2. Microbiota-Derived Butyrate in Colorectal Cancer
Short chain fatty acids (SCFAs) are weak organic acids with between two and five carbon molecules, including acetate (C2); propionate (C3); butyrate (C4); and valerate (C5), which are produced by the intestinal bacterial fermentation of mainly undigested dietary carbohydrates (especially resistant starches and dietary fibers)
[5]. However, they also result in small quantities from dietary and endogenous proteins, through a pathway that also produces toxic nitrogenous and sulfur metabolites, such as ammonia (from the conversion of the amino acid lysine into butyrate)
[6]. The concentration ratio in the colonic lumen of the three main SCFAs is about 3/1/1: 60% acetate, 25% propionate, and 15% butyrate, the last one being the preferred energy source utilized by epithelial cells from the colon, and only small proportions reach the portal vein or the systemic circulation
[5].
In vitro studies have demonstrated the important role of butyric acid in the prevention of CRC. An evaluation of HCT116 human CRC cells treated with butyric acid derivatives proved that apoptosis is induced in the cancer cells by activation of caspase-3 activity and induced cell cycle arrest
[7]. Moreover, SCFAs can also regulate the expression of inflammatory cytokines and chemokines by the colonic epithelial cells in different immune processes, having a role in gut homeostasis and promoting the integrity of the intestinal barrier
[8]. The effect of butyrate on the epithelial integrity has been confirmed also in animal models, where it contributes to the healing of colonic tissue at the anastomosis sites after surgery for CRC
[9].
Butyrate-producing bacteria are an abundant and phylogenetically diverse group of microorganisms, considered to be a functional group of Gram-positive anaerobic Firmicutes, which play an important role in maintaining a healthy gut, primarily through their production of butyrate
[5][10]. Two of the most numerically important groups are considered to be
Faecalibacterium prausnitzii, belonging to the
Clostridium leptum cluster (clostridial cluster IV) and
Eubacterium rectale/
Roseburia spp., belonging to the
Clostridium coccoides cluster (clostridial cluster XIVa)
[10]. There are two microbial enzymes responsible for the final synthesis of butyrate from two molecules of acetyl-CoA: butyryl-CoA transferase (dominant, formed by a variety of genera and species) and butyrate kinase (favored in proteolytic fermentation)
[11].
Increased butyrate production has often been hypothesized to be one of the beneficial effects of prebiotics and probiotics
[8]. One important prebiotic is represented by the resistant starch (the starch which escapes the digestion in the small intestine), which reduced colonic neoplasia in studies including carcinogen-treated rats but increased intestinal tumorigenesis in the genetically driven Apc1638N mouse model
[12]. The antineoplastic effect of resistant starch can be due to fermentation end-products, mainly butyrate
[12]. A mouse model study has shown that in the ones colonized with butyrate-producing bacterium the high fiber diet had a protective role, but not in the mice lacking a butyrate-producing bacterium
[13]. The same study also evaluated the protective effect in the case of mice colonized with a mutant strain of the butyrate-producing bacterium, harboring a deletion in the butyryl CoA synthesis operon which produces diminished levels of butyrate; the fiber diet had an attenuated protective effect with an intermediate tumor burden
[13]. Another way by which resistant starch, together with other insoluble fiber, may prevent the colonic neoplasm is by speeding the colonic transit, thus reducing the exposure of epithelial cells to ingested carcinogens
[14].
In a clinical trial (NCT03072641) which aimed to determine whether probiotic bacteria have a beneficial effect on the CRC-associated microbiota, researchers used dietary supplementation with
Bifidobacterium lactis Bl-04 and
Lactobacillus acidophilus NCFM and analyzed the microbiota composition in tissue and faeces samples, at baseline and after probiotics use
[15]. The results showed that patients with CRC who received probiotics had an increased abundance of butyrate-producing bacteria (especially
Faecalibacterium spp. and
Clostridiales spp.) in the samples from the tumor, non-tumor mucosa, and faeces, compared with the group that did not receive probiotics.
[15] Moreover, CRC-associated taxa (
Fusobacterium and
Peptostreptococcus) were less frequent in the fecal samples of patients who received probiotics, upholding the hypothesis that CRC-associated microbiota can be manipulated by specific probiotic strains and providing hope that the probiotics modulation of microbiota could be considered an integrative part of the therapeutic approach for CRC patients
[15].
The structure of microbiota in patients with CRC was described as significantly different from the one encountered in healthy individuals. There are several studies of butyryl-CoA: acetate CoA-transferase gene quantification from the gut microbial population and they all found that butyrate-producing bacteria (such as
Ruminococcus spp. and
Pseudobutyrivibrio ruminis) are reduced in the feces of CRC patients, pointing out the benefits of bacterial metabolites
[16][17][18]. An evaluation of diet and age suggested that these factors also influence the level of butyrate-producing bacteria in the gut—the older participants had significantly fewer copies of the butyryl-CoA:acetate CoA-transferase gene than young omnivores, while vegetarians showed the highest number
[19]. This fact may reflect the increased risk for CRC in the elderly, due to their low butyrate production capacity and the protective effect of a vegetarian diet against CRC.
In a study that aimed to describe how microbial functions may influence CRC development, researchers used stool profiling to identify intestinal microbiome and metabolome and analyze the different representation in humans with CRC, compared to healthy controls
[16]. They quantified several SCFAs from frozen stool samples, among which acetic and valeric acids were significantly higher in the feces from CRC patients. In contrast, butyric acid was significantly higher in the healthy samples, and propionic acid was detected in similar quantities between the two groups
[16]. Acetate can be turned into butyrate, but the proportional differences in these two SCFAs metabolites between CRC and healthy individuals may be explained by a reduction of gut bacteria that can perform this reaction in CRC samples. Otherwise, it may be a result of the conversion of butyrate into acetate, a degradation process that takes place under the acidic (low) colonic pH induced by the tumor
[16]. In CRC samples, significantly higher relative concentrations of isobutyric and isovaleric acid were observed as well, being products from the bacterial metabolism of branched-chain amino acids valine and leucine, also higher in CRC stool samples
[16]. Butyrate proved to be more than just a metabolite, having important cellular signaling roles as well, linked to epigenetic regulations of gene expression. Butyrate can regulate the expression of a large number of genes by direct interaction with transcription factors such as p53; retinoblastoma protein; Stat3; NF-kB; and estrogen receptors, which are critical epigenetic regulators and a new class of anticancer agents
[20]. Butyrate also has intracellular roles, like DNA methylation; histone methylation; hyperacetylation of nonhistone proteins; inhibition of histone phosphorylation; regulation of expression of micro-RNAs; and modulation of intracellular kinase signaling
[20]. Moreover, butyrate can act as agonist of a G-protein-coupled receptor found in the apical membrane of human colonic epithelial cells, GPR109A
[20].
There are two main forms of epigenetic changes encountered in CRC (and many other cancers), defined as chemical alterations to DNA or chromatin that do not affect the primary DNA sequence: those that directly modify DNA (DNA hypo- or hypermethylation) and those that modify DNA-binding proteins (histone modifications—methylation or demethylation, and the acetylation or deacetylation)
[21]. These changes alter the regulation and expression of genes and other DNA elements in a predictable fashion and are reversible, unlike changes to the genomic sequence
[21][22]. The enzymes that catalyze histone acetylation are called histone acetyltransferases (HATs) and the ones that catalyze the removal of an acetyl group from a histone are called histone deacetylases (HDACs), both playing a crucial role in the remodeling of chromatin
[22]. Their enzymatic activities induce structural alterations of histones, enabling access of transcription factors to a portion of DNA chromatin, influencing the transcription and expression of a given gene
[22].
Butyrate was the first identified endogenous inhibitor of HDAC, in 1977 and for more than two decades thereafter it was the only one available for research, with its primary target in the clinical development of cancer treatment
[23]. Butyrate acts as an inhibitor of HDAC and leads to hyperacetylation of histones
[20]. During the last decade, inhibition of HDACs by HDAC inhibitors (HDACIs) emerged as a target for specific epigenetic modification associated with cancer or other diseases (hemoglobinopathies; cystic fibrosis; X-linked adrenoleukodystrophy; muscular dystrophies; neurodegenerative disorders; systemic lupus erythematosus etc.). More than 20 substances have entered clinical studies by now, while some have already been approved (for example vorinostat orSuberAniloHydroxamic acid and romidepsin or depsipeptide for the treatment of cutaneous T-cell lymphoma or the drug panobinostat for the treatment of multiple myeloma)
[24].
Due to the fact that HDACs are key enzymes for regulating cell death and have a role in promoting carcinogenesis, HDACIs have been exploited for their role in cancer therapy and have been shown to regulate the survival of tumor-infiltrating T lymphocytes (TILs), by suppressing their apoptosis
[25]. HDACIs, particularly butyrate, also inhibit directly colon carcinogenesis, by decreasing the expression of cyclin B1 gene (a cell cycle promoter) in colon cancers cells, as shown in vitro
[26]. Moreover, co-administration of HDACIs and anti-CTLA4 (cytotoxic T-lymphocyte antigen 4) antibodies seems to act synergistically for the therapeutic effect, enhancing T-cell infiltration within the tumor and the anti-tumor immune response
[25]. Another mechanism by which microbiota-derived butyrate promotes cellular metabolism is the enhancement of memory potential in activated CD8
+ T cells (through increased oxidative and glycolytic activity, improved mitochondrial mass and membrane potential), with implications in immunotherapy and vaccination
[27].
The mentioned study of Belcheva et al. was conducted on APC
Min/+ (multiple intestinal neoplasia) mouse, a well-established animal model of human adenomatous polyposis, with MSH2 deficiency: an APC
Min/+MSH2
−/− mouse model of CRC
[28]. Alteration of the gut bacterial community structure with antibiotics led to decreased polyp numbers in the mice colons, without a reduction in the abundance of colonic bacteria, through a mechanism independent of both inflammation and DNA damage
[28]. Putting mice on a 7% low-carbohydrate diet led to substantial changes in the relative proportions of the bacterial phyla but did not alter the total bacterial abundance. Moreover, this diet reduced the number of polyps in the digestive tracts of APC
Min/+MSH2
−/− mice, in a similar amount achieved by treating mice with antibiotics
[28]. The antibiotic treatment and the low-carbohydrate diet had also numerous other effects observed in APC
Min/+MSH2
−/− mice: both interventions reduced the number of cells with DNA breaks, reduced Ki-67 expression, modulated and restored the nuclear β-catenin expression to that encountered in APC
Min/+MSH2
+/− mice, reduced the production of numerous metabolites (lactate, only butyrate from all SCFAs, uracil, xanthine etc.) of microbial fermentation, decreased three butyrate-producing families within the
Firmicutes phylum (
Clostridiaceae,
Lachnospiraceae and
Ruminococcaceae) without impacting total bacterial abundance, and reduced the gene copy number for butyryl-CoA transferase
[28]. All these results support the idea that the gut microbiota-related metabolome plays a crucial role in CRC by providing metabolites such as butyrate.
Histopathological studies have shown that adenomatous polyps appear through a top-down morphogenesis mechanism, from the apex to the bottom of the crypts, as the more time cells reside in the mucosa, the more chances exist for epigenetic alterations through carcinogens exposure, required for tumor formation
[29]. This process is in contradiction with the generally accepted statement that cancer cells derive from normal stem cells, as in the colon they exist near the base of the crypts. Therefore, in the superficial parts of the crypts, there are dysplastic epithelial cells which present a markedly abnormal pattern of Ki-67 proliferation and genetic alterations of APC locus (loss of heterozygosity) leading to functional changes in β-catenin expression and localization, these mutant clones being genetically unrelated to the cells from the bottom of the crypt
[29]. In the light of this particularity of the intestinal epithelium, it has been hypothesized that by influencing cell movement, profound effects on tumorigenesis may be obtained, since high-velocity cell loss represents an efficient way of eliminating cells that have acquired mutations and preventing irreversible cancerous phenotype by longer exposure to carcinogens
[30].
A study on murine APC
+/Min epithelial cells showed that in vitro, they are less motile than APC
+/+ cells and possess a disarranged actin cytoskeletal network, properties which make them more prone to acquiring additional genetic alterations and forming tumors
[30]. Treatment with two mM butyrate for 24 h was demonstrated to increase haptotaxis in both cellular lines, acting as a promoter of the migration of colonic cancerous epithelial cells. The effect was greater in the APC
+/Min cell line, as it was able to restore both motile function and actin cytoskeletal organization seen in APC
+/+ cells
[30]. The link between butyrate treatment and cytoskeleton assembly can be explained by its capacity for protein acetylation, which has a key role in these fibers’ function. Moreover, exposure to high concentrations of butyrate (5 mM) induced apoptosis in the mutated cells, measured by caspase-3-like activity
[30]. These results may explain the protective effect determined by butyrogenic diets on CRC carcinogenesis, by increasing colonocyte velocity and shortening the exposure of cells to carcinogens, especially in the cases with APC or the β-catenin gene mutations.
The association between fecal SCFAs concentrations and the efficacy of immunotherapy may emerge as a new biomarker to monitor patients undergoing treatment with programmed cell death-1 (PD-1) inhibitors. Nomura et al. recently evaluated 52 patients with solid tumors treated with nivolumab or pembrolizumab and concluded that those with higher concentrations of fecal SCFAs had a longer progression-free survival and also response to anti-PD-1 therapy
[31].
Some omega-3 fatty acids, like EPA and DHA, could have a potential adjuvant therapy role, thanks to their low toxicity profile and their capability to downregulate the expression of the efflux pump, P glycoprotein, in a doxorubicin-resistant variant of HT29 cells
[32]. Multiple other studies have evaluated their potential as adjuvant agents in chemotherapy, for example the association of EPA and a regimen of 5-fluorouracil (5-FU) and oxaliplatin can have a synergistic anti-cancer effect
[32]. Some researchers concluded that adding omega-3 fatty acids in chemotherapy could restore lipid stocks and potentially limit 5-FU side effects
[32].
Other studies have noticed an increased apoptosis of cancerous cells when adding EPA and DHA to 5-FU, oxaliplatin, and irinotecan
[32]. Dietary intake of omega3 fatty acids and chemotherapy might have a synergetic effect
[33]. Combined treatment of fish oil and 5-FU enhanced growth inhibition compared to cells exposed to either substance alone
[33].
3. Role of Omega-3 Fatty Acids in Regulating Butyrate-Producing Gut Microbiota
Consumption of omega-3-PUFA-rich diets has been demonstrated to be beneficial for health, supporting a good quality of life and ameliorating or preventing several disorders (cardiovascular, inflammatory, neurodegenerative diseases, diabetes mellitus, and cancer)
[34].
The effects of an omega-3-rich diet on gut microbiota were studied using animal models, proving there is a correlation between the two
[35]. Dietary omega-3-PUFAs are largely digested in the distal intestine by anaerobic bacteria such as
Bifidobacteria and
Lactobacilli, influencing the intestinal flora distribution, being shown to improve gut microbial dysbiosis by increasing probiotic species and butyric acid-producing bacteria, according to several studies conducted on humans
[36].
In a published case report, a healthy 45-year-old man who received 600 mg of omega-3 every day for 14 days had his feces sampled. Species diversity reduced after the intervention, although butyrate-producing bacteria increased.
Faecalibacterium prausnitzii and
Akkermansia spp. were found to be significantly reduced. There was found to be a remarkable increase in
Blautia, a genus whose reduction is associated with increased risk of CRC. After the 14-day washout, alterations in the gut flora were reversed, implying that the gut microbiota is a living, dynamic ecosystem that is subject to dietary changes. Therefore, increases in butyrate-producing bacteria may be responsible for some of omega-3’s health advantages
[37].
The increase in butyrate-producing bacteria may be influenced also by eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which together with prebiotic fermentable fibers may have protective effects against colonic neoplasm more due to increased apoptosis rather than decreased cell proliferation
[38]. EPA and DHA increase
Lactobacillus and reduces
Helicobacter and
Fusobacterium nucleatum [38]. The butyrate may be involved in colonocyte apoptosis through its effect to promote cellular oxidation, being able to produce cellular reactive oxygen species when metabolized
[38]. EPA and DHA can be incorporated in cell membranes and are susceptible to oxidation thanks to their high degree of unsaturation
[38]. Other direct roles of EPA and DHA on colorectal cancer cells include modulation of cyclooxygenase metabolism, alteration of lipid raft behavior, increase in lipid peroxidation, regulation of kinase pathways, induction of pro-apoptotic pathways, modulation of WNT/β-catenin pathway and others
[32].
Besides their direct roles, EPA and DHA have an effect on cancer cells through their metabolites, like resolvins, docosatriens and maresins
[39][40]. Resolvins (resolution phase interaction products), protectins, and maresins are endogenously generated from n-3 PUFAs
[38]. Resolvins are bioactive compounds with potent anti-inflammatory and immunoregulatory actions, and anti-carcinogenic compounds
[32][39][40].
Even though gut microbiota changes associated with omega-3-PUFAs are poorly understood, omega-3 fatty acids may aid in the treatment of colorectal cancer by increasing colon beneficial bacterial populations.
4. Conclusions and Future Perspectives
Given the important role of butyric acid in the prevention of CRC, therapies with exogenous SCFAs or prebiotic/probiotic administration to modulate bacterial metabolism in the gut are being proposed to reduce mucosal inflammation and induce apoptosis in cancer cells.
Omega-3-PUFAs may affect the balance of gut microorganisms, which may contribute to the occurrence and progression of CRC, particularly due to their ability to increase butyric acid-producing bacteria.
These discoveries may shed light on the mechanisms underlying omega-3-PUFAs’ impact on a variety of chronic conditions, as well as provide a framework for developing individualized medical treatments for CRC and other diseases. Supplementing the diet with omega-3 is likely to be a relevant potential mechanism for reducing CRC risk in a primary prevention setting, but it may also be appropriate for the possible use of omega-3-PUFAs as adjuvant treatment of CRC.