Crosslinking Protein Glutathionylation Mediated by O²-Arylated Bis-Diazeniumdiolate “Double JS-K”

Abstract

Attachment of glutathione (GSH) to cysteine residues in proteins (S-glutathionylation) is a reversible post-translational modification that can profoundly alter protein structure and function. Often serving in a protective role, e.g., by temporarily saving protein thiols from irreversible oxidation and inactivation, glutathionylation can be identified and semi-quantitatively assessed using anti-GSH antibodies, thought to be specific for recognition of the S-glutathionylation modification. Here we describe an alternate mechanism of protein glutathionylation in which the sulfur atoms of the GSH and the protein’s thiol group are covalently bound via a crosslinking agent, rather than through a disulfide bond. This form of thiol crosslinking has been shown to occur and confirmed by mass spectrometry at the solution chemistry level, as well as in experiments documenting the potent antiproliferative activity of the bis-diazeniumdiolate Double JS-K in H1703 cells in vitro and in vivo. The modification is recognized by the anti-GSH antibody as if it were authentic S-glutathionylation, requiring mass spectrometry to distinguish between them.

INTRODUCTION

Protein S-glutathionylation, the reversible oxidation of glutathione (GSH) and protein cysteine thiols to disulfide bonds, occurs naturally and increases as a consequence of oxidative stress.^{1, 2} It is recognized as a regulatory as well as protective mechanism, saving protein thiols from irreversible oxidative processes. Nitric oxide (NO) is a ubiquitous signaling molecule with a broad spectrum of actions in physiological and pathological processes. Arylated diazeniumdiolates are NO prodrugs that are broadly active against a number of rodent tumor models in vivo.^3–7 Certain members of this class have been reported to induce protein glutathionylation as a possible contributor to their mechanism of action.^{8, 9} The phenomenon was observed in cell lysates, based on immunodetection of GSH residues attached to proteins with antibodies specific for protein/GSH conjugates.

Here we compare the arylated mono-diazeniumdiolate JS-K (1, Scheme 1) to the related bis-diazeniumdiolate, Double JS-K (2, Scheme 1), in their ability to conjugate GSH to protein. The results reveal a previously unrecognized mechanism of protein modification displayed by certain members of the arylated diazeniumdiolate family.

Scheme 1.

Structures of JS-K and Double JS-K, two arylated diazeniumdiolate anticancer drug candidates compared in this investigation

MATERIALS AND METHODS

General

Compounds 1,¹⁰ 2,⁴ 4,⁴ and S-(2,4-dinitrophenyl)glutathione (DNP-SG)¹¹ were prepared using previously reported methods. 1,5-Difluoro-2,4-dinitrobenzene (DFDNB, 3), dithiothreitol (DTT), GSH, N-acetylcysteine (NAC), and all buffer components were purchased from Sigma-Aldrich (St. Louis, MO). Rabbit muscle actin was obtained from Cytoskeleton (Denver, CO). All HPLC solvents were purchased from Fisher (Waltham, MA).

Detection of Protein Glutathionylation by Immunoblotting

All cell lines (human non-small cell lung cancer (NSCLC) H1703, A549, and human leukemia U937) were obtained from American Type Culture Collection (Manassas, VA) and cultured according to the supplier’s protocol. Stock solutions (10 mM) of each compound were prepared in dimethyl sulfoxide (DMSO). Cells were treated with 1 µM compound, or an equal volume of DMSO as a control, for time points indicated in the figure legends. Cells were washed with PBS and prepared for immunoblotting by lysing in lysis buffer (25 mM Hepes buffer, pH 7.2, containing 150 mM NaCl, 10 mM MgCl², 1% NP-40 (nonyl phenoxypolyethoxylethanol), 0.25% sodium deoxycholate, 10% glycerol, 2.5 mM EDTA, and protease inhibitor cocktail (Roche Applied Science, Indianapolis, IN). Proteins in cell lysates were separated under nonreducing conditions on 4−12% NuPage bis-Tris gels (Invitrogen Life Technologies, Carlsbad, CA) and transferred to PVDF membranes (Invitrogen) at 30 V for 1.5 h. After overnight blocking in 5% nonfat milk in Tris-buffered saline containing 0.1% Tween-20 (TBS-T), the membranes were probed with anti-glutathione monoclonal antibodies (1:1000, Virogen, Watertown, MA) in 5% milk in TBS-T for 3 h at room temperature. After repeated washings in TBS-T, the blots were incubated for 60 min at room temperature with antimouse secondary antibody, 1:1000, conjugated with horseradish peroxidase (Cell Signaling Technology, Danvers, MA), in 5% milk in TBS-T, and then, after washing, developed using the chemiluminescence ECL kit (GE Healthcare, Piscataway, NJ). To prove equal loading, the blots were stripped with stripping buffer (Thermo Scientific, Rockford, IL) and reprobed with anti-β-actin rabbit polyclonal antibodies (Abcam, Inc., Cambridge, MA).

To detect apoptosis SDS-PAGE was run under standard reducing conditions and immunoblots were probed with anti-cleaved PARP antibody (Cell Signaling).

Intracellular Nitric Oxide Release

The intracellular level of nitric oxide after JS-K and Double JS-K treatment was quantified using the NO-sensitive fluorophore 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM diacetate) (Invitrogen). Cells growing on 6-well dishes (6 × 10⁵/well) were loaded with 2.5 µM DAF-FM diacetate in HBSS at 37 °C and 5% CO₂. After 30 min of incubation the cells were rinsed with HBSS to remove excess of probe, and fresh HBSS was added to the wells. JS-K or Double JS-K was added to the cells at 1 µM final concentration. After 30 min incubation the cells were scraped into HBSS and the fluorescence of the benzotriazole derivative formed on DAF-FM’s reaction with aerobic NO was analyzed using a Perkin Elmer LS50B luminescence spectrometer with the excitation source at 495 nm and emission at 515 nm. All experiments were performed at least three times, each time in triplicate.

Thiol Crosslinking with 1,5-Difluoro-2,4-dinitrobenzene (DFDNB)

GSH (4 mM) was suspended in 0.1 M phosphate buffer (pH 7.4) containing 50 µM diethylenetriaminepentaacetic acid (DTPA). To this solution was added DFDNB at a ratio of 2 moles of GSH per 1 mole of DFDNB, and the reaction mixture was held at room temperature for 10 min. This protocol was repeated with a 1:1 molar ratio of GSH and NAC followed by the addition of 1 molar equivalent of DFDNB. The reaction mixtures were analyzed via LC/MS using an Agilent 1100 series LC/Mass Selective Detector in positive ion mode with electrospray ionization (ESI) and a fragmentor voltage of 90 V. Separations were carried out on a Phenomenex Luna C18 column, 3 µm, 150 × 2.0 mm, with a gradient consisting of water and acetonitrile containing 0.1% formic acid.

In Vitro Metabolism of Double JS-K

H1703 cells were plated in 75-cm² flasks and incubated overnight at 37 °C, 5% CO₂. The cells were treated with 5 µM of Double JS-K and incubated for 5, 30 and 60 min. At each time point the cells were lysed via scraping in 800 µL of 10 mM HCl followed by successive rounds of freezing and thawing. To the lysate was added 200 µL of a 5% 5-sulfosalicylic acid solution. The precipitate was removed by centrifugation at 8000g for 10 min, and the supernatant was analyzed by LC/MS using a Thermoquest Surveyor HPLC coupled with a Finnigan LCQ Deca mass spectrometer. Positive ions were generated with an atmospheric pressure chemical ionization (APCI) source with a capillary voltage of 15 V and a corona discharge of 4 µA. Separations were performed on an Agilent Eclipse XDB-C18 5-µm 4.6 × 150 mm column at a flow rate of 1 mL/min under H₂O/acetonitrile/0.1% formic acid gradient conditions.

Actin Glutathionylation

A stock solution of actin, purified from rabbit skeletal muscle, was prepared in 0.1 M phosphate buffer (pH 7.4) containing 50 µM DTPA at a final concentration of 100 µM. A 3-µL aliquot was taken and diluted with 50 µL of the above mentioned phosphate buffer containing 5 mM DTT. After 10 min the DTT was removed via successive rounds of filtration using Millipore Microcon YM-10 centrifugation filters. The actin was eluted from the filtration unit in 45 µL of phosphate buffer. To this solution was added 3 µL of a 1 mM stock solution of GSH followed by the addition of 1 molar equivalent of DFDNB. The reaction was held at room temperature for 20 min, followed by LC/MS analysis using an Agilent 1100 series LC/Mass Selective Detector in positive ion mode with ESI and a fragmentor voltage of 250 V. Separations were carried out on a Phenomenex Jupiter C18 column, 5 µm, 150 × 2.0 mm, 300 A, with a gradient consisting of water and acetonitrile containing 0.1% formic acid.

Kinetics of Aromatic Substitution Reactions

Varying concentrations DTT were suspended in 0.1 M phosphate buffer (pH 7.4) with 50 µM DTPA. Each reaction was initiated by the addition of DNP-SG to a final concentration of 100 µM. The disappearance of DNP-SG was monitored by HPLC using an Agilent 1100 series solvent delivery system coupled with an Agilent 1100 series diode array detector. Separations were carried out on a Phenomenex Luna C18 column, 5 µm, 250 × 4.6 mm, under an isocratic mobile phase consisting of water and acetonitrile in a 1:1 ratio containing 0.1% formic acid. The product identity was confirmed using a Finnigan LCQ Deca mass spectrometer with APCI in positive ion mode.

Stabilization of Drug into Nanoparticles

Aqueous solution: PoloxamerR 188 and trehalose were dissolved in milli-Q water at the concentration of 9.5 and 12 mg/mL respectively. The aqueous solution was filtered through 0.2-µm syringe filter (Whatman Inc., Florham Park, NJ).

Organic solution: Block copolymer (PLA-b-PEG, 3.7k-b-5k, Lakeshore Biomaterials) was dissolved in 3.5 mL of tetrahydrofuran at a concentration of 8.8 mg/mL. The mixture was sonicated to get a clear solution. Double JS-K (at 8.8 mg/mL) was then added to the clear solution. The mixed solution was sonicated and filtered through a 0.2-µm syringe filter. Poly(D,L-lactide) (PLA) was synthesized by ring-opening polymerization of D,L-lactide in toluene, initiated by tin(II) 2-ethylhexanoate and n-octanol. Number-average molecular weight was determined by GPC (Mn = 3.6 k, polydispersity index = 1.27). For the control study, PLA (17.6 mg/mL) was used as the drug substitute. Nanoparticles were formulated using Flash NanoPrecipitation as previously described.¹²

Animal Studies

All animals used in this research project were cared for and used humanely according to the following policies: The U.S. Public Health Service Policy on Humane Care and Use of Animals (1996); the Guide for the Care and Use of Laboratory Animals (1996); and the U.S. Government Principles for Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training (1985). All Frederick animal facilities and the animal program are accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. Double JS-K, 2, was formulated in poly-lactide-b-poly(ethylene glycol) (PLA/PEG) nanoparticles, as described previously.¹²

Non-small cell lung cancer H1703 cells were injected at 5 × 10⁶ subcutaneously into a flank of 7-week-old female athymic NCr-nu/nu mice (Charles River). Animals were randomized and the drug injections were initiated when the tumors reached over 2 × 2 × 2 mm (typically 3.5 weeks). Animals were treated three times a week for four weeks with intravenous tail vein injections of either PLA/PEG nanoparticles, Double JS-K (64 µmol/kg in PLA/PEG nanoparticles), or saline only. Tumors were measured using a caliper twice a week, and the tumor volumes were calculated using a formula for ellipsoid volume, (π/6) × length × width × height. The non-parametric Mann-Whitney test was utilized for statistical comparisons of tumor volumes at each time point. Body weights were taken before each drug injection.

Immunohistochemistry

Immunohistochemical staining of 5-µm sections of the formalin-fixed paraffin-embedded xenograft tumors was carried out using anti-glutathione monoclonal antibody which detects glutathione-protein complexes (glutathione conjugated to proteins) (Virogen, Cat # 101-A-250, 1:200). This primary antibody was used with Dako ARK™ kit (Cat # K3954, Dako, Carpinteria, CA), according to the manufacturer’s protocol.

RESULTS

Differing Glutathionylating Abilities of JS-K (1) and Double JS-K (2)

Anticancer drug candidates of the arylated diazeniumdiolate class were designed to be activated according to the S_NAr mechanism outlined in Scheme 2. Thiol-containing biomolecules such as GSH attack the aryl ring carbon bearing the R₂N-N(O)=NO⁻(diazeniumdiolate) group to free the ion for spontaneous release of cytolytic NO while transferring the aryl ring to the thiol sulfur atom.¹⁰ In an effort to further characterize the notable glutathionylating ability reported earlier⁹ for arylated bis-diazeniumdiolates, we have screened a variety of representative structures, with the results shown in Figure 1 and Figure S1 of the Supporting Information. An important point that emerges from these data is illustrated in Figure 1A showing the comparison between JS-K (1) and Double JS-K (2) and highlighting the all-or-none difference between them in apparent glutathionylating activity. Other bis-diazeniumdiolates, compounds 3, 4, and 5, exhibit similar glutathionylating ability to double JS-K which begins rapidly upon treatment of the cells, Figure 1B. The glutathionylation signal derived from the treatment of cells with Double JS-K and related bis-diazeniumdiolates is much more extensive at lower concentrations than traditional glutathionylating agents such as kathon and diamide, Figure S2. The intracellular release of NO from Double JS-K is 1.5-fold greater than that of JS-K, measured in cells preloaded with the NO-sensitive probe, 4-amino-5-methylaminofluorescein diacetate (DAF-FM DA). This modest increase in intracellular NO release cannot account for the remarkable difference in the extent of protein glutathionylation between these two compounds.

Scheme 2.

Mechanism of arylated diazeniumdiolate activation by reaction with glutathione (GSH)

Figure 1.

Bis-diazeniumdiolates induce extensive non-specific apparent protein glutathionylation while mono-diazeniumdiolate JS-K does not. H1703 cells were treated with 1 µM compound for 1 h. Protein lysates (20 µg per lane) were resolved on 4–12% Bis-Tris Nu-PAGE gels, processed for immunoblotting and probed for protein/GSH conjugates with an anti-GSH antibody, as described in Materials and Methods. A) Double JS-K (2, right lane) induces extensive glutathionylation of cellular proteins while JS-K (1, left lane) does not. Note that the band on the left showing actin to be glutathionylated was also present in the untreated DMSO control, Figure S1, consistent with previous reports^{13, 14} of this protein being spontaneously Sglutathionylated in a variety of cultured cells. B) Close analogues of compound 2, compounds 3, 4, and 5, induce extensive protein/GSH conjugation at early time points (2 min and 15 min). C = DMSO control.

The Observed Glutathionylation Does Not Depend on NO Release

We postulated that this observed difference in glutathionylating ability follows from the fact that JS-K (1) is necessarily a monovalent electrophile while Double JS-K (2) has the potential to react with (and crosslink) two different thiol groups. To test this hypothesis, we replaced one or both of the two diazeniumdiolate leaving groups of Double JS-K with the even better leaving group fluoride to get 6 (DFDNB) and 7. Both showed substantial glutathionylating ability in H1703 cells. The results, shown in Figure 2, confirm the potent glutathionylating ability of a bivalent electrophile, including in the absence of a capability for NO release. These data point to the operation of an alternative mechanism by which bis-diazeniumdiolates can cause apparent protein glutathionylation, namely by crosslinking thiol groups through the dinitrophenyl ring, as shown in Scheme 3.

Figure 2.

H1703 cells were treated with 1 µM compound for 1 h. Equal amounts (20 µg) of cell lysate proteins were processed for immunoblotting and probed with an anti-glutathione/protein conjugate antibody. Like Double JS-K, difluoro compound 6 (middle lane) induces extensive glutathionylation of cellular proteins while the untreated DMSO control (left lane) does not. The signal intensity derived from compound 6 is slightly greater than that from compound 7, demonstrating that the nature of the leaving group and not the NO-generating capacity is what determines the extent of glutathionylation detected under these conditions.

Scheme 3.

An alternative mechanism of protein glutathionylation, crosslinking through a dinitrophenyl ring. X and Y represent any of a variety of leaving groups on the dinitrophenyl ring. RSH represents a non-protein thiol including but not limited to an additional molecule of glutathione. The shaded gray circle with the SH group symbolizes a solvent-exposed protein thiol residue.

To further explore the hypothesis that the observed immunoreactive glutathionylating activity is actually due to the postulated crosslinking mechanism, Double JS-K and in a separate set of reactions DFDNB were added to buffered solutions containing either 4 mM GSH or a 1:1 molar ratio of GSH: NAC and the ensuing reaction was monitored with HPLC/mass spectrometry. The results confirm that both Double JS-K (2) and DFDNB (6) are capable of covalently crosslinking biologically relevant thiols. In the GSH reactions, [M + H]⁺ = 779.2 and [M + 2H]²⁺ = 390.2 ions were detected, Figure S4A, consistent with a structure in which two GSH molecules are covalently linked through the dinitrophenyl ring. Similarly, an [M + H]⁺ = 635.1 ion was detected, Figure S4B, in the mixed thiol reactions, confirming the crosslinking of GSH and NAC through the dinitrophenyl ring (Scheme 4).

Scheme 4.

Thiol crosslinking of GSH (colored in red) and NAC (colored in blue) with either DFDNB or Double JS-K

Thiol Crosslinking Glutathionylation in Cultured Cells

Thiol crosslinking was also observed in H1703 NSCLC cells treated with Double JS-K. After treating the cells, extracted small molecules were analyzed for drug metabolites by LC/MS. After 5 min, Double JS-K was effectively absorbed into the cells and rapidly metabolized by glutathione, forming the aryl crosslinked bis-GSH adduct depicted in the top panel of Figure 3. This metabolite likely gives way to the aryl crosslinked dimercapturate detected after 30 min of treatment (Figure 3, bottom panel). The essentially irreversible consumption of reduced glutathione through GSH:GSH and protein:GSH crosslinking may result in a shift in the cellular GSH/GSSG redox couple toward a more oxidizing environment.⁵

Figure 3.

In vitro metabolism of Double JS-K. H1703 cells were treated with 5 µM Double JS-K. At varying time points the cells were lysed, under the conditions detailed in the Materials and Methods section, and the small molecule metabolites were analyzed by LC/MS. Double JS-K was not detected in either of the total ion current chromatograms of cells treated for 5 min (top panel) or 30 min (bottom panel). Instead, a bis-GSH crosslink was observed rapidly (top panel), with the bis-mercapturate crosslink forming over time (bottom panel).

Crosslinking Glutathionylation of Actin

To show that protein thiol groups can participate in such a crosslinking pathway, actin was suspended in a solution of GSH followed by the addition of DFDNB. The reaction mixture was analyzed by LC/MS. Figure 4 depicts both the total ion current (TIC) chromatogram (above) and the UV chromatogram with detection at 330 nm (below). The peak at 20.18 min corresponds to the retention time of unmodified actin. However, it showed UV absorption at 330 nm typical of a thiol-bound 2,4-dinitrophenyl (DNP) ring. Figure 5 contains the deconvoluted ESI/MS of the actin chromatographic peak before (top panel) and after (bottom panel) modification. As is consistent with the literature, the mass of actin was measured as 41,872 Da. The smaller peak in the control sample with a mass of 41,968 Da is consistent with the oxidation of three of actin’s thiol groups to sulfinic acids. Upon addition of DFDNB, the disappearance of the unmodified 41,872 Da mass was observed, concomitant with the formation of four higher mass peaks (Figure 5) at 42,179 Da, 42,346 Da, 42,529 Da, and 42,819 Da. The first peak’s mass is consistent with traditional glutathionylation through the formation of a disulfide bond, while the second peak reveals crosslinking between actin and GSH through the 2,4-dinitrophenyl ring. The third and fourth peaks have masses consistent with a dinitrophenol plus a DNP-SG adduct, and two DNP-SG adducts, respectively. These results directly confirm the existence of crosslinking glutathionylation of a protein.

Figure 4.

Aryl glutathionylation of actin by DFDNB. Actin was suspended in a buffered solution containing GSH and the reaction was initiated by the addition of DFDNB. After 20 min the reaction mixture was analyzed by LC/MS. The figure depicts both a TIC chromatogram (top) and a UV chromatogram at 330 nm (bottom). The major peak in the TIC chromatogram, which has a mass spectrum containing ions typical of a protein-ESI spectrum, also had an absorbance at 330 nm typical of a cysteine-bound dinitrophenyl group.

Figure 5.

This shows the deconvoluted mass spectrum of the actin peak in Figure 4 before (top panel) and after (bottom panel) the addition of DFDNB. The masses shown in the bottom panel are consistent with classical S-S glutathionylation (A), protein-GSH aryl crosslinking (B), dinitrophenol adduct formation (C), or combinations of these aryl ring modifications.

Crosslinking Protein Glutathionylation Can Be Reversed, but Slowly

To determine whether the aryl-sulfur bond can be cleaved, potentially providing a repair pathway for crosslinking glutathionylation, we dissolved DNP-SG in phosphate buffer (pH 7.4) containing various concentrations of DTT. The decay of DNP-SG was followed as a function of time by HPLC. Pseudo-first-order kinetic plots were obtained and the derived rate constants showed excellent linear dependence on the nucleophile concentration (Figure 6A). These plots yielded an observed second order rate constant of 5 × 10⁻⁴ M⁻¹s⁻¹. Although this result suggests that the modification may be reversible in the presence of adequately strong physiological nucleophiles, it is relatively sluggish when compared with the reduction of classical S₁-S₂ glutathionylation through disulfide exchange reactions.¹⁵ The reaction appears to be orders of magnitude slower than the attack of GSH on the arylated diazeniumdiolate during the initiating step of drug activation, a process that for JS-K has a second order rate constant³ of 1 × 10⁰ M⁻¹s⁻¹. TCEP, a water soluble phosphine known to efficiently reverse S-glutathionylation, proved to be significantly less reactive (25 fold) with DNP-SG than DTT, Figure S3.

Figure 6.

Reversibility of the aryl crosslinked GSH adduct. A) Second order rate plot of a reaction of DNP-SG (100 µM) with DTT in phosphate buffer at pH 7.4 containing 50 µM DTPA at 37 °C; k_obs = 5 × 10⁻⁴ M⁻¹s⁻¹R² = 0.9983. The reactions were initiated by the addition of DNP-SG and followed by HPLC. B) A549 cells were treated with 1 µM Double JS-K and lysed at various time points. Twenty micrograms of lysate protein were resolved by SDS-PAGE, imunoblotted, and probed for protein/GSH conjugates with the anti-GSH antibody (top panel) and an antibody specific for cleaved PARP (middle panel). In contrast to S-S glutathionylation, this protein/GSH conjugate is long lived, remaining relatively unchanged for up to 8 h. The decrease in protein/GSH conjugate signal is only seen upon the onset of apoptosis as indicated by the cleavage of PARP. The bottom panel depicts β-actin as a loading control.

As mentioned above, Double JS-K stimulates pronounced protein glutathionylation in cancer cells as detected by Western blot. The extent of this modification was followed as a function of time following a single dose of Double JS-K to determine its longevity in a cellular environment. In contrast to classical glutathionylating agents which produce relatively short-lived S₁-S₂ lutathionylation, Double JS-K treatment resulted in the formation of a robust protein glutathionylation signal detected soon after addition to the medium and remained unchanged for up to 8 h (Figure 6B). In fact, a decrease in this signal was only detected after apoptosis was fully engaged, as indicated by the cleavage of PARP. These observations suggest that Double JS-K leads to the formation of aryl crosslinking protein glutathionylation, which is stable over the course of the experiment, ultimately contributing to the induction of cell death through apoptotic processes.

In Vivo Results

We next confirmed that such reactions can occur in vivo. Double JS-K was incorporated into PEG-protected nanoparticles produced by Flash NanoPrecipitation using the block copolymers previously described.¹² H1703 non-small cell lung cancer cells were chosen for assessing the activity of 2 against xenografted tumors in athymic mice. The drug significantly reduced growth of xenograft tumors when compared with tumors in control animals (Figure 7). Importantly, the saline controls and nanoparticle controls did not differ significantly, which strongly suggests that the nanoparticle formulation is nontoxic, and the treatment with either vehicle or the drug did not affect body weight (see legend to Figure 7).

Figure 7.

Double JS-K treatment significantly reduced growth of H1703 non-small cell lung cancer cell xenografts. Animals were treated three times a week for five weeks with i.v. injections of either saline (n = 15), vehicle (formulation control) (n = 15), or Double JS-K (64 µmol/kg in the vehicle, n = 15). * P < 0.05; ** P < 0.01 (by Mann-Whitney test). The treatment did not affect body weights. The average body weights for saline control, formulation control, and drug-treated groups were 22.6 ± 0.27 g (n = 15), 22.6 ± 0.34 g (n = 15), and 22.3 ± 0.43 g (n = 15) (mean ± SE) at the beginning of the experiment. At the termination, the average weights of the control groups were 24.83 ± 0.37 g (n = 12) and 25.06 ± 0.34 g (n = 15), for saline and formulation controls, respectively. The mean weight of Double JS-K-treated animals was 25.62 ± 0.46 g (n = 13).

Blood taken from these animals at the termination of the study was partitioned and analyzed for metabolites. Both the aryl crosslinked bis-GSH adduct and the aryl crosslinked bis-mercapturate observed in the in vitro metabolism of Double JS-K (Figure 3) were detected in the plasma, as were remnants of the block copolymers used in the formulation. Interestingly, polymers from the formulation as well as the GSH metabolites were detected only in the plasma, not in the fraction containing the blood cells.

Immunohistochemical analysis of tumors from the Double JS-K-treated animals showed strong staining with antibodies recognizing glutathione conjugated to proteins (Figure 8), confirming that the drug is delivered to the tumor, generating pronounced protein glutathionylation. We found that sample extraction/processing conditions (prolonged high temperature incubations with dithiothreitol) tended to destroy the arylated protein crosslinks, Figure S5. For this reason, we rely on the immunohistochemistry studies as evidence for cross-linked arylation of native proteins in vivo.

Figure 8.

Immunostaining of xenograft tumor sections with anti-GSH antibodies (magnification 200x). Left panel: representative tumor from a control animal treated with PLA/PEG nanoparticles alone. Right panel: representative tumor from an animal treated with Double JS-K-containing nanoparticles. Note the strong staining that the Double JS-K induced.

DISCUSSION

S-glutathionylation is a post-translational modification wherein glutathione (GSH) is attached to a protein thiol through a disulfide linkage. This modification protects cellular thiols from irreversible oxidation and has been demonstrated to be a vital cellular signaling mechanism.¹ Altered S-glutathionylation levels are observed in several pathological conditions.^16–19

Aryl bis-diazeniumdiolates were created with the purpose of doubling the payload of NO, effectively delivering 4 mols of NO/mol of compound. An earlier study involving a variety of such agents⁹ showed that these compounds are potent protein glutathionylating agents.⁹ Here we have compared anticancer drug candidate JS-K, a mono-diazeniumdiolate, to a structurally related bis-diazeniumdiolate, Double JS-K, in their ability to conjugate GSH to protein.

Although both compounds effectively release NO in the cell, differing by 1.5-fold, only Double JS-K, the bis-diazeniumdiolate, caused significant non-specific protein glutathionylation as detected by Western blot. In fact, NO was found not to be necessary to produce a strong signal from the anti-GSH antibody under these conditions. The signal was instead dependent on the number of electrophilic centers on the aromatic ring. These bivalent electrophiles can effectively crosslink thiols through the dinitrophenyl ring. This includes crosslinking GSH to a protein thiol through an aryl ring, a modification which is detectable by the anti-GSH antibody. This modification is not readily reversible, persisting until apparent cell death pathways are observed.

Double JS-K also causes significant GSH arylation by way of crosslinking two molecules of GSH through the dinitrophenyl ring. The GSH/GSSG redox couple constitutes the major redox buffer in cells, the maintenance of which is crucial for cellular function and xenobiotic detoxification. The crosslinking of GSH by Double JS-K and the release of NO from the diazeniumdiolate anions in an aerobic environment may effectively deplete cells of GSH causing an imbalance in the GSH/GSSG redox couple. Similar effects have been demonstrated with JS-K in which GSH arylation and NO release lead to perturbations in the GSH/GSSG redox couple, resulting in a rise in the steady state levels of reactive oxygen species, and the triggering of stress signaling pathways and apoptosis.⁵ These effects may be exacerbated in the case of Double JS-K due to more extensive GSH arylation as well as delivering a higher NO payload. Double JS-K proved to be an effective anti-cancer agent in vivo.

Double JS-K, administered intravenously, significantly retarded the growth of H1703 non-small cell lung cancer xenografts in mice. Consistent with our in vitro metabolism studies, Double JS-K caused significant thiol crosslinking in vivo. Both the dinitrophenyl crosslinked GSH and dinitrophenyl crosslinked mercapturic acid were detected in blood samples extracted from Double JS-K treated mice. Tumors from the treated animals were also shown to contain a vast array of protein/GSH conjugates when compared with the control.

The results unveil a previously unrecognized modification of thiols by bivalent electrophiles of the arylated diazeniumdiolate class, adding to the list of established pathways of GSH depletion, NO-induced events including S-nitrosylation and classical S₁-S₂ glutathionylation, and protein S-arylation seen with their monovalent counterparts such as JS-K. It is likely that other bivalent electrophiles, including current chemotherapeutic agents,^20–22 can support the same type of antibody-reactive crosslinking glutathionylation demonstrated here, making it strongly advisable to check the immunohistochemically derived identification by mass spectrometry if distinctions between the crosslinking route and authentic S-glutathionylation are necessary.

ABBREVIATIONS

APCI

atmospheric pressure chemical ionization

DFDNB

1,5-difluoro-2,4-dinitrobenzene

DNP-SG

S-(2,4-dinitrophenyl)glutathione

DTPA

diethylenetriamine pentaacetic acid

ESI

electrospray ionization

GSH

glutathione

HPLC

high-performance LC

LC

liquid chromatography

MS

mass spectrometry

NAC

N-acetylcysteine

NO

nitric oxide

PARP

poly(ADP ribose) polymerase

PBS

phosphate-buffered saline

PLA

poly(D,L-lactide)

PLA/PEG

poly-lactide-b-poly(ethylene glycol)

TBS-T

Tween-20

TIC

total ion current

UV

ultraviolet