Natasha Jaiswal

Application of click chemistry towards an efficient synthesis of 1,2,3-1H-triazolyl glycohybrids as enzyme inhibitors

An efficient synthesis of novel 1,2,3-1H-triazolyl glycohybrids with two or more than two sugar units or a chromenone moiety via copper-catalysed azide-alkyne cycloaddition (CuAAC), a 1,3-dipolar cycloaddition of glycosyl azides to 2,3-unsaturated alkynyl glycosides or propargyloxy coumarins is described. The synthesised glycohybrids were screened for their α-glucosidase, glycogen phosphorylase and glucose-6-phosphatase inhibitory activities. A few of the glycohybrids showed promising inhibitory activities against these enzymes.

Pongamol from Pongamia pinnata stimulates glucose uptake by increasing surface GLUT4 level in skeletal muscle cells.

Skeletal muscle is the major site of postprandial glucose disposal and augmenting glucose uptake into this tissue may attenuate insulin resistance that precedes type 2 diabetes mellitus. Here, we investigated the effect of pongamol, an identified lead molecule from the fruits of Pongamia pinnata, on glucose uptake and GLUT4 translocation in skeletal muscle cells. In L6-GLUT4myc myotubes treatment with pongamol significantly promoted both glucose transport and GLUT4 translocation to the cell surface in a concentration-dependent manner, without changing the total amount of GLUT4 protein and GLUT4 mRNA, effects that were also additive with insulin. Cycloheximide treatment inhibited the effect of pongamol on GLUT4 translocation suggesting the requirement of new protein synthesis. The pongamol-induced increase in GLUT4 translocation was completely abolished by wortmannin, and pongamol significantly potentiated insulin-mediated phosphorylation of AKT (Ser-473). We conclude that pongamol-induced increase in glucose uptake in L6 myotubes is the result of an increased translocation of GLUT4 to plasma membrane, driven by a PI-3-K/AKT dependent mechanism.

4-Hydroxyisoleucine ameliorates fatty acid-induced insulin resistance and inflammatory response in skeletal muscle cells

The 4-hydroxyisoleucine (4-HIL), an unusual amino acid isolated from the seeds of Trigonella foenum-graecum was investigated for its metabolic effects to ameliorate free fatty acid-induced insulin resistance in skeletal muscle cells. An incubation of L6 myotubes with palmitate inhibited insulin stimulated-glucose uptake and -translocation of glucose transporter 4 (GLUT4) to the cell surface. Addition of 4-HIL strongly prevented this inhibition. We then examined the insulin signaling pathway, where 4-HIL effectively inhibited the ability of palmitate to reduce insulin-stimulated phosphorylation of insulin receptor substrate-1 (IRS-1), protein kinase B (PKB/AKT), AKT substrate of 160 kD (AS160) and glycogen synthase kinase 3β (GSK-3β) in L6 myotubes. Moreover, 4-HIL presented strong inhibition on palmitate-induced production of reactive oxygen species (ROS) and associated inflammation, as the activation of NF-κB, JNK1/2, ERK1/2 and p38 MAPK was greatly reduced. 4-HIL also inhibited inflammation-stimulated IRS-1 serine phosphorylation and restored insulin-stimulated IRS-1 tyrosine phosphorylation in the presence of palmitate, leading to enhanced insulin sensitivity. These findings suggested that 4-HIL could inhibit palmitate-induced, ROS-associated inflammation and restored insulin sensitivity through regulating IRS-1 function.

Fructose induces mitochondrial dysfunction and triggers apoptosis in skeletal muscle cells by provoking oxidative stress

Abstract Mitochondrial dysfunction in skeletal muscle has been implicated in the development of insulin resistance, a major characteristic of type 2 diabetes. There is evidence that oxidative stress results from the increased production of reactive oxygen species and reactive nitrogen species leads to mitochondrial dysfunction, tissue damage, insulin resistance, and other complications observed in type 2 diabetes. It has been suggested that intake of high fructose contributes to insulin resistance and other metabolic disturbances. However, there is limited information about the direct effect of fructose on the mitochondrial function of skeletal muscle, the major metabolic determinant of whole body insulin activity. Here, we assessed the effect of fructose exposure on mitochondria-mediated mechanisms in skeletal muscle cells. Exposure of L6 myotubes to high fructose stimulated the production of mitochondrial reactive oxygen species and nitric oxide (NO), and the expression of inducible NO synthase. Fructose-induced oxidative stress was associated with increased translocation of nuclear factor erythroid 2-related factor-2 to the nucleus, decreases in mitochondrial DNA content and mitochondrial dysfunctions, as evidenced by decreased activities of citrate synthase and mitochondrial dehydrogenases, loss of mitochondrial membrane potential, decreased activity of the mitochondrial respiratory complexes, and impaired mitochondrial energy metabolism. Furthermore, positive Annexin–propidium iodide staining and altered expression of Bcl-2 family members and caspases in L6 myotubes indicated that the cells progressively became apoptotic upon fructose exposure. Taken together, these findings suggest that exposure of skeletal muscle cells to fructose induced oxidative stress that decreased mitochondrial DNA content and triggered mitochondrial dysfunction, which caused apoptosis.

Karanjin from Pongamia pinnata induces GLUT4 translocation in skeletal muscle cells in a phosphatidylinositol-3-kinase-independent manner

Insulin-stimulated glucose uptake in skeletal muscle is decreased in type 2 diabetes due to impaired translocation of insulin-sensitive glucose transporter 4 (GLUT4) from intracellular pool to plasma membrane. Augmenting glucose uptake into this tissue may help in management of type 2 diabetes. Here, the effects of an identified antihyperglycemic molecule, karanjin, isolated from the fruits of Pongamia pinnata were investigated on glucose uptake and GLUT4 translocation in skeletal muscle cells. Treatment of L6-GLUT4myc myotubes with karanjin caused a substantial increase in the glucose uptake and GLUT4 translocation to the cell surface, in a concentration-dependent fashion, without changing the total amount of GLUT4 protein and GLUT4 mRNA. This effect was associated with increased activity of AMP-activated protein kinase (AMPK). Cycloheximide treatment inhibited the effect of karanjin on GLUT4 translocation suggesting the requirement of de novo synthesis of protein. Karanjin-induced GLUT4 translocation was further enhanced with insulin and the effect is completely protected in the presence of wortmannin. Moreover, karanjin did not affect the phosphorylation of AKT (Ser-473) and did not alter the expression of the key molecules of insulin signaling cascade. We conclude that karanjin-induced increase in glucose uptake in L6 myotubes is the result of an increased translocation of GLUT4 to plasma membrane associated with activation of AMPK pathway, in a PI-3-K/AKT-independent manner.

Synthetic studies in butenonyl C-glycosides: Preparation of polyfunctional alkanonyl glycosides and their enzyme inhibitory activity

A simple synthesis of phenyl butenoyl C-glycosides has been achieved by Aldol condensation of peracetylated glycosyl acetones with aromatic aldehydes followed by deacetylation with methanolic NaOMe. The selected butenoyl C-glycosides on conjugate addition of diethyl malonate resulted in polyfunctional alkanonyl glycosides in good yields. The butenoyl C- and alkanoyl C-glycosides were evaluated for their alpha-glucosidase, glucose-6-phosphatse and glycogen phosphorylase enzyme inhibitory activities in vitro. Three of the synthesized (3, 5 and 9) showed potent enzyme inhibitory activities as compared to standard drugs. Compounds 3, 5 and 9 were evaluated in vivo too displaying significant activity as compared to standard drugs acarbose and metformin.

Synthesis of novel triterpene and N-allylated/N-alkylated niacin hybrids as α-glucosidase inhibitors

Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia. α-Glucosidase (EC 3.2.1.20) inhibitors interfere with enzymatic action to slow down the liberation of d-glucose from oligosaccharides and disaccharides, resulting in delayed glucose absorption and decreased postprandial plasma glucose levels. In continuation of our drug discovery program on antidiabetic agents, we synthesized novel N-allylated/N-alkylated niacin and α-amyrin (4-9) and lupeol (12-16) hybrids and tested for their α-glucosidase inhibiting activity. Compounds 4-9 showed better activity profile than the marketed α-glucosidase inhibitor i.e. acarbose. Compound 4 possess the highest inhibitory action with IC50 of 5 μM. Kinetic and CD studies revealed that 4 inhibited the α-glucosidase in a noncompetitive manner and caused conformational changes in secondary structure of the enzyme protein.

Antidiabetic effect of Eclipta alba associated with the inhibition of alpha-glucosidase and aldose reductase

The antidiabetic effect of the ethanolic extract of Eclipta alba (EEA) on hyperglycaemia and diabetic nephropathy was investigated in streptozotocin-induced diabetic rats. Single-dose treatment of EEA to streptozotocin-induced diabetic rats lowered the blood glucose level by 17.6% (p < 0.05) at 250 mg kg−1 dose after 5 h post oral administration. Treatment of animals after 10 weeks of STZ-treatment with EEA (250 mg kg−1) for 21 days significantly reduced the elevated levels of blood glucose, %HbA1C, urea, uric acid and creatinine, and significantly increased the depressed serum insulin level. The extract exerted a significant inhibitory effect on alpha-glucosidase in a noncompetitive manner with an IC50 value of around 54 µg mL−1 and was found inhibitory to eye lens aldose reductase with an IC50 value of around 4.5 µg mL−1. The results suggest that EEA possesses antidiabetic effect associated with alpha-glucosidase and aldose reductase inhibition.

A convenient synthesis of novel pyranosyl homo-C-nucleosides and their antidiabetic activities

A series of pyranosyl homo-C-nucleosides have been synthesized by reaction of butenonyl C-glycosides (5a-5j, and 8) and cyanoacetamide in presence of t-BuOK followed by further modifications. The reaction proceeds by Michael addition of cyanoacetamide to the butenonyl C-glycosides and subsequent dehydrative cyclization and oxidative aromatization to give glycosylmethyl pyridones (6a-6j, 7a-7j, 9, and 10). The glycosylmethyl pyridones (6a-6e) on reaction with POCl(3) under reflux gave respective glycosylmethyl pyridines (11a-11e and 12a-12e) in good yields. The synthesized compounds were screened for their in vitro α-glucosidase, glucose-6-phosphatase and glycogen phosphorylase inhibitory activities. One of the pyridylmethyl homo-C-nucleoside, compound 11d, displayed 52% inhibition of glucose-6-phosphatase as compared to the standard drug sodium orthovanadate while compound 12a showed a significant antihyperglycemic effect of 17.1% in the diabetic rats as compared to the standard drug metformin.

Inhibition of Alpha-Glucosidase by Acacia nilotica Prevents Hyperglycemia along with Improvement of Diabetic Complications via Aldose Reductase Inhibition

Postprandial hyperglycemia is a prominent and early defect in diabetes and regulating blood glucose elevation may attenuate progression towards diabetes associated secondary complications. Here we investigated the alphaglucosidase inhibitory potential of the ethanolic extract of the stem bark of Acacia nilotica (EEAN). The EEAN showed a remarkable alpha-glucosidase inhibitory effect with IC50 value around 8 μg/ml. Kinetic studies revealed that the extract inhibited alpha-glucosidase in competitive manner and caused conformational changes in secondary structure of the enzyme protein. In vivo analysis showed that EEAN significantly suppresses the sucrose-induced postprandial glucose elevation in normal rats and exerts antihyperglycemic effect in streptozotocin (STZ)-induced diabetic rats in a dose-dependent fashion. Further, treatment of diabetic animals after 10 week of STZ-treatment with EEAN (250 mg/ kg) for 21 days, significantly reduced the elevated levels of blood glucose, %HbA1C, urea, uric acid and creatinine, and significantly increased the depressed plasma insulin level. The EEAN also showed inhibitory potential on aldose reductase activity with an IC50 of 7.5 μg/ml. The results suggest that EEAN possess antihyperglycemic activity through inhibition of alpha-glucosidase along with antidiabetogenic effect on polyol pathway through aldose reductase inhibition.