No abstract

GLP-1 secretion is stimulated by luminal oleic acid (OA), which crosses the cell membrane by an unknown mechanism. We hypothesized that L cell fatty acid transport proteins (FATPs) are essential for OA-induced GLP-1 release. Therefore, the murine GLUTag L cell model was used for immunoblotting, [3 H]OA uptake assay, and GLP-1 secretion assay as determined by radioimmunoassay following treatment with OA Ϯ phloretin, sulfo-N-succinimidyl oleate, or siRNA against FATP4. FATP4Ϫ/Ϫ and cluster-of-differentiation 36 (CD36) Ϫ/Ϫ mice received intraileal OA, and plasma GLP-1 was measured by sandwich immunoassay. GLUTag cells were found to express CD36, FATP1, FATP3, and FATP4. The cells demonstrated specific 3 H[OA] uptake that was dose-dependently inhibited by 500 and 1,000 M unlabeled OA (P Ͻ 0.001). Cell viability was not altered by treatment with OA. Phloretin and sulfo-N-succinimidyl oleate, inhibitors of protein-mediated transport and CD36, respectively, also decreased [ 3 H]OA uptake, as did knockdown of FATP4 by siRNA transfection (P Ͻ 0.05-0.001). OA dose-dependently increased GLP-1 secretion at 500 and 1,000 M (P Ͻ 0.001), whereas phloretin, sulfo-N-succinimidyl oleate, and FATP4 knockdown decreased this response (P Ͻ 0.05-0.01). FATP4 Ϫ/Ϫ mice displayed lower plasma GLP-1 at 60 min in response to intraileal OA (P Ͻ 0.05), whereas, unexpectedly, CD36 Ϫ/Ϫ mice displayed higher basal GLP-1 levels (P Ͻ 0.01) but a normal response to intraileal OA. Together, these findings demonstrate a key role for FATP4 in OAinduced GLP-1 secretion from the murine L cell in vitro and in vivo, whereas the precise role of CD36 remains unclear.