Human bitter perception correlates with bitter receptor messenger RNA expression in taste cells

Abstract

Background: Alleles of the receptor gene TAS2R38 are responsible in part for the variation in bitter taste perception of 6-n-propylthiouracil (PROP) and structurally similar compounds (eg, glucosinolates in cruciferous vegetables). At low concentrations, people with the PAV ("taster" amino acid sequence) form of TAS2R38 perceive these bitter compounds, whereas most with the AVI ("nontaster" amino acid sequence) form do not; heterozygotes (PAV/AVI) show the widest range of bitter perception.

Objectives: The objectives were to examine individual differences in expression of PAV-TAS2R38 messenger RNA (mRNA) among heterozygotes, to test the hypotheses that the abundance of allele-specific gene expression accounts for the variation in human bitter taste perception, and to relate to dietary intake of bitter-tasting beverages and foods.

Design: Heterozygous individuals (n = 22) provided psychophysical evaluation of the bitterness of PROP, glucosinolate-containing broccoli juice, non-glucosinolate-containing carrot juice, and several bitter non-TAS2R38 ligands as well as dietary recalls. Fungiform taste papillae were examined for allele-specific TAS2R38 expression by using quantitative polymerase chain reaction.

Results: PAV-TAS2R38 mRNA expression was measured in 18 of 22 heterozygous subjects. Relative expression varied widely and positively correlated with ratings of bitterness intensity of PROP (P = 0.007) and broccoli juice (P = 0.004) but not of the control solutions carrot juice (P = 0.26), NaCl (P = 0.68), caffeine (P = 0.24), or urea (P = 0.47). Expression amounts were related to self-reported recent and habitual caffeine intake (P = 0.060, P = 0.005); vegetable intake was too low to analyze.

Conclusions: We provide evidence that PAV-TAS2R38 expression amount correlates with individual differences in bitter sensory perception and diet. The nature of this correlation calls for additional research on the molecular mechanisms associated with some individual differences in taste perception and food intake. The trial was registered at clinicaltrials.gov as NCT01399944.

FIGURE 1.

Timeline of study procedures. AVI, “nontaster” amino acid sequence in TAS2R38; BP, blood pressure; gLMS, general labeled magnitude scale; PAV, “taster” amino acid sequence in TAS2R38; PROP, 6-n-propylthiouracil.

FIGURE 2.

Relation between TAS2R38 expression and bitterness ratings. Expression of the taster (PAV) form of TAS2R38 compared with bitterness ratings for PROP (A) and broccoli juice (B). Homozygous controls were not included in analyses but are shown as squares for reference: AVI/AVI, white squares; PAV/PAV, gray squares. AVI, “nontaster” amino acid sequence in TAS2R38; gLMS, general labeled magnitude scale; PAV, “taster” amino acid sequence in TAS2R38; PROP, 6-n-propylthiouracil.

FIGURE 3.

Relation between PAV-TAS2R38 expression and reported vegetable intake. Recent intake is shown on the left as either consumed or not consumed (yes or no). Habitual dietary intake is shown on the right as total servings per month. PAV, “taster” amino acid sequence in TAS2R38.

FIGURE 4.

Relation between PAV-TAS2R38 expression and reported caffeine intake. Recent caffeine intake is shown as milligrams consumed in 24 h (left), and habitual caffeine intake is shown as servings of caffeinated beverages per month (right). PAV, “taster” amino acid sequence in TAS2R38.

FIGURE 5.

Multivariate analysis. Arrows point to outcome measures. Dietary caffeine intake reflects recent and habitual intake. PAV, “taster” amino acid sequence in TAS2R38; PROP, 6-n-propylthiouracil.