The Fas-associated factor 1, FAF1, is a protein, which was first identified as an interaction partner of the death receptor Fas. Not much is known about the function of FAF1, but it has been found that it is able to potentiate Fas-induced apoptosis in cell lines. To clarify the role of FAF1 in human cancer, a number of tumors from different organs were screened for expression of the protein, and it was only found reduced in gastric carcinoma tissue. Thus, 58 human gastric carcinomas were collected, and the expression of FAF1 was analyzed by Western blotting and in a few cases also by immunohistochemistry. The hypothesis was that since FAF1 is able to potentiate apoptosis, it would likely be reduced in the gastric carcinomas in order for them to escape apoptosis. We found that FAF1 was reduced in 50% (29/58) of the gastric carcinomas analyzed as compared to non-neoplastic gastric mucosa from the same patients. 26 of the investigated carcinomas contained signet ring cells, and FAF1 was significantly reduced in 69% of these (p=0.017), whereas it was only reduced in 34% of the carcinomas without signet ring cells. The observed reduction of FAF1 was predominantly caused by proteolytic cleavage of the protein. Additionally, 31 colorectal carcinomas were analyzed for expression of FAF1. Here, FAF1 was only reduced in 16% of the carcinomas when compared to non-neoplastic colorectal mucosa. Our findings support the hypothesis that FAF1 is reduced in gastric carcinomas compared to non-neoplastic tissue, and there was a significant relation between FAF1 reduction and content of signet ring cells in the gastric carcinomas. Also, the reduction of FAF1 is likely to be specific for gastric cancer, which might be due to the fact that signet ring cells are most frequently found in gastric cancers.

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