Bitter stimuli induce Ca²⁺ signaling and CCK release in enteroendocrine STC-1 cells: role of L-type voltage-sensitive Ca²⁺ channels

Abstract

We previously demonstrated the expression of bitter taste receptors of the type 2 family (T2R) and the ␣-subunits of the G protein gustducin (G␣ gust) in the rodent gastrointestinal (GI) tract and in GI endocrine cells. In this study, we characterized mechanisms of Ca 2ϩ fluxes induced by two distinct T2R ligands: denatonium benzoate (DB) and phenylthiocarbamide (PTC), in mouse enteroendocrine cell line STC-1. Both DB and PTC induced a marked increase in intracellular [Ca 2ϩ (3,13,19,60). Molecular sensing by GI cells plays a critical role in the control of multiple fundamental functions, including digestion, food intake, and metabolic regulation. Although these fundamental control systems have been known for a considerable time, the initial molecular recognition events that sense the chemical composition of the luminal contents have remained poorly understood. The gustatory system has been selected during evolution to detect nonvolatile nutritive and beneficial (sweet) compounds as well as potentially harmful (bitter) substances (24, 34). In particular, bitter taste has evolved as a central warning signal against the ingestion of potentially toxic substances, including plant alkaloids and other environmental toxins (21, 65). Specialized neuroepithelial taste receptor cells, organized within taste buds in human and rodent lingual epithelium, expressed a family of bitter taste receptors (referred as T2Rs) (1, 6, 46). These putative taste receptors belong to the guanine nucleotide-binding regulatory protein (G protein)-coupled receptor (GPCR) superfamily (1), which are characterized by seven transmembrane ␣-helices (32). Extensive genetic and biochemical evidence indicate that specific G proteins, gustducin and transducin, mediate bitter and sweet gustatory signals in the taste buds of the lingual epithelium (47,48,62,63,73). More recently, phospholipase C␤ 2 (PLC␤ 2 ) and TRPM5, a member (melastatin subtype 5) of the transient receptor potential (TRP) family (49), have been linked to bitter and sweet signal transduction (55, 56, 81). There is evidence for the activation of multiple second messenger pathways and ion channels in individual taste cells (1, 82). Clearly, taste signal transduction is complex and multifactorial and there is still much that is unknown about individual taste cell regulation.Outside the tongue, expression of the ␣-subunit of gustducin (G␣ gust ) has been also localized to gastric (28, 75) and pancreatic (27) cells, suggesting that a taste-sensing mechanism may also exist in the digestive system. Indeed, we demonstrated the expression of members of the bitter taste receptors of the T2R family in the mouse and rat GI tract and in mouse and rat enteroendocrine cells in culture (74,75). More recently, these results have been confirmed (45) and extended to the expression of the sweet taste receptors of the T1R family (15). Collectively, these findings demonstrated the expression of taste signal transduction pathways in cells of the GI tract of mice and rats.The intracellular signal transd...

TRPA1 channel activation induces cholecystokinin release via extracellular calcium

“…The L-type voltage-gated calcium channel blockers nimodipine and nifedipine have been previously shown to totally block the L-type voltage-gated channels activated by various stimuli in STC-1 cells in the same or even lower concentrations that used in this study [18,19,23,24]. Blocking the L-type voltage-gated calcium channels only partially (by 36%) reduced the AITC induced CCK release suggesting that voltage-gated calcium channels contribute to some extent to the calcium influx in response to AITC stimulation, yet the major proportion of the increased calcium levels is due to calcium influx via TRPA1.…”

Nasal chemosensory cells use bitter taste signaling to detect irritants and bacterial signals

“…S3D). The noninnervated enteroendocrine cells release bioactive peptides rather than neurotransmitters in response to T2R activation (37,38). Despite these differences, the T2R/ gustducin/TrpM5-expressing epithelial cells in the different tissues are similar in extending microvillous processes to the top of the epithelium where the cells will have access to the luminal content of each particular organ.…”

Bitter taste receptors and α-gustducin regulate the secretion of ghrelin with functional effects on food intake and gastric emptying

“…The recent identification of taste receptors and their downstream signaling molecules in the GI mucosa suggests a role for these receptors in the functional detection of nutrients in the gut, which may in turn initiate a hormonal or neural cascade pathway (11). Taste receptors are also present in the tongue epithelium and consist of two G protein-coupled receptor families, T1R and T2R.…”

“…The presence of multiple transcripts corresponding to specific bitter taste receptors has been demonstrated in the upper GI tract of rodents and in a mouse GI enteroendocrine cell line, STC-1 (17). It has been reported that the bitter agonists, phenylthiocarbamide (PTC) and denatonium benzoate (DB), induce the release of cholecystokinin (CCK) and GLP-1 from STC-1 cells (11,18).…”