EVIDENCE OF 2 SEPARATE MECHANISMS FOR THE DECREASE IN ARYL SULFOTRANSFERASE ACTIVITY IN RAT-LIVER DURING EARLY STAGES OF 2-ACETYLAMINOFLUORENE-INDUCED HEPATOCARCINOGENESIS

Enzymatic and immunohistochemical experiments were conducted to evalua te the mechanistic basis for the downregulation of the important detox ication/bioactivation enzyme aryl sulfotransferase IV (AST IV) during 2-acetylaminofluorene (2AAF)-induced hepatocarcinogenesis. To distingu ish between possible genotoxic and cytotoxic actions of 2AAF, three di fferent dietary protocols were used in these experiments: group 1 rece ived 2AAF for 12 wk, group 2 received 2AAF for 3 or 6 wk and then a co ntrol diet lacking xenobiotics for 3 or 6 wk, and group 3 received 2AA F for 3 or 6 wk and then phenobarbital for 3 or 6 wk. When hepatic AST IV activity was assessed, N-hydroxy-2AAF sulfotransferase activity wa s found to decrease 80-90% in response to 2AAF feeding, but activity r ecovered to essentially normal levels in the livers of rats subsequent ly placed on either control diets or diets with phenobarbital, suggest ing a reversible cytotoxic mechanism for loss of AST IV activity. Howe ver, when liver sections from the rats were evaluated immunohistochemi cally, two distinct patterns were detected for the downregulation of A ST IV activity. In the livers of rats administered only 2AAF (group 1) , a general pattern of overall downregulation of AST IV expression was observed throughout the liver and among most but not all newly develo ped nodules. In tissue sections from rats initially fed 2AAF and then placed on a control diet (group 2) or a diet with phenobarbital (group 3), the nodules continued to show low levels of AST IV expression, wh ile expression in the areas surrounding nodules returned to the normal , high levels. In addition, among those rats fed 2AAF for just 3 wk an d then control diet or diet containing phenobarbital for 6 wk, only ra ts fed phenobarbital developed altered foci that stained weakly for AS T IV expression. These results show that there were two kinds of 2AAF- mediated decrease in hepatic AST IV activity: a general overall loss o f AST IV expression dependent on administration of 2AAF and reversible upon removal of 2AAF from the diet and a loss of AST IV expression am ong newly developed liver foci and nodules that persisted in the absen ce of 2AAF administration and appeared to be a property of 2AAF-induce d subpopulations of cells. These patterns may correspond, respectively , to cytotoxic and genotoxic mechanisms of 2AAF action. (C) 1994 Wiley -Liss, Inc.