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CHAPTER THREE
The Role of Short-Chain Fatty
Acids in Health and Disease
Jian Tan, Craig McKenzie, Maria Potamitis, Alison N. Thorburn,
Charles R. Mackay
1
, Laurence Macia
1
Department of Immunology, Monash University, Clayton, Victoria, Australia
1
Corresponding authors: e-mail address: charles.mackay@monash.edu; laurence.macia@monash.edu
Contents
1. Introduction 92
1.1 The production of SCFAs 92
1.2 Transport of SCFAs 96
2. SCFA Sensing and Signal Transduction 97
2.1 HDAC inhibitors 97
2.2 G-protein-coupled receptors 99
3. Varied Functions of SCFAs 103
3.1 Anti-inflammatory and antitumorigenic roles 103
3.2 SCFAs and antimicrobial activities 106
3.3 SCFAs and gut integrity 107
4. Integrative View of the Gut Microbiota, SCFAs, and Disease 109
5. Perspective 112
References 112
Abstract
There is now an abundance of evidence to show that short-chain fatty acids (SCFAs) play
an important role in the maintenance of health and the development of disease. SCFAs
are a subset of fatty acids that are produced by the gut microbiota during the fermen-
tation of partially and nondigestible polysaccharides. The highest levels of SCFAs are
found in the proximal colon, where they are used locally by enterocytes or transported
across the gut epithelium into the bloodstream. Two major SCFA signaling mechanisms
have been identified, inhibition of histone deacetylases (HDACs) and activation of
G-protein-coupled receptors (GPCRs). Since HDACs regulate gene expression, inhibition
of HDACs has a vast array of downstream consequences. Our understanding of SCFA-
mediated inhibition of HDACs is still in its infancy. GPCRs, particularly GPR43, GPR41, and
GPR109A, have been identified as receptors for SCFAs. Studies have implicated a major
role for these GPCRs in the regulation of metabolism, inflammation, and disease. SCFAs
have been shown to alter chemotaxis and phagocytosis; induce reactive oxygen species
(ROS); change cell proliferation and function; have anti-inflammatory, antitumorigenic,
and antimicrobial effects; and alter gut integrity. These findings highlight the role of
Advances in Immunology, Volume 121 #2014 Elsevier Inc.
ISSN 0065-2776 All rights reserved.
http://dx.doi.org/10.1016/B978-0-12-800100-4.00003-9
91
SCFAs as a major player in maintenance of gut and immune homeostasis. Given the vast
effects of SCFAs, and that their levels are regulated by diet, they provide a new basis to
explain the increased prevalence of inflammatory disease in Westernized countries, as
highlighted in this chapter.
1. INTRODUCTION
There is increasing evidence implicating the gut microbiota as critical
contributors to host health and gut/immune homeostasis. This may be
achieved, at least in part, through the release of short-chain fatty acids
(SCFAs), which are the main bacterial metabolites produced following
the fermentation of dietary fiber and resistant starches by specific colonic
anaerobic bacteria. SCFAs are a subset of saturated fatty acids containing
six or less carbon molecules that include acetate, propionate, butyrate, pen-
tanoic (valeric) acid, and hexanoic (caproic) acid. Recent advances in the
study of SCFAs, especially acetate, propionate, and butyrate, have
highlighted their effects on various systems both at cellular and molecular
levels. Indeed SCFAs or their deficiency may affect the pathogenesis of a
diverse range of diseases, from allergies and asthma to cancers, autoimmune
diseases, metabolic diseases, and neurological diseases.
1.1. The production of SCFAs
SCFAs are carboxylic acids defined by the presence of an aliphatic tail of two to
six carbons. Although SCFAs can be produced naturally through host meta-
bolic pathways particularly in the liver, the major site of production is the colon
which requires the presenceof specific colonicbacteria explaining their absence
in germ-free mice (Hoverstad & Midtvedt, 1986). Acetate (C2), propionate
(C3), and butyrate (C4), beingthe major SCFA released through fermentation
of fiber and resistant starches, are mostly released in the proximal colon in very
high concentrations (70–140 mM) while their concentrations are lower in the
distal colon (20–70 mM) and in thedistal ileum (20–40 mM) (Wong, de Souza,
Kendall, Emam, & Jenkins, 2006). The molar ratio of acetate, propionate, and
butyrate production in the colon is 60:25:15, respectively (Tazoe et al., 2008),
although proportions can vary depending on factors such as diet, microbiota
composition, site of fermentation, and host genotype (Hamer et al., 2008).
Butyrate is mostly utilized by colonocytes while acetate and propionate reach
the liver through the portal vein. Propionate is subsequently metabolized by
hepatocytes while acetate either remains in the liver or is released systemically
92 Jian Tan et al.
to the peripheral venous system (Pomare, Branch, & Cummings, 1985). Thus,
only acetate is usually detectable in peripheral blood. Extensive research has
highlighted the beneficial effects of SCFAs on health, detailed below in this
chapter. Health authorities have thus established a recommended daily intake
of fiber, which according to the World Health Organization is 20 g per
1000 kcal consumed (in adults) and this quantity is reached through the daily
consumption of grains as well as 400 g per day of fresh fruits and vegetables
(www.who.int). Notably, the typical consumption of fiber in most Western
countries is much less than this (King, Mainous, & Lambourne, 2012)and
consumption of fiber is inversely related to premature death from all causes
of disease (Park, Subar, Hollenbeck, & Schatzkin, 2011)
1.1.1 Substrates for SCFA production
Indigestible saccharides are the major substrates leading to SCFA produc-
tion. Polysaccharides are subdivided into three categories: starch, starch-like,
and nonstarch polysaccharides (NSPs). Starch, such as amylose, and starch-
like polysaccharides, such as glycogen, consist of polymers of glucose linked
by alpha 1–4 and alpha 1–6 glycosidic bonds. These bonds are broken down
by salivary, pancreatic, and intestinal brush barrier enzymes and are thus
digestible by mammals. Under healthy conditions, starch and starch-like
polysaccharides are fully digested in the small intestine yielding glucose.
Polysaccharides that are undigested or partially digested in the small intestine
are able to undergo a process of fermentation by specific colonic anaerobic
bacteria leading to the release of SCFAs in addition to gases and heat. These
polysaccharides are called fermentable polysaccharides and are subclassified
as NSPs, or dietary fibers, and resistant starch (RS). Depending on their
degree of solubility, fibers are subclassified into insoluble or soluble fibers
and in both cases are found in plant cell walls. Cellulose and lignin are exam-
ples of insoluble fibers while pectin substances or gums forming a gel in
water are classified as soluble fibers. Insoluble fibers are highly fermentable
and hence generate greater quantities of SCFA in the colon while soluble
fibers have a rather low fermentability but increase fecal bulking and
decrease colonic transit time. RS can be subdivided into four types: physi-
cally trapped starch (in coarse grains), RS granules naturally rich in amylose
(i.e., raw potato flour), retrograded starch (i.e., cooked and cooled potato),
and chemically modified starch (i.e., processed foods) (Englyst, Kingman, &
Cummings, 1992). RS is considered as the most powerful butyrogenic
substrate where fermentation of RS in vitro as well as in vivo generally results
in a significant higher level of butyrate production compared to NSP
93The Role of Short-Chain Fatty Acids in Health and Disease
(Englyst et al., 1992). Oligosaccharides, defined by a short chain of mono-
saccharide units, such as galactooligosaccharides, fructooligosaccharides,
mannanoligosaccharides, and chitooligosaccharides are also substrates for
SCFAs (Pan, Chen, Wu, Tang, & Zhao, 2009). Finally, to a lesser extent,
some SCFAs such as isobutyrate and isovalerate are produced during the
catabolism of branched chain amino acids valine, leucine, and isoleucine
and intermediate of fermentation in the microbiota such as lactate or ethanol
can also be metabolized into SCFA (Macfarlane & Macfarlane, 2003).
1.1.2 Mechanism of SCFA production
The process involved in the production of SCFAs from fiber involves com-
plex enzymatic pathways that are active in an extensive number of bacterial
species. The most general pathway of SCFA production in bacteria is via the
glycolytic pathway, although certain groups of bacteria such as the
Bifidobacteria can utilize the pentose phosphate pathway to produce
the same metabolites (Cronin, Ventura, Fitzgerald, & van Sinderen, 2011;
Macfarlane & Macfarlane, 2003). Other pathways utilizing a variety of sub-
strates are also able to produce SCFAs. Radioisotope analysis by Miller and
Wolin (1996) demonstrated that a major pathway of acetate production by
bacteria was via the oxygen-sensitive Wood–Ljungdahl pathway and is reg-
arded as the most efficient pathway of acetate production (Fast &
Papoutsakis, 2012). Using similar methods they show that propionate was
generally generated by a carbon dioxide fixation pathway while butyrate
was most commonly formed by conventional acetyl-Scoenzyme
A condensation (Miller & Wolin, 1996). Other pathways, such as the
Bifidobacterium pathway (fructose-6-phosphate phosphoketolase pathway)
found in the Bifidobacterium genus are able to utilize monosaccharides in a
unique manner to ultimately generate SCFAs (Pokusaeva, Fitzgerald, & van
Sinderen, 2011). These results suggest that different species possessing spe-
cific enzymes are involved in the production of the various SCFAs. Indeed,
the Wood–Ljungdahl pathway is typically found in acetate-producing bac-
teria (known as acetogens) where the majority are of the Firmicutes phylum
(Ragsdale & Pierce, 2008). On the other hand, the major groups involved in
the production of butyrate are of the Cytophaga and Flavobacterium group
belonging to the Bacteroidetes phylum (Guilloteau et al., 2010). Specific
species of bacteria characterized by their high levels of butyrate production
include Clostridium leptum,Roseburia species, Faecalibacterium prausnitzii, and
Coprococcus species belonging to both the Firmicutes and Bacteroidetes phyla
(Guilloteau et al., 2010).
94 Jian Tan et al.
The production of SCFAs is a highly complex and dynamic process. For
example, butyrate and propionate may be degraded into the smaller two car-
bon chain acetate by sulfate- or nitrate-reducing acetogenic bacteria such as
Acetobacterium,Acetogenium,Eubacterium,andClostridium species (Westermann,
Ahring, & Mah, 1989). However, increased proportion of butyrate-
producing or -consuming species such as F. prausnitzii and Roseburia species
can reverse this process (Duncan et al., 2004). Such interactions can involve
the mutualistic production of SCFAs asdemonstrated by the cocolonization of
Bacteroides thetaiotaomicron and Eubacterium rectale where acetate produced by
B. thetaiotaomicron acted as a substrate for butyrate generation by E. rectale
(Mahowald et al., 2009).
In addition to enzymatic requirements, expression of protein transporters
is also imperative for SCFA production. For example, the presence of ATP-
binding cassette (ABC) transporters in Bifidobacterium longum is crucial for the
uptake and transport of substrates, such as fructose, required for acetate pro-
duction (Davidson & Chen, 2004; Fukuda et al., 2011). Another trans-
porter, the PEP translocation group or the phosphotransferase system
(PTS) is able to transport carbohydrates which can be subsequently metab-
olized to produce SCFAs (Postma, Lengeler, & Jacobson, 1993; Zoetendal
et al., 2012). Genomic analysis revealed that Bacteroidetes possesses more
polysaccharide-degrading enzymes but less ABC transporters and fewer
PTS than the Firmicutes (Mahowald et al., 2009) suggesting that despite
having the machinery to produce SCFAs they might not efficiently uptake
the substrate necessary for their production. However, Firmicutes may be
excellent scavenger of acetate through their ABC transporters and can
uptake acetate to produce butyrate and propionate as fermentative
by-products. It has therefore been hypothesized that the two predominant
phyla could exist in a balance whereby acetate from Bacteroidetes is used to
produce butyrate and propionate by Firmicutes (Mahowald et al., 2009).
Therefore, the complex and delicate interaction within the microbiota
may also control the proportion and levels of SCFAs in the gut lumen.
Accordingly, prebiotics (agents favoring the growth of beneficial bacteria)
or probiotic (introduction of beneficial bacteria) agents altering such balance
may modulate the production of SCFAs.
1.1.3 Manipulation of SCFA production via modulation of microbiota
Dietary changes can alter the composition of the gut microbiota in as little as
a day (Wanders, Graff, & Judd, 2012) and even minute alteration of dietary
factors such as fiber content could shape microbial communities (Donohoe
95The Role of Short-Chain Fatty Acids in Health and Disease
et al., 2011). The biggest issue presented by a Western diet typically high in
fat and digestible saccharides is that nutrients are mostly absorbed in the duo-
denum leaving very few substrates for the colonic bacteria. Consequently,
this results in dysbiosis, the impairment of microbiota composition and
increased susceptibility to inflammatory diseases such as inflammatory bowel
diseases (IBDs) or colon cancer. On the other hand, in rural areas where the
diet is closer to the Paleolithic diet comprising of fruit and vegetables
enriched in fibers and RS, the prevalence of these inflammatory diseases
is low while SCFA and presence of SCFA-producing bacteria are signifi-
cantly more elevated (De Filippo et al., 2010). These data aligns with a “diet
hypothesis” which suggests that adequate intake of fiber promotes a healthy
microbiota that significantly reduces the prevalence of inflammatory dis-
eases, notably through the release of SCFA (Macia et al., 2012;
Maslowski & Mackay, 2011). Despite intense public health efforts to pro-
mote the beneficial effects of a healthy diet in Western countries, the inci-
dence of obesity and inflammatory diseases are still increasing suggesting that
other approaches must be explored. One alternative could be to provide
food supplements such as the prebiotic inulin-type fructans, which have
been shown to promote Bifidobacteria at the expense of Roseburia species
and of Clostridium cluster XIVa in mice (Dewulf et al., 2011). The other
alternative would be to directly introduce a cocktail of beneficial bacteria
including the SCFA producer Bifidobacteria into solution, such as yogurt,
similar to how some currently available probiotics products are consumed.
One study has shown that gavage of mice with B. longum increased the pro-
duction of acetate (Xiong et al., 2004) and reduced their susceptibility to
infection. Another study showed that mice inoculated with VSL#3 (com-
mercial formula containing eight naturally occurring probiotic strains of
bacteria) showed protection against acute DSS-induced colitis (Mennigen
et al., 2009). This suggests that even if all the mechanisms behind the use
of probiotics are not fully understood, such as their rate of survival or site
of action, they remain to be a very promising therapeutic strategy.
1.2. Transport of SCFAs
As discussed, while the majority of SCFAs are generated and utilized within
the vicinity of the gut, a small proportion of propionate and acetate reaches
the liver where they can be used as substrates for the energy-producing tri-
carboxylic acid cycle and efficiently metabolized to produce glucose. A small
percent of SCFAs in the gut exists as unionized forms and can directly cross
96 Jian Tan et al.
the epithelial barrier. However, most exists in an ionized state and requires
specialized transporters for their uptake. Therefore, the passage of the major-
ity of SCFAs across the mucosa involves active transport mediated by two
main receptors: the monocarboxylate transporter 1 (MCT-1) and the
sodium-coupled monocarboxylate transporter 1 (SMCT-1) receptors. Both
MCT-1 and SMCT-1 are highly expressed on colonocytes and also along
the entire gastrointestinal tract including the small intestine and the cecum
(Iwanaga, Takebe, Kato, Karaki, & Kuwahara, 2006). Additionally, MCT-1
is also highly expressed on lymphocytes suggesting the importance of intra-
cellular SCFA uptake by these cells (Halestrap & Wilson, 2012). Addition-
ally, SMCT-1 is expressed on the kidney and thyroid gland. SMCT-1 binds
SCFAs in order of affinity butyrate >propionate >acetate (Ganapathy,
Gopal, Miyauchi, & Prasad, 2005). Unabsorbed SCFAs are excreted.
2. SCFA SENSING AND SIGNAL TRANSDUCTION
The ability of SCFAs to modulate biological responses of the host
depends on two major mechanisms. The first involves the direct inhibition
of histone deacetylases HDACs to directly regulate gene expression. Intrin-
sic HDAC inhibitor (HDACi) activity is particularly characteristic of the
SCFAs butyrate and propionate. The second mechanism for SCFA effects
is signaling through G-protein-coupled receptors (GPCRs). The major
GPCRs activated by SCFAs are GPR41, GPR43, and GPR109A.
2.1. HDAC inhibitors
Acetylation of lysine residues within histones induces gene activation by facil-
itating the access of transcription factors to promoter regions (MacDonald &
Howe, 2009). HDACs remove acetyl groups from histones (Kim & Bae,
2011); as such, inhibition of HDAC activity or expression can increase gene
transcription by increasing histone acetylation. SCFAs inhibit HDAC activity,
and may therefore alter gene expression in a wide variety of cells.
Of all the SCFAs, butyrate is considered to be the most potent inhibitor of
HDAC activity. Indeed, butyrate exhibits a stronger HDAC inhibitory activity
than propionate as demonstrated in both HeLa (Boffa, Vidali, Mann, & Allfrey,
1978) and colon cancer cell lines whereas acetate appeared to have very little or
no or effect (Hinnebusch, Meng, Wu, Archer,& Hodin, 2002; Kiefer, Beyer-
Sehlmeyer, & Pool-Zobel, 2006; Waldecker, Kautenburger, Daumann,
Busch, & Schrenk, 2008). However, this lack of effect on HDACs by acetate
may be tissue dependent, since others have shown that acetate can inhibit
97The Role of Short-Chain Fatty Acids in Health and Disease
HDACs. In one study, treatment of hepatoma tissue (Sealy & Chalkley, 1978)
with acetate, propionate, or butyrate leads to a global increase in histone acet-
ylation. In the same vein,orally administered acetate has been shown to inhibit
both HDAC2 activity and protein expression in the rodent brain (Soliman &
Rosenberger, 2011). Thus, HDAC inhibition by SCFAs depends not only on
the type of SCFA but also on which tissue or cell type they are acting.
2.1.1 Mechanism behind SCFA-mediated HDAC inhibition
While the exact mechanism behind SCFA inhibition of HDACs is not
known, SCFAs might either act directly on HDACs by entering into the
cells via transporters or indirectly through the activation of GPCRs (see
below). Transporters such as SMCT-1 could be good candidates. Indeed,
expression of SMCT-1 was required for butyrate- and propionate-induced
blockade of murine dendritic cell development, which correlated with a
global increase in HDAC inhibition and DNA acetylation (Singh et al.,
2010). Thus, the transport of SCFAs into cells via SMCT-1 may account
for the observed global inhibition of HDACs by propionate and butyrate
and the subsequent blockade of enzymatic activity. Direct inhibitory activity
of SCFAs on HDACs has been highlighted by the fact that while one buty-
rate molecule is a noncompetitive inhibitor that does not interfere with the
binding of HDACs to their substrates, two molecules of butyrate may com-
petitively occupy the hydrophobic cleft of the active site of HDACs
(Cousens, Gallwitz, & Alberts, 1979). This is similar to the action of the
well-characterized HDACi trichostatin A (TSA) (Davie, 2003).
Apart from a direct effect of SCFAs on HDACs, another interesting
hypothesis is that they may have an indirect effect through GPCRs. Indeed,
activation of GPR41 in Chinese hamster ovary cell lines suppressed histone
acetylation possibly through the inhibition of HDACs (Wu, Zhou, Hu, &
Dong, 2012). Thus, GPR41 but also GPR43 or GPR109 might contribute
to HDAC inhibition mediated by SCFAs. Whether the SCFAs directly or
indirectly block HDAC activation remains elusive and extensive research
will be necessary to clarify these points.
2.1.2 Immunological relevance of SCFA-mediated HDAC inhibition
When SCFA-mediated HDAC inhibition can be established or associated, the
overwhelming result is an anti-inflammatory immune phenotype (Table 3.1).
Indeed, treatment of human macrophages with 1 mM of acetate in vitro signif-
icantly reduced their global HDAC activity and increased global histone acet-
ylation correlating with decreased production of inflammatory cytokines IL-6,
98 Jian Tan et al.
IL-8, and TNFa(Kendrick et al., 2010). Similarly, butyrate and propionate
decreased LPS-induced TNFaproduction in vitro from human peripheral
blood mononuclear cells (PMBCs) in a similar manner to TSA (Usami et al.,
2008). These results suggest an active control of the release of proinflammatory
cytokines by SCFAs through HDAC inhibition in both rodents and humans.
Activation of NF-kB is one of the major pathways involved in the release of
inflammatory cytokines (Hayden, West, & Ghosh, 2006). Butyrate and propi-
onate were shown to reduce NF-kB activity in PBMCs in a similar manner to
TSA (Usami et al., 2008) suggesting that the anti-inflammatory effect of SCFAs
might be mediated through the modulation of NF-kB via HDAC inhibition.
However, a direct effect of these SCFAs on histone acetylation in PMBCs has
not been shown. Finally, global inhibition of HDAC activity was also observed
in rodent neutrophils after addition of acetate, propionate, or butyrate in vitro
with increasing strength, respectively (Vinolo et al., 2011). In monocytes, buty-
rate and propionate, but not acetate, decreased LPS-induced TNFaexpression
and NOS expression in rodent neutrophils (Vinolo et al., 2011). This suggests
that acetate might not mediate its anti-inflammatory effects through HDAC
inhibition but rather through GPCR activation, as we have reported
(Maslowski et al., 2009). Finally, HDAC inhibition by SCFAs is not restricted
to cells of the innate immune system. Lymphocytes, in particular regulatory
T cells (Tregs), may also be affected by HDAC inhibition. Indeed, HDACinhi-
bition, particularly HDAC9, increased expression of the forkhead box P3
(Foxp3) transcription factor in mice, which subsequently increased prolifera-
tive and functional capabilities of Tregs (Lucas et al., 2009; Tao et al., 2007).
In vitro, addition of butyrate on human Treg was shown to moderately diminish
their proliferation while increased their inhibitory capacities on T cell prolifer-
ation through a CTLA-4-mediated mechanism (Akimova et al., 2010). Fur-
thermore, effector CD4
þ
T cells could be anergized via the HDACi
activities of butyrate, which occurred independently of Treg (Fontenelle &
Gilbert, 2012). Although global HDAC activity is often associated with
SCFA-mediated immunomodulation, specific HDAC inhibition or expres-
sion is rarely investigated and provides an avenue for further research.
2.2. G-protein-coupled receptors
2.2.1 GPR43
GPR43, also known as free fatty acid receptor 2 (FFA2/FFAR2), is the pri-
mary receptor for the SCFA acetate. GPR43 recognizes an extensive range of
SCFAs including propionate, butyrate, caproate, and valerate and while pro-
pionate was reported to be the most potent activator of GPR43, acetate is the
99The Role of Short-Chain Fatty Acids in Health and Disease
Table 3.1 HDAC specific immunomodulation of the immune system
HDAC No. Immunological function References
HDAC1 •Reduces TNF-induced NF-kB-dependent reporter gene expression via direct
interaction with corepressor p65 and p50
•Repression of IL-12 expression
•Increases expression of NF-kB-independent genes
Ashburner, Westerheide, and
Baldwin (2001), Zhong, May, Jimi,
and Ghosh (2002), Viatour et al.
(2003), and Lu et al. (2005)
HDAC2 •Reduces TNF-induced NF-kB-dependent reporter gene expression inde-
pendent of interaction with p65
•Repression of major histocompatibility class II transactivator (CIITA) activity
and subsequent repression of activation in macrophages
Ashburner et al. (2001) and Kong,
Fang, Li, Fang, and Xu (2009)
HDAC3 •Repression of NF-kB signaling by sequestration of NF-kB to the cytoplasm
•Increases expression of NF-kB-independent genes
•Required for inflammatory gene expression in macrophages
•Increased HDAC3 is associated with reduced apoptotic T lymphocytes from a
reduction in p53 expression (tumor suppressor)
Chen, Fischle, Verdin, and Greene
(2001), Viatour et al. (2003), and
Zhang, Shi, Wang, and Sriram
(2011)
HDAC7 •Transcriptionally represses macrophage genes during B cell development
•Enhances Foxp3 function
•Histone deacetylation of the Foxp3 promoter
Bruna Barneda-Zahonero et al.
(2013), Li et al. (2007), and Lal and
Bromberg (2009)
HDAC8 •Induces apoptosis of T cell lymphoma dependent on phospholipase C-g1
signaling
Balasubramanian et al. (2008)
HDAC9 •Inhibits proliferation and suppressive function of Tregs and is downregulated
during TCR stimulation of Tregs
•HDAC9 knock-out mice have increased numbers of Tregs compared to WT.
Tao et al. (2007)
HDAC11 •Regulates IL-10 expression from APCs
•Increasing HDAC11 caused an increase in IL-10 and promoted the restoration
of responsiveness in tolerant CD4
þ
T cells
•Reducing HDAC11 increased IL-10 expression in APCs and impaired antigen-
specific T cell responses
Villagra et al. (2009)
most selective (Le Poul et al., 2003). GPR43 expression has been identified
along the entire gastrointestinal tract, including cells of both the immune
and nervous system. In the intestinal tract, GPR43 is highly expressed on
intestinal peptide YY (PYY) and glucagon-like peptide 1 (GLP-1)
(Tolhurst et al., 2012) producing endocrine L-cells of the ileum and colon
(Vangaveti, Shashidhar, Jarrod, Baune, & Kennedy, 2010) as well as on col-
onocytes and enterocytes of the small and large intestine. Direct infusion of
SCFAs in the colon of rats and rabbits induced the release of PYY, possibly
through their binding on GPR43, that exerted anorexigenic effects
(Roelofsen, Priebe, & Vonk, 2010) and GPR43 knock-out (Gpr43
/
) mice
have decreased SCFA-induced release of GLP-1, a key hormone controlling
insulin release (Tolhurst et al., 2012). While SCFAs might modulate body
weight via central effects by reducing food intake through secretion of
PYY and GLP-1, they can also directly act in periphery on the adipose tissue.
Indeed, high fat diet has been shown to upregulate GPR43 expression in sub-
cutaneous adipose tissue in parallel with increased fatstorage in adipocytes. On
the other hand, supplementation of the diet with inulin-type fructans, fer-
mentable carbohydrates, blunted the weight gain and the overexpression of
GPR43 due to high fat feeding, suggesting that SCFAs might modulate adi-
posity (Dewulf et al., 2011). Moreover, inhibition of GPR43 expression in
the adipocyte cell line 3T3-L1 using small interfering RNA inhibited their
differentiation suggesting a possible role of GPR43 in adipocyte development
(Dewulf et al., 2013). While RS consumption in rats leads to activation of the
hypothalamic anorexigenic pathway shown by the increased expression of
proopiomelanocortin in the arcuate nucleus, GPR43 does not seem to be
expressed in the arcuate nucleus or other region of the hypothalamus
(Sleeth, Thompson, Ford, Zac-Varghese, & Frost, 2010). More broadly, to
our knowledge, there is no report of GPR43 expression in the central or
peripheral nervous system.
In the immune system, GPR43 is expressed on eosinophils, basophils
(Le Poul et al., 2003), neutrophils, monocytes, dendritic cells (Cox et al.,
2009; Le Poul et al., 2003), and mucosal mast cells (Karaki et al., 2008)
suggesting a broad role of SCFAs in immune responses. It is highly expressed
in murine hemopoietic tissues such as the bone marrow and spleen suggesting
the potential role for GPR43 in modulating the development or differentia-
tion of immune cells (Maslowski et al., 2009; Senga et al., 2003).
Finally, a recent study has shown the expression of GPR43 in
myometrium and fetal membranes after the onset of labor and a significant
upregulation of GPR43 in preterm fetal membranes with evidence of
101The Role of Short-Chain Fatty Acids in Health and Disease
infection. This study also suggests an anti-inflammatory role of SCFAs
through GPR43 that may reduce the risk of preterm labor induced by path-
ogens (Voltolini et al., 2012). This anti-inflammatory role of GPR43 is in
accordance with our findings on the exacerbated inflammatory phenotypes
of Gpr43
/
mice in colitis and arthritis models (Maslowski et al., 2009).
2.2.2 GPR41
Identified at the same time as GPR43, GPR41, also known as free fatty acid
receptor 3 (FFA3/FFAR3), is a receptor for acetate and propionate and to a
lesser degree butyrate. Like GPR43, it also recognizes other SCFAs includ-
ing caproate and valerate, but to a lesser degree. GPR41 is expressed in the
colonic mucosa in PYY but not GPR43-expressing cells. GPR41 is also
expressed in the colonic smooth muscle and SCFAs induce phasic contrac-
tion of these muscles in a GPR41-dependent manner with the following
order of potency: propionate butyrate >acetate (Tazoe et al., 2009).
SCFAs stimulate sympathetic activation through GPR41 activation by
acting on the sympathetic ganglion. This effect is abolished under fasted
conditions by ketone bodies (Kimura et al., 2011). Based on these results,
GPR41 agonists could be used as potential antiobesity therapeutics.
Moreover, the expression of GPR41 in adipose tissue and its potency to
induce the release of the anorexigenic hormone leptin when activated by SCFAs
confirms its beneficial effects on body weight (Xiong et al., 2004). The former
findings are still controversial as Hong and colleagues did not find GPR41 expres-
sion on adipocytes and suggest that this effect on leptin release is mediated
through GPR43. Langerhans cells in the pancreas also express GPR41 but its
functional role in these cells is unknown. Finally, GPR41 is expressed in spleen
and in PBMC but its role on immune cells remains uninvestigated.
2.2.3 GPR109A
GPR109a, also known as Niacin receptor 1, is a high affinity niacin (Vitamin
B3) receptor and related to its low affinity analog GPR109B, which is only
expressed in humans. Although niacin is the primary ligand of GPR109A,
physiological concentrations of niacin do not reach a threshold required to
activate the receptor (Wanders et al., 2012). However, butyrate is a suitable
candidate ligand with the ability to bind GPR109A with low affinity in mil-
limolar concentration (Thangaraju et al., 2009). GPR109A transcript is highly
expressed in adipocytes but declines with age (Thangaraju et al., 2009). To a
lesser extent, GPR109A is also expressed on immune cells such as dermal den-
dritic cells, monocytes, macrophages, and neutrophils (Wanders et al., 2012).
102 Jian Tan et al.
Activation of GPR109A in adipocytes has been shown to suppress lipolysis
and lowering of plasma-free fatty acid levels (Kang,Kim,&Youn,2011).
The role of GPR109A in immune responses, and gut homeostasis, is yet to
be reported. A summary of the major SCFA receptors, associated ligand,
and their functions is presented in Table 3.2.
3. VARIED FUNCTIONS OF SCFAs
SCFAs, particularly butyrate, are key promoters of colonic heath and
integrity. Butyrate is the major and preferred metabolic substrate for colo-
nocytes providing at least 60–70% of their energy requirements necessary for
their proliferation and differentiation (Suzuki et al., 2008). As such, colo-
nocytes of germ-free mice, deficient in SCFAs, are highly energy deprived,
as indicated by decreased expression of key enzymes involved in fatty acid
metabolism in mitochondria (Tazoe et al., 2008). Consequently, these cells
have a marked deficit of mitochondrial respiration, as shown by a decreased
NADH/NAD
þ
ratio, in ATP production as well as of oxidative phosphor-
ylation, which can lead to autophagy. Addition of butyrate to colonocytes
isolated from germ-free mice normalized this deficit (Donohoe et al., 2011).
Apart from being a major energy source for colonocytes, SCFAs in the gut
perform various physiological functions including dictating colonic mobility,
colonic blood flow, and gastrointestinal pH, which can influence uptake and
absorption of electrolytes and nutrients (Tazoe et al., 2008). These effects
could be mediated through the activation of GPCRs as discussed earlier.
Finally, the physiological roles of SCFAs are broader than a local effect on
the gut on enterocytes and on digestive function; they indeed play major
immunological roles both systemically and locally in the gut that will be fur-
ther expanded in the following sections.
3.1. Anti-inflammatory and antitumorigenic roles
SCFAs are well known for their anti-inflammatory functions by modulating
immune cell chemotaxis, reactive oxygen species (ROS) release as well as
cytokine release. Butyrate elicits anti-inflammatory effects via inhibition
of IL-12 and upregulation of IL-10 production in human monocytes
(Saemann et al., 2000), repressing production of proinflammatory molecules
TNFa, IL-1b, nitric oxide (Ni et al., 2010), and reduction of NF-ĸB activity
(Ni et al., 2010; Segain et al., 2000). The active suppression of NF-ĸB activ-
ity was shown by all three major SCFAs in order of potency being
butyrate >propionate >acetate in Colo320DM cells (Tedelind, Westberg,
103The Role of Short-Chain Fatty Acids in Health and Disease
Table 3.2 Summary of the major short-chain fatty acids-activated GPCR including its ligand, expression, and function
GPCR Ligands Expression Roles Reference(s)
GPR41 SCFA (C2–C7)
Formate, acetate,
propionate, butyrate,
and pentanoate
Adipocytes, various immune
cells, and enteroendocrine
L cells
Leptin production,
sympathetic activation
Kimura et al. (2011) and Xiong
et al. (2004)
GPR43 SCFA (C2–C7)
Formate, acetate,
propionate, butyrate,
and pentanoate
Adipocytes, various Immune
cells, enteroendocrine L cells,
gut epithelium, fetal membrane
Anorexigenic effects via
secretion of PYY and GLP-1,
anti-inflammatory, and
antitumorigenic
Cherbut et al. (1998),
Maslowski et al. (2009), Tang,
Chen, Jiang, Robbins, and Nie
(2011), Suzuki, Yoshida, and
Hara (2008), Tazoe et al. (2008),
Le Poul et al. (2003), Cox et al.
(2009), and Voltolini et al.
(2012)
GPR109a SCFAs (C4–C8),
particularly butyrate
Nicotinate
Adipocytes, various immune
cells, intestinal epithelial cells,
upregulated in hepatocytes
during inflammation, epidermis
in squamous carcinoma
High-density lipoprotein
metabolism, cAMP reduction
in adipocytes, DC trafficking,
anti-inflammatory, and
antitumorigenic
Li, Hatch, et al. 2010, Li, Millar,
Brownell, Briand, and Rader
(2010),Bermudez et al. (2011),
Ingersoll et al. (2012),
Thangaraju et al. (2009), and
Wanders et al. (2012)
Kjerrulf, & Vidal, 2007). Suppression of NF-ĸB activity and also TNFapro-
duction by SCFAs is also commonly observed in LPS-activated PMBCs
such as neutrophils (Aoyama, Kotani, & Usami, 2010). This is consistent
with the findings that butyrate could inhibit high mobility group box-1
(Aoyama et al., 2010), a nuclear transcription factor downstream of
NF-ĸB signaling involved in eliciting inflammatory roles and promoting cel-
lular proliferation that could promote cancer (Tang, Kang, Zeh Iii, & Lotze,
2010). Furthermore, butyrate (and also propionate) could induce apoptosis
of neutrophils in nonactivated and LPS- or TNFa-activated neutrophil apo-
ptosis by caspase-8 and caspase-9 pathways (Aoyama et al., 2010).
Under inflammatory conditions, addition of acetate has been shown to
inhibit human neutrophil migration toward C5a or fMLP in a GPR43-
dependent manner as phenylacetamide, a human GPR43 agonist mimicked
these effects (Vinolo et al., 2011). In vivo, migration of neutrophils toward the
peritoneum was exacerbated in Gpr43
/
mice when mice were challenged
with C5a or fMLP, confirming the critical role of GPR43 as regulator of cell
chemotaxis. It is, however, puzzling that under noninflammatory conditions,
SCFAs attract both mouse and human neutrophils through a mechanism
involving GPR43 activation (Le Poul et al., 2003; Maslowski et al., 2009;
Vinolo et al., 2009). This illustrates the dual effects of SCFAs on chemotaxis
and the phenomenon that SCFAs might attract inflammatory cells under basal
conditions requires further investigation. SCFAs can enforce the epithelial
barrier by affecting the mucus layer, epithelial cell survival, as well as tight
junction proteins, and will be discussed in a later section of this chapter. SCFAs
might enforce this epithelial barrier by increasing the infiltration of immune
cells in the lamina propria. Themost common immune mechanism known to
induce content leakage from the gut is through the release of neutralizing IgA;
however, the increase in phagocytes in the lamina propria might also be an
important unexplored mechanism. Other than suppressing neutrophil func-
tions, butyrate (and to a degree acetate and propionate) can inhibit IL-2 pro-
duction and lymphocyte proliferation in culture (Cavaglieri et al., 2003).
SCFAs not only modulate cell migration but also their activity. As dis-
cussed earlier SCFAs are potent anti-inflammatory mediators, by inhibiting
the release of proinflammatory cytokines from macrophages and neutro-
phils. Acetate was shown to promote the release of ROS when added on
mouse neutrophils by activating GPR43 (Maslowski et al., 2009). ROS
are efficient bactericidal factors involved in the clearance of pathogens.
Thus, SCFAs might be key regulators of inflammatory diseases by tightly
controlling the migration of immune cells toward inflammatory sites as well
105The Role of Short-Chain Fatty Acids in Health and Disease
as modulating their activation state, enabling accelerated pathogen clearance
through ROS activation. As discussed earlier, all these processes would
decrease host injury, which would not only allow for the survival of the host
but also for survival of the SCFA-producing bacteria.
Butyrate has been associated with anticancer activity on a variety of human
cancer cell lines. Treatment of human hepatoma cells in vitro increased expres-
sion of cell cycle inhibitory genes and appeared to reverse malignant pheno-
type, which has been associated with a reduction in telomerase activity via
HDAC inhibition (Nakamura et al., 2001; Wakabayashi et al., 2005). Telo-
merase activity can maintain cancer cellproliferation, therebyproviding a pos-
sible target for butyrate-induced antitumor effects. Furthermore, activation of
GPR109a on human colon cancer cells by butyrate has been associated with
increased apoptosis independent of HDAC inhibition and increased expres-
sion of the butyrate transporter MCT-1 (Borthakur et al., 2012; Thangaraju
et al., 2009). Butyrate-induced GPR109a activation may directly inhibit
colon cancer growth by inducing apoptosis or may act indirectly via increased
MCT-1 expression and subsequent increase of butyrate transport into the cell.
Expression of the butyrate transporter SMCT-1 on colon cancer cells is essen-
tial for its antitumorigenic function and correlates with global increases to his-
tone acetylation (Gupta, Martin, Prasad, & Ganapathy, 2006). In addition,
SMCT-1 is downregulated in human colon cancer cells, further accentuating
the role of SMCT-1 in colon cancer (Miyauchi, Gopal, Fei, & Ganapathy,
2004). SMCT-1 may therefore transport butyrate into colonic cells and pre-
vent development of a cancerous phenotype, though the involvement of
HDAC inhibition remains largely unknown. Even if the mechanisms behind
the beneficial role of SCFAs on cancer are not fully understood, it is widely
accepted that intake of fiber lowers risk of cancer, especially colorectal cancer.
The analysis of 25 studies demonstrated that cereals and whole grain intake
was associated with reduced risk of colorectal cancer supporting the potential
beneficial role of SCFAs in cancer (Aune et al., 2011).
3.2. SCFAs and antimicrobial activities
Free fatty acids (such as medium- and short-chain fatty acids) exhibit intrin-
sic broad-spectrum antimicrobial activity and are used as such in the agricul-
ture industry. For example, propionate is routinely used as an antimicrobial
additive in food (Arora, Sharma, & Frost, 2011) while in vivo administration
of butyrate is used to control Salmonella infections (Fernandez-Rubio et al.,
2009). Several key mechanisms were attributed to the antimicrobial
106 Jian Tan et al.
activities of free fatty acids including disruption of osmotic and pH balance,
nutrient uptake, and energy generation and their working concentrations
were well below the toxicity threshold to host cells (Dewulf et al., 2011).
This was shown in a study by Hong et al. (2005) demonstrating that formic
acid, acetate, propionate, butyrate, and hexanoic acid exerted various bio-
cidal (lethal) or biostatic (growth inhibitory) effects on oral microorganisms
at concentrations as low as micromolar. Propionate and hexanoic acid can
also exert antimicrobial activities by promoting host antimicrobial peptide
expression (Alva-Murillo, Ochoa-Zarzosa, & Lopez-Meza, 2012). Simi-
larly, host defense peptides of the innate immune system were potently
induced by oral treatment of butyrate and were responsible for the clearance
of Salmonella enteritidis without triggering a proinflammatory response indi-
cated by a lack of IL-1bproduction (Sunkara, Jiang, & Zhang, 2012). In
humans, the activity of cathelicidin, an antimicrobial agent released by poly-
morphonuclear leukocytes was induced by butyrate, possibly via its HDAC
inhibitory activities (Kida, Shimizu, & Kuwano, 2006). A recent study has
shown that the antimicrobial activities of individual SCFAs were relatively
inert toward species of bacteria that produced them but were otherwise
potent toward other microorganisms (Alva-Murillo et al., 2012). Therefore,
the production of SCFAs themselves may play a significant role in the shap-
ing of the gut microbial ecology; however, the precise effects of SCFAs on
bacterial selection require further investigation.
3.3. SCFAs and gut integrity
Gut integrity is an essential factor in maintaining mucosal homeostasis. It is
ensured by an efficient separation between the gut luminal contents and the
host, which is partly due to an effective epithelial barrier. Disruption of gut
integrity has been attributed to various intestinal diseases such as inflamma-
tory bowel disease, celiac diseases, irritable bowel syndrome (Voltolini et al.,
2012), and colorectal cancer (Tolhurst et al., 2012). It is interesting to note
that alteration of gut integrity seems to have much broader health implica-
tions than locally in the gut. Indeed, a phenomenon called “leaky gut,” char-
acterized by increased gut permeability, is associated with diseases such as
asthma or type 1 diabetes (T1D) showing that an effective physical separa-
tion of host tissues from the gut microbiota is critical for general health.
A layer of mucus forms a physical barrier that separates the epithelium
from the luminal environment, and this contributes to gut integrity by lim-
iting physical access of bacteria to the epithelium, thus limiting prospects for
107The Role of Short-Chain Fatty Acids in Health and Disease
breach and inflammation (Tolhurst et al., 2012). Mucus is comprised of
secretory (MUC2, MUC5A/B, MUC6) and epithelial membrane-bound
(MUC1, MUC3A/B, MUC4, MUC12, MUC13, MUC15, MUC16,
and MUC17) mucin glycoproteins (Cherbut et al., 1998; Tolhurst et al.,
2012). Deficiencies in mucins exacerbate various intestinal diseases such
as mucositis but can be remediated via oral supplementation of butyrate,
which modulates gut permeability (Ferreira et al., 2012). Consistent with
this, supplementation of either butyrate or propionate could induce both
MUC2 mRNA expression and MUC2 secretion in human goblet-like cell
line LS174T (Burger-van Paassen et al., 2009) suggesting that SCFAs might
be critical bacterial products promoting gut integrity. However, whether the
mechanisms behind these effects are through HDAC inhibition or via the
stimulation of GPR41, GPR43, or GPR109 has not been elucidated.
Functional tight junction proteins, such as ZO-1 and occludin between
epithelial cells, are also required for maintaining gut integrity by limiting gut
permeability (Balda & Matter, 2008). As mentioned earlier, increased gut
permeability is a common feature in diseases such as food allergy and asthma
(Hijazi et al., 2004; Perrier & Corthesy, 2011), however, whether it is the
cause or the consequence of these diseases remains largely unresolved.
In vitro, butyrate supplementation to Caco-2 cell monolayers enhances
the transepithelial resistance (TER), which is a marker of gut integrity, by
accelerating the assembly of tight junction proteins ZO-1 and occludin
dependent on AMPK activation without altering their expression levels
(Tolhurst et al., 2012). In vivo, mice treated with B. longum, a probiotic strain
of bacteria that releases large amounts of acetate, decreased the translocation
of Shiga toxin from enterohemorrhagic Escherichia coli O157:H7 toward the
bloodstream and thus increased survival (Xiong et al., 2004). In vitro, this
study shows that while acetate per se did not affect the TER of Caco-2 cells,
it did increase their survival when they were coinfected with this pathogen
resulting in increased gut integrity.
Finally, it has been shown in numerous studies that obesity or inflamma-
tory bowel disease, that dysbiosis is associated with increased gut permeabil-
ity. These conditions are probably associated with much lower concentrations
of SCFAs in both the GI tract and the blood. Apart from acting on the epi-
thelial layer, SCFAs might promote gut integrity by maintaining symbiosis.
Indeed, by lowering the luminal pH, SCFAs can directly promote the growth
of symbionts, and on the other hand inhibit growth of pathobionts (Roy,
Kien, Bouthillier, & Levy, 2006). However, some opportunistic pathobionts
have evolved to take advantage of the presence of SCFAs. Indeed it has been
108 Jian Tan et al.
shown that butyrate promotes virulence gene factor expression in pathogenic
E. coli and thus, colonize the colon where levels of butyrate are the highest
(Nakanishietal.,2009). Furthermore, SCFAs (particularly butyrate) could
also induce the production of flagella and regulate its motility function in
enterohemorrhagic E. coli (Herold, Paton, Srimanote, & Paton, 2009;
Tobe, Nakanishi, & Sugimoto, 2011).
From an evolutionary point of view, it is not surprising that beneficial
bacteria protect the host, notably by maintaining gut homeostasis to ensure
their own survival. Our view is that vertebrates have evolved systems that
allow bacterial metabolites such as SCFAs to regulate immunity and gut
physiology. Expression of GPR43 on innate/inflammatory immune cells
and the gut epithelium is an excellent example of this relationship. In
Western countries where consumption of dietary fiber is low, boosting
the levels of SCFAs appears as a promising new approach to promote gut
integrity and homeostasis. SCFAs or HDAC/GPR43 agonists might find
uses to treat or prevent a broad range of diseases from cancers to allergies
and autoimmune diseases.
4. INTEGRATIVE VIEW OF THE GUT MICROBIOTA,
SCFAs, AND DISEASE
The incidence of both inflammatory and autoimmune diseases has
increased dramatically in Westernized countries over the past several
decades. While both genetic and environmental factors influence the induc-
tion of such diseases, the contribution of diet and the relevance of SCFAs
have only been appreciated recently. The effect of SCFAs on various inflam-
matory and autoimmune diseases will be discussed below.
IBDs such as Crohn’s disease (CD) and ulcerative colitis (UC) are char-
acterized by inflammation of the gastrointestinal tract and colonic mucosa.
The induction of IBDs is multifactorial with genetic, environmental, and
microbial components. The increased incidence of IBD in developed coun-
tries over the last 20 years is too rapid to be explained by genetic changes.
However, what has dramatically changed over the last 20 years is the life-
style, particularly the introduction of Western style diets, which are gener-
ally low in fiber, and rich in fat and digestible sugars. Thus, “Western” diets
could be driving this increase of IBD in Western countries (Shapira,
Agmon-Levin, & Shoenfeld, 2010).
As mentioned previously, changes in diet can lead to rapid changes in the
composition of gut microbiota, which in turn could influence the relative
109The Role of Short-Chain Fatty Acids in Health and Disease
amounts of the different SCFAs produced. Observations in both mice and
humans support the link between diet, SCFA production via the gut micro-
biota, and IBDs. Indeed, metagenomic analyses of fecal bacteria have shown
significant dysbiosis in patients suffering from CD or UC, where there is a
lower representation of Bacteroidetes and Firmicutes, typical commensal
bacterial species, especially Clostridial clusters IV (C. leptum subgroup)
and XIVa (Clostridium coccoides subgroup) compared to healthy individuals
(Frank et al., 2007). Whether this dysbiosis is causative or a consequence
of IBD is unknown, however, targeting the microbiota through antibiotic
treatments has shown promising results by decreasing bacterial infiltration to
tissues. Combined treatment with probiotics and prebiotics also appears
beneficial in IBD, however, the use of anti-, pro-, and prebiotics as treat-
ments for IBD is yet to be fully established (Sartor, 2004). An emerging
and promising therapeutic approach is fecal transplantation, which has been
highly successful in some Clostridium difficile-infected patients (Borody,
Brandt, Paramsothy, & Agrawal, 2013; Brandt, 2012), as well as some
UC patients (Damman, Miller, Surawicz, & Zisman, 2012).
In mouse models, the role of the microbiota in the development of
DSS-induced colitis, a mouse model of ulcerative colitis, has been demon-
strated. While under SPF conditions, IL-10-deficient mice developed exac-
erbated colitis, whereas they were protected under germ-free conditions
(Sellon et al., 1998). These results suggest that IL-10-deficient mice have
a colitogenic microbiota. Although it has not been shown in humans, we
can speculate that patients with IBD may also house a colitogenic microbiota
that if transmitted from mother to child at birth may confer susceptibility to
CD (Akolkar et al., 1997).
Interestingly, in parallel with the dysbiosis, two studies have shown that
IBD was correlated with lower levels of SCFAs in feces by nuclear magnetic
resonance spectroscopy (Marchesi et al., 2007) and by HPLC with acetate
(162.0 mM/g), propionate (65.6 mM/g), and butyrate (86.9 mM/g) in the
feces of IBD patients compared to healthy individuals (209.7, 93.3, and
176.0 mM/g, respectively) (Huda-Faujan et al., 2010). Given these differ-
ences, SCFAs may play an important role in the pathogenesis of IBD. How-
ever, the stage of these diseases at which SCFAs are lowered, before the first
signs of inflammation, early signs, or once the diseases are clearly established,
remains unknown. Sabatino et al. (2005) explored the therapeutic effect of
administering butyrate orally to patients with CD. Administration of 4 g of
butyrate per day for 8 weeks via an enteric-coated tablet induced clinical
improvement and remission in 53% of patients where butyrate successfully
110 Jian Tan et al.
downregulated mucosal levels of NF-kB and IL-1b. Mouse studies have also
shown that SCFAs were beneficial in colitis as mice treated with butyrate
had reduced inflammation in their colonic mucosa with reduced neutrophil
infiltration (Vieira et al., 2012) and treatment with acetate had similar ben-
eficial effects (Maslowski et al., 2009). Moreover, lack of SCFA signaling
through GPR43 in Gpr43
/
mice exacerbated the development of colitis
(Maslowski et al., 2009). Thus, normalizing levels of SCFAs as well as
remediating dysbiosis may have synergistic and beneficial effects in the treat-
ment of IBD.
The beneficial anti-inflammatory effects of SCFAs extend beyond the
gut. Indeed, Brown et al. (2011) completed a metagenomic analysis of the
gut microbiome of T1D matched case–control subjects. 16S rRNA
sequencing revealed a larger proportion of bacterial species producing
butyrate in controls compared to individuals suffering from T1D. This
confirms the notion that in healthy individuals, the presence of
butyrate-producing bacteria might maintain gut integrity, while in T1D
patients, nonbutyrate-producing bacteria impede the synthesis of mucin,
which could lead to increased gut permeability. In rats, oral treatment with
butyrate during the preweaning period tended to delay the development of
diabetes (Li,Hatch,etal.,2010) suggesting that butyrate might play a role. In
this study, only one dose of butyrate was investigated, thus alternative dosing
strategies and perhaps in combination with other SCFAs such as acetate
would be necessary to draw firmer conclusions about the effects of SCFAs
on diabetes development. Moreover, analysis of fecal microbiota revealed
that Myd88
/
NOD mice, which are protected from diabetes development
under SPF conditions, had an increase in Bacteroidetes species when housed
under SPF conditions (Wen et al., 2008). Bacteroidetes produce large
amounts of SCFAs, thus protection from T1D in these Myd88
/
NOD
mice under SPF conditions could be via the anti-inflammatory effects pro-
vided by SCFAs. Similarly, fecal microbiota of patients suffering from rheu-
matoid arthritis (RA), another autoimmune disease, revealed that RA
patients had significantly less Bifidobacteria and Bacteroides species com-
pared to patients suffering from fibromyalgia, a noninflammatory musculo-
skeletal disease (Vaahtovuo, Munukka, Korkeamaki, Luukkainen, &
Toivanen, 2008). Thus, low levels of SCFAs might contribute or result from
the development of RA; however, prospective studies that assess the produc-
tion of SCFAs in RA patients as well as other inflammatory diseases would
be of great interest to determine if a defect in SCFA levels contributes to
disease onset.
111The Role of Short-Chain Fatty Acids in Health and Disease
Finally, Bo
¨ttcher et al. (2000) compared the production of SCFAs in aller-
gic and nonallergic children and found that allergic infants had lower levels of
propionate, acetate, and butyrate in their feces compared to nonallergic indi-
viduals. This may account for the observation that Gpr43
/
mice exhibit
exacerbated development of allergic airway inflammation (Maslowski et al.,
2009). These results suggest that SCFAs might play a protective role in allergic
disease. This would support a diet/fiber deficiency model (Maslowski et al.,
2009) for the increase in inflammatory diseases in Western countries.
5. PERSPECTIVE
The incidence of autoimmunity, IBD, and allergy has increased dramat-
ically inWestern and Westernized countries. This increase parallels a decrease in
the consumption of fiber and indigestible starches. Carefully designed studies
are now required to evaluate the effect of diet, independent of other possible
contributing factors (i.e., hygiene, infection, sunlight, etc.). These studies will
be critical for determining the role of diet, particularly fiber and SCFAs, in the
development of Western diseases. If indeed diet and the resulting changes to the
gut microbiota underlie certain Western lifestyle diseases, then there is enor-
mous potential for prevention or correction through diet, probiotics, or new
drugs targeting metabolite sensing mechanisms such as HDACs or GPCRs.
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