Homeostatic activation of aryl hydrocarbon receptor by dietary ligands dampens cutaneous allergic responses by controlling Langerhans cells migration

Mechanisms underlying aryl hydrocarbon receptor-driven divergent macrophage function

Macrophages play an essential role in the innate immune system by differentiating into functionally diverse subsets in order to fight infection, repair damaged tissues, and regulate inappropriate immune responses. This functional diversity stems from their ability to adapt and respond to signals in the environment, which is in part mediated through aryl hydrocarbon receptor (AHR)-signaling. AHR, an environmental sensor, can be activated by various ligands, ranging from environmental contaminants to microbially derived tryptophan metabolites. This review discusses what is currently known about how AHR-signaling influences macrophage differentiation, polarization, and function. By discussing studies that are both consistent and divergent, our goal is to highlight the need for future research on the mechanisms by which AHR acts as an immunological switch in macrophages. Ultimately, understanding the contexts in which AHR-signaling promotes and/or inhibits differentiation, proinflammatory functions, and immunoregulatory functions, will help uncover functional predictions of immunotoxicity following exposure to environmental chemicals as well as better design AHR-targeted immunotherapies.

Ginsenoside F2-Mediated Intestinal Microbiota and Its Metabolite Propionic Acid Positively Impact the Gut-Skin Axis in Atopic Dermatitis Mice

Atopic dermatitis (AD) is a complex inflammatory skin disease induced by multiple factors. AD can also cause intestinal inflammation and disorders of the gut microbiota. Ginseng is a kind of edible and medicinal plant; its main active components are ginsenosides. Ginsenosides have a variety of anti-inflammatory effects and regulate the gut microbiota; however, their role in AD and the underlying mechanisms are unclear. In this study, we found that intragastric administration of ginsenoside F2 improved AD-like skin symptoms and reduced inflammatory cell infiltration, serum immunoglobulin E levels, and mRNA expression of inflammatory cytokines in AD mice. 16s rRNA sequencing analysis showed that ginsenoside F2 altered the intestinal microbiota structure and enriched the short-chain fatty acid-producing microbiota in AD mice. Metabolomic analysis revealed that ginsenoside F2 significantly increased the propionic acid (Pa) content of feces and serum in AD mice, which was positively correlated with significant enrichment of Parabacteroides goldsteinii and Lactobacillus plantarum in the intestines. Pa inhibits inflammatory responses in the gut and skin of AD mice through the G-protein-coupled receptor43/NF-κB pathway, thereby improving skin AD symptoms. These results revealed, for the first time, the mechanism by which ginsenoside F2 improves AD through the Pa (a metabolite of intestinal microbiota)-gut-skin axis.

Tumor suppressive activity of AHR in environmental arsenic-induced carcinogenesis

The aryl hydrocarbon receptor (AHR) is a highly conserved pleiotropic transcription factor that senses environmental pollutants, microbial products, and endogenous ligands. The transcriptional targets of AHR include phase I and phase II detoxification enzymes, as well as numerous signaling molecules that affect a wide spectrum of biological and biochemical processes in a manner of cellular context-dependent. In this review, we systematically assess the latest discoveries of AHR in carcinogenesis with an emphasis on its tumor suppressor-like property that represses the expression of genes in oncogenic signaling pathways. Additionally, we outline recent progress in our studies on the interaction among AHR, TGFb and NRF2 in cellular responses to arsenic and malignant transformation. Our findings indicate that AHR antagonized TGFb and NRF2, suggesting that AHR could serve as a potential tumor suppressor in arsenic-induced carcinogenesis. Notably, while AHR can exhibit both oncogenic and tumor-suppressive properties in cancer development and the generation of the cancer stem-like cells (CSCs), the tumor suppressor-like effect of AHR warrants further extensive exploration for the prevention and clinical treatment of cancers.