A decrease in the levels of butyrate-producing gut bacteria has been found in patients with high cardiovascular risk. In mice, bacterial butyrate can prevent atherosclerosis by increasing gut barrier function secondary to the upregulation of tight junction genes.

A depletion in the levels of butyrate-producing gut bacteria has been found in patients with symptomatic atherosclerotic plaques and type 2 diabetes. Gut bacteria-derived metabolites may be behind the role of the gut microbiome in cardiovascular health, as the intermediate bacterial metabolite trimethylamine in the production of trimethylamine N-oxide (TMAO) promotes atherosclerosis in animal models and is associated with cardiovascular risk in humans. However, the role of butyrate-producing bacteria in the development of atherosclerosis is unclear.

A new mice study, led by Dr. Federico E. Rey from the Department of Bacteriology at the University of Wisconsin-Madison (Wisconsin, USA), shows that bacterial butyrate prevents atherosclerosis by maintaining gut barrier function.

In order to explore the role of butyrate in atherosclerosis, the researchers colonized a germ-free transgenic mouse model of atherosclerosis—apolipoprotein E-deficient mice—with defined microbial communities that had different genetic backgrounds. Eight human gut bacteria with a limited capacity to generate butyrate were tested, with or without the known butyrate producer Roseburia intestinalis.

The 16S ribosomal ribonucleic acid (rRNA) sequencing of fecal samples identified a correlation between butyrate-producing gut bacteria and small atherosclerotic plaque sizes in the aorta of mice susceptible to atherosclerosis. Among butyrate producers, Roseburia species showed the strongest negative correlation with atherosclerotic lesion size.

A diet high in plant polysaccharides and colonization with gut bacteria that have a reduced capacity for producing butyrate, together with R. intestinalis, led to a four-fold increase in butyrate levels in the cecum and reduced atherosclerotic plaque sizes, compared with colonization without R. intestinalis. In contrast, the effects of R. intestinalis and derived butyrate on cecal short-chain fatty acid levels and atherosclerotic plaques were not observed when mice were administered a diet low in complex plant polysaccharides. These results show that diet is crucial to the athero-protective effects of R. intestinalis and may explain the opposing microbial effects on atherogenesis development of TMAO (mediated by a diet rich in eggs and red meat, containing choline, phosphatidylcholine and L-carnitine) compared with butyrate (mediated by a diet rich in fiber).

A series of experiments revealed that the mechanisms behind the athero-protective effects of butyrate secondary to the colonization with R. intestinalis include:

An increase in cecal butyrate levels and a decrease in acetate levels secondary to acetate utilization by butyrate-producing bacteria (without changes in plasma levels).

An increase in gut barrier function secondary to the upregulation of tight junction genes—such as claudin3 and claudin 4—by butyrate.

A decrease in the aortic inflammation response secondary to a lower translocation of lipopolysaccharide and other endotoxins, which decreased expression of pro-inflammatory molecules such as tumor necrosis factor-alfa and vascular cell adhesion molecule 1 near plaques in the aorta and macrophage infiltration into plaques.

The induction of T cells was not behind the anti-inflammatory and atherosclerotic protective effects of butyrate as it did not affect T regulatory cells in the aorta or in para-aortic lymph nodes. These findings are in contrast with previous research in mice showing the contribution of T regulatory cells to reducing the severity of ischemic strokes.

Regarding how butyrate maintained gut barrier function in the transgenic mouse model of atherosclerosis, Kasahara et al. found that a diet rich in dietary fiber and colonization with Roseburia was associated with an upregulation of genes enriched for fatty-acid oxidation function. Furthermore, R. intestinalis affected histone post-translational modification responses in the colon but not in the aorta, suggesting that the effects of butyrate-producing bacteria on atherosclerosis could be mediated by epigenetic changes restricted to the colon.

The anti-atherosclerotic properties of butyrate were also confirmed when apolipoprotein E-deficient mice colonized with non-butyrate producers were provided with tributyrin—a triglyceride analogue of butyrate that is metabolized to butyrate by pancreatic lipases—through their diet. As observed with R. intestinalis, tributyrin supplementation inhibited the development of atherosclerosis, lipid deposition and macrophage accumulation in the plaque, without affecting blood lipids and T cells.

Altogether, these experimental findings suggest causal evidence linking colonization with butyrate-producing gut bacteria and small atherosclerotic plaques, with butyrate as the direct mediator. As a result, butyrate-producing gut microbes might be relevant mediators involved in explaining the benefits of dietary fiber for better cardiometabolic health. It should be kept in mind that butyrate protective effects are dictated by diet, which highlights the fact that commensal gut microbes are neither good nor bad per se, and that context (diet and epigenetic changes in this study) also matters.

Reference:

Kasahara K, Krautkramer KA, Org E, et al. Interactions between Roseburia intestinalis and diet modulate atherogenesis in a murine model. Nat Microbiol. 2018; 3(12):1461-71. doi: 10.1038/s41564-018-0272-x.