Butyrate is a naturally occurring monocarboxylate, produced in the lumen of the colon by microbial fermentation of complex carbohydrates that escape digestion in the small intestine. It serves as the principal metabolic fuel for colonic epithelial cells, and exerts a variety of effects important to intestinal health and function. This brief discussion focuses on the route, role and regulation of butyrate transport in the large intestine, with particular emphasis on the significance of butyrate transport to the ability of butyrate to modulate expression of genes important to the processes maintaining colonic tissue homoeostasis.

Introduction

Adaptation to the nutritional environment through the modulation of gene expression is an important requirement for all living cells. It is strikingly observed in the absorptive cells of the intestinal epithelium. These cells are exposed to a luminal environment that varies considerably with diet, and not surprisingly therefore, adapt to these changes by regulating their uptake of nutrients from the intestinal lumen [1]. This is achieved through the modulation of expression of specialized nutrient transporters resident in the enterocyte plasma membrane, and serves to ensure that acquisition of available nutrients is both economic and appropriate to requirements [2].

Although the regulation of nutrient transporter expression is becoming well established, a major challenge that remains is to gain an insight into the specific significance of this control. This is particularly important for the transport of nutrients that perform functions beyond their recognized role in energy provision. Butyrate is such a nutrient, and as such, is key to many aspects of intestinal health and function. This brief discussion focuses on the role, regulation and importance of butyrate transport to its modulation of gene expression in the large intestine.

Butyrate production and role in the large intestine

Butyrate is a naturally occurring monocarboxylate often referred to as an SCFA (short-chain fatty acid). It is produced, along with acetate and propionate, in the lumen of the large intestine by bacterial fermentation of plant-derived dietary fibre and resistant starch that escape hydrolysis in the upper digestive tract [3]. Considerable evidence suggests that these SCFAs, particularly butyrate, are fundamental to the health of the normal colonic mucosa. Indeed, butyrate serves as the principal source of energy for colonocytes [4], plays a role in suppressing mucosal inflammation [5], and is essential for the maintenance of homoeostasis in the colonic epithelium [6,7]. In addition, butyrate exhibits a range of anti-tumourigenic effects on many cancer cells lines. These effects include the induction of cell-cycle arrest, differentiation and apoptosis [8,9] and are associated with specific changes in gene expression. In colonic cells, these butyrate-responsive genes include the cell-cycle inhibitor p21Waf1/Cip1 [10], a number of cyclins [9,11] and members of the Bcl-2 family [12].

Mechanistically, the ability of butyrate to regulate gene expression is often attributed to its induction of histone hyperacetylation through inhibition of histone deacetylase [13]. However, it is clear that butyrate also induces a variety of other changes within both the nucleus and cytoplasm. These include the hyperacetylation of non-histone proteins [14], DNA methylation [15] and the modulation of intracellular kinase signalling [16,17]. This multiplicity of effects may underlie the ability of butyrate to modulate gene expression at several levels including transcription [10,18,19], mRNA stability [18] and elongation [20], and makes it probable that the response to butyrate is complex, involving multiple distinct mechanisms/pathways.

Butyrate transport in the large intestine

Historically, the entry of butyrate and other SCFA into the cells of the intestine was supposed to be via simple non-ionic diffusion of the free acid. However, given that at the colonic luminal pH (pH 7) butyrate (pKa 4.7) exists predominantly in its anionic form, it is not surprising that it is now widely acknowledged that butyrate is transported across the colonocyte luminal membrane via a specific carrier-mediated system; exchanging butyrate for an intracellular anion (HCO3 or OH) [19,21,22]. Evidence indicates that the protein that mediates this transport is MCT1 (monocarboxylate transporter 1). Accordingly, it has been demonstrated that: (i) MCT1 is expressed in human colon; (ii) the functional protein is located on the luminal membrane of colonic absorptive cells; and (iii) it transports butyrate [2325].

MCT1 is the prototype of the SLC16 family of MCTs that mediate the transport of a range of monocarboxylates across the plasma membrane of a variety of cell types [26,27]. Although in the lumen of the colon the major substrates are normally acetate, propionate and butyrate, in other tissues a number of metabolically important monocarboxylates are transported by MCTs [27]. At least 14 family members have been identified to date; however, most remain functionally uncharacterized, and only MCT1 has been demonstrated to be expressed at the protein level in colonic epithelial cells [28]. Recently, it has been reported that SLC5A8, a member of the sodium solute symporter gene family, encodes a protein that is able to transport monocarboxylates in a sodium-coupled (electrogenic) manner when expressed in Xenopus laevis oocytes [29]. However, neither its expression at the protein level nor its ability to mediate monocarboxylate transport in colonic epithelial cells has been demonstrated. Moreover, none of the previous studies has demonstrated sodium-coupled butyrate transport in colonic cells [19,24].

Regulation of butyrate transport

The central role of butyrate in cellular metabolism and the maintenance of colonic tissue homoeostasis make an understanding of the regulation of its transport particularly important. Expression of MCT1 is up-regulated by its substrate, butyrate, and this is reflected functionally as increased butyrate transport [18]. Nuclear run-on assays indicate that the regulation occurs at the level of transcription; however, regulation of mRNA stability is also involved [18]. The genomic organization of the MCT1 gene has been determined, the gene promoter isolated and the cis-acting elements required for basal transcription shown to reside within a −70/+213 proximal region of the gene promoter [30]. In addition, the 5′-flanking region contains potential binding sites for a variety of transcription factors with known association to butyrate's action in the colon. The significance of these potential regulatory regions to act as specific butyrate response elements is the subject of ongoing investigation.

Significance of butyrate transport

Given that many of the cellular effects of butyrate are concentration-dependent, the ability of butyrate to exert these effects may depend on its intracellular concentration. By extension, factors affecting the intracellular accumulation of butyrate have the potential to influence its availability to modulate gene expression and, hence, processes such as proliferation, differentiation and apoptosis. Accordingly, we have hypothesized that MCT1, and the regulation of its expression, may play an important role in the maintenance of tissue homoeostasis in the colonic epithelium [18]. Moreover, we have provided support for this suggestion with the demonstration that MCT1 expression is significantly reduced during the transition from normality to malignancy in the human colon [28]. As such, we further propose that this decline in transporter expression may result in reduction in the intracellular availability of butyrate required to regulate the expression of genes associated with the processes maintaining tissue homoeostasis in the colonic mucosa. To test this hypothesis directly, and so examine the significance of our previous findings, we have employed the technique of RNAi (RNA interference) [31] to inhibit specifically MCT1 expression, and have assessed the consequences of this inhibition to the ability of butyrate to exert its effects on target gene expression and cellular function in vitro. Using this approach, we have found that inhibition of MCT1 expression and, hence, butyrate uptake, has profound inhibitory effects on the ability of butyrate to regulate expression of key target genes, p21Waf1/Cip1, intestinal alkaline phosphatase and cyclin D1, and their associated processes of cellular proliferation and differentiation. Conversely, inhibition of MCT1 expression does not affect the ability of butyrate to modulate expression of Bcl-xL and Bak and this is reflected in a corresponding lack of effect on butyrate induction of apoptosis.

Collectively, these results demonstrate the important contribution of MCT1 to the ability of butyrate to induce cell-cycle arrest and differentiation, and suggest fundamental differences in the mechanisms by which butyrate modulates specific aspects of cell function. It is, therefore, evident that the identification of the detailed regulatory and signalling networks by which butyrate exerts its effects in the colon may have both nutritional and clinical significance.

Research Colloquia: Research Colloquia at BioScience2004, held at SECC Glasgow, U.K., 18–22 July 2004. Edited by M. Bouvier (Montreal, Canada), G. Milligan (Glasgow, U.K.), V. O'Donnell (Cardiff, U.K.), M. Brand (MRC-Dunn Human Nutrition Unit, Cambridge, U.K.), M. Schweizer (Heriot-Watt University, Edinburgh, U.K.), R. Insall (Birmingham, U.K.), A. Ridley (Ludwig Institute for Cancer Research, London, U.K.) and M. Sutcliffe (Leicester, U.K.). The first eight papers featured in this Section were presented as a part of the GPCR Regulation and Signalling Research Colloquium, incorporating the GPCR–Ion Channel Interactions Pfizer-Sponsored Research Colloquium.

Abbreviations

     
  • MCT

    monocarboxylate transporter

  •  
  • SCFA

    short-chain fatty acid

The financial support of the Biotechnology and Biological Sciences Research Council and The Wellcome Trust is gratefully acknowledged.

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