Phloridzin reduces synovial hyperplasia and inflammation in rheumatoid arthritis rat by modulating mTOR pathway

Rheumatoid arthritis (RA) is an inflammatory autoimmune disease and management of it still a challenge. Given report evaluates protective effect of phlorizin on RA and also postulates the molecular mechanism of its action. Bovine type II collagen (CIA) and Freund's incomplete adjuvant (1:1 and 1 mg/ml) was administered on 1st and 8th day of protocol to induce RA in rats and treatment with phlorizin 60 and 120 mg/kg was started after 4th week of protocol. Level of inflammatory cytokines and expression of proteins were estimated in phlorizin treated RA rats. Moreover in-vitro study was performed on Fibroblast-like synoviocytes (FLSs) and effect of phlorizin was estimated on proliferation, apoptosis and expression of mTOR pathway protein after stimulating these cell lines with Tumour Necrosis Factor alpha (TNF-α). Data of study suggest that phlorizin reduces inflammation and improves weight in CIA induced RA rats. Level of inflammatory cytokines in the serum and expression of Akt/PI3K/mTOR proteins in the join tissue was reduced in phlorizin treated RA rats. Phlorizin also reported to reverse the histopathological changes in the joint tissue of RA rats. In-vitro study supports that phlorizin reduces proliferation and no apoptotic effect on TNF-α stimulated FLSs. Expression of Akt/PI3K/mTOR proteins also downregulated in phlorizin treated TNF-α stimulated FLSs. In conclusion, phlorizin protects inflammation and reduces injury to the synovial tissues in RA, as it reduces autophagy by regulating Akt/PI3K/mTOR pathway.

[From the discovery of phlorizin (a Belgian story) to SGLT2 inhibitors]

Most physicians do not know, or do not remember, the name of phlorizin. Hence this molecule has a major historical importance because it was the precursor of gliflozins, a new class of oral antidiabetic drugs with recent therapeutic perspectives beyond diabetes. This article recalls the history of phlorizin: its discovery in the 19th century by De Koninck and Stas, the demonstration of its ability to induce glucosuria and reduce hyperglycaemia by von Mering, its use to demonstrate the concept of glucose toxicity by the team of DeFronzo and finally the development of selective (phlorizin being not selective) sodium-glucose cotransporter type 2 inhibitors (gliflozins) which block glucose reabsorption in renal tubules. Gliflozins have increasing therapeutic indications, not only in type 2 diabetes, but also in cardiology and nephrology among non-diabetic people with heart failure or renal insufficiency.

SGLT2 Inhibitors as a Therapeutic Option for Diabetic Nephropathy

Diabetic nephropathy (DN) is a major cause of end-stage renal disease (ESRD) worldwide. Glycemic and blood pressure (BP) control are important but not sufficient to attenuate the incidence and progression of DN. Sodium–glucose cotransporter (SGLT) 2 inhibitors are a new class of glucose-lowering agent suggested to exert renoprotective effects in glucose lowering-dependent and independent fashions. Experimental studies have shown that SGLT2 inhibitors attenuate DN in animal models of both type 1 diabetes (T1D) and type 2 diabetes (T2D), indicating a potential renoprotective effect beyond glucose reduction. Renoprotection by SGLT2 inhibitors has been demonstrated in T2D patients with a high cardiovascular risk in randomized controlled trials (RCTs). These favorable effects of SGLT2 inhibitors are explained by several potential mechanisms, including the attenuation of glomerular hyperfiltration, inflammation and oxidative stress. In this review article, we discuss the renoprotective effects of SGLT2 inhibitors by integrating experimental findings with the available clinical data.

Targeting renal glucose reabsorption to treat hyperglycaemia: the pleiotropic effects of SGLT2 inhibition

Healthy kidneys filter ∼160 g/day of glucose (∼30% of daily energy intake) under euglycaemic conditions. To prevent valuable energy from being lost in the urine, the proximal tubule avidly reabsorbs filtered glucose up to a limit of ∼450 g/day. When blood glucose levels increase to the point that the filtered load exceeds this limit, the surplus is excreted in the urine. Thus, the kidney provides a safety valve that can prevent extreme hyperglycaemia as long as glomerular filtration is maintained. Most of the capacity for renal glucose reabsorption is provided by sodium glucose cotransporter (SGLT) 2 in the early proximal tubule. In the absence or with inhibition of SGLT2, the renal reabsorptive capacity for glucose declines to ∼80 g/day (the residual capacity of SGLT1), i.e. the safety valve opens at a lower threshold, which makes it relevant to glucose homeostasis from day-to-day. Several SGLT2 inhibitors are now approved glucose lowering agents for individuals with type 2 diabetes and preserved kidney function. By inducing glucosuria, these drugs improve glycaemic control in all stages of type 2 diabetes, while their risk of causing hypoglycaemia is low because they naturally stop working when the filtered glucose load falls below ∼80 g/day and they do not otherwise interfere with metabolic counterregulation. Through glucosuria, SGLT2 inhibitors reduce body weight and body fat, and shift substrate utilisation from carbohydrates to lipids and, possibly, ketone bodies. Because SGLT2 reabsorbs sodium along with glucose, SGLT2 blockers are natriuretic and antihypertensive. Also, because they work in the proximal tubule, SGLT2 inhibitors increase delivery of fluid and electrolytes to the macula densa, thereby activating tubuloglomerular feedback and increasing tubular back pressure. This mitigates glomerular hyperfiltration, reduces the kidney's demand for oxygen and lessens albuminuria. For reasons that are less well understood, SGLT2 inhibitors are also uricosuric. These pleiotropic effects of SGLT2 inhibitors are likely to have contributed to the results of the EMPA-REG OUTCOME trial in which the SGLT2 inhibitor, empagliflozin, slowed the progression of chronic kidney disease and reduced major adverse cardiovascular events in high-risk individuals with type 2 diabetes. This review discusses the role of SGLT2 in the physiology and pathophysiology of renal glucose reabsorption and outlines the unexpected logic of inhibiting SGLT2 in the diabetic kidney.

Phloretin-induced cytoprotective effects on mammalian cells: A mechanistic view and future directions

Phloretin (C15 H14 O5 ), a dihydrochalcone flavonoid, is mainly found in fruit, leaves, and roots of apple tree. Phloretin exerts antioxidant, anti-inflammatory, and anti-tumor activities in mammalian cells through mechanisms that have been partially elucidated throughout the years. Phloretin bioavailability is well known in humans, but still remains to be better studied in experimental animals, such as mouse and rat. The focus of the present review is to gather information regarding the mechanisms involved in the phloretin-elicited effects in different in vitro and in vivo experimental models. Several manuscripts were analyzed and data raised by authors were described and discussed here in a mechanistic manner. Comparisons between the effects elicited by phloretin and phloridzin were made whenever possible, as well as with other polyphenols, clarifying questions about the use of phloretin as a potential therapeutic agent. Toxicological aspects associated to phloretin exposure were also discussed here. Furthermore, a special section containing future directions was created as a suggestive guide towards the elucidation of phloretin-related actions in mammalian cells and tissues.

Beneficial effects of phlorizin on diabetic nephropathy in diabetic db/db mice

AIMS

This study observes the effects of phlorizin on diabetic nephrology in db/db diabetic mice and explores possible underlying mechanisms.

METHODS

Sixteen diabetic db/db mice and eight age-matched db/m mice were divided into three groups: vehicle-treated diabetic group (DM group), diabetic group treated with phlorizin (DMT group) and normal control group (CC group). Phlorizin was given in normal saline solution by intragastric administration for 10 weeks. Differentially expressed proteins in three groups were identified using iTRAQ quantitative proteomics and the data were further analyzed with ingenuity pathway analysis.

RESULTS

The body weight and serum concentrations of fasting blood glucose (FBG), advanced glycation end products (AGEs), total cholesterol, triglycerides, blood urea nitrogen, creatinine and 24-h urine albumin were increased in the DM group compared to those of the CC group (P<0.05), and they were decreased by treatment with phlorizin (P<0.05). Morphologic observations showed phlorizin markedly attenuated renal injury. Phlorizin prevented diabetic nephropathy by regulating the expression of a series of proteins involved in renal and urological disease, molecular transport, free radical scavenging, and lipid metabolism.

CONCLUSIONS

Phlorizin protects mice from diabetic nephrology and thus may be a novel therapeutic approach for the treatment of diabetic nephrology.

SGLT2-inhibitors: a novel class for the treatment of type 2 diabetes introduction of SGLT2-inhibitors in clinical practice

Treatment of type 2 diabetes (T2DM) continues to present challenges, with significant proportion of patients failing to achieve and maintain glycemic targets. Despite the availability of many oral antidiabetic agents, therapeutic efficacy is offset by side effects such as weight gain and hypoglycemia. Therefore, the search for novel therapeutic agents with an improved benefit-risk profile continues. Recent research has focused on the kidney as a potential therapeutic target, especially because maximal renal glucose reabsorption is increased in T2DM. Under normal physiological conditions, nearly all filtered glucose is reabsorbed in the proximal tubule of the nephron, principally via the sodium-glucose cotransporter 2 (SGLT2). SGLT2-inhibitors are a new class of oral antidiabetics, which reduce hyperglycemia by increasing urinary glucose excretion independently of insulin secretion or action. Clinical results are promising with significant lowering of HbA1c without increased risk of hypoglycemia, reduction of body weight and reduction of systolic blood pressure. Dapagliflozin is the first highly selective SGLT2-inhibitor approved by the European Medecine Agency. Canagliflozin and empagliflozin are undergoing phase III trials. Actual safety issues are an increased risk for genital- and urinary tract infections and a possible increased risk for bladder and breast cancer. This led to refusal of dapagliflozin by the Food and Drug Administration (FDA). A large randomized control trial is therefore warranted by the FDA. This review provides an overview of the current evidence available so far on the therapeutic potential of the SGLT2-inhibitors for the treatment of T2DM.

Selective SGLT2 inhibition by tofogliflozin reduces renal glucose reabsorption under hyperglycemic but not under hypo- or euglycemic conditions in rats

To understand the risk of hypoglycemia associated with urinary glucose excretion (UGE) induced by sodium-glucose cotransporter (SGLT) inhibitors, it is necessary to know the relationship between the ratio of contribution of SGLT2 vs. SGLT1 to renal glucose reabsorption (RGR) and the glycemic levels in vivo. To examine the contributions of SGLT2 and SGLT1 in normal rats, we compared the RGR inhibition by tofogliflozin, a highly specific SGLT2 inhibitor, and phlorizin, an SGLT1 and SGLT2 (SGLT1/2) inhibitor, at plasma concentrations sufficient to completely inhibit rat SGLT2 (rSGLT2) while inhibiting rSGLT1 to different degrees. Under hyperglycemic conditions by glucose titration, tofogliflozin and phlorizin achieved ≥50% inhibition of RGR. Under hypoglycemic conditions by hyperinsulinemic clamp, RGR was reduced by 20-50% with phlorizin and by 1-5% with tofogliflozin, suggesting the smaller contribution of rSGLT2 to RGR under hypoglycemic conditions than under hyperglycemic conditions. Next, to evaluate the hypoglycemic potentials of SGLT1/2 inhibition, we measured the plasma glucose (PG) and endogenous glucose production (EGP) simultaneously after UGE induction by SGLT inhibitors. Tofogliflozin (400 ng/ml) induced UGE of about 2 mg·kg⁻¹·min⁻¹ and increased EGP by 1-2 mg·kg⁻¹·min⁻¹, resulting in PG in the normal range. Phlorizin (1,333 ng/ml) induced UGE of about 6 mg·kg⁻¹·min⁻¹ and increased EGP by about 4 mg·kg⁻¹·min⁻¹; this was more than with tofogliflozin, but the minimum PG was lower. These results suggest that the contribution of SGLT1 to RGR is greater under lower glycemic conditions than under hyperglycemic conditions and that SGLT2-selective inhibitors pose a lower risk of hypoglycemia than SGLT1/2 inhibitors.

Effects of phlorizin on vascular complications in diabetes db/db mice

BACKGROUND

Diabetic macrovascular complications are important causes of cardiovascular and cerebrovascular diseases and also one of the major causes of morbidity and mortality in patients with type 2 diabetes mellitus (T2DM). Phlorizin has been reported to be effective in reducing the blood glucose level in diabetic mellitus, while little is known about its effects on vascular complications. This study aimed to observe the effects of phlorizin on the aorta of diabetes db/db mice and explore its mechanism.

METHODS

Diabetic db/db mice (n = 16) and age-matched db/m mice (n = 8) were divided into three groups: normal control group (CC group, db/m mice, n = 8), untreated diabetic group (DM group, db/db mice, n = 8) and diabetic group treated by phlorizin (DMT group, db/db mice, n = 8). Phlorizin (20 mg/kg body weight) was given in normal saline solution intragastrically for 10 weeks. Animals were weighed weekly. At the 10th weekend, all mice were fasted overnight and then sacrificed. Fasting blood was collected, and the aortas were dissected. The blood samples were analyzed for fasting blood glucose (FBG), serum advanced glycation end products (AGEs), malondialdehyde (MDA) and superoxide dismutase (SOD) activity, the aortic ultrastructure was studied.

RESULTS

The weight and serum concentration of FBG, AGEs, and MDA in the DM group were higher than that in the CC group (P < 0.01), and they were significantly lower in the DMT group (P < 0.05). Serum SOD activity was lower than that in the CC group (P < 0.01), and it is significantly higher in the DMT group (P < 0.05). The severity of aorta damage in the DMT group was less than that in the DM group.

CONCLUSIONS

Phlorizin protected the db/db mice from diabetic macrovascular complications, attributed to the decreasing of blood glucose and AGEs level, and its antioxidant potential. This study may provide a new natural medicine for treating diabetic macrovascular complications.

A new class of drug for the management of type 2 diabetes: sodium glucose co-transporter inhibitors: 'glucuretics'

BACKGROUND

Type 2 diabetes is a common, chronic disease with a prevalence that is increasing at epidemic proportions. Management involves advice on lifestyle changes, oral anti-hyperglycaemic agents and/or insulin. The kidneys play a major role in the regulation of glucose, re-absorbing 99% of the plasma glucose filtered through the renal glomeruli tubules. The glucose transporter, SGLT2, which is found primarily in the S1 segment of the proximal renal tubule accounts for 90% of glucose re-absorption. Competitive inhibition of SGLT2 induces glucosuria in a dose dependent manner and appears to have beneficial effects on glucose regulation in individuals with type 2 diabetes. O-glucoside phlorozin is the model substance for SGLT2 inhibitors: various O-, C-, N- and S-glucosides with varying affinity and specificity have been synthesised.

AIMS

The aim of this review is to describe the background, the mechanism of action and the possible role for sodium glucose co-transporter inhibitors in the treatment of diabetes.

MATERIALS AND METHODS

Databases, including MEDLINE, COCHRANE, EMBASE and EBM reviews were searched for literature relating to sodium glucose transport inhibitors and improvements in glycaemic control in patients with diabetes.

RESULTS

The data suggest that sodium glucose transport inhibitors significantly improve glycaemic control by increasing glucosuria. Some studies described significant reductions in weight and improvement in blood pressure. The most common side effect was infection involving the urinary and genital tracts.

CONCLUSIONS

Sodium glucose co-transport inhibitors appear to be an effective line of treatment, well tolerated and could be a further drug class in the armamentarium available for the management of type 2 diabetes.

Phloridzin reduces blood glucose levels and improves lipids metabolism in streptozotocin-induced diabetic rats

Phloridzin is the specific and competitive inhibition of sodium/glucose cotransporters in the intestine (SGLT1) and kidney (SGLT2). This property which could be useful in the management of postprandial hyperglycemia in diabetes and related disorders. Phloridzin is one of the dihydrochalcones typically contained in apples and in apple-derived products. The effect of phloridzin orally doses 5, 10, 20 and 40 mg/kg body weight on diabetes was tested in a streptozotocin-induced rat model of diabetes type 1. From beneficial effect of this compound is significant reduction of blood glucose levels and improve dyslipidemia in diabetic rats. As a well-known consequence of becoming diabetic, urine volume and water intake were significantly increased. Administration of phloridzin reduced urine volume and water intake in a dose-dependent manner. Phloretin decreases of food consumption, as well as a marked lowering in the weight. In conclusion, this compound could be proposed as an antihyperglycemic and antihyperlipidemic agent in diabetes and potential therapeutic in obesity.

Changes of CCAAT enhancer-binding proteins (CEBPs) in the lung of streptozotocin-induced diabetic rats

CCAAT enhancer-binding proteins (CEBPs) play key roles in the metabolic regulation, cell transformation, and inflammation. However, the expression and/or functions of CEBPs in rats with hyperglycemia are still unclear. In the present study, we investigated the changes of CEBPs protein in lung of the diabetic rats. The levels of C/EBPβ and C/EBPδ protein were decreased in the lung isolated from streptozotocin-induced diabetic rats (STZ-diabetic rats) as compared with that of normal rats. Exogenous insulin at the dose sufficient to normalize the plasma glucose of STZ-diabetic rats reversed the protein levels of C/EBPβ and C/EBPδ in lung after a 4-day treatment. Similar results were also observed in STZ-diabetic rats that received the treatment of phlorizin to reverse the plasma glucose level for 4 days. Otherwise, the protein level of C/EBPα in lung of the STZ-diabetic rats was similar as the normal rats. Also, the level of C/EBPα protein in lung of the STZ-diabetic rat was not significantly changed by correction of plasma glucose by exogenous insulin or phlorizin. In addition, we also cultured human lung cells (A-549) and rat lung cells (L2) in varies concentration of D-glucose and L-glucose to identify the effect of glucose in expression of C/EBPs. The obtained results suggest that increase of plasma glucose is related to the lower expression of C/EBPβ and C/EBPδ proteins in the lung of STZ-diabetic rats. The changes of expression of C/EBPβ and C/EBPδ are not caused by changes of osmolarity but by D-glucose itself.

Role of glucotoxicity and lipotoxicity in the pathophysiology of Type 2 diabetes mellitus and emerging treatment strategies

Type 2 diabetes mellitus is a disease characterized by persistent and progressive deterioration of glucose tolerance. Both insulin resistance and impaired insulin secretion contribute to development of Type 2 diabetes. However, whilst insulin resistance is fully apparent in the pre-diabetic condition, impairment of insulin secretion worsens over the time, being paralleled by a progressive decline in both pancreatic B-cell function and B-cell mass. Intense research has identified a number of genetic variants that may predispose to impaired B-cell function, but such predisposition can be precipitated and worsened by toxic effects of hyperglycaemia (glucotoxicity) and elevated levels of free fatty acids (lipotoxicity). All these aspects of the pathogenesis of Type 2 diabetes are discussed in this review. Moreover, treatments that target reduction in glucotoxicity or lipotoxicity are outlined, including emerging strategies that target the role of glucagon-like peptide 1 and sodium glucose co-transporter 2.

Nonobese, insulin-deficient Ins2Akita mice develop type 2 diabetes phenotypes including insulin resistance and cardiac remodeling

Although insulin resistance has been traditionally associated with type 2 diabetes, recent evidence in humans and animal models indicates that insulin resistance may also develop in type 1 diabetes. A point mutation of insulin 2 gene in Ins2(Akita) mice leads to pancreatic beta-cell apoptosis and hyperglycemia, and these mice are commonly used to investigate type 1 diabetes and complications. Since insulin resistance plays an important role in diabetic complications, we performed hyperinsulinemic-euglycemic clamps in awake Ins2(Akita) and wild-type mice to measure insulin action and glucose metabolism in vivo. Nonobese Ins2(Akita) mice developed insulin resistance, as indicated by an approximately 80% reduction in glucose infusion rate during clamps. Insulin resistance was due to approximately 50% decreases in glucose uptake in skeletal muscle and brown adipose tissue as well as hepatic insulin action. Skeletal muscle insulin resistance was associated with a 40% reduction in total GLUT4 and a threefold increase in PKCepsilon levels in Ins2(Akita) mice. Chronic phloridzin treatment lowered systemic glucose levels and normalized muscle insulin action, GLUT4 and PKCepsilon levels in Ins2(Akita) mice, indicating that hyperglycemia plays a role in insulin resistance. Echocardiography showed significant cardiac remodeling with ventricular hypertrophy that was ameliorated following chronic phloridzin treatment in Ins2(Akita) mice. Overall, we report for the first time that nonobese, insulin-deficient Ins2(Akita) mice develop type 2 diabetes phenotypes including peripheral and hepatic insulin resistance and cardiac remodeling. Our findings provide important insights into the pathogenesis of metabolic abnormalities and complications affecting type 1 diabetes and lean type 2 diabetes subjects.

Increase of peroxisome proliferator-activated receptor delta gene expression in the lungs of streptozotocin-induced diabetic rats

Peroxisome proliferator-activated receptors (PPARs) play key roles in the regulation of energy homeostasis and inflammation while the agonists of PPARalpha and PPARgamma are recently used for therapy in clinic. However, functions of PPARdelta are still unclear. In the present study, we investigated the changes of PPARdelta gene expression in the lung of diabetic rats. The mean level of mRNA transcripts encoding PPARdelta was increased in the lung isolated from streptozotocin-induced diabetic rats (STZ-diabetic rats) to about 1.6-fold of that in normal rats. Exogenous insulin at the dose sufficient to normalize the plasma glucose of STZ-diabetic rats reversed the mRNA level of PPARdelta in lungs after a 4-day treatment. Similar results were also observed in STZ-diabetic rats that received the treatment of phlorizin to reverse the plasma glucose level for 4 days. Otherwise, the protein level of PPARdelta was higher in the lung of STZ-diabetic rats than that in normal rats. Treatment with exogenous insulin or phlorizin reversed this elevated protein level of PPARdelta in the lung of STZ-diabetic rats to near the normal level. The obtained results suggest that increase of plasma glucose is responsible for the higher gene expression of PPARdelta in the lung of STZ-diabetic rats.

Prevention of bone loss by phloridzin, an apple polyphenol, in ovariectomized rats under inflammation conditions

Aging and sex hormones related changes lead to inflammatory and oxidant conditions, which are involved in the pathogenesis of osteoporosis. Recent studies have suggested that polyphenols may exert a protective effect in such conditions. We assessed the effect of phloridzin (Phlo), a flavonoid exclusively found in apple, on bone metabolism in ovariectomized (OVX) or sham-operated (SH) rats with and without inflammation. Six-month-old Wistar rats were allocated to two equal groups that received either a control diet or a diet supplemented with 0.25% Phlo for 80 days. Three weeks before necropsy, inflammation was induced by subcutaneous injection of talc in 10 animals of each group. At necropsy, ovariectomy decreased both total (T-BMD) and metaphyseal (M-BMD) femoral bone mineral density (P < 0.01). Inflammation conditions, checked by an increase in the spleen weight and alpha1-acid glycoprotein concentration in OVX rats, exacerbated the decrease in T-BMD (g/cm2) (as well as M-BMD) observed in castrated animals (P < 0.05). Daily Phlo intake prevented ovariectomy-induced bone loss in conditions of inflammation as shown by T-BMD and M-BMD (P < 0.05). At the diaphyseal site, BMD was improved by Phlo in OVX rats with or without inflammation (P < 0.05). These results could be explained by changes in bone remodeling as the increased urinary deoxypyridinoline excretion in OVX and OVXinf animals was prevented by the polyphenol-rich diet (P < 0.001), while plasma osteocalcin concentration was similar in all experimental groups. In conclusion, Phlo consumption may provide protection against ovariectomy-induced osteopenia under inflammation conditions by improving inflammation markers and bone resorption.

Dynamic changes in {beta}-cell mass and pancreatic insulin during the evolution of nutrition-dependent diabetes in psammomys obesus: impact of glycemic control

Recent studies ascribe a major role to pancreatic beta-cell loss in type 2 diabetes. We investigated the dynamics of beta-cell mass during diabetes evolution in Psammomys obesus, a model for nutrition-dependent type 2 diabetes, focusing on the very early and the advanced stages of the disease. P. obesus fed a high-calorie diet for 26 days developed severe hyperglycemia, beta-cell degranulation, and markedly reduced pancreatic insulin content. Reducing calories for 7 days induced normoglycemia in 90% of the animals, restoring beta-cell granulation and insulin content. To dissociate effects of diet from blood glucose reduction, diabetic animals received phlorizin for 2 days, which normalized glycemia and increased the pancreatic insulin reserve to 50% of control, despite a calorie-rich diet. During diabetes progression, beta-cell mass decreased initially but recovered spontaneously to control levels, despite persistent hyperglycemia. Strikingly, however, beta-cell mass did not correlate with degree of hyperglycemia or pancreatic insulin content. We conclude that reduced insulin reserve is the main cause of diabetes progression, whereas irreversible beta-cell mass reduction is a late event in P. obesus. The rapid recovery of the pancreas by phlorizin-induced normoglycemia implies a causal relationship between hyperglycemia and islet dysfunction. Similar mechanisms could be operative during the evolution of type 2 diabetes in humans.

Phloridzin improves hyperglycemia but not hepatic insulin resistance in a transgenic mouse model of type 2 diabetes

The chronic hyperglycemia that occurs in type 2 diabetes may cause deterioration of beta-cell function and insulin resistance in peripheral tissues. Mice that express a dominant-negative IGF-1 receptor, specifically in skeletal muscle (MKR mice), exhibit severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyper-glycemia. To determine the role of hyperglycemia in the worsening of the diabetes state in these animals, MKR mice were treated with phloridzin (PHZ), which inhibits intestinal glucose uptake and renal glucose reabsorption. Blood glucose levels were decreased and urine glucose levels were increased in response to PHZ treatment in MKR mice. PHZ treatment also increased food intake in MKR mice; however, the fat mass was decreased and lean body mass did not change. Serum insulin, fatty acid, and triglyceride levels were not affected by PHZ treatment in MKR mice. Hyperinsulinemic-euglycemic clamp analysis demonstrated that glucose uptake in white adipose tissue was significantly increased in response to PHZ treatment. Despite the reduction in blood glucose following PHZ treatment, there was no improvement in insulin-stimulated whole-body glucose uptake in MKR mice and neither was there suppression of endogenous glucose production by insulin. These results suggest that glucotoxicity plays little or no role in the worsening of insulin resistance that occurs in the MKR mouse model of type 2 diabetes.

Glucose toxicity and the development of diabetes in mice with muscle-specific inactivation of GLUT4

Using cre/loxP gene targeting, transgenic mice with muscle-specific inactivation of the GLUT4 gene (muscle GLUT4 KO) were generated and shown to develop a diabetes phenotype. To determine the mechanism, we examined insulin-stimulated glucose uptake and metabolism during hyperinsulinemic-euglycemic clamp in control and muscle GLUT4 KO mice before and after development of diabetes. Insulin-stimulated whole body glucose uptake was decreased by 55% in muscle GLUT4 KO mice, an effect that could be attributed to a 92% decrease in insulin-stimulated muscle glucose uptake. Surprisingly, insulin's ability to stimulate adipose tissue glucose uptake and suppress hepatic glucose production was significantly impaired in muscle GLUT4 KO mice. To address whether these latter changes were caused by glucose toxicity, we treated muscle GLUT4 KO mice with phloridzin to prevent hyperglycemia and found that insulin-stimulated whole body and skeletal muscle glucose uptake were decreased substantially, whereas insulin-stimulated glucose uptake in adipose tissue and suppression of hepatic glucose production were normal after phloridzin treatment. In conclusion, these findings demonstrate that a primary defect in muscle glucose transport can lead to secondary defects in insulin action in adipose tissue and liver due to glucose toxicity. These secondary defects contribute to insulin resistance and to the development of diabetes.

Vanadate restores glucose 6-phosphate in diabetic rats: a mechanism to enhance glucose metabolism

Vanadate mimics the metabolic actions of insulin. In diabetic rodents, vanadate also sensitizes peripheral tissues to insulin. We have analyzed whether this latter effect is brought about by a mechanism other than the known insulinomimetic actions of vanadium in vitro. We report that the levels of glucose 6-phosphate (G-6-P) in adipose, liver, and muscle of streptozotocin-treated (STZ)-hyperglycemic rats are 77, 50, and 58% of those in healthy control rats, respectively. Normoglycemia was induced by vanadium or insulin therapy or by phlorizin. Vanadate fully restored G-6-P in all three insulin-responsive peripheral tissues. Insulin did not restore G-6-P in muscle, and phlorizin was ineffective in adipose and muscle. Incubation of diabetic adipose explants with glucose and vanadate in vitro increased lipogenic capacity three- to fourfold (half-maximally effective dose = 11 +/- 1 microM vanadate). Lipogenic capacity was elevated when a threshold level of approximately 7.5 +/- 0.3 nmol G-6-P/g tissue was reached. In summary, 1) chronic hyperglycemia largely reduces intracellular G-6-P in all three insulin-responsive tissues; 2) vanadate therapy restores this deficiency, but insulin therapy does not restore G-6-P in muscle tissue; 3) induction of normoglycemia per se (i.e., by phlorizin) restores G-6-P in liver only; and 4) glucose and vanadate together elevate G-6-P in adipose explants in vitro and significantly restore lipogenic capacity above the threshold of G-6-P level. We propose that hyperglycemia-associated decrease in peripheral G-6-P is a major factor responsible for peripheral resistance to insulin. The mechanism by which vanadate increases peripheral tissue capacity to metabolize glucose and to respond to the hormone involves elevation of this hexose phosphate metabolite and the cellular consequences of this elevated level of G-6-P.

Rat tissue collagen modified by advanced glycation: correlation with duration of diabetes and glycemic control

Collagenous proteins are especially prone to nonenzymatic glycation, because they contain several dibasic amino acid residues with free amino groups, have a very slow turnover rate, and are exposed to ambient levels of glucose. The aim of this study was to determine the time-dependent course of advanced glycation process in diabetic rats in relation to glycemic control and duration of diabetes, compared to age-matched controls. Immunochemical assay with antibodies to advanced glycation end products (AGE) was first developed to qualitatively detect and quantify the AGE formed in rat tendon and aortic collagen. Individual collagen samples were extracted by extensive pepsin and collagenase digestion. The amount of AGE was measured by competitive ELISA and results were expressed as AGE U/mg collagen. Diabetic rats showed a significant increase in AGE content in aortic collagen at 20 weeks (n = 6, 206.6 +/- 16.7 U/mg collagen) compared with that measured at 4 and 12 weeks (n = 6, 110 +/- 12.8 U/mg collagen, and n = 13, 184.9 +/- 12.3 U/mg collagen at 4 and 12 weeks, respectively; p < 0.001 between 20 weeks and 4 weeks; p < 0.01 between 20 weeks and 12 weeks). The amount of AGE in tendon collagen of diabetic rats increased from 1.9 +/- 0.38 U/mg at 4 weeks to 11.2 +/- 6.1 U/mg collagen at 20 weeks, p < 0.001. Considerable disparity was observed in the intensity of glycation between aortic and tendon collagen. AGE-content per mg of aortic collagen was several-fold to that found in tendon collagen (p < 0.001). To investigate the effect of glycemic control on the advanced glycation process, total aortic AGE-collagen content was compared between untreated diabetic rats (D; n = 13, 184.9 +/- 12.3 U/mg) and diabetic rats treated for 12 weeks with insulin (DI; n = 6, 133.9 +/- 10.7 U/mg), or phlorizin (DP; n = 6, 132.4 +/- 8.9 U/mg), or by a combination of insulin/phlorizin (DIP; n = 6, 124.3 +/- 6.5 U/mg). In spite of therapy used, all groups of diabetic animals had a significantly higher aortic AGE-collagen content than those in the nondiabetic control group (C: n = 8, 104.6 +/- 14.9 U/mg) of the same age (D, DI, DP, DIP vs. C, p < 0.001). Comparison between the mean levels of glycated hemoglobin (D: 5.62 +/- 0.38 % vs. C: 1.7 +/- 0.05%) and mean AGE levels in the studied group of animals yielded a very good exponential correlation (r = 0.89, p < 0.001). Glycation-derived late-stage collagen modification was detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and by immunoblotting confirmed to contain (an) AGE-structure(s). Our study provides strong immunochemical evidence of AGE formation in vivo during hyperglycemia, and of their temporal association with structural alterations of extracellular matrix proteins. The advanced glycation process is retarded and reduced in intensity, but not completely abolished, by glycemia regulation with, or independently of, insulin.

Leptin restores euglycemia and normalizes glucose turnover in insulin-deficient diabetes in the rat

Leptin has been shown to improve insulin sensitivity and glucose metabolism in normoinsulinemic healthy or obese rodents. It has not been determined whether leptin may act independently of insulin in regulating energy metabolism in vivo. The present study was designed to examine the effects of leptin treatment alone on glucose metabolism in insulin-deficient streptozotocin (STZ)-induced diabetic rats. Four groups of STZ-induced diabetic rats were studied: 1) rats treated with recombinant methionine murine leptin subcutaneous infusion with osmotic pumps for 12-14 days (LEP; 4 mg x kg(-1) x day(-1), n = 10); 2) control rats infused with vehicle (phosphate-buffered saline) for 12-14 days (VEH; n = 10); 3) pair-fed control rats given a daily food ration matching that of LEP rats for 12-14 days (PF; n = 8); and 4) rats treated with subcutaneous phloridzin for 4 days (PLZ; 0.4 g/kg twice daily, n = 10). Phloridzin treatment normalizes blood glucose without insulin and was used as a control for the effect of leptin in correcting hyperglycemia. All animals were then studied with a hyperinsulinemic-euglycemic clamp (6 mU x kg(-1) x min(-1). Our study demonstrates that leptin treatment in the insulin-deficient diabetic rats restored euglycemia, minimized body weight loss due to food restriction, substantially improved glucose metabolic rates during the postabsorptive state, and restored insulin sensitivities at the levels of the liver and the peripheral tissues during the glucose clamp. The effects on glucose turnover are largely independent of food restriction and changes in blood glucose concentration, as evidenced by the minimal improvement of insulin action and glucose turnover parameters in the PF and PLZ groups. Our results suggest that the antidiabetic effects of leptin are achieved through both an insulin-independent and an insulin-sensitizing mechanism.

Glucose rapidly decreases plasma membrane GLUT4 content in rat skeletal muscle

We have previously demonstrated that chronic hyperglycemia per se decreases GLUT4 glucose transporter expression and plasma membrane content in mildly streptozotocin- (STZ) diabetic rats (Biochem. J. 284, 341-348, 1992). In the present study, we investigated the effect of an acute rise in glycemia on muscle GLUT4 and GLUT1 protein contents in the plasma membrane, in the absence of insulin elevation. Four experimental groups of rats were analyzed in the postabsorptive state: 1. Control rats. 2. Hyperglycemic STZ-diabetic rats with moderately reduced fasting insulin levels. 3. STZ-diabetic rats made normoglycemic with phlorizin treatment. 4. Phlorizin-treated (normoglycemic) STZ-diabetic rats infused with glucose for 40 min. The uniqueness of the latter model is that glycemia can be rapidly raised without any concomitant increase in plasma insulin levels. Plasma membranes were isolated from hindlimb muscle and GLUT1 and GLUT4 proteins amounts determined by Western blot analysis. As predicted, STZ-diabetes caused a significant decrease in the abundance of GLUT4 in the isolated plasma membranes. Normalization of glycemia for 3 d with phlorizin treatment restored plasma membrane GLUT4 content in muscle of STZ-diabetic rats. A sudden rise in glycemia over a period of 40 min caused the GLUT4 levels in the plasma membrane fraction to decrease to those of nontreated STZ-diabetic rats. In contrast to the GLUT4 transporter, plasma membrane GLUT1 abundance was not changed by the acute glucose challenge. It is concluded that glucose can have regulatory effect by acutely reducing plasma membrane GLUT4 protein contents in rat skeletal muscle. We hypothesize that this glucose-induced downregulation of plasma membrane GLUT4 could represent a protective mechanism against excessive glucose uptake under hyperglycemic conditions accompanied by insulin resistance.

Treatment of streptozotocin-induced diabetic rats with vanadate and phlorizin prevents the over-expression of the liver insulin receptor gene

Administration of vanadate, an insulinomimetic agent, has been shown to normalize the increased number of insulin receptors in the liver of streptozotocin-induced diabetic rats. In the present study, the effects of vanadate on various steps of expression of the liver insulin receptor gene in diabetic rats have been analyzed and compared with those of phlorizin, a glucopenic drug devoid of insulinomimetic properties. Livers of rats killed 23 days after streptozotocin injection showed a 30-40% increase in the number of cell surface and intracellular insulin receptors, a 50-90% increase in the levels of 9.5 and 7.5 kb insulin receptor mRNA species, and a 20% decrease in the relative abundance of the A (exon 11-) insulin receptor mRNA isotype. Daily administration of vanadate or phlorizin from day 5 to day 23 prevented the increase in insulin receptor number and mRNA level, and vanadate treatment also normalized receptor mRNA isotype expression. Unlike observations in vivo, vanadate and phlorizin differentially affected the expression of the insulin receptor gene in Fao hepatoma cells. Vanadate treatment (0.5 mmol/l for 4 h) decreased the levels of the 9.5 and 7.5 kb insulin receptor transcripts by at least twofold, without affecting the relative abundance of the A insulin receptor mRNA isotype. In contrast, phlorizin treatment (5 mmol/l for 4 h) slightly increased or did not affect the levels of the 9.5 and 7.5 kb insulin receptor transcripts respectively, and increased by twofold the relative expression of the A insulin receptor mRNA isotype. It is suggested that, although mediated in part by a reversal of hyperglycemia, normalization of liver insulin receptor gene expression by vanadate treatment in diabetic rats may also involve a direct inhibitory effect of this drug on gene expression.

Effect of partial pancreatectomy on beta-cell mass in the remnant pancreas of Wistar fatty rats

Wistar fatty rat, which has been established by transferring the fa gene of Zucker fatty rat to the Wistar Kyoto rat, has many features in common with human NIDDM. It exhibits hyperglycemic obesity with hyperinsulinemia and insulin resistance. It is unclear, however, whether a defect in the beta-cell proliferation is related to the onset of diabetes mellitus together with insulin resistance in this model rat. To determine this, we compared non-fasting plasma glucose levels, insulin content and beta-cell mass in the remnant pancreas of Wistar fatty rats with those in their diabetic-resistant lean counterparts after a 70% partial pancreatectomy. We also examined whether such a defect, if present, could be improved by either phlorizin or nicotinamide. We further investigated if there were any differences in these parameters between the phenotypically identical but genotypically different Wistar lean rats with a gene type of homogeneous Fa/Fa and that of heterogeneous Fa/fa. Male rats, 6 weeks of age, were allocated at random into two groups: 70% pancreatectomy (Px) and sham-pancreatectomy (sham). A sustained hyperglycemia was evident in the Px Wistar fatty rats after surgery, which was accompanied by a reduction of insulin content and beta-cell mass in the remnant pancreas. The changes in insulin content and beta-cell mass were unaffected by restoration of normoglycemia, induced by phlorizin injection. The administration of nicotinamide partially ameliorated the sustained hyperglycemia by a slight but not significant increase in beta-cell mass. No discernible difference in the above parameters was observed between the Wistar lean rats with Fa/Fa and those with Fa/fa. These findings suggest that Wistar fatty rats have a poor capacity for proliferation of pancreatic beta-cells, which causes the onset of overt diabetes along with insulin resistance due to extreme obesity.

Experimental study on the treatment of diabetes by phloridzin in rats

Male rats at six weeks of age were divided into 5 groups at random: in group I, the rats with diabetes received 70% pancreatectomy; group II had sham-operation serving as controls; diabetic rats in group III were treated with phloridzin; In group IV rats received sham-operation and phloridzin treatment and group V were phloridzin-treated diabetic rats to be studied after discontinuance of phloridzin. 70 days after surgery, the weights and insulin contents of operated remnant pancreas were markedly higher than the expected value of 30%, reaching 44% (48.2% +/- 15.2%), demonstrating that the remnant pancreas still had capacities of compensatory regeneration and proliferation capacities. Phloridzin-treated diabetic rats completely returned to normal in terms of oral glucose tolerance and insulin sensitivity. Discontinuation of phloridzin treatment in diabetic rats resulted in the recurrence of insulin resistance. These results suggested that normalization of hyperglycemia could ameliorate insulin resistance under diabetic conditions.

Improved glucose tolerance restores insulin-stimulated Akt kinase activity and glucose transport in skeletal muscle from diabetic Goto-Kakizaki rats

The serine/threonine kinase Akt (protein kinase B [PKB] or related to A and C protein kinase [RAC]) has recently been implicated to play a role in the signaling pathway to glucose transport. However, little is known concerning the regulation of Akt activity in insulin-sensitive tissues such as skeletal muscle. To explore the role of hyperglycemia on Akt kinase activity in skeletal muscle, normal Wistar rats or Goto-Kakizaki (GK) diabetic rats were treated with phlorizin. Phlorizin treatment normalized fasting blood glucose and significantly improved glucose tolerance (P < 0.001) in GK rats, whereas in Wistar rats, the compound had no effect on glucose homeostasis. In soleus muscle from GK rats, maximal insulin-stimulated (120 nmol/l) Akt kinase activity was reduced by 68% (P < 0.01) and glucose transport was decreased by 39% (P < 0.05), compared with Wistar rats. Importantly, the defects at the level of Akt kinase and glucose transport were completely restored by phlorizin treatment. There was no significant difference in Akt kinase protein expression among the three groups. At a submaximal insulin concentration (2.4 nmol/l), activity of Akt kinase and glucose transport were unaltered. In conclusion, improved glucose tolerance in diabetic GK rats by phlorizin treatment fully restored insulin-stimulated activity of Akt kinase and glucose transport. Thus, hyperglycemia may directly contribute to the development of muscle insulin resistance through alterations in insulin action on Akt kinase and glucose transport.

The efficacy of phloridzin and phloretin on tumor cell growth

This study utilized phloridzin (P1) and its aglucone phloretin (P2), two known inhibitors of glucose transmembrane transport, to inhibit tumor cell growth in vivo. The efficacy of hydrazine sulfate as an anticachexic agent was also evaluated. Utilizing the rat mammary adenocarcinoma and Fischer bladder cell carcinoma cell lines, it has been shown that the i.p. administration of P1 and P2 can produce significant differences in mean tumor diameters as compared to the untreated controls.

Changes in uncoupling protein and GLUT4 glucose transporter expressions in interscapular brown adipose tissue of diabetic rats: relative roles of hyperglycaemia and hypoinsulinaemia

We have studied the time course and relative effects of hypoinsulinaemia and hyperglycaemia on concentrations of uncoupling protein (UCP) and glucose transporter (GLUT4) and their mRNAs in brown adipose tissue (BAT) during the early phase of diabetes induced by streptozotocin. Two days after intravenous injection of streptozotocin, plasma insulin concentration was at its lowest and glycaemia was higher than 22 mmol/l. After 3 days, a 60% decrease in BAT UCP mRNA concentration and a 36% decrease in UCP was observed. Concomitantly, there was an 80% decrease in GLUT4 mRNA and a 44% decrease in GLUT4 levels. When hyperglycaemia was prevented by infusing phlorizin into diabetic rats, BAT UCP mRNA and protein levels were further decreased (respectively 90% and 60% lower than in control rats). In contrast, the marked decreases in GLUT4 mRNA and protein concentrations in BAT were similar in hyperglycaemic and normoglycaemic diabetic rats. Infusion of physiological amounts of insulin restored normoglycaemia in diabetic rats, and BAT UCP and GLUT4 mRNA and protein concentrations were maintained at the level of control rats. When insulin infusion was stopped, a 75% decrease in BAT UCP mRNA level and a 75% decrease in GLUT4 mRNA level were observed after 24 h, but UCP and GLUT4 concentrations did not decrease. This study shows that insulin plays an important role in the regulation of UCP and GLUT4 mRNA and protein concentrations in BAT. Hyperglycaemia partially prevents the rapid decrease in concentration of UCP and its mRNA observed in insulinopenic diabetes whereas it did not affect the decrease in GLUT4 mRNA and protein concentration. It is suggested that UCP is produced by a glucose-dependent gene.

Effects of hyperglycemia on glucose transporters of the muscle: use of the renal glucose reabsorption inhibitor phlorizin to control glycemia

Individuals with non-insulin dependent or insulin-dependent diabetes mellitus present insulin resistance in peripheral tissues. This is reflected in a subnormal whole body insulin-dependent glucose utilization, largely dependent on skeletal muscle. Glucose transport across the cell membrane of this tissue is rate limiting in the utilization of the hexose. Therefore, it is possible that a defect exists in insulin-dependent glucose transport in skeletal muscle in diabetic states. This review focuses on two questions: is there a defect at the level of glucose transporters in skeletal muscle of diabetic animal models, and is this a consequence of abnormal insulin or glucose levels? The latter question arises from the fact that these parameters usually vary inversely to each other. Glucose transport into skeletal muscle occurs by two membrane proteins, the GLUT1 and GLUT4 gene products. By subcellular fractionation and Western blotting with isoform-specific antibodies, it was determined that isolated plasma membranes (PM) contain GLUT4 and GLUT1 proteins at a molar ratio of 3.5:1 and that an intracellular fraction (internal membranes; IM) different from sarcoplasmic reticulum contains only GLUT4 transporters. The IM furnishes transporters to the PM in response to insulin. Both transporter isoforms bind cytochalasin B in a D-glucose-protectable fashion. In streptozocin-induced diabetes of the rat with normal fasting insulin levels and marked hyperglycemia, the number of cytochalasin B-binding sites and of GLUT4 proteins diminishes in the PM whereas the GLUT1 proteins increase to a new ratio of about 1.5:1 GLUT4:GLUT1. In the IM, the levels of GLUT4 protein drop, as does the cellular GLUT4 mRNA. To investigate if these changes are associated with hyperglycemia, glucose levels were corrected back to normal values for a 24-h period with sc injections of phlorizin to block proximal tubule glucose reabsorption. This treatment restored cytochalasin B binding, restored GLUT4 and GLUT1 values back to normal levels in the PM, and partly restored cytochalasin B binding but not GLUT4 levels in the IM, consistent with only a partial recovery of GLUT4 mRNA. It is concluded that GLUT4 protein in the PM correlates inversely whereas GLUT1 protein correlates directly with glycemia. It is proposed that the decrease in GLUT4 levels is a protective mechanism, sparing skeletal muscle from gaining glucose and experiencing diabetic complications, albeit at the expense of becoming insulin resistant.(ABSTRACT TRUNCATED AT 400 WORDS)

Correction of hyperglycemia with phlorizin normalizes tissue sensitivity to insulin in diabetic rats

Insulin resistance is characteristic of the diabetic state. To define the role of hyperglycemia in generation of the insulin resistance, we examined the effect of phlorizin treatment on tissue sensitivity to insulin in partially pancreatectomized rats. Five groups were studied: group I, sham-operated controls; group II, partially pancreatectomized diabetic rats with moderate glucose intolerance; group III, diabetic rats treated with phlorizin to normalize glucose tolerance; group IV, phlorizin-treated controls; and group V, phlorizin-treated diabetic rats restudied after discontinuation of phlorizin. Insulin sensitivity was assessed with the euglyemic hyperinsulinemic clamp technique in awake, unstressed rats. Insulin-mediated glucose metabolism was reduced by approximately 30% (P less than 0.001) in diabetic rats. Phlorizin treatment of diabetic rats completely normalized insulin sensitivity but had no effect on insulin action in controls. Discontinuation of phlorizin in phlorizin-treated diabetic rats resulted in the reemergence of insulin resistance. These data demonstrate that a reduction of beta-cell mass leads to the development of insulin resistance, and correction of hyperglycemia with phlorizin, without change in insulin levels, normalizes insulin sensitivity. These results provide the first in vivo evidence that hyperglycemia per se can lead to the development of insulin resistance.

Correction of hyperglycemia with phloridzin restores the glucagon response to glucose in insulin-deficient dogs: implications for human diabetes

In insulin-deprived alloxan-induced diabetic dogs with severe hyperglycemia and marked hyperglucagonemia, glucagon was not suppressed by intravenous infusion of glucose at a progressively increasing rate up to 24 mg/kg of body weight per min. However, when the hyperglycemia was corrected by phloridzin, a blocker of renal tubular glucose reabsorption, the hyperglucagonemia was readily suppressed by as little as 2 mg of glucose per kg/min. Direct perfusion of phloridzin into the isolated pancreas of nondiabetic dogs had no effect on the in vitro glucagon response to increments in glucose. However, in pancreata isolated from dogs whose glucose levels had been lowered by phloridzin pretreatment, in vitro glucagon suppression in response to glucose increments was more than twice that of controls. This enhancing effect of phloridzin treatment was completely abolished by giving an intravenous infusion of glucose for the 5 hr prior to surgery for isolation of the pancreas. It is concluded that (i) alpha cells have a glucose-sensing system that is independent of insulin and beta cells, and (ii) this system is reversibly attenuated by hyperglycemia. Thus, hyperglycemia, a metabolic consequence of islet cell dysfunction, may be a self-exacerbating inducer of further islet cell dysfunction, a possibility with implications for human diabetes.