Bitter tasting agonist signaling through the T2Rs in the tongue represents an evolutionarily conserved aversive response to prevent the ingestion of bitter-tasting dietary toxins. T2Rs are also expressed in endocrine cells of the mammalian gut (6
), where they have been hypothesized to stimulate gut hormone secretion to prevent the absorption of potentially toxic compounds that are swallowed despite their bitter taste in the mouth. The T2Rs are unique single-exon GPCRs; there are 25 predicted functional receptors in the human genome, and while the mouse has homolog of all 25, there are approximately 10 additional functional T2Rs in the mouse genome (1
Bitter tasting agonist binding to T2Rs activates a signaling cascade through a selective Gα subunit, α-gustducin, leading to the stimulation of phospholipase Cβ2 (PLCβ2) (26
). In the tongue, activation of PLCβ2 leads to production of the 2 intracellular messengers inositol-1,4,5-trisphosphate and diacylglycerol and ultimately leads to the gating of the transient receptor potential protein and activation of the gustatory nerve (26
). In enteroendocrine cell lines, bitter tasting agonist signaling increases intracellular Ca2+
concentrations and stimulated CCK secretion (13
). Moreover, gastric infusion of the bitter tasting agonist denatonium benzoate delayed gastric emptying and elicited aversion to a flavored drink in rats (29
), and because CCK delays gastric emptying and decreases food intake, it could play a direct role in the gut sensing mechanism to help limit the absorption of diet-derived bitter tasting toxins (30
In this study, we identified promoters for T2R genes as novel binding targets of the cholesterol-sensitive SREBP-2 transcription factor. At first, the physiological relevance of this binding was unclear, as our ChIP-chip analysis was performed on liver tissue where expression of T2Rs was undetectable. However, we showed that SREBP-2 activates T2R gene expression in the enteroendocrine cell line STC-1, and agonist-responsive signaling through one receptor, T2R138, was enhanced in an SREBP-2–dependent fashion. We also demonstrated that T2R138 localized to enteroendocrine cells in the proximal intestine, where mRNA levels for many of the individual T2Rs were increased when mice were fed a diet supplemented with L/E, which increases nuclear SREBP-2 activity. Additionally, when a mixture of T2R agonists was introduced into the gastrointestinal tract of mice directly by oral gavage, plasma levels of CCK were elevated in the L/E treatment group.
Taken together with other published observations, our data indicate SREBP-2 might be associated with dietary sensing of bitter-tasting constituents of food that are potentially toxic. We suggest that the increased SREBP-2 activity associated with a low-cholesterol diet may be important to increase the sensitivity of the intestinal T2R signaling system. This would make the gut more responsive to the presence of potentially toxic compounds that escape the initial aversive response in the mouth.
In considering this possibility, it is interesting to note that a naturally low-cholesterol diet is high in plant material, which is inherently richer in bitter and potentially toxic compounds relative to a high-cholesterol diet composed of significant amounts of animal flesh. Thus, on a low-cholesterol diet, there is an increased need to absorb the relatively small amounts of essential fats and fat-soluble vitamins that are present and to prevent the absorption of potentially toxic/bitter substances in plant-derived foods (31
The well-described hormonal actions of CCK are consistent with a role in limiting absorption of dietary toxins while maximizing absorption of essential lipid components that are only provided in limited amounts when ingesting a plant-enriched diet (30
). CCK both reduces appetite and slows gastric emptying, which would on the one hand reduce further consumption of the toxic meal and, on the other, ensure that the food that does make it into the stomach has more time to be expelled through emesis, a protective response that is also induced by bitter receptor signaling. Additionally, CCK stimulates gall bladder contraction, and the associated increase in bile acids entering the intestine may aid in the breakdown and absorption of complex carbohydrates and other plant matter that is difficult to digest. This would also facilitate the absorption of low levels of essential fats and fat-soluble vitamins. Both the T2Rs and SREBP pathway are conserved in humans, so it is likely that the mechanism described here is relevant to human physiology as well. Since the combination drug Vytorin, which is a mixture of ezetimibe and a statin, is being widely used to treat hypercholesterolemia in humans, it might be possible to evaluate the link between SREBP-2 and T2Rs in humans.
SREBPs were first identified as key regulators of cholesterol and fatty acid metabolism in mammals (35
). Subsequent studies indicate that they are more globally involved in cell-environment interactions, including apoptosis in response to bacterial toxin (37
) and phagocytosis (38
). In fission yeast, SREBPs have evolved to sense oxygen levels (39
) and Cph2, a bHLH protein closely related to SREBPs, is involved in pseudohyphal growth in Candida albicans
), which occurs in response to environmental cues. Taken together with our current studies, these observations indicate that SREBPs have evolved to play a fundamental role across the Eukarya in the interactions between organisms and their environment.