Protein‐arginine deiminase 2 suppresses proliferation of colon cancer cells through protein citrullination

Abstract

Expression of the gene for protein‐arginine deiminase 2 ( PADI 2 ) has been shown to be downregulated in colon cancer, with such downregulation being indicative of a poor prognosis in individuals with this disease. We have now examined the expression of PADI 2 in matched colon cancer and normal colon tissue specimens as well as in colon cancer cell lines. We found that isoform 1 of PADI 2 is the predominant isoform in colon tissue and is downregulated during colon carcinogenesis. Immunohistochemical analysis showed that PADI 2 is expressed in normal colonic epithelial cells. Overexpression of PADI 2 isoform 1 suppressed the proliferation of colon cancer cells in vitro in association with increased protein citrullination. Expression of a catalytically inactive mutant (C647A) of PADI 2 or of PADI 2 isoform 2 did not induce such effects, indicating that the protein citrullination activity of PADI 2 is required for inhibition of cell growth. The growth defect induced by PADI 2 was not attributable to increased apoptosis but rather was accompanied by arrest of cell cycle progression in G 1 phase. Finally, we detected citrullinated proteins in normal colon tissue by immunoblot analysis. Our data thus suggest that PADI 2 suppresses the proliferation of colonic epithelial cells through catalysis of protein citrullination, and that downregulation of PADI 2 expression might therefore contribute to colon carcinogenesis.