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Scientific Publications


J Pharmacol Exp Ther. 2016 Nov;359(2):300-309. Epub 2016 Sep 8.
Identification and Characterization of New Chemical Entities Targeting Apurinic/Apyrimidinic Endonuclease 1 for the Prevention of Chemotherapy-Induced Peripheral Neuropathy.
Kelley MR1Wikel JH2Guo C2Pollok KE2Bailey BJ2Wireman R2Fishel ML2Vasko MR2.

Chemotherapy-induced peripheral neuropathy (CIPN) is a potentially debilitating side effect of a number of chemotherapeutic agents. There are currently no U.S. Food and Drug Administration-approved interventions or prevention strategies for CIPN. Although the cellular mechanisms mediating CIPN remain to be determined, several lines of evidence support the notion that DNA damage caused by anticancer therapies could contribute to the neuropathy. DNA damage in sensory neurons after chemotherapy correlates with symptoms of CIPN. Augmenting apurinic/apyrimidinic endonuclease (APE)-1 function in the base excision repair pathway reverses this damage and the neurotoxicity caused by anticancer therapies. This neuronal protection is accomplished by either overexpressing APE1 or by using a first-generation targeted APE1 small molecule, E3330 [(2E)-2-[(4,5-dimethoxy-2-methyl-3,6-dioxo-1,4-cyclohexadien-1-yl)methylene]-undecanoic acid; also called APX3330]. Although E3330 has been approved for phase 1 clinical trials (Investigational New Drug application number IND125360), we synthesized novel, second-generation APE1-targeted molecules and determined whether they would be protective against neurotoxicity induced by cisplatin or oxaliplatin while not diminishing the platins’ antitumor effect. We measured various endpoints of neurotoxicity using our ex vivo model of sensory neurons in culture, and we determined that APX2009 [(2E)-2-[(3-methoxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)methylidene]-N,N-diethylpentanamide] is an effective small molecule that is neuroprotective against cisplatin and oxaliplatin-induced toxicity. APX2009 also demonstrated a strong tumor cell killing effect in tumor cells and the enhanced tumor cell killing was further substantiated in a more robust three-dimensional pancreatic tumor model. Together, these data suggest that the second-generation compound APX2009 is effective in preventing or reversing platinum-induced CIPN while not affecting the anticancer activity of platins.

Neural Regen Res. 2017 Jan; 12(1): 72–74.
Challenges and opportunities identifying therapeutic targets for chemotherapy-induced peripheral neuropathy resulting from oxidative DNA damage
Mark R. Kelley, Ph.D.* and Jill C. Fehrenbacher

Abstract on chemotherapy-induced peripheral neuropathy (CIPN)
Incidence, prevalence, and consequences: Up to 90% of cancer patients experience CIPN at some point during or after anticancer treatment (Seretny et al., 2014). Although CIPN is second to hematologic toxicities in regards to the frequency of incidence, there are currently no approved treatments to prevent or treat CIPN, thus the neurotoxicity can be dose-limiting for some patients (Vasko et al., 2016). In addition, CIPN can persist following discontinuation of the drug: up to 40% of cancer patients continue to struggle with CIPN five years after treatment ends (Vasko et al., 2016) — and 10% remain symptomatic after more than 20 years. Thus, CIPN directly affects cancer survivorship, quality of life, and may limit future treatment options if cancer recurs (Vasko et al., 2016).

F. Shah, D. Logsdon, R.A. Messmann, J.C. Fehrenbacher, M.L. Fishel, and M.R. Kelley. (2017)
Exploiting the APE1-Ref-1 node in cancer signaling and other diseases: from bench to clinic. NPJ Precision Oncology June 2017; 1:19. doi:10.1038/s41698-017-0023-0.
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.

M.R. Kelley, J.H. Wikel, C. Guo, K.E. Pollok, B.J. Bailey, R. Wireman, M.L. Fishel, and M.R. Vasko. (2016)
Identification and Characterization of new chemical entities targeting apurinic/apyrimidinic endonuclease 1 for the prevention of chemotherapy-induced peripheral neuropathy. J Pharmacol Exp Ther. 2016 Nov;359(2):300-309. Epub 2016 Sept 8. DOI:10.1124/jpet.116.235283. PMCID: PMC5074487

Kelley MR, Fehrenbacher JC. (2017)
Challenges and opportunities identifying therapeutic targets for chemotherapy-induced peripheral neuropathy resulting from oxidative DNA damage. Neural Regen Res 2017;12:72-4. PMCID: PMC5319244

Mol Cancer Ther. 2016 Aug 17. pii: molcanther.0253.2016. [Epub ahead of print]
Regulation of HIF1α under Hypoxia by APE1/Ref-1 Impacts CA9 Expression: Dual-Targeting in Patient-Derived 3D Pancreatic Cancer Models.
Logsdon DP, Grimard M, Luo M, Shahda S, Jiang Y, Tong Y, Yu Z, Zyromski N, Schipani E, Carta F, Supuran CT, Korc M, Ivan M, Kelley MR, Fishel ML.
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.

J Biol Chem. 2015 Jan 30;290(5):3057-68. doi: 10.1074/jbc.M114.621995. Epub 2014 Dec 9.
Apurinic/apyrimidinic endonuclease/redox factor-1 (APE1/Ref-1) redox function negatively regulates NRF2.
Fishel ML, Wu X, Devlin CM, Logsdon DP, Jiang Y, Luo M, He Y, Yu Z, Tong Y, Lipking KP, Maitra A, Rajeshkumar NV, Scandura G, Kelley MR, Ivan M.
Apurinic/apyrimidinic endonuclease/redox factor-1 (APE1/Ref-1) (henceforth referred to as Ref-1) is a multifunctional protein that in addition to its base excision DNA repair activity exerts redox control of multiple transcription factors, including nuclear factor κ-light chain enhancer of activated B cells (NF-κB), STAT3, activator protein-1 (AP-1), hypoxia-inducible factor-1 (HIF-1), and tumor protein 53 (p53). In recent years, Ref-1 has emerged as a promising therapeutic target in cancer, particularly in pancreatic ductal carcinoma. Although a significant amount of research has centered on Ref-1, no wide-ranging approach had been performed on the effects of Ref-1 inhibition and transcription factor activity perturbation. Starting with a broader approach, we identified a previously unsuspected effect on the nuclear factor erythroid-related factor 2 (NRF2), a critical regulator of cellular defenses against oxidative stress. Based on genetic and small molecule inhibitor-based methodologies, we demonstrated that repression of Ref-1 potently activates NRF2 and its downstream targets in a dose-dependent fashion, and that the redox, rather than the DNA repair function of Ref-1 is critical for this effect. Intriguingly, our results also indicate that this pathway does not involve reactive oxygen species. The link between Ref-1 and NRF2 appears to be present in all cells tested in vitro, noncancerous and cancerous, including patient-derived tumor samples. In particular, we focused on understanding the implications of the novel interaction between these two pathways in primary pancreatic ductal adenocarcinoma tumor cells and provide the first evidence that this mechanism has implications for overcoming the resistance against experimental drugs targeting Ref-1 activity, with clear translational implications.

PLoS One. 2012;7(10):e47462. doi: 10.1371/journal.pone.0047462. Epub 2012 Oct 19.
APE1/Ref-1 regulates STAT3 transcriptional activity and APE1/Ref-1-STAT3 dual-targeting effectively inhibits pancreatic cancer cell survival.
Cardoso AA, Jiang Y, Luo M, Reed AM, Shahda S, He Y, Maitra A, Kelley MR, Fishel ML.
Pancreatic cancer is a largely incurable disease, and increasing evidence supports strategies targeting multiple molecular mediators of critical functions of pancreatic ductal adenocarcinoma cells. Intracellular redox state modulates the activity of various signal transduction pathways and biological processes, including cell survival, drug resistance and responsiveness to microenvironmental factors. Recently, it has been shown that the transcription factor STAT3 is under redox control, but the mechanisms involved in its regulation are unknown. Here, we demonstrate for the first time that STAT3 DNA binding and transcriptional activity is directly regulated by the redox function of the APE1/Ref-1 endonuclease, using overexpression and redox-specific mutational strategies, and gene knockdown. Also, pharmacological blockade of APE1/Ref-1 by the redox-selective inhibitor E3330 abrogates STAT3 DNA binding. Since APE1/Ref-1 also exerts redox control on other cancer-associated transcription factors, we assessed the impact of dual-targeting of STAT3 signaling and APE1/Ref-1 redox on pancreatic cancer cell functions. We observed that disruption of APE1/Ref-1 redox activity synergizes with STAT3 blockade to potently inhibit the proliferation and viability of human PDAC cells. Mechanistically, we show that STAT3-APE1/Ref-1 dual targeting promotes marked tumor cell apoptosis, with engagement of caspase-3 signaling, which are significantly increased in comparison to the effects triggered by single target blockade. Also, we show that STAT3-APE1/Ref-1 dual blockade results in significant inhibition of tumor cell migration. Overall, this work demonstrates that the transcriptional activityof STAT3 is directly regulated by the redox function of APE1/Ref-1, and that concurrent blockade of STAT3 and APE1/Ref-1 redox synergize effectively inhibit critical PDAC cell functions.

Mol Cancer Ther. 2011 Sep;10(9):1698-708. doi: 10.1158/1535-7163.MCT-11-0107. Epub 2011 Jun 23.
Impact of APE1/Ref-1 redox inhibition on pancreatic tumor growth.
Fishel ML, Jiang Y, Rajeshkumar NV, Scandura G, Sinn AL, He Y, Shen C, Jones DR, Pollok KE, Ivan M, Maitra A, Kelley MR.
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.

Cancer Invest. 2010 Nov;28(9):885-95. doi: 10.3109/07357907.2010.512816.
Reduced expression of DNA repair and redox signaling protein APE1/Ref-1 impairs human pancreatic cancer cell survival, proliferation, and cell cycle progression.
Jiang Y, Zhou S, Sandusky GE, Kelley MR, Fishel ML.
Pancreatic cancer is a deadly disease that is virtually never cured. Understanding the chemoresistance intrinsic to this cancer will aid in developing new regimens. High expression of APE1/Ref-1, a DNA repair and redox signaling protein, is associated with resistance, poor outcome, and angiogenesis; little is known in pancreatic cancer. Immunostaining of adenocarcinoma shows greater APE1/Ref-1 expression than in normal pancreas tissue. A decrease in APE1/Ref-1 protein levels results in pancreatic cancer cell growth inhibition, increased apoptosis, and altered cell cycle progression. Endogenous cell cycle inhibitors increase when APE1/ Ref-1 is reduced, demonstrating its importance to proliferation and growth of pancreatic cancer.

DNA Repair (Amst). 2008 Feb 1;7(2):177-86. Epub 2007 Oct 31.
Knockdown of the DNA repair and redox signaling protein Ape1/Ref-1 blocks ovarian cancer cell and tumor growth.
Fishel ML, He Y, Reed AM, Chin-Sinex H, Hutchins GD, Mendonca MS, Kelley MR.
Apurinic endonuclease 1/redox effector factor-1 (Ape1/Ref-1 or Ape1) is an essential protein with two distinct functions. It is a DNA repair enzyme in the base excision repair (BER) pathway and a reduction-oxidation (redox) signaling factor maintaining transcription factors in an active reduced state. Our laboratory previously demonstrated that Ape1 is overexpressed in ovarian cancer and potentially contributes to resistance. Therefore, we utilized siRNA technology to knockdown protein levels of Ape1 in ovarian cancercell line, SKOV-3x. Knocking Ape1 down had dramatic effects on cell growth in vitro but was not due to an increase in apoptosis and at least partially due to an extension in transit time through S-phase. Similarly, human ovarian tumor xenografts with reduced levels of Ape1 protein demonstrated a dramatic reduction in tumor volume (p<0.01) and also statistically significant (p=0.02) differences in (18)F-fluorodeoxyglucose (FDG) uptake indicating reduced glucose metabolism and cellular proliferation. Ape1’s role in DNA repair and redox signaling is important to our basic understanding of ovarian cancer cell growth and these findings strongly support Ape1 as a therapeutic target.

J Pharmacol Exp Ther. 2016 Sep 8. pii: jpet.116.235283. [Epub ahead of print]
Identification of new chemical entities targeting APE1 for the prevention of chemotherapy-induced peripheral neuropathy (CIPN).
Kelley MR, Wikel JH, Guo C, Pollok KE, Bailey BJ, Wireman R, Fishel ML, Vasko MR
Chemotherapy-induced peripheral neuropathy (CIPN) is a potentially debilitating side effect of a number of chemotherapeutic agents that does not have any FDA-approved interventions or prevention strategies. Although the cellular mechanisms mediating CIPN remain to be determined, several lines of evidence support the notion that DNA damage may be a causative factor in neuropathy induced by a number of cancer therapies. Therapies including platinum agents and ionizing radiation cause DNA damage in sensory neurons and augmenting key steps in the base excision repair (BER) pathway reverses this damage. Neuronal protection is provided by overexpressing APE1 as well as using a first generation targeted APE1 small molecule E3330 (also called APX3330). Accordingly, we determined whether novel second-generation APE1 targeted molecules would be protective against neurotoxicity-induced by cisplatin or oxaliplatin while not diminishing the anti-tumor effect of the platins. We determined using our ex vivo model of sensory neurons in culture measuring various endpoints of neurotoxicity that APX2009 is an effective small molecule that is neuroprotective against cisplatin and oxaliplatin-induced toxicity of sensory neurons. APX2009 also demonstrated a strong tumor cell killing effect in tumor cells. Additionally, the enhanced tumor cell killing was further shown in a more robust 3D pancreatic tumor model. Together, these data suggest that APX2009 is effective in preventing or reversing platinum-induced CIPN, while not affecting the anti-cancer activity of platins.

PLoS One. 2014 Sep 4;9(9):e106485. doi: 10.1371/journal.pone.0106485. eCollection 2014.
Role of the DNA base excision repair protein, APE1 in cisplatin, oxaliplatin, or carboplatin induced sensory neuropathy.
Kelley MR, Jiang Y, Guo C, Reed A, Meng H, Vasko MR.
Although chemotherapy-induced peripheral neuropathy (CIPN) is a dose-limiting side effect of platinum drugs, the mechanisms of this toxicity remain unknown. Previous work in our laboratory suggests that cisplatin-induced CIPN is secondary to DNA damage which is susceptible to base excision repair (BER). To further examine this hypothesis, we studied the effects of cisplatin, oxaliplatin, and carboplatin on cell survival, DNA damage, ROS production, and functional endpoints in rat sensory neurons in culture in the absence or presence of reduced expression of the BER protein AP endonuclease/redox factor-1 (APE1). Using an in situ model of peptidergic sensory neuron function, we examined the effects of the platinum drugs on hind limb capsaicin-evoked vasodilatation. Exposing sensory neurons in culture to the three platinum drugs caused a concentration-dependent increase in apoptosis and cell death, although the concentrations of carboplatin were 10 fold higher than cisplatin. As previously observed with cisplatin, oxaliplatin and carboplatin also increased DNA damage as indicated by an increase in phospho-H2AX and reduced the capsaicin-evoked release of CGRP from neuronal cultures. Both cisplatin and oxaliplatin increased the production of ROS as well as 8-oxoguanine DNA adduct levels, whereas carboplatin did not. Reducing levels of APE1 in neuronal cultures augmented the cisplatin and oxaliplatin induced toxicity, but did not alter the effects of carboplatin. Using an in vivo model, systemic injection of cisplatin (3 mg/kg), oxaliplatin (3 mg/kg), or carboplatin (30 mg/kg) once a week for three weeks caused a decrease in capsaicin-evoked vasodilatation, which was delayed in onset. The effects of cisplatin on capsaicin-evoked vasodilatation were attenuated by chronic administration of E3330, a redox inhibitor of APE1 that serendipitously enhances APE1 DNA repair activity in sensory neurons. These outcomes support the importance of the BER pathway, and particularly APE1, in sensory neuropathy caused by cisplatin and oxaliplatin, but not carboplatin and suggest that augmenting DNA repair could be a therapeutic target for CIPN.

Mutat Res. 2015 Sep;779:96-104. doi: 10.1016/j.mrfmmm.2015.06.010. Epub 2015 Jun 26.
APE1, the DNA base excision repair protein, regulates the removal of platinum adducts in sensory neuronal cultures by NER.
Kim HS, Guo C, Thompson EL, Jiang Y, Kelley MR, Vasko MR, Lee SH.
Peripheral neuropathy is one of the major side effects of treatment with the anticancer drug, cisplatin. One proposed mechanism for this neurotoxicity is the formation of platinum adducts in sensory neurons that could contribute to DNA damage. Although this damage is largely repaired by nuclear excision repair (NER), our previous findings suggest that augmenting the base excision repair pathway (BER) by overexpressing the repair protein APE1 protects sensory neurons from cisplatin-induced neurotoxicity. The question remains whether APE1 contributes to the ability of the NER pathway to repair platinum-damage in neuronal cells. To examine this, we manipulated APE1 expression in sensory neuronal cultures and measured Pt-removal after exposure to cisplatin. When neuronal cultures were treated with increasing concentrations of cisplatin for two or three hours, there was a concentration-dependent increase in Pt-damage that peaked at four hours and returned to near baseline levels after 24h. In cultures where APE1 expression was reduced by ∼ 80% using siRNA directed at APE1, there was a significant inhibition of Pt-removal over eight hours which was reversed by overexpressing APE1 using a lentiviral construct for human wtAPE1. Overexpressing a mutant APE1 (C65 APE1), which only has DNA repair activity, but not its other significant redox-signaling function, mimicked the effects of wtAPE1. Overexpressing DNA repair activity mutant APE1 (226 + 177APE1), with only redox activity was ineffective suggesting it is the DNA repair function of APE1 and not its redox-signaling, that restores the Pt-damage removal. Together, these data provide the first evidence that a critical BER enzyme, APE1, helps regulate the NER pathway in the repair of cisplatin damage in sensory neurons.

DNA Repair (Amst). 2011 Sep 5;10(9):942-52. doi: 10.1016/j.dnarep.2011.06.004. Epub 2011 Jul 8.
The repair function of the multifunctional DNA repair/redox protein APE1 is neuroprotective after ionizing radiation.
Vasko MR, Guo C, Thompson EL, Kelley MR.
Although exposure to ionizing radiation (IR) can produce significant neurotoxicity, the mechanisms mediating this toxicity remain to be determined. Previous studies using neurons isolated from the central nervous system show that IR produces reactive oxygen species and oxidative DNA damage in those cells. Because the base excision DNA repair pathway repairs single-base modifications caused by ROS, we asked whether manipulating this pathway by altering APE1 expression would affect radiation-induced neurotoxicity. In cultures of adult hippocampal and sensory neurons, IR produces DNA damage as measured by phosphorylation of histone H2A.X and results in dose-dependent cell death. In isolated sensory neurons, we demonstrate for the first time that radiation decreases the capsaicin-evoked release of the neuropeptide CGRP. Reducing APE1 expression in cultured cells augments IR-induced neurotoxicity, whereas overexpressing APE1 is neuroprotective. Using lentiviral constructs with a neuronal specific promoter that selectively expresses APE1s different functions in neurons, we show that selective expression of the DNA repair competent (redox inactive) APE1 constructs in sensory neurons resurrects cell survival and neuronal function, whereas use of DNA-repair deficient (redox active) constructs is not protective. Use of an APE1 redox-specific inhibitor, APX3330, also facilitates neuronal protection against IR-induced toxicity. These results demonstrate for the first time that the repair function of APE1 is required to protect both hippocampal and DRG neuronal cultures–specifically neuronal cells–from IR-induced damage, while the redox activity of APE1 does not appear to be involved.

Exp Hematol. 2010 Dec;38(12):1178-88. doi: 10.1016/j.exphem.2010.08.011. Epub 2010 Sep 6.
Inhibition of the redox function of APE1/Ref-1 in myeloid leukemia cell lines results in a hypersensitive response to retinoic acid-induced differentiation and apoptosis.
Fishel ML, Colvin ES, Luo M, Kelley MR, Robertson KA.
The standard of care for promyelocytic leukemia includes use of the differentiating agent all-trans retinoic acid (RA) and chemotherapy. RA induces cell differentiation through retinoic acid receptor (RAR) transcription factors. Because redox mechanisms influence how readily transcription factors bind to DNA response elements (RARE), the impact of small molecule (E3330) inhibition of the redox regulatory protein, apurinic-apyrimidinic endonuclease/redox effector factor (APE1/Ref-1) on RAR DNA binding and function in RA-induced myeloid leukemia cell differentiation and apoptosis was investigated.

The redox function of APE1 was studied using the small molecule inhibitor E3330 in HL-60 and PLB acute myeloid leukemia cells. Electrophoretic mobility shift assays were employed to determine effect of inhibitor on APE1/Ref-1 redox signaling function. Trypan blue assays, Annexin-V/propidium iodide and CD11b staining, and real-time polymerase chain reaction analyses were employed to determine survival, apoptosis, and differentiation status of cells in culture.

RARα binds to its RARE in a redox-dependent manner mediated by APE1/Ref-1 redox regulation. Redox-dependent RAR-RARE binding is blocked by E3330, a small molecule redox inhibitor of APE1/Ref-1. Combination treatment of RA + E3330 results in a profound hypersensitivity of myeloid leukemia cells to RA-induced differentiation and apoptosis. Additionally, redox inhibition by E3330 results in enhanced RAR target gene, BLR-1, expression in myeloid leukemia cells.

The redox function of APE1/Ref-1 regulates RAR binding to its DNA RAREs influencing the response of myeloid leukemia cells to RA-induced differentiation. Targeting of APE1/Ref-1 redox function may allow manipulation of the retinoid response with therapeutic implications.

Clin Cancer Res. 2001 Apr;7(4):824-30.
Elevated and altered expression of the multifunctional DNA base excision repair and redox enzyme Ape1/ref-1 in prostate cancer.
Kelley MR, Cheng L, Foster R, Tritt R, Jiang J, Broshears J, Koch M.
The DNA base excision repair pathway is responsible for the repair of cellular alkylation and oxidative DNA damage. A crucial step in the BER pathway involves the cleavage of baseless sites in DNA by an apurinic/apyrimidinic or baseless (AP) endonuclease (Ape1/ref-1), which is a multifunctional enzyme that acts not only as an AP endonuclease but also as a redox-modifying factor for a variety of transcription factors including Fos, Jun, paired box containing genes (PAX), nuclear factor-kappaB, hypoxia-inducible factor alpha (HIF-1alpha), HIF-like factor (HLF), p53, and others. The expression of Ape1/ref-1 in prostate has not been characterized previously. Ape1/ref-1 nuclear immunohistochemistry levels, scored for intensity as 1+, 2+, or 3+, were 91, 3, and 6% in benign hypertrophy (BPH), 0, 42, and 58% in prostatic intraepithelial neoplasia (PIN) and 3, 30, and 67% in prostate cancer, respectively, clearly showing an increase in Ape1/ref-1 nuclear staining in the PIN and cancer compared with BPH. Furthermore, the level of cytoplasmic staining of Ape1/ref-1 in cancer and PIN were elevated (42 and 36%, respectively) compared with BPH (5%). There was no correlation with prostate-specific antigen values or doubling times to Ape1/ref-1 levels. In conclusion, we have demonstrated that Ape1/ref-1 is dramatically elevated in prostate cancer, the level of staining of Ape1/ref-1 increases from low in BPH to intense in PIN and cancer, and there is an increase in the amount of Ape1/ref-1 in the cytoplasm of PIN and cancer compared with BPH. Given these results, we conclude that Ape1/ref-1 may be a diagnostic marker for earlyprostate cancer and play a role, through its repair, redox, or both functions, in the physiology of the early development of prostate cancer.

Mol Cancer Ther. 2004 Jun;3(6):679-86.
Human apurinic endonuclease 1 (APE1) expression and prognostic significance in osteosarcoma: enhanced sensitivity of osteosarcoma to DNA damaging agents using silencing RNA APE1 expression inhibition.
Wang D, Luo M, Kelley MR.
Osteosarcoma is the most common highly malignant bone tumor with primary appearance during the second and third decade of life. It is associated with a high risk of relapse, possibly resulting from a developed resistance to chemotherapy agents. As a means to overcome osteosarcoma tumor cell resistance and/or to sensitize tumor cells to currently used chemotherapeutic treatments, we examined the role ofhuman apurinic endonuclease 1 (APE1) in osteosarcoma tumor cell resistance and prognosis. Sixty human samples of archived conventional (intramedullary) osteosarcoma were analyzed. APE1 protein was elevated in 72% of these tissues and among those with a known clinical outcome, there was a significant correlation between high APE1 expression levels and reduced survival times. The remaining 28% of samples showed low expression of APE1. Given that APE1 was overexpressed in osteosarcoma, we decreased APE1 levels using silencing RNA (siRNA) targeting technology in the osteosarcoma cell line, human osteogenic sarcoma (HOS), to enhance chemo- and radiation sensitivity. Using siRNA targeted technology of APE1, protein levels were reduced by more than 90% within 24 hours, remained low for 72 hours, and returned to normal levels at 96 hours. There was also a clear loss of APE1 endonuclease activity following APE1-siRNA treatment. A decrease in APE1 levels in siRNA-treated human osteogenic sarcoma cells led to enhanced cell sensitization to the DNA damaging agents: methyl methanesulfonate, H(2)O(2), ionizing radiation, and chemotherapeutic agents. The findings presented here have both prognostic and therapeutic implications for treating osteosarcoma. The APE1-siRNA results demonstrate the feasibility for the therapeutic modulation of APE1 using a variety of molecules and approaches.

Clin Cancer Res. 2003 Oct 15;9(13):4689-94
Studies of apurinic/apyrimidinic endonuclease/ref-1 expression in epithelial ovarian cancer: correlations with tumor progression and platinum resistance.
Freitas S, Moore DH, Michael H, Kelley MR.
A crucial step in the DNA base excision repair pathway involves the cleavage of an apurinic/apyrimidinic site by an apurinic/apyrimidinic endonuclease (APE). The major APE in mammalian cells is APE/ref-1, a multifunctional enzyme that acts not only as a DNA repair enzyme but as a redox-modifying factor for a variety of transcription factors. The purpose of this study is to determine whetherAPE/ref-1 expression differs with ovarian cancer progression and metastasis and in platinum-resistant disease.
Ovarian tissue sections were obtained from the Cooperative Human Tissue Network for studies of APE/ref-1 expression and the metastatic process and from our institutional Department of Pathology for studies of APE/ref-1 expression and platinum resistance. Tissue microsections from formalin-fixed, paraffin-embedded specimens of epithelial ovarian cancers were stained using a monoclonal antibody to APE/ref-1 followed by standard immunohistochemical techniques. Slides were then analyzed for the percentage of positively staining nuclei as well as staining intensity using a blinded coding system.
All epithelial ovarian cancers expressed APE/ref-1. There were no significant differences in the percentage or intensity of nuclear staining in primary tumors from patients with early- versus advanced-stage disease or in primary tumors versus metastasis from patients with advanced disease. Both platinum-sensitive and platinum-refractory tumors demonstrated a range from minimal to high intensity staining nuclei with a median value of 2+ staining on a scale of 0-3+. The median value for the percentage of nuclei involved was 70% in the platinum-sensitive group and 90% in the platinum-refractory group (P = 0.118).
APE/ref-1 expression is ubiquitous among epithelial ovarian cancers and is unaltered with the metastatic process. APE/ref-1 expression does not appear to differ between platinum-sensitive and platinum-refractory ovarian cancers and thus is not a useful biomarker for platinum resistance. Combined with evidence that APE/ref-1 expression and function may not be equivalent in all cell types and tissues, future work will investigate APE/ref-1 as a potential therapeutic target.

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