Sodium-dependent glucose transporter protein as a potential therapeutic target for improving glycemic control in diabetes

Special Article

Sodium-dependent glucose transporter protein as a potential therapeutic target for improving glycemic control in diabetes Carmen Castaneda-Sceppa and Francisco Castaneda Glucose is transported across the cell membrane by two different types of glucose transporters: glucose-facilitated transporters and sodium-dependent glucose transport (SGLT) proteins. Regulation of SGLT activity (namely, inhibition of SGLT1 and SGLT2 activity and stimulation of SGLT3 activity) represents a potential means of managing hyperglycemia and diabetes, thus preventing complications of diabetes. The purpose of the present review is to discuss the role of SGLT proteins in the pathophysiology of diabetes and to describe the mechanisms by which these transporters may be used for glycemic control and the treatment of diabetes. The regulatory processes involved in SGLT-mediated glucose uptake are also described briefly. This information provides new insight into the complementary mechanisms involved in the regulation of SGLT-mediated glucose transport as well as a basis for further investigation. nure_423

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© 2011 International Life Sciences Institute

INTRODUCTION Diabetes mellitus is prevalent worldwide, affecting approximately 171 million people. Its incidence continues to rise, with the total number of diabetics projected to reach 300 million by the year 2025.1 Lifestyle modification accompanied by therapy with a single daily oral antihyperglycemic agent achieves target glycemic goals only infrequently, and, if it does, the effect is usually not sustained.2,3 In contrast, the combination of lifestyle change and intensive diabetes treatment with insulin therapy and/or oral medications can effectively treat type 2 diabetes and prevent its long-term complications.4 Given that the prevalence of type 2 diabetes is a major public health concern due to associated morbidity and mortality, the development of new pharmacological therapies to control blood glucose levels is crucial. Sodium-dependent glucose transport (SGLT) proteins represent an excellent target for the development of innovative therapies to achieve glycemic control and to better manage diabetes. SGLT proteins were first postulated as a possible therapeutic target for the control of

blood glucose levels in 1996 by Tsujihara et al.5 These investigators, along with Hongu et al., described the possibility of using SGLT inhibitors as antidiabetic agents on the basis of experimental studies in rats.5–7 Since then, regulation of glucose disposal through SGLT proteins has been considered an important biological target that can be used for the development of innovative therapies to effectively manage diabetes and prevent its long-term complications.8 The objective of this manuscript is to review the role of SGLT proteins in the pathophysiology of diabetes and to describe the mechanisms by which these transporters may be used for glycemic control and treatment of diabetes. PATHOPHYSIOLOGY OF DIABETES MELLITUS Diabetes is characterized by hyperglycemia due to either reduced insulin secretion from pancreatic b cells (type 1 diabetes) or resistance to insulin action (type 2 diabetes). Approximately 90–95% of the people diagnosed with diabetes have type 2 diabetes.9

Affiliations: C Castaneda-Sceppa is with the Bouve College of Health Sciences, Northeastern University, Boston, MA, USA. F Castaneda is with the Klinik Herzberg, Herzberg am Harz, Germany. Correspondence: F Castaneda, Klinik Herzberg, Dr-Frössel-Allee, 37412 Herzberg am Harz, Germany. E-mail: [email protected], Phone: +49-5521-866-275, Fax: +49-5521-866-459. Key words: diabetes treatment, diabetes-related complications, hyperglycemia, sodium-glucose transport proteins 720

doi:10.1111/j.1753-4887.2011.00423.x Nutrition Reviews® Vol. 69(12):720–729

Insulin resistance represents the principal factor in the development of type 2 diabetes.10 Modifiable risk factors involved in the development of insulin resistance and type 2 diabetes include obesity, physical inactivity, and unhealthy eating habits.11 Insulin resistance usually precedes glucose abnormalities and facilitates the progression of impaired fasting glucose or impaired glucose tolerance to type 2 diabetes by accelerating b-cell functional deterioration.12–14 The initial response to insulin resistance is compensatory hyperinsulinemia.15,16 As long as the b cells can compensate for the reduced insulin sensitivity by increasing insulin synthesis, glucose metabolism remains normal. As b-cell function deteriorates further, as occurs characteristically in individuals genetically predisposed to type 2 diabetes, glucose intolerance develops. Therefore, insulin resistance is the initial pathogenic mechanism leading to failure of pancreatic b cells to secrete enough insulin to overcome insulin resistance.17 As a consequence, hyperglycemia becomes a clinical manifestation. Chronic hyperglycemia resulting from an imbalance between insulin requirements and insulin availability leads to progressive impairment of insulin secretion and to insulin resistance in peripheral tissues (referred to as glucose toxicity). Moreover, chronic hyperglycemia represents the major risk factor for complications of diabetes, including heart disease, retinopathy, nephropathy, and neuropathy.18 Hyperglycemia increases the filtered load of glucose in the glomerulus,19 resulting in mesangial cell hypertrophy.20 Diabetic nephropathy is characterized by damage to glomeruli and tubular epithelium due to increased glucose fluxes.21,22 Other complications of diabetes, including neuropathy and retinopathy, result from microvascular injury of small blood vessels supplying nerves and the retina, respectively.23,24 The development and progression of these complications depends on the magnitude of hyperglycemia, with the subsequent increased expression of reactive oxygen species shown to lead to oxidative damage and mitochondrial dysfunction.25,26 The central role of hyperglycemia in the development and progression of the complications of diabetes has been reported extensively. Therefore, in order to prevent and/or minimize the development of diabetic microvascular and macrovascular complications, glycemic control is mandatory.27 GLYCEMIC CONTROL IN THE TREATMENT OF DIABETES Glycemic control can be achieved using many classes of oral antihyperglycemic agents in addition to lifestyle modifications that include increased physical activity28 and consumption of an appropriate diet.29,30 The role of Nutrition Reviews® Vol. 69(12):720–729

physical activity in the prevention and treatment of diabetes has been demonstrated in different clinical studies.31,32 In addition, physical activity reduces the incidence of diabetes in persons at high risk.33 Acute and chronic endurance exercise has been shown to be beneficial for patients with type 2 diabetes31,33–36 and to regulate the expression of glucose-facilitated transport (GLUT) protein 4 (GLUT4).37–39 Strength training has also been shown to improve glycemic control and glucose uptake in people with40–43 and without44,45 type 2 diabetes. Some studies using animal models of insulin resistance and human studies in people with glucose intolerance or type 2 diabetes have demonstrated that physical activity and exercise enhances insulin action in skeletal muscle. This effect has been attributed to an increase in the expression of GLUT4.46,47 There are, however, some reports suggesting that GLUT4 content48 and gene expression44,49,50 in skeletal muscle may not be affected by exercise. In contrast, studies using mice have reported that physical activity increases the expression of GLUT4 in skeletal muscle.51 Resistance exercise training enhances insulin sensitivity and improves glucose uptake in patients with type 2 diabetes mellitus.41,43,52 Moreover, progressive resistance training improves glycemic control and the abnormalities associated with metabolic syndrome among high-risk older adults with poorly controlled type 2 diabetes.40 Diabetes treatment with lifestyle modification accompanied by a single daily oral antihyperglycemic agent infrequently achieves target glycemic goals, and, if it does, the effect is usually not sustained.53,54 A number of oral antidiabetic drugs, including metformin, sulfonylurea, thiazolidinediones, and insulin, have been shown to be equally effective in improving glycemic control. However, most patients who were prescribed these drugs failed to achieve their glycemic targets,55 suggesting a therapy that combines drugs with different mechanisms of action may be more appropriate.4,56,57 In addition to supplementation with exogenous insulin, several hypoglycemic drugs are usually prescribed to treat diabetes. These therapies are aimed at reducing insulin resistance by using glitazones, increasing endogenous insulin production by using sulfonylureas and meglitinides, reducing hepatic glucose production by using biguanides, restricting postprandial glucose absorption by using alpha-glucosidase inhibitors, and/or improving the effectiveness of circulating insulin by using thiazolidinediones.8,54 If diabetes therapy is applied when b-cell function is reduced by approximately 50% of normal, monotherapy has been shown to improve glycemic control, achieving glycosylated hemoglobin goals of less than 7.0%.53 In contrast, if b-cell function is significantly reduced (more than 50% of normal), the combination of thiazolidinedione with other therapeutic agents 721

shown either to increase insulin availability (such as insulin secretogogues or exogenous insulin) or to provide additional improvement in insulin resistance (such as metformin) is indicated.58 Despite the availability of these and other antidiabetic drugs, treatment of diabetes is problematic because of the complexities associated with achieving glycemic control.2 Although each individual oral antihyperglycemic agent may be able to improve glucose metabolism, it is still difficult to maintain good glycemic control in most diabetic patients.53 Therefore, therapeutic agents that reduce glucose toxicity and improve glucose transport across the cell membranes, such as that achieved with SGLT proteins, represent potential treatments to effectively manage hyperglycemia (i.e., glucose toxicity) and diabetes. GLUCOSE TRANSPORT Cellular glucose uptake requires transport proteins because glucose does not freely permeate the plasma membrane.59 The glucose transport proteins are divided into two groups: GLUT proteins and SGLT proteins. GLUT proteins allow the transport of glucose down its concentration gradient,60 while SGLT proteins transport glucose against its concentration gradient.61 The transport of glucose into epithelial cells is mediated by a secondary active cotransport system, or SGLT, driven by a sodium gradient generated by the Na+/K ⫾ATPase. Glucose accumulated in the epithelial cell is further transported into the blood across the membrane by diffusion facilitated by GLUT proteins. The GLUT proteins belong to the SLC2 gene family and are composed of 13 members, including the GLUT1 through the GLUT12 proteins and the H(⫾)-myoinositol cotransporter.62 GLUT1 transports glucose across the endothelial cells of the blood-brain barrier, GLUT2 is localized in the basolateral membrane of intestine and kidney epithelial cells, and GLUT4 transports glucose into the skeletal muscle. This effect is characterized by an insulin-dependent regulation.63 Exercise, the major physiological activator of muscle glucose transport, regulates the expression of GLUT4 in skeletal muscle51,64 and induces the translocation of GLUT4 from the intracellular pool to the plasma membrane.65 SGLT protein belongs to the sodium-dependent glucose cotransporter family SLCA5.66 Two different SGLT isoforms, SGLT1 and SGLT2, have been identified to mediate renal tubular glucose reabsorption in humans. Both of them are characterized by their unique substrate affinity.67 Although both of them show 59% homology in their amino acid sequence, they are functionally different. SGLT1 transports glucose as well as galactose and is expressed both in the kidney and in the intestine, while 722

SGLT2 is found exclusively in the S1 and S2 segments of the renal proximal tubule.61 As a consequence, glucose filtered in the glomerulus is reabsorbed into the renal proximal tubular epithelial cells by SGLT2, a low-affinity/ high-capacity system in S1 and S2 tubular segments. Much smaller amounts of glucose are recovered by SGLT1, which functions as a high-affinity/low-capacity system in the distal segment of the tubule.61 This is why the research on the therapeutic effect of SGLT proteins has been focused on SGLT2. Another member of the SGLT protein family is SGLT3. SGLT3 is an insulin-independent glucose transporter expressed mainly in skeletal muscle.68 It was first described as SAAT-pSGLT2 due to its similarities with the other components of SGLT2 in the kidney of pigs.69 It has since been renamed SGLT3 after it was found in the DNA sequence of the human chromosome 22.70 SGLT3 is a low-affinity glucose transporter and is highly selective for D-glucose. SGLT3 is expressed at the neuromuscular junction in skeletal muscle and in the enteric nervous system. In both locations, SGLT3 co-localizes with the acetylcholine receptor.71 The expression of SGLT3 in skeletal muscle is stimulated by resistance training.72 Both GLUT and SGLT proteins are saturated under normal glucose concentrations. Therefore, changes in their expression, translocation, or activity levels are needed in order for these transport proteins to take up excessive quantities of glucose under conditions of hyperglycemia, such as diabetes.20 SODIUM-DEPENDENT GLUCOSE TRANSPORT PROTEINS SGLT1 plays a pivotal role in the translocation of sugar across epithelial cells in the small intestine and in the renal proximal tubule. In these epithelial membranes, glucose transport is active and requires the coupling of cellular energy metabolism with transepithelial translocation.73 The sodium-dependent glucose cotransporter is the site where such coupling occurs. The coupling is not direct, i.e., there is no hydrolysis of ATP involved as in other so-called primary active transport events such as those mediated by ion-translocating ATPases.74 Instead, the transporter uses the energy “stored” in an ion gradient to transport glucose against its concentration difference. This is referred to as secondary active transport and occurs in unicellular and multicellular organisms. In mammalian species, sodium is the most prominent ion gradient across the cell membrane. The secondary active transport of organic substances such as sugars, amino acids, carboxylic acids, and inorganic ions (chloride and phosphate) involves the simultaneous movement of one, two, or three sodium ions, mostly in the same direction as the substrate in a symport mode.67 Additionally, for vecNutrition Reviews® Vol. 69(12):720–729

torial transcellular transport, an asymmetry of the cell must be established, so that the plasma membrane facing one compartment contains different transporters than the membrane facing the other compartment into which translocation occurs.75 Therefore, the sodium gradient across the intestinal brush border is generated by sodiumpotassium-stimulated ATPase, a primary active ion transporter that removes sodium from the cell interior in exchange for potassium ions. As a result, D glucose can accumulate in the epithelial cells uphill from the intestinal or renal tubular lumen at a concentration higher than that in blood. Glucose leaves the cell according to its concentration difference via a carrier-mediated, sodiumindependent, passive movement.61 Glucose absorption in the human gut occurs in the first segments, where there is high affinity and velocity, and ceases in the colon. Glucose required for intracellular metabolism in the gut enters the intestinal cells from the cell side exposed to the blood. Studies on the presence of SGLT1 mRNA and protein expression in the various intestinal segments have confirmed the intraintestinal distribution of SGLT1. It should be noted that glucose transport in the intestine is also a mode of absorbing sodium across the epithelium. Therefore, the enteral application of a solution containing sodium and glucose is one of the most effective ways to compensate for fluid and electrolytes losses in cases of diarrhea.61 The other main organ where active sugar transport occurs is the kidney. In the early part of the renal proximal tubule, reabsorption of filtered glucose against a small gradient occurs, while in the late part, residual glucose is removed against a steep concentration difference.76 Transport studies in the early and late segments of the proximal tubule, as well as vesicle studies, showed that the kidney contains two sodium-dependent glucose cotransporters that differ in their stoichiometry for sodium. These differences have been confirmed by studies showing that one sodium and one glucose molecule are translocated together across the luminal membrane in the early proximal tubule, whereas in the late part of the tubule, two sodium ions are translocated with only one sugar molecule. These transporters also differ in their affinity for D glucose: in the early part of the tubule, the apparent transport affinity (Km) is about 2 mmol, while in the late part the affinity is higher (with a Km of < 0.5 mmol).76 The two sodium-dependent glucose cotransporters also exhibit different substrate specificity, as shown by the ability to detect D-galactose in the early part of the tubule but not in the late part.61 The sodium-dependent glucose cotransporter found in the late part of the tubule is similar to that in the intestine and is referred to as SGLT1, whereas the transporter found in the early renal proximal tubule is referred to as SGLT2. Nutrition Reviews® Vol. 69(12):720–729

The inherited disease of familial renal glucosuria is caused by a lack of SGLT2 in the early proximal tubule.77 Interestingly, in patients with this disorder, the reabsorptive capacity of the late proximal tubule seems to be sufficient to maintain normal plasma glucose levels. The only symptom observed in patients with familial renal glucosuria is an increase in urinary glucose excretion.78 In contrast, individuals who lack SGLT1 are found to have glucose and galactose malabsorption, accompanied by severe diarrhea, sodium, and fluid losses.61

REGULATION OF THE ACTIVITY OF SGLT PROTEINS Different mechanisms are involved in the regulation of SGLT1 and SGLT2 activity. In addition to blood glucose levels,79–81 protein kinases and calcitriol have been associated with regulation of SGLT1 activity (Fig. 1). One of the most extensively analyzed effects of protein kinases is that of serine/threonine protein kinase A (PKA) and protein kinase C (PKC). The regulatory effect of protein kinases on SGLT proteins can occur in a direct or an indirect manner.82 The direct regulatory effect of protein kinases on membrane transport results in kinetic changes in the transporter or changes in the carrier turnover number. In contrast, the indirect regulatory effect leads to an increased amount of the transporter present in the plasma membrane.82 Studies on oocytes expressing rabbit SGLT1 (rbSGLT1)83,84 have demonstrated that PKA and PKC regulate SGLT1-mediated glucose transport by augmenting the expression of SGLT1 in the plasma membrane.85 Specifically, there is a direct effect of PKA-mediated phosphorylation on the conformation of SGLT1.86 PKC indirectly regulates SGLT1 function by increasing the transporter trafficking from the intracellular pool to the plasma membrane via endo- or exo-cytosis.83 The indirect regulatory effect of SGLT proteins by PKC has been corroborated by the finding that mutation of the putative phosphorylation sites of PKC does not cause any effect on sodium-dependent alpha-methyl glucoside uptake in Chinese hamster ovary cells transiently expressing rbSGLT1 treated with PKC activators or inhibitors.87 Additionally, regulation of SGLT1 by PKC seems to involve intracellular signaling pathways triggered by second messengers. Specifically, it has been demonstrated that regulation of alpha-methyl glucoside uptake induced by PKC was mediated by at least two distinct intracellular signal cascades, the first involving MAPK (p38/MAPK, MEK/MAPK, and JNK pathways) and the second involving PI3K/Akt/mTOR signaling.87 This suggests that PKC regulates SGLT1 activity via a complex intracellular mechanism involving sorting and transcriptional effects. The involvement of these two complementary mecha723

Figure 1 Regulation of SGLT1 through protein kinase A (PKA), protein kinase C (PKC), and calcitriol in Chinese hamster ovary G6D3 cells.

nisms of action, namely stimulation and inhibition, are required for the regulation of SGLT1 by PKC. Vitamin D has been shown to be involved in the regulation of different biological processes, including cell growth and differentiation88 and apoptosis,89 as well as in certain physiological mechanisms, such as hormone secretion,90,91 T-cell proliferation and cytokine production,92 and immunosuppressive response.93 The active metabolite of vitamin D, calcitriol or 1a,25dihydroxyvitamin D3, acts through two different mechanisms of action. The first one mediates gene expression and is known as the genomic source of action of vitamin D.92,94–96 The second mechanism by which calcitriol regulates rbSGLT1 activity is through a rapid, extranuclearinitiated mechanism of action that involves intracellular signaling pathways, known as the nongenomic effect of vitamin D.97 In this, calcitriol activates PKC, which in turn acts as a membrane-bound receptor for calcitriol. Additionally, calcitriol regulates rbSGLT1 activity through a rapid, extranuclear-initiated mechanism of action stimulated by MAPK and inhibited by PI3K/Akt/mTOR.97 Activation of these transcription factors increases mRNA expression in calcitriol-treated cells, with subsequent increments in the amount of rbSGLT1 transcribed. Sub724

sequently, rbSGLT1 was found to be translocated into the plasma membrane, suggesting a regulatory effect on SGLT proteins similar to that seen with PKC. The effect of calcitriol on the activation of intracellular signaling pathways has also been reported in cultured rat intestinal cells,98 Chinese hamster ovary cells,99 skeletal muscle cells,100 and rabbit renal proximal tubule cells.94,101 Taken together, these data demonstrate that the regulatory mechanisms of action of SGLT1 are complex and involve several different factors, including PKA, PKC, and calcitriol. However, more research is needed to better understand these mechanisms of action. There is limited information examining the regulation of SGLT3 in skeletal muscle upon exercise. Individuals with uncontrolled type 2 diabetes (characterized by poor glycemic control and sustained hyperglycemia) undergoing moderate- to high-intensity resistance exercise training for 16 weeks40 exhibited a significant increase in SGLT3 transcript and protein levels in skeletal muscle tissue.72 SGLT3 was also preferentially localized in the plasma membrane of muscle fibers. Moreover, the change in SGLT3 transcript levels in the vastus lateralis muscle was significantly and positively correlated with glucose uptake, as measured by the change in muscle glycogen Nutrition Reviews® Vol. 69(12):720–729

stores. The observed increase in muscle glycogen stores after 16 weeks of resistance exercise (as compared with that in a control group not undergoing resistance exercise) suggests an increase in glucose transport across the cell membrane, in this case the sarcolema. This is believed to be the first clinical study examining the association between SGLT3 expression and glycemic control in human subjects undergoing resistance exercise training.72 The findings of this study suggest that SGLT3, an insulinindependent glucose transporter, is activated with exercise and may play a significant role in glycemic control with muscle contraction. The mechanisms of action and regulation of SGLT3 associated with these findings are not well understood and require further investigation. However, the functional significance of SGLT3 on reducing hyperglycemia (glucose toxicity) and improving insulin resistance is the subject of ongoing research.

CONTROL OF BLOOD GLUCOSE LEVELS BY SGLT PROTEINS Control of hyperglycemia is of utmost importance in the treatment of diabetes and its complications.102 In recent years, novel approaches for the treatment of diabetes suggest that affecting glucose absorption in the intestine and/or glucose reabsorption in the kidney might be a possible strategy to control blood glucose levels. As a result, inhibitors of glucose absorption have been developed to inhibit hydrolysis of sucrose and lactose by disaccharidases in the intestinal lumen. Examples of inhibitors that have been successfully introduced in the market include acarbose (Precose R® or Glucobay R®), voglibose (Basen R®), and miglitol (Glyset R®).103 These agents inhibit alpha-glucosidase in the brush border of the small intestine. Inhibition of these enzymes reduces the rate of digestion of complex carbohydrates and, consequently, glucose absorption in the small intestine is reduced. Treatment with alpha-glucosidase inhibitors in patients with diabetes results in a decreased postprandial plasma glucose level of about 10 mg/dL and a hemoglobin A1C level of about 0.5%.8,54,104 However, these beneficial effects of glycemic control are accompanied by gastrointestinal side effects such as abdominal discomfort, flatulence, and diarrhea. For these reasons, the clinical application of these agents is limited. As the understanding of glucose transport at the molecular level has progressed, inhibitors of the transport molecule itself have been synthesized, some of which are currently undergoing preclinical and clinical trials. The emphasis has been placed on specific inhibitors of glucose reabsorption by the kidney. Some of the specific therapeutic effects of SGLT1 (intestine) and SGLT2 (kidney) transport proteins are described below. Nutrition Reviews® Vol. 69(12):720–729

THERAPEUTIC EFFECTS OF SGLT PROTEINS Various O- and C-glycosides with high affinity for and specificity to inhibit SGLT2 have been synthesized. The rationale for the development of these synthetic inhibitors of SGLT2 is based on several assumptions. First, under physiological conditions, more than 90% of the glucose filtered in the glomeruli is reabsorbed in the S1 segment of the proximal tubule through the low-affinity/ high-capacity SGLT2 transporters.67,105 Second, abnormally high cellular glucose uptake resulting in glucose toxicity contributes to hyperglycemia, insulin resistance, and complications of diabetes.106 Third, glycemic control prevents or minimizes the development of diabetic complications.27 Studies of the O-glycoside phlorizin have shown phlorizin’s ability to inhibit SGLT2 by suppressing glucose reabsorption in the renal proximal tubule – the last site of glucose reabsorption – which in turn leads to glucosuria and reduced hyperglycemia.5,107,108 Phlorizin blocks the action of SGLT2 because it binds to the aglucone binding site and the sugar-binding/translocation site of the SGLT2 molecule.96,109 Since the initial observations of the inhibitory effects of phlorizin on SGLT2, a new and promising alternative for the treatment of diabetic patients has been proposed.107,110 Currently, synthetic glycosides are undergoing clinical trials and have been shown to successfully increase urinary glucose excretion and to decrease blood glucose levels without the danger of hypoglycemia seen with other medications for type 2 diabetes. Despite the potential antidiabetic effects of phlorizin, this glycoside may not be suited for the treatment of diabetes because of its low absorption rate, which is due to an increased hydrolysis of the O-glycosidic bond by intestinal disaccharidases. As a consequence, the C- and S-glycosides have been synthesized. Dapagliflozin, a C-aryl-glycoside, is a specific SGLT2 inhibitor for which the most clinical data are available.111–118 It has demonstrated sustained, dose-dependent glucosuria over 24 h with once-daily dosing in clinical trials.111,119–121 In addition, S-glycosides have high renal specificity that is associated with a strong competitive inhibitory effect of the sodium-dependent glucose cotransporter system mediated by SGLT2.122 Exercise, the major physiological activator of glucose transport in muscle, regulates GLUT4 expression in muscle51,64 by inducing the translocation of GLUT4 from the intracellular pool to the plasma membrane.123–125 It has been suggested that muscle contraction stimulates glucose transport by an insulin-independent mechanism. However, sustained insulin deficiency leads to a decreased number of glucose transporters, resulting in decreased responsiveness of glucose transport to both insulin and muscle contractions.64,126 People with type 2 725

diabetes have been shown to have defective insulinstimulated glucose transport in peripheral tissues, including GLUT4 expression in skeletal muscle.127 In contrast, as described above, SGLT3-mediated glucose transport is characterized by an insulin-independent mechanism of action that is not affected by insulin resistance and is enhanced by exercise,72 thus making it a promising target for the development of therapeutic agents for glycemic control.

CONCLUSION The present review describes the role and possible mechanisms of action by which SGLT proteins contribute to glycemic control and the treatment of diabetes.Activation of PKA stimulates SGLT1-mediated uptake of glucose directly as well as indirectly. The indirect effect results from a change in the sorting of the transporter between intracellular compartments and the plasma membrane, which results in an increased number of transporters in the plasma membrane. The direct effect results from phosphorylation of SGLT1 at the PKA consensus site. Phosphorylation induces conformational changes that alter the functional properties of the transporter. PKC regulates rbSGLT1-mediated glucose uptake indirectly by means of a change in the sorting of the transporter between intracellular compartments and the plasma membrane, which results in an increased number of transporters in the plasma membrane. In addition, the activation of intracellular signaling pathways by PKA and PKC, triggered by second messenger molecules, results in subsequent transcriptional regulation of rbSGLT1 mRNA expression. Calcitriol is also involved in the regulation of SGLT1 activity. This regulation is the result of a rapid, extranuclear-initiated (nongenomic) mechanism of action stimulated by MAPK and inhibited by PI3K/Akt/ mTOR. A new functional activity of SGLT3 found in human skeletal muscle after 16 weeks of resistance exercise training suggests that SGLT proteins may be a promising noninsulin-stimulated glucose transport system to improve glucose uptake. The information provided here offers new insight into the complementary mechanisms involved in the regulation of SGLT-mediated glucose transport. Further investigation of SGLT-mediated glucose transport as a novel therapy for glycemic control and the treatment of diabetes is warranted. Acknowledgments Declaration of interest. The authors have no relevant interests to declare. 726

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