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Video of caspase 3 activity used to capture images used in quantification of caspase activation, indicating apoptosis induction. Representative video of UC3 cells treated with APX2009 as red indicator dye generated from Caspase 3 activity is shown overlayed with brightfield image for each treatment. Duration of the video is the 72 hour duration of the experiment, as quantified and shown in Figure 2. Representative stills are shown in supplementary figure 3.

CA9 protein levels are increased under hypoxia, but APE1 levels are not

Confirmation of APE1/Ref-1 and STAT3 expression in 3d and monolayer conditions in RP-B-O1 and RP-B-O2 cells. Basal growth conditions as described in the methods for each cell line were used for monolayer and 3D conditions. No significant differences in expression relative to actin were observed in either condition. Representatuive blot is shown in [A], and quantified data as the ratio of each protein to actin from 3 determinations is shown in [B,C].

APE1/Ref-1 protein expression affects CA9 mRNA levels in additional PDAC cell lines

Confirmation of HIF-1 deletion in MEF cells

Supplementary Figure 2 from Impact of APE1/Ref-1 Redox Inhibition on Pancreatic Tumor Growth

APE1/Ref-1 interactions with STAT3 and NFκB are stimulated by IL-6 and TNFα (respectively) under normoxic conditions

Video of caspase 3 activity used to capture images used in quantification of caspase activation, indicating apoptosis induction. Representative video of UC3 cells treated with DMSO control red dye by itself, Duration of the video is the 72 hour duration of the experiment, as quantified and shown in Figure 2. Representative stills are shown in supplementary figure 3.

Video of caspase 3 activity used to capture images used in quantification of caspase activation, indicating apoptosis induction. Representative video of UC3 cells treated with APX2009 as red indicator dye generated from Caspase 3. Duration of the video is the 72 hour duration of the experiment, as quantified and shown in Figure 2. Representative stills are shown in supplementary figure 3.

Suppl Fig S1-S4

Cell viability analysis used for normalization of pH data

Supplementary Methods, Figure Legends 1-3 from Impact of APE1/Ref-1 Redox Inhibition on Pancreatic Tumor Growth

Supplementary figure legends and table

Pancreatic ductal adenocarcinoma (PDAC) is the fourth 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 multifunctional 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 a 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 the 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 three-dimensional tumor coculture 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. Mol Cancer Ther; 15(11); 2722–32. ©2016 AACR.

Pancreatic cancer is especially a deadly form of cancer with a survival rate less than 2%. Pancreatic cancers respond poorly to existing chemotherapeutic agents and radiation, and progress for the treatment of pancreatic cancer remains elusive. To address this unmet medical need, a better understanding of critical pathways and molecular mechanisms involved in pancreatic tumor development, progression, and resistance to traditional therapy is therefore critical. Reduction–oxidation (redox) signaling systems are emerging as important targets in pancreatic cancer. AP endonuclease1/Redox effector factor 1 (APE1/Ref-1) is upregulated in human pancreatic cancer cells and modulation of its redox activity blocks the proliferation and migration of pancreatic cancer cells and pancreatic cancer-associated endothelial cells in vitro. Modulation of APE1/Ref-1 using a specific inhibitor of APE1/Ref-1′s redox function, E3330, leads to a decrease in transcription factor activity for NFκB, AP-1, and HIF1α in vitro. This study aims to further establish the redox signaling protein APE1/Ref-1 as a molecular target in pancreatic cancer. Here, we show that inhibition of APE1/Ref-1 via E3330 results in tumor growth inhibition in cell lines and pancreatic cancer xenograft models in mice. Pharmacokinetic studies also show that E3330 attains more than10 μmol/L blood concentrations and is detectable in tumor xenografts. Through inhibition of APE1/Ref-1, the activity of NFκB, AP-1, and HIF1α that are key transcriptional regulators involved in survival, invasion, and metastasis is blocked. These data indicate that E3330, inhibitor of APE1/Ref-1, has potential in pancreatic cancer and clinical investigation of APE1/Ref-1 molecular target is warranted. Mol Cancer Ther; 10(9); 1698–708. ©2011 AACR.

APE1 expression correlates with decreased survival in PDAC and CA9 is upregulated in PDAC

Additional methods including synthesis information for one of the inhibitors used and methods used for TCGA survival analysis found in supplemental figure 7.

Bladder cancer is the ninth most common cause of cancer-related deaths worldwide. Although cisplatin is used routinely in treating bladder cancer, refractory disease remains lethal for many patients. The recent addition of immunotherapy has improved patient outcomes; however, a large cohort of patients does not respond to these treatments. Therefore, identification of innovative molecular targets for bladder cancer is crucial. Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a multifunctional protein involved in both DNA repair and activation of transcription factors through reduction–oxidation (redox) regulation. High APE1/Ref-1 expression is associated with shorter patient survival time in many cancer types. In this study, we found high APE1/Ref-1 expression in human bladder cancer tissue relative to benign urothelium. Inhibition of APE1/Ref-1 redox signaling using APE1/Ref-1–specific inhibitors attenuates bladder cancer cell proliferation in monolayer, in three-dimensional cultures, and in vivo. This inhibition corresponds with an increase in apoptosis and decreased transcriptional activity of NF-κB and STAT3, transcription factors known to be regulated by APE1/Ref-1, resulting in decreased expression of downstream effectors survivin and Cyclin D1 in vitro and in vivo. We also demonstrate that in vitro treatment of bladder cancer cells with APE1/Ref-1 redox inhibitors in combination with standard-of-care chemotherapy cisplatin is more effective than cisplatin alone at inhibiting cell proliferation. Collectively, our data demonstrate that APE1/Ref-1 is a viable drug target for the treatment of bladder cancer, provide a mechanism of APE1/Ref-1 action in bladder cancer cells, and support the use of novel redox-selective APE1/Ref-1 inhibitors in clinical studies.Significance:This work identifies a critical mechanism for APE1/Ref-1 in bladder cancer growth and provides compelling preclinical data using selective redox activity inhibitors of APE1/Ref-1 in vitro and in vivo.

Structures and NMR analysis of novel ref-1 inhibitors APX2009 and APX2014. A. APX3330; NMR and HPLC as previously published.15-18 B. APX2009; HPLC: HPLC were performed using an Alltech Alltima column C18 5u, 250 x 5.6 mm, flow 1 mL/min at 40 .C. Elution was with a mobile phase of 15:10:75 water:A1:methanol where A1 was made using 700 mL of water, 300 mL methanol and 3mL trimethylamine to which phosphoric acid was added to bring the pH to 3.4. HPLC analysis showed a purity of >99%. NMR: 300 MHz NMR -conforms to structure. NMR (CDCl3) Î' 8.15 (m, 2H), 7.75 (m,2H), 6.2 (s, 1H), 4.1 (s, 3H), 3.6 (br d, 4H), 2.2 (t, 2H), 1.4 (m, 4H), 1.25 (br d, 4H), 0.85(t, 3H). B. APX2014; HPLC: HPLC analysis was performed as for APX2009, but using a Zorbax SB Phenyl C18 5u, 250 x 4.6 mm column. Elution was with a mobile phase of 25:10:65 water:A1:methanol where A1 was made using 700 mL of water, 300 mL methanol and 3mL trimethylamine to which phosphoric acid was added to bring the pH to 3.4. Flow 1 mL/min at 40 .C. and showed a purity of >99%. NMR: 300 MHz NMR -conforms to structure. NMR (CDCl3) Î' 8.8 (br s,1H), 8.1 (m, 2H), 7.75 (m, 2H), 6.7 (s, 1H), 4.15 (s, 3H), 3.9 (s, 3H), 2.2 (m, 2H), 1.4 (m, 2H), 0.85 (t, 3H).

Supplementary Figure 1 from Impact of APE1/Ref-1 Redox Inhibition on Pancreatic Tumor Growth

Confirmation of cell death induction by caspase activity assay and PARP cleavage. Representative images for caspase activity via red fluorescence [A] for the data calculated and shown in Figure 2, each with matching brightfield images. Representative examples for T24 and UC3 cells are shown form cells treated with the growth-inhibitor EC50 concentrations for each drug, in each cell line. PARP cleavage was used to confirm the induction of apoptotic mechanisms by western blotting for cleaved PARP1 [B], with representative cell experiments in which UC3 cells were treated with EC50 and EC90 concentrations of each drug.

Legend for supplementary videos.

APE1/Ref-1 localization in human tumors. Additional examples of APE1/Ref-1 immunofluorescence in human bladder cancer specimens at 10X and 40X. Lower power (10X) [A] demonstrates transition from nuclear localized APE1/Ref-1 in the epithelial compartment (Epi) to cytosolic staining in the invasive regions of a human tumor, in the muscularis (Musc) in a single image, with high power (40x) views showing the cellular localization [B,C]. Additional examples in high power are shown in [D], from 3 different specimens (1-3), exclusively nuclear staining in the epithelial compartment (Epi) and cytosolic staining in the invasive satellite lesions (INV).

APE1/Ref-1 interactions with HIF1α and STAT3 are stimulated by hypoxia in Cancer-Associated Fibroblast (CAF) cells

Figure Legends for Supplemental Figures

APE1/Ref-1 (apurinic/apyrimidinic endonuclease 1, APE1 or APEX1; redox factor-1, Ref-1) is a dual-functional enzyme with crucial roles in DNA repair, reduction/oxidation (redox) signaling, and RNA processing and metabolism. The redox function of Ref-1 regulates several transcription factors, such as NF-κB, STAT3, HIF-1α, and others, which have been implicated in multiple human diseases, including ocular angiogenesis, inflammation, and multiple cancers. To better understand how APE1 influences these disease processes, we investigated the effects of APEX1 knockdown (KD) on gene expression in human retinal endothelial cells. This abolishes both DNA repair and redox signaling functions, as well as RNA interactions. Using RNA-seq analysis, we identified the crucial signaling pathways affected following APEX1 KD, with subsequent validation by qRT-PCR. Gene expression data revealed that multiple genes involved in DNA base excision repair, other DNA repair pathways, purine or pyrimidine metabolism signaling, and histidine/one carbon metabolism pathways were downregulated by APEX1 KD. This is in contrast with the alteration of pathways by APEX1 KD in human cancer lines, such as pancreatic ductal adenocarcinoma, lung, HeLa, and malignant peripheral nerve sheath tumors. These results highlight the unique role of APE1/Ref-1 and the clinical therapeutic potential of targeting APE1 and pathways regulated by APE1 in the eye. These findings provide novel avenues for ocular neovascularization treatment.

BackgroundPancreatic cancer is a complex disease with a desmoplastic stroma, extreme hypoxia, and inherent resistance to therapy. Understanding the signaling and adaptive response of such an aggressive cancer is key to making advances in therapeutic efficacy. Redox factor-1 (Ref-1), a redox signaling protein, regulates the conversion of several transcription factors (TFs), including HIF-1α, STAT3 and NFκB from an oxidized to reduced state leading to enhancement of their DNA binding. In our previously published work, knockdown of Ref-1 under normoxia resulted in altered gene expression patterns on pathways including EIF2, protein kinase A, and mTOR. In this study, single cell RNA sequencing (scRNA-seq) and proteomics were used to explore the effects of Ref-1 on metabolic pathways under hypoxia.MethodsscRNA-seq comparing pancreatic cancer cells expressing less than 20% of the Ref-1 protein was analyzed using left truncated mixture Gaussian model and validated using proteomics and qRT-PCR. The identified Ref-1’s role in mitochondrial function was confirmed using mitochondrial function assays, qRT-PCR, western blotting and NADP assay. Further, the effect of Ref-1 redox function inhibition against pancreatic cancer metabolism was assayed using 3D co-culture in vitro and xenograft studies in vivo.ResultsDistinct transcriptional variation in central metabolism, cell cycle, apoptosis, immune response, and genes downstream of a series of signaling pathways and transcriptional regulatory factors were identified in Ref-1 knockdown vs Scrambled control from the scRNA-seq data. Mitochondrial DEG subsets downregulated with Ref-1 knockdown were significantly reduced following Ref-1 redox inhibition and more dramatically in combination with Devimistat in vitro. Mitochondrial function assays demonstrated that Ref-1 knockdown and Ref-1 redox signaling inhibition decreased utilization of TCA cycle substrates and slowed the growth of pancreatic cancer co-culture spheroids. In Ref-1 knockdown cells, a higher flux rate of NADP + consuming reactions was observed suggesting the less availability of NADP + and a higher level of oxidative stress in these cells. In vivo xenograft studies demonstrated that tumor reduction was potent with Ref-1 redox inhibitor similar to Devimistat.ConclusionRef-1 redox signaling inhibition conclusively alters cancer cell metabolism by causing TCA cycle dysfunction while also reducing the pancreatic tumor growth in vitro as well as in vivo.

Background MPNST is a rare soft-tissue sarcoma that can arise from patients with NF1. Existing chemotherapeutic and targeted agents have been unsuccessful in MPNST treatment, and recent findings implicate STAT3 and HIF1-α in driving MPNST. The DNA-binding and transcriptional activity of both STAT3 and HIF1-α is regulated by Redox factor-1 (Ref-1) redox function. A first-generation Ref-1 inhibitor, APX3330, is being tested in cancer clinical trials and could be applied to MPNST. Methods We characterised Ref-1 and p-STAT3 expression in various MPNST models. Tumour growth, as well as biomarkers of apoptosis and signalling pathways, were measured by qPCR and western blot following treatment with inhibitors of Ref-1 or STAT3. Results MPNSTs from Nf1-Arfflox/floxPostnCre mice exhibit significantly increased positivity of p-STAT3 and Ref-1 expression when malignant transformation occurs. Inhibition of Ref-1 or STAT3 impairs MPNST growth in vitro and in vivo and induces apoptosis. Genes highly expressed in MPNST patients are downregulated following inhibition of Ref-1 or STAT3. Several biomarkers downstream of Ref-1 or STAT3 were also downregulated following Ref-1 or STAT3 inhibition. Conclusions Our findings implicate a unique therapeutic approach to target important MPNST signalling nodes in sarcomas using new first-in-class small molecules for potential translation to the clinic.

Apurinic/apyrimidinic (AP) endonuclease-reduction/oxidation factor 1 (APE1/Ref-1, also called APE1) is a multifunctional enzyme with crucial roles in DNA repair and reduction/oxidation (redox) signaling. APE1 was originally described as an endonuclease in the Base Excision Repair (BER) pathway. Further study revealed it to be a redox signaling hub regulating critical transcription factors (TFs). Although a significant amount of focus has been on the role of APE1 in cancer, recent findings support APE1 as a target in other indications, including ocular diseases [diabetic retinopathy (DR), diabetic macular edema (DME), and age-related macular degeneration (AMD)], inflammatory bowel disease (IBD) and others, where APE1 regulation of crucial TFs impacts important pathways in these diseases. The central responsibilities of APE1 in DNA repair and redox signaling make it an attractive therapeutic target for cancer and other diseases.

Market drugs, such as Food and Drug Administration (FDA) or European Medicines Agency (EMA)-approved drugs for specific indications provide opportunities for repurposing for newer therapeutics. This potentially saves resources invested in clinical trials that verify drug safety and tolerance in humans prior to alternative indication approval. Protein arginine methyltransferase 5 (PRMT5) overexpression has been linked to promoting the tumor phenotype in several cancers, including pancreatic ductal adenocarcinoma (PDAC), colorectal cancer (CRC), and breast cancer (BC), making PRMT5 an important target for cancer therapy. Previously, we showed that PRMT5-mediated methylation of the nuclear factor (NF)-κB, partially contributes to its constitutive activation observed in cancers. In this study, we utilized an AlphaLISA-based high-throughput screening method adapted in our lab, and identified one FDA-approved drug, Candesartan cilexetil (Can, used in hypertension treatment) and one EMA-approved drug, Cloperastine hydrochloride (Clo, used in cough treatment) that had significant PRMT5-inhibitory activity, and their anti-tumor properties were validated using cancer phenotypic assays in vitro. Furthermore, PRMT5 selective inhibition of methyltransferase activity was confirmed by reduction of both NF-κB methylation and its subsequent activation upon drug treatment. Using in silico prediction, we identified critical residues on PRMT5 targeted by these drugs that may interfere with its enzymatic activity. Finally, Clo and Can treatment have exhibited marked reduction in tumor growth in vivo. Overall, we provide basis for pursuing repurposing Clo and Can as anti-PRMT5 cancer therapies. Our study offers potential safe and fast repurposing of previously unknown PRMT5 inhibitors into clinical practice.

Cisplatin can induce peripheral neuropathy, which is a common complication of anti-cancer treatment and negatively impacts cancer survivors during and after completion of treatment; therefore, the mechanisms by which cisplatin alters sensory neuronal function to elicit neuropathy are the subject of much investigation. Our previous work suggests that the DNA repair activity of APE1/Ref-1, the rate-limiting enzyme of the base excision repair (BER) pathway, is critical for neuroprotection against cisplatin. A specific role for 8-oxoguanine DNA glycosylase-1 (OGG1), the glycosylase that removes the most common oxidative DNA lesion, and putative coordination of OGG1 with APE1/Ref-1 in sensory neurons, has not been investigated. We investigated whether inhibiting OGG1 glycosylase activity with the small molecule inhibitor, TH5487, and/or APE1/Ref-1 endonuclease activity with APE Repair Inhibitor III would alter the neurotoxic effects of cisplatin in sensory neuronal cultures. Sensory neuron function was assessed by calcitonin gene-related peptide (CGRP) release, as a marker of sensitivity and by neurite outgrowth. Cisplatin altered neuropeptide release in an inverse U-shaped fashion, with low concentrations enhancing and higher concentrations diminishing CGRP release. Pretreatment with BER inhibitors exacerbated the functional effects of cisplatin and enhanced 8oxo-dG and adduct lesions in the presence of cisplatin. Our studies demonstrate that inhibition of OGG1 and APE1 endonuclease activity enhances oxidative DNA damage and exacerbates neurotoxicity, thus limiting oxidative DNA damage in sensory neurons that might alleviate cisplatin-induced neuropathy.

Therapies with the mechanistic target of rapamycin complex 1 (mTORC1) inhibitors are not fully curative for tuberous sclerosis complex (TSC) patients. Here, we propose that some mTORC1-independent disease facets of TSC involve signaling through redox factor-1 (Ref-1). Ref-1 possesses a redox signaling activity that stimulates the transcriptional activity of STAT3, NF-kB, and HIF-1α, which are involved in inflammation, proliferation, angiogenesis, and hypoxia, respectively. Here, we demonstrate that redox signaling through Ref-1 contributes to metabolic transformation and tumor growth in TSC cell model systems. In TSC2-deficient cells, the clinically viable Ref-1 inhibitor APX3330 was effective at blocking the hyperactivity of STAT3, NF-kB, and HIF-1α. While Ref-1 inhibitors do not inhibit mTORC1, they potently block cell invasion and vasculature mimicry. Of interest, we show that cell invasion and vasculature mimicry linked to Ref-1 redox signaling are not blocked by mTORC1 inhibitors. Metabolic profiling revealed that Ref-1 inhibitors alter metabolites associated with the glutathione antioxidant pathway as well as metabolites that are heavily dysregulated in TSC2-deficient cells involved in redox homeostasis. Therefore, this work presents Ref-1 and associated redox-regulated transcription factors such as STAT3, NF-kB, and HIF-1α as potential therapeutic targets to treat TSC, where targeting these components would likely have additional benefits compared to using mTORC1 inhibitors alone.

In the realm of DNA repair, base excision repair (BER) protein, APE1/Ref-1 (Apurinic/Apyrimidinic Endonuclease 1/Redox Effector - 1, also called APE1) has been studied for decades. However, over the past decade, APE1 has been established as a key player in reduction-oxidation (redox) signaling. In the review by Caston et al. (The multifunctional APE1 DNA repair-redox signaling protein as a drug target in human disease), multiple roles of APE1 in cancer and other diseases are summarized. In this Review, we aim to expand on the contributions of APE1 to various diseases and its effect on disease progression. In the scope of cancer, more recent roles for APE1 have been identified in cancer cell metabolism, as well as chemotherapy-induced peripheral neuropathy (CIPN) and inflammation. Outside of cancer, APE1 signaling may be a critical factor in inflammatory bowel disease (IBD) and is also an emergent area of investigation in retinal ocular diseases. The ability of APE1 to regulate multiple transcription factors (TFs) and therefore multiple pathways that have implications outside of cancer, makes it a particularly unique and enticing target. We discuss APE1 redox inhibitors as a means of studying and potentially combating these diseases. Lastly, we examine the role of APE1 in RNA metabolism. Overall, this article builds on our previous review to elaborate on the roles and conceivable regulation of important pathways by APE1 in multiple diseases.

Proliferative diabetic retinopathy (PDR), neovascular age-related macular degeneration (nvAMD), retinopathy of prematurity (ROP) and other eye diseases are characterized by retinal and/or choroidal neovascularization, ultimately causing vision loss in millions of people worldwide. nvAMD and PDR are associated with aging and the number of those affected is expected to increase as the global median age and life expectancy continue to rise. With this increase in prevalence, the development of novel, orally bioavailable therapies for neovascular eye diseases that target multiple pathways is critical, since current anti-vascular endothelial growth factor (VEGF) treatments, delivered by intravitreal injection, are accompanied with tachyphylaxis, a high treatment burden and risk of complications. One potential target is apurinic/apyrimidinic endonuclease 1/reduction-oxidation factor 1 (APE1/Ref-1). The multifunctional protein APE1/Ref-1 may be targeted via inhibitors of its redox-regulating transcription factor activation activity to modulate angiogenesis, inflammation, oxidative stress response and cell cycle in neovascular eye disease; these inhibitors also have neuroprotective effects in other tissues. An APE1/Ref-1 small molecule inhibitor is already in clinical trials for cancer, PDR and diabetic macular edema. Efforts to develop further inhibitors are underway. APE1/Ref-1 is a novel candidate for therapeutically targeting neovascular eye diseases and alleviating the burden associated with anti-VEGF intravitreal injections.

Bladder cancer is the ninth most common cause of cancer-related deaths worldwide. Although cisplatin is used routinely in treating bladder cancer, refractory disease remains lethal for many patients. The recent addition of immunotherapy has improved patient outcomes; however, a large cohort of patients does not respond to these treatments. Therefore, identification of innovative molecular targets for bladder cancer is crucial. Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a multifunctional protein involved in both DNA repair and activation of transcription factors through reduction–oxidation (redox) regulation. High APE1/Ref-1 expression is associated with shorter patient survival time in many cancer types. In this study, we found high APE1/Ref-1 expression in human bladder cancer tissue relative to benign urothelium. Inhibition of APE1/Ref-1 redox signaling using APE1/Ref-1–specific inhibitors attenuates bladder cancer cell proliferation in monolayer, in three-dimensional cultures, and in vivo. This inhibition corresponds with an increase in apoptosis and decreased transcriptional activity of NF-κB and STAT3, transcription factors known to be regulated by APE1/Ref-1, resulting in decreased expression of downstream effectors survivin and Cyclin D1 in vitro and in vivo. We also demonstrate that in vitro treatment of bladder cancer cells with APE1/Ref-1 redox inhibitors in combination with standard-of-care chemotherapy cisplatin is more effective than cisplatin alone at inhibiting cell proliferation. Collectively, our data demonstrate that APE1/Ref-1 is a viable drug target for the treatment of bladder cancer, provide a mechanism of APE1/Ref-1 action in bladder cancer cells, and support the use of novel redox-selective APE1/Ref-1 inhibitors in clinical studies. Significance: This work identifies a critical mechanism for APE1/Ref-1 in bladder cancer growth and provides compelling preclinical data using selective redox activity inhibitors of APE1/Ref-1 in vitro and in vivo.

APE1/Ref-1 (also called Ref-1) has been extensively studied for its role in DNA repair and reduction-oxidation (redox) signaling. The review titled: “The multifunctional APE1 DNA repair-redox signaling protein as a drug target in human disease” by Caston et. al. summarizes the molecular functions of Ref-1 and the role it plays in a number of diseases, with a specific focus on various types of cancer [1]. Previous studies have demonstrated that Ref-1 plays a critical role in regulating specific transcription factors (TFs) involved in a number of pathways, not only in cancer, but other disease indications as well. Disease indications of particular therapeutic interest include retinal vascular diseases such as diabetic retinopathy (DR), diabetic macular edema (DME), and neovascular age-related macular degeneration (nvAMD). While Ref-1 controls a number of TFs that are under redox regulation, three have been found to directly link cancer studies to retinal diseases; HIF-1α, NF-κB and STAT3. HIF-1α controls the expression of VEGF for angiogenesis while NF-κB and STAT3 regulate a number of known cytokines and factors involved in inflammation. These pathways are highly implicated and validated as major players in DR, DME and AMD. Therefore, findings in cancer studies for Ref-1 and its inhibition may be translated to these ocular diseases. This report discusses the path from cancer to the potential treatment of retinal disease, the Ref-1 redox signaling function as a possible target, and the current small molecules which have been identified to block this activity. One molecule, APX3330, is in clinical trials, while the others are in preclinical development. Inhibition of Ref-1 and its effects on inflammation and angiogenesis makes it a potential new therapeutic target for the treatment of retinal vascular diseases. This commentary summarizes the retinal-relevant research that built on the results summarized in the review by Caston et. al. [1].

Ref-1/APE1 (Redox factor-1, Ref-1; Apurinic/apyrimidinic endonuclease 1, APE1) is a redox signaling enzyme that biochemically converts several transcription factors (TFs), such as NF-kB, STAT3, HIF-1α from an oxidized to a reduced state, allowing TFs to bind DNA and activate various biological responses such as cell growth and inflammation. Ref-1 and its transcriptional targets are highly active in Pancreatic Ductal Adenocarcinoma (PDAC) which often presents with tumor metastasis and therapy resistance. The redox activity of Ref-1 is determined by the redox status of the relevant cysteine residues (Cys), particularly Cys residue 65 (C65), which is critical among seven Cys residues for Ref-1’s redox activity. Using CRISPR editing in PDAC cells, we changed the C65 to Ala (C65A) in Ref-1 to evaluate the effects of loss of Ref-1 redox activity on cell signaling pathways and how Ref-1 activity regulates critical transcription factors. Using Ref-1 redox inhibitor, APX3330, we previously investigated interactions between Ref-1 and APX3330 in a cell-free system in which Ref-1 underwent conformational changes in the presence of APX3330. This resulted in complete exposure of all seven Cys residues causing oxidation of the Ref-1 protein and redox signaling inactivity. However, how these events translate in the cell has not been completely established. Using a gel-based mobility shift assay, we examined the redox-modified Cys residues of Ref-1 in response to APX3330 in PDAC cell lines over time. We observed a gradual increase in labeling of Cys in Ref-1 following APX3330 treatment between 10min to 6h which returned to control levels at 48h in both wild-type control (Cas9) and C65A-expressing cells. This reduction in the redox status of Ref-1 was likely due to unfolding alteration like the cell-free system. To confirm this, both cell lines were treated with N-ethylmaleimide (NEM) prior to cell lysing, which blocks free thiols in the protein. We detected dramatic decreases in reduced forms of Ref-1 after APX3330 treatment (between 10min-6h) in both cell lines, demonstrating that C65 is not essential for the unfolded conformational changes of Ref-1. However, C65A-Ref-1 protein demonstrated a distinctly different redox pattern upon APX3330 treatment in comparison to WT Ref-1 in the Cas9 control line. C65A-expressing cells also had reduced mitochondrial function compared to Cas9 control PDAC cells. In vivo studies demonstrated significant reduction in tumor size, weight, and growth in C65A PDAC cells compared to the Cas9 control lines, further demonstrating the critical role of C65 in the Ref-1 redox signaling and downstream pathways including metabolism and proliferation. In conclusion, we demonstrate direct interactions between Ref-1 and APX3330 in PDAC cells and subsequent biological results confirm a critical involvement of Ref-1-C65 in the redox signaling and tumor phenotype. Citation Format: Mahmut Mijit, Olivia Babb, Silpa Gampala, Randall Wireman, Millie M. Georgiadis, Melissa L. Fishel, Mark R. Kelley. Inhibition of Ref-1/APE1 redox activity with APX3330 enhances Ref-1/APE1 protein unfolded conformation in human PDAC cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2366.