Calcium absorption by Cav1.3 induces terminal web myosin II phosphorylation and apical GLUT2 insertion in rat intestine

Physiological functions of glucose transporter-2: From cell physiology to links with diabetes mellitus

Glucose is a sugar crucial for human health since it participates in many biochemical reactions. It produces adenosine 5'-triphosphate (ATP) and nucleosides through glucose metabolic and pentose phosphate pathways. These processes require many transporter proteins to assist in transferring glucose across cells, and the most notable ones are glucose transporter-2 (GLUT-2) and sodium/glucose cotransporter 1 (SGLT1). Glucose enters small intestinal epithelial cells from the intestinal lumen by crossing the brush boundary membrane via the SGLT1 cotransporter. It exits the cells by traversing the basolateral membrane through the activity of the GLUT-2 transporter, supplying energy throughout the body. Dysregulation of these glucose transporters is involved in the pathogenesis of several metabolic diseases, such as diabetes. Natural loss of GLUT-2 or its downregulation causes abnormal blood glucose concentrations in the body, such as fasting hypoglycemia and glucose tolerance. Therefore, understanding GLUT-2 physiology is necessary for exploring the mechanisms of diabetes and targeted treatment development. This article reviews how the apical GLUT-2 transporter maintains normal physiological functions of the human body and the adaptive changes this transporter produces under pathological conditions such as diabetes.

Ion channels and transporters regulate nutrient absorption in health and disease

Ion channels and transporters are ubiquitously expressed on cell membrane, which involve in a plethora of physiological process such as contraction, neurotransmission, secretion and so on. Ion channels and transporters is of great importance to maintaining membrane potential homeostasis, which is essential to absorption of nutrients in gastrointestinal tract. Most of nutrients are electrogenic and require ion channels and transporters to absorb. This review summarizes the latest research on the role of ion channels and transporters in regulating nutrient uptake such as K+ channels, Ca2+ channels and ion exchangers. Revealing the mechanism of ion channels and transporters associated with nutrient uptake will be helpful to provide new methods to diagnosis and find potential targets for diseases like diabetes, inflammatory bowel diseases, etc. Even though some of study still remain ambiguous and in early stage, we believe that ion channels and transporters will be novel therapeutic targets in the future.

Vitamin D and malabsorptive gastrointestinal conditions: A bidirectional relationship?

This paper is one of the outcomes of the 5th International Conference "Controversies in Vitamin D" held in Stresa, Italy from 15 to 18 September 2021 as part of a series of annual meetings which was started in 2017. The scope of these meetings is to discuss controversial issues about vitamin D. Publication of the outcomes of the meeting in international journals allows a wide sharing of the most recent data with the medical and academic community. Vitamin D and malabsorptive gastrointestinal conditions was one of the topics discussed at the meeting and focus of this paper. Participants to the meeting were invited to review available literature on selected issues related to vitamin D and gastrointestinal system and to present their topic to all participants with the aim to initiate a discussion on the main outcomes of which are reported in this document. The presentations were focused on the possible bidirectional relationship between vitamin D and gastrointestinal malabsorptive conditions such as celiac disease, inflammatory bowel diseases (IBDs) and bariatric surgery. In fact, on one hand the impact of these conditions on vitamin D status was examined and on the other hand the possible role of hypovitaminosis D on pathophysiology and clinical course of these conditions was also evaluated. All examined malabsorptive conditions severely impair vitamin D status. Since vitamin D has known positive effects on bone this in turn may contribute to negative skeletal outcomes including reduced bone mineral density, and increased risk of fracture which may be mitigated by vitamin D supplementation. Due to the immune and metabolic extra-skeletal effects there is the possibility that low levels of vitamin D may negatively impact on the underlying gastrointestinal conditions worsening its clinical course or counteracting the effect of treatment. Therefore, vitamin D status assessment and supplementation should be routinely considered in all patients affected by these conditions. This concept is strengthened by the existence of a possible bidirectional relationship through which poor vitamin D status may negatively impact on clinical course of underlying disease. Sufficient elements are available to estimate the desired threshold vitamin D level above which a favourable impact on the skeleton in these conditions may be obtained. On the other hand, ad hoc controlled clinical trials are needed to better define this threshold for obtaining a positive effect of vitamin D supplementation on occurrence and clinical course of malabsorptive gastrointestinal diseases.

Artocarpus lacucha Extract and Oxyresveratrol Inhibit Glucose Transporters in Human Intestinal Caco-2 Cells

Reduction of intestinal glucose absorption might result from either delayed carbohydrate digestion or blockage of glucose transporters. Previously, oxyresveratrol was shown to inhibit α-glucosidase, but its effect on glucose transporters has not been explored. The present study aimed to assess oxyresveratrol-induced inhibition of the facilitative glucose transporter 2 and the active sodium-dependent glucose transporter 1. An aqueous extract of Artocarpus lacucha, Puag Haad, which is oxyresveratrol-enriched, was also investigated. Glucose transport was measured by uptake into Caco-2 cells through either glucose transporter 2 or sodium-dependent glucose transporter 1 according to the culture conditions. Oxyresveratrol (40 to 800 µM) dose-dependently reduced glucose transport, which appeared to inhibit both glucose transporter 2 and sodium-dependent glucose transporter 1. Puag Haad at similar concentrations also inhibited these transporters but with greater efficacy. Oxyresveratrol and Puag Haad could help reduce postprandial hyperglycemic peaks, which are considered to be most damaging in diabetics.

Intestinal Calcium Absorption

In this article, we focus on mammalian calcium absorption across the intestinal epithelium in normal physiology. Intestinal calcium transport is essential for supplying calcium for metabolism and bone mineralization. Dietary calcium is transported across the mucosal epithelia via saturable transcellular and nonsaturable paracellular pathways, both of which are under the regulation of 1,25-dihydroxyvitamin D₃ and several other endocrine and paracrine factors, such as parathyroid hormone, prolactin, 17β-estradiol, calcitonin, and fibroblast growth factor-23. Calcium absorption occurs in several segments of the small and large intestine with varying rates and capacities. Segmental heterogeneity also includes differential expression of calcium transporters/carriers (e.g., transient receptor potential cation channel and calbindin-D_9k ) and the presence of favorable factors (e.g., pH, luminal contents, and gut motility). Other proteins and transporters (e.g., plasma membrane vitamin D receptor and voltage-dependent calcium channels), as well as vesicular calcium transport that probably contributes to intestinal calcium absorption, are also discussed. © 2021 American Physiological Society. Compr Physiol 11:1-27, 2021.

Small intestinal glucose and sodium absorption through calcium-induced calcium release and store-operated Ca2+ entry mechanisms

BACKGROUND AND PURPOSE

Luminal glucose enhances intestinal Ca²⁺ absorption through apical Ca_v 1.3 channels necessary for GLUT2-mediated glucose absorption. As these reciprocal mechanisms are not well understood, we investigated the regulatory mechanisms of intestinal [Ca²⁺ ]_cyt and SGLT1-mediated Na⁺ -glucose co-transports.

EXPERIMENTAL APPROACH

Glucose absorption and channel expression were examined in mouse upper jejunal epithelium using an Ussing chamber, immunocytochemistry and Ca²⁺ and Na⁺ imaging in single intestinal epithelial cells.

KEY RESULTS

Glucose induced jejunal I_sc via Na⁺ -glucose cotransporter 1 (SGLT1) operated more efficiently in the presence of extracellular Ca²⁺ . A crosstalk between luminal Ca²⁺ entry via plasma Ca_v 1.3 channels and the ER Ca²⁺ release through ryanodine receptor (RYR) activation in small intestinal epithelial cell (IEC) or Ca²⁺ -induced Ca²⁺ release (CICR) mechanism was involve in Ca²⁺ -mediated jejunal glucose absorption. The ER Ca²⁺ release through RyR triggered basolateral Ca²⁺ entry or store-operated Ca²⁺ entry (SOCE) mechanism and the subsequent Ca²⁺ entry via Na⁺ /Ca²⁺ exchanger 1 (NCX1) were found to be critical in Na⁺ -glucose cotransporter-mediated glucose absorption. Blocking RyR, SOCE and NCX1 inhibited glucose induced [Na⁺ ]_cyt and [Ca²⁺ ]_cyt in single IEC and protein expression and co-localization of STIM1/Orai1, RyR1 and NCX1 were detected in IEC and jejunal mucosa.

CONCLUSION AND IMPLICATIONS

Luminal Ca²⁺ influx through Ca_v 1.3 triggers the CICR through RyR1 to deplete the ER Ca²⁺ , which induces the basolateral STIM1/Orai1-mediated SOCE mechanism and the subsequent Ca²⁺ entry via NCX1 to regulate intestinal glucose uptake via Ca²⁺ signalling. Targeting these mechanisms in IEC may help to modulate blood glucose and sodium in the metabolic disease.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Protein Recovery from Underutilised Marine Bioresources for Product Development with Nutraceutical and Pharmaceutical Bioactivities

The global demand for dietary proteins and protein-derived products are projected to dramatically increase which cannot be met using traditional protein sources. Seafood processing by-products (SPBs) and microalgae are promising resources that can fill the demand gap for proteins and protein derivatives. Globally, 32 million tonnes of SPBs are estimated to be produced annually which represents an inexpensive resource for protein recovery while technical advantages in microalgal biomass production would yield secure protein supplies with minimal competition for arable land and freshwater resources. Moreover, these biomaterials are a rich source of proteins with high nutritional quality while protein hydrolysates and biopeptides derived from these marine proteins possess several useful bioactivities for commercial applications in multiple industries. Efficient utilisation of these marine biomaterials for protein recovery would not only supplement global demand and save natural bioresources but would also successfully address the financial and environmental burdens of biowaste, paving the way for greener production and a circular economy. This comprehensive review analyses the potential of using SPBs and microalgae for protein recovery and production critically assessing the feasibility of current and emerging technologies used for the process development. Nutritional quality, functionalities, and bioactivities of the extracted proteins and derived products together with their potential applications for commercial product development are also systematically summarised and discussed.

Starch Polysaccharides in the Human Diet: Effect of the Different Source and Processing on its Absorption

Starch is the main source of carbohydrates in human diet. It is widely used in food processing and non-food industrial applications. The effects on starch digestion and absorption in humans are reviewed in relation to the starch composition, sources, plant genetic variation, food processing and cooking. The impact of food industrial processing and starch modification on the digestibility of starch containing foods and on gut microbiota are discussed. Considering that the resistant starch (RS) fraction escaped from the small intestine is fermented in large intestine, all the variables that influence starch digestibility and absorption must be taken into account when discussing about healthy properties of fibers. Future trends in food industries are aimed to increase the RS fraction in processed foods in order to improve nutritional quality as well as to clarify the influence of RS3 and RS4 on gut microbiota.

Effects of Food Components That Activate TRPA1 Receptors on Mucosal Ion Transport in the Mouse Intestine

TRPA1 is a ligand-activated cation channel found in the intestine and other tissues. Components of food that stimulate TRPA1 receptors (phytonutrients) include allyl isothiocyanate, cinnamaldehyde and linalool, but these may also act at other receptors. Cells lining the intestinal mucosa are immunoreactive for TRPA1 and Trpa1 mRNA occurs in mucosal extracts, suggesting that the TRPA1 receptor is the target for these agonists. However, in situ hybridisation reveals Trpa1 expression in 5-HT containing enteroendocrine cells, not enterocytes. TRPA1 agonists evoke mucosal secretion, which may be indirect (through release of 5-HT) or direct by activation of enterocytes. We investigated effects of the phytonutrients on transmucosal ion currents in mouse duodenum and colon, and the specificity of the phytonutrients in cells transfected with Trpa1, and in Trpa1-deficient mice. The phytonutrients increased currents in the duodenum with the relative potencies: allyl isothiocyanate (AITC) > cinnamaldehyde > linalool (0.1 to 300 μM). The rank order was similar in the colon, but linalool was ineffective. Responses to AITC were reduced by the TRPA1 antagonist HC-030031 (100 μM), and were greatly diminished in Trpa1−/− duodenum and colon. Responses were not reduced by tetrodotoxin, 5-HT receptor antagonists, or atropine, but inhibition of prostaglandin synthesis reduced responses. Thus, functional TRPA1 channels are expressed by enterocytes of the duodenum and colon. Activation of enterocyte TRPA1 by food components has the potential to facilitate nutrient absorption.

A computer model simulating human glucose absorption and metabolism in health and metabolic disease states

A computer model designed to simulate integrated glucose-dependent changes in splanchnic blood flow with small intestinal glucose absorption, hormonal and incretin circulation and hepatic and systemic metabolism in health and metabolic diseases e.g. non-alcoholic fatty liver disease, (NAFLD), non-alcoholic steatohepatitis, (NASH) and type 2 diabetes mellitus, (T2DM) demonstrates how when glucagon-like peptide-1, (GLP-1) is synchronously released into the splanchnic blood during intestinal glucose absorption, it stimulates superior mesenteric arterial (SMA) blood flow and by increasing passive intestinal glucose absorption, harmonizes absorption with its distribution and metabolism. GLP-1 also synergises insulin-dependent net hepatic glucose uptake (NHGU). When GLP-1 secretion is deficient post-prandial SMA blood flow is not increased and as NHGU is also reduced, hyperglycaemia follows. Portal venous glucose concentration is also raised, thereby retarding the passive component of intestinal glucose absorption.   Increased pre-hepatic sinusoidal resistance combined with portal hypertension leading to opening of intrahepatic portosystemic collateral vessels are NASH-related mechanical defects that alter the balance between splanchnic and systemic distributions of glucose, hormones and incretins.The model reveals the latent contribution of portosystemic shunting in development of metabolic disease. This diverts splanchnic blood content away from the hepatic sinuses to the systemic circulation, particularly during the glucose absorptive phase of digestion, resulting in inappropriate increases in insulin-dependent systemic glucose metabolism.  This hastens onset of hypoglycaemia and thence hyperglucagonaemia. The model reveals that low rates of GLP-1 secretion, frequently associated with T2DM and NASH, may be also be caused by splanchnic hypoglycaemia, rather than to intrinsic loss of incretin secretory capacity. These findings may have therapeutic implications on GLP-1 agonist or glucagon antagonist usage.

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters

The absorption of glucose is electrogenic in the small intestinal epithelium. The major route for the transport of dietary glucose from intestinal lumen into enterocytes is the Na⁺/glucose cotransporter (SGLT1), although glucose transporter type 2 (GLUT2) may also play a role. The membrane potential of small intestinal epithelial cells (IEC) is important to regulate the activity of SGLT1. The maintenance of membrane potential mainly depends on the activities of cation channels and transporters. While the importance of SGLT1 in glucose absorption has been systemically studied in detail, little is currently known about the regulation of SGLT1 activity by cation channels and transporters. A growing line of evidence suggests that cytosolic calcium ([Ca²⁺]_cyt) can regulate the absorption of glucose by adjusting GLUT2 and SGLT1. Moreover, the absorption of glucose and homeostasis of Ca²⁺ in IEC are regulated by cation channels and transporters, such as Ca²⁺ channels, K⁺ channels, Na⁺/Ca²⁺ exchangers, and Na⁺/H⁺ exchangers. In this review, we consider the involvement of these cation channels and transporters in the regulation of glucose uptake in the small intestine. Modulation of them may be a potential strategy for the management of obesity and diabetes.

Ankyrin-B directs membrane tethering of periaxin and is required for maintenance of lens fiber cell hexagonal shape and mechanics

Periaxin (Prx), a PDZ domain protein expressed preferentially in myelinating Schwann cells and lens fibers, plays a key role in membrane scaffolding and cytoarchitecture. Little is known, however, about how Prx is anchored to the plasma membrane. Here we report that ankyrin-B (AnkB), a well-characterized adaptor protein involved in linking the spectrin-actin cytoskeleton to integral membrane proteins, is required for membrane association of Prx in lens fibers and colocalizes with Prx in hexagonal fiber cells. Under AnkB haploinsufficiency, Prx accumulates in the soluble fraction with a concomitant loss from the membrane-enriched fraction of mouse lenses. Moreover, AnkB haploinsufficiency induced age-dependent disruptions in fiber cell hexagonal geometry and radial alignment and decreased compressive stiffness in mouse lenses parallel to the changes observed in Prx null mouse lens. Both AnkB- and Prx-deficient mice exhibit disruptions in membrane organization of the spectrin-actin network and the dystrophin-glycoprotein complex in lens fiber cells. Taken together, these observations reveal that AnkB is required for Prx membrane anchoring and for maintenance of lens fiber cell hexagonal geometry, membrane skeleton organization, and biomechanics.

Luminal glucose does not enhance active intestinal calcium absorption in mice: evidence against a role for Ca(v)1.3 as a mediator of calcium uptake during absorption

Intestinal Ca absorption occurs through a 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-regulated transcellular pathway, especially when habitual dietary Ca intake is low. Recently the L-type voltage-gated Ca channel, Cav1.3, was proposed to mediate active, transcellular Ca absorption in response to membrane depolarization caused by elevated luminal glucose levels after a meal. We tested the hypothesis that high luminal glucose could reveal a role for Cav1.3 in active intestinal Ca absorption in mice. Nine-week-old male C57BL/6 J mice were fed AIN93G diets containing either low (0.125%) or high (1%) Ca for 1 week, and Ca absorption was examined by an oral gavage method using a 45Ca-transport buffer containing 25 mmol/L of glucose or fructose. Transient receptor potential vanilloid 6 (TRPV6), calbindin D9k (CaBPD9k), and Cav1.3 messenger RNA (mRNA) levels were measured in the duodenum, jejunum, and ileum. TRPV6 and CaBPD9k expressions were highest in the duodenum, where active, 1,25(OH)2D3-regulated Ca absorption occurs, whereas Cav1.3 mRNA levels were similar across the intestinal segments. As expected, the low-Ca diet increased renal cytochrome p450-27B1 (CYP27B1) mRNA (P = .003), serum 1,25(OH)2D3 (P < .001), and Ca absorption efficiency by 2-fold with the fructose buffer. However, the glucose buffer used to favor Cav1.3 activation did not increase Ca absorption efficiency (P = .6) regardless of the dietary Ca intake level. Collectively, our results show that glucose did not enhance Ca absorption and they do not support a critical role for Cav1.3 in either basal or vitamin D-regulated intestinal Ca absorption in vivo.

Pharmacology and physiology of gastrointestinal enteroendocrine cells

Gastrointestinal (GI) polypeptides are secreted from enteroendocrine cells (EECs). Recent technical advances and the identification of endogenous and synthetic ligands have enabled exploration of the pharmacology and physiology of EECs. Enteroendocrine signaling pathways stimulating hormone secretion involve multiple nutrient transporters and G protein-coupled receptors (GPCRs), which are activated simultaneously under prevailing nutrient conditions in the intestine following a meal. The majority of studies investigate hormone secretion from EECs in response to single ligands and although the mechanisms behind how individual signaling pathways generate a hormonal output have been well characterized, our understanding of how these signaling pathways converge to generate a single hormone secretory response is still in its infancy. However, a picture is beginning to emerge of how nutrients and full, partial, or allosteric GPCR ligands differentially regulate the enteroendocrine system and its interaction with the enteric and central nervous system. So far, activation of multiple pathways underlies drug discovery efforts to harness the therapeutic potential of the enteroendocrine system to mimic the phenotypic changes observed in patients who have undergone Roux-en-Y gastric surgery. Typically obese patients exhibit ∼30% weight loss and greater than 80% of obese diabetics show remission of diabetes. Targeting combinations of enteroendocrine signaling pathways that work synergistically may manifest with significant, differentiated EEC secretory efficacy. Furthermore, allosteric modulators with their increased selectivity, self-limiting activity, and structural novelty may translate into more promising enteroendocrine drugs. Together with the potential to bias enteroendocrine GPCR signaling and/or to activate multiple divergent signaling pathways highlights the considerable range of therapeutic possibilities available. Here, we review the pharmacology and physiology of the EEC system.

Does apical membrane GLUT2 have a role in intestinal glucose uptake?

It has been proposed that the non-saturable component of intestinal glucose absorption, apparent following prolonged exposure to high intraluminal glucose concentrations, is mediated via the low affinity glucose and fructose transporter, GLUT2, upregulated within the small intestinal apical border. The evidence that the non-saturable transport component is mediated via an apical membrane sugar transporter is that it is inhibited by phloretin, after exposure to phloridzin. Since the other apical membrane sugar transporter, GLUT5, is insensitive to inhibition by either cytochalasin B, or phloretin, GLUT2 was deduced to be the low affinity sugar transport route. As in its uninhibited state, polarized intestinal glucose absorption depends both on coupled entry of glucose and sodium across the brush border membrane and on the enterocyte cytosolic glucose concentration exceeding that in both luminal and submucosal interstitial fluids, upregulation of GLUT2 within the intestinal brush border will usually stimulate downhill glucose reflux to the intestinal lumen from the enterocytes; thereby reducing, rather than enhancing net glucose absorption across the luminal surface. These states are simulated with a computer model generating solutions to the differential equations for glucose, Na and water flows between luminal, cell, interstitial and capillary compartments. The model demonstrates that uphill glucose transport via SGLT1 into enterocytes, when short-circuited by any passive glucose carrier in the apical membrane, such as GLUT2, will reduce transcellular glucose absorption and thereby lead to increased paracellular flow. The model also illustrates that apical GLUT2 may usefully act as an osmoregulator to prevent excessive enterocyte volume change with altered luminal glucose concentrations.

The role of SGLT1 and GLUT2 in intestinal glucose transport and sensing

Intestinal glucose absorption is mediated by SGLT1 whereas GLUT2 is considered to provide basolateral exit. Recently, it was proposed that GLUT2 can be recruited into the apical membrane after a high luminal glucose bolus allowing bulk absorption of glucose by facilitated diffusion. Moreover, SGLT1 and GLUT2 are suggested to play an important role in intestinal glucose sensing and incretin secretion. In mice that lack either SGLT1 or GLUT2 we re-assessed the role of these transporters in intestinal glucose uptake after radiotracer glucose gavage and performed Western blot analysis for transporter abundance in apical membrane fractions in a comparative approach. Moreover, we examined the contribution of these transporters to glucose-induced changes in plasma GIP, GLP-1 and insulin levels.

In mice lacking SGLT1, tissue retention of tracer glucose was drastically reduced throughout the entire small intestine whereas GLUT2-deficient animals exhibited higher tracer contents in tissue samples than wild type animals. Deletion of SGLT1 resulted also in reduced blood glucose elevations and abolished GIP and GLP-1 secretion in response to glucose. In mice lacking GLUT2, glucose-induced insulin but not incretin secretion was impaired. Western blot analysis revealed unchanged protein levels of SGLT1 after glucose gavage. GLUT2 detected in apical membrane fractions mainly resulted from contamination with basolateral membranes but did not change in density after glucose administration.

SGLT1 is unequivocally the prime intestinal glucose transporter even at high luminal glucose concentrations. Moreover, SGLT1 mediates glucose-induced incretin secretion. Our studies do not provide evidence for GLUT2 playing any role in either apical glucose influx or incretin secretion.

Interaction of gypsum and the rhizome of Anemarrhena asphodeloides plays an important role in anti-allergic effects of byakkokakeishito in mice

Gypsum is a crude mineral drug used in the formulas of Japanese kampo medicine and traditional Chinese medicine. The present study aimed to evaluate the anti-allergic effect of byakkokakeishito extract (BKT), which consists of gypsum (natural hydrous calcium sulfate), Anemarrhena Rhizome (rhizome of Anemarrhena asphodeloides), Cinnamon Bark (bark of trunk of Cinnamomum cassia), Oriza Seed (seed of Oryza sativa), and Glycyrrhiza (root and stolon of Glycyrrhiza uralensis), and to clarify the role of gypsum in the formula. We prepared BKT by boiling a mixture of various quantities of gypsum and fixed amounts of the other four crude drugs in water. We evaluated the anti-allergic activity of the formulations using three different murine models of allergy: contact dermatitis induced by painting hapten onto skin; allergic dermatitis-like symptoms induced by cutaneous injection of mite-antigen; and skin passive cutaneous anaphylaxis (PCA) reaction using ovalbumin as antigen. The calcium content in the various BKT samples was dose-dependently increased up to 60 g/day of human dosage. BKT significantly suppressed the allergic symptoms in the three different experimental models. The effect of BKT was augmented by increasing the gypsum dosage only in the PCA reaction model. The extract prepared from a mixture of Anemarrhena Rhizome and gypsum exhibited an effect comparable to that of BKT. BKT exhibits an anti-allergic effect in several animal models, which may provide experimental evidence for the clinical use of BKT in allergic diseases. Gypsum may augment the anti-allergic activity of BKT, presumably through increasing intestinal absorption of Anemarrhena Rhizome-derived active constituents.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Regulation of epithelial calcium transport by prolactin: from fish to mammals

Among the reported ∼300 biological actions, the established role of prolactin (PRL) is to act as a vertebrate hypercalcemic hormone that regulates epithelial calcium transport in several organs, such as the gills, intestine, and kidney. In fish, PRL stimulates the branchial calcium transport by increasing the activity of Ca(2+)-ATPase. Although this calciotropic hormone also induces hypercalcemia in amphibians, reptiles and birds, little has been known regarding the underlying mechanism. In contrast, the effects of PRL on the epithelial calcium transport in mammals are well documented. In rodents, PRL has been shown to stimulate the renal tubular calcium reabsorption and intestinal calcium absorption, the latter of which is mediated by the PRL-induced upregulation of calcium transporter gene expression and activities. Recently, we demonstrated that the duodenal calcium absorption in lactating rats was markedly enhanced by the suckling-induced PRL surge, presumably to provide calcium for milk production. The cellular and molecular mechanisms of the PRL-stimulated calcium transport in mammals have been elaborated in this review.

Effect of the artificial sweetener, acesulfame potassium, a sweet taste receptor agonist, on glucose uptake in small intestinal cell lines

INTRODUCTION

Sweet taste receptors may enhance glucose absorption.

AIM

This study aimed to explore the cell biology of sweet taste receptors on glucose uptake.

HYPOTHESIS

Artificial sweeteners increase glucose uptake via activating sweet taste receptors in the enterocyte to translocate GLUT2 to the apical membrane through the PLC βII pathway.

METHODS

Caco-2, RIE-1, and IEC-6 cells, starved from glucose for 1 h were pre-incubated with 10 mM acesulfame potassium (AceK). Glucose uptake was measured by incubating cells for 1 to 10 min with 0.5-50 mM glucose with or without U-73122, chelerythrine, and cytochalasin B.

RESULTS

In Caco-2 and RIE-1 cells, 10 mM AceK increased glucose uptake by 20-30 % at glucose >25 mM, but not in lesser glucose concentrations (<10 mM), nor at 1 min or 10 min incubations. U-73122 (PLC βII inhibitor) inhibited uptake at glucose >25 mM and for 5 min incubation; chelerythrine and cytochalasin B had similar effects. No effect occurred in IEC-6 cells. Activation of sweet taste receptors had no effect on glucose uptake in low (<25 mM) glucose concentrations but increased uptake at greater concentrations (>25 mM).

CONCLUSIONS

Role of artificial sweeteners on glucose uptake appears to act in part by effects on the enterocyte itself.

Translocation of transfected GLUT2 to the apical membrane in rat intestinal IEC-6 cells

AIM

In this study, we transfected the full length cDNA of glucose transporter 2 (GLUT2) into IEC-6 cells (which lack GLUT2 expression) to investigate GLUT2 translocation in enterocytes. The purpose of this study was to investigate cellular mechanisms of GLUT2 translocation and its signaling pathway.

METHODS

Rat GLUT2 cDNA was transfected into IEC-6 cells. Glucose uptake was measured by incubating cell monolayers with glucose (0.5-50 mM), containing (14)C-D-glucose and (3)H-L-glucose, to measure stereospecific, carrier-mediated and passive uptake. We imaged GLUT2 immunoreactivity by confocal fluorescence microscopy. We evaluated the GLUT2 inhibitor (1 mM phloretin), SGLT1 inhibitor (0.5 mM phlorizin), disrupting microtubular integrity (2 μM nocodazole and 0.5 μM cytochalasin B), protein kinase C (PKC) inhibitors (50 nM calphostin C and 10 μM chelerythrine), and PKC activator (50 nM phorbol 12-myristate 13-acetate: PMA).

RESULTS

In GLUT2-IEC cells, the K(m) (54.5 mM) increased compared with non-transfected IEC-6 cells (7.8 mM); phloretin (GLUT2 inhibitor) inhibited glucose uptake to that of non-transfected IEC-6 cells (P < 0.05). Nocodazole and cytochalasin B (microtubule disrupters) inhibited uptake by 43-58% only at glucose concentrations ≥25 and 50 mM and the 10-min incubations. Calphostin C (PKC inhibitor) reproduced the inhibition of nocodazole; PMA (a PKC activator) enhanced glucose uptake by 69%. Exposure to glucose increased the GFP signal at the apical membrane of GLUT-1EC cells.

CONCLUSION

IEC-6 cells lacking GLUT2 translocate GLUT2 apically when transfected to express GLUT2. Translocation of GLUT2 occurs through glucose stimulation via a PKC-dependent signaling pathway and requires integrity of the microtubular skeletal structure.

The regulation of K- and L-cell activity by GLUT2 and the calcium-sensing receptor CasR in rat small intestine

Intestinal enteroendocrine cells (IECs) secrete gut peptides in response to both nutrients and non-nutrients. Glucose and amino acids both stimulate gut peptide secretion. Our hypothesis was that the facilitative glucose transporter, GLUT2, could act as a glucose sensor and the calcium-sensing receptor, CasR, could detect amino acids in the intestine to modify gut peptide secretion. We used isolated loops of rat small intestine to study the secretion of gluco-insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1) and peptide tyrosine tyrosine (PYY) secretion stimulated by luminal perfusion of nutrients or bile acid. Inhibition of the sodium-dependent glucose cotransporter 1 (SGLT1) with phloridzin partially inhibited GIP, GLP-1 and PYY secretion by 45%, suggesting another glucose sensor might be involved in modulating peptide secretion. The response was completely abolished in the presence of the GLUT2 inhibitors phloretin or cytochalasin B. Given that GLUT2 modified gut peptide secretion stimulated by glucose, we investigated whether it was involved in the secretion of gut peptide by other gut peptide secretagogues. Phloretin completely abolished gut peptide secretion stimulated by artificial sweetener (sucralose), dipeptide (glycylsarcosine), lipid (oleoylethanolamine), short chain fatty acid (propionate) and major rat bile acid (taurocholate) indicating a fundamental position for GLUT2 in the gut peptide secretory mechanism. We investigated how GLUT2 was able to influence gut peptide secretion mediated by a diverse range of stimulators and discovered that GLUT2 affected membrane depolarisation through the closure of K+(ATP)-sensitive channels. In the absence of SGLT1 activity (or presence of phloridzin), the secretion of GIP, GLP-1 and PYY was sensitive to K+(ATP)-sensitive channel modulators tolbutamide and diazoxide. L-amino acids phenylalanine (Phe), tryptophan (Trp), asparagine (Asn), arginine (Arg) and glutamine (Gln) also stimulated GIP, GLP-1 and PYY secretion, which was completely abolished when extracellular Ca2+ was absent. The gut peptide response stimulated by the amino acids was also blocked by the CasR inhibitor Calhex 231 and augmented by the CasR agonist NPS-R568. GLUT2 and CasR regulate K- and L-cell activity in response to nutrient and non-nutrient stimuli.

Nerveless and gutsy: intestinal nutrient sensing from invertebrates to humans

The increasingly recognized role of gastrointestinal signals in the regulation of food intake, insulin production and peripheral nutrient storage has prompted a surge of interest in studying how the gastrointestinal tract senses and responds to nutritional information. Identification of metabolically important intestinal nutrient sensors could provide potential new drug targets for the treatment of diabetes, obesity and gastrointestinal disorders. From a more fundamental perspective, the study of intestinal chemosensation is revealing novel, non-neuronal modes of communication involving differentiated epithelial cells. It is also identifying signalling mechanisms downstream of not only canonical receptors but also nutrient transporters, thereby supporting a chemosensory role for “transceptors” in the intestine. This review describes known and proposed mechanisms of intestinal carbohydrate, protein and lipid sensing, best characterized in mammalian systems. It also highlights the potential of invertebrate model systems such as C. elegans and Drosophila melanogaster by summarizing known examples of molecular evolutionary conservation. Recently developed genetic tools in Drosophila, an emerging model system for the study of physiology and metabolism, allow the temporal, spatial and high-throughput manipulation of putative intestinal sensors. Hence, fruit flies may prove particularly suited to the study of the link between intestinal nutrient sensing and metabolic homeostasis.

Mechanisms of glucose uptake in intestinal cell lines: role of GLUT2

BACKGROUND

GLUT2 is translocated to the apical membrane of enterocytes exposed to glucose concentrations >∼50 mM. Mechanisms of GLUT2-mediated glucose uptake in cell culture models of enterocytes have not been studied.

AIM

To explore mechanism(s) of glucose uptake in 3 enterocyte-like cell lines.

METHODS

Glucose uptake was measured in Caco-2, RIE-1, and IEC-6 cell lines using varying concentrations of glucose (0.5-50 mM). Effects of phlorizin (SGLT1 inhibitor), phloretin (GLUT2 inhibitor), nocodazole and cytochalasin B (disrupters of cytoskeleton), calphostin C and chelerythrine (PKC inhibitors), and phorbol 12-myristate 13-acetate (PKC activator) were evaluated.

RESULTS

Phlorizin inhibited glucose uptake in all 3 cell lines. Phloretin inhibited glucose uptake in Caco-2 and RIE-1 cells. Starving cells decreased glucose uptake in Caco-2 and RIE-1 cells. Glucose uptake was saturated at >10 mM glucose in all 3 cell lines when exposed briefly (<1 min) to glucose. After exposure for >5 min in Caco-2 and RIE-1 cells, glucose uptake did not saturate and K(m) and V(max) increased. This increase in glucose uptake was inhibited by phloretin, nocodazole, cytochalasin B, calphostin C, and chelerythrine. PMA enhanced glucose uptake by 20%. Inhibitors and PMA had little or no effect in the IEC-6 cells.

CONCLUSION

Constitutive expression of GLUT2 in the apical membrane along with additional translocation of cytoplasmic GLUT2 to the apical membrane via an intact cytoskeleton and activated PKC appears responsible for enhanced carrier-mediated glucose uptake at greater glucose concentrations (>20 mM) in Caco-2 and RIE-1 cells. IEC-6 cells do not appear to express functional GLUT2.

CaV1.3 channels and intracellular calcium mediate osmotic stress-induced N-terminal c-Jun kinase activation and disruption of tight junctions in Caco-2 CELL MONOLAYERS

We investigated the role of a Ca(2+) channel and intracellular calcium concentration ([Ca(2+)](i)) in osmotic stress-induced JNK activation and tight junction disruption in Caco-2 cell monolayers. Osmotic stress-induced tight junction disruption was attenuated by 1,2-bis(2-aminophenoxyl)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-mediated intracellular Ca(2+) depletion. Depletion of extracellular Ca(2+) at the apical surface, but not basolateral surface, also prevented tight junction disruption. Similarly, thapsigargin-mediated endoplasmic reticulum (ER) Ca(2+) depletion attenuated tight junction disruption. Thapsigargin or extracellular Ca(2+) depletion partially reduced osmotic stress-induced rise in [Ca(2+)](i), whereas thapsigargin and extracellular Ca(2+) depletion together resulted in almost complete loss of rise in [Ca(2+)](i). L-type Ca(2+) channel blockers (isradipine and diltiazem) or knockdown of the Ca(V)1.3 channel abrogated [Ca(2+)](i) rise and disruption of tight junction. Osmotic stress-induced JNK2 activation was abolished by BAPTA and isradipine, and partially reduced by extracellular Ca(2+) depletion, thapsigargin, or Ca(V)1.3 knockdown. Osmotic stress rapidly induced c-Src activation, which was significantly attenuated by BAPTA, isradipine, or extracellular Ca(2+) depletion. Tight junction disruption by osmotic stress was blocked by tyrosine kinase inhibitors (genistein and PP2) or siRNA-mediated knockdown of c-Src. Osmotic stress induced a robust increase in tyrosine phosphorylation of occludin, which was attenuated by BAPTA, SP600125 (JNK inhibitor), or PP2. These results demonstrate that Ca(V)1.3 and rise in [Ca(2+)](i) play a role in the mechanism of osmotic stress-induced tight junction disruption in an intestinal epithelial monolayer. [Ca(2+)](i) mediate osmotic stress-induced JNK activation and subsequent c-Src activation and tyrosine phosphorylation of tight junction proteins. Additionally, inositol 1,4,5-trisphosphate receptor-mediated release of ER Ca(2+) also contributes to osmotic stress-induced tight junction disruption.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

Is prolactin the cardinal calciotropic maternal hormone?

To produce offspring, mothers require a large amount of calcium for fetal growth and milk production. Increased calcium demand leads to enhanced intestinal calcium absorption and stockpiling of bone calcium in pregnancy prior to demineralization in lactation. These coordinated events must be carefully organized by calciotropic hormone(s), but the classical hormones, namely 1,25-dihydroxyvitamin D(3), parathyroid hormone and calcitonin, do not appear to be responsible. Plasma prolactin (PRL) levels are elevated during pregnancy and, in view of the presence of PRL receptors in gut, bone and mammary glands, as well as recent evidence of the stimulatory effects of PRL on intestinal calcium transport, bone resorption and mammary calcium secretion, we postulate that PRL is the cardinal calciotropic hormone during pregnancy and lactation.

Absence of evidence of translocation of GLUT2 to the apical membrane of enterocytes in everted intestinal sleeves

INTRODUCTION

Traditional models of intestinal glucose absorption confine GLUT2 to the basolateral membrane. Evidence suggests that GLUT2 is translocated to the apical membrane when the enterocyte is exposed to high luminal glucose concentrations.

HYPOTHESIS

GLUT2 translocates to the apical membrane by a PKC signaling mechanism dependent on activity of SGLT1 and the cellular cytostructure.

METHODS

Transporter-mediated glucose uptake was studied in rat jejunum using everted sleeves under seven conditions: Control, SGLT1 inhibition (phlorizin), GLUT2 inhibition (phloretin), both SGLT1 and GLUT2 inhibition, PKC inhibition (calphostin C or chelerythrine), and disruption of cellular cytostructure (nocodazole). Each condition was tested in iso-osmotic solutions of 1, 20, or 50 mM glucose for 1 or 5 min incubations (n = 6 rats each).

RESULTS

Control rats exhibited a saturable pattern of uptake at both durations of incubation. Phlorizin (P ≤ 0.006 each) inhibited markedly and phloretin (P ≤ 0.01 each) inhibited partially glucose uptake in all concentrations and time. Phloretin and phlorizin together completely inhibited uptake (P = 0.004 each). Calphostin C, chelerythrine, and nocodazole had little effect on glucose uptake at either 1 or 5 min. Inhibition of SGLT1 led to near complete cessation of transporter-mediated glucose uptake, while GLUT2 inhibition led to partial inhibition, suggesting some constitutive expression of GLUT2 in the apical membrane. Disruption of PKC signaling or cytoskeletal integrity partially inhibited transporter-mediated glucose uptake only in 1 mM glucose, suggesting a non-specific effect.

CONCLUSIONS

Under these conditions, it does not appear that GLUT2 is translocated to the apical membrane on the cellular cytostructure in response to PKC signaling.

Transepithelial calcium transport in prolactin-exposed intestine-like Caco-2 monolayer after combinatorial knockdown of TRPV5, TRPV6 and Ca(v)1.3

The milk-producing hormone prolactin (PRL) increases the transcellular intestinal calcium absorption by enhancing apical calcium uptake through voltage-dependent L-type calcium channel (Ca(v)) 1.3. However, the redundancy of apical calcium channels raised the possibility that Ca(v)1.3 may operate with other channels, especially transient receptor potential vanilloid family calcium channels (TRPV) 5 or 6, in an interdependent manner. Herein, TRPV5 knockdown (KD), TRPV5/TRPV6, TRPV5/Ca(v)1.3, and TRPV6/Ca(v)1.3 double KD, and TRPV5/TRPV6/Ca(v)1.3 triple KD Caco-2 monolayers were generated by transfecting cells with small interfering RNAs (siRNA). siRNAs downregulated only the target mRNAs, and did not induce compensatory upregulation of the remaining channels. After exposure to 600 ng/mL PRL, the transcellular calcium transport was increased by ~2-fold in scrambled siRNA-treated, TRPV5 KD and TRPV5/TRPV6 KD monolayers, but not in TRPV5/Ca(v)1.3, TRPV6/Ca(v)1.3 and TRPV5/TRPV6/Ca(v)1.3 KD monolayers. The results suggested that Ca(v)1.3 was the sole apical channel responsible for the PRL-stimulated transcellular calcium transport in intestine-like Caco-2 monolayer.

Enhancement of calcium transport in Caco-2 monolayer through PKCzeta-dependent Cav1.3-mediated transcellular and rectifying paracellular pathways by prolactin

Previous investigations suggested that prolactin (PRL) stimulated the intestinal calcium absorption through phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK) signaling pathways. However, little was known regarding its detailed mechanisms for the stimulation of transcellular and voltage-dependent paracellular calcium transport. By using Ussing chamber technique, we found that the PRL-induced increase in the transcellular calcium flux and decrease in transepithelial resistance of intestinal-like Caco-2 monolayer were not abolished by inhibitors of gene transcription and protein biosynthesis. The PRL-stimulated transcellular calcium transport was completely inhibited by the L-type calcium channel blockers (nifedipine and verapamil) and plasma membrane Ca(2+)-ATPase (PMCA) inhibitor (trifluoperazine) as well as small interfering RNA targeting voltage-dependent L-type calcium channel Ca(v)1.3, but not TRPV6 or calbindin-D(9k). As demonstrated by (45)Ca uptake study, PI3K and PKC, but not ROCK, were essential for the PRL-enhanced apical calcium entry. In addition, PRL was unable to enhance the transcellular calcium transport after PKC(zeta) knockdown or exposure to inhibitors of PKC(zeta), but not of PKC(alpha), PKC(beta), PKC(epsilon), PKC(mu), or protein kinase A. Voltage-clamping experiments further showed that PRL markedly stimulated the voltage-dependent calcium transport and removed the paracellular rectification. Such PRL effects on paracellular transport were completely abolished by inhibitors of PI3K (LY-294002) and ROCK (Y-27632). It could be concluded that the PRL-stimulated transcellular calcium transport in Caco-2 monolayer was mediated by Ca(v)1.3 and PMCA, presumably through PI3K and PKC(zeta) pathways, while the enhanced voltage-dependent calcium transport occurred through PI3K and ROCK pathways.

An energy supply network of nutrient absorption coordinated by calcium and T1R taste receptors in rat small intestine

T1R taste receptors are present throughout the gastrointestinal tract. Glucose absorption comprises active absorption via SGLT1 and facilitated absorption via GLUT2 in the apical membrane. Trafficking of apical GLUT2 is rapidly up-regulated by glucose and artificial sweeteners, which act through T1R2 + T1R3/alpha-gustducin to activate PLC beta2 and PKC betaII. We therefore investigated whether non-sugar nutrients are regulated by taste receptors using perfused rat jejunum in vivo. Under different conditions, we observed a Ca(2+)-dependent reciprocal relationship between the H(+)/oligopeptide transporter PepT1 and apical GLUT2, reflecting the fact that trafficking of PepT1 and GLUT2 to the apical membrane is inhibited and activated by PKC betaII, respectively. Addition of L-glutamate or sucralose to a perfusate containing low glucose (20 mM) each activated PKC betaII and decreased apical PepT1 levels and absorption of the hydrolysis-resistant dipeptide L-Phe(PsiS)-L-Ala (1 mM), while increasing apical GLUT2 and glucose absorption within minutes. Switching perfusion from mannitol to glucose (75 mM) exerted similar effects. c-glutamate induced rapid GPCR internalization of T1R1, T1R3 and transducin, whereas sucralose internalized T1R2, T1R3 and alpha-gustducin. We conclude that L-glutamate acts via amino acid and glucose via sweet taste receptors to coordinate regulation of PepT1 and apical GLUT2 reciprocally through a common enterocytic pool of PKC betaII. These data suggest the existence of a wider Ca(2+) and taste receptor-coordinated transport network incorporating other nutrients and/or other stimuli capable of activating PKC betaII and additional transporters, such as the aspartate/glutamate transporter, EAAC1, whose level was doubled by L-glutamate. The network may control energy supply.

Sugar absorption in the intestine: the role of GLUT2

Intestinal glucose absorption comprises two components. One is classical active absorption mediated by the Na+/glucose cotransporter. The other is a diffusive component, formerly attributed to paracellular flow. Recent evidence, however, indicates that the diffusive component is mediated by the transient insertion of glucose transporter type 2 (GLUT2) into the apical membrane. This apical GLUT2 pathway of intestinal sugar absorption is present in species from insect to human, providing a major route at high sugar concentrations. The pathway is regulated by rapid trafficking of GLUT2 to the apical membrane induced by glucose during assimilation of a meal. Apical GLUT2 is therefore a target for multiple short-term and long-term nutrient-sensing mechanisms. These include regulation by a newly recognized pathway of calcium absorption through the nonclassical neuroendocrine l-type channel Cav1.3 operating during digestion, activation of intestinal sweet taste receptors by natural sugars and artificial sweeteners, paracrine and endocrine hormones, especially insulin and GLP-2, and stress. Permanent apical GLUT2, resulting in increased sugar absorption, is a characteristic of experimental diabetes and of insulin-resistant states induced by fructose and fat. The nutritional consequences of apical and basolateral GLUT2 regulation are discussed in the context of Western diet, processed foods containing artificial sweeteners, obesity, and diabetes.

AICAR activates AMPK and alters PIP2 association with the epithelial sodium channel ENaC to inhibit Na+ transport in H441 lung epithelial cells

Changes in amiloride-sensitive epithelial Na(+) channel (ENaC) activity (NP(o)) in the lung lead to pathologies associated with dysregulation of lung fluid balance. UTP activation of purinergic receptors and hydrolysis of PIP(2) via activation of phospholipase C (PLC) or AICAR activation of AMP-activated protein kinase (AMPK) inhibited amiloride-sensitive Na(+) transport across human H441 epithelial cell monolayers. Neither treatment altered alpha, beta or gamma ENaC subunit abundance (N) in the apical membrane indicating that the mechanism of inhibition was via a change in channel open state probability (P(o)). We found that UTP depleted PIP(2) abundance in the apical membrane whilst activation of AMPK prevented the binding of beta and gamma ENaC subunits to PIP(2.) The association of PIP(2) with the ENaC subunits is required to maintain channel activity via P(o). Thus, these data show for the first time that AICAR activation of AMPK inhibits Na(+) transport via a mechanism that perturbs the PIP(2)-ENaC channel interaction to alter P(o). In addition, we show that dissociation of PIP(2) from ENaC together with activation of AMPK further reduced Na(+) transport by a secondary effect that correlated with ENaC subunit internalization. Thus, when PIP(2)-ENaC subunit interactions were compromised, ENaC protein retrieval was initiated, indicating that AMPK can modulate ENaC P(o) and N.

The L-type calcium ion channel cch1 affects ascospore discharge and mycelial growth in the filamentous fungus Gibberella zeae (anamorph Fusarium graminearum)

Cch1, a putative voltage-gated calcium ion channel, was investigated for its role in ascus development in Gibberella zeae. Gene replacement mutants of CCH1 were generated and found to have asci which did not forcibly discharge spores, although morphologically ascus and ascospore development in the majority of asci appeared normal. Additionally, mycelial growth was significantly slower, and sexual development was slightly delayed in the mutant; mutant mycelia showed a distinctive fluffy morphology, and no cirrhi were produced. Wheat infected with Deltacch1 mutants developed symptoms comparable to wheat infected with the wild type; however, the mutants showed a reduced ability to protect the infected stalk from colonization by saprobic fungi. Transcriptional analysis of gene expression in mutants using the Affymetrix Fusarium microarray showed 2,449 genes with significant, twofold or greater, changes in transcript abundance across a developmental series. This work extends the role of CCH1 to forcible spore discharge in G. zeae and suggests that this channel has subtle effects on growth and development.

GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-betal and plasma glucose concentration

AIMS/HYPOTHESIS

GLUT2 is the main renal glucose transporter upregulated by hyperglycaemia, when it becomes detectable at the brush border membrane (BBM). Since glucose-induced protein kinase C (PKC) activation in the kidney is linked to diabetic nephropathy, we investigated the effect of glycaemic status on the protein levels of PKC isoforms alpha, betaI, betaII, delta and epsilon in the proximal tubule, as well as the relationship between them and changes in GLUT2 production at the BBM.

METHODS

Plasma glucose concentrations were modulated in rats by treatment with nicotinamide 15 min prior to induction of diabetes with streptozotocin. Levels of GLUT2 protein and PKC isoforms in BBM were measured by western blotting. Additionally, the role of calcium signalling and PKC activation on facilitative glucose transport was examined by measuring glucose uptake in BBM vesicles prepared from proximal tubules that had been incubated either with thapsigargin, which increases cytosolic calcium, or with the PKC activator phorbol 12-myristate,13-acetate (PMA).

RESULTS

Thapsigargin and PMA enhanced GLUT-mediated glucose uptake, but had no effect on sodium-dependent glucose transport. Diabetes significantly increased the protein levels of GLUT2 and PKC-betaI at the BBM. Levels of GLUT2 and PKC-betaI correlated positively with plasma glucose concentration. Diabetes had no effect on BBM levels of alpha, betaII, delta or epsilon isoforms of PKC.

CONCLUSIONS/INTERPRETATION

Enhanced GLUT2-mediated glucose transport across the proximal tubule BBM during diabetic hyperglycaemia is closely associated with increased PKC-betaI. Thus, altered levels of GLUT2 and PKC-betaI proteins in the BBM may be important factors in the pathogenic processes underlying diabetic renal injury.

Sweet taste receptors in rat small intestine stimulate glucose absorption through apical GLUT2

Natural sugars and artificial sweeteners are sensed by receptors in taste buds. T2R bitter and T1R sweet taste receptors are coupled through G-proteins, alpha-gustducin and transducin, to activate phospholipase C beta2 and increase intracellular calcium concentration. Intestinal brush cells or solitary chemosensory cells (SCCs) have a structure similar to lingual taste cells and strongly express alpha-gustducin. It has therefore been suggested over the last decade that brush cells may participate in sugar sensing by a mechanism analogous to that in taste buds. We provide here functional evidence for an intestinal sensing system based on lingual taste receptors. Western blotting and immunocytochemistry revealed that all T1R members are expressed in rat jejunum at strategic locations including Paneth cells, SCCs or the apical membrane of enterocytes; T1Rs are colocalized with each other and with alpha-gustducin, transducin or phospholipase C beta2 to different extents. Intestinal glucose absorption consists of two components: one is classical active Na+-glucose cotransport, the other is the diffusive apical GLUT2 pathway. Artificial sweeteners increase glucose absorption in the order acesulfame potassium approximately sucralose > saccharin, in parallel with their ability to increase intracellular calcium concentration. Stimulation occurs within minutes by an increase in apical GLUT2, which correlates with reciprocal regulation of T1R2, T1R3 and alpha-gustducin versus T1R1, transducin and phospholipase C beta2. Our observation that artificial sweeteners are nutritionally active, because they can signal to a functional taste reception system to increase sugar absorption during a meal, has wide implications for nutrient sensing and nutrition in the treatment of obesity and diabetes.

Apical GLUT2 and Cav1.3: regulation of rat intestinal glucose and calcium absorption

We have proposed a model of intestinal glucose absorption in which transport by SGLT1 induces rapid insertion and activation of GLUT2 in the apical membrane by a PKC betaII-dependent mechanism. Since PKC betaII requires Ca(2+) and glucose is depolarizing, we have investigated whether glucose absorption is regulated by the entry of dietary Ca(2+) through Ca(v)1.3 in the apical membrane. When rat jejunum was perfused with 75 mM glucose, Ca(2+)-deplete conditions, or perfusion with the L-type antagonists nifedipine and verapamil strongly diminished the phloretin-sensitive apical GLUT2, but not the phloretin-insensitive SGLT1 component of glucose absorption. Western blotting showed that in each case there was a significant decrease in apical GLUT2 level, but no change in SGLT1 level. Inhibition of apical GLUT2 absorption coincided with inhibition of unidirectional (45)Ca(2+) entry by nifedipine and verapamil. At 10 mM luminal Ca(2+), (45)Ca(2+) absorption in the presence of 75 mM glucose was 2- to 3-fold that in the presence of 75 mM mannitol. The glucose-induced component was SGLT1-dependent and nifedipine-sensitive. RT-PCR revealed the presence of Ca(v)beta(3) in jejunal mucosa; Western blotting and immunocytochemistry localized Ca(v)beta(3) to the apical membrane, together with Ca(v)1.3. We conclude that in times of dietary sufficiency Ca(v)1.3 may mediate a significant pathway of glucose-stimulated Ca(2+) entry into the body and that luminal supply of Ca(2+) is necessary for GLUT2-mediated glucose absorption. The integration of glucose and Ca(2+) absorption represents a complex nutrient-sensing system, which allows both absorptive pathways to be regulated rapidly and precisely to match dietary intake.