The number of individuals diagnosed with type 2 diabetes mellitus, which is caused by insulin resistance and/or abnormal insulin secretion, is increasing worldwide, creating a strong demand for the development of more effective anti-diabetic drugs. However, animal-based screening for anti-diabetic compounds requires sacrifice of a large number of diabetic animals, which presents issues in terms of animal welfare. Here, we established a method for evaluating the anti-diabetic effects of compounds using an invertebrate animal, the silkworm, Bombyx mori. Sugar levels in silkworm hemolymph increased immediately after feeding silkworms a high glucose-containing diet, resulting in impaired growth. Human insulin and 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), an AMP-activated protein kinase (AMPK) activator, decreased the hemolymph sugar levels of the hyperglycemic silkworms and restored growth. Treatment of the isolated fat body with human insulin in an in vitro culture system increased total sugar in the fat body and stimulated Akt phosphorylation. These responses were inhibited by wortmannin, an inhibitor of phosphoinositide 3 kinase. Moreover, AICAR stimulated AMPK phosphorylation in the silkworm fat body. Administration of aminoguanidine, a Maillard reaction inhibitor, repressed the accumulation of Maillard reaction products (advanced glycation end-products; AGEs) in the hyperglycemic silkworms and restored growth, suggesting that the growth defect of hyperglycemic silkworms is caused by AGE accumulation in the hemolymph. Furthermore, we identified galactose as a hypoglycemic compound in jiou, an herbal medicine for diabetes, by monitoring its hypoglycemic activity in hyperglycemic silkworms. These results suggest that the hyperglycemic silkworm model is useful for identifying anti-diabetic drugs that show therapeutic effects in mammals.

We previously reported that sugar levels in the silkworm hemolymph, i.e., blood, increase immediately (within 1 h) after intake of a high-glucose diet, and that the administration of human insulin decreases elevated hemolymph sugar levels in silkworms. In this hyperglycemic silkworm model, however, administration of pioglitazone or metformin, drugs used clinically for the treatment of type II diabetes, have no effect. Therefore, here we established a silkworm model of type II diabetes for the evaluation of anti-diabetic drugs such as pioglitazone and metformin. Silkworms fed a high-glucose diet over a long time-period (18 h) exhibited a hyperlipidemic phenotype. In these hyperlipidemic silkworms, phosphorylation of JNK, a stress-responsive protein kinase, was enhanced in the fat body, an organ that functionally resembles the mammalian liver and adipose tissue. Fat bodies isolated from hyperlipidemic silkworms exhibited decreased sensitivity to human insulin. The hyperlipidemic silkworms have impaired glucose tolerance, characterized by high fasting hemolymph sugar levels and higher hemolymph sugar levels in a glucose tolerance test. Administration of pioglitazone or metformin improved the glucose tolerance of the hyperlipidemic silkworms. These findings suggest that the hyperlipidemic silkworms are useful for evaluating the hypoglycemic activities of candidate drugs against type II diabetes.

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Graphical abstract Thinned Irwin mango fruits were collected at the immature stage and lyophilized. Hot-water extract from thinned immature mango (TIMEx) had no toxic effect on SD rats given daily oral doses of 500, 1000 or 2500 mg/kg for 90 days. No significant alterations in hematological and serum chemical parameters, urinalysis, food consumption, body weight gain, or absolute and relative organ weights were noted in any group. In both sexes, a daily dose up to 2500 mg/kg body weight indicates a NOAEL.

The thinned immature fruit of the mango tree (Mangifera indica “Irwin”) are regarded as waste products. In this study, we evaluated the effects of daily consumption of a hot-water extract of thinned immature mango fruits (TIMEx) on the dyslipidemia of apolipoprotein E-deficient (ApoE−/−) mice. ApoE−/− mice and wild-type BALB/c mice were fed a 20% fat diet containing 0%, 0.1%, or 1.0% TIMEx for 8 weeks. Their body mass, food intake, and water consumption were unaffected by the TIMEx. The 1.0% TIMEx supplementation significantly reduced serum triglyceride, but not total cholesterol concentration. This effect was significant in ApoE−/− mice, but less marked under normal conditions in wild-type mice. In addition, the circulating concentrations of three hormones that regulate metabolism, resistin, leptin, and glucose-dependent insulinotropic polypeptide, were reduced by TIMEx consumption, which may be involved in its effect to prevent hypertriglyceridemia. However, none of the concentrations of TIMEx reduced the size of atherosclerotic plaque lesions. In conclusion, daily consumption of TIMEx ameliorates hypertriglyceridemia but not hypercholesterolemia in genetically predisposed mice.