Butyrate is known as one of the most important metabolites produced through bacterial fermentation in the gut. The biosynthesis of butyrate is done in two metabolic pathways. The first of which regards the phosphorylation of butyryl-CoA to butyryl-phosphate and then the transformation to butyrate with enzyme butyrate kinase. The other metabolic pathway regards the transformation of butyryl-CoA to acetate and then to butyric acid (Liu et al., 2018). It has a wide range of beneficial properties including immunomodulatory functions and anti-tumor activity (Wu et al., 2018; Fu et al., 2019; Siddiqui and Cresci, 2021). It affects immune system by, among others, regulating the expression of pro- and anti-inflammatory mediators. Butyrate inhibits the expression of pro-inflammatory IL-1β and IL-6 while promoting the expression of anti-inflammatory cytokine IL-10 (Hui et al., 2019). Additionally, butyrate promotes the anti-tumor immunity of CD8+ T cells (He et al., 2021). The activity, proliferation, and apoptosis of multiple immune cells may also be directly regulated by butyrate (Danne and Sokol, 2021). Zhou et al. have shown that butyrate is produced by F. prausnitzii, and it maintains Th17/Treg balance (Zhou et al., 2018). Clostridium butyricum, which has probiotic properties, is also known as a butyrate-producing bacterium (Chen et al., 2020; Stoeva et al., 2021). In the Li et al. study, it was observed that butyrate constrained neutrophil functions as well as ameliorated mucosal inflammation in case of inflammatory bowel disease (Li et al., 2021). Butyrate provides anti-inflammatory effects and increases apoptosis in cancer cells (Jaye et al., 2022). Butyrate and proprionate are more effective in inhibiting HT29 cell growth compared to acetate (Jaye et al., 2022). Moreover, butyrate is a main source of energy for colonocytes (Fu et al., 2019). Additionally, it influences mucosal immunity and maintains gut microbiota stability, providing gut microbial homeostasis (Wu et al., 2018; Fu et al., 2019).

Epithelial cells have G-protein-coupled receptors (GPCRs), which are the receptor of butyrate (Hanus et al., 2021). It modulates cellular functions by the above-mentioned GPCRs, which are expressed in multiple cells and tissues (for instance, adipose tissue, monocytes, neutrophils, B and T lymphocytes, and colonic myeloid cells) (Siddiqui and Cresci, 2021). In adipose tissues, butyrate promotes production and secretion of leptin; thus, it may be mentioned as a body weight regulator (Coppola et al., 2021). Butyrate enhances epithelial cell proliferation and the mucus layer, and improves tight junctions (Peng et al., 2009). Notably, tight junctions are intercellular junctions essential for maintaining epithelial barrier integrity (Otani and Furuse, 2020). The claudin family of membrane proteins are a significant part of tight junctions (Otani and Furuse, 2020). The mucus layer is made up of butyrate, mucins, immunoglobulins, and glycoproteins (Zhang et al., 2019). It should be emphasized that butyrate upregulates the expression of the MUC2 gene; thus, it promotes the synthesis of mucins, which protect the epithelial cells from lumen toxins (Zhang et al., 2019). Bacteria such as Fusobacterium nucleatum, Bacteroides fragilis, and Escherichia coli are known colorectal cancer-associated pathogens, and they contribute to the dysfunction of intestinal barrier integrity (Kaźmierczak-Siedlecka et al., 2020; Ahmad Kendong et al., 2021). Butyrate increases the expression of claudin-1 and Zonula Occludens-1; thus, it is important in the context of maintenance of intestinal barrier integrity (Hajjar et al., 2021). In the Feng et al. study (regarding Caco-2 monolayers), it has been revealed that acetate (0.5 mM), proprionate (0.01 mM), and butyrate (0.01mM) both alone or in combination significantly increase transepithelial electrical resistance (Feng et al., 2018). They protect intestinal barrier function by inhibition of NLRP3 (cytosolic protein) inflammasome (Feng et al., 2018; Sharma and Kanneganti, 2021). Nevertheless, according to Huang et al., butyrate is the main SCFA that alleviates intestinal barrier dysfunction by downregulating the level of claudin-2 (Huang et al., 2021). Moreover, the authors concluded that claudin-2 is the major target of butyrate (Huang et al., 2021). The improvement of the intestinal barrier after butyrate supplementation was also confirmed in the Zhao et al. study regarding 48 rats with severe acute pancreatitis and intra-abdominal hypertension (Zhao et al., 2020). In this study, C. butyricum (butyrate-producing probiotic bacteria) and butyrate were given orally. It was noted that rats that consumed C. butyricum or butyrate had reduced intestinal injury and decreased the plasma level of inflammatory cytokines, diamine oxidase, and lipopolysaccharide (Zhao et al., 2020).

To summarize this part, butyrate is a key microbial metabolite that extremely affects the intestinal barrier function, and it seems to be needed for colorectal cancer patients in which increased intestinal permeability is noted.

Butyrate can induce the apoptosis of colorectal cancer cells (Bordonaro, 2020). In the Xiao et al. study, the apoptosis of colon cancer cells (HCT116) was detected using flow cytometry (Xiao et al., 2014). These cells were exposed to NaB at a dose of 10 mmol/L per 24 h. Previously, colon cancer cells were also treated with ERK inhibitor or siRNA. It was noted that NaB modulates ERK and sphingosine kinase 2 and consequently induces apoptosis of colon cancer cells (Xiao et al., 2014). The anti-carcinogenic effect of butyrate on colon cancer cells (SW480) was also confirmed in the Elimrani et al. study (Elimrani et al., 2015). Similarly, Roy et al. have shown that butyrate at a dose of 2.5–20 mM (and also carnitine) induces apoptosis of colon cancer cells and inhibits Caco-2 cell proliferation (Roy et al., 2009). Taking together the results of the above-mentioned studies, it can be concluded that butyrate may provide anti-carcinogenic effects.

Recently, it was shown that butyrate may improve the efficiency of radiotherapy (Park et al., 2020). Moreover, butyrate may enhance the irinotecan effect; thus, it may enhance the effect of chemotherapy (Encarnação et al., 2018). Similarly, the results of another study revealed that butyrate promotes the effects of 5-fluorouracil on cancerous colonocytes (Geng et al., 2021). It is estimated that approximately 40% of patients who underwent chemotherapy developed mucositis (Ferreira et al., 2012). Butyrate can reduce the side effects of treatment with 5-fluorouracil, which was shown in a mouse model study with mucositis induced by chemotherapy (Ferreira et al., 2012).

The reduction of butyrate-producing bacteria is observed in colorectal cancer patients (Wang et al., 2012). The intracellular concentration of butyrate in colonic cells is regulated by transporters, such as monocarboxylate transporter 1 (MCT1), sodium-coupled monocarboxylate transporter 1 (SMCT1), and breast cancer resistance protein (BCRP) (Gonçalves and Martel, 2016). Notably, the alterations of these transporters’ expression may occur in colorectal cancer patients (Gonçalves and Martel, 2016). During the discussion of transporters MCT1 and SMCT1, the term “butyrate paradox” was mentioned. The basis of this paradox is found deeply in epigenetics (Salvi and Cowles, 2021). In case of differentiated intestinal epithelial cells, butyrate is oxidized and utilized as a fuel (energy), and in this time, it is not able to inhibit histone deacetylase (HDAC), whereas cancerous colon cells use preferentially glucose as fuel instead of butyrate (Salvi and Cowles, 2021). Then, butyrate accumulates and acts as an inhibitor of HDAC. There is an observed prevalence of glycolytic metabolism over oxidative phosphorylation in these cells. The modification of histone (H3), which is induced by butyrate, is linked to the activation of genes participating in both cell cycle inhibition and apoptosis (Salvi and Cowles, 2021).

The summary of butyrate properties is presented in Figure 1 .

NaB is able to provide an anti-tumor effect in the context of colorectal cancer. Notably, there are several mechanisms by which NaB may be involved in this process (Wang et al., 2020). In the Xi et al. study, the effect of NaB on colorectal cancer cells (lines SW480, LOVO, HCT116, and HCT8) was investigated (Xi et al., 2021). The results of this study explore the anti-cancer activity of this component; i.e., NaB is able to induce apoptosis as well as inhibit colorectal cancer cell proliferation (Xi et al., 2021). In another study, the impact of NaB on colorectal cell lines (HCT116 and SW480) was also assessed (Zhou et al., 2019). The authors used next-generation RNA sequencing. It was noted that 7,192 genes were differently expressed in cells treated with NaB in comparison with untreated cells. Recently, it was also observed that NaB is able to inhibit colorectal cancer cells’ (lines HCT116 and LOVO) migration through enhancement of miR-200c expression-mediated downregulation of Bmi-1 (Xu et al., 2018). Notably, in another study it was noted that the miR-200c/FUT4 axis prevents the proliferation of colon cancer cells (Cong et al., 2021). This effect is obtained by downregulation of the Wnt/β-catenin pathway (Cong et al., 2021). Interestingly, Sanaei et al. investigated the effect of NaB on both AsPC-1 (pancreatic cancer) and HCT-116 (colon cancer) cell lines (Sanaei and Kavoosi, 2022). It was observed that NaB increased p16INK4a, p14ARF, and p15INK4b, and reduced class I and II HDACs (Sanaei and Kavoosi, 2022). The authors also reported that colon cancer cell lines were more sensitive to NaB than pancreatic cell lines (Sanaei and Kavoosi, 2022). Notably, p16INK4a is a tumor suppressor protein (Serrano, 1997). It was observed that the loss of p14ARF expression is related to more advanced tumors, which has been confirmed in the Kim et al. study (Kim et al., 2020) ( Table 1 ).

Thioredoxin and thioredoxin reductase are important modulators of tumor development (Zhang et al., 2017). Wang et al. have reported that NaB significantly inhibited the growth of colorectal cancer cells as well as decreased the expression of thioredoxin-1 (Trx-1) protein in these cells (Wang et al., 2020). Notably, these results were not observed in normal colon epithelial cells and that can explain the selective inhibition of cell growth by NaB (Wang et al., 2020). Trx-1 is defined as a small redox-active protein (Shao et al., 2020; Liu et al., 2022). The downregulation of Trx-1 seems to be important in the context of colorectal cancer (Wang et al., 2020). It can be associated with inflammation and oxidative stress (Shao et al., 2020). According to the results obtained in the Shao et al. study, Trx-1 can also be a potential therapeutic target in case of sepsis, due to the fact that it plays a significant role in inflammation and oxidative stress (Shao et al., 2020).

The deficiency of folic acid alters the cytosine methylation in DNA (Lu et al., 2008). Recently, Lu et al. investigated the role of folic acid and NaB in the prevention of colorectal cancer in a mouse model study (Lu et al., 2008). The results of this study have shown that the lower level of p21WAF1 gene expression was found in colorectal cancer samples compared to normal colorectal mucosa. The administration of NaB beneficially increased the level of p21WAF1 mRNA and p21WAF1 protein. Thus, it can prevent tumorigenesis in a mouse model of colorectal cancer induced by 1,2-dimethylhydrazine (Lu et al., 2008).

AMPK regulates glucose and cholesterol metabolism (Shackelford and Shaw, 2009). Liver kinase B1 (LKB1) is a tumor suppressor gene on human chromosome 19p13. LKB1 controls both cell metabolism and oxidative stress, and together with AMPK, it controls cell growth (Shackelford and Shaw, 2009; Ciccarese et al., 2019). NaB induces autophagy in colorectal cancer cells by LKB1/AMPK signaling, which has been demonstrated in the Luo et al. study (Luo et al., 2019). The authors concluded that NaB may be a novel target for colorectal cancer patients (Luo et al., 2019).

The supplementation with NaB can beneficially affect gut microbiota. In the Ma et al. study, the effect of NaB on modulation of gut microbiota in mice with colorectal cancer liver metastasis was investigated (Ma et al., 2020). The composition of gut microbiota was assessed using 16S rRNA gene sequencing. It was observed that NaB beneficially altered gut microbiota. Moreover, it modulated immune system by decreasing Treg cells and increasing NK cells as well as T helper cells (Ma et al., 2020). In another animal model (C57BL/6J mice) study, it was revealed that exercise and NaB supplementation reversed metabolic dysfunctions, which have been induced by high-fat diet (p < 0.05), and inhibited the amount of microbes producing lipopolysaccharide (p = 0.001) (Yu et al., 2019).