Short-chain fatty acids and their effect on dendritic cells

Short-chain fatty acids (SCFA) are metabolites produced from microbiota fermentation.  SCFAs have been subject to extensive investigation in attempts to delineate the pathways and mechanisms that underlie health outcomes from a species-host relationship.  Many have investigated the role of SCFAs in intestinal pathways or intestinal dysbiosis-driven disease (e.g. liver disease), but little is known as to how these metabolites interact with biological components of the immune system and blood stream.  Specifically, it is important to learn about this as some immune cells, such as dendritic cells (DC), patrol the blood stream to sense and respond to bacterial metabolites and present these pathogens to other immune cells in the lymph nodes.  To investigate further, a European conglomerate of researchers examined the interaction between SCFAs and DCs at the molecular level.

PCR was first performed on human-derived DCs to characterize protein expression patterns of SCFA receptors on these cells, as SCFAs are postulated to be ligands for G-coupled protein receptors (supported by the PCR).  Next, still using human-derived DCs the researchers determined that individual SCFAs were shown to have different effects, more so on mature DCs.  Butyrate and propionate in particular strongly modulated gene expression in both immature and mature human DCs.  The researchers next conducted an ingenuity pathway analysis based on differential gene expression which determined that propionate and butyrate modulate leukocyte (white blood cell) trafficking.  On top of this, SCFA significantly tempered release of an array of pro-inflammatory cytokines.  Lastly, butyrate and propionate were shown to inhibit the expression of lipopolysaccharide-induced cytokines to support a strong anti-inflammatory effect.

Together, these results suggest that metabolites derived from microbiota fermentation and metabolism and differentially modulate inflammatory response by way of dendritic cell interaction.  These findings illustrate another key component to the host-species relationship, and provide more evidence as to the scale and important of a healthy synergistic relationship between host and microbiome as the interactions are involved in disease-prone pathways that require careful molecular regulation (e.g. inflammation and immunity).  This in vitro study was a good first step in this investigation.  

Asthma could be brought on by maternal diet and lack of bacterial metabolites

Asthma has become increasingly prevalent in Western societies, and while many theories have been explored as to the reason for this rise in prevalence, many are beginning to explore connections between dietary intake and associations with the microbiome as a manifestation for this malady.  High fat, low fiber diets – which are common in the West – are associated with high rates of asthma.  Investigators in Australia sought to explore this relationship further by understanding the cellular underpinnings of these associations.  Specifically, they explored whether or not high fiber diets in mice could suppress the onset of Allergenic airway disease (ADD -i.e. asthma).  Furthermore, maternal fiber intake was also examined to see what affects would result for the progeny when challenged with asthma inducing conditions.  They published the results in Nature Communications.

Using 16S sequencing the researchers first confirmed that the high fiber diet shaped gut microbiome composition in mice.  Specifically, a significant difference was observed between control diet and no fiber diet.  Bacteroidetes were highly abundant in mice that were fed the high fiber diet, including high acetate producing Bacteroides acidifaciens strain, while Proteobacteria were found abundant in the no fiber diet.  High fiber diet mice also displayed higher levels of short-chain fatty acids, metabolic products of the gut microbiota that provide overall positive health benefits. 

Turning next to the pathology, experimenters were first able to validate that HDM did indeed induce AAD, as confirmed by inflammatory cells and signal markers found in the bronchoalveolar fluid of mice.  Indeed, mice that were on the high fiber diet did not develop AAD symptoms.  Interestingly, this was also shown in control animals who were administered HDM but were provided acetate (a short-chain fatty acid) in their drinking water. 

Mice were then bred and split into three dietary groups based on diet, a control group, high fiber group, and no fiber group.  Allergenic airway disease (AAD) was induced using a house-dust mite (HDM) model which replicates certain aspects of human asthma.  Diets were provided three weeks prior to sensitizing the animals to HDM, and AAD was evaluated after 16 days following 15-day HDM exposure.

Pregnant mice were also subjected to the three different diet regiments in the previous experiment.  The offspring were born and given a control diet, but after 6 weeks they were administered AAD.  The mice that were born from mothers on the high fiber diet did not develop AAD into adulthood, demonstrating that maternal diet can suppress AAD in adult offspring.  Interestingly, these findings were correlated with human data that demonstrated that high fiber diets in mothers’ in late-stage pregnancy was correlated to high acetate in serum samples.  Maternal acetate levels above median levels of samples taken was associated with significantly less visits to the general practitioner for wheezing complaints and/or asthmatic incidences in their children.    

Increasing numbers of studies are showing similar patterns that behaviors of the mother can affect microbiome transfer to progeny, consequently affecting the health and development of the offspring.  One of these important factors as we have seen is the diet of the mother.  As further evidence is uncovered as to the importance of high fat diets and specifically the diet of the mother, it will be important to have conversations on the best way to educate the public about this evidence as well as implement recommendations for dietary habits during pregnancy. 

Americans swap foods with Africans and their microbiomes follow – fiber, fat and cancer risk

Phuto pap and porridge, a traditional South African, high fiber, meal.

Despite having similar genetic backgrounds, African Americans are thirteen times more likely to develop colon cancer than rural South Africans.  Indeed, environmental factors, rather than genetics, are thought to be the major factor in developing colon cancer, because recent immigrants’ children’s risk is more similar to where they are living than to their parents’ homeland.  This environmental risk could be primarily caused by a number of factors, such as antibiotic use or drug use, but many scientists believe that diet, and its influence on the microbiome, is primarily responsible.  As it turns out, rural Africans eat much more fiber (almost 5x more) and much less fat (almost 3x less) than African Americans, and these differences have drastic effects on the microbiomes of their hosts.  Not only are the most abundant bacterial species different, but the major metabolites vary greatly as well.  Scientists from the University of Pittsburgh came up with the clever idea of swapping the foods of rural South Africans and African Americans, to investigate how this dietary intervention would affect each group’s microbiomes and risk for colon cancer.  They published the results of their study in Nature Communications last week.

The researchers studied 20 middle aged African American men and 20 middle aged rural South African men.  They each had their microbiomes and colons studied for two weeks while eating their normal diets, and then again for two weeks after swapping diets.  Initially, the Americans had microbiomes dominated by Bacteroides and the Africans by Prevotella.  After the diet though, they noticed a rapid shift in these populations, and it corresponded to an increase in colonic inflammation for the Africans and decrease in the Americans.  In addition, an increase in butyrate, the short chained fatty acid (SCFA) that is thought to be beneficial to health, followed the fiber diet as well, and a decrease was associated with eating the high fat diet; this makes sense, as butyrate is produced as a metabolite of fiber fermentation by the microbiome.  Interestingly, prior to the diet change a top-level analysis of all the metabolic end products of the microbiome showed that Africans produced more of every single one studied except for choline, which is related to heart disease.  Many of the metabolites studied, including choline, followed their diet switch, and were produced according to the food eaten, rather than the person eating it.  Perhaps most importantly, secondary bile acids, which are produced by the microbiome and may be carcinogenic and an important cause of colon cancer, followed the diet as well.  Africans, who produced much fewer secondary bile acids than Americans while consuming their regular diet, had a 400% increase in production after the diet switch, and vice versa for the Americans, who had a 70% decrease.

This study really illustrates the importance of diet on the output of the microbiome.  These metabolites can directly influence our health, and may be more important to our well-being than the bacteria that produce them.  According to this study, it appears that eating more veggies and less fat, something that parents have been saying for a long time, fits in with our understanding of the microbiome.  As Erica Sonnenburg said in our podcast 3 weeks ago, “Feed your microbiome at every meal!”

Gut bacteria may prevent kidney injury

Scientists have found that short chain fatty acids (SCFAs), a product of gut bacteria, may protect the kidneys from acute kidney injury (AKI), a condition with high mortality rates that can also lead to other very serious kidney diseases. AKI is often caused by something called ischemia reperfusion injury, an injury resulting from a loss of oxygen to the tissue (ischemia) and a rush of blood back to the site (reperfusion). This instigates a cascade of events resulting in several immune cell populations accumulating at the site of the injury, causing inflammation and kidney damage.

Because AKI is a result of inflammation and because SCFAs are known to have anti-inflammatory effects, scientists in Brazil hypothesized that treatment with SCFAs could ameliorate kidney function. The results published in the Journal of the American Society of Nephrology were the first to show the protective role of SCFAs in kidney ischemia reperfusion injury (IRI). They found that when the three main SCFAs (acetate, propionate, and butyrate) were administered to mice undergoing this IRI injury, they protected the kidney from undergoing AKI.  As suspected, the SCFAs prevented an autoimmune response which resulted in less inflammation and apoptosis (cell death).

Acetate was the SCFA that was most protective to the kidney, so in another experiment the scientists administered acetate-producing bacteria to the mice.  Bifidobacterium adolescentis and Bifidobacterium longum were administered separately and each did produce acetate, as evidenced by increased acetate levels in the mice's feces. They found that these mice were protected from kidney IRI and therefore the bacteria were effective. They did note, though,  that it is unlikely the bacteria colonized the gut, so further investigation is needed.  

This study provides another example of probiotics preventing conditions that may have resulted in serious injury and even death. The bacteria in this study are already used in probiotics to treat other diseases, and so repurposing them for kidney disease should be possible.  The study also describes the anti-inflammatory effects of SCFAs, which we have written extensively about in this blog.