Further study of the subcellular localization of a preneoplastic antigen in hyperplastic liver nodules and primary hepatocellular carcinoma induced in rats by acetylaminofluorene has been pursued. Antisera against four subcellular fractions, cytosol, smooth and rough endoplasmic reticulum, and free polysomes, were obtained in rabbits and were used to assay for the presence of the antigen by immunodiffusion. The preneoplastic antigen, Antigen 1, appearing as a sharp precipitin line, is located predominantly, if not exclusively, in the smooth endoplasmic reticulum fraction of hyperplastic nodules and hepatomas. It appears after about 3 weeks of 2-acetylaminofluorene feeding and persists throughout the carcinogenic process in nodules and hepatomas. A second antigen, antigen 2, appears in the smooth endoplasmic reticulum of nodules after 13 weeks of 2-acetylaminofluorene feeding but gradually disappears on discontinuation of exposure to the carcinogen after 20 weeks. This antigen, appearing as a more diffuse precipitin line, becomes demonstrable in rough endoplasmic reticulum as well but only after stripping off the ribosomes. The presence and distribution of preneoplastic antigen during carcinogenesis as revealed by immunodiffusion was similar when a more sensitive assay, microcomplement fixation, was used. The preneoplastic antigen appears to be a potentially useful marker for alterations in the smooth endoplasmic reticulum that may be related to the development of liver cancer.

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Supported in part by research grants from the National Cancer Institute of Canada and by Contract NO1-CP-33262 from the National Cancer Institute.

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