Fe-Curcumin Nanozyme-Mediated Reactive Oxygen Species Scavenging and Anti-Inflammation for Acute Lung Injury.

Pneumonia, such as acute lung injury (ALI), has been a type of lethal disease that is generally caused by uncontrolled inflammatory response and excessive generation of reactive oxygen species (ROS). Herein, we report Fe-curcumin-based nanoparticles (Fe-Cur NPs) with nanozyme functionalities in guiding the intracellular ROS scavenging and meanwhile exhibiting anti-inflammation efficacy for curing ALI. The nanoparticles are noncytotoxic when directing these biological activities. Mechanism studies for the anti-inflammation aspects of Fe-Cur NPs were systematically carried out, in which the infected cells and tissues were alleviated through downregulating levels of several important inflammatory cytokines (such as TNF-α, IL-1β, and IL-6), decreasing the intracellular Ca2+ release, inhibiting NLRP3 inflammasomes, and suppressing NF-κB signaling pathways. In addition, we performed both the intratracheal and intravenous injection of Fe-Cur NPs in mice experiencing ALI and, importantly, found that the accumulation of such nanozymes was enhanced in lung tissue (better than free curcumin drugs), demonstrating its promising therapeutic efficiency in two different administration methods. We showed that the inflammation reduction of Fe-Cur NPs was effective in animal experiments and that ROS scavenging was also effectively achieved in lung tissue. Finally, we revealed that Fe-Cur NPs can decrease the level of macrophage cells (CD11bloF4/80hi) and CD3+CD45+ T cells in mice, which could help suppress the inflammation cytokine storm caused by ALI. Overall, this work has developed the strategy of using Fe-Cur NPs as nanozymes to scavenge intracellular ROS and as an anti-inflammation nanodrugs to synergistically cure ALI, which may serve as a promising therapeutic agent in the clinical treatment of this deadly disease. Fe-Cur NP nanozymes were designed to attenuate ALI by clearing intracellular ROS and alleviating inflammation synergistically. Relevant cytokines, inflammasomes, and signaling pathways were studied.

Introduction

The COVID-19 pandemic has resulted in more than five million deaths worldwide so far, causing lung complications such as pneumonia and, in more severe cases, leading to acute respiratory distress syndrome or acute lung injury (ALI).[1,2] Among them, ALI represents one of the most severe forms of the viral infection, bringing damage to alveolar epithelium and lung capillary endothelial cells, overwhelming pulmonary inflammation, and pulmonary edema as well as refractory hypoxemia, which may consequently develop into respiratory failure in critically ill patients.[3−6] Pathogenesis studies reveal that the progression of ALI is directly related to the excessive generation of reactive oxygen species (ROS) causing oxidative injury in the inflamed lungs even throughout the entire body.[4,7]

Current therapies of ALI mainly rely on inhaling vasodilator gases like nitric oxide (NO) for increasing blood flow in ventilated areas (to relieve hypoxemia)[8] or injecting pharmaceutical drugs including nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors and glucocorticoids to downregulate inflammatory cytokines.[9,10] Nevertheless, they are limited by their inefficiency to fundamentally neutralize the overproduced ROS in inflamed lung areas, thus resulting in low objective response in treatment, poor prognosis, and high treatment cost.[11,12] Therefore, developing a multifunctional nanomedicine with ROS scavenging ability and, at the same time, with anti-inflammation capacity would be a promising strategy to treat ALI.

Nanozymes are nanomaterial-based catalysts with enzyme mimicking properties. They are recognized as promising enzyme alternatives in biomedicine, mainly due to their high enzymatic activity, designability, physiological stability, and low production cost.[13−16] Among the diversified nanozymes, Fe-based biocatalysts have attracted specific attention in diagnosis and disease treatment benefiting from their biosafety and efficiency in clearing intracellular ROS.[17−19] In this work, we coordinated iron with an anti-inflammatory drug curcumin to prepare Fe-curcumin-based nanoparticles (Fe-Cur NPs) to achieve synergistic ROS scavenging and anti-inflammation for treating ALI for the first time. In particular, we systematically studied the mechanisms of our Fe-Cur NP treatment both in vitro and vivo. Results demonstrated that the secretion of several important inflammation cytokines like tumor necrosis factor-α (TNF-α) and interlukin-6 (IL-6) could be downregulated, inflammasomes such as a nucleotide-binding and oligomerization domain (NOD)-like receptor 3 (NLRP3) were suppressed, and ALI-related high intracellular Ca2+ levels were controlled. Further treating mice experiencing ALI, we found that the Fe-Cur NPs could be applied in two independent administration methods, through intratracheal injection (i.t.) or though intravenous injection (i.v.). The intracellular ROS scavenging as well as inflammation control were realized in vivo as well, and lung tissues were almost back to normal after such nanozyme treatment, demonstrating the effectiveness and promise of our Fe-Cur NPs for curing ALI and helping more COVID-19-infected patients to survive.

Results and Discussion

Preparation and Characterization of the Fe-Cur NPs

The Fe-Cur NPs were synthesized by adding curcumin to an FeCl3 and poly(vinylpyrrolidone) (PVP) mixture (dispersed in methanol) in a dropwise manner, in which the PVP was used to promote the formation of Fe-Cur NPs with better water dispersity (upper panel of Figure a). Upon reaction, the solution turned from yellow to deep-dark, indicating that the Fe3+ was coordinated with phenol groups of curcumin. The morphology and elemental mapping of Fe-Cur NPs were characterized by transmission electron microscope (TEM), showing the copresence of C, O, and Fe elements and their even distribution (Figure b, S1). X-ray diffraction (XRD) further indicated that the Fe-curcumin nanoconjugates were amorphous (Figure S2a). In addition, the particles also exhibited good stability within 1 week in water, phosphate buffered saline (PBS), and cell culture medium (Dulbecco’s modified eagle’s medium or DEME), with close hydrodynamic size at ∼12 nm (Figure c). The UV–vis adsorption wavelength of the particles was around 400 nm (Figures d and S3); and X-ray photoelectron spectroscopy (XPS) results demonstrated two strong binding energy peaks at 711 and 724 eV for Fe 2p3/2 and 2p1/2 respectively (Figure e),[20] as well as the C-related bonds in curcumin (Figure f).[21] All these data together confirmed that the Fe was associated with curcumin in our prepared nanoparticles.

Figure 1

Synthesis and characterization of Fe-Cur NPs. (a) Schematic illustration of Fe-Cur NP synthesis and Fe-Cur NP based treatment for ALI in mice. (b) TEM mapping of Fe-Cur NPs, showing the main elements in nanoparticles. (c) Hydrodynamic sizes of Fe-Cur NPs in different solutions (determined by DLS). (inset) Dispersity and stability of NPs in water, PBS, and DMEM. (d) UV spectra of Fe-Cur NPs. (e and f) XPS spectra of Fe and C in Fe-Cur NPs, respectively.

ROS Scavenging Ability of Fe-Cur NPs in Vitro

During the progression of ALI and its related inflammatory disorders, ROS are generated to regulate inflammation; however, excessive ROS are harmful and can lead to severe lung tissue injury.[9] Therefore, clearing excessive ROS is a strategy to attenuate ALI and the inflammation response in lungs. Since the Fe-based nanomaterials can be used as nanozymes for scavenging ROS,[13] and polyphenols such as curcumin are recognized as natural antioxidants to reduce inflammation,[22,23] we first studied the catalytic activities of our Fe-Cur NPs toward various substrates, ranging from 1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) to Methylene Blue (MB). Notably, nearly 90% DPPH and ABTS were oxidized by adding 25 μg/mL Fe-Cur NPs, indicating their quite effective ROS scavenging ability (Figure a,b). To understand the catalytic mechanisms, we then reacted the nanomaterials with MB, which can digest •OH generated during Fenton reactions.[24] As expected, more MB substrates were oxidized when gradually increasing the concentration of Fe-Cur NPs (Figure c), reaching a plateau of ∼80% (Figure S2b).

Figure 2

ROS scavenging ability of Fe-Cur NPs in vitro. (a) DPPH radical scavenging and (b) ABTS radical scavenging ratio of the Fe-Cur NPs. (c) Concentration-dependent catalysis of NPs toward MB based on its UV–vis absorption. (d) ROS fluorescence intensity in J774A.1 cells, determined by flow cytometry. (e) Statistical results of d. (f) Cell viability of J774A.1 cells under H2O2 with/without Fe-Cur NPs. (g) ROS level in cells (ROS stained by DCFH2-DA and analyzed by fluorescence microscopy). (h) ROS level in J774A.1 cells under H2O2 with/without Fe-Cur NPs, detected by flow cytometry. (i) Statistical results of h. **P < 0.01 and ***P < 0.001.

To further investigate the ROS clearance in cells, we stimulated J774A.1 cells by lipopolysaccharide (LPS)/ATP to induce intracellular ROS,[25,26] followed by Fe-Cur NPs treatment. The ROS level in cells was reflected by using a flow cytometry assay, in which cells with more ROS generation (screened by 2′-7′dichlorofluorescin diacetate, DCFH2-DA) would be more fluorescent. After LPS/ATP treatment, the cells emitted higher fluorescence (Figure d,e). Although the curcumin drug alone failed to scavenge the ROS yet even further provoked ROS enhancement, the Fe-Cur NPs were quite effective to help the J774A.1 cells get back to normal. Additionally, the ROS scavenging effect of the Fe-Cur NPs was further confirmed in H2O2-induced J774A.1 macrophage cells. As shown in Figure f, Fe-Cur NPs attenuated H2O2-induced cell death, in the presence of up to 80 μM H2O2. Similarly, staining the ROS by DCFH2-DA after H2O2 incubation, we also observed the significant ROS clearance ability of the Fe-Cur NPs according to the fluorescence microscopy and flow cytometry (Figures g–i). Collectively, these data indicated that Fe-Cur NPs have strong ROS scavenging ability in vitro.

Fe-Cur NPs Exhibited an Anti-Inflammation Effect in LPS-Stimulated Macrophage Cells

Since the biosafety of the nanozymes is critical for developing them to cure ALI, we next studied the cytotoxicity of the Fe-Cur NPs by 4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and, then, investigated relevant protein expressions by Western blotting assay and immunofluorescence methods (Figure a). As shown in Figure b, although the LPS/ATP stimulation deteriorated cell viability, after treating the weak cells by 0.1 μg/mL Fe-Cur NPs, the cell health was almost recovered, demonstrating the nontoxic nature of the Fe-Cur NPs.

Figure 3

Anti-inflammatory effects of Fe-Cur NPs on LPS/ATP-induced J774A.1 cells. (a) Schematic illustration of the experiment carried out here. (b) Cell viability of J774A.1 cells determined by MTT assays. (c and d) TNF-α and IL-6 levels were detected by ELISA. (e) Indicated protein expression detected by Western blotting assay. (f) IL-1β levels detected by ELISA kits. (g) Plasmids of OFPSpark-NEK7 and EGFP-NLRP3 activation determined by immunofluorescence. (h) Cleaved-caspase-1 activation determined by immunofluorescence. (i) Indicated protein expression detected by Western blotting assay. ##P < 0.01, ###P < 0.001 vs control group; *P < 0.05, **P < 0.01, and ***P < 0.001 vs LPS/ATP alone group.

The levels of several inflammatory cytokines involved in ALI progression, such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), are next determined due to their positive correlation with the disease development.[27−29] As indicated by Figure c,d, the LPS/ATP-stimulation increased TNF-α and IL-6 levels in J774A.1 cells, while Fe-Cur NP treatment effectively decreased their expression, being close to the TNF-α and IL-6 expression amount in cells without LPS/ATP incubation. Importantly, the curcumin drug itself was also useful in downregulating the related cytokines, yet it was not as good as the Fe-Cur NPs, indicating the possible synergistic effect of the conjugates in scavenging ROS and suppressing cytokines for relieving inflammation.

Moreover, in the deterioration of ALI, inflammasomes especially the nucleotide-binding and oligomerization domain (NOD) like receptor 3 (NLRP3) type are also involved in cytoplasm responding to pathogens infections.[30−32] Therefore, we next checked the NLRP3 inflammasome and its related cell components to better understand the anti-inflammatory mechanisms of the Fe-Cur NPs. First, after LPS/ATP stimulation, the NLRP3 inflammasome and the expression of cleaved caspase-1 as well as mature IL-1β were all overexpressed, leading to neutrophil recruitment (Figure e,f). At the same time, an important component of NLRP3, called NIMA-related kinase 7 (NEK7),[33,34] was stimulated; another important cell signaling pathway in inflammation known as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) was activated (Figure g).[35,36] Then, by having the Fe-Cur NPs or curcumin drug, these cell elements were all mediated back to normal, verifying the efficacy of this nanozyme in anti-inflammation.

To visualize the anti-inflammatory process in living cells, we also carried out immunofluorescence assays, by using NEK7 or NLRP3 plasmid transfected J774A.1 cells. The immunofluorescence results confirmed that the Fe-Cur NPs were useful to decrease the fluorescence intensity in the cytoplasm (Figure h,i). Together, these data demonstrated the ability of Fe-Cur NPs in eliminating inflammation, mainly through inhibiting the secretion of TNF-α and IL-6 and the suppression of NLRP3 inflammasome and NF-κB pathway.

Fe-Cur NPs Downregulated Intracellular Ca2+ Levels

Calcium ions (Ca2+) have a critical regulatory role in inflammatory response, and studies have shown that elevated intracellular Ca2+ concentrations could trigger mitochondrial destabilization, therefore leading to the production of ROS and activating the NLRP3 inflammasome to accelerate the inflammation.[37−39] Based on these findings, we are interested in whether the Fe-Cur NPs can also mediate abnormal Ca2+ levels for ALI. To this end, the same treatment was carried out on J774A.1 cells by LPS/ATP, and a Ca2+-related essential lipid phosphatidylinositol biphosphate (PIP2) was studied (Figure a). The PIP2 pathway on the membrane can control the Ca2+ release out of the endoplasmic reticulum.[40,41] As indicated by flow cytometry assays (Figure b,c), the level of Ca2+ significantly increased in LPS/ATP treatment cells, while Fe-Cur NPs suppressed the Ca2+ release, indicating their activity in downregulating the intracellular Ca2+ concentration. We further examined the level of calcium by using EGFP-Ca2+ plasmid transfected J774A.1 cells, and immunofluorescence showed a consistent trend (Figure d).

Figure 4

Fe-Cur NPs downregulated the intracellular Ca2+ level. (a) Schematic illustration of the experiment carried out here. (b) Fluorescence intensity of Ca2+ assessed by flow cytometry. (c) Relative intensity of b. (d) Plasmid of EGFP-Ca2+ fluorescence determined by immunofluorescence. (e) Expression of PIP2, PLCγ2, DAG Lipaseα, and IP3 Receptor1 detected by Western blotting assay. ###P < 0.001 vs control group; *P < 0.05, **P < 0.01, and ***P < 0.001 vs LPS/ATP alone group.

Next, the expression of calcium-related proteins was also determined by Western blotting. The results displayed that LPS/ATP upregulated the protein expression involved in the PIP2 pathways (such as the PIP2, and its hydrolyzed product diacylglycerol (DAG) and inositol triphosphate (IP3)[42]), but the Fe-Cur NPs or curcumin drug can again decrease those protein levels (Figure e). Therefore, the effectiveness of the Fe-Cur NPs in alleviating inflammation was also confirmed by studying the Ca2+-related cell components, which was very consistent with our aforementioned results.

Delivering Fe-Cur NPs for Curing Mice Experiencing ALI

Since ALI can be a deadly disease that deteriorates or even deprives lung function, and the excessive ROS level along with various inflammatory responses during its progression offer us an opportunity to restore cell health, scavenging ROS together with reducing inflammation could be a potential therapy for ALI.[9,43,44] After demonstrating its effectiveness in vitro, we then wanted to investigate in vivo performance in two independent administration methods, through intratracheal injection (i.t.) or intravenous injection (i.v.). We first studied the biodistribution of Fe-Cur NPs in different tissues of mice, by using these two different injection methods (Figure a). The biodistributions of Fe-Cur NPs were detected by UPLC-MS/MS.[45,46] In intratracheal injection, interestingly, the inhaled Fe-Cur NPs and curcumin drug were notably accumulated more in the lungs than in the other organs, and the nanoparticles were even more effective than the free drug molecule (Figure b), mainly due to the better penetrating ability of Fe-Cur NPs (Figure S4). Then, for the traditional intravenous injection, compared with free curcumin drug, Fe-Cur NPs also showed enhanced accumulation in the lung, consistent with the results of i.t. injection (Figure c). Probably due to prolonged blood circulation and the enhanced permeability and retention (EPR) effect, the nanomedicine selectively accumulated to the inflammatory site.[47]

Figure 5

Fe-Cur NP treatment for mice experiencing ALI. (a) Experiment schedule of ALI mice study. (b) Distribution rate of Fe-Cur NPs and curcumin in heart, liver, spleen, lung, and kidney at 4 h, through intratracheal injection (i.t.). (c) Distribution rate of Fe-Cur NPs and curcumin in heart, liver, spleen, lung, and kidney at 4 h, through intravenous injection (i.v.). (d) H&E staining of the lung tissues in i.t. and i.v. treated groups, respectively (H&E, original magnification, 200×). DEX (5 mg/kg, i.p.) was used as a positive control. (e) Blood concentration versus time profiles for Fe-Cur NPs following tail vein injection to rats. (f) Mouse lungs were weighed and used to calculate lung index. Data are means ± SEM (n = 3–5). *P < 0.05, **P < 0.01, and ***P < 0.001 vs LPS alone group.

Encouraged by these results, we further evaluated the therapeutic effect of Fe-Cur NPs after i.t. and i.v. administration respectively using LPS/ATP-induced ALI in mice as a model. After sacrificing the mice, their lung tissues were collected. First, hematoxylin and eosin (H&E) staining experiments were carried out to show that the LPS/ATP-induced ALI mice were indeed exhibited destructed alveoli structures, interstitial exudation of alveoli, and infiltration of inflammatory cells (Figure d). Based on that, free curcumin drug showed moderately relieved lesions in lung tissues, while Fe-Cur NPs exhibited obviously higher therapeutic efficiency in ALI mice, in which the alveoli structures in lung tissues were largely recovered. This result is also consistent with the positive control group treated by dexamethasone (DEX) (Figure d). Then the blood circulation half-life of Fe-Cur NPs after i.v. injection was evaluated, and he result showed that the fast half-life is 0.03260 h and the slow half-life is 13.44 h (Figure e). Furthermore, the lung index increased in LPS-induced ALI mice, Fe-Cur NPs have a better effect on decreasing the lung index compared to administration of free curcumin drug (Figure f).

To demonstrate the lung function of mice experiencing ALI after Fe-Cur NP treatment, the resistance of lung (RL), resistance of expiration (Re), and respiratory lung compliance (Cdyn) levels in mice were then detected using an AniRes2005 lung function test system. Results showed that the Fe-Cur NPs could obviously reverse the lung function index, while curcumin alone had little effect on lung function (Figures a–c). Considering that the Fe-Cur NPs have a ROS scavenging effect in vitro and that the production of ROS contributes to lung injury,[48] we also studied the effect of Fe-Cur NPs on scavenging ROS in vivo, by using a ROS kit and immunofluorescence of lung tissues. As vividly shown in Figure g (fluorescence intensities were quantified in Figure d), the Fe-Cur NPs demonstrated significant ROS scavenging ability both after i.t. and i.v. administration, and they were more effective than free curcumin drugs. In addition, the level of malondialdehyde[49] and myeloperoxidase,[12] which are important to defend the neutralization of ROS production, increased significantly in ALI mice; however, Fe-Cur NP treatment decreased their levels, validating the ability of such nanozymes in removing ROS in mice bodies (Figure e,f).

Figure 6

Fe-Cur NP treatment for the mice experiencing ALI. (a) Resistance of lung (RL), (b) resistance of expiration (Re), and (c) respiratory lung compliance (Cdyn) levels were detected using an AniRes2005 lung function test system. (d–f) Lung tissue of mice was collected to determine the level of ROS, MDA, and MPO with an ELISA kit. (g) Fluorescence images of ROS stained lung slices. ###P < 0.001 vs control group; *P < 0.05, **P < 0.01, and ***P < 0.001 vs LPS/ATP alone group.

Furthermore, we also determined whether the lung inflammatory cytokine storm could be attenuated by Fe-Cur NPs in ALI mice. The levels of TNF-α, IL-1β, and IL-6 expressions in lung tissues were determined by ELISA kit, and we found that the Fe-Cur NPs both decreased their levels in lung tissues after i.t. and i.v. administration, and this was also as effective as the model drug DEX (Figures a–c). The Fe-Cur NPs were effective in decreasing TNF-α, IL-1β, and IL-6 levels in serum and bronchoalveolar lavage fluid (BALF) after i.t. and i.v. administration (Figures S5a–f). Taken together, it is worth knowing that the downregulating inflammatory factors of Fe-Cur NPs are better than that of the curcumin drug in vivo (both in i.t. and in i.v.). Since the infiltration of immune cells facilitates the inflammation cytokine storm in pneumonia mainly through affecting the macrophages and T cells in the lung,[50] we detected the macrophage cells (CD11bloF4/80hi) in the lungs of ALI mice. The macrophage cells were activated and shift to M1 phenotype, while Fe-Cur NPs decreased the level of CD11bloF4/80hi cells compared with mice treated with Cur both after i.t. and i.v. administration (Figure d,e). We further detected the CD3+CD45+ T cells, which was significantly lower in Fe-Cur NPs treated mice than in those treated with Cur after i.v. administration (Figure f,g). Finally, the potential in vivo toxicity of Fe-Cur NPs was evaluated. The blood routine data for Fe-Cur NPs treated mice were all found to be normal (Figures a–i). And as observed in H&E staining imaging, the Fe-Cur NPs have almost no toxicity on the heart, liver, spleen, lung, and kidney (Figure j). These results together demonstrated that the Fe-Cur NPs are effective and safe in treating the ALI.

Figure 7

Fe-Cur NP treatment for the mice experiencing ALI. (a–c) Lung tissue of mice was collected to determine the inflammatory cytokines TNF-α, IL-1β, and IL-6 with an ELISA kit. (d) Representative plots of CD11bloF4/80hi cells as a percentage of the total CD45+ cell population (e) and corresponding quantification results in all cells (d) after various treatments. (f) Representative plots of CD3+ in CD45+ cells after various treatments. (g) Statistical significance of f. ###P < 0.001 vs control group; *P < 0.05, **P < 0.01, and ***P < 0.001 vs LPS/ATP alone group.

Figure 8

Toxicity of Fe-Cur NPs in vivo. (a–i) Blood panel data of normal mice (blank) and mice post Fe-Cur NP injection at different time points (1, 14, and 30 days). (j) H&E staining of heart, liver, spleen, lung, kidney after Fe-Cur NP treatment at different time points (1, 14, and 30 days).

Conclusion

In summary, we have developed a nanozyme Fe-Cur NPs, for the first time, to clear intracellular ROS and synergistically alleviate inflammation during the progression of ALI. To better understand the nanozyme-based disease treatment, we systematically studied important inflammatory cytokines (such as TNF-α and IL-6), NLRP3 inflammasome, intracellular Ca2+-related signaling pathways, and infiltration of immune cells. The Fe-Cur NPs both showed in vitro and in vivo that the ROS production and inflammation mediators could be effectively suppressed. The Ca2+ level indicating inflammation was also downregulated after treatment. Interestingly, we found that both the intratracheal and intravenous injection of Fe-Cur NPs in mice experiencing ALI were effective, therefore providing two independent treatment strategies in relieving this deadly disease. This work may also help to decrease the number of deaths during the COVID-19 pandemic.