Feedback control of AHR signalling regulates intestinal immunity

Aryl hydrocarbon receptor (AHR): "pioneer member" of the basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family of "sensors" of foreign and endogenous signals

The basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family comprises many transcription factors, found throughout all three kingdoms of life; bHLH/PAS members "sense" innumerable intracellular and extracellular "signals" - including endogenous compounds, foreign chemicals, gas molecules, redox potential, photons (light), gravity, heat, and osmotic pressure. These signals then initiate downstream signaling pathways involved in responding to that signal. The term "PAS", abbreviation for "per-Arnt-sim" was first coined in 1991. Although the mouse Arnt gene was not identified until 1991, evidence of its co-transcriptional binding partner, aryl hydrocarbon receptor (AHR), was first reported in 1974 as a "sensor" of foreign chemicals, up-regulating cytochrome P450 family 1 (CYP1) and other enzyme activities that usually metabolize the signaling chemical. Within a few years, AHR was proposed also to participate in inflammation. The mouse [Ah] locus was shown (1973-1989) to be relevant to chemical carcinogenesis, mutagenesis, toxicity and teratogenesis, the mouse Ahr gene was cloned in 1992, and the first Ahr(-/-) knockout mouse line was reported in 1995. After thousands of studies from the early 1970s to present day, we now realize that AHR participates in dozens of signaling pathways involved in critical-life processes, affecting virtually every organ and cell-type in the animal, including many invertebrates.

Structural Basis for Aryl Hydrocarbon Receptor-Mediated Gene Activation

The aryl hydrocarbon receptor (AHR) and the AHR nuclear translocator (ARNT) constitute a heterodimeric basic helix-loop-helix-Per-ARNT-Sim (bHLH-PAS) domain containing transcription factor with central functions in development and cellular homeostasis. AHR is activated by xenobiotics, notably dioxin, as well as by exogenous and endogenous metabolites. Modulation of AHR activity holds promise for the treatment of diseases featuring altered cellular homeostasis, such as cancer or autoimmune disorders. Here, we present the crystal structure of a heterodimeric AHR:ARNT complex containing the PAS A and bHLH domain bound to its target DNA. The structure provides insights into the DNA binding mode of AHR and elucidates how stable dimerization of AHR:ARNT is achieved through sophisticated domain interplay via three specific interfaces. Using mutational analyses, we prove the relevance of the observed interfaces for AHR-mediated gene activation. Thus, our work establishes the structural basis of AHR assembly and DNA interaction and provides a template for targeted drug design.

T-cell expression of AhR inhibits the maintenance of pTreg cells in the gastrointestinal tract in acute GVHD

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that affects the function and development of immune cells. Here, we show that recipient mice receiving AhR^-/- T cells have improved survival and decreased acute graft-versus-host disease (aGVHD) in 2 different murine allogeneic bone marrow transplant (BMT) models. We also show that CD4⁺ T cells lacking AhR demonstrate reduced accumulation in secondary lymphoid tissue because of low levels of proliferation 4 days after BMT. Additionally, we found a significant increase in the quantity of peripherally induced regulatory donor T (pT_reg) cells in the colon of recipients transplanted with AhR^-/- T cells 14 days after transplant. Blockade of AhR using a clinically available AhR antagonist greatly enhanced the in vitro generation of inducible T_reg (iT_reg) cells from naïve CD4⁺ human T cells. We have identified AhR as a novel target on donor T cells that is critical to the pathogenesis of aGVHD.

Meat Intake and the Dose of Vitamin B3 - Nicotinamide: Cause of the Causes of Disease Transitions, Health Divides, and Health Futures?

Meat and vitamin B3 - nicotinamide - intake was high during hunter-gatherer times. Intake then fell and variances increased during and after the Neolithic agricultural revolution. Health, height, and IQ deteriorated. Low dietary doses are buffered by 'welcoming' gut symbionts and tuberculosis that can supply nicotinamide, but this co-evolved homeostatic metagenomic strategy risks dysbioses and impaired resistance to pathogens. Vitamin B3 deficiency may now be common among the poor billions on a low-meat diet. Disease transitions to non-communicable inflammatory disorders (but longer lives) may be driven by positive 'meat transitions'. High doses of nicotinamide lead to reduced regulatory T cells and immune intolerance. Loss of no longer needed symbiotic 'old friends' compounds immunological over-reactivity to cause allergic and auto-immune diseases. Inhibition of nicotinamide adenine dinucleotide consumers and loss of methyl groups or production of toxins may cause cancers, metabolic toxicity, or neurodegeneration. An optimal dosage of vitamin B3 could lead to better health, but such a preventive approach needs more equitable meat distribution. Some people may require personalised doses depending on genetic make-up or, temporarily, when under stress.

The Gut, Its Microbiome, and Hypertension

PURPOSE OF THE REVIEW

Evidence is rapidly accumulating implicating gut dysbiosis in hypertension (HTN). However, we are far from understanding whether this is a cause or consequence of HTN, and how to best translate this fundamental knowledge to advance the management of HTN. This review aims to summarize recent advances in the field, illustrate the connections between the gut and hypertension, and establish that the gut microbiota (GM)-gut interaction is centrally positioned for consideration as an innovative approach for HTN therapeutics.

RECENT FINDINGS

Animal models of HTN have shown that gut pathology occurs in HTN, and provides some clues to mechanisms linking the dysbiosis, gut pathology, and HTN. Circumstantial evidence links gut dysbiosis and HTN. Gut pathology, apparent in animal HTN models, has not been fully investigated in hypertensive patients. Objective evidence and an understanding of mechanisms could have a major impact for new antihypertensive therapies and/or improved applications of current ones.

Hydroxystilbenes and methoxystilbenes activate human aryl hydrocarbon receptor and induce CYP1A genes in human hepatoma cells and human hepatocytes

Natural polyphenol resveratrol (trihydroxystilbene) is a partial agonist of human aryl hydrocarbon receptor AhR, thereby, displaying a plethora of biological effects. Biological activities of metoxylated and hydroxylated stilbenes were studied in the past. The aim of the current study was to describe the effects of 13 different hydroxy- and methoxystilbenes, including their cis/trans isomers on the transcriptional activity of AhR and the expression of CYP1A genes in hepatic cancer cells HepG2 and in primary human hepatocytes. Techniques of gene reporter assays, qRT-PCR, Simple Western blotting by Sally Sue™ and electrophoretic mobility shift assay EMSA were employed. All compounds activated AhR, but their efficacies, potencies and dose-response profiles differed substantially. The strongest activators of AhR and inducers of CYP1A1 in HepG2 cells were DMU-212 ((E)-3,4,5,4´-tetramethoxystilbene), trans-piceatannol, cis-piceatannol, trans-trismethoxyresveratrol and trans-pinostilbene. While DMU-212 and trans-trismethoxyresveratrol also induced CYP1A1 and CYP1A2 in primary human hepatocytes, the effects of trans-piceatannol, cis-piceatannol and trans-pinostilbene weaned off. On the other hand, trans-4-methoxystilbene was strong CYP1A inducer in hepatocytes but not in HepG2 cells. Differences between effects of stilbenes in HepG2 cells and human hepatocytes are probably due to the extensive phase I and phase II xenobiotic metabolism in human hepatocytes. The data obtained may be of toxicological relevance.

Papers of note in Nature (7640)

This week’s articles show how intestinal immune cell populations are controlled and how epithelial cells recognize oncogenic neighbors.