Proteomic analyses identify ARH3 as a serine mono-ADP-ribosylhydrolase.

ADP-ribosylation is a posttranslational modification that exists in monomeric and polymeric forms. Whereas the writers (e.g. ARTD1/PARP1) and erasers (e.g. PARG, ARH3) of poly-ADP-ribosylation (PARylation) are relatively well described, the enzymes involved in mono-ADP-ribosylation (MARylation) have been less well investigated. While erasers for the MARylation of glutamate/aspartate and arginine have been identified, the respective enzymes with specificity for serine were missing. Here we report that, in vitro, ARH3 specifically binds and demodifies proteins and peptides that are MARylated. Molecular modeling and site-directed mutagenesis of ARH3 revealed that numerous residues are critical for both the mono- and the poly-ADP-ribosylhydrolase activity of ARH3. Notably, a mass spectrometric approach showed that ARH3-deficient mouse embryonic fibroblasts are characterized by a specific increase in serine-ADP-ribosylation in vivo under untreated conditions as well as following hydrogen peroxide stress. Together, our results establish ARH3 as a serine mono-ADP-ribosylhydrolase and as an important regulator of the basal and stress-induced ADP-ribosylome.

New Quantitative Mass Spectrometry Approaches Reveal Different ADP-ribosylation Phases Dependent On the Levels of Oxidative Stress.

Oxidative stress is a potent inducer of protein ADP-ribosylation. Although individual oxidative stress-induced ADP-ribosylated proteins have been identified, it is so far not clear to which extent different degrees of stress severity quantitatively and qualitatively alter ADP-ribosylation. Here, we investigated both quantitative and qualitative changes of the hydrogen peroxide (H2O2)-induced ADP-ribosylome using a label-free shotgun quantification and a parallel reaction monitoring (PRM) mass spectrometry approach for a selected number of identified ADP-ribosylated peptides. Although the major part of the basal HeLa ADP-ribosylome remained unchanged upon all tested H2O2 concentrations, some selected peptides change the extent of ADP-ribosylation depending on the degree of the applied oxidative stress. Low oxidative stress (i.e. 4 µm and 16 µm H2O2) caused a reduction in ADP-ribosylation of modified proteins detected under untreated conditions. In contrast, mid to strong oxidative stress (62 µm to 1 mm H2O2) induced a significant increase in ADP-ribosylation of oxidative stress-targeted proteins. The application of the PRM approach to SKOV3 and A2780, ovarian cancer cells displaying different sensitivities to PARP inhibitors, revealed that the basal and the H2O2-induced ADP-ribosylomes of SKOV3 and A2780 differed significantly and that the sensitivity to PARP inhibitors correlated with the level of ARTD1 expression in these cells. Overall, this new PRM-MS approach has proven to be sensitive in monitoring alterations of the ADP-ribosylome and has revealed unexpected alterations in proteins ADP-ribosylation depending on the degree of oxidative stress.