Cytotoxic components from the leaves of Erythrophleum fordii induce human acute leukemia cell apoptosis through caspase 3 activation and PARP cleavage.

Cassaine diterpenoids as erythrofordins A-C (1-3), pseudo-erythrosuamin (4), and erythrofordin U (5) isolated from the leaves of Vietnamese Erythrophleum fordii Oliver were tested cytotoxic activity against human leukemia cancer cells. The results showed that these metabolites exhibited dose-dependent cytotoxicity against human leukemia HL-60 and KG cells with IC50 values ranging from 15.2 ± 1.5 to 42.2 ± 3.6 µM. Treatment with erythrofordin B led to the apoptosis of HL-60 and KG cells due to the activation of caspase 3, caspase 9, and poly (ADP-ribose) polymerase (PARP). Erythrofordin B significantly increased Bak protein expression, but downregulated the anti-apoptotic protein Bcl-2, in HL-60 cells. In silico results demonstrated that erythrofordin B can bind to both the procaspase-3 allosteric site and the PARP-1 active site, with binding energies of -7.36 and -10.76 kcal/mol, respectively. These results indicated that the leaves of Vietnamese E. fordii, which contain cassaine diterpenoids, can induce the apoptosis of human leukemia cancer cells.

In vitro and computational studies showed that perezone inhibits PARP-1 and induces changes in the redox state of K562 cells.

Cancer is one of the leading causes of morbidity and mortality worldwide. This disease is characterized by uncontrolled growth and proliferation of abnormal cells with a high probability to develop metastasis. Recently, it was demonstrated that perezone, a sesquiterpene quinone, is capable to induce cell death in leukemia (K562), prostate (PC-3), colorectal (HCT-15) and lung (SKLU-1) cancer cell lines; however, its mechanism of action is unknown. Therefore, in this study, in vitro and computational studies were performed to determine the mechanism of action of perezone. Firstly, changes in K562 cell viability, as well as changes in the redox status of the cell in response to treatment with several concentrations of perezone were analyzed. The type of cell death induced, and the modification of the cell cycle were determined. In addition, MD simulations and docking studies were performed to investigate the interaction of perezone with seven regulators of the apoptotic process. Finally, the ability of perezone to inhibit PARP-1 was evaluated by in vitro studies. K562 cells treated with perezone exhibited decreased viability and more oxidized status, being this effect concentration-dependent. In addition, the increase of G0/G1 phase of cell cycle and apoptosis were observed. According to the performed computational studies conducted, perezone showed the highest affinity to PARP-1 enzyme being this complex the most stable due to the presence of a small and deep cavity in the active site, which allows perezone to fit deeply by forming hydrogen bonds and hydrophobic interactions, which drive this interaction. The activity of perezone as PARP-1 inhibitor was corroborated with an IC50 = 181.5 µM. The pro-apoptotic action of perezone may be related to PARP-1 inhibition and changes in the redox state of the cell. The obtained results allowed to understand the biological effect of perezone and, consequently, these could be employed to develop novel PARP-1 inhibitors.

Forced Self-Modification Assays as a Strategy to Screen MonoPARP Enzymes.

Mono(ADP-ribosylation) (MARylation) and poly(ADP-ribosylation) (PARylation) are posttranslational modifications found on multiple amino acids. There are 12 enzymatically active mono(ADP-ribose) polymerase (monoPARP) enzymes and 4 enzymatically active poly(ADP-ribose) polymerase (polyPARP) enzymes that use nicotinamide adenine dinucleotide (NAD+) as the ADP-ribose donating substrate to generate these modifications. While there are approved drugs and clinical trials ongoing for the enzymes that perform PARylation, MARylation is gaining recognition for its role in immune function, inflammation, and cancer. However, there is a lack of chemical probes to study the function of monoPARPs in cells and in vivo. An important first step to generating chemical probes for monoPARPs is to develop biochemical assays to enable hit finding, and determination of the potency and selectivity of inhibitors. Complicating the development of enzymatic assays is that it is poorly understood how monoPARPs engage their substrates. To overcome this, we have developed a family-wide approach to developing robust high-throughput monoPARP assays where the enzymes are immobilized and forced to self-modify using biotinylated-NAD+, which is detected using a dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA) readout. Herein we describe the development of assays for 12 monoPARPs and 3 polyPARPs and apply them to understand the potency and selectivity of a focused library of inhibitors across this family.

Identification of regulators of poly-ADP-ribose polymerase inhibitor response through complementary CRISPR knockout and activation screens.

Inhibitors of poly-ADP-ribose polymerase 1 (PARPi) are highly effective in killing cells deficient in homologous recombination (HR); thus, PARPi have been clinically utilized to successfully treat BRCA2-mutant tumors. However, positive response to PARPi is not universal, even among patients with HR-deficiency. Here, we present the results of genome-wide CRISPR knockout and activation screens which reveal genetic determinants of PARPi response in wildtype or BRCA2-knockout cells. Strikingly, we report that depletion of the ubiquitin ligase HUWE1, or the histone acetyltransferase KAT5, top hits from our screens, robustly reverses the PARPi sensitivity caused by BRCA2-deficiency. We identify distinct mechanisms of resistance, in which HUWE1 loss increases RAD51 levels to partially restore HR, whereas KAT5 depletion rewires double strand break repair by promoting 53BP1 binding to double-strand breaks. Our work provides a comprehensive set of putative biomarkers that advance understanding of PARPi response, and identifies novel pathways of PARPi resistance in BRCA2-deficient cells.