SEQUENTIAL FUNCTIONAL AND MORPHOLOGICAL ALTERATIONS DURING HEPATOCARCINOGENESIS INDUCED IN RATS BY FEEDING OF A LOW-DOSE OF 2-ACETYLAMINOFLUORENE

The early cellular events in liver carcinogenesis were studied in Fisc her-344 male rats that either were fed 200 ppm 2-acetylaminofluorene ( AAF) for up to 10 wk or were fed the carcinogen for 8 wk followed by m aintenance for an additional 24 wk. By 1 wk of exposure, AAF caused a reduction in the number of glutamine synthetase (GS)-positive centrilo bular hepatocytes, an increase in DNA synthesizing hepatocytes in the central areas of the hepatic lobules, and a shift from multinucleated to mononucleated hepatocytes, although overt hepatocellular necrosis w as not evident. By 3 wk, altered hepatocellular foci characterized by deficiencies in iron storage (IS-) and collagen production and by expr ession of gamma-glutamyl transferase (GGT(+)) and placental-type gluta thione transferase (PGT(+)) activity appeared. Single PGT(+) cells wer e also found. During continued exposure, foci increased in number, siz e, and total area with the increases escalating between 8 and 10 wk of exposure. Cessation of AAF exposure at 8 wk resulted in a slight decr ease in the number of foci after a further 6 wk of maintenance, but wi th continued maintenance for another 6 and 12 wk, the number again inc reased. IS- characterized the majority of foci during carcinogen admin istration, whereas after cessation of exposure, GGT(+) and PGT(+) foci predominated. None of the foci were positive for GS. After AAF exposu re for 10 wk, a few neoplasms developed and greater numbers occurred a fter maintenance for a further 24 wk of rats exposed for 8 wk. We conc lude the following: (a) the low dose of AAF caused subtle alterations in function and proliferation of normal hepatocytes and converted hepa tocytes into focus cells; (b) reduction of the GS(+) area is a sensiti ve indicator of cytotoxicity of AAF; (c) the development of some foci at an early stage depends on a promoting action of AAF, which ceased w hen the carcinogen was withdrawn, allowing some foci to undergo revers ion; (d) a strong linkage exists in expression of IS-, GGT(+), and PGT (+) in foci; (e) the carcinogenic process accelerates in the absence o f any indication of increased cytotoxicity by AAF; and (f) under the c onditions of this study, no GS(+) foci, adenomas, and carcinomas were found, indicating that no carcinogen-induced expression of GS occurred in these lesions and that GS expression is not linked to other phenot ypic abnormalities.