Mitochondrial apurinic/apyrimidinic endonuclease 1 enhances mtDNA repair contributing to cell proliferation and mitochondrial integrity in early stages of hepatocellular carcinoma

Abstract

Background Hepatocellular carcinoma (HCC) is the leading cause of primary liver cancers. Surveillance of individuals at specific risk of developing HCC, early diagnostic markers, and new therapeutic approaches are essential to obtain a reduction in disease-related mortality. Apurinic/apyrimidinic endonuclease 1 (APE1) expression levels and its cytoplasmic localization have been reported to correlate with a lower degree of differentiation and shorter survival rate. The aim of this study is to fully investigate, for the first time, the role of the mitochondrial form of APE1 in HCC. Methods As a study model, we analyzed samples from a cohort of patients diagnosed with HCC who underwent surgical resection. Mitochondrial APE1 content, expression levels of the mitochondrial import protein Mia40, and mtDNA damage of tumor tissue and distal non-tumor liver of each patient were analyzed. In parallel, we generated a stable HeLa clone for inducible silencing of endogenous APE1 and re-expression of the recombinant shRNA resistant mitochondrially targeted APE1 form (MTS-APE1). We evaluated mtDNA damage, cell growth, and mitochondrial respiration. Results APE1’s cytoplasmic positivity in Grades 1 and 2 HCC patients showed a significantly higher expression of mitochondrial APE1, which accounted for lower levels of mtDNA damage observed in the tumor tissue with respect to the distal area. In the contrast, the cytoplasmic positivity in Grade 3 was not associated with APE1’s mitochondrial accumulation even when accounting for the higher number of mtDNA lesions measured. Loss of APE1 expression negatively affected mitochondrial respiration, cell viability, and proliferation as well as levels of mtDNA damage. Remarkably, the phenotype was efficiently rescued in MTS-APE1 clone, where APE1 is present only within the mitochondrial matrix. Conclusions Our study confirms the prominent role of the mitochondrial form of APE1 in the early stages of HCC development and the relevance of the non-nuclear fraction of APE1 in the disease progression. We have also confirmed overexpression of Mia40 and the role of the MIA pathway in the APE1 import process. Based on our data, inhibition of the APE1 transport by blocking the MIA pathway could represent a new therapeutic approach for reducing mitochondrial metabolism by preventing the efficient repair of mtDNA.

Ref-1 redox activity alters cancer cell metabolism in pancreatic cancer: exploiting this novel finding as a potential target

“…As mitochondria can also generate reactive oxygen species (ROS) that damage nuclear and mitochondrial DNA, the role of Ref-1’s BER function was hypothesized to also be important in the observed phenotype, as seen in the experiments with Ref-1 siRNA knockdown. Several studies including ours demonstrated that the repair role of Ref-1 (APE1) is essential to maintain the integrity of mitochondria after oxidative stress [ 24 , 87 , 88 ]. Ref-1’s redox regulation of transcription factors leading to gene expression changes that ultimately regulate mitochondrial function has been less explored.…”

Molecular Mechanisms Regulating the DNA Repair Protein APE1: A Focus on Its Flexible N-Terminal Tail Domain

“…Moreover, translocation of APE1 from the cytoplasm to the mitochondria, where it is thought to be involved mtDNA repair [65,72], is mediated by ROS signalling and Arg301 methylation and depends on Tom20 [73]. Once in the intermembrane space, which is thought to represent a storage site, a rise of mitochondrial ROS triggers a rapid translocation of APE1 into the matrix, through the Tim23 complex [74].…”

Role of Oxidative DNA Damage and Repair in Atrial Fibrillation and Ischemic Heart Disease

“…Some bi-functional glycosylases also possess AP lyase activity that cuts the phosphodiester bond of DNA and creates a single-strand break [ 84 ]. APE1, located prominently in nucleus as well as in the mitochondrial matrix, has the capacity of creating termini specific for the newly to-be-inserted bases, which significantly activates PARP1 [ 85 , 86 ]. As the response to DNA damage, ADP-ribosylation by PARP1 triggers the recruitment of various components of the BER complex, including scaffold protein X-ray repair cross-complementing protein 1, bifunctional polynucleotide kinase, and gap-filling DNA polymerase beta (POLB) and DNA ligase III (LIG3) [ 87 ].…”