) was applied to the one ovary to maximally preserve its structural integrity. DMBA-induced mutagenesis was additionally combined with repetitive gonadotropin hormone stimulation to induce multiple cycles of active proliferation of the ovarian surface epithelium. Animals were treated in three arms of different doses of DMBA alone or followed by hormone administration. Comparison of the DMBA-treated ovaries with the contralateral control organs revealed the presence of epithelial cell origin lesions at morphologically distinct stages of preneoplasia and neoplasia. Their histopathology and path of dissemination to other organs are very similar to human ovarian cancer. Hormone cotreatment led to an increased lesion severity, indicating that gonadotropins may promote ovarian cancer progression. Point mutations in the Tp53 and Ki-Ras genes were detected that are also characteristic of human ovarian carcinomas. Additionally, an overexpression of estrogen and progesterone receptors was observed in preneoplastic and early neoplastic lesions, suggesting a role of these receptors in ovarian cancer development. These data indicate that this DMBA animal model gives rise to ovarian lesions that closely resemble human ovarian cancer and it is adequate for additional studies on the mechanisms of the disease and its clinical management.

The clinical potential of PARP-1 inhibitors has been recognized > 10 years ago, prompting intensive research on their pharmacological application in several branches of medicine, particularly in oncology. However, natural or acquired resistance of tumors to known PARP-1 inhibitors poses a serious problem for their clinical implementation. Present study aims to reignite clinical interest to PARP-1 inhibitors by introducing a new method of identifying highly potent inhibitors and presenting the largest known collection of structurally diverse inhibitors. The majority of PARP-1 inhibitors known to date have been developed as NAD competitors. NAD is utilized by many enzymes other than PARP-1, resulting in a trade-off trap between their specificity and efficacy. To circumvent this problem, we have developed a new strategy to blindly screen a small molecule library for PARP-1 inhibitors by targeting a highly specific rout of its activation. Based on this screen, we present a collection of PARP-1 inhibitors and provide their structural classification. In addition to compounds that show structural similarity to NAD or known PARP-1 inhibitors, the screen identified structurally new non-NAD-like inhibitors that block PARP-1 activity in cancer cells with greater efficacy and potency than classical PARP-1 inhibitors currently used in clinic. These non-NAD-like PARP-1 inhibitors are effective against several types of human cancer xenografts, including kidney, prostate, and breast tumors in vivo. Our pre-clinical testing of these inhibitors using laboratory animals has established a strong foundation for advancing the new inhibitors to clinical trials.

During the last few years, Poly(ADP-ribose)Polymerase (PARP) proteins became a very popular target for anti-cancer treatment. Many PARP inhibitors have been generated and tested by pharmacological industry. However, most of them were designed to disrupt the DNA-dependent PARP1 protein activation pathway and were based on a competition with NAD for a binding site on PARP molecule and, therefore, on disruption of PARP-mediated enzymatic reaction. This limitation resulted in a discovery of mainly nucleotide-like PARP1-inhibitors which may target not only PARP, but also other pathways involving NAD and other nucleotides. Here, we describe a strategy for the identification PARP inhibitors that target a different pathway, the histone H4 dependent PARP1 activation. Besides the identification of NAD competitors in a small molecules collection, this approach allows finding novel classes of PARP inhibitors that specifically disrupt H4 based PARP activation.