The human colonic monocarboxylate transporter Isoform 1: its potential importance to colonic tissue homeostasis.
OBJECTIVE
Butyrate serves as the major source of energy for colonic epithelial cells, and has profound effects on their proliferation, differentiation, and apoptosis. Transport of butyrate across the colonocyte luminal membrane is mediated by the monocarboxylate transporter, MCT1; the expression of which is down-regulated dramatically during colon carcinogenesis. We have proposed that the decline in MCT1 expression during colon carcinogenesis may reduce the intracellular availability of butyrate required to regulate expression of genes associated with the processes maintaining tissue homeostasis within the colonic mucosa.


METHODS
To test this hypothesis we used the technique of RNA interference to inhibit MCT1 expression specifically, and determined the consequences of this inhibition on the ability of butyrate to exert its recognized effects in vitro using flow cytometry, immunofluorescence, Northern analysis, and Western analysis.


RESULTS
We show 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 (p21), intestinal alkaline phosphatase (IAP), and cyclin D1, and their associated processes of proliferation and differentiation. In contrast, inhibition of MCT1 expression had no effect on the ability of butyrate to modulate expression of either bcl-XL or bak, and this was reflected in a corresponding lack of effect on butyrate induction of apoptosis.


CONCLUSIONS
Collectively, these results show the importance 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.
Cuff
Mark
M
Department of Veterinary Preclinical Sciences, University of Liverpool, Liverpool, United Kingdom.
Dyer
Jane
J
Jones
Mark
M
Shirazi-Beechey
Soraya
S
D000469:Alkaline Phosphatase / Q000235:genetics; D000951:Antigens, Neoplasm / Q000235:genetics; D017209:Apoptosis / Q000187:drug effects; D002087:Butyrates / Q000494:pharmacology; D002453:Cell Cycle / Q000187:drug effects; D018797:Cell Cycle Proteins / Q000235:genetics; D002454:Cell Differentiation / Q000187:drug effects; D002460:Cell Line; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D019938:Cyclin D1 / Q000235:genetics; D050759:Cyclin-Dependent Kinase Inhibitor p21; D058851:GPI-Linked Proteins; D015870:Gene Expression / Q000187:drug effects; D019073:HT29 Cells; D006706:Homeostasis; D006801:Humans; D027501:Monocarboxylic Acid Transporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D034622:RNA Interference; D027981:Symporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000099233:Monocarboxylate Transport Protein 1
D000469:Alkaline Phosphatase; D000951:Antigens, Neoplasm; D002087:Butyrates; D018797:Cell Cycle Proteins; D019938:Cyclin D1; D050759:Cyclin-Dependent Kinase Inhibitor p21; D058851:GPI-Linked Proteins; D027501:Monocarboxylic Acid Transporters; D027981:Symporters; D000099233:Monocarboxylate Transport Protein 1; C493956:CDKN1A protein, human; C063088:ALPI protein, human