The Human Monocarboxylate Transporter MCT1: Gene Structure and Regulation
M. A. Cuff, S. P. Shirazi-Beechey
2005
American Journal of Physiology - Gastrointestinal and Liver Physiology
To the Editor: We read with concern the recent paper by Hadjiagapiou et al. (4) that examines the regulation of the monocarboxylate transporter isoform 1 (MCT1) gene promoter in Caco-2 cells and would like to make a number of comments with regard to 1) errors in their data and 2) misquotation and misrepresentation of prior publications by others in this important study area. MCT1 is the prototype of a family of proteins that play an important role in the transport of monocarboxylates across the
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... cell membrane in a variety of cell types (5). MCT1 is particularly prominent in the colonic epithelium where it functions to transport short-chain fatty acids (most notably, butyrate) from the lumen across the colonocyte luminal membrane (7, 8). We demonstrated several years ago that MCT1 abundance is markedly downregulated during the transition from normality to malignancy in the colon and, more recently, that its expression is required for butyrate to exert many of its effects in cultured colonic epithelial cells (1, 6). Not surprisingly, the regulation of MCT1 expression in the intestine is an important area of study. We have previously reported that human colonic MCT1 expression is responsive to its substrate, butyrate, and that this regulation involves the dual control of transcription and mRNA stability (2). We have also determined the site of transcription initiation, isolated the MCT1 gene promoter, and demonstrated the MCT1 gene structure to comprise five exons interrupted by four introns. The first of these introns occurs in the 5'untranslated region (UTR) encoding DNA, spans greater than 26 kb, and accounts for more than one-half of the entire MCT1 transcription unit (3). Given that the isolation of the human MCT1 gene promoter was first published in full more than three years ago (3) and the sequence submitted to the European Molecular Biology Laboratories database (Accession no. AJ438944), it is surprising that the first part of the paper by Hadjiagapiou et al. focuses on repetition of this isolation. This aside, a major concern is the inaccurate description by Hadjiagapiou et al. of where the MCT1 promoter DNA sequence is located in relation to the genome. The authors specifically and repeatedly state that the transcriptional start site is located only 281 bp upstream from the translational start site (i.e., the MCT1 coding region) and this is reinforced and illustrated by the contiguous DNA sequence shown in their Fig. 1 . We want make it clear that these data are incorrect and ignore the large (26 kb) intron that interrupts the MCT1 5Ј-UTR (3). The presence of this intron is clearly indicated by a database blast of the MCT1 5Ј-RACE sequence, which shows that the sequence of the MCT1 message and that of the MCT1 genomic DNA diverge 45 bp upstream of the MCT1 coding region and only reconverge ϳ26 kb upstream in the genomic DNA. Moreover, this arrangement is described prominently in our prior publication detailing the first isolation of the MCT1 gene promoter and determination of the gene structure of human MCT1 (3). It is also confirmed by the latest human genome-sequencing assembly. Accordingly, it is unfortunate that Hadjiagapiou et al. appear to have overlooked this and report that the MCT1 promoter lies Ͼ26 kb downstream from where it is actually located in the genome. We feel that this is misleading and will of course confuse future study of the regulation of MCT1 expression. We also want to make readers aware of a number of factual errors that Hadjiagapiou et al. report regarding our previously published data of the isolation and characterization of the MCT1 gene promoter. First, Hadjiagapiou et al. state that the MCT1 gene promoter was isolated from AA/C2 cells. We assume Hadjiagapiou et al. meant to refer to AA/C1 colonic epithelial cells; however, this too is incorrect because the MCT1 promoter was in fact isolated by our screening of a chromosome 1-specific genomic DNA cosmid library derived directly from human tissue (3). Second, Hadjiagapiou et al. report that our determination of the MCT1 transcription initiation site employed total RNA from AA/C1 cells, and they use this to rationalize their assignment of the transcriptional start site to an alternative downstream location in Caco-2 cells. This is also incorrect. Our determination of the MCT1 transcription initiation site employed total RNA derived directly from human colonic tissue and not AA/C1 cells. Once again, this is clearly detailed in the paper they refer to (3). We hope the above information and comments will serve to clarify the scientific record, reduce any confusion for those interested in the regulation of MCT1 expression, and so facilitate advancing future research in this area. REFERENCES 1. Cuff M, Dyer J, Jones M, and Shirazi-Beechey SP. The human colonic monocarboxylate transporter isoform 1: its potential importance to colonic tissue homeostasis. Gastroenterology 128: 676 -686, 2005. 2. Cuff MA, Lambert DW, and Shirazi-Beechey SP. Substrate-induced regulation of the human colonic monocarboxylate transporter, MCT1. J Physiol 539: 361-371, 2002. 3. Cuff MA, and Shirazi-Beechey SP. The human monocarboxylate transporter, MCT1: genomic organization and promoter analysis. Biochem Biophys Res Commun 292: 1048 -1056, 2002. 4. Hadjiagapiou C, borthakur A, dahdal R, Gill R, Malakooti J, Ramaswamy K, and Dudeja PK. Role of USF1 and USF2 as potential repressor proteins for human intestinal monocarboxylate transporter 1 promoter. Am J Physiol Gastrointest Liver Physiol 228: G1118 -G1126, 2005. 5. Halestrap AP and Price NT. The proton-linked monocarboxylate transporter (MCT) family: structure, function and regulation. Biochem J 343: 281-299, 1999. 6. Lambert DW, Wood IS, Ellis A, and Shirazi-Beechey SP. Molecular changes in the expression of human colonic nutrient transporters during the transition from normality to malignancy. Br J Cancer 86: 1262-1269, 2002. 7. Ritzhaupt A, Ellis A, Hosie KB, and Shirazi-Beechey SP. The characterization of butyrate transport across pig and human colonic luminal membrane.
doi:10.1152/ajpgi.00278.2005
pmid:16227529
fatcat:vpinpffajnhwzdqg5vv4tjltvq