Purification of ginseng rare sapogenins 25-OH-PPT and its hypoglycemic, antiinflammatory and lipid-lowering mechanisms

Background

Panax ginseng Meyer has been used as a nourishing edible herb in East Asia for thousands of years. 25-OH-PPT was first discovered as a natural rare triterpenoid saponin in ginseng stems and leaves by our group. Research found that it showed strong inhibitory effects on α-glucosidase and protein tyrosine phosphatase 1B, and protected cardiocytes (H9c2) through PI3K/Akt pathway.

Methods

In the research, in order to optimize the 25-OH-PPT enrichment process, optimal macroporous resins and optimal purification conditions were studied. Meanwhile, the hypoglycemic effect and mechanism of 25-OH-PPT were evaluated by using STZ to establish insulin-dependent diabetic mice and the spontaneous type 2 diabetes DB/DB mice.

Results and Conclusion

Research found that 25-OH-PPT can reduce blood glucose and enhance glucose tolerance in STZ model mice. It increases insulin sensitivity by upregulating GLUT4 and AMPK in skeletal muscle, and activating insulin signaling pathways. In DB/DB mice, 25-OH-PPT achieves hypoglycemic effects mainly by activating the insulin signaling pathway. Meanwhile, through the influence of liver inflammatory factors and lipids in serum, it can be seen that 25-OH-PPT has obvious anti-inflammatory and lipid-lowering effects. These results provide new insights into the study of ginseng as a functional food.

The liver surface as a favorable site for islet cell sheet transplantation in type 1 diabetes model mice

INTRODUCTION

Islet transplantation is one of the most promising therapeutic approaches for patients with severe type 1 diabetes mellitus (T1DM). Transplantation of engineered islet cell sheets holds great potential for treating T1DM as it enables the creation of stable neo-islet tissues. However, a large mass of islet cell sheets is required for the subcutaneous transplantation to reverse hyperglycemia in diabetic mice. Here, we investigated whether the liver surface could serve as an alternative site for islet cell sheet transplantation.

METHODS

Dispersed rat islet cells (0.8 × 106 cells) were cultured on laminin-332-coated thermoresponsive culture dishes. After 2 days of cultivation, we harvested the islet cell sheets by lowering the culture temperature using a support membrane with a gelatin gel. We transplanted two recovered islet cell sheets into the subcutaneous space or onto the liver surface of severe combined immunodeficiency (SCID) mice with streptozocin-induced diabetes.

RESULTS

In the liver surface group, the non-fasting blood glucose level decreased rapidly within several days after transplantation. In marked contrast, the hyperglycemia state was maintained in the subcutaneous space transplantation group. The levels of rat C-peptide and insulin in the liver surface group were significantly higher than those in the subcutaneous space group. An immunohistological analysis confirmed that most of the islet cells engrafted on the liver surface were insulin-positive. The CD31-positive endothelial cells formed vascular networks within the neo-islets and in the surrounding tissues. In contrast, viable islet cells were not found in the subcutaneous space group.

CONCLUSIONS

Compared with the subcutaneous space, a relatively small mass of islet cell sheets was enough to achieve normoglycemia in diabetic mice when the liver surface was selected as the transplantation site. Our results demonstrate that the optimization of the transplantation site for islet cell sheets leads to significant improvements in the therapeutic efficiency for T1DM.

Intestinal invalidation of the glucose transporter GLUT2 delays tissue distribution of glucose and reveals an unexpected role in gut homeostasis

OBJECTIVE

Intestinal glucose absorption is orchestrated by specialized glucose transporters such as SGLT1 and GLUT2. However, the role of GLUT2 in the regulation of glucose absorption remains to be fully elucidated.

METHODS

We wanted to evaluate the role of GLUT2 on glucose absorption and glucose homeostasis after intestinal-specific deletion of GLUT2 in mice (GLUT2^ΔIEC mice).

RESULTS

As anticipated, intestinal GLUT2 deletion provoked glucose malabsorption as visualized by the delay in the distribution of oral sugar in tissues. Consequences of intestinal GLUT2 deletion in GLUT2^ΔIEC mice were limiting body weight gain despite normal food intake, improving glucose tolerance, and increasing ketone body production. These features were reminiscent of calorie restriction. Other adaptations to intestinal GLUT2 deletion were reduced microvillus length and altered gut microbiota composition, which was associated with improved inflammatory status. Moreover, a reduced density of glucagon-like peptide-1 (GLP-1) positive cells was compensated by increased GLP-1 content per L-cell, suggesting a preserved enteroendocrine function in GLUT2^ΔIEC mice.

CONCLUSIONS

Intestinal GLUT2 modulates glucose absorption and constitutes a control step for the distribution of dietary sugar to tissues. Consequently, metabolic and gut homeostasis are improved in the absence of functional GLUT2 in the intestine, thus mimicking calorie restriction.

The F-actin modifier villin regulates insulin granule dynamics and exocytosis downstream of islet cell autoantigen 512

Objective

Insulin release from pancreatic islet β cells should be tightly controlled to avoid hypoglycemia and insulin resistance. The cortical actin cytoskeleton is a gate for regulated exocytosis of insulin secretory granules (SGs) by restricting their mobility and access to the plasma membrane. Prior studies suggest that SGs interact with F-actin through their transmembrane cargo islet cell autoantigen 512 (Ica512) (also known as islet antigen 2/Ptprn). Here we investigated how Ica512 modulates SG trafficking and exocytosis.

Methods

Transcriptomic changes in Ica512^−/− mouse islets were analyzed. Imaging as well as biophysical and biochemical methods were used to validate if and how the Ica512-regulated gene villin modulates insulin secretion in mouse islets and insulinoma cells.

Results

The F-actin modifier villin was consistently downregulated in Ica512^−/− mouse islets and in Ica512-depleted insulinoma cells. Villin was enriched at the cell cortex of β cells and dispersed villin^−/− islet cells were less round and less deformable. Basal mobility of SGs in villin-depleted cells was enhanced. Moreover, in cells depleted either of villin or Ica512 F-actin cages restraining cortical SGs were enlarged, basal secretion was increased while glucose-stimulated insulin release was blunted. The latter changes were reverted by overexpressing villin in Ica512-depleted cells, but not vice versa.

Conclusion

Our findings show that villin controls the size of the F-actin cages restricting SGs and, thus, regulates their dynamics and availability for exocytosis. Evidence that villin acts downstream of Ica512 also indicates that SGs directly influence the remodeling properties of the cortical actin cytoskeleton for tight control of insulin secretion.

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Ica512-depletion reduces the genetic expression of the F-actin modifier villin.

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Villin-depletion enhances basal insulin granule mobility and exocytosis.

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Villin regulates the size of actin cages restraining insulin granules.

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Villin acts downstream of insulin granule cargo Ica512.

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The Ica512-villin genetic link enables granules to control cytoskeleton plasticity.

Enhanced insulin sensitivity in skeletal muscle and liver by physiological overexpression of SIRT6

Objective

Available treatment for obesity and type 2 diabetes mellitus (T2DM) is suboptimal. Thus, identifying novel molecular target(s) exerting protective effects against these metabolic imbalances is of enormous medical significance. Sirt6 loss- and gain-of-function studies have generated confounding data regarding the role of this sirtuin on energy and glucose homeostasis, leaving unclear whether activation or inhibition of SIRT6 may be beneficial for the treatment of obesity and/or T2DM.

Methods

To address these issues, we developed and studied a novel mouse model designed to produce eutopic and physiological overexpression of SIRT6 (Sirt6BAC mice). These mutants and their controls underwent several metabolic analyses. These include whole-blood reverse phase high-performance liquid chromatography assay, glucose and pyruvate tolerance tests, hyperinsulinemic-euglycemic clamp assays, and assessment of basal and insulin-induced level of phosphorylated AKT (p-AKT)/AKT in gastrocnemius muscle.

Results

Sirt6BAC mice physiologically overexpress functionally competent SIRT6 protein. While Sirt6BAC mice have normal body weight and adiposity, they are protected from developing high-caloric-diet (HCD)-induced hyperglycemia and glucose intolerance. Also, Sirt6BAC mice display increased circulating level of the polyamine spermidine. The ability of insulin to suppress endogenous glucose production was significantly enhanced in Sirt6BAC mice compared to wild-type controls. Insulin-stimulated glucose uptake was increased in Sirt6BAC mice in both gastrocnemius and soleus muscle, but not in brain, interscapular brown adipose, or epididymal adipose tissue. Insulin-induced p-AKT/AKT ratio was increased in gastrocnemius muscle of Sirt6BAC mice compared to wild-type controls.

Conclusions

Our data indicate that moderate, physiological overexpression of SIRT6 enhances insulin sensitivity in skeletal muscle and liver, engendering protective actions against diet-induced T2DM. Hence, the present study provides support for the anti-T2DM effect of SIRT6 and suggests SIRT6 as a putative molecular target for anti-T2DM treatment.

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“Sirt6BAC” mice overexpress SIRT6 in a physiological manner unattained previously.

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SIRT6 overexpression protects against aberrant glucose homeostasis.

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Sirt6BAC mice exhibit enhanced insulin sensitivity in skeletal muscle and liver.

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Pharmacologically enhancing SIRT6 activity may be a viable way to treat diabetes.

Mice lacking neutral amino acid transporter B(0)AT1 (Slc6a19) have elevated levels of FGF21 and GLP-1 and improved glycaemic control

OBJECTIVE

Type 2 diabetes arises from insulin resistance of peripheral tissues followed by dysfunction of β-cells in the pancreas due to metabolic stress. Both depletion and supplementation of neutral amino acids have been discussed as strategies to improve insulin sensitivity. Here we characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19) as a model to study the consequences of selective depletion of neutral amino acids.

METHODS

Metabolic tests, analysis of metabolite levels and signalling pathways were used to characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19).

RESULTS

Reduced uptake of neutral amino acids in the intestine and loss of neutral amino acids in the urine causes an overload of amino acids in the lumen of the intestine and reduced systemic amino acid availability. As a result, higher levels of glucagon-like peptide 1 (GLP-1) are produced by the intestine after a meal, while the liver releases the starvation hormone fibroblast growth factor 21 (FGF21). The combination of these hormones generates a metabolic phenotype that is characterised by efficient removal of glucose, particularly by the heart, reduced adipose tissue mass, browning of subcutaneous white adipose tissue, enhanced production of ketone bodies and reduced hepatic glucose output.

CONCLUSIONS

Reduced neutral amino acid availability improves glycaemic control. The epithelial neutral amino acid transporter B(0)AT1 could be a suitable target to treat type 2 diabetes.

Peripheral activation of the Y2-receptor promotes secretion of GLP-1 and improves glucose tolerance

The effect of peptide tyrosine-tyrosine (PYY) on feeding is well established but currently its role in glucose homeostasis is poorly defined. Here we show in mice, that intraperitoneal (ip) injection of PYY3-36 or Y2R agonist improves nutrient-stimulated glucose tolerance and enhances insulin secretion; an effect blocked by peripheral, but not central, Y2R antagonist administration. Studies on isolated mouse islets revealed no direct effect of PYY3-36 on insulin secretion. Bariatric surgery in mice, enterogastric anastomosis (EGA), improved glucose tolerance in wild-type mice and increased circulating PYY and active GLP-1. In contrast, in Pyy-null mice, post-operative glucose tolerance and active GLP-1 levels were similar in EGA and sham-operated groups. PYY3-36 ip increased hepato-portal active GLP-1 plasma levels, an effect blocked by ip Y2R antagonist. Collectively, these data suggest that PYY3-36 therefore acting via peripheral Y2R increases hepato-portal active GLP-1 plasma levels and improves nutrient-stimulated glucose tolerance.