Formaldehyde promotes tumor-associated macrophage polarizations and functions through induction of HIF-1α-mediated glycolysis.

Formaldehyde (FA) exposure has been positively correlated with many diseases including various types of cancers. However, the mechanisms of FA-related carcinogenesis are still unclear. Tumor-associated macrophages (TAMs) are the most abundant immune cells in tumor microenvironment, which is a heterogeneous population consist of both pro-inflammatory (M1) and immunosuppressive (M2) cells. TAMs are deeply involved in tumor development and progression. Our previous studies demonstrated that FA enhanced M1 polarization of macrophages through induction of HIF-1α-mediated glycolysis. To examine if TAM polarizations are also potentiated by FA, BALB/c nude mice were inoculated with A549 cells to develop subcutaneous tumors and exposed to 2.0 mg/m3 FA for 14 days. Significant increases of both M1 and M2 polarizations of TAMs were observed in tumor tissues of FA-exposed mice. After confirmation of the potentiation effects in RAW264.7 and THP-1-derived in vitro TAM models, FA at 25 and 50 µM was found to enhance TAM immunosuppressive functions and glycolytic metabolism. In addition, FA-induced glycolysis in TAMs was reversed by a specific HIF-1α inhibitor PX-478 at 5 µM, and suppression of glycolytic metabolism with a glucose analog 2-DG at 1 mM also alleviated FA-potentiated TAM functions, which indicated that FA induced TAM polarizations through the upregulation of HIF-1α-mediated glycolysis. These results illustrated a potential carcinogenic mechanism of FA through metabolic disturbance of tumor immunity, which could be utilized to develop preventative or therapeutic agents for FA-induced carcinogenesis and immune disorders.

Accumulation of Intracellular Ferrous Iron in Inflammatory-Activated Macrophages.

Macrophages are important innate immune cells which can be polarized into heterogeneous populations. The inflammatory-activated M1 cells are known to be involved in all kinds of inflammatory diseases, which were also found to be associated with dysregulation of iron metabolism. While iron overload is known to induce M1 polarization, the valence states of iron and its intracellular dynamics during macrophage inflammatory activation have not been identified. In this study, THP-1-derived macrophages were polarized into M1, M2a, M2b, M2c, and M2d cells, and intracellular ferrous iron (Fe(II)) was measured by our previously developed ultrasensitive Fe(II) fluorescent probe. Significant accumulation of Fe(II) was only observed in M1 cells, which was different from the alterations of total iron. Time-dependent change of intracellular Fe(II) during the inflammatory activation was also consistent with the expression shifts of transferrin receptor CD71, ferrireductase Steap3, and Fe(II) exporter Slc40a1. In addition, accumulation of Fe(II) was also found in the colon macrophages of mice with ulcerative colitis, which was positively correlated to inflammatory phenotypes, including the productions of NO, IL-1ß, TNF-α, and IL-6. Collectively, these results demonstrated the specific accumulation of Fe(II) in inflammatory-activated macrophages, which not only enriched our understanding of iron homeostasis in macrophages, but also indicated that Fe(II) could be further developed as a potential biomarker for inflammatory-activated macrophages.

Myricetin alleviates the formaldehyde-enhanced Warburg effect in tumor cells through inhibition of HIF-1α.

Myricetin is a flavonoid widely-distributed in foods with many beneficial health effects, which has been marketed in health products. Formaldehyde is an environmental carcinogen which can enhance the Warburg effect through the induction of human hypoxia-inducible factor 1 subunit alpha (HIF-1α), the primary regulator of cellular glycolysis. HIF-1α was verified as an important target in lung and ovarian tumors, which was also identified as a receptor for myricetin via molecular docking. The reinforced HIF-1α signaling, the Warburg effect and T cell suppression induced by 50 µM formaldehyde in both A549 and Caov-3 cells were dose-dependently attenuated by myricetin from 20 to 100 µM, and the attenuative effects were diminished by the stabilization of HIF-1α with deferoxamine. Exposure to 2.0 mg/m3 formaldehyde also stimulated tumor growth and elevated HIF-1α expression in tumor tissues of A549 xenograft mice, which were also alleviated by oral administration of 100 mg/kg myricetin. These results demonstrated that myricetin alleviated formaldehyde-enhanced Warburg effect in tumor cells through HIF-1α inhibition, which could be further developed as a therapeutic or complementary agent for formaldehyde-induced carcinogenesis.

Arsenite inhibits M2a polarization of macrophages through downregulation of peroxisome proliferator-activated receptor gamma.

Arsenite (As+3) is a group one human carcinogen, which has been associated with many diseases. Previous studies indicated that As+3 could inhibit wound healing and repair. M2a cells are known as tissue remodeling macrophages, which play an important role in wound repair process. Peroxisome proliferator-activated receptor gamma (PPAR-γ), a key regulator of lipid and glucose metabolism, was found to mediate the IL-4-dependent M2a polarization of macrophages. In the present study, As+3 induced dose-dependent inhibition of M2a polarization starting from 0.1 µM in THP-1-derived macrophages stimulated with 20 ng/mL IL-4. Increased lipid accumulation and decreased PPAR-γ expression were also observed in As+3-treated M2a macrophages. Rosiglitazone (RSG), a potent PPAR-γ agonist, alleviated the suppressions of PPAR-γ and M2a polarization induced by 2 µM As+3. Collectively, these results not only demonstrated that As+3 was able to inhibit polarization of M2a cells through PPAR-γ suppression, but also indicated that PPAR-γ could be utilized as a target for the prevention and treatment of As+3-induced immunotoxicity.