Correlation studies between the binding of aflatoxin B1 to chromatin components and the inhibition of RNA synthesis

Abstract

Aflatoxin B1 (AFB1) is a potent inhibitor of rat liver nuclear and nucleolar RNA synthesis. However, since after activation AFB1 binds to both DNA and chromosomal proteins, the question is which form of binding is responsible for the inhibition of RNA synthesis. Male Sprague-Dawley rats (200 g) were given i.p. injections of 10, 50, 100, 300 and 500 micrograms AFB1 per 100 g body wt containing 50 microCi [3H]AFB1 (sp. act. 25 Ci/mmol), and the animals sacrificed 2 h later. Liver nuclei, nucleoli and P-3 (a transcriptionally active subnucleolar fraction that is 3.4-fold enriched in active rDNA) were isolated and the binding of AFB1 to DNA and protein of each fraction was determined by DNase I digestion and 5% trichloracetic acid (TCA) hydrolysis. We found that the binding of AFB1 to both nuclear and nucleolar DNA plateaus at 300 micrograms AFB1 per 100 g body wt with values around 100 and 400 pmol AFB1 per mg nuclear and nucleolar DNA, respectively. On the other hand, the binding to protein is linear, although with different slopes, for both nuclear and nucleolar fractions even at 500 micrograms AFB1 per 100 g body wt, the highest dose used. Since AFB1 inhibition of nuclear and nucleolar RNA synthesis plateaus respectively at 60% and 90% inhibition levels at the dose of 300 micrograms AFB1 per 100 g body wt, these results suggest the binding of AFB1 to DNA, but not to protein, is responsible for the inhibition of RNA synthesis. Further support for this contention is obtained by comparing the binding and the inhibition data between P-3 and nucleoli. P-3 is three times more transcriptionally active than nucleoli and, as a result, is more sensitive to AFB1 inhibition. This greater sensitivity is reflected by the specific binding activity of AFB1 to P-3 DNA, which is greater than 50% higher than to nucleolar DNA. In contrast, this effect is not reflected by the specific binding activity of AFB1 to protein which is exactly the same for both fractions.