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IL-36α and Lipopolysaccharide Cooperatively Induce Autophagy by Triggering Pro-Autophagic Biased Signaling

Autophagy is an intracellular catabolic process that controls infections both directly and indirectly via its multifaceted effects on the innate and adaptive immune responses. It has been reported that LPS stimulates this cellular process, whereas the effect of IL-36α on autophagy remains largely unknown. We therefore investigated how IL-36α modulates the endogenous and LPS-induced autophagy in THP-1 cells. The levels of LC3B-II and autophagic flux were determined by Western blotting. The intracellular localization of LC3B was measured by immunofluorescence assay. The activation levels of signaling pathways implicated in autophagy regulation were evaluated by using a phosphokinase array. Our results showed that combined IL-36α and LPS treatment cooperatively increased the levels of LC3B-II and Beclin-1, stimulated the autophagic flux, facilitated intracellular redistribution of LC3B, and increased the average number of autophagosomes per cell. The IL36α/LPS combined treatment increased phosphorylation of STAT5a/b, had minimal effect on the Akt/PRAS40/mTOR pathway, and reduced the levels of phospho-Yes, phospho-FAK, and phospho-WNK1. Thus, this cytokine/PAMP combination triggers pro-autophagic biased signaling by several mechanisms and thus cooperatively stimulates the autophagic cascade. An increased autophagic activity of innate immune cells simultaneously exposed to IL-36α and LPS may play an important role in the pathogenesis of Gram-negative bacterial infections.

Lipopolysaccharide (LPS) is a powerful immunomodulatory molecule that contributes to the pathogenesis and clinical symptoms of infections caused by Gram-negative bacteria. LPS is the major structural component of the outer bacterial membrane and is composed of O antigen, poly/oligosaccharide core, and lipid A, termed endotoxin [20,21]. LPS is a pathogen-associated molecular pattern (PAMP) detected by sensor molecules located in the cytoplasmic and endosomal membranes as well as in the cytoplasm of cells [22][23][24]. Extracellular LPS is sensed and extracted from the bacterial outer membrane by the LPSbinding protein (LBP) and CD14 [22][23][24]. CD14 transfers monomeric LPS molecules to the Toll-like receptor 4 (TLR4) leading to the assembly of a supramolecular complex termed the myddosome. The LPS-TLR4 complexes can also be internalized into endosomes resulting in the formation of another complex called the triffosome. These platforms trigger a signaling cascade involving MAPKs and the activation of several transcription factors ( Figure 1B) [20,[22][23][24][25]. The activated transcription factors turn on the expression of various cellular genes encoding inflammatory mediators including cytokines [20,22,23]. In addition to TLR4, another group of cytoplasmic membrane receptors-the transient receptor potential (TRP) cationic channels-can bind LPS [20,26]. TRP cation channel subfamily V members 2 and 4 (TRPV2 and TRPV4) and TRPM7 also contribute to the LPSmediated activation of innate immune cells by triggering intracellular Ca2+ mobilization and secretion of nitric oxide [27,28]. Additionally, intracellular LPS within the cytoplasm of cells directly binds to the caspase activation and recruitment domains of caspase-4/5 in humans and caspase-11 in mice, which in turn leads to activation of inflammatory caspases, secretion of IL-1β and IL-18, as well as induction of pyroptosis [29]. In localized infections caused by Gram-negative bacteria, LPS-mediated activation of the immune response is protective by restricting bacterial invasion whereas the exaggerated inflammation seen in systemic infections is of pivotal pathogenetic and prognostic importance.
Autophagy is an intracellular metabolic process in which cytoplasmic target molecules are transferred to the lysosome for degradation and recycling [30][31][32]. Depending on the type of cargo delivery, three different forms of autophagy can be distinguished: (1) macroautophagy, (2) microautophagy, and (3) chaperon-mediated autophagy [32]. The difference among these types is how the constituents to be degraded are transported to the lysosome. Macroautophagy (hereafter referred to as autophagy) occurs under basal conditions and can be stimulated by environmental cues, nutrient starvation, growth factor depletion, various pathological conditions, infections, hypoxia, or pharmacological treatment [33]. The major element in the regulation of autophagy is the mechanistic target of rapamycin complex 1 (mTORC1) [34,35]. Autophagy inducers inhibit mTORC1, and thereby activate the autophagic cascade. Inhibition of mTORC1 results in the activation of the class III phosphatidylinositol 3-kinase (PI3KC3) complex. The PI3KC3 complex contains Beclin-1 protein [36,37]. The activated PI3KC3 induces local production of phosphatidylinositol-3-phosphate (PI3P) at the endoplasmic reticulum membrane and thereby promotes the generation of an omegasome from which the isolation membrane is generated [37][38][39]. PI3P recruits two ubiquitin-like conjugation systems to the omegasome, which form a covalent bond between the membrane-resident phosphatidylethanolamine (PE) and the microtubule-associated protein 1 light chain 3B-I (LC3B-I) protein [38,39]. The lipidated LC3B, termed LC3B-II, is essential for the elongation and closure of the phagophore membrane, trafficking of the autophagosomes, and their fusion with lysosomes [40][41][42]. The level of LC3B-II reflects the cellular autophagic activity and it is used as a marker to measure autophagy. Upon autophagy induction, LC3B translocates from the cytoplasm to autophagic membranes, and can be detected as fluorescent puncta ( Figure 1C) [41].
The autophagosomes eventually fuse with lysosomes, and the content of the autophagic cargo is thus degraded and made available for reuse [43,44]. Autophagy is essential for the maintenance of cellular homeostasis and plays a substantial role in pathological processes including bacterial infections [33,45]. Autophagic capture and delivery of bacteria to lysosomes functions as a protective cellular antimicrobial defense mechanism known as xenophagy [46]. Autophagy also controls bacterial infections indirectly via its multifaceted effects on the innate and adaptive immune responses [47,48]. mTORC1 signaling affects the maturation, metabolic activity, activation, and differentiation of innate immune cells [49,50]. The mTORC1 pathway can also stimulate the production of type I interferons (IFN), IL-10, and transforming growth factor-β, and downregulates the expression of IL-6, IL-12, IL-23, and TNF-α in monocytes, macrophages, and dendritic cells [51]. The autophagic machinery promotes the secretion of IL-1β, IL-18, and high-mobility group protein B1 and also facilitates antigen presentation to CD4 + and CD8 + T-cells. Conversely, immune processes may profoundly alter the cellular autophagic activity [47]. Ample evidence indicates that LPS induces autophagy via some mTOR-independent mechanisms. In response to binding of LPS to TLR4, the myddosomes and triffosomes recruit, whereas TRAF6 ubiquitinates Beclin-1, leading to the more efficient assembly of PI3KC3 with subsequent activation of the autophagic cascade [52,53].
Besides the signaling events elicited by pathogen-recognition receptors, cytokines may also modulate autophagy. IL-1α, IL-1β, IL-2, IL-6, IL-17, TNF-α, TWEAK (tumor necrosis factor-like weak inducer of apoptosis), and IFN-γ act as inducers whereas IL-4, IL-10, and IL-13 function as inhibitors of autophagy [54,55]. Recent data demonstrate the pro-autophagic effect of IL-36β and IL-36γ [56,57]. IL-36β enhanced the expression of key autophagy markers, LC3-II, Beclin-1, and p62, as well as elevated autophagic flux in CD4 + CD25 + Treg cells [57]. This IL-36 subtype played protective role in sepsis by diminishing the immunosuppressive activity of CD4 + CD25 + Tregs [57]. The signal transduction mechanisms involved in IL-36β-mediated induction of autophagy, however, have not been identified. IL-36γ triggered autophagy via WNT5A-induced noncanonical WNT signaling in macrophages infected with Mycobacterium tuberculosis [56]. Moreover, IL-36γ-induced autophagy promoted the killing of intracellular bacteria [56]. In contrast, the effect of IL-36α on the autophagic process has not yet been elucidated. Infections involve a variety of pathogen-related molecular pattern (PAMP)-cytokine recognition events that can profoundly affect the antimicrobial response of immune cells. Thus, we investigated the effect of IL-36α upon endogenous and LPS-induced autophagy. We report that IL-36α can synergize with LPS for the induction of the autophagic process in the THP-1 cell line by triggering pro-autophagic biased signaling.

Chemical Compounds
Human recombinant IL-36α (Biomol GmbH, Hamburg, Germany) was prepared in sterile distilled water and used at 10 ng/mL concentration in all experiments. Human recombinant IL-36Ra (Sigma-Aldrich, St. Louis, MO, USA) was prepared in sterile distilled water and used at 20-fold molar excess.
A stock solution of autophagy inhibitor bafilomycin A1 BFLA (Santa Cruz Biotechnology, Dallas, TX, USA) was prepared in dimethyl sulfoxide. BFLA was used at a concentration of 100 nM in all experiments.

Immunofluorescence Assay
Cytospin cell preparations were fixed in methanol acetone (1:1) for 10 min at −20 • C. The cells were treated with 1% bovine serum albumin in PBS for 30 min at 37 • C to block non-specific binding of the antibodies. To detect LC3B, the slides were stained with a 1:150 dilution of rabbit polyclonal antibody to LC3B (Sigma-Aldrich) for 1 h at 37 • C. To detect Beclin-1, the slides were stained with a 1:100 dilution of rabbit polyclonal antibody to Beclin-1 (Sigma-Aldrich) for 1 h at 37 • C. After washing with PBS, the samples were reacted with a 1:300 dilution of CF488A-conjugated anti-rabbit antibody (Sigma-Aldrich) for 1 h at 37 • C. The cells were visualized by confocal microscopy using an Olympus FV1000 confocal laser scanning microscope using UPLSAPO 60X (N.A. 1.35) oil immersion objective and 488 nm laser excitation with 500-600 nm detection range. LC3B-positive vacuoles were automatically quantified for each field after subtraction of the background level and establishment of an intensity threshold using Image J software (U.S. National Institutes of Health, Bethesda, MD, USA). The numbers of the LC3B-positive puncta were normalized by the numbers of cells in each field. An average of 500 cells was analyzed for each condition. The fluorescence intensity of LC3B was determined using the surface plot functions of the Image J software. The mean fluorescence intensity (MFI) method was used to quantify the fluorescent signal intensities of cells. ImageJ software was used to draw an outline around each cell, and the MFI was measured. The corrected total cell fluorescence (CTCF) was calculated via the following formula: CTCF = integrated density-(area of selected cell × mean fluorescence of background readings).

Western Blot Assays
The cells were homogenized in CytoBuster lysis buffer (Merck KGaA, Darmstadt, Germany), and the mixture was then centrifuged at 10,000 g for 10 min to remove cell debris. Protein concentrations of cell lysates were determined using the Bio-Rad protein assay (Bio-Rad Laboratories Inc., Hercules, CA, USA). Supernatants were mixed with Laemmli sample buffer and boiled for 3 min. Aliquots of the supernatants were resolved by SDS-PAGE and electrotransferred onto Immun-Blot polyvinylidene difluoride (PVDF) membranes (Bio-Rad Laboratories Inc.). The membranes were blocked in PBS containing 0.05% Tween 20, and 5% dried non-fat milk (Difco Laboratories Inc., Detroit, MI, USA). The pre-blocked blots were probed with the appropriate antibodies for 4 h in PBS containing 0.05% Tween 20, 1% dried non-fat milk and 1% bovine serum albumin (Sigma-Aldrich). Rabbit anti-LC3B (Sigma-Aldrich) and rabbit anti-β-actin (Sigma-Aldrich) primary antibodies were used at a 1:1000 dilution. Blots were then incubated for 2 h with peroxidase-conjugated antirabbit antibody (Sigma-Aldrich). Membranes were developed using a chemiluminescence detection system (GE Healthcare, Chicago, IL, USA). The blots were scanned and the relative band intensities were quantified using ImageJ software (U.S. National Institutes of Health, Bethesda, MD, USA) [58].

Phospho-Kinase Array Analysis
A human phospho-kinase array (R & D Systems Inc., Minneapolis, MN, USA) was used to measure the relative phosphorylation levels of 43 signaling molecules. Control cells and cultures treated with IL-36α and LPS alone or in combination for 30 min were homoge-nized in a lysis buffer and centrifuged for five min at 14,000× g. Protein concentrations of the supernatants were determined using a Bio-Rad protein assay (Bio-Rad Laboratories Inc.). From each treatment group, three individual samples containing 100-100 µg protein were combined. After blocking, 300 µg of protein was incubated overnight at 4 • C with each array comprising two technical replicates. After washing, the arrays were reacted with a cocktail of phospho-site-specific biotinylated antibodies for two hours at room temperature, carefully washed again, and incubated with streptavidin-peroxidase for 30 min at room temperature. Signals were developed using a chemiluminescence detection system. The arrays were scanned, spot densities of phospho-proteins were quantified using ImageJ software (U.S. National Institutes of Health, Bethesda, MD, USA) [58] and normalized to those of positive controls on the same membrane after subtraction of background values.

Statistical Analysis
Statistical significance was analyzed by one-way ANOVA followed by Tukey's or Sidak's multiple comparison post-hoc tests. All statistical analyses were performed using GraphPad Prism 6 software (GraphPad Software Inc., San Diego, CA, USA), and p values less than 0.05 were considered statistically significant.

The Effects of IL-36α and LPS on the Subcellular Localization of LC3B in the THP-1 Cell Line
To elucidate how IL-36α and LPS affect the basal autophagy, we treated THP-1 cells with IL-36α and LPS alone or in combination and measured the subcellular localization of LC3B.
Immunofluorescence assays could determine the intracellular localization of LC3B at the 6-h time point and demonstrated that the control cells and the cultures treated with LPS have faint cytoplasmic LC3B staining ( Figure 2A). Accordingly, the 3D surface plots revealed a few peaks of low height (Figure 2A). Cells treated with IL-36α displayed staining patterns characterized by faint, punctate LC3B staining (Figure 2A). In contrast, the cells treated with the combination of IL-36α and LPS displayed very bright LC3B staining, and the 3D surface plot consisted of numerous robust peaks (Figure 2A). This result indicates that IL-36α and LPS cooperatively increased the accumulation of LC3B-positive vacuoles.
IL-36 receptor antagonist (IL-36Ra) was used to investigate the role of IL-36α in the synergistic activation of autophagy elicited by the combined treatment with IL-36α and LPS. The cultures were pre-treated with IL-36Ra for 30 min, and then IL-36α or a double combination of IL-36α and LPS were added. The cultures treated either with IL-36Ra alone or in combination with IL-36α displayed a faint cytoplasmic LC3B staining ( Figure 2A). Cells treated with the triple combination of IL-36α-IL-36Ra-LPS likewise displayed staining patterns characterized by faint, punctate LC3B staining ( Figure 2A). Accordingly, the 3D surface plots revealed a few peaks of low height (Figure 2A).
To investigate the effects of IL-36α and LPS on autophagosome formation, the abundances of LC3B-positive vacuoles were determined at the 6-h time point. The average numbers of LC3B-positive vacuoles per cell in the control, IL-36α-, or LPS-treated cultures were 4.72, 6.73, and 2.8, respectively ( Figure 2B). Thus, the cells treated with IL-36α alone displayed increased abundances of autophagic vesicles, this difference, however, was not statistically significant. The average numbers of LC3B-positive vacuoles per cell in cultures treated with a combination of IL-36α and LPS were significantly higher than that observed in the control cultures (the average number of autophagosomes in the cultures treated with IL-36α-LPS was 10.97 versus 4.72 in the control, p < 0.0001) ( Figure 2B). Moreover, the cells treated with the triple combination of IL-36α-IL-36Ra-LPS exhibited significantly lower numbers of LC3B-positive vacuoles per cell than in the cultures treated with the IL-36α-LPS combination: The average number of autophagosomes in the cultures treated with IL-36α-IL-36Ra-LPS was 7.58 vs. 10.97 in cells treated with IL-36α-LPS, p < 0.05 ( Figure 2B). Thus, although IL-36α and LPS alone do not cause a significant alteration in the number of LC3B-positive vacuoles, the combination of IL-36α and LPS does significantly stimulate the accumulation of autophagosomes.

The Effects of IL-36α and LPS on the Levels of LC3B-I and LC3B-II
The effects of IL-36α and LPS on the levels of LC3B-I and LC3B-II were determined by Western blot analysis (Figures 3 and S1). The control THP-1 cells displayed endogenous expression of both the lipidated and the non-lipidated forms of LC3B at each time point ( Figure 3A, lanes 1-4). IL-36α-treated cells exhibited slightly higher LC3B-II levels at the 0.5-, 2-, and 6-h time points than controls ( Figure 3A, lanes 5-7). However, these alterations were not statistically significant ( Figure 3B). Likewise, there were no significant alterations in LPS-treated cells versus controls (Figure 3). In contrast, the simultaneous treatment of cells with IL-36α and LPS triggered a significant increase in the level of LC3B-II as compared with the controls (at the 0.5-, 6-, and 24-h time points the fold increases of LC3B-II levels in cells treated with IL-36α-LPS combination were 3.52, p < 0.01, 3.0, and 3.02, p < 0.05 for both, respectively) ( Figure 3A, lanes 13, 15 and 16, and Figure 3B). The LC3B-II level at the 2-h time point dropped compared to the values measured at 0.5, 6, and 24 h after the IL-36α and LPS combined treatment ( Figure 3). These data suggest that IL-36α and LPS alone do not increase the level of LC3B-II, whereas combined IL-36α and LPS treatment cooperatively stimulates the lipidation of LC3B with characteristic kinetics.

The Effects of IL-36α and LPS on the Autophagic Flux
Bafilomycin A1 (BFLA) is an inhibitor of autophagosome-lysosome fusion and lysosomal hydrolase activity and was used to investigate the autophagic flux (Figures 4 and S2). The cultures were incubated with IL-36α and LPS alone or in combination for 2 h and then treated with BFLA for another 4-h period just before the preparation of cell lysates. Compared with the control, BFLA increased the level of LC3B-II ( Figure 4A, lanes 1 and 2, respectively). The elevated LC3B-II level of the BFLA-treated cells indicates that this drug efficiently blocked the autophagic flux under the experimental conditions used. In the presence of BFLA, IL-36α triggered a higher increase in the level of LC3B-II than in the corresponding drug control ( Figure 4A, lanes 4 and 2, respectively). However, this alteration was not statistically significant ( Figure 4B). In contrast, compared with the BFLA control, LPS-acting singly or in combination with IL-36α-elicited a significant increase in the levels of LC3B-II of cells incubated in the presence of BFLA (the fold increases of LC3B-II levels in cells treated either with LPS alone or the IL-36α-LPS combination in the presence of BFLA were 6.22, and 6.47, p < 0.05 for both) ( Figure 4B). In the presence of BFLA, the cells treated with the IL-36α-LPS combination exhibited higher increases in the level of LC3B-II than cultures stimulated only with LPS. These data indicate that combined IL-36α and LPS treatment cooperatively stimulates the autophagic flux.

The Effects of IL-36α and LPS on the Level of Beclin-1
The effects of IL-36α and LPS on the level and intracellular localization of Beclin-1 were determined by immunofluorescence assay at the 6-h time point. The control cells showed a faint cytoplasmic Beclin-1 staining ( Figure 5A). Accordingly, the 3D surface plots revealed a few peaks of low height ( Figure 5A). In contrast, the cells treated with IL-36α and LPS alone or in combination displayed very bright Beclin-1 staining, and the 3D surface plots consisted of numerous robust peaks ( Figure 5A). Measurement of the staining intensities showed that IL-36α and LPS acting singly or in combination elicited significant increases as compared with the control (the CTCF values in cells treated with IL-36α, LPS, or IL-36α-LPS combination were 1.35, 1.66, or 1.8 vs. 1.0 in the control, p < 0.0001 for all, respectively) ( Figure 5B). Measurement of the abundances of Beclin-1-positive vacuoles likewise revealed that IL-36α and LPS acting singly or in combination triggered significant increases as compared with the control (the average numbers of Beclin-1-positive puncta in cells treated with IL-36α, LPS or IL-36α-LPS combination were 5.86, 8.18, or 13.55 vs. 2.72 in the control, p < 0.01, p < 0.0001 and p < 0.0001, respectively) ( Figure 5C). These data indicate that IL-36α and LPS cooperatively elevate the level of Beclin-1.

Discussion
Compelling evidence indicates that cellular autophagic and immune processes are highly intertwined and that their coordinated functioning is essential for the efficient protection of the human body against pathogenic Gram-negative bacteria [33,[45][46][47]59]. During infections, cytokines and molecules defined as PAMP act simultaneously to activate partially overlapping signaling pathways. The combined effect may differentially regulate cellular autophagic activity. Thus, this study investigated the impact of IL-36α upon endogenous and LPS-induced autophagy.
To study the autophagic activity of THP-1 cells treated with IL-36α and LPS alone or in combination, we determined the intracellular distribution of LC3B and measured LC3B lipidation as well as the autophagic flux [60]. These experiments demonstrated that the cells treated with IL-36α alone displayed increased abundances of autophagic vesicles, elevated endogenous LC3B-II levels, and stimulated autophagic flux; these differences, however, were not statistically significant (Figures 2-4). Recent observations indicated that IL-36β and IL-36γ activate the autophagic process in primary murine CD4 + CD25 + Treg cells [57] and human macrophages [56], respectively. There may be several explanations for the weaker pro-autophagic effect of IL-36α revealed in our present study such as differences between IL-36 subtypes and different sensitivities of various cell types to this cytokine. Consistent with previous findings [52,53,61], our results demonstrated that LPS significantly increased autophagosome synthesis (Figure 3). The combination of IL-36α and LPS raised the intensity level of LC3B staining and cooperatively stimulated the translocation of this protein into autophagic vesicles (Figure 2). Supporting this observation, we found that IL-36Ra significantly inhibited the pro-autophagic effect of the IL-36α/LPS combined treatment ( Figure 2). The IL-36α-LPS combination elevated LC3B-II and decreased LC3B-I levels indicating that the lipidation of LC3B is highly stimulated (Figure 3). Finally, our experiments showed that in cultures treated with the combination of IL-36α and LPS, the autophagic flux is increased considerably by this cytokine/PAMP combination (Figure 4). These results suggest that IL-36α and LPS cooperatively stimulate autophagy.
To investigate the effect of IL-36α and LPS on the activation of some signaling pathways, we determined the phosphorylation levels of protein kinases implicated in autophagy regulation ( Figure 6). Our studies have shown that IL-36α increased the phosphorylation of Akt1/2/3 (S473), PRAS40 (T246), mTOR (S2448), WNK1 (T60), and some Src as well as STAT family kinases such as Src, Lyn, STAT2, STAT3, and STAT5a/b. PRAS40 is a negative regulator of mTORC1 [34,62]. Akt-and mTORC1-mediated phosphorylation of PRAS40 results in its dissociation from mTORC1 that in turn alleviates inhibition of mTORC1 and blocks induction of the autophagic cascade [34,62]. An interesting recent study revealed that IL-36β stimulates mTORC1 via the PI3K/Akt, IκB kinase and MyD88 pathways [63]. Our experiments demonstrate that, like IL-36β, IL36α activates mTORC1 via the PIK/Akt pathway. Moreover, we suggest that the Akt-mediated activation of mTOR involves PRAS40. Other previous studies indicated that WNK1 acts as an autophagy inhibitor by interfering with the activation of AMPK and PI3KC3 [64]. STAT3 has been shown to regulate autophagy in localization-and context-dependent manners and can elicit both pro-autophagic and anti-autophagic effects [65]. In light of these observations, our data suggest that in the early phase of IL-36α signaling, both anti-and pro-autophagic pathways are activated. Our observations show that LPS increases phosphorylation of Akt1/2/3, Src as well as STAT5a/b and decreases the level of phospho-AMPKα1 in THP-1 cells; this is fully consistent with previous reports [66,67]. Interestingly, the phosphorylation pattern of cells incubated with IL36α and LPS differed from the signatures detected either in IL36α-or LPS-treated cells. We found that the IL36α/LPS combined treatment increased phosphorylation of STAT5a/b, had minimal effect on the Akt/PRAS40/mTOR pathway, and reduced the levels of phospho-Yes, phospho-FAK, and phospho-WNK1. Thus, the combined treatment of IL-36α and LPS appears to dampen PI3K/Akt/mTOR, FAK and WNK1 signaling. The TLR4 signal transduction network is known to be kept under strict control by multiple mechanisms including positive and negative crosstalk regulations that maintain the integrity of immune cells by preventing excessive inflammation [68]. The negative regulators acting through the activation of transcription play a primary role in the late phase of TLR and IL-36 signaling; their role in the early stage thus can be excluded. Important studies, however, revealed that PI3K has an essential role in the safety mechanism controlling the early-stage of TLR4-mediated signaling [69]. PI3K can suppress TLR4 signaling by altering the availability of phosphatidylinositol-(4,5)bisphosphate (PIP 2 ) at the cytoplasmic membrane and hence can modulate the intracellular localization of adaptors, the magnitude of activation, and signal output [70]. Based on these observations, we suggest that the combinatorial effect of IL-36α/LPS may exert an excessive PI3K activation, which-while not suspending the inhibition of downstream events of autophagy-can significantly reduce it by decreasing the Akt-mediated activation of mTORC1.
Previous studies have demonstrated that Beclin-1 plays a pivotal role in the autophagic process. Beclin-1 was shown to interact with Bcl-2 and Bcl-X L , which suppresses autophagosome biogenesis. The release of Beclin-1 is a prerequisite for the formation of a functional PI3KC3 complex. Phosphorylation events and TRAF6-mediated ubiquitination of Beclin-1 may destabilize Beclin-1-Bcl-2/Bcl-X L association and abrogate BECN1-BCL2/Bcl-X L interaction. Some IL-36α and LPS signaling intermediates have the potential to regulate the functional activity of Beclin-1 [71,72]. Thus, we investigated the effect of IL-36α and LPS on Beclin-1 protein. Our results showed that IL-36α and LPS acting singly or in combination elevated the staining intensities and increased the abundances of Beclin-1-positive vacuoles ( Figure 5). The IL-36α/LPS treatment was again more efficient than IL-36α or LPS alone. These data further support the notion that IL-36α and LPS cooperatively promote the autophagic process because increased Beclin-1 levels were shown to correlate with enhanced autophagy [60].
Our results suggest a hypothetical model for the mechanism of the enhanced proautophagic effect observed in cells treated with IL-36α and LPS simultaneously. The IL-36α/LPS combination reduces the activation level of the PI3K/Akt/mTORC1 axis by triggering rapid depletion of PIP 2 at the cytoplasmic membrane. As a result, mTORmediated inhibition of autophagy is alleviated. Some components of the IL-36α and LPS signaling networks are known to induce autophagy. MyD88 binds directly whereas TRAF3 and TRAF6 ubiquitinate the Beclin-1 protein, thus disrupting the interaction between Beclin-1 and Bcl-2 [52,53]. This results in increased oligomerization of Beclin-1, activation of the PI3KC3 complex, and initiation of autophagosome formation [52,53]. The IL-36α/LPS combination increases the activation level of PI3KC3 complex directly and subsequently stimulates autophagy. Thus, this cytokine/PAMP combination triggers pro-autophagic biased signaling by several mechanisms and thereby cooperatively stimulates the autophagic cascade ( Figure S4). Previous studies have shown that bacteria affect the autophagic cascade and, conversely, autophagy influences the infection process. The bacteria studied so far all interact with the autophagic machinery but in different ways. The structural components, PAMPs, and exotoxins of several bacteria induce autophagy. However, some bacteria can effectively prevent autophagic recognition, inhibit autophagy initiation and maturation of autophagosomes or block the fusion of lysosomes with autophagosomes, while others hijack the autophagic compartment to support their intracellular survival.
Our data indicate that cytokines may modify the pro-autophagic effect of bacterial PAMPs. An increased xenophagic activity of innate immune cells exposed to IL-36α and LPSfunctioning as part of the cell-autonomous defense system-may play a protective role in the pathogenesis of infections caused by Gram-negative bacteria [73,74]. Our data indicate that cytokines may modify the pro-autophagic effect of bacterial PAMPs. An increased xenophagic activity of innate immune cells exposed to IL-36α and LPS-functioning as part of the cell-autonomous defense system-may play an important role in the pathogenesis of infections caused by Gram-negative bacteria.
In conclusion, our results demonstrate that the IL-36α/LPS triggers pro-autophagic biased signaling by several mechanisms and thereby stimulates the autophagic cascade cooperatively in the THP-1 cell line. However, some limitations of this investigation, such as the need for additional experimental evidence that can corroborate the synergistic effect of this cytokine/PAMP combination in other cell types, have yet to be addressed. Moreover, further studies are needed to investigate the effect of increased autophagic activity on the functions of innate immune cells treated with IL-36α and LPS simultaneously.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/biomedicines9111541/s1, Figure S1: IL-36α and LPS cooperatively increase the level of LC3B-II., Figure S2: IL-36α and LPS cooperatively stimulate the autophagic flux. Figure S3: Differential phospho-kinase array profiles of cells treated with IL-36α and LPS. Figure S4: IL36α and LPS cooperatively induced autophagy by multiple mechanisms. Table S1: List of the detected phospho-proteins by using the Proteome ProfilerTM human phospho-kinase array kit.