Acid Hydrolyzed Silk Peptide Consumption Improves Anti-Diabetic Symptoms by Potentiating Insulin Secretion and Preventing Gut Microbiome Dysbiosis in Non-Obese Type 2 Diabetic Animals

Oral sericin ameliorates type 2 diabetes through passive intestinal and bypass transport into the systemic circulation

ETHNOPHARMACOLOGICAL RELEVANCE

Boiled silkworm cocoons have been used to treat 'Xiaoke disease' (diabetes mellitus) recorded in Chinese medicine for over 800 years. In recent years, it has been found that the active substance silk sericin (SS) has therapeutic benefits in treating type 2 diabetes mellitus (T2DM). SS promotes pancreatic islet signalling, the proliferation of pancreatic islet cells, and insulin secretion. It is inferred that SS enters the bloodstream after oral administration and plays a role in the body's circulation. As a natural protein, SS needs to resist digestion by proteases in the gastrointestinal tract and cross the gastrointestinal barrier after oral administration. It is currently unclear how SS crosses the gastrointestinal barrier and whether it exerts therapeutic effects on T2DM by entering the circulation.

AIM OF THE STUDY

To study how SS crosses the gastrointestinal barrier and whether it enters the body circulation to exert a therapeutic effect on T2DM.

MATERIALS AND METHODS

SS was extracted from silkworm cocoons using an alkaline method with sodium carbonate. The antidigestive capacity of SS was detected using SDS-PAGE gel electrophoresis experiments. The mode of uptake and translocation of orally consumed SS in vivo was analysed using the AP-side to BL-side and BL-side-AP-side translocations, apparent Permeability coefficient (Papp), and Exocytosis rates (ER). The study compared the differences between the adSS group and the adSS + EDTA group by using Ethylenediaminetetraacetic acid (EDTA) to separate the tight junctions between Caco-2 cells. The aim was to analyze whether the transport mode of oral filaggrin proteins in vivo could be absorbed by bypass transport. By administering SS through oral and intraperitoneal injection to type 2 diabetic mice, we measured its concentration in the blood, as well as blood glucose and insulin levels, to determine its effectiveness in treating diabetes and its ability to enter the body's circulation for treatment.

RESULTS

The molecular weight of SS decreased from 10k∼25 kDa to 10k∼15 kDa after in vitro simulated gastrointestinal fluid digestion, indicating its good antidigestive properties. The apparent Papp was greater than 1 × 10-6 cm·s-1, and the ER was between 0.5 and 1.5, indicating that adSS was well-absorbed and mainly passively transported. The Caco-2 cell model showed that the addition of EDTA promoted the transport of adSS, resulting in significantly larger Papp and ER values, indicating that adSS was absorbed by bypass transport. After oral administration of SS, the concentration of SS in the blood was lower than after intraperitoneal injection, which is 60% of intraperitoneal administration. Mice with a T2DM model who were administered SS for 5 weeks showed significant improvement in insulin resistance and glucose tolerance. Additionally, the pancreatic tissue appeared more regular. In the treatment of T2DM, injections of SS have been shown to be more effective than oral administration. Both oral and intraperitoneal injections have been partially involved in the circulation.

CONCLUSIONS

SS is enzymatically cleaved by proteolytic enzymes in the gastrointestinal tract. The smaller molecules are partially absorbed into the body's circulation through passive and paracrine transport, exerting a therapeutic effect on T2DM.

Evaluating bio-physicochemical properties of raw powder prepared from whole larvae containing liquid silk of the domestic silkworm

The domestic silkworm, Bombyx mori, has been widely used in silk production for centuries. It is also used as a bioreactor by the textile and pharmaceutical industries to mass produce recombinant bioactive proteins containing silk-based materials. Furthermore, silkworms are well-known as a source of food and have also been orally administered to prevent and treat several human disorders. In this study, we aimed to investigate the inherent bio-physicochemical properties of edible silkworms to accurately evaluate their clinical and nutritional potential. We prepared raw powder from whole larvae of silkworm. The yield rate of the powder derived from dried larvae was almost 100% (98.1-99.1% in replicates). As "percentage yield" translates to "Budomari" in Japanese, this raw powder was named "B100rw." We further prepared B100dn that was denatured through autoclaving. Thereafter, we examined whether B100rw sustained the original bio-physicochemical properties by comparing it with B100dn. There was no significant difference in nutritional content between B100rw and B100dn. B100rw contained proteins derived from silkworm larvae and mulberry leaves, whereas the proteins of B100dn were mostly degraded. On measuring the enzymatic activity of both powders using trehalase as an indicator enzyme, B100rw was found to maintain trehalase activity. B100rw also maintained a random coil conformation, similar to that of liquid silk. This suggested that B100rw sustained the unique bio-physicochemical properties of living larvae. These findings may facilitate the development of novel food products or orally administered vaccines.

Bombyx mori cocoon as a promising pharmacological agent: A review of ethnopharmacology, chemistry, and biological activities

Silk cocoon, naturally produced by silkworms scientifically named Bombyx mori L. (Lepidoptera, Bombycidae), is one of the well-known medicinal agents with several therapeutic activities. The present study aims to review the various aspects of the silk cocoon, including chemical composition, traditional uses, biological and biotechnological activities, and toxicological issues, to provide a scientific source for scholars. For this purpose, Electronic databases including PubMed, Scopus, Google Scholar, Web of Science, and traditional literature, were searched up to December 2021. According to the historical data, silk farming is acknowledged as one of the most ancient agricultural findings. The silk is generally composed of 75–83% fibroin, 17–25% sericin, and 1–5% non-sericin components, including secondary metabolites, wax, pigments, carbohydrates, and other impurities. Flavonoids, especially quercetin and kaempferol, alkaloids, coumarin derivatives, and phenolic acids, are among the secondary metabolites isolated from the silk cocoon. In recent years the biological properties of the silk cocoon, especially its major proteins, namely fibroin and sericin, have drawn special attention. Scientific literature has investigated several pharmacological effects of the silk cocoon and its ingredients, including cardioprotective, antioxidant, anticancer, antidiabetic, antihyperlipidemia, gastroprotective, as well as ameliorated skin health activities. In addition, it has been extensively taken into consideration in drug delivery and tissue engineering study fields. Furthermore, its toxicity is in acceptable range.

Investigation of the Repairing Effect and Mechanism of Oral Degraded Sericin on Liver Injury in Type II Diabetic Rats

In the sericulture and silk production industry, sericin is discharged in the degumming wastewater, resulting in a large amount of wasted natural protein and environmental pollution. This study investigated the effect of degraded sericin recovered by the Ca(OH)₂–ultrasound degumming method (a green process) on liver injury in T2D rats. After 4 weeks of dietary sericin supplementation, the liver masses and organ coefficients of the T2D rats improved compared with those of the model rats that were not fed sericin. Oral sericin activated the damaged PI3K/AKT/AMPK pathway to enhance glycogen synthesis, accelerate glycolysis, and inhibit gluconeogenesis. The protein expression levels of the inflammatory factors NF-κB, IL-6, and TNF-α in the T2D model group were up to two times higher than in the normal group. However, all three T2D groups that received oral sericin showed significant decreases in these factors to the level found in the normal group, indicating that inflammation in the body was significantly reduced. These results show that the sericin protein might improve glycogen synthesis, accelerate glycolysis, and inhibit gluconeogenesis by enhancing the anti-oxidation capability and reducing inflammatory reactions. Therefore, sericin recovered by Ca(OH)₂ degradation has potential use in the development of functional health foods that can lower blood sugar.

PAYCS Alleviates Scopolamine-Induced Memory Deficits in Mice by Reducing Oxidative and Inflammatory Stress and Modulation of Gut Microbiota-Fecal Metabolites-Brain Neurotransmitter Axis

The bioactive peptide PAYCS (Pro-Ala-Tyr-Cys-Ser) identified from anchovy hydrolysates has been reported to be positive in memory alleviation. The gut microbiota-brain axis plays a vital role in brain functions, which could be affected by nutritional supplementation. Herein, we found that PAYCS at different concentrations (PAYCS-L and PAYCS-H) showed various improving effects in behavioral tests and alleviation effects on oxidative as well as inflammatory stress in the scopolamine-induced AD mouse model. The 16S rRNA results illustrated that PAYCS-L altered the ratio of Bacteroidetes/Firmicutes and PAYCS treatment elevated the relative abundance of Cacteroidaceae and Prevotellaceae. Notably, administration of PAYCS significantly upregulated memory-related metabolites and neurotransmitters. Overall, PAYCS-L reversed memory deficits of amnesiac mice partially via the modulation of gut microbiota-metabolites-brain neurotransmitter axis. For PAYCS-H, functions might be involved in the reversal of oxidative and inflammatory impairments in the liver and serum, which was also associated with the changed intestinal microbiota and fecal metabolites.

Understanding the mechanism underlying the anti-diabetic effect of dietary component: a focus on gut microbiota

Diabetes has become one of the biggest non-communicable diseases and threatens human health worldwide. The management of diabetes is a complex and multifaceted process including drug therapy and lifestyle interventions. Dietary components are essential for both diabetes management and health and survival of trillions of the gut microbiota (GM). Herein, we will discuss the relationship between diets and GM, the mechanism linking diabetes and gut dysbiosis, and the effects of dietary components (nutrients, phytochemicals, probiotics, food additives, etc.) on diabetes from the perspective of modulating GM. The GM of diabetic patients differs from that of health individuals and GM disorder contributes to the onset and maintenance of diabetes. Studies in humans and animal models consolidate that dietary component is a key regulator of diabetes and increasing evidence suggests that the alteration of GM plays a salient role in dietary interventions for diabetes. Given that diabetes is a major public health issue, especially that diabetes is linked with a high risk of mortality from COVID-19, this review provides compelling evidence for that targeting GM by dietary components is a promising strategy, and offers new insights into potential preventive or therapeutic approaches (dietary and pharmacological intervention) for the clinical management of diabetes.

Products of Sericulture and Their Hypoglycemic Action Evaluated by Using the Silkworm, Bombyx mori (Lepidoptera: Bombycidae), as a Model

Simple Summary

The use of invertebrates as animal models is gaining attention within the scientific community due to numerous advantages during the development of the experiments, low cost of rearing, and fewer ethical problems. The well-documented biology of the silkworm (Bombyx mori) makes this insect an ideal candidate to be used in different fields of research. In this study, we demonstrated the feasibility of using the silkworm to evaluate the hypoglycemic action of various products of sericulture included in the diet after promoting glucose or sucrose-induced hyperglycemia in silkworms. The postprandial antihyperglycemic activity of fibroin, sericin, and powder made from pupae of silkworms is confirmed. These natural products are therefore ideal candidates for the prevention and treatment of diabetes, obesity, and other lifestyle-related diseases.

Abstract

Sericulture generates different natural products with potential medical applications. Silk peptides, worms, or even pupae are commonly employed in traditional Asian medicine with a wide variety of purposes, and some scientific work has been focused on their antidiabetic properties. This work evaluates the postprandial antihyperglycemic activity of fibroin, sericin, and powder made from either larvae or pupae of silkworms, and Bombyx mori L. (Lepidoptera: Bombycidae), employing the silkworm itself as an animal model. The results indicate a reduction in the glucose levels in hemolymph after sucrose or glucose-induced hyperglycemia when these products are included in the diet of the worms.

Long-term silk peptide intake promotes skeletal muscle mass, reduces inflammation, and modulates gut microbiota in middle-aged female rats

Aging alters body composition to induce sarcopenia, particularly in women, but the mechanism remains unclear. We hypothesized that silk peptide(SP) intake could prevent an age-related decrease in muscle mass and strength in middle-aged female rats and explored the action mechanism. After the acute intake of SP and defatted soybean peptides, serum concentrations of amino acids were measured in ten middle-aged rats in each group. Forty 12-month-old female Sprague-Dawley rats were fed a high-fat and high-carbohydrate diet for 12 weeks including 0.5 g casein/kg body weight(BW)/day(Aged), 0.15 g SP plus 0.35 g casein/kg BW/day(Low-SP), 0.5 g SP/kg BW/day(High-SP), or 40 mg metformin plus 0.5 g casein/kg BW/day(Metformin). Ten rats aged 7-week old(Young) had the same treatment as the Aged-group. The body composition, grip strength, glucose metabolism, intestinal tissue morphology, and gut microbiota were also determined. After an acute consumption, total amino acids were more quickly absorbed and maintained at higher levels in SP than soybean peptides. Lean body mass(LBM) and grip strength were lower in the Aged-group than the Young and Low-SP groups, and the High-SP regimen increased these parameters as much as the Young-group. Serum concentrations and mRNA expression of TNF-α in the gastrocnemius and quadriceps muscles were higher in the Aged-group than the Young-group, whereas SP intake reduced their serum levels and skeletal muscles. Glucose and insulin tolerance indicated that insulin resistance was elevated in the Aged-group compared to the Young-group, while Low-SP and High-SP alleviated them as much as the Young-group. High-SP increased serum propionate and butyrate concentrations compared to the Aged-group. SP intake increased the relative abundance of Bacteroides and Prevotella and decreased Blautia and Clostridium in the feces. In conclusion, SP intake protects against a decrease in lean body mass and grip strength in middle-aged female rats. The protection was partly related to maintaining higher serum concentrations of total amino acids after SP consumption and decreasing inflammation and insulin resistance through gut microbiota modulation.

Ketogenic diet alleviates colitis by reduction of colonic group 3 innate lymphoid cells through altering gut microbiome

Accumulating evidence suggests that ketogenic diets (KDs) mediate the rise of circulating ketone bodies and exert a potential anti-inflammatory effect; however, the consequences of this unique diet on colitis remain unknown. We performed a series of systematic studies using a dextran sulfate sodium (DSS) animal model of inflammatory colitis. Animals were fed with a KD, low-carbohydrate diet (LCD), or normal diet (ND). Germ-free mice were utilized in validation experiments. Colon tissues were analyzed by transcriptome sequencing, RT2 profiler PCR array, histopathology, and immunofluorescence. Serum samples were analyzed by metabolic assay kit. Fecal samples were analyzed by 16S rRNA gene sequencing, liquid chromatography–mass spectrometry and gas chromatography–mass spectrometry. We observed that KD alleviated colitis by altering the gut microbiota and metabolites in a manner distinct from LCD. Quantitative diet experiments confirmed the unique impact of KD relative to LCD with a reproducible increase in Akkermansia, whereas the opposite was observed for Escherichia/Shigella. After colitis induction, the KD protected intestinal barrier function, and reduced the production of RORγt⁺CD3⁻ group 3 innate lymphoid cells (ILC3s) and related inflammatory cytokines (IL-17α, IL-18, IL-22, Ccl4). Finally, fecal microbiota transplantation into germ-free mice revealed that the KD- mediated colitis inhibition and ILC3 regulation were dependent on the modification of gut microbiota. Taken together, our study presents a global view of microbiome-metabolomics changes that occur during KD colitis treatment, and identifies the regulation of gut microbiome and ILC3s as novel targets involving in IBD dietary therapy.

Hepatic Steatosis Alleviated in Diabetic Mice upon Dietary Exposure to Fibroin via Transgenic Rice: Potential STAMP2 Involvement in Hepatocytes

Many benefits of silk protein fibroin (SPF) have been suggested in biomedical applications; and notably, significant SPF effects have been observed for metabolic syndromes that are directly linked to insulin resistance, such as type 2 diabetes mellitus (T2DM). Based on our previous findings, we believe that SPF from spiders exhibits outstanding glucose-lowering effects in diabetic BKS.Cg-m+/+Lepr^db mice. In order to evaluate the dietary effects of SPF in diabetic animals, we generated several lines of transgenic rice (TR) that expresses SPF, and the feeding of TR-SPF to diabetic animals decreased blood glucose levels, but did not change insulin levels. Western blot analyses of hepatic proteins showed that AMP-activated protein kinase (AMPK) expression and phosphorylation both decreased in TR-SPF-fed groups, compared with controls. This finding suggests that the glucose-lowering effects in this diabetic animal model might be AMPK-independent. In contrast, six-transmembrane protein of prostate 2 (STAMP2) was upregulated after TR-SPF exposure. Together with STAMP2, the Akt protein phosphorylation increased after TR-SPF exposure, which indicates that STAMP2 leads to Akt phosphorylation and thus increases insulin sensitivity in hepatocytes. Importantly, the hepatic steatosis that was seen in the liver of diabetic mice was remarkably alleviated in TR-SPF-fed mice. Hepatocytes that were immunopositive for STAMP2 were overwhelmingly observed in hepatic tissues from TR-SPF-fed mice compared to the control. Taken together, these results suggest that feeding diabetic mice with TR-SPF upregulates STAMP2 expression and increases Akt phosphorylation in hepatic tissues and thus potentially alleviates insulin resistance and hepatic steatosis.

Silk Fibroin Promotes the Regeneration of Pancreatic β-Cells in the C57BL/KsJ-Leprdb/db Mouse

Diabetes mellitus is a chronic metabolic disease, and its progression leads to serious complications. Although various novel therapeutic approaches for diabetes mellitus have developed in the last three decades, its prevalence has been rising more rapidly worldwide. Silk-related materials have been used as anti-diabetic remedies in Oriental medicine and many studies have shown the effects of silk fibroin (SF) in both in vitro and in vivo models. In our previous works, we reported that hydrolyzed SF improved the survival of HIT-T15 cells under high glucose conditions and ameliorated diabetic dyslipidemia in a mouse model. However, we could not provide a precise molecular mechanism. To further evaluate the functions of hydrolyzed SF on the pancreatic β-cell, we investigated the effects of hydrolyzed SF on the pancreatic β-cell proliferation and regeneration in the mouse model. Hydrolyzed SF induced the expression of the proliferating cell nuclear antigen (PCNA) and reduced the apoptotic cell population in the pancreatic islets. Hydrolyzed SF treatment not only induced the expression of transcription factors involved in the pancreatic β-cell regeneration in RT-PCR results but also increased neurogenin3 and Neuro D protein levels in the pancreas of those in the group treated with hydrolyzed SF. In line with this, hydrolyzed SF treatment generated insulin mRNA expressing small cell colonies in the pancreas. Therefore, our results suggest that the administration of hydrolyzed SF increases the pancreatic β-cell proliferation and regeneration in C57BL/KsJ-Leprdb/db mice.

Effect of Dietary Silk Peptide on Obesity, Hyperglycemia, and Skeletal Muscle Regeneration in High-Fat Diet-Fed Mice

Obesity is associated with excess body fat accumulation that can cause hyperglycemia and reduce skeletal muscle function and strength, which characterize the development of sarcopenic obesity. In this study, we aimed to determine the mechanism whereby acid-hydrolyzed silk peptide (SP) prevents high-fat diet (HFD)-induced obesity and whether it regulates glucose uptake and muscle differentiation using in vivo and in vitro approaches. Our findings demonstrate that SP inhibits body mass gain and the expression of adipogenic transcription factors in visceral adipose tissue (VAT). SP also had an anti-diabetic effect in VAT and skeletal muscle because it upregulated glucose transporter type 4 (GLUT4) and uncoupling protein 3 (UCP3) expression. Furthermore, SP reduced ubiquitin proteasome and promoted myoblast determination protein 1 (MyoD)/myogenic factor 4 (myogenin) expression, implying that it may have potential for the treatment of obesity-induced hyperglycemia and obesity-associated sarcopenia.