Type 2 diabetes is induced by an imbalance between insulin resistance and insulin secretion and does not occur unless β-cell function cannot compensate for insulin resistance [1
]. Insulin resistance is the major pathophysiology for type 2 diabetes in Caucasians, as β-cell function is sufficient to overcome insulin resistance. However, patients eventually fail to compensate for insulin resistance because increased insulin secretion does not normalize serum glucose levels by reducing insulin resistance [1
]. However, failure of β-cell function occurs more frequently in Asians than in Caucasians. Insulin secretion capacity is associated with β-cell mass, which tends to be lower in Asians [2
]. Therefore, impaired β-cell function is a major risk factor for developing type 2 diabetes in East Asians. Therapeutic agents for Asian type 2 diabetes need to not only enhance insulin sensitivity but also potentiate β-cell function and mass.
Blume (GEB) is a plant in the Orchidaceae family that has been used traditionally to treat convulsions, ischemia, Alzheimer’s disease, tremors, and vertigo [3
]. It contains many phenolic compounds, including gastrodin [4-(β-d
-gucopyranosyl) benzyl alcohol], 4-hydroxybenzyl alcohol, 4-hydroxybenzaldehyde, 4-hydroxy-3-methoxy benzaldehyde, and vanillin, which are small phenolic compounds that can pass through the blood brain barrier (BBB) where they have antioxidant, anti-inflammatory, and anti-angiogenic activities to enhance neurological disorders, including ischemic stroke and Alzheimer’s disease [3
]. Previous studies have demonstrated that Alzheimer’s disease is associated with brain insulin resistance, which is interrelated with type 2 diabetes [7
]. Alzheimer’s disease is type 3 diabetes or brain-specific type 2 diabetes [7
]. Moreover, the potentiation of insulin signaling in the hypothalamus and hippocampus improves peripheral glucose and energy metabolism. Thus, beneficial therapeutic agents against type 2 diabetes may be beneficial against Alzheimer’s disease, and vice versa
. Therefore, GEB may be beneficial for preventing and alleviating the symptoms of type 2 diabetes.
However, GEB has not been studied for its therapeutic effect in type 2 diabetes. In our previous study, GEB decreased visceral fat mass by increasing fat oxidation and lowering food intake, resulting in enhanced insulin sensitivity in diet-induced obese male rats [9
]. Vanillin and 4-hydroxybenzaldehyde increase insulin-stimulated glucose uptake by lowering fat accumulation in 3T3-L1 adipocytes [9
]. However, the activity of these compounds to stimulate insulin-stimulated glucose uptake was much less than that of rosiglitazone in a cell-based study [9
]. These results suggest that GEB may improve glucose metabolism by activating energy use in obese animals and that the improvement is related to indirect activation of the hypothalamus. No study has investigated the effect of GEB on alleviating type 2 diabetic symptoms. It is difficult to speculate whether GEB may be a hypoglycemic agent in non-obese type 2 diabetic animals, which represent the characteristics of type 2 diabetes in Asians.
We hypothesized that long-term administration of GEB would have a hypoglycemic effect by improving insulin resistance and β-cell function in a non-obese type 2 diabetic animal model. We tested the hypothesis by determining peripheral insulin resistance using the euglycemic hyperinsulinemic clamp assay and measuring insulin secretion capacity using the hyperglycemic clamp assay in partially pancreatectomized (Px) rats, a non-obese and insulin-insufficient type 2 diabetic animal model. We also measured insulin signaling in the hypothalamus to determine brain insulin sensitivity. Px rats are well represented the clinical characteristics of Asian type 2 diabetes.
The brain, particularly the hypothalamus, plays an important role regulating energy and glucose metabolism [22
]. In addition, Alzheimer’s disease and type 2 diabetes share impaired insulin signaling [7
]. Alzheimer’s disease is associated with hippocampal insulin resistance, whereas type 2 diabetes develops due to insulin resistance in peripheral tissues, including the liver, adipose tissue, and islets [7
]. Thus, insulin signaling in the hypothalamus and hippocampus are connected to peripheral glucose metabolism. GEB may improve type 2 diabetic symptoms as it has been traditionally used for brain-related diseases, such as convulsions, ischemia, Alzheimer’s disease, and tremors. The purpose of the study was to investigate whether long-term consumption of GEB would improve type 2 diabetic symptoms by reducing insulin resistance and potentiating β-cell function and mass in partial Px rats.
When Asians increase insulin resistance, they exhibit normal insulin levels or hypoinsulinemia and easily progress from glucose intolerance to type 2 diabetes [24
]. Px rats are a good model to examine the relationship between β-cell function and insulin resistance and β-cell expansion since they are a non-obese and insulin-insufficient type 2 diabetic model with characteristics relevant to Asian type 2 diabetes. Px rats exhibit a similar phenotype to Asian type 2 diabetes [24
]. After removing 90% of the pancreas, the pancreas regenerates up to 40%–50% of the intact pancreas and insulin secretion capacity is about 50%–60% of the non-diabetic rats [18
]. Overnight-fasting serum glucose levels were over 130–150 mg/dL and post-prandial serum glucose levels about 230–280 mg/dL. They showed moderate diabetic status. Thus, Px rats are non-obese and insulin insufficient type 2 diabetic animal model.
GEB has been reported to have neuroprotective activities to prevent and/or alleviate dizziness, epilepsy, stroke, and dementia. GEB contains small compounds, such as gastrodin, 4-hydroxybenzyl alcohol, 4-hydroxybenzaldehyde, 4-hydroxy-3-methoxybenzaldehyde, and vanillin, which can pass through the BBB, suggesting that it acts in the brain [6
]. The brain, particularly the hypothalamus, regulates energy and glucose metabolism [7
]. The brain receives neural inputs of glucose status from the periphery, and neurons directly sense glucose levels. When glucose levels rise, glucose-responsive neurons increase firing through an ATP-sensitive K+
channel, whereas glucose-sensitive neurons decrease firing [22
]. Both neurons are related to changes in food intake, sympathoadrenal activity, and energy expenditure in the states of extreme hyperglycemia and hypoglycemia [27
]. Glucose responsive neurons are hyper-responsive to glucose in rats with diet-induced obesity and insulin-dependent diabetes [27
]. Thus, brain glucose sensing differs in obese and diabetic states. Modulation of brain glucose metabolism can relieve the symptoms of obesity and type 2 diabetes [28
]. As GEB has actions against dementia, which is associated with brain insulin resistance, GEB may alter brain insulin resistance. Brain insulin resistance is also associated with peripheral insulin resistance, particularly in the liver [23
]. Therefore, GEB may influence peripheral glucose metabolism in patients with type 2 diabetes. In the present study, GEB potentiated hypothalamic insulin signaling (pAkt → pGSK-1β) in parallel with improving hepatic insulin resistance in partial Px rats. GEB may regulate peripheral glucose metabolism through brain insulin signaling.
GEB is known to improve energy metabolism possibly by enhancing leptin signaling [29
]. In our previous study GEB reduced weight gain and epididymal fat pads in diet-induced obese rats by improving hypothalamic leptin signaling and reducing NPY and AgRP expression [9
]. However, the present study GEB did not lower in body weight and epididymal fat pads in diabetic Px rats, although GEB tended to reduce daily energy intake and increase daily energy expenditure. This discrepancy between two studies was related to differences in the animal models. Diabetic Px rats showed hyperglycemia with insulin insufficiency in the present study, whereas diet-induced obese rats exhibited hyperinsulinemia with normoglycemia. Diabetic Px rats are non-obese rats and they lose weight gain when the diabetic symptoms are severe. Although GEB improved energy metabolism in both studies, it maintained body weight and body fat in diabetic Px rats by enhancing glucose utilization, possibly due to decreased urinary glucose excretion.
Type 2 diabetes is induced by impaired insulin sensitivity and secretion. When insulin resistance is initially induced in the liver, skeletal muscles, and adipose tissues by certain conditions, such as obesity, stress, inflammation, and aging; however, insulin secretion from pancreatic β-cells is sufficient to maintain normoglycemia and, thus, compensates for insulin resistance [1
]. Insulin insufficiency leads to type 2 diabetes in an increased insulin-resistant state. The present study showed that whole-body glucose infusion rates representing whole-body insulin resistance increased. Increased glucose infusion rates in GEB-H without whole body glucose uptake was related to decreased hepatic glucose output, indicating that GEB-H was lower hepatic insulin resistance. Surprisingly, glucose uptake in skeletal muscles was higher in GEB-H than the control although whole body glucose uptake slightly increased but not significantly different. The proportion of glucose uptake in skeletal muscles may not be large enough to influence whole body glucose uptake. These results suggested that GEB influenced glucose metabolism in both the liver and skeletal muscles but GEB improved insulin sensitivity in the liver more than skeletal muscles.
In this study, Px rats had 50%–60% glucose-stimulated insulin secretion capacity of the rats with an intact pancreas [18
]. However, as the decrease in insulin secretion itself takes longer to induce type 2 diabetes, a high-fat diet was fed to exacerbate glucose homeostasis and require more insulin secretion by increasing insulin resistance [20
]. Thus, the potentiation of glucose-stimulated insulin secretion is necessary to improve glucose homeostasis in partial Px rats. As Asians have lower insulin secretion capacity [2
], partial Px rats are an optimal model with which to explore therapeutic herbs for treating Asians. Our results show that GEB potentiated glucose-stimulated insulin secretion. GEB has been shown to enhance phosphorylation of both phosphatidylinositol 3-kinase (PI3K) and cAMP-responsive element binding protein (CREB) and to increase brain-derived neurotrophic factor in HT22 hippocampal cells. As islets are also neuronal cells, GEB may increase PI3K and CREB phosphorylation [30
], which is involved in glucose-stimulated insulin secretion. In addition, administering 4-hydroxybenzyl methyl ether (10 mg/kg, p.o.) isolated from GEB, significantly increases phosphorylation of cortical and hippocampal protein kinase A (PKA)/CREB [31
]. The present study showed that GEB might improve glucose-stimulated insulin secretion, possibly by 4-hydroxybenzyl alcohol and 4-hydroxybenzaldehyde, and the pathway to potentiate insulin secretion might be similar to that used by neurons.
Potentiated insulin secretion should be combined with increased β-cell mass because β-cell mass plays an important role in maintaining insulin secretion. Sulphonylureas have been used to treat diabetes by increasing insulin secretion by closing KATP
-channels to trigger calcium influx [32
]. They release insulin not only under high glucose but also under low glucose conditions without increasing β-cell mass [32
]. Eventually, sulphonylureas fail to normalize serum glucose levels. However, new insulin secretagogues, such as exenatide, not only induce significant increases in serum insulin but also increase the number of insulin- and GLP-1-positive cells [33
]. GLP-1 receptor agonists, such as exenatide, lower glucose concentration by augmenting insulin secretion and suppressing glucagon release [34
]. In addition, exenatide maintains β-cell mass by protecting β-cell apoptosis [34
]. Exenatide activates the cAMP, PKB, and PKA signal transduction pathways to activate PI3K and its downstream mitogen-activated protein kinase/extracellular regulated kinase. Thus, GLP-1 receptor agonists are better insulin secretagogues than sulphonylureas. Our results show that GEB increased glucose-stimulated insulin secretion by increasing β-cell mass and proliferation. Thus, GEB might be a good hypoglycemic agent, particularly for treating non-obese type 2 diabetes with insulin insufficiency.