Molecular and Histological Responses to AOM in Apex1 Haploinsufficient Mice Reveal a Protective Role of Ape1 in Liver Tissue

Strong evidence indicates that in liver diseases such as cirrhosis and hepatocellular carcinoma, there is a reduction of mitochondrial components, particularly mitochondrial DNA (mtDNA), thus suggesting that loss of mtDNA integrity plays an essential role in the development of liver pathologies. However, little is known about mtDNA repair's contribution to maintaining mtDNA and the prevention of liver-related diseases. We hypothesize that APE1, the main apurinic/apyrimidinic endonuclease that participates in base excision repair-mediated mtDNA repair, is required for the maintenance of mtDNA integrity after liver injury. To test this hypothesis, we used a mouse model with a heterozygous null mutation in the Apex1 gene, which encodes APE1. Liver tissue was evaluated for molecular and histological effects after treatment with the alkylating agent azoxymethane (AOM), a carcinogen widely used for the induction of colorectal cancer in rodents. AOM is bioactivated in the liver, thus this organ could represent a primary target of AOM action and a model for liver injury. We treated WT and Apex1 ^+/− mice with AOM (10 mg/kg body weight) once a week for 4 weeks and liver tissue was harvested 24 weeks after the first dose. Using a PCR-based approach, we observe a 3.2-fold increase in mtDNA damage and a concomitant 55% decrease in mtDNA abundance only in tissue from Apex1 ^+/− mice. To study the bioenergetics status of liver tissues after AOM treatment, we determined the relative protein levels of ATP5β (an oxidative phosphorylation marker) and GAPDH (a glycolysis marker). We observed a 1.5-fold increase and a 2.5-fold increase in the ATP5β/GAPDH in WT and Apex1 ^+/− mice, respectively, indicative of increased oxidative phosphorylation in response to AOM-induced alkylation damage. The noted alterations occur within significant histological transformations, including increased nuclear inclusions and ductular proliferation in liver tissue triggered by AOM in both strains of mice. In contrast, indicators of inflammation and hepatocyte injury, such as portal inflammation and fibrosis, were attenuated only in Apex1 ^+/− mice. In summary, these findings underscore the pivotal role of APE1 in the response of liver tissue to AOM-induced liver damage.