Identification and characterization of a monocarboxylate transporter (MCT1) in pig and human colon: its potential to transport L-lactate as well as butyrate

1. Oligonucleotide primers based on the human heart monocarboxylate transpo rter (MCT1) cDNA sequence were used to isolate a 544 bp cDNA product from h uman colonic RNA by reverse transcription-polymerase chain reaction (RT-PCR ). The sequence of the RT-PCR product was identical to that of human heart MCT1. Northern blot analysis using the RT-PCR product indicated the presenc e of a single transcript of 3.3 kb in mRNA isolated from both human and pig colonic tissues. Western blot analysis using an antibody to human MCT1 ide ntified a specific protein with an apparent molecular mass of 40 kDa in pur ified and well-characterized human and pig colonic lumenal membrane vesicle s (LMV). 2. Properties of the colonic lumenal membrane L-lactate transporter were st udied by the uptake of L-[U-C-14]lactate into human and pig colonic LMV. L- Lactate uptake was stimulated in the presence of an outward-directed anion gradient at an extravesicular pH of 5.5. Transport of L-lactate into anion- loaded colonic LMV appeared to be via a proton-activated, anion exchange me chanism. 3. L-Lactate uptake was inhibited by pyruvate, butyrate, propionate and ace tate, but not by Cl- and SO42-. The uptake of L-lactate was inhibited by ph loretin, mercurials and alpha-cyano-4-hydroxycinnamic acid (4-CHC), but not by the stilbene anion exchange inhibitors, 4,4'-diisothiocyanostilbene-2,2 '-disulphonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene2,2'-disul phonic acid (SITS). 4. The results indicate the presence of a MCT1 protein on the lumenal membr ane of the colon that is involved in the transport of L-lactate as well as butyrate across the colonic lumenal membrane. Western blot analysis showed that the abundance of this protein decreases in lumenal membrane fractions isolated from colonic carcinomas compared with that detected in the normal healthy colonic tissue.