The role of lipolysis in human orosensory fat perception

Abstract

Taste perception elicited by food constituents and facilitated by sensory cells in the oral cavity is important for the survival of organisms. In addition to the five basic taste modalities, sweet, umami, bitter, sour, and salty, orosensory perception of stimuli such as fat constituents is intensely investigated. Experiments in rodents and humans suggest that free fatty acids represent a major stimulus for the perception of fat-containing food. However, the lipid fraction of foods mainly consists of triglycerides in which fatty acids are esterified with glycerol. Whereas effective lipolysis by secreted lipases (LIPs) liberating fatty acids from triglycerides in the rodent oral cavity is well established, a similar mechanism in humans is disputed. By psychophysical analyses of humans, we demonstrate responses upon stimulation with triglycerides which are attenuated by concomitant LIP inhibitor administration. Moreover, lipolytic activities detected in minor salivary gland secretions directly supplying gustatory papillae were correlated to individual sensitivities for triglycerides, suggesting that differential LIP levels may contribute to variant fat perception. Intriguingly, we found that the LIPF gene coding for lingual/gastric LIP is not expressed in human lingual tissue. Instead, we identified the expression of other LIPs, which may compensate for the absence of LIPF.

Keywords: G protein-coupled receptor; free fatty acid; lipase; taste; triglyceride; von Ebner’s gland.

Fig. 1.

Threshold concentration of fatty stimuli and stimulation of hGPR40 and hGPR120 with a long-chain monounsaturated fatty acid and its triglyceride. A: Detection threshold concentrations determined for the fatty and scratchy oral sensation induced by triolein, oleic acid, and oleyl alcohol, respectively, in the TFL matrix. B: HEK 293T cells stably expressing Gα16Gust44 were transiently transfected with GPRs or empty vector as indicated, and stimulated with 3 μM C18:1 (18) and the corresponding triglyceride glyceryl trioleate at a concentration of 3 μM. Changes in fluorescence after substance application were monitored. Scale: y axis, 100 relative light units; x axis, 100 sec.

Fig. 2.

Expression analysis of human and mouse lingual LIPF by RT-PCR. A: cDNAs, originating from human and mouse CV and stomach (Sto), were used as a template to study the expression of LIPF. For controlling the cDNA quality, Gapdh was amplified. B: The presence of cDNA from the VEGs was controlled by amplifying the sequence of the secreted enzymes, Amy1C (amylase) and Vegp. The presence of cDNA from the chief cells of the fundus was controlled by amplifying the specific secreted enzyme, pepsinogen (PEP). M, molecular weight marker (Gene Ruler DNA ladder mix; Fermentas); CV, amplification of cDNA from human (h)CV or mouse (m)CV, respectively; Sto, amplification of cDNA from human (h)Sto or mouse (m)Sto; +, reaction including RT; −, control without RT; N, H₂O negative control.

Fig. 3.

Cellular expression of lingual LIPF in human and mouse VEG by in situ hybridization. Ten micron thick cryostat cross-sections of CV containing taste buds (not shown) and VEGs were hybridized with digoxigenin-labeled antisense (A, E) or sense (B, F) riboprobes specific for human (A, B) or mouse (E, F) lingual LIPF. To control the presence of VEGs, sequential sections were hybridized with antisense (C, G) or sense (D, H) riboprobes specific for human (C, D) and mouse (G, H) lingual amylase. Scale bar = 100 µM.

Fig. 4.

Lipolytic activity in human minor salivary gland secretions. Release of free oleic acid (A) and recovery of glyceryl trioleate (B) from filter paper vehicles (1 cm²) loaded with glyceryl trioleate (5.14 μmol/cm²) in the absence and presence of the LIP inhibitor tetrahydrolipstatin and placed on the human foliate papillae. The recovery of glyceryl trioleate (B) after 150 s is referenced on the levels of glyceryl trioleate found after a short incubation of 30 s.

Fig. 5.

Individual trioleate perception and oleic acid release by lipolytic activity. Individuals’ distribution of threshold concentrations (A) and LIP activity (B) expressed as amount of free oleic acid released from glyceryl trioleate (5.14 μmol/cm²) for five glyceryl trioleate-sensitive subjects showing detection thresholds <0.65 mmol/l (group A) and five less sensitive subjects with detection thresholds >1.0 mmol/l (group B).

Fig. 6.

Schematic representation of human LIPs and expression analyses of human LIPK, LIPM, and LIPN in circumvallate tissue. A: Phylogenetic tree and schematic of human acid LIP family members. aa, amino acids. Positions of residues corresponding to the leader peptide and the catalytic triad are labeled. cDNAs originating from the CV of two different human subjects were used as templates to study the expression of LIPK, LIPM, and LIPN (B), as well as LIPJ and PNLIP (C). For comparison and as controls, LIPF, α-amylase, or VEGP cDNAs were amplified. M, molecular weight marker (Gene Ruler DNA ladder mix; Fermentas); hCV, amplification of cDNA from human CV; TCF, amplification of cDNA from human tissue surrounding the CV without taste cells; +RT, reaction including RT; −RT, control without RT; N, H₂O negative control. Pos, cDNA of positive control tissue; LIPJ, testis; PNLIP, pancreas.

Fig. 7.

In situ hybridization of LIPK, LIPM, and LIPN on human taste tissue. Ten micron cryostat cross-sections of human VEGs located below the taste epithelium (TE) containing CV were hybridized with digoxigenin-labeled riboprobes specific for LIPK, LIPM, and LIPN. A: An overview of the tissue hybridized with an antisense probe specific for LIPN. Open arrowheads point to signals in basal epidermal layers, filled arrowheads point to minor salivary glands. Scale bar = 500 μm. B: Signals in the minor salivary glands obtained with antisense and sense probes for LIPK, LIPM, and LIPN. Note the absence of staining in the sections hybridized with sense probes. Filled arrowheads indicate some selected positive cells. Scale bar = 100 μm (insets = 20 μm).