Identification and quantification of DNA repair protein poly(ADP ribose) polymerase 1 (PARP1) in human tissues and cultured cells by liquid chromatography/isotope-dilution tandem mass spectrometry

Inhibition of human APE1 and MTH1 DNA repair proteins by dextran-coated γ-Fe2O3 ultrasmall superparamagnetic iron oxide nanoparticles

Aim: To quantitatively evaluate the inhibition of human DNA repair proteins APE1 and MTH1 by dextran-coated γ-Fe₂O₃ ultrasmall superparamagnetic iron oxide nanoparticles (dUSPIONs). Materials & methods: Liquid chromatography-tandem mass spectrometry with isotope-dilution was used to measure the expression levels of APE1 and MTH1 in MCL-5 cells exposed to increasing doses of dUSPIONs. The expression levels of APE1 and MTH1 were measured in cytoplasmic and nuclear fractions of cell extracts. Results: APE1 and MTH1 expression was significantly inhibited in both cell fractions at the highest dUSPION dose. The expression of MTH1 was linearly inhibited across the full dUSPION dose range in both fractions. Conclusion: These findings warrant further studies to characterize the capacity of dUSPIONs to inhibit other DNA repair proteins in vitro and in vivo.

Identification and Quantification of Apurinic/Apyrimidinic Endonuclease 1 in Human Peripheral Blood Leukocytes by Liquid Chromatography/Isotope-Dilution High Resolution Mass Spectrometry

Background Increasing evidence in recent years highlights the predictive and prognostic importance of the expression of DNA repair proteins in cancer treatment. Generally, western-blotting or immunohistochemical staining methods are often used to determine the expression of DNA repair proteins. These methods might cause misleading results such as binding to nonspecific molecules by cross-reaction or false negativity as a result of the inability of antibodies to bind; absolute quantitations of proteins can not be performed. Purpose In this study, an analytical measurement technique was developed for human apurinic/apyrimidinic endonuclease 1 (hAPE1) protein for identification and absolute quantification in human leukocyte sample using high resolution mass spectrometry (HR-MS) with the targeted proteomics based approach. Methods Sample preparation was performed by using density gradient centrifugation and total protein extraction cartridges. hAPE1 was analyzed by liquid chromatography isotope-dilution-HR-MS (LC-HR-MS). A fully 15N-labeled analogue of hAPE1 was used for the quantitative measurements. Results Six peptides were identified, which matched to a subset of the theoretically predicted tryptic peptides of hAPE1. Mass accuracy was calculated as

The Ubiquitin Ligase TRIP12 Limits PARP1 Trapping and Constrains PARP Inhibitor Efficiency

PARP inhibitors (PARPi) cause synthetic lethality in BRCA-deficient tumors. Whether specific vulnerabilities to PARPi exist beyond BRCA mutations and related defects in homology-directed repair (HDR) is not well understood. Here, we identify the ubiquitin E3 ligase TRIP12 as negative regulator of PARPi sensitivity. We show that TRIP12 controls steady-state PARP1 levels and limits PARPi-induced cytotoxic PARP1 trapping. Upon loss of TRIP12, elevated PARPi-induced PARP1 trapping causes increased DNA replication stress, DNA damage, cell cycle arrest, and cell death. Mechanistically, we demonstrate that TRIP12 binds PARP1 via a central PAR-binding WWE domain and, using its carboxy-terminal HECT domain, catalyzes polyubiquitylation of PARP1, triggering proteasomal degradation and preventing supra-physiological PARP1 accumulation. Further, in cohorts of breast and ovarian cancer patients, PARP1 abundance is negatively correlated with TRIP12 expression. We thus propose TRIP12 as regulator of PARP1 stability and PARPi-induced PARP trapping, with potential implications for PARPi sensitivity and resistance.

Role of Mycoplasma Chaperone DnaK in Cellular Transformation

Studies of the human microbiome have elucidated an array of complex interactions between prokaryotes and their hosts. However, precise bacterial pathogen-cancer relationships remain largely elusive, although several bacteria, particularly those establishing persistent intra-cellular infections, like mycoplasmas, can alter host cell cycles, affect apoptotic pathways, and stimulate the production of inflammatory substances linked to DNA damage, thus potentially promoting abnormal cell growth and transformation. Consistent with this idea, in vivo experiments in several chemically induced or genetically deficient mouse models showed that germ-free conditions reduce colonic tumor formation. We demonstrate that mycoplasma DnaK, a chaperone protein belonging to the Heath shock protein (Hsp)-70 family, binds Poly-(ADP-ribose) Polymerase (PARP)-1, a protein that plays a critical role in the pathways involved in recognition of DNA damage and repair, and reduces its catalytic activity. It also binds USP10, a key p53 regulator, reducing p53 stability and anti-cancer functions. Finally, we showed that bystander, uninfected cells take up exogenous DnaK-suggesting a possible paracrine function in promoting cellular transformation, over and above direct mycoplasma infection. We propose that mycoplasmas, and perhaps certain other bacteria with closely related DnaK, may have oncogenic activity, mediated through the inhibition of DNA repair and p53 functions, and may be involved in the initiation of some cancers but not necessarily involved nor necessarily even be present in later stages.