Butyrate-producing human gut symbiont, Clostridium butyricum, and its role in health and disease

Abstract

Clostridium butyricum is a butyrate-producing human gut symbiont that has been safely used as a probiotic for decades. C. butyricum strains have been investigated for potential protective or ameliorative effects in a wide range of human diseases, including gut-acquired infection, intestinal injury, irritable bowel syndrome, inflammatory bowel disease, neurodegenerative disease, metabolic disease, and colorectal cancer. In this review we summarize the studies on C. butyricum supplementation with special attention to proposed mechanisms for the associated health benefits and the supporting experimental evidence. These mechanisms center on molecular signals (especially butyrate) as well as immunological signals in the digestive system that cascade well beyond the gut to the liver, adipose tissue, brain, and more. The safety of probiotic C. butyricum strains appears well-established. We identify areas where additional human randomized controlled trials would provide valuable further data related to the strains' utility as an intervention.

Keywords: Clostridium butyricum; butyrate; cancer; immunity; inflammation; intestinal barrier; irritable bowel syndrome; metabolic disease; neurodegeneration; short chain fatty acid.

Figure 1.

The intestinal barrier separates the intestinal lumen (where microorganisms reside) from host tissues. The gut barrier is composed of three layers: the outermost mucus layer acts as a physical barrier against invading microorganisms; AMPs and antibodies (e.g. IgA) form a chemical barrier. Subsequent to the mucosal layer is the epithelium: a monolayer of epithelial cells mechanically linked by TJ proteins (e.g. claudin, occludin, JAMS, ZO-1). TJs regulate paracellular transport, selectively allowing the passage of small molecules. The epithelium also harbors the mucus-secreting goblet cells (not shown), hormone-secreting enteroendocrine cells (not shown), and several other immune cell types (e.g. intraepithelial lymphocytes). Finally, the lamina propria is a layer of connective tissue beneath the epithelium. It contains the majority of immune cells: innate immune cells such as macrophages, dendritic cells, and mast cells, and adaptive immune cells, such as T cells and antibody-producing plasma B cells. The epithelial and immune cells produce cytokines, or signaling molecules, to communicate across the layers of the intestinal barrier. (Figure adapted from König et al. 2016)

Figure 2.

C. butyricum strains improve gut barrier function by increasing the thickness of the mucosal layer and increasing expression of TJ proteins. Increased expression of TJ proteins may also result from C. butyricum’s stimulation of IL-17 production from intraepithelial T cells. This improved barrier results in decreased permeation of LPS and pathogenic bacteria into host tissue and blood

Figure 3.

C. butyricum strains modulate the immune system to present a tolerant and anti-inflammatory signature. Possibly via butyrate signaling and/or an LTA-activated TLR2-dependent pathway, C. butyricum stimulates TGF-β and IL-10 secretion from APCs (dendritic cells and macrophages), Tregs and intestinal epithelial cells. Increased levels of butyrate, TGF-β, and IL-10 contribute to Treg differentiation. An increased population of Tregs and IL-10 inhibit differentiation of Th17 cells that induce IL-17-mediated inflammation, and Th2 cells that induce an allergic response from eosinophils, mast cells, and plasma B cells. Dotted lines indicate indirect effect

Figure 4.

Signal transductions of C. butyricum-activated Akt pathways and their impacts in neurodegenerative disorders and metabolic diseases. A) Treatment of various neurodegenerative animal models with C. butyricum activates the Akt pathway in the brain via increasing BDNF and GLP-1. BDNF binding to its receptor, TrkB, or GLP-1 binding to GLP-1 R in the brain activates Akt via phosphorylation. Downstream of phosphorylated Akt, elevation of Bcl2 and downregulation of Bax in the mitochondria leads to inhibition of caspase-3-mediated apoptosis. As a result, subjects treated with C. butyricum are protected from neuronal death and damage. B) C. butyricum treatment of diabetic models, presumably via increased GLP-1 signaling and insulin sensitivity, results in increased phosphorylation of IRS-1 and the downstream activation of Akt in metabolic organs such as the liver, adipose tissue, and skeletal muscle. Activation of Akt pathway suppresses gluconeogenesis by increasing expression of G6Pase and PCK1, and induces glucose uptake by upregulation of GLUT4 and UCP1. Therefore, C. butyricum treatment of diabetic individuals may be effective in improvement of insulin sensitivity and glucose homeostasis via activation of Akt pathway

Figure 5.

The proposed beneficial effects of C. butyricum strains in the gut, pancreas, liver, and adipose tissue of individuals with metabolic disease. C. butyricum increases the concentration of SCFA as well as the abundance of butyrate-producing bacteria in the gut. The increased secretion of GLP-1 in the colon and the bloodstream can be attributed to the increased SCFA receptor signaling via GPR41 and GPR43. GLP-1 has pleiotropic effects across many organs in reducing appetite, slowing gastrointestinal motility, decreasing gluconeogenesis and increasing glucose uptake, and most importantly, increasing secretion of insulin from the pancreas. As such, in the liver C. butyricum increases insulin signaling (pIRS-1, pAkt, PPAR-γ, GLUT4) and decreases gluconeogenesis (G6Pase and PCK1). Serum markers of hepatic lipid storage such as triglyceride, LDL, and TC are decreased, indicating improved lipid metabolism. Concurrently, the markers of hepatic damage (ALT, AST and ALP) are decreased as well. In the adipose tissue, C. butyricum upregulates genes involved in mitochondrial fatty acid oxidation, indicating enhanced fatty acid metabolism: PPAR-γ, CPT1α, and NRF2. Finally, pro-inflammatory cytokines TNF-α, IL-1β, and MCP1 are decreased in the adipose tissue. Such anti-inflammatory effect may be associated with the direct effect of C. butyricum-produced metabolites such as butyrate, indirect effects of butyrate such as increased GLP-1 signaling and insulin secretion, and/or the decreased gut inflammation due to healthier gut epithelium evidenced by increased tight junction proteins claudin-1 and occludin and increased levels of serum LPS. Additionally, the population of Tregs and the level of anti-inflammatory cytokine IL-10 are increased due to C. butyricum, contributing to the reduced inflammation in the other organs, allowing proper insulin secretion and energy metabolism

Figure 6.

Proposed mechanism of action of beneficial C. butyricum strains across animal models of A) colorectal cancer (CRC) and B) bladder cancer. A) C. butyricum increases expression of SCFA receptors GPR43 and GPR109A in colonic and intestinal epithfiguelial cells. Activation of GPR43 eventually leads to an increased expression of p21^WAF and cell cycle arrest in cancer cells. Again likely via activation of GPR43 and GPR109A, C. butyricum triggers a decrease in anti-apoptotic proteins Bcl-2, and an increase in pro-apoptotic protein Bax, resulting in the apoptosis of cancer cells. Moreover, C. butyricum inhibits NF-κB signaling (TLR4-MyD88-NF-κB) and decreases certain proinflammatory factors (IL-22, IL-6, TNF-ɑ), potentially leading to a decrease in inflammation-associated carcinogenesis. Finally, C. butyricum may also act to decrease CRC development by inhibiting the Wnt signaling pathway (β-catenin, cyclinD1, c-Myc, Survivin etc.). Cross-talk among these pathways is likely, but not yet fully elucidated. B) C. butyricum treatment of PMNs stimulates their release of TRAIL, a cytokine that specifically induces apoptosis in tumor cells