Previous work has shown a consistent fall in S-adenosyl-l-methionine (SAM) in the liver of diethylnitrosamine-initiated rats, during the development of preneoplastic lesions, in persistent nodules (PNs), and hepatocellular carcinomas. The injection of SAM into rats causes the reconstitution of the SAM pool, coupled with growth restraint, remodeling, and apoptosis of preneoplastic cells, and inhibits the development of PNs and hepatocellular carcinomas. To evaluate if SAM treatment causes a long-term prevention of preneoplastic and neoplastic liver lesions or merely causes a delay in their development, we evaluated the effect of a relatively short SAM treatment on the development of preneoplastic and neoplastic lesions in a long-term study. Male Wistar rats were subjected to initiation with diethylnitrosamine, followed by selection and then by the administration of phenobarbital for 16 weeks. After selection, the rats were given i.m. injections of a purified SAM preparation (384 µmol/kg/day) for 24 weeks. In SAM-treated rats, a decrease in the incidence of PNs was found 6, 14, and 24–28 months after initiation. At the end of SAM treatment the number of PNs per rat liver, nodule diameter, and labeling and mitotic indices of nodular cells decreased considerably in control rats. Nodule diameter started to increase rapidly again only 8 months after arresting SAM treatment, when complete recovery of DNA synthesis in nodular cells occurred. The majority of nodules present in the liver 6–28 months after initiation belonged to the clear and acidophilic cell types, with lower percentages of mixed cell and basophilic cell types. A decrease in basophilic nodules occurred in SAM-treated rats. Fourteen and 24–28 months after initiation hepatocellular carcinoma incidence was 11 of 12 and 10 of 10 in control rats, respectively, and only 1 of 12 and 3 of 11 in SAM-treated rats. At the 24th–28th month all control rats had tumors identified as 2 poorly differentiated carcinomas, 6 trabecular carcinomas, or 3 adenocarcinomas, while only 2 relatively small trabecular carcinomas and 1 small glandular tumor developed in SAM-treated rats. In 3 of 11 SAM-treated rats, but in none of the control rats, leukemic infiltration of liver occurred 24–28 months after initiation. Leukemic infiltration of the spleen occurred in 5 and 3 control and SAM-treated rats, respectively. These results show that a 6-month treatment with SAM causes a great loss in the ability of initiated preneoplastic cells to evolve to cancer for a period of time (28 months) roughly corresponding to two-thirds of the rat life span. Preneoplastic and neoplastic lesions which escape from the SAM chemopreventive effect undergo a relatively slow progression to more malignant lesions. Our results envisage the possibility of preventing human liver tumors in populations at risk.

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Supported by grants from Associazione Italiana Ricerca sul Cancro, MURST (Programs 40% and 60%) and RAS (Ass. Programmazione).

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