The variations of S-adenosyl-L-methionine content modulate hepatocyte growth during phenobarbital promotion of diethylnitrosamine-induced rat liver carcinogenesis.

A decrease in liver S-adenosyl-L-methionine (SAM) content and an increase in ornithine decarboxylase (ODC) activity occurred between the 2nd and the 5th week after starting 2-acetylaminofluorene (AAF) feeding in diethylnitrosamine (DENA)-initiated rats. These rats then received a 0.05% phenobarbital (PB)-containing diet for 18 weeks after the end of AAF feeding. Two weeks after starting AAF, an increase in the hepatocyte labeling index (LI) also occurred in gamma-glutamyl-transpeptidase (GGT)-positive foci and surrounding tissue. LI returned to control values in a few days in surrounding tissue, while it remained high for at least 4 weeks in the foci. Analogous changes were observed, but for a shorter period of time, in the rats subjected to partial hepatectomy (PH) plus AAF, in which no GGT-positive foci developed. Twenty-four weeks after starting AAF, 30% of the liver was occupied by visible nodules. ODC activity and LI were high and SAM was low in nodules, but they were near to control values in surrounding liver. SAM administration reconstituted the liver SAM pool, inhibited ODC activity, and prevented visible nodule development. SAM inhibition of ODC activity occurred in vitro only after preincubation with liver homogenate and was enhanced by adenine, an inhibitor of methylthioadenosine (MTA) phosphorylase. MTA addition to the reaction of mixture for ODC determination was inhibitory. The SAM decrease in both liver and nodules was coupled with a decrease of MTA content. SAM administration caused MTA accumulation in the liver. It is suggested that liver SAM content by influencing MTA level, could be a rate-limiting factor for growth and promotion, through a modulation of polyamine synthesis.