Ge Circle, Toronto, ON M5S 1A8, Canada; [email protected] (S.T.); [email protected] (D.H.); [email protected] (D.M.G.) Division of Biosciences, Faculty of Mathematics and Organic Sciences, University of Oslo, Blindernveien 31, 0371 Oslo, Norway; [email protected] (A.M.); [email protected] (J.H.A.) Correspondence: [email protected]; Tel.: +47-22-851-102 Current affiliation: Terry Fox Laboratory, British Columbia Cancer Agency, 675 West 10th Avenue, Vancouver, BC V5Z 1L3, Canada. Current affiliation: Norwegian Research Center, NORCE, Mekjarvik 12, 4070 Randaberg, Norway.Citation: Rasmussen, M.; Tan, S.; Somisetty, V.S.; Hutin, D.; Olafsen, N.E.; Moen, A.; Anonsen, J.H.; Grant, D.M.; Matthews, J. PARP7 and Mono-ADP-Ribosylation Negatively Regulate BRD4 list estrogen Receptor Signaling in Human Breast Cancer Cells. Cells 2021, 10, 623. https:// doi.org/10.3390/cells10030623 Academic Editor: Herwig Sch er Received: 13 January 2021 Accepted: 10 March 2021 Published: 11 MarchAbstract: ADP-ribosylation is usually a post-translational protein modification catalyzed by a household of proteins generally known as poly-ADP-ribose polymerases. PARP7 (TIPARP; ARTD14) can be a mono-ADPribosyltransferase HIV-2 custom synthesis involved in several cellular processes, like responses to hypoxia, innate immunity and regulation of nuclear receptors. Since previous studies suggested that PARP7 was regulated by 17-estradiol, we investigated no matter whether PARP7 regulates estrogen receptor signaling. We confirmed the 17-estradiol-dependent increases of PARP7 mRNA and protein levels in MCF-7 cells, and observed recruitment of estrogen receptor to the promoter of PARP7. Overexpression of PARP7 decreased ligand-dependent estrogen receptor signaling, when therapy of PARP7 knockout MCF-7 cells with 17-estradiol resulted in elevated expression of and recruitment to estrogen receptor target genes, in addition to elevated proliferation. Co-immunoprecipitation assays revealed that PARP7 mono-ADP-ribosylated estrogen receptor , and mass spectrometry mapped the modified peptides to the receptor’s ligand-independent transactivation domain. Coimmunoprecipitation with truncated estrogen receptor variants identified that the hinge region in the receptor is necessary for PARP7-dependent mono-ADP-ribosylation. These outcomes imply that PARP7-mediated mono-ADP-ribosylation might play an important part in estrogen receptor good breast cancer. Search phrases: PARP7; ARTD14; TIPARP; mono-ADP-ribosylation; estrogen receptor ; poly ADP-ribose polymerase; breast cancerPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.1. Introduction The poly-ADP-ribose polymerase (PARP) household consists of 17 enzymes that use nicotinamide adenine dinucleotide (NAD+ ) as a substrate to transfer ADP-ribose onto themselves and target proteins [1,2]. This activity depends upon the conserved histidinetyrosine-glutamate (HYE) catalytic triad motif, while the glutamate residue is absent in 11 from the protein members of the family, suggesting that they differ in their catalytic activity [3,4]. The majority of PARPs catalyze the transfer of one ADP-ribose monomer, a method referred to as mono-ADP-ribosylation [2]. A number of bacterial toxins exert their pathogenic mechanisms by acting as mono-ADP-ribosyltransferases (mARTs), including diphtheria [5], giving rise towards the option nomenclature diphtheria-toxin-like ADP-ribosyltransferases (ARTDs). Usually, ADP-ribosylation can alter a tar.