Human proteins are subjected to more than 200 known post-translational modifications (PTMs) (e.g., phosphorylation, glycosylation, ubiquitination, S-nitrosylation, methylation, Nacetylation, and citrullination) and these PTMs can alter protein structure and function with consequent effects on the multitude of pathways necessary for maintaining the physiological homeostasis. When dysregulated, however, the enzymes that catalyze these PTMs can impact the genesis of countless diseases. In this review, we will focus on protein citrullination, a PTM catalyzed by the Protein Arginine Deiminase (PAD) family of enzymes. Specifically, we will describe the roles of the PADs in both normal human physiology and disease. The development of PAD inhibitors and their efficacy in a variety of autoimmune disorders and cancer will also be discussed.

Inflammatory bowel diseases (IBDs), mainly Crohn's disease and ulcerative colitis, are dynamic, chronic inflammatory conditions that are associated with an increased colon cancer risk. Inflammatory cell apoptosis is a key mechanism for regulating IBD. Peptidylarginine deiminases (PADs) catalyze the posttranslational conversion of peptidylarginine to peptidylcitrulline in a calcium-dependent, irreversible reaction and mediate the effects of proinflammatory cytokines. Because PAD levels are elevated in mouse and human colitis, we hypothesized that a novel small-molecule inhibitor of the PADs, i.e., chloramidine (Cl-amidine), could suppress colitis in a dextran sulfate sodium mouse model. Results are consistent with this hypothesis, as demonstrated by the finding that Cl-amidine treatment, both prophylactic and after the onset of disease, reduced the clinical signs and symptoms of colitis, without any indication of toxic side effects. Interestingly, Cl-amidine drives apoptosis of inflammatory cells in vitro and in vivo, providing a mechanism by which Cl-amidine suppresses colitis. In total, these data help validate the PADs as therapeutic targets for the treatment of IBD and further suggest Cl-amidine as a candidate therapy for this disease.

Resveratrol is a naturally occurring polyphenol that exhibits pleiotropic health beneficial effects, including anti-inflammatory, cardio-protective, and cancer-protective activities. It is recognized as one of the more promising natural molecules in the prevention and treatment of chronic inflammatory and autoimmune disorders. Ulcerative colitis is an idiopathic, chronic inflammatory disease of the colon associated with a high colon cancer risk. Here, we used a dextran sulfate sodium (DSS) mouse model of colitis, which resembles human ulcerative colitis pathology. Resveratrol mixed in food ameliorates DSS-induced colitis in mice in a dose-dependent manner. Resveratrol significantly improves inflammation score, downregulates the percentage of neutrophils in the mesenteric lymph nodes and lamina propria, and modulates CD3⁺ T cells that express tumor necrosis factor-α and IFN-γ. Markers of inflammation and inflammatory stress (p53 and p53-phospho-Ser¹⁵) are also downregulated by resveratrol. Because chronic colitis drives colon cancer risk, we carried out experiments to determine the chemopreventive properties of resveratrol. Tumor incidence is reduced from 80% in mice treated with azoxymethane (AOM) + DSS to 20% in mice treated with AOM + DSS + resveratrol (300 ppm). Tumor multiplicity also decreased with resveratrol treatment. AOM + DSS–treated mice had 2.4 ± 0.7 tumors per animal compared with AOM + DSS + 300 ppm resveratrol, which had 0.2 ± 0.13 tumors per animal. The current study indicates that resveratrol is a useful, nontoxic complementary and alternative strategy to abate colitis and potentially colon cancer associated with colitis. Cancer Prev Res; 3(4); 549–59. ©2010 AACR.

Chronic inflammation is associated with a high cancer risk. At the molecular level, free radicals and aldehydes, produced during chronic inflammation, can induce deleterious gene mutation and posttranslational modifications of key cancer-related proteins. Other products of inflammation, including cytokines, growth factors, and transcription factors such as nuclear factor κB, control the expression of cancer genes (e.g., suppressor genes and oncogenes) and key inflammatory enzymes such as inducible nitric oxide synthase and cyclooxygenase-2. These enzymes in turn directly influence reactive oxygen species and eicosanoid levels. The procancerous outcome of chronic inflammation is increased DNA damage, increased DNA synthesis, cellular proliferation, disruption of DNA repair pathways and cellular milieu, inhibition of apoptosis, and promotion of angiogenesis and invasion. Chronic inflammation is also associated with immunosuppression, which is a risk factor for cancer. Current treatment strategies for reactive species overload diseases are frequently aimed at treating or preventing the cause of inflammation. Although these strategies have led to some progress in combating reactive species overload diseases and associated cancers, exposure often occurs again after eradication, treatment to eradicate the cause fails, or the treatment has long-term side effects. Therefore, the identification of molecules and pathways involved in chronic inflammation and cancer is critical to the design of agents that may help in preventing the progression of reactive species overload disease and cancer associated with disease progression. Here, we use ginseng as an example of an antiinflammatory molecule that targets many of the key players in the inflammation-to-cancer sequence.

Sphingolipid metabolism is driven by inflammatory cytokines. These cascade of events include the activation of sphingosine kinase (SK), and subsequent production of the mitogenic and proinflammatory lipid sphingosine 1-phosphate (S1P). Overall, S1P is one of the crucial components in inflammation, making SK an excellent target for the development of new anti-inflammatory drugs. We have recently shown that SK inhibitors suppress colitis and hypothesize here that the novel SK inhibitor, ABC294640, prevents the development of colon cancer. In an azoxymethane (AOM)/dextran sulfate sodium (DSS) mouse model, there was a dose-dependent decrease in tumor incidence with SK inhibitor treatment. The tumor incidence (number of animals with tumors per group) in the vehicle, ABC294640 (20 mg/kg) and ABC294640 (50 mg/kg) groups were 80, 40 and 30%, respectively. Tumor multiplicity (number of tumors per animal) also decreased from 2.1 ± 0.23 tumors per animal in the AOM + DSS + vehicle group to 1.2 ± 0 tumors per animal in the AOM + DSS + ABC294640 (20 mg/kg) and to 0.8 ± 0.4 tumors per animal in the AOM + DSS + ABC294640 (50 mg/kg) group. Importantly, with ABC294640, there were no observed toxic side effects. To explore mechanisms, we isolated cells from the colon (CD45-, representing primarily colon epithelial cells) and (CD45+, representing primarily colon inflammatory cells) then measured known targets of SK that control cell survival. Results are consistent with the hypothesis that the inhibition of SK activity by our novel SK inhibitor modulates key pathways involved in cell survival and may be a viable treatment strategy for the chemoprevention colitis-driven colon cancer.