2.1. Synthesis and Characterization of PLNPCu
The LNPCu can quickly assemble by mixing polyethyleneimine (PEI), copper ions (Cu2+) and lactate oxidase (LOX) under gentle stirring at room temperature (see SI for the detailed description of the preparation). The driving force for the assembly was mainly electrostatic interaction and coordination among Cu2+, PEI and LOX. The obtained LNPCu have the hydrodynamic size of 115 ± 5.0 nm with a narrow polydispersity index (PDI) of 0.14 by dynamic light scattering (DLS) (Fig. S1d).
To improve the circulation stability and biocompatibility, the surface of the LNPCu was shielded with polyethylene glycol derivates with the double end group of aldehyde (CHO-PEG-CHO) through pH-responsive Schiff base (named as PLNPCu). As shown in the 1H NMR spectra, the disappearance (at pH 7.4) and reappearance (at pH 6.5) of the hydrogen peak for aldehyde groups in the dotted area suggested that the acid-responsive labiality of Schiff bases formed by PEI with aldehyde groups of OHC-PEG-CHO (Fig. S4). PLNPCu showed well-dispersed uniform spherical structures (Fig. 1a) with a visible PEG shell at pH 7.4 by transmission electron microscopy (TEM) (Fig. 1b). The hydrodynamic size of PLNPCu was 218 ± 3.0 nm (PDI 0.06), which was consistent well with the TEM results (Fig. 1d). Notably, PLNPCu exhibited the compression of zeta potential comparing with LNPCu (27.07 mV vs. 18.07 mV) (Fig. 1e). However, the rebound of zeta potential from 18.07 mV to 27.50 mV (Fig. 1e) occurred at the weakly acidic condition (at pH 6.5), indicating PEG shell could be shed from the surface of PLNPCu to exposure of the positive charged amino. More intuitively, the shellless PLNPCu upon incubated in H2O at pH 6.5 could be viewed in TEM images, as was shown in Fig. 1C and Fig. S1a.
Further, the size of PLNPCu was stable in neutral PBS solution within 48-hour monitoring (Fig. 1f) accompanied by a low release of LOX (26.25% ± 2.676), the content of LOX was quantified with the Bradford Protein Assay Kit according to the standard curve (Fig. S3), while the moderate LOX release behavior was found at pH 6.5 (44.74% ± 4.456) (Fig. 1h). The morphology of the PLNPCu at pH 5.5 was also explored (Fig. S1b), which suggested the potential for effective function exertion in the intracellular milieu. Apparently, the introduction of copper ion (with entrapment efficiency of ∼81.58% determined by inductively coupled plasma mass spectrometry (ICP-MS)) may facilitate LOX loading in LNPCu compared with the LNP with a smaller size (Fig. S1c) which could be attributed to the coordination of metal ions (LOX%: 226.6 ± 0.518 vs. 219.7 ± 1.237 µg·mg−1) (Fig. 1g).
2.2. Evaluation of PLNPCu as lactate treatment plant in vitro
PLNPCu, referred to as extra/intracellular lactate treatment plant, for lactate consumption and tumor cell inhibition were investigated. As shown in Scheme 2, the lactate treatment plant elicited a chain of the response of processing lactate to antipersonnel reactive oxygen species (ROS) in the tumor cell physiology and tumor microenvironments, such as acidic microenvironment and the relatively high GSH/GSSG ratios.[18, 19] Briefly, the entire process can be categorized into three stages. First, the local high lactate concentration is formed by PEI trapping of lactate; second, lactate reacts with oxygen to produce hydrogen peroxide (H2O2) under the catalysis of LOX; finally, the Fenton-like reaction between copper ions and enzymatic product (H2O2) from the second step produced a large number of hydroxyl radicals (·OH). The verification for the above three stages was investigated as follows.
Inspired by the cationic polymer of PEI with a large number of primary amino groups, we speculated that these positive charged groups could electrostatically interact with lactate and adsorb it to the PLNPCu nanosystem. We thus carefully studied the adsorption behavior of PLNPCu. Considering the LOX may influence the determination of lactate absorption of PLNPCu, bovine serum albumin (BSA) was used to replace LOX, regarding their high similarity in structure and properties (isoelectric point, pI 4.7 vs. 4.6, molecular weight≈66 kDa vs. 80 kDa) except for catalysis for lactate, and was fabricated according to the synthetic procedure of LNPCu and PLNPCu (named as BNPCu and PBNPCu, respectively). Both zeta potential and hydrodynamic size of BNPCu and PBNPCu were consistent with PLNPCu (Fig. S2b). Subsequently, the PEI, BNPCu, and PBNPCu at different pH with equimolar amount of PEI were mixed with NaL respectively and shaken overnight at 37 ℃. Followed by fully dialyzed for 4 h and the dialysate of different groups was collected for lactate analysis. The high lactate adsorption ratio of BNPCu (19.54% ± 0.559) and PBNPCu (pH=6.5) (13.56% ± 0.0284) were consistent with that of PEI solution (17.24% ± 1.07), while the PBNPCu (4.369% ± 0.0583) and control groups (5.804% ± 0.429) at pH 7.4 showed significantly lower levels of lactate adsorption compared to above three groups (Fig. 2a). The above results verified that the exposure of PEI on the PLNPCu nanosystem played an important role in lactate recruitment, and the detachable PEG shell contributed to the shielding effect in the neutral milieu.
To evaluate the lactate consumption-ability of the nanoparticles, LNPCu and BNPCu were added into NaL solution respectively. The concentration of lactate in the mixed solution containing LNPCu decreased with time compared to BNPCu (Fig. 2b), which demonstrated the maintenance of the catalytic activity of LOX. We further studied the degradation of lactate over a short time frame (10 or 20 min) at different pH. Surprisingly, after being incubated for 20 min, the lactate degradation rate of LNPCu was almost four-fold higher than the free LOX group at pH 7.4 (26.16% ± 0.102 vs. 6.727% ± 0.0298), even reached the ~43 times level in 10 min of reaction (18.71% vs. 0.429%). And LNPCu exhibited an excellent catalytic activity even at pH 5.5 (7.977% ± 0.122) (Fig. 2c). Immediately after, the degradation ratio of lactate following 2 h exposure to LNPCu was studied. The result showed that the catalytic activity of LNPCu at different pH values (pH 7.4, 6.5, and 5.5) was significantly higher than the respective values of free LOX groups (Fig. 2d). It was speculated that the discrepancy was likely because of the adsorption effect of PEI in favor of shortening the “lactate-hunting” for LOX, and local high concentration of lactate was more favorable for the performance of enzymatic hydrolysis reaction. The above results indicated that the lactate adsorption effect of PEI contributed to the enhance of lactate degradation at different pH.
Based on the above exciting results, the third process in the lactate treatment plant that Fenton-like reaction to generate hydroxyl radical by reacting H2O2 with Cu+ was explored. The concentration of H2O2 was quantified via the Hydrogen Peroxide Assay Kit according to the standard curve (Fig. S5). The LOX solution was mixed with NaL solution and thoroughly reacted for 1 h at 37 ℃ and then the H2O2 concentration of the supernatant was detected. As shown in Fig. S6, H2O2 was barely detected without NaL, which has excluded the effect of residual H2O2 background. And the generation of H2O2 increased along with increasing NaL concentration. This directly demonstrated the LOX catalyzed reaction of Lactate + O2 — pyruvate + H2O2. After that, the generation of H2O2 produced by LNPCu at different pH conditions (pH 7.4, 6.5 and 5.5) compared with free LOX solution was recorded. The H2O2 concentration was comparable in both groups which demonstrated carrier and pH change did not show interference in this process (Fig. 2e and Fig. S7). The methylene blue (MB) degradation was used to validate the Fenton-like reaction occurrence. We simulated the process of intracellular high levels of GSH (5-10 mM) induced the conversion of cupric ions (Cu2+) to cuprous form (Cu+),[18] briefly, GSH was added into the supernatant of BNPCu and LNPCu solution respectively for Cu+ generation, then the H2O2 and NaL solution were added in turn. The appropriate feeding amount of H2O2, GSH and NaL was identified from references.[20, 21] The reduction in absorbance of MB showed that additional lactate evoked a huge elevation of toxic ·OH in the orange group (with lactate addition) compared to light green groups (without lactate addition) (Fig. 2f). It was proven that the Cu+ resulted from GSH reduction can react with H2O2 to produce ·OH since the ·OH-induced MB indicator degradation caused the change in absorbance.[20] All the results supported a well-functioning lactate treatment plant in vitro.
2.3. Intracellular behaviors of PLNPCu
As breast tumors exhibit immunosuppression and Warburg phenotype (with strong aerobic glycolysis induced lactate buildup), 4T1 cell line was chosen for further study.[22] We first examined the cytotoxicity of PLNPCu on 4T1 cells. As shown in Fig. 3a, PLNPCu at both low and high concentrations induced high cytotoxicity compared with the free LOX group. The higher cell viability upon treatment with PLNPCu (pH=7.4) suggested the protective role of the PEG shell at low concentrations. Once the acid-sensitive reaction was triggered, LNPCu and PLNPCu (pH=6.5) showed similar high efficacy for killing 4T1 cells (Fig. S9). Besides, PBNPCu exhibited relatively low cytotoxicity even at high concentrations (2.5 µg·mL−1) (Fig. S8), which demonstrated that the establishment of an intracellular lactate treatment plan plays a critical role in the efficient killing of tumor cells. Then, the cellular uptake of LNPCu was examined by flow cytometry. 4 h of incubation with LNPCu@FITC appeared to reach the saturation of cellular uptake for comparable fluorescence intensity as that of 3 h, indicating that the optimal incubation time (Fig. S10). Particularly, both LNPCu and PLNPCu (pH=6.5) had similar high cellular uptakes in 4T1 cells compared to free LOX and PEG-shielded nanoparticles (PLNPCu pH=7.4) (Fig. 3b). Taken together, the superior cellular uptake of detached-shell PLNPCu and establishment of the intracellular lactate treatment plant account for the toxicity of PLNPCu to 4T1 cells.
Subsequently, the lactate consumption effect of PLNPCu was evaluated at the cellular level. Notably, the PLNPCu (pH=6.5) group showed the lowest levels of lactate among the experimental groups (Fig. 3c), suggesting that the lactate adsorption caused by PEI exposure on the surface of PLNPCu promoted the lactate consumption process, which was consistent with the functions verification results in vitro. It also showed the potent inhibition of intra/extracellular lactate. The ·OH generation induced by PLNPCu was investigated using 2′,7′-dichlorofluorescein diacetate (DCFH-DA) as a ROS indicator. From the fluorescent microscopy images, 4T1 cells treated with free LOX and PBS showed negligible fluorescence. And the faint fluorescence in PLNPCu (pH=7.4) group might be induced by the endocytosis of a small number of PLNPCu (Fig. 3d). In contrast, stronger green fluorescence was observed in PLNPCu (pH=6.5) treated cancer cells (Fig. 3d), indicating the relatively high ·OH levels rather than H2O2. Since the mean fluorescence intensity (MFI) in PLNP (pH=6.5) (the detailed information of PLNP were shown in Fig. S2a) treated 4T1 cells was insufficient as well (Fig. S11). Consistent with fluorescent microscopy images, flow cytometry results also demonstrated the critical role in triggering ·OH formation of copper ions in PLNPCu (Fig. 3e, f). Overall, PLNPCu showed the specific function to convert the lactate into the toxic ·OH product through the intracellular Fenton-like reaction, which indicated the successful construction of the intracellular lactate treatment plant.
2.4. Immunomodulatory effects on macrophages and ICD induction effects of PLNPCu in vitro
·OH, a highly toxic type of ROS causes oxidative damages to lipids, proteins, and DNA which in turn result in the apoptosis of tumor cells.[23] The apoptosis and necrosis induced by PLNPCu of 4T1 cells were evaluated by annexin V-FITC and propidium iodide (PI) staining assay. The flow cytometry results showed that the percentage of late apoptosis and necrosis in the PLNPCu (pH=6.5) group (56.15%), which was nearly 2-fold than other groups (Control ~35.32%, LOX ~34.63%, PLNPCu pH=7.4 ~36.16%), demonstrating the build of the lactate treatment plant in tumor cells and the excellent chemodynamic therapy (CDT) performance (Fig. 4a, b).
We further investigated the immunogenic cell death (ICD) level of 4T1 cells induced by the lactate treatment plant. As three representative markers of ICD, the expression of CRT, the efflux of HMGB1 from the tumor cell nucleus, and the secretion of ATP were evaluated, were critical for activating the adaptive immune response and anti-tumor T cell immunity.[24] As the results were shown in Fig. 4c and d, the CRT expression on tumor surface after being treated with PLNPCu pH=6.5 increased to 68.57%, which was considerably 1.29-fold higher than that in the control group. However, the positive rate of CRT in the LOX group (~55.40%) was insignificantly increased compared with controls, indicating that intracellular H2O2 was insufficient to trigger ICD. A similar result was reconfirmed in the immunofluorescence (Fig. 4e). Additionally, HMGB1 are normally localized in the cellular nucleus, while the weak green fluorescence at cell nuclei in PLNPCu pH=6.5, demonstrating the large quantities of HMGB1 extracellular released compared with the PBS treatment cells (Fig. 4f). For ATP secretion, PLNPCu treatment resulted in a higher ATP in the cellular supernatant (Fig. 4g). Notably, it was Fenton-like chemistry driven by cuprous ions that were the main reason for ICD, based on the results of the lower ATP release in PLNP pH=6.5 group compared with PLNPCu pH=6.5 (Fig. S12). The evidence above suggested that the intracellular lactate treatment plant activated by PLNPCu converted to lactate to anti-tumor ·OH, thereby producing considerable effects on ICD induction in tumor cells, paving the way for anti-tumor immune responses.
As one of the components of innate immunity in the tumor microenvironment (TME), macrophages were the major immune component of leukocyte infiltration in the tumor.[25] Unfortunately, lactate efflux associated with lactate accumulation and acid microenvironment suppressed proliferation and inhibited macrophage M1 polarization.[3, 26] The markers (CD80, CD206) of RAW264.7 macrophages were analyzed by flow cytometric analysis to determine the polarization after stimulating by the PLNPCu. 1 µg·mL−1 of LOX was chosen as the safe dose concentration according to the results of the cytotoxicity experiments (Fig. S13). The results showed that expression of CD80+ (M1 macrophages marker) on the cell surface significantly upregulated with the time increased of PLNPCu stimulation. Especially, it was up to 9.22% after 24 h of stimulation, which was 3-fold higher than the control group (Fig. 5a, c). However, the RAW264.7 macrophages displayed an insignificant change in M2-like polarization via analyzing the expression of intracellular CD206 (Fig. 5b, d). Again, immunofluorescence images revealed that the PLNPCu induced significant upregulation of the M1 biomarker CD80. (Green fluorescence signal of FITC labeled CD80, Fig. 5e). Collectively, these results demonstrated that PLNPCu induced macrophage M1 polarization in vitro through the consumption of extracellular lactate, which potentiated the antitumoral immune response.
2.5. Distribution and antitumor effects of PLNPCu in vivo
PLNPCu@Cy7 was prepared to investigate the distribution of the PLNPCu in 4T1-bearing mice. The major organs (i.e., heart, liver, spleen, lung, and kidney) and tumors were collected within 1 h after tail vein injection of agents (Cy7-labeled LOX, PLNPCu@Cy7 respectively) for ex vivo fluorescence imaging analysis. As shown in Fig. S14, free LOX indicated a non-specific distribution across the normal organs other than the tumor. In contrast, PLNPCu@Cy7 were significantly enriched in tumor tissues, which might be attributed to the enhanced permeability and retention (EPR) effect. And we observed the enrichment of PLNPCu@Cy7 in the liver was significantly lower than the free LOX, which decreased the immune-related adverse effects due to a nonspecific enrichment of LOX in the liver. In conclusion, these findings suggested that PLNPCu improved tumor targeting efficiency and accumulation, thereby decreasing the non-specific tissue distribution.
The antitumor effect and induction of systemic antitumor immune responses of PLNPCu were investigated in the 4T1 tumor-bearing mouse model (a weakly immunogenic tumor) (Fig. 6a). After 18 d of administration (five i.v. injections totally), the PLNPCu group exhibited the most effective tumor inhibition, while tumors showed similarly rapid growth in PBS and LOX groups (Fig. 6b, c). Specifically, PLNPCu demonstrating 88% tumor inhibition, the LOX only achieved 17%, and the moderate inhibitory of PLNP (37%) (Fig. S15). The tumor volume indicated that PLNPCu effectively inhibited tumor growth in vivo. The same tendencies of body weight changes (Fig. 6d) and the no obvious tissue damage in H&E staining supported the safety of the PLNPCu treatment in vivo (Fig. S16).
The effects of the lactate treatment plant PLNPCu established in the tumor treatment were further discussed in more detail. Tumor tissues were collected and the intratumoral lactate content was detected after five treatments with PBS, LOX, PLNP, and PLNPCu. The PLNPCu showed the lowest relative level of lactate (the fold of PBS) in the tumor, at only 0.48, which even reduced to half of the amount for the PBS or LOX group. The result of PLNP (0.79) indicated that thoroughly consumption and transition of intracellular lactate could counteract the active lactate metabolism in tumors, reflecting in the down-regulation of intra/extracellular lactate levels (Fig. 6e). In general, PLNPCu showed more effective lactate exhaustion than LOX or PLNP attributed to the effective tumor accumulation of nanoparticles and the well-functioning lactate treatment plant.
Although these experiments yielded good results, lactate consumption was only the first step of the lactate treatment plant. The generation of ·OH was quantified to validate the plant working properly. The generation of intracellular ROS in the tumor was detected by flow cytometry with the DCFH-DA probe. The MFI of the intracellular ROS in the PLNPCu group was 3 times higher than that of the PBS. There was no significant difference between the LOX and PLNP groups and, markedly, both were lower than the PLNPCu group (Fig. 7c, d). The above results confirmed that the PLNPCu triggered the massive generation of intracellular ROS in 4T1 tumor cells. We estimated that the ROS was therefore mainly toxic ·OH that has a high affinity for DCFH-DA binding rather than H2O2.[27] This conclusion was based on the minor amounts of ROS generation in PLNP group, specifically, the third step of the lactate treatment plant which is the Fenton-like reaction was disrupted due to the absence of copper ion. Incidentally, the limited cell uptake and the insufficient toxic ROS generation of free LOX also demonstrated the positive role of the continuous-operated lactate treatment plant in tumor inhibition. The abundant bright red fluorescence was observed in CLSM images of frozen tumor tissue sections, consistent with the flow cytometry results respectively (Fig. 7e). Together, the lactate treatment plant establishment triggered by PLNPCu exhibited the most effective lactate exhaustion and tumor inhibition.
2.6. Anti-tumor immune activation in vivo
Encouraged by the well-functioning lactate treatment plant in tumor, we subsequently monitored its anti-tumor immune responses. For this purpose, the typical cell types of cellular immunity (T cells) and innate immunity (macrophages) were detected. The percentage of cytotoxic T lymphocytes (CTL, CD3+CD8+T cells) infiltration in tumors after PLNPCu treatment was significantly increased compared with the PBS (4.06-fold higher) and PLNP (2.09-fold higher). There was a non-significant CTL proportion difference between LOX and PLNP groups. Although the slight upregulation of CTL was found in LOX (36.2%), PLNPCu showed the highest CTL proportion (1.30 times of LOX group) (Fig. 7b). The same results were observed in CLSM images, in which the copious green fluorescent signal at 488 nm confirmed higher CD8+T cell infiltration in PLNPCu group (Fig. 7a). Furthermore, a typical cytokine (IFN-γ) of immune activation in tumor tissues was detected. As expected, tumor tissues of PLNPCu group presented the highest level of IFN-γ after treatment (Fig. S17). As it is well known, lactate production by cancer cells affects M2-like TAM polarization, which was associated with tumor growth, metastasis, and immunosuppression.[3, 28] According to tumor infiltration macrophages analysis, M2 macrophages showed no obvious changes in number among all groups, while there was a significant increase in M1-like macrophages in PLNP treatment (Fig. S18). All the above results suggested that immune response activation at the tumor site was triggered through the establishment of the “lactate treatment plant” by PLNPCu.
The systemic anti-tumor immune response induced by PLNPCu was also explored. The spleen, containing abundant immune cells, is the largest immune organ and the immune center of the body in supporting anti-tumor immune response activation.[29] We analyzed the phenotype and quantity of spleen T lymphocytes. As shown in the flow cytometry results, CD8+T cells significantly increased in PLNPCu treatment group, the proportion of CD4+ T cells showed to be downregulated (Fig. 8a). Remarkably, PLNPCu increased the ratio of CD8+/CD4+T cells from 0.47 of PBS to 0.69 (Fig. 8d). Dendritic cells (DCs) present the tumor antigen to T cells, playing critical roles in pathogen sensing and initiation of anti-tumor immune responses. The DC maturation in draining lymph nodes (DLNs) including tumor DLNs was analyzed via flow cytometry. By qualitative and quantitative assays, the expressions of the DC maturation marker MHC-II, CD86, and CD80 were up-regulated, confirming PLNPCu promotes DC maturation in DLNs (Fig. 8b, c, e and f). Together, all the immune analysis results suggesting that PLNPCu activated the immune system for efficient inhibition of tumor cell growth.
