Insulin activates epithelial sodium channel (ENaC) via phosphoinositide 3-kinase in mammalian taste receptor cells

The Concentration of Salivary Extracellular Vesicles Is Related to Obesity

BACKGROUND AND AIMS

Saliva is essential for the proper dilution and distribution of taste molecules on the tongue. It harbors extracellular vesicles (EVs), which mediate cell-cell communication. Changes in the composition of salivary EVs may arise under obese conditions and may potentially be involved in taste sensation and dysregulated eating behavior. Therefore, this study addresses the relationship between the size and concentration of salivary EVs and metabolic shifts in obesity or factors of taste sensation.

MATERIALS AND METHODS

A total of 119 participants in the Obese Taste Bud (OTB) Study were included, who performed a standardized taste test, underwent taste bud density assessment, and were phenotypically characterized for anthropometrics, blood- and saliva adipokine levels, and various metabolic factors. Utilizing size exclusion chromatography followed by ultrafiltration, EVs were extracted from 2 mL of actively secreted saliva. EVs were characterized using nanoparticle tracking analyses, Western blot, and scanning transmission electron microscopy. Finally, group comparisons and bivariate correlation analyses were conducted.

RESULTS

Among the total cohort, the median size of salivary EVs was 190.05 nm, and the overall concentration ranged from 1.4 × 107 to 1.76 × 109 per mL of saliva. The size range and concentration of EVs per mL are negatively correlated (p = 0.0002, r = -0.264). Comparing lean participants (mean rank of 45.98) with those presenting obesity (mean rank of 34.46), a significant difference in the salivary EV content was observed (p = 0.029). Body weight, BMI, arm and calf circumferences, as well as the percentage of body fat were all negatively related to the concentration of EVs in all study participants (all p < 0.05, r > -0.2). No associations were found between the EV parameters and taste perception but serum alkaline phosphatase levels were negatively correlated (p = 0.007, r = -0.284) and adiponectin serum levels were positively correlated to the EV concentration (p = 0.036, r = 0.208).

CONCLUSION

The current study provides evidence for the relation between salivary EVs and anthropometric as well as metabolic parameters of obesity. This can provide the basis for further research on the cargo of salivary EVs and how they may influence taste sensation, and may elucidate their potential connection to altered eating habits in obesity.

Endocrinology of Taste with Aging

Taste is one of our five primary senses, and taste impairment has been shown to increase with aging. The ability to taste allows us to enjoy the food we eat and to avoid foods that are potentially spoiled or poisonous. Recent advances in our understanding of the molecular mechanisms of taste receptor cells located within taste buds help us decipher how taste works. The discoveries of "classic" endocrine hormones in taste receptor cells point toward taste buds being actual endocrine organs. A better understanding of how taste works may help in reversing taste impairment associated with aging.

A secret of salivary secretions: Multimodal effect of saliva in sensory perception of food

Saliva plays multifunctional roles in oral cavity. Even though its importance for the maintenance of oral health has long been established, the role of saliva in food perception has attracted increasing attention in recent years. We encourage researchers to discover the peculiarity of this biological fluid and aim to combine the data concerning all aspects of the saliva influence on the sensory perception of food. This review presents saliva as a unique material, which modulates food perception due to constant presence of saliva in the mouth and thanks to its composition. Therefore, we highlight the salivary components that contribute to these effects. Moreover, this review is an attempt to structure the effects of saliva on perception of different food categories, where the mechanisms of salivary impact in perception of liquid, semi-solid, and solid foods are revealed. Finally, we emphasize that the large inter-individual variability in salivary composition and secretion appear to contribute to the fact that everyone experiences food in their own way. Therefore, the design of the sensory studies should consider the properties of volunteers' saliva and also carefully monitor the experimental conditions that affect salivary composition and flow rate.

Aging-Related Metabolic Dysfunction in the Salivary Gland: A Review of the Literature

Aging-related salivary dysfunction commonly induces the poor oral health, including decreased saliva flow and dental caries. Although the clinical significance of the salivary glands is well-known, the complex metabolic pathways contributing to the aging-dysfunction process are only beginning to be uncovered. Here, we provide a comprehensive overview of the metabolic changes in aging-mediated salivary gland dysfunction as a key aspect of oral physiology. Several metabolic neuropeptides or hormones are involved in causing or contributing to salivary gland dysfunction, including hyposalivation and age-related diseases. Thus, aging-related metabolism holds promise for early diagnosis, increased choice of therapy and the identification of new metabolic pathways that could potentially be targeted in salivary gland dysfunction.

Recent Advances in Neural Circuits for Taste Perception in Hunger

Feeding is essential for survival and taste greatly influences our feeding behaviors. Palatable tastes such as sweet trigger feeding as a symbol of a calorie-rich diet containing sugar or proteins, while unpalatable tastes such as bitter terminate further consumption as a warning against ingestion of harmful substances. Therefore, taste is considered a criterion to distinguish whether food is edible. However, perception of taste is also modulated by physiological changes associated with internal states such as hunger or satiety. Empirically, during hunger state, humans find ordinary food more attractive and feel less aversion to food they usually dislike. Although functional magnetic resonance imaging studies performed in primates and in humans have indicated that some brain areas show state-dependent response to tastes, the mechanisms of how the brain senses tastes during different internal states are poorly understood. Recently, using newly developed molecular and genetic tools as well as in vivo imaging, researchers have identified many specific neuronal populations or neural circuits regulating feeding behaviors and taste perception process in the central nervous system. These studies could help us understand the interplay between homeostatic regulation of energy and taste perception to guide proper feeding behaviors.

Differences in taste and smell perception between type 2 diabetes mellitus patients and healthy controls

BACKGROUND AND AIMS

The senses of taste and smell are essential determinants of food choice, which in turn may contribute to the development of chronic diseases, including diabetes. Although past studies have evaluated the relationship between type 2 diabetes mellitus (DM2) and senses disorders, this relationship remains controversial. In this study, we evaluated taste and smell perception in DM2 patients and healthy controls (HC). Moreover, we analyzed the association of chemosensory impairments with anthropometric and clinical outcomes (e.g. Body Mass Index (BMI), Fasting blood glucose (FBG), drugs, cardiovascular diseases (CVD), and hypertension) in DM2 patients.

METHODS AND RESULTS

The study included 94 DM2 patients and 244 HC. Taste recognition for 6-n-propylthiouracil (PROP), quinine, citric acid, sucrose, and sodium chloride (NaCl) compounds was assessed using a filter paper method, while smell recognition of 12 odorants was performed using a Sniffin' sticks test. We found that a higher percentage of DM2 patients showed identification impairment in salt taste (22% vs. 5%, p-value<0.0009) and smell recognition (55% vs. 27%, p-value = 0.03) compared to HC. We also observed that 65% of hypertensive DM2 subjects presented smell identification impairment compared to 18% of non-hypertensive patients (p-value = 0.019). Finally, patients with impairments in both taste and smell showed elevated FBG compared to patients without impairment (149.6 vs.124.3 mg/dL, p-value = 0.04).

CONCLUSION

The prevalence of taste and smell identification impairments was higher in DM2 patients compared to HC, and a possible relationship with glycemic levels emerged.

Association of Salivary Content Alteration and Early Ageusia Symptoms in COVID-19 Infections: A Systematic Review

Coronavirus disease 2019 (COVID 19) is a major threat to the health and prosperity of human life at present. It has resulted in loss of thousands of lives globally and has brought countries to the brink of economic, social, and health collapse. A major issue of this infection is the ease with which it transmits through salivary droplets and its survival for long durations outside the body. Therefore, its early detection is critical in prevention, diagnostic, and management efforts of COVID-19 patients. Loss of taste and smell is one of the early symptoms reported in these patients and the virus is abundantly found in the salivary secretion of the infected symptomatic and asymptomatic patients. Infection and inflammation of salivary glands are common among viral infections, particularly in the early stages, which lead to salivary composition changes. Chemosensory sensation of taste is critically dependent on the salivary flow rate and its inorganic constituents, protein levels, specific 3',5'-cyclic adenosine monophosphate and 3',5'-cyclic guanosine monophosphate levels, ghrelins, pH levels, and enzymes. Therefore, the question arises, "Does COVID-19 infection alter the salivary components and composition leading to early transient symptoms of Ageusia and hypogeusia?" This review shows association of the COVID-19 and Ageusia, in addition to the early viral infection of salivary glands and possible changes in salivary flow and content. Therefore, suggesting a potential association between early ageusia in COVID-19 infection and salivary compositional changes.

Taste Receptor Cells in Mice Express Receptors for the Hormone Adiponectin

The metabolic hormone adiponectin is secreted into the circulation by adipocytes and mediates key biological functions, including insulin sensitivity, adipocyte development, and fatty acid oxidation. Adiponectin is also abundant in saliva, where its functions are poorly understood. Here we report that murine taste receptor cells (TRCs) express specific adiponectin receptors and may be a target for salivary adiponectin. This is supported by the presence of all three known adiponectin receptors in transcriptomic data obtained by RNA-seq analysis of purified circumvallate (CV) taste buds. As well, immunohistochemical analysis of murine CV papillae showed that two adiponectin receptors, ADIPOR1 and T-cadherin, are localized to subsets of TRCs. Immunofluorescence for T-cadherin was primarily co-localized with the Type 2 TRC marker phospholipase C β2, suggesting that adiponectin signaling could impact sweet, bitter, or umami taste signaling. However, adiponectin null mice showed no differences in behavioral lick responsiveness compared with wild-type controls in brief-access lick testing. AAV-mediated overexpression of adiponectin in the salivary glands of adiponectin null mice did result in a small but significant increase in behavioral lick responsiveness to the fat emulsion Intralipid. Together, these results suggest that salivary adiponectin can affect TRC function, although its impact on taste responsiveness and peripheral taste coding remains unclear.

Consequences of Obesity on the Sense of Taste: Taste Buds as Treatment Targets?

Premature obesity-related mortality is caused by cardiovascular and pulmonary diseases, type 2 diabetes mellitus, physical disabilities, osteoarthritis, and certain types of cancer. Obesity is caused by a positive energy balance due to hyper-caloric nutrition, low physical activity, and energy expenditure. Overeating is partially driven by impaired homeostatic feedback of the peripheral energy status in obesity. However, food with its different qualities is a key driver for the reward driven hedonic feeding with tremendous consequences on calorie consumption. In addition to visual and olfactory cues, taste buds of the oral cavity process the earliest signals which affect the regulation of food intake, appetite and satiety. Therefore, taste buds may play a crucial role how food related signals are transmitted to the brain, particularly in priming the body for digestion during the cephalic phase. Indeed, obesity development is associated with a significant reduction in taste buds. Impaired taste bud sensitivity may play a causal role in the pathophysiology of obesity in children and adolescents. In addition, genetic variation in taste receptors has been linked to body weight regulation. This review discusses the importance of taste buds as contributing factors in the development of obesity and how obesity may affect the sense of taste, alterations in food preferences and eating behavior.

Effects of insulin signaling on mouse taste cell proliferation

Expression of insulin and its receptor (IR) in rodent taste cells has been proposed, but exactly which types of taste cells express IR and the function of insulin signaling in taste organ have yet to be determined. In this study, we analyzed expression of IR mRNA and protein in mouse taste bud cells in vivo and explored its function ex vivo in organoids, using RT-PCR, immunohistochemistry, and quantitative PCR. In mouse taste tissue, IR was expressed broadly in taste buds, including in type II and III taste cells. With using 3-D taste bud organoids, we found insulin in the culture medium significantly decreased the number of taste cell and mRNA expression levels of many taste cell genes, including nucleoside triphosphate diphosphohydrolase-2 (NTPDase2), Tas1R3 (T1R3), gustducin, carbonic anhydrase 4 (CA4), glucose transporter-8 (GLUT8), and sodium-glucose cotransporter-1 (SGLT1) in a concentration-dependent manner. Rapamycin, an inhibitor of mechanistic target of rapamycin (mTOR) signaling, diminished insulin's effects and increase taste cell generation. Altogether, circulating insulin might be an important regulator of taste cell growth and/or proliferation via activation of the mTOR pathway.

Sleep, food cravings and taste

OBJECTIVE

Taste is influenced by factors from our environment, psychology, and from our own physiological state. The objective of the study was to determine whether sleep influences our sense of taste or our cravings for food.

METHOD

57 healthy panelists, predominantly of college age, submitted to sleep tracking, and subsequently underwent a series of sensory tests, using basic prototypic tastants, as well as real foods. Panelists were also evaluated to quantify food cravings, using both the Leeds Food Preference Questionnaire, and the Control of Eating Questionnaire.

RESULTS

Umami (p = 0.025, F = 5.301) and sour (p = 0.037, F = 4.591) taste were intensified in those rating sleepiness higher, while this group also reported higher implicit wanting for high fat sweet foods (p = 0.011, Wald chi-sq = 14.937). Craving for sweet or savory also associated with a number of measures of taste response to real foods.

CONCLUSIONS

Results imply that a lack of sleep may induce cravings for unhealthy foods, and that foods high in umami or sour taste may be experienced differently due to alterations in taste function. Results imply that feeding behavior may be influenced by a lack of sleep, acting at least partially through our sense of taste.

Angiotensin II type 2 receptor inhibits expression and function of insulin receptor in rat renal proximal tubule cells

Both renin-angiotensin systems and insulin participate in kidney-involved blood pressure regulation. Activation of angiotensin II type 2 receptor (AT₂R) decreases sodium reabsorption in renal proximal tubule (RPT) cells, whereas insulin produces the opposite effect. We presume that AT₂R has an inhibitory effect on insulin receptor expression in RPT cells, which may affect renal sodium transport and therefore be of physiological or pathological significance. Our present study found that activation of AT₂R inhibited insulin receptor expression in a concentration and time-dependent manner in RPT cells from Wistar-Kyoto (WKY) rats. In the presence of a protein kinase C (PKC) inhibitor (PKC inhibitor peptide 19-31, 10^-6 mol/L) or a phosphatidylinositol 3 kinase inhibitor (wortmannin, 10^-6 mol/L), the inhibitory effect of AT₂R on insulin receptor was blocked, indicating that both PKC and phosphatidylinositol 3 kinase were involved in the signaling pathway. There was a linkage between AT₂R and insulin receptor which was determined by both laser confocal microscopy and coimmunoprecipitation. However, the effect of AT₂R activation on insulin receptor expression was different in RPT cells from spontaneously hypertensive rats (SHRs). Being contrary to the effect in WKY RPT cells, AT₂R stimulation increased insulin receptor in SHR RPT cells. Insulin (10^-7 mol/L, 15 minutes) enhanced Na⁺-K⁺-ATPase activity in both WKY and SHR RPT cells. Pretreatment with CGP42112 decreased the stimulatory effect of insulin on Na⁺-K⁺-ATPase activity in WKY RPT cells, whereas pretreatment with CGP42112 increased it in SHR RPT cells. It is suggested that activation of AT₂R inhibits insulin receptor expression and function in RPT cells. The lost inhibitory effect of AT₂R on insulin receptor expression may contribute to the pathophysiology of hypertension.

Cyclic-AMP regulates postnatal development of neural and behavioral responses to NaCl in rats

During postnatal development rats demonstrate an age-dependent increase in NaCl chorda tympani (CT) responses and the number of functional apical amiloride-sensitive epithelial Na+ channels (ENaCs) in salt sensing fungiform (FF) taste receptor cells (TRCs). Currently, the intracellular signals that regulate the postnatal development of salt taste have not been identified. We investigated the effect of cAMP, a downstream signal for arginine vasopressin (AVP) action, on the postnatal development of NaCl responses in 19-23 day old rats. ENaC-dependent NaCl CT responses were monitored after lingual application of 8-chlorophenylthio-cAMP (8-CPT-cAMP) under open-circuit conditions and under ±60 mV lingual voltage clamp. Behavioral responses were tested using 2 bottle/24h NaCl preference tests. The effect of [deamino-Cys1, D-Arg8]-vasopressin (dDAVP, a specific V2R agonist) was investigated on ENaC subunit trafficking in rat FF TRCs and on cAMP generation in cultured adult human FF taste cells (HBO cells). Our results show that in 19-23 day old rats, the ENaC-dependent maximum NaCl CT response was a saturating sigmoidal function of 8-CPT-cAMP concentration. 8-CPT-cAMP increased the voltage-sensitivity of the NaCl CT response and the apical Na+ response conductance. Intravenous injections of dDAVP increased ENaC expression and γ-ENaC trafficking from cytosolic compartment to the apical compartment in rat FF TRCs. In HBO cells dDAVP increased intracellular cAMP and cAMP increased trafficking of γ- and δ-ENaC from cytosolic compartment to the apical compartment 10 min post-cAMP treatment. Control 19-23 day old rats were indifferent to NaCl, but showed clear preference for appetitive NaCl concentrations after 8-CPT-cAMP treatment. Relative to adult rats, 14 day old rats demonstrated significantly less V2R antibody binding in circumvallate TRCs. We conclude that an age-dependent increase in V2R expression produces an AVP-induced incremental increase in cAMP that modulates the postnatal increase in TRC ENaC and the neural and behavioral responses to NaCl.

Comparison of fluorescence probes for intracellular sodium imaging in prostate cancer cell lines

Sodium (Na⁺) ions are known to regulate many signaling pathways involved in both physiological and pathological conditions. In particular, alterations in intracellular concentrations of Na⁺ and corresponding changes in membrane potential are known to be major actors of cancer progression to metastatic phenotype. Though the functionality of Na⁺ channels and the corresponding Na⁺ currents can be investigated using the patch-clamp technique, the latter is rather invasive and a technically difficult method to study intracellular Na⁺ transients compared to Na⁺ fluorescence imaging. Despite the fact that Na⁺ signaling is considered an important controller of cancer progression, only few data using Na⁺ imaging approaches are available so far, suggesting the persisting challenge within the scientific community. In this study, we describe in detail the approach for application of Na⁺ imaging technique to measure intracellular Na⁺ variations in human prostate cancer cells. Accordingly, we used three Na⁺-specific fluorescent dyes-Na⁺-binding benzofuran isophthalate (SBFI), CoroNa™ Green (Corona) and Asante NaTRIUM Green-2 (ANG-2). These dyes have been assessed for optimal loading conditions, dissociation constant and working range after different calibration methods, and intracellular Na⁺ sensitivity, in order to determine which probe can be considered as the most reliable to visualize Na⁺ fluctuations in vitro.

A High-Throughput Automated Microfluidic Platform for Calcium Imaging of Taste Sensing

The human enteroendocrine L cell line NCI-H716, expressing taste receptors and taste signaling elements, constitutes a unique model for the studies of cellular responses to glucose, appetite regulation, gastrointestinal motility, and insulin secretion. Targeting these gut taste receptors may provide novel treatments for diabetes and obesity. However, NCI-H716 cells are cultured in suspension and tend to form multicellular aggregates, preventing high-throughput calcium imaging due to interferences caused by laborious immobilization and stimulus delivery procedures. Here, we have developed an automated microfluidic platform that is capable of trapping more than 500 single cells into microwells with a loading efficiency of 77% within two minutes, delivering multiple chemical stimuli and performing calcium imaging with enhanced spatial and temporal resolutions when compared to bath perfusion systems. Results revealed the presence of heterogeneity in cellular responses to the type, concentration, and order of applied sweet and bitter stimuli. Sucralose and denatonium benzoate elicited robust increases in the intracellular Ca(2+) concentration. However, glucose evoked a rapid elevation of intracellular Ca(2+) followed by reduced responses to subsequent glucose stimulation. Using Gymnema sylvestre as a blocking agent for the sweet taste receptor confirmed that different taste receptors were utilized for sweet and bitter tastes. This automated microfluidic platform is cost-effective, easy to fabricate and operate, and may be generally applicable for high-throughput and high-content single-cell analysis and drug screening.

Taste perception, associated hormonal modulation, and nutrient intake

It is well known that taste perception influences food intake. After ingestion, gustatory receptors relay sensory signals to the brain, which segregates, evaluates, and distinguishes the stimuli, leading to the experience known as "flavor." It is well accepted that five taste qualities – sweet, salty, bitter, sour, and umami – can be perceived by animals. In this review, the anatomy and physiology of human taste buds, the hormonal modulation of taste function, the importance of genetic chemosensory variation, and the influence of gustatory functioning on macronutrient selection and eating behavior are discussed. Individual genotypic variation results in specific phenotypes of food preference and nutrient intake. Understanding the role of taste in food selection and ingestive behavior is important for expanding our understanding of the factors involved in body weight maintenance and the risk of chronic diseases including obesity, atherosclerosis, cancer, diabetes, liver disease, and hypertension.

Regulatory Effects of Ca2+ and H+ on the Rat Chorda Tympani Response to NaCl and KCl

Modulatory effects of pHi and [Ca(2+)]i on taste receptor cell (TRC) epithelial sodium channel (ENaC) were investigated by monitoring chorda tympani (CT) responses to NaCl and KCl at various lingual voltages, before and after lingual application of ionomycin and with 0-10mM CaCl2 in the stimulus and rinse solutions adjusted to pHo 2.0-9.7. 0.1 and 0.5M KCl responses varied continuously with voltage and were fitted to an apical ion channel kinetic model using the same parameters. ENaC-dependent NaCl CT response was fitted to the same channel model but with parameters characteristic of ENaC. A graded increase in TRC [Ca(2+)]i decreased the ENaC-dependent NaCl CT response, and inhibited and ultimately eliminated its pH sensitivity. CT responses to KCl were pHi- and [Ca(2+)]i-independent. Between ±60 mV applied lingual potential, the data were well described by a linear approximation to the nonlinear channel equation and yielded 2 parameters, the open-circuit response and the negative of the slope of the line in the CT response versus voltage plot, designated the response conductance. The ENaC-dependent NaCl CT response conductance was a linear function of the open-circuit response for all pHi-[Ca(2+)]i combinations examined. Analysis of these data shows that pHi and [Ca(2+)]i regulate TRC ENaC exclusively through modulation of the maximum CT response.

Molecular mechanisms of taste recognition: considerations about the role of saliva

The gustatory system plays a critical role in determining food preferences and food intake, in addition to nutritive, energy and electrolyte balance. Fine tuning of the gustatory system is also crucial in this respect. The exact mechanisms that fine tune taste sensitivity are as of yet poorly defined, but it is clear that various effects of saliva on taste recognition are also involved. Specifically those metabolic polypeptides present in the saliva that were classically considered to be gut and appetite hormones (i.e., leptin, ghrelin, insulin, neuropeptide Y, peptide YY) were considered to play a pivotal role. Besides these, data clearly indicate the major role of several other salivary proteins, such as salivary carbonic anhydrase (gustin), proline-rich proteins, cystatins, alpha-amylases, histatins, salivary albumin and mucins. Other proteins like glucagon-like peptide-1, salivary immunoglobulin-A, zinc-α-2-glycoprotein, salivary lactoperoxidase, salivary prolactin-inducible protein and salivary molecular chaperone HSP70/HSPAs were also expected to play an important role. Furthermore, factors including salivary flow rate, buffer capacity and ionic composition of saliva should also be considered. In this paper, the current state of research related to the above and the overall emerging field of taste-related salivary research alongside basic principles of taste perception is reviewed.

Basolateral P2X₄channels stimulate ENaC activity in Xenopus cortical collecting duct A6 cells

The polarized nature of epithelial cells allows for different responses to luminal or serosal stimuli. In kidney tubules, ATP is produced luminally in response to changes in luminal flow. Luminal increases in ATP have been previously shown to inhibit the renal epithelial Na⁺ channel (ENaC). On the other hand, ATP is increased basolaterally in renal epithelia in response to aldosterone. We tested the hypothesis that basolateral ATP can stimulate ENaC function through activation of the P2X₄receptor/channel. Using single channel cell-attached patch-clamp techniques, we demonstrated the existence of a basolaterally expressed channel stimulated by the P2X₄agonist 2-methylthio-ATP (meSATP) in Xenopus A6 cells, a renal collecting duct principal cell line. This channel had a similar reversal potential and conductance to that of P2X₄channels. Cell surface biotinylation of the basolateral side of these cells confirmed the basolateral presence of the P2X4 receptor. Basolateral addition of meSATP enhanced the activity of ENaC in single channel patch-clamp experiments, an effect that was absent in cells transfected with a dominant negative P2X₄receptor construct, indicating that activation of P2X₄channels stimulates ENaC activity in these cells. The effect of meSATP on ENaC activity was reduced after chelation of basolateral Ca²⁺ with EGTA or inhibition of phosphatidylinositol 3-kinase with LY-294002. Overall, our results show that ENaC is stimulated by P2X₄receptor activation and that the stimulation is dependent on increases in intracellular Ca²⁺ and phosphatidylinositol 3-kinase activation.

Rapid elevation of sodium transport through insulin is mediated by AKT in alveolar cells

Abstract Alveolar fluid clearance is driven by vectorial Na(+) transport and promotes postnatal lung adaptation. The effect of insulin on alveolar epithelial Na(+) transport was studied in isolated alveolar cells from 18-19-day gestational age rat fetuses. Equivalent short-circuit currents (ISC) were measured in Ussing chambers and different kinase inhibitors were used to determine the pathway of insulin stimulation. In Western Blot measurements the activation of mediators stimulated by insulin was analyzed. The ISC showed a fast dose-dependent increase by insulin, which could be attributed to an increased ENaC (epithelial Na(+) channel) activity in experiments with permeabilized apical or basolateral membrane. 5-(N-Ethyl-N-isopropyl)amiloride inhibition of ISC was not affected, however, benzamil-sensitive ISC was increased in insulin-stimulated monolayers. The application of LY-294002 and Akti1/2 both completely blocked the stimulating effect of insulin on ISC. PP242 partly blocked the effect of insulin, whereas Rapamycin evoked no inhibition. Western Blot measurements revealed an increased phosphorylation of AKT after insulin stimulation. SGK1 activity was also increased by insulin as shown by Western Blot of pNDRG1. However, in Ussing chamber measurements, GSK650394, an inhibitor of SGK1 did not prevent the increase in ISC induced by insulin. The application of IGF-1 mimicked the effect of insulin and increased the ENaC activity. In addition, an increased autophosphorylation of the IGF-1R/IR was observed after insulin stimulation. We conclude that insulin rapidly increases epithelial Na(+) transport by enhancing the activity of endogenous ENaC through activation of PI3K/AKT in alveolar cells.

Regulation of ENaC in mice lacking renal insulin receptors in the collecting duct

The epithelial sodium channel (ENaC) is one of the central effectors involved in regulation of salt and water homeostasis in the kidney. To study mechanisms of ENaC regulation, we generated knockout mice lacking the insulin receptor (InsR KO) specifically in the collecting duct principal cells. Single-channel analysis in freshly isolated split-open tubules demonstrated that the InsR-KO mice have significantly lower ENaC activity compared to their wild-type (C57BL/6J) littermates when animals were fed either normal or sodium-deficient diets. Immunohistochemical and Western blot assays demonstrated no significant changes in expression of ENaC subunits in InsR-KO mice compared to wild-type littermates. Insulin treatment caused greater ENaC activity in split-open tubules isolated from wild-type mice but did not have this effect in the InsR-KO mice. Thus, these results suggest that insulin increases ENaC activity via its own receptor affecting the channel open probability. To further determine the mechanism of the action of insulin on ENaC, we used mouse mpkCCDc14 principal cells. Insulin significantly augmented amiloride-sensitive transepithelial flux in these cells. Pretreatment of the mpkCCDc14 cells with phosphatidylinositol 3-kinase (LY294002; 10 μM) or mTOR (PP242; 100 nM) inhibitors precluded this effect. This study provides new information about the importance of insulin receptors expressed in collecting duct principal cells for ENaC activity.

Metabolic hormones in saliva: origins and functions

The salivary proteome consists of thousands of proteins, which include, among others, hormonal modulators of energy intake and output. Although the functions of this prominent category of hormones in whole body energy metabolism are well characterized, their functions in the oral cavity, whether as a salivary component, or when expressed in taste cells, are less studied and poorly understood. The respective receptors for the majority of salivary metabolic hormones have been also shown to be expressed in salivary glands (SGs), taste cells, or other cells in the oral mucosa. This review provides a comprehensive account of the gastrointestinal hormones, adipokines, and neuropeptides identified in saliva, SGs, or lingual epithelium, as well as their respective cognate receptors expressed in the oral cavity. Surprisingly, few functions are assigned to salivary metabolic hormones, and these functions are mostly associated with the modulation of taste perception. Because of the well-characterized correlation between impaired oral nutrient sensing and increased energy intake and body mass index, a conceptually provocative point of view is introduced, whereupon it is argued that targeted changes in the composition of saliva could affect whole body metabolism in response to the activation of cognate receptors expressed locally in the oral mucosa.

The migratory capacity of human trophoblastic BeWo cells: effects of aldosterone and the epithelial sodium channel

Aldosterone is a key regulator of the epithelial sodium channel (ENaC) and stimulates protein methylation on the β-subunit of the ENaC. We found that aldosterone (100 nM) promotes cellular migration in a wound-healing model in trophoblastic BeWo cells. Here, we tested if the positive influence of aldosterone on wound healing is related to methylation reactions. Cell migration and proliferation were measured in BeWo cells at 6 h, when mitosis is still scarce. Cell migration covered 12.4, 25.3, 19.6 and 45.1 % of the wound when cultivated under control, aldosterone (12 h), 8Br-cAMP and aldosterone plus 8Br-cAMP, respectively. Amiloride blocked the effects of aldosterone alone or in the presence of 8Br-cAMP on wound healing. Wound healing decreased in aldosterone (plus 8Br-cAMP) coexposed with the methylation inhibitor 3-deaza-adenosine (3-DZA, 12.9 % reinvasion of the wound). There was an increase in wound healing in aldosterone-, 8Br-cAMP- and 3-DZA-treated cells in the presence of AdoMet, a methyl donor, compared to cells in the absence of AdoMet (27.3 and 12.9 % reinvasion of the wound, respectively). Cell proliferation assessed with the reagent MTT was not changed in any of these treatments, suggesting that cellular migration is the main factor for reinvasion of wound healing. Electrophysiological studies showed an increase in ENaC current in the presence of aldosterone. This effect was higher with 8Br-cAMP, and there was a decrease when 3-DZA was present. AdoMet treatment partially reversed this phenomenon. We suggest that aldosterone positively influences wound healing in BeWo cells, at least in part through methylation of the ENaC.

Globular adiponectin protects H9c2 cells from palmitate-induced apoptosis via Akt and ERK1/2 signaling pathways

Background

Cardiomyocytes apoptosis is an important contributor to myocardial dysfunction and heart failure. Adiponectin has cardioprotective effects, potential mechanisms behind it are not clear in cardiomyocytes. The purpose of the study was to investigate whether adiponectin can block palmitate-induced apoptosis and the underlying biochemical mechanism in H9c2 cells.

Methods

H9c2 cells were treated with palmitate presence or absence of 2.5 μg/mL globular adiponectin. The effect on the cell viability of H9c2 cells was evaluated using MTT assay, and cell apoptosis was determined by Hoechst 33342 staining. Protein expression was measured using the western blot method.

Results

Our results showed that the palmitate treatment induced apoptosis in H9c2 cells, which was associated with increasing the level of cleaved caspase-3 and cleaved PARP. Meanwhile, palmitate-induced apoptosis increased the protein level of p-ERK1/2, and decreased the protein level of p-Akt significantly. However, levels of both of these proteins were restored to the normal when pretreated with adiponectin, and followed with the decrease of cleaved caspase-3 and cleaved PARP. In line with these results, the protective effect of adiponectin can be blocked by PI3K/Akt inhibitor LY294002, and palmitate-induced apoptosis can be attenuated by ERK1/2 inhibitor U0126.

Conclusions

Taken together, the present study demonstrated that adiponectin protects H9c2 cells from palmitate-induced apoptosis via PI3K/Akt and ERK1/2 signaling pathways. Our results reveal a link between adiponectin and cardiomyocytes apoptosis, suggesting that adioponectin may be a promising therapeutic for the treatment of lipotoxicity cardiomyopathy.

Sodium-retaining effect of insulin in diabetes

Insulin has long been hypothesized to cause sodium retention, potentially of enough magnitude to contribute to hypertension in obesity, metabolic syndrome, and Type II diabetes. There is an abundance of supportive evidence from correlational analyses in humans, acute insulin infusion studies in humans and animals, and chronic insulin infusion studies in rats. However, the absence of hypertension in human insulinoma patients, and negative results for sodium-retaining or blood pressure effects of chronic insulin infusion in a whole series of dog studies, strongly refute the insulin hypothesis. We recently questioned whether the euglycemic, hyperinsulinemia model used for most insulin infusion studies, including the previous chronic dog studies, was the most appropriate model to test the renal actions of insulin in obesity, metabolic syndrome, and Type II diabetes. In those circumstances, hyperinsulinemia coexists with hyperglycemia. Therefore, we tested the sodium-retaining effect of insulin in chronically instrumented, alloxan-treated diabetic dogs. We used 24 h/day intravenous insulin infusion to regulate plasma insulin concentration. Induction of diabetes (∼400 mg/dl) caused sustained natriuresis and diuresis. However, if we clamped insulin at baseline, control levels, i.e., prevented it from decreasing, then the sustained natriuresis and diuresis were completely reversed, despite the same level of hyperglycemia. We also found that 24 h/day intrarenal insulin infusion had the same effect in diabetic dogs but had no sodium-retaining action in normal dogs. This new evidence that insulin has a sodium-retaining effect during hyperglycemia may have implications for maintaining sodium balance in uncontrolled Type II diabetes.

Chronic intrarenal insulin replacement reverses diabetes mellitus-induced natriuresis and diuresis

We showed recently that sustained natriuresis in type 1 diabetic dogs was attributed to the decrease in insulin rather than the hyperglycemia alone. The sodium-retaining action of insulin appeared to require hyperglycemia, and it completely reversed the diabetic natriuresis and diuresis. This study tested whether the sodium-retaining effect was attributed to direct intrarenal actions of insulin. Alloxan-treated dogs (D; n=7) were maintained normoglycemic using 24-h/d IV insulin replacement. After control measurements, IV insulin was decreased to begin a 6-day diabetic period. Blood glucose increased from 84±6 mg/dL to an average of 428 mg/dL on days 5 and 6, sodium excretion increased from 74±8 to 98±7 meq/d over the 6 days, and urine volume increased from 1645±83 to 2198±170 mL/d. Dir dogs (n=7) were subjected to the same diabetic regimen, but, in addition, insulin was infused continuously into the renal artery at 0.3 mU/kg per minute during the 6-day period. This did not affect plasma insulin. Blood glucose increased from 94±10 mg/dL to an average of 380 mg/dL on days 5 and 6, but sodium excretion averaged 76±5 and 69±8 meq/d during control and diabetes mellitus, respectively. The diuresis also was prevented. Glomerular filtration rate increased only in Dir dogs, and there was no change in mean arterial pressure in either group. This intrarenal insulin infusion had no effect on sodium or volume excretion in normal dogs. Intrarenal insulin replacement in diabetic dogs caused a sustained increase in tubular reabsorption that completely reversed diabetic natriuresis. Insulin plus glucose may work to prevent salt wasting in uncontrolled type 2 diabetes mellitus.

Glucose transporters and ATP-gated K+ (KATP) metabolic sensors are present in type 1 taste receptor 3 (T1r3)-expressing taste cells

Although the heteromeric combination of type 1 taste receptors 2 and 3 (T1r2 + T1r3) is well established as the major receptor for sugars and noncaloric sweeteners, there is also evidence of T1r-independent sweet taste in mice, particularly so for sugars. Before the molecular cloning of the T1rs, it had been proposed that sweet taste detection depended on (a) activation of sugar-gated cation channels and/or (b) sugar binding to G protein-coupled receptors to initiate second-messenger cascades. By either mechanism, sugars would elicit depolarization of sweet-responsive taste cells, which would transmit their signal to gustatory afferents. We examined the nature of T1r-independent sweet taste; our starting point was to determine if taste cells express glucose transporters (GLUTs) and metabolic sensors that serve as sugar sensors in other tissues. Using RT-PCR, quantitative PCR, in situ hybridization, and immunohistochemistry, we determined that several GLUTs (GLUT2, GLUT4, GLUT8, and GLUT9), a sodium-glucose cotransporter (SGLT1), and two components of the ATP-gated K(+) (K(ATP)) metabolic sensor [sulfonylurea receptor (SUR) 1 and potassium inwardly rectifying channel (Kir) 6.1] were expressed selectively in taste cells. Consistent with a role in sweet taste, GLUT4, SGLT1, and SUR1 were expressed preferentially in T1r3-positive taste cells. Electrophysiological recording determined that nearly 20% of the total outward current of mouse fungiform taste cells was composed of K(ATP) channels. Because the overwhelming majority of T1r3-expressing taste cells also express SUR1, and vice versa, it is likely that K(ATP) channels constitute a major portion of K(+) channels in the T1r3 subset of taste cells. Taste cell-expressed glucose sensors and K(ATP) may serve as mediators of the T1r-independent sweet taste of sugars.