Regulation of monocarboxylate transporter 1 (MCT1) promoter by butyrate in human intestinal epithelial cells: Involvement of NF‐κB pathway

MCT1 is a member of the proton-linked monocarboxylate transporter (MCT) family, which is known to comprise at least 14 isoforms in mammals [Halestrap and Meredith, 2004]. Of these, four isoforms (MCT1-MCT4) have been structurally and functionally well characterized [Halestrap and Meredith, 2004]. We have recently demonstrated the presence of various MCT isoforms along the length of the human intestinal epithelium [Gill et al., 2005]. The MCTs have been shown to catalyze the proton-linked transport of short-chain substituted monocarboxylates such as lactate, and pyruvate, ketone bodies, and also the short chain fatty acids (SCFA) [Ritzhaupt et al., 1998; Halestrap and Meredith, 2004]. The major SCFAs produced during bacterial fermentation of dietary fiber in the gut are acetate, propionate and butyrate. At the colonic luminal pH, these SCFAs exist almost entirely as anions, and require a carrier protein for cellular entry. Studies from our laboratory and others have clearly demonstrated that butyrate is transported across the human colonocyte luminal membrane via (MCT1) [Ritzhaupt et al., 1998; Hadjiagapiou et al., 2000; Stein et al., 2000]. The involvement of MCT1 in SCFA transport in bovine caecum and caprine rumen has been recently reported [Kirat and Kato, 2006; Kirat et al., 2006]. Although controversies exist regarding precise membrane localization of MCT1 in the intestine of human, and other species, its role as a major route of SCFA transport is widely recognized. Our recent report showing that inhibition of luminal butyrate uptake following enteropathogenic Eschericia coli (EPEC) infection was associated with a decreased level of surface MCT1 in Caco-2 cells also emphasizes importance of MCT1 in luminal SCFA transport [Borthakur et al., 2006]. SCFAs, particularly butyrate are excellent energy substrates for colonic epithelial cells and possess trophic effects on the colon [Cook and Sellin, 1998]. SCFAs are also known to stimulate water and NaCl absorption [Binder and Mehta, 1989] and blood flow [Mortensen et al., 1990] in the colon. They play an important role in homeostasis of colonic mucosa by inducing pathways of cell maturation, including cell cycle arrest, differentiation, and apoptosis [Barnard and Warwick, 1993; Heerdt et al., 1997]. Reduced luminal availability and impaired intracellular oxidation of butyrate have been implicated in the pathogenesis of colonic disorders such as ulcerative colitis [Cook and Sellin, 1998]. Butyrate is known to modulate expression of an array of genes including the cell cycle inhibitor p21 [Archer et al., 1998], cyclins [Siavoshian et al., 2000], and the BCL2 family [Hague et al., 1997]. Butyrate has been shown to stimulate NHE3 gene promoter [Kiela et al., 2001], and to up-regulate MCT1 mRNA, and protein levels [Cuff et al., 2002]. Because of the pivotal role of butyrate in cellular metabolism, and colonic tissue homeostasis, an understanding of the regulation of its transport into colonic tissues is important. However, very little information is available on the transcriptional regulation of MCT1. We and others have recently reported cloning and characterization of the promoter region of MCT1 gene [Cuff and Shirazi-Beechey, 2002; Hadjiagapiou et al., 2002; Hadjiagapiou et al., 2005]. We have also shown that USF proteins, particularly USF2, act as repressor of the basal promoter activity of MCT1 gene [Hadjiagapiou et al., 2005]. A recent report showed up-regulation of MCT1 and MCT4 mRNA by testosterone in rat skeletal muscle [Enoki et al., 2006]. In another study, it has been suggested that MCT1 expression, and activity are up-regulated by butyrate via involvement of both transcriptional and post-transcriptional mechanisms [Cuff et al., 2002]. However, to date, detailed mechanisms of transcriptional regulation of MCT1 expression by butyrate are not known. The ability of butyrate to modulate gene expression is often attributed to histone hyperacetylation through inhibition of histone deacetylases [Riggs et al., 1977; Sealy and Chalkley, 1978; Grunstein, 1997]. Histone acetylation causes a relaxed state of the chromosome(s) rendering it more accessible to transcription factors [Grunstein, 1997]. Despite the extensive knowledge about histone acetylation, the relationship between butyrate and transcriptional activation remains relatively unclear. The multiple effects of butyrate are implicated in modulation of gene expression at several levels including transcription, mRNA stability, and elongation [Cuff and Shirazi-Beechey, 2004]. In the present study, we have investigated the mechanism of up-regulation of MCT1 gene transcription by butyrate at the promoter level. We have shown that butyrate markedly stimulates MCT1 promoter activity and also provided evidence for the involvement of NF-κB mediated pathway in stimulatory effect of butyrate on MCT1 promoter.