The effects of streptozotocin diabetes on sodium-glucose transporter (SGLT1) expression and function in rat jejunal and ileal villus-attached enterocytes

Effect of Type 2 Diabetes and Impaired Glucose Tolerance on Digestive Enzymes and Glucose Absorption in the Small Intestine of Young Rats

The reactions of intestinal functional parameters to type 2 diabetes at a young age remain unclear. The study aimed to assess changes in the activity of intestinal enzymes, glucose absorption, transporter content (SGLT1, GLUT2) and intestinal structure in young Wistar rats with type 2 diabetes (T2D) and impaired glucose tolerance (IGT). To induce these conditions in the T2D (n = 4) and IGT (n = 6) rats, we used a high-fat diet and a low dose of streptozotocin. Rats fed a high-fat diet (HFD) (n = 6) or a standard diet (SCD) (n = 6) were used as controls. The results showed that in T2D rats, the ability of the small intestine to absorb glucose was higher in comparison to HFD rats (p < 0.05). This was accompanied by a tendency towards an increase in the number of enterocytes on the villi of the small intestine in the absence of changes in the content of SGLT1 and GLUT2 in the brush border membrane of the enterocytes. T2D rats also showed lower maltase and alkaline phosphatase (AP) activity in the jejunal mucosa compared to the IGT rats (p < 0.05) and lower AP activity in the colon contents compared to the HFD (p < 0.05) and IGT (p < 0.05) rats. Thus, this study provides insights into the adaptation of the functional and structural parameters of the small intestine in the development of type 2 diabetes and impaired glucose tolerance in young representatives.

Mechanisms of Glucose Absorption in the Small Intestine in Health and Metabolic Diseases and Their Role in Appetite Regulation

The worldwide prevalence of metabolic diseases such as obesity, metabolic syndrome and type 2 diabetes shows an upward trend in recent decades. A characteristic feature of these diseases is hyperglycemia which can be associated with hyperphagia. Absorption of glucose in the small intestine physiologically contributes to the regulation of blood glucose levels, and hence, appears as a putative target for treatment of hyperglycemia. In fact, recent progress in understanding the molecular and cellular mechanisms of glucose absorption in the gut and its reabsorption in the kidney helped to develop a new strategy of diabetes treatment. Changes in blood glucose levels are also involved in regulation of appetite, suggesting that glucose absorption may be relevant to hyperphagia in metabolic diseases. In this review we discuss the mechanisms of glucose absorption in the small intestine in physiological conditions and their alterations in metabolic diseases as well as their relevance to the regulation of appetite. The key role of SGLT1 transporter in intestinal glucose absorption in both physiological conditions and in diabetes was clearly established. We conclude that although inhibition of small intestinal glucose absorption represents a valuable target for the treatment of hyperglycemia, it is not always suitable for the treatment of hyperphagia. In fact, independent regulation of glucose absorption and appetite requires a more complex approach for the treatment of metabolic diseases.

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.

Aerobic Exercise Training Decreases Hepatic Asprosin in Diabetic Rats

Asprosin, a novel hormone released from white adipose tissue, regulates hepatic glucose metabolism and is pathologically elevated in the presence of insulin resistance. It is unknown whether aerobic exercise training affects asprosin levels in type 1 diabetes mellitus (T1DM). The aim of this study was to determine whether (1) aerobic exercise training could decrease asprosin levels in the liver of streptozotocin (STZ)-induced diabetic rats and (2) the reduction in asprosin levels could induce asprosin-dependent downstream pathways. Five-week-old male Sprague–Dawley rats were randomly divided into control, STZ-induced diabetes (STZ), and STZ with aerobic exercise training groups (n = 6/group). T1DM was induced by a single dose of STZ (65 mg/kg intraperitoneally (i.p.)). The exercise group was made to run on a treadmill for 60 min at a speed of 20 m/min, 4 days per week for 8 weeks. Aerobic exercise training reduced the protein levels of asprosin, PKA, and TGF-β but increased those of AMPK, Akt, PGC-1β, and MnSOD. These results suggest that aerobic exercise training affects hepatic asprosin-dependent PKA/TGF-β and AMPK downstream pathways in T1DM.

Pre-treatment with Empagliflozin ameliorates Cisplatin induced acute kidney injury by suppressing apoptosis

Dose-limiting nephrotoxicity restricts Cisplatin use in high therapeutic doses. Empagliflozin showed a reno-protective effect in diabetic nephropathy. We investigated if Empagliflozin can ameliorate Cisplatin nephrotoxicity whether used prophylactically or therapeutically. Forty male Wistar rats were divided into 5 groups: (1) control; (2) Cisplatin-induced nephrotoxicity by single intraperitoneal dose; (3) Empagliflozin was given for 10 days before a single dose of Cisplatin; (4) a single dose of Cisplatin followed by Empagliflozin for 10 days; (5) received Empagliflozin only. Regular assessment of weight was done, biochemical evaluation for serum urea, creatinine, uric acid, albumin, and glucose was performed, kidney tissue nerve growth factor-β (NGF-β) and oxidative stress parameters were measured, kidneys were evaluated histopathologically and immunostained for caspase 3. Cisplatin significantly reduced body weight, NGF-β, and reduced glutathione, elevated urea, creatinine, and malondialdehyde with no effect on other serum biochemical parameters. Histopathologically, there was high acute tubular necrosis (ATN) score with strong immunostaining of caspase 3. The use of Empagliflozin significantly reduced urea and creatinine in both prophylactic and therapeutic, reduced ATN score in the prophylactic group associated with minimal staining of caspase 3 and elevated reduced glutathione. In conclusion, prophylactic Empagliflozin protected against Cisplatin-induced acute kidney injury mainly via anti-apoptotic effect.

LX2761, a Sodium/Glucose Cotransporter 1 Inhibitor Restricted to the Intestine, Improves Glycemic Control in Mice

LX2761 is a potent sodium/glucose cotransporter 1 inhibitor restricted to the intestinal lumen after oral administration. Studies presented here evaluated the effect of orally administered LX2761 on glycemic control in preclinical models. In healthy mice and rats treated with LX2761, blood glucose excursions were lower and plasma total glucagon-like peptide-1 (GLP-1) levels higher after an oral glucose challenge; these decreased glucose excursions persisted even when the glucose challenge occurred 15 hours after LX2761 dosing in ad lib-fed mice. Further, treating mice with LX2761 and the dipeptidyl-peptidase 4 inhibitor sitagliptin synergistically increased active GLP-1 levels, suggesting increased LX2761-mediated release of GLP-1 into the portal circulation. LX2761 also lowered postprandial glucose, fasting glucose, and hemoglobin A1C, and increased plasma total GLP-1, during long-term treatment of mice with either early- or late-onset streptozotocin-diabetes; in the late-onset cohort, LX2761 treatment improved survival. Mice and rats treated with LX2761 occasionally had diarrhea; this dose-dependent side effect decreased in severity and frequency over time, and LX2761 doses were identified that decreased postprandial glucose excursions without causing diarrhea. Further, the frequency of LX2761-associated diarrhea was greatly decreased in mice either by gradual dose escalation or by pretreatment with resistant starch 4, which is slowly digested to glucose in the colon, a process that primes the colon for glucose metabolism by selecting for glucose-fermenting bacterial species. These data suggest that clinical trials are warranted to determine if LX2761 doses and dosing strategies exist that provide improved glycemic control combined with adequate gastrointestinal tolerability in people living with diabetes.

Effects of Xylanase Supplementation and Citric Acid on Performance, Ileal Nutrients Digestibility, and Gene Expression of Intestinal Nutrient Transporters in Broilers Challenged with Clostridium perfringens

This study was conducted to evaluate the effects of xylanase and citric acid (CA) on growth, digesta pH, ileal populations of Clostridium perfringens and lactic acid bacteria, ileal nutrient digestibility, digestive enzyme activity, and mRNA expression of intestinal nutrient transporters in starter broilers challenged with C. perfringens. The experiment was conducted in a 2×2 factorial arrangement with two levels of CA (0 and 30 g/kg) and 2 levels of xylanase (0 and 200 mg/kg). Each of the four dietary treatments was fed to six replicate pens (15 birds/pen) between 0 and 21 d of age. Dietary CA significantly increased ADFI and ADG; meanwhile, xylanase addition led to a substantial reduction in FCR (P<0.05). No differences in digesta pH, C. perfringens counts, or quantity of lactic acid bacteria were found between the treatments. Xylanase supplementation increased AME values (P<0.01) and ileal digestibility of CP (P<0.05) in challenged birds. The inclusion of CA also increased the AME (P<0.01), and tended to increased ileal CP digestibility (P=0.085). Xylanase supplementation increased α-amylase, trypsin, and sucrose activity in the jejunum (P<0.01). Dietary CA significantly increased (P<0.01) villi length as well as the villus length to crypt depth ratio in jejunum segments. The jejunal mRNA expression of sodium glucose co-transporter 1 (SGLT1) and H+-dependent peptide transporter 1 (PepT1) were upregulated by xylanase supplementation (P<0.01). The results suggest that dietary CA can promote growth as well as improve intestinal morphology and AME in birds challenged with necrotic enteritis. This study shows that xylanase supplementation improved FCR and AME in birds independent of C. perfringens infection; it also elevated the apparent ileal digestibility of CP, digestive enzyme activities, and mRNA expression of nutrient transporters in challenged birds.

Sodium-glucose cotransport

PURPOSE OF REVIEW

Sodium-glucose cotransporters (SGLTs) are important mediators of glucose uptake across apical cell membranes. SGLT1 mediates almost all sodium-dependent glucose uptake in the small intestine, while in the kidney SGLT2, and to a lesser extent SGLT1, account for more than 90% and nearly 3%, respectively, of glucose reabsorption from the glomerular ultrafiltrate. Although the recent availability of SGLT2 inhibitors for the treatment of diabetes mellitus has increased the number of clinical studies, this review has a focus on mechanisms contributing to the cellular regulation of SGLTs.

RECENT FINDINGS

Studies have focused on the regulation of SGLT expression under different physiological/pathophysiological conditions, for example diet, age or diabetes mellitus. Several studies provide evidence of SGLT regulation via cyclic adenosine monophosphate/protein kinase A, protein kinase C, glucagon-like peptide 2, insulin, leptin, signal transducer and activator of transcription-3 (STAT3), phosphoinositide-3 kinase (PI3K)/Akt, mitogen-activated protein kinases (MAPKs), nuclear factor-kappaB (NF-kappaB), with-no-K[Lys] kinases/STE20/SPS1-related proline/alanine-rich kinase (Wnk/SPAK) and regulatory solute carrier protein 1 (RS1) pathways.

SUMMARY

SGLT inhibitors are important drugs for glycemic control in diabetes mellitus. Although the contribution of SGLT1 for absorption of glucose from the intestine as well as SGLT2/SGLT1 for renal glucose reabsorption has been comprehensively defined, this review provides an up-to-date outline for the mechanistic regulation of SGLT1/SGLT2.

Multifaceted interplay among mediators and regulators of intestinal glucose absorption: potential impacts on diabetes research and treatment

Glucose is the prominent molecule that characterizes diabetes and, like the vast majority of nutrients in our diet, it is absorbed and enters the bloodstream directly through the small intestine; hence, small intestine physiology impacts blood glucose levels directly. Accordingly, intestinal regulatory modulators represent a promising avenue through which diabetic blood glucose levels might be moderated clinically. Despite the critical role of small intestine in blood glucose homeostasis, most physiological diabetes research has focused on other organs, such as the pancreas, kidney, and liver. We contend that an improved understanding of intestinal regulatory mediators may be fundamental for the development of first-line preventive and therapeutic interventions in patients with diabetes and diabetes-related diseases. This review summarizes the major important intestinal regulatory mediators, discusses how they influence intestinal glucose absorption, and suggests possible candidates for future diabetes research and the development of antidiabetic therapeutic agents.

Involvement of the Niacin Receptor GPR109a in the LocalControl of Glucose Uptake in Small Intestine of Type 2Diabetic Mice

Niacin is a popular nutritional supplement known to reduce the risk of cardiovascular diseases by enhancing high-density lipoprotein levels. Despite such health benefits, niacin impairs fasting blood glucose. In type 2 diabetes (T2DM), an increase in jejunal glucose transport has been well documented; however, this is intriguingly decreased during niacin deficient state. In this regard, the role of the niacin receptor GPR109a in T2DM jejunal glucose transport remains unknown. Therefore, the effects of diabetes and high-glucose conditions on GPR109a expression were studied using jejunal enterocytes of 10-week-old m+/db and db/db mice, as well as Caco-2 cells cultured in 5.6 or 25.2 mM glucose concentrations. Expression of the target genes and proteins were quantified using real-time polymerase chain reaction (RT-PCR) and Western blotting. Glucose uptake in Caco-2 cells and everted mouse jejunum was measured using liquid scintillation counting. 10-week T2DM increased mRNA and protein expression levels of GPR109a in jejunum by 195.0% and 75.9%, respectively, as compared with the respective m+/db control; high-glucose concentrations increased mRNA and protein expression of GPR109a in Caco-2 cells by 130.2% and 69.0%, respectively, which was also confirmed by immunohistochemistry. In conclusion, the enhanced GPR109a expression in jejunal enterocytes of T2DM mice and high-glucose treated Caco-2 cells suggests that GPR109a is involved in elevating intestinal glucose transport observed in diabetes.

Upregulation of ACE2-ANG-(1-7)-Mas axis in jejunal enterocytes of type 1 diabetic rats: implications for glucose transport

The inhibitory effects of the angiotensin-converting enzyme (ACE)-ANG II-angiotensin type 1 (AT₁) receptor axis on jejunal glucose uptake and the reduced expression of this system in type 1 diabetes mellitus (T1DM) have been documented previously. The ACE2-ANG-(1-7)-Mas receptor axis is thought to oppose the actions of the ACE-ANG II-AT₁ receptor axis in heart, liver, and kidney. However, the possible involvement of the ACE2-ANG-(1-7)-Mas receptor system on enhanced jejunal glucose transport in T1DM has yet to be determined. Rat everted jejunum and Caco-2 cells were used to determine the effects of ANG-(1-7) on glucose uptake and to study the ACE2-ANG-(1-7)-Mas receptor signaling pathway. Expression of target gene and protein in jejunal enterocytes and human Caco-2 cells were quantified using real-time PCR and Western blotting. T1DM increased jejunal protein and mRNA expression of ACE2 (by 59 and 173%, respectively) and Mas receptor (by 55 and 100%, respectively) in jejunum. One millimolar ANG-(1-7) reduced glucose uptake in jejunum and Caco-2 cells by 30.6 and 30.3%, respectively, effects that were abolished following addition of 1 μM A-779 (a Mas receptor blocker) or 1 μM GF-109203X (protein kinase C inhibitor) to incubation buffer for jejunum or Caco-2 cells, respectively. Finally, intravenous treatment of animals with ANG-(1-7) significantly improved oral glucose tolerance in T1DM but not control animals. In conclusion, enhanced activity of the ACE2-ANG-(1-7)-Mas receptor axis in jejunal enterocytes is likely to moderate the T1DM-induced increase in jejunal glucose uptake resulting from downregulation of the ACE-ANG II-AT₁ receptor axis. Therefore, altered activity of both ACE and ACE2 systems during diabetes will determine the overall rate of glucose transport across the jejunal epithelium.

SGLT2 inhibitors: molecular design and potential differences in effect

The physiological and pathological handling of glucose via sodium-glucose cotransporter-2 (SGLT2) in the kidneys has been evolving, and SGLT2 inhibitors have been focused upon as a novel drug for treating diabetes. SGLT2 inhibitors enhance renal glucose excretion by inhibiting renal glucose reabsorption. Consequently, SGLT2 inhibitors reduce plasma glucose insulin independently and improve insulin resistance in diabetes. To date, various SGLT2 inhibitors have been developed and evaluated in clinical studies. The potency and positioning of SGLT2 inhibitors as an antidiabetic drug are dependent on their characteristic profile, which induces selectivity, efficacy, pharmacokinetics, and safety. This profile decides which SGLT2 inhibitors can be expected for application of the theoretical concept of reducing renal glucose reabsorption for the treatment of diabetes. I review the structure and advancing profile of various SGLT2 inhibitors, comparing their similarities and differences, and discuss the expected SGLT2 inhibitors for an emerging category of antidiabetic drugs.

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Local RAS

The concept of a circulating RAS is well established and known to play an endocrine role in the regulation of fluid homeostasis (see Section 4.1, Chapter 4). However, it is more appropriate to view the RAS in the contemporary notion as an “angiotensin-generating system”, which consists of angiotensinogen, angiotensin-generating enzymes, and angiotensins, as well as their receptors. Some RASs can be termed as “complete”, having renin and ACE involved in the biosynthesis of angiotensin II peptide, i.e. in a renin and/or ACE-dependent manner which is exemplified in the circulating RAS. On the other hand, some RAS can be termed as “partial”, having alternate enzymes to renin and ACE, such as chymase and ACE2 (see Section 4.3, Chapter 4) available for the generation of angiotensin II and other bioactive angiotensin peptides in the biosynthetic cascade, i.e. in a renin and/or ACE-independent manner. Complete vs. partial RASs can be exemplified in the so-called intrinsic angiotensin-generating system or local RAS; for example, a local and functional RAS with renin and ACE-dependent but a renin-independent pathway have been indentified in the pancreas and carotid body, respectively. In the past two decades, local RASs have gained increasing recognition especially with regards to their clinical importance. Distinct from the circulating RAS, these functional local RASs exist in such diverse tissues and organs as the pancreas, liver, intestine, heart, kidney, vasculature, carotid body, and adipose, as well as the nervous, reproductive, and digestive systems. Taken into previous findings from our laboratory and others together, Table 5.1 is a summary of some recently identified local RASs in various levels of tissues and organs.

Diabetes mellitus and expression of the enterocyte renin-angiotensin system: implications for control of glucose transport across the brush border membrane

Streptozotocin-induced (Type 1) diabetes mellitus (T1DM) in rats promotes jejunal glucose transport, but the trigger for this response remains unclear. Our recent work using euglycemic rats has implicated the enterocyte renin-angiotensin system (RAS) in control of sodium-dependent glucose transporter (SGLT1)-mediated glucose uptake across the jejunal brush border membrane (BBM). The aim of the present study was to examine whether expression of enterocyte RAS components is influenced by T1DM. The effects of mucosal addition of angiotensin II (AII) on [(14)C]-D-glucose uptake by everted diabetic jejunum was also determined. Two-week diabetes caused a fivefold increase in blood glucose level and reduced mRNA and protein expression of AII type 1 (AT(1)) and AT(2) receptors and angiotensin-converting enzyme in isolated jejunal enterocytes. Angiotensinogen expression was, however, stimulated by diabetes while renin was not detected in either control or diabetic enterocytes. Diabetes stimulated glucose uptake into everted jejunum by 58% and increased the BBM expression of SGLT1 and facilitated glucose transporter 2 (GLUT2) proteins, determined by Western blotting by 25% and 135%, respectively. Immunohistochemistry confirmed an enhanced BBM expression of GLUT2 in diabetes and also showed that this was due to translocation of the transporter from the basolateral membrane to BBM. AII (5 microM) or L-162313 (1 microM), a nonpeptide AII analog, decreased glucose uptake by 18% and 24%, respectively, in diabetic jejunum. This inhibitory action was fully accountable by an action on SGLT1-mediated transport and was abolished by the AT(1) receptor antagonist losartan (1 microM). The decreased inhibitory action of AII on in vitro jejunal glucose uptake in diabetes compared with that noted previously in jejunum from normal animals is likely to be due to reduced RAS expression in diabetic enterocytes, together with a disproportionate increase in GLUT2, compared with SGLT1 expression at the BBM.

Sodium tungstate decreases sucrase and Na+/D-glucose cotransporter in the jejunum of diabetic rats

Sodium tungstate reduces glycemia and reverts the diabetic phenotype in several induced and genetic animal models of diabetes. Oral administration of this compound has recently emerged as an effective treatment for diabetes. Here we examined the effects of 30 days of oral administration of tungstate on disaccharidase and Na+/D-glucose cotransporter (SGLT1) activity in the jejunum of control and streptozotocin-induced diabetic rats. Diabetes increased sucrase-specific activity in the jejunal mucosa but did not affect the activity of lactase, maltase, or trehalase. The abundance and the maximal rate of transport of SGLT1 in isolated brush-border membrane vesicles also increased. Tungstate decreased sucrase activity and normalized SGLT1 expression and activity in the jejunum of diabetic rats. Furthermore, tungstate did not change the affinity of SGLT1 for d-glucose and had no effect on the diffusional component. In control animals, tungstate had no effect on disaccharidases or SGLT1 expression. Northern blot analysis showed that the amount of specific SGLT1 mRNA was the same in the jejunum from all experimental groups, thereby indicating that changes in SGLT1 abundance are due to posttranscriptional mechanisms. We conclude that the antidiabetic effect of tungstate is partly due to normalization of the activity of sucrase and SGLT1 in the brush-border membrane of enterocytes.

Involvement of an enterocyte renin-angiotensin system in the local control of SGLT1-dependent glucose uptake across the rat small intestinal brush border membrane

There is increasing evidence that locally produced angiotensin AII (AII) regulates the function of many tissues, but the involvement of enterocyte-derived AII in the control of intestinal transport is unknown. This study examined whether there is a local renin-angiotensin system (RAS) in rat villus enterocytes and assessed the effects of AII on SGLT1-dependent glucose transport across the brush border membrane (BBM). Gene and protein expression of angiotensinogen, ACE, and AT(1) and AT(2) receptors were studied in jejunal and ileal enterocytes using immunocytochemistry, Western blotting and RT-PCR. Mucosal uptake of d-[(14)C]glucose by everted intestinal sleeves before and after addition of AII (0-100 nm) to the mucosal buffer was measured in the presence or absence of the AT(1) receptor antagonist losartan (1 microm). Immunocytochemistry revealed the expression of angiotensinogen, ACE, and AT(1) and AT(2) receptors in enterocytes; immunoreactivity of AT(1) receptor and angiotensinogen proteins was especially pronounced at the BBM. Expression of angiotensinogen and AT(1) and AT(2) receptors, but not ACE, was greater in the ileum than the jejunum. Addition of AII to mucosal buffer inhibited phlorizin-sensitive (SGLT1-dependent) jejunal glucose uptake in a rapid and dose-dependent manner and reduced the expression of SGLT1 at the BBM. Losartan attenuated the inhibitory action of AII on glucose uptake. AII did not affect jejunal uptake of l-leucine. The detection of RAS components at the enterocyte BBM, and the rapid inhibition of SGLT1-dependent glucose uptake by luminal AII suggest that AII secretion exerts autocrine control of intestinal glucose transport.

Intestinal phosphate absorption in a model of chronic renal failure

Hyperphosphatemia is an important consequence of chronic renal failure (CRF). Lowering of the plasma phosphate concentration is believed to be critical in the management of patients with CRF, especially those on dialysis. Reports of the effect of CRF on the intestinal handling of phosphate in vitro have been conflicting; but what happens in vivo has not been studied. What effect a reduction in the dietary phosphate intake has on intestinal phosphate absorption in CRF in vivo is unclear. In this study, we have used the in situ intestine loop technique to determine intestinal phosphate absorption in the 5/6-nephrectomy rat model of CRF under conditions of normal and restricted dietary phosphate intake. In this model of renal disease, we found that there is no significant change in the phosphate absorption in either the duodenum or jejunum regardless of the dietary phosphate intake. There was also no change in the expression of the messenger RNA of the major intestinal phosphate carrier the sodium-dependent-IIb transporter. Furthermore, we found no change in the intestinal villus length or in the location of phosphate uptake along the villus. Our results indicate that in CRF, unlike the kidney, there is no reduction in phosphate transport across the small intestine. This makes intestinal phosphate absorption a potential target in the prevention and treatment of hyperphosphatemia.

Inhibition of intestinal and renal Na+-glucose cotransporter by naringenin

Reduction in glucose uptake constitutes a possible means of controlling diabetic hyperglycemia. Using purified intestinal brush border membrane vesicles and everted intestinal sleeves, we have demonstrated that naringenin, a flavonoid present in citrus fruits and juices, significantly inhibited glucose uptake in the intestine. In addition, naringenin also elicited inhibitory actions towards glucose uptake in renal brush border membrane vesicles. Naringin, a glycoside of naringenin, was totally inactive in these aspects. Naringenin exhibited moderate inhibitory action on glucose uptake in rabbit intestinal brush border membrane vesicles, and showed strong inhibitory action in rat everted intestinal sleeves. The IC(50) values were 205.9 and 2.4 micromol/l, respectively. Lineweaver-Burk analysis demonstrated that naringenin inhibited glucose uptake in rat everted intestinal sleeves in a competitive manner with a K(i) value of 1.1 micromol/l. Glucose uptake activities in both the intestinal and renal brush border membrane vesicles of diabetic rats were significantly higher than in normal rats. Naringenin (500 microM) reduced glucose uptake by more than 60% in both the intestinal and renal brush border membrane vesicles of diabetic rats to a level similar to that of the normal rats. The IC(50) values of naringenin in the renal brush border membrane vesicles of normal and diabetic rats were 323.9 and 166.1 micromol/l, respectively. These results suggest that inhibition of intestinal glucose uptake and renal glucose reabsorption explains, in part at least, the in vivo antihyperglycemic action of naringenin and its derivatives. The possible application of these natural compounds in controlling hyperglycemia warrants further investigations.

Sodium-dependent glucose transporter reduces peroxynitrite and cell injury caused by cisplatin in renal tubular epithelial cells

Cisplatin causes nephropathy accompanied by two types of cell death, necrosis and apoptosis, according to its dosage. The mechanisms of necrosis are still unclear. In this study, we examined how high doses of cisplatin induce cell injury and whether a high affinity sodium-dependent glucose transporter (SGLT1) has a cytoprotective function in renal epithelial LLC-PK(1) cells. Cisplatin decreased in transepithelial electrical resistance (TER) and increased in the number of necrotic dead cells in a time dependent manner. Phloridzin, a potent SGLT1 inhibitor, enhanced both TER decrease and increase of necrotic dead cells caused by cisplatin. Cisplatin increased in the intracellular nitric oxide, superoxide anion and peroxynitrite productions. Phloridzin enhanced the peroxynitrite production caused by cisplatin. The intracellular diffusion of ZO-1 and TER decrease caused by cisplatin were inhibited by N-nitro-l-arginine methyl ester, a nitric oxide synthase inhibitor. Protein kinase C was not involved in the cisplatin-induced injury. 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrinato iron (III) and reduced glutathione, peroxynitrite scavengers, inhibited the cisplatin-induced ZO-1 diffusion, TER decrease, and increase of necrotic dead cells. These results suggest that peroxynitrite is a key mediator in the nephrotoxicity caused by high doses of cisplatin. SGLT1 endogenously carries out the cytoprotective function by the reduction of peroxynitrite production.

Modelling molecular mechanisms controlling sequential gene expression in differentiating mammalian enterocytes

The gut epithelium represents a continuous developmental system in which cell proliferation in intestinal crypts is followed by the sequential expression of digestive and absorptive functions as enterocytes migrate out of crypts to the tips of intestinal villi. We have developed a mathematical model in the present work to mimic these sequential aspects of enterocyte differentiation. Using this model allows the characteristics of lactase expression to be ascribed to transcriptional control. In the case of a glucose transporter, however, it became necessary to assume an additional translational control that decreased exponentially as enterocytes migrated along villi. The suggestion that this type of modelling is useful in predicting which set of enterocytes is likely to use translation or transcription to control gene expression is also discussed.

Mucosal inflammation in a genetic model of spontaneous type I diabetes mellitus

The BioBreeding (BB) rat provides a model of spontaneous type I diabetes mellitus that closely resembles the human disease. Diabetes-prone BB rats demonstrate increased intestinal permeability prior to the development of insulinitis. Studies suggest that alterations in intestinal permeability can lead to increased intestinal inflammatory activity. Diabetes-prone (BBdp) and diabetes-resistant (BBdr) BB rats were examined at 45 days and at >70 days of age following the development of clinical disease (BBd). In separate experiments, tissue was assayed for myeloperoxidase (MPO) or fixed for histological assessment and immunohistochemistry. Blood was obtained for leukocyte MPO measurements and morphological assessment of circulating leukocytes. MPO activity was significantly elevated in the distal small intestine of 45-day-old BBdp rats. In contrast, at >70 days of age, MPO activity was significantly increased throughout the small intestine of BBd and non-diabetic BBdp rats. Subsequently, all measurements were performed in >70-day-old rats. An increase in inflammatory infiltrate was noted in the distal small intestine of BBd rats by light microscopy. Infiltrating cells were identified as bands (a maturing cell type of the neutrophil lineage) and mature neutrophils. The findings suggest diabetes susceptibility is associated with an increase in intestinal inflammatory activity.

Regulation of jejunal glucose transporter expression by forskolin

We have investigated the effects of forskolin on enterocyte membrane expression of the glucose transporters, SGLT1 and GLUT2, which are thought to be the main entry and efflux pathways for glucose, respectively. Forskolin treatment increased SGLT1 but decreased GLUT2 expression in mid and lower villus enterocytes. No change in transporter expression was noted in upper villus cells. Likewise, cyclic AMP levels were raised in mid and lower but not upper villus cells. The implications of these data for glucose transport are discussed.

Expression of monosaccharide transporters in intestine of diabetic humans

Noninsulin-dependent diabetes mellitus (NIDDM) is an increasingly common disease, which brings a number of life-threatening complications. In rats with experimentally induced diabetes, there is an increase in the capacity of the intestine to absorb monosaccharides. We have examined the activity and the expression of monosaccharide transporters in the intestine of patients suffering from NIDDM. Na(+)-dependent D-glucose transport was 3.3-fold higher in brush-border membrane (BBM) vesicles isolated from duodenal biopsies of NIDDM patients compared with healthy controls. Western analysis indicated that SGLT1 and GLUT5 protein levels were also 4.3- and 4.1-fold higher in diabetic patients. This was associated with threefold increases in SGLT1 and GLUT5 mRNA measured by Northern blotting. GLUT2 mRNA levels were also increased threefold in the intestine of diabetic patients. Analysis of other BBM proteins indicated that the activity and abundance of sucrase and lactase were increased by 1.5- to 2-fold and the level of the structural proteins villin and beta-actin was enhanced 2-fold in diabetic patients compared with controls. The increase in the capacity of the intestine to absorb monosaccharides in human NIDDM is due to a combination of intestinal structural change with a specific increase in the expression of the monosaccharide transporters SGLT1, GLUT5, and GLUT2.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Acute and chronic exposure of rat intestinal mucosa to dextran promotes SGLTI-mediated glucose transport

BACKGROUND

The intestinal handling of dextran, an alpha-1,6-linked glucose polymer, is poor compared with starch, and some ingested dextran might therefore reach the lower small intestine. As luminal sugar up-regulates SGLT1 (sodium-dependent glucose transporter) locally, we report the effects of a dextran-enriched diet on jejunal and ileal brush border membrane (BBM) glucose uptake.

METHODS

Rats were maintained on a diet containing 65% maltodextrin or 32.5% maltodextrin + 32.5% dextran (10 kD or 40 kD) for 8-10 days, and the kinetics of phlorizin-sensitive [3H]-glucose uptake by purified BBM vesicles was determined.

RESULTS

Ingestion of 40-kD but not 10-kD dextran increased Vmax for jejunal and ileal glucose uptake (+64.3% and +61.8% respectively, both P < 0.02). The transport response to 40-kD dextran was in keeping with lower levels of expired H2 at the end of the feeding period. High-performance liquid chromatography (HPLC) analysis of luminal contents indicated extensive hydrolysis of ingested dextran. Finally, 3-h jejunal exposure to 40-kD dextran in vivo increased the Vmax for glucose uptake by jejunal BBM.

CONCLUSION

It is likely that increased SGLT1-mediated glucose uptake after short or longer term mucosal exposure to dextran results from luminal dextran per se or a hydrolysis product. The clinical implications of this up-regulation are discussed.

Cholecystokinin decreases intestinal hexose absorption by a parallel reduction in SGLT1 abundance in the brush-border membrane

The dual lumenaly and vascularly perfused small intestine was used to determine the mechanism by which cholecystokinin octapeptide (CCK-8) decreases the rate of glucose absorption. With CCK-8 in the vascular perfusate the rate of 3-O-methyl-D-glucose absorption decreased, whereas the rate of D-fructose absorption was unaffected. The substrate pool size within the tissue during steady-state transport, in the presence and absence of CCK-8, was estimated by compartmental analysis of the 3-O-methyl-D-glucose washout into the vascular bed. When CCK-8 was included in the vascular perfusate, the absorptive cell pool size decreased when compared with untreated tissue. Both the steady-state hexose absorption data and the washout studies indicated that the locus of action of CCK-8 was the SGLT1 transporter located in the brush-border membrane. The SGLT1 protein abundance in isolated brush-border membranes, as quantified by Western blotting, showed a decrease that paralleled the decrease in the steady-state transport rate induced by CCK-8. These results indicate that CCK-8 diminishes the rate of intestinal hexose absorption by decreasing SGLT1 protein abundance in the brush-border membrane of the rat jejunum and therefore provides evidence for acute enteric hormonal regulation of the rate of glucose absorption across the small intestine.

Transcriptional and translational control over sodium-glucose-linked transporter (SGLT1) gene expression in adult rat small intestine

We have measured SGLT1 mRNA content and SGLT1-mediated glucose transport at different positions along the small intestine of control and streptozotocin diabetic rats and shown both parameters to be similar but higher in jejunal compared with ileal tissue. No such correlation was seen when comparing measurements of SGLT1 mRNA along jejunal villi with previous estimates of SGLT1 protein and SGLT1-mediated glucose transport [Debnam et al., Eur. J. Physiol. 430 (1995) 151-159]. This is the first time it has been possible to directly relate these three aspects of SGLT1 gene expression in a single species. Results are discussed in terms of a possible time rather than positional control over translation of SGLT1 mRNA.

Streptozotocin diabetes and the expression of GLUT1 at the brush border and basolateral membranes of intestinal enterocytes

Changes in membrane expression of sodium-dependent glucose transporter (SGLT1) and glucose transporter isoform (GLUT2) protein have been implicated in the increased intestinal glucose transport in streptozotocin-diabetes. The possible involvement of GLUT1 in the transport response, however, has not previously been studied. Using confocal microscopy on tissue sections and Western blotting of purified brush border membrane (BBM) and basolateral membrane (BLM), we have examined enterocyte expression of GLUT1 in untreated and in 1 and 21 day streptozotocin diabetic rats. In control enterocytes, GLUT1 was absent at the BBM and detected at low levels at the BLM. Diabetes resulted in a 4- to 5-fold increased expression of GLUT1 at the BLM and the protein could also be readily detected at the BBM. Insulin treatment of diabetic rats increased GLUT1 level at the BBM but was without effect on expression of the protein at the BLM.

Rapid enhancement of brush border glucose uptake after exposure of rat jejunal mucosa to glucose

BACKGROUND

Increased jejunal glucose transport after ingestion of carbohydrate rich diets may reflect higher concentrations of lumenal glucose. Normal processing of carbohydrate causes wide fluctuations in glucose concentration in the jejunal lumen and this raises the question of whether the high lumenal concentrations seen at peak digestion affect glucose uptake.

AIMS

To study the effects of 30 minute exposure of rat jejunal mucosa to glucose on sodium-glucose transporter (SGLT1) mediated glucose transport across the brush border membrane.

METHODS

Jejunal mucosa was exposed in vitro or in vivo to 25 mM glucose or 25 mM mannitol for 30 minutes. In addition, isolated villus enterocytes were incubated with mannitol or glucose for the same time. Brush border membrane vesicles were isolated from these preparations and phlorizin sensitive 3H-D-glucose accumulation was measured.

RESULTS

Lumenal glucose in vivo significantly enhanced SGLT1 mediated glucose uptake by 49.2-57.2%. For jejunal loops in vitro, the increase was 32.0-85.2%. Kinetic analysis disclosed a 50% greater Vmax for glucose uptake in each preparation. The facilitated and passive components of uptake were, however, unaffected by prior exposure to glucose. Incubation of villus enterocytes with 25 mM glucose did not influence glucose uptake by brush border membranes. Finally, exposure of intact mucosa to 20 mM galactose, a nonmetabolised sugar also transported by SGLT1, did not alter glucose transport.

CONCLUSIONS

Lumenal glucose promotes glucose transport by brush border membrane within 30 minutes. An intact mucosa is necessary for upregulation and evidence suggests that the response is mediated by locally acting mechanisms.