Evidence of two separate mechanisms for the decrease in aryl sulfotransferase activity in rat liver during early stages of 2‐acetylaminofluorene–induced hepatocarcinogenesis

Abstract

Enzymatic and immunohistochemical experiments were conducted to evaluate the mechanistic basis for the downregulation of the important detoxication/bioactivation enzyme aryl sulfotransferase IV (AST IV) during 2-acetylaminofluorene (2AAF)-induced hepatocarcinogenesis. To distinguish between possible genotoxic and cytotoxic actions of 2AAF, three different dietary protocols were used in these experiments: group 1 received 2AAF for 12 wk, group 2 received 2AAF for 3 or 6 wk and then a control diet lacking xenobiotics for 3 or 6 wk, and group 3 received 2AAF 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 was found to decrease 80-90% in response to 2AAF feeding, but activity recovered to essentially normal levels in the livers of rats subsequently placed on either control diets or diets with phenobarbital, suggesting a reversible cytotoxic mechanism for loss of AST IV activity. However, when liver sections from the rats were evaluated immunohistochemically, two distinct patterns were detected for the downregulation of AST 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 developed 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, while expression in the areas surrounding nodules returned to the normal, high levels. In addition, among those rats fed 2AAF for just 3 wk and then control diet or diet containing phenobarbital for 6 wk, only rats fed phenobarbital developed altered foci that stained weakly for AST IV expression. These results show that there were two kinds of 2AAF-mediated decrease in hepatic AST IV activity: a general overall loss of AST IV expression dependent on administration of 2AAF and reversible upon removal of 2AAF from the diet and a loss of AST IV expression among newly developed liver foci and nodules that persisted in the absence of 2AAF administration and appeared to be a property of 2AAF-induced subpopulations of cells. These patterns may correspond, respectively, to cytotoxic and genotoxic mechanisms of 2AAF action.

Loss of nuclear p53 protein in preneoplastic rat hepatocytes is accompanied by Mdm2 and Bcl-2 overexpression and by defective response to DNA damage in vivo

“…Our present data indicate that resistance of preneoplastic GST7-7 ϩ hepatocytes towards growth inhibitory effects of 2-AAF may not be the consequence of their decreased bioactivation of this compound but rather be caused by failure to express nuclear p53. The decrease in sulfotransferase activity responsible for bioactivation of the 2-AAF metabolite N-OH-AAF in these cells during 2-AAF-induced hepatocarcinogenesis 48 may instead be secondary to the lack of p53-mediated growth arrest, as we have shown previously that N-OH-AAF sulfotransferase activity is down-regulated during the cell cycle. 49 In conclusion, we have found that the lack of nuclear p53 in focal hepatocytes might be related to nuclear exclusion of p53 by Mdm2 and/or Bcl-2.…”