TRPM5 is critical for linoleic acid-induced CCK secretion from the enteroendocrine cell line, STC-1

Abstract

Fatty acidinduced stimulation of enteroendocrine cells leads to release of the hormones such as cholecystokinin (CCK) that contribute to satiety. Recently, the fatty acid activated G protein-coupled receptor GPR120 has been shown to mediate long-chain unsaturated free fatty acidinduced CCK release from the enteroendocrine cell line, STC-1, yet the downstream signaling pathway remains unclear. Here we show that linoleic acid (LA) elicits membrane depolarization and an intracellular calcium rise in STC-1 cells and that these responses are significantly reduced when activity of G proteins or phospholipase C is blocked. LA leads to activation of monovalent cation-specific transient receptor potential channel type M5 (TRPM5) in STC-1 cells. LA-induced TRPM5 currents are significantly reduced when expression of TRPM5 or GPR120 is reduced using RNA interference. Furthermore, the LA-induced rise in intracellular calcium and CCK secretion is greatly diminished when expression of TRPM5 channels is reduced using RNA interference, consistent with a role of TRPM5 in LA-induced CCK secretion in STC-1 cells. GPR120; cholecystokinin; calcium; patch-clamp recording INGESTED FAT, SPECIFICALLY in the form of free fatty acids (FFAs), activates enteroendocrine cells (EECs) in the proximal small intestine, which initiates a number of physiological functions including satiety, potentiating insulin release, and delaying gastric emptying (7,9,14). FFA-induced satiety is mainly mediated by secretion of satiety peptides including cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1) by specialized EECs (type I) present in preabsorptive sites in the intestine (6,7,14,24,28). Recently, the fatty acid-activated G protein-coupled receptor, GPR120, has been shown to mediate long-chain unsaturated FFA-induced CCK and GLP-1 release from the enteroendocrine cell line, STC-1, yet the downstream signaling pathway from GPR120 activation to CCK secretion remains unclear (9, 26). Several groups have shown that FFA-induced CCK release in STC-1 cells is dependent on increased intracellular calcium. Moreover, removal of extracellular calcium and/or application of the L-type voltage-gated calcium channel (VGCC) blocker nicardipine significantly reduced the FFA-induced intracellular calcium rise and CCK secretion in STC-1 cells (23,26). Thus, it is reasonable to postulate from these observations that polyunsaturated fatty acids (PUFAs) may depolarize STC-1 cells by activating GPR120 and initiating membrane depolarization thereby activating L-type VGCCs leading to an elevation of intracellular calcium concentration and CCK secretion. The link between GPR120 and the development of the receptor potential (i.e., depolarization) remains unknown.One candidate for producing the FFA-induced receptor potential in nutrient-responsive cells is transient receptor potential channel type M5 (TRPM5). TRPM5 is a calcium-activated, nonselective, monovalent cation-permeable channel that is highly expressed in taste receptor cells (TRCs) and is required for sweet, bitter...

β-Lactoglobulin-linoleate complexes: In vitro digestion and the role of protein in fatty acid uptake

“…The ability of STC-1 cells to produce and secrete CCK is thought not to require internalisation of the FA as an external receptor seems to induce CCK secretions (Choi et al, 2007, Tanaka et al, 2008, Shah et al, 2011. After 4 h exposure to the studied protein/FA complexes, no effect on CCK secretion was detected, whereas mRNA CCK levels differed between samples.…”

“…Relatively little is known concerning the mechanisms whereby nutrients influence CCK synthesis and secretion. Several studies have shown that peptides and FA can use an external receptor to induce CCK secretions (Choi et al, 2007, Tanaka et al, 2008, Shah et al, 2011. In vivo and in vitro studies indicate that proteins increase CCK secretion (Beucher et al, 1994, Cordier-Bussat et al, 1997, Nishi et al, 2001).…”

TRPM5-dependent amiloride- and benzamil-insensitive NaCl chorda tympani taste nerve response

“…It is suggested that the sensory attributes of complex-tasting divalent salts are also mediated by TRPM5 and TRPV1 channels (42). TRPM5 is also essential for fat taste (23) and for linoleic acid-induced CCK secretion from the enteroendocrine cell line STC-1 (44). TRPM5 is a highly temperature-sensitive, heatactivated channel that may underlie known effects of temperature on perceived taste, including the enhanced sweetness perception at high temperatures, in humans (46).…”

Increased Levels of Extracellular Dopamine in the Nucleus Accumbens and Amygdala of Rats by Ingesting a Low Concentration of a Long-Chain Fatty Acid

“…They have indicated that certain GPCRs were present upstream of this transduction mechanism. Additionally, Shah et al 37) have demonstrated that, although not present in taste cells but in STC-1, an enteroendocrine cell line, TRPM5 played a critical role in cholecystokinin (CCK) release by linoleic acid stimulation. The stimulation of GPR120 was necessary in the release of glucagon-like peptide-1 (GLP-1) 38) and CCK 39) with LCFA from the same cell line.…”