Monocarboxylate transporters 1 and 4: expression and regulation by PPARα in ovine ruminal epithelial cells

In the intact rumen epithelium, isoforms 1 and 4 of the monocarboxylate transporter (MCT1 and MCT4) are thought to play key roles in mediating transcellular and intracellular permeation of short-chain fatty acids and their metabolites and in maintaining intracellular pH. We examined whether both MCT1 and MCT4 are expressed at mRNA and protein levels in ovine ruminal epithelial cells (REC) maintained in primary culture and whether they are regulated by peroxisome proliferator-activated receptor-α (PPARα). Because both transporters have been characterized to function coupled to protons, the influence of PPARα on the recovery of intracellular pH after l-lactate exposure was evaluated by spectrofluorometry. MCT1 and MCT4 were detected using immunocytochemistry both at the cell margins and intracellularly in cultured REC. To test regulation by PPARα, cells were exposed to WY 14.643, a selective ligand of PPARα, for 48 h. The subsequent qPCR analysis resulted in a dose-dependent upregulation of MCT1 and PPARα target genes, whereas response of MCT4 was not uniform. Protein expression of MCT1 and MCT4 quantified by Western blot analysis was not altered by WY 14.643 treatment. l-Lactate-dependent proton export was blocked almost completely by pHMB, a specific inhibitor of MCT1 and MCT4. However, l-lactate-dependent, pHMB-inhibited proton export in WY 14.643-treated cells was not significantly altered compared with cells not treated with WY 14.643. These data suggest that PPARα is particularly regulating MCT1 but not MCT4 expression. Extent of lactate-coupled proton export indicates that MCT1 is already working on a high level even under unstimulated conditions.