Sodium-glucose cotransport

The diversified function and potential therapy of ectopic olfactory receptors in non-olfactory tissues

Olfactory receptors (ORs) are mainly distributed in olfactory neurons and play a key role in detecting volatile odorants, eventually resulting in the production of smell perception. Recently, it is also reported that ORs are expressed in non-olfactory tissues including heart, lung, sperm, skin, and cancerous tissues. Interestingly, ectopic ORs are associated with the development of diseases in non-olfactory tissues. For instance, ectopic ORs initiate the hypoxic ventilatory responses and maintain the oxygen homeostasis of breathing in the carotid body when oxygen levels decline. Ectopic ORs induce glucose homeostasis in diabetes. Ectopic ORs regulate systemic blood pressure by increasing renin secretion and vasodilation. Ectopic ORs participate in the process of tumor cell proliferation, apoptosis, metastasis, and invasiveness. Ectopic ORs accelerate the occurrence of obesity, angiogenesis and wound-healing processes. Ectopic ORs affect fetal hemoglobin levels in sickle cell anemia and thalassemia. Finally, we also elaborate some ligands targeting for ORs. Obviously, the diversified function and related signal pathway of ectopic ORs may play a potential therapeutic target in non-olfactory tissues. Thus, this review focuses on the latest research results about the diversified function and therapeutic potential of ectopic ORs in non-olfactory tissues.

Common variation in the sodium/glucose cotransporter 2 gene SLC5A2 does neither affect fasting nor glucose-suppressed plasma glucagon concentrations

AIM

Inhibition of sodium/glucose cotransporter 2 (SGLT2), the key transport protein in renal glucose reabsorption, promotes glucose excretion and represents a new concept in the therapy of type-2 diabetes. In addition, SGLT2 inhibition elevates circulating glucagon concentrations and enhances hepatic glucose production. Since SGLT2 is expressed in human pancreatic α-cells and regulates glucagon release, we tested whether common variants of the SGLT2 gene SLC5A2 associate with altered plasma glucagon concentrations in the fasting state and upon glucose challenge.

METHODS

A study population of 375 healthy subjects at increased risk for type-2 diabetes, phenotyped by a 5-point oral glucose tolerance test (OGTT) and genotyped for recently described SLC5A2 tagging single nucleotide polymorphisms (SNPs), was selected for plasma glucagon measurements.

RESULTS

After adjustment for gender, age, body mass index, and insulin sensitivity, the four tagging SNPs (rs9924771, rs3116150, rs3813008, rs9934336), tested separately or as genetic score, were neither significantly nor nominally associated with plasma glucagon concentrations at any time during the OGTT, with the inverse AUC of glucagon or the glucagon fold-change during the OGTT (p ≥ 0.2, all). Testing for genotype-related differences in the time course of the glucagon response using MANOVA did also not reveal any significant or nominal associations (p ≥ 0.5, all).

CONCLUSION

We could not obtain statistically significant evidence for a role of common SLC5A2 variants in the regulation of glucagon release in the fasting state or upon glucose challenge. Moreover, the reported nominal effects of individual SLC5A2 variants on fasting and post-challenge glucose levels may probably not be mediated by altered glucagon release.

Saving the sweetness: renal glucose handling in health and disease

Glucose homeostasis is highly controlled, and the function of the kidney plays an integral role in this process. The exquisite control of blood glucose relies, in part, on renal glucose filtration, renal glucose reabsorption, and renal gluconeogenesis. Particularly critical to maintaining glucose homeostasis is the renal reabsorption of glucose; with ~162 g of glucose filtered by the kidney per day, it is imperative that the kidney have the ability to efficiently reabsorb nearly 100% of this glucose back in the bloodstream. In this review, we focus on this central process, highlighting the renal transporters and regulators involved in both the physiology and pathophysiology of glucose reabsorption.

Effect of Dapagliflozin Treatment on Fluid and Electrolyte Balance in Diabetic Rats

AIM

This study evaluates the effect of dapagliflozin, a SGLT2 inhibitor, on fluid or electrolyte balance and its effect on urea transporter-A1 (UT-A1), aquaporin-2 (AQP2) and Na-K-2Cl cotransporter (NKCC2) protein abundance in diabetic rats.

METHODS

Diabetes mellitus (DM) was induced by injection of streptozotocin into the tail vein. Serum Na+, K+, Cl- concentration, urine Na+, K+, Cl- excretion, blood glucose, urine glucose excretion, urine volume, urine osmolality and urine urea excretion were analyzed after the administration of dapagliflozin. UT-A1, AQP2 and NKCC2 proteins were detected by western blot.

RESULTS

Dapagliflozin treatment decreased blood glucose concentration by 38% at day 7 and by 47% at day 14 and increased the urinary glucose excretion rate compared with the untreated diabetic animals. Increased 24-hour urine volume, decreased urine osmolality and hyponatremia, hypokalemia and hypochloremia observed in diabetic rats were attenuated by dapagliflozin treatment. Western blot analysis showed that UT-A1, AQP2 and NKCC2 proteins are upregulated in DM rats over control rats; dapagliflozin treatment results in a further increase in inner medulla tip UT-A1 protein abundance by 42% at day 7 and by 46% at day 14, but it did not affect the DM-induced upregulation of AQP2 and NKCC2 proteins.

CONCLUSION

Dapagliflozin treatment augmented the compensatory changes in medullary transport proteins in DM. These changes would tend to conserve solute and water even with persistent glycosuria. Therefore, diabetic rats treated with dapagliflozin have a mild osmotic diuresis compared to nondiabetic animals, but this does not result in an electrolyte disorder or significant volume depletion.

A Renal Olfactory Receptor Aids in Kidney Glucose Handling

Olfactory receptors (ORs) are G protein-coupled receptors which serve important sensory functions beyond their role as odorant detectors in the olfactory epithelium. Here we describe a novel role for one of these ORs, Olfr1393, as a regulator of renal glucose handling. Olfr1393 is specifically expressed in the kidney proximal tubule, which is the site of renal glucose reabsorption. Olfr1393 knockout mice exhibit urinary glucose wasting and improved glucose tolerance, despite euglycemia and normal insulin levels. Consistent with this phenotype, Olfr1393 knockout mice have a significant decrease in luminal expression of Sglt1, a key renal glucose transporter, uncovering a novel regulatory pathway involving Olfr1393 and Sglt1. In addition, by utilizing a large scale screen of over 1400 chemicals we reveal the ligand profile of Olfr1393 for the first time, offering new insight into potential pathways of physiological regulation for this novel signaling pathway.

Sodium glucose cotransporter 2 inhibition in the diabetic kidney: an update

PURPOSE OF REVIEW

The sodium glucose cotransporter 2 (SGLT2) reabsorbs most of the glucose filtered by the kidneys. SGLT2 inhibitors reduce glucose reabsorption, thereby lowering blood glucose levels, and have been approved as new antihyperglycemic drugs. Although the therapeutic strategy is very promising, many questions remain.

RECENT FINDINGS

Using validated antibodies, SGLT2 expression was localized to the brush border of the early proximal tubule in the human kidney and was found upregulated in genetic murine models of type 1 and 2 diabetes. SGLT2 may functionally interact with the Na/H exchanger NHE3 in the proximal tubule. SGLT1-mediated reabsorption explains the fractional renal glucose reabsorption of 40-50% during SGLT2 inhibition. SGLT2 is expressed on pancreatic alfa cells where its inhibition induces glucagon secretion. SGLT2 inhibition lowers glomerular filtration rate in hyperfiltering diabetic patients consistent with the tubular hypothesis of diabetic hyperfiltration. New data indicate a potential of SGLT2 inhibition for renal medullary hypoxia and ketoacidosis, but also for blood glucose effect-dependent and independent nephroprotective actions, renal gluconeogenesis inhibition, reduction in cardiovascular mortality, and cancer therapy.

SUMMARY

The findings expand and refine our understanding of SGLT2 and its inhibition, have relevance for clinical practice, and will help interpret ongoing clinical trials on the long-term safety and cardiovascular effects of SGLT2 inhibitors.

An evaluation of US patent 2015065565 (A1) for a new class of SGLT2 inhibitors for treatment 1 of type II diabetes mellitus

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is a growing and serious global health problem. Pharmacological inhibition of the sodium-glucose cotransporter-2 (SGLT2; SLC5A2) increases urinary glucose excretion, decreasing plasma glucose levels in an insulin-independent manner. Agents that inhibit SGLT2 have recently become available for clinical therapy of T2DM.

AREAS COVERED

The patent claims a new class of SGLT2 inhibitors: derivatives of dioxa-bicyclo[3.2.1]octane-2,3,4-triol (including ertugliflozin; PF-04971729). The invention describes the design, synthesis and pharmacological tests related to ertugliflozin, which could ultimately lead to efficacious therapy for T2DM alone or in combination with other anti-diabetic agents.

EXPERT OPINION

Ertugliflozin is likely to be of great clinical significance in the near future. Continued analysis of ertugliflozin derivatives to now validate safe and efficacious treatment of T2DM in a larger number of clinical subjects over an extended period is needed to further support clinical utility. Identification, and discussion, of likely contra-indications is also needed.