Importance of lipolysis in oral cavity for orosensory detection of fat

Lingual lipase is usually secreted from von Ebner's glands, although there is great variation between species. Lingual lipase is thought to be an auxiliary enzyme for fat digestion and absorption in mammals; however, the reason for lipolysis in the oral cavity is not known. We focused on the gustatory sense and investigated the significance of lingual lipase in the perception of a fat taste by using orlistat, a potent lipase inhibitor. Five-minute two-bottle preference tests demonstrated that the addition of orlistat diminished the preference for triacylglycerides but not for free fatty acids. Radioactive triolein applied on rats' circumvallate papilla revealed that lingual lipase was released continuously to generate significant amounts of fatty acids and other lipolytic products within 1-5 s, which was enough time to taste fat. These findings suggest that lingual lipase is released to perceive the taste of triacylglycerides and to find nutritive lipids in food.     

Oro-sensory perception of dietary lipids: New insights into the fat taste transduction

The sense of taste informs the organism about the quality of ingested food. Five basic taste modalities, e.g., sweet, sour, bitter, salty and umami have so far been identified. Recent compelling evidence from rodent and human studies raise the possibility for an additional sixth taste modality devoted to the perception of lipids. Recent studies strongly suggest that lingual CD36, being implicated in the perception of dietary fat, may act as a gustatory lipid sensor. Knocking down of CD36 gene decreases the spontaneous preference for long chain fatty acids (LCFA) in mice subjected to a free choice situation. Lingual CD36, after activation by LCFA, is able to trigger specific signalling mechanisms, e.g., increase in free intracellular calcium concentrations, ([Ca²⁺]i), phosphorylation of protein-tyrosine kinase (PTK) and release of the neurotransmitters like serotonin and nor-adrenaline into synaptic clefts. This signalling cascade is likely responsible for physiologic responses, induced by the detection of lipids in the oral cavity (i.e., lingual fat preference and cephalic phase of digestion). This review provides recent insights into the molecular mechanisms involved in the oro-sensory perception of lipids.     

Fatty acid detection during food consumption and digestion: Associations with ingestive behavior and obesity

The inability of humans to adequately regulate fat consumption is a salient contributor to the development of obesity. The macronutrients, fat, protein and carbohydrate, within foods are detected at various stages of consumption, during which their digestive products, fatty acids, amino acids and sugars, interact with chemosensory cells within the oral epithelium (taste receptor cells) and gastrointestinal (GI) tract (enteroendocrine cells). This chemoreception initiates functional responses, including taste perception, peptide secretion and alterations in GI motility, that play an important role in liking of food, appetite regulation and satiety. This review will summarize the available evidence relating to the oral and GI regulation of fat intake and how chemoreception at both locations is associated with digestive behavior, satiety and weight regulation.     

Measuring Oral Fatty Acid Thresholds,Fat Perception,Fatty Food Liking,and Papillae Density in Humans

Emerging evidence from a number of laboratories indicates that humans have the ability to identify fatty acids in the oral cavity, presumably via fatty acid receptors housed on taste cells. Previous research has shown that an individual''s oral sensitivity to fatty acid, specifically oleic acid (C18:1) is associated with body mass index (BMI), dietary fat consumption, and the ability to identify fat in foods. We have developed a reliable and reproducible method to assess oral chemoreception of fatty acids, using a milk and C18:1 emulsion, together with an ascending forced choice triangle procedure. In parallel, a food matrix has been developed to assess an individual''s ability to perceive fat, in addition to a simple method to assess fatty food liking. As an added measure tongue photography is used to assess papillae density, with higher density often being associated with increased taste sensitivity.     

Lipoprotein lipase in plasma after an oral fat load: relation to free fatty acids.

Lipoprotein lipase (LPL) releases fatty acids from triglyceride-rich lipoproteins for use in cellular metabolic reactions. How this hydrolysis, which occurs at the vascular endothelium, is regulated is poorly understood. A fatty acid feedback system has been proposed by which accumulation of fatty acids impedes LPL-catalyzed hydrolysis and dissociates the enzyme from its endothelial binding sites. We examined this hypothesis in humans who were subjected to an oral fat tolerance test of a mixed-meal type. Plasma triglycerides, free fatty acids, and LPL activity were measured before and repeatedly during a 12-h period after intake of the fat load. Since soybean oil with a high content of linoleic fatty acid was the source of triglycerides, a distinction could be made between endogenous free fatty acids (FFA) and FFA derived directly from lipolysis of postprandial triglyceride-rich lipoproteins. Mean LPL activity was almost doubled (P less than 0.01) 6 h after intake of the oral fat load. The rise in LPL activity was accompanied by an increase of plasma triglycerides and linoleic free fatty acids (18:2 FFA), but not of total plasma FFA, which instead displayed a heterogeneous pattern with essentially unchanged mean levels. The postprandial response of LPL activity largely paralleled the postprandial responses of 18:2 FFA and triglycerides. The highest degree of parallelism was seen between postprandial 18:2 FFA and LPL activity levels. Furthermore, the integrated response (area under the curve, AUC) for plasma measurements of LPL correlated with the AUC for 18:2 FFA (r = 0.40, P less than 0.05), but not with the AUC for plasma triglycerides (r = 0.21, ns). The high degree of parallelism and significant correlation between postprandial plasma LPL activity and 18:2 FFA support the hypothesis of fatty acid control of endothelial LPL during physiological conditions in humans.     

Dietary fatty acids promote sleep through a taste‐independent mechanism

Consumption of foods that are high in fat contribute to obesity and metabolism‐related disorders. Dietary lipids are comprised of triglycerides and fatty acids, and the highly palatable taste of dietary fatty acids promotes food consumption, activates reward centers in mammals and underlies hedonic feeding. Despite the central role of dietary fats in the regulation of food intake and the etiology of metabolic diseases, little is known about how fat consumption regulates sleep. The fruit fly, Drosophila melanogaster, provides a powerful model system for the study of sleep and metabolic traits, and flies potently regulate sleep in accordance with food availability. To investigate the effects of dietary fats on sleep regulation, we have supplemented fatty acids into the diet of Drosophila and measured their effects on sleep and activity. We found that flies fed a diet of hexanoic acid, a medium‐chain fatty acid that is a by‐product of yeast fermentation, slept more than flies starved on an agar diet. To assess whether dietary fatty acids regulate sleep through the taste system, we assessed sleep in flies with a mutation in the hexanoic acid receptor Ionotropic receptor 56D, which is required for fatty acid taste perception. We found that these flies also sleep more than agar‐fed flies when fed a hexanoic acid diet, suggesting the sleep promoting effect of hexanoic acid is not dependent on sensory perception. Taken together, these findings provide a platform to investigate the molecular and neural basis for fatty acid‐dependent modulation of sleep.     

Oral lipase activities and fat-taste receptors for fat-taste sensing in chickens

It has been reported that a functional fat-taste receptor, GPR120, is present in chicken oral tissues, and that chickens can detect fat taste in a behavioral test. However, although triglycerides need to be digested to free fatty acids to be recognized by fat-taste receptors such as GPR120, it remains unknown whether lipase activities exist in chicken oral tissues. To examine this question, we first cloned another fat-taste receptor candidate gene, CD36, from the chicken palate. Then, using RT-PCR, we determined that GPR120 and CD36 were broadly expressed in chicken oral and gastrointestinal tissues. Also by RT-PCR, we confirmed that several lipase genes were expressed in both oral and gastrointestinal tissues. Finally, we analyzed the lipase activities of oral tissues by using a fluorogenic triglyceride analog as a lipase substrate. We found there are functional lipases in oral tissues as well as in the stomach and pancreas. These results suggested that chickens have a basic fat-taste reception system that incorporates a triglycerides/oral-lipases/free fatty acids/GPR120 axis and CD36 axis.     

Lipid composition of the fat body and haemolymph and its relation to lipid release in Oncopeltus fasciatus

Lipid composition of the fat body and haemolymph of male milkweed bug, Oncopeltus fasciatus, was determined. Triglycerides were the predominant lipids of the fat body while diglycerides accounted for the major lipid in the haemolymph. Sterols, sterol esters, and non-esterified fatty acids were present in both fat body and haemolymph besides triglycerides and diglycerides. Only traces of monoglycerides were detected.Gas chromatographic analysis of the fatty acids revealed a difference in the fatty acid composition between fat body and haemolymph glycerides and sterol esters. Oleate and linoleate were the predominant unsaturated fatty acids in both fat body and haemolymph lipids and in the milkweed seeds as well.When fat body was labelled in vivo and in vitro with ¹⁴C-palmitate, the fatty acid was incorporated largely into the triglycerides. When the prelabelled fat body was incubated with a medium containing haemolymph the fat body released lipids mainly as diglycerides. Some radioactivity was observed in the triglycerides and non-esterified fatty acids also.Electrophoretic analysis of the incubation medium containing the haemolymph revealed that the released lipids were bound to three haemolymph lipoprotein bands. Lipid mobilization, release, and transport in Oncopeltus are discussed in relation to studies on other insects.     

Survival of the fattest: fat babies were the key to evolution of the large human brain

In the past 2 million years, the hominid lineage leading to modern humans evolved significantly larger and more sophisticated brains than other primates. We propose that the modern human brain was a product of having first evolved fat babies. Hence, the fattest (infants) became, mentally, the fittest adults. Human babies have brains and body fat each contributing to 11-14% of body weight, a situation which appears to be unique amongst terrestrial animals. Body fat in human babies provides three forms of insurance for brain development that are not available to other land-based species: (1) a large fuel store in the form of fatty acids in triglycerides; (2) the fatty acid precursors to ketone bodies which are key substrates for brain lipid synthesis; and (3) a store of long chain polyunsaturated fatty acids, particularly docosahexaenoic acid, needed for normal brain development. The triple combination of high fuel demands, inability to import cholesterol or saturated fatty acids, and dependence on docosahexaenoic acid puts the mammalian brain in a uniquely difficult situation compared with other organs and makes its expansion in early humans all the more remarkable. We believe that fresh- and salt-water shorelines provided a uniquely rich, abundant and accessible food supply, and the only viable environment for evolving both body fat and larger brains in human infants.     

Mechanism of fat taste perception: Association with diet and obesity

Energy homeostasis plays a significant role in food consumption and body weight regulation with fat intake being an area of particular interest due to its palatability and high energy density. Increasing evidence from humans and animal studies indicate the existence of a taste modality responsive to fat via its breakdown product fatty acids. These studies implicate multiple candidate receptors and ion channels for fatty acid taste detection, indicating a complex peripheral physiology that is currently not well understood. Additionally, a limited number of studies suggest a reduced ability to detect fatty acids is associated with obesity and a diet high in fat reduces an individual's ability to detect fatty acids. To support this, genetic variants within candidate fatty acid receptors are also associated with obesity reduced ability to detect fatty acids. Understanding oral peripheral fatty acid transduction mechanisms and the association with fat consumption may provide the basis of novel approaches to control development of obesity.     

Differential Candida albicans lipase gene expression during alimentary tract colonization and infection

The human pathogenic fungus Candida albicans, which can reside as a benign commensal of the gut, possesses a large family of lipase encoding genes whose extracellular activity may be important for colonization and subsequent infection. The expression of the C. albicans lipase gene family (LIP1-10) was investigated using a mouse model of mucosal candidiasis during alimentary tract colonization (cecum contents) and orogastric infection. LIPs4-8 were expressed in nearly every sample prepared from the cecum contents and infected mucosal tissues (stomach, hard palate, esophagus and tongue) suggesting a maintenance function for these gene products. In contrast, LIPs1, 3, and 9, which were detected consistently in infected gastric tissues, were essentially undetectable in infected oral tissues. In addition, LIP2 was expressed consistently in cecum contents but was undetectable in infected oral tissues suggesting LIP2 may be important for alimentary tract colonization, but not oral infection. The host responded to a C. albicans infection by significantly increasing expression of the chemokines MIP-2 and KC at the site of infection. Therefore, differential LIP gene expression was observed during colonization, infection and at different infected mucosal sites.     

Comparative understanding of UTS2 and UTS2R genes for their involvement in type 2 diabetes mellitus

Several reports have shown that urotensin 2 (UTS2) and its receptor (UTS2R) are involved in glucose metabolism and insulin resistance, which lead to development of type 2 diabetes mellitus (T2DM) in humans. In the present study, we annotated both bovine UTS2 and UTS2R genes and identified 5 single nucleotide polymorphisms (SNPs) for the former gene and 14 mutations for the latter gene. Four mutations were genotyped on a Wagyu x Limousin reference population, including 6 F₁ bulls, 113 F₁ dams and ~250 F₂ progeny. Among 12 phenotypes related to fat deposition and fatty acid composition, we observed that the UTS2 gene was significantly associated with the amount of skeletal saturated fatty acids, while its receptor (UTS2R) gene had significant effects on amounts of saturated and monounsaturated fatty acids, Δ⁹ desaturase activity for converting 16:0 into 16:1, muscle fat (marbling) score and Longissimus Dorsi muscle area. However, in this population, these markers were not associated with subcutaneous fat depth or percent kidney, pelvic and heart fat. We also found that mutations in the promoter regions altered the promoter activities in both genes and coding SNPs might affect the mRNA stability in the UTS2R gene. Overall, our present study provides the first evidence that both UTS2 and UTS2R genes regulate skeletal muscle fat accumulation and fatty acid metabolism, thus indicating their potential pathological functions related to obesity and T2DM in humans.     

The oral lipid sensor GPR120 is not indispensable for the orosensory detection of dietary lipids in mice

Implication of the long-chain fatty acid (LCFA) receptor GPR120, also termed free fatty acid receptor 4, in the taste-guided preference for lipids is a matter of debate. To further unravel the role of GPR120 in the “taste of fat”, the present study was conducted on GPR120-null mice and their wild-type littermates. Using a combination of morphological [i.e., immunohistochemical staining of circumvallate papillae (CVP)], behavioral (i.e., two-bottle preference tests, licking tests and conditioned taste aversion) and functional studies [i.e., calcium imaging in freshly isolated taste bud cells (TBCs)], we show that absence of GPR120 in the oral cavity was not associated with changes in i) gross anatomy of CVP, ii) LCFA-mediated increases in intracellular calcium levels ([Ca²⁺]_i), iii) preference for oily and LCFA solutions and iv) conditioned avoidance of LCFA solutions. In contrast, the rise in [Ca²⁺]_i triggered by grifolic acid, a specific GPR120 agonist, was dramatically curtailed when the GPR120 gene was lacking. Taken together, these data demonstrate that activation of lingual GPR120 and preference for fat are not connected, suggesting that GPR120 expressed in TBCs is not absolutely required for oral fat detection in mice     

INTESTINAL TRIGLYCERIDE ABSORPTION IN THE RAT : An Electron Microscopical Study

This report provides information on the morphology of fat absorption in rat intestinal epithelial cells. Three types of experiments were performed: (a) intubation of corn oil into fasted rats, (b) injection of physiological fatty-chyme prepared from fat-fed donor rats into ligated segments of jejunum of fasted animals, and (c) administration of electron-opaque particles in corn oil and markers given concurrently with the fat. These results support the hypothesis that fat is absorbed by selective diffusion of monoglycerides and fatty acids from micelles rather than by pinocytosis of unhydrolized triglycerides. Evidence is presented that the pits between the microvilli, previously believed to function in the transport of fat, are not involved in this process. Instead they appear to contribute their contents to lysosomes in the apical cytoplasm. Arguments are offered that the monoglycerides and fatty acids diffuse from the micelle while the latter is associated with the microvillous membrane of the absorptive cell. These micellar components penetrate the plasma membrane and diffuse into the cytoplasmic matrix where they encounter the SER. Triglyceride synthesis occurs in the SER and results in the deposition of fat droplets within its lumina. The synthesis of triglycerides and their sequestration into the SER establishes an inward diffusion gradient of monoglycerides and fatty acids.     

Analysis of the promoter region of the gene LIP1 encoding triglyceride lipase from Fusarium graminearum

Triglyceride lipases catalyze the reversible degradation of glycerol esters with long-chain fatty acids into fatty acids and glycerol. In silico analysis of 5′-end flanking sequence of the gene LIP1 encoding a triglyceride lipase from the wheat head blight pathogen Fusarium graminearum revealed the presence of several cis-regulatory elements. To delineate the function of these regulatory elements, we constructed a series of deletion mutants in the LIP1 promoter region fused to the open reading frame of a green fluorescent protein (GFP) and assayed the promoter activity. Analysis of GFP expression levels in mutants indicated that a 563-bp promoter sequence was sufficient to drive the expression of LIP1 and regulatory elements responsible for the gene induction were located within the 563-372 bp region. To further investigate the regulatory elements, putative cis-acting elements spanned within the 563-372 bp region were mutated using a targeted mutagenesis approach. A CCAAT box, a CreA binding site, and a fatty acid responsive element (FARE) were identified and confirmed to be required for the basal expression of LIP1, glucose suppression and fatty acid induction, respectively.     

Competition between Food Particles and Rumen Bacteria in the Uptake of Long-chain Fatty Acids and Triglycerides

S ummary . Suspensions of mixed rumen bacteria were incubated with long-chain fatty acids and their corresponding triglycerides in the presence and absence of the food particles present in the rumen. The uptake of free fatty acids and triglycerides by the bacteria was due largely to physical adsorption. With increasing unsaturation of the free fatty acids, less adsorption occurred and in all instances the presence of food particles reduced greatly the extent to which the fatty acids were adsorbed on the bacteria. The triglycerides were adsorbed to the bacteria to a much less extent than their corresponding free fatty acids, more of the substrate remaining in the supernatant fraction. The presence of food particles had little effect on the extent to which the triglycerides became adsorbed to the bacteria, but reduced greatly the amount of triglyceride present in the supernatant fraction. The decrease in amount of free fatty acid associated with the bacteria and the decrease in the amount of triglyceride in the supernatant liquid when food particles were present, was wholly accounted for by the increase in amount of these substrates associated with the food particles. The implications of these findings with respect to the metabolism of lipids in the rumen is discussed.