Trimetallic Nitride Endohedral Fullerenes Carboxyl-Gd₃N@C₈₀: A New Theranostic Agent for Combating Oxidative Stress and Resolving Inflammation

Abstract

Antioxidative and anti-inflammatory effects of trimetallic nitride endohedral fullerenes carboxyl-Gd₃N@C₈₀, a newly developed magnetic resonance imaging (MRI) contrast agent, were investigated. All hydrochalarone and carboxyl-functionalized fullerenes showed effective radical (hydroxyl and superoxide anion) scavenging, whereas the carboxyl-Gd₃N@C₈₀ more efficiently attenuated lipopolysaccharide (LPS) induced oxidative stress in macrophages. Carboxyl-Gd₃N@C₈₀ also suppressed LPS-elicited mRNA expression of pro-inflammatory inducible nitric oxide synthase and tumor necrosis factor-alpha, and upregulated antioxidative enzyme axis Nrf2 and heme oxygenase-1, possibly via ERK but not AKT signaling pathways. Therefore, carboxyl-Gd₃N@C₈₀ held a great promise in becoming a novel theranostic nanoplatform for simultaneously deliver MRI contrast and therapeutic functions to inflammation-related diseases.

Graphical Abstract

Oxidative stress is characterized by increased level of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS). This may be caused by impaired antioxidant enzyme activity, low concentrations of antioxidant, and excessive ROS production. Under the normal physiological condition, ROS play essential roles in maintaining redox signaling balance and controlling cell survival. However, high levels of ROS may induce alteration or damage to DNA, proteins, and lipids, which has been implied in the pathogenesis of a number of diseases,^1–3 including cancer, diabetic mellitus, cardiovascular, neurodegeneration, skeletal muscle degeneration, osteoarthritis, and intervertebral disc degeneration. Antioxidative supplements, enzymes, nanoparticles, and inhibitors for ROS-generating nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) oxidases have become potential therapeutic interventions to combat oxidative stress via scavenging/neutralizing ROS directly and/or inhibiting downstream deleterious effects of ROS. Among them, nanoparticles with intrinsic ROS scavenging, and antioxidant properties exhibit superior pharmacological profiles to other antioxidants via enhanced absorption, bioavailability, and readily functionalization for targeted delivery. These nanoparticles include mesoporous silica, cerium oxide, and fullerene.

Fullerene is a molecule of carbon in the form of a hollow sphere with diversified applications, such as photovoltaic and electronic devices, surface coating material, and antioxidant.⁴ Shortly after the discovery of fullerenes in the 1990s, endohedral metallofullerenes (EMF) and trimetallic nitride endohedral fullerenes (TNT-EMF) were recognized for their multifunctional capabilities in biomedical applications. Functionalized gadolinium-loaded fullerenes, Gd@C₈₂ and Gd₃N@C₈₀, attracted much attention as a potential new nanoplatform for next-generation magnetic resonance imaging (MRI) contrast agents, given their inherent higher ¹H relaxivity than most commercial contrast agents.^5–13 The fullerene cage is an extraordinarily stable species which makes it extremely unlikely to break and release the toxic Gd metal ions into the bioenvironment. In addition, radiolabeled metals could be encapsulated in this robust carbon cage to deliver therapeutic irradiation.^14–16 In the past decade, we has endeavored to discover a series of functionalized Gd₃N@C₈₀ for MRI detection of various pathological conditions, such as chronic osteomyelitis and brain cancer.^13,17,18

Fullerene is a powerful antioxidant due to delocalization of the π-electrons over the carbon cage, which can readily react with free radicals and subsequently delivers a cascade of downstream possessions in numerous biomedical applications.⁴ Numerous studies have demonstrated the free radical scavenging capabilities, to such a degree that fullerenes have been described as “free radical sponges”.¹⁹ Functionalized C₆₀ and Gd@C₈₂ have been reported to quench radicals efficiently.^8,12,20 However, the therapeutic potential of functionalized Gd₃N@C₈₀ has not been investigated. If these TNT-EMF exhibit antioxidative and anti-inflammatory properties, similar as their nonmetal or monometal counterparts, they would hold great promise as a novel class of theranostic agent in combating oxidative stress and resolving inflammation, given their inherent MRI applications.

In the current study, we performed chemical and biological screening on three Gd₃N@C₈₀-based derivatives, including Gd₃N@C₈₀(OH)₃₀(CH₂CH₂COOH)₂₀ (carboxyl-Gd₃N@C₈₀)¹⁷ (Figure 1a) and hydrochalarone-1 (HyC-1-Gd₃N@C₈₀), hydrochalarone-3 (HyC-3-Gd₃N@C₈₀), (Gd₃N@C₈₀-Rx, where R = [N(OH) (CH₂CH₂O)_nCH₃]_x, n = 1, 3, 6 and x = 10–22)¹⁰ (Figure 1b) to assess their radical scavenging, antioxidative, and anti-inflammatory properties. The related characterization of these three compounds were reported in the earlier papers.^10,17 We demonstrated intriguing results on structure–activity relationship, and investigated molecular and cellular mechanisms regarding the biological/therapeutic performance of the most promising fullerene carboxyl-Gd₃N@C₈₀ using an in vitro macrophage model. First of all, we utilized electron paramagnetic resonance (EPR) techniques to evaluate the capability of carboxyl-Gd₃N@C₈₀ (Figure 1a), HyC-1-Gd₃N@C₈₀ and HyC-3-Gd₃N@C₈₀ (Figure 1b) to eliminate ROS in a cell free system, which provided direct evidence in their radical scavenging capabilities. As hydroxyl radical (•OH) and superoxide radical anion (O₂^•−), are the most common ROS in the body, they were chosen as model in this experiment. Due to the relatively low sensitivity of EPR detection and short-lived biological free radicals, exogenous spin traps were used. The EPR assay was based on the competition between the trapping agents, and functionalized fullerenes for radicals. Hydroxyl radicals were generated by the classical Fenton reaction, which involves the reaction of FeSO₄ and H₂O₂. The concentration of H₂O₂ was 200 µM, DEPMPO was 500 µM, which was 9 times higher than the metallofullerene derivatives (56 µM). The superoxide radical anion was generated by using the xanthine/xanthine oxidase system. The concentration of trapping agent, BMPO, was about 500 times concentrated than metallofullerenes. As shown in Figure 1c, d, a portion of radicals were quenched by the functionalized metallofullerenes, and the residues were captured by the trapping agents to yield adducts of DEPMPO–OH and BMPO-OOH, respectively. After treatment with Gd₃N@C₈₀-based nanoparticles, EPR profiles of both DEPMPO–OH and BMPO-OOH were significantly declined compared with the control. The inhibitory effects of three Gd₃N@C₈₀ derivatives were further summarized in Figure 1e. Although carboxyl-Gd₃N@C₈₀ exhibited the highest scavenging capability toward hydroxyl radicals among all three fullerenes, there was no difference between HyC-1-Gd₃N@C₈₀ and HyC-3-Gd₃N@C₈₀ in hydroxyl radical elimination. These three Gd₃N@C₈₀-based derivatives exhibited similar quenching ability for superoxide radical anions. Such desired but slightly varied radical scavenging properties of these functionalized Gd₃N@C₈₀ could be attributed to their different surface functionalization. After chemical modification, some of the π-system of fullerene framework is altered by replacing a cage carbon with another unit. The active hydroxyl radical could attack the electron-deficient areas on the carbon cage surface or be stabilized by forming hydrogen bonds with the proximate hydroxyl protons of functionalized Gd₃N@C₈₀. There were more hydroxyl groups on the cage of carboxyl-Gd₃N@C₈₀ as compared to HyC-1-Gd₃N@C₈₀ and HyC-3-Gd₃N@C₈₀ (30 vs. 10–22), making the carboxyl-Gd₃N@C₈₀ a more efficient nanoparticle on quenching hydroxyl radicals. Our observation is also consistent with previous reports that modification the fullerene cage with tris-malonic acid leads to electron deficient areas on the surface which facilitate binding of superoxide radical anions.²¹ In addition, binding of a second superoxide radical anions to an adjacent electron deficient area might result in the destruction of this type of radicals to produce H₂O₂, and regeneration of the functionalized metallofullerene in a reaction similar to that catalyzed by superoxide dismutase. The number of –C–O– groups on this three Gd₃N@C₈₀-based derivatives were similar (20 vs. 10–22), hence leading to a similar scavenging effect of superoxide radical anions for all three fullerenes.

Figure 1.

Radical scavenging properties of three trimetallic nitride endohedral fullerenes. Structures of (a) carboxyl-Gd₃N@C₈₀, (b) HyC-1-Gd₃N@C₈₀ and HyC-3-Gd₃N@C₈₀. (c) Mechanism of the hydroxyl radical production by Fenton reaction and capture by DEPMPO. EPR spectra of the hydroxyl radicals captured by DEPMPO with and without Gd₃N@C₈₀ derivatives. Ultrapure water was used as a control. (d) Mechanism of superoxide radical production by xanthine/xanthine oxidase (XOD) system and capture by BMPO. EPR spectra of superoxide radicals captured by BMPO with and without Gd₃N@C₈₀ derivatives. PBS was used as a control. (e) Table summary of scavenging capabilities of hydroxyl radical and superoxide radical anion by three Gd₃N@C₈₀ derivatives, carboxyl-Gd₃N@C₈₀, HyC-1-Gd₃N@C₈₀, and HyC-3-Gd₃N@C₈₀.

Although ROS signaling is a complicated, cell- and tissue-specific, physiological and pathophysiological context, restoring ROS hemostasis has been a well-respected therapeutic strategy to combat oxidative stress and a cascade of pro-inflammatory events.²² To confirm our cell-free EPR assays on radical scavenging, an in vitro macrophage cell model (Raw 264.7 cells with or without LPS stimulation) was adopted for biological characterization. First of all, we observed little cytotoxicity of all three nanoparticles at 0.1, 1, and 10 µM concentration for up to 24 h with or without LPS treatment (Figures S1 S2). Subsequently, a fluorescence assay was developed by incorporating an intracellular ROS sensitive dye 2′,7′ dichlorodihydrofluorescein diacetate (H₂DCFDA) in a 96-well plate. A well-known antioxidative enzyme heme oxygenase-1 (HO-1) inducer protoporphyrin IX cobalt chloride (CoPP) was used as a positive control. Briefly, cells were preincubated with various concentration of nanoparticles (0.1, 1, and 10 µM) for 20 h and treated with or without LPS (100 ng/mL) for additional 4 h. The green fluorescence emitted from intracellular space indicated the abundance of ROS. As shown in Figure 2, LPS dramatically stimulated intracellular ROS production by ~4-fold compared to non-LPS control (*p < 0.05 vs control). The positive control CoPP pretreated cells showed a significant inhibitory effect on ROS production in Raw 264.7 cells (Figure 2d) via its known effect in promoting antioxidative enzyme HO-1 activity.²³ Carboxyl-Gd₃N@C₈₀ demonstrated a similar inhibitory trend in alleviating LPS-induced ROS (Figure 2a) in a dose-dependent manner. In contrast, the ROS inhibitory effect of hydrochalarone functionalized HyC-1 (Figure 2b) and HyC-3 (Figure 2c) was not as efficient as carboxyl-Gd₃N@C₈₀ even at 10 µM. This data corroborated with EPR hydroxyl radical scavenging data, which suggested that carboxyl-Gd₃N@C₈₀ would be an optimal candidate warranting further investigation. Therefore, we chose carboxyl-Gd₃N@C₈₀ for further biological evaluation.

Figure 2.

Biological screening of trimetallic nitride endohedral fullerenes for quenching LPS-induced reactive oxygen species (ROS). Raw 264.7 cells were treated with nanoparticles at various concentration (0, 0.1, 1, and 10 µM) in serum-free medium for 20 h, stimulated with or without LPS (100 ng/mL) for another 4 h, followed by intracellular ROS staining with 5 µM 2′,7′ dichlorodihydrofluorescein diacetate (H₂DCFDA) and fluorescence plate reading at E_x/E_m of 495/525 nm. (a) Carboxyl-Gd₃N@C₈₀ exhibited dose-dependent efficacy in attenuating LPS-induced excessive ROS, in a similar fashion as (d) the positive control protoporphyrin IX cobalt chloride (CoPP), a well-known heme oxygenase-1 inducer. In contrast, (b) HyC-1 and (c) HyC-3 showed no or little reduction in LPS-elicited ROS generation. (e) Representative fluorescence images of intracellular ROS illustrated robust ROS-scavenging activity of carboxyl-Gd₃NC80. Image taken at ×200 magnification. Scale bar represented 100 µm. *p < 0.05 vs non-LPS control, ^#p < 0.05 vs LPS-treated groups.

Both inducible nitrite oxide synthase (iNOS) and tumor necrotic factor-alpha (TNF-α) are pro-inflammatory mediators, which are dramatically up-regulated during LPS-induced inflammatory responses in macrophages, therefore downregulation of iNOS and TNF-α have been considered as a therapeutic maker for anti-inflammatory agents. HO-1 is an endogenous antioxidative microsomal enzyme whose upregulation showed a protective role (such as immune-modulation) in combating oxidative stress and resolving inflammation.²⁴ Nrf2 is a basic leucine zipper (bZIP) protein that regulates the expression of antioxidative proteins, such as HO-1.²⁵ To examine the antioxidative and anti-inflammatory effect of carboxyl-Gd₃N@C₈₀, we pretreated Raw 264.7 cells with nanoparticle at 3.5 µM for 20 h and LPS for another 4 h. Total RNA were isolated, reverse-transcribed to cDNA, and analyzed with real-time PCR. As shown in Figure 3, LPS significantly promoted the mRNA expression of iNOS (**p < 0.01) and TNF-α compared to the control (***p < 0.001). Carboxyl-Gd₃N@C₈₀ treatment markedly suppressed the upregulated iNOS and TNF-α mRNA. In contrast to LPS that possesses no impact on the gene expression of HO-1 (p > 0.05. vs. control), carboxyl-Gd₃N@C₈₀ significantly elevated mRNA level of HO-1 (***p < 0.0001 vs control; ^###p < 0.0001 vs LPS). In addition, Griess assay to assess nitrite (stable end-product of released NO) in cell media at a later time point illustrated carboxyl-Gd₃N@C₈₀ inhibited LPS-elicited nitric oxide release significantly (Figure S3), consistent with iNOS mRNA expression and prior intracellular ROS screening (Figure 2a). Our immunofluorescence staining data of Nrf2 expression in various treatment groups suggested carboxyl-Gd₃N@C₈₀ effectively elevated the global Nrf2 expression in presence or absence of LPS (***p < 0.001 vs control, ^###p < 0.001 vs LPS) linking its protective function for antioxidative and anti-inflammatory uses (Figure 3d, e). Primer sequences are listed in Table S1.

Figure 3.

Carboxyl-Gd₃N@C₈₀ protected Raw 264.7 cells from LPS-induced oxidative stress and upregulated pro-inflammatory cytokine via elevating Nrf2 expression. Cells were pretreated with carboxyl-Gd₃N@C₈₀ (3.5 µM) for 20 h before treatment of LPS (100 ng/mL) for 4 h. (a) Real-time reverse transcription polymerase chain reaction (RT-PCR) suggested LPS stimulation significantly increased mRNA expression of inducible nitric oxide synthase (iNOS) (**p < 0.01 vs control), whereas carboxyl-Gd₃N@C80 dramatically reversed such induction (^###p < 0.001 vs LPS). (b) Similarly, the upregulation of pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-α) (***p < 0.001 vs control) upon LPS treatment was significantly alleviated by carboxyl-Gd₃N@C₈₀ (^##p < 0.01 vs LPS). (c) The antioxidative enzyme heme oxygenase-1 (HO-1) was significantly enhanced by carboxyl-Gd₃N@C₈₀ (***p < 0.001 vs control, ^###p < 0.001 vs LPS) but it was not altered by LPS stimulation. (d) Immunofluorescence staining of Nrf2, a key antioxidative enzyme regulator, was dramatically increased upon nanoparticle treatment in the present and absence of LPS, whereas no significant intensity difference was observed in LPS-treated cells. (e) Mean fluorescence intensity analysis of d suggested carboxyl-Gd₃N@C₈₀ effectively upregulated the protective Nrf2 protein expression (***p < 0.001 vs control, ^###p < 0.001 vs LPS). Experiments were repeated three times (n = 3). Scale bar in d represented 100 µm.

Mitogen-activated protein kinase MAPK (p38, ERK1/2, JNK) signaling play crucial roles in regulating pro-inflammatory cytokines and mediators such as TNF-α and iNOS during the course of inflammatory response. In particular, activation of ERK1/2 has been reported to stimulate TNF-α transcription and control the transport of TNF-α mRNA from the nucleus to the cytoplasm.²⁶ To further elucidate the molecular and cellular pathways regarding how carboxyl-Gd₃N@C₈₀ regulated oxidative stress and inflammation in macrophages, phosphorylation of ERK, and Akt proteins were studied using Western blotting. As shown in Figure 4, LPS significantly elevated the phosphorylation of both ERK (***p < 0.001) and Akt (*p < 0.05) compared to control groups, whereas carboxyl-Gd₃N@C₈₀ (3.5 µM) markedly reduced phospho-ERK (^###p < 0.001) with no significant effect on the change of phospho-Akt (p > 0.05) compared to LPS groups. These results suggested that carboxyl-Gd₃N@C₈₀ protected macrophages from oxidative stress and inflammatory response primarily via downregulating LPS-induced phosphorylation of ERK but not AKT. Since activation of ERK is an early event in ROS signaling of macrophages activation, predominant regulation of this pathway by carboxyl-Gd₃N@C₈₀ would benefit substantial downstream intracellular targets to lessen oxidative stress and resolve inflammation.

Figure 4.

Carboxyl-Gd₃N@C₈₀ rescued Raw 264.7 cells from LPS-induced oxidative stress via ERK and Akt pathways. (a) Representative immunoblotting analysis of phosphor-ERK, total ERK, phosphor-Akt, and total Akt of cell lysates subjected to different treatments. Cells were preincubated with carboxyl-Gd₃N@C₈₀ (3.5 µM) for 20 h and treated with or without LPS (100 ng/mL) for another 4 h. (b) Quantification of pERK/tERK (n = 4) intensity showed carboxyl-Gd₃N@C₈₀ significantly reversed LPS-induced ERK phosphorylation. (c) Quantitation analysis of pAkt/tAkt (n = 4) exhibited some increase in p-Akt upon LPS stimulation, which was not significantly reduced by carboxyl-Gd₃N@C₈₀ pretreatment. Note, *p < 0.05, ***p < 0.001 vs control; ^###p < 0.001 vs LPS group.

To the best of our knowledge, we report the first discovery here that functionalized Gd₃N@C₈₀ possessed robust radical scavenging properties in solution and suppressed LPS-induced ROS in macrophage cell model, which was predominantly correlated to varied surface functionalization of metallofullerene. Carboxyl-Gd₃N@C₈₀ significantly attenuated LPS-induced iNOS and TNF-α mRNA expression, and correspondingly increased expression antioxidative enzyme HO-1 mRNA and its regulatory protein Nrf2. Such antioxidative and anti-inflammatory effects might be regulated via ERK but not Akt signaling pathways. In summary, we prove the concept that trimetallic nitride endohedral fullerenes, especially carboxyl-Gd₃N@C₈₀ hold great promise in becoming a novel class of theranostic agent against the oxidative stress and inflammation, in combination with their inherited MRI applications.