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Abstract

The anti-diabetic intestinal L cell hormone, glucagon-like peptide-1 (GLP-1), enhances glucose-dependent insulin secretion and inhibits gastric emptying. 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. The murine GLUTag L cell model was therefore used for immunoblotting, (3)H-OA uptake assay and GLP-1 secretion assay as determined by radioimmunoassay following treatment with OA plus/minus phloretin, sulfo-N-succinimidyl oleate or siRNA against FATP4. FATP4(-/-) and Cluster-of-Differentiation (CD36)(-/-) 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 which was dose-dependently inhibited by 500 and 1000 µM unlabeled OA (P<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<0.05-0.001). OA dose-dependently increased GLP-1 secretion at 500 and 1000 µM (P<0.001), while phloretin, sulfo-N-succinimidyl oleate and FATP4 knockdown decreased this response (P<0.05-0.01). FATP4(-/-) mice displayed lower plasma GLP-1 at 60 min in response to intra-ileal OA (P<0.05) while, unexpectedly, CD36(-/-) mice displayed higher basal GLP-1 levels (P<0.01) but a normal response to intra-ileal OA. Together, these findings demonstrate a key role for FATP4 in OA-induced GLP-1 secretion from the murine L cell in vitro and in vivo, while the precise role of CD36 remains unclear.