Effects of chronic feeding of 2-acetylaminofluorene on nuclear populations in rat liver

Abstract

Male Sprague-Dawley rats were fed 0.04% 2-acetylaminofluorene (AAF) in the diet for 6 weeks. At weekly intervals, rats were killed and liver nuclei were separated into two fractions, one sedimenting through 2.3 m sucrose (class N-I nuclei), and the second sedimenting through 1.6 m sucrose but not 2.3 m sucrose (class N-II nuclei). During AAF administration, the percentage of DNA found in class N-I nuclei decreased from the control value of 86% to 68% at 2 weeks and dropped to 1.4% by 3 weeks, then fluctuated between 10–40% at 4–6 weeks. There were corresponding increases in the amounts of DNA recovered in the class N-II nuclei. The early change in the distribution of DNA between the two nuclear fractions occurred in the absence of any appreciable change in net liver DNA content. In order to study the ploidy of nuclei in these two liver fractions, it was necessary to modify the fractionation procedure somewhat. Two classes of rat liver nuclei were again isolated, one pelleting in 2.3 m sucrose (H nuclei) and the second class sedimenting through 1.6 m and 1.8 m sucrose and banding at the 1.8 m–2.3 m sucrose interface (L nuclei). In normal adult rat liver, the H and L nuclear fractions appeared to be equivalent to the class N-I and N-II nuclei, respectively. However, in younger animals, the L nuclear fraction was the major fraction and the percentage of nuclei found in the L fraction decreased as the animals grew. Both the H and L nuclear fractions contained diploid, tetraploid, and octaploid nuclei but the degree of polyploidy was greater in the H fraction. Concomitant with the change in distribution of nuclei between the H and L fractions with increasing age, there was a progressive increase in the degree of polyploidy in the H fraction. These observations suggested that the shift of liver nuclei from the L fraction to the H fraction was associated with the maturation of hepatocytes. Male and female Sprague-Dawley rats were fed 0.04% AAF and killed at weekly intervals for 12 weeks. The percent distribution of the number of nuclei found in the H and L fractions of livers from control male rats was 75% and 25% respectively, and these values dropped to 3% and 97%, respectively, by 6 weeks of AAF feeding. The degree of polyploidy in the livers of male rats ingesting 0.04% AAF decreased progressively during the initial 6 weeks, going from control values of 40–50% polyploid nuclei to about 20% polyploid nuclei by 6 weeks. Alterations in nuclear populations in livers of the AAF-fed female rats, which are resistant to the induction of liver cancer with AAF, were minimal. The possible basis for alterations in nuclear populations of rat liver during AAF feeding is discussed.