5F). Previous studies have shown that long-lived Little mice HDAC activity assay have increased levels of genes involved in the xenobiotic detoxification and that crossing these mice with FXR KO mice corrected their expression.17 We performed western
blot analysis and found a four- to five-fold elevation of FXR in 24- to 36-month-old Little mice (Fig. 6A,B). It has been shown that the frequency of liver tumors increases with age and reaches around 30% at the age of 24 months.5 However, Little mice do not develop liver cancer with age. Therefore, we tested the hypothesis that high levels of FXR in old Little mice protect the liver from development of cancer. WT and Little mice were treated with DEN, and liver tumors were examined 35-36 weeks after DEN injection. We examined five WT mice and five Little mice and found that all WT animals developed advanced liver cancer, whereas only two Little mice find more had few tumor nodules of a very small size (Fig. 6C). Three other Little mice did not have liver cancer. Examination of liver sections via hematoxylin and eosin staining revealed that the livers of WT mice contained multiple diverse nodules of proliferating hepatocytes, including enlarged cells with moderate anisonucleosis on the left and a cluster of small, uniform, deeply
basophilic cells on the right (Fig. 6D). In contrast, livers of Little mice treated with DEN showed unremarkable architecture and cytology, with uniform hepatocytes containing minimal cytoplasmic lipid and glycogen. We found that the number of replicating hepatocytes increased significantly in WT mice (up to 25%-30%), while around 5% of hepatocytes were BrdU-positive
in the livers of Little mice (Fig. 6E,F). These data show that Little mice are resistant to the development of liver cancer after DEN treatment. We next determined the molecular mechanisms by which Little mice are protected from liver cancer. A recent report showed that gankyrin causes degradation of the liver-specific transcription factor hepatocyte nuclear factor 4α (HNF4α).22 Therefore, we included this Endonuclease protein in our studies. We found that gankyrin was elevated and that it caused reduction of C/EBPα, Rb, HNF4α, and p53 in control WT mice (Fig. 7A,B). FXR was slightly reduced in WT mice; however, in Little mice, FXR levels remained at high levels, leading to the lack of activation of the gankyrin and to no reduction of C/EBPα, Rb, HNF4α, or p53. The reduction of the tumor repressor proteins in WT mice took place on the levels of protein degradation, since levels of mRNA were not changed significantly (Fig. 7C). To determine whether gankyrin is responsible for the degradation of tumor suppressor proteins, we examined interactions of these proteins with gankyrin. In these experiments, we used up to 1 mg of nuclear extracts for the co-immunoprecipitation studies.