Disruptions in DNA repair pathways predispose cells to accumulating DNA damage. A growing body of evidence indicates that tumors accumulate progressively more mutations in DNA repair proteins as cancers progress. DNA repair mechanisms greatly affect the response to cytotoxic treatments, so understanding those mechanisms and finding ways to turn dysregulated repair processes against themselves to induce tumor death is the goal of all DNA repair inhibition efforts. Inhibition may be direct or indirect. This burgeoning field of research is replete with promise and challenge, as more intricacies of each repair pathway are discovered. In an era of increasing concern about healthcare costs, use of DNA repair inhibitors can prove to be highly effective stewardship of R&D resources and patient expenses.

Reduction-oxidation factor 1-apurinic/apyrimidinic endonuclease (Ref-1/APE1) is a critical node in tumor cells, both as a redox regulator of transcription factor activation and as part of the DNA damage response. As a redox signaling protein, Ref-1/APE1 enhances the transcriptional activity of STAT3, HIF-1α, nuclear factor kappa B, and other transcription factors to promote growth, migration, and survival in tumor cells as well as inflammation and angiogenesis in the tumor microenvironment. Ref-1/APE1 is activated in a variety of cancers, including prostate, colon, pancreatic, ovarian, lung and leukemias, leading to increased aggressiveness. Transcription factors downstream of Ref-1/APE1 are key contributors to many cancers, and Ref-1/APE1 redox signaling inhibition slows growth and progression in a number of tumor types. Ref-1/APE1 inhibition is also highly effective when paired with other drugs, including standard-of-care therapies and therapies targeting pathways affected by Ref-1/APE1 redox signaling. Additionally, Ref-1/APE1 plays a role in a variety of other indications, such as retinopathy, inflammation, and neuropathy. In this review, we discuss the functional consequences of activation of the Ref-1/APE1 node in cancer and other diseases, as well as potential therapies targeting Ref-1/APE1 and related pathways in relevant diseases. APX3330, a novel oral anticancer agent and the first drug to target Ref-1/APE1 for cancer is entering clinical trials and will be explored in various cancers and other diseases bringing bench discoveries to the clinic.

Pancreatic ductal adenocarcinoma (PDAC) is the 4th leading cause of cancer-related mortality in the United States. Aggressive treatment regimens have not changed the disease course, and the median survival has just recently reached a year. Several mechanisms are proposed to play a role in PDAC therapeutic resistance, including hypoxia, which creates a more aggressive phenotype with increased metastatic potential and impaired therapeutic efficacy. AP Endonuclease-1/Redox Effector Factor 1 (APE1/Ref-1) is a multi-functional protein possessing a DNA repair function in base excision repair and the ability to reduce oxidized transcription factors, enabling them to bind to their DNA target sequences. APE1/Ref-1 regulates several transcription factors involved in survival mechanisms, tumor growth, and hypoxia signaling. Here, we explore the mechanisms underlying PDAC cell responses to hypoxia and modulation of APE1/Ref-1 redox signaling activity, which regulates the transcriptional activation of hypoxia inducible factor 1 alpha (HIF1α). Carbonic anhydrase IX (CA9) is regulated by HIF1α and functions as part of the cellular response to hypoxia to regulate intracellular pH, thereby promoting cell survival. We hypothesized that modulating APE1/Ref-1 function will block activation of downstream transcription factors, STAT3 and HIF1α, interfering with hypoxia-induced gene expression. We demonstrate APE1/Ref-1 inhibition in patient-derived and established PDAC cells results in decreased HIF1α–mediated induction of CA9. Furthermore, an ex vivo 3D tumor co-culture model demonstrates dramatic enhancement of APE1/Ref-1-induced cell killing upon dual-targeting of APE1/Ref-1 and CA9. Both APE1/Ref-1 and CA9 are under clinical development, therefore these studies have the potential to direct novel PDAC therapeutic treatment.

No abstract

The essential base excision repair protein, apurinic/apyrimidinic endonuclease 1 (APE1), plays an important role in redox regulation in cells and is currently targeted for development of cancer therapeutics. One compound that binds APE1 directly is (E)-3-(2-(5,6-dimethoxy-3-methyl-1,4-benzoquinonyl))-2-nonyl propenoic acid (E3330). Here, we revisit the mechanism by which this negatively charged compound interacts with APE1 and inhibits its redox activity. At high concentrations (mM), E3330 interacts with two regions in the endonuclease active site of APE1, as mapped by hydrogen/deuterium exchange mass spectrometry. However, this interaction lowers the melting temperature of APE1 consistent with a loss of structure in APE1, as measured by both differential scanning fluorimetry and circular dichroism. These results are consistent with other findings that concentrations of E3330 greater than 100 μM are required to inhibit APE1’s endonuclease activity. To determine the role of E3330’s negatively charged carboxylate in redox inhibition, we converted the carboxylate to an amide by synthesizing (E)-2-((4,5-dimethoxy-2-methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)methylene)-N-methoxy-undecanamide (E3330-amide), a novel uncharged derivative. E3330-amide has no effect on the melting temperature of APE1, suggesting that it does not interact with the fully-folded protein. However, E3330-amide inhibits APE1’s redox activity in in vitro EMSA redox and cell-based transactivation assays producing lower IC50 values as compared to E3330, 8.5 μM vs. 20 μM and 7 μM vs. 55 μM, respectively. Thus, E3330’s negatively charged carboxylate is not required for redox inhibition. Collectively, our results provide additional support for a mechanism of redox inhibition involving interaction of E3330 or E3330-amide with partially unfolded APE1.