Apurinic/apyrimidinic endonuclease/redox effector factor-1 (APE/Ref-1) is a multifunctional protein involved both in DNA base excision repair and redox regulation. Studies have suggested that abnormal Ref-1 levels and/or activities are associated with tumor progression and sensitivities to treatment, but no direct evidence has yet been published regarding the role of Ref-1 in malignant transformation. We utilized the well-documented tumor promotor-sensitive JB6 mouse epithelial cell model as well as new transformants [by ultraviolet light B (UVB), H2O2 or Cd] to study this phenomenon. Significant increases of reactive oxygen species (ROS) were observed in JB6P+ and all the transformants compared with promotor-resistant JB6P- cells. These increases were paralleled by a sustained elevation of Ref-1 expression. Further analysis exhibited a strong inverse correlation between oxidative DNA lesions [8-oxodeoxyguanosine (8-oxo-dG)] and Ref-1 levels in all JB6 cells. Notably, apoptosis occurred after knock-down of Ref-1 by small interfering RNA (siRNA)] demonstrated by a approximately 2-fold increase of Annexin V-positive JB6P+ cells. Ref-1 depletion also inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced anchorage-independent growth of JB6P+ by 40% and reduced the colony numbers of JB6P+/H2O2 and JB6P+/Cd cells. Mechanistic studies revealed that Ref-1 reduction was associated with an increase of intracellular ROS levels and a marked decrease of activator protein-1 (AP-1) transcription activities in JB6P+/H2O2 cells. This is the first report of the novel role of Ref-1 in cellular transformation. Based on the data presented here, we propose that induction of Ref-1, serving as an adaptive response to elevated ROS, plays a critical role in transformation and protects cells from excess ROS stresses through both DNA repair and activation of transcription factors such as activator protein-1.

Hepatocellular carcinoma (HCC) is responsible for a third of the estimated cancer-caused deaths worldwide. To deeply understand the mechanisms controlling HCC progression is of primary importance to develop new approaches for treatment. Apurinic/apyrimidinic endonuclease-1/redox effector factor 1 (APE/Ref-1) has been uncovered elevated in various types of cancer, including HCC. Additionally, HCC progression is always correlated with elevated copper (Cu). Our previous data demonstrated that Cu treatment initiated APE/Ref-1 expression and its downstream targets. Therefore, we hypothesized that APE/Ref-1 may be involved in HCC progression through mediating the effect of Cu to its signaling cascades. Following different treatments, human HCC cell line (Hep3B) and immortalized non-malignant hepatocyte cell line (THLE3) were analyzed to explore the role of APE/Ref-1 signaling pathway. Unstained human tissue microarrays (TMA) were subjected to IHC analysis to study the relationship between APE/Ref-1 expression and clinic features. APE/Ref-1 was upregulated in HCC cells consistent with the strong expression of APE/Ref-1 in HCC tissue microarray. Greater cytoplasmic accumulation of APE/Ref-1 was found in poorly differentiated and more aggressive tumors. Also we provide evidence to show that APE/Ref-1 signaling pathway stimulates cellular proliferation, enhances anti-apoptosis, and facilitates metastasis through experimental knockdown of APE/Ref-1 using siRNA in Hep3B cells or overexpressing APE/Ref-1 in THLE3 cells. These results define a novel role of APE/Ref-1 in HCC progression as being an important mediating and potentiating molecule, and also provide a basis for further investigations utilizing appropriate APE/Ref-1 inhibitors in combination with chemo-drugs for HCC treatment.