Modulation of gut microbiota contributes to curcumin-mediated attenuation of hepatic steatosis in rats

Curcumin attenuates collagen-induced inflammatory response through the "gut-brain axis"

BACKGROUND

Previous studies have demonstrated that oral administration of curcumin exhibited an anti-arthritic effect despite its poor bioavailability. The present study aimed to explore whether the gut-brain axis is involved in the therapeutic effect of curcumin.

METHODS

The collagen-induced arthritis (CIA) rat model was induced by immunization with an emulsion of collagen II and complete Freund's adjuvant. Sympathetic and parasympathetic tones were measured by electrocardiographic recordings. Unilateral cervical vagotomy (VGX) was performed before the induction of CIA. The ChAT, AChE activities, and serum cytokine levels were determined by ELISA. The expression of the high-affinity choline transporter 1 (CHT1), ChAT, and vesicular acetylcholine transporter (VAChT) were determined by real-time PCR and immunohistochemical staining. The neuronal excitability of the vagus nerve was determined by whole-cell patch clamp recording.

RESULTS

Oral administration of curcumin restored the imbalance between the sympathetic and parasympathetic tones in CIA rats and increased ChAT activity and expression of ChAT and VAChT in the gut, brain, and synovium. Additionally, VGX eliminated the effects of curcumin on arthritis and ACh biosynthesis and transport. Electrophysiological data showed that curcumin markedly increased neuronal excitability of the vagus nerve. Furthermore, selective α7 nAChR antagonists abolished the effects of curcumin on CIA.

CONCLUSIONS

Our results demonstrate that curcumin attenuates CIA through the "gut-brain axis" by modulating the function of the cholinergic system. These findings provide a novel approach for mechanistic studies of anti-arthritic compounds with low oral absorption and bioavailability.

The Problem of Curcumin and Its Bioavailability: Could Its Gastrointestinal Influence Contribute to Its Overall Health-Enhancing Effects?

Curcumin, from the spice turmeric, exhibits anti-inflammatory, antioxidant, anticancer, antiviral, and neurotrophic activity and therefore holds promise as a therapeutic agent to prevent and treat several disorders. However, a major barrier to curcumin's clinical efficacy is its poor bioavailability. Efforts have therefore been dedicated to developing curcumin formulations with greater bioavailability and systemic tissue distribution. However, it is proposed in this review that curcumin's potential as a therapeutic agent may not solely rely on its bioavailability, but rather its medicinal benefits may also arise from its positive influence on gastrointestinal health and function. In this review, in vitro, animal, and human studies investigating the effects of curcumin on intestinal microbiota, intestinal permeability, gut inflammation and oxidative stress, anaphylactic response, and bacterial, parasitic, and fungal infections are summarized. It is argued that positive changes in these areas can have wide-ranging influences on both intestinal and extraintestinal diseases, and therefore presents as a possible mechanism behind curcumin's therapeutic efficacy.

NAFLD, Helicobacter species and the intestinal microbiome

Non-alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease worldwide. It is well-accepted that gut dysbiosis is associated with NAFLD, however, there is some conflicting evidence regarding the nature of these alterations. Infection with Helicobacter species, mainly H. pylori, has also been associated with increased NAFLD risk, however, some studies have failed to reproduce this finding. Further studies including large study samples and standardised procedures for microbiota analyses, H. pylori detection and NAFLD diagnostic criteria, are required. The mechanisms involving Helicobacter species and the intestinal microbiome in NAFLD pathogenesis appear to be part of the multiple-hit theory, in which increased intestinal permeability, inflammatory responses, altered choline, bile acids and carbohydrate metabolism, production of short-chain fatty acids, urea cycle and urea transport systems, altered maintenance of hepatic γδT-17 cells, insulin resistance, hormones secreted by the adipose tissue, metabolic hormones, bacterial metabolites and Helicobacter toxins, are all implicated.

New Insights into the Mechanisms of Chinese Herbal Products on Diabetes: A Focus on the "Bacteria-Mucosal Immunity-Inflammation-Diabetes" Axis

Diabetes, especially type 2, has been rapidly increasing all over the world. Although many drugs have been developed and used to treat diabetes, side effects and long-term efficacy are of great challenge. Therefore, natural health product and dietary supplements have been of increasing interest alternatively. In this regard, Chinese herbs and herbal products have been considered a rich resource of product development. Although increasing evidence has been produced from various scientific studies, the mechanisms of action are lacking. Here, we have proposed that many herbal monomers and formulae improve glucose homeostasis and diabetes through the BMID axis; B represents gut microbiota, M means mucosal immunity, I represents inflammation, and D represents diabetes. Chinese herbs have been traditionally used to treat diabetes, with minimal side and toxic effects. Here, we reviewed monomers such as berberine, ginsenoside, M. charantia extract, and curcumin and herbal formulae such as Gegen Qinlian Decoction, Danggui Liuhuang Decoction, and Huanglian Wendan Decoction. This review was intended to provide new perspectives and strategies for future diabetes research and product.

Regulative effects of curcumin spice administration on gut microbiota and its pharmacological implications

Curcumin, the major active component of turmeric (Curcuma longa), is widely used as a spice and food-coloring agent, and also exhibits multiple biological activities. However, as curcumin has poor systemic bioavailability its pharmacology remains to be elucidated. Owing to the high concentration of curcumin in the gastrointestinal tract after oral administration, we hypothesize that it may exert regulative effects on the gut microbiota. We investigated the regulative effects of oral curcumin administration on the gut microbiota of C57BL/6 mice and found that curcumin significantly affected the abundance of several representative families in gut microbial communities, including Prevotellaceae, Bacteroidaceae, and Rikenellaceae. Considering the pathogenic associations between gut microbiota and many diseases, the present findings may help us to interpret the therapeutic benefits of curcumin.