Regulation of Brain‐Derived Neurotrophic Factor Levels by the Long Chain Free Fatty Acid Receptors FFAR1 and FFAR4 in the Enteric Endocrine and Glial Cells

Abstract

Recent deorphanization of several G‐protein‐coupled receptors as endogenous receptors for free fatty acids (FFAs) has increased our understanding of the importance of chemosensing in the gut and the role of FFAs in the regulation of host defense mechanisms, energy metabolism, gastrointestinal motility, and secretion. There are five distinct FFA receptors (FFARs: FFAR1, FFAR2, FFAR3, FFAR4 and GPR84) that differ in molecular structure, ligand specificity, expression pattern, and functional properties. Both FFAR1 and FFAR4 are activated by the long chain fatty acid linolenic acid (LA). Our previous studies showed predominant expression of FFAR4 in enteroendocrine cells (STC‐1 cells) and predominant expression of FFAR1 in glial cells (rat glial cell line). In STC‐1 cells long chain fatty acids preferentially activate FFAR4, whereas in glial cells long chain fatty acids preferentially activate FFAR1. Neurotrophins such as brain‐derived neurotrophic factor (BDNF) are essential for the development and integrity of the enteric nervous system and play an important role in the regulation of gastrointestinal functions including motility and secretion. However, the role of FFARs in the regulation of BDNF levels is not known.AimTo test the hypothesis that activation of FFAR1 and FFAR4 by long chain FFAs induces an increase in BDNF levels in enteroendocrine and glial cells.MethodsSTC‐1 and rat glial cells were cultured in medium containing DMEM‐10. Co‐localization of BDNF with FFAR1 or FFAR4 was examined using GFP‐tagged BDNF. The responses to LA in the presence or absence of antagonists of FFAR1 (GW1100) and FFAR4 (AH7614) were measured as a change in number of BDNF positive cells by immunocytochemistry and change in BDNF protein by enzyme‐linked immunosorbent assay (ELISA).ResultsImmunocytochemical studies showed co‐localization of BDNF with FFAR4 in STC‐1 cells and with FFAR1 in glial cells. Treatment of STC‐1 cells with LA (100 mM) significantly increased the number of BDNF positive cells (4‐fold) (P<0.0001) and BDNF protein levels: the increase was blocked by a selective FFAR4 antagonist (AH7614, 100 μM), but not by a selective FFAR1 antagonist (GW1100, 10 μM). The LA‐induced increase in BDNF levels was abolished in STC‐1 cells transfected with FFAR4 siRNA. These results suggest that, in STC‐1 cells, LA selectively activates FFAR4 to increase BDNF levels. Treatment of glial cells with LA (100 mM) significantly increased the number of BDNF positive cells (2.5‐fold) (P<0.0001) and BDNF protein levels: the increase was blocked by a selective FFAR1 antagonist, but not by a selective FFAR4 antagonist. LA‐induced increase in BDNF protein levels was abolished in glial cells expressing FFAR1 siRNA. These results suggest that, in glial cells, LA selectively activates FFAR1 to increase BDNF levels.ConclusionIn enteroendocrine cells (STC‐1), long chain fatty acids increase BDNF content via FFAR4, whereas in glial cells they increase BDNF content via FFAR1. This is consistent with predominant expression of FFAR4 in STC‐1 cells and predominant expression of FFAR1 in in glial cells.Support or Funding InformationSupported by DK15564, DK28300, and DK34153.