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1 Medicine, Univ. of Illinois at Chicago, Chicago, IL, USA; Research, Jesse Brown VA Medical Center, Chicago, IL, USA
* To whom correspondence should be addressed. E-mail: pkdudeja{at}uic.edu.
Recent studies from our laboratory and others have demonstrated the involvement of monocarboxylate transporter 1 (MCT1) in the luminal uptake of short chain fatty acids (SCFA) in the human intestine. Functional studies from our laboratory have previously demonstrated kinetically distinct SCFA transporters on the apical and basolateral membranes (BLM) of the human colonocytes. While apical SCFA uptake is mediated by MCT1 isoform, the molecular identity of the BLM SCFA transporter(s) and whether this transporter is encoded by another MCT isoform is not known. The present studies were designed to assess the expression and membrane localization of different MCT isoforms in the human small intestine and colon. Immunoblotting was performed utilizing the purified apical and basolateral Recent studies from our laboratory and others have demonstrated the involvement of monocarboxylate transporter 1 (MCT1) in the luminal uptake of short chain fatty acids (SCFA) in the human intestine. Functional studies from our laboratory have previously demonstrated kinetically distinct SCFA transporters on the apical and basolateral membranes (BLM) of the human colonocytes. While apical SCFA uptake is mediated by MCT1 isoform, the molecular identity of the BLM SCFA transporter(s) and whether this transporter is encoded by another MCT isoform is not known. The present studies were designed to assess the expression and membrane localization of different MCT isoforms in the human small intestine and colon. Immunoblotting was performed utilizing the purified apical and basolateral membranes from human intestinal mucosa obtained form organ donor intestine. Immunohistochemistry studies were done on paraffin-embedded sections of human colonic biopsy samples. Immunoblotting studies detected a protein band of ~39 kDa for MCT1, predominantly in the apical membranes. The relative abundance of the MCT1 mRNA and protein increased along the length of the human intestine. MCT4 (54 kDa) and MCT5 (54 kDa) isoforms showed basolateral localization and were highly expressed in the distal colon. Immunohistochemical studies confirmed that hMCT1 antibody labeling was confined to the apical membranes, whereas MCT5 antibody staining was restricted to the basolateral membranes of the colonocytes. We speculate that distinct MCT isoforms may be involved in SCFA transport across the apical or basolateral membranes in polarized colonic epithelial cells.
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