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COL4A3 protein belongs to type IV collagen family and is closely linked to kidney diseases and cancer. To clarify the roles of COL4A3 in gastric carcinogenesis and subsequent progression, its expression was examined by immunohistochemistry on tissue microarrays containing gastric carcinomas, adjacent intestinal metaplasia, pure intestinal metaplasia, and gastritis. Gastric carcinoma tissue and cell lines were studied for COL4A3 expression by Western blotting and reverse transcription--polymerase chain reaction. We found that COL4A3 was differentially expressed in GES-1, AGS, BGC-823, GT-3 TKB, HGC-27, KATO-III, MGC-803, MKN28, MKN45, SCH, SGC-7901, and STKM-2 at both messenger RNA and protein levels. Carcinomas showed statistically lower COL4A3 expression than matched nonneoplastic mucosa (P < .05). Expression was strong in intestinal metaplasia in comparison with gastritis and carcinoma (P < .05). There was greater COL4A3 expression in carcinoma than gastritis (P < .05). Expression of COL4A3 protein was positively correlated with tumor size, lymphatic invasion, venous invasion, and TNM stage (P < .05). There was more COL4A3 expression in diffuse than in intestinal-type carcinomas regardless of invasion into the muscularis propria (P < .05). Histologically, all signet ring cell (n = 43) and mucinous (n = 12) carcinomas showed COL4A3 expression. Kaplan-Meier analysis indicated that COL4A3 expression was negatively associated with a favorable prognosis of overall, advanced, and intestinal-type gastric carcinomas (P < .05). Aberrant COL4A3 expression might play an important role in the pathogenesis and subsequent progression of gastric carcinoma. COL4A3 overexpression might be used as a marker of gastric intestinal metaplasia and mucinous and signet ring cell carcinoma. [ABSTRACT FROM AUTHOR]