Abstract
The effects of aflatoxin B1 (AFB1) and sterigmatocystin (STC) on the hormonal induction of liver-specific enzymes and the binding capacity of glucocorticoid receptor (GR) were observed with cultured mammalian cells, H4-II-E, which possess many functional enzymes of mature liver cells. The liver-specific enzymes such as tyrosine aminotransferase (TAT), phosphoenol-pyruvate carboxykinase (PEPCK), glutamic-oxaloacetic transaminase (GOT), and glutamic-pyruvic transaminase (GPT), were induced by various hormones such as hydrocortisone (HC), insulin, and dibutyryl cAMP (Bt2 cAMP) in various degrees. The 50% inhibi-tion concentrations (IC50, μg/ml) of AFB1 to HC-dependent induction of TAT, GPT, and PEPCK were 0.2, 0.055, and 2.0, respectively. Whereas, insulin-dependent induc-tion of TAT was slightly inhibited by AFB1. As for STC, the IC50 for HC induction of TAT was 3.5 μg/ml, and no inhibitory effect on insulin-dependent induction was observed. Quick-blot analysis, which showed a marked reduction of the HC-induced synthesis of TAT mRNA, indicated that AFB1 inhibited a transcriptional step. In the cell-free system, both mycotoxins showed no direct inhibitory effect on the formation of [3H]triamcinolone acetonide (TA)-GR complex, its activation and binding to nuclear acceptor sites. The inhibition of this GR-complex formation was observed only when the mycotoxins were activated by S9 system. In the whole cell system pretreated with AFB1, the formation of cytosolic [3H]TAGR complex was greatly reduced, and the release of nuclear GR complex was promoted. STC caused no marked reduction of the cytosolic GR complex. Kinetic analysis revealed that the cytosolic GR receptor was markedly impaired by AFBI, and the capacity of nuclear GR acceptor sites was greatly reduced by STC. The present data indicated marked differences between the carcinogenic bisfuranoids, AFB1 and STC, in regards to the hormonal induction of liver-specific enzymes and the function of cytosolic/nuclear GRs, and the impairment of cellular factors (enzymes) which regulates GR recycling system was suggested.
