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Table of Contents
ORIGINAL ARTICLE
Year : 2020  |  Volume : 63  |  Issue : 1  |  Page : 7-14

Distinct patterns of interleukin-12/23 and tumor necrosis factor α synthesis by activated macrophages are modulated by glucose and colon cancer metabolites


1 Graduate Institute of Physiology, National Taiwan University College of Medicine, Taipei; Department of Food Science and Biotechnology, National Chung Hsing University, Taichung, Taiwan
2 Graduate Institute of Physiology, National Taiwan University College of Medicine, Taipei, Taiwan

Date of Submission16-Oct-2019
Date of Acceptance04-Dec-2019
Date of Web Publication7-Feb-2020

Correspondence Address:
Prof. Linda Chia-Hui Yu
Graduate Institute of Physiology, National Taiwan University College of Medicine, Suite 1020, 1 Jen-Ai Rd. Sec. 1, Taipei
Taiwan
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Source of Support: This study was supported by grants from the Ministry of Science and Technology (MOST 107-2320-B-005-001, 107-2320-B-002-041-MY3, 106-2320-B-002-017, and 105-2811-B-002-014) and National Taiwan University (NTU-CDP-105R7798 and NTU-CCP-106R890504)., Conflict of Interest: None


DOI: 10.4103/CJP.CJP_75_19

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  Abstract 

Chronic inflammation is a major risk factor for colitis-associated colorectal carcinoma (CRC). Macrophages play a key role in altering the tumor microenvironment by producing pro-inflammatory and anti-inflammatory cytokines. Our previous studies showed that glucose metabolism conferred death resistance for tumor progression and exerted anti-inflammatory effects in ischemic gut mucosa. However, the effect of glucose and cancer metabolites in modulating macrophage cytokine profiles remains poorly defined. We used an in vitro system to mimic intestinal microenvironment and to investigate the roles of glucose and cancer metabolites in the cross-talk between carcinoma cells and macrophages. Human monocyte-derived THP-1 macrophages were stimulated with bacterial lipopolysaccharide (LPS) in the presence of conditioned media (CM) collected from human CRC Caco-2 cells incubated in either glucose-free or glucose-containing media. Our results demonstrated that glucose modulated the macrophage cytokine production, including decreased LPS-induced pro-inflammatory cytokines (i.e., tumor necrosis factor [TNF]α and interleukin [IL]-6) and increased anti-inflammatory cytokine (i.e., IL-10), at resting state. Moreover, glucose-containing CM reduced the macrophage secretion of TNFα and IL-8 but elevated the IL-12 and IL-23 levels, showing an opposite pattern of distinct pro-inflammatory cytokines modulated by cancer glucose metabolites. In contrast, LPS-induced production of macrophage inflammatory protein-1 (a macrophage-derived chemoattractant for granulocytes) was not altered by glucose or CM, indicating that resident macrophages may play a more dominant role than infiltrating granulocytes for responding to cancer metabolites. In conclusion, glucose metabolites from CRC triggered distinct changes in the cytokine profiles in macrophages. The downregulation of death-inducing TNFα and upregulation of Th1/17-polarizing IL-12/IL-23 axis in macrophages caused by exposure to cancer-derived glucose metabolites may contribute to tumor progression.

Keywords: Colonic neoplasms, cytokines, glycolytic metabolism, intestinal epithelial cells, macrophage, tumorigenesis


How to cite this article:
Huang CY, Yu LC. Distinct patterns of interleukin-12/23 and tumor necrosis factor α synthesis by activated macrophages are modulated by glucose and colon cancer metabolites. Chin J Physiol 2020;63:7-14

How to cite this URL:
Huang CY, Yu LC. Distinct patterns of interleukin-12/23 and tumor necrosis factor α synthesis by activated macrophages are modulated by glucose and colon cancer metabolites. Chin J Physiol [serial online] 2020 [cited 2020 Feb 26];63:7-14. Available from: http://www.cjphysiology.org/text.asp?2020/63/1/7/277954


  Introduction Top


Patients with inflammatory bowel disease (IBD) may develop colitis-associated colorectal carcinoma (CRC) later in life, of which the risk is positively correlated with disease duration, extent, and severity of inflammation and is distinct from sporadic colorectal cancer.[1],[2] The main element for cancer progression in IBD patients is chronic inflammation signified by an imbalance between pro-inflammatory and anti-inflammatory cytokine production.[3],[4] Over the past two decades, pro-inflammatory cytokines such as tumor necrosis factor (TNF)α and interleukin (IL) IL-12/IL-23 had been linked to the pathogenesis of chronic inflammation and served as therapeutic targets for the clinical management of IBD.[5],[6],[7] The blockade of pro-inflammatory cytokines by neutralizing antibodies has also been suggested as preventive measures for colitis-associated CRC development or as a combinational therapy for tumor regression.[8],[9],[10],[11]

Resident macrophages are one of the most abundant types of immune cells in the colon and are juxtaposing beneath the epithelial layer.[12],[13] Macrophages acting along with the epithelia-transformed tumor cells which generate and integrate signals reciprocally can either be beneficial or deleterious for patient prognosis.[13] The highly plastic macrophages are classified on the basis of in vitro activators and cytokine-producing profiles. The M1 macrophages, when stimulated by bacterial lipopolysacharride (LPS), are characterized by the production of pro-inflammatory cytokines. The elevated cytokine TNFα, IL-6, and IL-8 levels during acute inflammation are responsible for triggering leukocyte diapedesis to exert microbicidal activities and clearance of dying host cells.[11],[14] Moreover, TNFα is known for the induction of cell apoptosis and necrosis.[14] The IL-12/IL-23 axis promotes the differentiation of naïve T lymphocytes to Th1 and Th17 cells which are associated with chronic inflammation and advanced colon cancer stages.[15],[16],[17] Otherwise, the M2 macrophages are characterized by the production of IL-10 and transforming growth factor-β for the maintenance of tissue homeostasis during steady states and display wound healing and tumor-promoting functions.

Poor prognosis of CRC was related to high dietary glycemic load, fasting hyperglycemia, and diabetes.[18],[19],[20],[21] Recent evidence showed that hyperglycemia and high sugar retention in the intestinal lumen enhanced colon tumor growth in mouse models.[22] Warburg's effects refer to the phenomenon that high rates of glucose are fermented to lactate in tumors even in the presence of oxygen, hence the term aerobic glycolysis.[23] Our previous studies have demonstrated that glucose metabolism conferred death resistance in colorectal cancer cells under hypoxia and chemotherapy[24],[25],[26],[27] and in intestinal epithelial cells after ischemic challenges.[28],[29],[30],[31] These data indicated that glucose not only served as an energy source for cell proliferation but also provided anti-death mechanisms – the two main hallmarks of cancers. Recent studies showed that tumor cell-derived glycolytic lactate signals activated macrophages to a protumorigenic state,[32] suggesting that anaerobic glucose metabolism in cancer cells may be a driving force for macrophage polarization. Whether glucose and colon cancer metabolites modulate the patterns of pro-inflammatory and anti-inflammatory cytokines secreted by macrophages remains poorly defined.

Our hypothesis is that glucose per se or soluble factors derived from epithelia-transformed cancer cells with high glucose metabolism may impact on the cytokine profiles of macrophages. In the current study, we used an in vitro cell culture system to mimic the intestinal microenvironment and to investigate the role of glucose on modulating the cross-talk between carcinoma cells and macrophages.


  Materials and Methods Top


Cell lines and cell culture

The human colorectal carcinoma (CRC) cell line Caco-2 and the human peripheral blood acute monocytic leukemia cells THP-1 were purchased from ATCC/Bioresource Collection and Research Center (Manassas, VA, USA).[33],[34],[35] Human CRC cell line Caco-2 cells were maintained in standard Dulbecco's modified Eagle's medium (DMEM; Invitrogen, Grand Island, NY, USA) containing 5 mM glucose. The media was supplemented with 10% fetal bovine serum (FBS), 15 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and antibiotics (100 U/mL penicillin and 0.1 mg/mL streptomycin) (Sigma, St. Louis, MO, USA). The cells were used between passages 21 and 27. Human monocytic THP-1 cells were maintained in a culture medium consisting of DMEM containing 5 mM glucose, 10% FBS, 15 mM HEPES, and antibiotics. Both types of cells grew at 37°C with 5% CO2 under a humidified atmosphere.

Differentiation of human THP-1 monocytes to macrophages

Human THP-1 monocytes were seeded at a density of 5 × 105 cells/cm2 onto 24-well cell culture plates (Costar, Corning, NY, USA). The cells were stimulated to differentiate into macrophage (MØ) with 20 ng/ml phorbol 12-myristate 13-acetate (PMA) (Sigma-Aldrich, St. Louis, MO, USA) for 48 h.

Lipopolysaccharide challenge

The PMA-differentiated THP-1 cells were exposed to 100 ng/ml LPS (from  Escherichia More Details coli O26:B6, Sigma) in cell culture media containing 0, 5, and 25 mM glucose for 24 h, and the cytokine production was determined as described below. The glucose concentrations were chosen based on the experimental results from a previous research.[24],[29],[30] In some settings, lactic acid (Sigma #L6402) was added to LPS-activated THP-1 cells for 24 h, and the cell culture medium was collected for cytokine measurement.

Preparation of colorectal cancer-derived conditioned medium for macrophage culturing

Human CRC Caco-2 cells were seeded at a density of 3 × 105 cells/cm2 onto 6-well cell culture plates (Costar, Corning, NY, USA) for 1 week at 37°C in a humidified atmosphere with 5% CO2 and 95% air. For the collection of colorectal cancer-derived conditioned medium (CM), Caco-2 cells were grown to confluency and were washed thoroughly for one time with glucose-free standard DMEM before culturing for 8 h in standard media containing 0 or 25 mM glucose. The standard DMEM used in the study was pyruvate- and glucose-free. The Caco-2-derived CM was centrifuged to remove cell debris and stored in aliquots at −20°C until use. The Caco-2-derived CM was added to PMA-differentiated THP-1 cell cultures prior to LPS challenge for 24 h. The cytokine production by THP-1 cells was determined as described below.

Cytokine detection

The levels of TNFα, IL-6, IL-8, IL-10, IL-12, IL-23, and macrophage inflammatory proteins (MIPs)-1 were measured by using enzyme-linked immunosorbent assay development kits (PeproTech, NJ, USA) according to the manufacturer's instructions. To measure the cytokine levels, microplates were coated overnight with capture antibodies. The plates were blocked with phosphate-buffered saline containing 1% bovine serum albumin for 1 h and washed. The sample and standard solutions were added and incubated for 2 h. The biotinylated antigen-affinity detection antibodies were incubated for another 2 h. After washing, avidin-horseradish peroxidase conjugate was added for 30 min followed by incubation with 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) liquid substrate for color development. Absorbance will be measured at 405 nm with correction set at 650 nm. The cytokine levels in the cell culture medium were expressed in pg/mg of protein.[36],[37]

Statistical analysis

All values were expressed as mean ± standard error of mean, and the means were compared by one-way analysis of variance (ANOVA) followed by a Student–Newman–Keul test or ANOVA followed by an Fisher's least significant difference test using Prism GraphPad (San Diego, CA, USA) or Sigma Plot software (San Jose, CA, USA). Statistical significance was established at P < 0.05.


  Results Top


Glucose-attenuated lipopolysaccharide-induced pro-inflammatory and chemoattractant cytokine production by macrophages in a dose-dependent manner

Human monocyte-derived THP-1 macrophages were challenged with LPS (100 ng/ml) under various glucose concentrations. The levels of TNFα [Figure 1]a, IL-6 [Figure 1]b, and IL-8 [Figure 1]c after LPS challenge were quantified in THP-1 macrophages under 0, 5, or 25 mM glucose conditions. The THP-1 macrophages showed a significant increase of pro-inflammatory and chemoattractant cytokine production following LPS stimulation compared with untreated control cells [Figure 1]. High concentrations of extracellular glucose during exposure to LPS attenuated the pro-inflammatory and chemoattractant cytokine production by THP-1 macrophages in a dose-dependent manner [Figure 1]a, [Figure 1]b, [Figure 1]c]. There was no significant difference in the amounts of pro-inflammatory and chemoattractant cytokine production in cells between different glucose concentrations in untreated control groups [Figure 1].
Figure 1: Presence of glucose dose dependently reduced the proinflammatory cytokine production in lipopolysaccharide-activated macrophages. Differentiated THP-1 macrophages were cultured in standard media containing 0, 5, or 25 mM glucose (G0, G5, and G25, respectively). The cells were incubated in untreated (CON) condition or exposed to lipopolysaccharide (100 ng/ml) for 24 h, and the proinflammatory cytokine levels were measured. (a) Tumor necrosis factor-α, (b) interleukin-6 and (c) interleukin-8 production increased upon lipopolysaccharide exposure. The lipopolysaccharide-induced proinflammatory cytokine production was attenuated by glucose in a dosedependent manner. *P < 0.05 versus respective CON groups; #P < 0.05 versus lipopolysaccharide + G0 groups (n = 8/groups).

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Colorectal cancer-derived conditioned medium decreased pro-inflammatory cytokine production by macrophages

Culture supernatants were collected from Caco-2 cells incubated in either glucose-free or glucose-containing media, and the CM were compared to standard media for their effects on macrophages. The THP-1 macrophages cultured in standard or CM were stimulated with LPS for 24 h and assessed for the production of pro-inflammatory cytokines. LPS induced significant amounts of TNFα (1424.2 ± 67.9 pg/ml) and IL-6 (634.5 ± 15.2 pg/ml) in THP-1 macrophages in glucose-free standard medium [Figure 2]a and [Figure 2]b]. The presence of glucose caused a ~35% reduction of both TNFα (935.5 ± 125.3 pg/ml) and IL-6 (403.8 ± 4.5 pg/ml) levels in the LPS-activated macrophages under standard medium. When bathed in either glucose-free or glucose-containing Caco-2-derived CM, a further reduction (~70%) of LPS-induced cytokine production, i.e., TNFα (311.3 ± 38.0 pg/ml) and IL-6 (267.9 ± 20.5 pg/ml), was observed in THP-1 cells [Figure 2]a and [Figure 2]b].
Figure 2: Colorectal cancer-derived conditioned medium and glucose decreased the levels of pro-inflammatory cytokines in lipopolysaccharideactivated macrophages. Differentiated THP-1 macrophages cultured with standard medium or Caco-2-derived conditioned medium were activated with 100 ng/ml lipopolysaccharide for 24 h. The control (CON) group of THP-1 cells was untreated in standard medium. The THP-1 cells in standard medium groups were incubated in either glucose-free (φ) or glucose-containing (G, 25 mM) medium. The THP-1 cells in the conditioned medium groups were incubated with either glucose-free (φ) or glucose-containing (G, 25 mM) culture media obtained from Caco-2 cells. Levels of (a) tumor necrosis factor-α and (b) interleukin-6 were determined in macrophage cultures. *P < 0.05 versus respective standard medium groups; #P < 0.05 versus respective φ groups treated with lipopolysaccharide. The difference between the lipopolysaccharide and CON groups was statistically significant (P < 0.05), and the statistical labels were omitted from the figures for clarification of the results (n = 8/groups).

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Glucose and colorectal cancer-derived conditioned medium reduced the chemoattractant synthesis in macrophages

Macrophage-produced IL-8 and MIP-1 act as chemoattractants for neutrophils and granulocytes in response to signals at the site of injury or infection.[38] We next evaluated the levels of IL-8 and MIP-1 in LPS-activated THP-1 macrophages. Unlike TNFα and IL-6, the presence of glucose had no effect on LPS-induced IL-8 and MIP-1 production in THP-1 macrophages cultured in standard media. When THP-1 cells were cultured in Caco-2-derived CM, the IL-8 production caused by LPS was significantly attenuated compared to values in the standard medium [Figure 3]a. Moreover, a lower level of IL-8 production was noted in THP-1 cells incubated with glucose-containing than glucose-free CM [Figure 3]a. Despite a trend of decrease in MIP-1 production under Caco-2-derived CM, the values were not significantly different from those in the standard media [Figure 3]b.
Figure 3: Effects of glucose and colorectal cancer-derived conditioned medium on the chemokine production of lipopolysaccharide-activated macrophages. Differentiated THP-1 macrophages activated with 100 ng/ml lipopolysaccharide were cultured with standard medium or Caco- 2-derived conditioned medium. The control (CON) group of THP-1 cells was untreated in standard medium. THP-1 cells in standard medium groups were incubated in either glucose-free (φ) or glucose containing (25 mM) medium. The THP-1 cells in the conditioned medium groups were incubated with either glucose-free (φ) or glucose-containing (G, 25 mM) culture media obtained from Caco-2 cells. Levels of (a) interleukin-8 and (b) macrophage inflammatory proteins-1α were determined in macrophage cultures. *P < 0.05 versus respective standard medium groups; #P < 0.05 versus respective φ groups treated with lipopolysaccharide. The difference between the lipopolysaccharide and CON groups was statistically significant (P < 0.05), and the statistical labels were omitted from the figures for clarification of the results (n = 8/groups).

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Glucose and colorectal cancer-derived conditioned medium increased the production of interleukin-12/interleukin-23 in macrophages

As shown in [Figure 3]a and [Figure 3]b, LPS induced both IL-12 and IL-23 production by THP-1 macrophages in standard medium. Presence of glucose enhanced the LPS-induced IL-12 and IL-23 production compared to those under glucose-free standard media. When cultured in Caco-2-derived CM, the IL-12 and IL-23 cytokine levels were 3- to 4-fold higher than the levels under standard media [Figure 4]a and [Figure 4]b]. Furthermore, a higher level of IL-12 and IL-23 production in LPS-activated macrophages was observed in glucose-containing CM compared to glucose-free CM [Figure 4]a and [Figure 4]b].
Figure 4: Colorectal cancer-derived conditioned medium and glucose increased the production of interleukin-12/23 in lipopolysaccharideactivated macrophages. Differentiated THP-1 macrophages activated with 100 ng/ml lipopolysaccharide were cultured with standard medium or Caco-2-derived conditioned medium. The control (CON) group of THP-1 cells was untreated in standard medium. THP-1 cells in standard medium groups were incubated in either glucose-free (φ) or glucose containing (25 mM) medium. The THP-1 cells in the conditioned medium groups were incubated with either glucose-free (φ) or glucose-containing (G, 25 mM) culture media obtained from Caco-2 cells. Levels of (a) interleukin-12 and (b) interleukin-23 were determined in macrophage cultures. *P < 0.05 versus respective standard medium groups; #P < 0.05 versus respective φ groups treated with lipopolysaccharide. The difference between the lipopolysaccharide and CON groups was statistically significant (P < 0.05), and the statistical labels were omitted from the figures for clarification of the results (n = 8/groups).

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Anti-inflammatory cytokine secretion from macrophages was upregulated by glucose or lipopolysaccharide but not by cancer metabolites

IL-10 is known as an anti-inflammatory cytokine for the regulation of intestinal homeostasis. The presence of glucose without LPS stimulation increased the IL-10 production in THP-1 macrophages [Figure 5]. LPS challenge stimulated IL-10 production by macrophages in standard media with or without glucose, of which the level is similar to the upregulating effect by glucose alone [Figure 5]. The amount of IL-10 production in THP-1 macrophages was comparable between groups under standard media and Caco-2-derived CM [Figure 5].
Figure 5: Glucose and lipopolysaccharide induced anti-inflammatory cytokine production in macrophages, whereas colorectal cancer-derived conditioned medium had no effect. Differentiated THP-1 macrophages activated with 100 ng/ml lipopolysaccharide were cultured with standard medium or Caco-2-derived conditioned medium. The control (CON) group of THP-1 cells was untreated in standard medium. THP-1 cells in standard medium groups were incubated in either glucose-free (φ) or glucose-containing (25 mM) medium. The THP-1 cells in the conditioned medium groups were incubated with either glucose-free (φ) or glucosecontaining (G, 25 mM) culture media obtained from Caco-2 cells. Levels of interleukin-10 were determined in macrophage cultures. *P < 0.05 versus CON+φ (n = 8/groups).

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Lactic acid attenuated lipopolysaccharide-induced pro-inflammatory cytokine production secretion in a dose-dependent manner

Finally, to test whether the glycolytic by-product lactic acid played a role in the polarization of macrophages, various concentrations of lactic acid were added to the LPS-activated THP-1 cells. Extracellular lactic acid attenuated the pro-inflammatory cytokine production by THP-1 macrophages during exposure to LPS in a dose-dependent manner [Figure 6]a and [Figure 6]b].
Figure 6: Lactate attenuated pro-inflammatory cytokine secretion in lipopolysaccharide-activated macrophages. Differentiated THP-1 macrophages were cultured in standard media containing various concentrations of lactic acid. The cells were incubated in untreated (CON) condition or exposed to lipopolysaccharide (100 ng/ml) for 24 h, and the pro-inflammatory cytokine levels were measured. (a) Tumor necrosis factor-α and (b) interleukin-6 production increased upon lipopolysaccharide exposure. The lipopolysaccharide-induced pro-inflammatory cytokine production was attenuated by lactic acid in a dose-dependent manner. *P < 0.05 versus respective CON groups; #P < 0.05 versus lipopolysaccharide + lactic acid 0 mM groups (n = 8/groups).

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  Discussion Top


Macrophages are one of the most predominant tumor-associated immune cell types. Several cytokines secreted by macrophage have been linked to drive IBD and colitis-associated cancer pathogenesis. In this study, we first evaluated the influence of glucose per se in LPS-activated THP-1 cells and investigated the effect of CM from Caco-2 cells, incubated with or without glucose, on the cytokine production of macrophages. Our data showed that enhanced TNFα, IL-6, and IL-8 production by LPS stimulation was partially suppressed in the presence of glucose. THP-1 cells incubated with Caco-2-derived CM in high glucose showed a further decrease in the production of pro-inflammatory cytokines, TNFα, and IL-6. In contrast, LPS-induced IL-12 and IL-23 production by THP-1 cells was markedly increased when macrophages were incubated with Caco-2-derived CM in high glucose. The anti-inflammatory IL-10 production from THP-1 macrophages was upregulated by glucose or LPS, while incubation with Caco-2-derived CM did not increase the levels. Our novel findings suggested that downregulation of death-inducing TNF-α and upregulation of Th1/17-polarizing IL-12/IL-23 axis in macrophages caused by exposure to cancer-derived glucose metabolites may benefit the tumor to adapt the harsh microenvironment.

Macrophages act as a bridge between the innate and adaptive immune system. Macrophages elicited acute inflammatory responses and produced pro-inflammatory cytokines.[39] Overproduction of these pro-inflammatory cytokines has been linked to the pathogenesis of IBD as well as colitis-associated CRC.[40],[41] Recently, the discovery of antibodies targeting IL-12 and IL-23 has been addressed with potential clinical application in IBD for patients unresponsive to anti-TNF therapy.[6] IL-12 and IL-23 are heterodimeric cytokines which have a common IL-12p40 subunit and serve as the target of antibody neutralization.[6],[7],[42],[43],[44] In the current study, an opposite trend in IL-12/23 and TNFα production by macrophages was observed after treating with CRC-derived CM in high glucose. The CRC-derived glucose metabolites effectively reduced TNFα production, but significantly enhanced IL-12/23 production in response to LPS challenge, suggesting that the soluble factors produced by Caco-2 cells could exert distinct patterns of pro-inflammatory cytokine production in macrophages.

Resident macrophages, situated in the intestinal lamina propria underneath the epithelial layers, are capable of sensing and monitoring the environmental changes to maintain homeostasis or trigger pro-inflammatory response through metabolic changes.[45],[46] The M1-phenotype expressed pro-inflammatory cytokines, whereas the M2-phenotype produced anti-inflammatory cytokines. Reduction of phagocytosis and nitric oxide production in macrophage after long-term high glucose exposure was previously documented.[47] We here demonstrated that the presence of glucose induced anti-inflammatory IL-10 secretion from macrophages at resting state and downregulated the LPS-induced TNFα and IL-6 production in macrophages, which may be linked to promoting immune tolerance in a healthy gut microenvironment full of gut microbiota. Nevertheless, activation of macrophages and overexpression of pro-inflammatory cytokines to mount an acute-phase inflammation upon gut barrier break is necessary to exert free radical-mediated microbicidal activities accompanied by the clearance of dying host cells.

Cancer cells transformed from malignant epithelia may transduce different signals to the underlying macrophages in comparison to healthy colonocytes, in order to benefit tumor cell survival and growth. Several lines of evidence suggested that lactic acid is involved in the modulation of macrophage polarization. A recent study demonstrated that tumor-derived lactic acid induced M2-polarization of tumor-associated macrophages via upregulation of vegf and arg1 in a mechanism dependent on hypoxia-inducible factor 1α.[32] Several in vitro co-culture studies have been used to investigate the interactions between macrophages and CRC cells. An M2-like phenotype was induced when murine and human monocyte-derived macrophages were treated with CM from colon cancer cell lines (e.g., SW480, RKO, SW480, and Caco-2),[48],[49] which may be associated with the induction of c-myc in macrophages.[48] The results of our study revealed that glucose itself or Caco-2-derived glycolytic metabolites ameliorated TNFα production, but enhanced IL-12/IL-23 production. TNFα is known as an extrinsic inducer for stimulating apoptosis and necroptosis of infected and stressed cells.[14] The IL-12/23 axis mediates the Th1 and Th17 polarization, which is linked to advanced colon cancers.[6] It is plausible that cancer glycolytic metabolites modulated the macrophage cytokine production to attenuate death-inducing signals and to drive Th1/17 differentiation to facilitate tumor progression. It is noteworthy that cancer-derived glucose metabolites did not affect the anti-inflammatory cytokine IL-10 levels, which was already heightened by glucose alone, implicating that soluble factors from cancers actively downregulated the pro-inflammatory TNFα uncoupled to the antagonistic arm of IL-10. Moreover, chemokines such as MIP-1 for granulocyte infiltration were not altered by cancer-derived metabolites, suggesting that tumor cells mainly inhibited the functions of resident macrophages but had no effect on infiltrating granulocytes. Consistent with our data, a recent study highlighted the delicate mechanisms through which aerobic glycolytic lactate-derived lactylation of histone lysine residues induced macrophages to switch from M1 to M2-like characteristics by epigenetic modification, suggesting the potential of glycolytic metabolites in regulating macrophage polarization at a molecular level.[50] Overall, further mechanistic studies are warranted to elucidate the downstream signals to differentially regulate the secretion of pro-inflammatory cytokines in macrophages.


  Conclusion Top


Our novel findings demonstrated that CRC-derived glucose metabolites modulated the patterns of pro-inflammatory cytokine production in activated macrophages, of which opposite responses were observed in TNF-α and IL-12/23 levels. Further understanding of the link between CRC-derived soluble factors and glycolytic products in regulating macrophage functions would provide insights to the oncoimmunological pathogenesis of colitis-associated CRC.

Acknowledgment

We thank the technical assistance of First Research Core of National Taiwan University College of Medicine.

Financial support and sponsorship

This study was supported by grants from the Ministry of Science and Technology (MOST 107-2320-B-005-001, 107-2320-B-002-041-MY3, 106-2320-B-002-017, and 105-2811-B-002-014) and National Taiwan University (NTU-CDP-105R7798 and NTU-CCP-106R890504).

Conflicts of interest

There are no conflicts of interest.



 
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