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Evidence of two separate mechanisms for the decrease in aryl sulfotransferase activity in rat liver during early
D P Ringer1, B A Howell, T R Norton
1Biochemical Pharmacology Section, Samuel Roberts Noble Foundation, Inc., Ardmore, Oklahoma.
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.
Insights
Hepatocarcinogenesis involves two mechanisms for aryl sulfotransferase IV (AST IV) loss: reversible cytotoxic effects and persistent genotoxic effects in liver nodules. This study differentiates these impacts of 2-acetylaminofluorene (2AAF) on AST IV enzyme activity.
Area of Science:
- Hepatocarcinogenesis research
- Enzyme kinetics and regulation
- Toxicology and xenobiotic metabolism
Background:
- Aryl sulfotransferase IV (AST IV) is a key enzyme in detoxication and bioactivation pathways.
- Its downregulation is observed during 2-acetylaminofluorene (2AAF)-induced hepatocarcinogenesis, but the underlying mechanisms remain unclear.
- Distinguishing between cytotoxic and genotoxic effects of 2AAF is crucial for understanding liver cancer development.
Purpose of the Study:
- To elucidate the mechanistic basis for AST IV downregulation during 2AAF-induced hepatocarcinogenesis.
- To differentiate between cytotoxic and genotoxic actions of 2AAF on AST IV expression and activity.
- To investigate the reversibility of AST IV loss and its persistence in specific cellular subpopulations.
Main Methods:
- Enzymatic assays to measure hepatic AST IV activity.
- Immunohistochemical analysis to evaluate AST IV expression patterns in liver sections.
- Three dietary protocols involving 2AAF administration, withdrawal, and phenobarbital treatment in rats.
Main Results:
- Hepatic AST IV activity decreased significantly (80-90%) with 2AAF feeding but recovered upon 2AAF withdrawal or phenobarbital treatment, indicating a reversible cytotoxic effect.
- Immunohistochemistry revealed two patterns: general AST IV downregulation (reversible) and persistent low AST IV expression in liver nodules/foci (irreversible).
- Phenobarbital treatment induced altered foci with weak AST IV staining, suggesting a genotoxic mechanism contributing to specific cell subpopulation changes.
Conclusions:
- 2-Acetylaminofluorene (2AAF) induces two distinct types of hepatic AST IV decrease: a reversible, diet-dependent cytotoxic effect and an irreversible, genotoxic effect in developing liver nodules.
- These findings highlight the differential mechanisms of 2AAF toxicity and their specific impacts on AST IV expression during hepatocarcinogenesis.
- The results provide insights into the complex interplay between cytotoxic and genotoxic events in chemical-induced liver cancer.