Inhibition of the glutaredoxin and thioredoxin systems and ribonucleotide reductase by mutant p53-targeting compound APR-246

The tumor suppressor p53 is commonly inactivated in human tumors, allowing evasion of p53-dependent apoptosis and tumor progression. The small molecule APR-246 (PRIMA-1Met) can reactive mutant p53 in tumor cells and trigger cell death by apoptosis. The thioredoxin (Trx) and glutaredoxin (Grx) systems are important as antioxidants for maintaining cellular redox balance and providing electrons for thiol-dependent reactions like those catalyzed by ribonucleotide reductase and peroxiredoxins (Prxs). We show here that the Michael acceptor methylene quinuclidinone (MQ), the active form of APR-246, is a potent direct inhibitor of Trx1 and Grx1 by reacting with sulfhydryl groups in the enzymes. The inhibition of Trx1 and Grx1 by APR-246/MQ is reversible and the inhibitory efficiency is dependent on the presence of glutathione. APR-246/MQ also inhibits Trxs in mutant p53-expressing Saos-2 His-273 cells, showing modification of Trx1 and mitochondrial Trx2. Inhibition of the Trx and Grx systems leads to insufficient reducing power to deoxyribonucleotide production for DNA replication and repair and peroxiredoxin for removal of ROS. We also demonstrate that APR-246 and MQ inhibit ribonucleotide reductase (RNR) in vitro and in living cells. Our results suggest that APR-246 induces tumor cell death through both reactivations of mutant p53 and inhibition of cellular thiol-dependent redox systems, providing a novel strategy for cancer therapy.

cells 19,20 , and are required for the activity of ribonucleotide reductase (RNR) which provides deoxyribonucleotides for DNA replication and repair. Each turnover of the enzyme results in a disulfide in the buried active site which has to be reduced 21 . Trx and Grx systems have been recognized as the primary electron donors to reduce RNR 22 which is often induced in tumor cells 23 . The Trx system also reduces peroxiredoxins which control reactive oxygen species (ROS) and hydrogen peroxide levels 19 . Trx1 and Grx1 regulate apoptosis signal-regulating kinase1 (ASK1) by binding and inactivating the enzyme only in their reduced form 24 . Many transcription factors are controlled by reduced Trx1 and Grx1, including p53, nuclear factor-κB (NF-κB), and Activator protein 1 (AP-1) 19,25-27 .
The pro-inflammatory and anti-apoptotic properties of the Trx1 and Grx1 systems, as well as their redox capabilities, make them important for tumor cell survival. Tumor cells, in general, have a highly active glucose metabolism due to rapid proliferation, resulting in increased production of ROS 23 . The Trx1 and Grx1 systems are thus commonly upregulated in cancer cells to counteract the oxidative stress as a result of increased ROS production, which otherwise can lead to apoptosis or necrosis 23 . Trx1 expression was inversely correlated to the p53-dependent regulation of tumor growth in breast cancer 28 . Inhibition of Trx1 and Grx1 could, therefore, have pro-apoptotic effects in cancer cells 23,29 .
Here we investigated the potential interactions between APR-246/MQ and Trx1 and Grx1, and found that reaction of MQ with both Trx1 and Grx1 was associated with loss of free thiols in both proteins. Mass spectrometry confirmed that the inhibitory effect of MQ was due to its binding to thiols. The effect on Trxs was also evident in tumor cells where both Trx1 and Trx2 were modified. Moreover, we found that APR-246/MQ is a potent inhibitor of RNR, both in vitro and in living cells. These results provide further evidence for a potent effect of APR-246 on cellular redox regulation.

Materials and Methods
Proteins and reagents. Recombinant human Trx1 and Grx1, rat recombinant TrxR and polyclonal goat anti-Trx1 antibody were obtained from IMCO Corporation, Sweden (www.imcocorp.se). Anti-Trx2 was from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Recombinant yeast GR and all other chemicals and reagents were of analytical grade and obtained from Sigma-Aldrich. APR-246 and MQ were provided by Aprea Therapeutics AB (Stockholm, Sweden). Before using, APR-246 was freshly dissolved in DMSO and preheated at 90 °C for 10 min, and then diluted into the desired concentration.

Assessment of Grx1 system activity in vitro.
Grx activity assessment is based on the method of Coppo et al. 30 . Briefly, human recombinant Grx1 (0.1 µM) was pre-reduced in a 96-well microtiter plate for 30 min at 37 °C in the presence of 0.25 mM NADPH, 50 nM yGR, 50 µM GSH and 5 µg/mL BSA in KE buffer (0.1 M Potassium phosphate, pH 7.5 and 1 mM EDTA) in final concentrations. Indicated concentrations of APR-246 were added and after 60 min incubation the reaction was started by adding 20 µM Eosin-GSH labeled BSA (E-GS-BSA). Fluorescence was recorded at 545 nm emission after 520 nm excitation for 45 minutes using a Victor3 fluorescence plate reader (PerkinElmer, USA). The activity of Grx1 was obtained through calculation of the increasing fluorescence intensity over time due to the release of eosin-GSH. Assessment of Trx1 system activity in vitro. The method was based on a fluorescent microtiter method 31 . Human recombinant Trx1 (0.1 µM) was prereduced in a 96-well microtiter plate for 30 min at 37 °C in the presence of 50 nM TrxR and 0.25 mM NADPH in TE buffer (100 mM Tris, pH 7.5 and 10 mM EDTA) in final concentrations. Indicated concentrations of APR-246 were added and after 60 min incubation, 6 µM fluorescein isothiocyanate coupled to insulin (FITC-insulin) was added to start the reaction 31 . Fluorescence was recorded at 520 nm emission after 485 nm excitation for 30-45 minutes using a Victor3 fluorescence plate reader (PerkinElmer, USA). The activity of Trx1 was obtained by calculating the increasing fluorescence intensity over time within the linear area of the curve. Trx1 and Grx1 were reduced with 10 mM dithiothreitol (DTT) for 1 hour at 37 °C, and desalted using a NAP-5 column (GE Healthcare, Uppsala, Sweden) and eluted with TE buffer for Trx1 or KE buffer for Grx1. The absorbance of each fraction was measured with a spectrophotometer and the concentration was calculated for A 280 with a molar extinction coefficient of 8000 M −1 cm −1 for hTrx1 and 3230 M −1 cm −1 for hGrx1. A full reduction of Trx1 and Grx1 was determined with 6 M guanidine hydrochloride in 0.1 M Tris-Cl, pH 7.5 and 1 mM 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB) as described 32 . Absorbance was measured at 412 nm and the amount of free thiols was calculated with the molar extinction coefficient for DTNB (13600 M −1 cm −1 ). Fully reduced protein was incubated with indicated concentrations of APR-246 for one hour at 37 °C and desalted with Amicon Ultra Centrifugal Spinners with 3000 cutoff for 30 min at 14000 g to remove excess compound. Finally, the activities of Trx1 and Grx1 was determined with the fluorometric methods as described above.

Assessment of reduced Trx1 or Grx1 in vitro.
Reversibility and thiol dependence of APR-246 binding to Trx1 and Grx1. Reduced Trx1 and Grx1 were incubated with 240 and 1000 µM APR-246, respectively for one hour at 37 °C prior to assessment of protein thiol content with DTNB as described above. MQ-modified Trx1 was then incubated for one hour at 37 °C with either 10 mM DTT or 50 nM TrxR plus NADPH. Grx1 treated with preheated APR-246 was incubated with 10 mM DTT for one hour at 37 °C. After incubation, free DTT and NADPH were removed by desalting and protein thiol content was assessed once again to identify restoration of thiols in Trx1 and Grx1. Untreated Trx1 and Grx1 were used as controls. Mass spectrometry analysis. Pre-reduced recombinant hTrx1 and hGrx1 (50 µM) were incubated with 0.1 and 0.5 mM MQ, respectively, for one hour at 37 °C. Modified proteins were desalted with C4 ZipTip and then crystallized on a matrix-assisted laser desorption/ionization (MALDI) target plate with MALDI matrix prepared through dissolving 10 mg Sinapinic acid in 75% acetonitrile and 0.1% trifluoroacetic acid (TFA). MQ-modified Trx1 and Grx1 were identified using MALDI-TOF mass spectrometry (Voyager DE-Pro, AB SCIEX). Each spectrum was the result of 50 laser shots and myoglobin was used for external calibration.
Redox Western blot analysis of cellular Trx1 and Trx2. Detection of Trx1 and Trx2 redox states was carried out as described 33 , using p53 null Saos-2 and mutant p53-expressing Saos-2-His-273 cells. 19800 cells/cm 2 were plated on 15 cm dishes for 18 hours prior to 24 hours of treatment with 65 µM APR-246. The cells were then harvested for Trx redox state detection.
Total cellular glutathionylation detection. Cells were seeded into 6-well plates at 3 × 10 5 cells per well, and treated with different concentrations of APR-246 the following day. After 24 hours treatment, cells were washed and harvested in cold PBS and lysed in lysis buffer (25 mM Tris·HCl, pH 7.5, 100 mM NaCl, 2.5 mM EDTA, 2.5 mM EGTA, 20 mM NaF, 1 mM Na3VO4, 20 mM sodium ß-glycerophosphate, 10 mM sodium pyrophosphate, 0.5% Triton X-100) containing protease inhibitor cocktail (Roche) and 50 mM iodoacetamide (IAM, Sigma-Aldrich). After centrifugation, 25 µg of total proteins from the supernatant were separated by SDS-PAGE under the non-reducing condition and probed with an anti-glutathione antibody (VIROGEN Corporation). Anti-GAPDH (Santa Cruz) antibody was used as loading control.
Assay for ribonucleotide reductase activity in vitro. RNR  Incubation was carried out at 37 °C for 30 min. The reaction was terminated by the addition of 0.5 ml of 1 M HClO 4 and 50 µl of 20 mM dCMP was added as a carrier and placed in boiling water for 10 min to hydrolyze CDP and dCDP into CMP and dCMP. The protein precipitate was pelleted by centrifugation, the dCMP/[ 3 H]dCMP formed was isolated by ion exchange chromatography on Dowex-50 W columns, and the amount of radioactivity was quantified by liquid scintillation counter 33 . RNR was incubated with different concentrations of APR-246 or MQ for 30 min at 37 °C followed by the addition of the reaction mixture.
Assay for ribonucleotide reductase activity in cultured cells. Saos-2 and Saos-2-His-273 cells were treated with 12.5, 25, and 50 µM of APR-246 for 24 hours. The cells were scraped, spin down, and resuspended in HEPES buffer (100 mM HEPES, pH 7.6, 15 mM magnesium acetate and 10 mM DTT) with freshly added protease inhibitor (Roche Applied Science). The lysate was passed through five cycles of freeze-thaw and sonicated; debris was removed by centrifugation (13,000 rpm, 20 min, 4 °C) and the supernatant was assayed. Protein concentration was determined by the Bio-Rad protein assay. Assays were performed according to the method described by Lepoiver et al. [47] with some modifications using

Effects of GSH on the inhibition of glutaredoxin and thioredoxin systems by APR-246. In order
to stimulate conversion of APR-246 to MQ, we preheated APR-246 at 90 °C for 10 min. The effect of preheated APR-246 on the reducing activity of the Grx system was investigated using a novel fluorescence-based activity assay with Eosin-GSH labeled BSA (E-GS-BSA) as a substrate. Increasing inhibition of Grx1 was observed with decreasing concentrations of GSH (Fig. 1B). APR-246 had no significant inhibitory effect on the Trx system, while preheated APR-246 and MQ displayed an effective inhibition (Fig. 1C).
The results from the Grx activity assays prompted further examination of the reactivity of MQ with GSH. When 0.5 mM GSH was incubated with MQ and the thiol content assessed with DTNB, a reaction with a rate constant of 21.6 M −1 s −1 was observed (Fig. 1D). In a second experiment, the reactivity of NADPH-reduced TrxR, known to be inactivated by MQ (15), was tested with and without GSH. A complete inactivation of TrxR was obtained in the absence of GSH, whereas 1 mM GSH fully protected the enzyme (Fig. 1E). Thus, MQ reacts with GSH, in agreement with recent studies 16 .
Direct reaction of preheated APR-246 with Trx1 and Grx1. We then investigated if preheated APR-246 acted as an inhibitor of Trx1 and Grx1 specifically or through interaction with other components of the two systems, which was recently shown for TrxR1 15 . The IC 50 of the inhibition of the Trx system ( Fig. 2A) and the Grx system ( APR-246. The remaining components of the respective systems were then added to the mixture to measure the activity. Preheated APR-246 showed stronger inhibition of Trx1 (Fig. 2C) than of Grx1 (Fig. 2D). The IC 50 values for the inhibition of Trx1 and Grx1 were 5.40, and 57.7 µM, respectively, which is close to the IC 50 values for the inhibition of the whole Trx or Grx systems. This indicates that Trx1 and Grx1 are direct reactants with preheated APR246. Interestingly, the activity of Trx1 was fully restored after long-term kinetic measurements (data not shown), indicating that the initial inhibitory effect of preheated APR-246 is transient.
To examine the ability of MQ to form adducts with Trx1 and Grx1, we performed mass spectroscopy analysis. Pre-reduced Trx1 and Grx1 (50 µM) were incubated with MQ for one hour at 37 °C and then analyzed with MALDI-TOF mass spectrometry (Supplementary method). We used MQ 0.5 mM in order to override the reductant DTT in the protein preparation. The results show a clear mass shift of both Trx1 and Grx1 upon incubation with 0.5 mM MQ, indicating the formation of adducts (Supplementary Fig. 1A). Five peaks with multiple

Inhibition of Trx1 and Grx1 by preheated APR-246 is thiol-dependent and reversible.
To further study the reversibility of preheated APR-246-mediated inhibition of Trx1 and Grx1, we assessed the effect on the thiols in the two proteins. To determine if they could be fully restored after preheated APR-246 inhibition, preheated APR-246-treated Trx1 was incubated with either 50 nM TrxR and 0.25 mM NADPH (Fig. 3A) or 10 mM DTT (Fig. 3B); APR-246-treated Grx1 was also incubated with 10 mM DTT (Fig. 3C). Human Trx1 and Grx1 both contain five thiols. After the incubation, there were only 3-4 thiols in the control Trx1, indicating that it got partially recovered (Fig. 3A,B). Grx1 had five thiols, demonstrating that it is in a fully reduced form (Fig. 3A). Treatment with preheated APR-246 resulted in a decrease in the amount of free thiols in both Trx1 and Grx1 in a concentration-dependent manner. The number of free thiols was substantially recovered for both Trx1 and Grx1 after reduction with DTT, indicating that the inhibition was reversed by strong reductants. we used a redox Western blot method to detect changes in the redox status of Trx1 and Trx2. Saos-2 cells, either p53 null or carrying His-273 mutant p53, were treated with 65 µM APR-246 for 24 hours prior to blotting. We observed a significant loss of free thiols in both Trx1 and Trx2 in Saos-2-His-273 cells after treatment (Fig. 4). Similar results were obtained for Saos-2 cells, although the effect was much less pronounced. Quantification of band intensities show that most Trx1 protein has free thiols in untreated Saos-2 (78.8%) and Saos-2-His-273 (76.3%) control cells (Fig. 4).
However, treatment with APR-246 resulted in a clear decrease in the amount of free thiols in mutant p53-carrying Saos-2-His-273 cells, as the amount of fully reduced thiols was decreased from 76.3% to 17.0%, and as the amount of Trx1 without any free thiols (SH-0) increased with 8%, We obtained similar results for Trx2, where almost 50% of Trx2 in Saos-2 cells was in the fully reduced form and 63% in Saos-2-His-273 cells (Fig. 4). In Saos-2 cells, APR-246 treatment led to an increase in protein without any free thiols (SH-0) from 27% to 31%, and in Saos-2-His-273 cells, the increase was even more pronounced, from 17% to 45% of Trx2 without free thiols. These changes of Trx redox status correlate with induction of cell death in Saos-2 and Saos-2-His-273 cells, as previously described 15 .
Taken together, these results show a significant inactivation of both Trx1 and Trx2 in Saos-2 and Saos-2-His-273 cells upon treatment with APR-246, through MQ binding to free thiols. This effect is more visible in Saos-2-His-273 cells than in the p53 null Saos-2 cells.

Total glutathionylation in Saos-2 and Saos-2-His-273 cells. Protein glutathionylation is a reversible
formation of mixed-disulfide between glutathione and protein thiols, which is an important event involved not only in the protection of protein cysteines from irreversible oxidation but also in protein redox regulation. Since Grx1 is involved in the deglutathionylation process and is affected by APR-246, we treated Saos-2 and Saos-2-His-273 cells with different concentrations of APR-246 for 24 hours and analyzed total glutathionylation. As shown in Fig. 5 the total glutathionylation increased with the increasing concentration of APR-246.

APR-246 and MQ inhibit RNR activity.
In order to test whether APR-246 and MQ have a direct effect on ribonucleotide reductase (RNR) apart from Trx and Grx, we assessed the effect of APR-246 and MQ on RNR activity both in vitro and in cells. Both APR-246 and MQ inhibited RNR activity in vitro, but as expected, MQ was significantly more potent. MQ at 40 µM almost completely inhibited the activity (Fig. 6A). We also treated Saos-2 and Saos-2-His-273 cells with 12.5, 25, and 50 µM of APR246 for 24 hours at 37 °C and assessed RNR activity in the cell lysates. This revealed a dose-dependent inhibition of RNR by APR-246 in both Saos-2 p53 null and Saos-2-His-273 cells.

Discussion
The Trx and Grx systems are both commonly upregulated in tumor cells to compensate for the enhanced ROS production as a result of increased metabolism. The Trx system provides electrons to peroxiredoxins and methionine sulfoxide reductases (MSR) to protect cells against oxidative stress 34 . Either Trx1 or Grx1 are required for each turnover of RNR, the essential enzyme for deoxyribonucleotides synthesis during DNA replication and repair. Both redoxins are also known to have anti-apoptotic properties through interactions with several transcription factors and proteins, such as inhibition of the MAPK cascade by binding to ASK-1 in their reduced state 24,35 . Thessoulin and colleagues have shown that APR-246 (PRIMA-1 Met ) induces cell death in myeloma cells by GSH depletion 17 . We have shown that MQ can bind GSH and decrease GSH levels in ovarian cancer cells 16 . Since MQ will consume free GSH, all pressure in the cell to prevent oxidative stress will be put on the Trx system. However, TrxR is inactivated by MQ and converted to an NADPH oxidase that produces ROS 15 . Although GSH may act as a backup for TrxR to keep Trx in its reduced form 36 , it will be depleted by APR-246, leading to the dramatically increased ROS production due to the simultaneous inactivation of both major redox systems. If the cell contains mutant p53, APR-246 will reactivate mutant p53 and induce pro-apoptotic p53 target genes, which may lead to a further increase in ROS 37 .
We also observed a significant loss of free thiols in both Trx1 and Trx2 in APR-246-treated Saos-2-His-273 cells. The effect in p53 null Saos-2 cells was much less pronounced than in the mutant p53-expressing cells. It is likely that reactivation of overexpressed mutant p53 by APR-246, along with APR-246-mediated inhibition of TrxR1, leads to increased production of ROS, which makes Trx1 oxidized. Via peroxiredoxin, H 2 O 2 can inactive Trx1 forming a two-disulfide form 38 . Furthermore, mutant p53 has been shown to inhibit the activity of the antioxidant transcription factor Nrf2, which may lead to reduced expression of SLC7A11, a component of the x c cystine-glutamate antiporter, and thus decreased levels of GSH 39 . This provides an additional plausible explanation for the stronger effect on thiols in Trx1 and Trx2 in the mutant p53-expressing Saos-2 cells. The oxidation of Trx can lead to non-repaired DNA damage by loss of RNR activity, activate the MAPK pathway via ASK1, and further induce ROS production 40 . In combination with reactivation of mutant p53 by APR-246, the oxidation of Trx might facilitate p53-dependent apoptosis.
The depletion of GSH in the APR-246 treated cells was also confirmed by the result that increased total glutathionylation was observed in both cell lines in a dose-dependent manner. The increased glutathionylation is a sign of oxidative stress in the cells and is probably due to inhibition of Grx1 which can efficiently catalyze protein deglutathionylation (Fig. 5).
Thioredoxin and glutaredoxin systems provide the reducing power to RNR for balanced deoxyribonucleotide pools for DNA replication and repair. We carried out an in vitro RNR assay to test whether APR-246 via MQ has a direct effect on RNR activity. We found that both APR-246 and MQ inhibited RNR activity, with MQ being more potent (Fig. 6A). The ability of APR-246 itself to inhibit RNR in this experiment may be due to the generation of MQ in the APR-246 preparation. We also found that APR-246 inhibits RNR activity in Saos-2 cells. The inhibition was more potent in p53 null Saos-2 p53 cells (Fig. 6B,C).
In order to understand the mechanism for APR-246-induced loss of the thiols in Trx and Grx, we performed mass spectrometry analyses, which revealed direct binding of MQ to Trx1 and Grx1. These results provide solid confirmation that the decrease in thiol content observed in the proteins is indeed due to the formation of MQ adducts.
Inhibition of the thioredoxin system has been proposed as a novel anticancer approach 34,40,41 . Many compounds and clinical drugs have been shown to inhibit TrxR and lead to subsequent Trx oxidation 40,42,43 . Some Michael acceptor drugs can inactivate and convert the reductant TrxR into a pro-oxidant to cause the oxidation of Trx, the key determining factor for the cell fate 38,40,44 . APR-246 may also act in the same manner 15 .
Our analysis of Trx and Grx using redox Western blotting provided information about the fraction of each protein in the different redox states in the cells after APR-246 treatment. Human Trx1 contains two cysteines, Cys32 and Cys35, in the active site motif Trp-Cys-Gly-Pro-Cys, with three additional structural cysteines (Cys62, Cys69, and Cys73). After electron transfer to its substrate via the dithiol-disulfide exchange reaction, a disulfide bond is formed between Cys32 and Cys35, which is reduced by TrxR and NADPH. The three structural cysteines can be modified by glutathionylation or nitrosylation 38 . Cys73 may also be involved in Trx1 dimer formation through a Cys73-Cys73 disulfide bond. However, since our data show an inhibition of Trx activity by APR-246, there is a great probability that the shift in redox states observed in the redox Western blots also are results of MQ modifications. Loss of free thiols in Trx1 leads to the release of ASK1, which activates the c-Jun N-terminal kinase (JNK) and p38 MAP kinase pathways and in turn TNF-α-induced apoptosis 24 . The APR-246 structural analog PRIMA-1 has been shown to induce apoptosis through the JNK pathway, which synergizes with the apoptotic pathway induced by Trx1 oxidation via ASK1 release 45 . Thus, it is possible that the induction of the MAP kinase pathway by MQ could be a result of Trx1 inactivation, as shown in this study. Trx2 also induces apoptosis via ASK1 when being oxidized, as well as giving rise to increased ROS levels in the mitochondria, leading to release of cytochrome c and activation of caspase 3 and 9 46 . Interestingly, the APR-246-induced cell death was in part caspase 2-independent 45 . Our findings indicate that inhibition of Trx2 by APR-246 can be the explanation for this effect 47 .
A recent paper demonstrated that PARP-1 inhibitors sensitized head and neck squamous cell carcinoma cells to APR-246 by inactivation of TrxR and production of ROS 48 . The results showed that APR-246 may kill cancer cells independent of p53 48 .
In conclusion, we have shown that APR-246 reversibly inhibits Trx, Grx, and RNR, most likely through covalent binding of MQ to thiols. This, together with the previously reported inactivation of TrxR via modification of the selenocysteine residue in the active site and the fast reaction with GSH, will disrupt the redox balance in the cell and affect processes such as DNA replication and repair and neutralization of ROS. Thus, APR-246 targets both mutant p53 and cellular redox regulation, two Achilles' heels of tumor cells. Our data shed novel light on the mechanisms behind APR-246-induced apoptosis in cancer cells. Given that APR-246 is currently in clinical development, these results may have implications for its use as an anticancer agent.