Protease-activated receptors (PARs) are a unique family of G-protein coupled receptors. PAR4, the most recently identified PAR member, was reported to be overexpressed during the progression of colon and prostate cancers. Though PAR4 mRNA was detected in normal stomach, the role of PAR4 in gastric cancer has not been investigated. In this study, differential expression of PAR4 was measured by real-time PCR (n=28) and tissue microarrays (n=74). We showed that PAR4 was located from basal to middle portions of normal gastric mucosa. PAR4 expression was remarkably decreased in gastric cancer tissues as compared with matched noncancerous tissues, especially in positive lymph node or low differentiation cancers. Furthermore, methylation of the PAR4 promoter in cell lines was assessed by treatment with 5-aza-2'-deoxycytidine and genomic bisulfite sequencing. AGS and N87 human gastric cancer cell lines did not express PAR4, as compared to HT-29 human colon cancer cell line with significant PAR4 expression. Treatment with 5-aza-2'-deoxycytidine restored PAR4 expression in AGS and N87 cells, which exhibited significantly more 5-methylcytosines in the PAR4 promoter compared with HT-29 cells. Our results revealed that down-regulation of PAR4 expression occurs frequently in gastric cancers and exhibits association with more aggressive gastric cancer. Interestingly, the loss of PAR4 expression in gastric cancers may result from hypermethylation of the PAR4 promoter.
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