We have determined that widely used herbicides 2,4DP, 2,4D and MCPP potently inhibit the human T1R2+T1R3 receptor. We also found that clofibrate and related anti-lipid drugs such as bezafibrate and gemfibrosil also strongly inhibit the human T1R2+T1R3 receptor. We determined that these compounds act specifically on the human and not the rodent form of the sweet receptor. Furthermore, it is the transmembrane portion of human T1R3 that is targeted by these compounds. Because only old world monkeys and primates (including humans) have similar T1R3 receptors that would respond to lactisole and related compounds 14, 28, 29
most animal models would not have shown any effects through their T1R3 receptors, either in taste cells or gut endocrine cells.
From our measured IC-50s of fibrates and phenoxy herbicides on T1R3 we can draw some preliminary conclusions about structure-activity relationships (SARs). It appears that the longer and more branched aliphatic chains in fibrates weaken the inhibitory activity toward the T1R3. However, a certain length or branching of the chain may be required for optimal inhibition of T1R3: for example 2,4DP is 10-fold more potent than is the 2,4D. The only difference between the two structures is that propionic acid is replaced by acetic acid (). The inhibitory activity toward T1R3 is also affected by the modifications of the aromatic portion of the compounds. Naphthalene acetic acid, NAA, shows modest activity while its phenyl counterpart, PAA, is very weak, and the structures with indole rings (IAA, IPA) have no activity. Di-chloro substitutions (ortho or para) on the phenoxy group appear to improve T1R3 inhibitory potency in comparison to one single para-methoxy, single para-chloro substitutions, or double ortho-methyl/para-chloro substitutions (e.g. 2,4DP is 10X more potent than lactisole; and 2,4DP, and 2,4D gains 2-3-fold potency over MCPP and 4,CPA respectively). Tri-chloro substitutions do not have any further effect on T1R3 inhibitory activity (e.g. 2,4DP and 2,4,5TPP have about the same potency). The phenoxy-motif seems obligatory as its absence could be the reason that dicamba (and trichlorinated benzoic acid) do not inhibit T1R3 whereas 2,4D and 2,4,5TPP do.
T1R3’s role as a critical component of sweet and umami receptors in taste cells is well established 2, 3
. T1R3, like gustducin, has also been implicated in glucose-sensing functions of gut endocrine cells and thereby in glucose homeostasis 7
. From psychophysical self-experimentation conducted by one of the authors (BM) we found that clofibric acid potently inhibited both sweet and umami (MSG) taste in vivo (data not shown). We predict that fibrates and phenoxy herbicides would also inhibit T1R3 receptors within and outside of the taste system (e.g. enteroendocrine cells).
Fibrates are used in the treatment of many forms of hyperlipidemia: these compounds primarily lower triglyceride levels, modestly improve HDL and seem to improve insulin resistance when the dyslipidemia is associated with other features of the metabolic syndrome (hypertension and diabetes mellitus type 2) 30
. The therapeutic target of fibrates is thought to be nuclear peroxisome proliferator activated receptor-alpha (PPAR-α), whose activation leads to increased transcription of several genes involved in lipid metabolism 22
. Studies with PPAR agonists also reported lower plasma glucose, improved glucose tolerance, and enhanced insulin sensitivity 22
, although the mechanism of action of fibrates on glucose homeostasis remains unclear. Our results show that IC50s (30 to 100 µM) of clofibric and bezafibric acids for inhibiting the sweet receptor are comparable to their EC50s (50–55 µM) for activating PPAR-alpha 23
. Based on this calculation alone, the plasma level of clofibric acid attained in the course of the fibrate treatment would be sufficient to systemically block the T1R3 receptor. Thus, T1R3-containing receptors may be an important biological target of fibrates and could mediate certain of their effects on lipid metabolism and glucose homeostasis.
Phenoxy-auxin herbicides are synthetic herbicides that mimic the action of auxin plant hormones. These herbicides are very effective in killing broadleaf plants while leaving grasses largely unaffected, and are thus extensively used in crop agriculture and in landscape turf management 25
. Several of the phenoxy-herbicides tested here are among the most widely used, e.g. 2,4D. They have low soil sorption, high leachability, and are prone to enter the human food chain. Long-term biological effects of these compounds in humans are largely unknown and based on our studies their actions on T1R3-containing receptors would not have manifested in rodent models. We think it prudent to evaluate effects of acute and chronic exposure to these compounds specifically on human metabolism and development 31
T1R3 receptors are now known to be expressed in a number of tissues: taste cells, endocrine cells in the gastrointestinal tract, and cells in pancreas and testes 5, 32–34
. In rodents, T1R3, gustducin and other signaling molecules previously found in taste cells have been shown to be present in gut endocrine cells and implicated in nutrient sensing and regulation of glucose metabolism through release of intestinal hormones 7, 9, 10
. To date, comparable studies in humans or old-world monkeys have not been conducted. In view of the number of compounds used in agriculture, medicine and food industry that may affect activity of T1R receptors much more research needs to be done on the health-related effects of these compounds. The results presented here exemplify the need of testing chemicals intended for human use and/or consumption on tissues, cells and organisms with pharmacological targets similar or identical to humans. Based on our present work compounds that selectively act on T1R3 but not PPAR might be identified and prove useful in the treatment of metabolic syndrome, obesity and type II diabetes.