Differential Protein Citrullination in Human ER– and ER+ Tumor and Adjacent Healthy Breast Tissue

[Image: see text] Post-translational modification of arginine to citrulline is catalyzed by members of the peptidylarginine deiminase (PAD) family. Dysregulation of this catalysis is a significant driver of the pathogenesis of numerous inflammatory diseases, including cancer. However, dysregulation of PAD activity has not been examined in breast cancer with respect to hormone receptor status. In this study, we measured PAD enzyme levels using Western blotting and investigated protein citrullination using a mass spectrometry-based proteomics approach in primary estrogen receptor negative (ER−) or positive (ER+) breast tumor and matched adjacent normal tissue. Our findings reveal 72 and 41 citrullinated proteins in ER– tumor and adjacent healthy tissue, respectively, where 20 of these proteins are common between the two groups. We detected 64 and 49 citrullinated proteins in ER+ tumor and adjacent healthy tissue, respectively, where 32 proteins are common. Interestingly, upon comparison of ER– and ER+ tumor tissue, only 32 citrullinated proteins are shared between the two and the rest are unique to the tumor’s receptor status. Using the STRING database for protein–protein interaction network analysis, these proteins are involved in protein-folding events (i.e., heat shock proteins) in ER– samples and blood-clotting events (i.e., fibulin) in ER+ samples. Constituents of the extracellular matrix structure (i.e., collagen and fibrinogen) were found in both. Herein, we establish evidence that supports the role of this unique post-translational modification in breast cancer biology. Finally, to aid drug discovery against citrullination, we developed a liquid chromatography–ultraviolet method to measure PAD enzymatic activity and optimized glucagon-like peptide II to quantitatively measure the ability of PADs to citrullinate its substrate.